Triple-junction perovskite-perovskite-silicon solar cells with power conversion efficiency of 24.4

The recent tremendous progress in monolithic perovskite-based double-junction solar cells is just the start of a new era of ultra-high-efficiency multi-junction photovoltaics. We report on triple-junction perovskite-perovskite-silicon solar cells with a record power conversion efficiency of 24.4%. Optimizing the light management of each perovskite sub-cell (∼1.84 and ∼1.52 eV for top and middle cells, respectively), we maximize the current generation up to 11.6 mA cm-2. Key to this achievement was our development of a high-performance middle perovskite sub-cell, employing a stable pure-α-phase high-quality formamidinium lead iodide perovskite thin film (free of wrinkles, cracks, and pinholes). This enables a high open-circuit voltage of 2.84 V in a triple junction. Non-encapsulated triple-junction devices retain up to 96.6% of their initial efficiency if stored in the dark at 85 °C for 1081 h.

Inkjet-printed optical interference filters

Optical interference filters (OIFs) are vital components for a wide range of optical and photonic systems. They are pivotal in controlling spectral transmission and reflection upon demand. OIFs rely on optical interference of the incident wave at multilayers, which are fabricated with nanometer precision. Here, we demonstrate that these requirements can be fulfilled by inkjet printing. This versatile technology offers a high degree of freedom in manufacturing, as well as cost-affordable and rapid-prototyping features from the micron to the meter scale. In this work, via rational ink design and formulation, OIFs were fully inkjet printed in ambient conditions. Longpass, shortpass, bandpass, and dichroic OIFs were fabricated, and precise control of the spectral response in OIFs was realized. Subsequently, customized lateral patterning of OIFs by inkjet printing was achieved. Furthermore, upscaling of the printed OIFs to A4 size (29.7 × 21.0 cm²) was demonstrated.

Modeling and Fundamental Dynamics of Vacuum, Gas, and Antisolvent Quenching for Scalable Perovskite Processes

Hybrid perovskite photovoltaics (PVs) promise cost-effective fabrication with large-scale solution-based manufacturing processes as well as high power conversion efficiencies. Almost all of today's high-performance solution-processed perovskite absorber films rely on so-called quenching techniques that rapidly increase supersaturation to induce a prompt crystallization. However, to date, there are no metrics for comparing results obtained with different quenching methods. In response, the first quantitative modeling framework for gas quenching, anti-solvent quenching, and vacuum quenching is developed herein. Based on dynamic thickness measurements in a vacuum chamber, previous works on drying dynamics, and commonly known material properties, a detailed analysis of mass transfer dynamics is performed for each quenching technique. The derived models are delivered along with an open-source software framework that is modular and extensible. Thereby, a deep understanding of the impact of each process parameter on mass transfer dynamics is provided. Moreover, the supersaturation rate at critical concentration is proposed as a decisive benchmark of quenching effectiveness, yielding ≈ 10-3 - 10-1s-1 for vacuum quenching, ≈ 10-5 - 10-3s-1 for static gas quenching, ≈ 10-2 - 100s-1 for dynamic gas quenching and ≈ 102s-1 for antisolvent quenching. This benchmark fosters transferability and scalability of hybrid perovskite fabrication, transforming the "art of device making" to well-defined process engineering.

Discovering Process Dynamics for Scalable Perovskite Solar Cell Manufacturing with Explainable AI

Large-area processing of perovskite semiconductor thin-films is complex and evokes unexplained variance in quality, posing a major hurdle for the commercialization of perovskite photovoltaics. Advances in scalable fabrication processes are currently limited to gradual and arbitrary trial-and-error procedures. While the in situ acquisition of photoluminescence (PL) videos has the potential to reveal important variations in the thin-film formation process, the high dimensionality of the data quickly surpasses the limits of human analysis. In response, this study leverages deep learning (DL) and explainable artificial intelligence (XAI) to discover relationships between sensor information acquired during the perovskite thin-film formation process and the resulting solar cell performance indicators, while rendering these relationships humanly understandable. The study further shows how gained insights can be distilled into actionable recommendations for perovskite thin-film processing, advancing toward industrial-scale solar cell manufacturing. This study demonstrates that XAI methods will play a critical role in accelerating energy materials science.

