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Summary The field of all-polymer solar cells (all-PSCs) has experienced rapid development during the past few years, mainly driven by the design of efficient polymer acceptors (PAs). However, the polymer/polymer blend systems still lag far behind polymer/small molecule acceptor counterparts in power conversion efficiencies (PCEs). Here, we designed a near-infrared PA PY2F-T and paired it with polymer donor PM6 to fabricate all-PSCs with 15.0% PCE. Afterwards, PYT as the third component was introduced into the PM6:PY2F-T host system. Because of the complementary absorption bands and finely tuned microstructures of the ternary blend, the PCE is improved up to 17.2%, with the external quantum efficiency over 80% in visible and near-infrared spectral regions. Impressively, the ternary blend exhibited less energy loss, better light-soaking and photo-thermal stabilities than did the corresponding binary systems. This work promotes the development of high-performance ternary all-polymer systems and heralds a brighter future for accelerating the possible applications of all-PSCs.

Summary In spite of the great success in device efficiency resulting from the excessive design of photovoltaic materials, the stability and cost issues concerning basic commercial requirements of organic solar cells (OSCs) remain unresolved or controversial, slowing down the introduction of market-ready applications. We proposed an exciting discovery that as the active layer thickness decreases, the thermal stability increases gradually, even exceeding 100% of the initial efficiency under 150°C thermal stress. This trend was further confirmed by investigating thermodynamics and kinetics of morphology evolution for PM6:Y6 as a reference example. Extended research found that there is a similar correlation between photo-thermal stability and blend thickness. Consequently, a new reliable industrial figure of merit (i-FOM2.0), including four main factors: efficiency, photo-thermal stability, synthesis complexity, and active layer thickness, is presented here. Our work provides a promising trade-off strategy for reducing efficiency-stability-cost gap and accelerates the commercialization of OSCs.

Summary Recent advances in the development of polymer acceptors and the investigation of molecular mass have boosted the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) to approximately 11%. Here, a fused-aromatic-ring-constructed polymer acceptor PYT (Poly[(2,2′-((2Z,2′Z)-((12,13-bis(2-octyldodecyl)-3,9-diundecyl-12,13-dihydro[1,2,5]thiadiazolo[3,4e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]-indole-2,10-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile-alt-2,5-thiophene)]) is reported, while a series of PYT polymers with different molecular masses (designated as PYTL, PYTM, and PYTH) are prepared to fine-tune the molecular crystallinity and miscibility. Benefiting from the advantages of PYT series, which possess broad absorption with a narrow band of 1.40–1.44 eV and high absorption coefficients of over 1.00 × 105 cm−1, we investigated the blend miscibility and device performance of all-PSCs based on a wide-band-gap polymer donor, PM6. The PYTM-based all-PSCs exhibit an excellent PCE of 13.44%, outperforming those with PYTL (12.55%) and PYTH (8.61%). Our results provide insight into polymer acceptor backbone and molecular mass and suggest guidelines to rationally select polymers for all-PSCs.

Summary To date, organic solar cells (OSCs) with the development of photovoltaic materials have realized high power conversion efficiencies (PCEs) through the solution processing strategy with bulk heterojunction (BHJ) structure, but the BHJ morphology is difficult to control in large-scale fabrication of OSCs. Herein, we report an alternative film-forming technology known as layer-by-layer (LbL). As compared to its BHJ counterpart, LbL presents many unique advantages including controllable “p-i-n” morphology, good charge transport and extraction properties, and great universality. By using the LbL-bladed coating strategy, a high PCE of 16.35% was achieved in the PM6:Y6 OSCs. Notably, a large-area solar module of 11.52 cm2 with a geometrical fill factor of over 90% exhibited an outstanding PCE of 11.86%, which represents the highest efficiency of large-area solar modules. The results may pave the way for the fabrication of the photoactive layer in the future industrial production of OSCs.

