Chen Weijie

The preparation of perovskite solar cells by vacuum thermal evaporation processes has advantages in reproducibility, scalability, extendable applications, safety, and toxicity. In this review, materials and methods utilized for vacuum-processed perovskite solar cells are discussed, including precursor materials, the effect of the interlayer, and various deposition methods.

Perovskite solar cells (PSCs) based on inexpensive organic−inorganic hybrid semiconductors are considered a promising next-generation solar cell technology. For PSC commercialization, the further development of efficient and scalable fabrication methods is essential. To date, solution-based methods have been widely studied due to simplicity and cost-effectiveness. Despite the advantages, it is still necessary for developing vacuum-based methods as the alternative methods due to reproducibility and uniformity with in a large area. However, it is insufficiently studied for a systematic understanding of vacuum-based methods. To give helpful insight for understanding vacuum-based methods for PSC commercialization, this review introduces the precursor and charge transporting materials with the various preparation methods for vacuum-processed PSCs.

While ternary solar cells regularly outperform their binary counterparts, their device physics are poorly understood, hampering rational design. Herein, numerical modeling is compared to the literature and own experimental data to assess the merits of various conceptual models that have previously been proposed. It is found that only the cascade model provides a consistent picture.

Mixing a third compound into the active layer of an organic bulk heterojunction solar cell to form a ternary system has become an established way to improve performance. Various models, based on different assumptions regarding the active layer morphology and the energetics, have been proposed but there is neither consensus on the applicability of the various assumptions to different experimental systems, nor on the actual device physics of these, mostly qualitative, models. Kinetic Monte Carlo simulations are used to investigate the role of morphology and relative energy levels of the constituent materials. By comparing with experimental current–voltage characteristics, a consistent picture arises when the (minority) third compound is predominantly incorporated between the other (majority) compounds and has energy levels that are intermediate to those of the binary host. Even if morphologically imperfect, the resulting energy cascade promotes charge separation and reduces recombination, leading to higher fill factors and short-circuit current densities. The open-circuit voltage sits between that of the binary extremes, in agreement with data from an extensive literature review. This leads to selection criteria for third compounds in terms of energetics and miscibility that promote the formation of a cascade morphology in real and energy space.

Engineering of the central heteroatom in the chemical structure of enamine hole-transporting materials is presented, leading to the one-pot-synthesized low-cost hole-transporting material V1435 based on a nitrogen-containing triphenylamine central core to reach a power conversion efficiency of over 20% in perovskite solar cells, which is on par with reference spiro-OMeTAD.

Stabilizing the high-performing perovskite solar cells (PSCs) with low-cost and simply affordable hole-transporting materials (HTMs) has been identified as an ongoing ambitious challenge. Herein, a series of enamine-based HTMs having different central heteroatoms (C, N, O, and S) and a number of enamine branches is designed and synthesized. The impact of varied central heteroatom cores is investigated in-depth including thermal, photophysical, and photovoltaic properties. Importantly, molecularly engineered HTMs are obtained by a single condensation reaction without the need for expensive catalysts, inert reaction conditions, or tedious product purification. PSCs with a power conversion efficiency (PCE) of over 20% can be realized with the triphenylamine core HTM (V1435), a result comparable with spiro-OMeTAD. HTMs based on tetraphenylmethane (V1431) and diphenyl sulfide (V1434) cores give a slightly lower performance under similar device fabrication conditions. This work demonstrates how rational molecular engineering of a simple condensation approach can produce HTMs for high-performing PSCs without sacrificing the PCE.

Expensive picosecond/femtosecond laser technologies may solve the problem of considerable cell-to-module efficiency losses for perovskites, but it is not necessarily the best option for industrialization. This work demonstrates that a nanosecond pulse laser is able to deliver a high-quality interconnection and high geometric filling factor for perovskite solar modules with a certified efficiency of 21.07%.

Abstract

Overcoming cell-to-module (CTM) efficiency losses is indispensable to realize large-area high-efficiency perovskite photovoltaic devices for commercialization. Laser scribing technology is used to fabricate perovskite modules, but it does not seem to solve the problem of high-quality interconnection and high geometric filling factor (GFF), which are the prerequisites for overcoming CTM losses. In reality, what kind of laser technology is needed to fabricate high-efficiency perovskite solar modules is still an open question. Herein, this work demonstrates that a nanosecond pulse laser is able to deliver a reduced heat-affected zone due to the small thermal diffusion coefficient (D _t) of perovskite material, contributing to the accomplishment of a high GFF of up to 95.5%. In addition, the monolithic interconnection quality is improved by finely lifting off the capping layers on indium tin oxide and identifying the residue within the scribed area. As a result, a certified aperture area efficiency of 21.07% under standard 100 mW cm⁻² AM1.5G illumination is achieved with a high photovoltaic fill factor exceeding 80%. The present study provides guidance in overcoming key CTM efficiency losses in perovskite photovoltaic technology.

