The low carrier density in organic semiconductors leads to high resistivity and contact resistance in electronic devices. Doping has been implemented to solve these issues. We describe herein a molecular modification approach to increase the carrier density. A representative p-type organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), was modified with a pinacolborane (Bpin) group, a reactive functional group in the Suzuki-Miyaura cross-coupling reaction. The resulting Bpin-modified DNTT (Bpin-DNTT) has a low-lying HOMO energy level at the single molecular level (5.4 eV below the vacuum level) and excellent transistor characteristics with mobility of greater than 2 cm2 V-1 s-1. However, the Bpin-DNTT solid was easily oxidized upon exposure to ambient air, generating hole carriers. To clarify this unprecedented behavior, we investigated Bpin-DNTT in detail through single-crystal field-effect transistor (SC-FET), electron spin resonance (ESR) spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and theoretical calculations. The SC-FET and ESR spectra demonstrated that the surface of the Bpin-DNTT solid in air was readily oxidized, which was due to the significantly decreased ionization energy of 4.58 eV, confirmed by UPS. These results reveal the potential of the Bpin group to increase the carrier density in p-type organic semiconductors.

Achieving both low voltage loss and efficient charge generation remains a major challenge in advancing high-performance organic photovoltaics (OPVs). Here, we show that photovoltaic cells using PTNT1-F-a dithienonaphthobisthiadiazole (TNT)-based polymer recently developed by our group-exhibit a notably low nonradiative voltage loss (triangle Vnr) of 0.18 V, suggesting a minimal driving force for charge generation. Remarkably, when combined with a nonfullerene acceptor Y12, the PTNT1-F device achieved high photocurrents and charge generation efficiencies exceeding 80% of the theoretical limit-to the best of our knowledge, the highest reported for OPVs with similarly low triangle Vnr. PTNT1-F features a rigid, ordered backbone that preserves the density of states (DOS) upon blending with the acceptor. In contrast, reference polymers such as D18 and PM6 show significant DOS changes, emphasizing advantage of PTNT1-F in promoting hole delocalization and efficient charge dissociation even with limited driving force. These results offer valuable insights into designing polymer donors for simultaneous low voltage loss and efficient charge generation.

Improving the bulk quality of perovskite films is critical for achieving higher-performance photovoltaic devices. Chloride-containing additives, including lead chloride (PbCl2) and methylammonium chloride (MACl)─standard additives widely adopted in perovskite photovoltaics─are effective for controlling crystallization kinetics and grain morphology. However, the distinct impacts of different forms of chloride additives on nanoscale phase uniformity and luminescence homogeneity remain underexplored. Here, we provide new insights into how the choice and combination of chloride additives influence phase transitions and spatially uniform carrier dynamics within perovskite films. We demonstrate that strategically combining MACl and PbCl2 improves crystallinity and optoelectronic uniformity across dimensions spanning micrometers to millimeters. Leveraging these findings, we fabricated inverted (p-i-n) perovskite solar cells achieving certified quasi-steady-state efficiencies of 26.4% and 24.5% at device areas of 0.05 and 1 cm2, respectively. Furthermore, these devices exhibit robust operational stability, retaining 88% of their initial performance after 1200 h of continuous maximum power point tracking at elevated temperatures (65 °C) under simulated AM1.5G illumination. Our results elucidate the mechanistic differences between chloride additive forms, providing a viable strategy for advancing large-area, high-efficiency, and thermally stable perovskite photovoltaics.

