Silicon quantum dot superlattice solar cell structure including silicon nanocrystals in a photogeneration layer
- Shigeru Yamada^{1}Email author,
- Yasuyoshi Kurokawa^{1},
- Shinsuke Miyajima^{1} and
- Makoto Konagai^{1, 2}
https://doi.org/10.1186/1556-276X-9-246
© Yamada et al.; licensee Springer. 2014
Received: 19 February 2014
Accepted: 3 May 2014
Published: 20 May 2014
Abstract
The solar cell structure of n-type poly-silicon/5-nm-diameter silicon nanocrystals embedded in an amorphous silicon oxycarbide matrix (30 layers)/p-type hydrogenated amorphous silicon/Al electrode was fabricated on a quartz substrate. An open-circuit voltage and a fill factor of 518 mV and 0.51 in the solar cell were obtained, respectively. The absorption edge of the solar cell was 1.49 eV, which corresponds to the optical bandgap of the silicon nanocrystal materials, suggesting that it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of crystalline silicon.
PACS
85.35.Be; 84.60.Jt; 78.67.Bf
Keywords
Background
Over the past few years, many researchers have shown an interest in silicon nanostructures, such as silicon nanocrystals [1–4] and silicon nanowires [5–8] for solar cell applications. Since a silicon nanocrystal embedded in a barrier material can make carriers confined three-dimensionally, the absorption edge can be tuned in a wide range of photon energies due to the quantum size effect. Thus, it is possible to apply silicon nanocrystal materials or silicon quantum dot (Si-QD) materials to all silicon tandem solar cells [9], which have the possibility to overcome the Shockley-Queisser limit [10]. Moreover, it has been found that the weak absorption in bulk Si is significantly enhanced in Si nanocrystals, especially in the small dot size, due to the quantum confinement-induced mixing of Γ-character into the X-like conduction band states [11]. Therefore, Si-QD materials are one of the promising materials for the third-generation solar cells. Size-controlled Si-QDs have been prepared in an amorphous silicon oxide (a-SiO_{2}) [12], nitride (a-Si_{3}N_{4}) [13], carbide (a-SiC) [14–17], or hybrid matrix [18, 19], which is called as silicon quantum dot superlattice structure (Si-QDSL). In the case of solar cells, generated carriers have to be transported to each doping layer. Since the barrier height of an a-SiC matrix is relatively lower than that of an a-Si_{3}N_{4} or a-SiO_{2} matrix, the Si-QDSL using an a-SiC matrix has an advantage in carrier transport. Therefore, the development of the Si-QDSL solar cells using an a-SiC matrix is of considerable importance. There are a few researches fabricating Si-QDSL solar cells. Perez-Wurfl et al. reported that Si-QDSL solar cells with SiO_{2} matrix showed an open-circuit voltage (V_{oc}) of 492 mV. However, the clear evidence of the quantum size effect has not been reported from Si-QDSL solar cells [20]. In our previous work, Si-QDSLs with a-SiC matrix have been prepared by plasma-enhanced chemical vapor deposition (PECVD). The defect density in a Si-QDSL has been successfully reduced by hydrogen plasma treatment (HPT), and the shift of photoluminescence spectra from the Si-QDSLs has been confirmed by varying the diameter of the Si-QDs [2]. Moreover, it has been revealed that the oxygen-incorporation into the a-SiC matrix can suppress the formation of the leakage paths [21]. An V_{oc} of 518 mV has been obtained in a Si-QDSL solar cell with an amorphous silicon oxycarbide (a-Si_{1 - x - y}C_{ x }O_{ y }) matrix [1].
In this paper, we report the effect of oxygen addition on the formation of Si-QDs in a-Si_{1 - x - y}C_{ x }O_{ y }. Optical absorption coefficients of the Si-QDSL were also investigated. Si-QDSL solar cells were fabricated using the optimum oxygen concentration. In addition, the numerical analysis using the Bohm quantum potential (BQP) method was performed to simulate the electrical characteristics of fabricated solar cells.
