CH(3)NH(3)PbI(3) perovskite / silicon tandem solar cells: characterization based optical simulations

Enhancing perovskite-silicon tandem solar cells through numerical optical and electric optimizations for light management

We integrated optical and electrical numerical simulations to precisely investigate the effectiveness of using a pyramidal perovskite (Cs0.18FA0.82Pb(I,Br)3) nanostructured film as an example in perovskite-silicon tandem solar cells to reduce reflective losses and balance the current densities. Through our calculations, the PCE of tandem solar cells can be improved from 29.2% (the planar structures without texturing) to 36.1% in the best-performing textured tandem devices under the consistently calculated absorbed and EQE spectrum, where the predicted open-circuit voltage could reach over 2 V. These findings offer valuable theoretical insights for the advancement and optimization of perovskite-silicon tandem solar cells.

Towards High-Efficiency Photon Trapping in Thin-Film Perovskite Solar Cells Using Etched Fractal Metadevices

Reflective loss is one of the main factors contributing to power conversion efficiency limitation in thin-film perovskite solar cells. This issue has been tackled through several approaches, such as anti-reflective coatings, surface texturing, or superficial light-trapping metastructures. We report detailed simulation-based investigations on the photon trapping capabilities of a standard Methylammonium Lead Iodide (MAPbI3) solar cell, with its top layer conveniently designed as a fractal metadevice, to reach a reflection value R<0.1 in the visible domain. Our results show that, under certain architecture configurations, reflection values below 0.1 are obtained throughout the visible domain. This represents a net improvement when compared to the 0.25 reflection yielded by a reference MAPbI3 having a plane surface, under identical simulation conditions. We also present the minimum architectural requirements of the metadevice by comparing it to simpler structures of the same family and performing a comparative study. Furthermore, the designed metadevice presents low power dissipation and exhibits approximately similar behavior regardless of the incident polarization angle. As a result, the proposed system is a viable candidate for being a standard requirement in obtaining high-efficiency perovskite solar cells.

Self-Generated Buried Submicrocavities for High-Performance Near-Infrared Perovskite Light-Emitting Diode

Embedding submicrocavities is an effective approach to improve the light out-coupling efficiency (LOCE) for planar perovskite light-emitting diodes (PeLEDs). In this work, we employ phenethylammonium iodide (PEAI) to trigger the Ostwald ripening for the downward recrystallization of perovskite, resulting in spontaneous formation of buried submicrocavities as light output coupler. The simulation suggests the buried submicrocavities can improve the LOCE from 26.8 to 36.2% for near-infrared light. Therefore, PeLED yields peak external quantum efficiency (EQE) increasing from 17.3% at current density of 114 mA cm-2 to 25.5% at current density of 109 mA cm-2 and a radiance increasing from 109 to 487 W sr-1 m-2 with low rolling-off. The turn-on voltage decreased from 1.25 to 1.15 V at 0.1 W sr-1 m-2. Besides, downward recrystallization process slightly reduces the trap density from 8.90 × 1015 to 7.27 × 1015 cm-3. This work provides a self-assembly method to integrate buried output coupler for boosting the performance of PeLEDs.

High-performance parallel tandem MoTe2/perovskite solar cell based on reduced graphene oxide as hole transport layer

Recently, the impressive achievements accomplished in multijunction (tandem) perovskite solar cells have triggered a huge research effort to boost their performance. Here, using a three-dimensional (3D) finite element method (FEM) technique, we propose and investigate a parallel tandem PSCs consisting of two absorbing layers of MoTe2 and CH3NH3PbI3 with cascaded bandgaps to more efficiently use the near-infrared (NIR) solar spectrum. Endowed with a bandgap of about 1 eV, the MoTe2 layer in conjunction with a CH3NH3PbI3 layer is able to broaden the light absorption range of structure beyond the wavelength of 800 nm, up to 1200 nm. In addition to this, the MoTe2 material can not only appreciably harvest light even with a thickness as low as 20 nm due to their high absorption coefficient, but also make a perfect band alignment with the CH3NH3PbI3 layer. As a result, the proposed multijunction PCS yields a high power conversion efficiency (PCE) of 18.52% with a VOC of 0.83 V, Jsc of 26.25 mA/cm2, and FF of 0.84, which is considerably greater than its corresponding single-junction PSCs with PCE, VOC, Jsc, and FF of, 14.01%, 1.14 V, 15.20 mA/cm2, and 0.81, respectively. Furthermore, to mitigate the VOC loss caused by the low bandgap of MoTe2, we demonstrate an increase in VOC from 0.84 to 0.928 V and in PCE from 18.52% to 20.32%, when we replace a reduced graphene oxide (rGO) layer with Spiro-OMeTAD layer as a hole transport layer (HTL).

Deterministic fabrication of 3D/2D perovskite bilayer stacks for durable and efficient solar cells

Realizing solution-processed heterostructures is a long-enduring challenge in halide perovskites because of solvent incompatibilities that disrupt the underlying layer. By leveraging the solvent dielectric constant and Gutmann donor number, we could grow phase-pure two-dimensional (2D) halide perovskite stacks of the desired composition, thickness, and bandgap onto 3D perovskites without dissolving the underlying substrate. Characterization reveals a 3D-2D transition region of 20 nanometers mainly determined by the roughness of the bottom 3D layer. Thickness dependence of the 2D perovskite layer reveals the anticipated trends for n-i-p and p-i-n architectures, which is consistent with band alignment and carrier transport limits for 2D perovskites. We measured a photovoltaic efficiency of 24.5%, with exceptional stability of T₉₉ (time required to preserve 99% of initial photovoltaic efficiency) of >2000 hours, implying that the 3D/2D bilayer inherits the intrinsic durability of 2D perovskite without compromising efficiency.

