Abstract
A single molecular precursor solution is described for the deposition of CuIn(S,Se)2 (CIS) film onto Mo-coated glass substrates by spin coating, followed by annealing in Se atmosphere. Characterization of the films by X-ray diffraction, Raman spectroscopy and scanning electron microscopy demonstrates the formation of a highly homogeneous and compact 1.1 μm thick CIS layer, with a MoSe2 under-layer. Atomic force microscopy reveals the presence of spherical grains between 400 and 450 nm, featuring surface corrugation in the range of 30 nm. Film composition is found to be in close agreement with that of the precursor solution. Diffuse reflectance spectroscopy shows a direct band gap (Eg) of 1.36 eV. Intensity and temperature dependence photoluminescence spectra show characteristic features associated with a donor-acceptor pair recombination mechanism, featuring activation energy of 34 meV. Over 85 solar cell devices with the configuration Mo/CIS/CdS/i-ZnO/Al:ZnO/Ni-Al and an total area of 0.5 cm2 were fabricated and tested. The champion cell shows a power efficiency of 3.4% with an open circuit voltage of 521 mV and short circuit current of 14 mA/cm2 under AM 1.5 illumination and an external quantum efficiency above 60%. Overall variation in each of solar cell parameters remains below 10% of the average value, demonstrating the remarkable homogeneity of this solution processing method. To understand the limitation of devices, the dependence of the open-circuit voltage and impedance spectra upon temperature were analyzed. The data reveal that the CuIn(S,Se)2/CdS interface is the main recombination pathway with an activation energy of 0.79 eV as well as the presence of two "bulk" defect states with activation energies of 37 and 122 meV. We also estimated that the MoSe2 under-layer generates back contact barrier of 195 meV.
Highlights
Thin film technology is one of key contributor to global PV, occupying approximately 10% of the installed capacity
The latter method is very appealing from the processing point of view and has shown remarkable efficiencies of 13.0% and 14.7% for CIS and CIGS respectively, with dimethyl sulfoxide (DMSO) used as solvent.[20]
Impedance spectroscopy recorded as a function temperature demonstrated that key limiting factors in cell performance are the presence of defect states at the CIS/CdS interface and the formation of a Mo(S,Se)[2] layer during the thermal treatment
Summary
Thin film technology is one of key contributor to global PV, occupying approximately 10% of the installed capacity. Three different types of molecular precursors have been reported: (i) binary sulfides with excess chalcogen in hydrazine or thiolamine solvent mixtures demonstrating efficiencies of 15.2% and 8.0% for CIGS and CIS, respectively,[15,16] (ii) metal precursors followed by annealing under chalcogen atmosphere, where a maximum efficiency of 11.3% for CIGS is achieved,[17−19] and (iii) single solution containing all metal precursors and thiourea as sulfur source.[20] The latter method is very appealing from the processing point of view and has shown remarkable efficiencies of 13.0% and 14.7% for CIS and CIGS respectively, with dimethyl sulfoxide (DMSO) used as solvent.[20] In this contribution, we assess a new single molecular precursor based route employing dimethylformamide-isopropanol mixture (DMF-IPA) for the deposition of high quality CuIn(S,Se)[2] films. Impedance spectroscopy recorded as a function temperature demonstrated that key limiting factors in cell performance are the presence of defect states at the CIS/CdS interface and the formation of a Mo(S,Se)[2] layer during the thermal treatment
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