Contact resistivity analysis of different passivation layers via transmission line method measurements

Kökbudak, Gamze
Crystalline silicon (c-Si) homojunction solar cells constitute over 90% of the current photovoltaic market. Although the standard solar cells are cost effective and easy to process, their efficiency potential is unfortunately limited. Currently, more innovative cell concepts appeared with their high efficiency potential coupled with low costs. Since the recombination at surfaces and under metal contacts is one of the major obstacles against high conversion efficiencies, surface passivation has primary importance in solar cell design. However, the challenging part is reducing surface recombination and properly conducting electrical current simultaneously. To perform these requirements, depositing a thin interface oxide layer and a conductive thin film on top of it, under metal regions, namely passivation layer is a suitable solution. Simultaneously having low contact resistivity and recombination velocity is necessary for such structures. For this, different passivating contact structure have been applied by different research groups. The goal of this thesis is to analyze 3 different passivating contact structures in terms of contact resistivity. Electron beam (e-beam) evaporated in-situ doped (n) vi passivating contact, PECVD deposited in-situ doped (n) TOPCon passivating contact and LPCVD deposited and ex-situ doped (n) Poly-silicon passivating contact structures are the major type of investigated cell designs. The focus of this analysis is on the contact resistivity extraction of these layers. Oldest 1D-TLM contact resistivity extraction method coupled with the recently published 2D-TLM method is applied for all samples. Additional novel idea also presented in this work is applying a new contact resistivity evaluation method using 3D numerical simulations. This method could only be applied to a few samples within the scope of this thesis. The trade-off between the contact properties (ρcontact) and the passivation quality (iVOC) is investigated for various oxide layers obtained via different methods and post annealing temperature following passivation layer deposition. The methods of extracting contact resistivity are also compared. 900 °C annealed HNO3 sample shows as good contact resistivity as non-oxided sample with a contact resistivity of 0.9 mΩ•cm2 using 1D-TLM evaluation and 0.56 mΩ•cm2 using 2D-TLM evaluation. Differentiation of resistivity values between metal/TOPCon interface (ρc1) and TOPCon/bulk interface (ρc2) could be done via the 3D numerical simulation method with the help of plasma etching coupled with numerical simulations. ρc1 and ρc2 were found to be 0.1 and 0.25 mΩ•cm2 respectively for this specific sample. The 3D numerical simulation technique developed for contact resistivity analysis can be applied to a wide variety of structures with as few as possible assumptions. This work contributes to the research and development of high-efficiency silicon solar cells by providing new insights on the properties of passivating contacts. The methods of extracting contact resistivity are additionally compared and the most realistic evaluation method was also presented and performed on some of the samples. 
Citation Formats
G. Kökbudak, “Contact resistivity analysis of different passivation layers via transmission line method measurements,” M.S. - Master of Science, Middle East Technical University, 2017.