Thin Film Solar Cells Material Structure

Substrate

Thin-film solar cells devices are configured in either substrate or a superstrate structure. For superstrate configuration which is shown in Figure 2.1(a), the substrate is transparent and the contact is made by a conducting oxide coating on the substrate.

For substrate configuration which is shown in Figure 2.1(b), the substrate is metal or metallic coating on a glass/polymer substrate which also acts as the contact.

Transparent conducting oxide (TCO)

Transparent conducting oxides in general are n-type degenerate semiconductors with good electrical conductivity and high transparency in the visible spectrum. Thus, a low-resistance contact to the device and transmission of most of the incident light to the absorber layer is ensured. The conductivity of a TCO depends on the carrier concentration and mobility. An increase in the carrier concentration may result in enhanced free carrier absorption, which reduces the transparency of the TCO in the higher-wavelength region. Hence increasing the mobility by improving crystalline properties is considered to be the pathway for a good TCO.[9] Besides these optoelectronic properties, the mechanical, thermal, chemical, and plasma-exposure stability and passivity[10] of TCOs are important considerations. Studies have shown[11] that only ZnO-based TCOs can withstand H-bearing plasma and are also stable up to 800K. Therefore, ZnO-based materials are being increasingly used in thin film solar cells technologies. A number of reviews on TCOs have appeared in literature.[9,12-14]

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Window layer

The primary function of a window layer in a heterojunction is to form a junction with the absorber layer while admitting a maximum amount of light to the junction region and absorber layer; no photocurrent generation occurs in the window layer.[15]

For high optical throughput with minimal resistive loss the bandgap of the window layer should be as high as possible and as thin as possible to maintain low series resistance. It is also important that any potential ‘spike’ in the conduction band at the heterojuction be minimized for optimal minority carrier transport. Lattice mismatch (and consequent effects) at the junction is important for consideration for epitaxial or highly oriented layers. In the case of microcrystalline layers, mismatch varies spatially and thus the complicated effect, if any, averages out.

For a-Si solar cells, depending on device configuration, the n- or p-layer is very thin and acts like a window layer that allows all the photons to be transmitted to the i-region. Given the very high absorbance of these films, a very thin doped layer (~10 nm) is required. Alloy films such as a-SiC:H having excellent optical transparency and good photoconductivity have been used as the window layers.[16]

Absorber

Amorphous, micro/nanocrystalline and polycrystalline silicon Amorphous silicon is widely accepted as a thin-film solar cell material because: (a) it is abundant and non-toxic; (b) it requires low process temperature, enabling module production on flexible and low cost substrates; (c) the technological capability for large-area deposition exists; and (d) material requirements are low, 1–2 mm, due to the inherent high absorption coefficient compared with crystalline silicon. The high absorption of light results from the inherent high disorder, dangling bonds of the order of 1019/cm³, in the material so that all optical transitions are allowed. On the other hand, the

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disorder acts as recombination centers that severely reduce the carrier lifetime and pin the Fermi energy level so that the material cannot be doped either n- or p-type.

Incorporation of 10% hydrogen in the film during deposition greatly reduces the density of the defects to 1016/cm³, yielding a new and exotic material, a-Si:H which has a well-defined optical threshold (mobility gap) at 1.75 eV compared with the crystalline Si indirect bandgap at 1.1 eV. The reduction in the defect density makes the a-Si:H material suitable for doping and alloying with a range of materials and for junction device fabrication. However, the properties of the material and the junction device are severely affected by the light-induced creation of metastable defects, known as the Staebler–Wronski effect. Light-induced degradation of a-Si:H devices is partially tackled by reducing the a-Si:H layer thickness so that the photogenerated carriers need to move only a short distance before they reach the electrode. However, thinning down also results in lower light absorption and thus optical confinement techniques employing diffusely reflecting front and back contacts are required to increase effective layer thickness in order to absorb the photons. Over a period of time, extensive research and development work on deposition technique and device structure have resulted in development of single-junction and multijunction devices with high efficiency and moderately good stability. The a-Si alloy materials are no longer strictly classical amorphous materials with short-range order (<1 nm). Under suitable deposition conditions and strong hydrogen dilution, nanocrystalline and microcrystalline materials[17] are obtained. The existence of very small Si crystallites dispersed in amorphous matrix deposited by plasma enhanced chemical vapor deposition (PECVD) under high-H dilution was confirmed with infrared absorption and XRD measurements.[18] While the crystallite size and volume fraction are very small, these crystallites catalyze the crystallization of the remainder of the amorphous matrix upon annealing. Microcrystalline materials deposited by this method is found

