Chen Weijie

This review provides an up-to-date review about the small molecule interlayers (SMIs) in organic solar cells (OSCs). Compared to polymer interlayers, SMIs exhibit intrinsic advantages such as easy synthesis and purification, monodispersity, well-defined chemical structure, and high batch-to-batch reproducibility. Recently, various SMIs have been reported with landmark efficiencies of over 10% in both conventional and inverted OSCs, exhibiting promising potential in commercial application. In this review, the authors summarize the progress of SMIs from a device fabrication point of view, paying particular attention to the material categories, molecular design, preparation process, and applicable device structure. In addition, the working mechanisms of different SMIs are also discussed, including the structure–property relationships and the corresponding impact on device performance. Finally, a brief outlook is provided that includes opportunities and challenges in this emerging area.

Small molecule interlayers (SMIs) have attracted considerable attention in organic solar cells due to their simple syntheses, well-defined structures, and high batch-to-batch reproducibility. This article provides an overview of SMIs from a device fabrication point of view, focusing on the material categories, preparation methods, film properties, and applicable device configurations. The structure–property relationships and their impact on device performance are also discussed.

In this essay, the authors use two properly encapsulated high-efficiency mesoscopic perovskite solar cells (PSCs), which use a state-of-the-art perovskite composition (HC(NH₂)₂PbI₃)_0.85(CH₃NH₃PbBr₃)_0.15 with excess PbI₂ as the active layer, to demonstrate the potential effect of dynamical electroluminescence responses on the analysis and interpretation of PSCs electrical characteristic. The essay does not aim to determine how to overcome this issue, nor to investigate its physical/chemical origin, although tentative propositions are made; but rather, to warn researchers in the field about the interpretation and reporting the results obtained from luminescence imaging measurements and the effect of image collection timing on the results. This is a critical message since the authors predict that luminescence imaging techniques will soon become one of the key tools for PSCs characterization, both for long-term stability assessment and fabrication process optimization.

It is expected that luminescence imaging will become a key tool for perovskite solar cells (PSCs) stability assessment and fabrication process optimization, especially for large-area devices. Even state-of-the-art mesoscopic PSCs, with a small photocurrent hysteresis, show dynamic electroluminescence signal, which complicates immediate and accurate analysis of luminescence imaging measurements results. This article demonstrates this while focusing on the effect of image collection timing.

In article number 1700809 by Jung Inn Sohn, SeungNam Cha, and co-workers, flexible quantum dot solar cells mediated by a porous piezoelectric poly(vinylidenefluoride-trifluoroethylene) layer are presented for an advanced energy harvesting technology. An induced piezoelectric potential modulates junction properties of the solar cells, resulting in efficient transport and a reduced non-radiative recombination of photo-generated charge carriers and consequently significant improvements in flexible quantum dot solar cell performances.

In article number 1701436 by Bumjoon J. Kim and co-workers, a series of naphthalenediimide-based polymer acceptors with superior electron mobility and large dipole moment difference is developed by incorporating electron-withdrawing cyanovinylene groups into a polymer backbone. All-polymer solar cells based on these polymers generate outstanding power conversion efficiency of 7.4% with high fill factor (65%), by virtue of the high electron transport and efficient exciton dissociation with greatly suppressed charge recombination.

Facile electron injection and extraction are two key attributes desired in electron transporting layers to enhance the efficiency of planar perovskite solar cells. Herein it is demonstrated that the incorporation of alkali metal dopants in mesoporous TiO₂ can effectively modulate electronic conductivity and improve the charge extraction process by counterbalancing oxygen vacancies acting as nonradiative recombination centers. Moreover, sulfate bridges (SO₄²⁻) grafted on the surface of K-doped mesoporous titania provide a seamless integration of absorber and electron-transporting layers that accelerate overall transport kinetics. Potassium doping markedly influences the nucleation of the perovskite layer to produce highly dense films with facetted crystallites. Solar cells made from K:TiO₂ electrodes exhibit power conversion efficiencies up to 21.1% with small hysteresis despite all solution coating processes conducted under ambient air conditions (controlled humidity: 25–35%). The higher device efficiencies are attributed to intrinsically tuned electronic conductivity and chemical modification of grain boundaries enabling uniform coverage of perovskite films with large grain size.

