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Indium tin oxide (ITO) thin films with relatively high transparency and low absorption are prepared by glancing angle deposition (GLAD) method and their effect on the device performance of a-Si:H/μc-Si:H tandem thin film solar cells is theoretically investigated by applying the experimentally measured physical data of the fabricated films to the simulation parameters. The GLAD of ITO produces inclined porous columnar nanostructures due to the atomic shadowing effect. With increasing the incident flux angle, the columns are increasingly inclined, thus resulting in the improved transmission property as well as the decrease of the refractive index and extinction coefficient because of enhanced porosity within the film. Furthermore, the antireflection characteristics are improved over a wide wavelength range of 300-1100 nm. For a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the 0° ITO/80° ITO bi-layer structure, the conversion efficiency (η) of 13.6% is obtained from simulation under AM1.5g illumination, indicating an efficiency improvement compared to the device with the 0° ITO/0° ITO bi-layer structure (i.e. η = 12.58%).
©2011 Optical Society of America
1. Introduction
There has been great interest in the wide bandgap semiconductor of indium tin oxide (ITO) as a transparent conducting electrode for displays, electroluminescence devices, optical sensors, and photovoltaic devices because of its high optical transmittance in the visible region, low electrical resistivity, and stable chemical property [1–3]. For the fabrication of ITO thin films, a variety of methods including e-beam evaporation, chemical vapor deposition, sputtering, sol-gel process, and spray pyrolysis have been employed [4–8]. Recently, the glancing angle deposition (GLAD) which allows for control of morphological properties of nanocolumnar films has attracted much attention for device applications [9–11]. Among them, the e-beam evaporation would be desirable for use in the GLAD due to the strong shadowing property. Since the first report of the GLAD in 1886 [12], the technique has been demonstrated to deposit porous and sculptured thin films in various materials such as MgF2, SiO2, Si, TiO2, ITO, etc. [13–16]. The low-refractive-index (low-n) films prepared by GLAD are very promising for antireflective coatings, reflectors, and optical microresonators to enhance the device performance [17–19].
On the other hand, the enhancement in broadband light absorption or trapping, which is crucial especially in thin film solar cells, can improve the cell efficiency. In a-Si:H/μc-Si:H tandem thin film solar cells, it is necessary to match the current flowing through the top and bottom cells since the series connection of the tandem structure can limit the performance of a solar cell [20]. Additionally, transparent conducting oxide (TCO) films with a high transparency as well as a good electrical conductivity, e.g. ITO, are required as a transparent electrode layer. The morphology of ITO films may affect the performance of solar cells since their optical and electrical characteristics strongly depend on the morphological properties. Although there are some studies on the GLAD ITO films [15,16], very little work has been reported on the use of low-n ITO films in solar cell structures [17]. Also, numerical device simulation of multi-junction solar cells is required to analyze theoretically their characteristics. The solar cell structures have been often studied using the Silvaco ATLAS device simulator [21–23]. In this work, we investigated the effect of GLAD ITO films as a TCO layer, which were fabricated at different incident flux angles by using e-beam evaporator, on the cell efficiency of a-Si:H/μc-Si:H tandem thin film solar cells via the current matching. The experimentally obtained physical data of the fabricated GLAD ITO films were applied to the material parameters for the tandem solar cell simulation.
