Development of poly-Si(Cx) passivating contacts for high efficient c-Si based solar cells

Abstract

Carrier-selective passivating contacts (CSPC) are very promising contact structures for highly efficient silicon solar cell. They provide passivation of silicon surface and high carrier selectivity. So far, superior results have been achieved through the use of a stack of poly-Si with SiO2. The objective of this project is the optimization of an alternative passivation stack based on carbon alloyed poly-Si (poly-SiCx). The alloy is selected since carbon provides improved material resilience against blistering and wet-chemical stability in commonly used chemicals in the silicon industry. The optimized poly-SiCx contacts are implemented in fornt back contacted (FBC) solar cell. We improve the passivation quality of SiOx/poly-SiCx contacts optimizing several parameters. Our main focus is on the buffer layer optimization and its deposition methods comparing LPCVD and PECVD technique. We also turn our attention to identify the optimum annealing temperature and thickness of doped layers. Finally, we focus on the influence of the different capping layer composition on hydrogeneation process. With the optimization of the annealing temperature, and thickness of (i)a-Si layer deposited by LPCVD, we obtain high passivation quality on cell precursor structure: i-Voc of 713 mV, τeff of 2.14 ms and Jo of 9.5 fA/cm2. On the contrary, during the optimization of buffer layer deposited by PECVD, we firstly focus on the material properties and the bonds present in the deposited layers to minimize the hydrogen contact. After the optimization of the (i)a-Si:H layer thickness, deposition parameters and annealing temperature on (p)poly-SiCx symmetrical sample, we obtain the remarking results: i-Voc of 681 mV, τeff of 1.15 ms and Jo of 31.3 fA/cm2.
The potential of SiOx/poly-SiCx passivation contacts is checked on the device level of FBC solar cells. On FBC solar cells with buffer layer deposited by the PECVD, after an effective post annealing of the cell performed at a temperature of 350°C, we achieve the remarking parameters; Voc of 659 mV, Jsc of 34.36 mA/cm2, FF of 77.58 % and ηact of 17.56 %. We check also the the influence of different SiNx capping layers on the hydrogenation process. We obtain the best results on the FBC solar cell on which the capping layer is stoichiometric. These results, after the high-temperature port annealing, are Voc of 690 mV, Jsc of 36.18 mA/cm2, FF of 80.38 %, and ηact of 20.06 %. These are also the best FBC poly-SiCx results which we have obtained in this project.
The application of SiOx/poly-SiCx passivation contacts is investigated also in terms of the IBC solar cell concept. We proposed two alternative fabrication methods based on photolithography, which is crucial for contact formation of IBC solar cells. One of the fabrication methods focuses on a buffer layer deposited by LPCVD as this layer is of higher quality and homogeneity than the interlayer deposited by PECVD. The second developed method focuses on the buffer layer deposited by LPCVD and PECVD. Despite the lower passivation quality achieved on contact with the buffer layer deposited by PECVD, this method has a significant advantage because it requires fewer photolithography steps when compared to the fabrication method, which makes use only of buffer layers deposited by LPCVD. In the view of the photolithography processes, we have performed etching tests on (i)a-Si, (i)a-Si: H, and amorphous doped layers. Thanks to these tests, we have identified etching rates of these layers in various prepared poly-Si etching solutions.