ZiQi Sun

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.

Several challenges in organic photovoltaics have yet to be overcome including high power conversion efficiency, good processability, low cost, and excellent long-term stability. In article number 1700465, Jie Min and co-workers introduce a new merit factor (i-FOM) for material applied accessibility containing three parameters: synthetic complexity, device efficiency, and photostability. i-FOM approach can provide valuable insights for those attempting to realize the efficient evaluation of photovoltaic materials.

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.

Perovskite solar cells (PSCs) have made rapid advances in efficiency when fabricated as small-area devices. A key challenge is to increase the active area while retaining high performance, which requires fast and reliable measurement techniques to spatially resolve cell properties. Luminescence imaging-based techniques are one attractive possibility. A thermodynamic treatment of the luminescence radiation from MAPbI₃ and related perovskite semiconductors predicts that the intensity of luminescence emission is proportional to the electrochemical potential in the perovskite absorber, bringing with it numerous experimental advantages. However, concerns arise about the impact of the often-observed hysteretic behavior on the interpretation of luminescence-based measurements. This study demonstrates that despite their hysteretic phenomena, at steady-state perovskite solar cells are amenable to quantitative analysis of luminescence images. This is demonstrated by calculating the spatial distribution of series resistance from steady-state photoluminescence images. This study observes good consistency between the magnitude, voltage-dependence, and spatial distribution of series resistance calculated from luminescence images and from cell-level current–voltage curves and uncalibrated luminescence images, respectively. This method has significant value for the development of PSC process control, design and material selection, and illustrates the possibilities for large-area, spatially resolved, quantitative luminescence imaging-based characterization of PSCs.

This work demonstrates how steady-state luminescence imaging of perovskite solar cells can be used to quantify valuable device-level parameters. Electrically biased photoluminescence measurements are used to spatially resolve the series resistance of perovskite solar cells. Thus, the power of luminescence imaging to quantifiably characterisz large area perovskite solar cells in a very short time is demonstrated.

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.