An effective way to reduce energy loss and enhance open-circuit voltage in polymer solar cells based on a diketopyrrolopyrrole polymer containing three regular alternating units

A novel diketopyrrolopyrrole (DPP)-based conjugated polymer (PCDPP) was designed, synthesized and used as a donor material for polymer solar cells (PSCs). By increasing the planarity of polymer chains and reducing the energy loss in devices, we have simultaneously acquired a high short-circuit current (J_sc) and a large open-circuit voltage (V_oc) in PSCs based on PCDPP, which is a regular alternating ternary conjugated polymer. This polymer has a medium optical band gap (1.55 eV) with low-lying HOMO and LUMO energy levels. In addition, PCDPP exhibits a very good planarity from density functional theory (DFT) calculations and forms a fibrillar network in the active layer of solar cells. Because of these integrated favourable effects, PCDPP-based photovoltaic devices exhibit a high power conversion efficiency (PCE) of 9.02% which is among the highest values reported so far for devices based on DPP-containing polymers. More importantly, the V_oc of our PCDPP-based devices can reach as high as 0.86 V, which is much higher than that (<0.7 V) of high-efficiency solar cells based on other DPP polymers. These results provide a promising way to minimize the energy loss and to realize high V_oc and J_sc values at the same time in devices to obtain high power conversion efficiencies.