Advancing Organic Photovoltaics
Year: 2025
Type: article
Abstract: Organic photovoltaics (OPV) are envisioned as a key technology in the worldwide transition toward a decarbonized and sustainable supply of energy in the coming years. Net-zero carbon footprint energy technologies are imperative, and further scientific advances are urgently demanded to meet the greenhouse gas emission targets set by scientists and policymakers to mitigate the irreversible effects of climate change on Earth in time. OPV is well-positioned as a sustainable and environmentally-friendly energy technology, where functional devices can be realized entirely by low-cost and high-throughput printing methods thanks to the inherent solubility of organic semiconductors incorporating side chains. Over the past 25 years, the record power conversion efficiency (PCE) of OPV devices skyrocketed from ca. 3 % up to >20 %, yet further advances are needed to meet the demands of OPV up-scaling in terms of material development, environmentally-friendly processing, performance, and stability merits. These are precisely covered in the present special issue. The issue mostly gathers contributions submitted to the Symposium B: Advancing sustainable organic photovoltaics: from experiments and materials to applications and device models included in the program of the European Materials Research Society (E-MRS) Spring Meeting 2024 held in Strasbourg, France, and co-organized by Dr. Xabier Rodríguez-Martínez, Prof. Hongzheng Chen, Dr. Vida Turkovic and Prof. Feng Gao, who also serve as guest editors. The issue features 2 review articles, 1 perspective, and 25 research articles, with leading authors settled in renowned institutions worldwide (Figure 1). Hot topics covered in this issue include (i) the development of novel light harvesting materials for high performance and large area applications; (ii) the introduction of new insights on device physics, photophysics, simulations and machine learning approaches applied to OPV; (iii) the improved understanding toward the sustainable processing of OPV devices and their enhanced stability merits; and (iv) their integration into buildings and flexible forms. In regard to polymers, the review article signed by Dr. Bin Huang et al. (article number 202405451) critically analyzes the latest advances in the ternary polymerization strategy for OPV materials, namely, the synthesis of donor and acceptor copolymers including three types of building blocks in their conjugated backbone. Given the record PCEs attained in rigid (20.0 %) and flexible (18.0 %) form factors incorporating these novel copolymers, they are promising candidates to further boost the efficiency of OPVs. Inspired by luminescent polymers for OLEDs, the group of Prof. Yanming Sun (article number 202403747) presents a new polymer acceptor that includes a luminescent building block into its backbone. The incorporation of said unit is observed to comparatively enhance the electroluminescence external quantum efficiency and reduce the non-radiative energy losses of the corresponding all-polymer devices, resulting in a record PCE of 17.55 %. Multiple advances are presented in the issue in line with the development and benchmarking of asymmetric non-fullerene acceptors (NFAs). Prof. Alex K.-Y. Jen et al. (article number 202403121) present two new NFAs, termed C9BO and 2BO, which feature either two types or a single type of side-chains that accordingly make the NFAs either asymmetric or symmetric in regard to their molecular structure, respectively. Their results show that symmetric NFAs have a richer diversity of molecular packing orientations, thus reaching a slower rate and lower ratio of the spin-triplet state while lowering non-radiative voltage losses. Prof. Feng He et al. (article number 202406097) report the synthesis of four novel NFAs where the distribution of chlorine atoms at the (isometric) terminal groups is purposely tuned to be asymmetric. Upon blending with PBDB-TF as a donor, they identify that an increased chlorine dispersion improves the photocurrent while further branching of the NFA side chains boosts the PCE to over 19 %. Besides the symmetry considerations of NFAs, the group of Prof. Jianhui Hou (article number 202404482) presents a completely fused NFA structure termed as GS-20 with the ultimate goal of suppressing the need for either additives or annealing in the fabrication process of OPV. Their as-cast ternary OPV cells reach a maximum PCE of 19.0 % in small-area devices, and 13.5 % in blade-coated OPV modules, indicating suitability for up-scaling. The development of dimerized acceptors is also gaining momentum as they combine the high performance of small-molecule NFAs with the excellent thermal stability of polymers. Prof. Yongsheng Chen and Prof. Bin Kan co-led a study (article number 202404062) focused on the development of two new and highly miscible dimeric acceptors, namely, DC9-HD and DYSe-3, the latter incorporating selenium as a substituent of