First‐Principles Modeling of Point Defects and Complexes in Thin‐Film Solar‐Cell Absorber CuInSe₂

Abstract

as the composition of the sample changes from Cu-rich to Cu-poor, so that in the Cupoor samples only one peak is detected. Although only the Cu-poor material is important for actual devices Cu-rich samples give indispensable information for defect identification.First-principles calculations based on the density functional theory (DFT) can be used to obtain important, complementary information about point defects such as formation energies and charge transition levels. [6] A plethora of studies concerning defects in CuInSe 2 and CuGaSe 2 has been published over the last two decades. [7][8][9][10][11][12][13] The most recent DFT investigations have employed hybrid functionals, which can overcome the energy band gap problem plaguing the older studies and can thus also provide information about the defect level positions within the band gap. [9][10][11][12][13] The results of different calculations agree well with respect to general trends in formation energies of the most important defects, such as the copper vacancy (V Cu ), indium antisite on copper place (In Cu ), and copper interstitial (Cu int ). However, the results differ in some important cases. For instance, there are clearly different values for the ionization levels within the band gap for the copper antisite on the indium place (Cu In ) and indium vacancy (V In ). Based on the formation energy calculations, V Cu and Cu In are abundant acceptors, and are most probably responsible for some of the above-mentioned PL peaks, but it is unclear whether any native defect can be responsible for the third acceptor level.One important goal of the present work was to gain a perspective on the present unsatisfactory situation in modeling point defects in CIGSe and to approach the ultimate accuracy by which DFT is able to predict the properties of bulk crystalline materials. [14] First we carried out a detailed benchmarking of the first-principles computational scheme used. We checked effects due to the supercell size and shape, as well as those of the finite-size supercell correction scheme. We used also two very different implementations of the first-principles DFT method, which differ in describing valence-core electron interaction and electron wave functions (see below). After finding the computational parameters yielding accurate results, we calculated formation energies and charge transition levels for different acceptor candidates in CuInSe 2 . By carefully considering the relevant chemical potential limits, we were able to draw conclusions about the abundances of different defects. In addition to simple native defects, we have also considered a set of complexes formed by them. Our paper is organized as follows.

Electronic defects in Cu(In,Ga)Se2 : Towards a comprehensive model

“…IV B below). The majority of these defects can be expected to be combined into neutral defect complexes [22][23][24]. The density of remaining single defects is still high enough to cause electrostatic potential fluctuations, since their spatial distribution always shows some statistical fluctuations, causing spatially separated areas with different charges.…”

“…2. In fact, it has been argued that it is difficult to attribute the third acceptor in CuInSe 2 based on defect calculations [24]. It should be noted that in those CuGaSe 2 films where the third transition is observed as a spatially independent DA3 transition [69,70], it occurs only in very limited and separated spots about 1 μm across.…”

“…In recent years a number of new defect calculations have been performed for CuInSe 2 based on hybrid functionals [24,127,[135][136][137]. An overview of the predicted defect energies is given in Fig.…”

Efficiency Improvement of Near‐Stoichiometric CuInSe2 Solar Cells for Application in Tandem Devices

“…[9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes. [9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes.…”

“…

the formation of defect complexes and possibly even phases of ordered defect compounds. [9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes. It has been shown that stoichiometric Cu concentrations can be beneficial for absorber crystallinity, [13] defect density, [7,13] mobility, [14,15] and doping density.

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