Ligang Yuan

Research on compositional engineering can realize power conversion efficiency (PCE) over 25%. Interfacial engineering along with optimal perovskite solar cell device structure is expected to lead to stable and theoretical PCE over 30%.

Abstract

Discovery of the 9.7% efficiency, 500 h stable solid‐state perovskite solar cell (PSC) in 2012 triggered off a wave of perovskite photovoltaics. As a result, a certified power conversion efficiency (PCE) of 25.2% was recorded in 2019. Publications on PSCs have increased exponentially since 2012 and the total number of publications reached over 13 200 as of August 2019. PCE has improved by developing device structures from mesoscopic sensitization to planar p‐i‐n (or n‐i‐p) junction and by changing composition from MAPbI₃ to FAPbI₃‐based mixed cations and/or mixed anion perovskites. Long‐term stability has been significantly improved by interfacial engineering with hydrophobic materials or the 2D/3D concept. Although small area cells exhibit superb efficiency, scale‐up technology is required toward commercialization. In this review, research direction toward large‐area, stable, high efficiency PSCs is emphasized. For large‐area perovskite coating, a precursor solution is equally important as coating methods. Precursor engineering and formulation of the precursor solution are described. For hysteresis‐less, stable, and higher efficiency PSCs, interfacial engineering is one of the best ways as defects can be effectively passivated and thereby nonradiative recombination is efficiently reduced. Methodologies are introduced to minimize interfacial and grain boundary recombination.

A NH₄Cl additive is introduced in the preparation of AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5) perovskite, leading to highly vertically oriented tin‐based reduced‐dimensional perovskite films with enhanced efficiency and stability. Herein, under the effects of NH₄Cl additive, the optimized power conversion efficiency of tin‐based quasi‐2D perovskite solar cells increases from 4.19% to 8.71% with enhanced stability.

Abstract

Tin‐based perovskites have exhibited high potential for efficient photovoltaics application due to their outstanding optoelectrical properties. However, the extremely undesired instabilities significantly hinders their development and further commercialization process. A novel tin‐based reduced‐dimensional (quasi‐2D) perovskites is reported here by using 5‐ammoniumvaleric acid (5‐AVA⁺) as the organic spacer. It is demonstrated that by introducing appropriate amount of ammonium chloride (NH₄Cl) as additive, highly vertically oriented tin‐based quasi‐2D perovskite films are obtained, which is proved through the grazing incidence wide‐angle X‐ray scattering characterization. In particular, this approach is confirmed to be a universal method to deliver highly vertically oriented tin‐based quasi‐2D perovskites with various spacers. The highly ordered vertically oriented perovskite films significantly improve the charge collection efficiency between two electrodes. With the optimized NH₄Cl concentration, the solar cells employing quasi‐2D perovskite, AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5), as light absorbers deliver a power conversion efficiency up to 8.71%. The work paves the way for further employing highly vertically oriented tin‐based quasi‐2D perovskite films for highly efficient and stable photovoltaics.

Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted CH₃NH₃PbI₃ perovskite solar cells.

Abstract

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm⁻¹ for pristine PEDOT:PSS to 12.75 S cm⁻¹ for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.

Here, intermediate‐controlled crystal growth is reported via solvent tuning to prepare highly oriented 2D perovskite films with faster transport, longer carrier lifetime, and lower defect density.

Abstract

Reduced‐dimensional hybrid perovskite semiconductors have recently attracted significant attention due to their promising stability and optoelectronic properties. However, the issue of poor charge transport in 2D perovskites limits its application. Here, studies on intermediate‐controlled crystal growth are reported to improve charge carrier transport in 2D perovskite thin films. It is shown that the coordination strength of solvents with perovskite precursor affects the initial state of intermediate phase formation as well as the subsequent perovskite layer growth. Tuning the solvent composition with a mixture (5:5) of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) leads to the growth of highly orientated 2D perovskite films with much‐improved optoelectronic properties (faster transport by ≈50x, longer carrier lifetime by ≈4x, and lower defect density by ≈30x) than the film prepared with pure DMF. Consequently, perovskite solar cells based on DMF/DMSO (5:5) show >80% efficiency improvement than the devices based on pure DMF.