High-Performance and Stable Semi-Transparent Perovskite Solar Cells through Composition Engineering

Semi-transparent perovskite solar cells (ST-PeSCs) have tremendous potential as solar windows owing to their higher efficiency and visible transmittance. However, studies toward this application are still nascent, particularly in unraveling the interplay between how the perovskite composition impacts the achievable device performance and stability. Here, the role of A- and X-site modification in APbX3 perovskites is studied to understand their influence on these factors. Through detailed experimental and simulation work, it is found that a perovskite composition consisting of cesium (Cs) and formamidinium (FA) at the A-site delivers the best device performance over a range of band gaps, which are tuned by changes to the X-site anion. Using this optimized perovskite composition, power conversion efficiencies of 15.5% and 4.1% are achieved for ST-PeSCs with average visible transmittance values between 20.7% and 52.4%, respectively. Furthermore, the CsFA-based ST-PeSCs show excellent long-term stability under continuous illumination and heating. The stability of the precursor solutions across each of the studied compositions has also been considered, showing dramatic differences in the structural properties of the perovskites and their device performance for all mixed A-site compositions possessing the archetypal methyl ammonium species, while also confirming the superior stability of the CsFA precursor solutions.

In Situ Grown Nanocrystalline Si Recombination Junction Layers for Efficient Perovskite-Si Monolithic Tandem Solar Cells: Toward a Simpler Multijunction Architecture

The perovskite-Si tandem is an attractive avenue to attain greater power conversion efficiency (PCE) than their respective single-junction solar cells. However, such devices generally employ complex stacks with numerous deposition steps, which are rather unattractive from an industrial perspective. Here, we develop a simplified tandem architecture consisting of a perovskite n-i-p stack on a silicon heterojunction structure without applying the typically used indium-tin-oxide (ITO) recombination junction (RJ) layer between the top and bottom cells. It is demonstrated that an n-type hydrogenated nanocrystalline silicon (nc-Si:H) grown in situ on an amorphous silicon hole contact layer of the bottom cell acts as an efficient RJ layer, leading to a high open-circuit voltage (VOC) of >1.8 V and a PCE of 21.4% without optimizing the optical design. Compared to the tandem cell with an ITO RJ layer, the nc-Si:H RJ layer not only improves light management but also achieves a higher VOC due to superior contact properties with an overlying SnO2 electron transport layer of the perovskite top cell. Omitting the costly material and its deposition step offers the opportunity toward realizing industrially feasible high-efficiency tandem solar cells.

Ray-optics study of gentle non-conformal texture morphologies for perovskite/silicon tandems

We investigate gentle front side textures for perovskite/silicon tandem solar cells. These textures enhance the absorption of sunlight, yet are sufficiently gentle to allow deposition of an efficient perovskite top cell. We present a tandem solar cell with such gentle texture, fabricated by Kaneka corporation, with an efficiency as high as 28.6%. We perform an extensive ray-optics study, exploring non-conformal textures at the front and rear side of the perovskite layer. Our results reveal that a gentle texture with steepness of only 23° can be more optically efficient than conventional textures with more than double that steepness. We also show that the observed anti-reflective effect of such gentle textures is not based a double bounce, but on light trapping by total internal reflection. As a result, the optical effects of the encapsulation layers play an important role, and have to be accounted for when evaluating the texture design for perovskite/silicon tandems.

Nanostructured Top Contact as an Alternative to Transparent Conductive Oxides in Tandem Perovskite/c-Si Solar Cells

In the competition of solar cell efficiency, besides top-performance multijunction cells, tandem cells based on perovskites are also breaking efficiency records to enter into the 30% range. Their design takes advantage of the rapid development of perovskite cells, and the good sharing of the available spectrum between the perovskite, absorbing at short wavelengths, and the c-Si or similar lower band gap material, working at longer wavelengths. In this paper, we present a novel tandem solar cell that combines crystalline silicon (c-Si) and perovskites cells. We analyzed the device with computational electromagnetism based on the finite element method. Our design arranges the perovskite solar cell as a multilayer 1D grating, which is terminated with a gold thin film (top metallic contact). This multilayer nanostructure is placed on top of the c-Si cell and a thin protective dielectric layer of aluminum nitride covers the whole device. The short-circuit current of the perovskite cell is maximized by maintaining the current-matching conditions with the output from the c-Si cell. This optimization considers the geometrical parameters of the grating: period and thickness of the active layer of the perovskite cell. We compared the simulated short-circuit current of this device to the planar tandem solar cell with indium tin oxide (top contact). The comparison shows a slight increment, around 3%, of our device’s performance. Moreover, it has the potential capability to circumvent postprocessing procedures used with transparent contact oxides, which can reduce the device’s final efficiency. Furthermore, our proposed design can take advantage of photolithographic and nanoimprint techniques, enabling large-scale production at a relatively low cost.

Optoelectronic Simulation of Four-terminal All-inorganic CsPbI3/CZTSSe Tandem Solar Cell with High Power Conversion Efficiency

Tandem solar cells based on perovskite have been gaining ever-increasing attention for applications in photovoltaics. Here we stack the wide-bandgap CsPbI3 top subcell with the low-bandgap Kesterite Cu2ZnSnSxSe(4-x) (CZTSSe) bottom...

