Ligang Yuan

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600–950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 10⁵m^–1 cm^–1, low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10^–3 cm² V^–1 s^–1. Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.

A fused tris(thienothiophene)-based electron acceptor with strong near-infrared absorption and high electron mobility is designed, synthesized, and applied in as-cast organic solar cells and as-cast semitransparent organic solar cells, which exhibit efficiencies of 12.0% and 10.3%, respectively.

In this work, sidechain engineering on conjugated fused-ring acceptors for conformation locking is demonstrated as an effective molecular design strategy for high-performance nonfullerene organic solar cells (OSCs). A novel nonfullerene acceptor (ITC6-IC) is designed and developed by introducing long alkyl chains into the terminal electron-donating building blocks. ITC6-IC has achieved definite conformation with a planar structure and better solubility in common organic solvents. The weak electron-donating hexyl upshifts the lowest unoccupied molecular orbital level of ITC6-IC, resulting in a higher V_OC in comparison to the widely used ITIC. The OSCs based on PBDB-T:ITC6-IC reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V. The weaker π–π stacking induced by steric hindrance affords ITC6-IC with enhanced compatibility with polymer donors. The blend film treated with suitable thermal annealing exhibits a fibril crystallization feature with a good bicontinuous network morphology. The results indicate that the molecular design approach of ITC6-IC can be inspirational for future development of nonfullerene acceptors for high efficiency OSCs.

Conformation locking by introducing alkyl chains onto central electron-donating building blocks has been explored on fused-ring electron acceptor for high-performance nonfullerene organic solar cells. PBDB-T:ITC6-IC based devices treated with suitable thermal annealing reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V with a small energy loss.

Organic–inorganic lead halide perovskites have shown great future for application in solar cells owing to their exceptional optical and electronic properties. To achieve high-performance perovskite solar cells, a perovskite light absorbing layer with large grains is desirable in order to minimize grain boundaries and recombination during the operation of the device. Herein, a simple yet efficient approach is developed to synthesize perovskite films consisting of monolithic-like grains with micrometer size through in situ deposition of octadecylamine functionalized single-walled carbon nanotubes (ODA-SWCNTs) onto the surface of the perovskite layer. The ODA-SWCNTs form a capping layer that controls the evaporation rate of organic solvents in the perovskite film during the postthermal treatment. This favorable morphology in turn dramatically enhances the short-circuit current density of the perovskite solar cells and almost completely eliminates the hysteresis. A maximum power conversion efficiency of 16.1% is achieved with an ODA-SWCNT incorporated planar solar cell using (FA_0.83MA_0.17)_0.95Cs_0.05Pb(I_0.83Br_0.17)₃ as light absorber. Furthermore, the perovskite solar cells with ODA-SWCNT demonstrate extraordinary stability with performance retention of 80% after 45 d stability testing under high humidity (60–90%) environment. This work opens up a new avenue for morphology manipulation of perovskite films and enhances the device stability using carbon material.

In situ deposition of a capping layer of octadecylamine functionalized single-walled carbon nanotubes onto the surface of perovskite films generates beneficial effects including improved perovskite grain size, reduced ion migration, and water repellency. Consequently, improved efficiency, stability, and reduced hysteresis of perovskite solar cells (PSCs) are achieved. This work demonstrates the potential of carbon nanotubes in enhancing the performance of PSCs.

To achieve high-performance large-area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal-annealing-free process and thickness-insensitive photovoltaic properties. Here, an n-type self-doping fullerene electrolyte, named PCBB-3N-3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB-3N-3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB-3N-3I ETL also functions as light-harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB-3N-3I ETL-based inverted PSCs with a PTB7-Th:PC₇₁BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB-3N-3I ETL, yielding a PCE of 9.32% for the device with thick PCBB-3N-3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL-based PSCs reported so far. Importantly, the flexible PSCs with PCBB-3N-3I ETL also show robust bending durability that could pave the way for the future development of high-performance flexible solar cells.

An n-type doping fullerene electrolyte (PCBB-3N-3I) with high-content doping groups, resulting in high conductivity and well-matched energy levels, is synthesized. The inverted polymer solar cells with PCBB-3N-3I electron transport layer show a record efficiency in the flexible polymer solar cells with an extremely high bending durability and thickness-insensitive photovoltaic behavior.