Controlling Thin Film Morphology Formation during Gas Quenching of Slot-Die Coated Perovskite Solar Modules

Transferring record power conversion efficiency (PCE) >25% of spin coated perovskite solar cells (PSCs) from the laboratory scale to large-area photovoltaic modules requires significant advances in scalable fabrication techniques. In this work, we demonstrate the fundamental interrelation between drying dynamics of slot-die coated precursor solution thin films and the quality of resulting slot-die coated gas-quenched polycrystalline perovskite thin films. Well-defined drying conditions are established using a temperature-stabilized, movable table and a flow-controlled, oblique impinging slot nozzle purged with nitrogen. The accurately deposited solution thin film on the substrate is recorded by a tilted CCD camera, allowing for in situ monitoring of the perovskite thin film formation. With the tracking of crystallization dynamics during the drying process, we identify the critical process parameters needed for the design of optimal drying and gas quenching systems. In addition, defining different drying regimes, we derive practical slot jet adjustments preventing gas backflow and demonstrate large-area, homogeneous, and pinhole-free slot-die coated perovskite thin films that result in solar cells with PCEs of up to 18.6%. Our study reveals key interrelations of process parameters, e.g., the gas flow and drying velocity, and the exact crystallization position with the morphology formation of fabricated thin films, resulting in a homogeneous performance of corresponding 50 × 50 mm2 solar minimodules (17.2%) with only minimal upscaling loss. In addition, we validate a previously developed model on the drying dynamics of perovskite thin films on small-area slot-die coated areas of ≥100 cm2. The study provides methodical guidelines for the design of future slot-die coating setups and establishes a step forward to a successful transfer of solution processes towards industrial-scale deposition systems beyond brute force optimization.

Bright circularly polarized photoluminescence in chiral layered hybrid lead-halide perovskites

Hybrid perovskite semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while softness of their crystal lattice accommodates lattice strain to maintain high crystal quality with low defect densities, necessary for high luminescence yields. We report photoluminescence quantum efficiencies as high as 39% and degrees of circularly polarized photoluminescence of up to 52%, at room temperature, in the chiral layered hybrid lead-halide perovskites (R/S/Rac)-3BrMBA2PbI4 [3BrMBA = 1-(3-bromphenyl)-ethylamine]. Using transient chiroptical spectroscopy, we explain the excellent photoluminescence yields from suppression of nonradiative loss channels and high rates of radiative recombination. We further find that photoexcitations show polarization lifetimes that exceed the time scales of radiative decays, which rationalize the high degrees of polarized luminescence. Our findings pave the way toward high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

Enhancement of Amplified Spontaneous Emission by Electric Field in CsPbBr3 Perovskites

Perovskite gain materials can sustain continuous-wave lasing at room-temperature. A first step toward the unachieved goal of electrically excited lasing would be an improvement in gain when electrical stimulation is added to the optical. However, to date, electrical stimulation supplementing optical has reduced gain performance. We find that amplified spontaneous emission (ASE) in a CsPbBr₃ perovskite light-emitting diode (LED) held under invariant subthreshold optical excitation can be turned on/off by the addition/removal of an electric field. A positive bias voltage leads to a factor of 3 reduction in the optical ASE threshold, the cause of which can be attributed to an enhancement of the radiative rate. The slow components (10 s time scale) of the modulation in the photoluminescence and ASE when the voltage is changed suggest that the relocation of mobile ions trigger the increased radiative rate and observed lowering of ASE thresholds.

Monolithic Two-Terminal Perovskite/CIS Tandem Solar Cells with Efficiency Approaching 25

Monolithic two-terminal (2T) perovskite/CuInSe₂ (CIS) tandem solar cells (TSCs) combine the promise of an efficient tandem photovoltaic (PV) technology with the simplicity of an all-thin-film device architecture that is compatible with flexible and lightweight PV. In this work, we present the first-ever 2T perovskite/CIS TSC with a power conversion efficiency (PCE) approaching 25% (23.5% certified, area 0.5 cm²). The relatively planar surface profile and narrow band gap (∼1.03 eV) of our CIS bottom cell allow us to exploit the optoelectronic properties and photostability of a low-Br-containing perovskite top cell as revealed by advanced characterization techniques. Current matching was attained by proper tuning of the thickness and bandgap of the perovskite, along with the optimization of an antireflective coating for improved light in-coupling. Our study sets the baseline for fabricating efficient perovskite/CIS TSCs, paving the way for future developments that might push the efficiencies to over 30%.

Energy yield modelling of textured perovskite/silicon tandem photovoltaics with thick perovskite top cells

Recent advances in solution processing of micrometer-thick perovskite solar cells over textured silicon bottom solar cells allowed a new promising approach for the fabrication of 2T perovskite/silicon tandem photovoltaics, combining optimal light management in the textured bottom cell with the ease of solution processing. Detailed simulations are needed to assess the performances of this morphology configuration (thick perovskite configuration). In this work, in-depth optical and energy yield (EY) simulations are performed to compare the thick perovskite configuration with other relevant morphology configurations for 2T perovskite/silicon tandem photovoltaics. Under standard test conditions, the total photogenerated current of the thick perovskite configuration is 1.3 mA cm^-2 lower (-3.4% relative) than the one of the conformal perovskite on textured silicon configuration for non-encapsulated cells and only 0.8 mA cm^-2 (-2.1% relative) for encapsulated cells. Under realistic outdoor conditions, EY modelling for a wide range of locations shows that, while conformal perovskite on textured silicon configuration remains the optimal configuration, thick perovskite configuration exhibits a mere ∼2.5% lower annual EY. Finally, intermediate scenarios are investigated with the angle of the perovskite front-side texture differing from the silicon texture and critical angles for efficient light management in these configurations are identified.