Methods
Experimental method
The a-Si_{1 - x - y}C_{ x }O_{ y } matrix was deposited on a quartz substrate to investigate the fundamental optical properties such as Raman scattering spectrum, transmittance, and reflectance. The fabrication method is referred as follows. A 40-period-multilayer with silicon-rich hydrogenated amorphous silicon oxycarbide layers and hydrogenated amorphous silicon oxycarbide barrier layers was prepared on a quartz substrate by very high frequency PECVD method (VHF-PECVD). The source gases were silane (SiH_{4}), monomethylsilane (MMS), hydrogen (H_{2}), and carbon dioxide (CO_{2}). The flow rates of SiH_{4}, MMS, and H_{2} + CO_{2}; deposition pressure; substrate temperature; frequency; and plasma power were fixed at 3.3 , 1.3, and 47.4 sccm; 20 Pa; 60 MHz; 193 °C; and 13 mW/cm^{2}, respectively. The flow rate of CO_{2} was varied from 0 to 3.7 sccm. The mass flow controllers for SiH_{4} and CO_{2} were calibrated by N_{2}. A H_{2}-calibrated mass flow controller was used for MMS. During the deposition of a-Si_{1 - x - y}C_{ x }O_{ y } barrier layers, the flow of SiH_{4} gas was stopped. Subsequently, the samples were annealed at 900 °C for 30 min under a forming gas atmosphere to form Si-QDs in an a-Si_{1 - x - y}C_{ x }O_{ y } matrix. The target size of Si-QDs and barrier width were 5 and 2 nm, respectively. The concentrations of Si, C, and O in the barrier layer were measured by X-ray photoelectron spectroscopy (XPS). The crystallinity of Si-QDs was investigated by Raman scattering spectroscopy. The absorption coefficient of a Si-QDSL was estimated by the transmittance and the reflectance of a sample. The samples with uniform thickness were selected for the measurements, and one measurement was carried out for each measurement method and for each sample.
Numerical method
Parameters of each layer for calculations
Parameters | n-type poly-Si | Si-QD | a-Si_{1 - x- y}C_{ x }O_{ y } | p-type a-Si |
---|---|---|---|---|
Energy gap (eV) | 1.13 | 1.13 | 2.5 | 1.7 |
Electron affinity (eV) | 4.17 | 4.17 | 3.5 | 4.0 |
Carrier lifetime (s) | 1 × 10^{-15} | 1 × 10^{-10} | 1 × 10^{-10} | 1 × 10^{-6} |
Electron mobility (cm^{2}/Vs) | 1 | 1 | 1 | 1 |
Hole mobility (cm^{2}/Vs) | 0.1 | 0.1 | 0.1 | 0.1 |
Donor concentration (cm^{-3}) | 1 × 10^{19} | 0 or 1 × 10^{17} | - | - |
Accepter concentration (cm^{-3}) | - | - | - | 1 × 10^{19} |
Results and discussion
Optical properties of Si-QDSLs
Concentrations of Si, C, and O in a-Si _{ 1 - x - y } C _{ x } O _{ y } films with several CO _{ 2 } /MMS flow rate ratios
CO_{2}/MMS | Si (at.%) | C (at.%) | O (at.%) |
---|---|---|---|
0 | 44.6 | 37.9 | 17.5 |
0.3 | 40.3 | 34.6 | 25.1 |
1.5 | 34.2 | 33.2 | 32.6 |
3.0 | 31.9 | 28.3 | 39.8 |
These results indicate that the CO_{2}/MMS flow rate ratio should be below approximately 0.3 to form Si-QDs in the silicon-rich layers. According to the [22], the CO_{2}/MMS flow rate ratio should be higher than 0.3 to suppress the crystallization of a-SiC phase in the a-Si_{1 - x - y}C_{ x }O_{ y } barrier layers and the increment of the dark conductivity for the annealing temperature of 900°C. Although there is a trade-off between the promotion of the crystallization of Si-QDs and the suppression of the crystallization of a-SiC phase, the CO_{2}/MMS flow rate ratio of approximately 0.3 or the oxygen concentration of approximately 25 at.% is one of the optimal conditions. Therefore, the CO_{2}/MMS flow rate ratio of 0.3 is adopted for the solar cell fabrication in this study.
