Above 10% Efficient Earth-abundant Cu2ZnSn(S,Se)4 Solar Cells with Introducing Alkali Metal Fluoride Electron-Selective Contacts

• Main Author: Bandiyah Sri Aprillia
• Other Authors: Ruei-San Chen
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/87593524265190076569

碩士 === 國立臺灣科技大學 === 應用科技研究所 === 105 === Cu2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant and non-toxic alternative compound investigated for the purpose of replacing commercialized metal chalcogenides (i.e., CdTe and Cu(In,Ga)(S,Se)2) thin-film solar cells. The current efficiency of the CZTSSe champion solar cells is 12.6% with the absorber’s bandgap (Eg) of 1.13 eV, much lower than the 22.6% efficiency of its Cu(In,Ga)(S,Se)2 counterpart. This considerable performance disparity is caused by a large voltage deficit, 0.62 V, compared to the optical band gap. The VOC deficit issue in CZTSSe solar cells, influenced by band gap fluctuations from non-uniform S/Se distributions and Cu/Zn disorders, has been investigated. Besides band gap fluctuation, the contact loss due to mismatches of the work function is one of the key issues that limits VOC. In order to overcome this problem to boost the performance of CZTSSe based photovoltaics, many efforts have been applied to improving the quality of absorbers, band alignments, front and back interfaces/contacts. This thesis demonstrated highly efficient CZTSSe solar cells by simply introducing the interfacial alkali metal fluoride (AMF) layers (~ several nm NaF, and LiF) between the buffer layers (i.e., CdS) and the front transparent conductive electrodes (i.e., ITO) without extra ZnO layers to modify the ITO work function. Kelvin probe measurements confirm that the work function of the front ITO decreases from 4.82 eV to 3.39 eV (NaF) / 3.65 eV (LiF), which corresponds with the establishment of better Ohmic contacts and decreased contact resistances between CdS and ITO, from 1.99 Ω.cm2 to 0.40 Ω.cm2 (NaF) / 0.60 Ω.cm2 (LiF). They create a beneficial band alignment for electron collection (or hole blocking) on top electrodes which help to reduce interface recombination due to contact losses. According to the temperature-dependent current density-voltage measurement, the AMF based devices show reduced contact voltage loss, leading to larger implied VOC values of 900 meV, compared with the 740 meV of pristine devices. Consequently, all the solar cell parameters, including JSC, VOC, FF, RS, and RSH, are improved. Finally, the 10.4% efficient CZTSSe solar cells were achieved by simply introducing NaF as electron selective contacts.