Solution-processed chromium multioxide hole-selective heterojunction boosts power conversion efficiency and stability of perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) suffer from significant nonradiative recombination at perovskite/charge transport layer heterojunction, seriously limiting their power conversion efficiencies. Herein, solution-processed chromium multioxide (CrO_x) is judiciously selected to construct a MAPbI₃/CrO_x/Spiro-OMeTAD hole-selective heterojunction. It is demonstrated that the inserted CrO_x not only effectively reduces defect sites via redox shuttle at perovskite contact, but also decreases valence band maximum (VBM)-HOMO offset between perovskite and Spiro-OMeTAD. This will diminish thermionic losses for collecting holes and thus promote charge transport across the heterojunction, suppressing both defect-assisted recombination and interface carrier recombination. As a result, a remarkable improvement of 21.21% efficiency with excellent device stability is achieved compared to 18.46% of the control device, which is among the highest efficiencies for polycrystalline MAPbI₃ based n–i–p planar PSCs reported to date. These findings of this work provide new insights into novel charge-selective heterojunctions for further enhancing efficiency and stability of PSCs.

Lead acetate is introduced into the precursor to improve the α-phase stability of CsPbI₂Br, and the dopant-free PM6 is employed as hole transport layer to further optimize the charge transport, which collaboratively contribute to a power conversion efficiency of 33.68% for the indoor photovoltaic cell, along with a remarkable open-circuit voltage of 1.15 V, testing under a 1000 lux light-emitting diode illumination.

Abstract

All-inorganic CsPbI₂Br perovskite has attracted great attention due to the stable crystal structure and moisture resistance, and its 1.91 eV bandgap is close to the optimal bandgap of indoor artificial light sources, making it be the best candidate for the indoor photovoltaics (IPVs) to power a wide range of internet of things related electronic devices. Herein, we report on the preparation of CsPbI₂Br with α-phase and the improvement of its phase stability by adding lead acetate in the CsPbI₂Br precursor. A series of dopant-free conjugated polymers (P3HT, PBDB-T, and PM6) with different highest occupied molecular orbital energy levels are introduced as hole transport layers for building IPV devices. The PM6 based devices having better energy alignment with perovskite demonstrate best indoor photovoltaic performance, giving a remarkable open-circuit voltage of 1.15 V and high fill factor of 81.86% under 1000 lux (330 µW cm⁻²) light-emitting diode illumination, and finally realizing a decent power conversion efficiency of 33.68%. Our findings suggest that collaboratively optimize the CsPbI₂Br layer and hole transport layer is an effective approach to realize high performance IPVs.

Herein, a novel printed electrode deposition process is introduced to fabricate highly efficient, fully roll-to-roll coated perovskite solar cells. The dry press deposition method mitigates the risk of harmful solvent leaching and high temperature processing. Record device power conversion efficiencies of up to 16.7% are demonstrated for flexible, roll-to-roll coated perovskite solar cells with a vacuum-free electrode.

Abstract

Perovskite solar cells (PSCs) are attracting widespread attention due to their exceptional photovoltaic performance and their potential for large-scale production via low-cost, high-throughput roll-to-roll (R2R) methods. Full realization of this production approach requires replacement of the evaporated metal electrode commonly used in PSCs. Here, a novel vacuum-free R2R-compatible method is introduced to fabricate and deposit printed electrodes based on electrically conductive pastes, which avoids potential loss of PSC performance due to solvent migration from the pastes. Flexible R2R-fabricated PSCs with record power conversion efficiencies (PCEs) of up to 16.7% are produced by vacuum-free deposition of all functional layers, apart from the transparent conductive electrode. This performance compares very favorably with that of control flexible PSCs comprising an evaporated gold electrode, which displays PCEs of up to 17.4%. Furthermore, the PSCs comprising a printed electrode demonstrate outstanding operational and mechanical stability, with negligible loss of PCE after 24 h of continuous 1-sun illumination and retention of more than 90% of their initial PCE after 3000 cyclic bends.