Drying and Coating of Perovskite Thin Films: How to Control the Thin Film Morphology in Scalable Dynamic Coating Systems

Hybrid perovskite photovoltaics combine high performance with the ease of solution processing. However, to date, a poor understanding of morphology formation in coated perovskite precursor thin films casts doubt on the feasibility of scaling-up laboratory-scale solution processes. Oblique slot jet drying is a widely used scalable method to induce fast crystallization in perovskite thin films, but deep knowledge and explicit guidance on how to control this dynamic method are missing. In response, we present a quantitative model of the drying dynamics under oblique slot jets. Using this model, we identify a simple criterion for successful scaling of perovskite solution printing and predict coating windows in terms of air velocity and web speed for reproducible fabrication of perovskite solar cells of ∼15% in power conversion efficiency─in direct correlation with the morphology of fabricated thin films. These findings are a corner stone toward scaling perovskite fabrication from simple principles instead of trial and error optimization.

Emergence of Deep Traps in Long-Term Thermally Stressed CH3NH3PbI3 Perovskite Revealed by Thermally Stimulated Currents

Defect states are known to trigger trap-assisted nonradiative recombination, restricting the performance of perovskite solar cells (PSCs). Here, we investigate the trap states in long-term thermally stressed methylammonium lead iodide (MAPbI₃) perovskite thin films over 500 h at 85 °C employing thermally stimulated current measurements. A prominent deep trap level was detected with an activation energy of ∼0.459 eV in MAPbI₃ without being thermally stressed. Interestingly, upon the application of thermal stress, an additional deep trap level of activation energy ∼0.414 eV emerges and grows with thermal stress duration. After 500 h of thermal stress, the trap density was ∼10¹⁶ cm^-3. The trend of open-circuit voltage loss was in line with the trap density variation with thermal stress time, which elucidates the enhanced nonradiative recombination through these trap states. This work opens a path to understanding the mechanism behind long-term thermal instability and further inspires the development of strategies to minimize trap formation in PSCs.

Harvesting Sub-bandgap Photons via Upconversion for Perovskite Solar Cells

Lanthanide-based upconversion (UC) allows harvesting sub-bandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF₂:Yb³⁺, Er³⁺ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF₂:Yb³⁺, Er³⁺ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite's bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm² enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of sub-bandgap illumination, and at higher intensity, 2.09 mA/cm² enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF₂:Yb³⁺, Er³⁺ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity.

Annual energy yield of mono- and bifacial silicon heterojunction solar modules with high-index dielectric nanodisk arrays as anti-reflective and light trapping structures

While various nanophotonic structures applicable to relatively thin crystalline silicon-based solar cells were proposed to ensure effective light in-coupling and light trapping in the absorber, it is of great importance to evaluate their performance on the solar module level under realistic irradiation conditions. Here, we analyze the annual energy yield of relatively thin (crystalline silicon (c-Si) wafer thickness between 5 μm and 80 μm) heterojunction (HJT) solar module architectures when optimized anti-reflective and light trapping titanium dioxide (TiO₂) nanodisk square arrays are applied on the front and rear cell interfaces, respectively. Our numerical study shows that upon reducing c-Si wafer thickness down to 5 μm, the relative increase of the annual energy yield can go up to 23.3 %_rel and 43.0 %_rel for mono- and bifacial solar modules, respectively, when compared to the reference modules with flat optimized anti-reflective coatings of HJT solar cells.

In2O3:H-Based Hole-Transport-Layer-Free Tin/Lead Perovskite Solar Cells for Efficient Four-Terminal All-Perovskite Tandem Solar Cells

Narrow-band gap (NBG) Sn-Pb perovskites with band gaps of ∼1.2 eV, which correspond to a broad photon absorption range up to ∼1033 nm, are highly promising candidates for bottom solar cells in all-perovskite tandem photovoltaics. To exploit their potential, avoiding optical losses in the top layer stacks of the tandem configuration is essential. This study addresses this challenge in two ways (1) removing the hole-transport layer (HTL) and (2) implementing highly transparent hydrogen-doped indium oxide In₂O₃:H (IO:H) electrodes instead of the commonly used indium tin oxide (ITO). Removing HTL reduces parasitic absorption loss in shorter wavelengths without compromising the photovoltaic performance. IO:H, with an ultra-low near-infrared optical loss and a high charge carrier mobility, results in a remarkable increase in the photocurrent of the semitransparent top and (HTL-free) NBG bottom perovskite solar cells when substituted for ITO. As a result, an IO:H-based four-terminal all-perovskite tandem solar cell (4T all-PTSCs) with a power conversion efficiency (PCE) as high as 24.8% is demonstrated, outperforming ITO-based 4T all-PTSCs with PCE up to 23.3%.

