Influence of substrate texture on microstructure and photovoltaic performances of thin film polycrystalline silicon solar cells

doi:10.1016/S0022-3093(01)01083-3

Journal of Non-Crystalline Solids

Volumes 299–302, Part 2, April 2002, Pages 1152-1156

https://doi.org/10.1016/S0022-3093(01)01083-3 Get rights and content

Abstract

Thin film polycrystalline silicon (poly-Si) solar cells have been fabricated by plasma enhanced chemical vapor deposition on differently textured ZnO/Ag/SnO₂/glass substrates. Using a textured substrate with the root mean square (RMS) roughness (σ) of 38 nm, the conversion efficiency of 8.22% has been achieved due to improvement of long wavelength responses. Whereas using textured substrates with $σ>38 nm$ , (2 2 0) preferential growth is deteriorated, yielding decreases in both short circuit current and open circuit voltage. The field-dependent carrier collection behaviors reveal that the carrier diffusion length in the poly-Si layer on textured substrates with $σ>38 nm$ decreases due to the change in poly-Si microstructure.

Introduction

Intrinsic polycrystalline silicon (poly-Si) thin films have gathered a great deal attention as a promising photovoltaic material for a stable and highly efficient solar cell [1], [2], [3], [4], [5]. Since poly-Si is an indirect band gap material and has low optical absorption coefficient in visible-infrared region, the light trapping in poly-Si layer by textured substrate is one of the most important technical issues for achievement of high short circuit current. However, one needs to take into account the influence of substrate texture not only on the light trapping but also on poly-Si microstructure, because the substrate surface morphology dominates the preferential growth of poly-Si photovoltaic layer [5]. So far, only few studies have been examined for the optimum design of substrate texture to realize further improvement of the photovoltaic performances of poly-Si solar cells [1], [5].

For amorphous silicon solar cells, textured SnO₂ has been widely used as a front transparent conductive oxide (TCO) layer due to its controllability of surface morphology as well as good light trapping performance [6]. We expect that the textured SnO₂ could be utilized for the light trapping even in the substrate-type poly-Si solar cells as a novel back reflector with a structure of ZnO/Ag/SnO₂ triple layers.

In this work, a series of the poly-Si solar cells has been fabricated on differently textured ZnO/Ag/SnO₂/glass substrates and investigated concerning the influence of substrate texture on microstructure and photovoltaic performances. It has been demonstrated that the poly-Si microstructure strongly depends on the substrate surface morphology. Based on these results, we will discuss the correlation between poly-Si microstructure and transport properties in the poly-Si photovoltaic layer as a function of root mean square (RMS) roughness of substrate surface.

Section snippets

Experimental

We fabricated substrate-type poly-Si solar cells with a structure of Ag-grid/indium tin oxide (ITO)/p–i–n/back reflector/glass. A series of four substrates of different surface texture was prepared and then used as the back reflector of poly-Si solar cells. For these substrates, a highly reflective ZnO(1000 Å)/Ag(2000 Å) double layer was formed on differently textured SnO₂(8000 Å)/glass substrates (Asahi Glass Company). For comparison, ZnO/Ag/glass was also prepared as a flat substrate.

The

Results

Fig. 1 represents the roughness height distributions of the back reflectors used in this experiment, in which (a) corresponds to the surface of flat ZnO/Ag/glass and (b)–(e) correspond to that of textured ZnO/Ag/SnO₂/glass. The roughness height distribution of each substrate is well fitted by Gaussian function with different peak centers and standard deviations. The RMS roughness, σ, of these substrates are estimated as (a) 26 nm, (b) 34 nm, (c) 38 nm, (d) 46 nm and (e) 55 nm, respectively.

Discussion

As already mentioned in Section 1, the performance of back reflector plays an important role in obtaining high short circuit current of poly-Si solar cells. The key requirement for the back reflector is to realize the sufficient light trapping to enhance the path length of the incident light in thin film poly-Si photovoltaic layer, and reduce the weak-absorbable light getting out from the surface of solar cell. Although the highly textured ZnO/Ag/SnO₂ back reflector shows the excellent light

Conclusions

Although the highly textured back reflector shows excellent light trapping properties, it tends to deteriorate the poly-Si (2 2 0) preferential growth, yielding poor photovoltaic performances. When the RMS roughness of substrate increases over 38 nm, pronounced reduction in carrier diffusion length is observed, which is the main cause for low J_sc and V_oc. In this experiment, the textured ZnO/Ag/SnO₂/glass substrate with RMS roughness of 38 nm is found to be most suitable for poly-Si solar cells,

Acknowledgements

The authors thank the members of Asahi Glass Company for preparation of substrates.

