1. Introduction

Thin-film silicon solar cells such as hydrogenated amorphous silicon (a-Si:H), hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated microcrystalline silicon (μc-Si:H) solar cells have attracted considerable interest^[1-3]. In 2013, a high initial efficiency of 16.1% for the triple-junction a-Si:H/a-SiGe:H/μc-Si:H tandem solar cell was obtained by LG Electronics^[3]. Light-trapping managements, including textured front transparent conductive oxides (TCO), textured highly reflective back reflectors and/or highly reflective intermediate layers, play an important role on enhancing the short-circuit current density ( $J_{\rm sc}$ ) as well as conversion efficiency of thin film solar cells^[4-7]. ZnO thin films have been extensively developed by researchers around the world^[8-14]. It has been reported that textured surface zinc oxide (ZnO) TCO films applied as front electrodes exhibited good transparency, low resistivity, and excellent light scattering properties^[12-14]. To further enhance the light-trapping effect of ZnO-TCO films, the haze values for evaluating optical scattering should be increased in the wide spectrum region. Many research results indicated that the haze values of glass/ZnO-TCO are strongly influenced by their surface morphology. Several methods have been attempted to modulate the surface morphology through controlling the feature size or grains size, grains shape of ZnO films and the surface feature of plasma-etched glass substrates^{[13, 15-18]}. Multiscale transparent electrode architecture for efficient light management was also proposed by the IMT research group^[18], which combined the versatility of nanoimprint lithography, high carrier mobility of hydrogenated indium oxide, and light scattering properties of self-textured boron-doped ZnO (ZnO:B). Recently, M Konagai's research group developed "W-textured" and mountain-chain-like ZnO:B films on plasma-etched glass substrates with a relatively higher haze value^[19-21], which presented great potential in solar cells application.

Magnetron sputtering and metal-organic chemical vapor deposition (MOCVD) are the two main growth methods to obtain rough textured surface ZnO-TCO films, which typically exhibit crater-like surface and pyramid-like surface^{[12, 13, 22, 23]}. For MOCVD, it shows many great advantages due to its low cost, low deposition temperature, and the fact that it can be applicable to a large deposition. From 2006, Oerlikon Solar used low pressure MOCVD to fabricate large-area textured surface ZnO:B layers for application in Si-based thin film solar cells.

In the present work, we combined the micro-texture glass with high haze through the low cost frosting and nano-texture ZnO:B films so as to obtain high performance TCO glass substrates for thin film solar cells. To the best of our knowledge, the development of textured MOCVD grown ZnO:B films on low-cost wet chemically etched glass (i.e. the frosting etching technique) for thin film solar cells is firstly presented in this paper. The optimized high haze etched glass/ZnO:B was preliminarily applied in a-Si:H thin film solar cells.

2. Experimental details

Textured surface superwhite glass substrates were obtained using wet-chemical etching methods. The super-white glass substrates were treated by the frosting etching technique with frosting powder (Tang's Glass Consumptive Material Company) which contains fluoride. Mixing frosting powder and HCl generates HF, which reacts with the glass and produces soluble ionic salts such as NaF and slightly soluble compounds (Na₂SiF₆, Ca₂SiF₆, BaSiF₆ etc.), as well as insoluble residues (CaF₂ etc.). These slightly soluble compounds and insoluble residues cover parts of the glass surface, thus providing localized protection from etching. The glass surface therefore exhibits a“U-shaped”micro-texture. The treatment round (R) for glass substrate etching was varied from 0 to 8 R Pyramid-like ZnO:B films were deposited by the MOCVD technique on the flat glass and chemically etched glass substrates, i.e. flat glass/ZnO:B, etched glass-3R/ZnO:B, etched glass-6 R/ZnO:B, and etched glass-8 R/ZnO:B. In the MOCVD-ZnO:B process, high purity diethylzinc (DEZn) and water (H₂O) were used as resource materials, and diborane (B₂H₆) was used as an n-type dopant gas^[22]. MOCVD also presents good reproducibility due to premise separate gas flow control and stable thin film growth process and thus it can produce high quality ZnO thin films with a large area of above one square meter. Gradual doping growth (GDG) was adopted so as to obtain relatively larger grains while maintaining an appropriate sheet resistance^[24]. The deposition temperature was set at about 150℃. We have measured all the MOCVD-grown ZnO:B films on flat glass and chemically etched glass substrates and each thickness of ZnO:B film was about 2140 nm. The crater-like feature size of chemically etched glasses ranged from 5 to 20μm while the grain size of MOCVD-ZnO:B film was about 300-800 nm and thus in this case the thickness of thin films cannot be influenced. The MOCVD technique has the advantages of good uniformity and coverage. Pin type a-Si:H single-junction solar cells with an area of 0.25 cm² were fabricated by a cluster plasma enhanced chemical vapor deposition (PECVD) system on flat glass/ZnO:B and etched glass/ZnO:B substrates. The structure of solar cells was glass/textured ZnO:B/p-a-SiO:H/buffer/i-a-Si:H/n-a-Si:H/ZnO:B/Ag/Al. The i-a-Si:H layer thickness was about 300 nm.

