Silicon Based Thin Film Solar Cells

This short introduction presents the general situation of photovoltaic conversion, with emphasis on possible use of thin film solar cells.

The chapter presents a short review of the general principles of photovoltaic conversion and different types of thin films solar cells. The aim is to present how a device can evolve from the classical Si p-n junction, with its physical constraints limiting the conversion efficiency, to devices where higher absorption of the light and improved structural design result in higher conversion efficiency. It is now well recognized that these devices have to be based on thin film structures. As a consequence, we have to be able to design the whole stacked thin film structure, and within this, each film must be deposited with well-defined properties. Several deposition techniques can be used, and we will shortly discuss the so-called Plasma Enhanced Chemical Vapour Deposition (PECVD), considering the evolution of the film structure.

Plasma enhanced chemical vapor deposition technique plays a key role in the development of solar cells based on amorphous and microcrystalline silicon thin films. The deposition process depends strongly on physical and chemical interactions in the plasma. Subsequently, the film properties are dependent on different parameters such as power and frequency, the substrate temperature, the gas pressure and composition, the magnitude and the pattern of the gas flow, the electrode geometry, etc. The aim of this chapter is to discuss all effects of these parameters in detail.

Sputtering is one of the most widely used techniques for deposition of thin films. This chapter reviews the physical foundations of sputtering in a plasma and its application to the deposition of thin films. Different sputtering techniques are described and their advantages and disadvantages are highlighted.

Molecular Beam Epitaxy (MBE) represents a widely used growth technique to approach the basic research applied to the growth of semiconductor films and multilayer structures. The main features that distinguish the MBE from other growth techniques are the precise reproducibility of all parameters involved during the epitaxial process, the growth conditions far from thermodynamic equilibrium, and the possibility of controling the kinetic evolution of the outermost layers of the epitaxial film. Nowadays, MBE is also used to grow and investigate nanosized semiconducting materials, which are profoundly interesting for photovoltaic future applications as well.

The chapter is devoted to the Fourier transform infrared (FTIR) and Raman spectroscopy. The theory of both techniques has been briefly treated and the most widely used experimental apparatus have been described. The use of the FTIR and Raman spectroscopy for the characterization of silicon based films and the optimization of microcrystalline solar cell parameters, as examples of applications, have been reported. In particular, it has been shown that by means of FTIR it is possible to detect oxygen impurity in microcrystalline silicon, to individuate the device grade microcrystalline silicon for solar cells fabrication and to study the phase transition due to the thermal annealing of amorphous silicon carbon alloys (a-Si_1-xC_x:H) and a-Si_1-xC_x:H/SiC multilayers. Furthermore, it has been demonstrated that the Raman spectroscopy can be used for the optimization of J-V parameters of microcrystalline silicon solar cells, for the study of boron doping effect on the crystalline volume fraction in microcrystalline silicon carbon, for the investigation of crystallization process induced by thermal annealing in a-Si_1-xC_x:H/SiC multilayers, for determining the influence of the carbon alloying as well as the substrate on the crystalline volume fraction in nanostructured silicon carbon films and, finally, for the study on the medium range order in nanocrystalline silicon.

The chapter is devoted to the surface and structural characterisations of the materials used in photovoltaic applications for the determination of the topographic/morphological and structural properties.

In the first part of the chapter Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) are illustrated, with particular emphasis on the working principles. In the second part, some examples of application of these techniques to the silicon-based thin films solar cells are described. Recent and important experimental results obtained in these fields are examined and discussed, showing what kind of information they have provided.

Optical properties of semiconductors, dielectrics and metals play a key role in the development of thin film solar cells. While these devices belong to photon photodetectors both the photon and wave nature of light affects their performance. Some of these problems are discussed in this chapter. The efficiency of photogeneration of free carriers by light as well as its spatial distribution is taken into consideration. This distribution is strongly affected by light interfering in a structure of many thin films. The possible geometrical and optical inhomogeneities of the solar cell structure are discussed. A few methods of determining surface and averaged overall film thickness refractive indices of semiconductor materials are presented. Techniques for determining different components of absorption of light in semiconductors are also reported. Examples of optical methods useful for determining essential parameters of semiconductors (e.g., optical energy gap, carrier diffusion length, and parameters of electron states) are presented as well.

