1. Introduction

Thin film solar cells are a promising renewable energy source that can solve the energy shortage issue that our society will face in the near future because they have potential to reduce the cost of electricity by reducing the material costs [1–5]. However, a large fraction of sunlight is not converted into solar electricity in thin film solar cells because of incomplete light absorption originating from the thin absorption layer and the poor light absorption near the band gap of the photoactive material [1–6]. Hence, light trapping is crucial in thin film solar cells for enhancing light absorption by increasing the optical path length of incident sunlight and thereby improving the cell efficiency. Surface texturing is a common light trapping strategy for enhancing light absorption in solar cells [4–8]. However, surface texturing is difficult to realize in thin film solar cells and can lead to surface recombination losses associated with an increase in surface area and degrade the material quality, which results in deterioration of the cell efficiency [2–4]. Over the past decade, metallic nanoparticles (NPs), which provide light scattering over a broad angular range and the excitation of localized surface plasmon resonance, have attracted great attention for enhancing light absorption in thin film solar cells [1–4]. In plasmonic solar cells, metallic NPs are typically placed on the front surface of solar cell. However, this approach can lead to a decline in light absorption due to destructive interference between incident and scattered light below the plasmon resonance wavelength of the particles [1,3,4]. Recently, to avoid this problem, thin film solar cells with metallic NPs on the rear surface have been reported [3,9–11]. Unfortunately, there are still noneconomic barriers such as requirement of sophisticated equipment for metal evaporation and annealing as well as long process to realize cost-effective high-efficiency plasmonic thin film solar cells [9–11].

In this work, we focused on simplifying fabrication of metallic NPs on the rear surface of thin film solar cells. To achieve this goal, we fabricated silver (Ag) NPs on the rear surface of thin film hydrogenated amorphous silicon (a-Si:H) solar cells using spin-coating Ag ink, which can produce Ag NPs by a simple, fast, and inexpensive method without any sophisticated equipment [12,13]. We also aimed to optimize the process conditions to maximize the light absorption capability, which is closely related to the efficiency of solar cells, of thin film solar cells. Hence, optical properties of the solar cell depending on process conditions, including Ag ink concentration and sintering temperature of the as-coated Ag ink layer, were systematically investigated. A SiN_x spacer layer was placed between the thin film solar cell and the Ag NPs for electrical isolation [2], and the thickness dependent optical properties were also investigated.

2. Experimental details

Figure 1 illustrates the thin film a-Si:H solar cells with Ag NPs on the rear surface. The p-i-n structured thin film a-Si:H solar cells were separately deposited by turns on a transparent conductive oxide (SnO₂:F) covered glass substrate using a multi-chamber plasma-enhanced chemical vapor deposition (PECVD) system at 200°C. The film structure consisted of 20 nm-thick boron-doped p-type a-Si:H, 200 nm-thick intrinsic a-Si:H, and 20 nm-thick phosphorus-doped n-type a-Si:H. It is known that the light scattering and coupling efficiency increase when the metallic NPs lie as close as possible to the active region of the solar cell [9,10]. Hence, the Ag ink, which was composed of soluble Ag clusters that included Ag atoms of 10% wt., was directly spin-coated onto the rear surface of the solar cells and a subsequent sintering process was carried out on a hotplate for 5 min to transform the as-coated Ag ink layer into Ag NPs, as shown in the inset of Fig. 1. It is noteworthy that the fabrication process to produce Ag NPs on thin film solar cells was carried out through a simple process that took a short time without any complicated equipment. Thus, with this method, productivity can be increased and the fabrication cost can be considerably reduced. Details of the spin-coating Ag ink and an explanation of the sintering process can be found in the literature [12,13]. In this study, the Ag ink concentration (25%, 35%, and 50%) in a mixed solution of isopropanol and Ag ink and the sintering temperature of the as-coated Ag ink layer (150°C and 200°C) were carefully adjusted to maximize the light absorption because these conditions change the size, distribution, and surface coverage of the Ag NPs, which ultimately influence the light trapping efficiency [4].

 

Fig. 1 Schematic of the thin film a-Si:H solar cell with rear localized Ag NPs formed by spin-coating Ag ink without SiN_x spacer layer. The inset on the right shows the fabrication procedure to form Ag NPs by using spin-coating Ag ink and subsequent sintering process.