Recent advances in π-conjugated polymers have achieved high charge carrier mobilities by enhancing intrachain transport, particularly in devices such as organic field-effect transistors (OFETs), where charge transport occurs predominantly in-plane. However, for vertical devices such as organic photovoltaics (OPVs) where out-of-plane transport is required, efficient interchain charge transport becomes essential yet remains challenging. We previously reported a π-conjugated polymer PTNT2T, incorporating a newly developed π-extended dithienonaphthobisthiadiazole (TNT) unit, and primarily attributed its high charge carrier mobility in OFETs-despite its low crystallinity and wider π-π stacking distance-to enhanced intrachain transport arising from backbone rigidification. In this study, we demonstrate that, in addition to this intrachain contribution, the superior mobility of PTNT2T also arises from enhanced interchain transport enabled by strong core-to-core overlaps between TNT units. Importantly, these molecular-level contacts create intermolecular electronic connectivity that enables charge transport extending beyond crystalline domains into amorphous regions. Notably, PTNT2T exhibits substantially more efficient out-of-plane charge transport than the more crystalline reference polymer. This dual-pathway mechanism leads to exceptionally high fill factors in both fullerene- and nonfullerene-based OPVs, even with active layers as thick as 400 nm. Our findings demonstrate that strong core-to-core overlap enables efficient interchain charge transport even without high crystallinity, offering a new design strategy for π-conjugated polymers.

The diffusion length of photo-generated excitons in organic semiconductors is a fundamental parameter that governs photoelectronic processes in devices. However, the conventional electro-optical method for determining the exciton diffusion length typically requires multiple device fabrications and is subject to significant uncertainty, as several critical parameters are often unknown or assumed. Here, we present a single-device approach to simultaneously measure both the diffusion length and the charge-generation yield in organic semiconductors using a planar heterojunction structure. This methodology relies on fitting the external quantum efficiency spectra, whose shapes are strongly influenced by both the light-absorption profiles and the probability that excitons reach the charge-separation interfaces. We demonstrate that non-fullerene acceptors exhibit relatively long diffusion length, which correlates with the high power conversion efficiencies observed in organic photovoltaics. This approach enables unprecedented accuracy in probing the relationship between material structure and exciton diffusion length, providing a powerful tool for the rational design of high-performance organic semiconductor devices. The diffusion length of photo-generated excitons in organic semiconductors is crucial for optimizing photoelectronic device performance, yet traditional measurement methods are often imprecise. Here, the authors introduce a single-device technique using planar heterojunctions to accurately measure exciton diffusion length and charge-generation yield, revealing insights that enhance the design of efficient organic photovoltaics.

In this study, we introduce two volatile solid additives based on phenothiazine (PTz) derivatives with different N-substituents: ethyl (PTz-Et) and phenyl (PTz-Ph), into the PM6:Y6 photoactive layer system. The incorporation of PTz-Et and PTz-Ph enhances inter- and intramolecular stacking in neat PM6, neat Y6, and the blended PM6:Y6 films after thermal annealing, as evidenced by changes in absorption profiles. These improvements lead to more efficient exciton dissociation, optimized phase separation, balanced charge carrier transfer and transport, and reduced charge recombination, collectively enhancing the efficiency and stability of organic solar cells (OSCs). PTz-Et-treated PM6:Y6 blend achieves a remarkable enhancement in power conversion efficiency (PCE), increasing from 14.57% to 15.75%, along with improvements in short-circuit current (J SC) from 25.16 to 26.05 mA/cm2 and fill factor (FF) from 70.0% to 74.6%, and better device stability. This study demonstrates the effectiveness of PTz-based volatile solid additives and positions them as promising candidates for OSC commercialization.

The vertical component distribution is investigated in bulk-heterojunction (BHJ) type organic solar cells (OSCs) by combining photocrosslinking of donor polymers with layer-by-layer (LbL) deposition of acceptor molecules. Different concentrations of a tetradiazirine photocrosslinker controlled the crosslinker density of the polymer films, which in turn influenced the permeation behavior of acceptor molecules during LbL deposition. Time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and grazing incidence wide-angle X-ray scattering (GIWAXS) analyses revealed the effect of crosslinker density on the vertical distribution of donor and acceptor materials. Increasing crosslinker density during LbL processing produces distinct bilayer-like structures, with each layer having different component ratios. OSC performance is optimized at lower crosslink densities with the uniformly mixed structure, while higher densities reduce the donor-acceptor interface, thereby decreasing power conversion efficiency from 12.6% (0.3 wt.%) to 4.48% (2.0 wt.%). These findings challenge the previous assumption that molecular permeation during LbL deposition naturally results in continuous component gradients or p-i-n structures, which are proposed as an advantage of the LbL method over traditional BHJ structures.