References (9)

K Yamamoto
IEEE Trans. Electron Devices
(1999)
K Saito et al.
J Meier et al.
O Vetterl et al.
Mater. Res. Soc. Symp. Proc.
(2000)

There are more references available in the full text version of this article.

Cited by (64)

Spatial resolution and 2D chemical image of light-addressable potentiometric sensor improved by inductively coupled-plasma reactive-ion etching
2018, Sensors and Actuators, B: Chemical
Inductively coupled plasma reactive-ion etching (ICP-RIE) is first applied to decrease the thickness of a Si substrate for light-addressable potentiometric sensor (LAPS) characterization. A simple and reliable process flow with the self-aligned Al layer as a hard mask layer in ICP-RIE is used to fabricate a thin semiconductor substrate with Al back-side contact. A single-side polished P-type Si wafer with a thickness of 350 μm is decreased to 100 μm by ICP-RIE from the polished side. The thickness of the thin Si substrate and the dimensions of the designed patterns determined by ICP-RIE are verified using scanning electron microscopy (SEM) and an optical microscope (OM). A selective high-dielectric constant insulator, niobium oxide (NbOx), is directly deposited on the unpolished side of the thin Si substrate as the sensing membrane of a LAPS by reactive ratio frequency (rf) sputtering. A high photocurrent was found in the ICP-RIE LAPS sample under back-side illumination, particularly in high-frequency operation. Compared to an Si substrate without ICP-RIE, 3-fold and 6.5-fold higher photocurrents were achieved by the proposed LAPS with ICP-RIE measured at 1 and 50 kHz of light modulated frequency, respectively. The highest modulated frequency of illumination was 50 kHz for an acceptable photocurrent in the LAPS with ICP-RIE, which is sufficient for two-dimensional (2D) images acquired with high-speed scanning. The NbOx layer deposited on the unpolished Si surface could automatically generate higher pH sensitivity than conventional polished Si surfaces because of the higher surface roughness. The calculated pH sensitivity and linearity were 55.8 mV/pH and 99.6%, respectively, in a pH range from 2 to 12. No clear degradation of hysteresis was found for the LAPS with ICP-RIE. For the designed patterns of the Al hard mask layer, the spatial resolution of the 2D image of the LAPS with ICP-RIE could be obtained by the difference between dimensions observed by OM and the dimensions calculated from the photocurrent versus scanning length controlled by the X-Y stage. A pattern with a length of 250 μm generated by ICP-RIE was measured as 255 μm in the LAPS with scanning step of 5 μm. ICP-RIE is demonstrated to achieve a spatial resolution of 5 μm in the Si-based LAPS. Further reduction of the thickness of an Si substrate by ICP-RIE controlling and its ability to create a dynamic 2D image are suggested for future investigation in spatial resolution optimization.
Research and development efforts on texturization to reduce the optical losses at front surface of silicon solar cell
2016, Renewable and Sustainable Energy Reviews
Citation Excerpt :
Larger Si crystals are grown on rough TCO surfaces as opposed to the smooth surfaces, where it is deposited by electron beam [123]. In other situations, deterioration in mc-Si (220) preferential growth was observed for higher substrate roughness, which negatively increased carrier recombination rates in solar cell [124,125]. Table 6 presents several performance parameters for complete fabricated cell using textured Si.
Silicon wafer-based solar cell contributes to about 92% of the total production of photovoltaic cells. An average of 30% of the incident light is lost via reflection from the front surface of the silicon solar cell, thus reducing the cell's power conversion efficiency. Texturization is a process of producing the desired unevenness on the surface of solar cell. It is well known as a practical solution to the limitation. Front surface texture reduces cell reflectivity and contributes to more photocurrent generation within active materials. The research and development efforts to reduce the optical losses via texturization are reviewed in this paper. The mechanisms of optical loss reduction, desirable texture feature, methods of texturization, side effects of texturization, and its compatibility with other optical enhancements for crystal silicon cell are elaborated upon. Front surface texture is associated with minimizing optical loss, and negatively affecting carrier and electrical losses. The importance of texturization for crystalline silicon is briefly related with thin film amorphous silicon solar cell to fully encompass this topic. Lesson learned and conclusion is highlighted in the last section.
3D-printed concentrator arrays for external light trapping on thin film solar cells
2015, Solar Energy Materials and Solar Cells
Citation Excerpt :
For example, by texturing the surface of crystalline silicon (c-Si) solar cells the surface recombination velocity increases due to the enlarged surface area [3,4]. For other cells like nanocrystalline silicon (nc-Si) the growth of a solar cell on top of a textured scattering surfaces is challenging [5–7] while for organic solar cells texturing is less effective [8]. It is thus challenging to realize the full theoretical potential of internal light trapping for most solar cells and there is a need for better light trapping methods [9].
