Optical simulations of perovskite/silicon tandem solar cells show that nanotexturing both sides of the perovskite top cell yields the strongest antireflective effect. Cells with an intermediate texture in-between the perovskite and silicon sub cells perform comparably to configurations with a fully planar top cell. However, in experiment intermediate-textured solar cells perform slightly better than their planar counterparts. A numerical sensitivity analysis shows that this can be attributed to the thickness of a silicon oxide layer in-between the two sub cells: this thickness affects the optics for a fully planar top cell, but does not affect the performance for intermediate texturing.

Optical simulations of perovskite/silicon tandem solar cells show that nanotexturing both sides of the perovskite top cell yields the strongest antireflective effect. Cells with an intermediate texture in-between the perovskite and silicon sub cells perform comparably to configurations with a fully planar top cell. However, in experiment intermediate-textured solar cells perform slightly better than their planar counterparts. A numerical sensitivity analysis shows that this can be attributed to the thickness of a silicon oxide layer in-between the two sub cells: this thickness affects the optics for a fully planar top cell, but does not affect the performance for intermediate texturing.

Given the large variety of materials and architectures used in perovskite-based tandem solar cells, optical modelling and energy yield calculations are indispensable tools to optimize these devices. To minimize financial and energy payback times, the power output under realistic irradiation conditions need to be assessed for locations in various climatic zones. In response to this challenge, we have developed the software package “EYcalc” at KIT that allows rapid simulation of the complex tandem architectures. “EYcalc” is published and freely available at https://github.com/PerovskitePV/EYcalc. In this contribution, we will present our latest work on energy yield modelling addressing timely research questions of perovskite-based tandem photovoltaics. Our contribution will provide an overview of previous work on three-terminal, textured and bifacial perovskite/Si tandem solar cells [1-5] and elaborate in detail about identifying the optimal texture at the front side of textured perovskite/Si tandem solar cells with solution-processed thick (>1μm) perovskite top cell. Given recent advances in solution processing of thick perovskite cells of high power conversion efficiency, this approach represents a promising strategy for the fabrication of textured tandem solar cells with very good light management. We find that compared to a perfectly conformal deposition of the perovskite top cell, the thick solution-processed perovskite top cells exhibit an only 2-3% lower annual energy yield [6]. [1] R. Schmager et al., Opt. Express 27, A507 (2019). [2] M. Langenhorst et al., Prog. Photovoltaics Res. Appl. 27, 290-298 (2019). [3] J. Lehr et al., Sustain. Energy Fuels 2, 2754–2761 (2018). [4] Lehr, J. et al. Sol. Energy Mater. Sol. Cells 208, 110367 (2020). [5] F. Gota et al., Joule 4 (11), 2387-2403 (2020). [6] F. Gota et al., submitted (2021).

As silicon solar cells are getting close to their fundamental efficiency limit of 29.4%, perovskite/silicon tandem solar cells are the most promising candidate for a technology with higher efficiency potential. It is project to enter mass production within two years. Many companies looking to introduce perovskite/silicon tandems aim for an application in the residential rooftop market. For a successful market entry, this technology does not just need to demonstrate high efficiencies under Standard Testing Conditions (STC: 25°C, 1 kW/m², AM1.5g), but also high energy yields in different climates to be bankable to investors. However, the classical energy yield prediction tools (PVSyst, SAM, etc.) currently do not cover tandem devices. The research community has done several yield prediction studies for perovskite/silicon tandems. However, these studies are done with plane of array irradiance models, which assign homogenous irradiation to the whole module and cannot solve the irradiation situation in complex geometries typically surrounding modules on residential roofs tops with objects like chimneys and trees leading to partial shading. In this work, we introduce a new comprehensive model called “PVMD Toolbox”, which can simulate tandem PV systems as well as integrated PV Systems (IPV) in complex shading scenarios such as residential buildings. We combine wave optics-based generation profiles and a semiconductor software to simulate the optoelectrical properties of the tandem device. On a system level, we use ray tracing to determine irradiance and photon flux onto every solar cell in the PV system. Based on this we determine the temperature and IV curve of each solar cell for every hour of the year, which are then used in a lumped element model to calculate the yield of each module and factoring the inverter the AC yield is determined for two tandem systems in four different climates.

