Abstract
The solar cell structure of ntype polysilicon/5nmdiameter silicon nanocrystals embedded in an amorphous silicon oxycarbide matrix (30 layers)/ptype hydrogenated amorphous silicon/Al electrode was fabricated on a quartz substrate. An opencircuit 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:
Silicon nanocrystals; Silicon quantum dot; Solar cells; Quantum size effectBackground
Over the past few years, many researchers have shown an interest in silicon nanostructures, such as silicon nanocrystals [14] and silicon nanowires [58] for solar cell applications. Since a silicon nanocrystal embedded in a barrier material can make carriers confined threedimensionally, 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 (SiQD) materials to all silicon tandem solar cells [9], which have the possibility to overcome the ShockleyQueisser 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 confinementinduced mixing of Γcharacter into the Xlike conduction band states [11]. Therefore, SiQD materials are one of the promising materials for the thirdgeneration solar cells. Sizecontrolled SiQDs have been prepared in an amorphous silicon oxide (aSiO_{2}) [12], nitride (aSi_{3}N_{4}) [13], carbide (aSiC) [1417], or hybrid matrix [18,19], which is called as silicon quantum dot superlattice structure (SiQDSL). In the case of solar cells, generated carriers have to be transported to each doping layer. Since the barrier height of an aSiC matrix is relatively lower than that of an aSi_{3}N_{4} or aSiO_{2} matrix, the SiQDSL using an aSiC matrix has an advantage in carrier transport. Therefore, the development of the SiQDSL solar cells using an aSiC matrix is of considerable importance. There are a few researches fabricating SiQDSL solar cells. PerezWurfl et al. reported that SiQDSL solar cells with SiO_{2} matrix showed an opencircuit voltage (V_{oc}) of 492 mV. However, the clear evidence of the quantum size effect has not been reported from SiQDSL solar cells [20]. In our previous work, SiQDSLs with aSiC matrix have been prepared by plasmaenhanced chemical vapor deposition (PECVD). The defect density in a SiQDSL has been successfully reduced by hydrogen plasma treatment (HPT), and the shift of photoluminescence spectra from the SiQDSLs has been confirmed by varying the diameter of the SiQDs [2]. Moreover, it has been revealed that the oxygenincorporation into the aSiC matrix can suppress the formation of the leakage paths [21]. An V_{oc} of 518 mV has been obtained in a SiQDSL solar cell with an amorphous silicon oxycarbide (aSi_{1  x  y}C_{x}O_{y}) matrix [1].
In this paper, we report the effect of oxygen addition on the formation of SiQDs in aSi_{1  x  y}C_{x}O_{y}. Optical absorption coefficients of the SiQDSL were also investigated. SiQDSL 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 aSi_{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 40periodmultilayer with siliconrich hydrogenated amorphous silicon oxycarbide layers and hydrogenated amorphous silicon oxycarbide barrier layers was prepared on a quartz substrate by very high frequency PECVD method (VHFPECVD). 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 aSi_{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 SiQDs in an aSi_{1  x  y}C_{x}O_{y} matrix. The target size of SiQDs and barrier width were 5 and 2 nm, respectively. The concentrations of Si, C, and O in the barrier layer were measured by Xray photoelectron spectroscopy (XPS). The crystallinity of SiQDs was investigated by Raman scattering spectroscopy. The absorption coefficient of a SiQDSL 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.
The solar cells using SiQDSL as an absorber layer were also fabricated. The schematic of the solar cell structure is shown in Figure 1. The fabrication process is referred as follows. A phosphorusdoped hydrogenated amorphous silicon thin film was deposited on a quartz substrate by PECVD. The film was annealed at 900°C for 30 min under a forming gas, resulting in a polycrystalline silicon (ntype polySi) thin film. On the polySi layer, a 30period superlattice was deposited by VHFPECVD. The same deposition conditions as referred to above were used except the flow rates of CO_{2} and H_{2}, which were fixed at 0.4 and 47.0 sccm, respectively. Consequently, the sample was annealed at 900°C for 30 min to form SiQDs. The sample was exposed to hydrogen plasma to reduce dangling bond defects in the postannealed SiQDSL. After the treatment, a borondoped hydrogenated amorphous silicon (ptype aSi:H) was deposited by PECVD. An aluminum (Al) electrode was deposited by the evaporator on the sample. The electrode area of a solar cell was 0.00785 cm^{2}. The crosssectional structure of a solar cell was observed by transmission electron microscopy (TEM). The solar cells were characterized by dark IV characteristics and light IV characteristics under the illumination at AM1.5G, 100 mW/cm^{2}, and 25°C.
