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Enhancing exciton generation, promoting exciton dissociation, and facilitating charge transport and collection within the photoactive layer is crucial for improving the performance of organic photovoltaics (OPVs). In this study, we achieved an optimal vertical distribution of the insulating polymer polymethyl methacrylate (PMMA) in the active layer through sequential deposition. This appropriate distribution of PMMA inhibited the aggregation of the acceptor, and reduced the donor-acceptor phase separation, increased exciton generation, promoted exciton dissociation, and inhibited bimolecular recombination. OPVs based on PM6/PMMA/L8-BO achieved a maximum efficiency of 18.1%, surpassing that of devices without PMMA. By tuning the vertical segregation of multi-components and molecular packing behaviors, this study provides an efficient method for achieving high performance by synergistically enhancing exciton generation, promoting exciton dissociation, inhibiting bimolecular recombination, and facilitating charge collection through PMMA. Polymethyl methacrylate (PMMA) was optimally distributed vertically in the active layer through sequential deposition. With a maximum efficiency of 18.1%, OPVs based on PM6/PMMA/L8-BO outperformed PMMA-free devices.

Semitransparent organic photovoltaics (STOPVs) have gained wide attention owing to their promising applications in building-integrated photovoltaics, agrivoltaics, and floating photovoltaics. Organic semiconductors with high charge carrier mobility usually have planar and conjugated structures, thereby showing strong absorption in visible region. In this work, a new concept of incorporating transparent inorganic semiconductors is proposed for high-performance STOPVs. Copper(I) thiocyanate (CuSCN) is a visible-transparent inorganic semiconductor with an ionization potential of 5.45 eV and high hole mobility. The transparency of CuSCN benefits high average visible transmittance (AVT) of STOPVs. The energy levels of CuSCN as donor match those of near-infrared small molecule acceptor BTP-eC9, and the formed heterojunction exhibits an ability of exciton dissociation. High mobility of CuSCN contributes to a more favorable charge transport channel and suppresses charge recombination. The control STOPVs based on PM6/BTP-eC9 exhibit an AVT of 19.0% with a power conversion efficiency (PCE) of 12.7%. Partial replacement of PM6 with CuSCN leads to a 63% increase in transmittance, resulting in a higher AVT of 30.9% and a comparable PCE of 10.8%. © 2024

Low cost and printing friendly fabrication of organic solar cells (OSCs) require thick-film devices with simply structured photoactive molecules. Thus, achieving high power conversion efficiency (PCE) for non-fused ring acceptor-based devices with high thickness is of great significance. Herein, by transforming traditional blend casting method to emerging sequential deposition (SD) method, D18:A4T-16 active blend exhibits large efficiency improvement from 8.02% to 14.75% in 300 nm thick devices. Systematic morphological and photophysical characterizations showcase the effectiveness of SD processing in achieving sufficient donor/acceptor interpenetration and vertical stratification, which eliminates the dilemma of charge generation/transport in blend casting films. Meanwhile, D18 bottom layer is proven helpful in realizing fast evaporation of postdeposited poor solvent, resulting in naturally thickened active layer with well-regulated crystallization. Furthermore, a new index to emphasize thick-film devices based on nonfused ring acceptors, called figure-of-merit-X (FoM-X), has been defined. The SD processed D18:A4T-16 devices herein, with 300 nm, 500 nm, and 800 nm thicknesses possess leading FoM-X values. By sequential deposition of using poor solvent to treat bottom polymer donor layer, an outstanding figure-of-merit-X (low-cost and thick-film) organic solar cell is realized. The underlying mechanism is carefully characterized as a interpenetrating and vertical stratified donor-acceptor structure, where donor layer's thickness is the key factor. image