Rapid crystallization is demonstrated to be necessary in achieving high‐quality 2DRP perovskite films by comparing dimethylacetamide (DMAC), N,N‐dimethylformamide, and dimethyl sulfoxide solvents. The improved stability and efficiency are observed using DMAC due to the accelerating crystallization rate of 2DRP perovskite crystals.

Abstract

Due to the additional introduction of bulky organic ammonium and the competition between bulky organic ammonium and methyl ammonium in 2D Ruddlesden‐Popper (2DRP) perovskite, the crystallization process becomes complicated. Here, it is demonstrated that the rapid crystallization controlled by processing solvents plays an important role in achieving high‐quality 2DRP perovskite films. It is found that the processing solvents, e.g., dimethylacetamide (DMAC), N,N‐dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), with a different polarity and boiling point, have almost no effect on crystal structure and phase distribution but have a remarkable effect on crystallization kinetics, crystal growth orientation, and crystallinity of 2DRP perovskite. Compared to polar aprotic solvent DMF and DMSO with a high boiling point, DMAC with low polarity and a suitable boiling point has a weak coordination to lead and ammonium salts and is easy to escape during solution processing, which is able to accelerate the crystallization rate of 2DRP perovskite. Benefitting from the rapid crystallization enabled high‐quality 2DRP perovskite films, the best‐performing device with improved stability and a power conversion efficiency of 12.15% is obtained using DMAC solvent. These findings may give guidance for solvent engineering for highly efficient 2DRP perovskite solar cells in the future.

The efficiency and stability of 2D perovskite solar cells are synergistically improved through metal ion doping. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the NiO _x layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) layers are effectively improved with Cs ion doping.

Abstract

2D perovskites hold a great prospective to create highly efficient and stable solar cell devices. In order to explore their full potential, every component of 2D perovskite solar cells (PSCs) has to be carefully designed and engineered. Herein, the metal ion doping strategy is taken to optimize both the hole transport layers (HTLs) and the light absorbing layers of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) based 2D PSC devices. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the nickel oxide layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ layers are effectively improved with Cs ion doping. The synergistic incorporations of Cu and Cs ions have boosted the device power conversion efficiency to 13.92%, the highest for 2D PSCs based on inorganic HTLs. In addition, the inorganic nature of the Cu doped nickel oxide film and the high quality of the Cs doped 2D perovskite film also endow the PSC device with extraordinary humidity and thermal stabilities.

A compact TiO₂/WO₃ bilayer film is fabricated as electron transport layer (ETL) in perovskite solar cells. Compared to the single WO₃ layer, the bilayer efficiently covers the fluorine‐doped tin oxide (FTO), avoids the direct contact between perovskite and FTO, decreases the risk of recombination. Finally, the bilayer ETL based device achieves a superior power conversion efficiency of 20.14%.

Abstract

It is crucial to retard the carrier recombination and minimize the energy loss at the transparent electrode/electron transport layer (ETL)/perovskite absorber interfaces to improve the performance of the perovskite solar cells (PSCs). Here, a bilayered TiO₂/WO₃ film is designed as ETL by combining atomic layer deposition (ALD) technology and spin‐coating process. The ALD‐TiO₂ underlayer fills the fluorine‐doped tin oxide (FTO) valleys and makes the surface smoother, which effectively avoids the shunt pathways between perovskite layer and FTO substrate and thereby suppresses electron–hole recombination at the interface. Moreover, the presence of hydrophilic TiO₂ underlayer is helpful in forming a uniform and compact WO₃ layer which is beneficial for extracting electron from perovskite to ETL. Meanwhile, the lower valance band minimum level of TiO₂ relative to WO₃ can efficiently enhance the hole‐blocking ability. By employing the optimized TiO₂ (7 nm)/WO₃ bilayer as ETL, the resulting cell exhibits an obviously enhanced power conversion efficiency of up to 20.14%, which is much better than the single WO₃ or TiO₂ ETL based device. This work is expected to provide a viable path to design ultrathin and compact ETL for efficient PSCs.

