Metal nanoparticles (NPs) are incorporated in solar cells during the formation of front or back contacts to improve light absorption via the scattering of excitation light at their surface plasmon resonance (SPR) or localized SPR (LSPR). Here, we demonstrate LSPR-promoted improvement in the efficiency of CdS/CdTe solar cells fabricated by physical vapor deposition by incorporating different quantities of chemically synthesized 200-nm Au NPs in the CdTe layer. The J–V characteristics, external quantum efficiencies, absorption spectra, and cell efficiencies of these devices are compared. This study can guide future research on enhancing the CdS/CdTe solar cell performance using the plasmon effect.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Polycrystalline thin-film CdTe with a near-optimum bandgap and high absorption coefficient that can be relatively easily produced is a leading semiconducting candidate for photovoltaic applications. However, its low light absorption in the near bandgap range, the limited availability of Te, and high energy consumed in the formation of large grains of CdTe have been major concerns for further development of CdTe photovoltaic devices for future terawatt-scale power generation. Thus, methods have been proposed to reduce the cost of raw materials by reducing the thickness of the absorbing layer in order to reduce the unit cost per watt through the minimization of the energy consumption during the production of these cells [1,2]. One method for reducing the thickness of the absorber layer is to use metallic nanostructures to improve the solar cell performance . Metallic nanostructures such as those of Au, Ag, and other metals are potential candidates for improving the light absorption of devices incorporated with such nanostructures because of the localized surface plasmon resonance (LSPR) of these nanostructures, which contributes to a significant enhancement of local electromagnetic fields. Thus, the optical properties of devices containing metallic nanostructures can be improved [4–7]. The strong interactions of metallic NPs with incident light due to the dipole oscillation of localized surface plasmons increase the effective optical path length, light trapping, and energy conversion efficiency of the cells [4,8]. As metals have the ability to accommodate high surface charges, metallic nanostructures have been specially designed to facilitate strong subwavelength light scattering, which can significantly improve the light trapping capability of thin-film cells by coupling with semiconductor waveguide modes [9,10].
The enhancement of the performance of, several types of solar cells, such as crystalline Si wafer cells with various thicknesses , organic cells , and dye-sensitized cells , using plasmonic light-trapping has been reported by several groups. Despite the progresses made in terms of the design and optimization of plasmonic structures for solar cell applications, nearly all previous research efforts have focused on Si-based photovoltaics [13,14], while very few comprehensive studies have been reported for CdTe thin-film plasmonic solar cells [1,5]. For example, Spalatu et al. discussed the plasmonic effect of devices fabricated using a CdTe layer deposited by the close-spaced sublimation (CSS) method .
Conventionally, highly efficient devices are obtained only at temperatures above 500 °C via CSS or vapor transport deposition . The advantages of these processes are that Cd and Te atoms can be crystallized at a larger grain size under the high energy provided by the substrate. Further, Salavei et al. proposed solar cells of flexible CdTe films deposited on ultra-thin glass using a low-temperature physical vapor deposition (PVD) process . Evidently, maintaining a low temperature during the deposition of CdTe films via PVD has several advantages such as low energy consumption, reduction of the substrate stress, and use of a variety of substrates including polymer films, which cannot be achieved in the high-temperature process.
In this study, we used a simple solution-based method to coat an array of spherical Au NPs on the surface of CdTe films fabricated by a PVD deposition method with less energy consumption than that of the conventional CSS. The LSPR effect is demonstrated through various experiments including the absorption and external quantum efficiency (EQE) measurements. The enhancement of the light absorption of the device facilitated by the incorporated Au NPs leads to enhanced photocurrent of the device. To the best of our knowledge, this is the first report on the plasmonic effect of a CdTe device fabricated by PVD.
