Solar Cell Ion Implantation Outdiffusion Barrier
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Solution Overview
Problem
The high-temperature annealing process for solar cells with phosphorus doped Back Surface Field (BSF) leads to deep diffusion and loss of doping agent, resulting in increased contact resistance and potential short-circuits, due to the phosphorus evaporating and causing autodoping issues.
Innovation Solution
A cap impermeable to oxygen is deposited on the rear side of the solar cell before the annealing step to prevent phosphorus outdiffusion, which can be left as an anti-reflection/passivation layer or etched back afterwards, and a layer that injects vacancies into silicon during annealing to slow down phosphorus diffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-temperature annealing is performed to cure the phosphorus doped BSF, then the doping is activated, but phosphorus diffuses deeply and evaporates causing increased contact resistance and potential short-circuits
Solution Approach 1:
A cap layer is deposited on the rear side surface before the annealing process to prevent phosphorus outdiffusion during the high-temperature treatment. This preliminary protective action ensures that the phosphorus remains in the desired depth range while still being activated by the annealing process.
Solution Approach 2:
The cap layer acts as an intermediary barrier between the phosphorus dopant and the external environment during annealing. It mediates the process by allowing thermal energy to activate the phosphorus while blocking the phosphorus atoms from diffusing out and evaporating.
2Reliability
If high-temperature annealing is performed to cure the phosphorus doped BSF, then the doping is activated, but Free-Carrier-Absorption increases lowering quantum efficiency
Solution Approach 1:
The cap layer is deposited before annealing to control the diffusion process. By preventing excessive phosphorus diffusion, it ensures that the BSF profile remains within optimal depth ranges, thereby maintaining low Free-Carrier-Absorption and high quantum efficiency while still achieving proper doping activation.
3Manufacturing precision
If ion implantation is used to create different doping profiles for boron and phosphorus, then precise doping control is achieved, but the annealing process causes autodoping and short-circuits
Solution Approach 1:
The cap layer is deposited on the rear side before annealing to prevent phosphorus outdiffusion and autodoping. This maintains the precise doping profile achieved through ion implantation while ensuring electrical stability by preventing short-circuits between emitter and BSF.
Solution Approach 2:
The cap layer serves as an intermediary barrier that preserves the distinct doping profiles of boron and phosphorus during annealing. It prevents the mixing and autodoping effects that would otherwise compromise the electrical stability and create short-circuit paths.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method maintains a stable BSF profile, reduces Free-Carrier-Absorption, and prevents autodoping, enhancing the quantum efficiency and contact resistance of the solar cells without adding significant process complexity.
Implementation Method 1
depositing a cap onto the rear side after implantation of the BSF and before the curing step. The cap prevents the outdiffusion of phosphorus
Implementation Method 2
The curing of the implanted boron emitter requires an annealing step at very high temperature (>1000° C.) in inert atmosphere
Implementation Method 3
a first doping region is formed by ion implantation of a first dopant and in the second surface of the semiconductor substrate a second doping region is formed by ion implantation or thermal indiffusion of a second dopant
Data Source
AI summary
The invention relates to a method for producing a solar cell (1) from crystalline semiconductor material, wherein a first doping region (5) is formed by means of ion implantation (S2) of a first dopant in a first surface (3a) of a semiconductor substrate (3), and a second doping region (7) is formed by means of ion implantation (S3) or thermal indiffusion of a second dopant in the second surface (3b) of the semiconductor substrate. After the doping of the second surface, a cap (9b) acting as an outdiffusion barrier for the second dopant is applied and an annealing step (S4) is subsequently carried out.