2.1.

Bifacial Silicon PV

Section Author: Fatima Toor (University of Iowa)

In recent years, bifacial silicon (Si) photovoltaics (PV) technology has been growing in commercial PV systems because it results in higher performance yield and lower levelized cost of energy (LCOE) compared with conventional monofacial PV technology.⁴ Figure 1 illustrates the difference between monofacial and bifacial solar cell architectures. While traditional monofacial Si cells have an aluminum (Al)-based back surface field (BSF) that covers the entire backside of the cell precluding any light absorption on the backside, bifacial Si cells have point contacts that allow for backside light collection and absorption in the cells.

Fig. 1

Schematic of (a) monofacial and (b) bifacial solar cells.

The latest International Technology Roadmap for Photovoltaic (ITRPV) report⁵ suggests that Al BSF cell concept is expected to be phased out within 2023. The report predicts that mature concepts of diffused and passivated pn junction Si solar cells will be further used in the mainstream commercial PV panels with different rear side passivation technologies, such as passivated emitter and rear cell (PERC),⁶ passivated emitter, rear locally-diffused (PERL),⁷ passivated emitter, rear totally diffused (PERT),⁸ and tunnel oxide passivated contact (TOPCon).⁹ The ITRPV report suggests that in 2022 the market share of PERC p-type mono-Si was 80%, however the share of p-type mono-Si PERC will decrease to about 10% within the next ten years since upcoming, promising new cell technologies are using n-type material. TOPCon on n-material, using tunnel oxide passivation stacks at the rear side, will gain market share from about 10% in 2022 up to 60% within the next ten years. Based on the report’s analysis, TOPCon on n-type is expected to become the dominant cell concept after 2025.

All these cell architectures (PERC/PERL/PERT/TOPCon) support bifacial PV cell and module architecture. Therefore, not surprisingly, given the trends in advanced Si solar cell architectures, the ITRPV report predicts that by 2033, bifacial cells will make up 90% of the Si PV installations. Another notable trend presented in the ITRPV report is that, while in 2023, about 65% of modules are monofacial modules, the share of bifacial modules will grow to about 70% within the next years as shown in Fig. 2. This is because bifacial Si solar cells can be used in bifacial modules as well as in conventional, monofacial modules.

Fig. 2

ITRPV 2022-2033 predicted trends for monofacial and bifacial modules with bifacial Si solar cells.

The bifacial PV project supported by the US Department of Energy (DOE)¹⁰ performed and published extensive optical modeling based on RADIANCE¹¹ to determine the performance gains of bifacial modules installed at various tilt angles, with different row spacing, and ground albedo.^12–20 The team showed the effect of installation parameters on bifacial gain in energy (BGE) defined as: $\mathrm{BGE}=({\mathrm{E}}_{\mathrm{b}}/{\mathrm{E}}_{\mathrm{m}})-1$, where ${\mathrm{E}}_{\mathrm{b}}$ and ${\mathrm{E}}_{\mathrm{m}}$ are the energy yield of the bifacial and monofacial module, respectively. This value shows the energy gain in using bifacial PV system over the equivalent monofacial system (with the same installation parameters). The study’s results indicate that bifacial PV arrays generate lower energy than expected due to horizon blocking and large shadowing area cast by the modules on the ground. For albedo of 21%, the center module in a large array generates up to 7% less energy than a single bifacial module – this single bifacial module is typically utilized for lab certification of BGE of bifacial modules. The team also proposed draft recommendations for IEC 60904 to include bifacial PV module testing so that bifacial PV gain measurements can be standardized.²¹ The standard has since been formalized and adopted by the international community as IEC TS 60904-1-2:2019. As of 2023, the standard has gone through a further revision with added modifications for dual-simulator measurements to be re-published by the end of the year.

2.2.

Black Silicon

Section Authors: Ville Vähänissi (Aalto University) and Hele Savin (Aalto University)

Nanostructured silicon, often called black silicon due to its dark appearance, eliminates surface reflectance over a wide wavelength range [Fig. 3(a)] due to gradual change in refractive index^23–25 and increases the absorption of long wavelengths due to extended optical path.^26–29 Therefore, black silicon is a promising alternative to minimize optical losses in PV applications. However, due to the rough nanoscale morphology, electrical passivation of such surfaces has been considered challenging. In 2012 this problem was tackled with atomic layer deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ that provides a conformal coating [Fig. 3(b)] combined with a high-quality interface with silicon.^30,31 Indeed, it was demonstrated that surface passivation was equally good in black silicon as in the planar counterparts.^32,33 The successful surface passivation of black silicon was also proven later in actual solar cells (IBC) resulting in an external quantum efficiency (EQE) close to unity in a broad wavelength range (300 nm-1000 nm) and a conversion efficiency well above 20%.³⁴

Fig. 3

(a) Reflectance of black silicon and acidic-textured silicon with ${\mathrm{SiN}}_{\mathrm{x}}$ ARC as a function of wavelength.²² (b) Scanning electron microscope image of black silicon surface coated with a 50 nm atomic layer deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ surface passivation layer using Beneq TFS-500 system.

In PERC and TOPCON cells (or any front contact cell type) emitter formation brings an additional challenge as it needs to be made on the nanostructured surface. The dopant profile is difficult to control via a conventional diffusion process. Consequently, dopant diffusion on enhanced surface areas often results in too heavy Auger and SRH recombination inside the nanostructures.^35–42 This problem has recently been overcome via a more controlled doping method, i.e. using ion-implantation, which has resulted in emitter saturation currents less than $20\mathrm{fA}/{\mathrm{cm}}^2$ ⁴³ combined with EQEs even above 100% in UV.⁴⁴

While the original motivation to use black silicon in PV has been the improved optics, other benefits have been discovered later on. Firstly, the nanotexturing does not seem to have any substrate crystallinity related limitations,³³ e.g., it works well also for the diamond-wire cut multicrystalline wafers that are difficult to texturize with conventional acidic solutions. This benefit is valid for all commonly known black silicon fabrication methods, i.e., dry etching,^45–47 metal assisted chemical etching,^48–55 and laser processing.^56–61 Another benefit is related to metal impurity gettering, i.e., black silicon has been shown to have a higher gettering efficiency for iron as compared to planar surfaces.⁶² The same phenomenon is expected to take place for other mobile impurities as well. Quite surprisingly, black Si can also reduce the harmful light-induced degradation although the exact mechanism is still under debate.^63,64

While the aforementioned fundamental limitations regarding integration of black silicon into solar cells have been overcome in the laboratory environment, industrial viability raises other challenges. These include possibly harsh handling of the nanostructured surfaces in the production line, potential contact resistance issues when using nonconformal screen printing, as well as preserving excellent optics during module encapsulation. There are preliminary results showing that all these issues can be tackled efficiently in industrial production (Fig. 4).²² However, more extensive cost and lifecycle analysis would be required for black silicon to become the mainstream technology in PV. Fortunately, there are pioneers in the field who are already mass-producing black silicon solar cells. At the moment, metal assisted chemical etching seems to be the most viable fabrication method for the commercial cells as it requires fewer changes to the existing manufacturing lines.

Fig. 4

Dry-etched black silicon IBC solar panel without any antireflection coating fabricated in Naps Solar Systems, Finland. The energy conversion efficiency of 18.1% and 19.9% of the whole IBC module and the best individual cell of the module, respectively, demonstrate that fragile nanostructures can withstand standard module fabrication steps.⁶⁵

3.1.

Cu(In,Ga)Se₂ PV: Fundamentals and Efficiency Limits

Section Author: Lorelle Mansfield (National Renewable Energy Laboratory and RASEI)

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) solar cells have the highest record efficiency among commercialized thin-film PV technologies at 23.6%.⁶⁶ CIGS has the benefits of thin films,⁶⁷ such as low materials costs, low embodied carbon, and varied form factors. The substrate design of most cells (Fig. 5) allows fabrication on flexible plastics and metal foils, which leads to a multitude of potential applications. It is suitable for architectural solar, military deployment, space power, and consumer applications. Flexible substrates also allow for roll-to-roll fabrication enabling rapid manufacturing of high-efficiency devices.

Fig. 5

Cross-section of a typical CIGS-based device.

CIGS technology has experienced several technological advances in recent history. One of the most impactful has been post-deposition treatments (PDT) with alkali metal fluorides, such as potassium fluoride (KF), rubidium fluoride (RbF), and cesium fluoride (CsF) which have significantly improved open-circuit voltage (${\mathrm{V}}_{\mathrm{OC}}$) and efficiency of devices, even those fabricated at lower temperatures. Lower temperature processing can be further enhanced through alloying CIGS with small amounts of silver (Ag), which also results in smoother surfaces, improved crystallinity, and increased carrier concentrations. Traditionally, chemical-bath deposited cadmium sulfide (CdS) was used as a buffer layer, whereas now zinc oxysulfide (Zn(O,S)) and other more transparent and more manufacturable (e.g. sputtered) layers are being employed. Research directions being explored include CIGS for tandem solar cells. Bandgap variations are key to efficient tandems, and the CIGS-based material system can be tuned from 1 eV to around 2.5 eV by alloying in the form of ${(\mathrm{Ag},\mathrm{Cu})(\mathrm{In},\mathrm{Ga})(\mathrm{Se},\mathrm{S})}_2$ (Fig. 6). Existing high-efficiency low-bandgap CIGS cells are excellent bottom cells for perovskite and CdTe top cells, and improved wide-bandgap CIGS top cells could even be paired with Si bottom cells.

