Chen Weijie

An organic–inorganic halide CH₃NH₃SnI₃ perovskite with significantly improved structural stability is obtained via pressure-induced amorphization and recrystallization. In situ high-pressure resistance measurements reveal an increased electrical conductivity by 300% in the pressure-treated perovskite. Photocurrent measurements also reveal a substantial enhancement in visible-light responsiveness. The mechanism underlying the enhanced properties is shown to be associated with the pressure-induced structural modification.

The role of excitons on the amplifications of lead halide perovskites has been explored. Unlike the photoluminescence, the intensity of amplified spontaneous emission is partially suppressed at low temperature. The detailed analysis and experiments show that the inhibition is attributed to the existence of exciton and a quantitative model has been built to explain the experimental observations.

The molecular structure of pyridine derivatives is critical to perovskite solar cell performance, especially stability. Most of the pyridine additives easily form complexes with perovskite. A new pyridine additive with a long alkyl chain substituted at its o-position does not corrode perovskite. The stability of devices containing this additive is the highest among the investigated cells.

Influence of light exposure on cesium lead halide nanostructures has been explored. A discovery of photon driven transformation (PDT) in 2D CsPbBr₃ nanoplatelets is reported, in which the quantum-confined few-monolayer nanoplatelets will convert to bulk phase under very low irradiation intensity (≈20 mW cm⁻²). Benefiting from the remarkable emission color change during PDT, the multicolor luminescence photopatterns and facile information photo-encoding are established.

Organic solar cells (OSCs) and organic-inorganic metal halide perovskite solar cells (pero-SCs) have been regarded as two promising photovoltaic technologies. The recent advances with power conversion efficiency over 10% and 20% have been realized in OSCs and pero-SCs, respectively. The fullerene derivatives play important role as acceptor materials in OSCs and cathode buffer layer (CBL) materials in OSCs and pero-SCs. Here, we provide a comprehensive overview of recent progresses and perspectives of the functional fullerene derivatives as acceptor materials and CBLs for OSCs and pero-SCs.

Fullerene derivatives play a very important role as acceptor materials in organic solar cells (OSCs), cathode buffer layer materials in OSCs and perovskite solar cells (pero-SCs). Here, a comprehensive overview of recent progresses and perspectives of functional fullerene derivatives as acceptor materials and buffer layer materials for OSCs and pero-SCs is provided.

Spray-coating is a versatile coating technique that can be used to deposit functional films over large areas at speed. Here, spray-coating is used to fabricate inverted perovskite solar cell devices in which all of the solution-processible layers (PEDOT:PSS, perovskite, and PCBM) are deposited by ultrasonic spray-casting in air. Using such techniques, all-spray-cast devices having a champion power conversion efficiency (PCE) of 9.9% are fabricated. Such performance compares favorably with reference devices spin-cast under a nitrogen atmosphere that has a champion PCE of 12.8%. Losses in device efficiency are ascribed to lower surface coverage and reduced uniformity of the spray-cast perovskite layer.

Spray-coating is a versatile coating technique that can be used to deposit functional films over large areas at speed. Here, the authors fabricate inverted perovskite solar cell devices in which all of the solution-processible layers are deposited by ultrasonic spray-casting in air leading to all-spray-cast devices having a champion power conversion efficiency of 9.9%.

Hybrid organic–inorganic halide perovskites have emerged at the forefront of solution-processable photovoltaic devices. Being the perovskite precursor mixture a complex equilibrium of species, it is very difficult to predict/control their interactions with different substrates, thus the final film properties and device performances. Here the wettability of CH₃NH₃PbI₃ (MAPbI₃) onto poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer is improved by exploiting the cooperative effect of graphene oxide (GO) and glucose inclusion. The glucose, in addition, triggers the reduction of GO, enhancing the conductivity of the PEDOT:PSS+GO+glucose based nanocomposite. The relevance of this approach toward photovoltaic applications is demonstrated by fabricating a hysteresis-free MAPbI₃ solar cells displaying a ≈37% improvement in power conversion efficiency if compared to a device grown onto pristine PEDOT:PSS. Most importantly, V_OC reaches values over 1.05 V that are among the highest ever reported for PEDOT:PSS p-i-n device architecture, suggesting minimal recombination losses, high hole-selectivity, and reduced trap density at the PEDOT:PSS along with optimized MAPbI₃ coverage.

The synergistic effect of graphene oxide and glucose in improving the conduction properties of polymer electrolyte poly(3,4-ethyl­enedioxythiophene):poly­(styrenesul­fonate) and modifying the sensible interface of perovskite solar cells is reported. This method allows obtaining hysteresis-free and high V_OC CH₃NH₃PbI₃ devices displaying a ≈37% improvement in power conversion efficiency, evidencing minimal recombination losses and very efficient charge extraction at the electrodes.

The majority of hole-transporting layers used in n-i-p perovskite solar cells contain 4-tert butylpyridine (tBP). High power-conversion efficiencies and, in particular, good steady-state performance appears to be contingent on the inclusion of this additive. On the quest to improve the steady state efficiencies of the carbon nanotube-based hole-transporter system, this study has found that the presence of tBP results in an extraordinary improvement in the performance of these devices. By deconstructing a prototypical device and investigating the effect of tBP on each individual layer, the results of this study indicate that this performance enhancement must be due to a direct chemical interaction between tBP and the perovskite material. This study proposes that tBP serves to p-dope the perovskite layer and investigates this theory with poling and work function measurements.

The effect of the hole-transporter additive 4-tert Butylpyridine (tBP) on the device performance of perovskite solar cells is investigated. The additive is shown to improve the steady-state efficiency of perovskite solar cells independent of the hole-transport material. A direct interaction between tBP and the perovskite absorber is identified as being responsible for the observed improvement.

