Efficient polythiophene/polyfluorene copolymer bulk heterojunction photovoltaic devices: Device physics and annealing effects

Here the influence of annealing on the operational efficiency of all-polymer solar cells based on blends of the polymers poly(3-hexylthiophene) (P3HT) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2 ''-diyl) (F8TBT) is investigated. Annealing of completed devices is found to result in an increase in power conversion efficiency from 0.14 to 1.20%, while annealing of films prior to top electrode deposition increases device efficiency to only 0.19% due to a lowering of the open-circuit voltage and short-circuit current. By studying the dependence of photocurrent on intensity and effective applied bias, annealing is found to increase charge generation efficiency through an increase in the efficiency of the separation of bound electron-hole pairs following charge transfer. However, unlike many other all-polymer blends, this increase in charge separation efficiency is not only due to an increase in the degree of phase separation that assists in the spatial separation of electron-hole pairs, but also due to an order of magnitude increase in the hole mobility of the P3HT phase. The increase in hole mobility with annealing is attributed to the ordering of P3HT chains evidenced by the red-shifting of P3HT optical absorption in the blend. We also use X-ray photoelectron spectroscopy (XPS) to study the influence of annealing protocol on film interface composition. Surprisingly both top and bottom electrode/blend interfaces are enriched with P3HT, with the blend/top electrode interface consisting of more than 95% P3HT for as-spun films and films annealed without a top electrode. Films annealed following top electrode deposition, however, show an increase in F8TBT composition to similar to 15%. The implications of interfacial composition and the origin of open-circuit voltage in these devices are also discussed.