Fig. 6

$(\mathrm{Ag},\mathrm{Cu})(\mathrm{In},\mathrm{Ga}){(\mathrm{Se},\mathrm{S})}_2$ material system with bandgaps of ternary compounds.

Although CIGS-based PV are efficient and stable, several challenges have delayed their ubiquitous adoption. As could be expected, initial CIGS installations were not as robust as subsequent generations, which led to an incorrect assumption that CIGS products were inherently inferior solar cells. Unfortunately, some potential investors still hold those views, and CIGS research and development funding has decreased compared to other thin-film technologies, particularly in the US. To date, CIGS technology has suffered from a greater gap in efficiency between record cells (23.6%) and record modules (19.2%) than other commercialized PV technologies. Closing this gap could be accomplished through using wider bandgap absorbers, designing new ways to assemble cells into modules, and developing better transparent conductive contacts.⁶⁸ Unlike silicon technology, CIGS has yet to experience the full benefit of economies of scale. With the complexity of processing required to fabricate the absorber material, most companies have proprietary manufacturing equipment. That relegates the technology to smaller markets where premium prices are accepted for products with specific requirements such as flexibility and high specific power. Lower-cost processing methods and equipment designs have the potential to revolutionize the industry and bring CIGS to multi-GW scale.

3.2.

CdTe PV: Fundamentals and Efficiency Limits

Section Author: Matthew O. Reese (National Renewable Energy Laboratory)

CdTe is the most scaled thin-film PV technology. This is enabled by low material costs, high-throughput deposition, robust process space, low degradation, and low embodied energy. While CdTe is directly competing at utility-scale with Si, it has done so with relatively low demonstrated cell record efficiency, having only achieved $\sim 2/3$ of its detailed balance limit. The current collection has been well-engineered, such that it is near the theoretical limit. Further monofacial improvements will largely stem from advances enabling improved photovoltage.

CdTe devices have eliminated the parasitically-absorbing CdS emitter with a deleterious “cliff” in its band conduction band offset (CBO), replacing it with a wider bandgap oxide, such as the tunable ${\mathrm{Mg}}_{\mathrm{x}}{\mathrm{Zn}}_{1-\mathrm{x}}\mathrm{O}$ enabling a small “spike” in CBO with reduced recombination.⁶⁹ Furthermore, the absorber transitioned from the congruently-sublimating, binary CdTe absorber to a graded ternary Cd(Se,Te). This has changed the effective device bandgap from 1.5 eV in pure CdTe (which is present now at the back) to $\sim 1.4\mathrm{eV}$ at the front. This increases the theoretical and effective current density in devices [Fig. 7(a)], while raising the overall theoretical efficiency limit by $\sim 1\%$ absolute. These changes have dramatically improved achievable minority carrier lifetimes from a few ns to $>100\mathrm{ns}$.^71,72 Due to this increase in lifetime, device photovoltage has seen a modest increase even though the bandgap has decreased $\sim 100\mathrm{mV}$ [Fig. 7(b)].

Fig. 7

(a) CdTe PV certified record cell current densities and (b) open-circuit voltages, relative to the detailed balance limit for the bandgap of CdTe (1.5 eV) and a bandgap of 1.4 eV, which is typical of Se-alloyed material. Data from the semi-annual publication up to Version 61 of “Solar cell efficiency tables” (Ref. 70).

While CdTe has demonstrated scalability and low cost, it must demonstrate continued efficiency improvement. Over the past several years, work has revolved around changing the defect (doping) chemistry to achieve this. Initial demonstrations with single crystals demonstrated an improvement in stable carrier concentration of $\sim 100\mathrm{x}$ using a group V (GrV) dopant to replace traditional Cu to explicitly improve ${\mathrm{V}}_{\mathrm{OC}}$.⁷³ While GrV dopants have improved stability over Cu due to much lower diffusion and a reduction in complexing with Cl,^74,75 much improved ${\mathrm{V}}_{\mathrm{OC}}$ and efficiency over Cu-doped polycrystalline thin-film devices have yet to be realized. This is for a few reasons. First, the large increase in absorber carrier concentration collapses the space charge region, making devices more sensitive to front interface recombination.⁷⁶ Second, long lifetime from Se incorporation makes devices more sensitive to back interface recombination. Third, GrV-doped polycrystalline absorbers, while capable of high carrier concentration, have low activation, leading to significant losses from voltage fluctuations.⁷⁷ Increasing ${\mathrm{V}}_{\mathrm{OC}}$ requires simultaneous improvements on multiple fronts. However, in less than eight years, insight from single crystals is influencing GW-scale decisions. GrV-doped CdTe PV promises increased energy yield due to its increased stability over its Cu-doped predecessor, with demonstrated similar initial efficiencies that are poised to increase over the next few years as these challenges are overcome.

3.3.

CdTe PV: Commercial Perspective

Section Authors: Ella Wassweiler (First Solar) and Gang Xiong (First Solar)

CdTe is the most commercially successful thin film PV technology. In 2022, CdTe module production capacity has exceeded 9 GW annually and is expected to reach 21 GW by 2025. At the start of 2023, First Solar, the largest CdTe solar module manufacturer, reached a milestone of 50GW accumulated production. Thin film CdTe manufacturing has an inherently better cost structure compared to Si. In addition, CdTe has an energy yield advantage due to its lower temperature coefficient, better spectra response, and slower long term degradation rate. These advantages allow CdTe to compete with Si in utility-scale markets. Roughly 40% of the US utility-scale PV market was served by CdTe in 2019.

Currently the record CdTe solar cell efficiency is 22.3%.⁷⁸ Module efficiency has exceeded 19%. The state-of-art CdTe solar cell structure is outlined in Fig. 8. The absorber typically consists of a graded ${\mathrm{CdSe}}_{\mathrm{x}}{\mathrm{Te}}_{1-\mathrm{x}}$ alloy. A buffer layer such as ${\mathrm{Zn}}_{1-\mathrm{x}}{\mathrm{Mg}}_{\mathrm{x}}\mathrm{O}$ can be inserted between the transparent conducting oxide (TCO) and absorber. With a small positive conduction band offset between ${\mathrm{Zn}}_{1-\mathrm{x}}{\mathrm{Mg}}_{\mathrm{x}}\mathrm{O}$ and absorber, carrier recombination at the front interface can be suppressed. ZnTe is a good back contact to CdTe with relatively low barrier height ($\sim 0.2\mathrm{eV}$).⁷⁹

Fig. 8

State-of-art CdTe solar cell structure.

The absorber is typically p-type, traditionally doped by Cu with carrier concentration between ${10}^{14}$ and ${10}^{15}{\mathrm{cm}}^{-3}$. Increasing carrier concentration is a major pathway to further boost CdTe solar cell efficiency.

Carrier concentrations greater than ${10}^{16}{\mathrm{cm}}^{-3}$ have been demonstrated with group-V dopants such as As or P. For example, 22% As-doped CdTe devices have been reported.⁸⁰ Compared to Cu-doped devices, As doping has additional energy yield advantages including even lower long-term degradation rate and lower temperature coefficients.⁸¹ The paradigm shift from Cu to As-doping, provided a new platform and opportunities to further improve CdTe solar cell performance. In order to do so, it is suggested that the reduction of non-radiative recombination in the device stack, as well as reduction of potential fluctuation in the absorber are of critical importance.^77,80

First Solar’s Series 7 module is $2.8{\mathrm{m}}^2$ in size and has total area efficiency of up to 19.3%. CdTe modules are fabricated using a fully integrated inline process, with glass-in and module-out within 4 hours. Compared to Si, CdTe solar module has lower energy payback time and lower environmental footprint.

Improving module efficiency is the most effective way to reduce the cost per watt for manufacturing and levelized cost of electricity of a PV system. Bifacial CdTe solar cells have also been demonstrated.⁸² There is great synergy between cell efficiency and bifaciality improvement, as both require further reduction of back contact recombination loss. With less back contact recombination, absorber thickness can also be effectively reduced.

4.1.

III-V Tandem PV

Section Authors: Meghan N. Beattie (University of Ottawa) and Karin Hinzer (University of Ottawa)

Bandgap engineering of III-V materials and vertical integration of multiple subcells in tandem devices has resulted in the highest efficiency PV. III-V semiconductors have high absorption, and many compositions have direct bandgaps. Multiple pn junction subcells absorb a larger portion of the solar spectrum and reduce thermalization losses compared to single junction devices. The higher energy photons from the solar spectrum are absorbed by the largest bandgap subcells at the top of the tandem stack, while the lower energy photons are transmitted through the top subcells to be absorbed by the smaller bandgap subcells below. Figure 9 displays common approaches.

Fig. 9

Schematic showing four different design architectures for III-V tandem solar cells. ARC, antireflection coating; MM, metamorphic.