Conjugated polymers (CPs) with donor and acceptor units exhibit a flexible conformation due to the rotation of the single bond of the bridge groups which greatly hinders a precise study of their photodetection properties. Moreover, the modulation mechanism related to complex factors of CPs molecular conformation on photodetection performance is not clear, causing CP-based organic photodiodes (OPDs) to generally suffer from high dark current and low specific detectivity. In this work, we synthesized two novel PBQx-i-4F and PBQx-o-4F CPs by combining fluorinated BDT with DTQx units using i- and o-alkylated side chain-based thiophene linkages, respectively. Quantum simulation and photo-physics study revealed that PBQx-i-4F displayed a dominant trans conformation, a more planar molecule structure, and high utilization efficiency of photoinduced excitons. The OPDs based on PBQx-i-4F exhibited higher on–off ratios by over two orders of magnitude and higher detectivity (Dsh*) by nearly one order of magnitude compared with those based on PBQx-o-4F. PBQx-i-4F with the i-alkyl substitution on the thiophene linkage displayed enhanced planarity, resulting in more ordered packing and larger population of face-on orientation in film, where decreased trap density and trap-assisted recombination led to reduced dark current and improved charge transport. In summary, our work provides an insight into the relationship between the fine-tuning of the molecular conformation, planarity, and packing of CPs and their photodetection properties in OPDs. © 2024 Elsevier B.V.

Additive engineering has been regarded as a valuable approach for enhancing the power conversion efficiencies (PCEs) of organic solar cells (OSCs). However, the effective solid additive molecules, especially electron-deficient building blocks, that enhance the PCEs of OSCs are still limited. Herein, two cyano-functionalized highly electron-deficient building blocks, namely 3,6-dibromopyrazine-2-carbonitrile (CNPz) and 3,6-dibromopyrazine-2,5-dicarbonitrile (DCNPz), which feature simple structures and facile synthesis are employed as solid additives to optimize the performance of OSCs. It is found that CNPz can modulate the intermolecular interactions and improve the molecular packing, which is beneficial for charge generation, transport, and collection. As a result, a 19.67% PCE is achieved in the PTQ10/m-BTP-PhC6 binary devices, ranking amongst the highest values for OSCs. Furthermore, the general applicability of solid additives is demonstrated in other organic photovoltaic systems. These findings offer valuable insights into designing and developing novel electron-deficient solid additives to boost the efficiency of OSCs. © 2024 The Royal Society of Chemistry.

Organic solar cell (OSC) development continues to demonstrate impressive device efficiency improvements. However, the materials synthetic simplicity essential to industrialization remains seriously lacking attention, imparting inferior performance records in low-cost devices. Hence, low bandgap and completely non-fused electron acceptors (CNFEAs) having simple molecular structures are investigated herein. In contrast to typically explored fused-ring acceptors with smaller backbone conformational variations, minimizing the interface recombination sites through a greater extent of localized domains is identified as more critical in CNFEAs, leading to remarkable fill factors (FFs) approaching 75%, among the highest currently realized for low-cost systems. However, this comes with diminishing charge generation efficiency. The general ternary blend optimization strategy modifying the morphology of host components is limited in preserving such remarkably high FFs. To suppress the trade-off while keeping notable FFs, a new perspective of constructing functionally bridged components based on optical, electronic, and thermodynamic properties is introduced here. Specifically, charge generation is unrestrained from the host acceptor localized domains through the introduction of a "bridge" component while also taking advantage of the configuration to channel polarons toward the efficient transport moieties of the host components. Accordingly, this work incubates understanding-guided optimizations toward the advancement of more practical devices. Organic solar cell technology continues to display breakthrough milestones, yet the development of low-cost photoactive materials remains to prevent marketplace realization. Herein, the strategy of constructing functional sites allowing the more selective optimization for exciton dissociation and free charge transport is illustrated to be a tool in advancing the performance of low-cost materials with fully non-fused thiophene rings.imag

Though the application-promising photovoltaic technology named organic solar cells (OSCs) have been close to 20% benchmark power conversion efficiency (PCE) within fabrication friendly single-junction devices, these achievements are enabled by polymer donors based on benzodithiophene cores, requiring toxic production steps. Whilst, the bio-renewable benzo[1,2-b:4,5-b′]difuran unit constructed polymer donors cannot yield comparable efficiency, though their lower steric hindrance is widely appreciated. OSC field has paid great attention on optimizing their performance by chemistry design, yet the device engineering is relatively neglected compared to what have been done on the benzodithiophene side. Here we report a new dual additive strategy of simultaneously applying volatile (2-CN) and non-volatile (MF) solid additives to reduce non-radiative voltage loss and boost charge generation, via an occupying evaporated left vacancies in polymer matrix process. Consequently, the target system D18-Fu:L8-BO’s efficiency is promoted to 19.11%, representing the cutting-edge level of this research topic.

