Highly Efficient CsPbBr3 Planar Perovskite Solar Cells via Additive Engineering with NH4SCN

Bulk and surface defect manipulation of the ZnO ETL for all-inorganic CsPbBr3 perovskite solar cells

The electron transport layer (ETL) in traditional CsPbBr3 perovskite solar cells (PSCs) without a hole transport layer (HTL) presents the capability to transport electrons and block hole transport, which radically affects the photovoltaic performance of PSCs. However, ZnO ETL prepared using the classic sol-gel method exhibits obvious drawbacks, such as serious interfacial recombination reactions, inducement of oxygen vacancies (VO) and zinc interstitials (Zni). Herein, we demonstrate that alkali metal chloride (e.g. KCl), serving as the passivating agent for the surface and bulk phase, can promote surface modification and doping in the ZnO ETL, respectively. Experimental results show that the interaction between K+ and Zn2+, and the occupation of VO by Cl-, suppresses the internal defect states of the ZnO films, which enhances the crystal coordination between ZnO and CsPbBr3 and improves the film morphology and the quality of the upper perovskite (PVK) films. Experimental PSCs based on the doping approach achieved the highest power conversion efficiency (PCE) of 9.22%, which ranks the highest PCE of the (FTO/ITO)/ZnO/CsPbBr3/carbon structure. Moreover, the unpackaged devices of the two experimental PSCs could maintain 97.15% and 74.76% of the original PCE after being exposed for 28 days in the ambient environment, demonstrating the powerful effect of KCl on the regulation of surface and bulk phase defects in the ZnO ETL.

Multifunctional Self-Assembled Ionic Liquid Modified Rigid and Flexible Substrates for Efficient Simple-Structured Perovskite Solar Cells

The advancement of electron transport layer (ETL)-free perovskite solar cells (PSCs) is crucial for the commercialization of PSCs. At present, the slow electron extraction and significant carrier recombination, related to the energy-level alignment at the FTO/perovskite interface, restrict the performance of ETL-free PSCs. The facile modification of bottom electrodes is pivotal for tackling these issues and stimulating the photovoltaic potential of perovskite. Herein, a cost-competitive and neoteric 1-hydroxyethyl-3-methylimidazolium chloride, [HOEtMIM]Cl, ionic liquid is employed to modify the surface of rigid and flexible electrodes, and thus enable an energetically well-aligned interface with perovskite layer via the electric dipole effects. The resulting barrier-free FTO/perovskite contact can tremendously ameliorate the electron extraction and collection, with mitigated nonradiative interfacial carrier recombination loss. Additionally, the lone pair on the nitrogen of the imidazole group passivates the surface defects of perovskite layers, and the chloride anion plays a role in the crystallinity improvement of perovskite. Leveraged by the [HOEtMIM]Cl modification, the resulting ETL-free rigid and flexible devices deliver an outstanding power conversion efficiency of 19.60 % and 15.57 %, along with the ameliorated hysteresis and long-term tenability. This finding highlights the drastic potential of the engineered [HOEtMIM]Cl in manufacturing stable and high-performance ETL-free PSCs for their scaled-up production.

Regulatable Phase Manipulation-Enhanced Polarization and Conductance Loss Enabling Hierarchical 3D Microsphere-like MoS2 with Efficient Microwave Absorption

Molybdenum disulfide (MoS2) has become a new type of microwave absorption (MA) material due to the abundant functional groups and defects, high polarization effect, and controllable structural design. However, the development of MoS2 has been limited by its inherently low conductance losses and imperfect impedance matching. This study employs ammonium ion (NH4+) intercalation as a phase manipulation strategy to enhance dielectric loss and form heterogeneous structures by incorporating highly conductive 1T phase into the 2H-MoS2 crystal phase. Additionally, the implementation of CTAB as a soft template agent for constructing layered three-dimensional microsphere structures improves impedance matching. The experimental findings demonstrate that the MA performance of MoS2 can be effectively regulated by controlling the 1T phase content and morphological structure design. It is worth noting that A-MoS2-2 possesses excellent multifrequency absorption capability. A-MoS2-2 has a minimum reflection loss (RL) of -53 dB at a coating thickness of 1.99 mm and an effective absorption bandwidth (EAB) of 5.6 GHz at a thinner coating thickness of 1.77 mm. This work improves the MA properties of MoS2 by introducing metallic phases and unique structural design, which opens up new ideas for the development of MA materials.

