CdTe Double Heterostructures for High Voc Solar Cells
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Solution Overview
Problem
CdTe-based solar cells face challenges in achieving high open-circuit voltage (Voc) and efficiency due to short bulk carrier lifetimes, high interface recombination velocity, and low p-type doping levels, which limit the maximum achievable Voc and efficiency.
Innovation Solution
The implementation of CdTe/MgxCd1-xTe double-heterojunction structures grown using Molecular Beam Epitaxy (MBE) on InSb substrates, with MgxCd1-xTe barrier layers confining minority carriers and providing effective passivation, allowing for longer carrier lifetimes and reduced interface recombination velocity, and the use of p-type a-SiCx:H contact layers to enhance Voc and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CdTe solar cell structures are used, then manufacturing is simpler, but open-circuit voltage and efficiency are limited due to short bulk carrier lifetimes and high interface recombination velocity
Solution Approach 1:
The solar cell is divided into distinct functional layers: a CdTe absorber layer sandwiched between wider-bandgap barrier layers (such as CdZnTe or CdMgTe). This segmentation isolates the absorber from direct contact with electrodes, reducing interface recombination and enabling higher open-circuit voltages while maintaining manageable manufacturing complexity through modular layer construction.
Solution Approach 2:
Barrier layers with wider bandgaps are introduced as intermediary structures between the CdTe absorber and the electrodes. These intermediary layers act as selective contacts that block minority carriers while allowing majority carriers to pass, thereby reducing interface recombination velocity and extending effective carrier lifetime without requiring complex doping schemes.
2Reliability
If p-type doping is increased in CdTe absorber, then built-in potential increases, but doping challenges and material quality deteriorate
Solution Approach 1:
The barrier layers serve as intermediaries that enable the use of heavily doped contact regions without compromising the quality of the intrinsic or lightly-doped absorber region. The wider-bandgap barrier layers provide a transition zone that facilitates carrier extraction while maintaining low defect density in the absorber, thus achieving high built-in potential without sacrificing material quality.
Solution Approach 2:
Different regions of the device are assigned different doping levels and material compositions optimized for their specific functions: the absorber layer maintains low doping for high carrier lifetime, while the barrier/contact layers are heavily doped for efficient carrier extraction. This local optimization allows high built-in potential to be achieved without degrading overall material quality.
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 results in CdTe solar cells with Voc exceeding 1 V and efficiency of up to 17%, surpassing previous records, with improved bulk carrier lifetime and reduced interface recombination velocity, enabling higher Voc and efficiency while addressing the challenges of p-type doping and interface recombination.
Implementation Method 1
MgxCd1-xTe barrier layers confining minority carriers
Implementation Method 2
providing effective passivation, allowing for longer carrier lifetimes and reduced interface recombination velocity
Implementation Method 3
high-quality monocrystalline CdTe epitaxially grown on InSb (001) substrate with the use of Molecular Beam Epitaxy (MBE)
Implementation Method 4
CdTe possesses the preferable optical properties for photovoltaic (PV) applications: a near optimum bandgap of 1.5 eV, and a high absorption coefficient of over 15,000 cm−1 at the band edge
Data Source
AI summary
Devices converting light to electricity (such as solar cells or photodetectors) including a heavily-doped p-type a-SiCy:H and an i-MgxCd1-xTe/n-CdTe/N—Mg0.24Cd0.76Te double heterostructure (DH), with power conversion efficiency of as high as 17%, Voc as high as 1.096 V, and all operational characteristics being substantially better than those of monocrystalline solar cells known to-date. The a-SiCy:H layer is configured to enable high built-in potential while, at the same time, allowing the doped absorber to maintain a very long carry lifetime. In comparison, similar undoped CdTe/MgxCd1-xTe DH designs reveal a long carrier lifetime of 3.6 μs and an interface recommendation velocity of 1.2 cm/s, which are lower than the record values reported for GaAs/Al0.5Ga0.5As (18 cm/s) and GaAs/Ga0.5In0.5P (1.5 cm/s) DHs.