Ligang Yuan

Two new wide-bandgap D–A–π copolymer donor materials, PBDT-2TC and PBDT-S-2TC, based on benzodithiophene and asymmetric bithiophene with one carboxylate (2TC) substituent are synthesized by a facile approach for fullerene-free organic solar cells (OSCs). The combination of one carboxylate-substituted thiophene with one thiophene bridge in the backbone substantially reduces the steric hindrance, thereby favoring a planar geometry for efficient charge transport and molecular packing. A reasonable highest-occupied-molecular-orbital energy level in relation to that of the acceptor and balanced hole and electron transport are observed for both polymers. This asymmetric structure unit is flexible and versatile, allowing the absorption, energy levels, and morphology of the blend films to be tailored. Fullerene-free OSCs based on PBDT-S-2TC:ITIC achieve a high power conversion efficiency of 10.12%. More impressively, a successful nonhalogen solvent-processed solar cell with 9.55% efficiency is also achieved, which is one of the highest values for a fullerene-free OSC processed using an ecofriendly solvent.

New wide-bandgap D–A–π copolymers based on an asymmetric bithiophene with one carboxylate substituent were synthesized. The asymmetric structure unit is flexible and versatile, which allows the absorption, energy levels and morphology of the blend films to be adjusted easily. D-A-p copolymers produced a high power conversion efficiency of 10.0% for halogen solvent-processed OSCs and 9.55% for non-halogen solvent-processed devices.

Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most efficient and stable perovskite solar cells reported, so far they are prepared exclusively by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. In this report, thermal vacuum deposition with multiple sources (3 and 4) is used to prepare for the first time, multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability. The importance of the control over the film morphology is highlighted, which differs substantially when these compounds are vacuum processed. Avenues to improve the morphology and hence the performance of fully vacuum processed multications/anions perovskite solar cells are proposed.

Multiple-source (up to 4) thermal vacuum deposition is used to prepare for the first time multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability.

A new parameter, known as imbalance factor (Δ), is used to quantify the impacts of field dependent carrier mobilities on the performance of bulk heterojunction (BHJ) cells. BHJ solar cells with higher acceptor contents possess smaller Δ, and higher FFs when compared to control devices optimized by PCEs. These FF-optimized cells enjoy enhanced thermal and operational stabilities.

In the past years, hybrid perovskite materials have attracted great attention due to their superior optoelectronic properties. In this study, the authors report the utilization of cobalt (Co²⁺) to partially substitute lead (Pb²⁺) for developing novel hybrid perovskite materials, CH₃NH₃Pb_1-xCo_xI₃ (where x is nominal ratio, x = 0, 0.1, 0.2 and 0.4). It is found that the novel perovskite thin films possess a cubic crystal structure with superior thin film morphology and larger grain size, which is significantly different from pristine thin film, which possesses the tetragonal crystal structure, with smaller grain size. Moreover, it is found that the 3d orbital of Co²⁺ ensures higher electron mobilities and electrical conductivities of the CH₃NH₃Pb_1-xCo_xI₃ thin films than those of pristine CH₃NH₃Pb₄ thin film. As a result, a power conversion efficiency of 21.43% is observed from perovskite solar cells fabricated by the CH₃NH₃Pb_0.9Co_0.1I₃ thin film. Thus, the utilization of Co, partially substituting for Pb to tune physical properties of hybrid perovskite materials provides a facile way to boost device performance of perovskite solar cells.

The utilization of cobalt (Co²⁺) to partially substitute lead (Pb²⁺) for developing novel hybrid perovskite materials and perovskite solar cells is reported. A power conversion efficiency of 21.43% is observed from perovskite solar cells fabricated by the CH₃NH₃Pb_0.9Co_0.1I₃ thin film, which is due to it possessing a cubic crystal structure with superior thin film morphology and larger grain size.

