掺杂硅薄膜层的显微拉曼光谱分析及其异质结硅晶片太阳能电池的可行性

Hydrogenated doped silicon thin films deposited using RF (13.56 MHz) PECVD were studied in detail using micro  Raman spectroscopy to investigate the impact of doping gas flow, film thickness, and substrate type on the film characteristics. In particular, by deconvoluting the micro Raman spectra into amorphous and crystalline components, qualitative and quantitative information such as bond angle disorder, bond length, film stress, and film crystallinity can be determined. By selecting the optimum doped silicon thin film deposition conditions, and combining our p-doped and  n-doped silicon thin films in different heterojunction structures, we demonstrate both (i) an efficient field effect passivation and (ii) further improvement to c-Si/a-Si:H(i) interface defect density with observed improvement in implied  open-circuit voltage VOC and minority carrier lifetimes across all injections levels of interest. In particular, the heterojunction structure (a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(p)) demonstrates a minority carrier lifetime of 2.4 ms at  an injection level of 1015 cm⁻³ , and a high implied open-circuit voltage of 725 mV. Simulation studies reveal a strong  dependence of the interface defect density Dit on the heterojunction silicon wafer solar cell performance, affected by the  deposition conditions of the overlying doped silicon thin film layers. Using our films, and a fitted Dit of 5 × 1010 cm⁻² ·eV⁻¹ , we demonstrate that a solar cell efficiency of ~22.5% can be potentially achievable.

Heterojunction with intrinsic thin layer (HIT) solar cells  has proven to be a suitable candidate for cost reduction in  industrial high efficiency crystalline silicon (c-Si) wafer  solar cells, due to the significantly lower thermal budget  requirements, allowing the usage of much thinner  wafers (<100 µm). In fact, Panasonic has recently attained record conversion efficiencies of 24.7%  for a  wafer size of 101.8 cm²  and 98 µm thick. To achieve  higher conversion efficiency of heterojunction silicon  wafer solar cells, the optimisation of the heavily doped  thin-film emitter and back surface field (BSF) layers are  important.

Moreover, it was found through a series of H₂ effusion  experiments , that the addition of doped silicon thin  films may lower the passivation quality due to Fermi energy dependent Si-H bond rupture in the a-Si:H films,  for either type of doping. This can result in silicon dangling bonds creation, counteracting intentional doping of  the a-Si:H matrix and reduce the field effect passivation.  Increasing doping concentration is also associated with  increased defect densities, which results in Fermi level  pinning, and linked to enhanced recombination at the  a-Si:H/c-Si interface . Hence, the optimisation of the  doped silicon thin film layers is essential to its application in heterojunction silicon wafer solar cells. Despite  the challenges, the successful application of doped microcrystalline silicon thin film to heterojunction silicon  wafer solar cells has been demonstrated. Various  characterisation techniques are available for silicon thin  film layers. In this paper, we highlight how the usage of  μ-Raman spectroscopy has been instrumental in our optimisation process to achieve device quality doped silicon  thin films, while not degrading the underlying intrinsic buffer layer and its c-Si/a-Si:H(i) interface quality.

Firstly, to investigate the impact of doping concentration on the film, the doping gas flow (B₂H₆ and PH₃) has  been varied from 1 sccm to 4 sccm, while maintaining  the film thickness at ~40 nm as seen in Tables 1 and 2 below. Secondly, we investigate the impact of increasing  thickness for one of the chosen deposition conditions on  the μ-Raman results. Thirdly, given that the heterojunction silicon wafer solar cell typically consists of a symmetrical thin intrinsic a-Si:H layer for passivation  before the deposition of the thin doped layers, we also  investigate the impact of substrate type on doped silicon  thin film growth (i.e.: intrinsic a-Si:H and glass substrates). The doped silicon thin film on an a-Si:H substrate is prepared by depositing a thin intrinsic a-Si:H  layer of ~5 nm on a microscopic glass substrate before  depositing the doped layers of ~10 nm with different hydrogen dilution ratios R = H₂/SiH₄. In all depositions,  the RF power density, deposition pressure p, and substrate temperature T were maintained at 0.07 W/cm2 , 1.9  Torr, and 180˚C respectively.  

Fig1

Our extracted film crystallinity and conductivity as a  function of doping flow provides an alternative support  to the above claims, and highlights the importance of  doping flow optimisation amid other deposition parameters. The optimum doping gas flow for the p-doped silicon thin film and n-doped silicon thin film was found to  be B₂H₆ = 2.2 sccm and PH₃ = 2 sccm respectively. At  such doping flow, a sufficiently high conductivity for the  p-doped and n-doped thin film is achievable at 1.8 S/cm  and 16 S/cm respectively. Increasing doping flow beyond  the optimum point will only increase non-electrically  active dopant atoms, which can act as recombination  centers for minority carriers in a solar cell device, hence  reducing the current collection efficiency.

Secondly, with the chosen deposition conditions as per  sample P6 with a B₂H₆ doping gas flow of 2.0 sccm,  several p-doped silicon thin film samples were deposited  on microscopic glass substrates with varying thickness  from 20 nm up till 100 nm in 20 nm steps. Figure 7 shows the µ-Raman spectra of these samples, while Figure 8 shows the corresponding plot of film crystallinity  and conductivity. It is clear from Figures 7 and 8 that as  the film thickness increase from 20 nm to 100 nm, the  film evolves from a purely amorphous phase into a microcrystalline phase in which film crystallinity χc increases from 1% to 42%. This change correlates well  with an increase in conductivity from 0.06 to 4.8 S/cm.