Using the alkali element potassium (K) for n-type doping of mesoporous TiO₂ yields better solar cell performance by improving the interface and reducing the trap states. The improved device performance is attributed to simultaneous doping (K⁺) and surface engineering (SO₄ ²⁻) of the metal oxide electron-transporting layer thereby improving the electrical conductivity as well as forming a smooth and robust interface via a sulfate bridge.

In this contribution, a facile and universal method is successfully reported to fabricate perovskite solar cells (PSCs) with enhanced efficiency and stability. Through dissolving functional conjugated polymers in antisolvent chlorobenzene to treat the spinning CH₃NH₃PbI₃ perovskite film, the resultant devices exhibit significantly enhanced efficiency and longevity simultaneously. In-depth characterizations demonstrate that thin polymer layer well covers the top surface of perovskite film, resulting in certain surface passivation and morphology modification. More importantly, it is shown that through rational chemical modification, namely molecular fluorination, the air stability and photostability of the perovskite solar cells are remarkably enhanced. Considering the vast selection of conjugated polymer materials and easy functional design, promising new results are expected in further enhancement of device performance. It is believed that the findings provide exciting insights into the role of conjugated polymer in improving the current perovskite-based solar cells.

By dissolving conjugated polymer in antisolvent, the antisolvent is updated to antisolution in perovskite solar cell fabrication. Benefiting from a favorable arrangement of polymer layers, the device containing polymer exhibits improved photovoltaic efficiency (18.7%) and long-term stability. More importantly, it is shown that molecular fluorination of these functional polymers enhances device performance further.

In p-i-n planar perovskite solar cells (pero-SCs) based on methylammonium lead iodide (MAPbI₃) perovskite, high-quality MAPbI₃ film, perfect interfacial band alignment and efficient charge extracting ability are critical for high photovoltaic performance. In this work, a hydrophilic fullerene derivative [6,6]-phenyl-C₆₁-butyric acid-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB-OEG) is introduced as additive in the methylammonium iodide precursor solution in the preparation of MAPbI₃ perovskite film by two-step sequential deposition method, and obtained a top-down gradient distribution with an ultrathin top layer of PCBB-OEG. Meanwhile, a high-quality perovskite film with high crystallinity, less trap-states, and dense-grained uniform morphology can well grow on both hydrophilic (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)) and hydrophobic (polytriarylamine, PTAA) hole transport layers. When the PCBB-OEG-containing perovskite film (pero-0.1) is prepared in a p-i-n planar pero-SC with the configuration of ITO/PTAA/pero-0.1/[6,6]-phenyl-C₆₁-butyric acid methyl ester/Al, the device delivers a promising power conversion efficiency (PCE) of 20.2% without hysteresis, which is one of the few PCE over 20% for the p-i-n planar pero-SCs. Importantly, the pero-0.1-based device shows an excellent stability that can retain 98.4% of its initial PCE after being exposed for 300 h under ambient atmosphere with a high humidity, and the flexible pero-SCs based on pero-0.1 also demonstrate a promising PCE of 18.1%.

It is demonstrated that a new strategy of two-step method provides a simple way to develop high-quality perovskite film. The perovskite solar cells (pero-SCs) show a high power conversion efficiency (PCE) of 20.2% with an excellent device stability. This strategy can also be suitable for fabricating flexible pero-SC giving a promising PCE of 18.1%.

