Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case ofCu2ZnSnS4

Abstract

The theoretical models of radiative recombinations in both CuIn 1−x Ga x Se 2 chalcopyrite and Cu 2 ZnSnS 4 kesterite, and related compounds, were revised. For heavily doped materials, electrons are free or bound to large donor agglomerates which hinders the involvement of single donors in the radiative recombination channels. In this work, we investigated the temperature and excitation power dependencies of the photoluminescence of Cu 2 ZnSnS 4 -based solar cells in which the absorber layer was grown through sulphurization of multiperiod structures of precursor layers. For both samples the luminescence is dominated by an asymmetric band with peak energy at ∼1.22 eV, which is influenced by fluctuating potentials in both conduction and valence bands. A value of ∼60 meV was estimated for the root-mean-square depth of the tails in the conduction band. The radiative transitions involve the recombination of electrons captured by localized states in tails of the conduction band with holes localized in neighboring acceptors that follow the fluctuations in the valence band. The same acceptor level with an ionization energy of ∼280 meV was identified in both absorber layers. The influence of fluctuating potentials in the electrical performance of the solar cells was discussed.

Nanometre-scale optical property fluctuations in Cu2ZnSnS4 revealed by low temperature cathodoluminescence

“…6c and reveals a broad featureless peak centred at~917 nm (1.35 eV), similar to previous photoluminescence measurements (e.g. [8,9]). The broadness of the luminescence peak, even at 10 K temperature, makes it difficult to analyse any variations in the spectra between different sample regions using the traditional method of Gaussian curve fitting.…”

“…A key difference between CZTS and CIGS is the extensive band tailing in the former [3,5]. Band tailing in CZTS is related to: (i) Urbach tails in absorption and quantum efficiency spectra [5], (ii) broadening of Raman vibration modes [6,7], (iii) S-shaped temperature dependence of the photoluminescence peak [8,9] and (iv) an anomalously long carrier lifetime at low temperature [3,10]. Band tailing is common in highly doped, highly compensated semiconductors and is due to statistical fluctuations in the local dopant (i.e.…”

The influence of the degree of Cu-Zn disorder on the radiative recombination transitions in Cu2ZnSn(S,Se)4 solar cells

“…4. With increasing temperature (from T � 10 K to �130 K) a red-shift of the peak energy of about 15-20 meV can be observed for the two states of order, which is in accordance with other publications [7]. A precise determin ation of the actual shift is not possible, since the H20 vapor absorption affects the spectral region around the PL peak energy.…”

“…For the two states of Cu-Zn order, a clear blue-shift of the peak energy of about 40 me V is observed with increasing excitation power (starting from �6 11 W to � 10 m W). This shift is in the same order of magnitude as reported elsewhere [7]. For excitation powers above � 10 m W a second contribution to the photolumi nescence spectra appears on the high-energy side, which is energetically near the band gap measured by ER.…”

“…A summary of some of these recombination models can be found in the review of Teixeira et al [7]. The luminescence observed in CZTSSe is energetically far below the band gap and is often attributed to donor-acceptor pair transitions involving electro static potential fluctuations (so called quasi donor-acceptor pair transitions) [8].…”