Transparent double-period electrode with effective light management for thin film solar cells

Abstract

Aiming to significantly enhance solar cell efficiency through light management, we designed and fabricated transparent electrodes with a double-period structure. After demonstrating a significant increase of quantum efficiency in short wavelengths with a small-period self-textured ZnO : B, and in long wavelengths with large-period texture-etched ZnO : Al, respectively, we made the double-period transparent solar cell electrode by integrating self-textured ZnO : B and texture-etched ZnO : Al on μc-Si : H solar cells, using conventional physical and chemical vapor deposition techniques. Our investigation indicates that the double-period electrode can play an important role towards improving light absorption and trapping, thus effectively increasing the efficiency and reducing the cost of electricity from the solar cells.

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Light management is an effective method to improve the conversion efficiency of thin film solar cells.^1–11 The front electrode in solar cells is expected to efficiently couple light into the absorber layer and enable effective light trapping.^12–16

To achieve extended absorption of the solar spectrum, light trapping is required over a broad spectral range. Light trapping allows one to use a thinner absorber layer due to the enhancement of the effective light path,^17,18 and schemes of light trapping based on nanomaterials, photonic crystals, and gratings have been attempted.^19–30 While there are many reports of the enhancement of current density due to scattering-mediated light trapping, the optimal surface morphology for optimizing light trapping needs a lot more work. Here, we focus on the transparent front electrode of thin film solar cells, which acts not only to extract carriers without significant resistive losses, but also to couple light into the absorber layer and enable effective light trapping.^{12–16,31,32} For the μc-Si : H solar cells, the transparent front electrode is traditionally based on employing either the natively pyramidal texture of ZnO : B prepared by low-pressure chemical vapor deposition (LP-CVD),^33,34 or the craterlike texture of sputtered ZnO : Al³⁵ or ZnO : Ga³⁶ obtained after wet-etching in hydrochloric acid solution. Doped zinc oxide (ZnO) is currently a key transparent conducting material for thin film photovoltaic devices, due to a combination of its transparency and electrical property, as well as its abundance, non-toxicity and low cost.^37,38

On the platform of μc-Si : H solar cells, we fabricated self-textured ZnO : B through LP-CVD growth, and texture-etched ZnO : Al through a combination of sputtering and wet-etch processes. The AFM images of Fig. 1a and b demonstrate that the self-textured ZnO : B layer has a pyramidal texture and the texture-etched ZnO : Al layer has a craterlike texture. Here, we assume that the randomly textured surface can be approximated by a quasi-periodic surface structure. The studies of Haase et al. showed that this assumption is valid.³⁹Fig. 1c shows the EQE enhancement at low wavelengths (λ < 600 nm) of the cell deposited on a small-period self-textured ZnO : B substrate which can improve coupling of the short wavelength light due to its approximate periodic surface structure.³⁹ The above results are in a good agreement with the theoretical simulation by R. Dewan et al. using the Finite Difference Time Domain (FDTD) approach.⁴⁰ We conclude that the smaller texture features on the substrate are beneficial for increasing the short wavelength spectral response by effective optical coupling. Presumably arising from an increased optical path length due to multi-scattering, the texture-etched ZnO : Al device with large-period texture exhibits distinctly enhanced EQE at long wavelengths (λ > 600 nm), as compared to self-textured small period ZnO : B devices.

Fig. 1 Atomic force microscope (AFM) images of the single-period (SP) front electrode and external quantum efficiencies (EQEs) of the devices. (a) and (b) are AFM images of the Type A and Type B substrate. (c) EQEs for the self-textured boron doped zinc oxide (ZnO : B, Type A) film and texture-etched aluminum doped zinc oxide (ZnO : Al, Type B) film in the wavelength range 400–1100 nm.