the sulfur atoms on the outer ring of the fused-ring core. Their ternary blends with PM6 as donor concomitantly benefit from low open-circuit voltage (Voc) loss and long exciton diffusion lengths to achieve a PCE of 19.4 %. Importantly, these devices were able to retain more than 80 % of their PCE after 800 h of thermal stress at 65°C. Further progress on dimeric acceptors is reported by Prof. Feng Liu and colleagues (article number 202404567), who critically assess the role played by the central linking units connecting the two acceptor molecules together on the device performance. Their results indicate that the use of 3,4-ethylene dioxythiophene rather than electron-withdrawing or electron-neutral linkers has the best photovoltaic performance of 18.21 % PCE with a T80 of 970 h (being T80 the time required to reach 80 % of the initial PCE). The work led by Prof. Lijian Zuo et al. (article number 202403162) exploits an in situ toolbox (photoluminescence and UV–vis spectroscopy measurements) to study the phase-separation kinetics of the layer-by-layer processing approach that leads to a record PCE of 19.7 % in PM6:BTP-eC9 blends. In this case, the processing is observed to start by gelation of the underlying polymer donor layer, followed by the diffusion of the NFA and its intercalation to establish a favorable donor:acceptor phase separation. An improved understanding of the vertical distribution of donor and acceptor materials across the BHJ is given by Prof. Harald Ade and co-workers (article number 202404962). His team studies the vertical distribution of the BHJ in inverted and conventional device architectures in two model systems: PTQ10:BTP-eC9 and PM6:BTP-eC9. Their results show how, through the addition of PC71BM to the (ternary) BHJ, the efficiency of the inverted and conventional devices based on PTQ10 becomes very similar as a result of a uniform vertical composition distribution. Several studies also report on the use of additives to optimize the OPV performance to unprecedented levels. Exemplarily, advances toward printed blade-coated modules are provided by Prof. Hongzheng Chen and co-workers (article number 202403132), who study the role played by liquid and solid additives on the archetypal PM6:L8-BO binary system. In particular, her group introduces the solid additive 2-iododiphenyl ether as a key component to preserve an adequate aggregation of the acceptor in large-area blade-coated films processed from a non-halogenated solvent (toluene). As a result, blade-coated modules with an efficiency exceeding 15 % and a total module area of 18.90 cm2 are demonstrated. Similarly, the group of Prof. Yanchun Han (article number 202404507) introduces a new removable solid additive, 2,5-dibromothiazol (DBrTz), to enhance the molecular aggregation of the PM6:L8-BO blend. Authors demonstrate that during the spontaneous volatilization of DBrTz in a pseudo-dry film, the aggregation of the L8-BO is purposely enhanced, therefore improving the morphology, charge generation, and transport to yield PCEs as high as 19.4 %. In line with the use of volatile solid additives, Prof. Hui Huang and colleagues (article number 202405205) design and synthesize two new compounds, namely HBT-1 and HBT-2, following the strategy of noncovalent conformational locks to stabilize and guide the self-organization of the donor and acceptor materials during the active layer drying and annealing processes. Their results pinpoint HBT-2 as a versatile solid additive with excellent compatibility with multiple active layers that is furthermore able to improve the PCE up to 20.01 % in the ternary blend D18:L8-BO:PY-C11. Additives not only serve to improve the performance of the OPV but also to form selective contacts, as demonstrated by Prof. Gitti L. Frey and colleagues (article number 202404531). In their work, authors investigate the spontaneous interlayer formation methodology based on the metal-induced additive migration method. That is enabled by adding polyethylene glycol oligomer additives into the BHJ and using thermally-evaporated Al contacts as top (electron-selective) electrodes. Interestingly, their results indicate that the crystalline structure of the BHJ is barely modified upon migration of the additive through the amorphous non-crystalline regions of the active layer, thus resulting in a versatile contact formation methodology for organic electronic devices in general. Dr. Maria Saladina, Prof. Carsten Deibel, and colleagues provide in their perspective (article number 202405889) a critical evaluation of the performance losses attributed to transport resistance, eventually leading to high fill factor (FF) losses. Authors present an experimental protocol to quantify said transport resistance and outline strategies to further minimize the charge collection losses, which were observed even in the highest efficiency OPV reported to date. In line with the understanding and quantification of device performance losses, Prof. Thomas Kirchartz et al. (article