The survival of the human species on Earth is a function of human health, proper climatic conditions, and their affecting factors. Due to heavy energy demands, there has been an increase in the consumption of fossil fuel, which has elevated the pollution levels leading to global warming. Thus, the development of an alternate eco-friendly power source is vital to sustain the life on Earth. The global requirement of power is estimated to grow exponentially every year. Solar cells are considered as a promising way to meet these energy crises and reduce pollution.1 Since the last few decades, a silicon-based solar cell acts as an ideal solution. The production of silicon-based solar cells has increased by 10 times from 2005 to 2013. Silicon-based solar cells have obstacles such as cost factor, complicated production procedure, and conversion efficiency far less than the optimum value. Perovskites solar cells (PSC), which have various technical advantages, are considered as a better replacement for silicon solar cells and have a high chance for commercialization. To avail this opportunity, PSC and their associated factors have been extensively investigated by researchers.
Currently, silicon-based solar cells have efficiencies over 15%. However, they have a high energy pay-back time and emission rate.2 Cost of module, balance of systems,1 and operation management, though reducing, is reaching a stagnation value far below the optimum. These drawbacks have generated demands for exploration of new material with high-performance capability. The field of conversion of solar energy to electrical energy has experienced a substantial boost due to development of perovskite-based solar cells. The reported high efficiency of solid-state-sensitized thin film (solar cell based on perovskite as light harvester)3 has encouraged investigation in this domain. Figure 1 shows the increment in research papers published in the last few years on perovskites. This increased research confirms that PSC has rapidly evolved as an answer to energy crises; presently (by 2018), it can achieve an efficiency of 23.3% (Table 1).8,9,12,13,15–17 Further, the National Renewable Energy Laboratory (NREL) graph shows the rapid pace of increment in efficiency achieved in the last few years.18
Development of various structure resulting in increase of efficiency.
|Year||Device structure||PCE (%)||Reference|
|2018||As certified by NREL||23.3||5|
|2017||FTO/TiO2/meso TiO2: perovskite composite/perovskite /PTAA/Au||22.1||6|
|2011||FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/redox liquid electrolyte/Pt||6.5||14|
|2009||FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/redox liquid electrolyte/Pt||3.8||13|
Despite such aggressive progress, the contribution of solar cells, to meet the world’s electricity requirement is less than 1%.19 In solar cells, light-to-electricity conversion efficiency is defined as a product of open circuit current density (), open circuit voltage (), and fill factor (FF). Research is going on to achieve power conversion efficiency (PCE) up to Shockley–Queisser limit, which is 33.5% for the single-junction solar cell.20 Theoretical limits as calculated for a conventional PSC employing absorber () are is 27.20 (), is 1.28 V, is 90.2, and is 31.4%.21 However, actual results concerning PCE are limited to 15.4%. It has also drawbacks such as lead toxicity, low operational stability, and large-scale production issues. Recently, Christians et al. achieved operational stability of , by tailoring the interfaces of PSC.22 Although it is a positive sign, perovskites have to accomplish the levelized cost of electricity for residential use as 5.0 cents per kWh and achieve a stable lifespan of at least 20 years by 2030 to achieve successful commercialization.23 Production of 1 kWh of energy from perovskites would incur a consumption of 64.77% embodied energy and 31.38% of incorporated materials. This wastage of energy and materials assures that there is enormous room for improvement.24 Figure 2 shows the blue-dotted curve and a solid circle, which indicates the scope for performance improvement.25 The following four conditions would enable a solar cell to maximize its power output.
• Obey Maxwell–Boltzmann statistics and exhibit a mobility of charge carrier sufficiently high reaching as high as a .26
• Internal conversion efficiency reaches 100%.
(where denotes I and Br) with a band gap of 1.5 to 2.3 eV,32 close to optimum gap, shows the best performance in PSC. The success of hybrid PSC is due to the properties of organic and inorganic components in it. Organic elements render stability for perovskites and facilitate self-assembly, whereas inorganic component produces extended meshwork by covalence and ionic interactions. This hybrid combination and their interactions allow precise crystalline structure formation. Efficient carrier mobility, (multi) ferroics,33,34 high absorption coefficient ( at 600 nm), long charge diffusion length,35–38 low nonradiative Auger recombination,39 and direct band gap properties define good performance and efficiency of perovskite. In addition, Perovskite materials show a strong PL with a narrow full width at half maximum.40,41
is the original perovskite mineral, which was discovered and named after Russian mineralogist Lev Perovski. In 1978, Dieter Weber was the first to show hybrid perovskites. They initially explored lead halide and later tin, bromide, and iodide. Incidentally, they did not incorporate any improvements as they considered it as another semiconductor. Perovskites started their journey as light absorbers in dye-sensitized solar cells (DSSC). O’Regan and Grätzel reported fabricating DSSC by use of a nanoscopic titanium dioxide () particle layer achieving PCE of 7.1%.42 PCE of a different PSC structure ranges from 17% to 20.1% (9.3% for single junction,43 20.1% for regular structure, 19% for planar heterojunction, 21.6% for mesoporous, 17% for planar heterojunction, and 18% for inverted planar heterojunction). New metal halide perovskite films were fabricated by a single step method utilizing a molar ratio of 1.05 between /formamidinium Iodine, providing efficiency of 20.8%.44 These films are used in light-emitting diodes,45,46 solution-processed photodetectors,47–49 and lasers.50 Transition metal oxide perovskites, such as (mott insulators) displays an indirect band gap of 1.08 eV. The d–d transitions and p-d transitions ensure adequate absorption and utilization in solar cells.51 Heterostructures, such as grown on with a direct band gap of 1.1 eV, are also investigated for higher PCE solar cells.52 Doped perovskite solar cell (PSC) gained importance, especially with hetrovalaent elements. Their energy states can be tuned and reported to achieve PCE of 15.6% (0.956 sun, AM 1.5).53 Roldan-Carmona et al.54 fabricated semitransparent thin PSC films that maintained near to device transparency of 30% and PCE above 6%.
First perovskite material , with a general formula , is called perovskite. All perovskites are not solar perovskite structured compounds. Figure 3 shows an ideal structure of perovskite. The “A” cation represents methylammonium cations (), M represents heavy metal cation, and “X” is a halogen—chlorine, bromine, iodine, or combination of three.56 The schematic structure of an ideal perovskite is shown in Fig. 4.55 has a distorted three-dimensional (3-D) structure that crystallizes in the tetragonal I4/mcm space group.
Lead methylammonium iodide perovskites can have three crystalline phases, which are temperature-dependent, namely orthorhombic, tetragonal, and cubic. An extensive investigation is on for control of metal halide hybrid structure from 3-D to two-dimensional (2-D),57,58 one-dimensional (1-D),59 and 0D with quantum confinement. Figures 5 and 6 show the XRD pattern and FESEM morphological image of conventional perovskite film, respectively, against different thermal annealing temperature ranging from 80°C, to 140°C.60 XRD pattern confirms the presence of perovskite with peaks at values of 14.2 deg (110), 28.5 deg (220), and 31.8 deg (310). The peak at 12.7 represents phase. Unlike , which has a distorted tetragonal crystal, has a cubic perovskite structure phase of space group at room temperature, along with variation in their energy levels as shown in Fig. 7.61 In addition, research for fabricating hybrid PSC with various organic cations and metal halide anions is on.
New structures such as double perovskites (), replace with nontoxic trivalent/monovalent metals, are reported to yield high efficiency.62 The family of PSC constitutes structures such as dye-sensitized cells, mesoporous structure, planar (p-i-n and n-i-p) structure, HTL and ETL free cells, flexible cells, and tandem cells. The prominent structures of solar cells are as follows.
