Challenges to the Stability of Active Layer Materials in Organic Solar Cells

Photodegradation of Organic Solar Cells under Visible Light and the Crucial Influence of Its Spectral Composition

While wavelength-dependent photodegradation of organic solar cells (OSCs) under visible light is typically discussed in terms of UV/blue light-activated phenomena, we recently demonstrated wavelength-dependent degradation rates up to 660 nm for PM6:Y6. In this study, we systematically investigated this phenomenon for a broad variety of devices based on different donor:acceptor combinations. We found that the spectral composition of the light used for degradation, tuned in a spectral range from 457 to 740 nm and under high irradiances of up to 30 suns, has a crucial influence on the device stability of almost all tested semiconductors. The relevance of this phenomenon was investigated in the context of simulated AM1.5 illumination with metal halide lamps and white LEDs. It is concluded that the current stability testing protocols in OSC research have to be adjusted to account for this effect to reveal the underlying physics of this still poorly understood mechanism.

Optoelectronic conversion and polarization hysteresis in organic MISM and MISIM devices with DA-type single-component molecules

Organic electronic devices offer various advantages, such as low cost and tunability. However, the organic semiconductors used in these devices have significant drawbacks, including instability in air and low carrier mobility. To address these challenges, we recently introduced organic MISM and MISIM (M = metal, I = insulator, S = semiconductor) devices, which effectively generate photo-induced displacement current and exhibit ferroelectric behavior. In previous studies, the S layer consisted of an organic donor-acceptor (DA) bilayer. In the present research, we fabricated MISM and MISIM devices using DA-type single-component molecules as the S layer and examined their photocurrent and polarization hysteresis. While the performance of these devices does not surpass that of DA bilayer devices, we discovered that DA-type single-component molecules can be utilized for photoelectric conversion and polarization trapping.

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy

In addition to the donor-acceptor nano phases, the intermixed phase within the organic blends is crucial for the photovoltaic performance and stability of the bulk-heterojunction organic solar cells (OSCs). Here, the intermixed phase of a representative M-PhS:BTP-eC9 all-small-molecule organic solar cell was investigated by a concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy method, where a shift of the absorption maximum wavelength was measured for the acceptor component with the increase of the acceptor concentration. The blend ratios of the acceptor to the donor in the intermixed phase, corresponding to the critical concentration for the formation of the acceptor nanophase (CAP), were determined to be 0.35, 0.20, and 0.15 for the as-cast, thermal annealing (TA), and the combined TA and solvent vapor annealing films. These results indicated that M-PhS and BTP-eC9 are kinetically well intermixed during spin coating, whereas TA and the following solvent annealing promote the crystallization of BTP-eC9 molecules out of the intermixed phase. The photovoltaic performance of the M-PhS:BTP-eC9 cells with different blend ratios was investigated. The formation of the BTP-eC9 nano phase in the blend film leads to stable VOC and fast increased JSC, which can be understood by the reduction of bimolecular charge recombination and the formation of electron transporting pathways within the photoactive layer. Similarly, the critical concentration for the formation of the donor phase was estimated to be 0.15 by measuring the stabilized VOC and increased JSC values of the cells with different donor blending ratios. More importantly, after a fast "burn-in" thermal degradation, the M-PhS:BTP-eC9 cell showed excellent thermal stability aging at 85 °C for over 1128 h, which is in good accordance with the unchanged intermixed phases measured by the UV-vis spectra of the annealed films. The current work demonstrates the feasibility of the spectroscopy method to investigate the intermixed phases for organic bulk-heterojunction solar cells and proves that all-small-molecule solar cells can be intrinsically very stable.

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.

