Chen Weijie

An open-circuit voltage (V_oc) of 1.57 V under simulated AM1.5 sunlight in planar MAPbBr₃ solar cells with carbon (graphite) electrodes is obtained. The hole-transport-material-free MAPbBr₃ solar cells with the normal architecture (FTO/TiO₂/MAPbBr₃/carbon) show little hysteresis during current–voltage sweep under simulated AM1.5 sunlight. A solar-to-electricity power conversion efficiency of 8.70% is achieved with the champion device. Accordingly, it is proposed that the carbon electrodes are effective to extract photogenerated holes in MAPbBr₃ solar cells, and the industry-applicable carbon electrodes will not limit the performance of bromide-based perovskite solar cells. Based on the analysis of the band alignment, it is found that the voltage (energy) loss across the interface between MAPbBr₃ and carbon is very small compared to the offset between the valence band maximum of MAPbBr₃ and the work function of graphite. This finding implies either Fermi level pinning or highly doped region inside MAPbBr₃ layer exists. The band-edge electroluminescence spectra of MAPbBr₃ from the solar cells further support no back-transfer pathways of electrons across the MAPbBr₃/TiO₂ interface.

An open-circuit voltage (V_oc) of 1.57 V under AM1.5 sunlight is obtained in hole-transport-materials-free planar MAPbBr₃ solar cells with carbon (graphite) electrodes. Compared to the large band offset between MAPbBr₃ and graphite, a small (≤0.43 V) voltage loss across the MAPbBr₃/graphite interface is measured. The band-edge electroluminescence from MAPbBr₃ devices supports no back transfer of electrons across the MAPbBr₃/TiO₂ interface.

Organic–inorganic hybrid perovskite has led to the development of new solar cells with outstanding efficiency. In perovskite solar cells (PSCs), perovskite is sandwiched between a working electrode (fluorine-doped tin oxide) and a counter electrode (gold, Au). In order to transport charges and block opposite charges, charge transport layers are inserted between perovskite and the electrodes. In particular, a hole transport layer is important because it generally prevents perovskite from exposure to air. Therefore, it is necessary to investigate dopant-free and hydrophobic polymeric hole transport materials (HTMs). In this study, a novel polymeric HTM (PTEG) is synthesized by controlling the solubility using a tetraethylene glycol group. The planar-PSC employing PTEG exhibits an efficiency of 19.8% without any dopants, which corresponds to the highest value reported to date. This study offers a fundamental strategy for designing and synthesizing various polymeric HTMs.

This study examines a highly efficient perovskite solar cell (PSC) that employs a dopant-free hole transport material (HTM). A polymeric HTM (PTEG) combined with a tetraethylene glycol group is synthesized and systematically characterized. Results indicate that the PSC employing PTEG exhibits the highest efficiency (19.8%) in the planar device.

The low power conversion efficiency (PCE) of tin-based hybrid perovskite solar cells (HPSCs) is mainly attributed to the high background carrier density due to a high density of intrinsic defects such as Sn vacancies and oxidized species (Sn⁴⁺) that characterize Sn-based HPSCs. Herein, this study reports on the successful reduction of the background carrier density by more than one order of magnitude by depositing near-single-crystalline formamidinium tin iodide (FASnI₃) films with the orthorhombic a-axis in the out-of-plane direction. Using these highly crystalline films, obtained by mixing a very small amount (0.08 m) of layered (2D) Sn perovskite with 0.92 m (3D) FASnI₃, for the first time a PCE as high as 9.0% in a planar p–i–n device structure is achieved. These devices display negligible hysteresis and light soaking, as they benefit from very low trap-assisted recombination, low shunt losses, and more efficient charge collection. This represents a 50% improvement in PCE compared to the best reference cell based on a pure FASnI₃ film using SnF₂ as a reducing agent. Moreover, the 2D/3D-based HPSCs show considerable improved stability due to the enhanced robustness of the perovskite film compared to the reference cell.

An all-tin-based perovskite solar cell with a record power conversion efficiency of 9% is reported for the first time. The outstanding performance is attributed to the fact that the addition of a trace amount of 2D tin perovskite initiates the homogenous growth of highly crystalline and oriented FASnI₃ grains at low temperature.

