G.H. Bauer

Spatially resolved photoluminescence measurements on Cu(In,Ga)Se2 thin films

Abstract We have measured the spatial dependence of photoluminescence in thin films of Cu-poor Cu(In,Ga)Se 2 with submicrometer resolution. We found large variations in the photoluminescence signal by more than a factor of 5 confined to domains much larger than the typical grain size of 1 μm in the films. Numerical simulation of the diffusion and recombination of photoexcited carriers shows that grain boundary recombination velocities of 10 6 cm/s may lead to approximately 40% variation in the photoluminescence signal. The large differences in photoluminescence efficiency are interpreted in terms of different material phases or defect structures in the off-stoichiometric quaternary chalcopyrite system. From the variation of the photoluminescence intensity at 70 K, we deduced a maximum lateral variation in the quasi-Fermi level splitting of 10 meV.

PVD of copper sulfide (Cu2S) for PIN-structured solar cells

Thin layers of chalcocite (Cu2S) have been deposited via physical vapour deposition using various pre- and post-treatment parameters. The electrical and morphological properties have been investigated by in situ XPS, SEM and XRD measurements. Calibrated photoluminescence experiments were performed to investigate the materials suitability as an absorber layer in thin-film solar cells. Measurements of annealed Cu2S layers on glass without any surface passivation showed an optical band gap of 1.25 eV as well as a splitting of the quasi-Fermi levels of 710 meV. This value exceeds the highest reported open-circuit voltage for Cu2S-based devices so far, which leads to the assumption that Cu2S has not been brought to its full potential yet. The band alignments for ZnO/Cu2S as well as Cu2S/Cu2O interfaces have been determined using in situ XPS interface experiments to suggest a novel device structure according to the favourable PIN-layout. First devices have been built, but show no efficiency due to shunting caused by the inferior morphology of the absorber layers.

The correlation between local defect absorbance and quasi-Fermi level splitting in CuInS2 from photoluminescence

Analogously with Cu(In,Ga)Se2, CuInS2 shows a high degree of spatial inhomogeneities in structural, optical and electronic properties on the length scale of grain sizes and above which is caused by the grainy structure and the inhomogeneous growth of absorber layers. To analyse these locally fluctuating magnitudes, spectrally resolved photoluminescence measurements with high lateral resolution (≤1μm) have been performed in a confocal microscope setup. Based on these data sets and on Plancks generalized law determination of the spatial variation in the splitting of the quasi-Fermi levels and access to the local absorbance is possible. A detailed analysis of these properties, crucial for the solar light conversion efficiency of a final cell, is made for a CuInS2 absorber layer for data obtained from statistically representative scan areas. A cross-correlation between the splitting of the local quasi-Fermi levels and the local absorbance of an absorber leads to the conclusion that the splitting of quasi-Fermi levels is strongly governed by the excess-carrier recombination via deep defects.

Detailed photoluminescence studies of thin film Cu2S for determination of quasi-Fermi level splitting and defect levels

We have studied chalcocite (Cu2S) layers prepared by physical vapor deposition with varying deposition parameters by calibrated spectral photoluminescence (PL) and by confocal PL with lateral resolution of Δu2009x≈0.9u2009μm. Calibrated PL experiments as a function of temperature T and excitation fluxes were performed to obtain the absolute PL-yield and to calculate the splitting of the quasi-Fermi levels (QFLs) μ=Ef,n−Ef,p at an excitation flux equivalent to the AM 1.5 spectrum and the absorption coefficient α(ℏω), both in the temperature range of 20u2009K≤T≤400u2009K. The PL-spectra reveal two peaks at E#1=1.17u2009eV and E#2=1.3u2009eV. The samples show a QFL-splitting of μ>700u2009meV associated with a pseudo band gap of Eg=1.25u2009eV. The high-energy peak shows an unexpected temperature behavior, namely, an increase of PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-sp...

Design of domain size and molecular interactions in organic semiconductors to control the emission yield of thin films

Perylene- and phthalocyanine- pigment molecules were systematically modified and consequences were studied for their solid state properties. Thin films (1 - 150 nm) were prepared by physical vapor deposition. Intermolecular interactions were probed by optical measurements in absorption and emission. Atomic force microscopy served to analyze the morphology of films. Different interactions among the molecules and with the substrate surfaces allowed to prepare either crystalline or amorphous films. Crystalline films of perylene pigments were typically characterized by strong chromophore coupling leading to a characteristic splitting, well- defined shifts of the optical absorption bands and emission mainly from excimer species whereas the chromophore coupling in amorphous films was suppressed sufficiently to provide a significantly increased optical emission yield from uncoupled monomer states. Temperature-dependent optical emission experiments are presented which allow a detailed discussion of monomer vs. excimer emission. Decoupling of the chromophores could be obtained by appropriate chemical substitutions at the aromatic core system of phthalocyanines and perylene pigments that led to strong deviations from planarity. This was achieved by the introduction of bulky substituents in the bay position of the aromatic perylene core and by changes in the coordination number of the central group in phthalocyanines. The strategy led to a strongly enhanced optical emission for both classes of materials. This could be obtained, however, either in an amorphous arrangement of the molecules or under conservation of crystallinity, both offering alternative advantages.