Single-component organic solar cells-Perspective on the importance of chemical precision in conjugated block copolymers

Organic photovoltaics (OPV) present a promising thin-film solar cell technology with particular benefits in terms of weight, aesthetics, transparency, and cost. However, despite being studied intensively since the mid 90's, OPV has not entered the mass consumer market yet. Although the efficiency gap with other thin-film photovoltaics has largely been overcome, active layer stability and performance reproducibility issues have not been fully resolved. State-of-the-art OPV devices employ a physical mixture of electron donor and acceptor molecules in a bulk heterojunction active layer. These blends are prone to morphological changes, leading to performance losses over time. On the other hand, in "single-component" organic solar cells, the donor and acceptor constituents are chemically connected within a single material, preventing demixing and thereby enhancing device stability. Novel single-component materials affording reasonably high solar cell efficiencies and improved lifetimes have recently emerged. In particular, the combination of donor and acceptor structures in conjugated block copolymers (CBCs) presents an exciting approach. Nevertheless, the current CBCs are poorly defined from a structural point of view, while synthetic protocols remain unoptimized. More controlled synthesis followed by proper structural analysis of CBCs is, however, essential to develop rational structure-property-device relations and to drive the field forward. In this perspective, we provide a short overview of the state-of-the-art in single-component organic solar cells prepared from CBCs, reflect on their troublesome characterization and the importance of chemical precision in these structures, give some recommendations, and discuss the potential impact of these aspects on the field.

Processability Considerations for Next-Generation Organic Photovoltaic Materials

The evolution of organic semiconductors for organic photovoltaics (OPVs) has resulted in unforeseen outcomes. This has provided substitute choices of photoactive layer materials, which effectively convert sunlight into electricity. Recently developed OPV materials have narrowed down the gaps in efficiency, stability, and cost in devices. Records now show power conversion efficiency in single-junction devices closing to 20%. Despite this, there is still a gap between the currently developed OPV materials and those that meet the requirements of practical applications, especially the solution processability issue widely concerned in the field of OPVs. Based on the general rule that structure determines properties, methodologies to enhance the processability of OPV materials are reviewed and explored from the perspective of material design and views on the further development of processable OPV materials are presented. Considering the current dilemma that the existing evaluation indicators cannot reflect the industrial processability of OPV materials, a more complete set of key performance indicators are proposed for their processability considerations. The purpose of this perspective is to raise awareness of the boundary conditions that exist in industrial OPV manufacturing and to provide guidance for academic research that aspires to contribute to technological advancements.

Oligomerized Fused-Ring Electron Acceptors for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have attracted wide research attention in the past decades. Very recently, oligomerized fused-ring electron acceptors (OFREAs) have emerged as a promising alternative to small-molecular/polymeric acceptor-based OSCs due to their unique advantages such as well-defined structures, batch reproducibility, good film formation, low diffusion coefficient, and excellent stability. So far, rapid advances have been made in the development of OFREAs consisting of directly/rigidly/flexibly linked oligomers and fused ones. In this Minireview, we systematically summarized the recent research progress of OFREAs, including structural diversity, synthesis approach, molecular conformation and packing, and long-term stability. Finally, we conclude with future perspectives on the challenges to be addressed and potential research directions. We believe that this Minireview will encourage the development of novel OFREAs for OSC applications.

Semitransparent Solar Cells Employing n-Type Graphene on LaVO3

To effectively utilize solar energy, semitransparent solar cells are essential in various fields such as building-integrated solar power generation and portable solar chargers. We report triethylenetetramine (TETA)-doped graphene (Gr) transparent conductive electrode (TCE)-based LaVO₃ semitransparent solar cells. To optimize the Gr TCE, we varied the TETA molar concentration (n_D) from 0.1 to 0.3 mM. TETA-doped Gr (TETA-Gr)/LaVO₃ semitransparent solar cells exhibit the highest 1.45% efficiency and 62% average visible transmittance at n_D = 0.2 mM. These results indicate that the TETA-Gr/LaVO₃ structure not only harvests solar energy in the ultraviolet-visible region but also exhibits translucency, thanks to the thin film. Thanks to its translucent properties, we improved the power conversion efficiency (PCE) to 1.99% by adding an Al reflective mirror to the semitransparent cells. Finally, the device's PCE loss is only within 3% for 3000 h in air, suggesting good durability.