Currently, constructing ternary organic solar cells (OSCs) and developing nonfullerene small molecule acceptors (n-SMAs) are two pivotal avenues to enhance the device performance. However, introducing n-SMAs into the ternary OSCs to construct thick layer device is still a challenge due to their inferior charge transport property and unclear aggregation mechanism. In this work, a novel wide band gap copolymer 4,8-bis(4,5-dioctylthiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-N-(2-hexyldecyl)-5,5′-bis(thiophen-2-yl)-2,2′-bithiophene-3,3′-dicarboximide (PDOT) is designed and blend of PDOT:PC₇₁BM achieves a power conversion efficiency (PCE) of 9.5% with active layer thickness over 200 nm. The rationally selected n-SMA based on a bulky seven-ring fused core (indacenodithieno[3,2-b]thiophene) end-capped with 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile groups (ITIC) is introduced into the host binary PDOT:PC₇₁BM system to extend the absorption range and reduce the photo energy loss. After fully investigating the morphology evolution of the ternary blends, the different aggregation behavior of n-SMAs with respect to their fullerene counterpart is revealed and the adverse effect of introducing n-SMAs on charge transport is successfully avoided. The ternary OSC delivers a PCE of 11.2% with a 230 nm thick active layer, which is among the highest efficiencies of thick layer OSCs. The results demonstrate the feasibility of using n-SMAs to construct a thick layer ternary device for the first time, which will greatly promote the efficiency of thick layer ternary devices.

A ternary organic solar cell with a thick photoactive layer is constructed by introducing a nonfullerene small molecule acceptor into the host binary system based on a novel wide band-gap donor polymer and PC₇₁BM, achieving high V_oc of 0.96 V and PCE of 11.2%, which exhibits significant application potential in further roll-to-roll production.

In article number 1700265, Annalisa Bruno and co-workers show that applying an electrical poling to precondition the SCs below 170 K, the MA⁺ cations can be efficiently oriented and I⁻ accumulation at the TiO₂/MAPbI₃ interface is facilitated. Both phenomena lead to improved charge transfer efficiency at the interface. In the cover, the MAPbI₃ and TiO₂ interface in a solar cell at low temperature under electrical poling is highlighted.

In article number 1700522, Xiao Wei Sun, Qichun Zhang, and co-workers report the design and synthesis of a new n-type small molecule with a sulfur-containing structure. Employing this small molecule as electron transport layer (ETL), highefficiency planar perovskite solar cells up to 18.1% are realized. This superior performance is mainly due to effective suppression of charge recombination at the perovskite/ETL interface.

A highly-ordered and crystalline Fe₂TiO₅ honeycomb nanotube photoanode is described by Jae Sung Lee and co-workers in article number 1702428. Fabricated by hybrid microwave annealing using ultrathin anodic aluminum oxide as a template, the device shows an outstanding water splitting activity after triple modification. The strategy paves the way for significantly improving the performance of other photoelectrode materials.

Each component layer in a perovskite solar cell plays an important role in the cell performance. Here, a few types of polymers including representative p-type and n-type semiconductors, and a classical insulator, are chosen to dope into a perovskite film. The long-chain polymer helps to form a network among the perovskite crystalline grains, as witnessed by the improved film morphology and device stability. The dewetting process is greatly suppressed by the cross-linking effect of the polymer chains, thereby resulting in uniform perovskite films with large grain sizes. Moreover, it is found that the polymer-doped perovskite shows a reduced trap-state density, likely due to the polymer effectively passivating the perovskite grain surface. Meanwhile the doped polymer formed a bridge between grains for efficient charge transport. Using this approach, the solar cell efficiency is improved from 17.43% to as high as 19.19%, with a much improved stability. As it is not required for the polymer to have a strict energy level matching with the perovskite, in principle, one may use a variety of polymers for this type of device design.

The doping of polymer additives into MAPbI₃ films is reported. This allows for enlarged MAPbI₃ crystal grains by controlling the crystallization processes and decreased trap-state densities by passivating the MAPbI₃ crystal grain boundaries. As a result, the device based on the formed continuous, few-defect, large MAPbI₃ grains demonstrates improved efficiency from 17.43 to 19.19%, and relatively improved moisture stability.