The standard approach uses lattice-matched materials for the entire device, minimizing dislocations in the crystal lattice. Lattice-matched subcells are interconnected using transparent tunnel diodes [Fig. 9(a)]. In this series-connected configuration, the device current is limited by the smallest subcell current. The current generated in each subcell is maximized by tuning layer thicknesses and bandgaps so that each subcell absorbs the same amount of light. The lattice-matching requirement leads to the use of a variety of materials from Ge for the bottom subcell to ternary and quaternary III-V alloys or nanostructures^83,84 for upper layers.

With limited semiconductor substrates to choose from, the lattice-matched approach has a limited material design space, which caps the efficiency. Several advanced strategies have been developed to overcome the lattice-matching constraint, enabling the development of all-III-V and III-V hybrid tandems. One such approach is wafer bonding, wherein subcells are grown on different substrates and directly bonded without an intermediate layer^85–87 [Fig. 9(b)]. A four-junction GaInP/GaInAs//GaInAsP/GaInAs wafer bonded device demonstrated the highest recorded solar cell efficiency to date: 47.6% under an irradiance of 665 suns.^70,86

Another approach uses a graded metamorphic buffer to increment the lattice constant from that of the substrate to that of the subcell material^84,88–90 [Fig. 9(c)]. Excellent crystal quality can be achieved for III-V semiconductors grown on metamorphic buffer layers. An example is this six-junction AlGaInP/AlGaAs/GaAs/GaInAs(3) inverted metamorphic cell with a peak efficiency of 47.1% under 143 suns.^89,91

A third technique to overcome lattice-matching involves mechanical stacking of fabricated subcells grown on different substrates using a transparent adhesive to join the upper and lower subcells^92–94 [Fig. 9(d)]. This technique allows for electrical configurations with two, three, or four terminals, overcoming current-matching constraints. This approach was used to fabricate a four terminal, three-junction mechanically stacked GaInP/GaAs//Si device having 35.9% efficiency under 1-sun illumination.^94,95

These architectures can be combined with subcell segmentation to further enhance efficiencies.⁹⁶ This design technique enables near-perfect current-matching using non-ideal absorber bandgaps by dividing each subcell into semitransparent pn junction segments. It is used in vertically segmented III-V photonic power converters (PPCs) that convert monochromatic laser light to electricity. Multijunction III-V PPCs with up to 30 absorbing segments have been demonstrated with efficiencies up to 66%.^97–100

Due to their highest efficiencies, long-lifetimes, radiation hardness, and low resistances, III-V multijunction PV power satellites, unmanned vehicles and concentrated solar systems. Table 1 lists selected III-V tandem cells of note with corresponding efficiencies from Green et al.⁷⁰

Table 1

Selected III-V tandem cells and efficiencies from solar cell efficiency tables (version 63).70

4.2.

III-V/Si Tandem Junction Solar Cells

Section Authors: Stephanie Essig (ipv, University of Stuttgart) and Emily L. Warren (National Renewable Energy Laboratory)

Research and development of III-V/Si tandem solar cells is motivated by the efficiency enhancement achievable with multijunction architectures and the low cost and current market dominance of Si. Theoretically, the detailed balance efficiency of Si-based two (2J) and three-junction (3J) solar cells reaches 45% and almost 50%, respectively.¹⁰⁷ From a detailed balance perspective, the ideal III-V top cell material for a Si-based 2J device is GaInP with a bandgap of 1.8 eV and record 1-sun single-junction efficiency of 22.0%.⁷⁰ Fabrication of III-V solar cells is based on epitaxy, yet the direct growth of III-Vs with suitable bandgap energies on Si is challenging due to a mismatch in lattice constant and thermal expansion.¹⁰⁸ Therefore, high threading dislocation densities occur, causing significant non-radiative recombination losses in the top cell, which limit the tandem cell performance.⁹⁰ To overcome this difficulty, alternative manufacturing methods, including mechanical stacking^109–112 and wafer-bonding,^85,113,114 of the III-V and Si cells, have been investigated.

The efficiency of monolithic III-V/Si dual-junction solar cells fabricated by metal organic vapor phase epitaxy (MOCVD) or molecular beam epitaxy (MBE) on Si has so far been limited to 25.0%.¹¹⁵ This record device comprises a 1.7 eV wide-bandgap GaAsP top cell on a graded GaAsP buffer, that effectively reduces the dislocation density. In comparison, 3J cells realized by direct epitaxy on Si have reached higher efficiencies up to 25.9%,¹⁰¹ but have not surpassed the record values for Si single-junction devices. Other possible ways to reduce the dislocation density and cost of polished surfaces include use of nanopatterned substrates, and dislocation filtering approaches such as strained superlattices and thermal cycle annealing.^116–118

In contrast, mechanically stacked and wafer-bonded III-V/Si tandems push the efficiency of Si-based PV beyond 30%. Their few-μm thick III-V top cells are grown lattice-matched on GaAs parent wafers and transferred to independently fabricated Si bottom cells. Hence, both the Si and III-V cells are manufactured using the optimum processes. Subsequent integration by wafer-bonding requires extremely clean, mirror-polished surfaces to allow atomic bond formation across the heterogeneous semiconductor interface.¹¹⁹ Improved current-matching of the subcells has recently raised the record efficiency of wafer-bonded GaInP/GaInAsP/Si 3J solar cells to 35.9%.⁸⁵ The comparably easier integration by mechanical stacking involves typically an adhesive or glue between the III-V and Si cell that can level out interface irregularities. When applying front and rear contacts on each subcell, the tandem device can also be operated in 4-Terminal (4T) configuration. Compared to the commonly used 2-Terminal configuration, 4T offer the advantage that top and bottom cells are operated independently. Hence no current-matching is needed, a wider range of top cell bandgap energies can be used, and the energy output depends less on the temperature and solar spectrum.¹²⁰ Mechanically stacked 4T devices reached record efficiencies of 32.8% for 2J (GaAs/Si; see Fig. 10) and 35.9% for 3J (GaInP/GaAs/Si) using both-sides contacted Si heterojuction cells.⁹⁴ Similar 3J efficiency of 35.4% was achieved when using a rear-contacted Si bottom cell.¹²¹ Devices have also been stacked using transparent conductive adhesives or metal nanoparticles, often with the goal of forming 3T tandems, and these devices have reached efficiencies of 27.3%⁷ (2J,TCA) and 30.8%¹²² (3J, Pd nanoparticles).

Fig. 10

External Quantum Efficiency and sketch of a 4T GaAs/Si tandem solar cell with 32.8% efficiency.⁹⁴

Despite the high efficiencies and good scalability of III-V/Si multijunction solar cells, they will only be successful if their costs on a US$/Watt level will approach those of commercial Si single-junction technology.⁹² Main cost drivers are the MOCVD deposition of the III-V top cell, and the III-V growth substrate.¹²³ Therefore, extensive research on cost reduction strategies like the use of hydride vapor phase epitaxy (HVPE)¹²³ instead of MOCVD and advanced lift-off¹²⁴ and substrate reuse¹²⁵ approaches are needed.¹²⁶

5.1.

Stability of Perovskite Solar Panels with Partial Shading

Section Authors: Isaac Gould (University of Colorado Boulder) and Michael D. McGehee (University of Colorado Boulder and RASEI)

In a solar panel, partial shading prevents only some of the cells from generating photocurrent. Since all cells must pass the same amount of current for the panel to generate power and the cells are in series with each other, the cells that are illuminated put the shaded cells into whatever reverse bias is needed for current matching to occur. Several research groups have tested perovskite cells in reverse bias and observed both short-term reversible and irreversible degradation problems.^127–131

Several interesting observations have been made on the reverse bias behavior of perovskite solar cells. When metal electrodes are used the current typically increases very suddenly and localized heating is observed with thermal imaging.^127,132 This behavior is attributed to the formation of a shunt caused by the metal penetrating through the perovskite. The extreme heating at the shunt causes permanent damage to most layers of the cell. In contrast, this behavior is not seen when transparent conducting oxides and carbon electrodes are used. In these cells there is an exponential increase in current that starts typically between -2 V and -6 V when sweeping the voltage to increasing negative voltages.^127–131 This current is enabled by tunneling of holes from the electron transport layer due to strong band bending associated with mobile ions redistributing to screen the applied field. After passing current through a cell in reverse bias for as little as one minute, the efficiency can be reduced by more than 50 %.^129,133 Much, but not all, of this efficiency recovers if the cell is exposed to one sun at a positive voltage for approximately 30 minutes.^129,133,134 The rapid degradation most likely occurs because holes oxidize iodide on the octahedral corners to form smaller neutral iodine species that can move to interstitial sites, creating a vacancy in the process.^135,136 With few electrons being present in reverse bias, the reverse reaction is slow. The iodine interstitials act as recombination centers, which dramatically reduces the efficiency of the cell. During the recovery process at positive voltage, the iodine interstitials are reduced and return to the octahedral corners. The recovery only occurs, however, if the iodine species stay in the perovskite film.^129,137,138 For this reason, internal barriers that confine the iodine to the perovskite are critically important.

There has been some progress towards improving reverse bias stability by using carbon electrodes.

