Ultra-thin stack of n-type hydrogenated microcrystalline silicon and silicon oxide front contact layer for rear-emitter silicon heterojunction solar cells

Abstract

We demonstrate the clear advantage of a n-type hydrogenated microcrystalline silicon (n-µc-Si:H) seed layer on the optoelectronic properties and crystallisation behaviour of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiO_x:H) front contact layers. The presence of a non-oxidic n-µc-Si:H seed layer can reduce the thickness and refractive index of the n-µc-SiO_x:H front layer significantly while maintaining a high degree of crystallisation and excellent conductivity. This leads to increase in short-circuit current density (J_sc) by 2.64% and open-circuit voltage (V_oc) by 0.56% in comparison to that of a device without the seed layer. The enhancement in J_sc can be attributed to the reduction in parasitic absorption loss in the extremely thin front layer. In addition, the improvement in V_oc can result from enhanced surface passivation of the wafer due to seed layer growth in very high hydrogen plasma environment which can play a role as the hydrogen post-plasma treatment. The low thickness of the n-µc-SiO_x:H front layer yields lower internal recombination losses. In conjunction with an optimised n-µc-Si:H seed layer and n-µc-SiO_x:H front layer, we obtained a high conversion efficiency value of 21.8% with V_oc of 727 mV, J_sc of 39 mA/cm², and FF of 77% among the fabricated cells in laboratory.

Introduction

Silicon heterojunction (SHJ) solar cells owe their progress in performance to excellent passivation effects on both sides of the crystalline silicon wafer surface. SHJ solar cells have been in the research forefront for more than twenty years due to the following advantages: a) simple and cost-effective production process without the need for patterning steps, b) high open-circuit voltage (V_oc) in comparison with the traditional homojunction crystalline silicon cells, and c) good performance stability [1], [2]. High efficiencies far over 24% have been reported for the SHJ cell until now [3], [4], [5]. Numerous efforts have been made to improve the SHJ solar cells further by focusing on the following aspects: 1) reducing carrier recombination loss by high-quality surface passivation layer [6], [7]; 2) reducing parasitic absorption loss at the front side by optimising the front grid silver electrode with higher aspect ratio and conductivity and/or developing ultra-thin wide-gap front contact layers; and, 3) reducing the material cost and carrier collection loss by using thinner wafers [8], [9]. Among these, losses due to optical parasitic absorption at the front contact layer pose an enormous challenge which limits the performance of SHJ solar cells [10], [11].

Generally, in the front junction configuration, p-type doped layers are situated on the front side to collect those minority charge carriers that are generated close to their collecting contact in an efficient manner. This is really necessary in the case of short-lifetime absorber materials with small carrier diffusion lengths of minority charge carriers [1], [12]. However, with good surface passivation of hydrogenated amorphous silicon (a-Si:H) materials as also high-quality absorber wafers available today, the use of a p-type front contact layer in SHJ solar cells is less demanding as the minority charge carriers can diffuse effectively to their collecting electrode regardless of the location of the electrode [13]. In addition, a Schottky barrier is formed between the p-type front contact and transparent conducting oxide (TCO) layers due to the inherent weak conductivity of the front surface field layer [14], [15]. A thick and/or highly doped p-layer, formed at the expense of optical transparency, is normally able to eliminate this barrier and thus forming a suitable front surface field for hole collection. However, this results in parasitic absorption and carrier recombination at the junction which adversely affects the solar cell parameters, V_oc, fill factor (FF), and short-circuit current density (J_sc). It appears that the contact formation for hole collection at the front side is a delicate task [16]. Efforts made in this direction saw the evolution of a new SHJ device architecture, in which the p-contact layer is located on the rear side, popularly known as the rear-emitter silicon heterojunction (RE-SHJ) solar cell [17], [18].

In RE-SHJ configuration, a thicker and/or highly conducting p-contact layer can be used at rear side regardless of any major transparency concerns. The use of the RE-SHJ design relaxes the strict requirement of the front TCO material and, at the same time, avails more optical transmission into the bulk absorber (wafer) through a very thin, n-type front layer without seriously impairing conductivity [13], [19]. Standard RE-SHJ solar cells use n- and p-doped hydrogenated amorphous silicon (n-a-Si:H and p-a-Si:H) as carrier-selective front and back contact layers, respectively [20]. The efficient internal electric field which helps to separate and collect carrier charges depends strongly on the net doping of these doped layers. However, a highly uniform degree of doping is hardly ever achieved with a-Si:H material because large dopant quantities cause accumulation of defect density. This has a detrimental effect on uniform doping and even likely to deteriorate the passivation quality at wafer interfaces [21]. Additionally, the low bandgap of the a-Si:H material hinders the transparency of the solar spectrum in the short wavelength region. A material with wide bandgap and high conductivity appears most suitable to alleviate this problem.

Hydrogenated microcrystalline silicon (µc-Si:H) is found to be highly attractive as a front contact layer in thin film silicon solar cells [22], [23] as well as SHJ solar cells [24], [25]; this is owing to its high doping efficiency thanks to a crystalline phase and low absorption coefficient in the short wavelength region in comparison to the a-Si:H counterpart. By incorporating oxygen, a bi-phase microcrystalline silicon oxide (µc-SiO_x:H) material with columnar crystalline silicon phase mixed in an amorphous silicon oxide matrix phase and considerably improved transparency can be obtained without significantly affecting vertical conductivity [13], [24], [26]. In such a bi-phase material, high vertical conductivity is achieved throughout crystalline phase while the amorphous oxide phase is responsible for enhanced transparency. A challenge in realising µc-SiO_x:H as the front contact layer in SHJ solar cells is the formation of ultra-thin layers with high-crystalline quality and efficient doping without impairing the a-Si:H passivation layers below. It requires delicate control of the initial nucleation processes to limit the incubation phase during crystal growth of n-µc-SiO_x:H layer.

In this work, we investigate a stack of front contact layers for RE-SHJ solar cells including a very thin n-µc-Si:H layer grown under high hydrogen dilution and at low plasma power density in a favourable nucleation environment for subsequent growth of n-µc-SiO_x:H layers. This study helped us to evaluate the optimum thickness of n-µc-SiO_x:H layers that are endowed with high crystalline fraction and electrical conductivity. The analysis of solar cell parameters, namely short-circuit current density (J_sc) in the short wavelength region, open-circuit voltage (V_oc), and fill factor (FF) based on a stack of seed-crystalline layer and ultra-thin n-µc-SiO_x:H front layer is presented.