Radically super: A radical‐bearing redox‐active polymer, poly(1‐oxy‐2,2,6,6‐tetramethylpiperidin‐4‐yl methacrylate) (PTMA), significantly enhances the durability of an organo‐lead halide perovskite layer in a solar cell without decreasing the photovoltaic conversion performance. PTMA could work both as an eliminating agent of superoxide anion radical and as a carrier mediator in the perovskite under irradiation.

Abstract

A small amount of a radical‐bearing redox‐active polymer, poly(1‐oxy‐2,2,6,6‐tetramethylpiperidin‐4‐yl methacrylate) (PTMA), incorporated into the photovoltaic organo‐lead halide perovskite layer significantly enhanced durability of both the perovskite layer and its solar cell and even exposure to ambient air or oxygen. PTMA acted as an eliminating agent of the superoxide anion radical formed upon light irradiation on the layer, which can react with the perovskite compound and decompose it to lead halide. A cell fabricated with a PTMA‐incorporated perovskite layer and a hole‐transporting polytriarylamine layer gave a photovoltaic conversion efficiency of 18.8 % (18.2 % for the control without PTMA). The photovoltaic current was not reduced in the presence of PTMA in the perovskite layer probably owing to a carrier conductivity of PTMA. The incorporated PTMA also worked as a water‐repelling coating for providing humidity‐resistance to the organo‐lead halide perovskite layer.

The efficiency and stability of 2D perovskite solar cells are synergistically improved through metal ion doping. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the NiO _x layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) layers are effectively improved with Cs ion doping.

Abstract

2D perovskites hold a great prospective to create highly efficient and stable solar cell devices. In order to explore their full potential, every component of 2D perovskite solar cells (PSCs) has to be carefully designed and engineered. Herein, the metal ion doping strategy is taken to optimize both the hole transport layers (HTLs) and the light absorbing layers of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) based 2D PSC devices. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the nickel oxide layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ layers are effectively improved with Cs ion doping. The synergistic incorporations of Cu and Cs ions have boosted the device power conversion efficiency to 13.92%, the highest for 2D PSCs based on inorganic HTLs. In addition, the inorganic nature of the Cu doped nickel oxide film and the high quality of the Cs doped 2D perovskite film also endow the PSC device with extraordinary humidity and thermal stabilities.

Hole‐transport material based on dibenzo[b,d]thiophene (DBTMT) is synthesized with low costs. A champion power conversion efficiency of the optimized p–i–n planar perovskite solar cells based on dopant‐free DBTMT reaches 21.12% with a high fill factor of 83.25%, due to good hole‐transport properties and the passivation effect of DBTMT.

N ²,N ²,N ⁸,N ⁸‐tetrakis(4‐(methylthio)phenyl)dibenzo[b,d]thiophene‐2,8‐diamine (DBTMT) is synthesized from three commercial monomers for application as a promising dopant‐free hole‐transport material (HTM) in perovskite solar cells (pero‐SCs). The intrinsic properties (optical properties and electronic energy levels) of DBTMT are investigated, proving that DBTMT is a suitable HTM for the planar p–i–n pero‐SCs. The champion power conversion efficiency (PCE) of the optimized pero‐SCs (with structure as ITO/pristine DBTMT/MAPbI₃/C₆₀/BCP/Ag) reaches 21.12% with a fill factor (FF) of 83.25%, which is among the highest PCEs and FFs reported for planar p–i–n pero‐SCs based on dopant‐free HTMs. The Fourier‐transform infrared spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy spectra of MAPbI₃ and DBTMT–MAPbI₃ films demonstrate that there is an interaction between DBTMT and MAPbI₃ at the interface through the sulfur atoms in DBTMT to passivate the defects, which is corresponding to the higher FF and PCE of the corresponding device.

The crystallization process of perovskite inside a mesoscopic scaffold is revealed and a possible model of crystallization is proposed. By applying a solvent evaporation controlled crystallization method, ideal crystallization in the mesoscopic structure is achieved. As a result, a stabilized power conversion efficiency of 16.26% based on a printable mesoscopic perovskite solar cell is achieved.

Abstract

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.

Perovskite/Si tandem solar cells offer a feasible and promising approach to further reduce solar electricity costs by promising higher efficiency than their single‐junction counterparts. Prospects for achieving over 30% efficient monolithic perovskite/Si tandems in the near term using a combination of literature review with original results from numerical optoelectronic simulations are presented in this progress report.