An efficient perovskite photovoltaic-thermoelectric hybrid device is demonstrated by integrating the hole-conductor-free perovskite solar cell based on TiO₂/ZrO₂/carbon structure and the thermoelectric generator. The whole solar spectrum of AM 1.5 G is fully utilized with the ≈1.55 eV band gap perovskite (5-AVA)_x(MA)_1−xPbI₃ absorbing the visible light and the carbon back contact absorbing the infrared light. The added thermoelectric generator improves the device performance by converting the thermal energy into electricity via the Seebeck effect. An optimized hybrid device is obtained with a maximum point power output of 20.3% and open-circuit voltage of 1.29 V under the irradiation of 100 mW cm⁻².

By utilizing the whole AM 1.5 G solar spectrum energy, an efficient perovskite photovoltaic-thermoelectric hybrid device is demonstrated by integrating the perovskite solar cell based on carbon electrode and the thermoelectric generator. An optimized hybrid device is obtained with a maximum point power output of 20.3% and open-circuit voltage of 1.29 V under the irradiation of 100 mW cm⁻².

The application of conjugated polymer and fullerene water-based nanoparticles (NP) as ecofriendly inks for organic photovoltaics (OPVs) is reported. A low bandgap polymer diketopyrrolopyrrole–quinquethiophene (PDPP5T-2) and the methanofullerene PC₇₁BM are processed into three types of nanoparticles: pristine fullerene NPs, pristine polymer NPs, and mixed polymer:fullerene NPs, allowing the formation of bulk heterojunction (BHJ) composites with different domain sizes. Mild thermal annealing is required to melt the nanospheres and enable the formation of interconnected pathways within mixed phases. This BHJ is accompanied by a shrinkage of film, whereas the more compact layers show enhanced mobility. Consistently reduced recombination and better performance are found for mixed NP, containing both, the polymer and the fullerene within a single NP. The optimized solar cell processed by ultrasmall NPs delivers a power conversion efficiency of about 3.4%. This is among the highest values reported for aqueous processed OPVs but still lacks performance compared to those being processed from halogenated solvents. Incomplete crystallization is identified as the main root for reduced efficiency. It is nevertheless believed that postprocessing does not cut attraction from printing aqueous organic NP inks as a trendsetting strategy for the reliable and ecofriendly production of organic solar cells.

The correlation between microstructure and device physics in water processed nanoparticulate organic photovoltaics is investigated. Bulk heterojunction composites with largely different domain sizes are determined by pristine nanoparticle formation, which significantly influence the mobility-lifetime product and nongeminate recombination in the nanoparticle-based solar cells.

As the race toward higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport toward higher power efficiency has been urgently demanded. In this study, a hidden role of A-site cations of PSCs in carrier transport, which has been largely neglected is unraveled, i.e., tuning the Fröhlich electron–phonon (e–ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Fröhlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ions. The coordination to I⁻ alleviates electron–phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses low electron–phonon coupling in several promising organic cations including hydroxyl–ammonium cation (NH₃OH⁺), hydrazinium cation (NH₃NH₂⁺) and possibly Li⁺ solvating methylamine (Li⁺∙∙∙NH₂CH₃), on a par with methyl–ammonium cations. A new perspective on the role of A-site cations could help in improving power efficiency and accelerating the application of PSCs.

A hidden role of A-site cations in perovskite solar cells in steering Fröhlich polaron coupling is disclosed. Design principles suggest that A-site cations need to be close to halides and to maximize the coordination to halides. Based on first principles and many-body theory, organic cations such as NH₃OH⁺, LiNH₂CH₃⁺, and NH₃F⁺ are predicted to be promising.

