Nanometre-scale optical property fluctuations in Cu2ZnSnS4 revealed by low temperature cathodoluminescence

in Solar Energy Materials and Solar Cells (2018), 174

Nanometre-scale optical property fluctuations in Cu2ZnSnS4 revealed by low temperature cathodoluminescence

in Solar Energy Materials and Solar Cells (2017), 174

Quantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response

in Solar Energy Materials and Solar Cells (2014), 123

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6 ... [more ▼]

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6 % power conversion efficiency. The loss of short circuit current density for samples with increasing ZnSe content is consistent with an overall reduction of spectral response, pointing to a ZnSe current blocking behavior. A feature in the spectral response centered around 3 eV was identified and attributed to light absorption by ZnSe. A model is proposed to account for additional collection of the carriers generated underneath ZnSe capable of diffusing across to the space charge region. The model satisfactorily reproduces the shape of the spectral response and the estimated ZnSe surface coverage is in good qualitative agreement with analysis of the Raman spectral mapping. The model emphasizes the importance of the ZnSe morphology on the spectral response, and its consequences on the solar cell device performance. [less ▲]

Phase segregation in Cu2ZnSnSe4 thin films for photovoltaic applications. The effects of precursor microstructure and selenium activity during selenization of electrodeposited metallic precursors.

Doctoral thesis (2014)

This thesis develops the synthesis of thin film copper-zinc-tin-selenide kesterite absorber layers, Cu2ZnSnSe4 (CZTSe), suitable for solar cell applications. The kesterites are highly sensitive to ... [more ▼]

This thesis develops the synthesis of thin film copper-zinc-tin-selenide kesterite absorber layers, Cu2ZnSnSe4 (CZTSe), suitable for solar cell applications. The kesterites are highly sensitive to stoichiometry deviations and most of the kesterite growth technologies suffer from the segregation of secondary phases, which can be detrimental to the performance of the solar cell. This thesis examines the phase segregation occurring during CZTSe absorber growth during a two-step electrodeposition – annealing process. A method towards reducing phase segregation in the CZTSe absorber is developed by manipulating the microstructure of Cu-Sn-Zn precursors and the annealing conditions. A power conversion efficiency improvement from 0 % to 5.94 % was possible by changing the microstructure of the metallic precursor from a bi-layered with Zn-rich alloy on the top to a matrix-type. This is explained by the higher reactivity of Zn with selenium vapour than Cu or Sn. The synthesis route used in this study consisted of three stages: sequential electrodeposition of Cu, Sn and Zn layers, soft pre-alloying of the resulting metallic stacks, and thermal annealing in the presence of selenium vapour. The pre-alloying step is introduced in order to develop uniform microstructure to the precursors by formation of Cu-Sn and Cu-Zn alloys. Depending on the stacking order of the metals in the electrodeposited precursors, two types of microstructure are developed during the pre-alloying process: a bi-layered structure with the Cu-Zn alloy on the top and the Cu-Sn alloy on the bottom and a matrix-type microstructure with the Zn-rich phase in the Sn-rich matrix. It is shown that the precursor microstructure determines the further segregation of phases during CZTSe growth. Using a rapid thermal annealing system (RTP) it is possible to resolve the mechanism of CZTSe formation at short time intervals. Identification and location of secondary phases is studied by four characterization techniques: XRD, Raman spectroscopy, SEM/EDX and SIMS. The influence of the partial pressure of selenium above the precursor on the CZTSe growth is investigated by theoretical computation and compared to experimental results. It is shown that the rate-limiting step of kesterite formation is the arrival of selenium species from the source to the precursor sample. This is correlated to the thermodynamic feasibility of the metal- vapour phase selenium reactions, since the selenium partial pressure above the sample will determine which phases are prone to form. The studies of the CZTSe formation mechanism show that Zn is the first metal to react with selenium vapour. This explains why the presence of Zn on the precursor surface enhances the ZnSe phase segregation during the CZTSe growth. [less ▲]

The Effect of Soft Pre-Annealing of Differently Stacked Cu-Sn-Zn Precursors on the Quality of Cu2ZnSnSe4 Absorbers

in Materials Research Society Symposia Proceedings. (2013), 1538