If strategies for preventing degradation in reverse bias are not found, then engineering solutions for preventing degradation due to partial shading will be needed. Wolf et al. have analyzed different panel designs and suggested that perovskite-silicon tandems can more readily be protected with bypass diodes because the high breakdown strength of silicon enables many tandems to be protected with one bypass diode.¹³⁹ It is not clear how the current could be routed from the electrodes in a monolithically integrated panel (made with scribes) to a bypass diode on the side. The voltage drop would be quite large if current had to travel the length of a panel through a transparent conducting oxide. In contrast, it is easy to run current from silicon or perovskite-silicon tandem cells with metal ribbon to bypass diodes, which are typically incorporated into the junction box.

It is likely that bypass diodes can be used to protect perovskite-silicon tandems. Research will be needed to find ways to protect other types of perovskite solar cells to prevent degradation due to partial shading.

5.2.

Perovskite/Silicon Tandem Solar Cells

Section Authors: Aditya Chaudhary (ipv, University of Stuttgart) and Stephanie Essig (ipv, University of Stuttgart)

Since crystalline silicon (c-Si) solar cells approach their theoretical efficiency limit, there is an increasing interest in c-Si-based tandem PV. Stacking a wider bandgap metal-halide perovskite (${\mathrm{ABX}}_3$) cell onto a c-Si cell offers the potential to reach conversion efficiency over 30%, and to reduce levelized costs of electricity.^140,141 There are three architectures (Fig. 11) in which a tandem cell can be realized, i.e., 2-terminal (2T), 3-terminal (3T) and 4-terminal (4T). All these architectures have their own advantages as well as limitations. Owing to the easy industrial realization and lower requirements on the balance of system, 2T architecture is touted to be the most promising.¹⁴²

Fig. 11

Comparison of perovskite/c-Si tandem solar cells with 2-,3- and 4-terminal configuration.

The first results of 2T perovskite/c-Si tandem cells, in which the top and bottom cells are connected in series, were published in 2015.¹⁴³ From then on research has focused on the optimum device architecture, stability improvement and the defect-free large-area deposition of the top perovskite cell on c-Si.^144–146

To reach efficiencies over 30% with 2T tandem cells for operation temperatures between 20 and 100°C, a perovskite top cell with bandgap energy in the range of 1.65-1.73 eV is needed,¹⁴⁷ which can be realized by varying the ${\mathrm{ABX}}_3$ composition.¹⁴⁸ Furthermore, maximum energy yield requires that the perovskite cell coats the textured c-Si bottom cell conformally^149,150 which poses a challenge for solution-based deposition methods of the perovskite layers. In recent time, strategies have been developed to circumvent this issue, such as reducing the c-Si cell pyramid size or using innovative surface textures, increasing the thickness of perovskite film or application of self-assembled monolayers (SAM).^151–153

Recently, record efficiencies of 33.7% and 33.9% were reported for 2T perovskite/c-Si tandem cells by a group from KAUST, Saudi Arabia, and LONGi, China, respectively.¹⁵⁴ For the 4T architecture, a record tandem cell efficiency of 30.3% was demonstrated in early 2023 by the group from The Australian National University.¹⁵⁵ In contrast to the monolithic 2T devices, the top and bottom cells of 4T tandems are manufactured separately and then mechanically stacked.¹⁵⁰ This decouples the cells electrically, and hence there is no need for current matching of the sub-cells. Efficiencies of 3T tandem cells are still below 20%, nevertheless this architecture has great potential as the third contact allows the excess photo current of a subcell to be used, which would be lost in a 2T configuration.^156,157

Current world records for perovskite/c-Si tandem cells have been achieved with small cell areas ($\le 4{\mathrm{cm}}^2$), almost a factor of 50 less than commercial c-Si solar cells. Upscaling to an area as large as c-Si cells without losses in efficiency is the foremost challenge. Current research suggests that the most scalable production techniques for the perovskite cells are CVD and solution-based methods, such as slot-die coating, blade coating, inkjet printing and screen printing.^158–160

Apart from upscaling, degradation of the perovskite top cells¹⁶¹ is considered the major obstacle to commercialization of perovskite/c-Si tandem cells. To be competitive with the established c-Si single-junction modules, stable operation for a minimum of 20-25 years is needed. Yet, a recent study by a group from KAUST showed, that employing 2D/3D perovskite heterojunctions significantly improves the damp heat-stability and allows to pass industrial test protocols for PV modules.¹⁶²

5.3.

Perovskite Silicon Tandem Cells and Modules

Section Authors: Daniel Kirk (Oxford Photovoltaics Ltd.) and Christopher Case (Oxford Photovoltaics Ltd.)

Mainstream solar cell technology (or technologies) has developed rapidly in terms of efficiency¹⁶³ concomitant with a huge increase in scale; global PV installations in 2022 were above 250GW and year-on-year growth is expected for decades to come. However, the giant strides taken in silicon technology development mean that it is now approaching its efficiency limit—perovskite-silicon tandem technology opens up huge new possibilities for higher efficiency devices and have become the subject of intense academic and industrial interest.

The majority of the high efficiency devices reported in the literature have been at small area¹⁴² with very few examples of commercially viable cell formats. Oxford PV has recently measured efficiency of $>28\%$ on full area tandem cells with $>250{\mathrm{cm}}^2$ area; the IV curve and details are shown in Fig. 12. This fundamentally proves there is no issue with transferring high performance from small area cells to large area, mass production. Moreover, the assembly of high efficiency cells into industrial scale modules has also been demonstrated, with module efficiency of $>23\%$ already shown.¹⁶⁴ Albeit there is still clearly work to do to increase the cell and module efficiency in line with small cell records, it is already true that perovskite-based tandem technology is class-leading in this area.

Fig. 12

Light-IV (solid line) and Dark-IV (dotted line) of $>28\%$ tandem cell. Image courtesy of Oxford PV.

As mentioned, more than 250GW of solar was installed in 2022—Oxford PV capacity for tandem cells is roughly 0.01% of this; the biggest challenge towards commercialization is to move towards production volumes where real economies of scale can be realised. The biggest barrier to this movement is procurement of tools capable suited for tandem cell application with very high throughput and short lead times. Concerted global effort on equipment provision will greatly accelerate progress towards multi-GW tandem production.

In the short term, one area of challenge is around measurement and certification of PVSK-Si tandem products. The technology for accurate measurement of full-size tandems exists but is not widely available. Work to define appropriate measurement systems and standards is active and has made much progress but adoption of steady-state lamps with advanced spectral control is a pre-requisite for classifying such modules ready for sale. Furthermore, such measurement is essential for certification of the modules as compliant with international standards. The most appropriate standards for tandem modules are the current silicon-oriented ones (IEC 61215 & IEC 61730) and conformity to these requirements is currently being tested, however there is a need for the community to scrutinize such standards and determine if further or different tests are required for this new technology. To ensure these standards are appropriate also requires a growing understanding of long-term performance of tandem devices in operation; deploying significant quantities of tandem modules in appropriate climatic zones is therefore of utmost importance.

6.1.

Nonfullerene Acceptors: Progress, Challenges, and Perspectives

Section Authors: Min Hun Jee (Korea University), Minyoung Lim (Korea University), and Han Young Woo (Korea University)

Over the past decade, significant progress has been achieved in organic photovoltaics (OPV), leading to the successful demonstration of power conversion efficiencies (PCEs) reaching nearly 20%,^165–167 which was benefitted from the development of the outstanding non-fullerene acceptor (NFA) of Y6 in 2019.¹⁶⁸ Numerous derivatives of Y6 and other NFAs have been reported, employing innovative strategies such as chlorination, conjugation-extension, and symmetry-breaking, etc.^169–172 Compared to fullerene acceptors, NFAs offer several distinct advantages, including facilely tunable optical and electronic structures, longer exciton diffusion length, smaller energy offsets for charge separation and reduced energetic disorder, leading to lower energy losses (Fig. 13). He et al.¹⁶⁶ reported an NFA of BTP-H2 to have a near-zero highest occupied molecular orbital (HOMO) offset with the polymer donor PM6, showing efficient hole transfer and small voltage loss to achieve PCEs of 18.5% for binary PM6:BTP-H2 and 19.2% for ternary PM6:BTP-H2:L8-BO blends. To address thermal, mechanical, and long-term stability for small molecular NFA-based OSCs, Yu et al. developed a vinylene-linker-based polymer NFA (PY-V-$\gamma$) featuring a coplanar and rigid molecular conformation with high mobility and reduced energetic disorder, achieving a 17.1% PCE with PM6 as a donor.¹⁷³ To combine the advantages of small molecular and polymerized NFA systems, Sun et al. reported a dimerized Y6-derived NFA with an electron-donating benzodithiophene linker (DYBO), successfully demonstrating both high efficiency ($\mathrm{PCE}>18\%$) and excellent stability (${\mathrm{t}}_{80\%}>\mathrm{6,000}$ hours).¹⁷⁴

Fig. 13

Design strategies and advantages of non-fullerene acceptors.

