JIANG ZHI XUAN

A new fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is utilized to passivate the surface of CsPbI₂Br perovskite for solar cells with enhanced efficiency as well as improved stability.

Surface modification is demonstrated as an efficient strategy to enhance the efficiency and stability of perovskite solar cells (PVSCs). Fluorinated organic ammonium salts featuring a strong hydrophobic nature are seldom used as passivation agents for the surface modification of CsPbI₂Br perovskites. Herein, a fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is incorporated into the surface of CsPbI₂Br perovskite for the first time. After the CFPEAI modification, the defects of CsPbI₂Br perovskite are significantly passivated with reduced trap densities. The best‐performance PVSC with CFPEAI modification shows an excellent power conversion efficiency (PCE) of 16.07% with a high fill factor (FF) of 84.65%, a short‐circuit current density (J _SC) of 15.45 mA cm⁻², and an open‐circuit voltage (V _OC) of 1.23 V. In contrast, the control PVSCs without the surface modification exhibit a lower PCE of 14.50% with a FF of 80.56%, a J _SC of 15.05 mA cm⁻², and a V _OC of 1.20 V. With CFPEAI passivation, the CsPbI₂Br perovskite film exhibits enhanced hydrophobicity, thereby leading to improved stability for the corresponding PVSC in comparison with the control PVSC without any surface modification.

Minimization of the gap states in halide perovskites is crucial for approaching the theoretical device limit. This work correlates the appearance of gap states in methylammonium lead halides and the presence of dimethylsulfoxide in the processing solution. The results suggest the link between the products of the dimethylsulfoxide–methylammonium reaction and the gap states. Replacement with a non‐reactive additive eliminates these gap states.

Abstract

Understanding the origin and distribution of electronic gap states in metal halide perovskite (MHP) thin films is crucial to the further improvement of the efficiency and long‐term stability of MHP‐based optoelectronic devices. In this work, the impact of Lewis‐basic additives introduced in the precursor solution on the density of states in the perovskite bandgap is investigated. Ultraviolet photoemission spectroscopy and contact potential difference measurements are conducted on MHP thin films processed from dimethylformamide (DMF)‐based solutions to which either no additive, dimethylsulfoxide (DMSO), or N‐methylpyrrolidine‐2‐thione (NMPT) is added. The results show the presence of a density of states in the gap of methylammonium lead halide films processed from DMSO‐containing solution. The density of gap states is either suppressed when the methylammonium concentration in mixed cation films is reduced or when NMPT is used as an additive, and eliminated when methylammonium (MA) is replaced with cesium or formamidinium (FA). These results are consistent with the notion that reaction products that result from DMSO reacting with MA⁺ in the precursor solution are responsible for the formation of gap states.

Introduction of multi‐cation hybrid halide perovskite quantum dots reduces ionic defects at the surface and grain boundaries via a solid‐state interdiffusion process. The enhanced moisture and photostability enable power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Abstract

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic–inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid‐state interdiffusion process using multi‐cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid‐state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05(MA_0.17FA_0.83)_0.95PbBr₃ (abbreviated as QDs‐Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

The photo‐effect on ion conduction in mixed cation and halide perovskites is studied. Unlike A‐site substitution, anion replacement is of great influence. In I‐Br mixtures the differences in hole localization and defect formation favor (reversible) photo‐demixing (the situation in the right part is simplified as the interstitial neutral iodine is further stabilized by ionic rearrangement, and the hole in the bromide is delocalized over several regular anions).