Mesoporous Structured Solar Cell
Mesoporous structured solar cell (MSSC) is a third-generation device with low fabrication cost. is the most common mesoporous layer in them. Though nanoparticles are the most commonly used in the mesoporous layer, success reports are available while employing nanosheets,63 nanorods, and nanofibers. MSSC consists of a single-phase triple-conducting oxide (TCO), blocking layer, mesoporous or scaffold, perovskite absorber, hole transport layer (HTL), and a metal electrode in its construction (Fig. 8).64 Usually, MSSC fabrication done by a two-step deposition method. The processing temperature is in the range of 150°C. However, highly efficient MSSC devices have a high sintering temperature of 500°C.65 During high-temperature sintering, the mesoporous layer allows rapid extraction of photo-induced electrons, which shorten the electron transport length. This shortening of length makes MSSC an efficient light harvester. However, they suffer from the various disadvantage of having low open-circuit voltage () and lower light absorbance at wavelengths greater than 700 nm. Mesoporous , which acts as an insulator, is found to be more stable than , under continuous illumination. introduces nonstoichiometry defects, such as titanium interstitials and oxygen vacancies, which causes trap sites.66 Encapsulation of layers can prevent moisture ingress and degradation. Etgar et al.63 recently reported that spin-casting a methylammonium lead iodide () solution on a 400-nm thick nanosheet would form a mesoscopic heterojunction. Evaporating a gold film on top of creates a back contact. It provided PCE of 5.5%, short circuit photocurrent , and open-circuit photovoltage under Am 1.5 solar light of .63
Film structure has a simple construction as it does not have a mesoporous scaffold or electronic layer in it, facilitating ease in fabrication. Figure 9 shows the architecture of n-i-p and p-i-n variants with different location of the charge transport layer, which is their primary difference.67 A simple planar junction designed between perovskite and HTM layer, without scaffolding, corresponds to a simple planar thin-film cell. The major problem in the fabrication of this structure is to avoid contact between HTM and blocking layer. Vapor deposition and dual-source vapor method excel in achieving this required feature. Docampo et al.68 applied and in planar heterojunction solar cells with as photoactive material. However, these and interlayers resulted in low photovoltaic performance due to poor coverage on the substrate by perovskite film.68 Alternatively, UV treatment was reported to enhance surface wetting properties with increased PCE by 7% on these materials. HCl improves crystallization and coverage of in thin film, which subsequently improves the morphology achieving a PCE of 14.8% for perovskite thin film.69 Planar heterojunction additionally assists in evaluating and testing of the new materials and their effect, for solar cell fabrication. was also experimented to replace as electron-selective contact. was spin coated and sintered at a much lower temperature of 200°C than . Its fabrication included the solvent vapor annealing, which crystallizes the residual and adds it at the interface of . PSC devices based on displayed high stability of holding the PCE of 13% till 700 h.70
HTL/ETL Free Cells
The fundamental task of any photovoltaic device is to create charge carriers, i.e., holes and electrons on the absorption of light energy. Further separating them at respective heterojunction and making them available at their respective electrodes. This separation depends on energy offsets versus exciton binding energy. It defines the formation of two primary interfaces, perovskite/electron transport layer (ETL) interface and perovskite/HTL interface. The architecture of a few PSCs does not have HTL or ETL layers. As these are not mandatory layers, numerous researchers have developed PSC without HTL71,72 or ETL with good PCE. Perovskite layer has Au deposited over it while fabricating the HTL-free PSC. In this, perovskite layer conducts the duty of absorber and hole conductor by forming a heterojunction with ETL. Aharon et al.71 have reported a two-step deposition method to fabricate highly optimized HTL-free PSC with a PCE of 10.85%, FF of 68%, and of 0.84 V. Their performance is strongly dependent on the width of the depletion layer formed by . Depletion layer supports charge separation and restricts back reaction from to . The width of the depletion layer is evaluated using Mott Schottky analysis.73 Figure 10 shows a schematic diagram of HTL-free PSC with their energy levels.71 Figure 11(b) shows ETL-free PSC in-which an insulator () replaces scaffold,64 which proves that perovskite material could effectively transport electrons without an ETL. Production cost for HTL-less cells is high, leading to an increase in device cost. Further, the presence of the HTL layer complicates the mechanism followed for extraction.
Tandem cells consist of two formats, i.e., four-terminal and two-terminal devices. Figure 12 shows the arrangement of four-terminal tandem device, which has two-terminal devices/cells. They are either mechanically stacked or monolithically integrated through tunneling junction. Baillie and his coworkers74 with copper indium gallium selenide, Asadpour et al.75 with a highly efficient a-Si:H/c-Si heterojunction bottom cell and Mailoa et al. using the c-Si bottom cell on perovskite/silicon multijunction solar cell have obtained PCE of 30%, 13.4%, and 13.7%, respectively. Due to the limitation of band gap mismatch and current matching constraints, a traditional tandem cell of Si and perovskites could only achieve PCE of 24% and 20%, respectively. Top cell absorbs high energy photons and bottom cell harness the remaining low-level energy photons. makes an excellent top layer in c-Si solar cells. Hydrogenated indium oxide, having high mobility, is used as a rear electrode to fabricate semitransparent PSCs.76 Fabrication of two-terminal tandem solar cells with perovskite as the top layer and kesterite copper zinc tin sulfide (CZTS) as bottom cells was reported to attain a PCE of only 4.6%. This reduction is primarily due to semitransparency of the top aluminum electrode, resulting in transmission loss.77 Thus, the highly efficient and transparent top layer is a very critical factor. Further, they should be immune to thermal instability and hygroscopicity. Numerous transparent electrodes used here are ultrathin metal such as Au,78 graphene,79 carbon nanotubes,80 and Ag nanowires.19 However, efficiency achieved was below 10%, and their contacts featured strong absorption in near-infrared (NIR). PSC that are individually efficient underperformed when used as tandem cells, due to sensitivity to a thickness of perovskite layer. Thus, bifacial tandem cell was fabricated, which utilizes scattered light back from the ground (Fig. 13) to achieve a PCE of .75 Further, they are immune to variations in perovskites layer thickness.
Fabrication is a prominent factor in PSC. Fabrication factors such as time, solvent mixtures, doping, and humidity have a marked effect on crystallization of perovskites.81 Further processing routes and solvents selected induce intermediate phases and influence device performance with film quality.81 Synthesis and analysis protocol for large-scale production and laboratory analysis are different. Most of the PSC that achieved high efficiency in the lab, on small devices, does not possess commercialization advantage. Large-scale production of PSC should be cost-effective, good stability, and maintain high performance. Figure 14 shows a one-step and two-step spin coating method. The perovskites precursor solution is thoroughly stirred and dropped on to the substrate and accelerated for evaporation of the solvent.83 In the second step, crystalline perovskites are achieved, after thermal annealing the sample, by removing the residual solvent. or when fabricated by spin coating led to the formation of scattered nanodots3 and extremely thin layer.64 General drawbacks such as poor quality film, small grain sizes, and dense pinholes can be controlled to a limited extent by additive engineering84,85 to obtain required grain sizes. This process has achieved a maximum PCE of 19.19%. Another major drawback of spin coating is the formation of nonuniform thickness of the film from center to edges. Two-step coating provides full coverage of with cube-like structure.82
Two-step spin-coated deposition method, as a valid mode for fabrication of PSC, was reported in 2013. It involves step-by-step spin coating of solution and solution of methylammonium iodide (MAI) on the substrate. Whole set up is thermally annealed at 100°C driving the interdiffusion of precursors. Single step method produces uncontrolled precipitation of perovskite, resulting in large morphological variations, which hampers the prospects for practical applications. Thus, in performance sequential deposition process,12 solution is first spin-coated on top of the substrate and then is dipped into a solution of MAI. The performance gets varied according to different soaking time, from 5 s to 2 h. The optimum soaking period was found to be 15 min, which resulted in enhancement of current density and voltage of the device from and 933 mV (5 s) to and 1036 mV (15 min),86 respectively. This method can perform successfully for nanostructured devices with planar heterojunction structured PSC.
