awn

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.

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.

The desired α‐FAPbI₃ perovskite phase is stabilized by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium) overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇‐based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. It showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress.

Abstract

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.

A ferroelectricity‐driven self‐powered ultraviolet photodetector employing a 2D hybrid perovskite ferroelectric (BPA)₂PbBr₄ (BPA=3‐bromopropylammonium) is presented. It shows strong polarization sensitivity, with a large polarization ratio of up to 6.8.

Abstract

Polarization‐sensitive ultraviolet (UV) photodetection is highly indispensable in military and civilian applications and has been demonstrated with various wide‐band photodetectors. However, it still remains elusive to achieve the self‐powered devices, which can be operated in the absence of external bias. Herein, for the first time, ferroelectricity‐driven self‐powered photodetection towards polarized UV light was successfully demonstrated in a 2D wide‐band gap hybrid ferroelectric (BPA)₂PbBr₄ (BPA=3‐bromopropylammonium) (1). We found that the prominent spontaneous polarization in 1 results in a bulk photovoltaic effect (BPVE) of 0.85 V, that independently drives photoexcited carriers separation and transport and thus supports self‐powered ability. This self‐powered detector shows strong polarization sensitivity to linearly polarized UV illumination with a polarization ratio up to 6.8, which is superior to that of previously reported UV‐polarized photodetectors (ZnO, GaN, and GeS₂).

C−C oxidative dimerizations of α‐aryl ketonitriles are realized by using zwitterionic ligand capped CsPbBr₃ perovskite QDs as catalysts under visible light illumination, via a radical mediated reaction pathway. High stereoselectivities of dl‐isomers were obtained owing to less steric hindrance during the bond formation process. Our study sheds new lights on using lead‐halide perovskite QDs as photocatalysts for stereoselective organic synthesis.

Abstract

Semiconductor quantum dots (QDs) have attracted tremendous attention in the field of photocatalysis, owing to their superior optoelectronic properties for photocatalytic reactions, including high absorption coefficients and long photogenerated carrier lifetimes. Herein, by choosing 2‐(3,4‐dimethoxyphenyl)‐3‐oxobutanenitrile as a model substrate, we demonstrate that the stereoselective (>99 %) C−C oxidative coupling reaction can be realized with a high product yield (99 %) using zwitterionic ligand capped CsPbBr₃ perovskite QDs under visible light illumination. The reaction can be generalized to different starting materials with various substituents on the phenyl ring and varied functional moieties, producing stereoselective dl‐isomers. A radical mediated reaction pathway has been proposed. Our study provides a new way of stereoselective C−C oxidative coupling via a photocatalytic means using specially designed perovskite QDs.

The optimized ratio of chlorinated organic salt benzyltriethylammonium chloride ([BZTAm]Cl) is helpful for the interfacial defect passivation at the perovskite/PC₆₁BM interface. The corresponding perovskite film treatment produces high‐quality film, suppresses nonradiative recombination, and promotes the energy levels matching, which results in remarkably improved device performance and environmental stability.

In perovskite solar cells (PSCs), the interfaces between perovskite film and charge transport layers have an enormous influence on the device performance and stability. Recently, it has been proven that the surface defect passivation of perovskite layer is an effective strategy to improve the device efficiency. Herein, an organic ammonium salt benzyltriethylammonium chloride ([BZTAm]Cl) is used as an ultra‐thin modification layer in perovskite films in MAPbI₃ PSCs for passivating the surface defects. The obtained results demonstrate that the [BZTAm]Cl modifier improves the crystallization/morphology of perovskite film and effectively aligns the energy levels with the corresponding charge‐transporting layers, suppressing the nonradiative recombination and reducing the trap state density. As a result, a champion device efficiency of 20.45% is achieved for optimized concentration of [BZTAm]Cl in comparison with 17.87% for the control device. Moreover, the unencapsulated device presents a good long‐term stability after aging in an ambient environment with 40–50% relative humidity conditions for 30 days.

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.

Stable oxide thin films on both sides of the MoO _X film are employed in MoO _X ‐heterocontacted solar cells on p‐type CZ silicon wafers. The SiO _X tunneling layer formed by UV/O₃ treatment and the ultrathin V₂O _X capping layer maintains the work function and hole selectivity of MoO _X at a higher level. The p‐Si/SiO _X /MoO _X /V₂O _X /ITO/Ag solar cell demonstrates a stable efficiency of 20.0%.

