Transparent conductive oxides (TCOs) play a major role as the front electrodes of thin-film silicon (Si) solar cells, as they can provide optical scattering and hence improved photon absorption inside the devices. In this paper we report on the surface texturing of aluminium-doped zinc oxide (ZnO:Al or AZO) films for improved light trapping in thin-film Si solar cells. The AZO films are deposited onto soda-lime glass sheets via pulsed DC magnetron sputtering. Several promising AZO texturing methods are investigated using diluted hydrochloric (HCl) and hydrofluoric acid (HF), through a two-step etching process. The developed texturing procedure combines the advantages of the HCl-induced craters and the smaller and jagged—but laterally more uniform—features created by HF etching. In the two-step process, the second etching step further enhances the optical haze, while simultaneously improving the uniformity of the texture features created by the HCl etch. The resulting AZO films show large haze values of above 40%, good scattering into large angles, and a surface angle distribution that is centred at around 30°, which is known from the literature to provide efficient light trapping for thin-film Si solar cells.
Transparent conductive oxide (TCO) front electrodes are extensively utilised in thin-film silicon (Si) solar cells. Compared to wafer-based Si solar cells, thin-film Si solar cells have a very thin absorber layer (about 200–300 nm for amorphous Si cells and about 1–3
Commonly used TCO materials include fluorine-doped tin oxide (FTO), tin-doped indium oxide (ITO), and zinc oxide doped with group III impurities (e.g., Al, Ga, and B) [
For magnetron sputtered AZO films, the surface texture is usually obtained through a wet-chemical etching process (resulting in crater-like features) in weak or diluted acids [
Therefore, a two-step texturing procedure based on etching in HCl and HF is tested in this study. The idea behind the two-step process is to improve the lateral uniformity of the surface texture and also to further enhance the light scattering via HF etching. In order to achieve a homogeneously textured surface with enhanced light scattering capabilities, in this work we systemically investigate (i) single-step HCl or HF etching as the reference, (ii) two-step etching procedure using HF and then HCl acid, and (iii) two-step etching procedure using HCl and then HF acid for the surface texturisation of AZO films. For these experiments, pulsed DC sputtered AZO films with high transmission (
AZO films were deposited onto planar A3 size (30 cm × 40 cm) soda-lime glass sheets by an inline multichamber magnetron sputter machine (Model Line 540 from FHR Anlagenbau GmbH) using a pulsed DC power supply [
The deposition was carried out in dynamic mode (i.e., moving substrate). During deposition, the glass sheet was vertically attached (i.e., portrait format) on a moving carrier and allowed to oscillate 18 times in front of the AZO sputter cathodes, at a moving speed of 10 mm/s. This multiple-pass deposition potentially reduces the formation of pinholes and thus benefits the subsequent etching process [
All the acids used for the AZO texturisation experiments were diluted into aqueous solutions and are represented via their volume fraction (vol%). Although diluted HF solutions can still attack the glass substrates, the reaction proceeds at a very slow speed and barely affects the glass flatness [
The optical properties of AZO films were characterised with a double-beam UV/Vis/NIR spectrophotometer featuring a 150 mm diameter integrating sphere (PerkinElmer, LAMBDA 950), which records the reflectance and transmittance spectra (diffuse and total) from 300 to 1200 nm before and after texturing the AZO films. The diffuse transmittance was measured by opening the reflectance port at the rear of the integrating sphere and thereby letting the specular light escape from the integrating sphere. The haze value was calculated via
The AZO film thicknesses were obtained by curve fitting of the measured spectral transmittances, using the commercial optical simulation software CODE [
Statistical analysis of the surface inclination angle of textured AZO samples was carried out based on the AFM measurements and a MATLAB image processing programme [
Definition of the vectors and surface inclination angle used in the MATLAB image processing method.
For comparison, two sets of etching experiments were carried out in this work. In the first set, AZO samples were textured using 1% HF for 20 s (step 1) and then further etched using 0.5% HCl (step 2) from 0 to 40 s (noted as HF/HCl etching). For the second set, AZO films were textured using 0.5% HCl for 20 s (step 1) and then further etched using 1% HF (step 2) from 0 to 40 s (noted as HCl/HF etching).
These as-grown AZO films show a low sheet resistance of 3.8 Ω/□ with a thickness of around 1.8
Variation of visible transmission (
In contrast to the electrical performance, the optical properties are quite sensitive to the etching sequence. The values measured with the haze meter (see Figure
((a) and (c)) Optical transmittance (solid line) and absorbance (dash line), and ((b) and (d)) haze as a function of wavelength for two-step textured AZO by (left) HF/HCl etching and (right) HCl/HF etching. Arrows indicate the trends for increasing duration of the second etching step.
