Optical management in high‐efficiency thin‐film silicon micromorph solar cells with a silicon oxide based intermediate reflector

Abstract

The micromorph solar cell concept consists of an optical and electrical series connection of a high-gap a-Si:H top cell and a low-gap µc-Si:H bottom cell. To minimize light-induced degradation, the thickness of the a-Si:H absorber should not exceed 300 nm. This constraint considerably limits the short-circuit current density (J sc ) on the top cell and, hence, the efficiency of the whole device. Therefore an intermediate reflecting layer (IRL) between the individual cells must be introduced to increase the current in the a-Si:H absorber [1][2][3][4].In this letter, we first analyze the light scattering properties of nano-textured transparent conductive oxide (TCO) layers used as front electrodes for micromorph cells deposited in the superstrate configuration (p-i-n). Photocurrents in individual state-of-the-art cells with a Si oxide based IRL (SOIR) are then compared for front TCOs with different surface morphologies and the observed differences are related to the optical characteristics of these TCOs.The three types of TCO (type-A, -B and -C) used in this study are as-grown surface textured ZnO films with two different doping levels obtained by low-pressure chemical vapour deposition (LPCVD) on Schott AF45 glass substrates. The thickness of the resulting layers is adjusted to obtain a sheet resistance below 10 Ω/sq. The root mean square value of their surface roughness (σ rms ) and the correlation length ξ of the textured surface are determined by atomic force microscopy. These characteristics, summarized in Table 1, depend on the thickness of the layers and on the duration of a plasma post-treatment [5] applied to their surface. Note that the type-B ZnO (without posttreatment), whose sharp V-shape structures prevent good electrical properties of the device [5], is presented only for the sake of light scattering comparisons. The low doping level used for deposition of the thick large-grain ZnO layers (type-B and -C) provides high transparency in the near infrared (NIR) spectral range because of reduced free carrier absorption (FCA) [6]. The diffuse transmittance in-air of the different TCOs, when light is normally incident to the glass side, is investigated by two methods. First, the haze factor for transmitted light H T = T dif /T tot is calculated from total and diffuse optical transmittance (T tot and T dif ) measurements carried out with a photo-spectrometer equipped with an integration-sphere. Second, the intensity per unit of solid angle scattered at an angle θ with respect to the direction of an incident laser beam (633 nm) is deIn the effort to increase the stable efficiency of thin film silicon micromorph solar cells, a silicon oxide based intermediate reflector (SOIR) layer is deposited in situ between the component cells of the tandem device. The effectiveness of the SOIR layer in increasing the photo-carrier generation in the a-Si : H top absorber is compared for p -i -n devices deposited on different rough, highly transparent, front ZnO layers. High haze and low doping level for the front ZnO stron...

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post‐oxidation

“…However, its major use is in micromorph tandem cells, which consist in a stacked of amorphous and microcrystalline cells [2] which can reach stabilized efficiencies above 11% [3,4]. In mc-Si:H cells, zones of porous material, called thereafter cracks, appear typically when the device is deposited on substrates with Vshape morphology (i.e., sharp and steep valleys) [5][6][7][8], as is usually needed to achieve good light trapping properties.…”

Modulated surface textures for enhanced light trapping in thin-film silicon solar cells

“…5,14,15 This scattering mechanism is characterized by a pronounced exponential decay in T D with increasing wavelength, which is typical for the commonly used randomly surface-textured substrates for thin-film solar cells.…”

“…2,3 Employing different absorber materials in multi-junction solar cells, such as amorphous silicon alloys and microcrystalline silicon, efficient light trapping is required at long wavelengths ͑up to 1100 nm͒. To meet this demand, TCO substrates with different surface textures have recently been developed and tested in solar cells, such as optimized wet-etched 4 or surface plasma-treated 5 zinc-oxide, double-textured tin-oxide, 6 and a combination of etched glass with zinc-oxide. 7 Even though the potential of using of high performance TCOs has been already investigated, 8 a physical explanation of why these textures result in high scattering properties is still missing.…”

Nanoimprint Lithography for High-Efficiency Thin-Film Silicon Solar Cells

“…[13][14][15] Light scattering at nanotextured interfaces provides a powerful and proven alternative to improve the optical performance of thinfilm silicon devices. [16][17][18][19][20][21][22][23][24][25][26][27] However, despite intensive experimental and theoretical efforts, neither the ideal interface morphology nor the ideal scattering characteristics have been identified to date. From an experimental point of view, one desires a method that allows one to evaluate and compare the light-trapping capabilities of specifically designed photonic nanostructures directly in the device.…”