The thicknesses were evaluated with a step profilometer (Kosaka Lab-ET200). The surface morphology was measured via atomic force microscopy (AFM, Seiko SPA 400) operated in contact mode and field emission scanning electron microscopy (FE-SEM, Zeiss Supra-550p) using an accelerating voltage of 15 kV. Root-mean-square roughness (RMS) of layers was quantified from the AFM with a scanning area of 20×20μm². Carrier concentrations, sheet resistances and electron mobilities were characterized by Hall measurement (Accent HL5500 PC) using the van der Pauw configuration. The haze values of glass/ZnO:B samples were defined as the ratio of diffuse transmittance (DT) to total transmittance (TT) measured with a UV-VIS-NIR spectrophotometer (Varian, Cary 5000). The solar cell performances, including open voltage ( $V_{\rm oc}$ ), short-circuit current density ( $J_{\rm sc}$ ), fill factor (FF) and conversion efficiency (Eff.), were measured by current-density versus voltage (J-V) characteristics under an AM1.5 solar simulator at 298 K (Wacom Solar simulator-WXS-156S-L2) and quantum efficiency (QE) in the wavelength range from 300 nm to 800 nm.

3. Results and discussion

3.1 Structural properties

Figure 1 shows typical SEM images of chemically etched superwhite glasses with different treatment rounds. From Figure 1, it was found that the surface morphology of un-treated glass was very smooth and the chemically etched glasses presented a large crater-like structure. With increasing the treatment round from 3 R to 6 R, the crater-like feature size increased from 5-10 to 10-20μm. In this case, the etched glass presented a homogeneous geometric structure in the surface. With further increasing the treatment round to 8 R, the etched glass with a crater-like structure became nonuniform, exhibiting a wide feature size distribution which ranged from about 5 to 20μm, and the surface presented more shallow craters. The RMS roughness of the chemically etched superwhite glass with 3-R treatment (i.e. etched glass-3 R) was 202 nm. For the plasma-etched glass, with an increase in the power densities and pressure, the surface of glass changed from irregular small craters to relatively large craters with a lateral feature size from 200 to 6000 nm^[20]. From the above experimental results, one can see that the surface texture of glasses can be easily adjusted by chemical etching methods and the rough surfaces would increase the light-scattering. Compared with plasma etching, wet chemical etching has the advantages of low cost and easy processing for mass production. The surface of plasma etched glass is rather sharper^[19] than that of wet chemically etched glass, which shows a smooth crater-like structure.

Figures 2(a)-2(h) exhibit typical SEM images of ZnO:B films on chemically etched superwhite glasses with different treatment rounds. From Figures 2(a), 2(c), 2(e) and 2(g), it can be clearly observed that the surface morphology of glass/ZnO:B changed significantly from conventional pyramid-like single texture on flat glass to micro/nano double texture with a mixture of pyramid-like grains from ZnO:B and a large crater-like feature from chemically etched glass. Pyramid-like grains from ZnO:B can be clearly observed in the enlarged images from Figures 2(b), 2(d), 2(f) and 2(h) and the surface morphology of ZnO:B films changed little. The pyramid-like feature size of ZnO:B films was about 300-800 nm. Therefore, the crater-like feature from glass and pyramid-like grains from ZnO:B films was preserved in the etched glass/ZnO:B samples. Figures 3(a)-3(c) show typical AFM images of flat glass/ZnO:B and etched glass/ZnO:B samples. The RMS roughness increased evidently from 66 nm of flat glass/ZnO:B to 160 nm of etched glass/ZnO:B, and nearly three times improvement was achieved. More efficient light scattering at the TCO and p-Si-layer interface can be expected when the etched glass/ZnO:B with high RMS roughness and large feature size is applied to thin-film solar cells. On the one hand, ZnO-TCO front contacts may act as an efficient antireflection coating due to the refractive index grading at the ZnO:B/Si interface^[4]. On the other hand, rough surface increases multi-directional spreading of scattered light in the solar cell and thus the optical paths, leading to higher absorption^[25].