2. Experimental details and design of solar cell structures
2.1. GLAD of ITO films
The ITO films were deposited on (100) Si and glass (Corning Eagle 2000) substrates with a size of 2 × 2 cm2 by e-beam evaporator using GLAD method at room temperature. The substrates were cleaned in acetone and methanol, rinsed in de-ionized water, and subsequently dried in a flowing nitrogen gas before the deposition. The target material was an ITO pellet with a composition of 90 wt.% In2O3 and 10 wt.% SnO2. The source was located 50 cm from the substrate with a pure oxygen flow rate of 10 sccm. The base pressure and process pressure in deposition chamber were 5 × 10−6 Torr and 2 × 10−5 Torr, respectively. To obtain the GLAD ITO films, the deposition was performed at different incident flux angles of θα = 0°, 40°, 60°, and 80° without rotating the substrate. Figure 1 shows the schematic diagram for depositing the ITO films by GLAD method. Although ITO vapor fluxes arrive at the substrate with an incident flux angle (θα), the film is deposited with an inclined column angle (θβ) during the deposition process, which produces a porous columnar microstructure or nanostructure. The film thickness and the deposition rate were kept at about 200 nm and 0.5 nm/s, respectively. The thickness of deposited films was controlled with a material-related calibration using a quartz crystal thickness monitor. The samples which were mounted on holders with different tilting angles were loaded into the chamber together to fabricate simultaneously the GLAD ITO films under the same deposition condition and chamber environment. After the deposition, the ITO films were annealed at 550 °C for 30 min in an air atmosphere furnace. The structural morphologies and thicknesses of the deposited films were observed by using scanning electron microscope (SEM, Carl Zeiss, LEO SUPRA 55). The crystallinity was analyzed by X-ray diffraction (XRD, Mac Science, M18XHF-SRA) measurements. The refractive index and extinction coefficient were determined by spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.). The optical transmittance and reflectance were measured by using UV-VIS-NIR spectrophotometer (Varian, Cary 5000). The electrical properties were measured by using Hall effect measurement system (Accent, HL5500PC).
2.2. Modeling of a-Si:H/μc-Si:H tandem thin film solar cell structures
For numerical modeling and simulation, the experimentally reported a-Si:H/μc-Si:H tandem thin film solar cell structure was employed [24]. First, we theoretically performed the optimization of a-Si:H/μc-Si:H tandem thin film solar cells via the current matching between the top and bottom cells under 1-sun AM1.5g (air mass 1.5 global, 100 mW/cm2) of ASTM standard spectrum with the incident light source angle of 90° from the solar cell top layer [25]. Then, the simulations of the optimized a-Si:H/μc-Si:H tandem thin film solar cells incorporated with GLAD ITO films as a TCO layer were performed by applying the experimentally measured ITO film results to the simulation parameters. Except for the fabricated ITO layer, the material parameters of the cell structure used in simulation were referred from Silvaco ATLAS [26]. The refractive index and extinction coefficient of constituent materials, i.e. important parameters for the simulation of solar cells, were referred from the refs [27–29]. From the simulation results, the device characteristics of the solar cells were obtained.
3. Results and discussion
3.1. Characteristics of GLAD ITO films
Figure 2 shows the XRD patterns of the ITO films deposited on glass substrate for different incident flux angles. The diffraction peaks, which are related to the cubic structure of the ITO with preferred orientations in the (211), (222), (400), (444), and (622) planes, were observed at around 2θ = 21.8°, 30.86°, 35.68°, 51.22°, and 60.78°, respectively, for θα = 0°. The positions and shapes of the XRD peaks of GLAD ITO films remain almost the same except for the width of the peaks. As the incident flux angle was increased, the measured diffraction peaks become lower and broader with a slight increase in the full-width at half maximum (FWHM) of the (222) diffraction peak. The FWHM value of the (222) peaks was increased from 0.2° to 0.34° with increasing the incident flux angle, indicating that the degree of crystallinity in the ITO film is gradually decreased for larger inclination angle. This can be explained by the fact that the diffusion of deposited atoms is disturbed even in the post-annealing process due to the shadowing effect during the GLAD. In GLAD, the c-axis of films is deflected from the substrate normal toward the direction of the incident angle. The grain size can be determined from the FWHM of the dominant XRD peak using the well-known Scherrer formula [30]. The estimated grain size of the fabricated ITO films was reduced from 41.2 nm to 24.2 nm with increasing the incident flux angle.
Fig. 2 XRD patterns of the ITO films deposited on glass substrate for different incident flux angles.
