Stability Of Organic Photovoltaics Research Articles

Spray-Coated Transition Metal Dichalcogenides as Hole Transport Layers in Inverted NFA-Based Organic Photovoltaics with Enhanced Stability under Solar and Artificial Light

In this study, we explored the potential of exfoliated transition metal dichalcogenides (TMDs) as innovative spray-coated hole transport layers (HTLs) in organic photovoltaics (OPVs), addressing the need for efficient and stable materials in solar cell technology. This research was motivated by the need for alternative HTLs that can offer enhanced performance under varying lighting conditions, particularly in indoor environments. Employing UV-visible absorption and Raman spectroscopy, we characterized the optical properties of MoS2, MoSe2, WS2, and WSe2, confirming their distinct excitonic transitions and direct bandgap features. The nanocrystalline nature of these TMDs, revealed through XRD patterns and crystallite size estimation using the Scherrer method, significantly contributes to their enhanced physical properties and operational efficiency as HTLs in OPVs. These TMDs were then integrated into OPV devices and evaluated under standard solar and indoor lighting conditions, to assess their effectiveness as HTLs. The results demonstrated that MoS2, in particular, displayed remarkable performance, rivalling traditional HTL materials like MoO3. It maintained high power conversion efficiency across a spectrum of light intensities, illustrating its versatility for both outdoor and indoor applications. Additionally, MoS2 showed superior stability over extended periods, suggesting its potential for long-term usage in OPVs. This study contributes significantly to the field of photovoltaic materials, presenting TMDs, especially MoS2, as promising candidates for efficient and stable OPVs in diverse lighting conditions, thereby broadening the scope of solar cell applications.

Spontaneous Compositional-Interfacial Co-Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron-Transporting Layer for Exceptionally Long-Term Stability of Photovoltaics.

The long-term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional-interfacial co-modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH4PF6) into antisolvent to form gradient structures through a simple one-step solvent engineering. With the assistance of the ion exchange reaction, NH4PF6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long-term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH4PF6 for preventing H2O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long-term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N2 (ISOS-D-1I), ambient air (ISOS-D-1), and 85°C (ISOS-D-2I) for 14016, 2500, and 1248h, respectively. These performances compare well with the state-of-the-art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.

Spray-Coated MoO3 Hole Transport Layer for Inverted Organic Photovoltaics.

This study focuses on the hole transport layer of molybdenum trioxide (MoO3) for inverted bulk heterojunction (BHJ) organic photovoltaics (OPVs), which were fabricated using a combination of a spray coating and low-temperature annealing process as an alternative to the thermal evaporation process. To achieve a good coating quality of the sprayed film, the solvent used for solution-processed MoO3 (S-MoO3) should be well prepared. Isopropanol (IPA) is added to the as-prepared S-MoO3 solution to control its concentration. MoO3 solutions at concentrations of 5 mg/mL and 1 mg/mL were used for the spray coating process. The power conversion efficiency (PCE) depends on the concentration of the MoO3 solution and the spray coating process parameters of the MoO3 film, such as flow flux, spray cycles, and film thickness. The results of devices fabricated from solution-processed MoO3 with various spray fluxes show a lower PCE than that based on thermally evaporated MoO3 (T-MoO3) due to a limiting FF, which gradually increases with decreasing spray cycles. The highest PCE of 2.8% can be achieved with a 1 mg/mL concentration of MoO3 solution at the sprayed flux of 0.2 mL/min sprayed for one cycle. Additionally, S-MoO3 demonstrates excellent stability. Even without any encapsulation, OPVs can retain 90% of their initial PCE after 1300 h in a nitrogen-filled glove box and under ambient air conditions. The stability of OPVs without any encapsulation still has 90% of its initial PCE after 1300 h in a nitrogen-filled glove box and under air conditions. The results represent an evaluation of the feasibility of solution-processed HTL, which could be employed for a large-area mass production method.

Tuning Surface Defect States in Sputtered Titanium Oxide Electron Transport Layers for Enhanced Stability of Organic Photovoltaics.