A different scenario is observed for the reversed two-step etching sequence (HCl/HF). In this case the spectral profiles show that, with increasing duration of the second etching step (HF), the AZO films gradually become more scattering. The HCl-etched AZO film shows a haze value of 22% at 600 nm wavelength, which further increases to 54% when the second step HF etching time increases to 40 s (see Figure
The SEM micrographs in Figures
SEM micrographs of AZO films textured with the two-step sequences (left = HF/HCl, right = HCl/HF). (Left) 1% HF etching for 20 s and then 0.5% HCl etching for (a) 0 s, (b) 10 s, (c) 20 s, (d) 30 s, and (e) 40 s. (Right) 0.5% HCl etching for 20 s and then 1% HF etching for (f) 0 s, (g) 10 s, (h) 20 s, (i) 30 s, and (j) 40 s. During the measurements the samples were tilted by 60° and the images were recorded with a magnification of 25000.
For the HCl/HF sequence the SEM micrographs show (see Figures
Figure
Measured AFM 2D images and MATLAB processed images of AZO films textured with ((a) and (e)) a single-step etch of HF for 20 s, ((b) and (f)) a single-step etch of HCl for 20 s, ((c) and (g)) a two-step etch of HF for 20 s and HCl for 20 s, and ((d) and (h)) a two-step etch of HCl for 20 s and HF for 20 s.
The feature height and angle distributions of the textured surface are then calculated from AFM and MATLAB simulations. Figures
((a) and (c)) Surface height and ((b) and (d)) angle distribution histograms derived from AFM measurements for two-step textured AZO films using (left) 1% HF etching for 20 s and then 0.5% HCl etching from 0 to 40 s, (right) 0.5% HCl etching for 20 s, and then 1% HF etching from 0 to 40 s.
Figures
In addition to the surface height and angle, the feature size also plays an essential role in the light scattering process. One of the most commonly used methods to describe the surface feature of textured AZO films is to use the power spectral density (PSD) function [
Figures
((a) and (c)) Power spectral density (PSD) function and ((b) and (d)) corresponding autocorrelation length and RMS roughness derived from AFM measurements for two-step textured AZO films using (left) 1% HF etching for 20 s and then 0.5% HCl etching from 0 to 40 s, (right) 0.5% HCl etching for 20 s, and then 1% HF etching from 0 to 40 s.
The PSD function can also provide an exact value to represent the surface morphology of the textured films, such as the root-mean-square (RMS) roughness (
Angular resolved scattering (ARS) defines the distribution of diffuse scattered light between 0° and 90°. The light intensity at an angle of 0° represents the specularly transmitted light (i.e., the nonscattered light). In this work, the ARS of the transmitted light for the textured AZO films was simulated based on the phase model by using experimental AFM images and the refractive indices of the two media (AZO/air interface) [
Angular resolved scattering (ARS) of transmission into air derived from AFM measurements for two-step textured AZO films using (a) 1% HF etching for 20 s and then 0.5% HCl etching from 0 to 40 s and (b) 0.5% HCl etching for 20 s and then 1% HF etching from 0 to 40 s.
A good correlation is observed between the calculated ARS results of Figure
In this paper, wet-chemical texturing of pulsed DC sputtered AZO films was investigated using several etching processes: (i) single-step etching in HCl or HF acid as the reference, (ii) two-step etching in HF acid and then HCl acid, and (iii) two-step etching in HCl acid and then HF acid. Although the texturing increases the sheet resistance value of the AZO films due to reduced film thickness, there is no much difference between these different etching methods.
Compared to standard HCl-etched AZO, it is advantageous to use two-step texturing to modify the surface morphology for better optical performance. The two-step textured AZO films, initially textured in HCl and then in HF (e.g., 20 or 30 s), which slightly modified the texture features, have stronger light scattering capabilities (haze values of above 40% and good ARS intensity at large scattering angles), while maintaining similar optical transmission (~70% transmission into air at 600 nm wavelength) and a good surface angle distribution (centred at around 30°). The two-step textured films also show better lateral texture uniformity than the standard HCl-etched films, owing to the HF etching step. Thus, it is expected that the two-step texturing method (etching in HCl acid and then HF acid) produces the best performing AZO films for thin-film Si solar cell applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). This research was supported by the NRF, Prime Minister’s Office, Singapore, under its Clean Energy Research Programme (CERP, Award no. NRF2011EWT-CERP001-019). Xia Yan acknowledges a Ph.D. scholarship jointly funded by SERIS and the NUS Graduate School for Integrative Sciences and Engineering (NGS).