Efficient Organic Photovoltaics Research Articles

Molecular Precision Engineering for Efficient Binary Organic Photovoltaics through Energy Level and Fibrillar Structure Modulation

AbstractAdjusting the energy levels and fibrillar morphology is paramount to enhancing the power conversion efficiency (PCE) of organic solar cells (OSCs). In the present study, an increase in the open‐circuit voltage (VOC) is facilitated through the elongation of the alkyl chain within AQx (namely AQx‐8), aiming to decrease the free volume ratio (FVR). This reduction in FVR attenuates electron‐phonon coupling, thereby augmenting emission efficiency and diminishing the non‐radiative energy loss (ΔEnr). To further refine the energy levels and morphological characteristics, the external undecyl chain of AQx‐8 is substituted with a shorter carbon chain and cyclohexane noted for its considerable steric hindrance (AQx‐H). This alteration significantly mitigates intermolecular aggregation, expands the bandgap, and elevates the lowest unoccupied molecular orbital (LUMO) energy level, culminating in an elevated VOC of 0.923 V in devices based on AQx‐H. Morphological analysis reveals that blends based on AQx‐H exhibit an enhanced multi‐length‐scale fibrillar structure, which is conducive to exciton dissociation and charge transport, thereby contributing to a high fill factor (FF) nearing 80%. Consequently, this study reports one of the highest binary PCEs documented, standing at 19.5% (with certification at 19.0%).

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Reducing Voltage Losses in Organic Photovoltaics Requires Interfacial Disorder Management

AbstractThanks to the introduction of non‐fullerene acceptors, efficiencies of organic photovoltaics are now approaching 20%. Closing the gap with inorganic photovoltaics requires minimizing voltage losses without penalizing charge extraction, for which microstructure control is crucial. However, the complex interplay between microstructure and charge generation, recombination, and extraction has so far not been unraveled. Here, a systematic study linking device performance to distinct microstructural features via machine learning is presented. Building bi‐layer devices allows to separately study the influence of aggregation and disorder on the energies and lifetimes of bulk and interfacial states. Unambiguous assignments of specific structural motifs to the device photophysics are thus possible. It is found that the control of aggregation‐caused quenching is decisive for the exciton splitting efficiency and thus the carrier generation. Furthermore, the static disorder at the donor–acceptor interface controls the nonradiative recombination by shifting the excited state population from the bulk toward the interface. Finally, the amount of disorder in the bulk is found decisive for charge extraction. The finding that charge generation, recombination, and extraction are controlled by distinct structural features, is the key to optimizing these motifs independently, which will pave the way for organic photovoltaics toward the detailed balance limit.

Versatile Self‐Assembled Monolayer Material Enables Efficient Organic Photovoltaic Devices and Modules

AbstractEfficient and stable hole transport layer (HTL) is an important component of organic photovoltaics (OPV). Herein, a novel self‐assembled monolayer material of 4PADCB (short for (4‐(7H‐dibenzo[c,g]carbazol‐7‐yl)butyl)phosphonic acid) as HTL for non‐fullerene (NF) OPV is reported. Compared to the widely used PEDOT:PSS HTL, the usage of 4PADCB not only enhances photon incident and charge generation but also effectively improves the morphology of the upper active layer, promotes molecular packing, and balances the carrier transport within active layer. As a result, the single‐junction binary D18:L8BO OPV, employing 4PADCB as HTL, achieves a high‐power conversion efficiency (PCE) of 19.02% with excellent storage stability. Furthermore, a 19.3 cm2 OPV module comprising seven sub‐cells demonstrates a maximum PCE of 15.44%, which is the highest value of reported efficiency for large‐area binary OPV modules. Meanwhile, the module is the first demonstration of an OPV module based on self‐assembled monolayer material. This work underscores the substantial potential for the application of 4PADCB in efficient NF OPV devices and high‐throughput large‐area modules.

Role of proton conducting polyelectrolyte on the organic photovoltaics efficiency

Role of proton conducting polyelectrolyte on the organic photovoltaics efficiency

Designing Thiadiazoloquinoxaline-Based Conjugated Polymers for Efficient Organic Photovoltaics: A DFT/TDDFT Study.

