Enhanced efficiency in plastic solar cells via energy matched solution processed NiO _x 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, modifi ed 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 (PC ₇₀ BM) 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.