Increasing the power conversion efficiency (PCE) of the two-dimensional (2D) perovskite-based solar cells (PVSCs) is really a challenge. Vertical orientation of the 2D perovskite film is an efficient strategy to elevate the PCE. In this work, vertically orientated highly crystalline 2D (PEA)₂(MA)_n–1Pb_nI_3n+1 (PEA= phenylethylammonium, MA = methylammonium, n = 3, 4, 5) films are fabricated with the assistance of an ammonium thiocyanate (NH₄SCN) additive by a one-step spin-coating method. Planar-structured PVSCs with the device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/(PEA)₂(MA)_n–1Pb_nI_3n+1/[6,6]-phenyl-C61-butyric acid methyl ester/bahocuproine/Ag are fabricated. The PCE of the PVSCs is boosted from the original 0.56% (without NH₄SCN) to 11.01% with the optimized NH₄SCN addition at n = 5, which is among the highest PCE values for the low-n (n < 10) 2D perovskite-based PVSCs. The improved performance is attributed to the vertically orientated highly crystalline 2D perovskite thin films as well as the balanced electron/hole transportation. The humidity stability of this oriented 2D perovskite thin film is also confirmed by the almost unchanged X-ray diffraction patterns after 28 d exposed to the moisture in a humidity-controlled cabinet (H_r = 55 ± 5%). The unsealed device retains 78.5% of its original PCE after 160 h storage in air atmosphere with humidity of 55 ± 5%. The results provide an effective approach toward a highly efficient and stable PVSC for future commercialization.

The advantage of phenylethylammonium (PEA⁺) in forming pinhole-free 2D (PEA)₂(methylammonium (MA))_n−1Pb_nI_3n+1 n = 3, 4, 5) perovskite film with vertical orientation and high crystallinity under assistance of an ammonium thiocyanate additive by one-step spin-coating method is demonstrated. The optimized planar-structured perovskite solar cell based on vertically oriented (PEA)₂(MA)₄Pb₅I₁₆ (n = 5) film presents the best power conversion efficiency of 11.01% with excellent stability.

In article number 1701935 by Taiho Park and co-workers, tetraethylene glycol (TEG) group is incorporated into a well-known high mobility polymer backbone to improve its solubility. The polymer, PTEG, exhibits high mobility and solubility in common organic solvents, showing the highest efficiency (19.8%) in the planar perovskite solar cell.

In article number 1701683, In Hwan Jung, Sung-Yeon Jang, and co-workers report the synthesis of ‘high-efficiency low-temperature processed perovskite solar cells’ by the combination of two techniques; the modification of solution-processed ZnO using a self-assembled monolayer, and the fabrication of perovskite layers by a sequential deposition method. This technique may pave the way to efficiently reducing the processing temperature of perovskite solar cells.

In article number 1702235, Taiho Park and co-workers demonstrate that the efficiency of a planar-type perovskite solar cell is increased by the addition of CuI ionic salt to the TiO₂ electron transport layer. Due to the characteristics of CuI (Visualized on the cover as yellow and red spheres), the electrons from perovskite layer (pink octahedra) show fast extraction to TiO₂ layer (blue and purple spheres).

This study proposes a novel strategy of controllable deamination of Co–NH₃ complexes in a system containing Ni(OH)₂ to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo₂O₄. Through this approach, ultrasmall (5 nm on average) and well-dispersed NiCo₂O₄ NPs without exotic ligands are obtained, which enables the formation of uniform and pin-hole free films. The tightly covered NiCo₂O₄ films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo₂O₄ NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo₂O₄ NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo₂O₄ as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand-free NPs HTLs to enable cost-effective photovoltaic devices.

An ultrasmall and well-dispersed ternary oxide of NiCo₂O₄ nanoparticles, achieved by a new strategy of controllable deamination of the Co–NH₃ complexes in a system containing Ni(OH)₂ , facilitate the formation of large perovskite grains, which is first applied as hole transport layer for highly efficient perovskite solar cells.

The recent rapid increase in efficiency of organic–inorganic perovskite solar cells (PSCs) has resulted in a need to develop a clear understanding of their stability and working mechanisms. In particular, it has been suggested that ion migration contributes to the commonly observed hysteresis in the current–voltage measurements of PSCs, but the rate of ion migration and its effects on the electronic properties of PSCs remain to be addressed. In this work, electron-beam-induced current (EBIC) is used to directly map changes in local current extraction in organic–inorganic PSCs under applied voltage. By combining EBIC mapping, standard current–voltage measurements, and external quantum efficiency measurements, it is shown that between the two potential roles that point defects play in device enhancement under voltage biasing, the effects caused by defect-mediated ion migration outweigh the effects from the filling of trap states caused by these defects. Evidence is also provided for ion migration preferentially at local features such as extended defects. The measured timescale of tens of seconds for migration across a full device imply that ion migration contributes indirectly to the electronic capacitance of perovskite devices through interface charging.

