Structural and optical properties of CdS/Cu(In,Ga)Se2 heterostructures irradiated by high-energy electrons*

Abstract

Thin films of Cu(In, Ga)Se 2 (CIGS) with a Ga/(Ga + In) ratio of ~0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E g ) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P 1 at ~0.91 eV and P 2 at ~0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu In ) and indium vacancies V In , respectively, as the simplest radiation-induced defects.Introduction. Fabrication of highly efficient solar cells based on Cu(In,Ga)Se 2 (CIGS) semiconducting films is a critical and simultaneously complicated scientific and technical problem of modern semiconducting solar energy [1,2]. The development and improvement of the technology for preparing solar cells using CIGS compounds with the chalcopyrite structure has recently enabled efficiency records (~19.9%) to be set for all known thin-film semiconducting materials [1]. The stable operation of solar cells based on CIGS compounds that are used under ordinary terrestrial conditions and, especially, that are exposed to penetrating radiation (high-energy electrons, protons, etc.) in near-earth space orbits to an even greater extent stimulate further scientific and technological developments of semiconducting solar photovoltaics based on these materials [1][2][3][4][5]. Such promising developments prompt investigations that determine the criteria for radiation resistance of CIGS semiconducting compounds and solar cells based on them to the action of high-energy particles and that establish the nature of the radiation-induced defects at the atomic level [3][4][5][6]. New knowledge about the radiation physics of defects in CIGS material will indubitably assist a deeper understanding of the geometric and electronic structure of not only radiation defects but also growth defects formed during formation of CIGS thin films that determine the perfection of the material crystal structure. Herein we describe experiments that establish the nature of the radiation defects in CdS/Cu(In,Ga)Se 2 /Mo (CdS/CIGS/Mo) heterostructures, on which solar cells with efficiency ~12-14% are based [4,7,8].Experimental. The studied CIGS films were sputtered onto glass substrates coated with a thin film of Mo with simultaneous co-evaporation of Cu, In, Ga,[7][8][9]. Layers of CdS (~50 nm thick) were deposited by the standard chemical method in the corresponding solutions. The studied CdS/CIGS/Mo/glass structures consisted of a CdS buffer layer (~50 nm), CIGS films (~1.5 μm thick), a contact layer of Mo (~0.8 μm thick), and the glass substrate (2 mm thick). The elemental compositio...

Determination of the structural and optical characteristics of Cu2ZnSnS4 semiconductor thin films

“…According to our estimations, the numerical value of the parameter j is ~4 meV. The energy j is commonly referred to as the energy of spatial fluctua tions of the crystal potential in heavily doped semi conductors, especially multicomponent ones, in which such fluctuations are due to high concentra tions of intrinsic structural defects [22][23][24]. It should be noted that the value of j = 4 meV determined for the CZTS films is much smaller than the value of j for the similar 1.3 eV PL band observed in CZTS single crys tals (~11 meV) [25] and CZTS polycrystalline powders (~15 meV) [19,26].…”

“…By analogy with what happens in Cu(InGa)Se 2 alloys, such a value of k is indicative of the possibility of radiative recombination via transitions in donor-acceptor pairs or by band-(deep level) optical transitions. We have found that, as the excitation power density is elevated from 0.11 to 1.5 W cm -2 , the PL band shifts to the high energy spectral region, to 1.151 eV; in the region of relatively low excitation levels (0.11-3 W cm -2 ), the energy shift of the ~1.151 eV band (E p ) under variation in the exci tation power density J can be described by the expo nential function J = , where j is the parameter corresponding to the energy shift with increasing excitation power by one order of magnitude [22,23]. According to our estimations, the numerical value of the parameter j is ~4 meV.…”

Influence of Chemical Composition Heterogeneity on the Spectral Position of the Fundamental Absorption Edge of Cu(In, Ga)Se2 Solid Solutions

“…The experiments showed that the ratio of elements in the CIGS thin fi lms varied from Ga/(Ga + In) ~ 0.28 on the surface to ~0.58 near the glass substrate. It is noteworthy that heterogeneous distributions of In and Ga with a preferential increase of the Ga concentration near the glass substrate or a contact layer of Mo on glass were previously observed [7,[14][15][16]. This must be considered when analyzing the physical properties of CIGS solid solutions.…”

“…Analysis of the fi lm composition by SAES showed that the average concentrations of the principal substituents Ga and In were also x = Ga/(Ga + In) ~ 0.45. Known data for E g as a function of x of CIGS solid solutions indicated that E g approximated by a parabolic function for x ~ 0.45 [10,20,23,24] was ~1.20-1.26 eV at room temperature; approximated by a linear function [10,15], ~1.32 eV. The difference of E g ~ 1.36 eV for CIGS solid solution fi lms grown in the present work and the expected E g ~ 1.20-1.26 eV for x ~ 0.45 that was approximated by a parabolic function was logically explained by the heterogeneous distribution of x along the fi lm thickness.…”

Люминесценция и стимулированное излучение поликристаллических пленок Cu(In,Ga)Se-=SUB=-2-=/SUB=-, осажденных методом магнетотронного напыления

“…Как известно, твердые растворы CIGSe являются прямозонными полупроводниками, обладающими высоким коэффициентом поглощения света α ∼ 10 5 см −1 , и поэтому светопоглощающие слои в СЭ формируются с толщиной ∼ 1−3 мкм [2,[4][5][6]. СЭ, созданные на основе твердых растворов CIGSe, обладают такими достоинствами, как незначительное падение мощности при повышении температуры, слабая чувствительность к затемнению [8], высокая радиационная стойкость [9][10][11]. Максимальный достигнутый коэффициент полезного действия (кпд) СЭ на основе твердых растворов CIGSe, сформированных на стеклянных подложках, составляет 22.6% [12].…”