Mechanisms of charge transfer and electronic properties of Cu2ZnGeS4 from investigations of the high field magnetotransport

Recent development of the thin film solar cells, based on quaternary compounds, has been focused on the Ge contain compounds and their solid solutions. However, for effective utilization of Cu2ZnGeS4, deeper investigations of its transport properties are required. In the present manuscript, we investigate resistivity, amp; 961; T , magnetoresistance and Hall effect in p type Cu2ZnGeS4 single crystals in pulsed magnetic fields up to 20 T. The dependence of amp; 961; T in zero magnetic field is described by the Mott type of the variable range hopping VRH charge transfer mechanism within a broad temperature interval of 100 200 K. Magnetoresistance contains the positive and negative components, which are interpreted by the common reasons of doped semiconductors. On the other hand, a joint analysis of the resistivity and magnetoresistance data has yielded series of important electronic parameters and permitted specification of the Cu2ZnGeS4 conductivity mechanisms outside the temperature intervals of the Mott VRH conduction. The Hall coefficient is negative, exhibiting an exponential dependence on temperature, which is quite close to that of amp; 961; T . This is typical of the Hall effect in the domain of the VRH charge transfer

Spectroscopic ellipsometry study of Cu2ZnSnS4 bulk poly-crystals

The linear optical properties of Cu2ZnSnS4 bulk poly-crystals have been investigated using spectroscopic ellipsometry in the range of 1.2-4.6 eV at room temperature. The characteristic features identified in the optical spectra are explained by using the Adachi analytical model for the interband transitions at the corresponding critical points in the Brillouin zone. The experimental data have been modeled over the entire spectral range taking into account the lowest E0 transition near the fundamental absorption edge and E1A and E1B higher energy interband transitions. In addition, the spectral dependences of the refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity values have been accurately determined and are provided since they are essential data for the design of Cu2ZnSnS4 based optoelectronic devicesThe research leading to the presented results was partially supported by the European Project INFINITE-CELL (Ref. H2020-MSCA-RISE-2017-777968, 2017–2021, www.infinitecell.eu) and the Spanish MINECO Projects “WINCOST” (ENE2016-80788-C5-2-R) and PHOTOMANA (TEC2015- 69916-C2-1-R). The authors from the Institute of Applied Physics appreciate the financial support from STCU 6224 and from the Institutional Project No. CSSDT 15.817.02.04

Mechanisms of conductivity and energy spectrum of near edge holes in Cu2ZnSnS4 powder samples

The resistivity, r T , of Cu2ZnSnS4 powder samples, obtained by a solid state reaction method from pure elements, exhibits an activated character. The Mott variable range hopping charge transfer is observed between T 100e230 K, followed by the conductivity due to activation of holes into the states above the mobility threshold when T is increased. Such behavior, accompanied by semi width W of the acceptor band, exceeding the mean acceptor energy, indicates overlap of the acceptor band states with those of the valence band, forming a joint near edge hole spectrum in conditions of the strong degree of the compensation. The details of this spectrum have been determined, including positions of the Fermi level, m, and the mobility threshold, as well as the density of the localized states at the Fermi level, g m . The acceptor concentration and the hole localization radius have been obtained, too. These parameters indicate the material to lie relatively far from the metal insulator transitio

