Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films

Abstract

In this work, we investigated structural, morphological, electrical, and optical properties from a set of Cu 2 ZnSnS 4 thin films grown by sulfurization of metallic precursors deposited on soda lime glass substrates coated with or without molybdenum. X-ray diffraction and Raman spectroscopy measurements revealed the formation of single-phase Cu 2 ZnSnS 4 thin films. A good crystallinity and grain compactness of the film was found by scanning electron microscopy. The grown films are poor in copper and rich in zinc, which is a composition close to that of the Cu 2 ZnSnS 4 solar cells with best reported efficiency. Electrical conductivity and Hall effect measurements showed a high doping level and a strong compensation. The temperature dependence of the free hole concentration showed that the films are nondegenerate. Photoluminescence spectroscopy showed an asymmetric broadband emission. The experimental behavior with increasing excitation power or temperature cannot be explained by donor-acceptor pair transitions. A model of radiative recombination of an electron with a hole bound to an acceptor level, broadened by potential fluctuations of the valence-band edge, was proposed. An ionization energy for the acceptor level in the range 29-40 meV was estimated, and a value of 172 ± 2 meV was obtained for the potential fluctuation in the valence-band edge.

Effects of sulphurization time on Cu2ZnSnS4 absorbers and thin films solar cells obtained from metallic precursors

“…1 0 has also been observed by Hall effect measurements in samples grown under very similar conditions [29]. Figure 7 shows the R/ √ vs. 1000/T curves for samples S10, S30 and S60.…”

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

“…Under these conditions, the electron and holes pairs are further spatially separated increasing the characteristic decay time with a large thermal activation energy (E H ). This model does not only explains the observed temperature behavior of the PPC characteristic decay time in our experiments, but it is also in agreement with the previously reported photoluminescence studies carried out in similar CZTS thin films [42]. The origin of the local potential fluctuations in CZTS is still unknown.…”

“…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. Therefore, the random distribution of charged and unscreened defects causes local electrostatic potential fluctuations, which in turn shifts the conduction and valence band edges below and above their positions in a pure uncompensated semiconductor, respectively.…”

“…We have previously shown that the low temperature optical emission from CZTS thin films present a broad and asymmetric band centered at 1.24 eV [42,44]. As temperature increases, the band shifts linearly to lower energies.…”

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

“…The PL peak position blue shifts by 10-20 meV/decade with increasing excitation power (inset of Fig. 5b), indicating strong compensation, which has been found to be a necessary prerequisite for good solar cell performance in chalcopyrite as well as kesterite-based thin film solar cells [45][46][47]. The observed PL bands in CZTS annealed samples can be attributed to quasi donor acceptor recombination [10].…”