CsPbI₂Br perovskite solar cells (PSCs) have received much attention because of the excellent thermal stability. In view of the existing problems of CsPbI₂Br PSCs, optimization methods including preparation process, additives, modification materials, carrier transport layers, application in tandem solar cells, and development of large area PSCs are summarized. Finally, the challenges and outlook of CsPbI₂Br PSCs are discussed.

Abstract

Over the past few years, all-inorganic perovskite solar cells (PSCs), especially CsPbI₂Br PSCs, have received much attention because of their excellent thermal stability and a suitable trade-off between light absorption and higher phase stability among the family of inorganic perovskites. In this progress report, the realization of highly efficient and stable CsPbI₂Br PSCs is summarized through preparation process, additive engineering, interface modification, and transport material selection. Furthermore, the application of CsPbI₂Br in tandem solar cells and its large-area development are highlighted. Finally, the challenges and outlook of CsPbI₂Br PSCs are discussed for further performance improvement and future practical deployment.

Highly-efficient green (λ_max = 515 nm) perovskite light-emitting diodes are developed by combining blends of the quasi-2D perovskite, PEA₂Cs₄Pb₅Br₁₆, and the wide bandgap organic semiconductor 2,7 dioctyl[1] benzothieno[3,2-b]benzothiophene, with different self-assembled monolayers as the hole-injecting interlayers.

Abstract

The high photoluminescence efficiency, color purity, extended gamut, and solution processability make low-dimensional hybrid perovskites attractive for light-emitting diode (PeLED) applications. However, controlling the microstructure of these materials to improve the device performance remains challenging. Here, the development of highly efficient green PeLEDs based on blends of the quasi-2D (q2D) perovskite, PEA₂Cs₄Pb₅Br₁₆, and the wide bandgap organic semiconductor 2,7 dioctyl[1] benzothieno[3,2-b]benzothiophene (C₈-BTBT) is reported. The presence of C₈-BTBT enables the formation of single-crystal-like q2D PEA₂Cs₄Pb₅Br₁₆ domains that are uniform and highly luminescent. Combining the PEA₂Cs₄Pb₅Br₁₆:C₈-BTBT with self-assembled monolayers (SAMs) as hole-injecting layers (HILs), yields green PeLEDs with greatly enhanced performance characteristics, including external quantum efficiency up to 18.6%, current efficiency up to 46.3 cd A⁻¹, the luminance of 45 276 cd m⁻², and improved operational stability compared to neat PeLEDs. The enhanced performance originates from multiple synergistic effects, including enhanced hole-injection enabled by the SAM HILs, the single crystal-like quality of the perovskite phase, and the reduced concentration of electronic defects. This work highlights perovskite:organic blends as promising systems for use in LEDs, while the use of SAM HILs creates new opportunities toward simpler and more stable PeLEDs.

Ruddlesden–Popper perovskites are a promising class of materials for photovoltaic devices with both high efficiency and stability. Control of material properties, such as crystallinity, vertical orientation, and phase distribution, is crucial to further advance this research field. This review explores molecular and additive engineering and novel fabrication techniques to achieve high efficiency solar cells.

Abstract

Quasi-2D Ruddlesden–Popper perovskites (RPPs) are promising candidates for stable and efficient solar cells. Even though photovoltaic devices based on these materials are still lagging behind traditional 3D perovskites, they have experienced a dramatic increase in power-conversion efficiency, recently reaching >20%. As knowledge develops, the toolbox of RPP researchers is steadily growing in terms of organic spacers, additives, and characterization methods. This review aims to describe the use of such a toolbox to achieve high efficiency solar cells. The use of additives, functionalized spacers, and novel fabrication techniques are explored to control morphology, crystallinity, interlayer interaction, and phase distribution. Moreover, methods to achieve the coveted phase purity in RPPs and its implications on both single- and multi-junction solar cells are discussed. By describing successful cases of high efficiency solar cells combined with in-depth knowledge of material properties, it is shown that there are still several open research questions that need exploring to further develop this fascinating class of perovskites.

Metal halide perovskite solar cells are devices based on various semiconductor heterojunctions that all play important roles in improving the device performance. A comprehensive review on the recent progress of device designs on perovskite/electron transport layer, perovskite/hole transport layer, and perovskite/perovskite heterojunctions is presented with a focus on device physics. Finally, conclusions and perspectives on this field are addressed.