Lifetime over 10000 hours for organic solar cells with Ir/IrOx electron-transporting layer

The stability of organic solar cells is a key issue to promote practical applications. Herein, we demonstrate that the device performance of organic solar cells is enhanced by an Ir/IrO_x electron-transporting layer, benefiting from its suitable work function and heterogeneous distribution of surface energy in nanoscale. Notably, the champion Ir/IrO_x-based devices exhibit superior stabilities under shelf storing (T₈₀ = 56696 h), thermal aging (T₇₀ = 13920 h), and maximum power point tracking (T₈₀ = 1058 h), compared to the ZnO-based devices. It can be attributed to the stable morphology of photoactive layer resulting from the optimized molecular distribution of the donor and acceptor and the absence of photocatalysis in the Ir/IrO_x-based devices, which helps to maintain the improved charge extraction and inhibited charge recombination in the aged devices. This work provides a reliable and efficient electron-transporting material toward stable organic solar cells.

On the interface reactions and stability of nonfullerene organic solar cells

Long-term stability is critical for organic solar cells (OSCs) for practical applications. Several factors affect the stability of OSCs, including materials stability, morphology stability of bulk-heterojunctions and interface stability. In this perspective, we focus on interface stability due to interfacial reactions between the emerging acceptor-donor-acceptor (A-D-A) type nonfullerene active layers and interfacial layers. The description covers the initial phenomena of interfacial instability, mechanism of interfacial reactions, and strategies adopted to suppress interfacial reactions between the nonfullerene active layers and interfacial layers. Methods to test and analyze the chemical instability of nonfullerene acceptors are also included. The C[double bond, length as m-dash]C vinyl linker between the donor moiety and acceptor moiety is chemically or photochemically reactive and is a weak point for interface stability. The interface stability of OSCs could be enhanced by reducing the reactivity of the C[double bond, length as m-dash]C vinyl linker or removing it directly, modifying the surface of interfacial layers, and developing other novel interfacial materials.

BODIPY Cored A-D-A'-D-A Type Nonfused-Ring Electron Acceptor for Efficient Polymer Solar Cells

In this work, boron dipyrromethene (BODIPY) is for the first time employed as electron-deficient core (A') to construct an A-D-A'-D-A type nonfused-ring electron acceptor (NFREA) for polymer solar cells (PSCs). Among, cyclopentadithiophene (CPDT) and fluorinated dicyanoindanone (DFIC) are involved as electron-donating (D) bridges and terminal A groups, respectively. Bearing with the steric BODIPY core, tMBCIC exhibits twisted configuration with dihedral angles >45^o  between BODIPY and CPDT bridges. Thus, compared with the BODIPY-free planar A-D-D-A structured bCIC, reduced aggregation, weakened intramolecular D-A interactions with up-shifted LUMO by 0.4 eV as well as blue-shifted absorption by up to 150 nm is observed in tMBCIC. Moreover, owing to the intrinsic large molar extinction coefficient from BODIPY, promoted light-harvest ability is achieved for tMBCIC, particularly in its blend films. Therefore, PSCs by using PBDB-T as donor, tMBCIC as NFREA afford superior power conversion efficiency (PCE) of 9.22% and higher open-circuit voltage (V_oc ) of 0.954 V compared to 4.47% and 0.739 V from bCIC-devices. Moreover, compared to other BODIPY-flanked electron acceptors (<5%) reported so far, BODIPY-cored tMBCIC realizes a remarkable progress in PCE. This article is protected by copyright. All rights reserved.

Morphological Stabilization in Organic Solar Cells via a Fluorene-Based Crosslinker for Enhanced Efficiency and Thermal Stability

Power conversion efficiencies (PCEs) and device stability are two key technical factors restricting the commercialization of organic solar cells (OSCs). In the past decades, though the PCEs of OSCs have been significantly enhanced, device instability, especially in the state-of-the-art nonfullerene system, still needs to be solved. In this work, an effective crosslinker (namely, DTODF-4F), with conjugated fluorene-based backbone and crosslinkable epoxy side-chains, has been designed and synthesized, which is introduced to enhance the morphological stabilization of the PM6:Y6-based film. This crosslinker with two epoxy groups can be in situ crosslinked into a stable network structure under ultraviolet radiation. We demonstrate that DTODF-4F, which acted as a third component, can promote the exciton dissociation rate and reduce traps/defects, finally resulting in the enhancement of efficiency. In particular, the OSC devices exhibit better stability under continuous heating owing to the morphology fixation of the bulk heterojunction. This work drives the development direction of morphological stabilization to further improve the performance and stability of OSCs.