Electron-beam induced current measurements are used to map local current extraction in organic–inorganic perovskite solar cells. Current extraction before and after prebiasing of organic–inorganic perovskite solar cell devices and quantification of corresponding current decay rates support a model of mixed ionic and electronic processes contributing to the dependence of perovskite solar cells on previous biasing conditions.

In article number 1701140, Pavel Troshin and co-workers, show that planar junction solar cells based on antimony (V) bromide complexes, demonstrate an external quantum efficiency of ≈80% and power conversion efficiency of ≈4%. The discovery of the first perovskite-like compound ABX₆ exhibiting good photovoltaic performance opens wide opportunities for rational design of novel hybrid semiconductor materials for advanced electronic and photovoltaic applications.

As rapid progress has been achieved in emerging thin film solar cell technology, organic–inorganic hybrid perovskite solar cells (PVSCs) have aroused many concerns with several desired properties for photovoltaic applications, including large absorption coefficients, excellent carrier mobility, long charge carrier diffusion lengths, low-cost, and unbelievable progress. Power conversion efficiencies increased from 3.8% in 2009 up to the current world record of 22.1%. However, poor long-term stability of PVSCs limits the future commercial application. Here, the degradation mechanisms for unstable perovskite materials and their corresponding solar cells are discussed. The strategies for enhancing the stability of perovskite materials and PVSCs are also summarized. This review is expected to provide helpful insights for further enhancing the stability of perovskite materials and PVSCs in this exciting field.

Perovskite solar cells have attracted much attention due to their low-cost fabrication and high efficiency, with a recently recorded power conversion efficiency of 22.1%. However, a crucial challenge for perovskite solar cells is stability. The various external causes of failure, such as moisture, heat, light, etc., and associated mechanisms of perovskite solar cells degradation from two aspects of perovskite layer and device structure are reviewed.

High quality perovskite films are achieved by blade coating lead acetate trihydrate sourced precursor solutions under harsh ambient conditions. The hydrate water forms a quasi-liquid nutrition pool for the cultivation of MAPbI3 grain domains via Ostwald ripening. The solar cells based on the perovskite films with grain domains of up to 100 µm, give champion efficiency of 16.4%.

Binary solvent additives treatment boosts the efficiency of PTB7:PCBM polymer solar cells to over 9.5%. A novel binary solvent additive combination of DPE:DIO is demonstrated, which combines the influence of DIO on the PC70BM dispersion together with the DPE on the PTB7 crystallization. As a result, a high efficiency polymer solar cell is obtained with a significant PCE enhancement from 6.96 to 9.25%, and high FF of over 70%.

Compared to the traditional one-step solution method, the evaporation-assisted solution (EAS) method could fabricate dense and pin-hole free CsSnI₃ film. The elimination of direct contact of hole and electron transporting materials can reduce the carrier recombination on the surface. As a result, a CsSnI₃ solar cell by EAS method with an efficiency of 2.23% is demonstrated.

Due to the fact that BDTTID has a crystalline-like structure while BDT3TID has a near-amorphous structure, the former-based photovoltaic devices show a higher efficiency with superior thermal stability than the latter-based ones. These results can inform future studies regarding the design of small donor molecules for OPV devices by demonstrating the importance of controlling their crystallinity rather than their absorption properties.

Power conversion efficiency (PCE) of planar Si/PEDOT:PSS solar cells is improved by incorporating metal NPs into the PEDOT:PSS layer. Both Ag and Au NPs induce a localized surface plasomonic resonance effect and bring better device performance. The dual LSPR effect of Au and Au NPs can bring broader light absorption, and thus a higher PCE.

This Al₂O₃ interlayer acts as an excellent insulating layer and interface decorator for electron transport material (ETM), which effectively reduces the recombination at ETM/perovskite and perovskite/counter electrode interfaces simultaneously, thus leading to an enhanced open-circuit voltage for hole-conductor-free carbon-based perovskite solar cell.