Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology** By Wanli Ma, Cuiying Yang, Xiong Gong, Kwanghee Lee, and Alan J. Heeger* 1. Introduction Solar-cell technology based on conjugated polymer±fullerene composites continues to be of interest as a potential source of renewable energy. [1±5] In particular, because of the light weight, mechanical flexibility, and potential for low-cost production of electronic devices fabricated from semiconducting polymers, high-efficiency polymer solar cells could have a major impact on energy needs. Although encouraging progress has been made in recent years with 3±4 % power conversion efficiencies under AM 1.5 (AM = air mass) illumination, [6] this efficiency is not yet sufficient for large-scale implementation. Moreover, concerns about instability at elevated temperatures have hin- dered the path toward commercialization. The need to improve the device efficiency and thermal stability requires state-of- the-art fabrication and treatment under more rigorously de- fined conditions. The device performance of a photovoltaic cell is character- ized by the short-circuit current (I sc ), open-circuit voltage (V oc ), and fill factor (FF). Higher values of these three parame- ters yield larger light-to-electricity power-conversion efficiency (g e ): g e = I sc V oc FF (1) Thus, the device efficiency can be enhanced by implementing fabrication procedures that influence I sc and FF (for a specific donor±acceptor system, V oc is determined by the energy level difference between the component materials of the compos- ite [7,8] ). Based upon the equivalent circuit of a photovoltaic cell, [9] the current density vs. voltage (I±V) characteristics can be de- scribed by the following equation: [10] I  I 0 expe UIR S nkT  1 UIR S R SH  I PH (2) where I 0 is the dark current, e is the electron charge, n is the diode ideality factor, U is the applied voltage, R S is the series resistance, R SH is the shunt resistance, and I PH is the photocur- rent. Thus, to obtain high short-circuit currents, I SC (U = 0 V), solar-cell devices must have small R S and large R SH . The fill factor can be written as: FF  V MPP I MPP V OC I SC (3) where the subscript MPP denotes the maximum power point. Thus, a large FF requires that the photocurrent rise abruptly as U approaches V oc (i.e., to obtain the maximum of the V MPP  I MPP product). This optimum condition can only be met when the photogenerated carriers are extracted without signifi- cant loss from recombination. Therefore, the fill factor is lim- ited by the carrier drift length, L d L d = lsE (4) where l is the carrier mobility, s is the carrier recombination lifetime, and E is the electric field; L d must be longer than the active layer thickness to prevent significant loss by recombina- tion. [11,12] Hence a high mobility or thin film is necessary for efficient charge-carrier extraction. One approach toward improving both I sc and FF is through postproduction heat treatment. Recent studies have demon- strated improved power-conversion efficiencies after thermally annealing polymer±fullerene composite solar cells at elevated temperatures (50±130 C). [6,13±16] In these earlier studies, the best results (g e » 3.5 %) were obtained after postproduction thermal annealing at 75 C for 5 min. [6] Here, we report that by applying the specific device-fabrica- tion conditions summarized in the Experimental section and postproduction annealing at 150C, bulk heterojunction solar Adv. Funct. Mater. 2005, 15, 1617±1622 DOI: 10.1002/adfm.200500211  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1617 ± [*] Prof. A. J. Heeger, W. Ma, C. Yang, Dr. X. Gong, Dr. K. Lee Institute for Polymers and Organic Solids University of California at Santa Barbara Santa Barbara, CA 93106±5090 (USA) E-mail: ajhe@physics.ucsb.edu [**] This research was supported by Konarka Technologies (Lowell, MA) and by the Air Force Office of Scientific Research, AFOSR, under FA9550±05±0139 and by AFOSR through the MURI Center (ªPolymer Smart Skinsº, F49620±01±10364), Charles Lee, Program Officer. By applying the specific fabrication conditions summarized in the Experimental section and post-production annealing at 150 C, polymer solar cells with power-conversion efficiency approaching 5 % are demonstrated. These devices exhibit remark- able thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by ther- mal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the im- proved contact to the electron-collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells. FULL PAPER