The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

Cu2ZnSn(SxSe1-x)4 (CZTSSe) photovoltaic absorbers could be the earth-abundant and low toxicity replacement for the already commercialized CuIn1-xGaxSe2 (CIGS) thin film technology. In order to make this possible, specific research efforts applied to the bulk, front and back interfaces must be performed with the aim of improving CZTSSe performance. In this paper the importance of back contact modification to obtain high efficiency Cu2ZnSnSe4 (CZTSe) solar cells and to increase a paramount and limiting parameter such as VOC is highlighted. Several Mo configurations (monolayer, bi-layer and tri-layer) with different electrical and morphological properties are investigated in CZTSe solar cells. An optimum tri-layer configuration in order to minimize overselenization of the back contact during thermal annealing while keeping reasonable electrical features is defined. Additionally, a thin intermediate MoO2 layer that results in a very effective barrier against selenization and innovative way to efficiently assist in the CZTSe absorber sintering is introduced. The use of this layer enhances grain growth and subsequently the efficiency of solar cells increases via major VOC and FF improvement. An efficiency increase from 7.2% to 9.5% is obtained using a Mo tri-layer with a 20 nm intermediate MoO2 layerThis research was supported by the Framework 7 program under the project KESTCELLS (FP7-PEOPLE-2012-ITN-316488), by MINECO (Ministerio de Economía y Competitividad de España) under the SUNBEAM project (ENE2013-49136-C4-1-R), and by European Regional Development Founds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC and the University of Barcelona belong to the M-2E (Electronic Materials for Energy) Consolidated Research Group and the XaRMAE Network of Excellence on Materials for Energy of the “Generalitat de Catalunya”. M.E-R. thanks the MINECO for the FPI-MINECO (BES-2011-045774), Y.S. for the PTA fellowship (PTA2012-7852-A), SG for the FPI fellowship (BES-2014-068533), M.P. for the MINECO postdoctoral fellow (FPDI-2013-18968), E.S. and R.C. for the “Ramon y Cajal” fellowship (RYC-2011-09212) and (RYC-2011-08521) respectively, and H.X. thanks the “China Scholarship Council” fellowship (CSC Nº 201206340113

Systematic compositional changes and their influence on lattice and optoelectronic properties of Cu2ZnSnSe4 kesterite solar cells

Kesterite thin films solar cells have been studied in recent years as a potential earth abundant alternative to other thin film technologies such as the ones based on the chalcopyrite Cu(In,Ga)Se2 (CIGS). However, the efficiency of the kesterite devices is still low and needs further development. In order to increase the performance of the kesterite solar cells, a better understanding of the influence of variations of compositions on the crystal lattice of the absorber layer is needed. This paper presents a study of how variations in the Cu content in the Cu2ZnSnSe4 (CZTSe) absorber layer affects the crystal structure and therefore changes the optoelectronic properties of the solar cell. Three sets of CZTSe absorbers with different Cu content were synthesised by sputtering of metallic targets followed by a rapid thermal selenisation process. An accurate characterisation of the lattice parameters of the unit cell is presented, showing that a decrease of the Cu content in the CZTSe absorber layer diminishes the unit cell volume. This decrease in volume is related to an increase in the concentration of [VCu+ZnCu] and the ordering of the Cu/Zn atoms in the (001) plane. The variations in the lattice of CZTSe induced by decreasing the Cu content cause an increase of the bandgap of absorber layer, which leads to an increase of the open circuit voltage (VOC) in the devices. The best solar cell was based on the absorber layer with the lowest Cu content and yielded an efficiency of 8.1% with a fill factor of 59.8%, Jsc=31.1 mA/cm2 and Voc=434 mV

Raman scattering quantitative assessment of the anion composition ratio in Zn(O,S) layers for Cd-free chalcogenide-based solar cells

This work reports the use of Raman scattering for the chemical characterization of Zn(O,S) layers that are being developed as a Cd-free alternative for the buffer layer in advanced chalcogenide solar cells. The nanometric thickness of these layers requests the use of resonant excitation conditions, which are strongly sensitive to the alloy composition. In this study Raman spectra were measured with different excitation wavelengths (325 nm, 532 nm) on a set of reference samples with chemical compositions covering the whole range from stoichiometric ZnS to stoichiometric ZnO. The results show the existence of a strong linear dependence of the frequency of the ZnO-like peak on the alloy composition, which provides a simple methodology for the quantitative assessment of the chemical composition in almost all the composition region. In the case of samples with a S-rich composition (0.5 ≤ S/(S + O) ≤ 1), the analysis of the relative intensity of the ZnS like peak allows for a complementary assessment of the S/(S + O) content ratio. The characterization of layers grown under conditions similar to those used for the fabrication of chalcogenide solar cells has allowed the demonstration of the viability of the proposed methodology for the non-destructive chemical assessment of these advanced buffer layers. © The Royal Society of Chemistry 2016.Peer ReviewedPostprint (published version

Complex Surface Chemistry of Kesterites Cu Zn Reordering after Low Temperature Postdeposition Annealing and Its Role in High Performance Devices

A detailed study explaining the beneficial effects of low temperature postdeposition annealing combined with selective surface etchings for Cu2ZnSnSe4 CZTSe based solar cells is presented. After performing a selective oxidizing surface etching to remove ZnSe secondary phases typically formed during the synthesis processes an additional 200 C annealing step is necessary to increase device performance from below 3 power conversion efficiency up to 8.3 for the best case. This significant increase in efficiency can be explained by changes in the surface chemistry which results in strong improvement of the CdS CZTSe heterojunction commonly used in this kind of absorber buffer window heterojunction solar cells. XPS measurements reveal that the 200 C annealing promotes a Cu depletion and Zn enrichment of the etched CZTSe absorber surface relative to the CZTSe bulk. Raman measurements confirm a change in Cu Zn ordering and an increase in defect density. Furthermore, TEM microstructural investigations indicate a change of grain boundaries composition by a reduction of their Cu content after the 200 C annealing treatment. Additionally, insights in the CdS CZTSe interface are gained showing a significant amount of Cu in the CdS buffer layer which further helps the formation of a Cu depleted surface and seems to play an important role in the formation of the pn heterojunctio