Hopping conduction and persistent photoconductivity in Cu₂ZnSnS₄thin films

Abstract

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu 2 ZnSnS 4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros-Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu 2 ZnSnS 4 . Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu 2 ZnSnS 4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu 2 ZnSnS 4 .the CZTS absorber layer and the Mo back contact have been pointed out as factors that reduce the solar cell efficiency [4,5]. Strong research efforts have been made in the study of different CZTS synthesis routes [3][4][5][6][7][8][9][10], CZTS structural and optical properties [2,[6][7][8], as well as in the development of CZTS solar cells [3-5, 9, 10]. However, very little is known about the electrical transport and photoconductivity properties of CZTS thin films and further studies are required.The study of the temperature dependence of electrical transport in semiconductor materials is very important for the knowledge of material parameters, the understanding of the transport and microscopic mechanisms of charge transfer, as well as for the optimization and design of solar cells. Very recently, we have shown that the radiative and non-radiative recombination mechanisms in CZTS thin films are strongly

Disorder and variable-range hopping conductivity in Cu2ZnSnS4 thin films prepared by flash evaporation and post-thermal treatment

“…This supports the validity of the microscopic parameter values obtained for these samples and collected in Table 2, being the values of N, E c , a and g () in a reasonable agreement with the corresponding data obtained for CZTS bulk [26] and film [27][28][29] samples, as well as for Cu 2 ZnSiSe 4 single crystals [30].…”

“…Such mechanism of the charge transfer has recently been established in CZTS single crystals [26] and films [27][28][29], as well as in Cu 2 ZnSiSe 4 single crystals [30], belonging to the same family of compounds. The upper border of the VRH conduction in the above materials, lying close to the room temperature, reflects the high lattice disorder [26][27][28][29][30]. Indeed, such disorder stimulates the localization of the conduction electrons, favoring the hopping conductivity [20].…”

Phase-transition-driven growth of compound semiconductor crystals from ordered metastable nanorods

“…When heating with a fast heating rate (above 2.6 K s -1 ), which leads to formation of larger grains, the films show increased conductivity, with an average of 2.5 ×10 -2 Ω -1 cm -1 (average from 14 samples). This value is comparable with conductivities observed for CZTS films prepared by sulfurization of metallic precursors [39], which is an approved method for fabrication of CZTS solar cells [40][41][42]. Above a heating rate of 2.6 K s -1 no systematic dependency on the heating rate could be observed.…”

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

“…Electrostatic local potential fluctuations will spatially separate the photo-excited free carriers reducing their probability of recombination giving rise to a PPC effect. These fluctuations could be caused by elevated doping levels and high degree of compensation in the films [10,40,41]. In highly compensated materials, such as the CZTS thin films [42,43], most defects are ionized and the free carrier concentration is low.…”

“…Cu 2 ZnSnS 4 thin films were prepared by sequential deposition of precursor thin layers of Zn, Sn and Cu on a soda-lime glass (SLG) substrate followed by a sulphurization step as reported elsewhere [10,30,31]. The precursors were deposited by DC magnetron sputtering and the sulphurization was done by thermal annealing in sulphur vapour atmosphere.…”