Figure 3 shows the top-view and cross-sectional SEM images of the deposited ITO films on Si substrate at incident flux angles of (i) θα = 0°, (ii) θα = 40°, (iii) θα = 60°, and (iv) θα = 80°, respectively. The thicknesses of deposited ITO films were 200 nm, 195 nm, 182 nm, and 156 nm at incident flux angles of θα = 0°, 40°, 60°, and 80°, respectively. Thus, the deposition rate of ITO films was decreased from 6.67 nm/min at θα = 0° to 5.17 nm/min at θα = 80°. This is caused by the inclined deposition tendency of the film due to the self-shadowing effect and the angular distribution of the deposition rate which follows the cosine-like distribution for the GLAD [31]. The void spacing between nanocolumns was also increased with increasing the incident flux angle. From the morphological changes, the effective surface area of ITO films is clearly dependent on the tilted angle [32]. As shown in Fig. 3, the increasingly inclined columnar nanostructure of ITO films was observed because of the enhanced self-shadowing effect and limited atom mobility for larger θα during the deposition process. The measured column angles of ITO films were θβ = 14.6 o, 28.2 o, and 45.8° for θα = 40 o, 60 o, and 80 o, respectively, which are smaller than those expected by the tangent rule for small θα or the cosine rule for large θα [33,34]. There exists the difference between the values estimated by the empirical equations and the experimentally measured results for the column inclination angles due to the difference in the deposition condition.
Fig. 3 Top-view and cross-sectional SEM images of the deposited ITO films on Si substrate at incident flux angles of (i) θα = 0°, (ii) θα = 40°, (iii) θα = 60°, and (iv) θα = 80°, respectively.
Figure 4 shows the measured (a) refractive index and (b) extinction coefficient of the GLAD ITO films on Si substrate in the wavelength range of 350-1100 nm. The measured refractive index and the estimated relative porosity of the GLAD ITO films as a function of incident flux angle at a wavelength of 633 nm are shown in the inset of Fig. 4(a). As the θα was increased, the refractive index of ITO films was decreased, rapidly above θα = 60°. The refractive index of ITO film was decreased from 2.19 to 1.35 at 350 nm of wavelength and from 1.76 to 1.28 at 1100 nm when the θα was changed from 0° to 80°. This is attributed to the increased porosity within the inclined columnar films by the GLAD. The porosity of the GLAD ITO films can be evaluated from a well-known relationship [9] of the refractive index of the ITO film at θα = 0°, the effective refractive index of the GLAD ITO film (i.e. θα = 40°, 60°, and 80°), and the volume fraction of the air, relative to the deposited film at θα = 0°. The refractive index values of ITO films were determined from the spectroscopic ellipsometry measurements. We assume that the normal deposited ITO film at θα = 0° (i.e. 0° ITO film) has zero porosity. The relative porosity of the GLAD ITO films at λ = 633 nm was increased from 8.03% at θα = 40° to 63.37% at θα = 80°. The extinction coefficient was also decreased with increasing the incident flux angle as shown in Fig. 4(b). This implies that the scattering loss in the porous ITO films is relatively small compared to the intrinsic material absorption. From the extinction coefficient, the optical energy bandgap (Eg) of the films can be estimated. The inset of Fig. 4(b) shows the (αhν)2 versus hν plots of GLAD ITO films. The absorption coefficient (α) was evaluated using α = 4πk/λ and (αhν)2 = A(hν-E g) [35], where k is the extinction coefficient of the film, λ is the wavelength, A is a constant that depends on the material, and hν is the photon energy. For GLAD ITO films, the Eg was increased from 3.93 eV at θα = 0° to 4.1 eV at θα = 80°. In this case, the blueshift in the absorption edge was observed because the oxygen content within the GLAD ITO films was increased.
Fig. 4 Measured (a) refractive index and (b) extinction coefficient of the GLAD ITO films on Si substrate in the wavelength range of 350-1100 nm. The inset of (a) shows the measured refractive index and the estimated relative porosity of the GLAD ITO films versus incident flux angle at 633 nm. The inset of (b) shows the (αhν)2 versus hν plots of GLAD ITO films.
