Performance of heterojunction p+ microcrystalline silicon n crystalline silicon solar cells

Abstract

We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition ͑PECVD͒ on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20-30 nm p ϩ c-Si:H layers with a considerably large crystalline volume fraction (ϳ22%) and good window properties were deposited on crystalline silicon under moderate PECVD conditions. The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer layer used. A large improvement of open circuit voltage is observed in these solar cells when a thin 2-3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature (ϳ100°C) is inserted between the microcrystalline and crystalline silicon ͓complete solar cell configuration: Al/(n)c-Si/buffer/p ϩ c-Si:H/ITO/Ag͔͒. Detailed modeling studies showed that the wide band-gap a-Si:H buffer layer is able to prevent electron backdiffusion into the p ϩ c-Si:H layer due to the discontinuity in the conduction band at the amorphous-crystalline silicon interface, thereby reducing the high recombination losses in the microcrystalline layer. At the same time, the discontinuity in the valence band is not limiting the hole exit to the front contact and does not deteriorate the solar cell performance. The defect density inside the crystalline silicon close to the amorphous-crystalline interface has a strong effect on the operation of the cell. An extra atomic hydrogen passivation treatment prior to buffer layer deposition, in order to reduce the number of these defects, did further enhance the values of V oc and fill factor, resulting in an efficiency of 12.2% for a cell without a back surface field and texturization.

High-efficiency Silicon Heterojunction Solar Cells: A Review

“…Tested alternatives to replace the a-Si:H stacks are (microcrystalline) silicon oxides [136,137,182,183] and carbides [148,[184][185][186]. Microcrystalline silicon has a lower but indirect bandgap and features a higher doping efficiency, making it an attractive material for emitter [52,[187][188][189][190] and BSF formation [189,[191][192][193] as well. Of note is that such films may also resolve possible contact problems between TCO-layers and doped films [192,194].…”

Understanding of Passivation Mechanism in Heterojunction c-Si Solar Cells

“…This effect has been attributed to better properties of an a-Si:H(i)/c-Si interface, compared with an a-Si:H(doped)/c-Si interface [2,11,12].…”

“…So far, a number of studies have reported the improvement of a-Si:H/c-Si cell performance by the incorporation of a-Si:H i-layer at the a-Si:H/c-Si heterointerface [2,11,12]. This effect has been attributed to better properties of an a-Si:H(i)/c-Si interface, compared with an a-Si:H(doped)/c-Si interface [2,11,12].…”

17% efficiency heterostructure solar cell based on p-type crystalline silicon

“…Particular treatments of the crystalline surface have been investigated, such as wet cleaning by HF solutions [4] passivation by hydrogen [1,2], dry cleaning by plasmas of CF 4 [5] and CF 4 +O 2 [6], and the use of a microcrystalline (lc) [3], or a-Si:H [1] thin layer as a buffer layers to improve the interface with the c-Si substrate.…”