Xing-Min Cai

The photovoltaic performance of ZnSnN2 homojunction has never been studied. Here, ZnSnN2 homojunction solar cells with Ag-doped ZnSnN2 as the p-type layer are fabricated with magnetron sputtering deposition, and the mechanism limiting the photovoltaic performance is revealed. The junction is abrupt. The current transport is dominated by thermionic emission below 320 K and generation-recombination (GR) in the space charge region in 330–380 K. Below 320 K, the short circuit current density and the open circuit voltage are limited by nonradiative interface recombination. The open circuit voltage loss also results from interfacial barrier height inhomogeneity, which obeys the Gaussian distribution model or the temperature fluctuation of interface barrier heights for holes. At higher temperatures, deep energy levels in the middle of the bandgap result in the GR in the space charge region. One shallow energy level at 0.13 eV and one interface energy level at 0.72 eV above the valence band maximum are observed. The former is likely induced by substitutional Ag-doping.

Cu₂O and ZnSnN₂ are non-toxic photocatalytic semiconductor materials with Earth-abundant elements. There is no work about Cu ₂ O\Al ₂ O ₃ \ZnSnN ₂ heterojunctions and here the heterojunctions with the structure of glass\Pt\Cu ₂ O\Al ₂ O ₃ \ZnSnN ₂ \InSn are prepared with magnetron sputtering. The dark and illuminated current density J versus the applied voltage V curves are characterized. The dark temperature-dependent JV measurements show that the current is dominated by thermionic emission at lower bias voltages but limited by space-charge at higher bias voltages. The barrier height of the heterojunction is non-uniform and follows the Gaussian distribution. The illuminated JV curve of the heterojunctions shows no improvement as compared with that without Al ₂ O ₃ since the insertion of Al ₂ O ₃ unfavorably increases the series resistance though favorably reduces the parallel conductance.

The p-type conduction of Ag-doped ZnSnN2 (AZTN) is studied with experiments and density functional theory (DFT) calculation. The experimental results show that doping ZnSnN2 with Ag introduces no impurity phase but reduces the optical bandgap. The AZTN samples are p-type conductive with hole density of 1015–1016 cm−3 and the p-type origin is likely the doped Ag atoms which very possibly substitute Zn or Sn in the cation sublattice. These observations are consistent with the DFT calculation. The p-type conduction of the AZTN samples is further revealed by the Ohmic contact to Pt and the Schottky contact to Ag. The latter is demonstrated with the rectifying IV curves and the energy band alignment measured with ultraviolet photoelectron spectroscopy. The current transport mechanism of the AZTN\Ag Schottky junction is thermionic emission.

There has been no experimental work on CuCrO₂-ZnSnN₂ heterojunctions (HJs), though theoretical work shows that their photoelectric conversion efficiency is around 20%. Here, CuCrO₂ thin films and p CuCrO₂-n ZnSnN₂ HJs are prepared by varying the sputtering power of the Cu-Cr alloy target while the other parameters are held constant. The as-deposited Cu_xCr_yO_z thin films are amorphous, with CuCrO₂ as the major phase. The CuCrO₂ thin films are p-type conductive, with an optical band gap of about 3.64–3.84 eV. The ZnSnN₂ thin films are wurtzite and n-type conductive. The dark current density J versus voltage V curve measurements show that all the HJs showed rectification, while only the samples deposited at 40 and 50 W had a photo-induced current. Further analysis shows the HJs deposited at 40 W have the lowest shunt conductance, saturation current density, and trap density, implying an effect of fabrication conditions on the properties of HJs.

To reveal the formation mechanism of Schottky barrier heights, inter-diffusion and chemical reaction between ZnSnN2 and the metals (Cu or Ag) are studied. Nitrogen and selective cation out-diffusion are observed, besides stronger Cu and weaker Ag in-diffusion. The Schottky barrier heights are consistent with the Schottky–Mott theory based on the improved work functions of ZnSnN2 layers and metals due to oxygen contamination. The Fermi level is pinned at about 1/3 of the forbidden bandgap below the conduction band bottom. Each layer in ZnSnN2\Ag is electronically homogeneous with the chemical reaction limited to the interface region. The ZnSnN2 layer in ZnSnN2\Cu is bulk inhomogeneous with one smaller work function from the minor phase resulting from Cu in-diffusion and suppressed oxygen out-diffusion besides the larger one from the original major phase, while the Cu layer is also bulk inhomogeneous, resulting from the higher chemical activity of Cu, which leads to the observed barrier height inhomogeneity and quenches the photovoltaic effect.

The trap and defect energy levels of ZnSnN2 and the current transport mechanism of its heterojunctions are studied. A shallow energy level at 105 meV below the conduction band minimum (Ec) of ZnSnN2 is detected and its possible origin is the intrinsic antisite defect of SnZn (Sn occupy the position of Zn in ZnSnN2), besides the traps located at 0.67, 1.03 and 1.06 to 1.21 eV below Ec. The interface states of ZnSnN2 heterojunctions form two discrete energy levels with one at Ec + 0.05 eV and another at Ec−0.03 eV. The current of ZnSnN2 heterojunctions is controlled by thermionic emission at relatively low bias voltage and limited by space charge at higher bias voltage. The barrier height of the heterojunctions is inhomogeneous, which obeys Gaussian distribution and possibly results from interface roughness.

ZnSnN₂ has potential applications in photocatalysis and photovoltaics. However, the difficulty in preparing nondegenerate ZnSnN₂ hinders its device application. Here, the preparation of low-electron-density nanocrystalline ZnSnN₂ and its device application are demonstrated. Nanocrystalline ZnSnN₂ was prepared with reactive sputtering. Nanocrystalline ZnSnN₂ with an electron density of approximately 10¹⁷ cm⁻³ can be obtained after annealing at 300 °C. Nanocrystalline ZnSnN₂ is found to form Schottky contact with Ag. Both the current I vs. voltage V curves and the capacitance C vs. voltage V curves of these samples follow the related theories of crystalline semiconductors due to the limited long-range order provided by the crystallites with sizes of 2–10 nm. The I−V curves together with the nonlinear C⁻²−V curves imply that there are interface states at the Ag-nanocrystalline ZnSnN₂ interface. The application of nanocrystalline ZnSnN₂ to heterojunction solar cells is also demonstrated.

Nondegerate ZnSnN₂ with mobility over 20 cm² V⁻¹ s⁻¹ was fabricated by improving the chemical potentional of nitrogen to tune the stoichiometry.