Surfaces and interfaces can often be complicated issues in solar cell devices. Here, three interfaces present in typical Cu(In,Ga)Se₂ (CIGS) solar cells, namely Mo/CIGS, CIGS/CIGS, and CIGS/buffer layer (near p-n junction) are focused upon. Remaining issues to improve CIGS solar cell performance are reviewed and discussed.

We proposed a compact image sensor that uses a combination of a matrix-driven Spindt-type field emitter array (FEA) and a CdTe-based photoconducting film. First of all, we investigated the effect of gamma irradiation of CdTe/CdS photodiode for radiation tolerant FEA image sensor. Taking into account the decrease in transmittance of the glass substrate, the CdS/CdTe photodiodes have sufficient tolerance to high gamma-ray exposure more than 1.7 MGy. Next, we investigated I-V characteristics under gamma-ray irradiation. The increase in current due to the gamma-ray absorption was observed. The increase rate in current at the voltage of -5 V was approximately 220 nA/cm² per unit dose rate (1 kGy/h).

The ternary thallium compound of TlInSe₂ shows the giant thermoelectric power in a low-temperature incommensurate phase below 410K, thus expected as a promising material for thermoelectric devices. TlInSe₂ exhibit a low dimensional nanostructure is Tl atoms arranged in one dimension, the optical anisotropy is measured. Similarly, TlInS₂ of Tl compounds exhibit a nanostructure that Tl is located in a two-dimensional, show the incommensurate phase at low temperatures. These material system, application to nano-devices and optical device materials also expected. In this work, we study electronic structures and optical properties of TlInSe₂ and TlInS₂ using the first-principles calculation and clarify the origins of their giant thermoelectric powers.

CuGaSe₂ thin films with different Cu/Ga ratio were studied using photoluminescence (PL) method. Measurement at 1.4 K shows asymmetrically broadened PL spectra for all the samples where PL-peaks shift to lower energy with a decrease of the Cu/Ga ratio. Excitation power and temperature dependent PL spectra were explained using donor to acceptor pair transition under the influence of potential fluctuation.

Cu₂SnS₃ (CTS) is considered to be a suitable candidate for future use as the absorber layer in thin-film solar cells. In this study, CTS films were formed by co-evaporation of Cu, Sn and S on alkali-free glass substrate (Corning EAGLE XG). After deposition, NaF was deposited on the CTS thin films. After NaF deposition, the NaF-deposited CTS films were annealed in a carbon container with sulfur. After annealing, the sample was dipped in deionized water to remove residual NaF. As the results, the grain size of the films with NaF deposition was larger than that of the films without NaF deposition, and the power conversion efficiency of the CTS thin film solar cells were improved by annealing with sodium deposition.

CZTS is expected to a thin film solar cell of low cost and environmental load by these characteristic. Surface treatment of CZTS thin films using Hypochlorous Acid (HClO) were carried out. After the reaction, there was no change in composition of CZTS. A change in the surface morphology was seen. Different from the results of surface treatment using pure water, bromine water and NH₄OH, oxygen is not removed from the CZTS surface using NaClO.

The compositionally adjusted Cu₂ZnSnS₄ (CZTS) poly-crystals with compositional uniformity were grown by solid-phase method using stoichiometrically defined CZTS, ZnS, SnS and S as source materials. The CZTS thin-films with compositional uniformity were successfully deposited on Soda lime glass substrates using pulse laser deposition (PLD) with the grown CZTS poly-crystal as a target. We confirmed that improving the composition uniformity of the CZTS target is effective for improving uniformity of the CZTS thin-film.

The relationship between energy band discontinuity and electrical properties of p-n heterointerface for tin monosulfide (SnS) based solar cells are investigated. Typical trap activation energy (Ea) value for Cd_0.7Zn_0.3S/SnS, and Mg_0.2Zn_0.8O/SnS-based solar cells were higher than the values for CdS/SnS, and Zn_0.7Sn_0.3O/SnS-based solar cells. These experimental result confirmed that an appropriate band structure and a high quality p-n interface are indispensable for the realization of high efficiency SnS based solar cells.