Thermal Stability and Cation Composition of Hybrid Organic-Inorganic Perovskites

One of the great challenges of hybrid organic-inorganic perovskite photovoltaics is the material's stability at elevated temperatures. Over the past years, significant progress has been achieved in the field by compositional engineering of perovskite semiconductors, e.g., using multiple-cation perovskites. However, given the large variety of device architectures and nonstandardized measurement protocols, a conclusive comparison of the intrinsic thermal stability of different perovskite compositions is missing. In this work, we systematically investigate the role of cation composition on the thermal stability of perovskite thin films. The cations in focus of this study are methylammonium (MA), formamidinium (FA), cesium, and the most common mixtures thereof. We compare the thermal degradation of these perovskite thin films in terms of decomposition, optical losses, and optoelectronic changes when stressed at 85 °C for a prolonged time. Finally, we demonstrate the effect of thermal stress on perovskite thin films with respect to their performance in solar cells. We show that all investigated perovskite thin films show signs of degradation under thermal stress, though the decomposition is more pronounced in methylammonium-based perovskite thin films, whereas the stoichiometry in methylammonium-free formamidinium lead iodide (FAPbI₃) and formamidinium cesium lead iodide (FACsPbI₃) thin films is much more stable. We identify compositions of formamidinium and cesium to result in the most stable perovskite compositions with respect to thermal stress, demonstrating remarkable stability with no decline in power conversion efficiency when stressed at 85 °C for 1000 h. Thereby, our study contributes to the ongoing quest of identifying the most stable perovskite compositions for commercial application.

Bimolecular and Auger Recombination in Phase-Stable Perovskite Thin Films from Cryogenic to Room Temperature and Their Effect on the Amplified Spontaneous Emission Threshold

Recently, continuous-wave (CW) lasing was demonstrated at room temperature in quasi-2D perovskites. For 3D films, CW lasing at room temperature remains challenging. Issues hampering 3D materials include the temperature dependence of the (a) distribution of carrier energies, (b) buildup of photoinduced nonradiative channels, and (c) rates of bimolecular versus Auger recombination. We study the latter in a phase-stable 3D perovskite using high-index substrates to completely suppress amplified spontaneous emission (ASE). The bimolecular recombination coefficient decreases from 80 to 290 K (from (6.4 to 1.1) × 10^-10 cm^-3 s^-1), whereas the Auger coefficient stays constant at 3 × 10^-29 cm^-6 s^-1. Above 250 K, the Auger rate exceeds the bimolecular rate at carrier densities corresponding to the ASE threshold. At lower temperatures, the decrease in the bimolecular rate coefficient with increasing temperature and the fraction of photoluminescence in the ASE band determine the temperature dependence of the ASE threshold.

Co-evaporation of CH3NH3PbI3: How Growth Conditions Impact Phase Purity, Photostriction, and Intrinsic Stability

Hybrid organic-inorganic perovskites are highly promising candidates for the upcoming generation of single- and multijunction solar cells. Despite their extraordinarily good semiconducting properties, there is a need to increase the intrinsic material stability against heat, moisture, and light exposure. Understanding how variations in synthesis affect the bulk and surface stability is therefore of paramount importance to achieve a rapid commercialization on large scales. In this work, we show for the case of methylammonium lead iodide that a thorough control of the methylammonium iodide (MAI) partial pressure during co-evaporation is essential to limit photostriction and reach phase purity, which dictate the absorber stability. Kelvin probe force microscopy measurements in ultrahigh vacuum corroborate that off-stoichiometric absorbers prepared with an excess of MAI partial pressure exhibit traces of low-dimensional (two-dimensional, 2D) perovskites and stacking faults that have adverse effects on the intrinsic material stability. Under optimized growth conditions, time-resolved photoluminescence and work functions mapping corroborate that the perovskite films are less prone to heat and light degradation.

Numerical study on the angular light trapping of the energy yield of organic solar cells with an optical cavity

A limiting factor in organic solar cells (OSCs) is the incomplete absorption in the thin absorber layer. One concept to enhance absorption is to apply an optical cavity design. In this study, the performance of an OSC with cavity is evaluated. By means of a comprehensive energy yield (EY) model, the improvement is demonstrated by applying realistic sky irradiance, covering a wide range of incidence angles. The relative enhancement in EY for different locations is found to be 11-14% compared to the reference device with an indium tin oxide front electrode. The study highlights the improved angular light absorption as well as the angular robustness of an OSC with cavity.