Dimensional Optimization of TiO2 Nanodisk Photonic Crystals on Lead Iodide (MAPbI3) Perovskite Solar Cells by Using FDTD Simulations

Perovskite solar cells (PSC) are currently exhibiting reproducible high efficiency, low-cost manufacturing, and scalable electron transport layers (ETL), which are becoming increasingly important. The application of photonic crystals (PC) on solar cells has been proven to enhance light harvesting and lead solar cells to adjust the propagation and distribution of photons. In this paper, the optimization of a two-dimensional nanodisk PC introduced in ETL with an organic-inorganic lead-iodide perovskite (methylammonium lead-iodide, MAPbI3) as the absorber layer was studied. A finite-difference time-domain (FDTD) simulation was used to evaluate the optical performance of PSC with various lattice constants and a radius of nanodisk photonic crystals. According to the simulation, the optimum lattice constant and PC radius applied to ETL are 500 nm and 225 nm, respectively. This optimum design enhances PSC absorption performance by more than 94% of incident light.

Effects of photon recycling and scattering in high-performance perovskite solar cells

Efficient external radiation is essential for solar cells to achieve high power conversion efficiency (PCE). The classical limit of 1/2n² (n, refractive index) for electroluminescence quantum efficiency (ELQE) has recently been approached by perovskite solar cells (PSCs). Photon recycling (PR) and light scattering can provide an opportunity to surpass this limit. We investigate the role of PR and scattering in practical device operation using a radiative PSC with an ELQE (13.7% at 1 sun) that significantly surpasses the classical limit (7.4%). We experimentally analyze the contributions of PR and scattering to this strong radiation. A novel optical model reveals an increase of 39 mV in the voltage of our PSC. This analysis can provide design principles for future PSCs to approach the Shockley-Queisser efficiency limit.

Electrical Pumping of Perovskite Diodes: Toward Stimulated Emission

The success of metal halide perovskites in photovoltaic and light-emitting diodes (LEDs) motivates their application as a solid-state thin-film laser. Various perovskites have shown optically pumped stimulated emission of lasing and amplified spontaneous emission (ASE), yet the ultimate goal of electrically pumped stimulated emission has not been achieved. As an essential step toward this goal, here, a perovskite diode structure that simultaneously exhibits stable operation at high current density (≈1 kA cm-2 ) and optically excited ASE (with a threshold of 180 µJ cm-2 ) is reported. This diode structure achieves an electroluminescence quantum efficiency of 0.8% at 850 A cm-2 , which is estimated to be ≈3% of the charge carrier population required to reach ASE in the same device. It is shown that the formation of a large angle waveguide mode and the reduction of parasitic absorption losses are two major design principles for diodes to obtain a positive gain for stimulated emission. In addition to its prospect as a perovskite laser, a new application of electrically pumped ASE is proposed as an ideal perovskite LED architecture allowing 100% external radiation efficiency.

Backside light management of 4-terminal bifacial perovskite/silicon tandem PV modules evaluated under realistic conditions

Perovskite/silicon tandem modules have recently attracted growing interest as a potential candidate for new generations of solar modules. Combined with a bifacial configuration it can lead to considerable energy yield improvement in comparison to conventional monofacial tandem solar modules. Optical modeling is crucial to analyze the optical losses of perovskite/silicon solar modules and achieve efficient light management. In this article we study the optical properties of four-terminal bifacial tandem modules, using metal-halide perovskite top solar cell and a conventional industrial crystalline silicon PERC bottom solar cell. We propose a method to analyze bifacial gains, improve back side light management and challenge it under realistic spectral conditions at several locations with various albedos. We show that both optimized designs for the back side show comparable advantages at all locations. These results are a good sign for the standardization of bifacial four-terminal perovskite/silicon modules.

Recent Progress in Interconnection Layer for Hybrid Photovoltaic Tandems

Hybrid tandem solar cells offer the benefits of low cost and full solar spectrum utilization. Among the hybrid tandem structures explored to date, the most popular ones have four (simple stacking design) or two (terminal/tunneling layer addition design) terminal electrodes. Although the latter design is more cost-effective than the former, its widespread application is hindered by the difficulty of preparing an interface between two solar cell materials. The oldest approach to the in-series bonding of two or more bandgap solar cells relies on the introduction of a tunneling layer in multijunction III-V solar cells, but it has some limitations, e.g., the related materials/technologies are applicable only to III-V and certain other solar cells. Thus, alternative methods of realizing junction contacts based on the use of novel materials are highly sought after. Here, the strategies used to realize high-performance tandem cells are described, focusing on interface control in terms of bonding two or more solar cells for tandem approaches. The presented information is expected to aid the establishment of ideal methods of connecting two or more solar cells to obtain the highest performance for different solar cell choices with minimized energy loss through the interface.

