Improved CZTSSe Solar Cell Performance by Grain Boundary Passivation

• Main Author: Naili Saidatin
• Other Authors: Ruei-San Chen
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/thx9mp

碩士 === 國立臺灣科技大學 === 應用科技研究所 === 106 === Thin film solar cells, based on Cu2ZnSn(S, Se)4 have attracted widespread interest, due to its high potential for a lower production cost than commercial solar cells. However, highest efficiency for the CZTSSe solar cell has only achieved 12.6% from IBM group in the USA and this is still lower than CIGS solar cell’s 21.7 % from ZSW (Zentrum für Sonnenenergie - und Wasserstoff-Forschung - or Center for Solar Energy and Hydrogen Research - Baden-Württemberg) group in Germany with similar structure. One of the critical issues in CZTSSe solar cells is Voc (open-circuit voltage) deficit. The Voc deficit in the CZTSSe solar cell due to Cu/Zn disorders and band gap fluctuation from non-uniform S/Se distribution have been investigated. In addition, several factors including low dielectric constant, large populations of defects, and un-passivated grain boundaries are issues that limit the Voc of CZTSSe solar cell. In this thesis, we enhance the Voc of CZTSSe solar cell without post-annealing treatments by inserting an alkaline earth fluoride (MgF2) to passivate grain boundaries (GBs) of CZTSSe surface. Direct and indirect effects of MgF2 at grain boundaries and near-interface region in the CZTSSe surface has been investigated in this study. We found that the electronic properties of the GBs are enhanced by MgF2 without post-annealing treatments. According to the scanning Kelvin probe microscopy results, the potential of the sample with MgF2 increases from 18.2 mV to 32.78 mV. This effect leads to a higher negative charge at GBs and forms an electric field passivation, reducing the electron-hole recombination. The results of admittance spectroscopy show that the defect energy level decreases from 220 meV to 133 meV, which is possibly caused by the electric field passivation from the MgF2 at the interface and reduction of the electron-hole recombination. Such mechanism can be also supported by a longer carrier lifetime for the sample with MgF2 (30.6 ns) than pristine sample (20.3 ns). As a consequence, the current density (Jsc) and open circuit voltage (Voc) can be both improved from 30.71 mA/cm2 to 32.61 mA/cm2 and 0.44 V to 0.46 V, respectively. Finally, a CZTSSe solar cell with 9.36 % efficiency was achieved by using 10nm MgF2 without post-annealing treatments.