3.2 Electrical and optical properties

Table 1 gives electrical properties of ZnO:B films on chemically etched superwhite glasses with different treatment rounds. From Table 1, it was observed that the electrical properties of ZnO:B films changed little with increasing the glass treatment round. The sheet resistance, resistivity, carrier concentration, and electron mobility of ZnO:B films were approximately 17.1-19.2 $\Omega$ , 3.7-4.1×10^-3 $\Omega$ $\cdot$ cm, 3.4-3.6×10¹⁹ cm^-3, and 42.7-46.8 cm²/(V $\cdot$ s), respectively. The electron mean-free path, in the range of a few nanometers, is much smaller than the typical grain size. Grain boundary scattering will not influence the optical mobility $\mu_{\mathrm{optic}}$ and only intragrain ionized impurity scattering will influence it. Note that in the case of the Hall effect measurement, electrons cross several grain boundaries, and the grain boundary density will influence the Hall mobility $\mu_{\mathrm{Hall}}$ . In lightly doped ZnO:B films (like $n\approx$ 3.8×10¹⁹), the observed differences between optical and Hall mobility values as a function of grain size are due to a grain barrier limited mobility^[26]. For our measured ZnO:B samples with $n\approx$ (3.4-3.6)×10¹⁹, it was suggested that the grain boundary scattering effects are the predominant mechanism in the ZnO:B films. Additionally, when the treatment round (R) for glass substrate was varied from 0 R to 8 R, the electron mobility of ZnO:B films was slightly decreased, which leads to relatively higher resistivity and sheet resistances. Slightly higher sheet resistances may increase the series resistance of solar cells and thus influence the fill factor FF. However, double-junction and multi-junction Si-based thin film solar cells present relatively lower short-circuit current density $J_{\rm sc}$ ( 9-11 mA/cm²)^[27], which will reduce the requirement of electrical performances of TCO layers. From the above results, one can see that the glass-substrate treatment using the chemical etching method allows the modification of the surface properties of glass/ZnO:B while maintaining their good electrical properties.

Figure 4 shows typical haze value curves of ZnO:B films on chemically etched superwhite glasses with different treatment rounds. One can see that the etched glass/ZnO:B samples presented higher haze values than the flat glass/ZnO:B in the wavelength range of 400-1800 nm. Rough etched glass/ZnO:B samples increased the optical path through multiple refractions and reflections in the interface between glass and ZnO:B film and the craters region. With increasing the treatment round, the haze values of etched glass/ZnO:B decreased, especially in the long-wavelength region. The double micro/nano textures are helpful to improve the light-scattering in the short-wavelength and long-wavelength ranges. The haze values at 550 and 850 nm wavelength of the etched glass-3 R/ZnO:B were 61% and 42%, respectively. The above results revealed that the glass-etching round is a key parameter to control the light-scattering properties of the etched glass/ZnO:B samples.

3.3 Application in Si thin film solar cells

Flat glass/ZnO:B and etched glass-3 R/ZnO:B samples were applied in p-i-n type a-Si:H thin film solar cells. Figure 5(a) shows the $I$ - $V$ curves of a-Si:H thin film solar cells with the flat glass/ZnO:B and etched glass-3 R/ZnO:B substrates. For ZnO:B film deposited on the chemically etched glass, a conversion efficiency of 7.22% ( $V_{\rm oc}$ : 0.841 V, $J_{\rm sc}$ : 14.79 mA/cm² and FF: 0.581) was obtained. The efficiency was obviously higher than that of a solar cell with ZnO:B film deposited on the flat glass, 6.88% ( $V_{\rm oc}$ : 0.853 V, $J_{\rm sc}$ : 13.89 mA/cm² and FF: 0.581), which was mainly due to the improved $J_{\rm sc}$ . Figure 5(b) shows the QE curves of a-Si:H thin film solar cells with the flat glass/ZnO:B and etched glass-3 R/ZnO:B substrates. The a-Si:H thin film solar cell deposited on etched glass-3 R/ZnO:B exhibited a relatively better spectral response than the sample deposited on flat glass/ZnO:B. It is well known that the absorption coefficient of a-Si:H material is high for short wavelength light, it strongly decreases towards longer wavelengths as the light energy approaches the optical band gap of 1.75 eV. In addition, ZnO:B front electrodes mainly absorb the short wavelength light with photo energy above 3.37 eV. Therefore, the improvement of QE took place in the wavelength region of 380-750 nm and was mainly attributed to higher haze values. Current densities calculated by integrating the incident photon to current conversion efficiency (IPCE) spectra are 13.83 and 14.73 mA/cm² for solar cell devices with flat glass/ZnO:B and etched glass-3 R/ZnO:B textured glass, respectively. These obtained values are almost equivalent to those measured $J_{\rm sc}$ dada in $I$ - $V$ curves. Furthermore, glass/ZnO:B-TCO layers should have appropriate surface roughness and morphology to achieve good light scattering and subsequent growth of Si thin films when fabricating p-i-n type Si-based thin film solar cells. Too high surface roughness will induce voids and/or cracks in the Si-based thin film solar cells and thus decrease the device performances.

4. Conclusions

The super-white glasses with crater-like feature size (5-20μm) can be obtained through the chemical etching treatment method. High haze textured surface etched-glass/ZnO:B with double micro/nano textures have been successfully achieved. The etched glass/ZnO:B samples presented a much stronger light-scattering capability than the conventional flat glass/ZnO:B and they showed equivalent electrical properties with flat glass/ZnO:B. Typical etched glass-3 R/ZnO:B exhibited high RMS roughness of 160 nm. The haze values at 550 and 850 nm wavelength of etched glass-3 R/ZnO:B sample were 61% and 42% respectively. The a-Si:H solar cell with the etched glass/ZnO:B substrate presented a relatively better spectral response and thus increased the conversion efficiency.