Efficient quasi-2D perovskite light-emitting diodes (PeLEDs) are realized by partially substituting methyl ammonium (MA) cations with phenylethyl ammonium cations in MAPbBr3. The excitons move to the crystals, which have the smallest bandgap, and recombine. The quasi-2D PeLED designed by T.-W. Lee and co-workers, as described on page 7515, shows higher luminance and current efficiency (2935 cd m⁻² and 4.90 cd A⁻¹, respectively) compared with 3D MAPbBr3 PeLEDs.

Doping of organic bulk heterojunction solar cells has the potential to improve their power conversion efficiency (PCE). Deconvoluting the effect of doping on charge transport, recombination, and energetic disorder remains challenging. It is demonstrated that molecular doping has two competing effects: on one hand, dopant ions create additional traps while on the other hand free dopant-induced charges fill deep states possibly leading to V _OC and mobility increases. It is shown that molar dopant concentrations as low as a few parts per million can improve the PCE of organic bulk heterojunctions. Higher concentrations degrade the performance of the cells. In doped cells where PCE is observed to increase, such improvement cannot be attributed to better charge transport. Instead, the V _OC increase in unannealed P3HT:PCBM cells upon doping is indeed due to trap filling, while for annealed P3HT:PCBM cells the change in V _OC is related to morphology changes and dopant segregation. In PCDTBT:PC70BM cells, the enhanced PCE upon doping is explained by changes in the thickness of the active layer. This study highlights the complexity of bulk doping in organic solar cells due to the generally low doping efficiency and the constraint on doping concentrations to avoid carrier recombination and adverse morphology changes.

Ultralow level doping (≈ppm) can increase the power conversion efficiency of organic solar cells. Trap states filling by free charges and trap creation by dopant ions have competing effects on carrier mobility and open circuit voltage thereby imposing constraints on the effectiveness of doping. Measurements are performed to study what electronic process dominates in different materials or fabrication conditions.

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground-state decay of the CT state is significantly slower than dissociation.

A detailed study of bimolecular recombination in a high efficiency organic solar cell, comprised of a liquid crystalline donor and PC71BM, is presented. Using multiple techniques, it is shown that the bimolecular recombination is nearly 150 times suppressed with respect to that predicted by Langevin theory. This reduction is attributed to an equilibrium between charge transfer states and free charges.

In the present work, a Pb-assisted two step method is successfully proposed to fabricate high-quality CH₃NH₃Sn_0.5Pb_0.5I₃ (MASn_0.5Pb_0.5I₃) perovskite film on the indium tin oxide (ITO) glass/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) substrate. The film shows regular crystalline grains with a flat and compact morphology as well as full coverage on the planar PEDOT:PSS substrate. Remarkably, corresponding devices ITO/PEDOT:PSS/MASn_0.5Pb_0.5I₃/C₆₀/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/Ag are fabricated with high reproducibility, achieving a high power conversion efficiency of 13.6%, which is, to the best of knowledge, the most efficient solar cell based on Sn-based perovskite.

A high efficiency inverted planar Sn-based perovskite solar cell is fabricated by utilizing a two-step solution processing technique. Through use of PbI₂ in combination with SnI₂, the Sn-based perovskite film quality is improved obviously. The lead contents are successfully reduced to 50% in the perovskite layer, and the power conversion efficiency of the best corresponding device reaches up to 13.6%.

On page 6562, K. T. Nam, H. W. Jang, and co-workers describe an ultralow electric field and high-ON/OFF-ratio resistive-switching behavior of solution-processed organo-metal halide perovskite (OHP) films with the rotational motion of the A-site molecular cation. Metal/OHP/metal cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10³ V cm⁻¹ for four distinguishable ON-state resistance levels.

On page 6695, X. Y. Gao, L.-S. Liao, and co-workers describe the fabrication of mixed Pb–In perovskite solar cells, using indium (III) chloride and lead (II) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of the perovskites with multiple ordered crystal orientations. This work demonstrates the possibility of substituting the Pb (II) by using In (III), which opens a broad route to fabricating alloy perovskite solar cells with mitigated ecological impact.

Efficient lead (Pb)-free inverted planar formamidinium tin triiodide (FASnI₃) perovskite solar cells (PVSCs) are demonstrated. Our FASnI₃ PVSCs achieved average power conversion efficiencies (PCEs) of 5.41% ± 0.46% and a maximum PCE of 6.22% under forward voltage scan. The PVSCs exhibit small photocurrent–voltage hysteresis and high reproducibility. The champion cell shows a steady-state efficiency of ≈6.00% for over 100 s.

A crosslinked organic hole-transporting layer (HTL) is developed to realize highly efficient and stable perovskite solar cells via a facile thiol-ene thermal reaction. This crosslinked HTL not only facilitates hole extraction from perovskites, but also functions as an effective protective barrier. A high-performance (power conversion efficiency: 18.3%) device is demonstrated to show respectable photo and thermal stability without encapsulation.

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high-performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p-type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro-OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short-circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro-OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.

Copper salts (cuprous thiocyanate or cuprous iodide) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene is used as the hole transport layer in planar perovskite solar cells by their good film conductivity and hole mobility. As a result, a maximum 18.02% power conversion efficiency is achieved with improved cell stability. 2D grazing incidence X-ray diffraction technique is utilized to probe the cell degradation process.

It is observed that degradation of organic bulk heterojunction is directly associated with degradation of solvent additive, 1,8-diiodooctane. The iodide impurities released from 1,8-diiodooctane react with fullerenes, making fullerene-iodide intermediate compounds which lead to fullerene oxidation and efficiency drop, irrespective of photooxidation of polymer in polymer/fullerene bulk heterojunctions. Replacing the additive with an iodine-free reagent significantly elongates a device lifetime.