Clean and renewable energy development is becoming frontier research for future energy resources, as renewable energy offers sustainable and environmentally friendly alternatives to non-renewable sources such as fossil fuels. Among various renewable energy sources, tremendous progress has been made in converting solar energy to electric energy by developing efficient organic photovoltaics. Organic photovoltaic materials comprising conjugated polymers (CP) with narrow optical energy gaps are promising candidates for developing sustainable sources due to their potentially lower manufacturing costs. Organic semiconductor materials with a high electron affinity are required for many optoelectronic applications. We have designed a series of organic semiconductors comprised of cyclopentadithiophene as a donor and thiadiazoloquinoxaline (TQ) as an acceptor, varying the π-conjugation and TQ-derivatives. We have employed density functional theory (DFT) and time-dependent DFT (TDDFT) to evaluate the designed CP's optoelectronic properties, such as optical energy gap, dipole moment, and absorption spectra. Our DFT/TDDFT result shows that the energy gap of CPs is lowered and redshifted in the absorption spectra if there is no insertion of conjugation units such as thiophene and selenophene between donor and acceptor. In addition, selenophene shows relatively better redshift behavior compared to thiophene. Our work also provides rational insight into designing donor/acceptor-based CPs for organic solar cells.

Enhancing the indoor performance of organic photovoltaic devices: interface engineering with an aminobenzoic-acid-based self-assembled monolayer

Significant advances in the performance of organic photovoltaic (OPV) devices can facilitate their use in internet of things applications. However, achieving excellent photostability and high efficiency using stable, efficient OPV devices in indoor settings is considerably difficult. To address this issue, a zinc oxide (ZnO) electron transport layer (ETL) was modified with a self-assembled monolayer of 4-aminobenzoic acid (ABA) in the present study, and the impact of this modification was correlated with the indoor performance of an OPV device with the PM6:L8-BO photoactive layer. The ABA-treated ZnO ETL exhibited a significant reduction in the work function (from 4.51 to 4.04 eV), surface roughness (from 0.201 to 0.177 nm), and hydrophilicity of an indium-tin-oxide electrode; this aided in selectively extracting charge carriers from the device and minimizing trap-assisted recombination losses. Additionally, the ABA treatment of the ZnO ETL considerably enhanced the electron mobility and recombination resistance. It reduced the trap density, thereby enabling the ZnO/ABA-based device to achieve improved performance. Consequently, the ZnO/ABA-based device exhibited a noteworthy 14.68% higher maximum power output than that of the device without any ZnO surface modification under 1000 lx halogen (HLG) illumination (P out, max = 354.48 and 309 µA cm−2, respectively). Moreover, under thermal illumination conditions (1000 lx HLG lighting), the ZnO/ABA-based device sustained ∼74% of its initial power conversion efficiency over 120 h, significantly higher than its ABA-free equivalent (∼55%).

Suppress energy loss to boost power conversion efficiency of organic photovoltaics with linear side chains modulation

The high energy loss (Eloss) in organic solar cells (OSCs) is being the one of major problem, which limit the power conversion efficiency (PCE) of OSCs. In this work, we designed and synthesized three perylene diimide (PDI)-based Acceptor’-Donor-Acceptor-Donor- Acceptor’ (A′-DAD-A′) small molecules namely anti-PDFC-Cx (C8, C10, and C12) by changing the length of linear chains on IDT cores to investigate how the length of linear side chain affects Eloss and the power conversion efficiencies (PCEs) of OSCs. The optical energy bandgap and stacking mode for three anti-PDFC-Cx (C8, C10, and C12) remain the same, which were not influenced by the different lengths of side chains. Increasing the length of side chains on IDT cores from C8 to C10 and C12 would improve the PCE of PM6: anti-PDFC-Cx based devices from 11.52% to 12.43% and 12.27%, particularly the Voc increasing from 0.98 V of anti-PDFC-C8 and 1.00 V of anti-PDFC-C10 to 1.02 V of anti-PDFC-C12. Eloss analysis indicated the value of Eloss reduced as the length increases from C8, to C12. These results demonstate linear side chain engineer is an effective method to reduce Eloss and realize high PCE on the bases of not modulating the stacking mode of acceptor.