Abstract

The article commences with a review focusing on three critical aspects of the perovskite/Si tandem technology: the evolution of efficiencies to date, comparisons of Si subcell choices, and the interconnection design strategies. Building on this review, a clear route is provided for minimizing optical losses aided by optical simulations of a recently reported high‐efficiency perovskite/Si tandem system, optimizations which result in tandem current densities of ≈20 mAcm⁻² with front‐side texture. The primary focus is on electrical modeling on the Si‐subcell, in order to understand the efficiency potential of this cell under filtered light in a tandem configuration. The possibility of increasing the Si subcell efficiency by 1% absolute is offered through joint improvements to the bulk lifetime, which exceeds 4 ms, and improves surface passivation quality to saturation current densities below 10 fA cm⁻². Polycrystalline‐Si/SiO _x passivating contacts are proposed as a promising alternative to partial‐area rear contacts, with the potential for further simplifying cell fabrication and improving device performance. A combination of optical modeling of the complete tandem structure alongside electrical modeling of the Si‐subcell, both with state‐of‐the‐art modeling tools, provides the first complete picture of the practical efficiency potential of perovskite/Si tandems.

Oxo‐functionalized graphene/dodecylamine is used to solve ion migration in cesium‐formamidinium‐methylammonium triple cation‐base perovskites. The ultra‐thin two‐dimensional network structure can wrap the crystals and reduce the ion migration of the perovskite film. The resulting devices deliver a power conversion efficiency of 21.1%, and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability.

Abstract

Mixed cation/halide perovskites have led to a significant increase in the efficiency and stability of perovskite solar cells. However, mobile ionic defects inevitably exacerbate the photoinduced phase segregation and self‐decomposition of the crystal structure. Herein, ultrathin 2D nanosheets of oxo‐functionalized graphene/dodecylamine (oxo‐G/DA) are used to solve ion migration in cesium (Cs)‐formamidinium (FA)‐methylammonium (MA) triple‐cation‐based perovskites. Based on the superconducting carbon skeleton and functional groups that provide lone pairs of electrons on it, the ultrathin 2D network structure can fit tightly on the crystals and wrap them, isolating them, and thus reducing the migration of ions within the built‐in electric field of the perovskite film. As evidence of the formation of sharp crystals with different orientation within the perovskite film, moiré fringes are observed in transmission electron microscopy. Thus, a champion device with a power conversion efficiency (PCE) of 21.1% (the efficiency distribution is 18.8 ± 1.7%) and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability, is reported. This work provides a simple method for the improvement of the structural stability of perovskite in solar cells.

Spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra, is employed to study degradation across perovskite and perovskite/silicon tandem solar cells. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap and intensity.

Abstract

Instability in perovskite solar cells is the main challenge for the commercialization of this solar technology. Here, a contactless, nondestructive approach is reported to study degradation across perovskite and perovskite/silicon tandem solar cells. The technique employs spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra. Parasitic absorption in other layers, carrier diffusion, and photon smearing phenomena are all demonstrated to have negligible effects on the extracted absorptivity. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap, and intensity. The technique is applied to study various common factors which induce and accelerate degradation in perovskite solar cells including air and heat exposure and light soaking. Finally, the technique is employed to extract the individual absorptivity component from the perovskite layer in a monolithic perovskite/silicon tandem structure. The results demonstrate the value of this approach for monitoring degradation mechanisms in perovskite and perovskite/silicon tandem cells at early stages of degradation and various fabrication stages.

Crystallizing perovskites on an ionic liquid‐modified SnO₂ substrate causes a shift of the perovskite Fermi level toward the conduction band and decreases the density of trap states in the perovskite. This results in a reduction of nonradiative recombination losses and, consequently, improved solar cell efficiencies.

