Organic Photovoltaic Performance Research Articles

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1–10 cm2), medium (10–100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as “fully printed/coated” devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

π-Extended Nonfullerene Acceptor for Compressed Molecular Packing in Organic Solar Cells To Achieve over 20% Efficiency.

Organic photovoltaics (OPVs) suffer from a trade-off between efficient charge transport and suppressed nonradiative recombination due to the aggregation-induced luminance quenching of organic semiconductors. To resolve this grand challenge, a π-extended nonfullerene acceptor (NFA) B6Cl with large voids among the honeycomb network is designed and introduced into photovoltaic systems. We find that the presence of a small amount of (i.e., 0.5 or 1 wt %) B6Cl can compress the molecular packing of the host acceptor L8-BO, leading to shortened π-π stacking distance from 3.59 to 3.50 Å (that will improve charge transport) together with ordered alkyl chain packing (that will inhibit nonradiative energy loss due to the suppressed C-C and C-H bonds vibrations), as validated by high-energy X-ray scattering measurements. This morphology transformation ultimately results in simultaneously improved JSC, FF, and VOC of OPVs. As a result, the maximum PCEs of PM6:L8-BO and D18:L8-BO are increased from 19.1 and 19.3% to 19.8 and 20.2%, respectively, which are among the highest values for single-junction OPVs. The university of B6Cl to increase the performance of OPVs is further evidenced in a range of polymer:NFA OPVs.

Impact of alloy‐like phase on energy loss mitigation in multi‐component organic photovoltaics

AbstractThe multi‐component strategy has proven effective in advancing the performance of organic photovoltaics (OPVs), enhancing photocurrent and fill factor through spectral complementarity and morphology optimization. However, the open‐circuit voltage (VOC) mechanism in multi‐component systems lacks systematic investigation. In this study, we explore the influence of alloy‐like phases on energy level distribution and energy loss mechanisms in multi‐component OPVs. Appropriate modulation of donor alloy‐like phases maintains the original intermolecular stacking, enhances component compatibility, reduces acceptor aggregation, and improves acceptor phase purity, mitigating non‐radiative recombination losses. Additionally, suitable alloy‐like phase modulation elevates charge transfer (CT) states, reducing the gap between CT and local exciton state, lowering reorganization energy, and alleviating radiative recombination loss below the bandgap. Through synergistic optimization (layer‐by‐layer method with solid additive), ternary devices based on Y6 acceptor achieve a notable 19.41% power conversion efficiency, offering new insights for the analysis of the energy loss of the multi‐component OPVs.

Diffusion‐Controlled Morphology Modification via Employing Host/Guest Acceptors to Improve the Stability of Organic Photovoltaics

AbstractTypical bulk‐heterojunction organic photovoltaics (OPV) suffer from instability due to the slowly diffusing small‐molecule acceptors (SMAs) under operational conditions. The originally well‐phase‐separated morphology will undergo SMA self‐aggregation to form over‐purified domains that will deteriorate the OPV performance. This issue can be partially alleviated in sequentially processed OPV cells with a planar‐mixed heterojunction absorber, which is intrinsically more robust due to larger domain sizes and a lower degree of mixing. To further enhance the morphological stability, a strategy is developed to inhibit the disordered molecular diffusion that leads to the loss of crystallinity of SMA domains. Two SMAs, C9‐4F and Y6‐OBO, are incorporated as guest acceptors to form an alloy‐like acceptor with the host acceptor BTP‐eC9. These alloy‐like acceptors improved the crystallinity of the acceptor phase by promoting more ordered molecular packing, effectively restraining the diffusion‐caused loss of SMA crystallinity upon illumination. This approach helps maintain favorable gradients and ultimately ensures OPV morphological stability. Consequently, the derived devices exhibited considerably enhanced stability, with the T80 lifetime of C9‐4F‐based devices reaching ≈790 h and a remarkable ≈1010 h for Y6‐OBO‐based devices, significantly surpassing the ≈450 h observed from binary devices.