Figure 5 shows the optical transmittance spectra of the GLAD ITO films on glass substrate for different incident flux angles. The optical transmission of the bare glass substrate was measured as a baseline. Then, the transmittance of only GLAD ITO films was estimated from the measurement results. There are oscillations in transmittance spectra of ITO films. This is resulting from the constructive or destructive light interference at the film interfaces. The oscillation period is closely related to the refractive index and thickness of ITO films. For inclined columnar structures, the morphological characteristics certainly have an influence on the optical properties of the ITO film. As the incident flux angle became larger, the average transmittance was increased from 79% at θα = 0° to 95% at θα = 80° at wavelengths of 280-1100 nm and the ultraviolet cut-off wavelength also was slightly shifted to the short wavelength region. This is probably attributed to the relatively high porosity and low absorption of the films with inclined nanocolumnar structures due to the shadowing effect as can be seen in Fig. 4. For theoretical investigation of the optical properties of GLAD ITO films, the simulations were performed by the rigorous coupled wave analysis (RCWA) method [36] using the experimentally measured ITO parameters. The insets of Fig. 5 show the simulation model (upper) and calculated transmittance spectra (lower) of GLAD ITO films for different incident flux angles. Although there is a slight discrepancy between the measured and calculated results in some wavelength regions because the surface roughness as well as the voids may exist in the fabricated films but no consideration was given in simulations, their tendency and average transmittance are not much different.
Fig. 5 Optical transmittance spectra of the GLAD ITO films on glass substrate for different incident flux angles. The insets show the simulation model (upper) and calculated transmittance spectra (lower) of corresponding film structures.
For solar cell applications, the GLAD ITO films were formed on the 0° ITO film because only porous films exhibit a seriously high resistivity and sheet resistance characteristics caused by the enhanced void [16]. Figure 6 shows (a) the resistivity and sheet resistance and (b) the measured reflectance spectra of the 0° ITO (200 nm)/GLAD ITO bi-layer structures as a function of incident flux angle. The thicknesses of GLAD ITO films were similar to those of Fig. 3. The insets of Fig. 6(a) shows the SEM images (upper) of the 0° ITO/0° ITO and 0° ITO/80° ITO bi-layer structures, respectively, and the carrier concentration and Hall mobility (lower) of the corresponding films at different incident flux angles. The resistivity and sheet resistance were increased from 4.94 × 10−4 Ω-cm and 12.46 Ω/sq for 0° ITO/0° ITO to 2.06 × 10−3 Ω-cm and 58.91 Ω/sq for 0° ITO/80° ITO, respectively, due to the increased porosity in the inclined structure as the incident flux angle was increased. For the same reason, the carrier concentration was decreased from 3.76 × 1020 cm−3 for 0° ITO/0° ITO to 1.37 × 1020 cm−3 for 0° ITO/80° ITO. With increasing the incident flux angle, the Hall mobility was decreased from 33.5 cm2/V-s for 0° ITO/0° ITO to 18.7 cm2/V-s for 0° ITO/80° ITO. This may be attributed to the increased grain boundary potential because the electron scattering was increased due to the smaller grain size for more inclined columnar film [37]. As expected, for the 0° ITO/GLAD ITO bi-layer structures, the electrical properties are not much degraded compared to the 0° ITO/0° ITO bi-layer structure, exhibiting the resistivity and sheet resistance values less than 2.06 × 10−3 Ω-cm and 58.91 Ω/sq, respectively.
Fig. 6 (a) Resistivity and sheet resistance and (b) measured reflectance spectra of the 0° ITO (200 nm)/GLAD ITO bi-layer structures as a function of incident flux angle. The insets of (a) show the SEM images (upper) of the 0° ITO/0° ITO and 0° ITO/80° ITO bi-layer structures, respectively, and the carrier concentration and Hall mobility (lower) of the corresponding films at different incident flux angles. The insets of (b) show the simulation model (left) and calculated reflectance spectra (right) of the corresponding film structures.
