Large-grained Cu 2 O photocathodes in a superstrate configuration on a F-doped SnO 2 (FTO) coated glass substrate are synthesized via two-step electrodeposition. Only submicrometer sized grains were obtained during single-step electrodeposition in the potential window (−0.31 to −0.7 V vs Ag/AgCl) of stable Cu 2 O formation. We observe reductive decomposition of the Cu 2 O to Cu metal in the potential range of −0.7 to −0.98 V; bulk reduction of Cu 2+ in the solution to Cu metal occurs only beyond −0.98 V. In the potential window of stable Cu 2 O deposition, only the growth of the few nuclei occurs until a certain time. Minimal nucleation on the pristine FTO sites occurs during this period of deposition. The time to secondary nucleation is ∼6 min at −0.31 V and ∼15 s at −0.37 V. Interrupting the deposition at −0.31 V after 6 min and increasing the potential to −0.37 V leads to uniform, large grains (∼3 μm) of Cu 2 O. Photoinduced conducting atomic force microscopy reveals shunting and the presence of sub-bandgap states at the grain boundaries of Cu 2 O. Also, the lower carrier concentration (∼10 16 cm −3 ) in the large-grained Cu 2 O film obtained from Mott−Schottky analysis suggests a lower rate of Auger recombination. Thus, lowering the grain boundary crosssection in the two-step deposited film leads to a 30% increase in photocurrent at 0.0 V vs RHE.

The disparity between theoretical estimate and experimentally achieved efficiency of Cu2O-based photovoltaic and photoelectrochemical devices is attributed to poor electrical transport in the material. Toward this, we study native point defects in single and polycrystalline Cu2O and their effect on charge carrier transport via temperature-dependent Hall measurement in a temperature range of 82–300 K. The temperature-dependent carrier concentration evinces the presence of two monovalent acceptors pertaining to VCu and VCu split. We find that the second acceptor level lies ∼80 meV above the first acceptor and is active above ∼200 K temperatures only. Interestingly, the compensation ratio (N D/N A) decreases with the grain boundary cross section (ΛGB) of the sample, from 0.07 for the sample with ΛGB = 0.45 ×10–3 μm–1 to 0.02 for the sample with ΛGB = 0.22 × 10–3μm–1. In polycrystalline samples, carrier scattering at grain boundaries governs the hole transport at low temperatures (T < 150 K). However, trapping of holes by the acceptor-like intrinsic defects is the major factor affecting the high-temperature mobility in both single and polycrystalline Cu2O.

The nucleation and growth mechanism of functional oxides have a direct bearing on the structural and electronic properties of the deposit. We study the effect of electrolyte pH and deposition potential on the nucleation and growth of Cu2O on polycrystalline metal oxide (FTO) & metal (Au) substrates. Modelling of the recorded current-time transients indicates that both instantaneous and progressive nucleation occur with growth limited by diffusion or lattice incorporation of electro-active species or both. The preferred orientation of Cu2O shows strong dependence on electrolyte pH. The films are (100) oriented on both substrates at pH 9 except at high applied potential on FTO where the orientation changes to (111). Interestingly, irrelevant of electrolyte pH, the grain size of Cu2O decreases with potential on FTO whereas it increases on Au substrates. We attribute this to a difference in the number of active nucleation sites between the two substrates. The nucleation and growth at pH 12 is observed to be dependent both on diffusion and lattice incorporation of electro-active species. Additionally, the films are primarily (111) oriented on both the substrates, which is correlated to the availability of OH– ions.

Additionally, the temperature dependent resistivity exhibits a negative slope that is characteristic of a semiconductor. Therefore, the measured electrical characteristics can be attributed to the electrodeposited Cu 2 O semiconductor film rather than the conductive substrate. This method can be applied for the Hall measurement of any other electrodeposited semiconductor by optimizing the line/space geometry of the conductive substrate.