Abstract

Lead halide perovskites are considered to be most promising photovoltaic materials. Highest efficiency and improved stability of perovskite solar cells have been achieved by using cation and anion mixtures. Experimental information on electronic and ionic charge carriers is key to evaluate device performance, as well as processes of photo‐decomposition and photo‐demixing which are observed in these materials. Here, we measure ionic and electronic transport properties and investigate various cation and anion substitutions with a special eye on their photo‐ionic effect, following our previous study on CH₃NH₃PbI₃, where we found that light enhances not only electronic but also ionic conductivities. We find that this phenomenon is very sensitive to the nature of the halide, while the cationic substitutions are less relevant. Based on the observation that the ionic conductivity enhancement found for iodide perovskites is significantly weakened by bromide substitution, we provide a chemical rationale for the photo‐demixing in mixed halide compositions.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

Two fully non‐fused acceptors are precisely designed and easily prepared. The side chain encapsulation can induce a planar molecular backbone conformation, endowing the acceptor with broad light absorption. Thermal annealing promotes molecular rearrangement to form J‐aggregates with even broader absorption and higher absorption coefficient. A PCE over 10 % is one of the highest PCE for fully non‐fused ring acceptors.

Abstract

Fused‐ring electron acceptors have made significant progress in recent years, while the development of fully non‐fused ring acceptors has been unsatisfactory. Here, two fully non‐fused ring acceptors, o‐4TBC‐2F and m‐4TBC‐2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o‐4TBC‐2F formed planar backbones, while m‐4TBC‐2F displayed a twisted backbone. Additionally, the o‐4TBC‐2F film showed a markedly red‐shifted absorption after thermal annealing, which indicated the formation of J‐aggregates. For fabrication of organic solar cells (OSCs), PBDB‐T was used as a donor and blended with the two acceptors. The o‐4TBC‐2F‐based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o‐4TBC‐2F gave a PCE of 10.26 %, which was much higher than those based on m‐4TBC‐2F at 2.63 %, and it is one of the highest reported PCE values for fully non‐fused ring electron acceptors.

Nature Energy, Published online: 31 August 2020; doi:10.1038/s41560-020-00684-7

Nature Energy, Published online: 31 August 2020; doi:10.1038/s41560-020-00689-2

Introduction of multi‐cation hybrid halide perovskite quantum dots reduces ionic defects at the surface and grain boundaries via a solid‐state interdiffusion process. The enhanced moisture and photostability enable power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Abstract

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic–inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid‐state interdiffusion process using multi‐cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid‐state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05(MA_0.17FA_0.83)_0.95PbBr₃ (abbreviated as QDs‐Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

The development of the first high‐performance, printable CsPbI₃ solar cells via an ambient blade‐coating technique is reported. High‐quality CsPbI₃ films are grown via the introduction of a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂. As a result, the additive‐treated perovskite solar cell delivers a power conversion efficiency (PCE) of 19%.

Abstract

All‐inorganic CsPbI₃ holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI₃ solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade‐coating at a low temperature (≤100 °C) in ambient conditions. High‐quality CsPbI₃ films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂, which reconciles the conflict between air‐flow‐assisted fast drying and low‐quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO₂ interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO₂ interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.

The search for nontoxic perovskite materials with long‐term stability provides new opportunities for optoelectronics. Alkali‐substituted compounds based on the double perovskite Cs₂AgBiBr₆ are scrutinized in terms of their photo‐physical and structural properties. Cation‐substitution entails an enhanced carrier recombination lifetime and, in combination with the tunability in the sensitivity to X‐rays, makes these materials suitable for applications in X‐ray detectors.

Abstract

Lead‐free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs₂AgBiBr₆ has emerged as a promising material, with suboptimal photon‐to‐charge carrier conversion efficiency, yet well suited for high‐energy photon‐detection applications. Here, the optoelectronic and structural properties of pure Cs₂AgBiBr₆ and alkali‐metal‐substituted (Cs_{1− x} Y _x )₂AgBiBr₆ (Y: Rb⁺, K⁺, Na⁺; x = 0.02) single crystals are investigated. Strikingly, alkali‐substitution entails a tunability to the material system in its response to X‐rays and structural properties that is most strongly revealed in Rb‐substituted compounds whose X‐ray sensitivity outperforms other double‐perovskite‐based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali‐substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron–acoustic phonon scattering is found compared to Cs₂AgBiBr₆. The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

Defect tolerance of metal‐halide perovskites is a commonly evoked concept to explain the development of high efficiency solar cells upon solution processing. However, moving the attention to solar cell stability, these materials seem to be defect intolerant. Further material engineering is needed to obtain a 100% defect tolerant materials platform.

