Photoluminescence and Photoluminescence Excitation Spectroscopy of Cu(In,Ga)Se2 Thin Films

The role of intrinsic point defects on radiative recombination in Cu(In,Ga)Se2 thin films was investigated by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Experiments were performed on device-grade polycrystalline layers and single crystal thin films. PL transitions identified by others as indicating a shallow state with an ionization energy of ~16 meV is proposed to be a transition into band tail states rather than a distinct shallow defect. The presence of deep levels contributing to radiative recombination does not necessarily preclude the material from producing a high efficiency device and may suggest the absence of dominant nonradiative recombination pathways. The band edge width as measured by PLE and the separation of this edge from defect states are suggested to be potentially effective indicators of the quality of a material. Luminescence that appears to be connected with the absence of Na in the growth process persists in high Ga alloy, Na containing materials, suggesting that Na may become ineffective in passivating or eliminating certain defects in high Ga material. INTRODUCTION Recent environmental and energy resource concerns have spurred interest in renewable energy technologies, particularly in the area of photovoltaic devices. Cu(In,Ga)Se2 (CIGS) and its alloys are leading choices for thin film photovoltaic absorber layers due to their high performance in devices, stability, optical absorption coefficient and tolerance to compositional variations and intrinsic defects. Interpretation and modeling of device results suggest that device performance is limited by ShockleyReed-Hall type recombination of photogenerated carriers in the space charge region [1]. Identification of the defects mediating this recombination has proven difficult and no particular defect has been shown to be responsible [2-4]. One of the most common defect characterization methods is photoluminescence (PL). PL has been performed on CIGS epitaxial layers [5-7], polycrystals [8] and bulk single crystals [9-10]. Only a few studies have incorporated photoluminescence excitation spectroscopy (PLE), with mixed results [6,11]. Almost as many PL studies have been performed on the wide gap compound CuGaSe2 as on CuInSe2. To date, there has been no fundamental difference identified between defect spectra of CuInSe2 and CuGaSe2 – the transition energies are different but the general behavior is relatively similar. Siebentritt et al. [12] have summarized composition-dependent PL and electrical transport measurements on such material. The authors identify two acceptor levels 60 meV and 100 meV above the valence band edge and one donor 12 meV below the conduction band edge resulting in freeand bound-exciton emissions. Transitions may Mater. Res. Soc. Symp. Proc. Vol. 1165 © 2009 Materials Research Society 1165-M03-05