After our recent demonstration of a 3D-printed external light trap on a small solar cell, we now consider its potential for large solar panels. An external light trap consists of a parabolic concentrator and a spacer that redirects the photons that are reflected by the solar cell back towards the solar cell. These retro-reflections enable higher absorptance and improved power conversion efficiency. Scaling a single external light trap such that it covers a large solar panel has disadvantages in terms of height and cost of the external light trap. These disadvantages can be overcome by deploying an array of concentrators as the top part of the external light trap. We present an optimization study of concentrator arrays for external light trapping. We fabricated 3D-printed external light traps with a square, hexagonal and circular compound parabolic concentrator to test their suitability for concentrator arrays. The 3D-printed traps were placed on top of an organic solar cell which resulted in a significant enhancement of the external quantum efficiency. The required transmittance of these concentrator arrays is calculated as a function of the parameters of both the concentrator and the solar cell. We compare the theoretical and experimentally determined optical performance of the different concentrators. Finally, the prospects of external light trapping are analyzed and we give guidelines for improvements of the external light trap design.
Stacked nanoparticle-transparent conductive oxide substrate combining high haze with low surface roughness for improvement of thin film Si solar-cell performance
2015, Thin Solid Films
Surface-textured morphology of transparent conductive oxide (TCO) substrates enhances light absorption in Si thin films by scattering and trapping light, which results in an increase in the current density of Si thin film solar cells. However, the highly textured morphology often induces the formation of structural defects within the Si layer, which decreases the photovoltaic performance parameters of solar cells such as the open-circuit voltage (V_OC) and fill factor (FF). To overcome this trade-off, we propose a flattened light-scattering TCO substrate based on a strongly light-scattering metal-oxide nanoparticle (NP) layer in conjunction with a surface TCO layer of low surface roughness. Individual layers of ZnO, TiO₂, and stacked TiO₂/ZnO metal-oxide nanoparticles (NP-TiO₂/NP-ZnO), which functioned as a light-scattering layer, were formed between a glass substrate and a low-resistivity Al-doped zinc oxide (AZO) layer to serve as substrates for Si thin film solar cells. After deposition of the AZO film, the nanoparticle AZO substrate demonstrated good flatness and high haze values, which were nearly equivalent to the haze values of the nanoparticle layers before coating by AZO. For thin film hydrogenated amorphous Si (a-Si:H) solar cells fabricated on the stacked AZO/NP-TiO₂/NP-ZnO substrate, the short-circuit current increased with no corresponding decrease in the V_OC or FF. We attribute this result to an improved spectral response in a wavelength range λ > 500 nm and a decrease in the structural defects within the Si layers.
ZnO based back reflectors with a wide range of surface morphologies for light trapping in n-i-p microcrystalline silicon solar cells
2014, Energy Procedia
Surface morphology of ZnO layers with as-deposited thickness between 550 to 1050 nm was varied over wide range by changing etching time. The surface morphology of these ZnO layers was measured by atomic force microscopy (AFM) and statistically analyzed in terms of root mean square roughness and the diameter and depth of the craters. The texture-etched ZnO covered with a Ag and ZnO layers were applied into n-i-p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The link between the morphology, the scattering properties of ZnO layers, and the solar cell performance is discussed.
Thin-film silicon solar cells applying optically decoupled back reflectors
2013, Materials Science and Engineering: B
Thin-film silicon solar cells often apply a metal back reflector (BR) separated from the silicon layers by a thin rear dielectric of thickness around 80 nm or a white paint combined with a thick rear dielectric of several micrometers. In this work, we investigate the optical performance of microcrystalline silicon (μc-Si:H) solar cells applying BRs of various topographies. In contrast to a standard 80 nm-ZnO/Ag BR design, for which the BR nearly strictly follows the texture of the underlying μc-Si:H layers, placing the Ag BR far from the μc-Si:H layers allows for a variation of the BR topography. Irrespective of the investigated BR topographies and also for a conventional white paint BR, long distances (of several micrometers) between the BR and the μc-Si:H layers are found to be detrimental for the light trapping. Optical simulations based on both rigorous and scalar scattering theory have been performed to understand the impact of the diverse BR designs on the optical cell performance.

View all citing articles on Scopus

View full text