Characterization of SiNx/a-Si:H crystalline silicon surface passivation under UV light exposure

Abstract

One of the most promising solution for crystalline silicon surface passivation in solar cell fabrication consists in a low temperature (b 400°C) Plasma Enhanced Chemical Vapor Deposition of a double layer composed by intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (SiN x ). Due to the high amount of hydrogen in the gas mixture during the double layer deposition, the passivation process results particularly useful in case of multi-crystalline silicon substrates in which hydrogenation of grain boundaries is very needed. In turn the presence of hydrogen inside both amorphous layers can induce metastability effects. To evaluate these effects we have investigated the stability of the silicon surface passivation obtained by the double layer under ultraviolet light exposure. In particular we have verified that this double layer is effective to passivate both p-and n-type crystalline silicon surface by measuring minority carrier high lifetime, via photoconductance-decay. To get better inside the passivation mechanisms, strongly connected to the charge laying inside the SiN x layer, we have collected the Infrared spectra of the SiN x /a-Si:H/c-Si structures and we have monitored the capacitance-voltage profiles of Al/SiN x /a-Si:H/c-Si Metal Insulator Semiconductor structures at different stages of UltraViolet (UV) light exposure. Finally we have verified the stability of the double passivation layer applied to the front side of solar cell devices by measuring their photovoltaic parameters during the UV light exposure.

Laser fired back contact for silicon solar cells

“…In particular we ascribe this effect to the large amount of positive charge density that lay inside the SiN x and at interface with a-Si:H. These charges are able to perform an inversion layer within the silicon wafer so strong that the Fermi level position, at the wafer back edge, is shifted close to the conduction band. This effect reduces the probability of recombination at the Si/a-Si:H interface [6].…”

“…Around the grid a 15 µm deep trench has been buried by CP4 wet etching at 4°C fixing the area of the cell to 4 cm 2 . To ensure passivation of both device surfaces a double thin layer of amorphous silicon and silicon nitride, a-Si: H/SiN x , deposited in a 13.56MHz plasma enhanced chemical vapor deposition (PECVD) system has been adopted [6]. In particular the a-Si:H layer has been deposited at 250°C, 750 mTorr and 15W RF power, 120 cm of 5% SiH 4 diluted in Ar.…”

“…It consists in two steps: 1) back Si surface passivation by an amorphous silicon/silicon nitride a-Si:H/SiN x double layer on the base of previous results demonstrated in Ref. [6]; 2) formation of an improved local BSF obtained by laser firing of Al and B, vacuum evaporated and deposited as a spin-on compound respectively. Local Al & B simultaneous diffusion through the passivation layers have been obtained using a Nd:YAG pulsed laser.…”

Back contacted a-Si:H/c-Si heterostructure solar cells

“…Then the wafer has been turned over to deposit the passivation/anti-reflection coating. We have preferred to passivate the sunward side by the (aSi:H/SiN x ) double layer instead of single SiN x layer, since it has been demonstrated to better passivate crystalline silicon surface with respect to the SiN x [11]. In turn, we lose part of the blue sun spectrum due to the a-Si:H absorption of the high energy photons.…”

High quality amorphous–crystalline silicon heterostructure prepared by grid-biased remote radio-frequency plasma enhanced chemical vapor deposition

“…Typically, amorphous-crystalline silicon heterostructures are prepared by depositing thin-layers of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) onto the surface of a c-Si wafer using either radio-frequency (RF) or very-high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD) [6][7][8][9]. The benefits of these deposition methods are that they allow for a-Si:H growth over large areas with high uniformity and reproducibility.…”