2D/3D Perovskite Photodetectors with High Response Frequency and Improved Stability Based on Thiophene-2-ethylamine and Dual Additives

Organic-inorganic lead halide perovskite materials have received great attention in recent years. However, the poor stability of these materials severely limits the commercial application of perovskite devices. Here, we used thiophene-2-ethylammonium iodide (TEAI) material as the organic spacer NH₄SCN and NH₄Cl as the dual additives to realize high-stability two-dimensional (2D)/three-dimensional (3D) perovskite thin films for perovskite photodetectors. Then, we investigated different effects of the dual additives on the orientation and crystallinity of the perovskite films. At room temperature, the optimized 2D/3D perovskite photodetectors exhibit good performance with high external quantum efficiency (EQE) (72%), large responsivity (0.36 A/W), high detectivity (2.46 × 10¹² Jones at the bias of 0 V), high response frequency (1.7 MHz), and improved stability (retains 90% photocurrent after 2000 h storage in RT and 10% RH conditions). Based on these devices, a dual-channel optical transport system and a light-intensity adder are achieved. The results of this study indicate that, with a simple process, the TEAI and dual-additives based 2D/3D perovskite photodetectors have promising applications in light-intensity adder and optical communication systems.

Efficient and Stable Carbon-Based Perovskite Solar Cells Enabled by Mixed CuPc:CuSCN Hole Transporting Layer for Indoor Applications

Perovskite solar cells (PSCs) are an innovative technology with great potential to offer cost-effective and high-performance devices for converting light into electricity that can be used for both outdoor and indoor applications. In this study, a novel hole-transporting layer (HTL) was created by mixing copper phthalocyanine (CuPc) molecules into a copper(I) thiocyanate (CuSCN) film and was applied to carbon-based PSCs with cesium/formamidinium (Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃) as a photoabsorber. At the optimum concentration, a high power conversion efficiency (PCE) of 15.01% was achieved under AM1.5G test conditions, and 32.1% PCE was acquired under low-light 1000 lux conditions. It was discovered that the mixed CuPc:CuSCN HTL helps reduce trap density and improve the perovskite/HTL interface as well as the HTL/carbon interface. Moreover, the PSCs based on the mixed CuPc:CuSCN HTL provided better stability over 1 year due to the hydrophobicity of CuPc material. In addition, thermal stability was tested at 85 °C and the devices achieved an average efficiency drop of approximately 50% of the initial PCE value after 1000 h. UV light stability was also examined, and the results revealed that the average efficiency drop of 40% of the initial value for 70 min of exposure was observed. The work presented here represents an important step toward the practical implementation of the PSC as it paves the way for the development of cost-effective, stable, yet high-performance PSCs for both outdoor and indoor applications.

Highly Efficient and Stable 2D Dion Jacobson/3D Perovskite Heterojunction Solar Cells

Heterostructures involving two-dimensional/three-dimensional (2D/3D) perovskites have recently attracted increased attention due to their ability to combine the high photovoltaic performance of 3D perovskites with the increased stability of 2D perovskites. Here we report ammonium thiocyanate (NH₄SCN) passivated 3D methylammonium lead triiodide (MAPbI₃) perovskite active layer and deposition of 2D perovskite capping layer using xylylene diammonium iodide (XDAI) organic cation. The 2D/3D perovskite heterojunction formation is probed by using FESEM and UPS spectroscopy. The NH₄SCN passivated MAPbI₃ perovskite has shown 19.6% PCE compared to the 17.18% PCE of pristine MAPbI₃ perovskite solar cells (PSCs). Finally, the champion 2D/3D perovskite heterojunction based solar cells have achieved the remarkable PCE of 20.74%. The increased PCE in 2D/3D PSCs is mainly attributed to the reduced defect density and suppressed nonradiative recombination losses. Moreover, the hydrophobic 2D capping layer endows the 2D/3D heterojunction perovskites with exceptional moisture, thermal and UV stability, highlighting the promise of highly stable and efficient 2D/3D PSCs.

Thiocyanate-Passivated Diaminonaphthalene-Incorporated Dion-Jacobson Perovskite for Highly Efficient and Stable Solar Cells