High field hopping magnetotransport in kesterites

Transport properties of the kesterite like single crystals of Cu2ZnSnS4, Cu2ZnSnxGe1 amp; 8722;xSe4 and Cu2ZnGeS4 are investigated in pulsed magnetic fields up to B amp; 8239; amp; 8239;20 amp; 8239;T. The Mott variable range hopping VRH conduction is established by investigations of the resistivity, amp; 961; T , in all the materials mentioned above within broad temperature intervals of amp; 916;Tv4 amp; 8239; amp; 8764; amp; 8239;50 150 amp; 8239;K, 50 250 amp; 8239;K and 100 200 amp; 8239;K, respectively. In addition, the Shklovskii Efros VRH conductivity below Tv2 amp; 8239; amp; 8764; amp; 8239;3 4 amp; 8239;K, the nearest neighbour hopping NNH charge transfer between T amp; 8239; amp; 8764; amp; 8239;250 320 amp; 8239;K and the conductivity by activation of holes on the mobility threshold at temperatures outside amp; 916;Tv4, respectively, are observed in these materials. In Cu2ZnSnS4, magnetoresistance MR contains only a positive contribution, connected mainly to a shrinkage of impurity wave functions by the magnetic field. At the same time, a negative contribution to MR, attributable to interference effects in VRH, is observed in Cu2ZnSnxGe1 amp; 8722;xSe4 and, especially, in Cu2ZnGeS4. The joint analysis of the MR and amp; 961; T data has yielded important electronic parameters of the materials. This includes widths of the acceptor band W and of the Coulomb gap amp; 916;, the NNH activation energy En, the localization radius a, the acceptor concentration NA and the density of the localized states at the Fermi level, g amp; 956; . A dramatic increase of a in Cu2ZnSnS4 with decreasing T is observed, whereas in Cu2ZnSnxGe1 amp; 8722;xSe4 all the parameter W, En, g amp; 956; , a and NA are non monotonic functions of x. Finally, in Cu2ZnGeS4 the Hall coefficient RH T is negative despite of the p type conduction , exhibiting the dependence close to that of amp; 961; T in the Mott VRH interva

High field magnetotransport in Cu2ZnGeS4 single crystals

The quaternary chalcogenides, attracting much attention in recent time as promising solar energy materials, permit an effective optimization of their composition by the Ge incorporation. In particular, this implies an interest to the Cu2ZnGeS4 compound, which utilization requires, however, a deeper understanding of its electronic properties in general. Here, we investigate magnetotransport of the p type Cu2ZnGeS4 single crystals, including resistivity, amp; 961; T , magnetoresistance MR and Hall effect, in pulsed magnetic fields up to B 20 T. The Mott variable range hopping charge transfer has been established within a broad temperature interval of T between amp; 8764;100 and 200 K by investigations of amp; 961; T in zero field. The positive and negative contributions to MR have been observed, attributing them to shrinkage of the impurity wave functions by the magnetic field and to the destructive interference of the hopping charge carriers, respectively. Observation of the negative Hall coefficient, RH T , exhibiting the dependence close to that of amp; 961; T , gives a strong support to the Mott conduction mechanism in our p type Cu2ZnGeS4 material. In addition, the conductivity, connected with thermal activation of holes on the mobility edge, Ec, has been identified both below and above the Mott conduction interval. Finally, the joint analysis of the amp; 961; T and MR data has yielded a series of important microscopic parameters. These include such details of the hole spectrum in the acceptor band, as its semi width, W, the density of localized states, g amp; 956; , at the Fermi level, amp; 956;, the positions of amp; 956; and Ec, as well as values of the localization radius of holes, a, and of the acceptor concentration, N

Spectroscopic ellipsometry study of Cu2Zn(GexSi1-x)Se4 bulk poly-crystals

5 pags., 3 figs., 2 tabs.The present study addresses to the synthesis and determination of the dielectric function of CuZn(GeSi)Se solid solutions with x = 0.4 and 0.8 over the range 1–4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E transition and high energy E and E transitions are identified. It is found that the band gap varies nonlinearly on composition in the CuZn(GeSi)Se alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells.This research was supported by the H2020 programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). The authors appreciate also the financial support from the Institutional Project ANCD 20.80009.5007.03.Peer reviewe

Spectroscopic ellipsometry study of Cu2ZnSn(SxSe1-x)4 bulk polycrystals

The pseudo dielectric function of Cu2ZnSn(SxSe1-x)4 [x = 0.35, 0.62, 0.81] bulk polycrystals is determined over the range 1.1–4.6 eV at room temperature from the analysis of spectroscopic ellipsometry data using the Adachi model. From the analysis, the lowest E0 transition and high energy E1A and E1B transitions are clearly identified, and used to follow the evolution of the pseudo dielectric function as a function of the composition. It is shown that the fundamental E0 and high energy E1A transitions can be tuned by increasing the sulfur content over a range of 0.3 eV. These results show the potential of the kesterite compounds for the design of efficient tailored photovoltaic solar cells