Rylene-Fullerene Hybrid an Emerging Electron Acceptor for High-Performing and Photothermal-Stable Ternary Solar Cells

Molecular carbon imides, especially extended perylene diimides (PDIs) have been the best wide-band-gap nonfullerene acceptors. Despite their excellent photothermal/chemical stability, flexible reaction sites, and unique photoelectronic properties, there is still a lack of fundamental understanding of their molecular characteristics as a third component. Here, generations of PDIs with distinctive molecular architecture, are deliberately screened out as the third component to PM6:Y6. Only a rylene-fullerene hybrid, S-Fuller-PMI, surprisingly boosts the fill factor (FF) of ternary organic solar cells (OSCs) to 0.77 from 0.72 for PM6:Y6 binary ones, and therefore the power conversion efficiency (PCE) of ternary cells is enhanced from 15.3% to 16.2%. Compared with highly-flexible rylene dimer and rigid multimer, S-Fuller-PMI exhibits higher electron mobility, favorable surface tension, and, therefore tailored compatibility with Y6. These formed Y6:S-Fuller-PMI alloys play as a morphological controller to improve charge separation and transport process. Simultaneously, the suppressed photothermal-induced traps, along with inherent enlarged entropy effect, endow the ternary OSCs still with ≈70% of initial PCE even after 500 h continuous illumination, whereas only 53% is left in their binary counterparts. These results provide new insight into the molecular design principle for distinctive molecular carbon imides as the third component for efficient and durable PM6:Y6-based OSCs.

Study of light-induced degradation of polymer: fullerene solar cells

In the present study, the role of active and transport layer materials in the degradation mechanism is studied using different device geometries.

Optimized Stoichiometry for CuCrO2 Thin Films as Hole Transparent Layer in PBDD4T-2F:PC70BM Organic Solar Cells

The performance and stability in atmospheric conditions of organic photovoltaic devices can be improved by the integration of stable and efficient photoactive materials as substituent of the chemically unstable poly (3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS), generally used as organic hole transport layer. Promising candidates are p-type transparent conductive oxides, which combine good optoelectronic and a higher mechanical and chemical stability than the organic counterpart. In this work, we synthesize Cu-rich CuCrO2 thin films by aerosol-assisted chemical vapour deposition as an efficient alternative to PEDOT:PSS. The effect of stoichiometry on the structural, electrical, and optical properties was analysed to find a good compromise between transparency, resistivity, and energy bands alignment, to maximize the photovoltaic performances., Average transmittance and bandgap are reduced when increasing the Cu content in these out of stoichiometry CuCrO2 films. The lowest electrical resistivity is found for samples synthesized from a solution composition in the 60-70% range. The optimal starting solution composition was found at 65% of Cu cationic ratio corresponding to a singular point in Hackee's figure of merit of 1 × 10-7 Ω-1. PBDD4T-2F:PC70BM organic solar cells were fabricated by integrating CuCrO2 films grown from a solution composition ranging between 40% to 100% of Cu as hole transport layers. The solar cells integrating a film grown with a Cu solution composition of 65% achieved a power conversion efficiency as high as 3.1%, representing the best trade-off of the optoelectronic properties among the studied candidates. Additionally, despite the efficiencies achieved from CuCrO2-based organic solar cells are still inferior to the PEDOT:PSS counterpart, we demonstrated a significant enhancement of the lifetime in atmospheric conditions of optimal oxides-based organic photovoltaic devices.