Two-dimensional (2D) metal halide perovskites have recently emerged as promising photovoltaic materials due to their superior ambient stability and rich structural diversity. However, power conversion efficiencies (PCEs) of the 2D perovskites solar cells (PSCs) still lag behind their three-dimensional (3D) counterpart, particularly due to the anisotropy in the charge carrier mobility and inhomogeneous energy landscape. A promising alternative is Dion-Jacobson (D-J) phase quasi-2D perovskite, where the bulky organic diammonium cations are introduced into inorganic frameworks to remove the weak van der Waals interactions between interlayers and to improve the open-circuit voltage (V_oc). Although the D-J phase 2D perovskite shows a homogeneous energy landscape and better charge transport, their poor crystallinity and existence of higher trap states remain a major challenge for the development of high-efficiency solar cells device. To address this issue, here, we report the eclipsed D-J phase 2D perovskite using 1,5-diaminonaphthalene cation and subsequently treated the film with ammonium thiocyanate (NH₄SCN) additive to further improve the film crystallinity, out-of-plane orientation, and carrier mobility. We observe that 2 mol NH₄SCN surface treatment in NDA-based D-J phase perovskite leads to better film morphology and improved crystallinity, as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Time-resolved photoluminescence (TRPL) spectroscopy and steady-state space charge limited current (SCLC) mobility measurement reveal a significant reduction of trap-assisted nonradiative recombination and improvement of carrier mobility in the thiocyanate-passivated perovskite. Consequently, the PCE of the NH₄SCN-treated (NDA)(MA)₃(Pb)₄(I)₁₃ perovskite device enhanced nearly 46% from 10.3 to 15.08%. We have further studied intensity-dependent J-V characteristics, which demonstrate the reduction of ideality factor, confirming the effective suppression of trap-assisted nonradiative recombination, consistent with the transient PL results. Electrochemical impedance spectroscopy (EIS) confirms the improved charge carrier transport in NH₄SCN additive-treated devices. Interestingly, our additive-engineered unsealed perovskite devices retained 75% of their initial efficiency after 1000 h of continuous storage under 60% relative humidity. This study opens up the strategy for developing high-efficiency and stable 2D perovskite solar cells.

Boosting the Performance of Self-Powered CsPbCl3-Based UV Photodetectors by a Sequential Vapor-Deposition Strategy and Heterojunction Engineering

All-inorganic CsPbCl₃ perovskite in ultraviolet (UV) detection is drawing increasing interest owing to its UV-matchable optical band gap, ultrahigh UV stability, and superior inherent optoelectronic properties. Almost all of the reported CsPbCl₃ photodetectors employ CsPbCl₃ nano- or microstructures as sensitive components, while CsPbCl₃ polycrystalline film-based self-powered photodetectors are rarely studied on account of the terrible precursor solubility. Herein, a novel sequential vapor-deposition technique is demonstrated to fabricate CsPbCl₃ polycrystalline film for the first time. High-quality CsPbCl₃ films with excellent optical, electronic, and morphological features are obtained. A self-powered photodetector based on the CsPbCl₃ film is constructed without any charge transport layer, showing a high UV detection performance. A thin p-type PbS buffer layer is further introduced to passivate the surface defects of the CsPbCl₃ layer and decrease the interfacial energy barrier by forming a type-II heterojunction, contributing to a faster hole extraction rate and a suppressed dark current level. The best-performing device achieves an ultrafast response time of 1.92 μs, an ultrahigh on/off ratio of 2.22 × 10⁵, and a responsivity of 0.22 A/W upon 375 nm UV illumination at 0 V bias. This comprehensive performance is the best among all of the CsPbCl₃ photodetectors reported to date. The as-prepared photodetectors also present an eminent UV irradiation and long-term durability in ambient air. Furthermore, a large-area and uniform 625-pixel UV image sensor is fabricated and attains a prominent imaging capability. Our work opens a new avenue for the scalable production of CsPbCl₃-based optoelectronics.

Chlorides, other Halides, and Pseudo-Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) are attracting a tremendous attention from the scientific community due to their excellent power conversion efficiency, low cost, and great promise for the future of solar energy. The best PSCs have already achieved a certified power conversion efficiency (PCE) of 25.5 % after an unprecedented rapid performance rise. However, high requirements with respect to large area, high-efficiency devices, and stability are still the challenges. Major efforts, especially for achieving a high degree of chemical control, have been made to reach these targets. The use of halide additives has played a critical role in improving the efficiency and stability. The present paper reviews the important breakthroughs in PSC technologies made by using halide additives, especially chloride, and pseudo-halide additives for the preparation of the perovskite layers, other layers, and interfaces of the devices. These additives help perovskite (PVK) crystallization and layer morphology control, grain boundary reduction, bulk and interface defects passivation, and so on. Normally, these halide additives play different roles depending on their categories and their location. Herein, recent progresses made due to additives employment in every possible layer of PSCs are reviewed, with focus on chloride, other halides, and pseudo-halides as additives in PVK films, halide additives in carrier transport layers, and at PVK-contact interfaces. Finally, an outlook of engineering of these additives in PSC progress is given.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

High-Efficiency, Low-Hysteresis Planar Perovskite Solar Cells by Inserting the NaBr Interlayer

With great research potential, the perovskite solar cells (PSCs) have been well developed in recent years, but there are still some urgent issues like efficiency and hysteresis defects that severely limit their commercialization. Interface modification is a significant measure to reduce defects and promote performance. In the article, an easy and effective strategy of modifying the electron transport layer (ETL) with NaBr is proposed to improve efficiency and reduce hysteresis. The charge carrier dynamics can be greatly optimized by diffusing NaBr on the ETL. The efficiency of the NaBr coated device can achieve 21.16%, which is extremely higher than the control one and shows low hysteresis behavior with a hysteresis index reduced from 0.135 to 0.025. The results indicate that the NaBr modification provides a novel strategy for preparing PSCs with high efficiency and low hysteresis.