We quantitatively investigated the anisotropic enhancement of hole mobility induced by structural modifications upon ion-exchange doping in highly uniaxially aligned poly(3,3 '''-didodecyl-quaterthiophene) (PQT-12) films. Large-area, uniform alignment was achieved via the floating film transfer method (FTM), giving a high dichroic ratio of 15. Anion-exchange p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) yielded a carrier density of 8.06 x 1020 cm-3 while preserving uniaxial order and improving crystallinity. Compared to undoped films, the hole mobility increased by 850-fold along the backbone direction and 1700-fold along the pi-pi stacking (perpendicular) direction, demonstrating that doping optimizes structural order for charge transport in both directions, with a more pronounced relative enhancement perpendicular to the chains. Using the stronger oxidant magic blue with LiTFSI further increased the doping level, achieving a higher conductivity of 251 S cm-1. These results reveal how ion-exchange doping and structural anisotropy cooperatively govern charge transport in aligned conjugated polymers.

Here, we reported the synthesis of an ortho-phenylene bridged cyclic tetra(benzo[c][1,2,5]thiadiazole) (3) by stepwise Suzuki-Miyaura couplings. Subsequent oxidation of 3 yielded a two-sided fused product 4, identified as an ortho-phenylene bridged cyclic triphenyleno[1,2-c:7,8-c']bis([1,2,5]thiadiazole) dimer. The structures of 3 and 4 were confirmed by high-resolution mass spectroscopies (HRMS) and NMR techniques. Their photophysical and electrochemical properties were fully characterized by ultraviolet-visible (UV-vis), fluorescence spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations.

Organic photovoltaics (OPVs) have recently achieved high short-circuit current densities (JSC) approaching 30 mA/cm2 with internal quantum efficiencies surpassing 90%. In comparison to their inorganic or perovskite counterparts, a distinguishing feature of OPVs is the involvement of singlet or charge-transfer excitons in photoelectron conversion. A deeper understanding of the charge generation process with these excitons is crucial to further enhance JSC while maintaining the open-circuit voltage and fill factor. In this perspective, we provide new insights into the charge generation mechanisms and their electric field dependence derived from investigations using planar heterojunction structures and their comparison to bulk heterojunction systems. We aim to foster open discussion and collaboration within the research community to address the aforementioned challenges.

Large batch-to-batch differences in the performance of organic photovoltaic (OPV) devices are a serious problem, often attributed to structural inconsistencies in semiconducting copolymers. This work uses X-ray photoelectron spectroscopy (XPS) to quantify the monomer unit ratio in widely used electron donor copolymers for OPVs, achieving high accuracy through optimized measurement conditions and calibration. The results show that the monomer unit ratios in different polymer batches deviate from the ideal 1:1 ratio and vary depending on the supplier. Additionally, we synthesize copolymers with intentionally altered unit ratios, which are accurately quantified by XPS. For both commercial and synthesized polymers, larger deviations from a 1:1 ratio are associated with reduced fill factor and open-circuit voltage in bulk-heterojunction OPVs. Quantitative XPS analysis of copolymer structures provides valuable insight into the relationship between variations in OPV performance and the structural integrity of the polymer across batches.

A fluoroalkyl-containing electron acceptor (Y-SSM) is designed and synthesized to control the orientation of the benchmark non-fullerene acceptor Y6 in thin films. Due to the low surface energy of the two fluoroalkyl chains at the terminal part of Y-SSM, it spontaneously segregates to the film surface during spin coating, forming a monolayer of edge-on oriented Y-SSM. The Y-SSM monolayer leads to crystallization of the underlying Y6 to induce a standing-up orientation in the bulk of the films, which is strikingly different from pure Y6 films that tend to be a face-on orientation. Solid evidence for standing-up Y6 in the film is provided by two-dimensional grazing incidence wide-angle X-ray scattering and optical anisotropy measurements based on variable angle spectroscopic ellipsometry. The surface Y-SSM can be partially removed by washing with hexane without disrupting the orientation of Y6, resulting in exposure of the standing-up Y6 on the surface. Organic photovoltaics based on a planar heterojunction structure with standing-up Y6 show a significant increase in short-circuit current density, reaching 2.5 times the value compared to face-on oriented Y6, due to the improved charge generation efficiency resulting from the different relative orientation with respect to PM6.