Morphologically engineered multi-component organic solar cells with stratified donor distribution and alloyed acceptors for enhanced efficiency and stability

The strategy of integrating multiple components within the bulk-heterojunction layer of organic photovoltaics (OPVs) has proven effective in enhancing device performance and demonstrates broad potential applications. Nonetheless, achieving precise control over the morphology in such a multifaceted system presents a significant challenge. In this work, we introduce an innovative sequential casting technique to fabricate highly efficient quaternary OPVs with a meticulously tailored morphology featuring layers of stratified donor distribution and composite alloyed acceptors. The layered configuration of D18/PTQ10, with distinct crystalline domains, establishes a dedicated hole-transport pathway, while the alloyed BTP-eC9:Y6-O acceptors are evenly dispersed across the layered D18/PTQ10 donor phase. This carefully crafted morphology presents a gradient and interpenetrated donor/acceptor phase separation at an ideal length scale, which facilitates exciton dissociation, minimizes energetic disorder, and mitigates recombination. As a result, a power conversion efficiency close to 19% with excellent operational stability (extrapolated T80 = 818 h) was achieved. This work offers valuable insights into the morphological engineering of multicomponent OPVs for improved performance.

Phase separation and domain crystallinity control enable open‐air‐printable highly efficient and sustainable organic photovoltaics

AbstractOrganic solar cells (OSCs) have emerged as a promising solution for sustainable energy production, offering advantages such as a low carbon footprint, short energy payback period, and compatibility with eco‐solvents. However, the use of hazardous solvents continues to dominate the best‐performing OSCs, mainly because of the challenges of controlling phase separation and domain crystallinity in eco‐solvents. In this study, we combined the solvent vapor treatment of CS2 and thermal annealing to precisely control the phase separation and domain crystallinity in PM6:M‐Cl and PM6:O‐Cl systems processed with the eco‐solvent o‐xylene. This method resulted in a maximum power conversion efficiency (PCE) of 18.4%, which is among the highest values reported for sustainable binary OSCs. Furthermore, the fabrication techniques were transferred from spin coating in a nitrogen environment to blade printing in ambient air, retaining a PCE of 16.0%, showing its potential for high‐throughput and scalable production. In addition, a comparative analysis of OSCs processed with hazardous and green solvents was conducted to reveal the differences in phase aggregation. This work not only underscores the significance of sustainability in OSCs but also lays the groundwork for unlocking the full potential of open‐air‐printable sustainable OSCs for commercialization.

Stretchable and transparent nanopillar arrays for high-performance ultra-flexible organic photovoltaics

Ultra-flexible organic photovoltaic (OPV) devices are gaining recognition due to their lightweight, mechanical flexibility, and high productivity potential and are emerging as a notable alternative power source. Significant enhancements in material design and device architecture have facilitated a near 20% of power conversion efficiencies (PCEs). To elevate PCE further, the integration of soft optical elements is pivotal, enabling optimized photon utilization by minimizing reflection and augmenting the optical path through diffraction. In this research, we showcase an ultra-flexible energy harvesting system embedding thin, stretchable nanopillar arrays made of poly(dimethylsiloxane). The unique diffraction and interference from nanopillar arrays amplify the photogeneration of exciton pairs, which, in turn, boosts the current density of PM6:Y6 based OPV devices from 25.46 to 28.01 mA/cm2, resulting in a PCEmax of 15.92%. Furthermore, the exceptional flexibility of this mold was demonstrated on an ultra-flexible PM6:Y6 device, enduring 100% compressive strain through 1000 repeated tests. These results propose an advantageous approach for improving OPV efficiency without necessitating alterations to the inherent components or structures of the device.

