Adjusting optical resonance thickness to increase the conversion efficiency of polymer solar cells

Abstract

The derivatives of C 60, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and 3-hexylthiophene (P3HT) were dissolved in DCB solvent, then spin coated into an active layer for polymer solar cells. The experimental parameters were studied carefully to obtain the optimum power conversion efficiency (PCE). The primary process for generation of photocurrent in an organic photovoltaic device is the generation of bound electron–hole pairs (excitons) by absorption of energy (photons) from the optical electric field. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the active layer. Improve organic optics absorb by insert organic layer (CuPc or C 60) at the active layer/Al interface. This research is divided into two components. First part, we use n-type C 60 as transmission layer. When an optimum thickness of C 60 is 5 nm, the J sc of polymer solar cell can be increased from 7.26 mA/cm 2 to 7.7 mA/cm 2. The V oc decrease is because the energy level of C 60 LUMO (lowest unoccupied molecular orbital) at 4.5 eV is higher than the 3.7 eV of PCBM. Second part, we use p-type CuPc as transmission layer. When an optimum thickness of CuPc is 3 nm, the short circuit photo-current density ( J sc) and open circuit voltage ( V oc) of polymer solar cell can be increased from 7.26 mA/cm 2 to 8.0 mA/cm 2 and 0.56–0.58 V, respectively. The reason is the same as C 60. The V oc increase is because the energy level of CuPc LUMO (lowest unoccupied molecular orbital) at 3.1 eV is lower than the 3.7 eV of PCBM. The J sc increase is because the 3 nm of CuPc leads to a constructive interference happened in the active layer and thus optical absorption increases. In this study we used 3 nm of CuPc at the active layer/Al interface to enhance the short circuit current density, and the efficiency was increased to 2.94%.