Efficient energy transfer mitigates parasitic light absorption in molecular charge-extraction layers for perovskite solar cells

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (≤10 nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Computational study of iron perovskite CH3NH3FeI3as an alternative to the lead perovskite CH3NH3PbI3for application in solar cells

Structural and optical properties of methylammonium iron iodide perovskite CH3NH3FeI3are studied at DFT-PBE(mBJ)/FP-LAPW + lo level of theory to assess feasibility of the replacement of the toxic lead with the non-toxic iron in the perovskite layer of solar cells. Starting from experimental crystal structure of the Pb perovskite, volume and aspect ratio (c/a) and atomic positions are optimized for the CH3NH3FeI3structure, and its electronic and optical characteristics are calculated. An index, measuring the raw optical performance of the light harvesting layer of a solar cell is introduced and calculated for the two Fe and Pb perovskites. Comparative values of this index shows that the iron perovskite CH3NH3FeI3has an acceptable optical performance, ∼61% that of the Pb perovskite CH3NH3PbI3. Analysis of the Brewster angles (θB) calculated for the TiO2/perovskite and perovskite/spiro interfaces shows that the Fe perovskite solar cell can have better optical harvesting performance by a factor of 1.32, which improves its comparative overall performance up to 80%. As a conclusion, application of iron perovskite CH3NH3FeI3is promising, especially due to its much lower costs and significantly alleviated environmental hazards of the incorporating solar cells.

Perovskite Solar Cell with Added Gold/Silver Nanoparticles: Enhanced Optical and Electrical Characteristics

Different perovskite materials, such as methylammonium lead triiodide MAPbI3, exhibit many outstanding and desirable properties in solar energy harvesting. In this paper, the enhancement of perovskite solar cells’ both optical and electrical characteristics through adding either gold (Au) or silver (Ag) nanoparticles (NPs) using different simulations was studied. The used plasmonic nanoparticles were found to be able to compensate for the low absorption of MAPbI3 in the visible with optical coupling resonance frequencies close to that spectrum. Optimal diameters of Au and Ag NPs were found and simulated, and their impact on different parameters such as transmission, absorption, reflection, external quantum efficiency (EQE), open circuit voltage, short-circuit current density, fill factor, and most importantly, efficiency of the perovskite solar cell, have been investigated.

Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions

Since single junction c-Si solar cells are reaching their practical efficiency limit. Perovskite/c-Si tandem solar cells hold the promise of achieving greater than 30% efficiencies. In this regard, optical simulations can deliver guidelines for reducing the parasitic absorption losses and increasing the photocurrent density of the tandem solar cells. In this work, an optical study of 2, 3 and 4 terminal perovskite/c-Si tandem solar cells with c-Si solar bottom cells passivated by high thermal-budget poly-Si, poly-SiOx and poly-SiCx is performed to evaluate their optical performance with respect to the conventional tandem solar cells employing silicon heterojunction bottom cells. The parasitic absorption in these carrier selective passivating contacts has been quantified. It is shown that they enable greater than 20 mA/cm2 matched implied photocurrent density in un-encapsulated 2T tandem architecture along with being compatible with high temperature production processes. For studying the performance of such tandem devices in real-world irradiance conditions and for different locations of the world, the effect of solar spectrum and angle of incidence on their optical performance is studied. Passing from mono-facial to bi-facial tandem solar cells, the photocurrent density in the bottom cell can be increased, requiring again optical optimization. Here, we analyse the effect of albedo, perovskite thickness and band gap as well as geographical location on the optical performance of these bi-facial perovskite/c-Si tandem solar cells. Our optical study shows that bi-facial 2T tandems, that also convert light incident from the rear, require radically thicker perovskite layers to match the additional current from the c-Si bottom cell. For typical perovskite bandgap and albedo values, even doubling the perovskite thickness is not sufficient. In this respect, lower bandgap perovskites are very interesting for application not only in bi-facial 2T tandems but also in related 3T and 4T tandems.

Refractory plasmonics enabling 20% efficient lead-free perovskite solar cells

Core-shell refractory plasmonic nanoparticles are used as excellent nanoantennas to improve the efficiency of lead-free perovskite solar cells (PSCs). SiO2 is used as the shell coating due to its high refractive index and low extinction coefficient, enabling the control over the sunlight directivity. An optoelectronic model is developed using 3D finite element method (FEM) as implemented in COMSOL Multiphysics to calculate the optical and electrical parameters of plain and ZrN/SiO2-modified PSCs. For a fair comparison, ZrN-decorated PSCs are also simulated. While the decoration with ZrN nanoparticles boosts the power conversion efficiency (PCE) of the PSC from 12.9% to 17%, the use of ZrN/SiO2 core/shell nanoparticles shows an unprecedented enhancement in the PCE to reach 20%. The enhancement in the PCE is discussed in details.

A monolithic nanostructured-perovskite/silicon tandem solar cell: feasibility of light management through geometry and materials selection

The use of several layers of different materials, taking advantage of their complementary bandgap energies, improves the absorption in multi-junction solar cells. Unfortunately, the inherent efficiency increment of this strategy has a limitation: each interface introduces optical losses. In this paper, we study the effects of materials and geometry in the optical performance of a nanostructured hybrid perovskite - silicon tandem solar cell. Our proposed design increases the performance of both subcells by managing light towards the active layer, as well as by minimizing reflections losses in the interfaces. We sweep both refractive index and thickness of the transport layers and the dielectric spacer composing the metasurface, obtaining a range of these parameters for the proper operation of the device. Using these values, we obtain a reduction in the optical losses, in particular they are more than a 33% lower than those of a planar cell, mainly due to a reduction of the reflectivity in the device. This approach leads to an enhancement in the optical response, widens the possibilities for the manufacturers to use different materials, and allows wide geometrical tolerances.