Great efforts toward developing novel and efficient hole-transporting materials are needed to further improve the device efficiency and enhance the cell stability of perovskite solar cells (PSCs). The poor film conductivity and the low carrier mobility of organic small-molecule-based hole-transporting materials restrict their application in PSCs. This study develops an efficient and stable hole-transporting material, tetrafluorotetracyanoquinodimethane (F₄-TCNQ)-doped copper phthalocyanine-3,4′,4′′,4′′′-tetra-sulfonated acid tetra sodium salt (TS-CuPc) via a solution process, in planar structure PSCs. The p-type-doped TS-CuPc film demonstrates improved film conductivity and hole mobility owing to the strong electron affinity of F₄-TCNQ. By the F₄-TCNQ tailoring, the composite film gives the highest occupied molecular orbital level as high as 5.3 eV, which is beneficial for hole extraction. In addition, the aqueous solution processed TS-CuPc:F₄-TCNQ precursor is almost neutral with good stability for avoiding the electrode erosion. As a result, the fabricated PSCs employing TS-CuPc:F₄-TCNQ as the hole-transporting material exhibit a power conversion efficiency of 16.14% in a p–i–n structure and 20.16% in an n–i–p structure, respectively. The developed organic small molecule of TS-CuPc provides the diversification of hole-transporting materials in planar PSCs.

p-Type-doped copper phthalocyanine-3,4′,4″,4″′-tetra-sulfonated acid tetra sodium salt (TS-CuPc) by tetrafluorotetracyanoquinodimethane (F₄-TCNQ) with improved film conductivity and hole mobility is realized via a solution process. The composite film is used as a hole-transporting layer in both p–i–n structure and n–i–p structure devices. A champion n–i–p structure device with a power conversion efficiency of 20.16% is obtained.

Charge transport layers play an important role in determining the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs). However, it has proven challenging to produce thin and compact charge transport layers via solution processing techniques. Herein, a hot substrate deposition method capable of improving the morphology of high-coverage hole-transport layers (HTLs) and electron-transport layers (ETLs) is reported. PSC devices using HTLs deposited on a hot substrate show improvement in the open-circuit voltage (V_oc) from 1.041 to 1.070 V and the PCE from 17.00% to 18.01%. The overall device performance is then further enhanced with the hot substrate deposition of ETLs as the V_oc and PCE reach 1.105 V and 19.16%, respectively. The improved performance can be explained by the decreased current leakage and series resistance, which are present in PSCs with rough and discontinuous HTLs and ETLs.

A hot-substrate deposition method contributes to the modification of both hole-transport layers and electron-transport layers with high coverage and uniform morphology. It is useful to reduce the current leakage and series resistance resulting from rough and discontinuous charge transport layers in perovskite solar cells. This strategy improves open-circuit voltage and power conversion efficiency of the device to 1.105 V and 19.16%, respectively.

Organic–inorganic hybrid perovskite solar cells (PSC) are promising third-generation solar cells. They exhibit good power conversion efficiencies and in principle they can be fabricated with lower energy consumption than many more established technologies. To improve the efficiency and long-term stability of PSC, organic molecules are frequently used as “interlayers.” Interlayers are thin layers or monolayers of organic molecules that modify a specific interface in the solar cell. Here, the latest progress in the use of interlayers to optimize the performance of PSC is reviewed. Where appropriate interesting examples from the field of organic photovoltaics (OPV) are also presented as there are many similarities in the types of interlayers that are used in PSC and OPV. The review is organized into three parts. The first part focuses on why organic molecule interlayers improve the performance of the solar cells. The second section discusses commonly used molecular interlayers. In the last part, different approaches to make thin and uniform interlayers are discussed.

Small molecules are increasingly being used as interlayers in perovskite solar cells to modify band energy offsets at an interface, to improve the morphology of active layers, and to enhance the long-term stability of the devices. This article reviews recent advances in the use of molecular interlayers in perovskite cells and introduces relevant examples from the field of organic solar cells.

Designing polymers that facilitate exciton dissociation and charge transport is critical for the production of highly efficient all-polymer solar cells (all-PSCs). Here, the development of a new class of high-performance naphthalenediimide (NDI)-based polymers with large dipole moment change (Δµ_ge) and delocalized lowest unoccupied molecular orbital (LUMO) as electron acceptors for all-PSCs is reported. A series of NDI-based copolymers incorporating electron-withdrawing cyanovinylene groups into the backbone (PNDITCVT-R) is designed and synthesized with 2-hexyldecyl (R = HD) and 2-octyldodecyl (R = OD) side chains. Density functional theory calculations reveal an enhancement in Δµ_ge and delocalization of the LUMO upon the incorporation of cyanovinylene groups. All-PSCs fabricated from these new NDI-based polymer acceptors exhibit outstanding power conversion efficiencies (7.4%) and high fill factors (65%), which is attributed to efficient exciton dissociation, well-balanced charge transport, and suppressed monomolecular recombination. Morphological studies by grazing X-ray scattering and resonant soft X-ray scattering measurements show the blend films containing polymer donor and PNDITCVT-R acceptors to exhibit favorable face-on orientation and well-mixed morphology with small domain spacing (30–40 nm).