Leveraging Bromine-Induced Large Stokes Shift in Pyrrolidinium Perovskite Nanoparticles for Improved Organic Photovoltaic Performance

Leveraging Bromine-Induced Large Stokes Shift in Pyrrolidinium Perovskite Nanoparticles for Improved Organic Photovoltaic Performance

Cover Picture: Molecular Structural Descriptor‐Assisted Machine Learning for Organic Photovoltaics with Perylenediimide Acceptors (BKCS 1/2024) Gyu‐Hee Kim, Keonho Yoon, Chihyung Lee, Minwoo Nam, Doo‐Hyun Ko

The cover picture illustrates the artificial intelligence model that predicts the performance of organic photovoltaics. This model predicts how organic photovoltaics will perform under 1 sun AM 1.5 G illumination by using only the molecular structure of the organic semiconductors within the bulk heterojunction photoactive layer. The organic semiconductors of the target organic photovoltaics comprise polymer donors and bay‐position‐linked diperylenediimide acceptors. More details are available in the article Gyu‐Hee Kim, Keonho Yoon, Chihyung Lee, Minwoo Nam, Doo‐Hyun Koimage

Probing Charge Transfer Character in Modern Donor/Acceptor Materials via Electroabsorption Spectroscopy.

We use electroabsorption (EA) spectroscopy to probe the charge transfer (CT) character in neat films and blends of donors and acceptors of interest for organic electronic applications. In particular, we compare the CT character in two polymer donor and non-fullerene acceptor blends, including 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-4F) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]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(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). Like classic polymer/fullerene blends, the blend based on IT-4F exhibits primarily first derivative-like EA features, suggesting localized exciton formation upon photoexcitation. However, the Y6-based blend has an EA spectrum that is dominated by second derivative-like features, consistent with CT character. We show that this signal originates primarily from Y6. We find that Y6 exhibits the highest dipole moment change (7.5 ± 2.5 D) of the molecules that comprise this study, consistent with a high degree of the CT character, and a relatively large polarization volume of 361 ± 70 Å3, consistent with strong electron delocalization. These results point to the origins of exceptional performance of organic photovoltaics (OPVs) based on Y6.

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.

Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films.

Domain morphology and composition, and the structure of interfaces between domains are key factors in the performance and stability of organic photovoltaics (OPVs) fabricated from polymer/small-molecule blends. In this study, we investigate the evolution of composition, phase-volume and interfacial roughness in model polymer/small-molecule bilayers, in response to thermal annealing. Polystyrene/fullerene mixing is studied as a function of annealing temperature, using in situ neutron reflectivity, in thin-film bilayer samples comprising pure component or mixed layers. Remarkably, we discover that thermal annealing at temperatures around or above the reported glass transition temperatures, Tg, of the components can result in extensive mass-transfer between layers, that has the superficial appearance of equilibration, but leaves the layer compositions, thicknesses, and/or the interfacial composition profile in a non-equilibrium state. This is not merely a case of slow kinetics near Tg, as subsequent heating to higher temperatures, followed by cooling, reveals pronounced hysteresis in these systems. This has important implications for the measurement of equilibrium compositions in polymer/small-molecule mixtures for OPV applications, and for device stability during operation.

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.

Highly-Efficient 2D Nonfullerene Acceptors Enabled by Subtle Molecular Tailoring Engineering.

The conjugate expansion of nonfullerene acceptors is considered to be a promising approach for improving organic photovoltaic performance because of its function in tuning morphological structure and molecular stacking behavior. In this work, two nonfullerene acceptors are designed and synthesized using a 2D π-conjugate expansion strategy, thus enabling the construction of highly-efficient organic solar cells (OSCs). Compared with YB2B (incorporating dibromophenanthrene on the quinoxaline-fused core), YB2T (incorporating dibromobenzodithiophene on the quinoxaline-fused core) has red-shifted spectral absorption and better charge transport properties. Moreover, the more orderly and tightly intermolecular stacking of YB2T provides the possibility of forming a more suitable phase separation morphology in blend films. Through characterization and analysis, the YB2T-based blend film is found to have higher exciton dissociation efficiency and less charge recombination. Consequently, the power conversion efficiency (PCE) of 17.05% is achieved in YB2T-based binary OSCs, while YB2B-based devices only reached 10.94%. This study demonstrates the significance of the aromatic-ring substitution strategy for regulating the electronic structure and aggregation behavior of 2D nonfullerene acceptors, facilitating the development of devices with superior photovoltaic performance.

EXAMINING THE OPERATION OF AN INVERSELY STRUCTURED ORGANIC PHOTOVOLTAIC SYSTEM USING LAMINATED SILVER ELECTRODES.

