Textured Substrate for High-Efficiency n-i-p μc-Si:H Solar Cells Guangtao Yang, René A. C. M. M. van Swaaij, Sergiy Dobrovolskiy and Miro Zeman

Delft University of Technology, PVMD-DIMES, P. O. Box 5031, 2600 GA Delft, the Netherlands

Abstract

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Index Terms

I. INTRODUCTION

Thin film silicon solar cells made of hydrogenated microcrystalline silicon (μc-Si:H) have lower fabrication cost than crystalline silicon based solar cells as their fabrication needs less energy and materials. The typical absorber layer thickness is less than 2 μm. At short wavelengths the absorption of μc-Si:H is large enough to absorb all the light. However, for long wavelengths not all light is absorbed. Effective light management for high-efficiency thin-film silicon solar cells is therefore an essential part for increasing the light absorption in the wavelength range from 600 nm to 1100 nm to enhance the current.

Many authors have reported methods to increase the effective absorption: reflective back contacts [1], photonic crystals [2], plasmonic nanoparticles [3] or dielectric gratings [4]. The aim is to decrease the cell reflectivity and increase the optical path length by deflecting incident photons into the

active silicon layer. Using a textured TCO layer is the most common way to achieve enhanced absorption. There are several techniques to obtain a textured TCO surface: natural growth of ZnO:B layer using LPCVD [5], laser texturing [6], or wet etching of flat sputtered ZnO:Al (AZO) [7]. However, a large feature size of a textured substrate induces a high micro-void density in the μc-Si:H intrinsic layer, which will deteriorate the solar-cell performance, especially the open-circuit voltage (Voc) and fill factor (FF) [8,9]. By carefully controlling the features of the substrate we can obtain a moderate textured surface with high haze factor without reduction of the Voc and FF [10].

In this paper, we present results of the effect of Ag deposition on highly textured AZO. This textured AZO is obtained by wet etching and has a broad distribution of feature sizes for effective light trapping in the long wavelength range. We will show that the evaporation of Ag changes the angle distribution of the back reflector. This combination of layers leads to a very broad angular intensity distribution (AID), which results in an increased short-circuit current density, Jsc, of n-i-p c-Si:H solar cells. Experimental results demonstrate that the developed back reflector (BR) increases short-circuit current density (Jsc) without reducing Voc and FF.

II. EXPERIMENTAL

The AZO films are deposited on Corning Eagle XG glass using a magnetron sputtering system with 2 wt.% Al2O3 doped ceramic ZnO target. After wet etching with 0.5% HCl for different times, atomic force microscopy (AFM) is used to scan the features. Then 300-nm thick Ag is evaporated on top of the etched AZO at a rate of 1 nm/s. Before μc-Si:H deposition, an 80-nm thick AZO is sputtered at room temperature to cover the Ag layer. On this back contact we deposit μc-Si:H single junction n-i-p solar cells with rf-PECVD in order to study the effect of the AZO morphology on the light trapping and the solar-cell performance. The solar-cell structure is as follows: 25-nm thick n-type a-Si:H layer, 1500-nm thick intrinsic μc-Si:H layer, and 15-nm thick p-type μc-Si:H layer. Then 80-nm thick ITO is sputtered as front TCO layer with an area of 0.16 cm2.

The surface morphology is studied by AFM. The optical properties of the Ag coated textured AZO BR (i.e., total reflection, haze in reflection, and AID of reflected light) are measured using an integrating sphere and the Angular Resolved Transmittance / Reflectance Analyzer in PerkinElmer Lambda 950 spectrophotometer. Experimental details of the measurements can be found elsewhere [11,12].

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III. MORPHOLOGY AND OPTICAL PROPERTIES OF BR

A. Influence substrate temperature on AZO roughness

The substrate temperature is a key parameter for controlling the quality of sputtered AZO and influences the effect of the wet etch [7]. We have varied the temperature at which the layers were deposited. All AZO layers have the same thickness of 1300 nm after deposition and an as-deposited rms roughness of ~ 2 nm. The angle distributions of textured AZO obtained from AFM scans, following the wet etch for an optimized time at which the rms roughness has stabilized, are shown in Fig. 1. The textured AZO sputtered at low substrate temperature (180 °C) has a broad angle distribution with the main peak at 25.5° and an average angle of 19.5°. For textured AZO sputtered at 300 °C there is one peak for the surface angle distribution at 22.0° and average angle of 18.6 °. The textured AZO sputtered at 250 °C gives the highest peak position and broadest value for surface angle distribution. It has a main peak at 32.2° and an average angle of 27.2°. Moreover, this AZO has a wide tail for shallower angles. A broader surface angle distribution indicates broader surface feature sizes, which is essential for scattering a wider wavelength range.