Abstract

Metal halide perovskite solar cells (PSCs) have become one of the most promising next-generation photovoltaic technologies due to their low-cost fabrication, solution processability, and superior optoelectronic properties. Although state-of-art PSCs demonstrate a power conversion efficiency record comparable to that of silicon solar cells, there are still many challenges toward commercialization. PSCs are devices based on various semiconductor heterojunctions that all play important roles in device performance. The device operation relies on a combination of multiple heterojunctions to offer a delicate control of photocarrier generation, separation, and transport to respective electrodes. Hence, advanced heterojunction design in PSCs is crucial for the further improvement of device performance. Notably, the conversion efficiency records for PSCs are mainly ascribed to optimized heterojunction engineering. Considering the significance of this topic, a comprehensive review of the recently developed heterojunction designs is presented. Following a brief introduction to PSC architectures, operation, and fundamental heterojunction design theories, the recent progress on perovskite/electron transport layer, perovskite/hole transport layer, and perovskite/perovskite heterojunction engineering is elaborated. Finally, conclusions and perspectives on this research field are addressed.

Fast synthesis of α-phase crystallized mixed-cation perovskite powder assisted with moisture in ambient air is developed. The significant role of moisture in introducing the solvation effect and the facet orientation change of PbI₂ is demonstrated by a combined experimental and theoretical investigation. Perovskite solar cells based on α-phase mixed-cation perovskite powder deliver an impressive PCE of 24.07 %.

Abstract

Phase-pure crystallised perovskite is considered an excellent precursor for fabricating high-stability perovskite films with minimal defects. However, currently available protocols for synthesising crystallised perovskites must be conducted in an inert atmosphere or in the presence of an organic solvent as the reaction medium, which hinders mass production. Here, we report the fast synthesis of α-phase-crystallised perovskite powder assisted by moisture in ambient air. Moisture can promote the reaction between PbI₂ and organic salts and facilitate complete phase transition, as demonstrated in a joint experimental and theoretical study. Perovskite solar cells with a power conversion efficiency of 24.07 % were achieved using phase-pure crystallised perovskite powder as the precursor. This ambient-air-compatible method opens new vistas to reproducible high-quality precursors for large-scale photovoltaic applications.

MXene nanosheets introduced to SnO₂-based perovskite solar cells maximize the interface matching between SnO₂ and perovskite and induce vertically aligned crystal growth. As a result, the photoelectric conversion efficiency (PCE) is improved by 15 %, reaching 23.07 %, and the short-circuit current is up to 25.07 mA cm⁻². In addition, an unencapsulated device maintains 90 % of its initial PCE after 500 h of storage in ambient air.

Abstract

Defects at the interfaces of perovskite (PVK) thin films are the main factors responsible for instability and low photoelectric conversion efficiency (PCE) of PVK solar cells (PSCs). Here, a SnO₂-MXene composite electron transport layer (ETL) is used in PSCs to improve interfacial contact and passivate defects at the SnO₂/perovskite interface. The introduced MXene regulates SnO₂ dispersion and induces a vertical growth of PVK. The lattice matching of MXene and perovskite suppresses the concentration of interfacial stress, thereby obtaining a perovskite film with low defects. Compared with SnO₂-based device, the PCE of SnO₂-MXene-based device is improved by 15 % and its short-circuit current is up to 25.07 mA cm⁻². Furthermore, unencapsulated device maintained about 90 % of its initial efficiency even after 500 h of storage at 30–40 % relative humidity in ambient air. The composite ETL strategy provides a route to engineer interfacial passivation between metal halide perovskites and ETLs.

A technique of sequential passivation with acetylacetone (ACAC) and ethylenediamine (EDA) was proposed. The ACAC treatment can enlarge the grain size, and the EDA treatment stabilizes the perovskite against oxidation. A 13.0 % efficiency with improved stability was reported, which is one of the top efficiencies and stabilities for tin halide perovskite-based solar cells.

Abstract

Lead-free tin perovskite solar cells (PKSCs) have attracted tremendous interest as a replacement for toxic lead-based PKSCs. Nevertheless, the efficiency is significantly low due to the rough surface morphology and high number of defects, which are caused by the fast crystallization and easy oxidization. In this study, a facile and universal posttreatment strategy of sequential passivation with acetylacetone (ACAC) and ethylenediamine (EDA) is proposed. The results show that ACAC can reduce the trap density and enlarge the grain size (short-circuit current (J _sc) enhancement), while EDA can bond the undercoordinated tin and regulate the energy level (open-circuit voltage (V _oc) enhancement). A promising 13 % efficiency is achieved with better stability. In addition, other combinations of diketones or amines are selected, with similar effects. This study provides a universal strategy to enhance the crystallinity and passivate defects while fabricating stable PKSCs with high efficiency.