The sequential vapor deposition process is very similar to the two-step spin coating method except that this method uses vapor deposition, which yields layered multistack thin films over large areas.17 MAI and were vapor deposited on a substrate as layers before thermal annealing. Yang and his coworkers used vapor-assisted solution process (VASP), also called a blended deposition method, to produce films as shown in Fig. 15.87 A solution of in -dimethylformamide was deposited on the substrate by spin coat method. Later, -coated substrate is annealed in MAI vapor at 150°C in a nitrogen atmosphere for the desired time to yield perovskite films. Though spin-coating techniques offer good thin film uniformity for lab-scale experiments, its drawbacks are concerning wastage of solution during coating, unsuitable for large-scale production, and low material transfer efficiency. However, vacuum and solution processes are two primary methods to fabricate perovskite films starting with inorganic and organic counterparts of PSC. Solution process results in the rough film surface. Vacuum process requires high energy for creation of vacuum conditions, and the flaky nature of MAI powder makes synthesis less controllable. VASP resulted in enhanced controllability of film quality. VASP method takes advantage of the characteristics of the hybrid nature of perovskite materials, particularly the low sublimation temperature of organic halides and fast reaction rate between inorganic and organic species. Full surface coverage, 100% precursor transformation, uniform grain structure, and print size up to are highlights of perovskite film derived from this approach. It is free of pinhole defects and yields an efficiency of 15.4%.17 Snaith and his coworkers synthesized PSC using dual-source vacuum deposition method. In this process, MAI and were preheated to 120°C and 325°C, respectively, and evaporated together using separate sources, to attain superior uniformity in films.17 Vapor-based deposition processes have few issues in comparison to solution-based deposition processes, such as an increase in manufacturing cost and capability to achieve increased efficiencies.88,89
Analysis showed that large crystal would have a noticeable effect on PCE, leading to the development of hot-casting technique. This method guarantees large output and ultrafast production.90 In this, precursor (iodide) is maintained at 70°C and is immediately transferred to spin coater as drops in less than 5 s. Rotating for 30 s assists in cooling and quenching it fast, results in a uniform film with large grain size. Figure 16 shows a schematic diagram of the hot casting process and displays the proportional relation between temperature and large grain formation. Figure 16(d) shows a comparative chart of grain size and processing temperature for the hot-casting method and conventional annealing method.91 The comparison confirms that the evaporation rate defines the formation of larger crystals. In 2015, Vak’s group have fabricated a fully printed planar-heterojunction PSC by slot-die coating.92,93
Large-scale perovskites production for roll-to-roll fabrication consisted of various methods, such as slot-die coating, knife coating, spray coating, screen printing, gravure printing, flexographic printing, and inkjet printing.94 Slot-die is a premetered coating process, in which mass flow determines the thickness of the film. Metered solution feed controls the formation of a uniform thickness of film stripes. In addition, transferring the complete solution to the substrate would lead to zero loss of solvent. This process involves two steps. The first step consists of heating the substrate at 70°C, which results in dense and glassy .12,94 The second approach replicates the quenching mechanism to avoid large crystals. It consists of high-pressure nitrogen flow allowing layers to dry faster. Spray coating and doctor blading are few methods that are advantageous when integrated with the roll-to-roll process. The genesis of spray coating is from polymer solar cell fabrication.95 It is combined with ultrasonic spray to enable large-scale production in room temperature. PSC achieved a PCE of 13% with active area on glass substrates96 by this method. Spray coating is reported to be a favorable method for fabricating tandem structural devices and mixed cation halide perovskites for achieving enhanced performance and stability.97 It utilizes minimum solid concentration for large-scale production but has a disadvantage of excessive restrictions on boiling points of solvents used. Inkjet process is programmable production process but has limitations induced due to ink properties.98
Doctor blading method has an advantage of slower evaporation time, cost-efficient, higher coverage area, better crystal quality, nil solution waste, and favorable for ambient temperature processing.99 It is reported to be highly compatible with the roll-to-roll production process for optoelectronic device fabrication.92,100,101 By applying blade coating for preparation of perovskite films, under ambient conditions, yields large crystalline domains, high quality, and moisture/air-resistant films. Slower evaporation time leads to better quality crystals and higher coverage. For perovskites, the formation of large crystalline domains is encouraged by a relatively slow solvent drying process, on the uniformly wet film formed by solution blading.102,103
Stability and degradation though have different meaning; they together define the issues and parameters, which affect existence, availability, and commercialization of PSC. Stability of PSC is our concern, as they degrade significantly after a few hundred hours of usage. Even after the necessary coating, they can sustain only 80% of initial efficiency after 500 h. Performance and efficiency of PSCs decrease, as degradation increases with time. Evaluation of stability is as per standard of International Electrotechnical Commission damp heat test (at 85°C, 85% relative humidity). Stable devices can maintain its performance with less than 10% reduction of PCE after 1000 h of exposure.104,105 Hybrid PSCs are still struggling to reach this goal. Significant factors affecting stability are sensitivity to moisture, ultraviolet exposure, thermal stress, and so forth. (Fig. 17).106,107 In addition, a prominent hurdle for commercialization of PSC is toxicity and safety issues due to lead usage.108 Optimum fabrication method for large-scale production of PSC is still under research. Material instability can be regulated utilizing cross-linking additives and compositional engineering.109 The best example is adding a combination of and in precursors,110 additionally using cation cascade, including Cs and Rb cations.7,111 Associated layers of PSC induce instability into PSC. Monitoring of deterioration by different performance parameters, such as , , FF, and , describe its stability status. Fabrication processes, light absorption, charge separation, charge carriers transport mechanisms of different layers influence performance parameters. Table 2 shows various methodologies undertaken to increase the stability factor. Unlike organic solar cells, the decay behavior of in hybrid PSC, , follows three steps. These three steps are a fast partial decay within 10 to , followed by an extremely slow decay between 1 and 100 ms, and then a final rapid decay on the 10-s timescale.130 Initial decay is continuous until 70% and holds at that sector for a hybrid PSC, whereas an organic solar cell decays very fast, within 1 s.
Methodologies to increase stability of PSC.
|Architecture||Type||Methodology||η (%)||Stability check result||Other advantages||References|
|Glass/FTO/c-TiO2/m-TiO2/H3PbI3/carbon||Hole conductor free||P-type NiO nanoparticles into carbon electrodes||13.26||Retained 85% of initial efficiency after 800 h in ambient atmosphere.||Easy electron transfer||112|
|FTO/TiO2/MAPbI3/HTM/Au(HTM = acylhydrazone-based porphyrin derivative)||Planar||Acylhydrazone-based porphyrin derivative used as HTM layer||17.8||Retained 65% of the initial PCE after 60 h for water stability. Maintained original PCE after 100 h for thermal stability.||Improved water stability with contact angles of water droplets at ∼107°C||113|
|Graphene/AU/spiro-OMeTAD/TiO2/perovskite/ZnO-MgO-EA+/FTO||Planar and mesoporous||Surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA)||22.1||Stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells at AM 1.5G illumination and 70% RH||No hysteresis||114|
