A direct measurement of higher photovoltage at grain boundaries in CdS/ CZTSe solar cells using KPFM technique

• Published in: Solar energy materials and solar cells 2018-08, Vol.183, p.34-40
• Main Authors: Vishwakarma, Manoj, Varandani, Deepak, Andres, Christian, Romanyuk, Yaroslav E., Haass, Stefan G., Tiwari, Ayodhya N., Mehta, Bodh R.
• Format: Article
• Language: English
• Subjects: Band bending; Boundaries; Cadmium sulfide; Carrier recombination; Circuits; Contamination; Copper; Copper zinc tin selenide; Cu2ZnSn(SSe)4; Defects; Grain and grain boundaries; Grain boundaries; Kesterite phase; Mapping; Microscopy; Open circuit voltage; Photovoltage; Photovoltaic cells; Recombination; Secondary phases; Solar cells; Surface potential; Zinc oxide
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2018.01.040

A direct mapping of photovoltage in a complete Mo/CZTSSe/CdS/ZnO/Al:ZnO solar cell device is carried out using Kelvin probe force microscopic (KPFM) measurements in surface and junction modes. Four cells having different values of open circuit voltage (VOC) have been studied, and nanoscale variation of photovoltage have been obtained from the difference of surface potential (SP) images in the two modes. The maps exhibit a higher photovoltage at grain boundaries in general. Observed SP image of pristine CZTSe layer reveals downward band bending resulting in reduction of carrier recombination, and thus, lower Jdark at grain boundaries (GB). The observed downward band bending resulting in low Jdark at GB in CZTSe layer is used to explain the increase in photovoltage (VOC) at the GB. KPFM measurements of CZTSe and CZTSe/CdS layers show that the surface potential of CZTSe layer and its variation can be affected by CdS deposition and further device processing, in addition to surface adsorption and contamination effects. The photovoltage mapping obtained from Kelvin probe force microscopic measurements on the final device without any interfering effects, is an important advantage of the present method. A direct mapping of photovoltages by KPFM measurements in surface (S) and junction (J) contact modes in the final PV device, results in histograms with two peaks corresponding to Grain (peak1) and GB (Peak2). [Display omitted] •Nanoscale photovoltage is studied in CZTSe and CZTSSe finished solar cell devices using KPFM technique.•The present method used two contact modes which nullifies the effect of AZO/i-ZnO layers during photovoltage mapping.•The nanoscale photovoltage measured by KPFM under light conditions co-relates well with macroscopic VOC of the device.•The heterogeneous distribution of photovoltages on grain and grain boundaries revealed in the present method.