Flexible Inkjet-Printed Triple Cation Perovskite X-ray Detectors

Flexible direct conversion X-ray detectors enable a variety of novel applications in medicine, industry, and science. Hybrid organic-inorganic perovskite semiconductors containing elements of high atomic number combine an efficient X-ray absorption with excellent charge transport properties. Due to their additional cost-effective and low-temperature processability, perovskite semiconductors represent promising candidates to be used as active materials in flexible X-ray detectors. Inspired by the promising results recently reported on X-ray detectors that are based on either triple cation perovskites or inkjet-printed perovskite quantum dots, we here investigate flexible inkjet-printed triple cation perovskite X-ray detectors. The performance of the detectors is evaluated by the X-ray sensitivity, the dark current, and the X-ray stability. Exposed to 70 kVp X-ray radiation, reproducible and highly competitive X-ray sensitivities of up to 59.9 μC/(Gy_aircm²) at low operating voltages of 0.1 V are achieved. Furthermore, a significant dark current reduction is demonstrated in our detectors by replacing spin-coated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) with sputtered NiO_x hole transport layers. Finally, stable operation of a flexible X-ray detector for a cumulative X-ray exposure of 4 Gy_air is presented, and the applicability of our devices as X-ray imaging detectors is shown. The results of this study represent a proof of concept toward flexible direct conversion X-ray detectors realized by cost-effective and high-throughput digital inkjet printing.

Nanostructured front electrodes for perovskite/c-Si tandem photovoltaics

The rise in the power conversion efficiency (PCE) of perovskite solar cells has triggered enormous interest in perovskite-based tandem photovoltaics. One key challenge is to achieve high transmission of low energy photons into the bottom cell. Here, nanostructured front electrodes for 4-terminal perovskite/crystalline-silicon (perovskite/c-Si) tandem solar cells are developed by conformal deposition of indium tin oxide (ITO) on self-assembled polystyrene nanopillars. The nanostructured ITO is optimized for reduced reflection and increased transmission with a tradeoff in increased sheet resistance. In the optimum case, the nanostructured ITO electrodes enhance the transmittance by ∼7% (relative) compared to planar references. Perovskite/c-Si tandem devices with nanostructured ITO exhibit enhanced short-circuit current density (2.9 mA/cm² absolute) and PCE (1.7% absolute) in the bottom c-Si solar cell compared to the reference. The improved light in-coupling is more pronounced for elevated angle of incidence. Energy yield enhancement up to ∼10% (relative) is achieved for perovskite/c-Si tandem architecture with the nanostructured ITO electrodes. It is also shown that these nanostructured ITO electrodes are also compatible with various other perovskite-based tandem architectures and bear the potential to improve the PCE up to 27.0%.

High-Brightness Perovskite Light-Emitting Diodes Using a Printable Silver Microflake Contact

Achieving efficient devices while maintaining a high fabrication yield is a key challenge in the fabrication of solution-processed, perovskite-based light-emitting diodes (PeLEDs). In this respect, pinholes in the solution-processed perovskite layers are a major obstacle. These are usually mitigated using organic electron-conducting planarization layers. However, these organic interlayers are unstable under applied bias in air and suffer from limited charge carrier mobility. In this work, we present a high brightness p-i-n PeLED based on a novel blade-coated silver microflake (SMF) rear electrode, which allows for a low-cost nanocrystalline ZnO inorganic electron-transporting layer to be used. This novel SMF contact is crucial for achieving high performance as it prevents the electrical shorting suffered when standard thermally evaporated silver rear contacts are used. The fabricated PeLEDs exhibit an excellent maximum luminance of 98,000 cd/m², a maximum current efficiency of 22.3 cd/A, and a high external quantum efficiency of 4.6% under 5.9 V forward bias. The SMF rear contact can be printed and scaled at low cost to large areas and applied to flexible devices.

Perovskite/Hole Transport Layer Interface Improvement by Solvent Engineering of Spiro-OMeTAD Precursor Solution

Perovskite solar cells (PSCs) are one of the most promising emerging energy-conversion technologies because of their high power conversion efficiencies (PCEs) and potentially low fabrication cost. To improve PCE, it is necessary to develop PSCs with good interfacial engineering to reduce the trap states and carrier transport barriers present at the various interfaces of the PSCs' architecture. This work reports a facile method to improve the interface between the perovskite absorber layer and the hole transport layer (HTL) by adding a small amount of acetonitrile (ACN) in the Spiro-OMeTAD precursor solution. This small amount of ACN dissolves the surface of the perovskite layer, allowing the formation of an interdiffusion structure between perovskite and Spiro-OMeTAD layers. This modification allows for an improved electrical contact, enhanced collection of holes, and reduced recombination losses at the interface between the perovskite and Spiro-OMeTAD layers and, consequently, enhances the PCE. A maximum PCE of 19.7% with low hysteresis and a steady-state power conversion efficiency of 19.0% is obtained for optimized PSCs.