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After optimizing the process conditions to maximize the light absorption in the solar cell, a SiN_x spacer layer was inserted between the solar cell and the Ag NPs in order to isolate the solar cell and the Ag NPs and avoid additional surface recombination losses due to the presence of the Ag NPs [2]. The SiN_x spacer layer was deposited on the rear surface of the solar cell using PECVD (Plasmalab 80 Plus, Oxford) at 200°C before placing the Ag NPs. The thickness of the SiN_x spacer layer was varied from 5 nm to 40 nm to find the appropriate thickness of an SiN_x spacer and investigate the effect of its thickness variation on the light absorption of the thin film a-Si:H solar cell with rear localized Ag NPs. Hemispherical reflectance and transmittance of the thin film a-Si:H solar cells having Ag NPs integrated at the rear side with and without a SiN_x spacer layer were measured using a UV–Vis-NIR spectrophotometer (Cary 500, Varian) with an integrating sphere at the near-normal incident angle of 8° in a wavelength range of 300-1200 nm and were used to derive the absorption spectra.

3. Results and discussion

Figures 2(a)-2(b) show the top-view field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi) images of the Ag NPs directly fabricated onto the rear surface of the solar cell and the surface coverage estimated using a public image processing program (ImageJ 1.42q, NIH), respectively, corresponding to the Ag ink concentration and the sintering temperature. It is clear that the size, distribution, and surface coverage of the Ag NPs are closely correlated with the Ag ink concentration and the sintering temperature. In Fig. 2(a), the Ag NPs formed at a sintering temperature of 200°C are isolated due to the sufficient surface energy of Ag for dewetting [13,14]. The Ag NPs formed at a sintering temperature of 150°C using 35% and 50% Ag ink concentrations are elongated owing to a thick Ag ink layer and insufficient surface energy of Ag for dewetting. It is also observed that the size and interspacing of the Ag NPs increase as the Ag ink concentration and the sintering temperature increase. As can be seen in Fig. 2(b), the surface coverage increases with a decreasing sintering temperature and an increasing Ag ink concentration.

 

Fig. 2 (a) Top-view SEM images of the Ag NPs formed directly on the rear surface of thin film a-Si:H solar cell and (b) surface coverage of the Ag NPs corresponding to various Ag ink concentrations and sintering temperatures.

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Figures 3(a)-3(b) show the measured hemispherical transmittance and reflectance spectra of the thin film a-Si:H solar cells with rear localized Ag NPs, respectively, corresponding to various Ag ink concentrations and sintering temperatures. The hemispherical transmittance and reflectance spectrum of bare solar cell are also shown as a reference. In Fig. 3(a), the transmittance of the bare solar cell begins to appreciably increase for wavelengths above ~500 nm. This means that the photons above ~500 nm were not fully absorbed by the solar cell in an optical path length. The transmittance of the solar cells with Ag NPs was reduced compared to that of the bare solar cell. This is because the portion of the light that reached the rear surface of the solar cell was scattered by the Ag NPs and absorbed by the solar cell. The transmittance decreased as the Ag ink concentration increased and further decreased as the sintering temperature decreased predominantly due to the increase of surface coverage, as can be seen in Figs. 2(a)-2(b). On average, the reflectance for samples sintered at 150°C was lower than that of the samples sintered at 200°C. This can be attributed to the further elongated optical path length, resulting in an increase of the light absorption (reduction of the reflection), due to the increased surface coverage (density of the Ag NPs) as the sintering temperature decreased. Among three different solar cells with the Ag NPs sintered at 150°C, the solar cell with the Ag NPs formed using an Ag ink concentration of 35% showed the lowest average reflectance of 11.6%.

 

Fig. 3 (a) Hemispherical transmittance and (b) reflectance spectra of the thin film a-Si:H solar cells with rear localized Ag NPs corresponding to various Ag ink concentrations and the sintering temperatures.