In order to obtain a stronger enhancement of broadband optical absorption, we designed and fabricated a transparent front electrode with integrated small- and large-period structures. This innovative transparent front electrode combines a large micro-scale ‘triangular groove’ structure as the bottom layer and a small nano-scale ‘triangular protrusion’ structure as the top layer, as schematically shown in Fig. 2a. For the top layer, intrinsic or doped ZnO about 300–600 nm in thickness was deposited by LP-CVD, which is convenient for modifying the periodicity through controlling the thickness.⁴¹ In the case of the bottom layer, aluminum-doped ZnO, around 1500 nm in thickness, was obtained using sputtering followed by wet-etching in a hydrochloric acid solution (0.5%). The large period for sputtered aluminum-doped ZnO can be actively controlled through deposition conditions and the wet-etching time.⁴² Atomic force microscope (AFM) images and the surface line profiles of these two types of substrate are also shown in Fig. 2. Fig. 2b and d demonstrate self-textured intrinsic ZnO with pyramidal texture, and texture-etched ZnO : Al with craterlike texture. Despite different surface morphologies, the surface line profile of the substrates exhibits an almost periodic arrangement with ‘triangular’ profiles (shown in Fig. 2c and e). Self-textured intrinsic ZnO shows the period of around 200 nm and texture-etched ZnO : Al of about 2000 nm.

Fig. 2 Atomic force microscope (AFM), scanning electron micrograph (SEM) images and surface line profiles of the single-period (SP) and double-period (DP) front electrode. (a) Schematic drawing of a cross section of the DP front electrode (L with large-scale surface morphology, S with small-scale surface morphology and G represents glass substrate). (b) AFM images of the intrinsic ZnO growth via LP-CVD. (c) Surface line profile of the same textured substrate, showing small period nano-scale ‘triangular protrusions’. (d) AFM images of the craterlike texture of sputtered ZnO : Al obtained after wet-etching in hydrochloric acid solution (0.5%). (e) Surface line profile of the same textured substrate, showing the ‘triangular grooves’ that are formed due to etching by a hydrochloric acid (HCl) solution. (f) Cross-sectional view of the DP transparent front electrode on the glass substrate. (g) and (h) AFM images of the DP front electrode. (i) Surface line profile of the same textured substrate, showing large period micro-scale ‘triangular grooves’ as the bottom layer and small nano-scale ‘triangular protrusions’ as the top interface layer.

The cross-sectional view of the DP transparent front electrode on a glass substrate is shown in Fig. 2f. The substrate was eagle XG glass with a thickness of 0.7 mm. The textured glass surface with a large-period triangular type structure was created by sputtering aluminium-doped zinc oxide (ZnO : Al) followed by wet-etching in HCl (0.5%); the size of the textured glass surface was around 2–6 μm. The textured glass substrate was then deposited with intrinsic ZnO into small-period triangular type structures by LP-CVD. Dense small-period intrinsic ZnO was grown on the top of the large-period textured glass surface. The intrinsic ZnO was perpendicular to the textured glass substrate, due to their anisotropic growth along the c-axis. The morphology of the intrinsic ZnO can be controlled by modifying growth parameters,⁴³ and its period and thickness were measured from the cross-section profile. As seen from Fig. 2f, the intrinsic ZnO has a period of approximately 200 nm and thickness of 550 nm.

AFM images and the surface line profile of the front electrode are shown in Fig. 2g-i. Surface texturing of the film leads to a surface morphology that combined large-period and small-period ‘triangular type’ structures. A surface line profile of the front electrode exhibits an approximate periodic arrangement with a large period of around 3000 nm and a top surface covered with a small period of around 300 nm. The difference between SEM and AFM comes from the different positions of the sample fabricated by the sputtering technique followed by the wet-etching process.

In order to illustrate the light trapping ability, hydrogenated amorphous silicon (a-Si : H) and hydrogenated microcrystalline silicon (μc-Si : H) tandem solar cells were deposited. This tandem device structure has an efficiency potential of 30% due to its combination of band gaps.⁴⁴ Since a-Si can efficiently absorb visible light and μc-Si can absorb some infrared light, a-Si : H–μc-Si : H tandem thin film solar cells extend the absorption of the solar spectrum. Both materials have a low absorption coefficient close to their band gap, so the transparent electrode substrate must therefore provide efficient light management for both subcells in a broad range of wavelengths (400–1100 nm). The basic structure of tandem solar cells is glass/front electrode(single-period or double-period front electrode)/p(μc-Si : H)/p(a-SiC : H)/i(a-Si : H)/n(μc-Si : H)/n(μc-SiO : H)/p(μc-Si : H)/i(μc-Si : H)/n(a-Si : H)/ZnO/Al. In addition, n-type μc-SiO : H was also used as an intermediate reflection layer.⁴⁵