Liquid Glass for Photovoltaics: Multifunctional Front Cover Glass for Solar Modules

Advanced optical concepts, making use of tailored microstructured front cover glasses, promise to reduce the losses encountered with encapsulated solar modules. However, implementing optical concepts into the conventional architecture of encapsulated solar modules and simultaneously maintaining high durability represent a severe technological challenge. The liquid glass technique offers a route to meet this challenge by enabling the implementation of these optical concepts directly into the durable front cover glass of solar modules. In this work, we demonstrate for the first time two showcases of texturing fused silica front cover glass, using the facile liquid glass technique: (I) multifunctional microcone textures that reduce front-side reflection losses by ∼80% compared to a planar reference, which correlates to an increase in short-circuit current density of encapsulated planar monocrystalline silicon heterojunction solar cells by 2.9 mA cm^-2, and exhibit strong hydrophilic behavior facilitating self-cleaning and (II) embedded freeform surface cloaks that redirect incident light away from the metallic contact grids of the solar cell and demonstrate a cloaking efficiency of ∼88%.

Light Management: A Key Concept in High-Efficiency Perovskite/Silicon Tandem Photovoltaics

The remarkable recent progress in perovskite photovoltaics affords a novel opportunity to advance the power conversion efficiency of market-dominating crystalline silicon (c-Si) solar cells. A severe limiting factor in the development of perovskite/c-Si tandems to date has been their inferior light-harvesting ability compared to single-junction c-Si solar cells, but recent innovations have made impressive headway on this front. Here, we provide a quantitative perspective on future steps to advance perovskite/c-Si tandem photovoltaics from a light-management point of view, addressing key challenges and available strategies relevant to both the 2-terminal and 4-terminal perovskite/c-Si tandem architectures. In particular, we discuss the challenge of achieving low optical reflection in 2-terminal cells, optical shortcomings in state-of-the-art devices, the impact of transparent electrode performance, and a variety of factors which influence the optimal bandgap for perovskite top-cells. Focused attention in each of these areas will be required to make the most of the tandem opportunity.

Coated and Printed Perovskites for Photovoltaic Applications

Hybrid organic-inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low-cost thin-film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small-area perovskite photovoltaics has surpassed many established thin-film technologies. However, the large-scale solution-based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high-quality, pin-hole-free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin-coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale-up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin-coated solar cells and to scale these efficiencies to large areas are highlighted.

Model for the Prediction of the Lifetime and Energy Yield of Methyl Ammonium Lead Iodide Perovskite Solar Cells at Elevated Temperatures

With the realization of highly efficient perovskite solar cells, the long-term stability of these devices is the key challenge hindering their commercialization. In this work, we study the temperature-dependent stability of perovskite solar cells and develop a model capable of predicting the lifetime and energy yield of perovskite solar cells outdoors. This model results from the measurement of the kinetics governing the degradation of perovskite solar cells at elevated temperatures. The individual analysis of all key current-voltage parameters enables the prediction of device performance under thermal stress with high precision. An extrapolation of the device lifetime at various European locations based on historical weather data illustrates the relation between the laboratory data and real-world applications. Finally, the understanding of the degradation mechanisms affecting perovskite solar cells allows the definition and implementation of strategies to enhance the thermal stability of perovskite solar cells.

Methodology of energy yield modelling of perovskite-based multi-junction photovoltaics

Energy yield (EY) modelling is an indispensable tool to minimize payback time of emerging perovskite-based multi-junction photovoltaics (PV) but it relies on many assumptions about device architecture and environmental conditions. Here, we propose a comprehensive framework that enables rapid simulation of complex architectures of perovskite-based multi-junction PV and detailed calculation of their power output under realistic irradiation conditions in various climatic zones. Applying the framework to perovskite/silicon multi-junction solar modules, we showcase the impact of tracking on energy losses arising from spectral variations. Moreover, we demonstrate the strong dependency of the EY of bifacial multi-junction solar modules on the albedo.

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm^-2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

Exposure-dependent refractive index of Nanoscribe IP-Dip photoresist layers

The refractive indices of photoresists used for direct laser writing (DLW) have been determined after exposure to ultraviolet (UV) light. However, it was anticipated that the refractive index will differ when applying a two-photon polymerization (TPP) process. In this Letter, we demonstrate that this is indeed the case. Making use of a guided mode coupling approach, we measure the dispersive real part of the refractive index (n) of a commercial photoresist (IP-Dip, Nanoscribe) at very high accuracy. Additionally, the imaginary part of the refractive index (k) is determined from absorption measurements for wavelengths in the range 300 to 1700 nm. TPP layers exhibit a significantly lower refractive index than their UV exposed bulk counterparts (Δn up to 0.01). Furthermore, when fabricating a TPP shell and UV exposing the interior, the refractive index of the shell will not change. This is an important consideration for optical component design and opens the possibility for low refractive index difference wave guiding.