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Figure 4(a) shows the absorption spectra of the thin film a-Si:H solar cells with rear localized Ag NPs corresponding to the Ag ink concentration and the sintering temperature. The absorption spectrum was calculated by using the measured transmittance and reflectance (absorption = 100% - transmittance - reflectance) [11]. The solar cells with Ag NPs clearly had higher absorption for wavelengths beyond ~500 nm than the bare solar cell because of the enhanced light trapping by the Ag NPs. The solar cells with the Ag NPs sintered at 150°C had higher absorption than the one with the Ag NPs sintered at 200°C due to their lower transmittance and reflectance, as can be seen in Figs. 3(a)-3(b). Among three different solar cells with the Ag NPs sintered at 150°C, the solar cell with the Ag NPs formed using an Ag ink concentration of 35% showed the best light absorption capability with an average absorption of 77.0%, which is 1.37 times larger compared to the bare solar cell (56.3%). Therefore, we consider that an Ag ink concentration of 35% and sintering temperature of 150°C are the optimal conditions for enhancing the light absorption of the thin film a-Si:H solar cell. The inset shows the calculated average absorption of the solar cells with Ag NPs corresponding to the Ag ink concentration and the sintering temperature. This result clearly reveals that the light absorption of thin film solar cell as well as the efficiency can be enhanced by integrating the spin-coated Ag NPs on the rear surface and can further be improved by tuning the size, distribution, and surface coverage of the Ag NPs (i.e., optimizing the process conditions). Figure 4(b) shows the absorption enhancement, which is the ratio of the absorption of the solar cell with Ag NPs to that of the bare solar cell, corresponding to the Ag ink concentration and the sintering temperature. The solar cells with Ag NPs exhibit a light absorption enhancement greater than 1 for wavelengths beyond ~500 nm. In particular, the As mentioned, a dielectric spacer layer is necessary to electrically isolate the solar cell and the metallic NPs to eliminate additional surface recombination losses [2]. However, spacing between the metallic NPs and the underneath substrate influences the coupling of the localized surface plasmons to the underneath substrate [9,10]. Hence, the influence of the SiN_x spacer thickness on the optical properties of the solar cells with rear localized Ag NPs was investigated. Figures 5(a)-5(b) show the measured hemispherical transmittance and reflectance spectra of the thin film a-Si:H solar cells with rear localized Ag NPs formed using the optimum process conditions for different SiN_x spacer thickensses. The solar cell without a SiN_x spacer layer is included for comparison. The solar cells with the SiN_x spacer show slightly decreased transmittance at the wavelengths above ~840 nm predominantly because of the increased reflectance, as can be seen in Fig. 5(b). This result indicates that much light was reflected from the interface between the solar cell and the SiN_x spacer before it reached the Ag NPs, which can elongate the optical path length and thereby increase the light absorption (decrease the reflection), due to their large refractive index difference (n_a-Si ~4.05 and n_SiNx ~1.99 at 840 nm). In Fig. 5(b), the reflectance resonance (~800 nm) peak is blue shifted (~700 nm) by inserting the SiN_x spacer [2]. It is also observed that the reflectance increased with an increase in the thickness of the SiN_x spacer [2,9]. The increase of reflectance was due to the decreased coupling of the localized surface plasmons to the underneath solar cell with an increased spacing between the solar cell and the Ag NPs [9,10]. Figure 5(c) shows the absorption spectra of the thin film a-Si:H solar cells with Ag NPs and a SiN_x spacer layer for various spacer thicknesses. The solar cells with the SiN_x spacer show enhanced absorption for wavelengths above ~500 nm compared to the bare solar cell but partially decreased absorption compared to the one without a SiN_x spacer because of increased reflectance. As the SiN_x spacer thickness increased from 0 nm to 40 nm, the average absorption gradually decreased from 77.0% to 74.5%, as can be seen in Fig. 5(d). This result is in agreement with previously reported results [9,11]. It is notable that only 0.2% of average absorption was reduced by placing a 5 nm-thick SiN_x spacer layer (i.e., the most appropriate thickness of the SiN_x spacer). This means that cost-effective high-efficiency thin film solar cells can be realized by integrating a very thin SiN_x spacer layer and spin-coated Ag NPs onto the rear surface of the solar cell.

 

Fig. 4 (a) Absorption spectra and (b) absorption enhancement of the thin film a-Si:H solar cells with rear localized Ag NPs corresponding to various Ag ink concentrations and sintering temperatures. The inset in Fig. 4(a) shows average absorption in the wavelength range of 300-1200 nm.

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Fig. 5 (a) Hemispherical transmittance, (b) reflectance, and (c) absorption spectra of the thin film a-Si:H solar cells with SiN_x spacer layer and rear localized Ag NPs corresponding to various thicknesses of the spacer layer. (d) Average absorption of the solar cells as a function of SiN_x spacer layer thickness. Schematic of the thin film a-Si:H solar cell with Ag NPs on SiN_x spacer is shown in the inset of Fig. 5(d).

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4. Conclusion

We employed spin-coated Ag NPs on the rear surface of thin film a-Si:H solar cells to improve the light absorption of the solar cell by a simple, fast, and inexpensive method. Our study demonstrated that light absorption of the solar cells with Ag NPs were correlated with the size and distribution as well as the surface coverage of the Ag NPs, which were varied by adjusting the Ag ink concentration and the sintering temperature. The highest average absorption of 77.0%, which is much larger than the bare solar cell (56.3%) in the wavelength range of 300-1200 nm, was achieved by directly integrating the Ag NPs using an Ag ink concentration of 35% and a sintering temperature of 150°C. We also confirmed that the thickness of a SiN_x spacer layer, which was placed between the solar cell and the Ag NPs for electric isolation, affects the light absorption capability of the solar cell. Although the highest average absorption was slightly reduced by 0.2% by integrating a thin SiN_x spacer layer of 5 nm, the thin film solar cell with a SiN_x spacer layer and spin-coated Ag NPs on the rear surface of the solar cell shows great potential for realizing cost-effective high-efficiency solar cells.