Radial n-i-p structure SiNW-based microcrystalline silicon thin-film solar cells on flexible stainless steel
© Xie et al.; licensee Springer. 2012
Received: 3 October 2012
Accepted: 30 October 2012
Published: 12 November 2012
Radial n-i-p structure silicon nanowire (SiNW)-based microcrystalline silicon thin-film solar cells on stainless steel foil was fabricated by plasma-enhanced chemical vapor deposition. The SiNW solar cell displays very low optical reflectance (approximately 15% on average) over a broad range of wavelengths (400 to 1,100 nm). The initial SiNW-based microcrystalline (μc-Si:H) thin-film solar cell has an open-circuit voltage of 0.37 V, short-circuit current density of 13.36 mA/cm2, fill factor of 0.3, and conversion efficiency of 1.48%. After acid treatment, the performance of the modified SiNW-based μc-Si:H thin-film solar cell has been improved remarkably with an open-circuit voltage of 0.48 V, short-circuit current density of 13.42 mA/cm2, fill factor of 0.35, and conversion efficiency of 2.25%. The external quantum efficiency measurements show that the external quantum efficiency response of SiNW solar cells is improved greatly in the wavelength range of 630 to 900 nm compared to the corresponding planar film solar cells.
KeywordsSilicon nanowires Microcrystalline Solar cells
Solar power as the richest clean energy is the most favorable substitution for biochemical energy resource, which would be exhausted in decades. Finding an effective and low-cost approach to harness solar energy is a key step to resolve the energy crisis . Silicon thin-film solar cell technologies are industrially proven, environmentally friendly, and without fundamental limitation in material supply. However, the conflict between light absorption and photogenerated charge extraction makes planar silicon thin-film solar cells with comparatively low efficiencies . Building radial junction thin-film solar cells on top of silicon nanowires (SiNWs) would enable a decoupling of the requirements for light absorption and carrier extraction into orthogonal spatial directions [3, 4]. Also, the natural-light-trapping structure of SiNWs allows enhanced optical anti-reflection and absorption in a wide spectrum range [5–11]. Thus, the SiNW-based thin-film solar cells would be a potential candidate for low-cost and high-efficiency solar cells. The numerical simulation by Pei et al. shows an 11.6% conversion efficiency of SiNW-based thin-film solar cells . The group of Yu has achieved some excellent experiment results in SiNW-based thin-film solar cells [13–15]. However, they almost focused on SiNW-based amorphous (a-Si:H) thin-film solar cells on glass substrates. In this work, we present SiNW-based microcrystalline (μc-Si:H) thin-film solar cells on flexible stainless steel substrates. The microcrystalline silicon film is an ideal light-absorbing material for solar cells due to its better stability under light soaking and stronger long-wavelength absorption compared to a-Si:H film . Also, the μc-Si:H with a lower bandgap is more suitable for bandgap match between n-type crystalline SiNWs and i-type absorption layer . In addition, flexible substrates make cells with a wider range of application.
Synthesis conditions for the n-type silicon nanowires
SiH4 + PH3 flow rate
6 sccm + 6 sccm
H2 flow rate
Radio frequency (13.56 MHz) power density
Acid treatment on n-type SiNWs
The as-synthesized n-type SiNW sample was taken out the PECVD chamber and dipped into 1% volume fraction hydrochloric acid for 10 min at room temperature. After the acid treatment, the sample was rinsed in deionized water for 10 min. Then, the air-dried sample was loaded into the PECVD chamber again for intrinsic and p-type layer coating.
SiNW-based μc-Si thin-film solar cell formation
Results and discussion
Morphology of n-type SiNWs and SiNW-based μc-Si:H thin-film solar cells
Representation of acid treatment
The crystalline volume fraction of i-type μc-Si:H layer
Optical and electrical characterization
Parameters of the cells for light-soaking I - V characters
SiNWs cell (without acid treatment)
SiNWs cell (acid treatment)
Planar film cell
Figure 6b shows the external quantum efficiency (EQE) of the SiNW-based μc-Si:H solar cells (red dot line for cells with acid treatment and green triangle line for cells without acid treatment) and planar μc-Si:H film solar cell (black square line). From the picture, the EQE of the SiNW-based cells has a definite improvement in the wavelength range of 630 to 900 nm that is due to the light-trapping effect of SiNWs. However, in the short wavelengths (from 400 to 630 nm), the SiNW-based cells show lower EQE than the corresponding planar film cell, which is inconsistent with the results of the optical reflectance spectrum shown in Figure 5. It may be explained as follows: first, the anti-reflection effect of SiNW-based cells in short wavelengths is not as good as in long wavelengths (refer to Figure 5); second, the interface of the n-type SiNWs and i-type μc-Si:H layer influences the collection of photo-induced electrons, especially the photo-induced electrons in the short-wavelength range.
We have produced a radial n-i-p structure SiNW-based μc-Si:H thin-film solar cell on stainless steel foil by plasma-enhanced chemical vapor deposition. The SiNW solar cell displays a very low optical reflectance over a broad range of wavelengths (400 to 1,100 nm) due to its natural anti-reflective structure. The modified open-circuit voltage, short-circuit current density, and conversion efficiency under AM 1.5 illumination were 0.48 V, 13.42 mA/cm2, and 2.25%, respectively. The EQE measurements show that the EQE response of SiNW solar cells is improved greatly in the wavelength range of 630 to 900 nm compared to the corresponding planar film solar cells. Further, we will focus on improving the SiNW solar cells by minimizing shunts, reducing contact resistance, and improving the open-circuit voltage.
external quantum efficiency
indium tin oxide
- J sc :
short-circuit current density
plasma-enhanced chemical vapor deposition
scanning electron microscope
- V oc :
H: Hydrogenated microcrystalline silicon.
This work was financially supported by the National High Technology Research and Development Program (863 Program) of China (no. 2011AA050504), the Knowledge Innovation Program of the Chinese Academy of Sciences (no. 1KGCX2-YW-383-1) and the Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (no. 12JG01).
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