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Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiNx films as emitters.

Wu PJ, Wang YC, Chen IC - Nanoscale Res Lett (2013)

Bottom Line: Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device.These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase.A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Materials Science and Engineering, National Central University, Jhongli 320, Taiwan. ichen@ncu.edu.tw.

ABSTRACT
Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiNx (Si-NCs/SiNx) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiNx films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.

No MeSH data available.


Dark current density-voltage characteristics of Si-NCs/sc-Si heterojunction solar cells. The inset shows the saturation current density J0 and ideality factor n as a function of the Rc value.
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Figure 8: Dark current density-voltage characteristics of Si-NCs/sc-Si heterojunction solar cells. The inset shows the saturation current density J0 and ideality factor n as a function of the Rc value.

Mentions: The P-doped Si-NCs/SiNx layers with various Rc values were fabricated on top of p-type sc-Si substrates for fabrication of Si heterojunction solar cells, as shown in the inset of Figure 5a. This study concentrates on basic Si-NCs/sc-Si heterojunction solar cells without the designs or processes to enhance the conversion efficiency, such as surface texturing, anti-reflection coating and back-surface field. The illuminated J-V curves corresponding to each sample are displayed in Figure 5a, and their open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and efficiency are shown in Figure 6 as a function of the N2/SiH4 flow ratio. The magnitude of Voc is generally correlated to the built-in potential (Vbi) of the junction, which could be influenced by the energy bandgap of the Si-NCs for the Si heterojunction solar cells. As shown in Figure 7, the Vbi of the P-doped Si-NCs/sc-Si heterojunction extracted from the capacitance-voltage characteristic increases from 0.77 to 1.95 V with increasing Rc value. This trend may be ascribed to the bandgap expansion of Si-NCs with the shrinkage of the Si-NC size, leading to an increase in Vbi at the junction, and thus, the Si heterojunction solar cell is expected to show a higher Voc as Rc increases. However, in this study, the Voc value is in the range of 0.49 to 0.5 for all Si heterojunction solar cells (shown in Figure 6), implying that Voc is quite insensitive to the Si-NC size. Figure 8 shows dark J-V curves for the solar cells with different Rc values. Both the saturation current density (J0) and the ideality factor (n) were extracted by fitting the dark J-V curves at intermediate voltages (approximately 0.4 to 0.5 V) using a diode equation J = J0exp(qV / nkT), where q is the electron charge, T is the temperature, and k is the Boltzmann constant [21]. As shown in the inset of Figure 8, the values of J0 and n are in the ranges of 1.5 × 10−6 to 5 × 10−6 A/cm2 and 2.5 to 3 for all heterojunction solar cells, respectively. The large n value (n > 2), together with the high J0, indicates that the recombination current contributes significantly to the conduction process in the cells, which may be caused by trap-assisted tunneling or field-assisted recombination at point defects [22,23]. It has been reported that formation of charged defects would occur in SiNx films after high-temperature annealing owing to the removal of hydrogen atoms [24,25]. Since the charged defect density in the annealed film should be proportional to the volume fraction of the SiNx matrix, we suggest that the increase in the charge defect density would increase the probability of trap-assisted tunneling and thus compensate the enhanced Vbi effect with increasing Rc value, leading to similar J0, as well as Voc for all heterojunction solar cells.


Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiNx films as emitters.

Wu PJ, Wang YC, Chen IC - Nanoscale Res Lett (2013)

Dark current density-voltage characteristics of Si-NCs/sc-Si heterojunction solar cells. The inset shows the saturation current density J0 and ideality factor n as a function of the Rc value.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4228313&req=5

Figure 8: Dark current density-voltage characteristics of Si-NCs/sc-Si heterojunction solar cells. The inset shows the saturation current density J0 and ideality factor n as a function of the Rc value.
Mentions: The P-doped Si-NCs/SiNx layers with various Rc values were fabricated on top of p-type sc-Si substrates for fabrication of Si heterojunction solar cells, as shown in the inset of Figure 5a. This study concentrates on basic Si-NCs/sc-Si heterojunction solar cells without the designs or processes to enhance the conversion efficiency, such as surface texturing, anti-reflection coating and back-surface field. The illuminated J-V curves corresponding to each sample are displayed in Figure 5a, and their open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and efficiency are shown in Figure 6 as a function of the N2/SiH4 flow ratio. The magnitude of Voc is generally correlated to the built-in potential (Vbi) of the junction, which could be influenced by the energy bandgap of the Si-NCs for the Si heterojunction solar cells. As shown in Figure 7, the Vbi of the P-doped Si-NCs/sc-Si heterojunction extracted from the capacitance-voltage characteristic increases from 0.77 to 1.95 V with increasing Rc value. This trend may be ascribed to the bandgap expansion of Si-NCs with the shrinkage of the Si-NC size, leading to an increase in Vbi at the junction, and thus, the Si heterojunction solar cell is expected to show a higher Voc as Rc increases. However, in this study, the Voc value is in the range of 0.49 to 0.5 for all Si heterojunction solar cells (shown in Figure 6), implying that Voc is quite insensitive to the Si-NC size. Figure 8 shows dark J-V curves for the solar cells with different Rc values. Both the saturation current density (J0) and the ideality factor (n) were extracted by fitting the dark J-V curves at intermediate voltages (approximately 0.4 to 0.5 V) using a diode equation J = J0exp(qV / nkT), where q is the electron charge, T is the temperature, and k is the Boltzmann constant [21]. As shown in the inset of Figure 8, the values of J0 and n are in the ranges of 1.5 × 10−6 to 5 × 10−6 A/cm2 and 2.5 to 3 for all heterojunction solar cells, respectively. The large n value (n > 2), together with the high J0, indicates that the recombination current contributes significantly to the conduction process in the cells, which may be caused by trap-assisted tunneling or field-assisted recombination at point defects [22,23]. It has been reported that formation of charged defects would occur in SiNx films after high-temperature annealing owing to the removal of hydrogen atoms [24,25]. Since the charged defect density in the annealed film should be proportional to the volume fraction of the SiNx matrix, we suggest that the increase in the charge defect density would increase the probability of trap-assisted tunneling and thus compensate the enhanced Vbi effect with increasing Rc value, leading to similar J0, as well as Voc for all heterojunction solar cells.

Bottom Line: Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device.These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase.A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Materials Science and Engineering, National Central University, Jhongli 320, Taiwan. ichen@ncu.edu.tw.

ABSTRACT
Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiNx (Si-NCs/SiNx) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiNx films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.

No MeSH data available.