Area-scaling of organic solar cells

We report on the performance of organic solar cells based on pentacene/C-60 heterojunctions as a function of active area. Devices with areas of 0.13 and 7 cm(2) were fabricated on indium-tin-oxide (ITO) coated glass. Degradation of the performance with increased area is observed and analyzed in terms of the power loss density concept. The various power loss contributions to the total series resistance (R(S)A) are measured independently and compared to the values of the series resistance extracted from the current-voltage characteristics using a Shockley equivalent circuit model. The limited sheet resistance of ITO is found to be one of the major limiting factors when the area of the cell is increased. To reduce the effects of series resistance, thick, electroplated, metal grid electrodes were integrated with ITO in large-area cells. The metal grids were fabricated directly onto ITO and passivated with an insulator to prevent electrical shorts during the deposition of the top Al electrode. By integrating metal grids onto ITO, the series resistance could be reduced significantly yielding improved performance. Design guidelines for metal grids are described and tradeoffs are discussed.