Semi-Transparent Perovskite Solar Cells with ITO Directly Sputtered on Spiro-OMeTAD for Tandem Applications

Perovskite silicon tandem solar cells have the potential to overcome the efficiency limit of single-junction solar cells. For both monolithic and mechanically stacked tandem devices, a semi-transparent perovskite top solar cell, including a transparent contact, is required. Usually, this contact consists of a metal oxide buffer layer and a sputtered transparent conductive oxide. In this work, semi-transparent perovskite solar cells in the regular n-i-p structure are presented with tin-doped indium oxide (ITO) directly sputtered on the hole conducting material Spiro-OMeTAD. ITO process parameters such as sputter power, temperature, and pressure in the chamber are systematically varied. While a low temperature of 50 °C is crucial for good device performance, a low sputter power has only a slight effect, and an increased chamber pressure has no influence on device performance. For the 5 × 5 mm² perovskite cell with a planar front side, a 105 nm thick ITO layer with a sheet resistance of 44 Ω sq^-1 allowing for the omission of grid fingers and a MgF₂ antireflection coating are used to improve transmission into the solar cells. The best device achieved an efficiency of 14.8%, which would result in 24.2% in a four-terminal tandem configuration.

Energy yield analysis of textured perovskite silicon tandem solar cells and modules

Perovskite silicon tandem solar cells combine potentially low production costs with the ability to surpass the efficiency limit of silicon single junction solar cells. Optical modeling and optimization are crucial to achieve this ambitious goal in the near future. The optimization should seek to maximize the energy yield based on realistic environmental conditions. This work analyzes the energy yield of perovskite silicon tandem solar cells and modules based on realistic experimental data, with a special focus on the investigation of surface textures at the front and rear side of the solar cell and its implication for reflection as well as parasitic absorption properties. The investigation reveals a 7.3%_rel higher energy yield for an encapsulated tandem cell with a textured front side compared with an encapsulated high efficiency single junction solar cell with 24.3% harvesting efficiency for irradiance data of the year 2014 in Freiburg/Germany.

Highly Efficient Semitransparent Perovskite Solar Cells for Four Terminal Perovskite-Silicon Tandems

Tandem solar cells (SCs) based on perovskite and silicon represent an exciting possibility for a breakthrough in photovoltaics, enhancing SC power conversion efficiency (PCE) beyond the single-junction limit while keeping the production cost low. A critical aspect to push the tandem PCE close to its theoretical limit is the development of high-performing semitransparent perovskite top cells, which also allow suitable near-infrared transmission. Here, we have developed highly efficient semitransparent perovskite SCs (PSCs) based on both mesoporous and planar architectures, employing Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ and FA_0.87Cs_0.13PbI₂Br perovskites with band gaps of 1.58 and 1.72 eV, respectively, which achieved PCEs well above 17 and 14% by detailed control of the deposition methods, thickness, and optical transparency of the interlayers and the semitransparent electrode. By combining our champion 1.58 eV PSCs (PCE of 17.7%) with an industrial-relevant low-cost n-type Si SCs, a four-terminal (4T) tandem efficiency of 25.5% has been achieved. Moreover, for the first time, 4T tandem SCs' performances have been measured in the low light intensity regime, achieving a PCE of 26.6%, corresponding to revealing a relative improvement above 9% compared to the standard 1 sun illumination condition. These results are very promising for their implementation under field-operating conditions.

Perovskite/Silicon Tandem Solar Cells: From Detailed Balance Limit Calculations to Photon Management

Energy conversion efficiency losses and limits of perovskite/silicon tandem solar cells are investigated by detailed balance calculations and photon management. An extended Shockley-Queisser model is used to identify fundamental loss mechanisms and link the losses to the optics of solar cells. Photon management is used to minimize losses and maximize the energy conversion efficiency. The influence of photon management on the solar cell parameters of a perovskite single-junction solar cell and a perovskite/silicon solar cell is discussed in greater details. An optimized solar cell design of a perovskite/silicon tandem solar cell is presented, which allows for the realization of solar cells with energy conversion efficiencies exceeding 32%.

Consistent Device Simulation Model Describing Perovskite Solar Cells in Steady-State, Transient, and Frequency Domain

A variety of experiments on vacuum-deposited methylammonium lead iodide perovskite solar cells are presented, including JV curves with different scan rates, light intensity-dependent open-circuit voltage, impedance spectra, intensity-modulated photocurrent spectra, transient photocurrents, and transient voltage step responses. All these experimental data sets are successfully reproduced by a charge drift-diffusion simulation model incorporating mobile ions and charge traps using a single set of parameters. While previous modeling studies focused on a single experimental technique, we combine steady-state, transient, and frequency-domain simulations and measurements. Our study is an important step toward quantitative simulation of perovskite solar cells, leading to a deeper understanding of the physical effects in these materials. The analysis of the transient current upon voltage turn-on in the dark reveals that the charge injection properties of the interfaces are triggered by the accumulation of mobile ionic defects. We show that the current rise of voltage step experiments allow for conclusions about the recombination at the interface. Whether one or two mobile ionic species are used in the model has only a minor influence on the observed effects. A delayed current rise observed upon reversing the bias from +3 to -3 V in the dark cannot be reproduced yet by our drift-diffusion model. We speculate that a reversible chemical reaction of mobile ions with the contact material may be the cause of this effect, thus requiring a future model extension. A parameter variation is performed in order to understand the performance-limiting factors of the device under investigation.

