Assessment of CuInS2 and CuInS2:(Zn,Mg) Solar Cells by Means of Photoluminescence Spectroscopy

摘要

Solar cells based on CuInS2 thin films still suffer from Voc limitation compared to the theoretically achievable value as expected from the optical band gap of 1.5 eV. The open circuit voltage can be significantly enhanced from about 700 mV to more than 800 mV via controlled Zinc or Magnesium doping (<1 at. %), whereas the efficiency remains mostly constant. The increase of the open circuit voltage occurs in spite of an extended red response. In order to investigate this phenomenon we used standard tools to perform the basic characterisation of photovoltaic cells (I/U and quantum efficiency (QE) measurements). As it is desirable for a solar cell production to determine the quality of the absorber with respect to the functionality of the later device we use photoluminescencespectroscopy (PL) at room temperature as such a controlling tool. PL spectra at room temperature of undoped CuInS2solar cells, made in a sequential process, are dominated by two main luminescent transitions. From the variation ofthe excitation energy we identify a broad emission as donor-acceptor-pair transition (1.15 eV) and a band edge nearemission as a band-band transition (1.51 eV). However, we find a new broad emission band at 1.35 eV in the spectraof Zn or Mg doped samples. In contradiction to the standard samples, where a less dominant broad sub-band gapemission is an indicator for high device quality, Zn-doped samples show an increase in the open circuit voltage for arising PL-intensity of the new emission. From results of Raman spectroscopy we attribute this behavior to a structuralchange of CuInS2:(Zn, Mg) induced by Zn (Mg) incorporation.