Copper diffusion in In2S3 and charge separation at In2S3/CuSCN and TiO2/In2S3 interfaces

Abstract

The concept of inorganic nanostructured solar cells consists of a very thin absorber layer sandwiched between highly structured electron and hole conductors. When a TiO2/In2S3/CuSCN nanocomposite heterostructure is illuminated with light, photogenerated electrons in In2S3 can be injected into the conduction band of TiO2 and holes into the valence band of CuSCN. Charge transfer at the interfaces is limited by the deposition parameters, band alignment and diffusion of Cu from CuSCN into In2S3, which was the focus of this work. TiO2 nanoparticles were screen printed onto SnO2:F (FTO)-coated glass substrates to give a layer of nanoporous (np) TiO2. In2S3 layers were deposited by thermal evaporation or ion layer gas reaction (ILGAR) methods producing Cl-free (In(acac)3 precursor) and Cl-containing (InCl3 precursor) layers. A spray-spin method was developed for deposition of CuSCN onto In2S3. Diffusion of Cu into In2S3 layers was investigated by Rutherford backscattering spectrometry (RBS) while charge transport mechanisms were studied with surface photovoltage (SPV) technique in the fixed capacitor configuration. The activation energy (Ea) for Cu diffusion in thermally evaporated and Cl-free ILGAR In2S3 layers was 0.30 and 0.24 eV, respectively but increased to between 0.72 and 0.78 eV for Cl-containing In2S3 with residual Cl concentrations of 7.8 – 13.8 at.%. The diffusion prefactor (D0) was six orders of magnitude higher for Cl-containing compared to Cl-free layers. The relationship between Ea and D0 was described by the Meyer-Neldel rule with a Meyer-Neldel energy of 40 meV. The presence of Cl has no significant influence on the structural properties of In2S3 but resulted in a modified diffusion mechanism for Cu diffusion. The photovoltage of In2S3/CuSCN samples decreased after annealing for longer than 2 min at 200°C. A defect band was formed near the interface where holes accumulated and electrons tunneled through traps to recombine. The minimum distribution of tail states and hence the lowest disorder was achieved for Cl-containing In2S3 layers. The conduction band offset at the np-TiO2/In2S3 interface was 0.05 and 0.30 eV for Cl-free and Cl-containing In2S3, respectively. Bulk or interface recombination mechanism dominated charge transport at the interface with Cl-free or Cl-containing In2S3, respectively. Table of