The realization of high open-circuit voltage (Voc) in organic solar cells (OSCs) mainly depends on the delicate donor (D) and acceptor (A) structures, meticulously optimized bulk heterojunction (BHJ) microstructure and functionalized interfacial materials. In this work, we demonstrate a spontaneous Voc gain in efficient OSCs without sacrificing the short-circuit current (Jsc) and fill factor (FF). Using a combination of device studies and morphological analysis, we point out that the spontaneous phenomenon occurred at the bottom of the BHJ layer, which is caused by [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acceptors spontaneously migrating to the BHJ/ZnO interface under ambient conditions. Furthermore, physical characterization determines the relation between morphology evolution and spontaneous Voc enhancement, which mainly results from the eliminated energetic disorder at the BHJ/ZnO interface. The investigation of surface energy parameters among selected components highlights the wetting coefficient as a dominant dynamic for this spontaneous phenomenon. Besides, a wide range of photovoltaic systems introduced for enabling parallel verification also confirm the effect of the interfacial surface energy on Voc spontaneous enhancement in inverted solar cells. These findings exemplify the importance of surface energy modification as a tool for improved interfaces of layered morphology, and open new routes to device interfacial optimization using novel design strategies of photoactive materials and interfacial materials. Such a spontaneous phenomenon may open a novel field for making materials work in an intelligent way in organic electronics.

The thermal stability of organic solar cells is critical for practical applications of this emerging technology. Thus, effective approaches and strategies need to be found to alleviate their inherent thermal instability. Here, we show a polymer acceptor-doping general strategy and report a thermally stable bulk heterojunction photovoltaic system, which exhibits an improved power conversion efficiency of 15.10%. Supported by statistical analyses of device degradation data, and morphological characteristics and physical mechanisms study, this polymer-doping blend shows a longer lifetime, nearly keeping its efficiency (t = 800 h) under accelerated aging tests at 150 oC. Further analysis of the degradation behaviors indicates a bright future of this system in outer space applications. Notably, the use of polymer acceptor as a dual function additive in the other four photovoltaic systems was also confirmed, demonstrating the good generality of this polymer-doping strategy., Thermal instability is a critical bottleneck for bulk heterojunction organic solar cells. Here Yang et al. use barely 1 wt% of a polymer acceptor as an additive to simultaneously improve the device efficiency and thermal stability of several state-of-the-art organic photovoltaic systems at high temperatures.

A major breakthrough in organic solar cells (OSCs) in the last thirty years was the development of the bulk heterojunction (BHJ) solution processing strategy, which effectively provided a nanoscale phase-separated morphology, aiding in the separation of Coulombically bound excitons and facilitating charge transport and extraction. Compared with the application of the layer-by-layer (LbL) approach proposed in the same period, the BHJ spin-coating technology shows overwhelming advantages for evaluating the performance of photovoltaic materials and achieving more-efficient photoelectric conversion. Thus, in this study, we have further compared the BHJ and LbL processing strategies via the doctor-blade coating technology because it is a roll-to-roll compatible high-throughput thin film fabrication route. We systematically evaluated multiple target parameters, including morphological characteristics, optical simulation, physical kinetics, device efficiency, and blend stability issues. It is worth emphasizing that our findings disprove the old stereotypes such as the BHJ processing method is superior to the LbL technology for the preparation of high-performance OSCs and the LbL approach requires an orthogonal solvent and donor/acceptor materials with special solubility. Our studies demonstrate that the LbL blade-coating approach is a promising strategy to effectively reduce the efficiency-stability gap of OSCs and even a superior alternative to the BHJ method in commercial applications.

In this work we introduced and constructed two different ternary solar cells based on J101:MeIC host films separately blended with two distinct near-IR non-fullerene acceptors, F-IXIC and Cl-IXIC. The J101:MeIC host devices with a PCE of 11.83% were improved to 14.08% by loading 40 wt% F-IXIC, but reduced to 9.32% by loading Cl-IXIC with the same content. Notably, the J101:F-IXIC and J101:Cl-IXIC binary devices showed the comparable PCEs of 9.44% and 9.45%, respectively. Importantly, the large difference of fill factor values (70.19% for F-IXIC-based device and 50.24% for Cl-IXIC-based device) drives us to carefully analyze the working mechanisms and morphological characteristics of these two ternary systems via the physical and morphological measurements and the surface energy calculations. The investigated results suggest that the interfacial surface energy parameters of the third component and host materials should be rationally and comprehensively considered in the design of ternary systems. More importantly, combining with the investigations of the photo- and thermal-stability issues of the corresponding ternary systems, we further found that localization driving force of sensitizers partially explained in terms of surface energy can rationally control the efficiency-stability gap of ternary solar cells.