Surface Passivation Materials for High-Efficiency Silicon Solar Cells

Introduction

Prologue

Since the first silicon p-n junction solar cell was successfully fabricated in Bell Labs, silicon solar cells have always been the dominant product in photovolta- ics (PV). In the early stage, the conversion efficiency of the silicon solar cells was mainly improved by classical semiconductor technologies like diffusion. Afterward, in order to improve the cell efficiency and reduce the fabrication cost, some techniques such as surface texturing, screen printing, passivated emitter and rear contact (PERC) solar cells, and firing process were introduced. These techniques played a key role and promoted the industrialization of Si solar cells. Nowadays, two types of silicon solar cells, namely, monocrystalline silicon (c-Si) and polycrystalline silicon solar cells, are the most important PV technology with a global market share of more than 80%. The efficiencies of the monocrystalline and polycrystalline silicon solar cells with conventional aluminum (Al) back surface field (BSF) are currently 18.5% and 19.8%, respectively [1-3]. Due to the demand for further enhancing the cell efficiency and reducing the fabrication cost, the major PV manufacturers such as SolarWorld, Trina Solar, Jinko Solar, Canadian Solar Inc., Hebei JA SOLAR, GCL Solar Energy, Wuxi Suntech, and Zhejiang Astronergy have been working in the research and development of various high-efficiency, low-cost c-Si solar cells such as heterojunction-intrinsic- thin film, integrated back contact, and Topcon c-Si solar cells [4-6].

Considering the development history of c-Si solar cells in recent decades, the key technical factors to improve the c-Si cell efficiency can be summarized as follows: photolithographically defined metallization, surface texturization, shallow junction, improvements in antireflection coatings, selective emitter, front-and-rear surface passivation, and elimination of optical shading losses. Among them, the surface passivation is the most crucial as it effectively suppresses the recombination of photogenerated carriers. This is also important for future thinner substrates. Extensive research and development have been made to reduce the surface recombination of c-Si, and some valuable device structures have been proposed. In the past few years, the recorded efficiency of c-Si solar cells is refreshed over and over again. In particular, PERC solar cells are the most promising for industrialization and commercialization owing to their compatibilities with the industrial p-type c-Si solar cell processes. It is estimated that PERC solar cells will gradually replace the traditional Al-BSF c-Si solar cells in the next few years. The evolution of the commercial conversion efficiencies of PERC silicon solar cells using each technology is shown in Figure 11.1. At present, LONGi’s PERC solar cells reach the conversion efficiency of 24.02%.

Device Structure

Conventional Structure

The conventional c-Si solar cells are fabricated using the full Al-BSF and screenprinting technologies. Figure 11.2 shows the structure and fabrication flow of the conventional c-Si solar cell, which is mostly fabricated in the 2010s [7].

The evolution of the conversion efficiencies of PERC c-Si solar cells with the years (2012-2019)

FIGURE 11.1 The evolution of the conversion efficiencies of PERC c-Si solar cells with the years (2012-2019).

Structure and fabrication flow for a conventional Al-BSF c-Si solar cell,

FIGURE 11.2 Structure and fabrication flow for a conventional Al-BSF c-Si solar cell, (a) Schematic diagram of the cross section of the solar cell, (b-f) Five core steps that are used for the fabrication of the basic Al-BSF c-Si cells.

The absorber material is based on p-type c-Si wafer. In general, the processes in the industrial screen-printing technology include five core steps, namely, texturing and surface cleaning, phosphorus dopant diffusion to form p-n junction, coating antireflective layer on the front side, printing Al paste on the rear to form BSF and silver (Ag) pastes on the front, and co-firing the printed pastes to form ohmic contacts to rear base and front emitter. A detailed description of the Al-BSF and screen-printing technologies can be found in several studies [8,9].

The surface of the front emitter is passivated by silicon nitride (SiNs), which also acts as an antireflective layer. During the co-firing process, A1 atoms are doped into c-Si to form a BSF that prevents minority carriers from recombination at the rear surface. No additional dielectric thin films are used for passivating the rear surface of c-Si cells.

Passivated Emitter and Rear Contact Structure

For a PERC solar cell as shown in Figure 11.3, the front and rear surfaces of the c-Si are passivated by dielectrics [10]. The rear dielectric layer is partly opened by laser, and then metal can be contacted to the rear surface of c-Si. Compared to a conventional Al-BSF c-Si solar cell, PERC solar cell has a higher conversion efficiency mainly due to the additional passivating dielectric layer on the rear side that avoids the recombination of minority carriers at the rear surface. In addition, the rear dielectric layer can reflect the long-wavelength light from the rear surface back to the device to increase light absorption.

A square image with zigzag on top presents the structure of PERC solar cells. The structure from top to bottom includes Ag front contact, silicon nitride anti-reflection coating (ARC) layer, n-type c-Si emitter, p-type c-Si base, dielectric rear passivation layer, laser openings, and A1 rear contact.

 
Source
< Prev   CONTENTS   Source   Next >