Abstract

Metal‐halide perovskites present exceptional optoelectronic properties such as large light absorption coefficients, long free charge carrier diffusion lengths with ambipolar character. They are apparently protected by what is often described as a “defect tolerance” which has allowed to achieve, relatively quickly, highly performing devices. Nevertheless, there also exists a “defect intolerance” when it is dealt with stability. Further rationalization of the passivation strategies, especially for complex chemical systems, will be beneficial to achieve a full materials library which can be the platform for an efficient and reliable technology.

A composite consisting of 1D cation‐doped TiO₂ brookite nanorod embedded by 0D fullerene is investigated as a top modification buffer for inverted perovskite photovoltaic (IP‐PV) cells. The resultant IP‐PV displays an efficiency exceeding 22% with a favorable stability. This work opens up more opportunities in expanding the material inventory for charge transfer layer in perovskite solar cells development and application.

Abstract

Simultaneously achieving high efficiency and high durability in perovskite solar cells is a critical step toward the commercialization of this technology. Inverted perovskite photovoltaic (IP‐PV) cells incorporating robust and low levelized‐cost‐of‐energy (LCOE) buffer layers are supposed to be a promising solution to this target. However, insufficient inventory of materials for back‐electrode buffers substantially limits the development of IP‐PV. Herein, a composite consisting of 1D cation‐doped TiO₂ brookite nanorod (NR) embedded by 0D fullerene is investigated as a top modification buffer for IP‐PV. The cathode buffer is constructed by introducing fullerene to fill the interstitial space of the TiO₂ NR matrix. Meanwhile, cations of transition metal Co or Fe are doped into the TiO₂ NR to further tune the electronic property. Such a top buffer exhibits multifold advantages, including improved film uniformity, enhanced electron extraction and transfer ability, better energy level matching with perovskite, and stronger moisture resistance. Correspondingly, the resultant IP‐PV displays an efficiency exceeding 22% with a 22‐fold prolonged working lifetime. The strategy not only provides an essential addition to the material inventory for top electron buffers by introducing the 0D:1D composite concept, but also opens a new avenue to optimize perovskite PVs with desirable properties.

2D perovskites are of great importance to increase both the efficiency and stability of perovskite interfaces. Motivated by the stronger halogen bond interaction, (5FBzAI)₂PbI₄ used as a capping layer in 3D/2D systems self‐organizes with an in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V _oc, and remarkable operational stability.

Abstract

Despite organic/inorganic lead halide perovskite solar cells becoming one of the most promising next‐generation photovoltaic materials, instability under heat and light soaking remains unsolved. In this work, a highly hydrophobic cation, perfluorobenzylammonium iodide (5FBzAI), is designed and a 2D perovskite with reinforced intermolecular interactions is engineered, providing improved passivation at the interface that reduces charge recombination and enhances cell stability compared with benchmark 2D systems. Motivated by the strong halogen bond interaction, (5FBzAI)₂PbI₄ used as a capping layer aligns in in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V _oc, a twofold improvement compared with its analogous monofluorinated phenylethylammonium iodide (PEAI) recently reported. This endows the system with high power conversion efficiency of 21.65% and extended operational stability after 1100 h of continuous illumination, outlining directions for future work.

Arising from the lubricant role of WS₂ nanoflakes between ETL and perovskite, the tensile strain in the perovskite film is released for a uniform perovskite with low defect density and high activation energy for ion migration. This significantly improves the device performance and stability.