B. Etching time

With increasing AZO etching time we can obtain larger feature sizes. In Fig. 2(a) and 2(b) two examples are shown of textured AZO sputtered at a substrate temperature of 250 °C and etched for 10 s and 45 s, respectively. We can see that after 10-s etching small valleys appear, which are considered to be initial etching points. After 45-s etching the AZO

Fig. 1. Statistical fraction of the surface exhibiting a given angle quantized into one degree increments, which is normalized to the maximum point.

Fig. 3. Statistical fraction of the surface exhibiting a given angle quantized into one degree increments, which is normalized to the maximum point for textured AZO sputtered at 250 °C.

Fig. 2. AFM scans of AZO sputtered at a substrate temperature of 250 °C and after wet etching for (a) 10 s (with scale of 10 m); (b) 45 s (with scale of 5 m); (c) and 300 nm Ag covered wet etched AZO for 45 s (with scale of 5 m). (d) The 1-D scans of AFM for all the three BR.

surface becomes very rough. This is also shown in Fig. 2(d), the 1-D scan of textured AZO after 10 s and 45 s etching. With increasing etching time the angle distribution of the textured surface becomes larger and broader, as is shown in Fig. 3.

C. Ag coverage

To form a back reflector, we evaporate 300 nm Ag on top of the 45 s texture-etched AZO surface. The AFM image is shown in Fig. 2(c) and the 1-D scan is shown in Fig. 2(d). We can see that after the evaporation of Ag we obtain small features superimposed on the large features of the etched AZO. This Ag coverage broadens the angle distribution of the textured AZO surface, as can be seen in Fig. 3.

D. Optical properties

In order to investigate the optical properties of the back reflector, we have measured the total reflectance and the haze in reflection of the textured AZO etched for 10 s and 45 s, and subsequently covered with Ag. As shown in Fig. 4(a) the back reflector with the optimized texture etched AZO has a very high total reflection of more than 90% in the long wavelength range and high reflective haze in the whole measured range. For the 10-s etched AZO the total reflectance is only around 80%, which we ascribe to plasmonic absorption of the small features after Ag deposition in the wavelength region above 500 nm. The reflective haze is also lower, due to the lower rms roughness of the AZO.

imes this maximum shifts to higher angles and peaks at 45° for the BR using the 45 s etched AZO. For each BR, the shape of the AIDs are nearly the same when varying the incident wavelength of the light from 400 nm to 800 nm (not shown here).

E. Solar-cell performance

With these BRs we made n-i-p c-Si:H solar cells with rf-PECVD in order to investigate the effect of the different BRs

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Fig. 5. The influence of the AZO etching time on the Voc and FF of solar cells with 1200 nm thick intrinsic layer deposited with VHF-PECVD and p-type and n-type μc-SiOx:H as doped layers.

on the solar-cell performance. The EQE curves in Fig. 4(c) clearly show that with longer etching time the cell has a higher EQE response in the long wavelength range, due to more light scattering into larger angles. The solar cell using the BR with 45 s etching gives the highest Jsc of 25.2 mA/cm2 and the highest efficiency of 7.68%. Although the Voc’s of our rf-PECVD cells are somewhat low, we did not see a reduction of the Voc and FF of solar cells with increasing rms roughness.

In order to study the effect of our BR on the electrical properties of the solar cells we fabricated n-i-p c-Si:H solar cells with VHF-PECVD on similar BRs as used for the rf-PECVD cells. This time the i-layer thickness is 1200 nm, and p-type and n-type μc-SiOx:H were used. The influence of the AZO etching time on the Voc and FF of the solar cells is shown in Fig. 5, and the corresponding EQE curves are shown in Fig. 6. We can see that although the spectral response and light absorption improves for a BR with very large features, the FF and Voc do not decrease. The best solar cell has an efficiency of 8.9%, with Voc = 0.500V, FF = 0.733 and Jsc = 24.4 mA/cm2. We ascribe this absence of a drop with roughness to the fact that the large features of the AZO become smoothened when covered with a thick Ag layer.

IV. CONCLUSION

In this paper, we use Ag covered wet etched AZO as back reflector for n-i-p c-Si:H solar cells. We studied the effect of sputtering temperature, etching time and Ag coverage on the morphology and optical properties of back reflector. We have shown that after Ag deposition small features superimposed on the roughness of the AZO are present on the BR. This BR has high reflectance and haze. And with increasing the AZO etching time the reflective light distributed on the large angles is increased. With this BR improved light scattering is

obtained for c-Si:H solar cells. The Voc and FF of these solar cells is not affected.

V. ACKNOWLEDGEMENT

The authors thank China Scholarship Council for the financial support, and Martijn Tijssen and Stefaan Heirman for their technical support.

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Fig. 6. The influence of the AZO etching time to EQE of solar cells with 1200 nm intrinsic layer deposited with VHF-PECVD and p- and n-type μc-SiOx:H as doped layers.