Spectral Dependence of Degradation under Ultraviolet Light in Perovskite Solar Cells

Perovskite solar cells (PSCs) demonstrate excellent power conversion efficiencies (PCEs) but face severe stability challenges. One key degradation mechanism is exposure to ultraviolet (UV) light. However, the impact of different UV bands is not yet well established. Here, we systematically study the stability of PSCs on the basis of a methylammonium lead iodide (CH₃NH₃PbI₃) absorber exposed to (i) 310-317 (UV-B range) and (ii) 360-380 nm (UV-A range), under accelerated conditions. We demonstrate that the investigated UV-B band is detrimental to the stability of PSCs, resulting in PCE degradation by more than 50% after an exposure period >1700 sun-hours. This finding is valid for architectures with a range of electron transport layers, including SnO₂, compact-TiO₂, electron-beam TiO₂, and nanoparticle-TiO₂. We also show that photodegradation is apparent for high, as well as for low illumination intensities of UV-B light, but not for illumination with UV-A wavelengths. Finally, we show that degradation of PSCs is preventable at the cost of a small fraction of photocurrent by using UV-filtering or luminescent downshifting layers.

Temperature Variation-Induced Performance Decline of Perovskite Solar Cells

This paper reports on the impact of outdoor temperature variations on the performance of organo metal halide perovskite solar cells (PSCs). It shows that the open-circuit voltage ( V_OC) of a PSC decreases linearly with increasing temperature. Interestingly, in contrast to these expected trends, the current density ( J_SC) of PSCs is found to decline strongly below 20% of the initial value upon cycling the temperatures from 10 to 60 °C and back. This decline in the current density is driven by an increasing series resistance and is caused by the fast temperature variations as it is not apparent for solar cells exposed to constant temperatures of the same range. The effect is fully reversible when the devices are kept illuminated at an open circuit for several hours. Given these observations, an explanation that ascribes the temperature variation-induced performance decline to ion accumulation at the contacts of the solar cell because of temperature variation-induced changes of the built-in field of the PSC is proposed. The effect might be a major obstacle for perovskite photovoltaics because the devices exposed to real outdoor temperature profiles over 4 h showed a performance decline of >15% when operated at a maximum power point.

Inkjet-printed perovskite distributed feedback lasers

We report on digitally printed distributed feedback lasers on flexible polyethylene terephthalate substrates based on methylammonium lead iodide perovskite gain material. The perovskite lasers are printed with a digital drop-on-demand inkjet printer, providing full freedom in the shape and design of the gain layer. We show that adjusting the perovskite ink increases the potential processing window and decreases the surface roughness of the active layer to less than 7 nm, which is essential for low lasing thresholds. Prototype inkjet-printed perovskite lasers processed on top of nanopatterned rigid as well as flexible substrates are demonstrated. Optimized perovskite gain layers printed on PET substrates demonstrated lasing and showed a linewidth of 0.4 nm and a lasing threshold of 270 kW/cm². In addition, printing of a distinct shape shows a high level of uniformity, demonstrated by a low spatial resolved full width half maximum variation over the whole printing area. These results reveal the possibilities of digital printed perovskite layers towards large-scale and low-cost laser applications of arbitrary shape.

Infiltrated photonic crystals for light-trapping in CuInSe2 nanocrystal-based solar cells

Solution processable nanocrystal solar cells combine the advantages of low-cost printing and wide range of accessible absorber materials, however high trap densities limit performance and layer thickness. In this work we develop a versatile route to realize the infiltration of a photonic crystal, with copper indium diselenide nanocrystal ink. The photonic crystal allows to couple incident light into pseudo-guided modes and thereby enhanced light absorption. For the presented design, we are able to identify individual guided modes, explain the underlying physics, and obtain a perfect match between the measured and simulated absorption peaks. For our relatively low refractive index layers, a 7% maximum integrated absorption enhancement is demonstrated.

Post passivation light trapping back contacts for silicon heterojunction solar cells

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 μm and additional anti-reflection coating, the short-circuit current density (J_SC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the J_SC is enhanced around 1.8 mA cm^-2 to 38.5 mA cm^-2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

3D-printed external light trap for solar cells

We present a universally applicable 3D-printed external light trap for enhanced absorption in solar cells. The macroscopic external light trap is placed at the sun-facing surface of the solar cell and retro-reflects the light that would otherwise escape. The light trap consists of a reflective parabolic concentrator placed on top of a reflective cage. Upon placement of the light trap, an improvement of 15% of both the photocurrent and the power conversion efficiency in a thin-film nanocrystalline silicon (nc-Si:H) solar cell is measured. The trapped light traverses the solar cell several times within the reflective cage thereby increasing the total absorption in the cell. Consequently, the trap reduces optical losses and enhances the absorption over the entire spectrum. The components of the light trap are 3D printed and made of smoothened, silver-coated thermoplastic. In contrast to conventional light trapping methods, external light trapping leaves the material quality and the electrical properties of the solar cell unaffected. To explain the theoretical operation of the external light trap, we introduce a model that predicts the absorption enhancement in the solar cell by the external light trap. The corresponding calculated path length enhancement shows good agreement with the empirically derived value from the opto-electrical data of the solar cell. Moreover, we analyze the influence of the angle of incidence on the parasitic absorptance to obtain full understanding of the trap performance. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.

