This paper describes the use, within p–i–n‐ and n–i–p‐type solar cells, of hydrogenated amorphous silicon (a‐Si:H) and hydrogenated microcrystalline silicon (μc‐Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma‐assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i‐layers are described. These properties are linked to the microstructure and hence to the i‐layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single‐junction amorphous and microcrystalline p–i–n‐type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc‐Si: H/a‐Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass‐production of thin‐film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin‐film silicon PV modules, especially in building‐integrated PV (BIPV) are shown. In this context, the energy yields of thin‐film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.

Tandem solar cells constructed from a crystalline silicon (c-Si) bottom cell and a low-cost top cell offer a promising way to ensure long-term price reductions of photovoltaic modules. We present a four-terminal tandem solar cell consisting of a methyl ammonium lead triiodide (CH3NH3PbI3) top cell and a c-Si heterojunction bottom cell. The CH3NH3PbI3 top cell exhibits broad-band transparency owing to its design free of metallic components and yields a transmittance of >55% in the near-infrared spectral region. This allows the generation of a short-circuit current density of 13.7 mA cm(-2) in the bottom cell. The four-terminal tandem solar cell yields an efficiency of 13.4% (top cell: 6.2%, bottom cell: 7.2%), which is a gain of 1.8%abs with respect to the reference single-junction CH3NH3PbI3 solar cell with metal back contact. We employ the four-terminal tandem solar cell for a detailed investigation of the optical losses and to derive guidelines for further efficiency improvements. Based on a power loss analysis, we estimate that tandem efficiencies of ∼28% are attainable using an optically optimized system based on current technology, whereas a fully optimized, ultimate device with matched current could yield up to 31.6%.

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...

In the present paper, the structural and electrical performances of microcrystalline silicon (lc-Si:H) single junction solar cells codeposited on a series of substrates having different surface morphologies varying from V-shaped to U-shaped valleys, are analyzed. Transmission electron microscopy (TEM) is used to quantify the density of cracks within the cells deposited on the various substrates. Standard 1 sun, variable illumination measurements (VIM) and Dark J(V) measurements are performed to evaluate the electrical performances of the devices. A marked increase of the reverse saturation current density (J 0 ) is observed for increasing crack densities. By introducing a novel equivalent circuit taking into account such cracks as non-linear shunts, the authors are able to relate the magnitude of the decrease of V oc and FF to the increasing density of cracks.

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A series of nip-type microcrystalline silicon (mc-Si:H) single-junction solar cells has been studied by electrical characterisation, by transmission electron microscopy (TEM) and by Raman spectroscopy using 514 and 633 nm excitation light and both top-and bottomillumination. Thereby, a Raman crystallinity factor indicative of crystalline volume fraction is introduced and applied to the interface regions, i.e. to the mixed amorphous-microcrystalline layers at the top and at the bottom of entire cells. Results are compared with TEM observations for one of the solar cells. Similar Raman and electrical investigations have been conducted also on pin-type mc-Si:H single-junction solar cells. Experimental data show that for all nip and pin mc-Si:H solar cells, the open-circuit voltage linearly decreases as the average of the Raman crystallinity factors for top and bottom interface regions increases.

The authors report on the fabrication of microcrystalline silicon p-i-n solar cells with efficiencies close to 10%, using glass coated with zinc oxide (ZnO) deposited by low pressure chemical vapor deposition (LPCVD).LPCVD front contacts were optimized for p-i-n microcrystalline silicon solar cells by decreasing the free carrier absorption of the layers and increasing the surface roughness. These modifications resulted in an increased current density of the solar cell but also in significantly reduced fillfactor (FF) and open-circuit voltage (Voc). In order to avoid these reductions, a new surface treatment of the ZnO is introduced. It transforms profoundly the surface morphology by turning the typical V-shaped valleys of the LPCVD ZnO into U-shaped valleys and by erasing from the surface small-sized pyramids and asperities. As a result, for fixed deposition parameters, the p-i-n microcrystalline silicon solar cell efficiency increased from 3.3% to 9.2%Further optimization of the microcrystalline silicon solar cell on this 'new' type of LPCVD ZnO front contact has led to an efficiency of 9.9%.

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...