Over 18.8% Efficiency of Layer‐By‐Layer Organic Photovoltaics Enabled by Ameliorating Exciton Utilization in Acceptor Layer

AbstractThe layer‐by‐layer (LbL) organic photovoltaics (OPVs) are constructed with wide‐bandgap donor PM1 and narrow‐bandgap acceptor L8‐BO. The exciton utilization near cathode is still challenging considering restricted diffusion distance of excitons and inability for transferring energy from L8‐BO to PM1. Herein, donor incorporation into acceptor layer (DIA) strategy is employed to improve exciton utilization near cathode. The efficiency of LbL OPVs can be improved from 18.02% to 18.81% by incorporating 10 wt% PM1 into L8‐BO layer, which is closely associated with efficient exciton separation into L8‐BO layer originated from more adequate donor/acceptor interface for faster charge transfer, as evidenced by magneto‐photocurrent and transient absorption results. The in situ test and morphological characterization clarify that molecular packing property can be improved benefited from prolonged aggregation and nucleation time of acceptor layer assisted by DIA strategy, contributing to more efficient charge transport and inhibited charge recombination in active layers. The thickness insensitive property of LbL OPVs can be also improved induced by DIA strategy, indicated by PCE retention value (82.2% vs. 74.0%) for PM1/L8‐BO:PM1 and PM1/L8‐BO OPVs when acceptor layer thickness increased to ≈180 nm. This work demonstrates the effectiveness of DIA strategy in improving efficiency and thickness tolerance of LbL OPVs.

Manipulating the functionality and structures of π-conjugated polymers utilizing intramolecular noncovalent interactions towards efficient organic photovoltaics.

Careful control of electronic properties, structural order, and solubility of π-conjugated polymers is central to the improvement of organic photovoltaic (OPV) performance. In this work, we designed and synthesized a series of naphthobisthiadiazole-quaterthiophene copolymers by systematically replacing the alkyl groups with ester groups and changing the position of the fluorine groups in the quaterthiophene moiety. These alterations lowered the HOMO and LUMO energy levels and systematically varied the combination of intramolecular noncovalent interactions such as O⋯S and F⋯S interactions in the backbone. More importantly, although the introduction of such noncovalent interactions often lowers the solubility owing to the interlocking of backbone linkages, we found that careful design of the noncovalent interactions afforded polymers with relatively high solubility and high crystallinity at the same time. As a result, the power conversion efficiency of OPV cells that used fullerene (PC61BM) and nonfullerene (Y12) as the acceptor was improved. Our work offers important information for the development of high-performance π-conjugated polymers for OPVs.

Enhancing the performance of indoor organic photovoltaics through precise modulation of chlorine density in wide bandgap random copolymers

This study demonstrates that the combination of wide bandgap random copolymers and PC71BM is an ideal pair for cost-effective, efficient indoor organic photovoltaics.

Design of Chlorinated Indaceno[1,2-b:5,6-b']dithiophene Acceptors toward Efficient Organic Photovoltaics.

Chlorinated modifications have been extensively employed to modulate the optoelectronic properties of π-conjugated materials. Herein, the Cl substitution in designing nonfullerene acceptors (NFAs) with various bandgaps is studied. Four narrow-bandgap electron acceptors (GS-40, GS-41, GS-42, and GS-43) were synthesized by tuning the electrostatic potential distributions of the molecular conjugated backbones. The optical absorption onset of these NFAs ranges from 900 to 1030 nm. Compared to the nonchlorinated analogue, the introduction of Cl atoms on the core of indaceno[1,2-b:5,6-b'] dithiophene (IDT) and π spacer results in an upward shift of the lowest unoccupied molecular orbital levels and induces a blue shift in the absorption spectra of the NFAs. This alteration facilitates achieving appropriate energy-level alignment and favorable bulk heterojunction morphology when blended with the widely used donor PBDB-TF. The PBDB-TF:GS-43-based solar cells show an optimal power conversion efficiency of 13.3%. This work suggests the potential of employing chlorine-modified IDT and thiophene units as fundamental building blocks for developing high-performance photoactive materials.

Unraveling the Structure-Property-Performance Relationships of Fused-Ring Nonfullerene Acceptors: Toward a C-Shaped ortho-Benzodipyrrole-Based Acceptor for Highly Efficient Organic Photovoltaics.

