Electrochemical characterization of CdSe-coated ZnO nanowire extremely-thin-absorber solar cells

Abstract

Four different CdSe-coated nanostructured ZnO ETA configurations as photoelectrochemical cells with polysulfide electrolyte were studied using both conventional and electrochemical characterization techniques. ETA configurations with different ZnO nanowire lengths of 500 nm and 1000 nm were varied with different CdSe absorber-layer thicknesses of 15 nm and 45 nm to examine the effects on PV performance, carrier transport, and carrier recombination. Linear-sweep voltammetry (J-V) measurements showed that longer ZnO nanowires with thinner CdSe absorber layers gave better PV performance with the 1000 nm length/15 nm CdSe thickness samples having the highest JSC ~4.4 mA/cm2, VOC ~0.38 V, Pmax ~0.52 mW/cm2, and second-highest FF ~0.32. Mott-Schottky (MS) analysis was performed on individual ETA-layer materials to obtain estimates of their ND and VFB for insight into how individual layers in an ETA cell can assist in carrier separation. MS results were shown to be irrespective of illumination, exposed area, or the electrolyte used. Annealed ZnO nanowires had an ND ~2x1019 cm-3, a VFB ~(-0.4) V. versus Ag/AgCl, and were observed to be n-type. MS analysis of planar CdSe showed it to be slightly n-type and gave parameter estimates of ND ~3x1017 cm-3 and VFB ~-1.1 V v. Ag/AgCl, which were also used to calculate its VBI to be ~0.4 V, and its depletion width, W to be ~44 nm. Carrier transport studies were performed using IMPS and photocurrent decay measurements to estimate the time constant for carrier transport, with the fastest observed for shorter nanowires and thicker CdSe absorber layers at ~10 μs. Carrier recombination studies were also performed using IMVS, photovoltage decay, and EIS measurements to estimate the time constant for carrier recombination, with the slowest estimated for the samples with 45 nm CdSe thickness samples at ~100 ms. Therefore, shorter nanowires with thicker CdSe absorber layers showed the best potential for improving carrier collection and reducing recombination but due to their lowered J-V performance, must be reconciled with light-absorption and nanowire-density considerations. It was recommended that future endeavors for optimizing the design of ETA cell configurations incorporate these electrochemical techniques to provide additional insight into carrier transport and recombination, which can supplement the design of ETA cells beyond using conventional characterization methods alone.%%%%M.S., Chemical Engineering – Drexel University, 2014