Although the efficiency of perovskite solar cells (PSCs) is close to crystalline silicon solar cells, the instability of perovskite, especially in humid condition, still hinders its commercialization. As an effective method to improve their stability, surface functionalization, by using hydrophobic molecules, has been extensively investigated, but usually accompanied with the loss of device efficiencies owing to their intrinsic electrical insulation. In this work, for the first time, it is demonstrated that 3-alkylthiophene-based hydrophobic molecules can be used as both water-resistant and interface-modified layers, which could simultaneously enhance both stability and performance significantly. Benefitting from their unique structures of thiophene rings, the π-electrons are highly delocalized and thus enhance the charge transfer and collection at the interface. The device based on 3-hexylthiophene treatment exhibits a champion energy conversion efficiency of 19.89% with a dramatic 10% enhancement compared with the pristine one (18.08%) of Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45 I_2.55-based PSCs. More importantly, the degradation of the long-term efficiency of unsealed device is less than 20% in Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45I_2.55-based PSCs after more than 700 h storage in air. This finding provides an avenue for further improvement of both the efficiency and stability of PSCs.

3-alkylthiophene derivatives are utilized as the multifunctional layer in perovskite solar cells for the first time, which demonstrates a power conversion efficiency of 19.89% with superb long-term stability. Optimized carrier migration and enhanced surface water resistance are responsible for the high performance and stability of the resulting devices.

Organic–inorganic hybrid perovskite solar cells (PVSCs) have become the front-running photovoltaic technology nowadays and are expected to profoundly impact society in the near future. However, their practical applications are currently hampered by the challenges of realizing high performance and long-term stability simultaneously. Herein, the development of inverted PVSCs is reported based on low temperature solution-processed CuCrO₂ nanocrystals as a hole-transporting layer (HTL), to replace the extensively studied NiO_x counterpart due to its suitable electronic structure and charge carrier transporting properties. A ≈45 nm thick compact CuCrO₂ layer is incorporated into an inverted planar configuration of indium tin oxides (ITO)/c-CuCrO₂/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/Ag, to result in the high steady-state power conversion efficiency of 19.0% versus 17.1% for the typical low temperature solution-processed NiO_x-based devices. More importantly, the optimized CuCrO₂-based device exhibits a much enhanced photostability than the reference device due to the greater UV light-harvesting of the CuCrO₂ layer, which can efficiently prevent the perovskite film from intense UV light exposure to avoid associated degradation. The results demonstrate the promising potential of CuCrO₂ nanocrystals as an efficient HTL for realizing high-performance and photostable inverted PVSCs.

A new, multifunctional CuCrO₂ hole-transporting layer is developed and incorporated into the inverted perovskite solar cells. The CuCrO₂ layer with superior electronic and optical properties is proved to not only enhance the photovoltaic performance but also improve the device photostability via the UV-blocking effect. Consequently, a high power conversion efficiency of 19.0% with enhanced device photostability is successfully demonstrated.

Lead tri-iodide methylammonium (MAPbI₃) perovskite polycrystalline materials show complex optoelectronic behavior, largely because their 3D semiconducting inorganic framework is strongly perturbed by the organic cations and ubiquitous structural or chemical inhomogeneities. Here, a newly developed time-dependent density functional theory-based theoretical formalism is taken advantage of. It treats electron–hole and electron–nuclei interactions on the same footing to assess the many-body excited states of MAPbI₃ perovskites in their pristine state and in the presence of point chemical defects. It is shown that lead and iodine vacancies yield deep trap states that can be healed by dynamic effects, namely rotation of the methylammonium cations in response to point charges, or through slight changes in chemical composition, namely by introducing a tiny amount of chlorine dopants in the defective MAPbI₃. The theoretical results are supported by photoluminescence experiments on MAPbI_3−mCl_m and pave the way toward the design of defect-free perovskite materials with optoelectronic performance approaching the theoretical limits.

How methylammonium cations and chlorine dopants heal defects in lead tri-iodide methylammonium (MAPbI₃) perovskites is proposed. Time-dependent density functional theory excited-state calculations in defective MAPbI₃ show crossovers from confined to extended states when varying the electrostatic environment around vacancies. Deep trap states are dynamically healed through collective rotation of the methylammonium cations and by introducing tiny amounts of chlorine dopants.

