Characterization of vacancy defects in Cu(In,Ga)Se2 by positron annihilation spectroscopy

M R M Elsharkawy,E E Abdel-Hady,D J Keeble,G S Kanda,M V Yakushev

doi:10.1063/1.4972251

M R M Elsharkawy, E E Abdel-Hady + Show 3 more

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https://doi.org/10.1063/1.4972251

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Abstract

The photovoltaic performance of Cu(In1-x,Gax)Se2 (CIGS) materials is commonly assumed to be degraded by the presence of vacancy-related defects. However, experimental identification of specific vacancy defects remains challenging. In this work we report positron lifetime measurements on CIGS crystals with x = 0, and x = 0.05, saturation trapping to two dominant vacancy defect types, in both types of crystal, is observed and found to be independent of temperature between 15–300 K. Atomic superposition method calculations of the positron lifetimes for a range of vacancy defects in CIS and CGS are reported. The calculated lifetimes support the assignment of the first experimental lifetime component to monovacancy or divacancy defects, and the second to trivacancies, or possibly the large In-Se divacancy. Further, the calculated positron parameters obtained here provide evidence that positron annihilation spectroscopy has the capability to identify specific vacancy-related defects in the Cu(In1-x,Gax)Se2 chalcogenides.

Highlights

The efficiency of thin film Cu(In1-x,Gax)Se2 (CIGS) chalcogenide solar cells continues to improve,[1,2] with recent gains resulting from the developments in alkali post deposition treatments.[2,3] The deposited CIGS layer is commonly p-doped by lattice defects.[2]
The electron-positron enhancement factor obtained from the data of Arponen and Pajanne,[37] both the original by parameterization by Boronski and Nieminen (BN),[38] described within the local density approximation (LDA), and with an expression obtained by Barbiellini and co-workers,[39,40] described within the generalized gradient approximation (GGA) were used
These results provide evidence that a combination of positron lifetime (Table I) and coincidence Doppler broadening spectroscopy (CDBS) (Figs. 2–4) measurements have the capability to identify specific vacancy related defects in Cu(In,Ga)Se2 chalcogenide materials

Summary

INTRODUCTION

The efficiency of thin film Cu(In1-x,Gax)Se2 (CIGS) chalcogenide solar cells continues to improve,[1,2] with recent gains resulting from the developments in alkali post deposition treatments.[2,3] The deposited CIGS layer is commonly p-doped by lattice defects.[2]. Positron lifetime measurements on CuInSe2 crystals have provided evidence for the perfect lattice, bulk, positron lifetime of approximately 235-240 ps.[22,24] It has not, been possible to clearly establish the vacancy defect lifetimes, a range of values have been reported.[20,21,22,23,25,26,27,28,29,30,31]. Atomic superposition method DFT calculated positron lifetimes are reported for vacancy related defects in CuInSe2 and CuGaSe2. These include calculations performed using available relaxed local geometries for the stable charge states of the monovacancy defects in CuInSe2.13 In consequence, the two experimental positron lifetimes, observed in both crystals, are assigned. The positron momentum distributions, obtained from the calculations, are found to provide the possibility of further differentiation of vacancy defect local environment

Experimental details

Computational details

RESULTS AND DISCUSSION

CONCLUSIONS