Analysis of Band Gap Fluctuations in Cu(In,Ga)Se 2 by Confocal Optical Transmission and Photoluminescence
- PDF / 320,739 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 195 Views
1012-Y08-03
Analysis of Band Gap Fluctuations in Cu(In,Ga)Se2 by Confocal Optical Transmission and Photoluminescence Levent Gütay and Gottfried H. Bauer Institute of Physics, Carl von Ossietzky University Oldenburg, Oldenburg, 26111, Germany
ABSTRACT We present new insights into inhomogeneities of Cu(In0.7,Ga0.3)Se2 absorbers by confocal optical transmission (OT) and spectrally resolved photoluminescence (PL) with lateral resolution in the one-micron regime. We carry out scans for PL and OT at identical positions and observe explicit local variations of the respective spectra. aThe PL-yield shows fluctuations in the suband few-µm regime. As shown in previous papers we extract local variations of the splitting of quasi-Fermi-levels of about 40-50meV, which limits the open circuit voltage of cells, and of the optical threshold of the absorber of some tens of meV related to the local optical band gap. Local transmission spectra similarly show lateral variations in the few-µm regime. From these spectra we calculate local absorption coefficients and absorbance functions which yield variations of the optical threshold of the absorber of 20-30meV, amounting to the same range as PL-results. Via Planck’s generalized law we are able to compute theoretical PL-spectra from the resulting absorbance functions. The discrepancies between measured and calculated hypothetical PLspectra are discussed with respect to diffusion of excess charge carriers and fluctuations of the band gap in the sub-µm-regime below the spatial resolution of the experiment. INTRODUCTION Chalcopyrite Cu(In1-xGax)Se2 (CIGSe), being one of the most promising absorbers for use in low cost, high efficiency solar cells, is subject of many publications. Although impressive efficiencies have been demonstrated for CIGSe cells [1], some fundamental issues which result from the grainy structure of the absorber layer are not sufficiently understood yet. Polycrystalline layers are governed by grain sizes in the µm-regime. Evidently some opto-electronic material parameters, which are of substantial importance for the macroscopic properties of a solar cell, show a distinct degree of inhomogeneity in the CIGSe compound, when analyzed with sufficient spatial resolution [2-8]. Those are inter alia the optical threshold of the absorber - say band gap; the quality of the photo-excited state, as splitting of quasi-Fermi-levels or local open circuit voltage; local current densities and velocities of minority carrier extraction coupled with the quality of the local electronic junction. On a more fundamental level, these effects which obviously affect the optoelectronic behaviour of a material are potentially caused by material inhomogeneities in terms of crystal size and crystal orientation, composition – meaning local concentration of Cu and/or Ga – and of course local defect concentration in the bulk and at interfaces. In previous publications we have already shown results for lateral inhomogeneities of quasiFermi-level splitting and of the optical threshold (band gap) of the
Data Loading...
