Comparative Study on Thermally Evaporated and Solution Processed Cathode Modifying Layers in Organic Solar Cells

Organic solar cells have been fabricated using cathode modifying layers of thermally evaporated bathophenanthroline (Bphen), ytterbium doped Bphen (Bphen:Yb), and solution processed (N,N-dimethyl-ammonium N-oxide) propyl perylene diimide (PDINO). Compared to pristine Bphen, Bphen:Yb shows higher electron mobility and thereby increases fill factor of device, demonstrating the weak n-doping of Yb in Bphen. As a result of Fermi level pinning, Bphen:Yb forms an ohmic contact with photoactive layer, underpinning efficient electron injection and extraction of device. Compared to conventional PDINO, despite lower electron mobility, Bphen:Yb enables increased optical absorption of device and smoother morphology of device, thereby improving power conversion efficiency of device. The current research points out that the integration of thermally evaporated weakly n-doped cathode modifying layer and solution processed photoactive layer is a promising method to fabricate high-efficiency and low-cost organic solar cells.