The tunable bandgap and excellent optoelectronic properties of mixed-halide perovskites make this an ideal material for application in (tandem) solar cells. Under light exposure, however, these materials show halide segregation. This results in pinning of the open-circuit voltage by the lowest bandgap regions, which limits the solar cell efficiency. A lot of effort has been taken to prevent photosegregation, but instability remains a problem. In this work, instead of preventing it, we make use of the phase segregation through a self-optimizing process to improve solar cell performance. By placing silica microspheres on a mixed-bromide/iodide perovskite film, incoming light is focused to a hotspot underneath the center of the sphere, leading to local phase segregation. Due to energy funneling to the lowest bandgap region, a local emitter is formed in the hotspot. The rules of reciprocity cause the light emitted from this region to be highly directional into the direction of the incoming light. The focusing of both the incoming and the outgoing light by the spheres results in an increase in both the short circuit current and the open-circuit voltage. The resulting solar cell efficiency is significantly higher compared to the same film without spheres. The system is self-optimized and emitters will always be formed at the optimal location underneath the spheres. Due to constant reconfiguration in the perovskite film, the system can realign itself with respect to the position of the sun, resulting in effective concentration without the need for mechanical solar tracking.

Perovskite-based solar cells are the subject of intense study today because of their promise in terms of high efficiency, easy and low-cost fabrication. To gain insight on the behavior of carriers inside the perovskite layer, time resolved photo luminescence (TR-PL) and time resolved fluorescence imaging (TR-FLIM) are used. However, owing to their long lifetimes (~1µs) and slow diffusion (D~1e-2cm2s-1) the acquired signals require specific care for interpretation. In a previous work, we showed how these properties can be exploited to derive a scaling-law for the normalized time derivative of the TR-PL signal just after the laser pulse 1/tau. This scaling links the derivative to the material parameters: interface and bulk non radiative recombination, radiative recombination, and diffusion. Our previous focus showed the impact of the laser fluence on the derivative and its use to obtain among others the external radiative recombination coefficient. In this work we extend the possibility of our previous technique to separate surface and bulk contributions using the impact of the laser wavelength on the scaling of 1/tau through its impact on the spatial distribution of photogenerated carriers. The absorption coefficient of the material at the laser wavelength plays a crucial role in the scaling. We use theoretical computations as well as drift-diffusion simulation to analyze the range of applicability of our technique. We apply experimentally our methodology on perovskite samples with a pulsed laser of varying wavelength. The aim is to determine quantitatively the bulk, front and bottom surface non radiative. We show experimental validation of the scaling on perovskite material and analyze how it can be combined with drift-diffusion simulation. We investigate both interfaces (bottom and front) by varying the illumination side and show how this technique allows for the quantitative comparison of non-radiative recombination at both interfaces. We discuss the experimental uncertainty.

