Influence of nanoscale morphology in small molecule organic solar cells

Organic Solar Cells were fabricated by co-evaporation of C-60:Cu-Phthalocyanine blend films in vacuum on poly(ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) coated indium-tin oxide substrates. At 80 nm blend film thickness external quantum efficiencies of up to 58% were recorded. Two different methods were applied to manipulate the morphology of the active film: Current treatment of solar cells improved the power conversion efficiency to about 2.0%. Treated cells were shown by scanning electron microscopy to contain agglomerates with approximately 30 nm diameter. As a second approach the influence of substrate temperature during deposition on the film morphology was examined by scanning probe microscopy. The morphology changed from amorphous films at room temperature to structure sizes of approximately 300 nin at 145 +/- 6 degrees C. Solar cells performance reached a power conversion efficiency of 2.1 % when deposited at a substrate temperature of 51.5 +/- 2.5 degrees C. We conclude that film morphologies with structures around 30 mn are advantageous for device efficiency as opposed to both amorphous films and larger structure sizes. (c) 2005 Elsevier B.V. All rights reserved.