|FTO/ALD-NiO/Cs0.05MA0.95PbI3/PCBM/BCP/ALD-AZO/Ag||Inverted PSC||Al-doped ZnO films act as efficient electron-transporting||18.45||Retains 86.7% of initial PSC for 500 h under 1 Sun illumination at 85°C||Negligible current–voltage hysteresis in this work||115|
|FTO/mesoscopic ZrO2/TiO2 infiltrated with /(5-aminovaleric acid)x(MA)1-xPbI3||Mesoporous||Uses a double layer of mesoporous TiO2 and ZNO||12.8||Near to initial PCE at AM 1.5 sunlight over 1008 h at room temperature||Hole-conducting layer is eliminated||116|
|Al/PCBM/2-D (BA)2(MA)3Pb4I13//poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS))/FTO-glass||Planar||Ruddlesden–Popper phases-layered 2-D perovskite films||4.73||Unencapsulated, it retains >60% efficiency for 2250 h under AM1.5G at 65% RH||Photovoltaic efficiency of 12.52%, no hysteresis, no deterioration when encapsulated||117|
|Fluorine-doped tin-oxide (FTO) /bl-TiO2/mp-TiO2/(FAPbI3)0.95(MAPbBr3)0.05/HTM/Au.||Film/planar||Fluorene-terminated hole-transporting material||High thermal stability maintaining almost 95% of its initial performance for >500 h at 60°C||High glass transition temperature average grain size suitable for electron extraction||118|
|FTO/polytriarylamine (PTAA)/Cs0.05 MA0.16 FA0.79 Pb(I0.83Br0.17)3/C (unsealed)||Planar||Stress modulation to increase stability||Stress of −130 to 130 MPa after 24 h of damp air aging at 25°C and 85% RH||Compressive load increases stability||119|
|FTO/NixMg1−x LiO/MAPbI3/PCBM/Ti(Nb)Ox/Ag (sealed)||Inverted planar||p-doped (p+) NixMg1–xO and n-doped (n+) TiOx||>15||Retained 90% of PCE at AM 1.5 G, 100 mW/cm2, 1000 h||No hysteresis||16|
|Glass/NDI-(P(NDI2DT-TTCN)/PEDOT:PSS/CH3NH3PbI3/ITO/glass||Inverted PSC||Naphthalene diimide (NDI)-based polymer along with dicyanothiophene (P(NDI2DT-TTCN)) used as ETL||17||Deteriorated less than PSC with PCBM at 25°C under ∼55% of RH for 100 h||Electron extraction ability forming a hydrophobic ETL surface. Excellent mechanical stability negligible hysteresis||120|
|Al/PC61BEH/CH3NH3PbI3/PEDOT:PSS/ITO||Inverted planar||Fullerene derivative with branched alkyl chain such as [6,6]-phenyl-C61, butyric acid 2-ethylhexyl ester (PC61BEH) as ETLs||15.19||Under AM 1.5G irradiation 45% to 50% humidity in air at room temperature without encapsulation||121|
|Glass/FTO/m-MTDATA/MAPbI3/C60/BCP/Cu||Inverted planar||4,4′,4′′-tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) as HTM layer||18.12||Retains 90% PCE in ambient temperature 30% RH for 1000 h.||Small hysteresis, large recombination resistance. Good water resistance and hydrophobicity with water contact angle of 83°C||122|
|ITO/SnO2/MAPbI3/spiro-OMeTAD/Ag (unsealed)||Planar||SnO2 layer as electron selective contact prepared by sinter-less spin-coating processes||13||PEC constant over 700 h at AM 1.5 G.||Grain size of PbI2 precursors can be controlled good thermal stability||70|
|ITO/TiO2-Cl/(Cs0.02FA0.98PbI3)0.97(MAPbBr3)0.03 /spiro-OMeTAD/Au||Film||Perovskite seeding growth (PSG) method for Cs-containing, high-quality FAPbI3-based PSC||21.5||Retain 60% of their initial efficiency after 140 h under one sun illumination.||123|
|ITO/PEDOT:PSS/FA0.75Cs0.25Sn0.5Pb0.5I3)/C60/BCP/Ag||Two terminal tandem solar cells||Tin content at 50% or below, using formamidinium and cesium at the A-site, and employing the postdeposition methylammonium chloride (MACl) vapor treatment||19.1||Unencapsulated maintains full PCE after 150 h at 85°Cover 80% of its initial PCE for over 300 h||Small bandgap PSC, low-cost, encapsulation technologies will be sufficient to prevent oxidation||124|
|Ag/spiro-OMeTAD/perovskites/MesoTiO2 and perovskites compact TiO2/FTO||3-D perovskite film||Distinct layered low-dimensional perovskite is engineered on top of the 3-D film||20.1||Retains 85% of initial PCE when subjected to one sun illumination for 800 h at 50°C||Negligible hysteresis, increased charge recombination resistance||125|
|TO/NiOx/CH3NH3PbI3/PCBM:C60/Zracac/Ag||p-i-n film||Post-device ligand treatment. Stitching effect||20.13||Unencapsulation 70% PCE maintained under ambient conditions after a 500-h maximum-power-point tracking test||Healing and improves PCE to 900%||126|
|Au/spiro-OMeTAD/2-D@3-D perovksite/mp-TiO2/c-TiO2/FTO||2-D/3-D perovskites||Highly oriented 2-D@3-D [(AVA)2PbI4@MAPbI3] perovskite structure combining||18.0||Device stability under inert is 90% of initial PCE for 32 days. Unencapsulated, retains 72% of initial PCE for 20 days under ambient conditions||Moisture stability at RH of ≈ 93%. Combines advantages of 2-D and 3-D perovskites||127|
|Al/PCBM/ZnO.np/Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br 0.1)3/NiOxnp/substrate/ITO||Flexible inverted PSCs||NiOx nanoparticles as HTL and ZnO nanoparticles as ETL||18.6||Over 85% of PCE is retained after 1000 h for thin film and over 90% after 500 h with a thick films.||Good stack development with negligible hysteresis||128|
|CsxFA1-xPbI3-[GaAA3]4(0 < x < 1)||Core/shell heterostructure||Gallium(III) acetylacetonate (GaAA3) as the precursor additive to in situ induce a metal–organic-complex monomolecular intermediate ([GaAA3]4)||18.24||Unencapsulated cell retains constant PCE over 18% for 5 h at 50% RH||Shows low defect-state density, strong luminescence and long lifetime of photogeneration charge carriers||129|
Half-life is the time required to reach half of the initial absorbance in set conditions. It determines the strength of decomposition, mainly due to RH. For RH of 98%, 80%, 50%, and 20%, the half-life of -based PSC films are estimated to be 4, 34, 1000, and 10,000 h, respectively. The half-life is found to be independent of film depth confirming that moisture diffuses into the bottom of the PSC layer along the grain boundaries.131 Methodology for evaluating stability is also an essential factor. Researchers at Aalto University have found that only a fraction of stability tests done on perovskite-based solar cells and DSSCs meet proper requirements.132 The team analyzed 261 aging tests, conducted on solar cells, and saw significant shortcomings in the procedure of reporting test results and testing methodology. These defects emphasized the importance of standardization of testing methodology. Presently, tests are conducted only on one cell and in dark region only.132 Many reports state that samples that are stable in humid conditions were found to be unstable in UV light.132 Only 15 tests conducted outdoors and three with cells connected, leading to erroneous results; checks need to be undertaken in real-world conditions in groups of several cells to bring in standardization.132 The same team has developed a new method, which saves time, effort, and could assess deterioration to the smallest range. PSC solar films, when they get heated, they turn from black to lead iodide color, which is yellow, displaying degradation. This degradation does not happen in its bulk form, due to the catalytic activity of moisture and reduced surface area as compared to films.132 PSC can be protected from moisture to a certain extent using PMMA [poly(methylmethacrylate)]. It also helps in nucleation and formation of smooth and fine films.133 The next section discusses various direct and indirect factors that affect stability.
Materials and structure are also essential factors that decides the stability of PSC. The mesoporous structure formed by -, -, or ZrO-coated substrates, exhibited the pore-filling problem, low stability, and degraded performance.17,64 Alteration of precursor influences the loss of organic material in PSC. This mass loss was reported to be 16%, 12%, and 5%, between temperatures of 250°C and 400°C134 in three steps. Under sunlight and device operating temperature (), undergoes a phase transition from tetragonal to cubic. A disordered character is shown by methylammonium ions during the cubic–tetragonal transition, whereas octahedron displays a displacive character.135,136 Various combinations of halogens, especially of Pb and Br, induce change and transformation in the crystal structure of . As changes from 0 to 0.13, crystal structure transforms from tetragonal to cubic form with a change in the band gap. Empherical representation by quadratic equation describes the dependence of band gap on .61 The low value of bowing parameter, 0.33, in the equation confirms high miscibility137,138 of and . This low value shows that would constitute the entire composition of the compound and enables convenient band gap tailoring. For subservient Br fraction (), PCE remained unchanged with improved stability factor when tested for 20 days at 55% RH. Diminished cross-section and transformation from tetragonal to cubic stage, due to higher Br content, has led to enhanced stability.