number 202403129) present in their report a model that allows exploiting the apparent photoshunt as an easily accessible quantitative probe of the charge collection losses in OPV, including the introduction of a new figure of merit for convenient benchmarking across active layers and device structures. On the other hand, Prof. Martijn Kemerink (article number 202405087) and colleagues study the roll-off of the Voc at cryogenic temperatures by adding a light intensity sweep to the usual temperature-dependent measurements. Accordingly, authors develop a kinetic analytical model that is able to explain the temperature dependence of Voc as charges getting trapped in intrinsic tail states of the Gaussian density of states of the active layer, i.e., without having to explicitly include extrinsic traps. Novel insights on device physics are provided by the group of Prof. Feng Gao (article number 202403675), who introduces a thorough study on the role played by thermal activation on the charge separation efficiency (CSE) in OPV resulting in a classification of donor:acceptor blends into three categories according to the offset (low, mid, high) between the lowest excited state and the charge-transfer state. Their study indicates that for mid-offset blends, both the CSE and the Voc can be concomitantly maximized under room temperature operation conditions, thus providing a quantifiable rationale to achieve optimized PCEs. On the other hand, Prof. Julien Glorenflot and colleagues (article number 202405322) shine light on the controversial measurements of exciton diffusion lengths in NFAs. They carefully study the exciton diffusion lengths in archetypal ITIC derivatives through a combination of transient absorption (TA) spectroscopy and Kinetic Monte Carlo (KMC) simulations. Authors find that the tight π–π packing of the ITIC lattice and a low disorder are key properties to enable long exciton diffusion lengths in pristine NFA films. The use of machine learning (ML) methods in OPVs is also highlighted in this special issue. In particular, the group of Prof. Derya Baran presents a ML approach to estimate the ionization energy and electron affinity of organic semiconductors solely from their simplified molecular input line entry system (SMILES) string (article number 202403707). Their approach exploits the crude estimation of properties (CEP) framework to optimize the predictive performance, exploiting a limited experimental dataset of only 60 data points, yet resulting in impressive cross-validated mean absolute errors of only 0.13 and 0.14 eV for the ionization energy and electron affinity predictions, respectively. Insights on the thickness-dependence of active layer materials for OPV are given in a study co-led by Prof. Mariano Campoy-Quiles and Dr. Xabier Rodríguez-Martínez, where ML tools are also invoked (article number 202405735). Therein, authors exploit a database of 720 inverted devices and 20 different donor:acceptor blends to study what makes thickness-tolerant OPV, a critical feature toward industrialization of the printed OPV technology. Through a combination of KMC simulations and ML methods, the authors discuss material and device features that should be fulfilled to enable competitive FFs and performance in thick active layer (>200 nm) devices. Advances in the stability of OPV materials and device interlayers are also covered in this issue. First, Prof. Taiho Park, Prof. Minjun Kim, and co-workers contribute with an in-depth review (article number 202501340) on the stability of OPV that includes the analysis of the burn-in losses and long-term degradation issues together with the underlying governing mechanisms (environmental factors and device interlayers). Authors also discuss indicators to mitigate instabilities and present analytical tools and methodologies to adequately evaluate and monitor OPV degradation. The group of Prof. Pavel A. Troshin (article number 202501179) contributes with an extensive and systematic investigation on how the intrinsic photostability of 240 conjugated polymers develops under UV light exposure. The researchers introduce stability descriptors assigned to the polymer building blocks and develop a model that is able to quantitatively predict the photodegradation behavior of amorphous and crystalline conjugated polymers. Theoretical calculations pinpoint the energetics of triplet states as a key property driving the observed photostability trends. In regard to device interlayers, the group of Prof. Maria Antonietta Loi (article number 202404981) presents a solution-processed NiOx interlayer functionalized with carbazole-based phosphonic acid derivatives for its use as a hole-transport layer in PM6:L8-BO OPV. Through a careful control of the Ni3+ species formed, the authors achieve a remarkable 80 % FF under 1 sun conditions after optimization of the dipole moment of the phosphonic acid-anchored derivatives. Another important contribution to the improvement of the stability and performance of