Development of Organic Polymer Interfacial Materials to Improve the Stability of Solution-Processed Photovoltaic Cells

Abstract

Photovoltaic cells have been considered as a promising alternative of clean and renewable energy production to solve serious environmental and energy problems. To commercialize the photovoltaic devices, high stability should be significantly considered because there are several issues limiting the stability of photovoltaic devices such as moisture, oxygen, reaction and diffusion of electrode, irradiation, heating, and mechanical stress. Interfacial materials could be one of the crucial strategies to enhance the stability of solar cells due to their key roles such as energy level tuning at the organic/electrode interface, improving charge transporting and electrode selectivity, introducing optical effects to enhance light-trapping, and modulating bulk heterojunction (BHJ) morphology evolution. Despite the frantic efforts to develop new organic interfacial materials, more stable materials are still required. In chapter 1, an overview of photovoltaics and interfacial materials in photovoltaics are generally explained. In chapter 2, rational designed small molecule semiconductors (SMs) were investigated for use as donors in organic solar cells through truncation approach. In general, SMs have many advantages over polymers including well-defined molecular structures, monodispersity, and no batch-to-batch dependence. Although SM semiconductors can be designed by truncation from polymers, such examples have rarely been reported. Herein we designed SM semiconductors by truncating a representative polymer, Poly[4-(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b’]dithiophen-2-yl)-alt-benzo[c][1,2,5]thiadiazole] (PBDTBT). Based on density functional theory (DFT) calculations, 2,1,3-benzothiadiazole (BT) was chosen as an electron acceptor subunit instead of thieno[3,4-c]pyrrole-4,6-dione (TPD). The SM semiconductors were end-capped with pyridine derivatives. In particular, the benzyloxypyridine-capped semiconductor (SM1) exhibited power conversion efficiency (PCE) of 1.92% which is higher than those shown by the corresponding polymer PBDTBT (0.90% and 1.71%). In chapter 3, newly designed polymers were applied to the dopant-free hole transporting materials (HTMs) in perovskite solar cells (PSCs) with maintaining high stability. To overcome device instability without deteriorating PCE, dopant-free HTMs are needed to separate the air-sensitive perovskite layer from extrinsic factors which induce its degradation. Herein we developed novel conjugate polymers of benzo[1,2-b:4,5-b′]dithiophene (BDT) and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) for use as HTMs without dopants. The pBDT-BODIPY polymer allows individual “dialing” of the HOMO or LUMO levels with small modifications to the molecular structure, enabling the study of the impact of the frontier molecular orbital on PSC performance. Different alkyl chains on BDT can minutely adjust the HOMO level, and meso-substituents on BODIPYs can selectively set the LUMO level of the resulting polymers. Application of BODIPY-containing polymer into the perovskite solar cell as an HTM leads to a high PCE value (16.02%) and exceptional solar cell stability shown by the fact that over 80% of its original PCE value maintained after 10 days under ambient air conditions. In chapter 4, a series of polyethyleneimine (PEI) derivatives were synthesized for use as cathode interfacial materials (CIMs) in organic solar cells. Despite an excellent contribution to the development of fullerene-based solar cells (FSCs), PEI as a representative CIM shows inferior stability in nonfullerene-based solar cells (NFSCs) due to reactive amines. To solve this problem, there are a few attempts. Nevertheless, a clear reason underlying the side reaction is not demonstrated yet, and more stable NFSCs and more facile synthesis of new CIMs are required. Here, we synthesized new PEI derivatives through a simple imine formation reaction to control work function values and a simple methylation reaction to shield reactivity of a secondary amine. To confirm the possibility for use as CIMs, the density functional theory (DFT) calculation and the fundamental characterization was conducted. These PEI derivatives applied to CIMs in FSCs and NFSCs. The highest power conversion efficiency (PCE) values are achieved 9.61% in FSCs and 12.34% in NFSCs, respectively, which is better performance compared to ZnO-based devices. Furthermore, the methylated CIMs-containing NFSCs show improved thermal stability as well as enhancement of the device performance after thermal treatment.