Development of high-performance transparent organic solar cells

Abstract

π-conjugated organic semiconductors have attracted attention because of their unique properties, such as solution processability, low weight, mechanical flexibility, and visible transparency. Consequently, semitransparent organic solar cells (ST-OSCs) are promising candidates for building-integrated photovoltaics (BIPV), vehicle-integrated photovoltaics (VIPV), and smart-window applications. To achieve high performance in ST-OSCs, both power conversion efficiency (PCE) and average visible transmittance (AVT) must be improved simultaneously; however, the intrinsic PCE–AVT trade-off limits further improvement. In this thesis, engineering of the photoactive layer and the transparent top electrode is pursued to improve AVT while limiting PCE reduction, thereby increasing light utilization efficiency (LUE = PCE × AVT), where larger values indicate a more favorable balance between transparency and power. Chapter 1 introduces a general overview of ST-OSCs. First, the chemical and electronic structure of π-conjugated organic semiconductors is reviewed. Next, device architecture and fundamental operating mechanisms of photovoltaics are outlined, including exciton generation, dissociation at donor–acceptor interfaces, charge transport, and charge collection. Finally, figures of merit used to evaluate transparent devices are summarized, and design strategies to mitigate the PCE–AVT trade-off are presented. Chapter 2 establishes a material design principle in which selenium substitution, combined with inner side-chain engineering, is used. Because selenium induces strong intermolecular Se⋯Se interactions, planarity, and extended effective conjugated length of photoactive materials, selenium-containing nonfullerene acceptors (NFAs) exhibit red-shifted absorption. Furthermore, side-chain engineering is used to control intermolecular stacking, charge mobility, crystallization, and the photoactive morphology. Accordingly, we newly synthesized a series of selenium-containing NFAs that are identical except for the inner alkyl side-chain: 2-butyloctyl (BO) versus 2-hexyldecyl (HD). Shorter inner side-chains show tighter packing, lower miscibility, higher electron mobility, and consequently higher short- circuit current with improved charge collection, whereas longer side-chains are associated with looser packing, higher miscibility, and reduced transport and current. In opaque devices, these behaviors result in a distinct current difference that correlates with side-chain length. Chapter 3 describes strategies for transparent top electrode engineering to enhance ST-OSC performance by precisely controlling the thickness of each layer. To increase the AVT of oxide/ultrathin metal/oxide (OMO) electrodes, the bottom oxide thickness is maintained at approximately 25–40 nm so that the two dielectric layers have nearly symmetric thicknesses and reinforce destructive interference in the photopic band. Compared with molybdenum oxide (MoO3), tungsten oxide (WO3) provides higher electrical conductivity and visible transparency while retaining an identical hole-transport mechanism. Therefore, WO3 enables a thicker bottom oxide and supports OMO multilayer electrodes with adequate, approximately symmetric dielectric thicknesses, leading to higher AVT. Guided by transfer-matrix optical simulations, optimized WO3/Ag/WO3-based ST-OSCs exhibit AVT of 46.7% and PCE of 7.0%, yielding an LUE of 3.27%, which is higher than that of MoO3/Ag/MoO3-based controls. Chapter 4 defines a photoactive layer engineering principle to enhance the performance of ST-OSCs. Because donors predominantly absorb in the visible region, reducing the donor ratio in the blend is a simple and effective strategy to increase AVT. However, PCE losses are caused by mobility imbalance and insufficient percolation, which increase recombination. To increase PCE while maintaining AVT, we introduce a hole- transporting and hole-selective small molecule, [4-(3,6-dimethyl-9H-carbazol-9- yl)butyl]phosphonic acid (Me-4PACz), as a photoactive additive into a low-donor-ratio bulk- heterojunction (BHJ). A small amount of Me-4PACz in the blend forms a self-organized hole- transporting layer (HTL) on top of the indium tin oxide electrode, serving as a hole-selective layer and reducing parasitic absorption losses. Furthermore, its higher solubility in chloroform than 2PACz and related derivatives leads to a substantial residence throughout the BHJ layer, thereby suppressing trap-assisted nonradiative recombination under a low-donor- ratio photoactive blend. This approach results in optimized ST-OSCs (HTL-free) with a high PCE of 10.70% and an AVT of 37.53%, achieving an impressive light utilization efficiency of 4.01% at a donor/acceptor ratio of 1:5. Chapter 5 provides an overall summary of the work and the conclusions drawn in this thesis.