The Effect of Side Chains of Polymer Donors on the Performance of Organic Photovoltaics
Converting solar energy into electricity provides an effective solution to the energy crisis the world is facing today. Organic Photovoltaics have shown potential to harness solar energy in a cost-effective way. Extensive progress has been achieved in understanding OPV photophysical phenomena and in identifying key factors needed to improve organic solar cell device power conversion efficiency (PCE). The newly synthesized polymer BDTP-Bz-1 and BDTP-Bz-2 composed of alternating benzotriazole and phenyl substituted benzodithiophene (Figure 1), yield 8% PCEs when blended in a bulk-hetero-junction (BHJ) structure with electron acceptor materials such as ITCC. The donor polymer BDTP-Bz-2 showed better device performance including short circuit current density (Jsc), open circuit voltage (Voc), and External Quentum Efficiency (EQE), which can contribute to its preference for the “face-on” polymer backbone orientation in which the π-conjugated polymer backbone planes lie parallel to the substrate surface, resulting in maximal contacts between organic photoactive materials and the ITO anode coated with electron transport layer (ZnO). This orientation of the π-π stacked structures would favor charge transport across the interface. By AFM analysis, it was revealed that the BDTP-Bz-2 : ITCC active layer showed lower surface roughness (10.6 nm) than BDTP-Bz-1 : ITCC active layer (12.5 nm). Lower surface roughness also helped form better interface contact between the active layer and the electrode.