New fluorine/chlorine regio-regular hetero-trihalogenated terminal was firstly synthesized and systematically employed to enhance single-crystal packing, improve film morphology, and boost device performance of the hetero-halogenated SMAs. The PM6 : Y-FClF achieved a remarkable PCE of 17.65 %, which is far better than that of Y-Cl and Y-FCl and is the highest efficiency for the hetero-halogenated SMAs-based binary OSCs.

Abstract

Herein, we synthesized new hetero-halogenated end groups with well-determined fluorinated and chlorinated substitutions (o-FCl-IC and FClF-IC), and synthesized regioisomer-free small molecular acceptors (SMAs) Y-Cl, Y-FCl, and Y-FClF with distinct hetero-halogenated terminals, respectively. The single-crystal structures and theoretical calculations indicate that Y-FClF exhibits more compact three-dimensional network packing and more significant π-π electronic coupling compared to Y-FCl. From Y-Cl to Y-FCl to Y-FClF, the neat films exhibit a narrower optical band gap and gradually enhanced electron mobility and crystallinity. The PM6 : Y-FClF blend film exhibits the strongest crystallinity with preferential face-on molecular packing, desirable fibrous morphology with suitable phase segregation, and the highest and balanced charge mobilities among three blend films. Overall, the PM6 : Y-FClF organic solar cells (OSCs) deliver a remarkable efficiency of 17.65 %, outperforming the PM6 : Y-FCl and PM6 : Y-Cl, which is the best PCE for reported hetero-halogens-based SMAs in binary OSCs. Our results demonstrate that difluoro-monochloro hetero-terminal is a superior regio-regular unit for enhancing the intermolecular crystal packing and photovoltaic performance of hetero-halogenated SMAs.

The exciton diffusion and charge transport of PM6:Y6-based organic solar cells are simultaneously enhanced by trans-bis(dimesitylboron)stilbene (BBS), and the PM6:Y6:BBS devices achieve a higher efficiency of 17.6% relative to that without BBS (16.2%).

Abstract

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

This article reviews the experimental and theoretical research of the thermal transport properties of halide perovskites since 2016. The microscopic behaviors of phonons are discussed along with the nonphononic descriptions. Also, the recent trends of the halide perovskites for thermal-related applications are presented.

Abstract

Halide perovskites have emerged as promising candidates for various applications, such as photovoltaic, optoelectronic and thermoelectric applications. The knowledge of the thermal transport of halide perovskites is essential for enhancing the device performance for these applications and improving the understanding of heat transport in complicated material systems with atomic disorders. In this work, the current understanding of the experimentally and theoretically obtained thermal transport properties of halide perovskites is reviewed. This study comprehensively examines the reported thermal conductivity of methylammonium lead iodide, which is a prototype material, and provides theoretical frameworks for its lattice vibrational properties. The frameworks and discussions are extended to other halide perovskites and derivative structures. The implications for device applications, such as solar cells and thermoelectrics, are discussed.

Polar species modification (PSM) is employed to reduce the defect capturing probability by strengthening the defect dielectric screening effect via increasing the dielectric constant of a perovskite film. The introduction of F₃EAI fills the vacancy defects at surface and also produces a hydrophobic umbrella with high resistance to humidity. PSM realizes a power conversion efficiency of 20.5% for CsPbI₃ perovskite solar cells.

Abstract

Nonradiative losses caused by defects are the main obstacles to further advancing the efficiency and stability of perovskite solar cells (PSCs). There is focused research to boost the device performance by reducing the number of defects and deactivating defects; however, little attention is paid to the defect-capture capacity. Here, upon systematically examining the defect-capture capacity, highly polarized fluorinated species are designed to modulate the dielectric properties of the perovskite material to minimize its defect-capture radius. On the one hand, fluorinated polar species strengthen the defect dielectric-screening effect via enhancing the dielectric constant of the perovskite film, thus reducing the defect-capture radius. On the other, the fluorinated iodized salt replenishes the I-vacancy defects at the surface, hence lowering the defect density. Consequently, the power-conversion efficiency of an all-inorganic CsPbI₃ PSC is increased to as high as 20.5% with an open-circuit voltage of 1.2 V and a fill factor of 82.87%, all of which are among the highest in their respective categories. Furthermore, the fluorinated species modification also produces a hydrophobic umbrella yielding significantly improved humidity tolerance, and hence long-term stability. The present strategy provides a general approach to effectually regulate the defect-capture radius, thus enhancing the optoelectronic performance.

Nature, Published online: 01 September 2022; doi:10.1038/s41586-022-05268-x