Fig. 3a shows photographs of two devices: a 300 nm thick a-Si : H top cell based on the SP front electrode (left), and a similar device using the DP front electrode (right). Fig. 3b shows photographs of two devices: a 2.8 μm thick a-Si : H–μc-Si : H tandem thin film solar cell using the DP front electrode (left), and a similar device based on a SP front electrode (right). The device with the DP front electrode looks black, suggesting enhanced absorption due to the suppression of reflection from the front surface.

Fig. 3 Photographs, total reflection, EQEs, simulation, and J–V curves for solar cells deposited on the single-period (SP) and double-period (DP) front electrodes. (a) Photographs of two devices: 300 nm thick a-Si : H top cells based on the SP front electrode (left), and the DP front electrode (right) with the same run during the PECVD process. (b) Photographs of two devices: 2.8 μm thick a-Si : H–μc-Si : H tandem thin film solar cells based on the DP front electrode (left), and the SP front electrode (right) with the same run during PECVD process. (c) The total device reflection is also plotted for the DP and SP front electrodes, respectively. (d) EQE curves of the top and bottom subcells of a-Si : H–μc-Si : H tandem thin film solar cells deposited on the DP front electrode and the SP front electrode. (e) EQE curves of the whole a-Si : H–μc-Si : H tandem thin film solar cells deposited on the DP front electrode and the SP front electrode. (f) J–V curves of a-Si : H–μc-Si : H tandem thin film solar cells deposited on the DP front electrode and the SP front electrode.

In order to quantitatively characterize the absorption of these samples, we carried out absolute hemispherical measurements with an integrating sphere. The reflection measurement was conducted over a broad range of wavelengths (400–1100 nm), which covers the whole spectrum that is useful for a-Si : H–μc-Si : H tandem thin film solar cells. The measured results are summarized in Fig. 3c. Between 400 and 930 nm, the reflection of the DP front electrode device is much lower than the SP front electrode device. The measured total reflection decreased to 5.6% at 600 nm, which is important for a-Si : H top cell. As a result, the total absorption, represented as one minus the reflection (1 − R), is enhanced for the DP front electrode device.

Generally, due to the high refractive index of a-Si : H, a part of the incident light is reflected back from the surface of a-Si : H and thus cannot be used to generate current in solar cell devices. Several groups have demonstrated broadband reflection suppression using nanostructures,^46–52 but few technologies can be applied to the thin film solar cells due to either restrictions on materials or complexity in the fabrication process. Without depositing any antireflection coatings (ARC), our DP front electrode combined large and small period arrays, providing good matching between a-Si : H and air through a gradual reduction of the effective refractive index. Because of the suppressed reflection, absorption is greatly improved over a broad range of wavelengths.

Fig. 3d shows the EQEs of the tandem solar cells based on the SP and the DP front electrode. Part of the current gain in the top cell for the device based on the DP front electrode is obtained in the 400–750 nm range. This is accompanied by an increase of J_sc from 10.11 mA cm⁻² to 11.24 mA cm⁻². In the case of the top cell, the increase in the current density for the DP front electrode is attributed to the small period as the top layer. For such a small period, the wavelength of the incident light is larger than the period of the ‘triangular protrusions’, which is similar to a triangular grating and acts as an effective refractive index gradient. The refractive index linearly increases from a refractive index of ZnO to the index of silicon thin film. As a consequence, the reflection at this particular interface is reduced and more light is coupled into the solar cell.^9–11 At the same time, short circuit current density in the bottom cell was also enhanced from 10.54 mA cm⁻² to 11.52 mA cm⁻². The reason comes from using the DP front electrode with a large period as the bottom layer to enhance light scattering, causing an increase of the optical path length. As a consequence, the DP front electrode reaches a higher total current density (22.76 mA cm⁻²) than that of the SP front electrode (20.65 mA cm⁻²), as shown in Fig. 3e, which improves both the short and long wavelength response. The DP front electrode design, consisting of a large period micro-scale ‘triangular groove’ structure as the bottom layer and a small nano-scale ‘triangular protrusion’ structure as the top layer, yields broadband wavelengths (400–1100 nm) and isotropic photocurrent enhancement.

