Stretched-exponential a-Si:H∕c-Si interface recombination decay

Abstract

The electronic properties of hydrogenated amorphous silicon ͑a-Si: H͒ relax following stretched exponentials. This phenomenon was explained in the past by dispersive hydrogen diffusion, or by retrapping included hydrogen motion. In this letter, the authors report that the electronic passivation properties of intrinsic a-Si: H/crystalline silicon ͑c-Si͒ interfaces relax following a similar law. Carrier injection dependent a-Si: H / c-Si interface recombination calculations suggest this originates from amphoteric interface state ͑or Si dangling bond͒ reduction, rather than from a field effect. These findings underline the similarity between a-Si: 3 Alternatively, c-Si surface dangling bond passivation may also be obtained by deposition of intrinsic hydrogenated amorphous silicon ͑a-Si: H͒ films. 4 As such films can, in addition, be doped relatively easily, they allow for the fabrication of electronically abrupt a-Si: H / c-Si heterojunctions.5 Since direct deposition of such doped films on c-Si can result in poor interface properties, typically, a few nanometer thin intrinisic a-Si: H͑i͒ buffer layer is inserted in between for a-Si: H / c-Si heterostructure solar cell fabrication.6 Impressive large area ͑Ͼ100 cm 2 ͒ energy conversion efficiencies ͑Ͼ22% ͒ have been reported for such devices.7 For this, atomically sharp a-Si: H / c-Si interfaces, i.e., where no epitaxial Si ͑epi-Si͒ film was grown on the wafer during film deposition, are considered to be essential. 8 This can be monitored in real time by various optical probes.9,10 On a related note, the excellent a-Si: H / c-Si interface passivation properties may also offer an explanation why surrounding a-Si: H tissue is crucial for device-grade microcrystalline Si.11 Nevertheless, the relation between processing parameters and the physical mechanism underlying the a-Si: H / c-Si interface passivation is not yet fully understood.Previously it was suggested that low-temperature annealing is beneficial for the interface passivation quality, provided that neither epi-Si at the interface 12 nor boron-doped overlayer 13,14 is present. In this letter, we show that under these conditions electronic passivation relaxation can be accurately described by stretched exponentials. It is observed that even moderate temperature ͑ഛ 180°C͒ annealing can yield an extremely low interface recombination activity. Carrier injection dependent recombination calculations suggest that the origin of this phenomenon is related to dangling bond reduction at the interface, rather than to a field effect.Consequently, similar to the SiO 2 / c-Si interface, 15 also here, a strong analogy between the nature of the defects present at the a-Si: H͑i͒ / c-Si interface and those in the a-Si: H͑i͒ bulk may possibly exist.For the experiments, 300 m thick ϳ3.0 ⍀ cm phosphorus-doped high quality float zone ͑100͒ FZ-Si wafers were used. Both substrate surfaces were mirror polished to eliminate the influence of substrate surface roughness on the passivation properties. 16 Surface cleaning simply consisted o...

Light-induced performance increase of silicon heterojunction solar cells

“…2 In recent years, a-Si:H layers also garnered significant attention, thanks to their excellent crystalline silicon (c-Si) surface passivation properties, even when only a few nm thin. [3][4][5][6][7][8] This property is exploited with remarkable success for passivating-contact fabrication in silicon heterojunction (SHJ) solar cells, [9][10][11][12][13][14][15][16][17][18][19][20][21][22] with reported conversion cell efficiencies as high as 26.3%. 23 For any solar cell technology, an important criterion for ultimate device performance is its stability under prolonged light exposure.…”

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

“…25 The a-Si:H/c-Si interface itself is also strongly affected by the hydrogen content and structure of the a-Si:H film. 10,26 Since hydrogen in the plasma plays such a critical role, it is clear that D can be a physically relevant parameter for assessing the quality of the passivating layers. A detailed description of the link between depletion and passivation through hydrogen chemical reactions is, however, beyond the scope of this paper.…”

“…It has been shown that a-Si:H can provide excellent passivation of c-Si surfaces. [7][8][9][10] However, to produce highefficiency devices, it is crucial to control the properties of the a-Si:H layers during deposition. This task is usually difficult for heterojunction solar cells, because the layers thicknesses have to be kept very low, in the order of 10-15 nm.…”

Towards a Better Understanding of Silicon Heterojunction Solar Cells

“…time dependent) hydrogen diffusion [3], or from retrapping included hydrogen motion [4]. For the a-Si:H(i) / c-Si interface, based on the latter interpretation, it has been argued that the annealing induced passivation may originate from a transfer of hydrogen from a higher hydride state in the a-Si:H film (close to the interface) to a monohydride cSi surface state [5]. Consequently, the a-Si:H(i)/c-Si interface passivation is likely due to chemical surface state passivation, rather than a field effect.…”