2. Experimental section
The devices were fabricated in a superstrate configuration on Corning 7059 glass coated with a 150-nm-thick indium tin oxide (ITO) layer. CdS films with thicknesses of 150 to 200 nm were deposited by PVD at a substrate temperature of 200 °C and deposition rate of 3 Å/s using high-purity CdS powder as the source. Then, a CdTe layer with a thickness less than 2 to 2.5 μm was sequentially deposited by PVD at a substrate temperature of 350 °C. The CdS/CdTe samples were dipped in a saturated solution of CdCl2 in methanol, then annealed in air at 330 °C for 10 min, and etched in a standard NP solution (H3PO4:HNO3:H2O) to remove chloride and other residues. Colloidal aqueous suspensions of chemically synthesized 200-nm-diameter Au NPs at different concentrations ranging from 0.13 to 2 wt.% were directly coated onto the CdTe layers of the cells to obtain cells with different amounts of Au NPs. The CdTe cell without Au NPs is used as the reference cell. Then, a Cu layer (~10 Å), an ITO layer (~2000 Å), and a Au film (~2000 Å) were consecutively deposited by DC sputtering on the Au NP arrays and then annealed at 190 °C. Cu is required to form a heavily p-doped film on the backside of the CdTe film to reduce the Schottky barrier prior to back contact formation. Figure 1(a) illustrates the device structure of the CdTe solar cells using Au NPs.
The current density–voltage (J–V) characteristics were evaluated under AM1.5 illumination at 100 mW/cm2. EQE spectra were acquired using a measurement system including a monochromator and a xenon lamp. Optical absorbance spectra were recorded using a UV/visible spectrometer. The surface images of the CdTe layer with Au NPs were captured using a field-emission scanning electron microscope (SEM).
3. Results and discussion
Figure 1(a) shows the schematic of the CdTe solar cell structure. The devices were fabricated with different surface coverage of 200-nm-diameter Au NPs. Figure 1(b) presents an SEM image of the ~200-nm Au NPs deposited on the CdTe surface. Typically, a 0.5 wt.% Au NP suspension provided an average surface coverage of ~50%. Further, as shown in Fig. 1(b), the NPs appeared at regular intervals rather than existing as isolated particles or doublets/multiplets. This type of arrangement is suitable for our study because the interaction between adjacent NPs interferes somewhat with the LSPR of the isolated metallic NPs, as demonstrated in other studies . Figure 1(c) shows the light current density (J) versus the bias voltage (V) of the CdTe solar cells with different contents of Au NPs. Note that the processing conditions such as the treatment temperature and the annealing time of CdCl2, CdS, and Cu layers were optimized for the CdS/CdTe samples before the deposition of Au NPs. As the NP content increases, the short-circuit current density (Jsc) gradually increases from 22.1 to 24.8 mA, and the device performance gradually improves, as reported previously . This is due to the increase in light absorption from the overlapping of the plasmon resonance of the Au NP. The device treated with 0.5 wt.% Au NP suspension shows improved performance because of the significantly improved Jsc. The samples obtained with 1 and 2 wt.% Au NP suspensions, however, show inferior performance to that obtained with 0.5 wt.% Au NPs. The J–V curves shift diagonally and the fill factor (FF) hardly changes after the incorporation of Au NPs into the cells. In general, when the device performance of a solar cell is improved by charge transport, the FF also increases simultaneously, similar to other factors. However, the stationary FF indicates that the improvement in the device performance is possibly due to the resonant field enhancement caused by the scattering and absorption of light by CdTe combined with Au NPs . Therefore, we consider that, as the Au NP content increases, the amount of light absorbed by the CdTe device is increased because of light trapping through the interaction of light with the NPs.