The high-performance Y6-based nonfullerene acceptors (NFAs) feature a C-shaped A-DA'D-A-type molecular architecture with a central electron-deficient thiadiazole (Tz) A' unit. In this work, we designed and synthesized a new A-D-A-type NFA, termed CB16, having a C-shaped ortho-benzodipyrrole-based skeleton of Y6 but with the Tz unit eliminated. When processed with nonhalogenated xylene without using any additives, the binary PM6:CB16 devices display a remarkable power conversion efficiency (PCE) of 18.32% with a high open-circuit voltage (Voc) of 0.92 V, surpassing the performance of the corresponding Y6-based devices. In contrast, similarly synthesized SB16, featuring an S-shaped para-benzodipyrrole-based skeleton, yields a low PCE of 0.15% due to the strong side-chain aggregation of SB16. The C-shaped A-DNBND-A skeleton in CB16 and the Y6-series NFAs constitutes the essential structural foundation for achieving exceptional device performance. The central Tz moiety or other A' units can be employed to finely adjust intermolecular interactions. The single-crystal X-ray structure reveals that ortho-benzodipyrrole-embedded A-DNBND-A plays an important role in the formation of a 3D elliptical network packing for efficient charge transport. Solution structures of the PM6:NFAs detected by small- and wide-angle X-ray scattering (SWAXS) indicate that removing the Tz unit in the C-shaped skeleton could reduce the self-packing of CB16, thereby enhancing the complexing and networking with PM6 in the spin-coating solution and the subsequent device film. Elucidating the structure-property-performance relationships of A-DA'D-A-type NFAs in this work paves the way for the future development of structurally simplified A-D-A-type NFAs.

Side‐Chain Engineering of Y‐Series Acceptors with Halogenated End‐Group for Efficient Indoor Organic Photovoltaics

AbstractDiverse design strategies for nonfullerene acceptors (NFAs) have aided the development of efficient indoor organic photovoltaics (OPVs) for Internet‐of‐things‐enabled gadgets. However, the side‐chain approach to Y‐series NFAs for fabricating indoor OPVs remains largely unexplored. In this study, the synthesis and photovoltaic characteristics of two side‐chain engineered NFAs with halogenated end‐groups (BTP‐PO‐4F and BTP‐PO‐4Cl) are investigated. NFAs with a Y6 core unit are modified with butyloctyl inner side chains and ethylhexyloxynaphthyl outer side chains. The extended conjugation moiety contributed to red‐shifted absorption, with the chlorination effect presenting a more pronounced influence than fluorination. Further, the outer moiety introduced a slight electron‐withdrawing effect on the Y6 core unit, resulting in energy‐level downshifts. An excellent power conversion efficiency (PCE) of 23.2% is achieved in fluorinated OPVs under a light‐emitting diode lamp (1000 lux). Moreover, integrating the Y6 NFA into the binary blend further enhanced the PCE, reaching 28.3% in PM6:Y6:halogenated NFA ternary OPVs. The panchromatic absorption, high crystallinity, and cascaded energy levels of the ternary blends contribute to achieving a sufficient spectral match and suppress trap‐assisted recombination, ultimately fostering a substantially enhanced fill factor and PCEs under indoor lighting.

Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation.

Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure-property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.

Highly efficient solution-processed organic photovoltaics enabled by improving packing behavior of organic semiconductors

Highly efficient solution-processed organic photovoltaics enabled by improving packing behavior of organic semiconductors

Revealing Design Rules for Improving The Photostability of Non‐Fullerene Acceptors from Molecular to Aggregation Level

AbstractWith the rapid increase in power conversion efficiency of organic photovoltaics due to the development of non‐fullerene acceptors (NFAs), prolonging the operational lifetime of devices becomes one of the critical prerequisites for commercial application. In this work, the degradation pathways of a wide range of state‐of‐the‐art NFA molecules via multiple spectroscopic techniques combined with density functional theory and molecular dynamics simulation are revealed. The structural confinement and molecular ordering are responsible for molecular conformational stability under illumination. More importantly, a very general trend is revealed that the origin of increased nonradiative decay under illumination is predominately in the aggregated states with strong intermolecular interactions while the intramolecular exciton dynamics are stable. The increased nonradiative decay correlates with reduced exciton diffusion length. This work provides vital information toward the design of intrinsically photo‐stable NFAs at the molecular level and the importance of aggregation control toward long‐term stable organic solar cells.