Abstract

Perovskite lattice distortion induced by residual tensile strain from the thermal expansion mismatch between the electron‐transporting layer (ETL) and perovskite film causes a sluggish charge extraction and transfer dynamics in all‐inorganic CsPbBr₃ perovskite solar cells (PSCs) because of their higher crystallization temperatures and thermal expansion coefficients. Herein, the interfacial strain is released by fabricating a WS₂/CsPbBr₃ van der Waals heterostructure owing to their matched crystal lattice structure and the atomically smooth dangling bond‐free surface to act as a lubricant between ETL and CsPbBr₃ perovskite. Arising from the strain‐released interface and condensed perovskite lattice, the best device achieves an efficiency of 10.65 % with an ultrahigh open‐circuit voltage of 1.70 V and significantly improved stability under persistent light irradiation and humidity (80 %) attack over 120 days.

Introduction of multi‐cation hybrid halide perovskite quantum dots reduces ionic defects at the surface and grain boundaries via a solid‐state interdiffusion process. The enhanced moisture and photostability enable power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Abstract

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic–inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid‐state interdiffusion process using multi‐cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid‐state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05(MA_0.17FA_0.83)_0.95PbBr₃ (abbreviated as QDs‐Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Herein, a bulk heterojunction structure hybridizing SnS QDs and CsPbBr₃ is designed as an absorber layer for solar cells. The prominent SnS QD–CsPbBr₃ structure improves the crystallinity of perovskite, enhances the charge transfer efficiency, and reduces the trap state density of the CsPbBr₃ film, thereby further boosting the photoelectric performance of perovskite solar cells.

Inorganic cesium lead halide perovskite solar cells (PSCs) have attracted great attention due to their remarkable thermal and moisture stability. However, the low photoelectric conversion efficiency (PCE) of inorganic PSCs due to the high trap state density, high carrier recombination rate, and poor carrier transport kinetics impedes their industrial applications. Herein, a remarkable bulk heterojunction structure coupling SnS quantum dots (QDs) with CsPbBr₃ is prepared for the first time via a facile spin‐coating process using a SnS‐QD‐dispersed toluene solution as an antisolvent. The introduction of SnS QDs provides extra crystallization sites and promotes the crystallization of perovskites along (100) and (200) faces. Meanwhile, the bulk heterojunction structure with matched energy structure can effectively reduce the trap state density of the perovskite film and improves the dynamic performance of carriers, suppressing the charge recombination rate and effectively boosting the PCE of solar cells. As a result, the optimal bulk heterojunction solar cell achieves a PCE of 8.01%, about 52% higher than that of the pristine solar cell. This work provides a low‐cost and facile strategy for preparing high‐performance bulk heterojunction PSCs and an effective method to boost the PCE by nontoxic QDs.

Graphene‐based materials have demonstrated tremendous potentials in boosting the efficiency and stability of planar perovskite solar cells (PSCs). A comprehensive overview that focused on the specific applications of graphene in planar PSCs has never been published before. Herein the recent applications of graphene in planar PSCs are systematically discussed and concluding perspectives on current challenges and future developments are proposed.

Metal halide perovskite solar cells (PSCs) have recently become the most promising new‐generation solar cells, with a breathtaking growth of efficiency from 3.8% to 25.2% in just one decade. Scientists have abandoned the traditional high‐temperature‐processed mesoscopic layer of the initial mesoscopic PSCs in designing and manufacturing planar PSCs. This new feature endows planar PSCs with possibilities of low‐temperature processibility and large‐scale production. Nevertheless, the advancement of planar PSCs remains limited by two bottlenecks: charge loss and device degradation. To address these two issues, researchers have adopted graphene‐based materials, which demonstrate tremendous potentials due to their superb optical transparency, outstanding carrier mobility, remarkable electrical conductivity, and superior physicochemical stability. Defects inside films and at interfaces are regulated by graphene, thereby contributing to more efficient charge extraction and suppressed charge recombination. The graphene protective layer enhances the moisture and heat stability of planar PSCs, thereby extending the lifetime of devices. Herein, the typical synthesis methods of graphene and the recent applications of graphene in planar PSCs are summarized and discussed. Furthermore, concluding perspectives on current challenges and the future development of graphene in planar PSCs are proposed.