Sulfurization of co-evaporated Cu2ZnSnSe4 thin film solar cells: The role of Na content << Other seminars
Name of the researcher: Leonor de la Cueva
Faculty of Sciences, UAM, Spain
Cu2ZnSn(S,Se)4 (CZTSSe) has been shown as a promising candidate as absorber layer for thin film solar cells. However, maximum efficiencies in the range of 13 % have been achieved . Sulfurization of the surface of the kesterite absorber layer may lead to enhanced device efficiency via a widening surface bandgap.
In this work, a relationship between the Na concentration and the further S incorporation in the CZTSe absorber layer is demonstrated . Na is added by a NaF precursor layer of 15 nm before CZTSe co-evaporation and/or by diffusion from the soda-lime glass (SLG) substrate. Higher Na concentrations result in bigger grain sizes, higher S/(S+Se) atomic ratios in CZTSSe layer and photovoltaic devices with higher VOC, as investigated by SEM, GIXRD, Raman and I-V measurements. Results using an alternative substrate as ceramic verify the importance of the control of the Na content, which alters the carrier concentrations and elements distribution with much higher S and Na concentrations at the surface in this particular case. By using this methodology, it is possible to tune the kesterite band gap energy by the creation of S-gradient through the absorber thin film (Figure 1). This may provide a new approach for the control of CZTSSe growth and development of high efficiency kesterite solar cells. Efficiencies of 5.5 % and 6.4 % are obtained using ceramic and SLG substrates respectively (without antireflection coatings and grids).
On the other hand, transparent conducting coatings are widely used as electrodes in optoelectronic devices like solar cells . Among the most important material are n-type oxide semiconductors (TCO) such as indium tin oxide (ITO) and fluorine or antimony dope tin dioxide (FTO, ATO). However, these materials are usually deposited by high cost physical deposition techniques . In the second part of this work, done within the project INFITE-CELL in the company UAB Modernios E-Technologijos during a stay in Vilnius by part of the authors, we have also studied and characterized the ITO deposition onto soda-lime glass substrates by spin-coating.