We show that SiO-based intermediate reflectors ͑SOIRs͒ can be fabricated in the same reactor and with the same process gases as used for thin-film silicon solar cells. By varying input gas ratios, SOIR layers with a wide range of optical and electrical properties are obtained. The influence of the SOIR thickness in the micromorph cell is studied and current gain and losses are discussed. Initial micromorph cell efficiency of 12.2% ͑V oc = 1.40 V, fill factor= 71.9%, and J sc = 12.1 mA/ cm 2 ͒ is achieved with top cell, SOIR, and bottom cell thicknesses of 270, 95, and 1800 nm, respectively.A micromorph tandem solar cell consists of a high-gap amorphous ͑a-Si: H͒ top cell and a low-gap microcrystalline silicon ͑ c-Si: H͒ bottom cell stacked on top of each other. The thickness of the a-Si: H cell has to be kept reasonably thin to minimize the impact of light-induced degradation, 1 and its current, therefore, generally limits the current of the tandem device. To overcome this issue, an intermediate reflecting layer ͑IRL͒ can be introduced between the two cells to increase the current of the top cell. 2 For an intermediate layer to act as a reflector, its refractive index n must be lower ͑typically n IRL Ϸ 2͒ than that of silicon ͑n Si = 3.8 at 600 nm͒ such as to produce a refractive index step that causes the reflection of light at the material interface. The layer which serves as IRL is required to be sufficiently conductive to avoid blocking current and as transparent as possible to minimize the current losses due to absorption of light outside the active layers. In the first attempts to realize this intermediate reflector, zinc oxide ͑ZnO͒ has been used. [2][3][4] In a recent study, the top-cell current could be increased by 2.8 mA/ cm 2 , using a 110-nm-thick ZnO IRL with a 180-nm-thick top cell. 3 When considering industrialization, there are, however, two main drawbacks of ZnO-based IRL: the need for an additional ex situ deposition step and an additional laser scribe for monolithic series interconnection to avoid lateral shunting of solar module segments. 5 Another group reported in situ fabrication of a different IRL but without specifying the material used. 6 In this paper, we propose the preparation of an IRL based on a "doped silicon oxide" material fabricated by plasma enhanced chemical vapor deposition ͑PECVD͒ in the same reactor as the solar cells. We demonstrate that it is possible to produce such a SiObased intermediate reflector ͑SOIR͒, with a refractive index close to 2 and electrical properties suitable for incorporation into micromorph devices.The SiO-based layers are deposited by very-high frequency PECVD at 110 MHz, 200°C, and with a power density of 0.01-0.1 W / cm 2 . Optical and electrical characterizations are performed on ϳ100-nm-thick layers deposited on glass. Optical reflectance and transmittance are measured with a Perkin-Elmer photospectrometer, type lambda 900, within a spectral range from 320 to 2000 nm. The refractive index n and the absorption coefficient ␣ are estimated by fitti...

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...

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The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm 2 cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

We present optical properties and microstructure analyses of hydrogenated silicon suboxide layers containing silicon nanocrystals (nc-SiO x :H). This material is especially adapted for the use as intermediate reflecting layer (IRL) in micromorph silicon tandem cells due to its low refractive index and relatively high transverse conductivity. The nc-SiO x :H is deposited by very high frequency plasma enhanced chemical vapor deposition from a SiH 4 /CO 2 /H 2 /PH 3 gas mixture. We show the influence of H 2 /SiH 4 and CO 2 /SiH 4 gas ratios on the layer properties as well as on the micromorph cell when the nc-SiO x :H is used as IRL. The lowest refractive index achieved in a working micromoph cell is 1.71 and the highest initial micromoph efficiency with such an IRL is 13.3 %.