Nonfullerene polymer solar cells (PSCs) are fabricated by using one wide bandgap donor PBDB-T and one ultranarrow bandgap acceptor IEICO-4F as the active layers. One medium bandgap donor PTB7-Th is selected as the third component due to the similar highest occupied molecular orbital level compared to that of PBDB-T and their complementary absorption spectra. The champion power conversion efficiency (PCE) of PSCs is increased from 10.25% to 11.62% via incorporating 20 wt% PTB7-Th in donors, with enhanced short-circuit current (J_SC) of 24.14 mA cm⁻² and fill factor (FF) of 65.03%. The 11.62% PCE should be the highest value for ternary nonfullerene PSCs. The main contribution of PTB7-Th can be summarized as the improved photon harvesting and enhanced exciton utilization of PBDB-T due to the efficient energy transfer from PBDB-T to PTB7-Th. Meanwhile, PTB7-Th can also act as a regulator to adjust PBDB-T molecular arrangement for optimizing charge transport, resulting in the enhanced FF of ternary PSCs. This experimental result may provide new insight for developing high-performance ternary nonfullerene PSCs by selecting two well-compatible donors with different bandgap and one ultranarrow bandgap acceptor.

Highly efficient ternary nonfullerene polymer solar cells (PSCs) are fabricated by employing two well-compatible donors with complementary absorption spectra and one ultranarrow bandgap acceptor. The power conversion efficiency and short-circuit current density of ternary PSCs are simultaneously increased to 11.62% and 24.14 mA cm⁻² by incorporating 20 wt% PTB7-Th due to the enhanced photon harvesting and optimized film morphology.

In the past few years, organic–inorganic metal halide ABX₃ perovskites (A = Rb, Cs, methylammonium, formamidinium (FA); B = Pb, Sn; X = Cl, Br, I) have rapidly emerged as promising materials for photovoltaic applications. Tuning the film morphology by various deposition techniques and additives is crucial to achieve solar cells with high performance and long-term stability. In this work, carbon nanoparticles (CNPs) containing functional groups are added to the perovskite precursor solution for fabrication of fluorine-doped tin oxide/TiO₂/perovskite/spiro-OMeTAD/gold devices. With the addition of CNPs, the perovskite films are thermally more stable, contain larger grains, and become more hydrophobic. NMR experiments provide strong evidence that the functional groups of the CNPs interact with FA cations already in the precursor solution. The fabricated solar cells show a power-conversion efficiency of 18% and negligible hysteresis.

Carbon nanoparticles are incorporated in perovskite solar cells as a degradation inhibitor and perovskite crystal size magnifier. Here, carbon nanoparticles are used with functional groups in the perovskite solution, which interact with formamidinium cations resulting in morphology tuning, increased hydrophobicity, and thermal stability.

Solution-processed organic–inorganic lead halide perovskite solar cells (PSCs) are considered as one of the most promising photovoltaic technologies thanks to both high performance and low manufacturing cost. However, a key challenge of this technology is the lack of ambient stability over prolonged solar irradiation under continuous operating conditions. In fact, only a few studies (carried out in inert atmosphere) already approach the industrial standards. Here, it is shown how the introduction of MoS₂ flakes as a hole transport interlayer in inverted planar PSCs results in a power conversion efficiency (PCE) of ≈17%, overcoming the one of the standard reference devices. Furthermore, this approach allows the realization of ultrastable PSCs, stressed in ambient conditions and working at continuous maximum power point. In particular, the photovoltaic performances of the proposed PSCs represent the current state-of-the-art in terms of lifetime, retaining 80% of their initial performance after 568 h of continuous stress test, thus approaching the industrial stability standards. Moreover, it is further demonstrated the feasibility of this approach by fabricating large-area PSCs (0.5 cm² active area) with MoS₂ as the interlayer. These large-area PSCs show improved performance (i.e., PCE = 13.17%) when compared with the standard devices (PCE = 10.64%).

Highly stable and efficient inverted planar perovskite solar cells are fabricated based on a molybdenum disulfide (MoS₂) hole extraction interlayer. The performance improvement is attributed to improved hole extraction, while the enhancement in the long-term stability is attributed to the stabilization of the hole transporting materials/perovskite interface, inhibiting the bulk degradation process of the MAPbI₃ structure itself.

Material instability has greatly prohibited the practical application of organic-inorganic hybrid perovskite solar cells (PSCs). Replacing organic-inorganic hybrid perovskites with inorganic perovskites in PSCs has been a promising resolution, and up to now much progress has been made. In this review, a systematical review of the most recent research and progress on inorganic PSCs is presented.

