Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers

We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a solution‐processed nickel oxide (NiO_x) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiO_x HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiO_x films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiO_x HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiO_x HTL is shown to provide superior contact properties by increasing the ITO/NiO_x contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiO_x interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):[6,6]‐phenyl‐C61 butyric acid methyl ester PCBM and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC₇₀BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.