Effects of Optical Interference and Annealing on the Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells

Polymer solar cells are of tremendous interests due to their attractive properties such as flexibility, ease of fabrication, low materials and energy budget. However, organic materials have short exciton diffusion length and poor charge mobility, which can greatly decrease the performance of polymer solar cells. These challenges can be effectively overcome through the use of the bulk heterojunction (HJ) structure because it can guarantee the effective exciton dissociation and carrier transport simultaneously if a proper bicontinuous interpenetrating network is formed in the active layer. Based on this structure, the performance of polymer solar cells has been improved steadily in the past decade. The performance of a polymer solar cell is mainly determined by the short-circuit current density (JSC), the open circuit voltage (VOC), and the fill factor (FF), given that η=JSCVOCFF/Pin (where η is power conversion efficiency, PCE, and Pin is the incident optical power density). VOC has a direct relationship with the offset energies between the highest occupied molecular orbital of Donor (D) material and the lowest unoccupied molecular orbital of Acceptor (A) material (Cheyns et al., 2008). Since the D and A materials are intimately mixed together in the bulk HJ structure and their interfaces distribute everywhere in the active layer, it is difficult to increase VOC by changing D/A interface property for a given material system (such as poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester, P3HT:PCBM). Thus the usually used optimization method is to improve JSC and FF. JSC greatly depends on the optical interference effect in polymer solar cells. Because of the very high optical absorption ability of organic materials, the active layer is very thin and typically from several ten to several hundred nanometers. This thickness is so thin compared to the incident light wavelength that the optical interference effect has to be carefully considered. Depending on the thicknesses and optical constants of the materials, the optical interference causes distinct distributions of the electric field and energy absorption density. Due to this effect, JSC shows an obvious oscillatory behavior with the variation of active layer thickness. In order to gain a high PCE, the active layer thickness needs to be well optimized according to the optical interference.