It was found that stressing the fully roll-to-roll printed photovoltaic modules by sunlight is crucial to achieve high open-circuit voltage and high efficient modules. Here, imaging techniques, such as electroluminescence imaging and lock-in thermography, are used to reveal the active material dependent processing and interface defects in roll-to-roll printing.

In this work, four donor (D)–acceptor (A) copolymers based on benzodithiophene (BDT) and benzothiadiazole (BT) with different alkylthiolated and/or fluorinated side chains are developed for efficient fullerene and nonfullerene polymer solar cells (PSCs). The synergistic effect of sulfuration and fluorination on the optical absorption, energy level, crystallinity, carrier mobility, blend morphology, and photovoltaic performance is investigated systematically. By incorporating sulfur atoms onto the side chains, a little blueshifted but significantly increased absorption can be obtained for PBDTS-FBT compared to PBDT-FBT. On the other side, a little more blueshifted but much stronger absorption and much lower-lying highest occupied molecular orbital (HOMO) level can be realized for PBDTF-FBT when introducing fluorine atoms instead of sulfur atoms. With the combination of both fluorination and sulfuration strategies, PBDTS-FBT exhibits the best absorption ability, lowest HOMO energy level, and highest crystallinity, which make PBDTSF-FBT devices show the highest power conversion efficiency (PCE) of 10.69% in fullerene PSCs and 11.66% in nonfullerene PSCs. The PCE of 11.66% is the best value for PSCs based on BT-containing copolymer donors reported so far. The results indicate that fluorination and sulfuration have a synergistically positive effect on the performance of D–A photovoltaic copolymers and their solar cell devices.

With the combination of fluorination and sulfuration strategies, new benzodithiophene (BDT)–benzothiadiazole (BT) copolymer donors are developed for improving the optical absorption, energy level, carrier mobility, and blend morphology. The fabricated fullerene and nonfullerene polymer solar cells exhibit high power conversion efficiencies of 10.69% and 11.66%.

Organolead trihalide perovskite MAPbI₃ shows a distinctive combination of properties such as being ferroelectric and semiconducting, with ion migration effects under poling by electric fields. The combination of its ferroelectric and semiconducting nature is used to make a light harvesting, self-powered tactile sensor. This sensor interfaces ZnO nanosheets as a pressure-sensitive drain on the MAPbI₃ film and once poled is operational for at least 72 h with just light illumination. The sensor is monolithic in structure, has linear response till 76 kPa, and is able to operate continuously as the energy harvesting mechanism is decoupled from its pressure sensing mechanism. It has a sensitivity of 0.57 kPa⁻¹, which can be modulated by the strength of the poling field. The understanding of these effects in perovskite materials and their application in power source free devices are of significance to a wide array of fields where these materials are being researched and applied.

The ferroelectric properties of MAPbI₃ films are coupled with a dynamic drain of ZnO nanosheets to make a self-powered tactile sensor that is operational with just light illumination for at least 72 h. The device is developed by decoupling the ion migration and ferroelectric effects in these films based on poling conditions.

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next-generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple-donor or multiple-acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple-donor or multiple-acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

This progress report provides an overview of the most impactful recent progress in high-performance organic solar cells based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components, focusing particularly on the interactions between different components from the perspective of morphology and photophysics.

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂Br absorber and polythiophene hole-acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (V_OC) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss.

The interfacial properties between CsPbI₂Br absorber and poly(3-hexylthiophene) (P3HT) hole-acceptor are improved by passivating the surface defects of CsPbI₂Br and reducing the energy disorder of P3HT. Consequently, a stable inorganic perovskite solar cell with high power conversion efficiency of 12.02% and minimal energy loss of 0.50 eV is obtained.

In the search for high efficiency organic solar cells, additives often play an important role in improving the film morphology. The vast majority of the currently used additives are liquids that, while often effective, evaporate or migrate over time lowering the stability of the device. Herein, Liu et al. (article No. 1700208) report a solid photoactive molecular mediator, namely N(BAI)₃, that could be employed to replace the liquid additives to tune the morphology of bulk heterojunction films for improved device performance. The N(BAI)₃ mediator not only resides in the active films to fine tune the phase morphology, but also contributes to the additional absorption of the active films, leading to ∼ 11% enhancement of power conversion efficiency of P3HT:PC₆₀BM devices. In-depth studies on the nanoscale morphologies using X-ray and neutron scattering techniques suggest that the use of 1 wt% N(BAI)₃ effectively tunes the packing of P3HT, presumably through balanced Π-interactions endowed by its large conjugated Π surface, and promote the formation of a PC₆₀BM-rich top interfacial layer. These findings open up a new way to tailor the phase morphology by photoactive molecular mediators in organic photovoltaics.