Abstract

Crystalline silicon heterojunction solar cells based on hole‐selective MoO _X contacts provide obvious merits in terms of the decent passivation and carrier selectivity but face the challenge of long‐term stability. With the aim to improve the performance and stability of solar cells with full area MoO _X /metal contacts, a SiO _X tunneling layer on silicon surface is intentionally formed by UV/O₃ treatment and an indium tin oxide (ITO) film is sputtered as a high‐work‐function electrode. Before ITO sputtering, an ultrathin V₂O _X capping layer is introduced to efficiently prevent MoO _X film from air exposure and the damage by sputtering bombardment. The insertion of SiO _X , V₂O _X , and ITO keeps the work function of MoO _X at a high level, which improves the hole selectivity as well as the stability of the contact. The p‐Si/SiO _X /MoO _X /V₂O _X /ITO/Ag solar cell demonstrates an efficiency of 20.0% with improved stability, which is the highest value for MoO _X heterocontacts class on p‐type silicon to date.

A formamidinium (FA)‐based quasi‐2D Ruddlesden–Popper (RP) perovskite, namely, (ThMA)₂(FA) _{n −1}Pb _n I_{3 n +1} (nominal n = 5), is successfully demonstrated with high photovoltaic performance by using an organic‐salt‐assisted crystal growth method. The optimized device exhibits a champion efficiency of 19.06%, which is a record for quasi‐2D RP perovskite solar cells with nominal n‐value lower than 6.

Abstract

Quasi‐2D Ruddlesden–Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi‐2D RP PSCs using 2‐thiophenemethylammonium as spacer with nominal n‐value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi‐2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)‐based quasi‐2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4‐(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG‐processed quasi‐2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI₃‐based

3D PSCs.

A unique strategy of “transparent hole‐transporting frameworks” is proposed. A hole‐transporting large‐bandgap polymer, PTAA, is employed to partially replace the polymer donors in the active layer. As a result, semitransparent organic photovoltaic devices with power conversion efficiencies ≈12% and average visible transmittances ≈20% are achieved both on rigid and flexible substrates.

Abstract

Thanks to the nature of molecular orbitals, the absorption spectra of organic semiconductors are not continuous like those in traditional inorganic semiconductors, which offers a unique application of organic photovoltaics (OPVs): semitransparent OPVs. Recently, the exciting progress of materials design has promoted the development of semitransparent OPVs. However, in the perspective of device engineering, almost all reported works reduce the thickness of back/reflected electrode to obtain high average visible transmittance (AVT), which is a trade‐off between power conversion efficiency (PCE) and the transmittance of the whole solar spectrum (visible and infrared), and therefore limit the further development. Herein, a unique strategy of “transparent hole‐transporting frameworks” is proposed. A hole‐transporting large‐bandgap polymer (poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA)) is employed to partially replace polymer donors in the active layer of PBDB‐T/Y1. PTAA is a p‐type polymer with a large bandgap of 2.9 eV; the partial substitution of PBDB‐T by PTAA reduces the absorption of the active layer only in the visible region, keeping the hole‐transporting pathways as well as the optimized film morphology. As a result, semitransparent OPVs with PCEs of 12% and AVTs of 20% are achieved, both on rigid and flexible substrates. To demonstrate the generality, this strategy is also used in three different active layers.

Ultralong charge‐carrier lifetimes >6 μs are achieved in polycrystalline halide perovskites by decorating the grain boundaries with a trace amount of electron‐rich anchors, which benefits from weak excitonic effects and the weakening of electron–phonon couplings in passivated films, fulfilling reduced voltage deficits and enhanced efficiencies in perovskite photovoltaics. This finding provides a new insight into realizing superior carrier properties of polycrystalline perovskite films and high‐performance perovskite optoelectronics.

