“Investigation of the Current Transport Mechanisms in Fire Through Dielectric Contact (FTDC) to Silicon Solar Cells by Spectroscopic Analyses”
Doctoral Advisor: Dr. Abasifreke Ebong
This thesis work investigates the current transport mechanisms at the Si/gridline interface of the screen-printed commercial silicon (Si) solar cells with lowly doped emitters. The use of lightly doped emitter is one way to improve the efficiency of the Si solar cell. In addition, developing the screen-printable Ag/Cu paste to reduce the cost of using 100% Ag paste is crucial to reach the cost-effective solar electricity. Hence, the contacts under investigation are, respectively, Ag/Si and Ag/Cu/Si, which are formed with screen-printable pastes consisting Ag powder (and or Cu at some wt%), glass frits and organic binders. For screen-printable contacts formed on heavily doped emitter, the contact resistance at the Ag/Si is always low but because of the shadowing and surface recombination losses, the cell efficiency is low. In this work, the Ag/Si interface on lightly doped emitter was first studied to elucidate the understanding of the transport mechanisms at the interface and then extended to the Ag/Cu/Si interface.
Optical and electrical characterization of the Ag/Si interface was carried out after the contact formation at high temperature. For the Ag/Si interface on the lightly doped emitter, a very thick interface glass layer (IGL) was measured, which according to carrier transport reported in the literature would result in a very high contact resistance. However, this was not the case here because this IGL was found to be conductive according the conductive AFM (c-AFM) I-V curve, which fitted a barrier height of only 0.1 eV. The Raman spectroscopy revealed some semimetal nano alloys (PbTe and Ag2Te) in the glass with very small bandgaps, and these semimetal alloys caused the IGL to be conductive. This led to the conclusion that, in the presence of these semimetal alloys, the contact behaves as ohmic as seen with the lithography and buried contacts which are pure metal semiconductor contacts.
The same characterization was extended to the screen-printable Ag/Cu paste interface with Si. In this case, different weight percent of Cu (0 wt%, 25 wt%, 50 wt%) was added to the Ag paste. It was observed that the higher wt% Cu led to the Cu-doping the glass frits and caused increased transition temperature (Tg) of the glass frits. The SEM showed that the increased Tg of the glass impeded the uniform spreading of the molten glass and resulted in poor wetting and etching of the SiNx. Moreover, the agglomerated Ag crystallites were found in the Si. According to the EDS combined with STEM analyses, the IGL acted as an effective diffusion barrier layer to prevent Cu atoms from diffusing into the Si emitter, which is the primary requirement for applying Cu in the Si solar cells. Further investigation of the Ag/Cu contacts with the c-AFM in conjunction with the SEM analysis revealed that the growth of Ag crystallites in the Si emitter is responsible for carrier conduction in the Ag/Cu/Si contacts.
Date: January 22, 2020