Herein, two emerging device optimization methods, solid additive and layer-by-layer (LBL) process, for organic solar cells (OSCs) are simultaneously studied. Through traditional blend cast and recently proposed identical solvent LBL cast, BDCB (2-monobromo-1,3-dichloro-bezene), a benzene derivative, is used to improve the device performance based on celebrity combination PM6:L8-BO. The results reveal that finely optimized BDCB concentration in PM6 solution can push the efficiency of LBL to 19.03% compared to blend cast with only 18.12% while the power conversion efficiency (PCE) changing trend is determined by BDCB's ratio in L8-BO's precursor. The morphology characterizations confirm there exists no significant stratification for LBL-processed devices, supported by a previously reported swelling-intercalation-phase separation (SIPS) model. Thereby, the solid additive's 2D optimization is considered a smart strategy for finely tuning the SIPS process, which results in various final morphology states. This work not only reports a cutting-edge efficiency for binary OSCs, but also new insight and deep understanding for LBL method-based morphology optimization strategy development.

Efficient exciton-to-charge generation from the introduction of non-fullerene acceptors (NFAs) has been an important breakthrough in organic solar cell (OSC) developments. However, low device fill factors (FFs) following significant free charge carrier recombination loss continue to undermine their marketplace potential. Previous studies have successfully uncovered the importance of donor and acceptor domains in promoting charge transport. However, the functionality of donor/acceptor (D/A) interfaces relevant to free charge recombination remains unclear despite such interfaces being present throughout the material. In this work, the disorder-induced uphill bulk-to-D/A interface transport energy landscape is unveiled to enhance the polaron recombination resistance thereby also mitigating the triplet state formation from back charge transfer. In simple words, increasing the interface disorder while keeping the purer domains highly ordered will impart greater bulk-to-interface differential energy which then serves as a recombination energy barrier. Herein, these are made possible by varying the NFA outer side chains from linear alkyls to bulkier 2D phenylalkyls which influence the donor–acceptor interaction and define the interfacial disorder. Meanwhile, the extent of interface disorder without tradeoff in geminate losses is dependent on electrostatics and nanomorphology driving efficient exciton dissociation. By understanding these interplays, remarkable FFs over 80% and power conversion efficiencies (PCEs) above 18% are revealed to remain accessible even with simple binary component device fabrication without additional components/treatments thereby maintaining the scalability. Consequently, the principles uncovered here will serve as the foundation to reach the optimum potential of binary and simple systems, a prerequisite to ultimately realize the most cost-effective OSC design strategies for practical applications.

Currently, nearly all high-efficiency organic photovoltaic devices use donor polymers based on the benzo-dithiophene (BDT) unit. To diversify the choices of building blocks for high-performance donor polymers, the use of benzo-difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF-based devices lagged behind those of BDT-based devices. In this study, a high efficiency (18.4%) is achieved using a BDF-based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18-Fu) and the aid of a solid additive (2-chloronaphthalene), which is the isomer of the commonly used additive 1-chloronaphthalene. These results revealed the significant effect of 2-chloronaphthalene in optimizing the morphology and enhancing the device parameters. This work not only provides a new building block that can achieve an efficiency comparable to dominant BDT units but also proposes a new solid additive that can replace the widely used 1-chloronaphthalene additive.

Using two structurally similar polymer acceptors in constructing high-efficiency ternary all-polymer solar cells is a widely acknowledged strategy; however, the focus thus far has not been on how polymer acceptor(s) would tune the aggregation of polymer donors, and furthermore film morphology and device performance (efficiency and stability). Herein, it is reported that matching of the celebrity acceptor PY-IT and the donor PBQx-TCl results in enhanced H-aggregation in PBQx-TCl, which can be finely tuned by controlling the amount of the second acceptor PY-IV. Consequently, the efficiency-optimized PY-IV weight ratio (0.2/1.2) leads to a state-of-the-art power conversion efficiency of 18.81%, wherein light-illuminated operational stability is also enhanced along with well-protected thermal stability. Such enhancements in the efficiency and operational and thermal stabilities of solar cells can be attributed to morphology optimization and the desired glass transition temperature of the target active layer based on comprehensive characterization. In addition to being a high-power conversion efficiency case for all-polymer solar cells, these enhancements are also a successful attempt for using combined acceptors to tune donor aggregation toward optimal morphology, which provides a theoretical basis for the construction of other types of organic photovoltaics beyond all-polymer solar cells. © 2024 Wiley-VCH GmbH.