Figure 1. The schematic of the fabricated SiQDSL solar cell structure.
Numerical method
The numerical calculations of the SiQDSL solar cells were performed using a twodimensional device simulator, Atlas ver. 5.18.3.R (Silvaco, Inc., Santa Clara, CA, USA). The device structure used for numerical calculations is shown in Figure 2. Quartz substrate/ntype polySi/30period SiQDSL (SiQDs embedded in aSi_{1  x  y}C_{x}O_{y})/ptype hydrogenated amorphous silicon (ptype aSi:H)/Al electrode structure was assumed in this simulation. The diameter of SiQDs and the gap between any two SiQDs were fixed at 5 and 2 nm, respectively. The BQP method [2226] was adopted to describe the quantum confinement effect and the quantum tunnel effect in the SiQDSL layer. The electrical transport in the SiQDSL was described by driftdiffusion equations and current continuity equations for electrons and holes. In the theory, the transport of carriers is influenced by the total potential of the potential characterizing the system and quantum potential. The definition of the effective quantum potential Q_{eff} is derived from a weighted average of the BQPs seen by all singleparticle wavefunctions, which can be expressed as
Figure 2. The structure of the SiQDSL solar cell for numerical calculations.
and
where Q_{eff,n} and Q_{eff,p} are the effective BQPs for the conduction band and the valence band, respectively. h is Planck's constant. n and p are the electron and hole concentrations, respectively. γ_{n} and γ_{p} are adjustable parameters for quantum confinement. In general, a threedimensional quantum system cannot be described on a twodimensional device simulator due to the difference of the quantum confinement effects between two and threedimensional systems. To take threedimensional quantum effect into twodimensional simulation, we adjusted the γ_{n} and γ_{p} parameters in the BQP model. The parameter values were determined to satisfy that the bandgap calculated from the BQP method is equal to the bandgap derived from threedimensional Schrödinger equations. The γ_{n} and the γ_{p} for the SiQDSL with 5nmdiameter SiQDs and 2nmthick aSi_{1  x  y}C_{x}O_{y} barrier layers were 4.0. In this simulation, the radiative recombinations, ShockleyReadHall (SRH) recombination [2729] and Auger recombination, were taken into account. Auger coefficients and effective masses of bulk Si were adapted for all layers. The other parameters are shown in Table 1. The bandgaps in the table do not affect optical absorption but carrier transport phenomenon. To take into account the phosphorus diffusion into the SiQDSL layer, a calculation with the donor concentration in SiQDs of 1 × 10^{17} cm^{3} was also performed. The light IV characteristics were calculated, assuming solar illumination of AM1.5G at 100 mW/cm^{2}. Additionally, the quantum efficiencies were calculated without bias light and bias voltage. An incident light was put into the solar cells from the quartz substrate side normally. The light intensity and the photogeneration rate were calculated based on the ray tracing method, where the SiQDSL was regarded as an optically homogeneous material, and the optical parameters from the spectroscopic ellipsometry measurement of the SiQDSL were used.
Table 1. Parameters of each layer for calculations
Results and discussion
Optical properties of SiQDSLs
The concentrations of Si, C, and O in aSi_{1  x  y}C_{x}O_{y} thin films were measured by the relative sensitivity factor (RSF) method. The concentrations of Si, C, and O for each CO_{2}/MMS flow rate ratio were shown in Table 2. The oxygen concentration and the deposition rate of the films depend on the CO_{2}/MMS flow rate ratio. The oxygen concentrations of the films prepared without CO_{2} gas and with the CO_{2}/MMS flow rate ratios of 0.3, 1.5, and 3.0 were 17.5, 25.1, 32.6, and 39.8 at.%, respectively. Oxygen was observed even in the asdeposited film prepared without flowing CO_{2} gas. This unintentionally incorporated oxygen is thought to be originating from the deposition atmosphere. The deposition rate is proportional to the oxygen concentration in the film, suggesting that the volume of the thin film increases with the oxygen incorporation.
