Homojunction cells

Several c-Si homojunction cell architectures are currently being studied by the PV community. These are mainly front and back side passivated architectures (PERC1, PERT2, PERL3 and TOPCON4) [1] with optimization of the doped areas under the passivation and under the metal contacts. 

Among these structures, we are interested in the poly-Si/SiOx/c-Si passivating contact, which aims to minimise the recombination losses linked to overdoping and to the direct contact between the metal contacts and the c-Si absorber. It should be remembered that this structure combined with an IBC5 architecture allows record efficiencies of 26.1% to be achieved today [2]. 

Electrical transport through poly-Si/SiOx/c-Si structures is currently the subject of numerous studies [3-7] due to the different manufacturing processes of the poly-Si, its annealing temperatures as well as the differences that may exist for the SiOx layer in terms of thickness and quality.

The c-AFM technique is one of the approaches we have favoured and implemented to map the local current through these structures under different polarisation and illumination conditions. The results showed that the strong lateral conduction of poly-Si does not allow the discrimination of conduction mechanisms, especially those related to the presence of pinholes [8]. A second approach by KPFM allowed us to observe inhomogeneities of the surface potential on the poly-Si layer. These inhomogeneities are only observable on samples with a passivation oxide (see Figure 1). These preliminary observations led us to conclude that these inhomogeneities are an indirect observation of the presence of pinholes [9, 10]. Simulations of dopant diffusion processes show that diffusion through a SiOx layer with pinholes can lead to a local variation of the CPD around the pinhole [11].

 

Figure 1: Illustration of a KPFM mapping performed on a structure: a) poly-Si/SiOx/c-Si and b) poly-Si/c-Si.

 

Références

[1] C. Battaglia, A. Cuevas, et S. De Wolf, Energy Environ. Sci., 9, 1552‑1576 (2016) ; https://doi.org/10.1039/C5EE03380B
[2] F. Haase et al., Sol. Energy Mater. Sol. Cells 186, 184–193 (2018) ; https://doi.org/10.1016/j.solmat.2018.06.020
[3] D. Tetzlaff’ et al., Solar Energy Materials and Solar Cells. 173, 106–110 (2017);  https://doi.org/10.1016/j.solmat.2017.05.041
[4] F. Feldmann et al., Solar Energy Materials and Solar Cells. 178, 15–19 (2018);  https://doi.org/10.1016/j.solmat.2018.01.008
[5] A. S. Kale et al., Applied Physics Letters. 114, 083902 (2019);  https://doi.org/10.1063/1.5081832
[6] N. Folchert et al., Solar Energy Materials and Solar Cells. 185, 425–430 (2018);  https://doi.org/10.1016/j.solmat.2018.05.046
[7] A. Morisset, Integration of poly-Si/SiOx contacts in silicon solar cells : Optimization and understanding of conduction and passivation properties [dissertation] (2019) Paris-Saclay University
[8] A. Morisset et al., Solar Energy Materials and Solar Cells 200 109912 (2019);  https://doi.org/10.1016/j.solmat.2019.109912
[9] C. Marchat et al., Proceeding of EU-PVSEC. 22-24 (2019);  https://doi.org/10.4229/EUPVSEC20192019-1AO.2.6
[10] J. Alvarez et al., Proc. SPIE (2020). 11288 (2020);  https://doi.org/10.1117/12.2540422
[11] C. Marchat, Caractérisation électrique et optoélectronique de matériaux et composants photovoltaïques par technique AFM [dissertation] (2020) Paris-Saclay University


 
1Passivated Emitter and Rear Cell;  2Passivated Emitter Rear Totally Diffused;  3Passivated Emitter Rear Locally-diffused; 4Tunnel Oxide Passivated Contact; 5Interdigitated Back Contact