The research looked into the performance of an organic photovoltaic (OPV) with an inverse structure and laminated silver electrodes. The goal was to investigate the properties of the PCMB:P3HT blend used in the active layer and how they relate to device performance. The findings from UV-Visible absorption spectroscopy, which examined the optical properties of the PCMB: P3HT blend, were presented in the results. The blend had high absorption between 584.55 and 623.33 nm and low absorption in the red and near-infrared regions of the spectrum. The transmittance spectra revealed low transmittance, especially between 400 and 635 nm. The absorption and extinction coefficients of the blended material were calculated and analysed. The absorption coefficient increased between 400 and 635 nm, while the extinction coefficient peaked at 617.81 nm. The optical energy bandgap of the blend was determined to be 1.94 eV. The photovoltaic characteristics of the solar cell were assessed using I-V and P-V measurements. The key parameters obtained were open-circuit voltage, short-circuit current, voltage at maximum power point, and current at maximum power point. The fill factor and efficiency of the solar cell were calculated to be approximately 0.6462 and 0.6989%, respectively. The research helps to better understand the performance characteristics of inverse structure OPVs with laminated silver electrodes. Further investigation and optimization of the device structure and materials may improve the fill factor and efficiency, thereby improving the performance and adoption of organic photovoltaics in renewable energy applications.

A2 -A1 -D-A1 -A2 -Type Nonfullerene Acceptors.

The A2 -A1 -D-A1 -A2 -type molecules consist of one electron-donating (D) core flanked by two electron-accepting units (A1 and A2 ) and have emerged as an essential branch of nonfullerene acceptors (NFAs). These molecules generally possess higher molecular energy levels and wider optical bandgaps compared with those of the classic A-D-A- and A-DA'D-A-type NFAs, owing to the attenuated intramolecular charge transfer effect. These characteristics make them compelling choices for the fabrication of high-voltage organic photovoltaics (OPVs), ternary OPVs, and indoor OPVs. Herein, the recent progress in the A2 -A1 -D-A1 -A2 -type NFAs are reviewed, including the molecular engineering, structure-property relationships, voltage loss (Vloss ), device stability, and photovoltaic performance of binary, ternary, and indoor OPVs. Finally, the challenges and provided prospects are discussed for the further development of this type of NFAs.

Correlation of Broad Absorption Band with Small Singlet-Triplet Energy Gap in Organic Photovoltaics.

Organic photovoltaics (OPV) are one of the most effective ways to harvest renewable solar energy, with the power conversion efficiency (PCE) of the devices soaring above 19% when processed with halogenated solvents. The superior photocurrent of OPV over other emerging photovoltaics offers more opportunities to further improve the efficiency. Tailoring the absorption band of photoactive materials is an effective way to further enhance OPV photocurrent. However, the field has mostly been focusing on improving the near-infrared region photo-response, with the absorption shoulders in short-wavelength region (SWR) usually being neglected. Herein, by developing a series of non-fullerene acceptors (NFAs) with varied side-group conjugations, we observe an enhanced SWR absorption band with increased side-group conjugation length. The underpinning factors of how molecular structures and geometries improve SWR absorption are clearly elucidated through theoretical modelling and crystallography. Moreover, a clear relationship between SWR absorption intensity and singlet-triplet energy gap is established. These collectively establish a comprehensive correlation from NFA structure to non-radiative recombination loss in OPV, affording a high PCE of 18.5% when processed with a non-halogenated green solvent.

Defect evolution of iodine vacancy and related strain modulation in all-inorganic halide perovskites

Vacancy related defects play a crucial role in optoelectronic properties and carrier transport for photovoltaic materials, especially for its structural evolution becoming non-radiative defects induced by strain. Thus far, the evolution phenomena of vacancy defects in halide perovskite triggered by energy or strain have not been systematically investigated. Herein, we study the change in defect levels occurred in different inorganic perovskite systems and the situation caused by strain in varied strength based on density functional theory calculations. We discover that VI deep levels are easily transformed from shallow levels due to the formation of Pb–Pb dimers and octahedral distortion in all-inorganic perovskites, especially in CsPbI3. Moreover, strain can be quantitatively applied to control the suppression or enhancement of the formation of dimer in CsBI3 (B = Pb/Ge) perovskites. Eventually, our calculation results unravel that the defect physics of VI defect and the formation mechanism of non-radiative center in all inorganic perovskites, which depends on the strain strength and the accompanying octahedral distortion. The strain modulation and its quantitation effect on defect evolution of dominant vacancy map a pioneering route toward fabricating high performance inorganic photovoltaics.