Table 2. Concentrations of Si, C, and O in aSi_{1  x  y}C_{x}O_{y }films with several CO_{2}/MMS flow rate ratios
The crystallization of SiQDs was investigated by Raman scattering spectroscopy. The Raman spectra of the SiQDSLs with the CO_{2}/MMS flow rate ratios of 0, 0.3, 1.5, and 3.0 are shown in Figure 3. A Raman spectrum was separated into three Gaussian curves. The peaks at approximately 430 and 490 cm^{1} are originating from the LO mode and TO mode of aSi phase, respectively [30]. These Gaussian curves are indicated by blue dashed lines. The peak at approximately 510 cm^{1} is originating from SiQDs. The Gaussian curve is indicated by green dashed line. As the CO_{2}/MMS flow rate ratio increases, the intensity of the peak from SiQDs becomes weaker compared with the peak from aSi phase. This indicates that the crystallization of SiQDs in the siliconrich layers is prevented by the oxygenincorporation, and the crystallization temperature of nanocrystalline silicon phase becomes higher [31].
Figure 3. The Raman spectra of the SiQDSLs with several CO_{2}/MMS flow rate ratios. (a) CO_{2}MMS = 0. (b) CO_{2}MMS = 0.3. (c) CO_{2}MMS = 1.5. (d) CO_{2}MMS = 3.
The absorption coefficient was estimated from the measurements of transmittance and reflectance. The absorption coefficients of the SiQDSLs with the CO_{2}/MMS flow rate ratios of 0, 0.3, 1.5, and 3.0 are shown in Figure 4. For both SiQDSLs with the CO_{2}/MMS flow rate ratios of 0 and 0.3, the absorption enhancement was observed below the photon energy of 2.0 eV. Moreover, the absorption enhancement becomes weaker as the CO_{2}/MMS flow rate ratio increases. This tendency corresponds to that of the intensity of the peak originating from SiQDs in the Raman scattering spectrum. Therefore, one can conclude that the absorption enhancement is due to the increment of the nanocrystalline silicon phase. Moreover, the absorption edge was estimated by the Tauc model [32]. The absorption edges of the SiQDSLs with the CO_{2}/MMS flow rate ratios of 0 and 0.3 were estimated at 1.48 and 1.56 eV, respectively. These values are similar to the optical gap of 5nmdiameter SiQDs in an aSiC matrix measured by photoluminescence spectrum [2]. On the other hand, the absorption edges of the SiQDSLs with the CO_{2}/MMS flow rate ratios of 1.5 and 3.0 were estimated at approximately 1.70 eV, which corresponds to the optical gap of aSi.
Figure 4. The absorption coefficients of the SiQDSLs with several CO_{2}/MMS flow rate ratios.
These results indicate that the CO_{2}/MMS flow rate ratio should be below approximately 0.3 to form SiQDs in the siliconrich layers. According to the [22], the CO_{2}/MMS flow rate ratio should be higher than 0.3 to suppress the crystallization of aSiC phase in the aSi_{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 tradeoff between the promotion of the crystallization of SiQDs and the suppression of the crystallization of aSiC 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.
IV characteristics of the fabricated solar cells
The crosssectional TEM images of the fabricated solar cell are shown in Figure 5. Figure 5a shows the image of the whole region of the solar cell. Figure 5b shows the magnified image of the SiQDSL layer in the solar cell. The thicknesses of the ntype polySi layer, the SiQDSL layer, and ptype aSi:H layer were approximately 530, 143, and 46 nm, respectively. The black region below the ntype polySi layer is a quartz substrate. The textured quartz substrate is used to prevent from peeling off the films during the thermal annealing. In Figure 5b, the yellow lines and orange circles indicate the interface between an aSi_{1  x  y}C_{x}O_{y} barrier layer and a SiQD layer, and SiQDs, respectively. This magnified image revealed that a SiQDSL layer including average 5nmdiameter SiQDs was successfully prepared.