The accurate measure of semiconductor electrical properties is a fundamental step for the design and the correct operation of any electronic device. The electrical performance of any device will depend on how the carriers move inside the semiconductor lattice. The measure of the resistivity, the concentration of shallow and deep states, the charge carrier mobility, etc., allow for the design of new and advanced functionalities and for improvement in current device technology. In this chapter we will give a brief overview of the main electronic transport coefficients and experimental techniques used to investigate semiconductor materials and the main solar cell parameters. We have limited our attention to the most common and reliable techniques. Our work has been organised into seven sections. The first and second sections define the conductivity and the mobility of any material in terms of its band structure and looks at some semiconductor properties and the material doping. The third and fourth sections illustrate the main scattering mechanisms of charge carriers in a semiconductor and several experimental techniques to measure thin film resistivity, respectively. The fifth section introduces the Hall effect and defines the Hall coefficient and the Hall mobility, with a description of an experimental method to measure these. In the sixth section we report a brief analysis of deep state defects and we describe the DLTS technique to reveal them in a semiconductor lattice. Finally, in the seventh section we describe the current-voltage technique commonly used to measure the main solar cell parameters.

The amorphous/crystalline silicon heterojunction solar cell fascinated the researchers since the beginning as a way to improve the efficiency of silicon based solar cell and to reduce the cost of the PV. Indeed with the aid of a heterojunction it is possible to achieve higher built-in voltage and higher cell open circuit voltage with respect to the homojunction. Moreover thin amorphous film technology, using low temperature processes (below 300°C), in principle allows the use of thinner silicon wafer that could suffer of high temperature step required in the homojunction approach to the cell efficiency. Many efforts have been spent by several research groups to address the main problems of the HJ device related to the silicon surface cleaning, defects at the interface, thin film doping, metal contact and device architecture. From ’90 years the progresses of SANYO results have driven the scientific community to achieve higher cell efficiency. Actually SANYO has demonstrated the possibility to fabricate heterojunction cell on c-Si wafer thinner than 100 μm, leading the amorphous crystalline silicon heterojunction concept toward thin film solar cell.

In this chapter we overview the heterojunction cell concept out lighting the problems encountered and the current understanding of the fundamental device physics to achieve highest efficiency.

The photovoltaic (PV) industry is growing, with rates well in excess of 30% a year over the last decade. World solar photovoltaic market installations are expected to reach 17.5 Gigawatt in 2010; however, PV contribution to global electricity generation is still negligible. The main challenge for a major contribution demands incremental reductions in €/Wp costs of PV modules. The traditional development of photovoltaics was based on crystalline-silicon wafer technology. However, the early 1970’s witnessed a new approach based on the possibility of growing silicon in the form of a thin-film on a given substrate. Several techniques are currently used to achieve this deposition, and they are mainly based on the chemical vapour deposition (CVD) technique. While amorphous siliconbased PV modules have dominated the thin-film market for over 20 years, recent industrial developments include tandem solar cells based on stacks of amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) film (“micromorph cells”). A basic question that already awaits an answer concerns the link between the microstructure of the material, transport properties of the intrinsic absorbing layers, as well as the electrical performance of solar cells. μc-Si:H is a complex material with a wide range of microstructures, depending both on deposition conditions and on the substrate material. This chapter will report on the state of the art and several aspects of material properties. Device physics will also be presented.

Light trapping plays a key role in solar cell to enhance light confinement and then light absorption within the cell. The reduction of the optical losses reflects in photovoltaic solar cell efficiency enhancement. To this aim different and combined strategies can be adopted. Firstly a reflectance reduction of sunlight impinging on the cell is mandatory. Then a cell surface texturing can be very helpful to reduce the reflection by increasing the chances of reflected light bouncing back onto the surface rather than surrounding air. To this purpose different approach can be followed such as substrate and/or single layer texturing. Moreover particular care should be paid to the rear side of the cell where the introduction of a reflecting mirror can produce an optical path length enhancement over a wide wavelength spectrum. Rightly combining the front side texturing and the rear side mirroring it is possible to enhance this pathlength up to 50 times the device thickness indicating that light bounces back and forth within the cell many times performing a light confinement. To this purpose Bragg reflector formed using thin film technology is one of the most promising approach to the back side reflector. Also multi-junction cell, such as tandem micromorph, can receive benefit to the light management by the introduction of inner reflector within the two cells to enhance the spectral separation between the two cells and reduce the front cell thickness where the metastability problem still remain an open request.

In this work we overview all these concepts as applied to thin film cell and to silicon based cell that actually are speedily moving toward thinner substrate making the light confinement a key point to cell efficiency enhancement and PV cost reduction.