Fully conjugated block copolymers enhance thermal stability of polymer blend solar cells

Fully conjugated donor-acceptor block copolymers are composed of covalently-linked conjugated polymers that can leverage self-assembly to realize an ordered microstructure in the active layer of photovoltaic devices. Providing a thermodynamic driving force for mesoscale structure formation may also provide enhanced morphological stability. In this work, a new donor-acceptor block copolymer is synthesized and added into polymer blend solar cells to examine their photovoltaic performance under thermal stress. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [W3,4-b]thiophene)-2-carboxylate-2-6-diyl)]-block-poly{[N,N′-bis(2-octyldodecyl) naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PTB7Th-b-PNDI) was synthesized through sequential Stille reactions. Photovoltaic performance of ternary blends comprised of PTB7Th and PNDI homopolymers with various amounts of block copolymers were examined. The addition of the block copolymer improves thermal stability, but with an initial drop in power conversion efficiency (PCE) that follows block copolymer content. Nevertheless, inclusion of 5 % of PTB7Th-b-PNDI by weight in the active layer can effectively reduce 30 % of thermal degradation of the photovoltaic devices under thermal stress at 110 °C for 168 h, without sacrificing initial PCE. Donor-acceptor block copolymers are expected to provide a broad, generalizable approach to enhance thermal stability of polymer blend solar cells.