Figure 5. The crosssectional TEM images of the fabricated solar cell structure. (a) The whole region image with the schematic of the structure and the thicknesses of each layer. (b) The magnified image of the SiQDSL layer in the solar cell.
Figure 6 shows the dark IV characteristics and the light IV characteristics of the solar cells with the CO_{2}/MMS flow rate ratio of 0 and 0.3 [1,3]. The diode properties were confirmed from the dark IV characteristics. The characteristics were evaluated by onediode model:
where I_{0}, n, R_{s}, and R_{sh} represent reverse saturation current density, diode factor, series resistance, and shunt resistance, respectively. According to the fitting of the dark IV characteristics of the oxygenintroduced SiQDSL solar cell, the reverse saturation current density, the diode factor, the series resistance, and the shunt resistance were estimated at 9.9 × 10^{6} mA/cm^{2}, 2.0, 2.3 × 10^{1} Ω cm^{2}, and 2.1 × 10^{4} Ω cm^{2}, respectively. The solar cell parameters of the light IV characteristics under AM1.5G illumination are summarized in Table 3. An V_{oc} of 518 mV was achieved. Compared with the V_{oc} of 165 mV with nonoxygenintroduced SiQDSL solar cells, the characteristics were drastically improved. The possible reasons for this improvement are due to the passivation effect of SiO phase on silicon quantum dots [33], and the reduction of the leakage current by the introduction of oxygen [21]. Figure 7 shows the internal quantum efficiency of the solar cell. The red line corresponds to the experimental internal quantum efficiency. The quantum efficiency decays to zero at approximately 800 nm, suggesting that the contribution is originating not from the ntype polySi but from the SiQDSL absorber layer.
Table 3. Solar cell parameters of the fabricated SiQDSL solar cells and the calculated by BQP method
Figure 7. Internal quantum efficiencies of fabricated solar cell and of that calculated by the BQP method.
Calculations of IV characteristics and quantum efficiencies
The light IV characteristics and the internal quantum efficiency of the SiQDSL solar cells with a doped SiQDSL layer and a nondoped SiQDSL 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 shortcircuit 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 SiQDSL layer. In an actual solar cell, the phosphorus concentration in the SiQDSL absorber layer is more than 1 × 10^{19} cm^{3} due to the hightemperature 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 SiQDSL layer increases. The phosphorus in the SiQDSL layer degrades the J_{sc} due to the reduction of the electrical field in the SiQDSL layer. Unfortunately, simulations were not possible when the dopant concentration in the SiQDSL 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 SiQDSL 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 ntype polySi layer. In this calculation, the surface roughness of the textured quartz substrate was not taken into account. The effective optical path length in the ntype 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 shortwavelength region is higher than that of the experimental because of the low optical absorption loss in the ntype polySi layer.
Even though the J_{sc} mismatch, the absorption edge can be estimated from the simulated quantum efficiency. The calculated quantum efficiencies at the longwavelength 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 SiQDSL estimated from the optical measurements. This indicates that the photogeneration in the SiQDSL solar cell is thought to be the contribution from SiQDs, 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 SiQDSLs were investigated, and the solar cell structure using the SiQDSL as an absorber layer was fabricated and characterized. From the measurements of the Raman spectra and the absorption coefficients of SiQDSLs, it was revealed that the absorption coefficient is enhanced by the crystallization of the SiQDs, and the crystallinity of SiQDs 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 SiQDSL solar cell was in agreement with that of the fabricated one. Moreover, the absorption edge of the SiQDSL 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.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
SY carried out the experiments and the calculations. MK supervised the work and finalized the manuscript. YK and SM participated in the design of the study and the instructions of the calculations, and helped draft the manuscript. All authors read and approved the final manuscript.
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.
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