Abstract

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge‐selective layers being where most recombination losses occur. In this study, a fluorinated ionic liquid (IL) is employed to modify the perovskite/SnO₂ interface. Using Kelvin probe and photoelectron spectroscopy measurements, it is shown that depositing the perovskite onto an IL‐treated substrate results in the crystallization of a perovskite film which has a more n‐type character, evidenced by a decrease of the work function and a shift of the Fermi level toward the conduction band. Photoluminescence spectroscopy and time‐resolved microwave conductivity are used to investigate the optoelectronic properties of the perovskite grown on neat and IL‐modified surfaces and it is found that the modified substrate yields a perovskite film which exhibits an order of magnitude lower trap density than the control. When incorporated into solar cells, this interface modification results in a reduction in the current–voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady‐state PCEs exceeding 20%.

In this work, it is demonstrated that the active material loading in all‐solid‐state batteries is largely controlled by the active material and solid electrolyte particle size ratio. A generalized guideline is provided for increasing active material loading via particle size optimization, and a composite cathode with >50 vol% active material loading is experimentally demonstrated.

Abstract

Low active material loading in the composite electrode of all‐solid‐state batteries (SSBs) is one of the main reasons for the low energy density in current SSBs. In this work, it is demonstrated with both modeling and experiments that in the regime of high cathode loading, the utilization of cathode material in the solid‐state composite is highly dependent on the particle size ratio of the cathode to the solid‐state conductor. The modeling, confirmed by experimental data, shows that higher cathode loading and therefore an increased energy density can be achieved by increasing the ratio of the cathode to conductor particle size. These results are consistent with ionic percolation being the limiting factor in cold‐pressed solid‐state cathode materials and provide specific guidelines on how to improve the energy density of composite cathodes for solid‐state batteries. By reducing solid electrolyte particle size and increasing the cathode active material particle size, over 50 vol% cathode active material loading with high cathode utilization is able to be experimentally achieved, demonstrating that a commercially‐relevant, energy‐dense cathode composite is achievable through simple mixing and pressing method.

This study reports a simultaneous contact and grain‐boundary passivation strategy in planar perovskite solar cells using SnO₂‐KCl composite as the electron transport layer. When applied to perovskite solar cells employing a composition of (FAPbI₃)_0.95(MAPbBr₃)_0.05, this strategy increases the open‐circuit voltage from 1.077 to 1.137 V and the corresponding efficiency from 20.2% to 22.2%.

Abstract

The performance of perovskite solar cells is sensitive to detrimental defects, which are prone to accumulate at the interfaces and grain boundaries of bulk perovskite films. Defect passivation at each region will lead to reduced trap density and thus less nonradiative recombination loss. However, it is challenging to passivate defects at both the grain boundaries and the bottom charge transport layer/perovskite interface, mainly due to the solvent incompatibility and complexity in perovskite formation. Here SnO₂‐KCl composite electron transport layer (ETL) is utilized in planar perovskite solar cells to simultaneously passivate the defects at the ETL/perovskite interface and the grain boundaries of perovskite film. The K and Cl ions at the ETL/perovskite interface passivate the ETL/perovskite contact. Meanwhile, K ions from the ETL can diffuse through the perovskite film and passivate the grain boundaries. An enhancement of open‐circuit voltage from 1.077 to 1.137 V and a corresponding power conversion efficiency increasing from 20.2% to 22.2% are achieved for the devices using SnO₂‐KCl composite ETL. The composite ETL strategy reported herein provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.

Highly efficient and stable perovskite solar cells are fabricated utilizing perovskite/Ag‐graphene in the active layer and SrTiO₃/Al₂O₃‐graphene in the electron transport layer. This composites‐based device not only improves charge transport and thermal‐ and photostability but also suppresses moisture penetration, ion migration, and recombination, resulting in remarkable long‐term stability, sustaining ≈93% of initial power conversion efficiency over 300 d under ambient conditions.