As can be seen in Fig. 3f, the V_oc, which is sensitive to the electrical quality of the junction, has a low value for the DP front electrode device, that is attributed to the small size sharp ZnO surfaces using the LP-CVD technique. The cracks and voids arise from the tip of the small size sharp ZnO within the intrinsic layer, and have been observed in silicon thin film solar cells grown on LP-CVD.⁵³ The appearance of cracks within the intrinsic layer may indicate a device with a high defect density, which then causes the low V_oc. However, the surface plasma treatment applied to the above front TCO for micromorph tandem solar cells increased the V_oc.⁵⁴ So, we believe that the V_oc value in this experiment can be improved by introducing plasma treatment.

The FF of tandem devices is a delicate parameter as it is strongly affected by the difference of J_sc between the top and bottom cells.⁵⁴ In general, the lowest FF of the tandem cells is obtained for the closest subcell current. Here, the FF difference between the DP front and the SP electrode cannot be attributed to a mismatch reduction (0.28 compared to 0.43 mA cm⁻²). The high FF value for the DP front electrode device can be partly attributed to the decrease of the series resistance from 23.9 Ω cm² to 3.85 Ω cm² calculated from the light J–V curves. For the low series resistance, the reason possibly arises from the improved interface between the DP transparent front electrode and the silicon thin film. Compared to the SP front electrode device, the conversion efficiency of the DP device has been improved by 26% from 6.71% to 8.45%, mainly obtained through increases in Jsc (11%) and FF (17%), which suggests that our approach allows for efficient light management and simultaneously enhances the efficiency.

It is also noted that an important feature of the fabrication technique of the DP front electrode is its compatibility with industrial in-line mass production. We used a simple and cost effective technique of sputtering and wet-etching followed by LP-CVD to form the DP front electrode simultaneously with a large and small period. Sputtering processes are very well controlled generally, especially for the ZnO deposition for solar cells.^32,35,36 In the case of LP-CVD, it has been successfully transferred to mass production in the photovoltaic industry.⁵⁵ The above results showed that it is possible to grow the DP electrode using a conventional technique of sputtering and LP-CVD for potential applications in photonics and high performance photovoltaic devices. A key innovation is that we have taken into account the advantage of large and small periods to enhance light absorption in solar cells. In addition, a simple and cost effective method has also been successfully employed to fabricate the DP front electrode.

In summary, based on the influence of transparent electrodes on the light absorption within microcrystalline silicon thin-film solar cells, we designed and fabricated a DP front electrode based on traditional growth techniques, which have shown large-area and low cost potential in industry. Compared to the SP front electrode, the DP front electrode could suppress the optical reflectance over a broad range of wavelengths due to a minimized reflection originating from the gradual transition of the effective refractive index, and enhanced light trapping resulting from large scale surface morphology. The DP front electrode photovoltaic device exhibits a broadband absorption enhancement, an 11% improvement in the short-circuit current density and a 26% improvement in the conversion efficiency. Since it does not require critical patterning and lithography, the DP front electrode was achieved by the simple, low-cost and scalable deposited methods. We believe that a strong potential exists for applications of other solar cells, including copper indium gallium diselenide (CIGS), cadmium telluride (CdTe), and dye-sensitized and organic solar cells, as well as for other optoelectronic applications.

