Ligang Yuan

Semitransparent organic solar cells (OSCs) show attractive potential in power-generating windows. However, the development of semitransparent OSCs is lagging behind opaque OSCs. Here, an ultralow-bandgap nonfullerene acceptor, “IEICO-4Cl”, is designed and synthesized, whose absorption spectrum is mainly located in the near-infrared region. When IEICO-4Cl is blended with different polymer donors (J52, PBDB-T, and PTB7-Th), the colors of the blend films can be tuned from purple to blue to cyan, respectively. Traditional OSCs with a nontransparent Al electrode fabricated by J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-Th:IEICO-4Cl yield power conversion efficiencies (PCE) of 9.65 ± 0.33%, 9.43 ± 0.13%, and 10.0 ± 0.2%, respectively. By using 15 nm Au as the electrode, semitransparent OSCs based on these three blends also show PCEs of 6.37%, 6.24%, and 6.97% with high average visible transmittance (AVT) of 35.1%, 35.7%, and 33.5%, respectively. Furthermore, via changing the thickness of Au in the OSCs, the relationship between the transmittance and efficiency is studied in detail, and an impressive PCE of 8.38% with an AVT of 25.7% is obtained, which is an outstanding value in the semitransparent OSCs.

A new nonfullerene acceptor, IEICO-4Cl, is designed to prepare semitransparent organic solar cells (OSCs), yielding a power conversion efficiency of 8.38% with an average visible transmittance of 25.7%, which is among the top results for semitransparent OSCs.

The development of molecular design of porphyrins and BODIPY small molecules and polymers for bulk heterojunction organic solar cells are reviewed and a guideline for the structure-performance relationship is provided.

With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2′-((2Z,2′Z)-((6,6′-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp³ carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 10⁵m⁻¹ cm⁻¹ in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.

A thieno[3,4-b]thiophene-based electron acceptor, NITI, featuring a 14-π-electron indenoindene core is designed and synthesized. Despite its twisted molecular geometry, NITI shows a low optical bandgap and a high molar extinction coefficient. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which represents an exciting progress in the design of new electron acceptors.

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.

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.

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.

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.

Methylammonium lead iodide (MAPbI₃) perovskites are organic–inorganic semiconductors with long carrier diffusion lengths serving as the light-harvesting component in optoelectronics. Through a substitutional growth of MAPbI₃ catalyzed by polar protic alcohols, evidence is shown for their substrate- and annealing-free production and use of toxic solvents and high temperature is prevented. The resulting variable-sized crystals (≈100 nm–10 µm) are found to be tetragonally single-phased in alcohols and precipitated as powders that are metallic-lead-free. A comparatively low MAPbI₃ yield in toluene supports the role of alcohol polarity and the type of solvent (protic vs aprotic). The theoretical calculations suggest that overall Gibbs free energy in alcohols is lowered due to their catalytic impact. Based on this alcohol-catalyzed approach, MAPbI₃ is obtained, which is chemically stable in air up to ≈1.5 months and thermally stable (≤300 °C). This method is amendable to large-scale manufacturing and ultimately can lead to energy-efficient, low-cost, and stable devices.

A substitutional growth method is introduced herein for the first time as a solution-phase technique, which produces tetragonally single-phased and metallic-lead-free methylammonium lead iodide perovskite crystals. In this approach, alcohols as polar protic solvents are found to catalyze the growth and nucleation of these high-symmetry crystals by facilitating large-scale and low-cost manufacturing for perovskite optoelectronics.

Two donor–acceptor (D–A) conjugated polymers composed of the same ratio of 5-fluorobenzothiadiazole and thiophene subunits are synthesized through different routes, providing a precisely regioregular (2TRR) and a random (2TRA) polymer structures. Detailed structural analyses indicate that the backbone of regioregular 2TRR has only one donor segment of bithiophene, while the backbone of random 2TRA consists of three different donor segments: thiophene, bithiophene, and terthiophene (in a ratio of 0.16:0.68:0.16). Synergetic contributions from these segments allow the “tetrapolymer” 2TRA to achieve more favorable film morphology and a higher hole-mobility relative to 2TRR. Consequently, the random polymer 2TRA achieves a substantially higher power conversion efficiency (8.8%) than the regioregular polymer 2TRR (5.1%). Notably, the “tetrapolymer” 2TRA is readily synthesized from two monomers, rather than through complex conventional preparation required for similar multipolymers. These findings provide a novel route toward the design and synthesis of multipolymeric materials and demonstrate their potential advantages in high-performance organic electronic applications.

Two conjugated polymers are synthesized using the same components in different sequences, regioregular and random. Readily synthesized with only two monomers, detailed structure analyses indicate that the random polymer 2TRA contains three distinct donor segments in its “tetrapolymer” backbone, granting a favorable morphology, higher hole-mobility, and better photovoltaic performance in blends with PC71BM than its regioregular analog (2TRR).

