Atomic‐Scale Observation of Oxygen Substitution and Its Correlation with Hole‐Transport Barriers in Cu2ZnSnSe4 Thin‐Film Solar Cells

Kesterite‐type Cu₂ZnSn(S,Se)₄ has been extensively studied over the past several years, with researchers searching for promising candidates for indium‐ and gallium‐free inexpensive absorbers in high‐efficiency thin‐film solar cells. Many notable experimental and theoretical studies have dealt with the effects of intrinsic point defects, Cu/Zn/Sn nonstoichiometry, and cation impurities on cell performance. However, there have been few systematic investigations elucidating the distribution of oxygen at an atomic scale and the correlation between oxygen substitution and charge transport despite unavoidable incorporation of oxygen from the ambient atmosphere during thin‐film fabrication. Using energy‐dispersive X‐ray spectroscopy, scanning transmission electron microscopy, and electron energy‐loss spectroscopy, the presence of nanoscale layers is directly demonstrated in which oxygen is substantially substituted for Se, near grain boundaries in polycrystalline Cu₂ZnSnSe₄ films. Density‐functional theory calculations also show that oxygen substitution remarkably lowers the valence band maximum and subsequently widens the overall bandgap. Consequently, anion modification by oxygen can make a major contribution to the formation of a robust barrier blocking the holes from bulk grains into grain boundaries, thereby efficiently attaining electron?hole separation. The findings provide crucial insights into achieving better energy conversion efficiency in kesterite‐based thin‐film solar cells through optimum control of oxidation during the fabrication process.     

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.     

Design‐to‐Device Approach Affords Panchromatic Co‐Sensitized Solar Cells

Data‐driven materials discovery has become increasingly important in identifying materials that exhibit specific, desirable properties from a vast chemical search space. Synergic prediction and experimental validation are needed to accelerate scientific advances related to critical societal applications. A design‐to‐device study that uses high‐throughput screens with algorithmic encodings of structure–property relationships is reported to identify new materials with panchromatic optical absorption, whose photovoltaic device applications are then experimentally verified. The data‐mining methods source 9431 dye candidates, which are auto‐generated from the literature using a custom text‐mining tool. These candidates are sifted via a data‐mining workflow that is tailored to identify optimal combinations of organic dyes that have complementary optical absorption properties such that they can harvest all available sunlight when acting as co‐sensitizers for dye‐sensitized solar cells (DSSCs). Six promising dye combinations are shortlisted for device testing, whereupon one dye combination yields co‐sensitized DSSCs with power conversion efficiencies comparable to those of the high‐performance, organometallic dye, N719. These results demonstrate how data‐driven molecular engineering can accelerate materials discovery for panchromatic photovoltaic or other applications.     

Scaling Up ITO‐Free Solar Cells

Indium‐tin‐oxide‐free (ITO‐free) polymer solar cells with composite electrodes containing current‐collecting grids and a semitransparent poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate) (PEDOT:PSS) conductor are demonstrated. The up‐scaling of the length of the solar cell from 1 to 6 cm and the effect of the grid line resistance are explored for a series of devices. Laser‐beam‐induced current (LBIC) mapping is used for quality control of the devices. A theoretical modeling study is presented that enables the identification of the most rational cell dimension for the grids with different resistances. The performance of ITO‐free organic solar cells with different dimensions and different electrode resistances are evaluated for different light intensities. The current generation and electric potential distribution are found to not be uniformly distributed in large‐area devices at simulated 1 Sun illumination. The generated current uniformity increases with decreasing light intensities.     

Development of Next‐Generation Organic‐Based Solar Cells: Studies on Dye‐Sensitized and Perovskite Solar Cells

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.     

Ternary Blended Fullerene‐Free Polymer Solar Cells with 16.5% Efficiency Enabled with a Higher‐LUMO‐Level Acceptor to Improve Film Morphology

Ternary approaches to solar cell design utilizing a small bandgap nonfullerene acceptor as the near infrared absorber to increase the short‐circuit current density always decreases the open‐circuit voltage. Herein, a highly efficient polymer solar cell with an impressive efficiency of 16.28 ± 0.20% enabled by an effective voltage‐increased ternary blended fullerene‐free material approach is reported. In this approach, the structural similarity between the host and the higher‐LUMO‐level guest enables the two acceptors to be synergized, obtaining increased open‐circuit voltage and fill factor and a small increase of short‐circuit current density. The same beneficial effects are demonstrated by using two host binary systems. The homogeneous fine film morphologies and the π–π stacking patterns of the host blend are well maintained, while larger donor and acceptor phases and increased lamellar crystallinity, increased charge mobilities, and reduced monomolecular recombination can be achieved upon addition of the guest nonfullerene acceptor. The increased charge mobilities and reduced monomolecular recombination not only contribute to the improved fill factor but also enable the best devices to be fabricated with a relatively thicker ternary blended active layer (110 vs 100 nm). This, combined with the absorption from the added guest acceptor, contribute to the increased short‐circuit current.     