Achieving light harvesting is crucial for the efficiency of the solar cell. Constructing optical structures often can benefit from micro-nanophotonic imprinting. Here, a simple and facile strategy is developed to introduce a large area grating structure into the perovskite-active layer of a solar cell by utilizing commercial optical discs (CD-R and DVD-R) and achieve high photovoltaic performance. The constructed diffraction grating on the perovskite active layer realizes nanophotonic light trapping by diffraction and effectively suppresses carrier recombination. Compared to the pristine perovskite solar cells (PSCs), the diffraction-grating perovskite devices with DVD obtain higher power conversion efficiency and photocurrent density, which are improved from 16.71% and 21.67 mA cm⁻² to 19.71% and 23.11 mA cm⁻². Moreover, the stability of the PSCs with diffraction-grating-structured perovskite active layer is greatly enhanced. The method can boost photonics merge into the remarkable perovskite materials for various applications.

Diffraction grating is introduced into a perovskite active layer via optical discs. The constructed architecture enhances light harvesting and photon-to-electron conversion efficiency by diffracting the incident light. Besides, it also improves the charge extraction process and suppresses electron–hole recombination. The diffraction-grating perovskite devices achieve a high power conversion efficiency of 19.71%.

The current work reports a high power conversion efficiency (PCE) of 9.54% achieved with nonfullerene organic solar cells (OSCs) based on PTB7-Th donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC) acceptor fabricated by doctor-blade printing, which has the highest efficiency ever reported in printed nonfullerene OSCs. Furthermore, a high PCE of 7.6% is realized in flexible large-area (2.03 cm²) indium tin oxide (ITO)-free doctor-bladed nonfullerene OSCs, which is higher than that (5.86%) of the spin-coated counterpart. To understand the mechanism of the performance enhancement with doctor-blade printing, the morphology, crystallinity, charge recombination, and transport of the active layers are investigated. These results suggest that the good performance of the doctor-blade OSCs is attributed to a favorable nanoscale phase separation by incorporating 0.6 vol% of 1,8-diiodooctane that prolongs the dynamic drying time of the doctor-bladed active layer and contributes to the migration of ITIC molecules in the drying process. High PCE obtained in the flexible large-area ITO-free doctor-bladed nonfullerene OSCs indicates the feasibility of doctor-blade printing in large-scale fullerene-free OSC manufacturing. For the first time, the open-circuit voltage is increased by 0.1 V when 1 vol% solvent additive is added, due to the vertical segregation of ITIC molecules during solvent evaporation.

Printed nonfullerene organic solar cells are investigated with a power conversion efficiency of 9.54% via incorporating a 1,8-diiodooctane additive for achieving a favorable nanoscale phase separation. The migration of nonfullerene acceptor molecules from bottom to top helps form the optimal donor/acceptor interface distribution, leading to the reduced exciton recombination and optimized electrical parameters.

In most current state-of-the-art perovskite solar cells (PSCs), high-temperature (≈500 °C)-sintered metal oxides are employed as electron-transporting layers (ETLs). To lower the device processing temperature, the development of low-temperature-processable ETL materials (such as solution-processed ZnO) has received growing attention. However, thus far, the use of solution-processed ZnO is limited because the reverse decomposition reaction that occurs at ZnO/perovskite interfaces significantly degrades the charge collection and stability of PSCs. In this work, the reverse decomposition reaction is successfully retarded by sulfur passivation of solution-processed ZnO. The sulfur passivation of ZnO by a simple chemical means, efficiently reduces the oxygen-deficient defects and surface oxygen-containing groups, thus effectively preventing reverse decomposition reactions during and after formation of the perovskite active layers. Using the low-temperature-processed sulfur-passivated ZnO (ZnO–S), perovskite layers with higher crystallinity and larger grain size are obtained, while the charge extraction at the ZnO/perovskite interface is significantly improved. As a result, the ZnO–S-based PSCs achieve substantially improved power-conversion-efficiency (PCE) (19.65%) and long-term air-storage stability (90% retention after 40 d) compared with pristine ZnO-based PSCs (16.51% and 1% retention after 40 d). Notably, the PCE achieved is the highest recorded (19.65%) for low-temperature ZnO-based PSCs.