Nonfullerene polymer solar cells (PSCs) based on polymer donors and nonfullerene small molecular acceptors (SMAs) have recently attracted considerable attention. Although much of the progress is driven by the development of novel SMAs, the donor polymer also plays an important role in achieving efficient nonfullerene PSCs. However, it is far from clear how the polymer donor choice influences the morphology and performance of the SMAs and the nonfullerene blends. In addition, it is challenging to carry out quantitative analysis of the morphology of polymer:SMA blends, due to the low material contrast and overlapping scattering features of the π–π stacking between the two organic components. Here, a series of nonfullerene blends is studied based on ITIC-Th blended with five different donor polymers. Through quantitative morphology analysis, the (010) coherence length of the SMA is characterized and a positive correlation between the coherence length of the SMA and the device fill factor (FF) is established. The study reveals that the donor polymer can significantly change the molecular ordering of the SMA and thus improve the electron mobility and domain purity of the blend, which has an overall positive effect that leads to the enhanced device FF for nonfullerene PSCs.

Morphological analysis reveals influence of the donor polymer on the structural/electronic properties of small molecular acceptor and the overall blend morphology. A direct correlation is found between the (010) coherence length of small molecular acceptor with device fill-factor and photocurrent density, which is in good agreement with the parameters reported for state-of-art high-efficiency nonfullerene polymer solar cells.

Using bromoantimonate (V) (N-EtPy)[SbBr₆] as an example, it is demonstrated that ABX₆ compounds can form perovskite-like 3D crystalline frameworks with short interhalide contacts, enabling advanced optoelectronic characteristics of these materials. The designed compound shows an impressive performance in planar junction solar cells delivering external quantum efficiency of ≈80% and power conversion efficiency of ≈4%, thus being comparable with the conventional perovskite material MAPbBr₃. The discovery of the first perovskite-like compound ABX₆ exhibiting good photovoltaic performance opens wide opportunities for rational design of novel perovskite-like semiconductor materials for advanced electronic and photovoltaic applications.

Planar junction solar cells based on the complex antimony (V) bromide (N-EtPy)[SbBr₆] reveal 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.

Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long-term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH₃NH₃PbI₃. It is observed that the aggregation of Li⁺/I⁻ tunes the energy level of the perovskite and induces n/p doping in CH₃NH₃PbI₃, which makes charge extraction quite efficient from perovskite to both NiO and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li⁺ and I⁻ modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.

This work highlights the positive effect of selective ionic doping in CH₃NH₃PbI₃, which benefits for the energy band alignment and perovskite solar cell performances.

Perovskite/silicon tandem solar cells are increasingly recognized as promi­sing candidates for next-generation photovoltaics with performance beyond the single-junction limit at potentially low production costs. Current designs for monolithic tandems rely on transparent conductive oxides as an intermediate recombination layer, which lead to optical losses and reduced shunt resistance. An improved recombination junction based on nanocrystalline silicon layers to mitigate these losses is demonstrated. When employed in monolithic perovskite/silicon heterojunction tandem cells with a planar front side, this junction is found to increase the bottom cell photocurrent by more than 1 mA cm⁻². In combination with a cesium-based perovskite top cell, this leads to tandem cell power-conversion efficiencies of up to 22.7% obtained from J–V measurements and steady-state efficiencies of up to 22.0% during maximum power point tracking. Thanks to its low lateral conductivity, the nanocrystalline silicon recombination junction enables upscaling of monolithic perovskite/silicon heterojunction tandem cells, resulting in a 12.96 cm² monolithic tandem cell with a steady-state efficiency of 18%.

A nanocrystalline silicon recombination junction is presented, which mitigates reflection losses in monolithic perovskite/silicon heterojunction tandem solar cells, resulting in efficiencies of up to 22.7% on an aperture area of 0.25 cm². Thanks to its low lateral conductance, this recombination junction enables tandem cell up-scaling, leading to a steady-state efficiency of 18% on an aperture area of 12.96 cm².

The device performance of polymer solar cells (PSCs) is strongly dependent on the blend morphology. One of the strategies for improving PSC performance is side-chain engineering, which plays an important role in controlling the aggregation properties of the polymers and thus the domain crystallinity/purity of the donor–acceptor blends. In particular, for a family of high-performance donor polymers with strong temperature-dependent aggregation properties, the device performances are very sensitive to the size of alkyl chains, and the best device performance can only be achieved with an optimized odd-numbered alkyl chain. However, the synthetic route of odd-numbered alkyl chains is costly and complicated, which makes it difficult for large-scale synthesis. Here, this study presents a facile method to optimize the aggregation properties and blend morphology by employing donor polymers with a mixture of two even-numbered, randomly distributed alkyl chains. In a model polymer system, this study suggests that the structural and electronic properties of the random polymers comprising a mixture of 2-octyldodecyl and 2-decyltetradecyl alkyl chains can be systematically tuned by varying the mixing ratio, and a high power conversion efficiency (11.1%) can be achieved. This approach promotes the scalability of donor polymers and thus facilitates the commercialization of PSCs.