Methods

ZnO deposition and characterization

The ZnO : Al films were deposited on the glass substrate (Corning Eagle XG) by pulsed direct current (PDC) magnetron sputtering. A sintered ceramic ZnO target with 2 wt.% Al₂O₃ was used. The glass substrates were ultrasonically cleaned sequentially in detergent and deionized water and finally dried with nitrogen gas. The distance between the target and the substrate was 50 mm. The working power was 460 W. The sputtering system was pumped down to a base pressure of 5 × 10⁻⁵ Pa using a turbo molecular pump. The polycrystalline AZO film about 1500 nm was first prepared at a working pressure of 3.3 mT and then etched in diluted HCl (0.5%) for 70 s to obtain a large period ‘triangular’ shape textured surface topography. Subsequently, the intrinsic ZnO films about 500 nm thickness were grown on the textured glass substrate using LP-CVD at 175 °C using (C₂H₅)₂Zn and H₂O as precursor gases. Cross sections of the films were observed by scanning electron microscopy (Carl Zeiss, SUPRA 55VP) photomicrographs. The surface morphology and roughness of the films was characterized by atomic force microscopy (NanoNavi-SPA400).

Solar cell deposition and characterization

The a-Si : H–μc-Si : H (Micromorph) tandem solar cells of size 5 × 5 mm² were deposited by plasma-enhanced chemical vapor deposition in a seven-chamber system (for the a-Si : H top cell) and cluster-tool system (for the μc-Si : H bottom cell). The micromorph tandem cells consist of an a-Si : H top cell with an intrinsic layer thickness of 300 nm and a μc-Si : H bottom cell with an intrinsic layer thickness of 2.5 μm. In addition, the μc-Si : H bottom cell was deposited using a very high frequency (70 MHz) at high rates (1.5 nm s⁻¹). Important techniques for controlling the μc-Si : H bottom cell performance includes the high crystalline interface layer⁵⁶ and the optimization of the solar cell structures with a ZnO : B (about 90 nm) back reflector.⁵⁷ A SiO_x interlayer was incorporated between the amorphous and microcrystalline subcells, as described in ref. 45, to serve as an intermediate reflector that enhances the absorption in the amorphous top cell. The open-circuit voltage (V_oc) and the fill factor (FF) of the solar cells are determined from the current density–voltage (J–V) characteristics, measured under an AM 1.5 spectrum under standard test conditions (25 °C, 1000 W m⁻²). External quantum efficiencies (EQEs) of the a-Si : H top and μc-Si : H bottom cells (EQE_top and EQE_bot) were measured under red and blue bias light illumination, respectively. The corresponding short-circuit current densities, J_sc top cell and J_sc bot cell, were calculated from the EQE curves by convolution with the photon flux of the global air mass 1.5 (AM1.5 g) solar spectrum. The summed short-circuit current density, J_sc sum = J_sc top + J_sc bot does not represent a real current flowing through the device, but is a useful quantity to characterize the absorption, that is, the light trapping in the cell, as it allows direct comparison with single junction cell current densities. The current from the limiting cell is taken for the calculation of the efficiency. The reflectance measurements on the solar cells were carried out with a photo spectrometer equipped with an integrating sphere (Carry 5000).

Acknowledgements

The authors would like to thank Samuel Mao (University of California at Berkeley) for improving the writing. The work was supported by the National Basic Research Program of China (Grant Nos.2011CBA00705, 2011CBA00706, 2011CBA00707), National Natural Science Foundation of China (60976051), Science and Technology Support Program of Tianjin (12ZCZDGX03600), and Program for New Century Excellent Talents in University of China (NCET-08-0295).

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Footnotes

† S. Yang deposited single junction μc-Si solar cells. X. Zhang conceived the ideas and coordinated the work. Y. Wang carried out the ZnO prepared by sputtering. L. Bai and B. Liu deposited bottom cell and measured the cells. X. Yang and H. Zhao fabricated LP-CVD ZnO. J. Fan prepared the top cell. Y. Wang characterized the ZnO. X. Zhang processed and performed the data analysis and wrote the manuscript. C. Wei was in charge of the PECVD system. Q. Huan was in charge of sputtering. X. Chen was in charge of LP-CVD. G. Wang was in charge of measurements. X. Zhang led the TCO and silicon layer activities. Y. Zhao supervised the work.

‡ The authors declare no competing financial interests.

§ These authors contributed equally to this work.

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