Double-Graded CdSeTe Thin Film for Bandgap and Phase Stability
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Solution Overview
Problem
CdTe thin film solar cells face challenges in optimizing the energy gap throughout the photoactive layer, particularly in forming a CdSeTe structure with an optimized energy gap at both the front and back sides, and high Se content can lead to a phase change from zinc-blende to wurtzite structure, negatively affecting performance.
Innovation Solution
A method involving the formation of a double-graded CdSeTe thin film structure with three layers of varying selenium content, where the first and third layers have higher energy gaps than the middle layer, allowing for a continuous energy gap transition, achieved through specific deposition and annealing processes, including co-deposition of cadmium, selenium, and tellurium, and subsequent temperature treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If high amounts of Se are included in the photoactive layer to optimize the energy gap, then the energy gap can be reduced for better light absorption, but this leads to a phase change from zinc-blende to wurtzite structure which negatively affects solar cell performance
Solution Approach 1:
The patent applies local quality by creating a double-graded CdSeTe thin film structure where the selenium concentration varies spatially - with higher Se content (w=0.25 to 0.4) in the middle layer for energy gap optimization, and lower Se content (w<0.25) in the outer layers to maintain zinc-blende phase stability. This local variation in composition allows simultaneous optimization of energy gap and phase structure in different regions of the same photoactive layer.
Solution Approach 2:
The patent uses composite materials by forming a multi-layered CdSeTe structure with varying selenium concentrations (different compositions of the same material system). The composite nature of having multiple layers with graded Se content allows the system to combine the benefits of low energy gap (from high Se regions) with phase stability (from low Se regions), achieving neither would be possible with a uniform composition.
2Use of energy by moving object
If a gradient in Se concentration is formed to optimize energy gap at front and back sides, then photovoltaic conversion efficiency can be improved, but the manufacturing process becomes complicated requiring multiple deposition and annealing steps
Solution Approach 1:
The patent applies segmentation by dividing the photoactive layer into three distinct sub-layers with different selenium concentrations (first layer with w<0.25, middle layer with w=0.25 to 0.4, third layer with w<0.25). This segmentation allows independent optimization of each layer's properties and simplifies the manufacturing process compared to attempting to create a continuous gradient in a single layer, as each layer can be deposited and annealed separately with controlled Se content.
Solution Approach 2:
The patent applies preliminary action by performing multiple annealing treatments at different stages - first annealing the initial CdTe layer, then depositing and annealing the middle layer with higher Se content, and finally depositing and annealing the outer layers. This stepwise preliminary treatment of each layer before final assembly ensures proper phase formation and Se distribution, reducing the need for complex post-processing and simplifying the overall manufacturing sequence.
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 approach enhances the energy gap difference at the p-n junction and reduces charge carrier recombination, improving the photovoltaic conversion efficiency by tailoring the energy gap along the entire CdSeTe thin film layer, while maintaining the zinc-blende phase even at higher selenium content.
Implementation Method 1
An optional temperature treatment step performed subsequently to the deposition processes, causes interdiffusion and reaction of atoms comprised in different deposited layers
Implementation Method 2
including high amounts of Se into CdTe may lead to a phase change of the CdSeTe layer from zinc-blende structure to wurtzite structure
Data Source
AI summary
The present invention proposes a method to form a double-graded CdSeTe thin film. The method comprises providing a base substrate, forming a first CdSewTe1-w layer having a first amount w1 of selenium in it, forming a second CdSewTe1-w layer having a second amount w2 of selenium in it and forming a third CdSewTe1-w layer having a third amount w3 of selenium in it. The second amount w2 lies in the range between 0.25 and 0.4, whereas each of the amounts w1 and w3 lies in the range extending from 0 to 1. According to the present invention, the energy gap in the first and the third CdSewTe1-w layers is equal to or higher than 1.45 eV and the energy gap in the second CdSewTe1-w layer lies in the range between 1.38 eV and 1.45 eV and is smaller than the energy gap in the first and the third CdSewTe1-w layers.