Interfacial modification of LiF-incorporated MnO2 mixed P3HT:PC60BM-based organic photoactive layer

Interfacial charge transfer in the photoactive layer plays an essential role in determining device performance in organic photovoltaics. Recently, numerous studies have reported interfacial modification by incorporating inorganic nanostructures into the bulk heterojunction photoactive layer, with the aim of increasing the interfacial area to improve charge transfer efficiency. However, there is limited understanding of in-depth defect analysis and the charge transfer mechanism. In this report, different weight percentages of LiF were incorporated into MnO2 (LMO) mixed photoactive layers. The robust relationship between charge transport dynamics and defect density of the device in the presence of LiF is elucidated. The results show that the photoactive layer with 10.0 wt% LMO exhibited a 41% improvement in device performance compared to that of pristine P3HT:PC60BM, with a power conversion efficiency (PCE), open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) of 2.43%, 0.52 V, 9.77 mA/cm2, and 0.48, respectively. The photovoltaic performance improvement can be attributed to (i) the efficient charge transfer properties in the 10.0 wt% LMO, as evidenced by the suppressed S 2p and intensified O=C in X-ray photoelectron spectroscopy (XPS) analysis, and (ii) the low carrier recombination due to the success of LiF as a surface passivation agent, as indicated by 53% photoluminescence quenching. This report provides a fundamental understanding and offers an alternative solution for designing high-efficiency organic photovoltaics in the near future.

Effect of terminal fluorination of indenoindene-cored non-fullerene molecular acceptors for photovoltaic application

Two new non-fullerene molecule acceptors (NFAs), IDTIC and IDTIC-4F, based on a 5,10-dihydroindeno[2,1-a]indene core and 4H-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) bridges, were designed and synthesized. The fluorine-terminated IDTIC-4F shows a larger bathochromic shift than the non-fluorinated IDTIC. When blending with a PBDB-T polymer donor to fabricate organic solar cells, the IDTIC and IDTIC-4F acceptors respectively showed power conversion efficiencies (PCEs) of 5.60% and 6.77%. The PBDB-T:IDTIC-4F device performed better on Jsc and FF, which was attributed to the morphological evolution due to terminal fluorinated design of NFAs. Comparatively, the terminal fluorinated IDTIC-4F contributes to the formation of a more uniform film with less traps, which improves the charge transport and reduces the bimolecular recombination within the device. The work demonstrates that terminal fluorination of NFAs is an efficient design to modifying the morphology and performance of non-fullerene organic photovoltaics.

Role of Fullerene ICxA and Non-fullerene Y6 in P3HT-based Ternary Organic Photovoltaics

In recent years, organic photovoltaics (OPVs) have emerged as a promising technology for producing renewable energy. Developing high-performance ternary OPVs requires the discovery of new compounds. In this comprehensive study, we investigated the role of fullerene ICxA and non-fullerene Y6 acceptors as third components in improving the performance of P3HT-based ternary OPVs. Our devices exhibited the good light-harvesting ability of the non-fullerene acceptor and the efficient electron mobility of the fullerene acceptor. Raman and X-ray diffraction (XRD) tests show that increasing the ICxA content increases the molecular order of the ternary thin films, whereas increasing the Y6 content reduces their crystallisation. Results showed the synergistic effects of the weight ratio and the additional component on OPV performance.

Optimization of Bulk Heterojunction Organic Photovoltaics

The performance of poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) organic photovoltaic (OPV) devices was found to be strongly influenced by environmental during preparation, thermal annealing conditions, and the material blend composition. We optimized laboratory fabricated devices for these variables. Humidity during the fabrication process can cause electrode oxidation and photo-oxidation in the active layer of the OPV. Thermal annealing of the device structure modifies the morphology of the active layer, resulting in changes in material domain sizes and percolation pathways which can enhance the performance of devices. Thermal annealing of the blended organic materials in the active layer also leads to the growth of crystalline for P3HT domains due to a more arrangement packing of chains in the polymer. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) acts as a hole transport layer in these P3HT:PCBM devices. Two commercially materials of PEDOT:PSS were utilizing in the optimization of the OPV in this research; high conductivity PEDOT:PSS-PH1000 and PEDOT:PSS-Al4083, which is specifically designed for OPV interfaces. It was demonstrated that OPVs were prepared with PEDOT:PSS-PH1000 have a less than the average performance of PEDOT:PSS-Al4083. The power conversion efficiency (PCE) decreased clearly with a reducing in masking area devices from 5 mm2 to 3.8 mm2 for OPVs based on PH1000 almost absolutely due to the reduced short circuit current (Jsc). This work provides a roadmap to understanding P3HT:PCBM OPV performance and outlines the preparation issues which need to be resolved for efficient device fabrication

Organic Photovoltaic Performance Resiliency: Role of Molecular Weight in a PM7 Derivative

Organic Photovoltaic Performance Resiliency: Role of Molecular Weight in a PM7 Derivative