Fully Solution‐Processed TCO‐Free Semitransparent Perovskite Solar Cells for Tandem and Flexible Applications

Semitransparent perovskite solar cells (st‐PSCs) have received remarkable interest in recent years because of their great potential in applications for solar window, tandem solar cells, and flexible photovoltaics. However, all reported st‐PSCs require expensive transparent conducting oxides (TCOs) or metal‐based thin films made by vacuum deposition, which is not cost effective for large‐scale fabrication: the cost of TCOs is estimated to occupy ≈75% of the manufacturing cost of PSCs. To address this critical challenge, this study reports a low‐temperature and vacuum‐free strategy for the fabrication of highly efficient TCO‐free st‐PSCs. The TCO‐free st‐PSC on glass exhibits 13.9% power conversion efficiency (PCE), and the four‐terminal tandem cell made with the st‐PSC top cell and c‐Si bottom cell shows an overall PCE of 19.2%. Due to the low processing temperature, the fabrication of flexible st‐PSCs is demonstrated on polyethylene terephthalate and polyimide, which show excellent stability under repeated bending or even crumbing.     

Crystallization Control of Ternary‐Cation Perovskite Absorber in Triple‐Mesoscopic Layer for Efficient Solar Cells

Controlling the crystallization of organic–inorganic hybrid perovskite is of vital importance to achieve high performing perovskite solar cells. The growth mechanism of perovskites has been intensively studied in devices with planar structures and traditional structures. However, for the printable mesoscopic perovskite solar cells, it is difficult to study the crystallization mechanism of perovskite owing to the complicated mesoporous structure. Here, a solvent evaporation controlled crystallization method to achieve ideal crystallization in the mesoscopic structure is provided. Combining results of scanning electron microscope and X‐ray diffraction, it is found that adjusting the evaporation rate of solvent can control the crystallization rate of perovskite and a model for the crystallization process during annealing in mesoporous structures is proposed. Finally, a homogeneous pore filling in the mesoscopic structure without any additives is successfully achieved and a stabilized power conversion efficiency of 16.26% using ternary‐cation perovskite absorber is realized. The findings will provide better understanding of perovskite crystallization in printable mesoscopic perovskite solar cells and pave the way for the commercialization of perovskite solar cells.     

GaSb‐Based Solar Cells for Full Solar Spectrum Energy Harvesting

In this work, a multijunction solar cell is developed on a GaSb substrate that can efficiently convert the long‐wavelength photons typically lost in a multijunction solar cell into electricity. A combination of modeling and experimental device development is used to optimize the performance of a dual junction GaSb/InGaAsSb concentrator solar cell. Using transfer printing, a commercially available GaAs‐based triple junction cell is stacked mechanically with the GaSb‐based materials to create a four‐terminal, five junction cell with a spectral response range covering the region containing >99% of the available direct‐beam power from the Sun reaching the surface of the Earth. The cell is assembled in a mini‐module with a geometric concentration ratio of 744 suns on a two‐axis tracking system and demonstrated a combined module efficiency of 41.2%, measured outdoors in Durham, NC. Taking into account the measured transmission of the optics gives an implied cell efficiency of 44.5%.     

High‐Performance,Transparent, Dye‐Sensitized Solar Cells for See‐Through Photovoltaic Windows

Dye‐sensitized solar cells (DSCs) have attracted great interest as one of the most promising photovoltaic technologies, and transparent DSCs show potential applications as photovoltaic windows. However, the competition between light absorption for photocurrent generation and light transmittance for obtaining high transparency limits the performance of transparent DSCs. Here, transparent DSCs exhibiting a high light transmittance of 60.3% and high energy conversion efficiency (3.66%) are reported. The strategy is to create a cocktail system composed of ultraviolet and near‐infrared dye sensitizers that selectively and efficiently harvest light in the invisible or low‐eye‐sensitivity region while transmitting light in high‐eye‐sensitivity regions. This new design provides a reasonable approach for realizing high efficiency and transparency DSCs that have potential applications as photovoltaic windows.     

Impact of Nanoscale Elemental Distribution in High‐Performance Kesterite Solar Cells

The effort to develop earth‐abundant kesterite solar cells has led to an approximate doubling of the power conversion efficiency over the past five years to 12.6%, primarily due to increases in short‐circuit current and fill factor; open‐circuit voltage has resisted similar change, limiting further efficiency improvement. In the present investigation, Auger nanoprobe spectroscopy, X‐ray/ultraviolet photoelectron spectroscopy, and device characterization are used to provide a comprehensive understanding of the role of grain boundaries and interfaces in limiting performance in kesterite‐based devices. High photovoltaic performance is found to correlate with grain boundaries that are Cu‐depleted and enriched with SnOx. The formation of this bulk‐like oxide at grain boundaries with type I band offset provides a unique effective passivation that limits electron‐hole recombination. Building on these new insights, photovoltaic device simulations are performed that show optimized electrostatic designs can compensate for bulk defects, allowing efficiencies closer to the theoretical limit.     

Current Collecting Grids for ITO‐Free Solar Cells

Indium‐tin‐oxide (ITO) free polymer solar cells prepared by ink jet printing a composite front electrode comprising silver grid lines and a semitransparent PEDOT:PSS conductor are demonstrated. The effect of grid line density is explored for a large series of devices and a careful modeling study enabling the identification of the most rational grid structure is presented. Both optical and light beam induced current (LBIC) mapping of the devices are used to support the power loss model and to follow the evolution of the performance over time. Current generation is found to be evenly distributed over the active area initially progressing to a larger graduation in areas with different performance. Over time coating defects also become much more apparent in the LBIC images.