Air-stable high efficiency perovskite solar cells are developed using sulfur-passivated ZnO electron transport layers. Sulfur passivation of ZnO effectively prevents the interfacial reverse reaction from perovskite to PbI₂, while the surface hydrophobicity of ZnO is increased. The results show that the quality of perovskite layers is improved and the interfacial charge recombination is reduced.

All-polymer solar cells (all-PSCs) are attractive as alternatives to fabricate thermally and mechanically stable solar cells, especially with recent improvements in their power conversion efficiency (PCE). In this work, efficient all-PSCs with near-infrared response (up to 850 nm) are developed using newly designed regioregular polymer donors with relatively narrow optical gap. These all-PSCs systems achieve PCEs up to 6.0% after incorporating fluorine into the polymer backbone. More importantly, these polymers exhibit absorbance that is complementary to previously reported wide bandgap polymer donors. Thus, the superior properties of the newly designed polymers afford opportunities to fabricate the first spectrally matched all-polymer tandem solar cells with high performance. A PCE of 8.3% is then demonstrated which is the highest efficiency so far for all-polymer tandem solar cells. The design of narrow bandgap polymers provides new directions to enhance the PCE of emerging single-junction and tandem all polymer solar cells.

By adopting D1-A-D2-A ternary structure, a pair of novel regioregular polymers, namely PBBSB and PBFSF, are synthesized. Benefiting from the new arrangement and molecular fluorination, the polymer exhibits relatively narrow optical gap, good intermolecular packing, and excellent charge transport. More importantly, it is shown that these functional donor polymers can achieve high efficiency in either single-junction or tandem all-polymer solar cells.

All-inorganic CsPbBrI₂ perovskite has great advantages in terms of ambient phase stability and suitable band gap (1.91 eV) for photovoltaic applications. However, the typically used structure causes reduced device performance, primarily due to the large recombination at the interface between the perovskite, and the hole-extraction layer (HEL). In this paper, an efficient CsPbBrI₂ perovskite solar cell (PSC) with a dimensionally graded heterojunction is reported, in which the CsPbBrI₂ material is distributed within bulk–nanosheet–quantum dots or 3D–2D–0D dimension-profiled interface structure so that the energy alignment is optimized in between the valence and conduction bands of both CsPbBrI₂ and the HEL layers. Specifically, the valence-/conduction-band edge is leveraged to bend with synergistic advantages: the graded combination enhances the hole extraction and conduction efficiency with effectively decreased recombination loss during the hole-transfer process, leading to an enhanced built-in electric field, hence a high V_OC of as much as 1.19 V. The profiled structure induces continuously upshifted energy levels, resulting in a higher J_SC of as much as 12.93 mA cm⁻² and fill factor as high as 80.5%, and therefore record power conversion efficiency (PCE) of 12.39%. As far as it is known, this is the highest PCE for CsPbBrI₂ perovskite-based PSC.

Here, a 3D–2D–0D multi-graded interface based on CsPbBrI₂ bulk, nanosheets, and quantum dots is first designed for CsPbBrI₂ perovskite solar cells. Such a multigraded surface favorably reduces the recombination at the CsPbBrI₂/poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] interface, resulting in a record stabilized power conversion efficiency of 12.39%, a nearly 20% increase compared with 10.38% for ungraded devices.

