The Investigation into the Effective Strategies for Improving the Performance of Organic Solar Cells

Abstract

Renewable energy has been developed to solve the serious issue of global climate change caused by increasing carbon dioxide emissions. Organic solar cells (OSCs) have attracted a lot of attention due to their potential for future energy source because of several advantages such as lightweight, low-cost production, and flexibility. Recently, the OSCs with high power conversion efficiency (PCE) over 18% have been realized with the introduction of non-fullerene acceptor (NFA) in which light absorption in near-infrared region (NIR) and adjustable chemical structure and energy level. However, higher PCE and better long-term stability are still required for the commercialization. Operation principle of OSCs is based on photo-induced charge transfer. Photo generated holes and electrons initially forms the excitons, and these excitons are dissociated at the interface of donor and acceptor. Thus, the effective dissociation of excitons has a significant influence on the performance of OSCs. Created excitons must move to the interface of donor and acceptor to be dissociated. If the phase of each donor and acceptor is larger than the exciton diffusion length, excitons can be recombined before reaching the donor-acceptor interface. Therefore, optimizing the well phase separated morphology of photoactive layer is one of the essential factors to achieve high efficiency. In addition, elimination of various trap sites, including charge transfer state and interface trap, is also an essential factor to obtain higher efficiency and better stability. Thus, in this study, various technologies such as ternary blend, additive engineering, and interfacial treatment have been applied to improve the performance and ensure the stability of OSCs. In first study, the ternary blend system was employed to diminish energy loss of a fullerene based OSCs. The photoactive layer with ternary blend system was formed by introducing the small molecule DRCN5T into binary system based on wide bandgap polymer donor PBDTTPD-HT and PC71BM. It was constructed cascading charge transfer by DRCN5T acting as a bridge and enabled indirect electron transfer from PBDTTPD-HT to PC71BM. As a result, a small amount of DRCN5T was introduced into PBDTTPD-HT:PC71BM to avoid the deep charge transfer state between PBDTTPD-HT and PC71BM, thereby increasing VOC and improving the efficiency of the device. In second study, the solid-solvent hybrid additive method was applied to the photoactive layer based on PM6:Y6 for simultaneously optimizing both the macroscopic donor-acceptor phase separation and the microscopic morphology such as π-π stacking and orientation of the inside phase. The solvent additive, 1-chloronaphthalene (CN) was used to optimize the macroscopic donor-acceptor phase separation and microscopic morphology was optimized using newly synthesized Star-A or Star-F with 3D structure as a solid additive. The effect of solid additive on morphology was confirmed through the measurements of grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS), which showed enhanced microscopic intermolecular π-π stacking within the phase as well as optimize the phase separation. Therefore, the performance of OSCs was improved by optimizing the morphology of photoactive layer using 1% of small amount of solid additive. In third study, in order to improve the performance of OSCs and ensure the long-term stability, the interfacial treatment method was applied. The deep trap generated at the interface between photoactive layer and metal oxide-based electron transport layer and shallow trap caused penetrated oxygen-containing defects increase trap-assisted recombination and reduce charge extraction efficiency, resulting in a decrease in VOC and long-term stability of the device. To overcome this issue, the interfacial defect between photoactive layer and electron transport layer was suppressed by chemical modification, thereby the device efficiency was improved to 17.43% and the stability was maintained almost 90% after 1200 h for air storage. In conclusion, by introducing various techniques, such as ternary blend system, additive engineering, and interfacial treatment to optimize the morphology of photoactive layer and suppressing the defect states, the OSCs with higher PCE and significantly improved stability were successfully demonstrated.