Abstract

For practical use of perovskite solar cells (PSCs) the instability issues of devices, attributed to degradation of perovskite molecules by moisture, ions migration, and thermal‐ and light‐instability, have to be solved. Herein, highly efficient and stable PSCs based on perovskite/Ag‐reduced graphene oxide (Ag‐rGO) and mesoporous Al₂O₃/graphene (mp‐AG) composites are reported. The mp‐AG composite is conductive with one‐order of magnitude higher mobility than mp‐TiO₂ and used for electron transport layer (ETL). Compared to the mp‐TiO₂ ETL based cells, the champion device based on perovskite/Ag‐rGO and SrTiO₃/mp‐AG composites shows overall a best performance (i.e., V _OC = 1.057 V, J _SC = 25.75 mA cm⁻², fill factor (FF) = 75.63%, and power conversion efficiency (PCE) = 20.58%). More importantly, the champion device without encapsulation exhibits not only remarkable thermal‐ and photostability but also long‐term stability, retaining 97–99% of the initial values of photovoltaic parameters and sustaining ≈93% of initial PCE over 300 d under ambient conditions.

Sum frequency generation vibrational spectroscopy, a state‐of‐the‐art nonlinear vibrational spectroscopy, is applied to elucidate molecular structure at buried halide perovskite interfaces in situ nondestructively with sub‐monolayer sensitivity. Molecular interfacial structures between different layers play an increasingly important, sometimes vital role in determining the overall performance in a halide perovskite device.

Abstract

As performance of halide perovskite devices progresses, the device structure becomes more complex with more layers. Molecular interfacial structures between different layers play an increasingly important role in determining the overall performance in a halide perovskite device. However, current understanding of such interfacial structures at a molecular level nondestructively is limited, partially due to a lack of appropriate analytical tools to probe buried interfacial molecular structures in situ. Here, sum frequency generation (SFG) vibrational spectroscopy, a state‐of‐the‐art nonlinear interface sensitive spectroscopy, is introduced to the halide perovskite research community and is presented as a powerful tool to understand molecule behavior at buried halide perovskite interfaces in situ. It is found that interfacial molecular orientations revealed by SFG can be directly correlated to halide perovskite device performance. Here how SFG can examine molecular structures (e.g., orientations) at the perovskite/hole transporting layer and perovskite/electron transporting layer interfaces is discussed. This will promote the use of SFG to investigate molecular structures of buried interfaces in various halide perovskite materials and devices in situ nondestructively with a sub‐monolayer interface sensitivity. Such research will help to elucidate structure–function relationships of buried interfaces, aiding in the rational design/development of halide perovskite materials/devices with improved performance.

Recent progress in first‐principles simulations of carrier recombination in halide perovskites is reviewed. Misunderstandings relating to the impact of the Rashba effect on radiative recombination are clarified. The origin of exceptionally strong Auger recombination and avenues for improved materials design are discussed. Critical analysis of the recombination mechanisms reveals fruitful directions for improving the performance of halide perovskites.

Abstract

In recent years, there have been remarkable developments in halide perovskites, which are used in highly efficient optoelectronic devices and exhibit intriguing materials physics. Detailed knowledge of carrier recombination mechanisms is essential for understanding their excellent performance and to further increase their efficiency. Obtaining such knowledge is challenging however, and different studies have reached divergent conclusions in some cases. This progress report outlines the critical developments in understanding the carrier recombination mechanisms in halide perovskites from a computational perspective. The primary focus is radiative and Auger recombination, since they have not been systematically assessed and discussed before, and a number of important issues have been actively debated. This comprehensive discussion of the carrier recombination mechanisms is aimed at establishing physically justified insights that can form the basis for better materials and devices design.

A power conversion efficiency of 13.36% in ternary organic photovoltaics is obtained by carefully picking materials with good compatibility and complementary absorption spectra, as well as well‐matched energy levels with efficient energy transfer.

Abstract

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT‐2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT‐2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT‐2F to T6Me. The efficient energy transfer from IT‐2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short‐circuit current density (J _SC) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT‐2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT‐2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

Abstract

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V _oc of 1.01 V and a J _sc of 17.89 mA cm⁻². Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

Eco‐compatible solvent‐processed organic photovoltaic cells with over 16% power conversion efficiency are achieved via modifying the flexible alkyl chains of BTP‐4F‐8. Combining with the polymer donor T1, over 14% power conversion efficiencies are obtained not only for using several kinds of greener solvents like o‐xylene, 1,2,4‐trimethylbenzene, and tetrahydrofuran but also for 1.07 cm² cells by the blade‐coating method.

