Nanojunction solar cells based on polycrystalline CdTe films grown on ZnO nanocones

We report the nanojunction effect on nanocone based solar cell consisting of 5 μm thick CdTe thin film as a p-type absorption layer and n-type ZnO nanocones as a minority electron collector. The morphology of the nanojunction, with embedding depth ~100 nm into CdTe matrix, was clearly revealed by transmission electron microscopy (TEM) and scanning TEM (STEM). The characteristics of nanojunction solar cell were compared with those of planar ZnO/CdTe structure in dark and under 100 mW (AM 1.5) illumination using a solar simulator. The nanojunction cell showed small reverse saturation current density in dark. Lower capacitance of the nanojunction from capacitance-frequency (C-f) measurement implies the existence of low defects at near the junction interfaces, which enables to lowerer junction recombination. The small junction area and low defects produces the higher open circuit voltage (V_oc) and fill factor (ff) than those of planar under light illumination. The fact that the performance of nanojunction based solar cell is not critically affected by interfacial defects will be very useful for polycrystalline or nanoparticle CdTe with many crystal defects. Our current conversion ~3.2 % nanocone was achieved by decreasing CdTe grain size, which is different from planar polycrystalline solar cells.