Amorphous silicon nanocone array solar cell
© Thiyagu et al; licensee Springer. 2012
Received: 28 November 2011
Accepted: 6 March 2012
Published: 6 March 2012
In the hydrogenated amorphous silicon [a-Si:H]-thin film solar cell, large amounts of traps reduce the carrier's lifetime that limit the photovoltaic performance, especially the power conversion efficiency. The nanowire structure is proposed to solve the low efficiency problem. In this work, we propose an amorphous silicon [a-Si]-solar cell with a nanocone array structure were implemented by reactive-ion etching through a polystyrene nanosphere template. The amorphous-Si nanocone exhibits absorption coefficient around 5 × 105/cm which is similar to the planar a-Si:H layer in our study. The nanostructure could provide the efficient carrier collection. Owing to the better carrier collection efficiency, efficiency of a-Si solar cell was increased from 1.43% to 1.77% by adding the nanocone structure which has 24% enhancement. Further passivation of the a-Si:H surface by hydrogen plasma treatment and an additional 10-nm intrinsic-a-Si:H layer, the efficiency could further increase to 2.2%, which is 54% enhanced as compared to the planar solar cell. The input-photon-to-current conversion efficiency spectrum indicates the efficient carrier collection from 300 to 800 nm of incident light.
KeywordsNanocone amorphous silicon solar cell
The solar energy is a renewable energy and is expected to alleviate the progress of global warming. However, the cost to produce electricity by solar energy harvesting is still higher than the traditional method, such as thermal power generation by burning coal and petroleum or the hydro-electrical power generation. Thin-film silicon solar cell is one of the candidates to achieve low cost requirement. However, to achieve low cost, the low-temperature process makes the thin-film silicon generally be in the form of microcrystalline or becomes amorphous. In this structure, large amounts of traps reduce the carrier's lifetime that limit the photovoltaic performance, especially the power conversion efficiency. The nanowire structure is proposed to solve the low efficiency problem . The light harvesting is along the wire and the carrier collection is along the radial direction. The path for photo-carrier collection and light harvesting is perpendicular. Longer wire could ensure that the solar light be harvested thoroughly while maintaining the efficient carrier collection. Among all materials, the hydrogenated amorphous silicon [a-Si:H] nanowire solar cell has been particularly investigated and predicted to have better photovoltaic performance over planar solar cell . Additional to the efficient carrier collection, the nanostructure surface was expected to have light-trapping behavior that could further increase the total amount of solar energy harvesting in a short nanowire [3–5]. These advantages cause the nanowire solar cell to largely improve the efficiency compared to planar solar cell. However, the randomly grown Si nanowires by bottom-up method are hardly to manufacture a good solar cell device that has higher photovoltaic performance on Si nanowire than planar one is rarely reported [6–8]. Moreover, the combination of the efficient carrier collection and light trapping limits the understanding of the electrical advantages of nanowire solar cell itself. In this work, we prepare the a-Si:H nanostructure by reactive-ion etching [9–12] through a closely packed nanosphere template . In previous study, the low aspect ratio nanocone a-Si:H has negligible light-trapping effect . The light illuminated through glass side instead of surface of the a-Si:H nanocone to make the effect of light-trapping negligible To explore the advantage of the nanostructure, four types of a-Si:H were used to fabricate a-Si:H solar cell. The first one is the planar solar cell without nanostructure used as a reference. The second one is an intrinsic-a-Si:H nanocone solar cell. The third is an intrinsic-a-Si:H nanocone solar cell with H2 plasma treatment for 10 min prior to N+-a-Si:H deposition. The last one is also an intrinsic-a-Si:H nanocone solar cell with additional 10 nm a-Si:H layer after H2 plasma treatment.
Results and discussion
, in which T is the absolute transmission, α is the absorption coefficient and d is the thickness of the intrinsic-a-Si:H layer. Figure 3d depicts the absorption coefficient of planar and nanocone a-Si:H layer. The absorption coefficient for the a-Si:H nanocone is approximately 5 × 105/cm at 500 nm which is slightly higher than the planar structure. The effect of the difference in total amount of light harvesting between planar and nanocone solar cell while exploring the carrier collection efficiency can be minimized by light illumination through the glass side.
The detailed photovoltaic properties of a-Si:H nanocone solar cell
Jsc (mA/cm2)/Rsh (Ω·cm2)
Voc (V)/Rs (Ω·cm2)
(H2 Plasma+10 nm i)
In this work, we propose an amorphous silicon [a-Si] solar cell with a nanocone-array structure which were implemented by PS nanospheres as template for RIE dry etching. The amorphous Si nanocone exhibits 5 × 105/cm absorption coefficient which is similar to the planar a-Si:H layer in our study. The proposed nanocone solar cell could have better carrier collection efficiency and implies an efficiency of 1.77% for a-Si nanocone solar cell which has 24% enhancement over planar solar cell (1.43%). With hydrogen plasma treatment and additional 10-nm a-SI:H layer, the efficiency further increased to 2.2%, which is 54% enhanced as compared to the planar solar cell. This indicates that the a-Si nanostructure could efficiently enhance the photocurrent of the thin-film solar cell.
input photon-to-electron conversion efficiency
short-circuit current density
power conversion efficiency
plasma enhanced chemical vapor deposition
reactive ion etching
scanning electron microscope.
We thank the National Science Council for the financial support under the grant NSC-99-ET-E-005-001-ET.
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