First, I will discuss the effects of photon reabsorption and surface scattering on the photoluminescence (PL) spectra and the extent of photon recycling in polycrystalline perovskite thin films. A novel curve-fitting model is introduced that, for the first time, allows the determination of the total photon escape probability from measurements of the external luminescence quantum efficiency and a PL spectrum using an integrating sphere setup [1]. Thereby, the large variety of reported externally observed PL spectra (asymmetric/red-shifted/broadened) for perovskite thin films can be explained simply by considering photon propagation and scattering. The model and assumptions are tested through various control experiments and Monte Carlo simulations. Applying the model to highly luminescent CH3NH3PbI3 films with external luminescence quantum efficiencies of up to 47.4% allows to correct previous estimates of the internal luminescence quantum efficiency of ~90% to a new benchmark of ~78% [1]. This emphasizes that there is still much scope for future material optimization of perovskite thin films, especially when integrated into a full solar cell stack where interface recombination still strongly limits the external luminescence quantum efficiency. In addition, I will discuss the effect of parasitically absorbing layers in state-of-the-art perovskite solar cell stacks. It will be shown that parasitic absorption should be minimized not only to maximize light absorption and below-bandgap transmission for perovskite-based tandem solar cell, but also to maximize the extraction of photons initially-trapped in the perovskite thin film. Only this will allow the open-circuit voltage of perovskite solar cells to approach the radiative limit. What is more, luminescent-coupling effects in perovskite-based tandem solar cells crucially depend on the extent of parasitic absorption of the internal luminescence in the perovskite top solar cell. In that regard, I will present first experimental and modelling results on the extent of luminescent-coupling in highly efficient two-terminal perovskite/Si and perovskite/CIGS tandem solar cells. [1] P. Fassl et al., Matter, 2021, 4, 1391–1412.

Inverted perovskite solar cells passivated via organic cations exhibit superior power conversion efficiency compared to un-passivated ones. These record efficiencies have been reached thanks to the use of large organic cations to passivate the interface between the perovskite absorber and the transport layers. Here, we study the optoelectronic properties and chemical structure of interface doped perovskite solar cells where large cations, namely Cl-PEAI and F-PEAI, were incorporated at both front and rear interfaces of the absorber. The effect of the cation addition led to an increase of all the main PV characteristic, reaching PCE values up to 23.7%. We combined steady-state and time-resolved multidimensional photoluminescence imaging techniques to probe the main optoelectronic and transport properties of such devices. We thus obtained quantitative maps for physical parameters such as Quasi Fermi Level Splitting (QFLS), Urbach Energy and surface recombination rate, which proved a homogenous passivation by Cl-PEAI and F-PEAI cations over the 3D surface. For instance, the front and rear surface recombination velocities are reduced by a factor of 6 to 8 for Cl-PEAI based samples and 5 to 10 for F-PEAI based samples. In addition, we identified interfacial passivation as the main mechanism leading to a clear improvement of the Voc which increases from 1.10 to 1.16 eV. Indeed, we noticed a clear increase in terms of QFLS only after the addition of the electron transport layer whereas only an increase in the range of 0.01-0.02 eV was observed for bare perovskite thin film with the cation on top. Mapping the opto-electronical properties showed their good spatial homogeneity, By linking optical and electrical measurements we highlight the benefits of this passivation method in maximising all the main photovoltaic characteristics and in approaching inverted perovskite solar cell theoretical limit.

Absorption coefficient rules photonic transport within solar cells and is key for optical optimization. By probing materials density of states, it offers insights in defects characterization of photovoltaic device. Moreover, obtaining this quantity locally would refine the interpretation of the photoluminescence (PL) maps. Its measurement raise challenges as far as classical spectroscopy signals depends on the whole stack structure. Also, its exponential decrease hinders its accurate determination near bandgap energy. When describing photoluminescence, light absorption influences both the generation function - at the excitation wavelength - and the photons emission and propagation - close to the bandgap energy. We take advantage of these dependances to explore different techniques to obtain either the absorptivity or the absorption coefficient and discuss the link between these two quantities. Transient photoluminescence excitation (PLE) measurement is used to determine the local absorptivity above the bandgap with multidimensional PL imaging techniques. We assess the impact of pulsed excitation on the measurement by modelling the both transient and continuous regime. In addition, absorptivity can be obtained near the optical bandgap directly from the PL spectrum. In this frame, low temperature measurements have already allowed the determination of absorption coefficient with ultra-low values. These different approaches are compared to reflection measurements classically used at the global scale. The limitations and application scope of the different absorption determination will be described, as well as how they relate to one another. Finally, application to PL imaging will be discussed.

