Spectral Calibrated and Confocal Photoluminescence of Cu 2 S Thin-Film Absorber
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Spectral Calibrated and Confocal Photoluminescence of Cu2S Thin-Film Absorber Hendrik Sträter1, Rudolf Brüggemann1, Sebastian Siol2, Andreas Klein2, Wolfram Jaegermann2, Gottfried H. Bauer1 1 Institut für Physik, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg, Germany 2 Fachbereich 11, Materialwissenschaft, Fachgebiet Oberflächenforschung, Technische Universität Darmstadt, D-64287 Darmstadt, Germany. ABSTRACT We have studied Cu2S absorber layers prepared by physical vapor deposition (PVD) by calibrated spectral photoluminescence (PL) and by confocal PL as function of temperature T and excitation fluxes to obtain the absolute PL-yield at an excitation flux equivalent to the AM1.5 spectrum and to calculate the splitting of the quasi-Fermi levels (QFL) μ = Ef,n-Ef,p and the absorption coefficient (E), both in the temperature range 20 K T 400 K. The PL-spectra reveal two peaks at E1 = 1.17 eV and E2 = 1.3 eV, of which the low energy peak is only detectable at temperatures T < 200 K. The samples show an impressive QFL-splitting of μ > 700 meV at 300 K associated with a pseudo band gap of Eg = 1.25 eV. The high energy peak shows an unexpected temperature behavior, namely an increase of the PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T from extrapolated T = 0K value of μ = 1.3 eV. The PL-yield versus temperature will be discussed in terms of different defect states in the band gap. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of the opto-electronic properties by confocal PL shows a strong correlation between the QFLsplitting, the Urbach energy EU and the optical band gap Eopt, respectively. INTRODUCTION Copper sulfide (Cu2S) is an abundant and non-toxic p-type semiconductor and thus a potential candidate for an absorber in thin film solar cells [1]. The Cu2S/CdS heterojunction was even the first reported thin film solar cell device [2], which later obtained efficiencies of 10% [3]. We present a detailed study of the excitation intensity and temperature dependent photoluminescence (PL) on the large and onto the microscopic scale. We use PL experiments as a contact-less technique to determine the radiative recombination of photo-generated excitation states within a semiconductor, which provides access to opto-electronic properties like the energy dependent absorption A(E), the splitting of quasi-Fermi levels μ = Ef,n - Ef,p and additionally information about defect levels within the band gap and the dynamics of transitions. Since the QFL-splitting can be interpreted as the limiting open-circuit voltage of a finally fabricated solar cell, Voc = μ/e-, PL measurements allow analyzing the condition of an absorber material before it is used as a solar cell. According to Planck's generalized law, the emitted photon flux or PL-yield YPL(E) a
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