I-V characteristics of the fabricated solar cells
Solar cell parameters of the fabricated Si-QDSL solar cells and the calculated by BQP method
Parameters | Experimental | Calculated | |
---|---|---|---|
Doped Si-QDSL | Non-doped Si-QDSL | ||
V_{oc} (mV) | 518 | 520 | 505 |
J_{sc} (mA/cm^{2}) | 0.34 | 3.98 | 4.96 |
FF | 0.51 | 0.61 | 0.69 |
Calculations of I-V characteristics and quantum efficiencies
The light I-V characteristics and the internal quantum efficiency of the Si-QDSL solar cells with a doped Si-QDSL layer and a non-doped Si-QDSL layer were simulated under AM1.5G illumination using the BQP method. The calculated solar cell parameters are shown in Table 3. Also, the calculated quantum efficiencies are shown in Figure 7. The simulated quantum efficiencies are multiplied by 0.12 for comparison with the experimental one. The calculated short-circuit current densities (J_{sc}) and quantum efficiencies are much higher than those of the experimental results. There are two possible reasons.
The first reason is due to the difference of the doping concentration in a Si-QDSL layer. In an actual solar cell, the phosphorus concentration in the Si-QDSL absorber layer is more than 1 × 10^{19} cm^{-3} due to the high-temperature annealing process [34]. From the simulations, the J_{sc} and the quantum efficiency in the whole wavelength region becomes lower if the phosphorus concentration in the Si-QDSL layer increases. The phosphorus in the Si-QDSL layer degrades the J_{sc} due to the reduction of the electrical field in the Si-QDSL layer. Unfortunately, simulations were not possible when the dopant concentration in the Si-QDSL was higher than 1 × 10^{17} cm^{-3} due to the convergence problem of the BQP calculations. It is expected that J_{sc} will decrease more if the dopant concentration becomes higher. We previously reported that the quantum efficiency in the whole wavelength region decreases as the dopant concentration in the Si-QDSL increases from experiments and the simulations using classical model [35], which is similar to the results of the BQP method. The second reason is due to the optical losses in the n-type poly-Si layer. In this calculation, the surface roughness of the textured quartz substrate was not taken into account. The effective optical path length in the n-type layer of the simulated structure should be shorter than that of the actual solar cell structure. As a result, the simulated quantum efficiency in the short-wavelength region is higher than that of the experimental because of the low optical absorption loss in the n-type poly-Si layer.
Even though the J_{sc} mismatch, the absorption edge can be estimated from the simulated quantum efficiency. The calculated quantum efficiencies at the long-wavelength region are in agreement with those of the experimental one. This suggests that the absorption edge of the solar cell can be theoretically reproduced using this simulation. Moreover, the absorption edge was estimated to be 1.49 eV, which is quite similar to the absorption edge of the Si-QDSL estimated from the optical measurements. This indicates that the photogeneration in the Si-QDSL solar cell is thought to be the contribution from Si-QDs, and it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of a crystalline silicon.
Conclusions
The fundamental optical properties of Si-QDSLs were investigated, and the solar cell structure using the Si-QDSL as an absorber layer was fabricated and characterized. From the measurements of the Raman spectra and the absorption coefficients of Si-QDSLs, it was revealed that the absorption coefficient is enhanced by the crystallization of the Si-QDs, and the crystallinity of Si-QDs is affected by the oxygen concentration in the superlattice. In addition, the solar cell characteristics were simulated by the BQP method. The absorption edge of the simulated Si-QDSL solar cell was in agreement with that of the fabricated one. Moreover, the absorption edge of the Si-QDSL solar cell was 1.49 eV, which is similar to the absorption edge estimated from the optical measurements. These results suggest that it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of a crystalline silicon.
Declarations
Acknowledgements
This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy Trade and Industry of Japan.
Authors’ Affiliations
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