Light-Induced Open Circuit Voltage Increase in Polymer Solar Cells with Ternary Bulk Heterojunction Nanolayers

We report a light-induced open circuit voltage (V-oc) increase in polymer solar cells with ternary bulk heterojunction (BHJ) layers that are composed of poly(3-hexylthiophene) (P3HT), poly[(4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,S-b']dithiophene)-2,6-diyl-alt--(N-2-ethylhexylthieno[3,4-c]pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD), and [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM). The ternary BHJ layers were prepared by varying the composition of donor polymers at a fixed ratio (1:1 by weight) of donor (P3HT + PBDTTPD) to acceptor (PC61BM). Results showed that V-oc was gradually increased under continuous illumination of solar light (100 mW/cm(2)) for ternary solar cells, whereas no V-oc increase was measured for binary solar cells without PBDTTPD. As a consequence, the power conversion efficiency (PCE) of ternary solar cells (except the highest PBDTTPD content) was rather higher after solar light illumination for 10 h, even though the binary solar cell exhibited significantly lowered PCE after 10 h illumination. The V-oc increase has been attributed to the lateral phase segregation between P3HT and PBDTTPD domains in the ternary BHJ layers under continuous illumination of solar light, as evidenced from the analysis result by Raman spectroscopy, atomic force microscopy, transmission electron microscopy, and synchrotron radiation grazing-incidence angle X-ray diffraction measurements.