To solve the stability issues of perovskite solar cells (PSC), here a novel interface engineering strategy that a versatile ultrathin 2D perovskite (5-AVA)₂PbI₄ (5-AVA = 5-ammoniumvaleric acid) passivation layer that is in situ incorporated at the interface between (FAPbI₃)_0.88(CsPbBr₃)_0.12 and the hole transporting CuSCN is reported. Surface analysis using X-ray photoelectron spectroscopy confirms the formation of 2D perovskite. Hysteresis is reduced by the interfacial 2D layer, which could be ascribed to improvement of interfacial charge extraction efficiency, associated with suppression of recombination. Moreover, introduction of the interface passivating layer enhances the moisture stability and photostability as compared to the control perovskite film due to hydrophobic nature of 2D perovskite. The unencapsulated device retains 98% of the initial power conversion efficiency (PCE) after 63 d under moisture exposure of about 10% in the dark. A PCE of the control device is boosted from 13.72 to 16.75% as a consequence of enhanced open-circuit voltage (V_oc) and fill factor along with slightly increased short-circuit current density (J_sc), which results from reduced trap states of (FAPbI₃)_0.88(CsPbBr₃)_0.12 as evidenced by enhanced carrier lifetimes and charge extraction. The perovskite/hole transport material interface engineering gives insight into simultaneous improvements of PCE and device stability.

A versatile ultrathin 2D perovskite (5-AVA)₂PbI₄ (5-AVA = 5-ammoniumvaleric acid) interlayer is in situ incorporated at the back contact interface between perovskite and CuSCN, which reduces current–voltage hysteresis and improves simultaneously power conversion efficiency (PCE) and stability. An unencapsulated device retains 98% of the initial PCE after 63 days under 10% relative humidity in the dark.

Organic/inorganic hybrid solar cells, typically mesoscopic and perovskite solar cells, are regarded as promising candidates to replace conventional silicon or thin film photovoltaics. There have been intensive investigations on the development of advanced materials for improved power conversion efficiencies, however, economical feasibilities and reliabilities of the organic/inorganic photovoltaics are yet to reach at a sufficient level for practical utilizations. In this study, cobalt nitride (CoN) nanofilms prepared by room-temperature vapor deposition in an inert N₂ atmosphere, which is a facile and highly reproducible procedure, are proposed as a low-cost counter electrode in mesoscopic dye-sensitized solar cells (DSCs) and a hole transport material in inverted planar perovskite solar cells (PSCs) for the first time. The CoN film successfully replaces conventional Pt in DSCs, resulting in a power conversion efficiency comparable to the ones based on Pt. In addition, PSCs employing the CoN manifest high efficiency even up to 15.0%, which is comparable to state-of-the-art performance in the cases of PSCs employing inorganic hole transporters. Furthermore, flexible solar cell applications of the CoN are performed in both mesoscopic and perovskite solar cells, verifying the advantages of the room-temperature deposition process and feasibilities of the CoN nanofilms in various fields.

CoN nanofilms prepared by room-temperature vapor deposition are applied as electrocatalysts and hole transport materials in organic/inorganic hybrid solar cells. The CoN counter electrode in place of Pt manifests a comparably high performance, and power conversion efficiency achieved in perovskite solar cells employing the CoN hole transporter is among the state-of-the-art results from inorganic hole transport materials.

The power conversion efficiency of perovskite solar cells (PSCs) has ascended from 3.8% to 22.1% in recent years. ZnO has been well-documented as an excellent electron-transport material. However, the poor chemical compatibility between ZnO and organo-metal halide perovskite makes it highly challenging to obtain highly efficient and stable PSCs using ZnO as the electron-transport layer. It is demonstrated in this work that the surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA) readily makes ZnO as a very promising electron-transporting material for creating hysteresis-free, efficient, and stable PSCs. Systematic studies in this work reveal several important roles of the modification: (i) MgO inhibits the interfacial charge recombination, and thus enhances cell performance and stability; (ii) the protonated EA promotes the effective electron transport from perovskite to ZnO, further fully eliminating PSCs hysteresis; (iii) the modification makes ZnO compatible with perovskite, nicely resolving the instability of ZnO/perovskite interface. With all these findings, PSCs with the best efficiency up to 21.1% and no hysteresis are successfully fabricated. PSCs stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells.

Surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine readily makes ZnO a very promising electron-transporting material for creating efficient, hysteresis-free and stable perovskite solar cells (PSCs). PSCs, stable in air for more than 300 h, are achieved when graphene is used to encapsulate the cells.

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution-processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing-incidence X-ray diffraction (GI-XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI₃ (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI₃ films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI-XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N-dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI₃-based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.

The real-time perovskite formation and conversion processes under various solvent vapor atmospheres are investigated through the in situ synchrotron-based grazing-incidence X-ray diffraction technique, providing direct insights into the film growth mechanism and offering practical guidance to improve film quality. By carefully tuning the dimethyl sulfoxide ratio, the best power conversion efficiency is obtained for the planar MAPbI₃-based devices.

Organometallic halide perovskites (OMHPs) have attracted broad attention as prospective materials for optoelectronic applications. Among the many anomalous properties of these materials, of special interest are the ferroelectric properties including both classical and relaxor-like components, as a potential origin of slow dynamics, field enhancement, and anomalous mobilities. Here, ferroelectric properties of the three representative OMHPs are explored, including FAPb_xSn_1–xI₃ (x = 0, x = 0.85) and FA_0.85MA_0.15PbI₃ using band excitation piezoresponse force microscopy and contact mode Kelvin probe force microscopy, providing insight into long- and short-range dipole and charge dynamics in these materials and probing ferroelectric density of states. Furthermore, second-harmonic generation in thin films of OMHPs is observed, providing a direct information on the noncentrosymmetric polarization in such materials. Overall, the data provide strong evidence for the presence of ferroelectric domains in these systems; however, the domain dynamics is suppressed by fast ion dynamics. These materials hence present the limit of ferroelectric materials with spontaneous polarization dynamically screened by ionic and electronic carriers.

Anomalous ferroelectric properties are reported in a series of organometallic halide perovskite compounds. Noncentrosymmetric polarization detected from second-harmonic generation provides a precondition for intrinsic polarization. A classical ferroelectric domain with difference in amplitude and phase is observed, and there are no frequency changes in band excitation piezoresponse force microscopy, ruling out topographic and microstructural artefacts. Finally, Kelvin probe force microscopy reveals that the ferroelectricity is hidden by ion migration that gives rise to ferroelectric-like responses.

Fast research progress on lead halide perovskite solar cells has been achieved in the past a few years. However, the presence of lead (Pb) in perovskite composition as a toxic element still remains a major issue for large-scale deployment. In this work, a novel and facile technique is presented to fabricate tin (Sn)-rich perovskite film using metal precursors and an alloying technique. Herein, the perovskite films are formed as a result of the reaction between Sn/Pb binary alloy metal precursors and methylammonium iodide (MAI) vapor in a chemical vapor deposition process carried out at 185 °C. It is found that in this approach the Pb/Sn precursors are first converted to (Pb/Sn)I₂ and further reaction with MAI vapor leads to the formation of perovskite films. By using Pb–Sn eutectic alloy, perovskite films with large grain sizes up to 5 µm can be grown directly from liquid phase metal. Consequently, using an alloying technique and this unique growth mechanism, a less-toxic and efficient perovskite solar cell with a power conversion efficiency (PCE) of 14.04% is demonstrated, while pure Sn and Pb perovskite solar cells prepared in this manner yield PCEs of 4.62% and 14.21%, respectively. It is found that this alloying technique can open up a new direction to further explore different alloy systems (binary or ternary alloys) with even lower melting point.

Sn-rich perovskite solar cells with large grains are fabricated from a Pb–Sn eutectic alloy in the liquid phase by using a chemical vapor deposition technique, resulting in a device power conversion efficiency of 14.04%, which is comparable with that of pure Pb devices and among the highest for Sn-rich binary Sn/Pb metal perovskite solar cells.