As shown in Fig. 6(b), the strong oscillations in reflectance spectra are resulting from the light interference effect as mentioned above, indicating high reflectance maxima of > 30%. The average reflectance was decreased from 17.1% for 0° ITO/0° ITO to 14.6% for 0° ITO/80° ITO. This reason is because the refractive index is gradually changed from air (nair = 1) to Si (nSi = 3.89) via the 0° ITO/GLAD ITO bi-layer structure as shown in Fig. 4(a). For the 0° ITO/80° ITO bi-layer structure, the maxima and number of the oscillation in reflectance spectrum were significantly reduced compare to the 0° ITO/0° ITO bi-layer structure, indicating the maximum value of ~26.4%. The simulation model (right) and calculated reflectance spectra (left) of the 0° ITO/GLAD ITO bi-layer structures are shown in the insets of Fig. 6(b). The calculated results were reasonably consistent with the measured results though there is a slight discrepancy at some wavelengths. For the 0° ITO/80° ITO bi-layer structure, the average reflectance of about 13.1% with the maxima of < 25.6% was obtained from the simulation results.
3.2. Simulation of a-Si:H/μc-Si:H tandem thin film solar cells
To study the influence of GLAD ITO films as a TCO layer in solar cells, the theoretical simulation of an a-Si:H/μc-Si:H tandem thin film solar cell structure, which was experimentally reported in Ref [24], was performed. The physical parameters of the main materials (i.e., a-SiC:H, a-Si:H, and μc-Si:H) that were used for this simulation are summarized in Table 1 .In order to obtain the optimized tandem solar cell structure, the current matching between the top and bottom cells is required. For the current matching method, the detailed description was given in our previous report [38]. In this simulation, the current matching condition was theoretically determined by varying the intrinsic layer thicknesses of 100-250 nm for top a-Si:H cell and of 1-2.5 μm for bottom μc-Si:H cell. Figure 7(a) shows the short circuit current density (Jsc) as a function of the intrinsic layer thickness in the top a-Si:H cell under AM1.5g illumination. The Jsc strongly relied on intrinsic layer thicknesses of both the cells. The maximum current matching point of Jsc = 11.98 mA/cm2 was obtained at the intrinsic layer thicknesses of 170 nm and 1.8 μm for top and bottom cells, respectively, thus leading to the improved conversion efficiency (η) of ~12.27% compared to η = 12% in the reference structure under AM1.5g illumination. The experimentally measured results of the GLAD ITO films at different incident flux angles were used to investigate the performance of the optimized a-Si:H/μc-Si:H tandem thin film solar cell structures as the TCO layer. The schematic diagram of optimized a-Si:H/μc-Si:H tandem thin film solar cell structure with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer used in this simulation is shown in Fig. 7(b).
Table 1. Physical Parameters of Main Materials Used for this Simulation
Fig. 7 (a) Jsc versus the intrinsic layer thickness in the top a-Si:H cell under AM1.5g illumination and (b) schematic diagram of optimized a-Si:H/μc-Si:H tandem thin film solar cell structure with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer used in this simulation.