Nonfullerene acceptors (NFAs) have dramatically improved the power conversion efficiency (PCE) of organic photovoltaics (OPV) in recent years; however, their device stability currently remains a bottleneck for further technological progress. Photocatalytic decomposition of nonfullerene acceptor molecules at metal oxide electron transport layer (ETL) interfaces has in several recent reports been demonstrated as one of the main degradation mechanisms for these high-performing OPV devices. While some routes for mitigating such degradation effects have been proposed, e.g., through a second layer integrated on the ETL surface, no clear strategy that complies with device scale-up and application requirements has been presented to date. In this work, it is demonstrated that the development of sputtered titanium oxide layers as ETLs in nonfullerene acceptor based OPV can lead to significantly enhanced device lifetimes. This is achieved by tuning the concentration of defect states at the oxide surface, via the reactive sputtering process, to mitigate the photocatalytic decomposition of NFA molecules at the metal oxide interlayers. Reduced defect state formation at the oxide surface is confirmed through X-ray photoelectron spectroscopy (XPS) studies, while the reduced photocatalytic decomposition of nonfullerene acceptor molecules is confirmed via optical spectroscopy investigations. The PBDB-T:ITIC organic solar cells show power conversion efficiencies of around 10% and significantly enhanced photostability. This is achieved through a reactive sputtering process that is fully scalable and industry compatible.

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.

Investigating the Photostability of Organic Photovoltaics for Indoor and Outdoor Applications

Organic photovoltaics (OPVs) show great promise for both outdoor and indoor applications. However, there remains a lack of understanding around the stability of OPVs, particularly for indoor applications. In this work, the photostability of the poly[(thiophene)‐alt‐(6,7‐difluoro‐2‐(2‐hexyldecyloxy)quinoxaline)]:2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile blend is investigated for both outdoor and indoor applications. Photostability is found to vary drastically with illumination intensity. Devices under high‐intensity white light‐emitting diode (LED) illumination, with their short‐circuit current density (JSC) matching JSC–EQE for AM1.5 G illumination, lose 42% of their initial performance after 30 days of illumination. Contrastingly, after almost 47 days of illumination devices under 1000 lux white LED illumination show no loss in performance. The poor photostability under 1 sun illumination is linked to the poor photostability of IDIC. Through Raman spectroscopy and mass spectrometry, IDIC is found to suffer from photoisomerization, which detrimentally impacts light absorption and carrier extraction. In this work, it is highlighted that under low light levels, the requirement of intrinsic material photostability may be less stringent.

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.

Evaluation of Ethylene-Vinyl Acetate, Methyl Methacrylate, and Polyvinylidene Fluoride as Encapsulating Materials for Perovskite-Based Solar Cells, Using the Low-Temperature Encapsulation Method in a Cleanroom Environment

In this article, the development of a stable perovskite-based photovoltaic device manufactured in a controlled environment, with humidity between 40 and 65%, and encapsulated is presented. Encapsulation using polymers like ethylene-vinyl acetate (EVA), polymethyl methacrylate (PMMA), and EVA combined with polyvinylidene fluoride (PVDF) was proposed due to the low curing temperatures, insulating properties, and simple deposition processes of these materials. Testing involved subjecting these materials to humidity, temperature, and UV irradiation, following the International Summit on Stability of Organic Photovoltaics (ISOS-T) protocols, and using a 24 W UV lamp. Characterization analyses were carried out using various technologies including digital microscopy, spectroscopic ellipsometry, Fourier-transform infrared spectroscopy and electrical simulations. The results indicate that EVA-encapsulated samples displayed higher stability and resistance against external factors compared to PMMA and EVA-PVDF. Specifically, the EVA-encapsulated samples maintained a 15.06% power conversion efficiency (PCE) after the thermal cycles were carried out, reducing only by 0.9% compared to pristine samples. Similarly, after 350 h of UV exposure, they retained a PCE of 13.90%, decreasing by just 9.58% compared to the initial value.

Trace Doping: Fluorine-Containing Hydrophobic Lewis Acid Enables Stable Perovskite Solar Cells.

With the rapid developments in perovskite solar cell (PSC), high efficiency has been achieved, but the long-term operational stability is still the most important challenges for the commercialization of this emerging photovoltaic technology. So far, bi-dopants Li-TFSI/t-BP doped hole-transporting materials (HTM) have led to state-of-the art efficiency in n-i-p PSCs. However, such dopants have several drawbacks in terms of stability, including the complex oxidation process, undesirable ion migration and ultra-hygroscopic nature. Herein, a fluorine-containing organic Lewis acid dopant bis(pentafluorophenyl)zinc (Zn-FP) with hydrophobic property and high migration barrier has been employed as a potential alternative to widely employedbi-dopants Li-TFSI/t-BP for PTAA. The resulting Zn-FP-based PSCs achieve a maximum PCE of 20.34% with J-V hysteresis-free. Specifically, the unencapsulated device exhibits significantly advancement of operational stability under the International Summit on Organic Photovoltaic Stability protocols (ISOS-L-1), maintaining over 90% of the original efficiency after operation for 1000 hours under continuous 1-sun equivalent illumination in N2 atmosphere in both forward and reverse J-V scan.

