Analysis of optical absorption in GaAs nanowire arrays
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In this study, the influence of the geometric parameters on the optical absorption of gallium arsenide [GaAs] nanowire arrays [NWAs] has been systematically analyzed using finite-difference time-domain simulations. The calculations reveal that the optical absorption is sensitive to the geometric parameters such as diameter [D], length [L], and filling ratio [D/P], and more efficient light absorption can be obtained in GaAs NWAs than in thin films with the same thickness due to the combined effects of intrinsic antireflection and efficient excitation of resonant modes. Optimized geometric parameters are obtained as follows: D = 180 nm, L = 2 μm, and D/P = 0.5. Meanwhile, the simulation on the absorption of GaAs NWAs for oblique incidence has also been carried out. The underlying physics is discussed in this work.
PACS: 81.07.Gf nanowires; 81.05.Ea III-V semiconductors; 88.40.hj efficiency and performance of solar cells; 73.50.Pz photoconduction and photovoltaic effects.
KeywordsGaAs Gallium Arsenide Effective Refractive Index Filling Ratio Single Nanowire
Semiconductor nanowire arrays [NWAs] are presently under intense research and development for next-generation solar cells due to their potential for lower cost and greater energy conversion efficiency compared to conventional thin film devices [1, 2, 3, 4]. Among semiconductor nanowires [NWs], gallium arsenide [GaAs] NWs show particular promise due to the superior electrical and optical properties of III to V materials. For example, the GaAs material system features a direct band gap and high absorption coefficient. This makes GaAs NWs prime candidates for future optoelectronic devices, just as bulk materials [5, 6, 7]. Recently, many advances have been reported in the fabrication of GaAs NW solar cells. For example, Czaban et al. observed a photovoltaic [PV] effect with a photoconversion efficiency of 0.83% from vertically oriented GaAs NWs grown on n-GaAs(111)B substrates . Colombo et al. reported a coaxial p-i-n single nanowire cell with an efficiency of 4.5% . These results illustrate that the efficiency of GaAs NW PV devices is much lower than that of thin film counterparts. There are still many problems to be resolved before GaAs NWs can become available for practical applications.
One of the main issues on nanowire solar cells is the determination of the nanowire geometry. It has been proved by many theoretical and experimental works that NWAs with well-defined geometric parameters such as diameter, length, and filling ratio exhibit a much more efficient light absorption in the solar spectrum [1, 2, 3, 4, 8, 9, 10, 11]. In this paper, the influence of geometric parameters on the optical absorption in GaAs NWAs is analyzed using finite-difference time-domain [FDTD] simulations . Optimized geometric parameters are obtained through the simulations. The underlying physics is discussed in this work.
Results and discussion
where f = πD 2 /4P 2 and nair and nGaAs are the refractive indexes of air and GaAs, respectively. Therefore, the effective refractive index of the NWA is much lower than that of the thin film counterparts, resulting in a perfect refractive index matching at the top interface between the air and NWAs, hence leads to good coupling of the incident light into the NWAs [14, 15, 16]. In long wavelength region, the results clearly indicate that longer NWs have higher absorptance due to the increased optical path length in the NWAs. For photovoltaic device applications, however, it should be noted that longer NWs would sacrifice efficient carrier extraction properties and lead to unnecessary material consumption. Hence, in the following simulations, we fixed the length of the NWs to L = 2 μm.
Figure 2b, d compares the reflectance, transmittance, and absorptance of NWAs with D/P = 0.4, 0.5, 0.6, and 0.8 for a fixed diameter of 180 nm. The calculated spectra reveal that the absorption is uniquely determined by the reflection and decreases with larger filling ratios in the visible wavelength region (λ < 700 nm). As seen from Figure 2d, only zero-order transmission exists owing to the high extinction coefficient of GaAs in these wavelengths. The trend of enhanced reflectance with the increased D/P as indicated in Figure 2c can be attributed to the heightened effective refractive index of the NWAs. In the long wavelength region, however, NWAs would undergo a significant transmission and reflection loss. The absorptance curve, shown in Figure 2b, tends to shift towards larger wavelengths as the filling ratio is increased. From these results, it can be concluded that the optimal filling ratio is determined by the trade-off between the reflection enhancement and light transmission suppression with the increase of D/P.
where ε″ is the imaginary part of the permittivity and E is the electric field intensity. Figure 3b shows the cross-sectional distribution of the optical generation rate in a single nanowire for a same incident wave power of 100 mW/cm2 with different wavelengths (λ = 400, 600, 800 nm). The optical generation rate for small wavelengths (e.g., 400 nm) is concentrated near the top and sides of the nanowire due to the strong wire-wire light scattering and short absorption length of GaAs at these photon energies. However, the generation rates for most of the solar spectrum (e.g., 600 and 800 nm) are concentrated near the core, demonstrating the internal absorption enhancement mode in the nanowire. Each nanowire acts as a nanoscale cylindrical resonator, which can trap light by multiple total internal reflections.
In summary, we have analyzed the optical properties of GaAs NWAs by FDTD simulations, which were found to be sensitive to the structural parameters such as wire diameter D, length L, and filling ratio D/P. The optimal results for the normal incidence are evaluated as D = 180 nm, L = 2 μm, and D/P = 0.5. Our calculation shows that the absorptance exceeds 90% in well-designed GaAs NWAs in the visible light region, which is much higher than that of thin films with the same thickness due to the combined effects of the intrinsic antireflection and efficient excitation of resonant modes. The simulated optical generation rates in a single GaAs nanowire for most of the solar spectrum are concentrated near the core, illustrating the internal wire absorption enhancement mode. For the oblique incidence, perfect antireflection properties for the coupling of oblique incident light into the NWAs are demonstrated at incident angles up to 60°, while the absorption declines as the incident angle is over 60° due to the large reflectance. Meanwhile, a higher absorption is observed in TM polarization than in TE polarization, which is attributed to the electric field component along the axis of TM polarization.
Financial support from the National Natural Science Foundation of China (No. 50872134) is gratefully acknowledged.
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