Understanding the Role of Side-Chain Structure On the Morphology and Performance of All-Conjugated Block Copolymer Photovoltaics

Understanding the role of side-chain structure on the morphology and performance of all-conjugated block copolymer photovoltaics

Kendall Smith, Yen-Hao Lin,  Bridget Stewart, Jorge Mok, and Rafael Verduzco

Rice University, Department of Chemical and Biomolecular Engineering, Houston, TX

All-conjugated block copolymers with donor (p-type) and acceptor (n-type) polymer blocks offer a route to ordered bulk heterojunctions through micro-phase segregation. Recent work demonstrated near 3 % power conversion efficiencies in poly(3-hexylthiophene) – block – poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (P3HT-b-PFTBT) block copolymers, but the optimal relationship between block copolymer solubility, thin film crystallinity, morphology, and optimal performance is unknown. Here, we explore the role of side-chain structure on block copolymer solubility, crystallization, and performance. Block copolymers are synthesized through a two-step route involving Grignard metathesis polymerization followed by Suzuki-Miyaura polycondensation.  A series of PFTBT polymers with varying side-chain lengths and with branched side-chains are investigated by grazing incidence X-ray scattering (GIXS). Crystallization and micro-phase segregation is investigated in a series of solution processed block copolymer thin films. Shorter side-chains increase crystallization and packing in the PFTBT domains, but lead to reduced solubility in organic solvents. Co-crystallization is observed in block copolymers with balanced block ratios, and the orienataion of polymer crystallites can be controlled (primarily face-on vs. primarily edge-on) through variation of the processing conditions. All-polymer and polymer-fullerene block copolymer blends are tested as the active layer in solution processed photovoltaics.This work demonstrates performance enhancements through systematic variation of block copolymer side-chain structure, which lead to changes in crystallization and self-assembly in block copolymer thin films.