Plasmas for texturing, cleaning, and deposition: towards a one pump down process for heterojunction solar cells

Abstract

Low temperature plasma deposition of a‐Si:H thin films has emerged as a promising alternative for high efficiency hetero junction (HJ) solar cells. In this work we study plasma processes for texturing and cleaning c‐Si wafers pursuing a low cost dry fabrication process of HJ solar cells. We have studied two independent plasma processes: i) Texturing of c‐Si wafers using SF6 ‐ O2 plasmas in a RIE system, in order to reduce the surface reflectance and therefore improve the light trapping. The effects of the RF power and gas ratio on the c‐Si surface texture have been studied in detail. Highly textured surfaces, with very low reflectance values (around 6% in the range of 300 – 1000 nm) have been achieved. ii) Etching of the native oxide and passivation of the c‐Si surface by plasma, in a standard RF PECVD system. We used SiF4 plasma with optimized conditions for an efficient native oxide removal, and without breaking the vacuum, 40 nm of a‐Si:H were deposited in order to passivate the c‐Si surface. High effective lifetime values were obtained (τeff ≈ 1.5 ms), providing high implicit open circuit voltages (Voc ≈ 0.713 V) and low surface recombination velocities (Seff < 9 cm s‐1). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin crystalline silicon solar cells based on epitaxial films grown at 165°C by RF-PECVD

“…Pursuing this approach, we have studied the growth of thick epitaxial layers and have found that the epitaxial growth can be sustained up to a thickness of 3 µm at such low-temperature conditions. Moreover, by using an SiF 4 plasma etching step on the wafer substrate before deposition, we have found that it is possible to produce a crystalline silicon layer having a porous interface with the crystalline silicon wafer, which allows to easily detach the epitaxial film from the substrate [8,9]. In this way it is possible to produce thin and flexible crystalline silicon wafers that could be used for electronic devices, in a similar way to the "smart-cut" process [10].…”

Feasibility of using thin crystalline silicon films epitaxially grown at 165 °C in solar cells: A computer simulation study

“…This knowledge is of utmost importance, since our final aim is to achieve a lift-off from the c-Si wafer allowing for the re-use of the latter. As far as the epi-Si film itself is concerned, it has already been demonstrated [6,7] that by using a SiF 4 plasma etching step on the wafer substrate before the deposition of the epi-Si film, it is possible to produce a crystalline silicon layer having a porous interface with the c-Si wafer, which allows the epi-Si film to be easily detached from the substrate. It is naturally desirable that after such detachment, the photo-current of the device remains more or less the same.…”