Side chain engineering of conjugated polymers for efficient all-polymer solar cells and eco-friendly solvent processable polymer solar cells

Abstract

Research objective set forth in this thesis is to develop efficient all-polymer solar cells (all-PSCs) and eco-friendly solvent processable polymer solar cells (eco-PSCs) via designing chemical structures of side chains in conjugated polymers. All-PSCs, which consists of a polymer donor ($P_D$) and a polymer acceptor ($P_A$) in the active layer, have emerged because of their promising benefits such as enhanced complementary light absorption from both $P_D$ and $P_A$, tunable energy levels of $P_D$ and $P_A$, and high photo-, thermal- and mechanical-stabilities. Despite these advantages, all-PSCs have suffered from low light-to-electricity conversion efficiency as compared to the conventional polymer/fullerene polymer solar cells, mainly due to undesirable polymer/polymer blend morphologies including large-scale phase separation of $P_D$ and $P_A$ with high χ-values, reduced chain orderings of PA in thin films, and poorly aligned interface orientation of $P_D$ and $P_A$. We demonstrated that those problems in all-PSCs can be overcome by careful side chain designs of both $P_D$ and $P_A$. For examples, a series of naphthalenedimide (NDI)-based polymer acceptors with different side alkyl chain lengths were designed and the impact of the side chain engineering on the polymer packing structures, the blend morphologies of polymer blends, and thus the resulting photovoltaic performances was investigated. In addition, we elucidated the importance of conjugated side chains in interfacial orientation of $P_D$ and $P_A$A via a rational design of model polymer donors and exhaustive morphology analyses by grazing incidence X-ray scattering and resonant soft X-ray scattering tools. Through these fundamental studies on the structure-morphology-performance relation, we could developed high-performance all-PSCs over 6.6% power conversion efficiency (PCE). In this thesis, we also tackled another important issue that polymer solar cell technology involves, that is, green solvents-based manufacturing of solar cell devices. Growing concerns on environment and human wellness encourage eco-friendly large-scale mass production of polymer solar cells, however, a large number of high-performance PSCs still requires the use of toxic halogenated, aromatic solvents including chloroform, chlorobenzene, toluene, etc. It is imperative to develop new photo-electroactive materials that can be processed with eco-friendly solvents such as ethanol or water. Therefore, based on the side chain engineering approach, we aimed to develop ethanol-soluble highly-crystalline p- and n-type photoactive materials. We have designed hydrophilic oligoethylene glycol (OEG) side chains and introduced them into efficient organic backbones for making them soluble in polar solvents. Ethanol-soluble conjugated polymers and fullerene derivatives with OEG side chains were synthesized (PPDT2FBTA polymer donor, Bis-$C_{60}$-A and $PC_{61}BO_{12}$), and indeed, the OEG-based materials were excellently dissolved in ethanol solvent. More strikingly, they exhibited good light absorption properties and crystalline properties. They were employed to fabricate eco-PSCs, and a PCE of 1.4% was achieved, which was the record photovoltaic performance among the developed eco-PCSs to date. A structure-property relationship in OEG-based materials was established in this study, which provides future challenges and important guidelines for design of novel efficient photoactive materials and devices.