Ultra-thin (0.5-10 nm) plasma-enhanced atomic layer deposited (PE-ALD) titanium oxide (TiO x ) films, deposited on indium-tin-oxide (ITO) contacts, are investigated as hole-blocking interlayers using conventional electrochemistry of select probe molecules, in blended heterojunction (P3HT:PCBM) organic photovoltaics (OPVs) and in conventional Al/TiO x /p-Si diode platforms. Even films as thin as 0.5 nm, which represent as few as 10 ALD cycles, begin to show hole blocking in the electrochemical experiments, and optimized rectification and power conversion efficiencies are seen for the diode and OPV platforms respectively at a thickness of ca. 3 nm. These results suggest a significant reactivity of the ALD precursors with the ITO substrate to form conformal films with properties which can normally only be achieved with much thicker TiO 2 films created from chemical vapor deposition or sol-gel solution processing. The performance of these PE-ALD TiO x layers is highly dependent on thickness. Up to ca. 3 nm these PE-ALD films remain amorphous, whereas for thicker layers (10 nm) grazing incidence X-ray diffraction shows a transition to the anatase structure, with an increase in both leakage current and reduction in shunt resistance in PV platforms. TiO 2 films can be quite attractive electron-selective, holeblocking interlayers in both PV and photoelectrochemical energy conversion platforms, but need to be thin, owing to their lower intrinsic conductivities. PE-ALD TiO 2 films appear to provide these capabilities, with strikingly optimized performance at very low thickness. † Electronic supplementary information (ESI) available: Film growth rate using spectroscopic ellipsometry (Fig. S1), chemical composition analysis using angle-resolved X-ray photoelectron spectroscopy ( Fig. S2 and S3), and local ideality factor of OPV devices (Fig. S4). See Fig. 6 Current-voltage (I-V) curves with C-AFM. (a) I-V curves of two distinguishable areas of 10 nm TiO x C-AFM images (inset image). (b) Average I-V curves of bright areas with 1, 3, and 10 nm TiO x films on glass/ITO substrates. This journal isView Article Online a Shunt resistance (R P ) is estimated from the inverted slope of J-V curves at À1 V. b Series resistance (R S ) is estimated from the inverted slope of light J-V curves at +1. c Ideality factor (n) is estimated from the dark J-V curves at 0.4 V. J. Mater. Chem. AThis journal is

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This report focuses on the evaluation of the electrochemical properties of both solution-deposited sol-gel (sg-ZnO) and sputtered (sp-ZnO) zinc oxide thin films, intended for use as electron-collecting interlayers in organic solar cells (OPVs). In the electrochemical studies (voltammetric and impedance studies), we used indium-tin oxide (ITO) over coated with either sg-ZnO or sp-ZnO interlayers, in contact with either plain electrolyte solutions, or solutions with probe redox couples. The electroactive area of exposed ITO under the ZnO interlayer was estimated by characterizing the electrochemical response of just the oxide interlayer and the charge transfer resistance from solutions with the probe redox couples. Compared to bare ITO, the effective electroactive area of ITO under sg-ZnO films was ca. 70%, 10%, and 0.3% for 40, 80, and 120 nm sg-ZnO films. More compact sp-ZnO films required only 30 nm thicknesses to achieve an effective electroactive ITO area of ca. 0.02%. We also examined the electrochemical responses of these same ITO/ZnO heterojunctions overcoated with device thickness pure poly(3-hexylthiophehe) (P3HT), and donor/acceptor blended active layers (P3HT:PCBM). Voltammetric oxidation/reduction of pure P3HT thin films on ZnO/ITO contacts showed that pinhole pathways exist in ZnO films that permit dark oxidation (ITO hole injection into P3HT). In P3HT:PCBM active layers, however, the electrochemical activity for P3HT oxidation is greatly attenuated, suggesting PCBM enrichment near the ZnO interface, effectively blocking P3HT interaction with the ITO contact. The shunt resistance, obtained from dark current-voltage behavior in full P3HT/PCBM OPVs, was dependent on both (i) the porosity of the sg-ZnO or sp-ZnO films (as revealed by probe molecule electrochemistry) and (ii) the apparent enrichment of PCBM at ZnO/P3HT:PCBM interfaces, both effects conveniently revealed by electrochemical characterization. We anticipate that these approaches will be applicable to a wider array of solution-processed interlayers for "printable" solar cells.