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An international research group has developed a novel technique that allows contacts to be created at low temperatures. New types of contact can improve cell efficiency by up to 3%.
Scientists in Australia and the United Kingdom have developed a novel method to construct localized back contacts in p-type silicon absorbers. The proposed process uses polycrystalline silicon (poly-Si) doped with aluminum (Al) and induced, formed at low temperatures.
“This approach avoids the high-temperature processes necessary for the formation of poly-Si and the toxic gases involved in in situ doping through chemical vapor deposition,” they stressed. “In systems where high-temperature contact fabrication could compromise upper cell materials – such as in III-V/Si multijunction solar cells formed by epitaxial growth – this technique offers a valuable alternative.”
The researchers constructed the contacts using p-Si wafers and p-Si substrates, on which they deposited a 45 nm layer of hydrogenated intrinsic amorphous silicon [a-Si:H(i)]. They then deposited 400 nm of Al contacts and annealed the samples at 190 °C for 5 minutes.
“Surface passivation is impaired due to the presence of Al dopants. Therefore, instead of using a full-area a-Si:H(i)/Al contact, a localized contact structure was chosen to combine the advantages of the superior surface passivation of a-Si:H(i) and the low contact resistivity introduced by Al-doped poly-Si,” the researchers explained.
Before annealing, the samples showed “insulator-like” behavior. However, once annealed, the sample showed a contact resistivity of 13.17 mO·cm2. When the quality of passivation was checked, the area directly beneath the Al contacts was significantly degraded after annealing.
“It should be noted that, however, the a-Si:H(i) layer still provides passivation for areas not covered by the Al contacts, despite the reduced passivation quality,” the academics highlighted. “After annealing, the calculated implicit open-circuit voltage drop is about 12.7 mV for the Al-free regions. The preserved passivation and reasonable voltage drop indicate the feasibility of applying this contact technique as localized passivated contacts.” .
Following these results, researchers moved forward in creating proof-of-concept solar cells. They were fabricated from p-Si PERC cell precursors with a conventional n-type emitter. The front surface was passivated with silicon nitride (SiNx), which also acted as an anti-reflective coating (ARC). Additionally, a 400 nm Al layer was deposited on top to form a full-area Al contact.
For the construction of the rear part, the posterior structure of the PERC cell precursor was removed. A 45 nm a-Si:H(i) layer was then deposited, followed by localized circular Al contacts. The process was followed by annealing at 190 °C for 5 minutes.
“For comparison, a reference cell based on the PERC precursor with a full-area Al back contact was also fabricated,” the scientists explain. “Power conversion efficiencies of 17% and 13.9% were obtained for cells with the a-Si:H(i)/Al contact and a full-area Al back contact, respectively, indicating improved of the absolute efficiency of 3%”.
Their findings were presented in the study “ Low-temperature aluminum doped and induced polysilicon and its application as partial rear contacts on p-type silicon solar cells. ” p-type silicon solar cells), published in Solar Energy Materials and Solar Cells . The group consisted of researchers from the University of New South Wales (UNSW) Sydney and the University of Oxford. |
