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Dielectric Films for Si Solar Cell Applications

Thin films of many dielectric materials have been used in the past for fabrication of solar cells and as a part of their device structure. However, current efforts to reduce solar cell costs in commercial production have led to simplification of cell design and fabrication. Use of self-aligning techniques has obviated the need for photolithography and conventional masking with dielectric films for cell fabrication. Currently, the most favored dielectric material in Si solar cell production is SiN:H, deposited by the plasma-enhanced chemical vapor deposition (PECVD) process. The SiN:H layer and its processing play multiple roles of serving as an antireflection coating, a surface passivating layer, a buffer layer through which metal is fired, and a means of transporting hydrogen into the bulk of the solar cell. In order to optimize the solar cell performance, the SiN:H layer must meet some conflicting demands. The various applications of the SiN:H layer in solar cell fabrication are described here.

INTRODUCTION

The use of dielectric films in Si solar cell technology is very similar to that in other microelectronic or optoelectronic devices. In general, the functions of dielectric films for solar cells may be categorized into two groups: (1) as a device component, needed for the operation of the device and integrated as a part of the cell structure, and (2) as an aid in fabrication of solar cells from wafers. As a device component, dielectric films can be used to control electronic and optical properties of interfaces, and hence that of the device. The relevant electronic parameter that is most influenced by dielectric films is the interface recombination (also referred to as surface passivation).1 The optical properties that can be altered by thin dielectric films are reflection and antireflection at the interfaces.2,3 Thin layers of SiO^sub 2^ are used for passivation of solar cell surfaces, resulting in surface recombination properties similar to those in metal oxide semiconductor (MOS) devices.4 Likewise, the materials used for antireflection coatings include SiO^sub 2^, TiO^sub 2^, ZnO, MgF, and SiN, which are also used in other optical and optoelectronic devices.8 Thus, there is a correspondence in the use of dielectric films in fabrication of solar cells. This is because, traditionally, Si solar cell technology has closely followed methods and concepts used in the semiconductor industry. Solar cell fabrication steps consisted of photolithography and other processes, such as etching, diffusion masking, and pre-Ohmic patterning, which were standard processes in semiconductor device fabrication.

Until recently, the general trend in photovoltaics (PV) has been to use films of different dielectrics of optimum properties for various applications. Thus, SiO^sub 2^ was used for surface passivation, other materials such as TiO^sub 2^, ZnO, MgF, and SiN films were selected for optical properties, and either SiN or SiO^sub 2^ was used for device fabrication. As the solar cell business matured, commercial technologies began to diverge from the more expensive microelectronics methods. While the laboratory (and high efficiency) solar cell fabrication methods and device structures continued to follow semiconductor technology, commercial solar cell technology weaned away from parental technologies, primarily to lower the device cost and increase throughput. Rapid increase in the production of PV energy and a need for cost reduction has necessitated streamlined procedures for solar cell fabrication. Consequently, the number of process steps used for device fabrication has been greatly reduced, with each new process step performing functions of many previous process steps. In addition, device fabrication has become more homogeneous within the PV industry. Because process steps such as masked diffusions and acid etching are expensive, current cell fabrication has replaced masking steps by self-aligned methods.

This paper will first present a brief review of various functions of dielectric materials used in the operation of a Si solar cell and discuss the basic concepts of dielectric films for optical and electronic design of solar cells. A brief discussion of the deposition methods will be given. Because newer commercial solar cell fabrication does not employ dielectric films in cell fabrication, we will not discuss this application. Finally, we will focus on the use of SiN as a versatile dielectric for solar cell applications. We will discuss the multifunctional use of SiN in.PV device fabrication. This paper is aimed at giving technical insight into the issues relating to the use of SiN in solar cells.

ROLE OF DIELECTRIC LAYERS IN SOLAR CELL OPERATION

The active part of a Si solar cell is often regarded as the Si wafer (with an N/P junction), because the absorption of the incident light and a concomitant generation of carriers occurs within Si. However, surfaces play an important role in optical or electronic operation of a solar cell, thereby influencing both the generation and collection of photocarriers. Indeed, the importance of the surfaces grows stronger as the thickness of the cell is reduced, which is done to lower the Si consumption and increase the cell efficiency. The optical and the electronic properties of the cell surfaces (and hence that of the device) can be controlled through deposition of suitable dielectric films. To examine the requirements of dielectric coatings needed for efficient operation of solar cells, consider the simple Si solar cell structure illustrated in Fig. 1. This figure shows the main regions of a cell, consisting of a p-type base and η-type emitter and front and back metallizations. In the normal operation of the cell, the cell is illuminated from the front side through a gridded contact, as shown in Fig. 1. Because solar illumination has a broad spectral range, light is absorbed through the entire cell-with shorter wavelengths absorbed near the surface and longer wavelengths deeper inside the cell. The absorbed light generates electron-hole pairs, which diffuse toward the interfaces. The minority carriers from both the η and the ρ regions that reach the junction are collected there. However, the carriers reaching the interfaces can experience recombination due to surface states, and are lost from the power-generation process.

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