Chen Weijie

In article number 1703070, Won Suk Shin, Bumjoon J. Kim, and co-workers describe the importance of the design of polymer donor-polymer acceptor pair to achieve complementary light absorption in all-polymer solar cells (all-PSCs). Synthetic donor-acceptor random copolymers with tunable light absorption characteristics were developed, which resulted in high performance all-PSC with a power conversion efficiency of 6.8%.

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%.

Solution-processed colloidal quantum dot (CQD) solar cells harvesting the infrared part of the solar spectrum are especially interesting for future use in semitransparent windows or multilayer solar cells. To improve the device power conversion efficiency (PCE) and stability of the solar cells, surface passivation of the quantum dots is vital in the research of CQD solar cells. Herein, inorganic CsPbI₃ perovskite (CsPbI₃-P) coating on PbS CQDs with a low-temperature, solution-processed approach is reported. The PbS CQD solar cell with CsPbI₃-P coating gives a high PCE of 10.5% and exhibits remarkable stability both under long-term constant illumination and storage under ambient conditions. Detailed characterization and analysis reveal improved passivation of the PbS CQDs with the CsPbI₃-P coating, and the results suggest that the lattice coherence between CsPbI₃-P and PbS results in epitaxial induced growth of the CsPbI₃-P coating. The improved passivation significantly diminishes the sub-bandgap trap-state assisted recombination, leading to improved charge collection and therefore higher photovoltaic performance. This work therefore provides important insight to improve the CQD passivation by coating with an inorganic perovskite ligand for photovoltaics or other optoelectronic applications.

Inorganic CsPbI₃ perovskite is epitaxially grown on the PbS colloidal quantum dot at a low temperature. Increased depletion width, lower trap density, reduced charge recombination, and enhanced built-in electric field within the solar cell are obtained by using the CsPbI₃-perovskited coated PbS colloidal quantum dot as a light absorbing material in the solar cell, resulting in high performance.

Few-layer ultrathin nanosheets and ultrasmall quantum dots of black phosphorus (BP) have attracted increasing research interest due to their fascinating properties including a tunable bandgap, high carrier mobility, and ambipolar conduction ability. These excellent properties together with their unique structures make BP derivatives promising candidates for a wide range of device applications. In this research news, the latest advancements in the synthesis, properties, and applications of BP and its derivatives are highlighted. In particular, the focus is on the use of these rising star materials for emerging solar cells, in terms of both theoretical predictions and experimental investigations. Finally, the authors' personal perspectives on potential future research directions are provided.

Few-layer phosphorus, often called phosphorene, is the most recent addition to the family of 2D nanomaterials. The material has several interesting properties including a tunable bandgap, high carrier mobility, and ambipolar conductivity. This article explores the current state of synthesis of few-layer black phosphorus derivatives and their successful application in various emerging solar cells.

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J_SC) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V_OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC₆₁BM), that achieves a high V_OC (0.9 V) with very low energy losses (E_loss = 0.52 eV) from the energy of absorbed photons, a respectable J_SC (13 mA cm⁻²), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC₆₁BM. These results hold promise that given the appropriate morphology, the J_SC, V_OC, and FF can all be improved, even with very low energetic offsets.

To realize organic photovoltaics with high open-circuit voltages and short-circuit currents, it is necessary to minimize energetic offsets between donor and acceptor semiconductors. This article describes a comprehensive study on charge recombination and generation in a system with very low energetic offsets yet relatively high performance, in order to identify the root cause for the limited fill factor.

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.

Charge carriers in methylammonium lead perovskite solar cell absorbers assume a combined (hyper-) polaronic state combining a classical Fröhlich polaron with a micelle-like arrangement of electric dipoles formed by the methylammonium ions. This “micellion” provides an extra contribution to the dielectric constant which screens defects but maintains mobility of the polaron. This is reported by Doru C. Lupascu and co-workers in article number 1700600. In the image, the structure of the hyperpolaron is shown.

The influence of solvent and processing additives on the pathways and rates of crystalline morphology formation for spin-coated semiconducting PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl]]) thin films is investigated by in situ grazing incidence wide-angle X-ray scattering (GIWAXS) and optical reflectance, to better understand polymer solar cell (PSC) optimization approaches. In situ characterization of PTB7 film formation from chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) solutions, as well as CB solutions with 1% and 3% v/v of the processing additives 1-chloronapthalene (CN), diphenylether (DPE), and 1,8-diiodooctane (DIO), reveals multiple crystallization pathways with: (i) single-solvent systems exhibiting rapid (<3 s) crystallization after a solvent boiling point-dependent film thinning transition, (ii) solvent + additive systems exhibiting different crystallization pathways and crystallite formation times from minutes (CN, DPE) to 1.5 h (DIO). Identifying crystalline intermediates has implications for bulk-heterojunction PSC morphology optimization via optimized spin-casting processes.

An in situ GIWAXS study of the crystalline evolution of PTB7 thin films during spin coating is performed. A divergence in intermediate aggregation states and film formation times determined by solvent additive choice is observed. This work shows how additive effects go beyond simply boiling point and the importance of the additive molecular structure on morphological evolution.

