Pei-Tzu Wu

Solar Cells Based on Block Copolymer Semiconductor Nanowires: Effects of Nanowire Aspect Ratio

The solution-phase self-assembly of nanowires (NWs) from diblock copolymer semiconductors, poly(3-butylthiophene)-block-poly(3-octylthiophene), of different block compositions gave crystalline NWs of similar width (13-16 nm) but a tunable average aspect ratio (length/width) of 50-260. The power conversion efficiency of bulk heterojunction solar cells comprising the diblock copolythiophene NWs and PC(71)BM was found to increase with increasing aspect ratio, reaching 3.4% at the highest average aspect ratio of 260. The space charge limited current mobility of holes in neat films of the copolymer NWs and in copolymer NWs/PC(71)BM films (∼1.0 × 10(-4) cm(2)/(V s)) was invariant with aspect ratio, reflecting the parallel orientation of the NWs to the substrate. The enhancement of photovoltaic efficiency with increasing aspect ratio of NWs was explained in terms of increased exciton and charge photogeneration and collection in the bulk heterojunction solar cells.

Dithienopyrrole–quinoxaline/pyridopyrazine donor–acceptor polymers: synthesis and electrochemical, optical, charge-transport, and photovoltaic properties

Soluble alternating copolymers of N-(3,4,5-tri-n-dodecyloxyphenyl)dithieno[3,2-b:2′,3′-d]pyrrole donor groups and 2,3-di-n-decylquinoxaline, 2,3-di-n-decylpyrido[3,4-b]pyrazine, 2,3,6,7-tetrakis(n-decyloxy)benzo[a,c]phenazine, or 2,3,6,7-tetrakis(n-decyloxy)dibenzo[f,h]pyrido[4,3-b]quinoxaline acceptors were synthesised using Stille coupling reactions. Experimental absorption maxima in THF range from 645 to 770 nm. These optical data, along with the results of quantum-chemical calculations and electrochemical measurements, show that, as expected, the pyridopyrazine moiety acts as a stronger acceptor than quinoxaline and that the extended species benzophenazine and dibenzopyridoquinoxaline are stronger acceptors than quinoxaline and pyridopyrazine, respectively. Modest average hole mobilities of up to ca. 3.0 × 10−4 cm2 V−1s−1 were obtained in field-effect transistors. Bulk heterojunction photovoltaic devices made from blends of the benzo[a,c]phenazine-based polymer with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (1 : 3 weight ratio) exhibited average power conversion efficiencies up to 1.4%.