A new and low-cost hole selective material, 2,6,14-tris(5’-(N,N-bis(4-methoxyphenyl) aminophenol-4-yl)-3,4-ethylenedioxythiophen-2-yl)-triptycene, i.e. TET, for efficient perovskite solar cells (PVSCs) has been synthesized and demonstrated in article No. 1700175. As a 2,2’,7,7’-Tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’- spirobifluorene (spiro-OMeTAD) derivative, TET inherits excellent optoelectronic properties and overcomes the major shortfalls of spiro-OMeTAD. Planar PVSCs with TET hole selective layers (HSLs) have achieved a maximum power conversion efficiency (PCE) of 19.1% under reverse voltage scan and steady-state efficiency of 18.6%, comparable with those (19.5% PCE and 19.0% steady-state efficiency) of PVSCs using spiro-OMeTAD HSLs. Importantly, these efficient PVSCs used very thin TET HSLs (about 30 nm) without the need of oxygen doping. Considering the lower lab synthesis and purification cost ($123/g vs $500/g) and thinner HSL (30 nm vs 200 nm), the cost for TET on a unit area of one device is significantly lower (25 times) than that for high-purity spiro-OMeTAD.

Significant development has been achieved in nonfullerene organic solar cells. However, most of the high-efficiency nonfullerene systems are composed of polymer donors and fused-ring acceptors, and only a few small molecule donors can work well. Herein, a new A–D–A small molecule donor named NDTSR with naphtho[1,2-b:5,6-b′]dithiophene (NDT) as building blocks is synthesized. Two energy levels well-matched fused-ring acceptors ITIC and IDIC are chosen to construct all-small-molecule solar cells with NDTSR, respectively. When mixed with IDIC, a high power conversion efficiency (PCE) of 8.05% is achieved, which is the highest efficiency for NDT-based small molecule donor. However, the NDTSR:ITIC system only exhibits a low PCE of 1.77%. The big difference in the performance of these two systems should be attributed to the different morphology and phase separation resulting from the crystallinity and aggregation ability of the acceptors. The results demonstrate that NDT-based small molecule is a promising candidate donor for all-small-molecule systems, while the crystallinity of fused-ring acceptors is a critical factor for optimizing the phase separation in the active layer.

An all-small-molecule nonfullerene solar cell is constructed with a novel small molecule NDTSR as donor, and ITIC and IDIC as acceptor, respectively. Through enhancing the crystallinity of acceptors, a high power conversion efficiency of 8.05% is obtained, which indicates that the crystallinity of the acceptor is a key factor for the performance of all-small-molecule solar cells.

Polymer aggregation plays a critical role in the miscibility of materials and the performance of all-polymer solar cells (APSCs). However, many aspects of how polymer texturing and aggregation affect photoactive blend film microstructure and photovoltaic performance are poorly understood. Here the effects of aggregation in donor–acceptor blends are studied, in which the number-average molecular weights (M_ns) of both an amorphous donor polymer, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PBDTT-FTTE) and a semicrystalline acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) are systematically varied. The photovoltaic performance is correlated with active layer microstructural and optoelectronic data acquired by in-depth transmission electron microscopy, grazing incidence wide-angle X-ray scattering, thermal analysis, and optical spectroscopic measurements. Coarse-grained modeling provides insight into the effects of polymer aggregation on the blend morphology. Notably, the computed average distance between the donor and the acceptor polymers correlates well with solar cell photovoltaic metrics such as short-circuit current density (J_sc) and represents a useful index for understanding/predicting active layer blend material intermixing trends. Importantly, these results demonstrate that for polymers with different texturing tendencies (amorphous/semicrystalline), the key for optimal APSC performance, photovoltaic blend morphology can be controlled via both donor and acceptor polymer aggregation.

The templating effects in morphology engineering by regulating aggregation are clearly demonstrated for the first time in all-polymer solar cells, where the morphology may be templated by the amorphous phase in one blend and by the semicrystalline phase in another, all dictated by the degree of polymer aggregation.

In article number 1701659, Jinsong Huang, Jiarong Lian, and co-workers propose a simple hot-substrate deposition method to prepare a thin film with higher coverage and improved uniformity. The hot substrate improves the adhesion of the solvent on the substrate and speeds its drying process to avoid the aggregation of the upmost molecules, so that both reduced current leakage and series resistance are simultaneously realized in perovskite solar cells.