Grating-based light trapping for solar cells has been investigated since 1989, when Kiess and Morf [1] proposed that concept. Over the years, various grating designs have been explored theoretically and experimentally. Different grating types were investigated, such as linear, crossed and hexagonal gratings. The periods of the investigated gratings typically are in the range between 300 nm and a few micrometers. While for small periods, only a few diffraction orders propagate in the semiconductor at very high angles with respect to the surface normal, gratings with larger periods excite a higher number of orders with a large variety of propagation angles. Gratings could improve the solar cell performance only in a few cases. One example is a III-V//Si triple junction solar cell developed at Fraunhofer ISE [2], in which a crossed grating with 1 µm period contributed to achieve the record efficiency of 35.9%. Although many researchers have investigated gratings for light trapping and have performed various optimizations, the results do not converge to a clear picture. For the optimization, basically two aspects need to be considered: (1) the path length enhancement due to the propagation angle and (2) the escape probability that is connected to the number of diffraction orders which are either trapped or within the escape cone. In this contribution, we present an analytical approach, which takes single path-length enhancement, escape probability, the divergence of solar radiation and the reciprocity of diffraction efficiencies into account. The basic principles of the approach are strongly related to works by Yablonovich [3], Green [4] and Mellor [5]. Based on rather simple assumptions concerning the distribution of the diffraction efficiencies, we show that for crossed and hexagonal gratings there are sweet spots in the period range leading to excellent light trapping, resulting in enhanced near infrared absorptance and photocurrent. One advantage of such a basic model is that fundamental interrelations of the grating parameters can be understood very well. [1] H. Kiess and R. Morf "Light Trapping In Solar Cells And Determination Of The Absorbed Energy By Calorimetry", Proc. SPIE 1149, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion VIII, (12 December 1989); DOI: 10.1117/12.962174. [2] P. Schygulla et al, "Two-Terminal III-V//Si Triple-Junction Solar Cell with Power Conversion Efficiency of 35.9 % at AM1.5g”, accepted for publication in Prog Photovoltaics. [3] E. Yablonovitch (1982): "Statistical ray optics”, J. Opt. Soc. Am. 72 (7), S. 899–907. [4] M. A. Green (2002): "Lambertian light trapping in textured solar cells and light-emitting diodes. analytical solutions”, In: Prog Photovoltaics 10, S. 235–241. [5] A. Mellor, et al (2011): "Upper limits to absorption enhancement in thick solar cells using diffraction gratings”, In: Prog Photovoltaics 19 (6), S. 676–687. DOI: 10.1002/pip.1086.

Theoretical solar cell efficiency limits increase with optical concentration of sunlight while the usage of expensive absorber material decreases. In practice, the efficiency of concentrator solar cells peaks already at a few hundred Suns and drops sharply thereafter due to a trade-off between shading losses due to the front metal grid and resistive heating losses of the cell. In this work we deposit a transparent polymer layer with V-shaped grooves, imprinted above the cell’s metal fingers, which redirects light away from the metal and onto the absorber material. This renders the contacts effectively transparent and breaks the trade-off, thus allowing for higher concentration ratios and higher efficiency. We will demonstrate how performance of a Si solar cell with 25% front metal grid coverage and an initial short-circuit current density (JSC) of 29.95 mA/cm2 can be improved to a JSC of 39.12 mA/cm2 after adding the patterned polymer, mitigating shading losses almost completely. Furthermore, we investigate the performance of this layer with respect to varying angles of incidence and find that as long as the V-grooves are parallel to the line that describes the motion of the sun across the hemisphere, there is no loss in performance as a function of angle. In fact, based on the electronic and optical angular performance of the designed structure, we obtain an effective shading of 0.6% for a solar cell with 25% front metal grid coverage at 1000 suns, for a realistic concentration geometry. Lastly, we plan on presenting experimental IV-data of highly efficient III-V cells with V-grooves under concentration.

