Nanomorphology control and novel materials studies in polymer/fullerene bulk heterojunction solar cells.

摘要

This dissertation focuses on improving the efficiency of polymer/fullerene bulk heterojunction solar cells from two directions: (i) understanding the nanomorphology-efficiency relationship and controlling the active layer morphology during its formation; (ii) developing new materials for both electron donors and electron acceptors in the polymer solar cells.;A solvent annealing approach is successfully demonstrated to control the morphology and increase device efficiency in poly(3-hexylthiophene):fullerene solar cells. A detailed evolution study on this approach using absorption, photoluminescence, external quantum efficiency, atomic force microscopy, and grazing incidence X-ray diffraction techniques leads to following understanding: the optimum nanomorphology must be a balance between large interfacial area for exciton dissociation and continuous pathways for carrier transportation. 4.4% efficiency is demonstrated in this system. Effects of solvent mixture on the nanoscale phase separation are studied further. The donor/acceptor components in the active layer can "intelligently" phase separate into an optimum morphology during the spin-coating process and no further treatment is necessary. Devices with the solvent mixture show about 10 times higher efficiency compared to those devices fabricated without the additive solvent fabricated under the same condition. A model and additive solvent selection rule are proposed to explain the phenomenon.;To address the absorption mismatch with solar spectrum, two novel low band gap copolymers containing 3-alkoxythiophene have been synthesized with the band gap of 1.64 and 1.77 eV, respectively. In addition, novel electron acceptors also hold great promise. A 50% increase in short-circuit current is demonstrated by using (6,6)-phenyl-C71-butyric acid methyl ester (C70-PCBM) to replace (6,6)-phenyl-C61-butyric acid methyl ester (PCBM). As the result, 2.4% power conversion efficiency is achieved for low band gap polymer based solar cells. Another low band gap polymer, an ester group modified polythieno[3,4-b]thiophene, is utilized to fabricate near-infrared photodetector with external quantum efficiency exceeding 38%, response bandwidth of 4 MHz, and the noise equivalent power of 3.85 x 10-12 W/Hz1/2 at 850 nm. An all-polymer based optocoupler with high current-density-transfer-ratio (1.5%) and fast response time (500 kHz) is also demonstrated in this work.