Abstract

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large‐area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco‐compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco‐compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP‐4F‐8 (also known as Y6) are modified and BTP‐4F‐12 is synthesized. Combining with the polymer donor PBDB‐TF, BTP‐4F‐12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB‐TF is replaced by T1 with better solubility, various eco‐compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm² OPV cells by the blade‐coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

The development and recent advances in the design of all‐organic photoelectrodes based on conjugated polymers and small molecules for water splitting are reviewed. The effects of the band energy structure, film morphology, and thickness on the photoelectrochemical properties are discussed. In addition, future directions are briefly presented.

Currently, photoelectrochemical water‐splitting research is dominated by inorganic and organic–inorganic hybrid photoelectrodes. Although organic semiconductors have several advantages over their inorganic counterparts, the development of photoelectrodes based on pure organic materials has been lagging for the last decade. Recent improvements in molecular design, synthesis, and processing of organic semiconductors, such as polythiophenes, graphitic carbon nitrides, conjugated acetylenic polymers, alternating donor–acceptor‐conjugated polymers, and N‐containing fused‐ring small molecules, significantly enhance the performance of these photolectrodes without added cocatalysts. Although this research has been conducted over the past few years, this overlooked topic still stays unexplored, with more efficient materials yet to be discovered. Herein, the early development and recent advances of exclusively organic photoelectrodes for water splitting are summarized and brief perspectives for future improvements are provided.

Using diluted poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer (HTL), Sn–Pb‐based low‐E _g perovskite solar cells (PSCs) with a maximum power conversion efficiency (PCE) of up to 19.58% and J _sc of 29.81 mA cm⁻² are achieved. Then, an all‐perovskite four‐terminal tandem cell with a PCE of 23.26% is demonstrated with this low‐E _g PSC as the bottom cell. This easy and effective approach also reduces the cost of devices.

Recently, Sn–Pb low‐bandgap (E _g) perovskite solar cells (PSCs) have attracted enormous interest as an ideal bottom cell for all‐perovskite tandem solar cells. However, due to the lack of high‐performance Sn–Pb low‐E _g PSCs, the development of all‐perovskite tandem solar cells is severely constrained. Herein, the performance of Sn–Pb low‐E _g (1.2 eV) PSC is improved significantly using diluted poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer with a maximum power conversion efficiency (PCE) up to 19.58% and short‐circuit current density of 29.81 mA cm⁻². The four‐terminal (4‐T) all‐perovskite tandem solar cell is constructed using an optical splitting system with this high‐efficient low‐E _g PSC as the bottom cell and a wide‐E _g (1.6 eV) PSC as the top cell. The best all‐perovskite 4‐T tandem solar cell shows a PCE of 23.26%.

Graphite carbon nitride (g‐C₃N₄) with various functional groups is developed and serves as an additive in perovskite solar cells (PSCs). This functional g‐C₃N₄ not only improves charge transport but also passivates the defects on grain boundaries in PVSCs. These features enhance the power conversion efficiency from 17.85% to 20.08% and demonstrate a feasible new concept of additives with multifunctions.

Passivation strategies are considered as one of the most efficient methods to suppress nonradiative recombination of organic–inorganic lead halide perovskite solar cells (PSCs), leading to tremendous photovoltaic performance. An innovative 2D polymer, graphitic carbon nitride (g‐C₃N₄), as well as various organic groups (amino, sulfonic, nitrato, and hydroxy group), are widely used as passivation agents, according to the previous reports. Anchoring g‐C₃N₄ and the aforementioned organic groups as additives in perovskite can both heal charged defects around the grain boundaries by passivating the charge recombination center. In addition, the crystalline quality can also be enhanced by the incorporation of g‐C₃N₄, leading to improved conductivity of perovskite light absorber films that is beneficial for benign charge extraction efficiency. Inspired by the underlining mechanisms, a series of novel passivation molecules, functionalized g‐C₃N₄ (F‐C₃N₄) with assorted organic groups, is designed herein, yielding a champion power conversion efficiency (PCE) of 20.08% for NO₃‐C₃N₄‐based p‐i‐n structure PSC, in comparison with that of PSC without passivation (17.85%). These findings present an efficient strategy to understand and design multiple facets of applications of novel passivation molecules to further improve the PCE of PSCs.