Substituting the postproduction treatment for bulk-heterojunction solar cells using chemical additives

Postproduction treatment of poly (3-alkylthiophene) based bulk-heterojunction solar cells is an important and required procedure in order to fabricate organic solar cells with highest power conversion efficiencies. Postproduction treatment, by means of annealing solar cells at elevated temperatures during which an external voltage is simultaneously applied, is not very suitable for large scale production schemes due to the need for well controlled environment and involvement of flexible substrates. A faster and easier method for improving the efficiency, like a simple addition of chemicals, would be highly desirable. We have studied the effect of alkyl thiol addition into bulk-heterojunction solar cells, based on poly(3-hexylthiophene) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) mixtures as reported first by Santa Barbara group [J. Peet, C. Soci, R.C. Coffin, T.Q. Nguyen, A. Mikhailovsky, D. Moses, G. C. Bazan, Appl. Phys. Lett. 89 (2006) 252105, J. Peet, J.Y. Kim, N.E. Coates, W.L Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497], which gives the same final results in even higher performance of organic solar cells through increased power conversion efficiency and thus substitutes the postproduction treatment. Red-shift in optical absorption is seen in the films with alkyl thiol, resembling the absorption of thermally annealed films. Based on steady-state current-voltage characteristics and transient charge carrier extraction by linearly increasing voltage (CELIV) measurements, the conductivity of thermally annealed and films with alkyl thiol is found to be an order of magnitude higher than in films spun from pristine blends of P3HT and PCBM. Charge carrier mobility measurements indicate significant increase in carrier mobility, consistent with the improved structural order and formation of interpenetrating network between the donor and acceptor in the bulk-heterojunction solar cells. (C) 2008 Elsevier B.V. All rights reserved.