The latest developments in bifacial solar cells at ECN part of TNO

The performance of current industrial solar cells is for a large part limited by recombination at metallic contacts. A solution to overcome this is to screen minority carriers from the metal by using so-called passivating carrier-selective contacts. Using carrier-selective contacts based on doped polysilicon layers combined with tunnel oxides, efficiencies well above 25% have already been reached on front and back contacted cells on laboratory scale. In this paper we will present our latest developments in bifacial cells featuring polysilicon carrier-selective passivating contacts which can be seen as an upgrade to bifacial cell architectures such as PERC+ and PERT. First, we will present the upscaling and application of these polysilicon passivating contacts to 6” bifacial cells featuring both n+ and p+polysilicon contacts (n+ and p+poly-Si), using industrial screen printed and fired metallization. The main challenges for such solar cells are keeping the contact recombination low, while applying a cost effective, industrial metallization approach. The contact mechanism of commercial silver and silver/aluminum pastes on both n+ andp+poly-Si layers has been extensively studied using scanning and tunneling electron microscopy (SEM, TEM) and Jo,contact and contact resistance characterization, using transmission line measurements (TLM). These experiments lead to a consistent description and understanding of the polysilicon passivating contacts and its interaction with the fire through metallisation. The contact recombination on n+poly-Si surfaces has been reduced down to 80 fA/cm2 while the contact resistance is kept low enough to enable FF > 79.5%. Voc as high as 683 mV has been obtained enabling initial cell efficiencies of 21.5%. Further optimization of the processing and non-contacted passivation of the polysilicon layer stack has yielded record low values for Jo on textured surfaces: we present Jo and implied Voc measurements with best values of <1 fA/cm2 and 741 mV for n+poly-Si, and ~8 fA/cm2 and 724 mV for p+poly-Si on textured n-Cz wafers, which are maintained after firing by improvement of the hydrogenation scheme.
In this study we also present our improvements of p+poly-Si passivation on industrially relevant textured surfaces by means of advanced hydrogenation schemes, involving novel dielectric stacked layers, and modification of the B-profile leading to a record passivation level of ~8 fA/cm2 and 724 mV before firing and ~9 fA/cm2 and 722 mV after firing. The results promote industrial implementation of p+poly-Si on 6’’ bifacial cells with excellent passivation properties and compatibility with screen-printed and fire-through metallization, especially as our latest results show Jo,contact being reduced down to 200 fA/cm2. Bifacial solar cells, have been manufactured with industry compatible processing with front-and-rear passivating contacts with efficiencies above 20% in the first attempt and this developments shows a potential of 23% with a bifaciality factor of 90%.