Efficient Light Management in a Monolithic Tandem Perovskite/Silicon Solar Cell by Using a Hybrid Metasurface

Solar energy is now dealing with the challenge of overcoming the Shockley–Queisser limit of single bandgap solar cells. Multilayer solar cells are a promising solution as the so-called third generation of solar cells. The combination of materials with different bandgap energies in multijunction cells enables power conversion efficiencies up to 30% at reasonable costs. However, interfaces between different layers are critical due to optical losses. In this work, we propose a hybrid metasurface in a monolithic perovskite-silicon solar cell. The design takes advantage of light management to optimize the absorption in the perovskite, as well as an efficient light guiding towards the silicon subcell. Furthermore, we have also included the effect of a textured back contact. The optimum proposal provides an enhancement of the matched short-circuit current density of a 20.5% respect to the used planar reference.

Perovskite-a Perfect Top Cell for Tandem Devices to Break the S-Q Limit

Up to now, multijunction cell design is the only successful way demonstrated to overcome the Shockley-Quiesser limit for single solar cells. Perovskite materials have been attracting ever-increasing attention owing to their large absorption coefficient, tunable bandgap, low cost, and easy fabrication process. With their rapidly increased power conversion efficiency, organic-inorganic metal halide perovskite-based solar cells have demonstrated themselves as the most promising candidates for next-generation photovoltaic applications. In fact, it is a dream come true for researchers to finally find a perfect top-cell candidate in tandem device design in commercially developed solar cells like single-crystalline silicon and CIGS cells used as the bottom component cells. Here, the recent progress of multijunction solar cells is reviewed, including perovskite/silicon, perovskite/CIGS, perovskite/perovskite, and perovskite/polymer multijunction cells. In addition, some perspectives on using these solar cells in emerging markets such as in portable devices, Internet of Things, etc., as well as an outlook for perovskite-based multijunction solar cells are discussed.

The Influence of Embedded Plasmonic Nanostructures on the Optical Absorption of Perovskite Solar Cells

The interaction of light with plasmonic nanostructures can induce electric field intensity either around or at the surface of the nanostructures. The enhanced intensity of the electric field can increase the probability of light absorption in the active layer of solar cells. The absorption edge of perovskite solar cells (PSCs), which is almost 800 nm, can be raised to higher wavelengths with the help of plasmonic nanostructures due to their perfect photovoltaic characteristics. We placed plasmonic nanoparticles (NPs) with different radii (20–60 nm) within the bulk of the perovskite solar cell and found that the Au nanoparticles with a radius of 60 nm increased the absorption of the cell by 20% compared to the bare one without Au nanoparticles. By increasing the radius of the nanoparticles, the total absorption of the cell will increase because of the scattering enhancement. The results reveal that the best case is the PSC with the NP radius of 60 nm.

Highly efficient MoOx-free semitransparent perovskite cell for 4 T tandem application improving the efficiency of commercially-available Al-BSF silicon

In this work, the fabrication of MoOx-free semitransparent perovskite solar cells (PSC) with Power Conversion Efficiencies (PCE) up to 15.7% is reported. Firstly, opaque PSCs up to 19.7% were fabricated. Then, the rear metal contact was replaced by a highly transparent and conductive indium tin oxide (ITO) film, directly sputtered onto the hole selective layer, without any protective layer between Spiro-OMeTAD and rear ITO. To the best of our knowledge, this corresponds to the most efficient buffer layer-free semitransparent PSC ever reported. Using time-resolved photoluminescence (TRPL) technique on both sides of the semitransparent PSC, Spiro-OMeTAD/perovskite and perovskite/TiO2 interfaces were compared, confirming the great quality of Spiro-OMeTAD/perovskite interface, even after damage-less ITO sputtering, where degradation phenomena result less important than for perovskite/TiO2 one. Finally, a 4-terminal tandem was built combining semitransparent PSC with a commercially-available Aluminium Back Surface Field (Al-BSF) silicon wafer. That silicon wafer presents PCE = 19.52% (18.53% after being reduced to cell size), and 5.75% once filtered, to generate an overall 4 T tandem efficiency of 21.18% in combination with our champion large semitransparent PSC of 15.43%. It means an absolute increase of 1.66% over the original silicon wafer efficiency and a 2.65% over the cut Si cell.

Effect of Silicon Surface for Perovskite/Silicon Tandem Solar Cells: Flat or Textured?

Perovskite and textured silicon solar cells were integrated into a tandem solar cell through a stacking method. To consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of two- and four-terminal structures by perovskite layers on crystal silicon wafers. The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7 to 0.8%), which resulted in increases in average incident photon to current conversion efficiency values (from 83.0 to 88.0%) and current density (from 13.7 to 14.8 mA/cm²). Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by four-terminal device, which was an increase of 2.4% from that of SHJ solar cells alone.

