Enhanced Performance of PDPP3T/ ${\rm PC}_{60}{\rm BM}$ Solar Cells Using High Boiling Solvent and UV - Ozone Treatment

Using higher temperature without any additives, poly(diketopyrrolopyrrole-terthiophene) was successfully dissolved with ${\rm PC}_{60}{\rm BM}$ in a commonly used high-boiling point solvent namely chlorobenzene (CB). Use of CB allowed us to heat the blend solution at a higher temperature, which leads to improved nanoscale phase separation between donor and acceptor, which is important for an efficient exciton dissociation. Devices were fabricated in both regular and inverted bulk heterojunction structure for comparison by spin coating an active layer. Inverted structure solar cells exhibited higher power conversion efficiency than regular structure with significant improvement in current density, possibly attributed to increased light absorption in the active layer due to less parasitic absorption in the PEDOT:PSS and the Ca layer, which were used in normal structures. Furthermore, ultraviolet (UV) ozone treatment of ZnO film improved the solar cell performance. A UV ozone treatment of 5 min was found to be the optimal time resulting in the highest device efficiency (4.45%) with short circuit current density of 9.3 ${\rm mA}/{\rm cm}^{2}$ , open circuit voltage of 0.69, and fill factor 69.5% without the need for any solvent additives.