New materials such as can resolve lead toxicity problems. Whole fabrication was conducted in environment as it decolorizes if exposed to the room environment. This decolorization is primarily due to oxidation of ions, which lead to the formation of oxides of Sn and residual perovskites material. Sn-based PSC shows an enhanced PCE of more than 6% and has a short diffusion length. It restricts the fabrication of devices with planar architecture. A combination of Sn with other halides, namely was developed to achieve PCE of 6%.139 Schoonman has explained the degradation in the absence of moisture.140 Crystal structure of 3-D perovskites incorporating corner-sharing octahedra is broken into 1-D double chains consisting of two adjacent octahedral (, monohydrate form) and subsequently into 0D isolated octahedral (, dihydrate form) by intercalation of . Structural degradation occurs due to ionization-simulated degradation; it happens when the hole of halide member, which the inner layers produce, migrates and reacts with negative iodine on the surface. The reports confirm that doping improves crystallization. doping, in planar PSC forming composite films, displayed active sites for crystal nucleus during annealing.141 Doping has an overwhelming influence on performance, due to various reasons. Primarily it is due to undesired band bending at ETL and HTL interfaces with perovskites, increase in Auger recombination, and traps/defects. For higher PCE, the Fermi level in perovskite should not be above (below) Fermi level of ETL (HTL) in equilibrium.25
Materials of PSC
Perovskite materials exhibit features such as direct band gap, high optical extinction coefficients ( to ) in the NIR-visible region, and high carrier mobility ( to ) making it a prospective member for light harvesting. A high optical extinction coefficient of perovskite makes it a good absorption of light, at low mesoporous film thickness. Tuning the band gap of perovskite materials is achieved by modifying the chemical composition of perovskite61 or by alloying different perovskites.142 In addition to other properties, perovskite materials also exhibit ambipolar charge transport property (can be n- or p-type), which conducts electrons and holes through them, due to small effective mass of both electrons.36,143–145 High dielectric constant leads to effective charge screening. By bandgap engineering through modification of the chemical composition of PSC, such as , the entire spectrum is utilized.146 Enhancement in PCE happens due to reduction in binding energy. The same was confirmed by incorporating core-shell in PSC and achieved PCE of 11.4%, ).146 On addition of , Sn halide perovskite exhibits photovoltaic properties. It attained a maximum photon to current conversion of 4.18% with 1060-nm-thick Sn layer, with a redshift at Sn thickness of 260 nm.142 Variation in prominent layers and adaptation of methodology can increase stability and PCE. Addition of 5% by volume of water into MAI solution of isopropyl alcohol assists in preferential crystallization in (110) plane, achieving a large grain size of . This method achieved a PCE of 12.42%.147
Electron conducting material layer
A layer that supports electron transport and aids in avoiding hole-electron recombination is called ECM. LUMO level should be lower than the conduction band of perovskites to achieve high electron transfer, and for perfect hole blocking HOMO, should be below perovskites. Thus, normal ETM in perovskites will have LUMO of and HOMO of . is the most established and favorite ECM layer. It is highly photocatalytic in the UV regime and thus is a prime member in inducing instability due to UV exposure.148 It provides a high surface area to perovskites loading and can exist with all nanostructures of 1-D, 2-D, and 3-D variants. Compact- results in lower PCE; however, ethanolamine (EA)-based solvent treatment of compact- would enhance the transport feature along with its defects. EA and ionic liquid molecules (ILMs) are spin coated on compact- modifying surface morphology of a planar perovskites.149 Various other useful solvents that reduce defects of metal oxide and energy barrier mismatch are conjugated polyelectrolyte,150 ILMs,151 a self-assembled monolayer,152,153 polar solvents, or alcohol/water-soluble-conjugated polymers.154,155 is unsuitable on flexible substrates and large-scale production due to high sintering temperature.64 ZnO, organic materials such as [6,6]-phenyl--butyric acid methyl ester (PCBM), and ICBA (indene bis adduct), as well as carbon nanoforms and polymers, are an equivalent replacement of with good performance data.156,157 Nanostructured ZnO in PSC can be fabricated at a low temperature (), making it viable for photovoltaic devices.10 It has good electron mobility along with energy alignment. However, ZnO nanostructured, quadruple the rate of degradation due to deprotonation reaction of the organic part in perovskite.158 PCE of 17.2% and enhancement of 23.2% compared to a nonadditive solution was observed by addition of nonhalogen additives such as acetate (, Ac-) salts (, NaAc) in solution.159 Modification of ZnO layer using ILMs, such as 1-benzyl-3-methylimidazolium chloride (benmim-Cl),151 elevates performance. Further, is doped with various metals to overcome drawbacks. was experimented to replace as ECM material to achieve a high PCE of 13%.70 In few devices, the layer is between Al anode and fullerene for ensuring proper electronic contacts. Further research has led to the formation of valence bands of lower binding energies, with Fermi levels, within the band gap, by inserting a monolayer between perovskite and contacts.160 The ability of to accept electrons results in high PCE, of 6.7%, along with enhancement of and . Planar devices using as ETM and NiO as HTM achieved a PCE of 9.6%. For polymer ETM, solubility is a major issue. Thus, polymer ETM deposition will affect performance and perovskites layer. In addition to , other ETM example consists of higher fullerene-like , . Fullerene and its derivative have increased PCE from 7.3% to 14.9% due to reduced trap density and lower degree of symmetry. However, due to nonequivalent double bonds, it leads to regioisomeric mixtures, which are difficult to separate - nanocomposites, as ETM, achieved PCE of 15.6%. Graphene quantum dots, due to their confinement in 3-D modify the band gap. Thus, incorporating these between and perovskites achieved PCE of 10.2% with a of 0.94 V and of .
Hole conducting material layer
The primary function of HTM is to extract charges selectively from perovskite and its interface. The basic advantage of this HCM is its ability to penetrate in mesoporous layer easily. However, it induces degradation in PSC film.161 Classification of HCM consists of small molecules, polymers, and inorganics. Small molecule HCM such as spiro-OMeTAD is a favorite for mesoporous and planar structured PSC. Electrolytes have an inbuilt problem of low ionic conductivity. HTM is less conductive than perovskite material (), leading to the requirement of thick HTM layer. High thickness is necessary to avoid pinholes and leakage, but in devices, it would lead to high series resistance and imbalance between series and shunt resistance. Thus, spiro-OMeTAD as solid-state HCM is introduced to nullify these limitations. However, multiplex synthetic approach and purification are the hindrances for its commercialization. Doping with lithiumbisimide (trifluoromethanesulfonyl) salt (LiTFSI), 4-tert-butylpyridine (TBP) and cobalt complex162 assists in improving its conductivity. Due to absorption, device instability increases and oxidized form of spiro-OMeTAD starts acting as a filter in the visible region at 520 nm.163 Spiro-OMeTAD shows an immediate initial decrease of 20% and, further, it declines steadily. New materials nicknamed EH44 are replacing spiro-OMeTAD, as it repels water and does not contain lithium.164 PEDOT:PSS is another proven organic HTL for planar structured devices. There is energy level misalignment between PEDOT: PSS and VB (5.0 eV) of (5.46 eV), which results in lower (0.80 V). A higher work function of HTL would lead to strong recombination at the grain boundary, very low PCE, and affect the of the device. Doping and addition of additives were used to remove the misalignment and increase efficiency. Improved HTL doping ratio of 4:1 in volume for has achieved PCE of 15.15%, of , of 0.96 V, and FF of 0.74 under AM 1.5 illumination.141 nanoparticles/toluene solution an inorganic hole transporting material for planar PSCs is reported to exhibit stable efficiency of 14.8% under illumination of 1 Sun ().165 Numerous reports conclude NiO as a good candidate for HTM in PSC device fabrication. It was able to achieve PCE of 16.40% with high (1.04 V) and FF (0.72) with negligible current-voltage hysteresis.166 The conjunction of mesoporous film and a hole conducting polymer led to the formation of PSC having 123% PCE under 1.5 Am.158 CuSCN as HTM, along with a buffer layer of thin mesoporous alumina () and an added coat of insulating polymethyl methacrylate on PSC, was utilized to achieve PCE of 13.3%. In addition, it displayed stability by retaining the efficiency of 77% after 1000 h at 85°C in air.161 Utilization of bifunctional polymer nanocomposites as HTLs for light harvesting is also recently investigated with positive results. A gold (Au) nanoparticle assists in attaining a fourfold enhancement of electrical conductivity and carrier mobility. Scattering effects of doped nanoparticles have led to a 25% improvement in PCE along with amplification of the photon absorption on the photoactive layer.167
Efficient lead halide PSCs use hole-blocking layers to help collection of photogenerated electrons and to achieve high open-circuit voltages. Deactivation of reactions at the surface of enhances stability, which is due to the blocking effect of this layer. Spiro-OMeTAD as HTL and TBP as additives to HTL would develop a reduction of perovskites film absorption. Introduction of montmorillonite layer between the perovskite and spiro-OMeTAD layers, as buffer, provides the solution for reduction in film absorption.168 Experiments showed that nonencapsulated devices had higher stability than encapsulated devices because oxygen would remove the surface states and ensuring better stability.66 Thus, oxygen is necessary for these devices. When replaces n-type , as an insulating layer in MSSC, devices achieve stability of 1000 h.169 Performance enhancement occurs when a hole-blocking layer is formed by Mg-doped , with a PCE of 12.28%; high FF and (1.08 V). This enhancement is due to the enlargement of the gap between valence band and conduction band for better optical transmission properties.170 Ke et al. have fabricated PSC without blocking layers, by growing perovskite directly in fluorine-doped tin oxide-coated.171 Solution method and ultraviolet–ozone treatment can form layers. Its planar architecture consists of Au/hole-transporting material//substrate. It achieved a PCE of 14% and of 1.06 V, which is at par with conventional PSC. This success is due to ultraviolet–ozone treatment and involvement of Cl in synthesis. These results confirm that is not an ultimate interfacial material for achieving high-performance PSCs.171
Electrode design for large-scale production is still a challenge. The electrode should ensure good catalytic activity, electron conduction, and high proton diffusion rates. A single-phase TCO is reported to have enhanced proton and charge transport capability for perovskites.141 Ag and Au are the favorite elements.172,173 However, due to the cost factor, Ag is a better alternative than Au.146 However, PSC with Ag as electrode has extremely low stability. Exposure of element to humidity () led to the formation of Agl, which deteriorates PCE. Agl transforms the electrode from a reflective metal to yellow color, which is highly influenced by the HTL layer. Antireflection coatings for front side of the transparent glass and plastic substrates are being explored extensively.174–177 Nanophotonic front and rear electrodes for improved light transmittance in the solar cell is desirable for long wavelengths range operation in tandem device PSC. Successful nanopatterns are being replicated into transparent substrates for large-scale applications at low cost.178 Long-term performance is a function of thermal stability of perovskite other layers in a photovoltaic device. Inorganic blocking layer and scaffold layer display appreciable stability compared to organic material in devices.