Optical modeling of wide-bandgap perovskite and perovskite/silicon tandem solar cells using complex refractive indices for arbitrary-bandgap perovskite absorbers

Wide-bandgap perovskites are attractive top-cell materials for tandem photovoltaic applications. Comprehensive optical modeling is essential to minimize the optical losses of state-of-the-art perovskite/perovskite, perovskite/CIGS, and perovskite/silicon tandems. Such models require accurate optical constants of wide-bandgap perovskites. Here, we report optical constants determined with ellipsometry and spectrophotometry for two new wide-bandgap, cesium-formamidinium-based perovskites. We validate the optical constants by comparing simulated quantum efficiency and reflectance spectra with measured cell results for semi-transparent single-junction perovskite cells and find less than 0.3 mA/cm² error in the short-circuit current densities. Such simulations further reveal that reflection and parasitic absorption in the front ITO layer and electron contact are responsible for the biggest optical losses. We also show that the complex refractive index of methylammonium lead triiodide, the most common perovskite absorber for solar cells, can be used to generate approximate optical constants for an arbitrary wide-bandgap perovskite by translating the data along the wavelength axis. Finally, these optical constants are used to map the short-circuit current density of a textured two-terminal perovskite/silicon tandem solar cell as a function of the perovskite thickness and bandgap, providing a guide to nearly 20 mA/cm² matched current density with any perovskite bandgap between 1.56 and 1.68 eV.

Optical modeling of structured silicon-based tandem solar cells and module stacks

Silicon-based tandem solar cells and modules are complex systems that require optical modeling for the optimization towards highest efficiencies. The fact that such devices typically incorporate surface structures of different optical regimes poses high requirements to the involved simulation tools. The OPTOS formalism is ideally suited to deal with such complexity. Within this work OPTOS is extended in order to calculate the layer resolved absorptance in silicon-based tandem solar cells and module stacks. After describing the relevant mathematical details, a good agreement between OPTOS absorptance simulation results and EQE measurements of the current 33.3% record efficiency III-V on silicon two-terminal tandem solar cell is found. Furthermore, a detailed loss analysis is performed for an exemplary perovskite silicon solar cell with and without module encapsulation. The comparison reveals a lower photocurrent density for the module stack due to increased reflectance and absorption in the EVA.

Optical design of spectrally selective interlayers for perovskite/silicon heterojunction tandem solar cells

Monolithic perovskite/c-Si tandem solar cells have the potential to exceed the Shockley-Queisser limit for single junction solar cells. However, reflection losses at internal interfaces play a crucial role for the overall efficiency of the tandem devices. Significant reflection losses are caused by the charge selective contacts which have a significantly lower refractive index compared to the absorber materials. Here, we present an approach to overcome a significant part of these reflection losses by introducing a multilayer stack between the top and bottom cell which shows spectrally selective transmission/reflection behavior. The layer stack is designed and optimized by optical simulations using transfer matrix method and a genetic algorithm. The incident sun light is split into a direct part and an isotropic diffuse part. The tandem solar cell with interlayer shows an absolute improvement of short-circuit current density of 0.82 mA/cm².

Design of perovskite/crystalline-silicon monolithic tandem solar cells

We present an optical model implemented in the commercial software SETFOS 4.6 for simulating perovskite/silicon monolithic tandem solar cells that exploit light scattering structures. In a first step we validate the model with experimental data of tandem solar cells that either use front- or rear-side textures and extract the internal quantum efficiency of the methyl-ammonium lead iodide (MALI) perovskite sub-cell. In a next step, the software is used to investigate the potential of different device architectures featuring a monolithic integration between the perovskite and silicon sub-cells and exploiting rear- as well as front-side textures for improved light harvesting. We find that, considering the available contact materials, the p-i-n solar cell architecture is the most promising with respect to achievable photocurrent for both flat and textured wafers. Finally, cesium-formamidinium-based perovskite materials with several bandgaps were synthetized, optically characterized and their potential in a tandem device was quantified by simulations. For the simulated layer stack and among the tested materials with bandgaps of 1.7 and 1.6 eV, the one with 1.6 eV bandgap was found to be the most promising, with a potential of reaching a power conversion efficiency of 31%. In order to achieve higher efficiencies using higher band-gap materials, parasitic absorptance in the blue spectral range should be further reduced.

The Effect of Dopant-Free Hole-Transport Polymers on Charge Generation and Recombination in Cesium-Bismuth-Iodide Solar Cells

The photovoltaic characteristics of CsBi₃ I₁₀ -based solar cells with three dopant-free hole-conducting polymers are investigated. The effect on charge generation and charge recombination in the solar cells using the different polymers is studied and the results indicate that the choice of polymer strongly affects the device properties. Interestingly, for the solar cell with poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1), the photon-to-current conversion spectrum is highly improved in the red wavelength region, suggesting that the polymer also contributes to the photocurrent generation in this case. This report provides a new direction for further optimization of Bi-halide solar cells by using dopant-free hole-transporting polymers and shows that the energy levels and the interaction between the Bi-halide and the conducting polymers are very important for solar cell performance.

Mixed Valence Perovskite Cs2 Au2 I6 : A Potential Material for Thin-Film Pb-Free Photovoltaic Cells with Ultrahigh Efficiency

New light is shed on the previously known perovskite material, Cs₂ Au₂ I₆ , as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs₂ Au₂ I₆ has an optimal band gap that is close to the Shockley-Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂ Au₂ I₆ a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂ Au₂ I₆ has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂ Au₂ I₆ will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future.

Field Performance versus Standard Test Condition Efficiency of Tandem Solar Cells and the Singular Case of Perovskites/Silicon Devices

Multijunction cells may offer a cost-effective route to boost the efficiency of industrial photovoltaics. For any technology to be deployed in the field, its performance under actual operating conditions is extremely important. In this perspective, we evaluate the impact of spectrum, light intensity, and module temperature variations on the efficiency of tandem devices with crystalline silicon bottom cells with a particular focus on perovskite top cells. We consider devices with different efficiencies and calculate their energy yields using field data from Denver. We find that annual losses due to differences between operating conditions and standard test conditions are similar for single-junction and four-terminal tandem devices. The additional loss for the two-terminal tandem configuration caused by current mismatch reduces its performance ratio by only 1.7% when an optimal top cell bandgap is used. Additionally, the unusual bandgap temperature dependence of perovskites is shown to have a positive, compensating effect on current mismatch.