For real industrialization of OPVs, some remaining challenges need to be carefully considered. To realize large-area OPV devices using high-throughput roll-to-roll printing processes,¹⁷⁵ it is essential to develop organic materials that can be processed using non-spin coating methods, utilizing environmentally friendly and green solvents. In the case of semi-transparent OPVs, there is a need to improve both PV performance and average visible transmittance by incorporating near-infrared absorbing materials into the active layer.^{173,175–177} Additionally, the use of flexible linkers can be explored to enhance both the PV efficiency and mechanical properties of the stretchable devices.^178,179 Moreover, it is crucial to develop cost-effective synthesis for mass production of NFAs. It is also crucial to address the issue of stability in OPVs in order to achieve commercial viability for large-area applications. While encapsulation can help mitigate extrinsic degradation of OPVs, there is a need for a fundamental solution to address the problem of low stability resulting from intrinsic factors such as thermal degradation and photoreaction. Therefore, when developing new high-performance NFAs, it is imperative to conduct extensive research on the impact of their chemical structures on the long-term stability of OPV devices under various conditions, including thermal stress and exposure to light in ambient air. By addressing these challenges, significant progress can be made towards making OPV commercially feasible.

6.2.

Optical Incoupling in Thin Film Organic PV Devices

Section Authors: Zheng Tang (Donghua University) and Olle Inganäs (Linköping University)

Organic photovoltaic (OPV) materials have poor transport properties for charge carriers, compared to classical inorganic PV materials. Thin active layers (<< 1 micrometer) are therefore necessary for OPV devices. This leads to optical losses due to reflection and parasitic absorption. OPV devices are thus easy targets for light trapping and optical incoupling.^180–182 With the recent OPV materials of high performance, increasing exciton diffusion lengths¹⁸³ and cost, stability is improved with thinner absorbers¹⁸⁴; thus optical incoupling is also relevant here to enable extremely thin film devices.

Strategies for improving light incoupling in the thin OPV devices include both macro- and mesoscale geometry modifications of device architecture, as well as micro- and nanopatterning of materials and devices, using physical effects from microcavity optics in multilayer structures, diffraction or scattering of light, and modification of incoming light with plasmonics. As a transparent electrode is necessary for light incoupling, a considerable effort has gone into making transparent anodes (cathodes), with decreased power dissipation or decreased reflection in the electrode facing incoming light, but not covered in this overview.

6.2.1.

Mode structure of light in multilayer devices

Solutions to Maxwell’s equation in the one dimensional device model of a stack of materials with different dielectric functions are now a standard tool for simulation of devices.¹⁸⁵ This transfer matrix method was used also to shape the electromagnetic power dissipation into the optical absorber in the stack, and used to design optical cavity structures.¹⁸⁶ Thin organic layers were also used simultaneously for exciton blocking and optical cavities.¹⁸⁷

When the active layers are thinner than optimal, cavity optimization become beneficial.¹⁸⁸ A sizable absorption enhancement over a wide wavelength range was realized for PV devices by designing new cavities, with the enhancement more pronounced for the absorption tail;¹⁸⁹ see Fig. 14.

Fig. 14

Optical cavity inverse design. (a) Schematic representation of the dielectric/metal (DML) layer optical cavity. (b) Left axis: Computed EQEs for devices with a standard ITO cell configuration (solid grey line), the DML cell configuration with an Ag front electrode of 7 nm (solid blue line), and 5 nm Ag (dashed blue line), an optical ergodic geometry configuration with a 99% wave-length independent reflectivity back mirror (dashed red line). Right axis: EQE percentage enhancement for DML cells with a 7 nm (solid green line) and a 5 nm (dashed green line) Ag front electrode relative to the optical ergodic geometry configuration. (c) Energy storing capacity (vertical axis) of the DML cavity configuration as a function of the wavelength and dielectric layer (TiO2) thickness. A dashed rectangle indicates the broadband energy storage region. Reproduced by permission from Liu et al., Ref. 189.

When tandem devices are built, the distribution of different wavelengths of exciting light between the active materials is critical, and with series connected devices, the balance of photocurrents from these must sometimes be found; here the microcavity structure is also of importance.^190,191

While optical cavity structures are often useful for narrowband organic photodiodes,¹⁹² they have smaller relevance for PV devices where the overriding aim is power generation, and with one cavity but many wavelengths of light.¹⁸⁸

8.1.

Indoor PV

Section Author: Lethy Krishnan Jagadamma (University of St Andrews)

One of the most emerging and thriving areas of PV is indoor light harvesting.^236–238 This advancement of indoor PV is spurred by the rapid development of the internet of things (IoT) and the urge to make this technology more sustainable and robust.²³⁹ The main difference between indoor PV and outdoor solar cells lies in the properties of the incident illumination spectra. The illumination spectra of indoor artificial light sources are in the visible spectra range of 400-700 nm with an irradiance of $\sim 0.05$ to $0.35\mathrm{mW}/{\mathrm{cm}}^2$ whereas the sun’s spectra range from UV to infrared with an irradiance of $100\mathrm{mW}/{\mathrm{cm}}^2$. The extremely low light intensity and higher photon energy for indoor PV demand new design guidelines in terms of the material chemistry (not only in the photoactive layer but also the functional charge extraction layers) and the device physics. The optimum bandgap is $\sim 1.9\mathrm{eV}$, and the theoretical power conversion efficiency (PCE) can be as high as 50-57% for indoor PV depending on whether the illumination source is a fluorescent lamp or a white LED.^240,241

The most promising PV materials so far for efficient indoor light harvesting, are amorphous silicon, dye-sensitized solar cells (DSSCs), organic PV, and halide perovskites. Compared to DSSCs, the latter two possess higher prospects of commercialization due to their large area solution processibility, tuneable of bandgap, flexibility, and light weight. The latest reported champion power conversion efficiency (for 900-1000 lux indoor illumination) for these PV are respectively 36 % (a-Si: H),²⁴² 25 % (DSSC),²⁴³ 30 % (OPV)²⁴⁴ and 40.1 % (from the halide perovskites).²⁴⁵ Despite these impressive PCEs, the gap between the theoretical and lab based PCE is still >20%.

To further advance the indoor PV field and its commercialization, the existing challenges need to be overcome. Presently, the field lacks a standardized illumination source. Considering the different indoor illumination levels (200 to 1500 lux), and artificial lights of white LEDs and fluorescent lamps, standardisation is not straightforward. However, reporting colour temperature, irradiance spectrum and irradiance of the indoor light source will make the comparison of the power conversion efficiency values from different research labs more meaningful. Validation of the measured short circuit current density (${\mathrm{J}}_{\mathrm{sc}}$) with that from the external quantum efficiency (EQE) measurement should also be encouraged for indoor PV. In the case of hybrid perovskite-based indoor PV, along with the J-V curves in both scan directions, reporting steady-state power conversion efficiency would enhance the reliability of measurements. To close the gap between the theoretical and experimental efficiency values, an overall improvement in all the PV performance parameters is necessary.²⁴⁶ However, the ${\mathrm{V}}_{\mathrm{oc}}$, loss needs immediate attention. Despite the large bandgap (1.8 to 2.0 eV) of the photoactive material, most of the reported indoor PV have a ${\mathrm{V}}_{\mathrm{oc}}$ of $\sim 0.8\mathrm{V}$ only (loss of more than 1V). Though compared to normal outdoor solar cells, indoor PV may require less stringent stability testing protocols, many are yet to be formulated. In the coming years, research on indoor PV is expected to be focused on, maximising its PCE, developing the stability protocols, integrating with the energy storage units and implementing them in real IoT applications (see Fig. 16).

Fig. 16

Sustainable IoT for buildings.

8.2.

Commercialization of Organic PV

Section Author: Stephan Kube (Heliatek GmbH)

Heliatek is the technology leader in organic PV, founded 2006 out of research in the field of organic electronics from TU Dresden and the University of Ulm. Researchers had developed a new generation of semiconductor materials built on carbon-based molecules from organic chemistry. With this new generation of materials, Heliatek was able to develop an innovative new solar technology called “organic photovoltaics” (OPV). Since 2006, Heliatek has developed new materials, new product concepts, and a proprietary “Roll-to-Roll” vacuum evaporation production process.

In 2022, Heliatek started to produce and sell its first commercial product HeliaSol. HeliaSol is an ultralight, flexible, and ultrathin solar film that can be easily glued to various building surfaces such as metal, glass, or concrete with the integrated backside adhesive. This makes HeliaSol the perfect choice for all applications where conventional solar modules reach their limits due to weight or surface restrictions (e.g., curved shapes). With a carbon footprint of less than 10 g ${\mathrm{CO}}_2\mathrm{e}/\mathrm{kWh}$, HeliaSol is based on the greenest of all solar technologies and is amongst the greenest of all electricity generation technologies. The reasons for this ultralow carbon footprint mainly come from the few materials required to manufacture solar films and the abundance of toxic heavy metals (such as cadmium or lead) and rare earths. The films only use 1 g of organic material per film and the films weigh in total less than 2 kg.

The solar films are produced in an innovative roll-to-roll evaporation production process, where Heliatek processes coils with a length of up to 2.6 km and a width of up to 1.3 meter. This allows us to produce different product dimensions. The first HeliaSol that Heliatek is producing has a dimension of 2 m length and 0.44 cm width, which has a compact and easy to handle format. These films have a power between 50 and 60 W.

Heliatek has already proven the versatile applications possibilities in projects around the world, where its façade solutions, building rooftop applications, or special projects like a wind turbine tower in Spain, show that with lightweight, flexible solar films, Heliatek is able to transfer surfaces into active green electricity generators.