The structural and electronic properties of random polymers comprising a mixture of commercially available alkyl chains can be systematically tuned and a power conversion efficiency up to 11.1% can be achieved, which is one of the highest values to date for polymer:fullerene solar cells. These random polymers are easier to scale up compared to that obtained using odd-numbered alkyl chains.

Newly developed benzo[1,2-b:4,5-b′]dithiophene (BDT) block with 3,4-ethylenedioxythiophene (EDOT) side chains is first employed to build efficient photovoltaic copolymers. The resulting copolymers, PBDTEDOT-BT and PBDTEDOTFBT, have a large bandgap more than 1.80 eV, which is attributed to the increased steric hindrance between the BDT and EDOT skeletons. Both copolymers possess the satisfied absorptions, low-lying highest occupied molecular orbital (HOMO) levels and high crystallinity. Using the fluorination strategy, PBDTEDOT-FBT exhibits a wider and stronger absorption and a deeper HOMO level than those of PBDTEDOT-BT. PBDTEDOT-FBT:[6,6]-Phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) blend also shows the higher hole mobility and better surface morphology compared with the PBDTEDOTBT:PC₇₁BM blend. Combination of above advantages, PBDTEDOT-FBT devices exhibit much higher power conversion efficiency (PCE) of 10.11%, with an improved open circuit voltage (V_oc) of 0.86 V, short circuit current densities (J_sc) of 16.01 mA cm⁻², and fill factor (FF) of 72.6%. This work not only provides a newly efficient candidate of BDT donor block modified with EDOT conjugated side chains, but also achieves high-performance large bandgap copolymers for polymer solar cells (PSCs) via the synergistic effect of fluorination and side chain engineering strategies.

Combination of fluorination and side chain engineering strategies, newly developed benzo[1,2-b:4,5-b′]dithiophene block with 3,4-ethylenedioxythiophene side chains is first employed to build the efficient large bandgap copolymers with efficiency of 10.11%.

In article number 1700566, Min Ju Cho, Dong Hoon Choi, and co-workers report a new conjugated widebandgap donor polymer, 3MT-Th, harmonized with an ITIC acceptor to enable the production of a polymer solar cell (PSC) with high efficiency of 9.73% under eco-friendly conditions using a non-halogenated solvent. This PSC also exhibits excellent shelf-life stability in air and good operational stability under continuous light illumination.

In article number 1700722, Aleksandra B. Djurišić, Zhu-Bing He, and co-workers investigate the cesium doped NiO film as highly transparent and conductive HTL for inverted perovskite solar cells. Cs dopant can significantly improve the conductivity of NiO and lower the work function, allowing better charge transfer and band alignment between perovskite and Cs doped NiO. Efficiency over 19% for the Cs doped devices is obtained and 90% of its initial performance is maintained after 80 days.

Organic–inorganic metal halide perovskites (e.g., CH₃NH₃PbI_3−xCl_x) emerge as a promising optoelectronic material. However, the Shockley–Queisser limit for the power conversion efficiency (PCE) of perovskite-based photovoltaic devices is still not reached. Nonradiative recombination pathways may play a significant role and appear as photoluminescence (PL) inactive (or dark) areas on perovskite films. Although these observations are related to the presence of ions/defects, the underlying fundamental physics and detailed microscopic processes, concerning trap/defect status, ion migration, etc., still remain poorly understood. Here correlated wide-field PL microscopy and impedance spectroscopy are utilized on perovskite films to in situ investigate both the spatial and the temporal evolution of these PL inactive areas under external electric fields. The formation of PL inactive domains is attributed to the migration and accumulation of iodide ions under external fields. Hence, we are able to characterize the kinetic processes and determine the drift velocities of these ions. In addition, it is shown that I₂ vapor directly affects the PL quenching of a perovskite film, which provides evidence that the migration/segregation of iodide ions plays an important role in the PL quenching and consequently limits the PCE of organometal halide-based perovskite photovoltaic devices.

Ion migration in organometal halide perovskites is directly observed in real-time using photoluminescence wide-field microscopy. A lateral electrode configuration is used to apply an electric field. Accumulating iodide ions quench the photoluminescence of individual crystalline domains starting from the positive electrode moving to the negative, which corresponds to iodide ions moving the opposite way. Impedance measurements confirm the respective migration.