A new solution based route for depositing Cu 2 ZnSnS 4 (CZTS) thin films is described, focusing on the effects of Sb and Na co-doping. X-ray diffraction and Raman spectroscopy confirm formation of the kesterite phase with a measurable improvement in crystallinity upon doping. A sharp band gap absorption edge at 1.4 eV is determined from diffuse reflectance measurements, whilst improvement in the photoluminescence yield and sharpening of the band-to-band emission spectra are observed in the presence of Na and Sb. The performance of devices with the configuration: glass/Mo/CZTS/CdS/i-ZnO/ ZnO:Al/Ni-Al and total area of 0.5 cm 2 is reported.Analysis of over 200 cells shows that introduction of Na and Sb leads to an increase of the average power conversion efficiency from 3.2±0.6 to 5.2±0.3%. The best cell with efficiency of 5.7% is obtained upon Na and Sb doping, featuring 14.9 mA cm -2 short-circuit current, 610 mV open circuit voltage and 63% fill factor under simulated AM 1.5 illumination. This performance ranks among the highest in pure sulphide CZTS cells. We propose that the improvement in crystallinity and cell performance is linked to the formation of alkali antimony chalcogenides flux during the annealing step, in addition to Sb and Na decreasing disorder in specific lattice positions of the CZTS unit cell.

A single molecular precursor solution is described for the deposition of CuIn(S,Se) (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 MoSe 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 (E) 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 cm 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/cm 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)/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 MoSe under-layer generates back contact barrier of 195 meV.

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 devices.

Understanding recombination losses in solution processed Cu2SnS3 solar cells with low temperature J–V, impedance measurements and DFT calculations.

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