To understand the effect of the enhanced photocurrent (short-circuit current) shown in Fig. 1(c), the EQE spectra of CdTe devices with varying amounts of Au NPs deposited using suspensions with different Au NP concentrations, 0 to 0.5 wt.%, are presented in Fig. 2(a). The spectral response of the solar cells was determined by measuring the photocurrent at each wavelength. The EQE was then calculated from the used illumination intensity as the fraction of incident photons that are converted to the electrical current . As the concentration of the Au NPs is increased, the EQE of the 0.5 wt.% NP sample is gradually improved by ~15% compared to that of the reference without Au NPs, with the corresponding Jsc being enhanced by 13% compared to that of the reference cell. We also observed good EQE for devices containing Au NPs at all studied wavelengths, and the EQE peaks began to dramatically decrease at a wavelength of 800 nm, near the band edge of CdTe. Particularly notable is the enhancement in the EQE in the spectral range of 550–600 nm, which is related to the LSPR of Au NPs shown in Fig. 3, for the devices with Au NPs. The significant EQE enhancement of the cells with NPs confirms the key role played by the Au NPs. Therefore, it is clear that this result is due to the enhanced resonant field caused by the stronger light absorption by the active layer, as shown in Fig. 1(c). To compare the optical properties of the CdTe samples incorporated with the plasmonic Au NPs, the normalized absorbance spectra of the CdTe surfaces were recorded (Fig. 2(b)). In these samples, Au NPs deposited using suspensions with different concentrations were scattered on the surface of the CdTe films. As the Au NP concentration is increased from 0 to 0.5 wt.%, the absorbance of the CdTe sample improves gradually. However, when the Au NP concentration is increased above 0.5 wt.%, the absorbance begins to decrease. As presented in Fig. 1(b), when a suspension with Au NP concentration higher than 0.5 wt.% is used, the scattering cross-sections (Qscat) of adjacent NPs deposited on the CdTe film are likely to overlap, and the interparticle interaction can negatively affect the localized oscillations of the isolated metallic NPs . Thus, the absorbance of devices incorporated with NPs operating in the LSPR mode is affected by the spacing between the metallic NPs, which differs according to the NP concentration. Therefore, the amount of Au NPs deposited on CdTe should be specially optimized.
As shown in Fig. 2 (b), the peaks are generally flat at most wavelengths. Further, due to the effect of SPR, the peak area increases because of the increased absorbance between 550 and 600 nm. This agrees well with the region of increased absorption in the 550 to 600 nm range shown in Fig. 3. In addition, the optical bandgap of the CdTe layer drops dramatically to ~780 nm. The LSPR wavelength (λspr) of the Au NPs with a diameter of 200 nm used for sample fabrication is normally located in the visible wavelength range (500–800 nm), which is the absorbance region of the CdTe solar cell. This condition is suitable for applying the light-trapping concept to the CdTe device, in which the light absorbance is especially diminished at longer wavelengths. Therefore, the incident light with λspr is mostly absorbed by the device owing to the interaction with Au NPs during its first pass through the CdTe cell, which enhances the optical path length at longer wavelengths .
Table 1 lists the main factors responsible for the enhancement of the performance of the CdTe solar cells incorporated with different amounts of Au NPs, as presented in Fig. 1(c). The conversion efficiency improved by ~14% from 7.9% (for a reference sample without Au NPs) to 9.0% (for a device treated with 0.5 wt.% Au NPs). In the case of the device fabricated with 0.5 wt.% Au NPs, Jsc increased to 24.8 mA/cm2 from 22.1 mA/cm2 for the reference sample. As shown in Fig. 2(b), the amount of light absorbed by the CdTe devices fabricated with 0 to 0.5 wt.% Au NPs increased compared to that of the reference sample, and the efficiencies of the devices improved proportionally.