High-performance polymer acceptors with high electron mobility and large dipole moment difference are developed by introducing electron-withdrawing cyanovinylene groups into naphthalenediimide-based polymers. All-polymer solar cells fabricated using these new polymer acceptors exhibit outstanding power conversion efficiencies of up to 7.4% and high fill factors (65%) as a result of efficient exciton dissociation and enhanced charge transport.

Bilayer graphene (BLG) comprises a 2D nanospace sandwiched by two parallel graphene sheets that can be used to intercalate molecules or ions for attaining novel functionalities. However, intercalation is mostly demonstrated with small, exfoliated graphene flakes. This study demonstrates intercalation of molybdenum chloride (MoCl₅) into a large-area, uniform BLG sheet, which is grown by chemical vapor deposition (CVD). This study reveals that the degree of MoCl₅ intercalation strongly depends on the stacking order of the graphene; twist-stacked graphene shows a much higher degree of intercalation than AB-stacked. Density functional theory calculations suggest that weak interlayer coupling in the twist-stacked graphene contributes to the effective intercalation. By selectively synthesizing twist-rich BLG films through control of the CVD conditions, low sheet resistance (83 Ω ▫⁻¹) is realized after MoCl₅ intercalation, while maintaining high optical transmittance (≈95%). The low sheet resistance state is relatively stable in air for more than three months. Furthermore, the intercalated BLG film is applied to organic solar cells, realizing a high power conversion efficiency.

Intercalation of MoCl₅ into large-area bilayer graphene (BLG) grown by chemical vapor deposition is performed. Twist stacking gives a much higher degree of MoCl₅ intercalation than AB stacking. A low sheet resistance with high optical transmittance is obtained by using twist-rich BLG. A transparent electrode suitable for use in organic solar cells is developed from this intercalated bilayer.

Colloidal quantum dots (QDs) are promising candidate materials for photovoltaics (PV) owing to the tunable bandgap and low-cost solution processability. Lead selenide (PbSe) QDs are particularly attractive to PV applications due to the efficient multiple-exciton generation and carrier transportation. However, surface defects arising from the oxidation of the PbSe QDs have been the major limitation for their development in PV. Here, a new passivation method for chlorinated PbSe QDs via ion exchange with cesium lead halide (Br, I) perovskite nanocrystals is reported. The surface chloride ions on the as-synthesized QDs can be partially exchanged with bromide or iodide ions from the perovskite nanocrystals, hence forming a hybrid halide passivation. Consistent with the improved photoluminescence quantum yield, the champion PV device fabricated with these PbSe QDs achieves a PCE of 8.2%, compared to 7.3% of that fabricated with the untreated QDs. This new method also leads to devices with excellent air-stability, retaining at least 93% of their initial PCEs after being stored in ambient conditions for 57 d. This is considered as the first reported PbSe QD solar cell with a PCE of over 8% to date.

PbSe quantum dots (QDs) with robust hybrid halide passivation are obtained via ion exchange with CsPbX₃ halide perovskite nanocrystals, resulting in significant improvement in their photoluminescence quantum yield. A champion solar cell fabricated with these passivated PbSe QDs can achieve an efficiency of over 8%, as well as excellent air-stability.

The introduction of oligomeric polystyrene (PS) side chains into the conjugated backbone is proven to enhance the processability and electronic properties of semiconducting polymers. Here, two series of donor and acceptor polymers are prepared with different molar percentages of PS side chains to elucidate the effect of their substitution arrangement on the all-polymer solar cell performance. The observed device performance is lower when the PS side chains are substituted on the donor polymer and higher when on the acceptor polymer, indicating a clear arrangement effect of the PS side chain. The incorporation of PS side chains to the acceptor polymer contributes to the decrease in phase separation domain size in the blend films. However, the reduced domain size was still an order of magnitude larger than the typical exciton diffusion length. A detailed morphological study together with the estimation of solubility parameter of the pristine PS, donor, and acceptor polymers reveals that the relative value of solubility parameter of each component dominantly contributes to the purity of the phase separated domain, which strongly impacts the amount of generated photocurrent and overall solar cell performance. This study provides an understanding of the design strategies to improve the all-polymer solar cells.

All-polymer solar cells consisting of polystyrene (PS) oligomer side chain attached donor and acceptor polymers are investigated to reveal the effect of PS side chain substituted arrangement. An increase in domain purity and decrease in domain size are observed when the PS side chain is attached to donor and acceptor polymers, respectively, which mainly results from the solubility parameter of the polymers.