Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector. (C) 2012 Optical Society of America

Comparison of periodic and random structures for scattering in thin-film microcrystalline silicon solar cells

Corsin Battaglia

Random structures are typically used for light trapping in thin-film silicon solar cells. However, theoretically periodic structures can outperform random structures in such applications. In this paper we compare random and periodic structures of similar shape. Both types of structure are based on atomic force microscopy (AFM) scans of a sputtered and etched ZnO layer. The absorption in a solar cell on both structures was calculated and compared to external quantum efficiency (EQE) measurements of samples fabricated on the random texture. Measured and simulated currents were found to be comparable. A scalar scattering approach was used to simulate random structures, the rigorous coupled wave analysis (RCWA) to simulate periodic structures. The length and height of random and periodic structures were scaled and changes in the photocurrent were investigated. A high height/length ratio seems beneficial for periodic and random structures. Very high currents were found for random structures with very high roughness. For periodic structures, current maxima were found for specific periods and heights. An optimized periodic structure had a period of Λ = 534 nm and a depth of d = 277 nm. The photocurrent of this structure was increased by 1.6 mA/cm2 or 15% relative compared to the initial (random) structure in the spectral range between 600 nm and 900 nm.

2012

Effect of Substrate Morphology Slope Distributions on Light Scattering, nc-Si:H Film Growth, and Solar Cell Performance

Chunmin Zhang

Thin-film silicon solar cells are often deposited on textured ZnO substrates. The solar-cell performance is strongly correlated to the substrate morphology, as this morphology determines light scattering, defective-region formation, and crystalline growth of hydrogenated nanocrystalline silicon (nc-Si:H). Our objective is to gain deeper insight in these correlations using the slope distribution, rms roughness (srms) and correlation length (lc) of textured substrates. A wide range of surface morphologies was obtained by Ar plasma treatment and wet etching of textured and flat-as-deposited ZnO substrates. The srms, lc and slope distribution were deduced from AFM scans. Especially, the slope distribution of substrates was represented in an efficient way that light scattering and film growth direction can be more directly estimated at the same time. We observed that besides a high sigma(rms), a high slope angle is beneficial to obtain high haze and scattering of light at larger angles, resulting in higher short-circuit current density of nc-Si:H solar cells. However, a high slope angle can also promote the creation of defective regions in nc-Si:H films grown on the substrate. It is also found that the crystalline fraction of nc-Si:H solar cells has a stronger correlation with the slope distributions than with srms of substrates. In this study, we successfully correlate all these observations with the solar-cell performance by using the slope distribution of substrates.

Amer Chemical Soc2014

Explaining Morphological and Electrical Features of Boron-doped Zinc Oxide to Tailor New Electrodes for Photovoltaics

Lorenzo Fanni

TCOs are a class of metal oxides that combine transparency to visible light with electrical conductivity. Each TCO is characterized by the trade-off of these two properties which can be tuned for a particular application. This thesis is dedicated to the investigation of one TCO material: boron doped zinc oxide (ZnO:B) deposited by low-pressure chemical vapor deposition (LP-MOCVD). Its main distinction is low absorptance which makes ZnO films deposited by LP-MOCVD ideal as transparent electrodes in thin-film solar cells. Although ZnO:B properties have previously been optimized for this application, the exact processes behind the film formation have not been fully described. This thesis substantially clarifies the processes of the film nucleation and growth evolution, and the mechanisms of incorporation of the B atoms into the ZnO. First, for non-intentionally doped ZnO we establish that the main deposition parameters influencing the film properties are the deposition temperature, the gas precursor ratio and the total gas flow. We experimented with these three parameters across a wide range of parameter values. The films deposited were characterized at different stages of their growth using atomic force microscopy, X-ray diffraction and automated crystallographic orientation mapping. These data reveal the dependence of the film preferential orientation on the deposition conditions. We propose a model based on the adsorbed atom mean free path to explain this dependence. Using deposition parameters learnt from this model, we control the preferential orientation during film growth to increase the grain sizes. Second, we analyze the conductivity to B-doped ZnO films. Boron atoms act as electron donors in ZnO, increasing the electrical conductivity and decreasing the transparency. This work quantifies how the B concentration and spatial distribution affect the film conductivity. Quantification of the B atoms incorporated in the film was performed using nuclear reaction analysis. We found that a high level of O precursor gas favors B incorporation in the film. Combining nano secondary ion mass spectroscopy and Kelvin probe force microscopy we demonstrate that the dopant atoms incorporate in only one of the two sides of each grain. This is a new observation because dopant atoms are commonly assumed to be uniformly distributed in the film. The transparency and electrical conductivity of ZnO depend also on the carrier mobility. The sources of carrier scattering are investigated for intrinsic (i.e. O vacancies and Zn interstitials) and extrinsic (B atoms) impurity concentrations. We observed that for the same B concentration the source of electron scattering depends on the concentration of intrinsic defects: for films with a lower concentration of intrinsic defects, the main source of scattering is the grain boundaries; for films with a higher concentration of intrinsic defects, the main sources are the mechanisms happening in the crystalline region. Finally, LP-MOCVD ZnO:B films were optimized using the previous results and successfully applied in four different types of solar cells: amorphous/microcrystalline silicon, amorphous silicon, copper indium gallium selenide and silicon heterojunction. The advantages of the developed films to these four cells include the simplification of the fabrication process, the reduction of reflectance and parasitic absorption, and a better understanding of carrier transport mechanisms through the device.

EPFL2016

Explaining Morphological and Electrical Features of Boron-doped Zinc Oxide to Tailor New Electrodes for Photovoltaics

Lorenzo Fanni

EPFL2016