Around 2014, scientists observed hysteretic current–voltage behavior of PSC.179,180 Hysteresis, which is a specific property, is formed likely due to slow transient effects, leading to an over or underestimation of solar cell efficiency. J–V hysteresis property of PSC is a function of dielectric response of perovskite material, mobile ions,181 and trap-state discharging.179 The shape of measured current–voltage (IV) curves and efficiency are function of scan direction, light source, delay in measurement time, and voltage bias conditions before measurement. It was established by various modeling and experimentation that hysteresis is weak when surface recombination is low and diffusion length is long.182 Improved crystallinity, better fabrication process, and improved contacts are going to continue to improve hysteresis. HTMs influence hysteresis and J–V curve. Cu-based, CZTS HTM express less hysteresis than with spiro-MeOTAD HTM.183 J–V hysteretic behavior of CuI-based devices with an efficiency of 7.5% is better than spiro-OMeTAD devices. Faster polarization relaxation was observed using EIS and OCVD for CuI-based devices.184 Recently, in addition to normal-type of hysteresis, an inverted hysteresis that exhibits an opposite behavior to the normal is also reported.185,186 The inverted hysteresis is due to interfacial charge extraction barriers formed by ionic defect accumulation and space charge build-up.187 The inverted hysteresis is noted to be a function of bias range and sweep rate. Different bias scanning conditions lead to a varying ionic movement at the interface of the ETH or HTL/perovskite, which assists in the tuning of these hysteresis.186
Deposition of fullerene layers onto perovskites, which assists in the elimination of photocurrent hysteresis, is reported.188 Methodology and accuracy for evaluating performance characteristics of PSCs are crucial. Hysteresis influences the evaluation of performance parameters such as I–V curves and . Measurement conditions such as sweep direction, start voltage, end voltage, hold time, and sweep time have an overwhelming effect.189 Inverted devices (p-i-n structured) are reported to have less hysteresis.182 Figure 18 shows that normalized time-dependent of the regular film has degraded to a higher rate than inverted structured PSC.190 Though moving of electron contact to the rear side is a possible explanation, detailed reasoning requires further investigation. Incorporation of a [6,6]-phenyl--butyric acid methyl ester (PCBM) interlayer and high-quality perovskite absorber fabricated by thermal evaporation–spin coating method produces a hysteresis-free planar perovskites.76
Ions accumulate at the ion-blocking interface. The “p” and “n” doping induce ion migration at ion-blocking interface. These doping will screen the applied voltage and alter charge collection capability, resulting in hysteresis. In addition, ion migration is responsible for unusual behaviors in perovskites materials such as photo-induced phase separation,191 photoinduced giant dielectric constant,192 and absence of transistor behavior at room temperature.193 In high efficient PSC cells, ion migration does not cause hysteresis.182 Ion migration within crystal structure,194 which is also a major contributing factor for hysteresis, has a profound effect on device stability.195 Experimental proof and test results show that ions migrate along the grain boundaries.196
Moisture acts as a catalyst, and its presence tends to induce hydrolysis107 in PSC, thereby converting PSC back to precursors, which is irreversible. Experiments to evaluate the stability of PSC against moisture were conducted in inert and other carrier gases to ascertain degradation exclusively due to moisture.12 When exposed to moisture, perovskite structure tends to hydrolyze, undergoing irreversible degradation and decomposing back into precursors. Highly hygroscopic converts to salts and , with = halide.197 Heat, electric field, and ultraviolet exposure198 accelerate this process. Figure 19 shows the mechanism of degradation:
Stability of PSC, due to moisture, requires evaluation in three modes. One is purely because of moisture, the second is under the influence of moisture along with light, and finally the third mode is a combination of moisture along with light and oxygen. Degradation of perovskites is reported to follow the mechanism of Grotthuss.199 In this, one water molecule reacts with perovskite material and degrades to HI and . Further, HI reacts with to form water, to initiate more deterioration. After exposing perovskite film to 80% RH for 2.5 h, the possibility of intermediate crystalline phase such as a hydrated compound exists. Grazing incidence x-ray diffraction (GIXRD) confirms the formation of intermediate phase. XRD pattern shows strong peaks of hydrate at 8.42 deg and 10.46 deg, which confirm the existence of intermediate phase. Exposure of solar cell to RH of 0% to 50% has led to very little degradation. However, under 90% RH, it showed a rapid decrease in PV performance efficiency from 12% to less than 1% in just 3 days. Third day values are , , and , .200 Increase in RH level beyond 55% resulted in a drastic increase in degradation rate. In addition to general characterization, a powerful method such as photothermal deflection spectroscopy evaluates the changes in absorption after exposing perovskites to moisture.201 In Fig. 20, shows absorption edge as , which shifts to 2.3 eV on exposure to moisture, this correspondence to formation, and decomposition of . Figure 21 shows PCE versus heterojunction solar cells variation based on (, 0.06, 0.20, 0.29) as per time, stored without encapsulation at room temperature with RH ranging from 35% to 55%.202 It also shows variation in UV–vis spectra as per moisture content and variation in the morphology. -based perovskites are less sensitive to moisture than -based Perovskites. However, the absorption capacity of pure is not comparable to that of .55,203 The first line of defense against the moisture is the top charge-transporting layer of the device. Resistance to moisture can be increased by inherently strengthening the bond between cation and metal halide.200 Few other approaches are inclusion of thin blocking layer between perovskites and HTL, such as ,169,204 moisture-blocking hole transporters,205 and hydrophobic carbon electrodes.2,116,206–209 Incorporation of 2-D PSC with large aromatic phenyl ethylammonium (PEA) cations could achieve average PCE of 5.5% with excellent stability against moisture. After exposure of these 2-D PSCs for more than 90 h at RH, they were stable and maintained 50% PCE of initial value.210 Incorporating polyethylenimine cations in 2-D PSC helps in attaining better film quality and resistance to moisture degradation with PCE of 8.77%.211 In addition, near-single-crystalline-type thin films are fabricated, which have, preferential out-of-plane alignment of crystallographic planes, good charge transport, and moisture resistance.117 Thus, 2-D PSC has better moisture resistance capability than the 3-D PSC.212,213
Solution-processed was immune to changes when exposed to . The decomposition on exposure to moisture or ambient air is due to .202 However, very low quantity acts as an n-dopant for , affecting the energy level. Proposed decomposition of eventually releases and HI as volatile species, leaving and making decomposition reaction irreversible. Reports confirm that moisture coupled with light directly led to decomposition of to , which was an irreversible reaction. Residuals such as and are formed easily under sunlight, instead of reversible hydrate in the dark, when exposed to an environment of 18 h at 60% RH.107 Though the analysis of stability in dark is a basic test, stability research under illumination is essential for evaluation of working efficiency and degradation factor. Unlike degradation in the dark, exposure to moisture in light leads to irreversible decomposition of to .
Light Spectrum (UV and Visible)
Mostly laboratory experiments are conducted under controlled light source such as LED. However, actual utilization involves sunlight with UV element, which accelerates degradation pattern. The element in PSC, which is photocatalytic in the UV regime, assists degradation.148 To further surprise, as shown in Fig. 22, degradation is higher in encapsulated devices than the unencapsulated devices for a conventional device.