Novel Diffusion‐Regulated Layering Methodology to Improve Blend Miscibility and Thermal Stability of Organic Photovoltaics

AbstractExtensive research on bulk‐heterojunction (BHJ) optimization has advanced organic photovoltaics (OPVs). However, the need for research addressing the issue of morphological instability and ensuring long‐term durability remains a priority. Herein, a diffusion‐governed morphological modification methodology via a sequential deposition (SD) process comprising ternary components with low miscibility is demonstrated. Sequential coating of a high glass transition temperature (Tg) material and a host binary blend induces a concentration difference between successively coated layers, allowing for effective blending of immiscible materials during solvent evaporation. The enhanced miscibility of the SD‐processed BHJ layer facilitates molecular interactions between the high Tg material and the host materials, thereby increasing the Tg of the BHJ blend. The SD‐processed OPVs exhibit superior photovoltaic performance and suppressed glass transition under thermal stress compared to reference OPVs fabricated via a conventional method. After 500 h of thermal aging at 85 °C, the SD‐BHJ OPV retains over 80% of its initial efficiency, whereas the reference device shows a drastic drop to below 80% of its initial efficiency after only 80 h. This study provides a step toward efficient, long‐term stable OPVs by overcoming the limitations of blend miscibility and poor thermal durability of conventional BHJ systems via a SD process.

Important Role of Additive in Morphology of Stretchable Electrode for Highly Intrinsically Stable Organic Photovoltaics

Important Role of Additive in Morphology of Stretchable Electrode for Highly Intrinsically Stable Organic Photovoltaics

Hexanary blends: a strategy towards thermally stable organic photovoltaics

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells.

The Mechanism of the Photodimerization of [60]Fullerene Derivatives: It is Still a Charge Separation Followed by a Back Transfer!

AbstractHerein, we present a systematic study of the photochemical behavior of a set of fullerene compounds bearing various organic addends attached to the fullerene cage. It has been shown that the yield of photodimerization products strongly depends on the structure and electronic properties of the material, primarily the electron affinity (acceptor strength) of the modified fullerene core. This finding being inconsistent with the conventional [2+2]cycloaddition mechanism and pointed to an alternative pathway. This proposed pathway involves photoinduced charge separation followed by back charge transfer events leading to the efficient generation of triplet excitons, which are responsible for the formation of dimerized species. The light‐induced charge separation pathway was confirmed by electron spin resonance spectroscopy and was supported by theoretical calculations. The revealed mechanism allows one to control the photochemical behavior of the fullerene derivatives via rational structural engineering: some of the compounds were shown to be fully resistant to photodimerization, which makes them promising candidates in the development of stable organic photovoltaics. Furthermore, this approach could be used to suppress back charge transfer in donor‐acceptor blends and hinder the formation of triplet excitons which represents one a major energy loss channel in the current generation of organic solar cells.

Unraveling the Degradation Pathway of Inverted Perovskite Solar Cells Based on ISOS-D-1 Protocol.

Perovskite solar cells (PSCs) have shown rapid development recently, whereas nonideal stability remains the chief obstacle toward commercialization. Thus, it is of utmost importance to probe the degradation pathway for the entire device. Here, the extrinsic stability of inverted PSCs (IPSCs) is investigated by using standard shelf-life testing based on the International Summit on Organic Photovoltaic Stability protocols (ISOS-D-1). During the long-term assessment of 1700h, the degraded power conversion efficiency is mainly caused by the fill factor (53% retention) and short-circuit current density (71% retention), while the open-circuit voltage still maintains 97% of the initial values. Further absorbance evolution and density functional theory calculations disclose that the perovskite rear-contact side, in particular for the perovskite/fullerene interface, is the predominant degradation pathway. This study contributes to understanding the aging mechanism and enhancing the durability of IPSCs for future applications.

Improvement of Encapsulation Technique of Organic Photovoltaics by UV-Curable Adhesive

This study develops a low-cost UV adhesive as a sealant and the associated encapsulation architecture, highly improving the stability of organic photovoltaic (OPV) device. In detail, this present study demonstrates how the composition and additive in the UV adhesive as a sealant affect the OPV stability. The face sealing and UV adhesive enable the OPV device to successfully meet three industrial criteria of accelerated tests of OPV stability: (1) T80 lifetime is more than 1000 h under the damp heat test of 65 0C/65% relative humidity (RH) environment, (2) T80 lifetime is more than 1000 h under the damp heat test of 85 0C/85% RH environment, and (3) T80 lifetime is larger than 1000 h under the continuous AM 1.5G solar illumination (i.e., light-soaking test).