Heliatek is funded by the Free State of Saxony, the Federal Republic of Germany, and the European Union. The company has more than 250 employees at the headquarters in Dresden and the research location in Ulm. Figure 17 shows the diverse applications of the company’s products globally.

Fig. 17

Clockwise from top left: HeliaSol solar film (rolled), production of solar films in Roll-to-Roll manufacturing process, façade installation in Korea, wind turbine tower in Spain, and biogas plant in Germany. Images courtesy of Heliatek, Samsung, Acciona, and RWE.

8.3.

Agrivoltaics: Status, Land Use Conflicts and Synergy with Organic Photovoltaics

Section Author: Harald Hoppe (Friedrich Schiller University Jena, CEEC Jena, and IOMC)

With the current expansion of renewable energy supply in large parts of the world, primarily through wind energy, energy crop cultivation, and PV, the land use required for this is also increasing inexorably.^247–249 Not in all regions can land that is otherwise unusable be used for this purpose, and thus resistance to such projects is also growing.^250,251 In western countries, for example, this has led to increased state regulation with the associated extensive bureaucracy for the realization of new regenerative power plants, which makes further expansion considerably more difficult. On the other hand, there are now not only competitive situations with traditional agriculture, but also land use conflicts between energy crop cultivation and photovoltaics in particular have become a priority problem, for example in Germany.²⁵²

However, a solution to these land use conflicts could be found in a dual use of the available land through agrivoltaics,²⁵³ a coexisting agriculture with PV installations, providing a solution to the water-food-energy nexus.²⁵⁴ Typical agrivoltaic system installation parameter considerations (presented in Fig. 18) include crop type, PV module installation angle, distance, and overlap on the crops, and the transparency of the modules. At present, more than 15 GW of installed capacity have already been realized worldwide, for which subsidy programs have even been set up in some countries.²⁵⁵

Fig. 18

The schematic shows the most important parameters of agrivoltaic installations. The crop type and its handling impacts, e.g., on the distance and height of installations. The angle, distance, and overlap determine the shadow or rain protection, while the transparency modulates the shadowing.

The majority of agrivoltaic systems are currently being based on the use of silicon solar cells, which is accompanied by an increased installation effort compared to classical PV installations. However, there are already first projects for the use of organic solar cells, respectively organic PV.^256–258

Organic solar cells have distinct advantages such as light weight and ease of integration, as well as the potential for very high average visible transmittance (AVT) due to their unique property of limited absorption bands within a spectral interval,^259–261 which potentially eliminates competition between PV and plant light use in greenhouses.^262,263 More research and development is needed,^264,265 however, in order to synthesis and mass-produce novel organic semiconductors that preferentially use the infrared (IR) section of the sun spectrum.

8.4.

Perovskites for Space PV Applications

Section Authors: Duong Nguyen Minh (National Renewable Energy Laboratory) and Joseph M. Luther (National Renewable Energy Laboratory and RASEI)

While the prospects of new PV technologies are growing rapidly for terrestrial terawatt-scale electricity generation,²⁶⁶ PV was first marketed as technology to power spacecraft such as satellites, and emerging PV technologies could provide substantial cost reductions for the emerging space market.²⁶⁷ The satellite industry is rapidly expanding due to the commercialization of launch opportunities (SpaceX and others). More than 10x the historical number of satellites are planned to launch in the upcoming 5 years.²⁶⁸ Traditionally, III-V, multijunction solar cells make up the bulk of space PV market because of their higher efficiency, improved radiation tolerance, and improved temperature characteristics compared to Si albeit at much higher cost. Thus, the goal for new PV technologies, like perovskites would be to provide low cost options (equivalent to terrestrial Si) that may match or improve upon the radiation tolerance and temperature characteristics of III-V technologies.^269,270 Perovskite fabrication advantages enable flexible roll-out solar arrays and furthermore, the potential to even manufacture solar cells in space.^267,270

Perovskites for space PV has already shown promising results.^271,272 Although the early demonstrations also reported the surprising high radiation tolerance of perovskites, the testing conditions are widely different.²⁷³ Part of the rapid increase in interest of perovskites globally was because they are shown to tolerate a higher density of defects (while retaining high efficiency) than other PV technologies.²⁷³ This is further beneficial because not only can they tolerate higher defect densities, the radiation produces less defects in perovskites from particles of relevant energy as shown by the slightly lower non-ionizing energy loss and slightly higher stopping range plotted in Figs. 19(a)–19(b). This property is ideally suited for radiation tolerance in space where interactions with charged particles can lead to atomic displacements or vacancies within the absorber layer. The future development of perovskite PV for space applications would benefit from a consistent ground-based radiation testing convention to accelerate the optimization of perovskite PV designs, because radiation tolerance may not only be an inherent property, but it may also be a design criterion to optimize with interfacial engineering and contact decisions. However, the well-developed radiation-testing protocol for conventional space PV technology may not be equally suitable for testing perovskite solar cells due to their soft lattices, thermal properties, and markedly different device architectures. Currently, the low-energy protons of 0.05-0.15 MeV were reported to cause atomic displacements and vacancies in unencapsulated devices,²⁷³ which effective probe the radiation interactions in perovskite solar cells.

Fig. 19

Development of perovskite solar cells for space applications. a-b) Fresh protocol testing for perovskite solar cells. a) Comparison of irradiation interaction to Si, InGaP, and perovskite PV technology. b) stopping ranges of protons for various absorbers. c-d) Development of encapsulation for perovskite solar cells. c) Simulated proton interaction with perovskite devices. d) ${\mathrm{SiO}}_{\mathrm{x}}$ barriers block proton irradiation to perovskites solar cells. Figure adapted with permission from Refs. 271 and 274.

However, there are still many hurdles to overcome for perovskites in space regarding thermal properties.²⁶⁷ Some orbits require aggressive thermal cycling tests (thousands of times more aggressive than IEC standards, and at wider and faster temperature swings), while other orbits may require constant operation indefinitely (no diurnal cycle), where the cells must be built to endure the temperature without convective heat transfer.

For either case (terrestrial or space), perovskites will need to be well-protected/packaged.²⁷⁵ While polymers and sealants encapsulation are likely to decompose under harsh radiations because of the weakening carbon bonds, the use of traditional cover glass could also be challenged for weight and cost savings.^267,275 Comparatively thin (1 μm) SiOx encapsulation (lighter than 99% of conventional barriers) was recently shown to protect perovskites against the various key critical stressors in both terrestrial and space environments, including protons [as shown in Figs. 19(c)–19(d)], alpha particles, atomic oxygen, and moisture, and provides a good starting point for more complete packaging ideas.²⁷⁴

9.1.

Progress towards the Hot Carrier Solar Cell

Section Authors: Ian R. Sellers (University at Buffalo), Vincent R. Whiteside (University of Oklahoma), and David K. Ferry (Arizona State University)

Since the pioneering work of Ross & Nozik,²⁷⁶ the hot carrier solar cell has been considered a potential protocol to circumvent the single gap limit.²⁷⁷ This limit suggests that practical losses due to transmission (of low energy photons) and thermalization (due to absorption of high energy photons) limit conventional semiconductor solar cells to $\sim 30\%$ power conversion efficiency.

Fig. 20

(a) Schematic of the typical extraction energy/voltage in a single gap solar cell. (b) Protocol for hot carrier operation using IV scattering. Hot carriers are rapidly transferred to satellite valleys then extracted via an energy matched collector at $V>V_{\mathrm{oc}}$. Figure created by Stephen J. Polly (RIT).

The proposal in Ref. 276 suggests that solar cells operating in excess of 60% might be realized, without the need for multijunction architectures.²⁷⁸ One scenario for the operation of a hot carrier solar cell will occur when carrier-carrier interactions have generated a steady state Maxwellian “hot carrier” distribution whose temperature is in excess of the lattice. Generally, this requires inhibiting the thermalization of high energy photogenerated carriers, then extracting these carriers at their higher energy.

Since its original inception in 1982,²⁷⁶ the hot carrier solar has received significant attention with much of the work related to inhibited carrier-phonon interaction or the creation of a so-called hot phonon bottleneck^279,280 in low-dimensional structures such as quantum wells. Alternative approaches involve the investigation of materials with large phononic band gaps²⁸¹ that may inhibit hot LO phonon dissipation such as InN,²⁸² AlAsSb,²⁸³ and/or HfN,²⁸⁴ amongst others. In addition to these, recent approaches seek to control carrier-phonon interactions by design in type-II superlattice structures.^285,286

While many of the early works in this area focused on spectroscopic analysis in both the CW^287–289 and ultrafast^290–292 regimes, recently hot carrier effects have also been observed in devices. Recent device results have shown: the presence of PV carrier extraction,^293,294 extremely high non-equilibrium carrier temperatures in quantum well devices,²⁹⁵ and selective hot carrier extraction in metallic absorbers.²⁹⁶ While these demonstrations show significant practical progress towards the realization, decoupling electron-phonon mediated heat generation remains challenging.