Figure 8(a) shows the J-V characteristics of the optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer under AM1.5g illumination. As the incident flux angle was increased, the Jsc was increased from 12.06 mA/cm2 for 0° ITO/0° ITO to 13.1 mA/cm2 for 0° ITO/80° ITO. As shown in Figs. 5 and 6, there is a compromise between the optical and electrical properties for porous nanocolumnar ITO films. The optical transmittance of the ITO film for 0° ITO/80° ITO was largely improved compared to that for 0° ITO/0° ITO, but the electrical properties were slightly degraded. The highest Jsc value was obtained for the solar cell structure with the 0° ITO/80° ITO bi-layer structure, maintaining open circuit voltage (Voc) values of ~1.37 V for 0° ITO/0-80° ITO. The fill factor (FF), which is related to the series resistance occurred in the regions of metal grid, TCO layer, and tunnel junction of the cells, was rarely changed from 76.14% for 0° ITO/0° ITO to 75.78% for 0° ITO/80° ITO [39,40]. Thus, the device exhibited the highest conversion efficiency of η = 13.6% for the solar cell structure incorporated with the 0° ITO/80° ITO bi-layer structure. The increase in the conversion efficiency of ~1% was achieved compared to η = 12.58% for the 0° ITO/0° ITO bi-layer structure. This improvement is due to the relatively low absorption as well as high transmittance of GLAD ITO film for 0° ITO/80° ITO because its electrical properties are only slightly degraded compared to those of the 0° ITO/0° ITO bi-layer structure. Moreover, the enhanced antireflection characteristics at the device surface over a wide wavelength range of 300-1100 nm help to improve the cell efficiency.
Fig. 8 (a) J-V characteristics and (b) EQE spectra of the optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer under AM1.5g illumination. The inset of (a) shows the photogeneration rate of optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/80° ITO bi-layer structure for incident light of 400 nm and 800 nm wavelengths under AM1.5g illumination.
The external quantum efficiency (EQE) spectra of the optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer under AM1.5g illumination are shown in Fig. 8(b). The integrated EQE spectrum with solar spectrum enables an estimation of the Jsc. The solar cell with the 0° ITO/80° ITO bi-layer structure exhibited overall highest EQE spectrum for both the top and bottom cells. At wavelengths of 400-640 nm, most of the light are absorbed and converted by the top a-Si:H cell, while the light absorption occurs at wavelengths of 640-1100 nm by the bottom μc-Si:H cell. This can be also confirmed by the photogeneration rate, which is generated by the incident light, in the solar cell structure. The photogeneration rates of optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/80° ITO bi-layer structure for incident light of 400 nm and 800 nm wavelengths under AM1.5g illumination are shown in the inset of Fig. 8(a). As shown in EQE spectra, at a wavelength of 400 nm, the photogeneration occurs only in the top a-Si:H cell region. However, at 800 nm, the photogeneration occurs only in the bottom μc-Si:H cell region because of the transparent a-Si:H (1.84 eV) layer to the incident light. The parameters of a-Si:H/μc-Si:H tandem thin film solar cells with 0° ITO/GLAD ITO bi-layer structures at different incident flux angles under AM1.5g illumination are summarized in Table 2 .
Table 2. Parameters of a-Si:H/μc-Si:H Tandem Thin Film Solar Cells with 0° ITO/GLAD ITO Bi-Layer Structures at Different Incident Flux Angles under AM1.5g Illumination
4. Conclusion
The porous, nanocolumnar ITO films were fabricated by e-beam evaporator using the GLAD method. By using the experimentally measured physical data of the fabricated GLAD ITO films as the simulation parameters, their effect on the device performance of optimized a-Si:H/μc-Si:H tandem thin film solar cells was theoretically investigated. With increasing the incident flux angle (θα), the inclined nanocolumnar ITO films exhibited lower refractive index and extinction coefficient due to the increase of porosity within the film by the shadowing effect during the GLAD process. The optical transmission characteristics were improved in the wavelength range of 280-1100 nm. The electrical properties of the 0° ITO/GLAD ITO bi-layer structures were not seriously degraded. Additionally, the GLAD ITO films enhanced the antireflection property by suppressing the surface reflection. From simulation results, for optimized a-Si:H/μc-Si:H tandem thin film solar cell structures with the 0° ITO/GLAD ITO bi-layer structures, the conversion efficiency was improved from 12.58% at 0° ITO/0° ITO to 13.6% at 0° ITO/80° ITO under AM1.5g illumination. These results may provide a better understanding of Si-based thin film solar cells with the GLAD low-n TCO layer.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0016930 and 2010-0025071).
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