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to have less defect density and are more stable against light degradation compared with a-Si. Recently developed improved efficiency materials consist of this heterogeneous mixture of the amorphous and an intermediate range order microcrystalline material. The laser[19] and rapid thermal annealing, and optically assisted metal-induced crystallization techniques[20] were also used to obtain a microcrystalline film from an amorphous film and to increase the grain size. New high-rate deposition technologies for polycrystalline Si films and innovative solar cell designs are being evolved to make reasonably efficient cells with thickness less than 25 mm at an acceptably high throughput. For example, crystalline silicon on glass (CSG) technology combines the low manufacturing cost of thin-film technology with the established strengths of silicon wafer technology.[21] Owing to the high conductivity of the silicon, no TCO was required for the current collection. With the assistance of light trapping technique, efficient modules (as high as 7%) have been fabricated on 2-mm-thick silicon films.[21] High rate deposition of poly crystalline silicon films can also by obtained by hot-wire CVD techniques.[22] Large-grain, 5–20 mm, polycrystalline silicon layers have been deposited at rates as high as 3 mm/min, using iodine vapour transport at atmospheric pressure[23]. Deposition of poly-Si for solar cell applications has been reviewed extensively in the literature.[24,25]

Microcrystalline and polycrystalline silicon films have lower optical absorption in contrast to the high optical absorption in a-Si. Thus, in the former case, light trapping is necessary to extract the photon energy efficiently.

Interfaces

Thin film solar cells are comprised of several layers of different semiconductors and metals, and thus the device has a large number of interfaces. Besides these surfaces/interfaces, submicrometer grain-size polycrystalline films have a high

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concentration of grain boundaries acting as internal interfaces. As far as possible, photovoltaic materials are selected to match lattice constant, electron affinity/work function and thermal expansion coefficient between the adjacent layers. However, the interface properties also get modified during device processing as a result of growth process involving the sequential deposition of multilayers at different deposition conditions. In addition, post-deposition treatments involving high-temperature annealing can alter the interface and intergrain properties. Interfacial defect states and chemical and metallurgical changes affect the optoelectronic transport properties of the device. As a result, the device parameters such as open-circuit voltage, current and fill factor can be modified significantly. Extensive scanning tunneling microscopy studies of interfaces and intergranular regions show clearly that they are not only active, but are also significantly different electronically from the bulk of the grains.

On the other hand, manipulation of the interfacial structure, chemistry and metallurgy provides a powerful tool to tailor/ engineer the Fermi level, bandgap, electric field and their gradients to improve the overall device performance. Both activation and passivation of grain boundaries have been effectively used in some devices.

Amorphous Si solar cells contain a number of interfaces, particularly the p/i and n/i interfaces in the p–i–n cell structure. According to the defect pool model,[26] there is a higher defect density near the interfaces compared with the bulk of the i-region.

This results in recombination processes being dependent on the position in the i-region. Improvements in the device properties by annealing under reverse bias can be explained by the movement of hydrogen to the interface and consequent reduction in the interface states.[27,28] The presence of interface states also causes open-circuit voltage limitation, particularly for the p/i interface.[29] In fact, modifying this interface by using a buffer layer increases Voc.[30] Since textured substrates are used in order to enhance the optical absorption, this affects the topography of all the layers,

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depending on deposition conditions, and causes interfacial roughness.[31] The resultant optical scattering for different wavelengths can cause improvements in the photoresponse of the devices.

Back contact

For amorphous silicon devices, the back contacts are formed on n-type semiconductor (and hence higher work function metals are not required) using Ag or Al. However the use of Ag or Al directly on n-type semiconductor will result in optical losses in the long-wavelength region. The long-wavelength response of the device will be improved if the n/metal interface reflects the long-wavelength radiation back to the cell. Thus the back contact for both p–i–n and n–i–p configuration is often formed with double-layer back reflector consisting of ZnO and Ag or Al. The low index ZnO layer will effectively increase the total internal reflection from the n- ZnO interface. The optical and electrical properties of the ZnO layer play a significant role in improving the back reflection properties and also the device performance.