High-efficiency organic photovoltaics for indoor and outdoor applications

Abstract

With the rapid development of the economy, non-renewable resources such as fossil fuels and natural gas are being depleted. One solution to maintain sustainable development is to utilize solar energy, which can be harvested directly through photovoltaic (PV) technology to produce electricity. In recent years, organic solar cells (OSCs)/organic photovoltaics (OPVs) in the PV field have attracted more and more attention due to their advantages of low cost, flexibility, light weight, and large area producibility using roll-to-roll technology. They are a promising candidate for realizing environmentally-friendly and low-cost PV technology that can convert irradiance into electrical power.
In addition to utilizing sunlight that is only available outdoors, artificial light supplied indoors can also be collected and converted into electricity. As the market for IoT nodes (such as sensors, watches, calculators, remote controls, hearing aids, and monitors) used in relatively mild indoor environments rapidly grows, the demand for artificial light energy harvesters to supply continuous and cordless power for indoor environments has emerged. When the illuminance is switched from 1-sun to dim light, the PV technology that adopts organic material as the photon capture layer is called indoor organic photovoltaics (IOPVs).
However, achieving a breakthrough in energy transfer efficiency, whether in indoor or outdoor environments, is still a challenge. To address this challenge, the aim of this study is to improve the power conversion efficiency (PCE) of the OPVs system by designing novel molecules that can be applied under different spectra, optimizing processing technology, and studying the underlying principles of efficiency improvement. In summary, the study can be divided into three main parts:
The first part focuses on improving device performance that can be applied under both outdoor and indoor environments by designing a novel molecule resulting in a compromise absorption spectrum and studying the influence of different processing technologies under outdoor and indoor conditions. A novel asymmetric acceptor TB-4Cl, modified from Y6, was designed and synthesized to create a trade-off absorption spectrum that can be applied under both artificial light and sunlight. Furthermore, processing technology optimization was carried out. Two different processing methods, conventional bulk-heterojunction (C-BHJ) and sequential deposition bulk-heterojunction (SD-BHJ), were compared to study their difference under sunlight and artificial light. The optimal devices of PM6:TB-4F produced PCE of 14.44% and 15.24% for C-BHJ and SD-BHJ, respectively, under the illumination of AM1.5. For 1000-lux LED illumination, devices showed PCE of 16.82% and 21.05% for C-BHJ and SD-BHJ, respectively. SD-BHJ showed comparable performance than C-BHJ under sunlight. However, the PCE was significantly increased by 25% for SD-BHJ compared to C-BHJ under artificial light because of the strong influence of trap-assisted recombination and dark current on PCE in the condition of low carrier density. Our result indicates that asymmetric molecule with blue-shifted spectrum fabricated by SD-BHJ can be a promising candidate to be applied in the indoor environment to harvest sunlight and artificial light simultaneously.
The second part focuses on improving the performance of IOPVs through the systematic modification of molecules to tune the intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) in order to achieve high PCEs under indoor environments. I carefully investigated the effects of asymmetric skeleton and non-chlorination on terminal groups by thoroughly evaluating the photovoltaic performance and analyzing the intra-CT and inter-CT processes. Two new acceptors with an A-D1A'D2-A structure were designed and synthesized to compare the performance of an asymmetric backbone with two specified end groups. At the condition of 1000-lux illumination, the TB-SCl acceptor with the halogenous end group, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene) molononitrile (1), coupled with PM6 acquired a PCE of 19.7%. The TB-S with non-halogenous end group, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) molononitrile (2), blended with PM6 yielded a higher PCE of 23.3%. In comparison, symmetric molecule BTP-2ThCl only produced a low PCE of 14.6%. The results indicate that tuning the intra-CT and inter-CT processes through reducing a fused ring to form an asymmetric skeleton and increasing the electronic density of the end group, based on the Y6 series molecule, is an effective strategy to increase the PCE of IOPVs. And our investigation can provide molecular design guidance for IOPVs.
The final part of the study aims to investigate the correlation among the eigen-properties of asymmetric skeleton non-fullerene (ASNF) molecules, the morphology of the active layer, and the photovoltaic performance of ternary organic photovoltaic (OPV) devices. Two wider bandgap ASNF acceptors, named TB-S1 and TB-S1-O, were synthesized based on TB-S. These three ASNF molecules have similar skeletons but different terminal groups or alkyl/alkoxy side chains which were incorporated into the host system PM6:BTP-eC9 as a third component, respectively. Interestingly, all the binary OPV devices for ASNF acceptors produced significantly higher V_OC than that of the PM6:BTP-eC9 system (0.949-1.120 vs. 0.837 V), however, only incorporating TB-S1-O into the host system had a positive effect of VOC improvement. The alkoxy substituted TB-S1-O showed a cascade energy level alignment, a good absorption complementarity and an excellent compatibility with PM6/BTP-eC9. Moreover, the PM6:BTP-eC9:TB-S1-O ternary film exhibited an ideal interpenetrating network, leading to reduced ∆Enonrad and enhanced charge transfer. As a result, the PCE was increased from 17.36% (binary film) to 18.14% (ternary film) when TB-S1-O was blended into the PM6:BTP-eC9 system due to the increase in VOC and JSC. In contrast, the incorporation of TB-S and TB-S1 formed an inferior morphology and thus failed to reduce voltage losses, resulting in a poorer PCE of 16.16% and 16.18%, respectively. This work reveals that alkoxy substitution on asymmetric backbone is an efficient method to construct the third component for high-performance ternary organic solar cells.

Date of Award	Jul 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Tao Wu (Supervisor), Bencan Tang (Supervisor) & Ziyi Ge (Supervisor)