Analysis of Charge Photogeneration as a Key Determinant of Photocurrent Density in Polymer: Fullerene Solar Cells

• DOI: 10.1002/adma.201002357
• Publication Date: 2010-09-08T13:42:08Z
• Authors: Tracey M. Clarke; Amy Ballantyne; Safa Shoaee; Ying W. Soon; Warren Duffy; Martin Heeney; Iain McCulloch; Jenny Nelson; James R. Durrant
• Keywords: Polymers; Electric Conductivity; 01 natural sciences; 7. Clean energy; Absorption; 0104 chemical sciences; Engineering; Electric Power Supplies; Physical Sciences and Mathematics; Solar Energy; Sunlight; Fullerenes

Signifi cant progress has been made in relating the voltage output of organic solar cells to materials’ properties, specifi cally to the energy difference between the donor ionisation potential and acceptor electron affi nity. [ 1–3 ] However, progress in predicting device photocurrent densities on the basis of materials or fi lm properties has proved much more problematic. Signifi cant attention has focused upon enhancing light-harvesting effi ciency by reducing the optical bandgap of the photoactive layer, as discussed in recent reviews. [ 4–6 ] Most models of device effi ciency have typically assumed a unity yield for exciton dissociation into separated charges, requiring only that the donor/acceptor LUMO level offset is greater than 0.3 eV (corresponding to the assumed exciton binding energy). In practice, these models have proved rather poor in predicting the photocurrent densities of real devices, even after processing optimization. [ 7 ] Whilst some materials (e.g. P3HT:PCBM) have indeed achieved photocurrent densities consistent with near unity internal quantum effi ciencies for photocurrent generation, most new materials (with some notable exceptions) evaluated for their performance in organic photovoltaic devices have yielded much lower photocurrent densities, and consequently poor device performance. [ 6 , 7 ] In this paper, we consider the extent to which such variations in photocurrent density can be largely understood in terms of the effi ciency of charge photogeneration. The key processes involved in charge photogeneration in organic bulk heterojunciton solar cells are illustrated in Figure 1 . By ‘charge photogeneration’ we refer to the overall process by which photon absorption leads to the generation of dissociated