In contrast, the degree of performance enhancement starts to decrease with an increase in the Au NP concentration above 0.5 wt.%, as shown in Fig. 2. From this result, it is reasonable to conclude that as the light of wavelength λ interacts with small (diameter d<<λ) metallic particles, LSPR excitation occurs because of the electron oscillations of the isolated metal particles, and the LSPR peak is observed as a distinct maximum peak in the absorbance spectrum of the Au NPs . The electromagnetic field associated with the LSPR mode is strongly confined to a very small region near the particle surface. According to the theoretical simulation by another group , the calculated spatial distribution of the electric field intensity of the Au NP/CdTe structure indicates significantly increased electric field amplitude, with interference patterns observed inside the CdTe active layer, indicating enhanced light scattering, which also contributes to the overall enhancement of the absorption. This is in good agreement with the experimental results presented in Figs. 1(c) and 2(a) and (b). To support the concept of the LSPR effect demonstrated experimentally, the normalized absorbance spectra of suspensions of Au NPs at various concentrations dropped on ITO/glass substrates are presented in Fig. 3. The scattering cross-section may vary depending on the underlying medium, but the absorption spectrum in Fig. 3 clearly shows the change in the absorption peaks with the concentration of Au NPs. Figure 3 shows that all optical spectra exhibit a distinctive absorption peak because of the LSPR effect. The apparent LSPR wavelength λspr, at the points marked by arrows in Fig. 3, is observed in the range of 550–600 nm in each spectrum. As mentioned above, the LSPR phenomenon is due to the collective oscillations of the conductive electrons confined to the metallic NP surface. The excitation of localized plasmons by the electromagnetic field produces strong light scattering and absorption when resonance occurs . Therefore, as the concentration of Au NPs is increased from 0 to 1.0 wt.%, the intensity of the peaks at the surface resonance wavelengths strongly increases. However, as shown in Fig. 2(b) and Table 1, when the concentration of Au NPs is increased to more than 0.5 wt.%, the performance parameters of the device including the efficiency, photocurrent, and light absorbance begin to decrease. This can be explained by considering that the overlapped Qscat due to the interaction between adjacent metal particles interferes with the LSPR effect. In contrast, as shown in Fig. 3, the λspr (LSPR wavelength) peaks show small wavelength shifts. According to the harmonic oscillator model of localized plasmonic excitations, the resonance frequency is proportional to the restoring force induced by the displaced electron gas on one side of the Au NP polarized by the incident light . As the amount of NPs increases, the amount of charge induced by the incident optical field increases, and the restoring force of the dense electron gas increases. Therefore, the increase in the Au NP concentration at regular intervals causes electromagnetic coupling and blue shift of the resonance wavelength, as shown in Fig. 3. In addition, we experimentally demonstrated that the enhancement of light absorption by the device is gradually strengthened by the LSPR phenomenon when the Au NP concentration is increased above 0.5 wt.%, as shown in Figs. 2(b) and 3. Note that our experimental results are in good agreement with the fact that the deposited Au NPs interact with the incident light. In addition, the observed blue shift of these resonant wavelengths is clearly reflected in the absorbance shown in Fig. 2(b). Figure 3 shows the increase in the optical absorption in the same region as that of the LSPR resonance band, suggesting that the improvement of the performance of actual devices is accompanied by progressive increase in Jsc and efficiency, as shown in Table 1.
We experimentally verified the enhancement of the electrical properties and improvement in the light absorption of a CdTe solar cell by the LSPR effect, by applying Au NPs to a CdTe solar cell with a thickness of less than 2.5 μm. Our results indicated ~14% improvement in the conversion efficiency and ~15% improvement in light absorption of the CdTe devices treated with 0.5 wt.% Au NP suspension in comparison with that of the reference cell (without Au NPs). Our results indicate that the experimental results are reasonable according to the increase in the concentration of Au NPs. This study demonstrates that the LSPR effect caused by the oscillations of metallic NPs is significantly affected by the intensity shift of the LSPR peaks. Meanwhile, when the Au NP concentration is increased to more than 0.5 wt.%, the performance of the devices gradually decreased because of the overlapping of the Qscat of adjacent particles that interact. In the future, we plan to maximize the LSPR effect for CdTe solar cells through a more detailed study of the concentration, size, shape, and spacing of metallic NPs. We expect that this study can serve as a reference for improving the performance of CdS/CdTe solar cells through the plasmonic effect.
National Research Foundation of Korea: Basic Science Research Programs (NRF-2017R1D1A1A02018517).
The authors declare that there are no conflicts of interest related to this article.
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