Recently, an alternative protocol has been proposed in which intervalley scattering is used to transfer high energy photogenerated carriers into the satellite valleys of the absorber layer of heterostructure solar cells.^297,298 In such valley PV solar cells, it is proposed that intervalley phonon scattering is harnessed to inhibit hot carrier-LO phonon interactions in a semiconductor. This process allows the transfer and stabilization of hot carriers via the efficient transfer of photogenerated carriers to the upper valleys in the absorber, a process by which radiative recombination is exponentially reduced. A schematic of this process in comparison to the typical operation of a single gap solar cell is shown in Fig. 20. Initial work in this area has shown proof of principle operation and efficient carrier scattering in the InGaAs/AlInAs system,^297,298 with work now focused on efficient carrier extraction across the absorber/barrier interface. Efficient upper valley extraction would support photo-voltages in excess of the absorber band gap and power conversion efficiencies²⁹⁹ well above that of the single gap limit.

10.1.

Luminescent Solar Concentrator PV: Performance

Section Authors: Angèle Reinders (Eindhoven University of Technology), Xitong Zhu (Eindhoven University of Technology), Wilfried van Sark (Utrecht University), and Michael G. Debije (Eindhoven University of Technology)

Luminescent solar concentrator photovoltaics (LSC-PV) act as spectral converters, optimizing spectral matching between the irradiance spectrum and peak of the response of PV cells.^300,301 An LSC-PV normally consists of a transparent optical lightguide (usually glass or a polymer such as poly [methyl methacrylate] [PMMA], polycarbonate [PC], or polydimethylsiloxane [PDMS]³⁰²) with embedded luminophores which redirects absorbed light as downshifted emission toward the edge(s) where it may be used to illuminate attached PV cells;^303–305 see Fig. 21 (top and middle). Spectral conversion and (in larger lightguides) total internal reflection (TIR) are used to improve the LSC-PV device’s efficiency.^306,307

The concept of LSC-PV was introduced four decades ago,^308–310 initially to reduce the high cost of PV modules,³¹¹ but now these colorful, transparent, visually appealing solar devices could contribute to the general acceptance of solar technologies in the built environment, even with modest efficiency, thanks to their greater design flexibility than conventional flat, rectangular PV modules; see Fig. 21 (bottom).

LSC-PV perform well under various irradiance conditions, particularly under diffuse light.³¹² Color (from purple to red hues)³¹³ is determined by the luminophores employed and may comprise organic dyes,^314–317 quantum dots (QDs),^{305,318–322} and/or rare-earth ions.^323–325 The optimal dye has broad spectral absorption,^326,327 high photoluminescence quantum yield (PLQY),^328–330 large Stokes shift,^331,332 good solubility in the host matrix material,^333,334 and good spectral overlap of the luminophore emission s and the spectral response of the PV.^335–337

The performance of LSC-PV devices is limited by several loss mechanisms, including attenuation by the lightguide material, scattering, escape cone losses,³³⁸ and reabsorption during the lightguiding process.^339,340 Several methods have been proposed to minimize loss mechanisms in lightguides and enhance the internal efficiency (${\eta }_{int}$) and power conversion efficiencies (${\eta }_{PCE}$).^{319,341–343} Further proposed designs include using different lightguide materials, geometric shapes (e.g., rectangular,³⁴⁴ square,^345,346 cylindrical,^347,348 and bent³⁴⁶), and different dimensions (from $1.5\mathrm{cm}\times 1.5\mathrm{cm}\times 0.1\mathrm{cm}$,³³¹ to $100\mathrm{cm}\times 150\mathrm{cm}\times 0.6\mathrm{cm}$,³⁴⁹) to numerous edge-mounted and or bottom-mounted PV cell technologies, including, GaInP,³⁵⁰ GaAs,³²¹ CdTe,³⁵⁰ CIGS,³⁵⁰ mc-Si,³⁵¹ c-Si,³⁵² a-Si,^353–355 p-Si,³⁵⁶ DSSC,³⁵⁷ and perovskite.³⁵⁸

Reported LSC-PV devices have shown various efficiencies.^340,343 The most frequently reported world record of ${\eta }_{LSC-PV}=7.1\%$ applies to a small-sized device ($5\mathrm{cm}\times 5\mathrm{cm}\times 0.5\mathrm{cm}$) with high efficiency GaAs solar cells edge attached to a PMMA containing a mixture of two organic dyes.³⁵⁹ However, most LSC-PV devices have modest efficiencies in the range of 1-5%. The average efficiency of LSC-PV devices has increased from 2.5% (1980–1985) to 5% (2005–2021); Fig. 22 summarizes LSC-PV devices with variety in host matrix, luminescent materials, solar cell technology, lightguide dimension, PV cell size, geometric gain (Gg), internal efficiency (${\eta }_{int}$), power conversion efficiency (${\eta }_{PCE}$), and other design configurations.

Note the efficiencies reported in Fig. 22 were not obtained using the same metrics or measurement procedures, as comprehensive standard protocols for performance reporting have only recently been introduced.^360,361 As such, previously reported LSC-PV device efficiencies were overly positive, ignoring transparency of the LSC, and cannot be used for comparisons of other efficiencies obtained under STCs.³⁶²

Fig. 21

(Top) Solar irradiance spectrum with an indication of conversion of a blue photon to the red part of the spectrum, (middle) a lightguide with a solar cell attached to one of its edges, (bottom) in pink, possible locations of application of LSC-PV devices in a dwelling.

Fig. 22

Reported power conversion efficiencies of luminescent solar concentrator PV (LSC-PV) devices over the past 43 years with edge-attached (black squares), bottom-attached (red circles), and both edge- and bottom-attached solar cells (blue triangles).

10.2.

Luminescent Solar Concentrator PV: Technology Status

Section Author: Vivian E. Ferry (University of Minnesota)

Luminescent solar concentrators (LSCs) have been proposed for PV applications for more than 40 years.³⁰⁹ LSCs are attractive devices for integration of solar panels into the built environment: composed of luminescent materials (dyes, colloidal quantum dots) embedded in a polymer matrix, their colorful/semi-transparent nature and ability to capture and concentrate diffuse sunlight makes them excellent candidate architectural materials.³⁴¹ Diffuse sunlight is absorbed and emitted by the luminophore, and directed onto an adjacent solar cell by total internal reflection. Historically, performance has been limited by significant optical losses, and especially re-absorption in large-area devices, since every absorption/emission event is an opportunity for losses from the non-unity quantum yield or the imperfect waveguide (escape cone, scattering); see Fig. 23.

Fig. 23

Estimated power conversion efficiency and light utilization efficiency for a tandem LSC (a) with various candidate top luminophores, and [(b),(c)] constant overall absorption of 10%. Figure adapted from Ref. 363.

The past 15 years have seen considerable new interest in LSCs, driven by new advances in light-emitting materials.³⁶⁴ For efficient operation, the luminophores must strongly absorb incident sunlight, have high quantum yields, and large Stokes shifts to avoid re-absorption of the luminescent light, and have low toxicity/earth-abundancy for large-scale application. Colloidal nanocrystals, such as II-VI particles,^{351,352,365,366} Si,^{331,367–369} carbon dots,³⁷⁰ perovskites,^371–374 and ternary alloys,^300,375 and metal-halide nanoclusters³⁷⁶ have all emerged as candidate materials,³⁶³ offering highly tunable and advantageous optical properties. However, these new materials must be dispersed in polymer matrices and waveguide materials^370,377 without significant degradation of optical properties, light scattering, or absorption losses by the matrix material, and large-scale, cost-effective synthesis and fabrication methods are needed.³⁶⁷ Additionally, as the quality of the luminophores improves, other aspects of optical design become increasingly important to the overall efficiency, such as photonic structures that trap and direct luminescence toward the PV cell.^{351,377–381} Recent calls to standardize efficiency metrics for LSCs should also be noted.³⁸²

There are other intriguing new developments in the LSC community, including the implementation of downconverting/upconverting materials, as opposed to the downshifting mechanism of a traditional LSC architecture. New application spaces for LSCs with PV are emerging,³⁸³ including the use of LSCs in greenhouses where the tailored transmission spectrum benefits crop growth.^384–387

Large-area and scalable fabrication remains an important challenge for LSCs. Techniques such as doctor blade coating^367,388 or spray coating³⁸⁹ are important for production, but may require materials advances for luminophore compatibility, as well as understanding of the process to create high quality, low haze materials. Similarly, photonic structures that assist in light-guiding must be manufacturable. Lifetime is critical to commercial applications, requiring luminescent composites that are stable against oxygen and moisture. Sustainable luminescent materials with low toxicity but excellent optical properties in the composite continue to need development.³⁹⁰ Commercialization may also require different form factors or design modifications to integrate with application-specific components, i.e. architectural glazing or displays.³⁹¹

Given the large number of design variables (shape, luminophore, concentration, matrix material, use of photonic structures, environment for implementation, and others), there is an opportunity to understand the design rules for LSCs from the perspective of the commercial application, i.e., accounting for sustainability, scalability, and the operating environment from the start, rather than designing for small-scale laboratory tests. Computational methods that accelerate this design process will likely play an essential role in both understanding and optimizing these future generations of LSCs.

10.3.