Interfacial Depletion Regions: Beyond the Space Charge Limit in Thick Bulk Heterojunctions

Space charge limited photocurrent is typically described as the limiting factor in carrier extraction efficiency for organic bulk heterojunctions with increasing thickness. It successfully characterizes the carrier extraction efficiency in these devices with thin to moderate thickness and dissimilar carrier mobilities. However, in this article we show that space charge limited photocurrent cannot solely explain the intensity dependent spectral response of extremely thick organic photovoltaics. In addition, interfacial depletion regions near the contacts contribute to the field distribution and carrier collection. Here, we describe charge collection efficiency with an optical p-i-n model, allowing for collection from band bending due to mobility-induced and interfacial-doping-induced space charge regions. We verify the model with up to 1400 nm thick spray-coated devices in both p-i-n (conventional) and n-i-p (inverted) architecture, including variations of thickness, illumination intensity, transport materials, and bifacial (semitransparent) devices.

Angular dependence of light trapping in nanophotonic thin-film solar cells

The angular dependence of light-trapping in nanophotonic thin-film solar cells is inherent due to the wavelength-scale dimensions of the periodic nanopatterns. In this paper, we experimentally investigate the dependence of light coupling to waveguide modes for light trapping in a-Si:H solar cells deposited on nanopatterned back contacts. First, we accurately determine the spectral positions of individual waveguide modes in thin-film solar cells in external quantum efficiency and absorptance. Second, we demonstrate the strong angular dependence of this spectral position for our solar cells. Third, a moderate level of disorder is introduced to the initially periodic nanopattern of the back contacts. As a result, the angular dependence is reduced. Last, we experimentally compare this dependence on the angle of incidence for randomly textured, 2D periodically nanopatterned and 2D disordered back contacts in external quantum efficiency and short-circuit current density.

Nanoscale observation of waveguide modes enhancing the efficiency of solar cells

Nanophotonic light management concepts are on the way to advance photovoltaic technologies and accelerate their economical breakthrough. Most of these concepts make use of the coupling of incident sunlight to waveguide modes via nanophotonic structures such as photonic crystals, nanowires, or plasmonic gratings. Experimentally, light coupling to these modes was so far exclusively investigated with indirect and macroscopic methods, and thus, the nanoscale physics of light coupling and propagation of waveguide modes remain vague. In this contribution, we present a nanoscopic observation of light coupling to waveguide modes in a nanophotonic thin-film silicon solar cell. Making use of the subwavelength resolution of the scanning near-field optical microscopy, we resolve the electric field intensities of a propagating waveguide mode at the surface of a state-of-the-art nanophotonic thin-film solar cell. We identify the resonance condition for light coupling to this individual waveguide mode and associate it to a pronounced resonance in the external quantum efficiency that is found to increase significantly the power conversion efficiency of the device. We show that a maximum of the incident light couples to the investigated waveguide mode if the period of the electric field intensity of the waveguide mode matches the periodicity of the nanophotonic two-dimensional grating. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes in a number of optoelectronic devices concerned with nanophotonic light-trapping as well as nanophotonic light emission.

Advancing tandem solar cells by spectrally selective multilayer intermediate reflectors

Thin-film silicon tandem solar cells are composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell, stacked and connected in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is required. Up to date, single-layer intermediate reflectors of limited spectral selectivity are applied to match the photocurrents of the top and the bottom cell. In this paper, we design and prototype multilayer intermediate reflectors based on aluminum doped zinc oxide and doped microcrystalline silicon oxide with a spectrally selective reflectance allowing for improved current matching and an overall increase of the charge carrier generation. The intermediate reflectors are successfully integrated into state-of-the-art tandem solar cells resulting in an increase of overall short-circuit current density by 0.7 mA/cm(2) in comparison to a tandem solar cell with the standard single-layer intermediate reflector.

Design of nanostructured plasmonic back contacts for thin-film silicon solar cells

We report on a plasmonic light-trapping concept based on localized surface plasmon polariton induced light scattering at nanostructured Ag back contacts of thin-film silicon solar cells. The electromagnetic interaction between incident light and localized surface plasmon polariton resonances in nanostructured Ag back contacts was simulated with a three-dimensional numerical solver of Maxwell's equations. Geometrical parameters as well as the embedding material of single and periodic nanostructures on Ag layers were varied. The design of the nanostructures was analyzed regarding their ability to scatter incident light at low optical losses into large angles in the silicon absorber layers of the thin-film silicon solar cells.