This phenomenon is due to electrons injected into are entrapped in deep-lying unoccupied sites.148 However, encapsulation along with UV-filter can enable attaining better results. Degradation is directly proportional to efficiency and follows a pattern of reduction in . A -free mesoporous structured solar cell is comparatively less degradable than MSSC with , which is confirmed by transient absorption spectroscopy. MSSC has a property of degradation being high at maximum power and rather low at open circuit condition. However, for practical applications installation of a UV filter on PSC overcomes the degradation process. Degradation is attributed to the instability problem214 and the catalytic role of metal oxide when exposed to the UV spectrum. A general degradation mechanism for UV exposure is shown in Fig. 23. On exposure to UV light, photogenerated holes that are generated by absorption of energy react with the oxygen radicals adsorbed at surface oxygen vacancies. Molecular oxygen is absorbed from these sites, leading to a formation of unoccupied deep surface traps sites and a free electron per site. These electrons will recombine with the excess of holes in doped-hole transporter. On photoexcitation of sensitizer, electrons can follow in two ways. In the first route, they get injected into conduction band by which they get deeply trapped and in other they directly enter into the deep surface traps. These deeply trapped electrons are not mobile and recombine readily with holes on hole transporter.148 The primary bottleneck that exists is transporting the charge. In quantum dots, polydispersity strongly affects transport. In quantum dot film, if few dots are of different sizes, then trap states manifest within the bandgap and lead to recombination. Similar is the case with 1-D carbon nanotubes. Three schemes are formulated to nullify the effects of UV. First being the reduction of trap sites or pacifying it and second is to undertake measures to avoid UV light reaching ETL (especially ). Finally, the third scheme is to replace scaffold with another material or make it free, which has been explored by Leijtens at el.148 The efficiency of PSC device reduces to almost 0% after 12 h of exposure in a nonencapsulated format. However, with the introduction of buffer layer in PSC, efficiency increased to of its initial value. This increase is due to the blocking layer, which blocks the UV-induced photocatalysis in , by providing a form of passivation. In addition, this extra layer assists in maintaining the crystalline structure of .215 A methodology such as usage of the downshifting molecule such as -(2-(6-chloros-tetrazin-3-yloxy)ethyl)-naphthalimide (NITZ) with a strategy to suppress UV from the source is also explored, for UV protection in PSC.216 They have a specialty to get excited in the UV spectrum and to emit into the visible spectrum, allowing UV photons utility.
Variation in efficiency and increase in temperature will result in amplification of deformities. These anomalies are due to deterioration in perovskites materials and behavior of HTM. It is understood that when the heat of the device ingresses into cell materials, mechanical stress of the materials at interfaces increases, thereby decreasing the lifetime of the PSC device. Accelerated aging test results showed that deteriorates much faster than , approximately after 60 min. High stability of Br is due to shorter and stronger bonds of Pb-Br compared to Pb-I.217 Heating of in environment at 90°C not only leads to the formation of 3-D perovskites structure but also results in a high rate of degradation at 100°C.218 Hybrid perovskite assists in maintaining stability, as inorganic part of hybrid perovskites increases thermal stability and its organic part increases the stability of device. Comparison of formaldinium lead trihalide and -based device reveals that the former is more stable as it deteriorates only after 290°C than the later, which decomposes at 230°C.219 In PSC, deterioration rate due to temperature increases in the presence of moisture. Reports reveal that at 85°C time of decomposition is 283, 208, and 133 min when RH is 12%, 57%, and 85%, respectively.220 Period of exposure is also an influential factor for evaluation and characterization by hard x-ray photoelectron spectroscopy instead of conventional XRD, because assessment is done on chemical content regardless of crystallinity of the sample. Mechanism of degradation is as follows:
Films are made independent of parameters such as moisture and oxygen by subjecting it to ultrahigh vacuum heating chamber, to evaluate effects purely due to temperature.221 I/Pb and N/Pb ratios define the characterization of films for degradation. Reduction of these ratios indicates degradation of perovskites into , as shown in Fig. 24. Variation in temperature induces a different degree of degradation in various layers of PSC. For example, perovskites made of scaffolded ZnO are found to be more stable than .222,223
To understand degradation on perovskites purely due to thermal effects, semifabricated PSC, without HTL, ETL, and the top electrode are tested.106 These semifinished planar structures undergo exposure to inert, , and ambient conditions for 24 h. Surprisingly, degradation in the form of delamination happens in an ambient environment rather than in . GIXRD and surface photovoltage measurements assisted in characterization and revealed that at temperature above 140°C and 160°C perovskite materials decomposed into in vacuum.224 Up to 150°C, an increase in the Braggs peak with enhancement of perovskite film occurs. From 150°C to 200°C, these peaks reduced due to formation and further heating to 330°C lead to complete decomposition into Pb.225 Empirical studies revealed that thermal annealing conducted for production of perovskite materials induces slight production, causing drop in efficiency. Heat treatment of layers such as ZnO before deposition lead to an increase in efficiency and thermal stability, due to reduction in deportation of methylammonium at the interface.223 Thermal stability is a highly potent factor that affects different layers especially the HTM. Initiation of crystallization and formation of large crystals causing morphological hole traps affecting charge transportation and effecting PCE was reported for PSC with spiro-OMeTAD as HTM.218 This crystalline process gets aggravated by additives or doping done in the layer to increase charge due to the hygroscopic nature of dopants.226 To achieve better stability, many new HTM materials were explored, such as PTAA and 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c] carbazole (KR131).227 Introduction of bulky aromatic PEA cations was reported to produce thermally stable 2-D PSC, which maintained 65% of its initial PSC after exposure to 100°C for 70 h.210 In addition, multidimensional 3-D/2-D perovskites with an intermediate dimension between them led to the formation of thermally stable PSC, due to reduced defect density.228
Fabrication and Preparation
Fabrication is a vital factor for sustaining stability in PSC. It primarily constitutes two elements such as the synthesis method used and the substrate precursor. Synthesis temperature and time strongly influence the morphology, stability, and performance of PSC. In one-step deposition process, perovskite solution is dispensed on the substrate while turning at a fixed velocity. Though excess perovskite is spun off, perovskites crystallize on evaporation of the solvent. Uncontrolled precipitation yields nonuniform large- and small-size crystals, resulting in poor performance and solar cell efficiency. Adhikari et al. have proposed the usage of an antisolvent method for better crystal formation and growth. It involves annealing conditions and defines charge transport capability, as measured by nanoscale Kelvin probe force microscopy (KPFM).229 Annealing temperature effect on is analyzed from 80°C to 140°C as shown in Fig. 5. The peak of at 12.7°C, in XRD, increases proportionally as per the annealing temperature. The annealing time is optimized to achieve correct crystal size, charge transport layer such as HTL formed CZTS nanoparticles (CZTS-30).230 Thermal annealing of films at 90°C, for short duration such as , would lead to increased crystallinity, eliminating residual solvent and achieving complete conversion of the precursor to crystal, in comparison to longer annealing time.231 Greater annealing time would initiate sublimation, so accurate monitoring of time is essential. For films, photonic curing technique is more effective than high-temperature annealing (), as it creates more conductive films.96 Photonic curing is also called pulse-thermal processing, which utilizes high-intensity plasma arc lamp. It delivers a peak sintering power up to for milliseconds and produces rapidly annealed thin films without damaging underlying plastic substrates. PEDOT:PSS-based PSC grown at room temperature without annealing was reported to exhibit a PCE of 14.27%.232 They have high crystallinity when subjected to 40% RH at room temperature.232
Air sensitivity of PSC elements, in conjunction with moisture sensitivity of perovskite absorbers, emphasizes the requirement of encapsulation of high level to attain a stable outdoor lifetime of over 25 years.3,64,61 Without encapsulation, highly efficient devices were reported to degrade rapidly, showed 80% degradation when subjected to ambient temperature.233 Usually, a thin glass coverslip is used to cover device, which is further sealed using a UV curable epoxy resin.148,169,234 Ramos et al.235 reported that single step high-temperature encapsulation () resulted in delamination of active and metallization layers in devices. A two-step encapsulation approach minimizes the defect due to mechanical and thermal stress. The first step in a pre-encapsulation process consists of making electrical contact and later embedding it in a polymer matrix to ensure mechanical stability. The next step involves laminating the pre-encapsulated device using low temperature, to retain good photovoltaic characteristics of PSC. During the process and after, samples are monitored electrically (J–V curves). There are two types of configuration for encapsulation. Han et al. have compared these two types,168 in first method, UV curable epoxy resin used to fill the area between silver contact and plain glass cover.131,236–238 The other configuration has an electrode, which has a top glass cover with a gap in between. Water absorbent, desiccant material was made to fill this gap, thus absorbing moisture that had penetrated through epoxy resin sealant.168 For 2-D PSC films, encapsulation becomes very critical. Therefore, plastic barrier films are used. It is available in two formats, namely partial and complete. They laminate commercial encapsulate adhesive on to films at 100°C. This type of encapsulation could only retard the ingress of moisture penetration and can maintain the operational performance of PSC steady for more than 1 year.131,236,239–241 Conductive tapes made of commercial carbon assist in achieving a dual purpose such as terminal contact with perovskite and encapsulation. Encapsulated devices are leak tested with helium gas to assure moisture penetration. Investigations reveal that incorporation of impermeable barrier layers can be a long-term remedy to eliminate the ingress of moisture and oxygen.