Organic solar cells (OSCs) have demonstrated over 19% power conversion efficiency (PCE) with the help of material innovation and device optimization. Co-working with newly designed materials, traditional solvent additives, 1-chloronaphthalene (CN), and 1,8-diodooctane (DIO) are still powerful in morphology modulation towards satisfying efficiencies. Here, we chose recently reported high-performance polymer donors (PM6 & D18-Fu) and small molecular acceptors (Y6 & L8-BO) as active layer materials and processed them by different conditions (CN or DIO or none). Based on corresponding 12 groups of device results, and their film morphology characterizations (both ex-situ and in-situ ones), the property-performance relationships are revealed case by case. It is thereby supposed to be taken as a successful attempt to demonstrate the importance and complexity of donor-acceptor-additive interaction, since the device performance and physics analyses are also tightly combined with morphology variation. Furthermore, ternary blend construction for PCE improvement provides an approaching 19% level and showcases the potential of understanding-guided-optimization (UGO) in the future of OSCs. © 2023 The Authors. Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd.

From the past years, the most commonly reported state-of-the-art binary bulk heterojunction organic solar cells (OSCs) are mostly based on mixtures of polymer donors and fullerene-free acceptors (polymer:NFA). However, along with it are a number of contradictory propositions, including (but not limited to) strategies to reduce energy loss and improve photocurrent generation through energy level alignments. Due to the resulting high similarity of molecular fragments from polymer:NFA heterojunctions, the effects of vertical molecular stratification are not yet well studied. Herein, the time-of-flight secondary ion mass spectrometry (ToF-SIMS) molecular depth profiling reveals a vertical stratification in PM6:IT-4Cl and illustrates how it can significantly influence the photovoltaic properties. The said inhomogeneity is also bound to introduce microstructure variations within device active layers. Consequently, it is systematically demonstrated how thin-film microstructures can influence optoelectronic properties, wherein important metrics (e.g., energy losses and molecular energy offsets) are highly dependent. Thus, the understanding from this work provides foundations for more precise development of strategies to further advance OSC technology in future studies.

Energy losses significantly reduce the open-circuit voltage among current state-of-the-art organic solar cells (OSCs), which limits the further enhancement of their power conversion efficiencies (PCEs). In this study, the bulk heterojunction blends of PM6 donor and halogenated nonfullerene acceptors (NFAs) display a trade-off between radiative energy losses, i.e., charge transfer state (CTS) radiative energy loss (ΔErad) and the loss associated with CTS formation from acceptor singlet excitons (ΔECTEL). Similarly, a trade-off between ΔErad and the nonradiative energy loss (ΔEnr) is found, reflecting a competition in radiative and nonradiative charge recombination pathways. Further, the energy levels of relaxed CTS (ECTEL) are shown to exhibit dependence on morphologically induced energetic traps, suggesting that it should not be associated merely to blend constituents. Interestingly, these correlations extend even to thermally degraded devices considered herein. Accordingly, this work provides further understandings of energy losses relevant to overcome the current limitations concerning NFA-based OSC developments.

Advancing non-fullerene acceptor (NFA) organic photovoltaics requires the mitigation of the efficiency-limiting processes. Acceptor end-group and side-chain engineering are two handles to tune properties, and a better understanding of their specific impact on the photophysics could facilitate a more guided acceptor design. Here, the device performance, energetic landscape, and photophysics of rhodanine and dicyanovinyl end-capped IDT-based NFAs, namely, O-IDTBR and O-IDTBCN, in PCE10-based solar cells are compared by transient optical and electro-optical spectroscopy techniques and density functional theory calculations. It is revealed how the acceptors’ quadrupole moments affect the interfacial energetic landscape, in turn causing differences in exciton quenching, charge dissociation efficiencies, and geminate versus non-geminate recombination losses. More precisely, it is found that the open circuit voltage (VOC) is controlled by the acceptors’ electron affinity (EA), while geminate and non-geminate recombination, and the field dependence of charge generation, rely on the acceptors’ quadrupole moments. The kinetic parameters and yields of all processes are determined, and it is demonstrated that they can reproduce the performance differences of the devices’ current–voltage characteristics in carrier drift-diffusion simulations. The results provide insight into the impact of the energetic landscape, specifically the role of the quadrupole moment of the acceptor, beyond trivial considerations of the donor–acceptor energy offsets.