Strategies To Control Solar Cell Performance

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As the basic energy source in renewable energy, solar energy has the most abundant reserves. The annual radiant energy of solar radiation on the ground is as high as 1.05*1018 kWh. Since the birth of photovoltaic cells in 1954, photovoltaic power generation technology has developed rapidly and plays an important role in energy development.

The hydrogenated-amorphous-silicon (a-Si:H) thin film is regarded as an important material in photovoltaic applications. When used as the window layer of an a-Si:H solar cell, a p-type layer with optimized design can reduce parasitic absorption at short wavelengths while improving the built-in potential (Vbi) of the solar cell; therefore, a p-type a-Si:H layer also has a considerable impact on the short-circuit current density (Jsc) and open-circuit voltage (Voc).

Holes can easily be hindered in the transport process for photogenerated carriers because the migration of holes is much lower than that of electrons. Using a p-type layer with high conductivity and wide optical bandgap is the key to optimizing the characteristics of the window layer. Furthermore, fluorine-doped tin oxide (FTO, SnO2: F) films, a transparent conductive oxide (TCO) material, is most commonly used as the transparent electrode in thin-film solar modules based on hydrogenated amorphous silicon (a-Si:H) as the light-absorbing material.

However, because the Sn4+ species in FTO can be destabilized and reduced to its metallic state (Sn0) upon exposure to a high-flux hydrogen plasma, p-nc-SiOx:H cannot be used in a-Si:H solar cells with the p-i-n configuration and FTO as the TCO. This destabilization and reduction of Sn4+ in FTO occur when the p-nc-SiOx:H film is deposited by a high-flux hydrogen plasma via radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD).

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In” Controlling performance of a-Si:H solar cell with SnO2:F front electrode by introducing dual p-layers with p-a-SiOx:H/p-nc-SiOx:H nanostructure,” a paper recently published in Solar Energy, Ren et al. proposed and applied consisting of the p-a-SiOx:H/p-nc-SiOx:H nanostructure with a FTO front electrode to improve the optical and electrical properties of the p side of an a-Si:H solar cell, without affecting the destabilization of Sn4+ in the FTO layer. Instead of using a single p-layer consisting of p-a-Si:H, p-a-SiOx:H, p-nc-Si:H, or p-nc-SiOx:H, the p-a-SiOx:H/p-nc-SiOx:H part of the structure formed the dual p-layers that served as the window layer of the cell. The silicon thin-film layers and a-Si:H solar cells were fabricated at a temperature of 210 °C by a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) in a four-chamber cluster system.

The structure and primary components of an a-Si:H solar cell are shown in Fig. 1(a) and (b). Fig. 1(c) shows a schematic illustration of a-Si:H solar-cell structures with different window layers. Type A and type B represent a-Si:H solar cells with a single p-layer (p-a-SiOx:H) and dual p-layers (p-a-SiOx:H/p-nc-SiOx:H), respectively. Source: https://doi.org/10.1016/j.solener.2018.07.035

The authors found that the wide optical bandgap of the p-nc-SiOx:H layer with low parasitic absorption could also block the electrons on the conduction-band edge at the p/i interface, thus preventing recombination with holes from the valence-band edge. These three factors could promote hole transport at the p/i interface, thus enhancing the blue-wavelength response of the a-Si:H solar cell. A suitable thick p-nc-SiOx:H layer with high dark conductivity and wide optical bandgap was required to enhance Voc and Jsc.

In order to identify the suitable thickness of p-nc-SiOx:H, the authors utilized a comparison analysis with p-nc-SiOx:H films of different thickness in the dual p-layers with p-a-SiOx:H/p-nc-SiOx:H nanostructure. The total thickness of the p-type window layer was kept at 14 nm, and the a-Si:H solar-cell performance in this work was optimized by controlling the thickness of the individual p-a-SiOx:H and p-nc-SiOx:H layers. The authors found that when the p-nc-SiOx:H layer was too thin (<7 nm), the initial growth process formed an amorphous incubation layer with high defect density. These defects deteriorated the p-i-n junction to increase Jdark of the a-Si:H solar cell with low FF. When the p-nc-SiOx:H layer was too thick (>7 nm), however, the p-a-SiOx:H layer could not prevent the deterioration of the FTO front electrode by the diffused hydrogen atoms, and the a-Si:H solar cell exhibited poor performance.

Finally, the variations in Eff showed a trend similar to that of the variations in FF, and the highest average Eff of 9.57% was obtained at dnc = 7 nm, representing an enhancement of 12.90% from the average Eff of the solar cell with a single p-layer.

These findings are described in the article entitled Controlling performance of a-Si:H solar cell with SnO2:F front electrode by introducing dual p-layers with p-a-SiOx:H/p-nc-SiOx:H nanostructure, recently published in the journal Solar Energy. This work was conducted by Ningyu Ren, Jun Zhu, Pengfei Shi, Qi Shan, Tiantian Li* from the Key Laboratory of Semiconductor Photovoltaic Technology at Universities of Inner Mongolia Autonomous Region, ChangChun Wei, Ying Zhao and Xiaodan Zhang* from the Nankai University.

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Cite this article as:
Tiantian Li. Strategies To Control Solar Cell Performance, Science Trends, 2018. Available at:
http://doi.org/10.31988/SciTrends.25228
*Note, DOIs are registered Friday weekly and therefore may not work until then.

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