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Influence of the Dopant Gas Precursor in P-Type Nanocrystalline Silicon Layers on the Performance of Front Junction Heterojunction Solar Cells

Christophe Ballif, Quentin Thomas Jeangros, Laurie-Lou Senaud, Mathieu Gérard Boccard, Jan Haschke, Luca Massimiliano Antognini, Jean Cattin, Julie Amandine Dreon, Mario Joe Lehmann, Vincent Philippe Paratte

Silicon heterojunction solar cells can employ p-type hydrogenated nanocrystalline silicon nc-Si:H(p) on their front side, since these can provide better transparency and contact resistance compared to hydrogenated p-type amorphous silicon layers. We investigate here the influence of trimethyl boron (TMB) and BF3 as dopant source on the layer properties and its performance in solar cells. Both gases enable high efficiencies but yield a different crystallinity and effective doping. A high BF3 flow lowers the series resistance through a low activation energy of dark lateral conductivity and maintains a high crystallinity. This allows fill factors up to 83%, however with the apparition of a parasitic absorption in the UV. A low TMB flow enables simultaneously a high crystallinity and a low activation energy. As an illustration of this layer potential, a 23.9%-certified efficiency is achieved with a 2 x 2 cm(2) screen-printed device. We finally suggest that similar transport versus transparency trade-offs can be reached for both dopant types for front junction application, while high BF3 flow allowing lower series resistance might be of interest when placed on the rear side.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

Influence of Light Soaking on Silicon Heterojunction Solar Cells With Various Architectures

Christophe Ballif, Bertrand Yves Paul Paviet-Salomon, Laurie-Lou Senaud, Matthieu Despeisse, Mathieu Gérard Boccard, Jan Haschke, Jean Cattin

In this article, we investigate the effect of prolonged light exposure on silicon heterojunction solar cells. We show that, although light exposure systematically improves solar cell efficiency in the case of devices using intrinsic and p-type layers with optimal thickness, this treatment leads to performance degradation for devices with an insufficiently thick (p) layer on the light-incoming side. Our results indicate that this degradation is caused by a diminution of the (i/p)-layer stack hole-selectivity because of light exposure. Degradation is avoided when a sufficiently thick (p) layer is used, or when exposure of the (p) layer to UV light is avoided, as is the case of the rear-junction configuration, commonly used in the industry. Additionally, applying a forward bias current or an infrared light exposure results in an efficiency increase for all investigated solar cells, independently of the (p)-layer thickness, confirming the beneficial influence of recombination on the performance of silicon heterojunction solar cells.

2021

Dopant-Free Bifacial Silicon Solar Cells

Christophe Ballif, Mathieu Gérard Boccard, Luca Massimiliano Antognini, Jean Cattin, Julie Amandine Dreon, Vincent Philippe Paratte, Sihua Zhong, Wenjie Lin, Hui Shen

Herein, challenges in the fabrication of full dopant-free bifacial silicon solar cells are discussed and efficient devices utilizing a MoO3/ indium tin oxide (ITO)/Ag hole-selective contact and ZnO/LiFx/Al electron-selective contacts with up to 79% short-circuit current bifaciality are demonstrated. The ZnO/LiFx/Al rear electron contact features a full-area ZnO antireflective coating and a LiFx/Al finger contact, allowing sunlight absorption from the back side, thus producing more overall power. The ZnO/LiFx/Al electron contacts with a thinner ZnO layer and a larger contact fraction display a better selectivity and a lower resistance loss. When considering rear-side irradiance of 0.15 sun, the dopant-free bifacial solar cell with 60 nm ZnO and 50% LiFx/Al metal contact fraction achieves a 3% estimated output power density improvement compared with its monofacial counterpart (21.0 mW cm(2) compared to 20.3 mW cm(2)) using the full-area back contact. Both the efficiency and bifaciality factor of this dopant-free device are still significantly lower than those of state-of-the-art devices relying on doped-silicon-based layers. The required improvement for this technology to become industry-relevant is discussed.

WILEY-V C H VERLAG GMBH2021