Spectrum Splitting PV Systems

Section Authors: Raymond K. Kostuk (University of Arizona) and Benjamin Chrysler (Amazon Lab)

High efficiency PV systems have many operational and financial advantages. High efficiency allows less land usage for the same electrical power generation and the ability to generate significant power in constrained areas. According to the detailed balance condition, high efficiency PV conversion can only be achieved by using multiple energy bandgap PV cells within the range of the solar energy illumination spectrum.^277,392 The most common way of doing this has been to stack multijunction cells in tandem with the topmost cell converting shorter wavelengths and the bottom most cell the longer wavelengths. This approach requires precise lattice matching between different bandgap materials, current matching, and is restricted to compatible materials. These factors increase cost and require high concentration optics for economic viability. An alternative approach is spectrum splitting in which an optical element separates the spectrum of the incident solar illumination and directs spectral components to an array of single junction cells that have high responsivity to different spectral bands [Fig. 24(b)]. In spectrum splitting systems the emphasis of the design shifts from the stacked multijunction cell to the design and fabrication of the optics that perform the spectral separation of the incident solar illumination. The concept of spectrum splitting PV systems is not new (dating back to 1955)³⁹³ and comprehensive reviews have been given by Imenes and Mills³⁹⁴ and Mojiri et al.³⁹⁵

During the past ten years, a variety of methods have been proposed for implementing spectrum splitting optics. These approaches include: cascaded dichroic reflection filters,^396,397 filter-mirror combinations, ^398–400 optimized surface relief diffraction gratings,^401,402 hologram-lens combinations,⁴⁰³ and volume holographic lenses.^404–406 All spectrum splitting systems require at least single axis tracking, however, single axis tracking is being used with many more efficient types of silicon cell modules and is therefore not considered a major impediment. The most robust and compact optical approaches are surface relief diffraction gratings (SRDGs) and volume holographic lenses (VHLs). Ultimate use of either of these techniques will depend on fabrication cost and durability. Volume holographic elements have been successfully incorporated into traditional PV module packages that have passed accelerated life testing for 25 years or more and provide some support for the viability of this technique. In addition, methods for replicating volume holographic elements and lenses have also been demonstrated (Fig. 25).^407,408

Recent work on spectrum splitting has led to an interesting result for optimal cell configurations.⁴⁰⁹ Figure 24 shows cell types: stacked multijunction, lateral separated single junction, and hybrid cells. A stacked multijunction PV cell is a form of absorption spectral filtering where each cell in the stack filters and converts a different spectral band of the incident sunlight to electrical power. The laterally separated cells completely rely on optics for achieving spectral separation, while the hybrid cell uses a combination of absorption and optical spatial spectral separation mechanisms. It was found that the hybrid approach incurs less overall system loss. This approach can also take advantage of recent perovskite-silicon tandem cell development.⁴¹⁰

Fig. 24

(a) Stacked, (b) lateral, and (c) hybrid configurations for spectrum-splitting PV modules. In stacked modules, PV cells are placed on top of each other either. Upper cells in the stack convert shorter wavelength light and pass longer wavelength light to the lower cells. The stack can be implemented as a monolithic multijunction cell or a set of mechanically stacked single-junction cells. In a lateral configuration, an optical element sends different parts of the solar spectrum to single-junction PV cells located in different areas of the module. In a hybrid configuration, an optical element sends different parts of the solar spectrum to different cell stacks where the solar spectrum is further subdivided. In general, cells can be connected in series, series-parallel combinations, or electrically independent configurations.

Another emerging applications is for combined PV conversion and bio-growth (agrivoltaics⁴¹¹) where a volume holographic element filters part of the spectrum to match the photosynthesis active region of the spectrum (PAR)⁴¹² and the remainder for electrical conversion.⁴¹³ The irradiance at the crop area can also be modified to further enhance plant growth. This approach can be applied to both open fields and greenhouse crops and provides dual use of much more land area with synergistic benefits for crop production.

Fig. 25

(a) An array of volume holographic lenses fabricated using a replication system.⁴¹⁴ (b) Spectral band separation using the VHL array shown in (a).

12.1.

Common Themes and Challenges

The diverse contributions to this report demonstrate the remarkable range of emerging PV technologies as well as developments in their applications. They also describe some of the challenges to widespread deployment. Some of these challenges are specific to particular technologies—for example, finding materials for lattice matching for III-V solar cells. However, it is interesting to see that there are also challenges that arise across many (or all) solar technologies. One of these recurring themes is translating lab solar cell performance to modules and on to the production line. This includes challenges of scaling up of coating, module design, and developing appropriate manufacturing tools and processes. It applies not only to solar cells themselves but also to ways of enhancing their efficiency—for example, through light management and photonic structures.

Manufacture is also closely associated with related issues of cost, energy of manufacture and environmental impact. Whilst cost has been recognised for a long time as crucial for determining extent of deployment, the articles show increasing attention being paid to energy payback time, i.e., the time for a solar cell to generate energy equal to that required for its manufacture. There is also growing concern to develop clean manufacturing processes—for example, by using “green” solvents, and to consider the environmental impact of both the process and the materials used. There is increasing awareness of the importance of life cycle analysis. Indeed, to become truly sustainable it will be necessary to develop a circular economy for PV.

Progress in stability is needed for many solar technologies, especially perovskites, organics, and dye-sensitized solar cells, with a 25-year lifetime in the field being desirable. There are grounds for optimism as in related fields, such as organic light-emitting diodes (OLEDs), tremendous progress in stability has been made and the best lifetimes of OLED pixels are much longer than 25 years. Aspects of advancing stability include understanding degradation mechanisms and developing appropriate encapsulation and packaging. The impact of shading also needs to be considered as a shaded solar cell can experience a substantial reverse bias due to its neighbours.

The exciting applications emerging bring their own challenges that need to be addressed. One such issue is developing appropriate testing protocols as the intensity, spectrum and temperature may differ greatly from the standard test conditions developed for silicon solar cells. For example, indoor applications use light that is at shorter wavelength and much less intense than most of the solar spectrum. Stability testing protocols will also need tailoring to the application. Particular applications place their own requirements on solar cells—for example, some large warehouses have flimsy roofs that would only be able to support lightweight solar cells. For agriphotovoltaics and building integration, the transmission (and reflection) spectrum of the solar cells is important.

Finally, the authors share a spirit of optimism regarding the future of PV technology, despite varying perspectives on the priorities and challenges involved (see Sec. 12.2). Overall, this report provides an overview of emerging PV approaches that show potential in helping to achieve the development of new materials, device concepts, and light management strategies to enable higher efficiencies and more scalable and sustainable manufacturing.

12.2.

Author Survey and Perspectives

As a way to gain a broader perspective on the field of emerging PV, the authors of this article were asked to complete an anonymous informal survey regarding their projections for the future directions of the field. 19 responses were received. Below is a list of the questions and summary of responses:

Q1: What do you think are the most near-term promising applications of emerging PV technology?

Responses (rank-ordered %): Agriculture (22%), PV-integrated parking structures (14%), Non-III-V in space (12%), IOT (12%), Automotive/mobile (12%), Semitransparent PV windows (10%), Indoor (8%), Other (8%)

Q2: When do you predict perovskites (all perovskite tandem or tandem with other materials) will reach commercialization at the level of 1 MW installation?

Responses (%): 1 - 3 years (26%), 3 - 10 years (47%), 10+ year (0%), Never (11%), No idea (16%)

Q3: When do you predict OPVs of any form will reach commercialization at the level of 1 MW installation?

Responses (%): 1 - 3 years (5%), 3 - 10 years (42%), 10+ year (32%), Never (11%), No idea (11%)

Q4: When do you predict DSSCs will reach commercialization at the level of 1 MW installation?

Responses (%): 1 - 3 years (5%), 3 - 10 years (11%), 10+ year (0%), Never (68%), No idea (16%)

Q5: Which of the “exotic” mechanisms for breaking the detailed balance limit will most likely be commercialized someday?

Responses (rank-ordered %): Photon up/down conversion (32%), Spectrum splitting (24%), Multiexciton generation (11%), None (11%), Hot carrier solar cells (8%), Intermediate band solar cells (5%), 2D absorbers (5%), Singlet fission (3%))

Q6: What PV materials are understudied and deserve more attention?

Responses (alphabetical order): Chalcogenides, Dilute nitrides, Inorganic semiconductors, Lead- and tin-free perovskites, Low bandgap materials, Mismatched materials, Non-toxic earth-abundant Inorganic materials, OPV, Solar concentrators, Thermophotovoltaics, Quantum dots

Q7: Which are the most pressing environmental and sustainability concerns of solar PV, which the community should focus on as technologies are being developed? Please rank the following 5 concerns.

Responses (rank, number): Recyclable solar cells (1^st, 4; 2^nd, 5; 3^rd, 3; 4^th, 5; 5^th, 2), Lifecycle analysis (1^st, 4; 2^nd, 1; 3^rd, 9; 4^th, 4; 1), Elemental availability (1^st, 5; 2^nd, 3; 3^rd, 2; 4^th, 5; 5^th, 3), Green chemistry for fabrication (1^st, 2; 2^nd, 7; 3^rd, 4; 4^th, 3; 5^th, 2), Land usage for sustainability (1^st, 4; 2^nd, 2; 3^rd, 1; 4^th, 1; 5^th, 11)