Postpassivation of Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 Perovskite Films with Tris(pentafluorophenyl)borane

Passivating defects to suppress recombination is a valid tactic to improve the performance of third-generation perovskite-based solar cells. Pb⁰ is the primary defect in Pb-based perovskites. Here, tris(pentafluorophenyl)borane is inserted between the perovskite and spiro-OMeTAD layer in SnO₂-based planar perovskite solar cells. The incorporation of tris(pentafluorophenyl)borane can effectively passivate Pb⁰ defects, decreasing recombination at the surface of the perovskite film. Additionally, the modification with tris(pentafluorophenyl)borane decreases the grain boundaries quantity in the perovskite film, enhancing the transportation capability of carriers. The resulting perovskite solar cell gets a high efficiency of 21.42%. While the reference device without tris(pentafluorophenyl)borane treatment acquires an efficiency of 19.07%. More importantly, the stability tests manifest that incorporating tris(pentafluorophenyl)borane in perovskite solar cells is conducive to the stability of the device.

Direct deposition of Sn-doped CsPbBr3 perovskite for efficient solar cell application

All inorganic carbon-based planar perovskites, particularly CsPbBr3, have attracted considerable attention due to their excellent stability against oxygen, moisture, and heat for photovoltaic utilization. However, the power conversion efficiency of carbon-based planar CsPbBr3 perovskite solar cells is mostly low, primarily because of the inferior film quality with undesirable crystallization and narrow light absorbance ranges. Herein, we develop a novel direct deposition approach combined with Sn doping to achieve highly efficient and stable carbon-based Sn-doped CsPbBr3 perovskite solar cells. Mass-scale Sn ion-doped CsPbBr3 perovskite powder was effectively synthesized and characterized via a facile strategy by adding hydrohalic acid in the CsBr, PbBr2 and SnBr2 precursor in a dimethyl sulfoxide solution. Moreover, using the as-synthesized CsPbBr3 and Sn-doped CsPbBr3 perovskite powder, PSCs were obtained via effective direct thermal evaporation. A smooth, constant and pinhole-free perovskite film was achieved with a configuration of FTO/TiO2/Sn:CsPbBr3/carbon. PSCs based on Sn:CsPbBr3 as an absorber and carbon as the HTM achieved an impressive power conversion efficiency of 8.95% compared to 6.87% for undoped CsPbBr3; moreover, it displayed admirable stability in an open-air atmosphere for an operational period of about 720 h without a noticeable negative result. The introduction of the Sn ion may advance the interface extraction of charge between the electric transport layer to the absorber layer and absorber to the carbon electrode. Accordingly, the Sn ion doping on CsPbBr3 during the synthesis phase and the direct evaporation paves a novel approach for intended photovoltaic applications.

Challenges and strategies relating to device function layers and their integration toward high-performance inorganic perovskite solar cells

Parallel to the flourishing of inorganic-organic hybrid perovskite solar cells (PSCs), the development of inorganic cesium-based metal halide PSCs (CsPbX₃) is accelerating, with power conversion efficiency (PCE) values of over 20% being obtained. Although CsPbX₃ possesses numerous merits, such as superior thermal stability and great potential for use in tandem solar cells, severe challenges remain, such as its phase instability, trap state density, and absorption range limitations, hindering further performance improvements and commercialization. This review summarizes challenges and strategies relating to each device functional layer and their integration for the purposes of performance improvement and commercialization, utilizing the fundamental configuration of a perovskite photo-absorption layer, electron transport layer (ETL), and hole transport layer (HTL ). In detail, we first analyze comprehensively strategies for designing high-quality CsPbX₃ perovskite films, including precursor engineering, element doping, and post-treatment, followed by discussing the precise control of the CsPbX₃ film fabrication process. Then, we introduce and analyze the carrier dynamics and interfacial modifications of inorganic ETLs, such as TiO₂, SnO₂, ZnO, and other typical organic ETLs with p-i-n configuration. The pros and cons of inorganic and organic HTLs are then discussed from the viewpoints of stability and band structure. Subsequently, promising candidates, i.e., HTL-free carbon-electrode-based inorganic CsPbX₃ PSCs, that meet the "golden triangle" criteria used by the PSC community are reviewed, followed by discussion of other obstacles, such as hysteresis and large-scale fabrication, that lie on the road toward PSC commercialization. Finally, some perspectives relating to solutions to development bottlenecks are proposed, with the attempt to gain insight into CsPbX₃ PSCs and inspire future research prospects.