Abstract

Lead halide perovskite films have witnessed rapid progress in optoelectronic devices, whereas polycrystalline heterogeneities and serious native defects in films are still responsible for undesired recombination pathways, causing insufficient utilization of photon‐generated charge carriers. Here, radiation‐enhanced polycrystalline perovskite films with ultralong carrier lifetimes exceeding 6 μs and single‐crystal‐like electron–hole diffusion lengths of more than 5 μm are achieved. Prolongation of charge‐carrier activities is attributed to the electronic structure regulation and the defect elimination at crystal boundaries in the perovskite with the introduction of phenylmethylammonium iodide. The introduced electron‐rich anchor molecules around the host crystals prefer to fill the halide/organic vacancies at the boundaries, rather than form low‐dimensional phases or be inserted into the original lattice. The weakening of the electron‐phonon coupling and the excitonic features of the photogenerated carriers in the optimized films, which together contribute to the enhancement of carrier separation and transportation, are further confirmed. Finally the resultant perovskite films in fully operating solar cells with champion efficiency of 23.32% are validated and a minimum voltage deficit of 0.39 V is realized.

The relatively large non‐radiative recombination voltage loss (ΔV _non‐rad) is the main challenge for the development of organic solar cells (OSCs). ΔV _non‐rad of OSCs can be effectively reduced to 0.16 V by adopting material combinations that deliver high E _CT (the energy of charge‐transfer state) and low ΔE _CT (energetic difference between singlet excited state and CT state), together with chlorination in donors.

Abstract

Compared with inorganic or perovskite solar cells, the relatively large non‐radiative recombination voltage losses (ΔV _non‐rad) in organic solar cells (OSCs) limit the improvement of the open‐circuit voltage (V _oc). Herein, OSCs are fabricated by adopting two pairs of D–π–A polymers (PBT1‐C/PBT1‐C‐2Cl and PBDB‐T/PBDB‐T‐2Cl) as electron donors and a wide‐bandgap molecule BTA3 as the electron acceptor. In these blends, a charge‐transfer state energy (E _CT) as high as 1.70–1.76 eV is achieved, leading to small energetic differences between the singlet excited states and charge‐transfer states (ΔE _CT ≈ 0.1 eV). In addition, after introducing chlorine atoms into the π‐bridge or the side chain of benzodithiophene (BDT) unit, electroluminescence external quantum efficiencies as high as 1.9 × 10⁻³ and 1.0 × 10⁻³ are realized in OSCs based on PBTI‐C‐2Cl and PBDB‐T‐2Cl, respectively. Their corresponding ΔV _non‐rad are 0.16 and 0.17 V, which are lower than those of OSCs based on the analog polymers without a chlorine atom (0.21 and 0.24 V for PBT1‐C and PBDB‐T, respectively), resulting in high V _oc of 1.3 V. The ΔV _non‐rad of 0.16 V and V _oc of 1.3 V achieved in PBT1‐C‐2Cl:BTA3 OSCs are thought to represent the best values for solution‐processed OSCs reported in the literature so far.

Softly deposited transparent MoO _x /Ag/WO _x composite films enable high‐performance semitransparent perovskite solar cells obtaining an optimal conversion efficiency of 15.40% along with 10.17% in the average visible‐light transmission simultaneously.

Abstract

Semitransparent solar cells play a crucial role in such typical photovoltaic applications as smart windows, transparent chargeable devices, tandem and bifacial devices, and so on. Relying on the commercial conductive transparent oxides as the front electrodes, the development of rear transparent electrode (RTE) is especially essential. Here, an efficient semitransparent perovskite solar cell (PSC) with the softly deposited transparent MoO _x /Ag/WO _x (MAW) as the rear electrode is demonstrated. MoO _x enables the continuously grown silver ultrathin film while the high‐refractive‐index WO _x capping layer can modulate the whole optical interference and enhance the light transmission throughout the device. As a result, depending on the MAW RTE, the semitransparent normal planar PSC owns the optimal conversion efficiency of 15.40% along with 10.17% in the average visible‐light transmission (AVT) simultaneously, which claims the best conversion efficiency of the semitransparent PSCs at such considerable AVT. Combining the optical characterizations, the bifacial performance test of the same device also reveals the uneven absorption due to the different optical interference depending on the light direction, as well as the typical parasitic absorption by the functional layers. This paper paves an alternative and promising way to fabricate high‐performance semitransparent optoelectronic devices in the future.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Sheng Dong, Tao Jia, Kai Zhang, Jianhua Jing, Fei Huang

The relatively large non‐radiative recombination voltage loss (ΔV _non‐rad) is the main challenge for the development of organic solar cells (OSCs). ΔV _non‐rad of OSCs can be effectively reduced to 0.16 V by adopting material combinations that deliver high E _CT (the energy of charge‐transfer state) and low ΔE _CT (energetic difference between singlet excited state and CT state), together with chlorination in donors.

