Water splitting with series-connected polymer solar cells

We investigate light-driven electrochemical water splitting with series-connected polymer solar cells using a combined experimental and modeling approach. The expected maximum solar-to-hydrogen conversion efficiency (η_STH) for light-driven water splitting is modeled for two, three, and four series-connected polymer solar cells. In the modeling, we assume an electrochemical water splitting potential of 1.50 V and a polymer solar cell for which the external quantum efficiency and fill factor are both 0.65. The minimum photon energy loss (E_loss), defined as the energy difference between the optical band gap (E_g) and the open-circuit voltage (V_oc), is set to 0.8 eV, which we consider a realistic value for polymer solar cells. Within these approximations, two series-connected single junction cells with E_g = 1.73 eV or three series-connected cells with E_g = 1.44 eV are both expected to give an η_STH of 6.9%. For four series-connected cells, the maximum η_STH is slightly less at 6.2% at an optimal E_g = 1.33 eV. Water splitting was performed with series-connected polymer solar cells using polymers with different band gaps. PTPTIBDT-OD (E_g = 1.89 eV), PTB7-Th (E_g = 1.56 eV), and PDPP5T-2 (E_g = 1.44 eV) were blended with [70]PCBM as absorber layer for two, three, and four series-connected configurations, respectively, and provide η_STH values of 4.1, 6.1, and 4.9% when using a retroreflective foil on top of the cell to enhance light absorption. The reasons for deviations with experiments are analyzed and found to be due to differences in E_g and E_loss. Light-driven electrochemical water splitting was also modeled for multijunction polymer solar cells with vertically stacked photoactive layers. Under identical assumptions, an η_STH of 10.0% is predicted for multijunction cells.