To elucidate the effects of main-chain sequence defects in donor-acceptor (D-A) copolymers on their charge-transport behavior, we synthesized bithiophene-quinoxaline (BTQ) copolymers via two complementary polymerization routes: conventional copolymerization by heterocoupling of two monomers and homopolymerization of a single monomer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry confirmed that the BTQ polymers synthesized by one-pot Stille coupling were free from sequence defects, whereas those obtained by conventional heterocoupling contained such defects. UV-vis absorption spectroscopy, photoemission yield spectroscopy (PYS), ultraviolet photoelectron spectroscopy (UPS), and grazing-incidence wide-angle X-ray scattering (GIWAXS) revealed that the absence of these defects led to enhanced backbone ordering, improved crystallinity, and a narrower highest occupied molecular orbital (HOMO) density of states (DOS). Organic field-effect transistors (OFETs) based on the homocoupled BTQ exhibited hole mobilities 3.5 times higher than those based on the heterocoupled BTQ. These results demonstrate that adopting a homocoupling route to minimize sequence defects is an effective strategy for uncovering the intrinsic electronic properties of π-conjugated polymers.

A rod-like molecule composed of biphenyl and a fluoroalkyl chain was synthesized, and its surface segregation behavior was investigated. This molecule spontaneously organized into a dense two-dimensional monolayer, termed surface segregated monolayer (SSM), on the surface of poly(3-hexylthiophene) (P3HT) film during spin-coating. The formation of SSM with high density and specific molecular orientation was confirmed by X-ray photoelectron spectroscopy, water contact angle measurement, and atomic force microscopy. The orientation of the molecular dipole moments within the SSM significantly increased the ionization potential, which was attributed to a vacuum level shift at the surface, as detected by ultraviolet photoelectron spectroscopy. The introduction of the SSM at the interface of organic semiconductor films led to an interfacial vacuum level shift, which could cause asymmetric current-voltage characteristics in the vertical hole conduction that varied with the direction of the interfacial dipole moments.

Ultraflexible organic photovoltaics have emerged as a potential power source for wearable electronics owing to their stretchability and lightweight nature. However, waterproofing ultraflexible organic photovoltaics without compromising mechanical flexibility and conformability remains challenging. Here, we demonstrate waterproof and ultraflexible organic photovoltaics through the in-situ growth of a hole-transporting layer to strengthen interface adhesion between the active layer and anode. Specifically, a silver electrode is deposited directly on top of the active layers, followed by thermal annealing treatment. Compared with conventional sequentially-deposited hole-transporting layers, the in-situ grown hole-transporting layer exhibits higher thermodynamic adhesion between the active layers, resulting in better waterproofness. The fabricated 3 μm-thick organic photovoltaics retain 89% and 96% of their pristine performance after immersion in water for 4 h and 300 stretching/releasing cycles at 30% strain under water, respectively. Moreover, the ultraflexible devices withstand a machine-washing test with such a thin encapsulation layer, which has never been reported. Finally, we demonstrate the universality of the strategy for achieving waterproof solar cells.