Abstract

Compared with inorganic or perovskite solar cells, the relatively large non‐radiative recombination voltage losses (ΔV _non‐rad) in organic solar cells (OSCs) limit the improvement of the open‐circuit voltage (V _oc). Herein, OSCs are fabricated by adopting two pairs of D–π–A polymers (PBT1‐C/PBT1‐C‐2Cl and PBDB‐T/PBDB‐T‐2Cl) as electron donors and a wide‐bandgap molecule BTA3 as the electron acceptor. In these blends, a charge‐transfer state energy (E _CT) as high as 1.70–1.76 eV is achieved, leading to small energetic differences between the singlet excited states and charge‐transfer states (ΔE _CT ≈ 0.1 eV). In addition, after introducing chlorine atoms into the π‐bridge or the side chain of benzodithiophene (BDT) unit, electroluminescence external quantum efficiencies as high as 1.9 × 10⁻³ and 1.0 × 10⁻³ are realized in OSCs based on PBTI‐C‐2Cl and PBDB‐T‐2Cl, respectively. Their corresponding ΔV _non‐rad are 0.16 and 0.17 V, which are lower than those of OSCs based on the analog polymers without a chlorine atom (0.21 and 0.24 V for PBT1‐C and PBDB‐T, respectively), resulting in high V _oc of 1.3 V. The ΔV _non‐rad of 0.16 V and V _oc of 1.3 V achieved in PBT1‐C‐2Cl:BTA3 OSCs are thought to represent the best values for solution‐processed OSCs reported in the literature so far.

The film quality of perovskite active layer is crucial for achieving high efficiency of perovskite solar cells. The antisolvent treatment method is a successful technique to improve the film quality. The fundamentals of antisolvent treatment, various antisolvent treatment methods, issues with antisolvents, and alternative methods are discussed.

Abstract

Organic–inorganic metal halide perovskite solar cells are emerging as potential solar energy harvesting tools and can be a tough competitor to already matured solar cell technologies. The success of perovskite solar cells is attributed to superior optoelectronic properties of perovskites, feasible synthesis process, and low fabrication cost. Though perovskite solar cells confront perovskite film quality related issues, such as rough surface, pinholes (which result in poor device performance) at the initial stages, many techniques have been developed to improve the perovskite film quality. Among these developed techniques, the antisolvent treatment method is certainly one of the most successful techniques till date. Antisolvent treatment increases the nucleus density during film formation to produce uniform and pinhole‐free perovskite film, which facilitates improved solar cell efficiency, low hysteresis, and stability. Interestingly, many of the best efficiency perovskite solar cells till date have been produced by the antisolvent treatment. This review discusses the fundamentals of antisolvent treatment, various aspects of antisolvent application on perovskite film, different issues with antisolvent usage, and alternatives techniques for perovskite film quality improvement.

A modified monomolecular layer strategy (m‐MLS) enables high‐quality perovskite films formation on the hydrophobic polymer hole transporting layer (HTL), and minimizes the ohmic loss induced by the HTL. The perovskite solar cells (PSCs) based on m‐MLS‐modified HTL (F‐PSCs) give a superior reproducibility and a champion efficiency of 19.7% with a fill factor of over 80%.

The hole transport materials that interact with the indium tin oxide (ITO) surface can be processed into monomolecular layers (MLs), which often exhibit different surface and electronic properties than their thin‐film counterparts. Herein, it is found that poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) films (R‐PTAA) can be easily processed into ML (M‐PTAA) due to the van der Waals interaction between ITO and PTAA. However, compared with R‐PTAA, the work function (WF) and conductivity of M‐PTAA are simultaneously reduced by the charge transfer at the ITO/PTAA interface. To address this issue, a modified monomolecular layer strategy (m‐MLS) is developed, where a small amount of 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) is introduced to enhance the interaction force between ITO and PTAA. PTAA treated by m‐MLS (F‐PTAA) has a hydrophilic physical surface, closely matching electronic energy level with the perovskite layer and smaller bulk resistance. As a result, the efficiency and reproducibility of perovskite solar cells (PSCs) are substantially improved. PSCs based on F‐PTAA demonstrated the highest power conversion efficiency (PCE) of 19.7% with a fill factor of over 80%. This study inspires the development of novel interface modification materials, and provides a simple and convenient direction for the fabrication of high‐performance and reproducible inverted PSCs with high fill factors.