Low‐Temperature Prepared ZnO Layer with Electron Beam Annealing Process for Enhancing the Environmental, Thermal, and Operational Stability of Organic Photovoltaics

Low‐Temperature Prepared ZnO Layer with Electron Beam Annealing Process for Enhancing the Environmental, Thermal, and Operational Stability of Organic Photovoltaics

Organic Photovoltaic Stability: Understanding the Role of Engineering Exciton and Charge Carrier Dynamics from Recent Progress.

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.

Low‐Temperature Prepared ZnO Layer with Electron Beam Annealing Process for Enhancing the Environmental, Thermal, and Operational Stability of Organic Photovoltaics

The commercialization of organic photovoltaics (OPVs) requires a high level of stability and a high‐power conversion efficiency (PCE). To satisfy these requirements, inverted OPVs with electron transport layers of ZnO obtained by electron beam annealing (EBA) are fabricated, and their properties are compared with those of a reference device with a thermal‐treated ZnO layer. Electrons are extracted from Ar plasma and accelerated by supplying a negative DC voltage; the ZnO layer is annealed using the energy from the accelerated electrons. The PCE of the OPV with a ZnO layer obtained by the optimal EBA process is 15.6%, which is 11.4% higher than that of the reference device. In addition, the OPV device retains 90.1% of its initial PCE after it is stored at room temperature for 30 days and 70.1% of the initial PCE after 280 h at 90 °C. Further, the operational stability is measured for 500 min at the maximum power point under 1 sun illumination in ambient air. The OPV with the ZnO layer treated under optimal EBA conditions retains 85.0% of the initial PCE and shows outstanding environmental and thermal stability.

In Situ Solution-Processed Submicron Thick SiOx Cy /a-SiNx (O):H Composite Barrier Film for Polymer:Non-Fullerene Photovoltaics.

Aiming to improve the environmental stability of organic photovoltaics, a multilayered SiOx Cy /a-SiNx (O):H composite barrier film coated with a hydrophobic perfluoro copolymer stop layer for polymer:non-fullerene solar cells is developed. The composite film is prepared by spin-coating of polysilicone and perhydropolysilazane (PHPS) following a densification process by vacuum ultraviolet irradiation in an inert atmosphere. The transformation of polysilicone and PHPS to SiOx Cy and a-SiNx (O):H is confirmed by Fourier transform infrared and energy-dispersive X-ray spectroscopy measurement. However, the as-prepared PHPS-derived silicon nitride (PDSN) can react with moisture in the ambient atmosphere, yielding microscale defects and a consequent poor barrier performance. Treating the incomplete PDSN with methanol vapor significantly densifies the film yielding low water vapor transmission rates (WVTRs)of 5.0 × 10-1 and 2.0 × 10-1 gm-2 d-1 for the one- and three-couple of SiOx Cy /a-SiNx (O):H (CON) composite films, respectively. By incorporating a thin hydrophobic perfluoro copolymer layer, the three-coupled methanol-treated CON film with a total thickness of 600nm shows an extremely low WVTR of 8.7 × 10-4 gm-2 d-1 . No performance decay is measured for the PM6:Y6 and PM6:L8-BO cells after such an encapsulation process. These encapsulated polymer cells show good stability storaged at 25°C/50% relative humidity, or under simulated extreme rainstorm tests.

Role of Nonfullerene Acceptor Impurities and Purification on the Efficiency and Stability of Organic Photovoltaics

The introduction of nonfullerene acceptors (NFAs) has pushed the power conversion efficiency and organic photovoltaics (OPV) device stability to new standards. In this aspect, removal of trace impurities from one purification stage to the next is frequently stressed throughout the synthesis of photoactive OPV materials and NFAs to obtain the highest‐purity material. However, detailed studies of the effect of purification on device performance are less reported. Herein, the role of NFA trace impurities on the optoelectronic characteristics and lifetime of resulting OPV devices is studied. The optimization of PBDB‐T:ITIC‐X devices, with various ITIC purity levels (X), has been thoroughly studied via a combination of photophysical, chemical, morphological, electrical, and optical characterization techniques, to shine light on the role of these impurities on device performance and lifetime. The findings suggest that, even in materials with larger concentrations of trace impurities, careful tuning can produce high efficiencies. Interestingly, the less‐pure materials lead to longer device lifetimes along with an enhancement in accumulative power generation by a factor 3, compared to the purest ITIC‐based devices. This demonstrates that selecting a material with the highest purity may not always be the best option for NFA OPV and that any positive effects of NFA purification must be carefully considered in light of both the device efficiency and stability.