Benefiting from the synergistic development of material design, device engineering, and the mechanistic understanding of device physics, the certified power conversion efficiencies (PCEs) of single-junction non-fullerene organic solar cells (OSCs) have already reached a very high value of exceeding 19%. However, in addition to PCEs, the poor stability is now a challenging obstacle for commercial applications of organic photovoltaics (OPVs). Herein, recent progress made in exploring operational mechanisms, anomalous photoelectric behaviors, and improving long-term stability in non-fullerene OSCs are highlighted from a novel and previously largely undiscussed perspective of engineering exciton and charge carrier pathways. Considering the intrinsic connection among multiple temporal-scale photocarrier dynamics, multi-length scale morphologies, and photovoltaic performance in OPVs, this review delineates and establishes a comprehensive and in-depth property-function relationship for evaluating the actual device stability. Moreover, this review has also provided some valuable photophysical insights into employing the advanced characterization techniques such as transient absorption spectroscopy and time-resolved fluorescence imagings. Finally, some of the remaining major challenges related to this topic are proposed toward the further advances of enhancing long-term operational stability in non-fullerene OSCs.

Multijunction photovoltaics (PVs) are gaining prominence owing to their superior capability of achieving power conversion efficiencies (PCEs) beyond the radiative limit of single-junction cells1-8, for which improving narrow-bandgap (NBG) tin-lead perovskites is critical for thin-film devices9. Here, with a focus on understanding the chemistry of tin-lead perovskite precursor solutions, we find that Sn(II) species dominate interactions with precursors and additives and uncover the exclusive role of carboxylic acid in regulating solution colloidal properties and film crystallization and ammonium in improving film optoelectronic properties. Materials that combine these two functional groups, amino acid salts, considerably improve the semiconducting quality and homogeneity of perovskite films, surpassing the effect of the individual functional groups when introduced as part of separate molecules. Our enhanced tin-lead perovskite layer allows us to fabricate solar cells with PCEs of 23.9%, 29.7% (certified 29.26%) and 28.7% for single-junction, double-junction and triple-junction devices, respectively. Our 1-cm2 triple-junction devices show PCEs of 28.4% (certified 27.28%). Encapsulated triple-junction cells maintain 80% of their initial efficiencies after 860 h maximum power point tracking (MPPT) in ambient. We further fabricate quadruple-junction devices and obtain PCEs of 27.9% with the highest open-circuit voltage of 4.94 V. This work establishes a new benchmark for multijunction PVs.

In this study, we designed and synthesized two nonfullerene acceptors (NFAs) based on 1H- and 2H-benzo[d]imidazole (BIz) isomers, named 1-OBIA and 2-OBIA, for organic photovoltaics (OPVs). Whereas 1-OBIA possesses an electron-donating amine-type nitrogen atom and an electron-withdrawing imine-type nitrogen atom, 2-OBIA possesses two electron-withdrawing imine-type nitrogen atoms in the benzimidazole moiety. Owing to its more electron-rich central core, 1-OBIA showed higher HOMO and LUMO energy levels than 2-OBIA. Photovoltaic cells based on 2-OBIA, when paired with a benchmark polymer donor, PM6, provided a power conversion efficiency of 10.3%, which was much higher than that of cells based on 1-OBIA (4.0%). This can be explained by the more efficient hole transfer for the PM6:2-OBIA blend than that for the PM6:1-OBIA blend. Studies on the morphology, thin-film structure, and charge carrier transport properties further validated the high photovoltaic performance of PM6:2-OBIA cells. This study demonstrates the interesting properties and potential of BIz-based NFAs for OPVs.

Achieving a good balance between the crystallinity and solubility is of great importance in the design of pi-conjugated polymers. Here, we synthesized a series of terpolymers in which small fractions of a planar V-shaped fused ring, triphenyleno[1,2-c:7,8-c']bis([1,2,5]thiadiazole) (TPTz), are incorporated into the linear-shaped backbone of a highly crystalline polymer based on naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole) (NTz), PNTz4T. Due to the "bending" structure induced by the TPTz unit, the terpolymers had greatly improved solubility by about 5-9 times compared to the linear-shaped PNTz4T. In particular, the terpolymers preserved the crystallinity of PNTz4T to a considerable extent with a higher fraction of face-on oriented domains in films. As a result, the highly solution-processable terpolymers exhibited high photovoltaic performances and slightly outperformed PNTz4T. Our results indicate that the incorporation of a V-shaped unit into a linearly shaped polymer is a good strategy to develop pi-conjugated polymers with high solubility and crystallinity.