The energy disorder that arises from colloidal quantum dot (CQD) polydispersity limits the open-circuit voltage (V_OC) and efficiency of CQD photovoltaics. This energy broadening is significantly deteriorated today during CQD ligand exchange and film assembly. Here, a new solution-phase ligand exchange that, via judicious incorporation of reactivity-engineered additives, provides improved monodispersity in final CQD films is reported. It has been found that increasing the concentration of the less reactive species prevents CQD fusion and etching. As a result, CQD solar cells with a V_OC of 0.7 V (vs 0.61 V for the control) for CQD films with exciton peak at 1.28 eV and a power conversion efficiency of 10.9% (vs 10.1% for the control) is achieved.

A new solution-phase ligand exchange method for preventing colloidal quantum dots fusion and etching is developed via judicious incorporation of reactivity-engineered additives. This method provides improved monodispersity in final colloidal quantum dot films and thus leads to the significant enhancement of open-circuit voltages in colloidal quantum dot solar cells.

All-inorganic cesium lead halide perovskite is suggested as a promising candidate for perovskite solar cells due to its prominent thermal stability and comparable light absorption ability. Designing textured perovskite films rather than using planar-architectural perovskites can indeed optimize the optical and photoelectrical conversion performance of perovskite photovoltaics. Herein, for the first time, this study demonstrates a rational strategy for fabricating carbon quantum dot (CQD-) sensitized all-inorganic CsPbBr₃ perovskite inverse opal (IO) films via a template-assisted, spin-coating method. CsPbBr₃ IO introduces slow-photon effect from tunable photonic band gaps, displaying novel optical response property visible to naked eyes, while CQD inlaid among the IO frameworks not only broadens the light absorption range but also improves the charge transfer process. Applied in the perovskite solar cells, compared with planar CsPbBr₃, slow-photon effect of CsPbBr₃ IO greatly enhances the light utilization, while CQD effectively facilitates the electron–hole extraction and injection process, prolongs the carrier lifetime, jointly contributing to a double-boosted power conversion efficiency (PCE) of 8.29% and an increased incident photon-to-electron conversion efficiency of up to 76.9%. The present strategy on CsPbBr₃ IO to enhance perovskite PCE can be extended to rationally design other novel optoelectronic devices.

Novel carbon quantum dot (CQD)-sensitized inorganic CsPbBr₃ inverse opal perovskite solar cells are fabricated for the first time. CsPbBr₃ inverse opal induces improved light utilization originating from the slow-photon effect with tunable photonic band gaps, while CQD helps to facilitate the charge transfer process, which jointly contributes to a greatly improved photoelectrical conversion efficiency with outstanding stability.

Solution-grown films of CsPbBr₃ nanocrystals imbedded in Cs₄PbBr₆ are incorporated as the recombination layer in light-emitting diode (LED) structures. The kinetics at high carrier density of pure (extended) CsPbBr₃ and the nanoinclusion composite are measured and analyzed, indicating second-order kinetics in extended and mainly first-order kinetics in the confined CsPbBr₃, respectively. Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr₃ indicates enhanced oscillator strength consistent with earlier published attribution of the sub-nanosecond exciton radiative lifetime in nanoprecipitates of CsPbBr₃ in melt-grown CsBr host crystals and CsPbBr₃ evaporated films.

Photoluminescence and electroluminescence in solution-grown films of CsPbBr₃ nanocrystals imbedded in Cs₄PbBr₆ are studied. Radiative recombination kinetics are second order in bulk CsPbBr₃ and first order in CsPbBr₃ nanoinclusions. Exciton absorption strength and sub-nanosecond lifetime imply enhanced oscillator strength in the confined form. A semiconductor that is dark in bulk lights up as a highly efficient and fast nanocomposite light emitter.

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.

Polymer self-assembly in solution prior to film fabrication makes solution-state structures critical for their solid-state packing and optoelectronic properties. However, unraveling the solution-state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution-state structure and the charge-transport properties in field-effect transistors. Here, for the first time, it is revealed that the thin-film morphology of a conjugated polymer inherits the features of its solution-state supramolecular structures. A “solution-state supramolecular structure control” strategy is proposed to increase the electron mobility of a benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)-based polymer. It is shown that the solution-state structures of the BDOPV-based conjugated polymer can be tuned such that it forms a 1D rod-like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution-state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm² V⁻¹ s⁻¹. This work demonstrates that “solution-state supramolecular structure” control is critical for understanding and optimization of the thin-film morphology and charge-transport properties of conjugated polymers.

A supramolecular self-assembly strategy is used to control the solution-state structure of a conjugated polymer. It is revealed that the thin-film morphology of the conjugated polymer inherits the features of their solution-state supramolecular structures. Through “solution-state supramolecular structure control”, the electron mobility of the polymer is boosted to 3.2 cm² V⁻¹ s⁻¹, nearly doubling the original performance.