Alternative approaches to random texturing to improve light coupling into solar cells are plentiful, and the understanding of dedicated photonic structures for this purpose deepens continuously. However, only few of the proposed designs are adopted by the PV industry due to lack of scalability. Here, we propose a disordered, nanophotonic ARC design based on cylinder-shaped scattering elements made of titanium dioxide that can be applied to practically any device, irrespective of its material or surface topography. Fabrication is based on a scalable bottom-up technique using colloidal self-assembly and industrial type heterojunction silicon solar cells of multiple square centimeter area are used as a platform to demonstrate feasibility. We experimentally show a broadband reduction of reflectance leading to a 5.1 %rel improvement of short-circuit compared to a reference cell with an optimized flat ARC (80 nm ITO). We use a theoretical model based on Born's first approximation to link the current increase to the arrangement of disks that is characterized by the structure factor S(q) of the disk array. The performance of our metasurface is also discussed within the framework of helicity preservation, which can be achieved at specific wavelengths for an isolated disk for illumination along the symmetry axis by tuning its dimensions, and in comparison to a simulated periodic metasurface.

Photon upconversion could enable to harvest parts of the solar spectrum that cannot be absorbed in conventional silicon solar cells. However, typical upconversion materials suffer from a low absorption cross-section and a low brightness. Using the sun as energy source, signal enhancement strategies for efficient photon upconversion are required, e.g. by the utilization of metasurfaces providing strongly enhanced electrical near-fields. Here, we placed β-NaYF4:Er3+ nanoparticles on large-area silicon metasurfaces designed to convert near-infrared (1550 nm) excitation light to visible luminescence. We observed a more than 2400-fold enhanced photon upconversion luminescence compared to a planar substrate. Optical simulations allowed attributing this result to the coupling of the excitation source with metasurface resonances at appropriate incident angles. Aiming at broadband applications such as solar energy conversion we also introduced a multi-layer metasurface design: Comparing multi-layer to conventional single-layer structures revealed that the resonances associated with enhanced near-fields are split into multiple different modes and are spectrally broadened. The findings not only permit the significant reduction of the excitation power densities required for photon upconversion of 1550 nm light, but also introduce a new approach for efficient upconversion under broadband excitation conditions. This can open new perspectives for applications of such materials in the third biological excitation window, telecommunication, and photovoltaics.

This presentation is for the upconversion for harvesting subbandgap photons in perovskite solar cells.

We introduce an optically resonant bulk heterojunction solar cell to study optoelectronic properties of nanostructured p-n junctions. The nanostructures yield strong light-matter interaction, as well as distinct charge-carrier extraction behaviour, which together improve the overall power conversion efficiency. We demonstrate a novel approach using high-resolution substrate conformal soft-imprint lithography technology in combination with state-of-the art ZnO nanoparticles to create a nanohole template in an electron transport layer. The nanoholes are infiltrated with PbS quantum dots (QDs) to form a nanopatterned depleted heterojunction. Optical simulations show the absorption per unit volume in the cylindrical QD absorber layer is enhanced by 19.5% compared to a planar reference. This is achieved for a square array of QD nanopillars of 330 nm height and 320 nm diameter, with a pitch of 500 nm on top of a residual QD layer of 70nm, surrounded by ZnO. Electronic simulations show that the patterning results in a current gain of 3.2 mA/cm2 and a slight gain in voltage, yielding an efficiency gain of 0.4%. Our simulations further show that the fill factor is highly sensitive to the patterned structure. This is explained by the electric field strength varying strongly across the patterned absorber. We outline a path towards further optimized optically resonant nanopattern geometries with enhanced carrier collection properties. We demonstrate a 0.74 mA/cm2 current gain for a patterned cell compared to a planar cell in experiments, owing to a much improved infrared response, as predicted by our simulations.