The effect of low pressure annealing on SnS thin films and solar cells grown by sulfurization was investigated to prevent the formation of extra phases. The SnS thin film grown by sulfurization is annealed in a vaccum. XRD and Raman measurement showed that this SnS thin film was without the formation of extra phases such as SnS₂ or Sn₂S₃.

We fabricated the Cu-Ag-Sn-S thin films by depositing the Cu-Ag-Sn precursor by vacuum evaporation method and sulfurizing it in H₂S atmosphere. By heat treatment at 350℃, peaks attributable to Ag₈SnS₆ crystal and Ag₄Sn₃S₈ crystal were observed. By increasing the heat treatment temperature, only Ag₈SnS₆ crystals were obtained.

Effects of substrate temperature for the deposited Cu₂SnS₃ thin films using vacuum evaporation method ware investigated. The diffraction peak attributable to the monoclinic Cu₂SnS₃ crystal was obtained at any substrate temperature. It is estimated that the band gap of thin films is 0.94-1.03eV. The voids in the thin films decrease as the substrate temperature increases.

Ag and SnS were sequentially vapor-deposited on a glass substrate by a vacuum deposition method and heat-treated at 350 to 500℃ for 1 hour in an H₂S atmosphere. From the XRD pattern, diffraction peaks of SnS crystal for precursor film, Ag₂S crystal for the thin films annealed at 350 to 400℃, and Ag₈SnS₆ crystal for the thin films annealed at 400 to 500℃ could be confirmed.

The SnS films were deposited on soda-lime glass substrate by sol-gel method. We have also confirmed that the thin films have a SnS structure after annealing in N₂ at 300℃. From the results of XPS measurements, it is found that Sn state is Sn²⁺ and Sn⁴⁺ in the films. In addition, XPS data show desorption of S atoms from the surface during annealing in vacuum. We have obtained the bandgap energy of 1.2 eV for the films annealed in N₂ and air atmosphere.

SnS thin films have been prepared by vacuum deposition and sulfurization process. Atomic content of SnS films was almost stoichiometry by sulfurization at 300℃ for 60 min. Also, ZnO:Al thin film have been prepared by vacuum deposition and annealing in Air. Then, high transmittance and low electrical resistivity was obtained after annealing in air at 260℃ for 45 min. We fabricated n-ZnO:Al/p-SnS on FTO/SLG substrate by these methods and measured electrical property diode made. As a result, diode characteristic was obtained with a threshold voltage of 0.5 V.

Bulk crystals of Cu₂Sn_1-xGe_xS₃, a candidate material for absorption layer of thin-film solar cells, were grown using chemical vapor transport with iodine. Resulting crystals had a composition ratio consistent with the charged ratio. XRD and Raman scattering results suggested that the crystals are mixed crystal between Cu₂SnS₃ and Cu₂GeS₃.

Visible-light transparent solar cells based on heterojunction of NiO and ZnO layer that absorbs only ultraviolet light were fabricated. The oxygen vacancy, which is the one of the origin of free carrier of ZnO, becomes a defect works as a recombination center in ZnO near the pn interface. In this study, we varied the Ar-O₂ gas supply ratio f(O₂) [f(O₂) = O₂/(Ar+O₂)] during ZnO sputtering to reduce defects in ZnO near the pn interface.

CuBr_1-xI_x thin films were prepared as a variable band gap transparent p-type semiconductor, by solution coating method which is a non-vacuum process. We confirmed the change in XRD patterns, exciton absorption energy, band gap energy, and ionization potential of CuBr_1-xI_x thin films with the change of mixing ratio x. From the results, band lineup of CuBr_1-xI_x was proposed.

The use of electrical discharge pulses is a new method to prepare thin films. When an electrical pulse is discharged between the electrode and the substrate, the electrode material, which is zinc and phosphor, moves to the substrate. We evaluated the resultant films with scanning electron microscopy (SEM) and X-ray diffraction (XRD).