This work discusses the fundamental understanding of the built‐in potential on efficient operation of the perovskite solar cells (PSCs) and the approach for enhancing the built‐in potential in the PSCs. The outcomes of this work are very inspiring, providing a commercially viable and cost‐effective approach for attaining high‐performance solution‐processable PSCs.

The performance of perovskite solar cells (PSCs) has been improved substantially over the past few years. However, the related fundamental understanding of improving the built‐in potential on the efficiency of the PSCs is still far from adequate. A combination of morphology, charge extraction, and built‐in potential studies would help us to gain an insight on efficient operation of the PSCs. Herein, the effect of the hybrid hole extraction layer (HEL), comprising a mixture of tungsten oxide (WO₃) and poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) (WO₃–PEDOT:PSS), on the growth of the perovskite photoactive layer and built‐in potential in PSCs is investigated using structural analyses, photoelectron spectroscopy, and transient photocurrent (TPC) measurements. It shows that the use of hybrid HEL is an effective approach for enhancing the built‐in potential across the photoactive layer in the PSCs, leading to >20% increase in power conversion efficiency as compared to that of a control PSC prepared using a pristine PEDOT:PSS HEL. PSCs with a higher built‐in potential are favorable for efficient cell operation, as manifested by the charge extraction analyses and TPC measurements.

A dual‐ion‐migration phenomenon and its underlying possible mechanism are reported for the lead‐free double perovskite Cs₂AgBiBr₆, where the diffusive behavior of both Ag and Br contribute significantly to the degradation of the perovskite thin‐film and long‐term operational stability of the Cs₂AgBiBr₆ solar cells.

Abstract

Lead‐free double perovskite Cs₂AgBiBr₆ has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs₂AgBiBr₆ material have been focusing on photovoltaic performance and potential applications, its long‐term stability and degradation mechanism are well under‐explored. Herein, high‐quality Cs₂AgBiBr₆ thin‐films are developed for lead‐free double perovskite solar cells with a decent efficiency of 1.91%. By exploring the ambient stability of these photovoltaic devices, it is found that the Cs₂AgBiBr₆ exhibits a unique dual‐ion‐migration phenomenon, where Ag and Br ions gradually diffuse through the hole‐transporting layer in the long‐term operation. This phenomenon leads to the degradation of the Cs₂AgBiBr₆ perovskite and subsequent device failure. Theoretical calculations indicate that low formation energies of the Ag and Br vacancies, and low diffusive energy barriers contribute to the dual‐ion‐migration effect. A possible mechanism involving a vacancy‐mediated ion‐migration is proposed to explain this phenomenon. These key findings are essential for halide double perovskites not only in providing a new knowledge base for further addressing the challenge of double perovskite stability, but also in extending their optoelectronic/electronic applications where mixed electronic, ionic and photonic properties may be desired.

By identifying the bulk polarization in the magnetic field under photoexcitation conditions, the existence of photoinduced bulk polarization is verified by removing the interferences of photoinduced mobile ions. In addition, the linear/circular polarization modulated photocurrent (ΔJ _sc) measurements indicate that the orbit–orbit interaction between excitons in hybrid perovskites can be tuned by bulk polarization through the dipole moment of organic cations.

Abstract

Photoinduced polarization and orbit–orbit interaction are important issues in hybrid perovskites toward developing optoelectronic functionalities. This paper identifies that photoinduced polarization occurs in hybrid perovskites with mixed‐cation methylammonium (MA)/formamidinium (FA) (MA _x FA_{(1− x )}PbI₃) by measuring bulk polarization at 1 MHz in a magnetic field. Interestingly, when the internal dipole moment is increased upon increasing the MA:FA ratio, the photoinduced dipolar polarization can be substantially enhanced, clarifying the controversial issue of whether photoexcitation can induce a dielectric polarization within dipolar polarization regime in hybrid perovskites. Furthermore, upon increasing photoinduced dipolar polarization, it is found that the intrinsic orbit–orbit interaction between excitons can be increased, revealed by monitoring photocurrent change (ΔJ _sc) upon switching the photoexcitation between linear and circular polarizations. This presents that organic cations are directly involved in the orbit–orbit interaction within band structures. Clearly, the studies provide an insightful understanding of the dipole moment effects on photoinduced dipolar polarization and orbit–orbit interaction between excitons in hybrid perovskites toward controlling the optoelectronic properties.