PSC (for ) has achieved laboratory PCE of 20.1% and an open-circuit voltage () of 1.1 V, which is below theoretical limit of 1.32 V.242 Loss of 200 mV is due to nonradiative recombination, which manifests itself at low external quantum yield () for electroluminescence, corresponding to .242,243 KPFM results of a single step and sequentially deposited films show that increase in potential barrier suppresses back recombination between electrons in and holes in perovskite.244 A blocking layer is very crucial in a conventional PSC for to inhibit charge-interception/recombination. Conversion of to occurs at solution interface of interface.86
Lead and Other Toxicity
Lead composition245 leading to toxicity108 and bioaccumulation in ecosystems246 hamper the commercialization of organic-inorganic lead halide-based PSC. A PSC with as absorber material would contain about one-third lead by weight. On exposure to rainwater, perovskite will completely dissolve in water leaving behind .75
Evaluations show that a PSC panel releases only 70 ppm of lead. This concentration is moderately low in comparison to the standards for low levels of contamination.247 Though it is undesirable and not catastrophic, there is no accepted safe threshold for lead exposure.
In additional, other elements such as iodine248,249 and methylamine250 also have a high degree of toxicity. nanoparticles get airborne and affect the respiratory system, mix with fluids of the body, and reach brain and nerves.251 Daniele et al. have reported the formation of PSC without lead using transition materials such as Cu. They developed series and confirmed green photoluminescence due to Cu+ ions formation. Mixing of halides and chlorine with it will ensure appreciable stability, as it highly deliquescent.252 Impedance spectroscopy was used to evaluate performance. Strong anisotropy and low absorption coefficient hamper these cells. The exact material combination decides the thickness of films and light harvesting requirement with limited diffusion length. Tin (Sn)253–257 is also considered an apt replacement of Pb as it belongs to group 14 metal, with four electrons in its outer shell. However, due to the bigger size of metal in group 14, it induces instability of 2+ oxidation state.139 Theoretical investigations reveal that I-5p orbitals of groups influence band-edge reconstruction. This leads to new conduction–band minimum at Brillouim zone, which facilitated the formation of lead-free perovskites.258 Lead-free perovskites films such as (, Br, Cl) is synthesized by vapor-assisted solution processing253,259 and hot injection method.255 Halide double perovskites utilize interplay between two metal ions to achieve this goal.260,261 Replacing sites with a combination of trivalent and monovalent cations has led to the formation of 3-D double perovskites.262,263
Stability Comparison of 2-D and 3-D PSC
A comparative study shows 2-D PSC is more stable, up to 1400 h than 3-D PSC, when tested under the light source of AM 1.51 without any UV filters.117 They retain about 80% of their original value, whereas 3-D PSC has a dramatic loss in efficiency. Thus, 2-D perovskites showed superior stability and water resistance.117,264,265 Quasi-2-D (-butylammonium)2(MA)2Pb3I10 has an efficiency of 12% with a drop in performance only after 2250 h.117 Grancini and coworkers have integrated 2-D/3-D perovskite to combine advantages of both. Their interface has a combined benefit of excellent stability, excellent charge transport of 2-D perovskites, and absorption quality of 3-D perovskites.266 The ability to collect ions to assemble into a stable 3-D perovskite structure is predicted by the Goldschmidt tolerance factor, “.” It is a stability indicator of a perovskite crystal, which describes the compatibility of ions with other materials in PSC. An ideal Goldschmidt tolerance factor should be below or equal to 1; for guanidinium, it is 1.03. For a perfect cubic perovskite structure . However, practically, one can expect ions to form a cubic 3-D structure if . However, when , it enables the formation of lower symmetry structures, such as tetragonal, rhombohedral, hexagonal, and orthorhombic.267 Recent research at EPFL has reported enlarging the stability of methylammonium lead iodide perovskites, by introducing large organic cation. A promising member such as guanidinium (CH6N3+) is an example of a large organic cation group. It enhances the material’s overall thermal and environmental stability for 3-D perovskites. Figure 25 shows the SEM image of 3-D PSC achieved efficiency over 19% () and sustaining its performance for 1000 h under continuous light illumination.268 Chen et al.269 have reported to achieve exceptional PCE for 2-D format using simple solution process and to achieve a vertical alignment of 2-D perovskite layers to electrodes. Figure 26 shows the vertical orientation of (101) planes and (202) planes parallel to the substrate.269
PSCs are an excellent and advantageous replacement for silicon solar cells. Positive features such as excellent absorption coefficient, ability to tune bandgap, ability to form flexible solar cells, compatibility with the solar spectrum, and good charge mobility make it a good performance solar cell. The hybrid feature of PSC, utilizing the advantages of organic and inorganic components in it, has enabled it to achieve efficiency at par with silicon solar cell and beyond. Perovskites absorber materials coupled with silicon solar cells, in a tandem cell format, could obtain a PCE above 30%. Replacing PSC with better HTM and innovative materials would enhance the PCE. Various combinations and materials are first tested in environment in the encapsulated form before further investigation and exploration. Major hindrances in its commercialization are its stability factor and toxicity. Our survey of the literature assures that continuous progress and development is going on to resolve these hindrances. In addition to the energy-related application, perovskites have an enhanced implication in numerous other domains such as flexible optoelectronics, supercapacitors, rechargeable batteries, and other hybrid systems that couple with photovoltaic cells. The futuristic application is to paste PSC films on buildings and skyscrapers enabling them to produce power and become self-sufficient. Testing methodology process is also wholly restructured to reduce research time on combinations, which cannot be commercialized. Recently, a new fast and low threshold photography method has been developed by Aalto University in Finland to detect even a slight disintegration in a perovskite structure. This method provides more reliable results than optical measurement devices and is less complicated with fewer labor requirements than conventional x-ray crystallography. Encapsulation methodology is also revamped to handle moisture ingress issues. As this adds to the cost of manufacturing, researchers have recently reported the fabrication of air-stable PSC in 2-D format. To date, PSC has achieved a high PCE of 23.3%. Development of innovative materials for perovskites and their combination with PSC to tailor a new increased PCE along with applications in other domains is still yet to be unlocked.
This work was supported by Science and Engineering Research Board, India (Project No. EMR/2016/002815).
Unni Krishnan is a PhD student in the Department of Nanotechnology at Sri Guru Granth Sahib World University (SGGSWU). He received his MTech degree in aerospace from Defence Institute of Armament Technology, Pune, India, in 2013. His current research interest includes semiconducting nanoparticle and their application.
Manjot Kaur is a PhD student in the Department of Nanotechnology at SGGSWU. She has completed her MTech degree with a gold medal in 2018. Her research interest includes development of innovative nanomaterial for semiconducting and biomedical applications.
Manjeet Kumar is a postdoctoral fellow at Incheon National University. He received his PhD from the DIAT-DRDO, Pune, India, in 2016. He completed his MTech degree in 2009 from Thapar University. From 2009 to 2010, he worked as a project associate at IIT, Delhi, India. His research interest includes chemical and physical processing of metal oxides, such as nanowires and functional thin/thick films for sensing applications, and study of interaction mechanism at gas–solid interfaces.
Akshay Kumar is an assistant professor in the Department of Nanotechnology at SGGSWU, India. He received his doctoral degree from Thapar University, India, in 2011, and master’s degree in applied physics from Punjabi University in 2005. He is the author and coauthor of more than 50 research papers and has written one book. His research interest includes structure property relation of nanostructured materials.