Cd(Se,Te) Thin-Film Annealing for Extreme Lateral Grain Growth
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Solution Overview
Problem
Polycrystalline thin film growth processes, such as for solar cells, are limited by small grain sizes due to rapid deposition techniques and non-ideal substrates, leading to increased grain boundaries that degrade performance by reducing carrier lifetime and mobility.
Innovation Solution
A method involving the deposition of a polycrystalline CdSexTe1-x alloy film on a substrate with a high surface energy interfacial layer, followed by annealing in a Se or Te atmosphere, which promotes grain growth up to 1000 μm in dimension, reducing grain boundaries and enhancing carrier transport.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If rapid deposition techniques are used to grow polycrystalline thin films, then deposition speed is improved, but grain size is limited to only a few microns
Solution Approach 1:
The patent changes the physical-chemical parameters of the deposition process by using close-spaced sublimation at controlled temperatures (200-400°C) with specific pressure conditions, allowing rapid deposition while achieving unusually large grain sizes up to 1000 microns through precise parameter optimization
Solution Approach 2:
The patent applies preliminary low-temperature annealing treatment to the deposited film before final processing. This preliminary action promotes grain growth and reduces grain boundary density, enabling the film to achieve large grain sizes that would not be possible through deposition alone
2Length of moving object
If annealing is performed at temperatures equal to or greater than substrate temperature to increase grain size, then grain size is improved, but re-evaporation of CdTe causes surface coarsening or complete film elimination
Solution Approach 1:
The patent performs annealing in an inert atmosphere (argon or vacuum) that prevents excessive oxidation and controls the chemical environment. This inert environment allows higher annealing temperatures to be used for grain growth without causing uncontrolled re-evaporation or surface degradation
Solution Approach 2:
The patent uses a two-stage approach where volatile components are allowed to evaporate controllably during annealing, then re-deposit onto the film surface. This discarding and recovering process actually improves film quality by removing defects while maintaining film integrity and promoting grain growth
3Reliability
If grain size is increased to reduce grain boundaries, then carrier lifetime is improved, but device complexity increases due to required processing steps
Solution Approach 1:
The patent merges the deposition and initial grain growth processes by using close-spaced sublimation followed immediately by in-situ low-temperature annealing. This combination achieves large grain sizes in a single integrated process sequence, reducing the number of separate processing steps and overall device complexity
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
The method achieves significantly larger grain sizes and improved carrier mobility, resulting in enhanced solar cell performance by increasing the grain-size-to-thickness ratio beyond previous limits, with minimal voiding and pinholes, and avoids the corrosive issues associated with CdCl2 treatments.
Implementation Method 1
annealing in a Se or Te atmosphere, which promotes grain growth up to 1000 μm in dimension
Implementation Method 2
The rate or speed at which grain growth occurs is determined by the barriers to the atomistic diffusion required for grain growth
Implementation Method 3
an over-pressure of Se, Te, or a mixture of Se and Te such that Se, Te, or the Se and Te mixture diffuses into the polycrystalline film of CdSexTe1-x alloy film
Implementation Method 4
Se, Te, or the Se and Te mixture diffuses into the polycrystalline film of CdSexTe1-x alloy film
Implementation Method 5
depositing an interfacial layer upon a substrate wherein the surface energy of the interfacial layer is greater than the surface energy of the CdSexTe1-x alloy film
Implementation Method 6
the surface energy of the interfacial layer is greater than 200 erg/cm2 upon the substrate
Implementation Method 7
the use of rapid deposition techniques, lower temperatures, and non-ideal substrates
Implementation Method 8
close-spaced sublimation
Data Source
AI summary
Disclosed herein are compositions and methods for making polycrystalline thin films having very large grains sizes and exhibiting improved properties over existing thin films.