Simultaneous Improvement of Efficiency and Stability of Organic Photovoltaic Cells by using a Cross-Linkable Fullerene Derivative

Improving power conversion efficiencies (PCEs) and stability are two main tasks for organic photovoltaic (OPV) cells. In the past few years, although the PCE of the OPV cells has been considerably improved, the research on device stability is limited. Herein, a cross-linkable material, cross-linked [6,6]-phenyl-C61-butyric styryl dendron ester (c-PCBSD), is applied as an interfacial modification layer on the surface of zinc oxide and as the third component into the PBDB-TF:Y6-based OPV cells to enhance photovoltaic performance and long-term stability. The PCE of the OPV cells that underwent the two-step modification increased from 15.1 to 16.1%. In particular, such OPV cells exhibited much better stability under both thermal and air conditions because of the decreased number of interfacial defects and stable interfacial and active layer morphologies. The results demonstrated that the introduction of a cross-linkable fullerene derivative into the interfacial and active layers is a feasible method to improve the PCE and stability of OPV cells.

Benzo[1,2-b:4,5-b']difuran Polymer-Based Non-Fullerene Organic Solar Cells: The Roles of Non-Fullerene Acceptors and Molybdenum Oxide on Their Ambient Stabilities and Processabilities

The ambient stability and processability of organic solar cells (OSCs) are important factors for their commercialization. Herein, we selected four benzo[1,2-b:4,5-b']difuran (BDF) polymers and two electron acceptors to examine the role of photovoltaic materials in the ambient stability. The investigations revealed that the MoO_x layer is the detrimental factor for the ambient stabilities. The penetration of MoO_x into the active layer and their interactions will strengthen the interface and form a favorable contact, hence leading to the increased photovoltaic performance, in which the efficiency loss induced by air was balanced out. As such, these BDF polymer-based non-fullerene (NF) OSCs possessed very promising ambient stabilities even after ∼1000 h with the almost maintained power conversion efficiencies (PCEs). These results drive us to further investigate the ambient processability of these NF-OSCs. The PCEs from the devices processed under ambient condition only possessed 0.3-2% loss compared to those devices under inert conditions, which suggest the significant potentials of BDF polymers to develop highly efficient and stable NF-OSCs for the practical applications.

Broadband photomultiplication organic photodetectors

Broadband photomultiplication organic photodetectors (PMOPDs) can be achieved with a double-layered active layer prepared from IEICO-4F : PBDB-T blend solutions with different weight ratios (1 : 1 or 3 : 100, wt/wt). The response range of the double-layered PMOPDs covers from 310 nm to 930 nm, determined by the photon harvesting range of the IEICO-4F : PBDB-T (1 : 1, wt/wt) layer. The IEICO-4F : PBDB-T (3 : 100, wt/wt) layer was used as a PM layer in the double-layered PMOPDs, achieving external quantum efficiency (EQE) more than 100% based on the work mechanism of trap-assisted hole tunneling injection. The trapped electrons in PBDB-T/IEICO-4F/PBDB-T near the Al electrode will makeinterfacial-band-bending to narrow the injection barrier, resulting in hole-tunneling-injection from the external circuit. The polymer PBDB-T can provide an efficient charge transport channel for the injected hole from the external circuit. The specific detectivity (D*) and responsivity (R) of the double-layered PMOPDs are 1.05 ± 0.03 × 1012 Jones and 0.94 ± 0.03 A W-1 at 810 nm under a -10 V bias, respectively.

High-Efficiency Thermal-Annealing-Free Organic Solar Cells Based on an Asymmetric Acceptor with Improved Thermal and Air Stability

The stability of organic solar cells (OSCs) is an urgent problem for commercialization. In this work, a novel asymmetric molecule TB-4Cl was designed and synthesized. Quantum chemical computations revealed that TB-4Cl has a larger dipole moment of 1.98 Debye than that of Y6, which can induce a stronger intermolecular interaction. Without thermal annealing, devices based on PM6:TB-4Cl achieved a higher efficiency of 14.67%. Impressively, all of the devices showed a negligible difference in power conversion efficiency (PCE) before and after thermal-annealing treatment. Compared to the unencapsulated PM6:Y6-based devices, PM6:TB-4Cl-based devices exhibited improved thermal and air stability, evidenced by retaining around 75% (TB-4Cl) and 60% (Y6) after being heated at 100 °C in nitrogen for 110 h and 65% (TB-4Cl) and 50% (Y6) in air for 92 h. This work indicates that an A-D₁A'D₂-A asymmetric molecule can be a promising candidate for achieving stable OSCs with high efficiency.