Low-Temperature Solution Processed Random Silver Nanowire as a Promising Replacement for Indium Tin Oxide

A low-temperature solution-based process for depositing silver nanowire (AgNW) networks for use as transparent conductive top electrode is demonstrated. These AgNWs when applied to Cu₂ZnSnS₄ solar cells outperformed indium tin oxide as the top electrode. Thinner nanowires allow the use of lower temperatures during processing, while longer wires allow lowered sheet resistance for the same surface coverage of NWs, enhancing the transmittance/conductance trade-off. Conductive atomic force microscopy and percolation theory were used to study the quality of the NW network at the microscale. Our optimized network yielded a sheet resistance of 18 Ω/□ and ∼95% transmission across the entire wavelength range of interest for a deposition temperature as low as of 60 °C. Our results show that AgNWs can be used for low-temperature cell fabrication using cheap solution-based processes that could also be promising for other solar cells constrained to low processing temperatures such as organic and perovskite solar cells.

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.

Numerical optical optimization of monolithic planar perovskite-silicon tandem solar cells with regular and inverted device architectures

We numerically maximize the achievable photocurrent density of planar perovskite-silicon tandem solar cells for different device architectures. For the optimizations we combine the transfer-matrix method with a simulated annealing algorithm. The optimizations are conducted within experimentally accessible and relevant layer-thickness ranges, which allows to extract applicable device guidelines. A comparison between regular and inverted tandem-cell designs reveals that a rear-emitter silicon heterojunction in combination with an inverted perovskite top-cell can yield a photocurrent, which is 1.4 mA/cm² higher than that of tandem cells with the usual polarity and a front-emitter silicon bottom cell. Switching from the regular to the inverse architecture leads to over 2% (absolute) gain in power conversion efficiency. Finally we show that an efficiency of 30.8% is achievable for such tandem cells with an optimized perovskite band-gap.

Halide Perovskites for Tandem Solar Cells

Perovskite solar cells have become one of the strongest candidates for next-generation solar energy technologies. A myriad of beneficial optoelectronic properties of the perovskite materials have enabled superb power conversion efficiencies (PCE) exceeding 22% for a single-junction device. The high PCE achievable via low processing costs and relatively high variability in optical properties have opened new possibilities for perovskites in tandem solar cells. In this Perspective, we will discuss current research trends in fabricating tandem perovskite-based solar cells in combination with a variety of mature photovoltaic devices such as organic, silicon, and Cu(In,Ga)(S,Se)₂ (CIGS) solar cells. Characteristic features and present limitations of each tandem cell will be discussed and elaborated upon. Finally, key issues for further improvement and the future outlook will be discussed.

Modeling and designing multilayer 2D perovskite / silicon bifacial tandem photovoltaics for high efficiencies and long-term stability

A key challenge in photovoltaics today is to develop cell technologies with both higher efficiencies and lower fabrication costs than incumbent crystalline silicon (c-Si) single-junction cells. While tandem cells have higher efficiencies than c-Si alone, it is generally challenging to find a low-cost, high-performance material to pair with c-Si. However, the recent emergence of 22% efficient perovskite photovoltaics has created a tremendous opportunity for high-performance, low-cost perovskite / crystalline silicon tandem photovoltaic cells. Nonetheless, two key challenges remain. First, integrating perovskites into tandem structures has not yet been demonstrated to yield performance exceeding commercially available crystalline silicon modules. Second, the stability of perovskites is inconsistent with the needs of most end-users, who install photovoltaic modules to produce power for 25 years or more. Making these cells viable thus requires innovation in materials processing, device design, fabrication, and yield. We will address these two gaps in the photovoltaic literature by investigating new types of 2D perovskite materials with n-butylammonium spacer layers, and integrating these materials into bifacial tandem solar cells providing at least 30% normalized power production. We find that an optimized 2D perovskite ((BA)₂(MA)₃(Sn_0.6Pb_0.4)₄I₁₃)/silicon bifacial tandem cell, given a globally average albedo of 30%, yields a normalized power production of 30.31%, which should be stable for extended time periods without further change in materials or encapsulation.

Detailed optical modelling and light-management of thin-film organic solar cells with consideration of small-area effects

We present detailed numerical and experimental investigation of thin-film organic solar cells with a micro-textured light management foil applied on top of the front glass substrate. We first demonstrate that measurements of small-area laboratory solar cells are susceptible to a significant amount of optical losses that could lead to false interpretation of the measurement results. Using the combined optical model CROWM calibrated with realistic optical properties of organic films and other layers, we identify the origins of these losses and quantify the extent of their influence. Further on, we identify the most important light management mechanisms of the micro-textured foil, among which the prevention of light escaping at the front side of the cell is revealed as the dominant one. Detailed three-dimensional simulations show that the light-management foil applied on top of a large-area organic solar cell can reduce the total reflection losses by nearly 60% and improve the short-circuit current density by almost 20%. Finally, by assuming realistic open-circuit voltage and especially the realistic fill factor that deteriorates as the absorber layer thickness is increased, we determine the optimal absorber layer thickness that would result in the highest power conversion efficiency of the investigated organic solar cells.