Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes over 30 years (original) (raw)
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Advanced Energy Materials, 2019
decades, heuristic approaches have been successfully used to enhance the performance of OSCs, including synthesis of new donor and acceptor molecules, [5-10] optimization of the interface and morphology, [11-15] ternary blending, [16-19] and device engineering. [20-23] As a result, power conversion efficiency (PCE) of OSCs has surpassed 15% for single-layer bulkheterojunction systems. [24,25] As the PCE of OSCs is getting closer to the requirements for commercial applications and competing technology, the most efficient OSCs also need to perform consistently or maintain low efficiency loss throughout their lifetimes. [26-28] To make organic photovoltaics commercially viable and competitive, researchers have been making efforts on characterizing, understanding, and rationally engineering the long-term stability of OSC devices. [26,29-38] Generally, the performance degradation of OSCs comes from the oxidation of electrodes, degradation of the interface layers, and changes in the morphology of the active layer. Among these factors, the oxidation of electrodes and the degradation of the interface layers are attributed to exposure to oxygen and moisture, [39,40] and these drawbacks can be largely prevented by encapsulation. [41,42] However, the intrinsic instability of active layer morphology driven by light, temperature, and thermodynamics cannot be prevented by encapsulation. In-depth studies were carried out to understand the stability of the active layers under multiple stresses. [43] For instance, McGehee and co-workers [29] reported that solar cells based on amorphous materials would suffer from open-circuit voltage (V OC) burnin degradation resulted from the impact of light-induced traps, and this degradation can be reduced by using materials with high degree of crystallinity. Brabec group [33] demonstrated that the light-induced [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) dimerization would lead to the short-circuit current density (J SC) loss after aging, while this dimerization can be inhibited by a high degree of polymer-fullerene mixing and it can also be reduced via increasing the crystallization of the fullerene domains. Brabec and co-workers [34] found that burnin degradation driven by low miscibility is the major short-time Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7-Th:IEICO-4F binary solar cells and to rationally use PC 71 BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO-4F in PTB7-Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC 71 BM in PTB7-Th of 48 vol% is well above the percolation threshold. At the same time, PC 71 BM is partly miscible in IEICO-4F suppressing crystallization of IEICO-4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7-Th:IEICO-4F binary solar cells, demonstrates the structure-function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor.
Toward reliable high performing organic solar cells: Molecules, processing, and monitoring
APL Materials, 2020
A steady surge in device efficiencies of organic solar cells (OSCs) along with improvement in associated features, such as stability and facile processing methods, is expected to provide a realistic, feasible commercial option. The introduction of high performing donor and acceptor molecules along with tailored buffer layers has provided the impetus for the resurgence of this field. Further options of ternary and tandem architectures of these OSC systems should push this technology to competitive levels. A major hurdle, which is expected when these devices are evaluated for long-term performance in all weather conditions, is the level of degradation. We examine and address these stability-limiting factors in this perspective article. Modifications in microstructure/morphology and interfaces with time and energy levels defining the molecules form some of the critical intrinsic degradation pathways. Various strategies that have been used to limit the associated pathways of degradation of the active layer will be discussed. One such strategy is electric field-assisted thermal annealing treatment, which concomitantly also brings in a favorable vertical phase segregated active layer morphology. We also emphasize the utility of photocurrent noise measurements to monitor the level of degradation and possibly forecast the trajectory of long-term performance of OSCs.
Lifetime Study of Organic Solar Cells with O-IDTBR as Non-Fullerene Acceptor
Frontiers in Energy Research, 2021
Organic solar cells (OSCs) have increased their power conversion efficiency above 18% thanks to the use of non-fullerene acceptors in binary or ternary blends or in tandem configurations. In this article, a study on the lifetime of P3HT:O-IDTBR bulk heterojunction OSCs on ITO-free flexible substrates is presented. A direct comparison of glass–glass and plastic–plastic encapsulation performance, with a special focus on its effect on the lifetime of the devices after degradation procedures, has been carried out complying with the ISOS protocols for organic photovoltaic devices. The manufactured OSCs with 1 cm2 active layer have power conversion efficiencies ranging from 1.9 to 3.4% depending on the encapsulant material, encapsulation process, and substrate. An exponential degradation rate has been found, with a similar functional behavior for glass and plastic differing in the degradation constants, which ranges from k = 0.01 to 0.002 h−1. Only in one case, the ISOS-T3 essay for plast...
This research provides a structure-property relation that sheds light on morphological stability of NF-OSCs by using the thermodynamic and the kinetic perspectives. We show that NF-OSCs can suffer from excessive amorphous-amorphous phase separation in the blends and crystallization of NF-SMA. The former Q9 instability channel can be eliminated in systems with an optimal miscibility, whereas the excessive phase separation in low miscibility systems and NF-SMA crystallization need to be suppressed through the utilization of polymers or NF-SMAs with low flexibility.
Implications of the device structure on the photo-stability of organic solar cells
Solar Energy Materials and Solar Cells, 2014
Small molecule organic solar cells (OSCs) are systematically studied for their photo-stability in an inert N 2 environment. In this work, 28-day dark, light-stress and heat-stress experiments are conducted on OSCs with strongly varied mixing ratios. Comparisons are made between the emerging, high performance Schottky device structure and the more widely studied (standard) BHJ structure. In both structures, light stress experiments result in a 10-15% loss in power conversion efficiency. Simultaneous heat-stress experiments demonstrate that these variations are not related to thermally induced changes. Photo-induced losses in open circuit voltages are observed for both device structures and are attributed to organic-electrode degradation. However, variations in the other photovoltaic parameters are more strongly dependent on the active layer composition and associated device structure. Schottky OSCs are shown to be slightly more resilient to variations in short circuit current compared to standard BHJ OSCs, but they suffer from losses in fill factor. Microsecond transient photocurrent and external quantum efficiency measurements are employed to show that these fill factor losses are due to increased recombination, associated with increased trap density by the exciton-induced degradation of the C 60 acceptor. This effect may be problematic for all Schottky OSCs due to their use of mixed layers with very high C 60 content. The choice of device architecture is thus shown to alter degradation mechanisms, and so it can have implications on the overall OSC photo-stability.