contact interlayers is provided by Prof. Ergang Wang and co-workers, who present a study (article number 202404297) on a novel passivation strategy for the sol-gel processed ZnO used as an electron transport layer in inverted OPV. Their approach consists of adding dextran as an interfacial passivator of the ZnO layer, which results in lower interfacial recombination losses, improved PCE, and T80 lifetimes exceeding 1500 h for fullerene, non-fullerene, and all-polymer OPV in inverted architecture. Important advances toward the sustainable fabrication of stable OPV devices are reported by Dr. Julianna Panidi and co-workers (article number 202405635), who present a thorough degradation study of blends including the low-cost polymer donors PTQ10 and FO6-T processed from 2-methyltetrahydrofuran as a biorenewable solvent. Their study raises attention to the FO6-T:Y12 blend as a promising active layer with an extrapolated T80 lifetime of over 2000 h under LED illumination due to its resilient morphology. For device and building integration in unconventional form factors, assessing flexible and semi-transparent OPV devices is mandatory. The mechanical properties of OPV were investigated by Prof. Zonglong Zhu, Prof. He Yan, and co-workers (article number 202404499), who replaced sulfur by selenium atoms at key positions in their new small molecular acceptors mPh4F-TT (TT) and mPh4F-TS (TS) and the corresponding polymer acceptor PYSe2F-T (PA). The incorporation of either TS or PA as guest material to the already preoptimized PM6/TT binary outperformed the efficiency of the corresponding binary and flexible devices. Nevertheless, the ternary blend incorporating PA was observed to be remarkably stable in terms of mechanical robustness. Advances in the semi-transparent OPV field for integration into buildings and windows are accordingly presented by Prof. Hin-Lap Yip and co-workers (article number 202404129). In their work, novel narrow-bandgap NFAs are incorporated into the active layer, and a high-throughput optical modeling toolbox is exploited to optimize the energy production and thermal management capabilities of the OPV to be integrated into semi-transparent windows. The authors estimate the potential reduction in annual energy demand gained through the incorporation of these devices into buildings as high as 60 % compared to existing technologies. In line with the up-scaling needs, Prof. Ross Hatton and co-workers (article number 202405148) propose a new type of transparent flexible electrode based on Zn grids as a low-cost replacement of the workhorse ITO, PEDOT:PSS, or Ag (grids) in OPV. Their fabrication approach combines condensation coefficient modulation, micro-contact patterning, and a seed layer to nucleate zinc condensation where grid lines are required. In summary, this special issue constitutes an excellent collection of the most recent advances underscoring the future development directions of the OPV technology with special focus on materials, processing, device physics, stability, and integration. Overall, they all underpin the potential of printed OPVs to consolidate as a sustainable and low-cost energy technology in the coming years. We are honored to serve as guest editors of this special issue and feel really thankful to authors, referees, and the Advanced Energy Materials editorial team for relying on us to craft such an insightful collection of research articles led by world-class OPV researchers. Xabier Rodríguez-Martínez is a UDC-Inditex InTalent associate researcher at the University of A Coruña in Spain. He obtained his PhD in 2020 at the Materials Science Institute of Barcelona for the high-throughput development of organic solar cells by combinatorial methods. He enrolled as a postdoc in Linköping University, Sweden, followed by a Humboldt postdoctoral fellowship at Heidelberg University, Germany. His research interests include organic and sustainable electronics for energy harvesting: photovoltaics and thermoelectrics. Hongzheng Chen is the Qiushi Distinguished Professor at Zhejiang University (ZJU) in China. She her studies at in She after her at the in and a Professor in She University of Science and University of and of and research interests focus on the development of organic and materials for including solar cells and Vida Turkovic is an Professor at She obtained her Dr. in from the University of Germany. In the University of and in was an associate research include the University of and research on approaches to the of organic and energy devices. She the For in Science and the 2020 In was a and in Feng Gao is a and of the at Linköping University in He has as a His group on semiconductors for devices. He obtained his from the University of in He the in and and the in both by the of
Source: Advanced Energy Materials
Authors: Xabier Rodríguez‐Martínez, Hongzheng Chen, Vida Turkovic, Feng Gao
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W4416945470
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