Silicon wafer solar cells continue to be the leading photovoltaic technology, and in many places are now providing a substantial portion of electricity generation. Further adoption of this technology will require processing that minimises losses in device performance. A fundamental mechanism for efficiency loss is the recombination of photo-generated charge carriers at the unavoidable cell surfaces. Dielectric coatings have been shown to largely prevent these losses through a combination of different passivation mechanisms. This review aims to provide an overview of the dielectric passivation coatings developed in the past two decades using a standardised methodology to characterise the metrics of surface recombination across all techniques and materials. The efficacy of a large set of materials and methods has been evaluated using such metrics and a discussion on the current state and prospects for further surface passivation improvements is provided.

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Dielectric surface passivation for silicon solar cells: A review

We examine the surface passivation properties of Al2O3 deposited on boron-doped planar ⟨100⟩ crystalline silicon surfaces as a function of the boron concentration. Both uniformly doped and diffused surfaces are studied, with surface boron concentrations ranging from 9.2 × 1015 to 5.2 × 1019 cm−3. Atmospheric pressure chemical vapor deposition and thermal atomic layer deposition are used to deposit the Al2O3 films. The surface recombination rate of each sample is determined from photoconductance measurements together with the measured dopant profiles via numerical simulation, using the latest physical models. These values are compared with calculations based on the interface properties determined from capacitance–voltage and conductance measurements. It is found that the fundamental surface recombination velocity of electrons, Sn0, which describes the chemical passivation of the interface, is independent of the surface boron concentration Ns for Ns ≤ 3 × 1019 cm−3, and in excellent agreement with values calculated from the interface state density Dit and capture coefficients cn and cp measured on undiffused boron-doped surfaces. We conclude that the physical properties of the Si–Al2O3 interface are independent of the boron dopant concentration over this range.

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Effect of boron concentration on recombination at the p-Si–Al2O3 interface

The book addresses the problem of passivation at the surface of crystalline silicon solar cells. More specifically, it reports on a high-throughput, industrially compatible deposition method for Al2O3, enabling its application to commercial solar cells. One of the main focus is on the analysis of the physics of Al2O3 as a passivating dielectric for silicon surfaces. This is accomplished through a comprehensive study, which moves from the particular, the case of aluminium oxide on silicon, to the general, the physics of surface recombination, and is able to connect theory with practice, highlighting relevant commercial applications.

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New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface

The interface microstructure of a silicon heterojunction (SHJ) solar cell was investigated. We found an ultrathin native oxide layer (NOL) with a thickness of several angstroms was formed on the crystalline silicon (c-Si) surface in a very short time (∼30 s) after being etched by HF solution. Although the NOL had a loose structure with defects that are detrimental for surface passivation, it acted as a barrier to restrain the epitaxial growth of hydrogenated amorphous silicon (a-Si:H) during the plasma-enhanced chemical vapor deposition (PECVD). The microstructure change of the NOL during the PECVD deposition of a-Si:H layers with different conditions and under different H2 plasma treatments were systemically investigated in detail. When a brief H2 plasma was applied to treat the a-Si:H layer after the PECVD deposition, interstitial oxygen and small-size SiO2 precipitates were transformed to hydrogenated amorphous silicon suboxide alloy (a-SiO(x):H, x ∼ 1.5). In the meantime, the interface defect density was reduced by about 50%, and the parameters of the SHJ solar cell were improved due to the post H2 plasma treatment.

Evolution of a Native Oxide Layer at the a-Si:H/c-Si Interface and Its Influence on a Silicon Heterojunction Solar Cell.

Ion Implantation for All-alumina IBC Solar Cells with Floating Emitter

We report an outstanding level of surface passivation for both n+ and p+ silicon by AlOx/SiNx dielectric stacks deposited in an inline plasma-enhanced chemical vapor deposition (PECVD) reactor for a wide range of sheet resistances. Extremely low emitter saturation current densities (J0e) of 12 and 200 fA/cm2 are obtained on 165 and 25 Ω/sq n+ emitters, respectively, and 8 and 45 fA/cm2 on 170 and 30 Ω/sq p+ emitters, respectively. Using contactless corona-voltage measurements and device simulations, we demonstrate that the surface passivation mechanism on both n+ and p + silicon is primarily due to a relatively low interface defect density of <;1011 eV-1cm-2 in combination with a moderate fixed negative charge density of (1-2) × 1012 cm-2. From advanced modeling, the fundamental surface recombination velocity parameter is shown to be in the order of 104 cm/s for PECVD AlOx/SiNx passivated heavily doped n+ and p+ silicon surfaces.

Serena Fen Lin

Papers

Progress in Surface Passivation of Heavily Doped n-Type and p-Type Silicon by Plasma-Deposited AlO $_{\bm x}$ /SiN $_{\bm x}$ Dielectric Stacks