Tunnel Junction Layer Resistance in Compound Photovoltaic Cells
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Solution Overview
Problem
Existing compound photovoltaic cells face inefficiencies due to high resistance at the tunnel junction interface, which hinders the flow of tunnel current and reduces conversion efficiency.
Innovation Solution
A compound photovoltaic cell design featuring a tunnel junction layer with a p+-type (Al)GaInAs layer and an n+-type InP layer, or a p+-type AlGaInAs layer and an n+-type InP layer, is implemented, reducing the resistance at the joint interface and enhancing tunnel current flow by degenerating the conduction and valence bands, allowing carriers to tunnel more effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional tunnel junction layer is used to connect photoelectric conversion cells, then the cell structure is simple, but the resistance at the joint interface is high which reduces conversion efficiency
Solution Approach 1:
The tunnel junction layer is segmented into multiple distinct layers: a first highly-doped layer (n-type or p-type) and a second highly-doped layer (opposite polarity) with each layer optimized for specific functions. This segmentation allows the first layer to provide strong doping for low resistance while the second layer facilitates effective tunneling, thereby reducing joint interface resistance and improving conversion efficiency without complicating the manufacturing process
Solution Approach 2:
Different regions of the tunnel junction layer are assigned different doping types and concentrations tailored to their specific functional requirements. The first highly-doped layer uses one polarity to establish low resistance contact, while the second highly-doped layer uses the opposite polarity to enable tunneling. This local optimization of material properties reduces overall interface resistance while maintaining structural simplicity
2Reliability
If the band gap of photoelectric conversion cells is increased to absorb broader wavelength sunlight, then energy conversion efficiency improves, but the lattice mismatch with the substrate increases making fabrication difficult
Solution Approach 1:
A buffer layer is introduced as an intermediary between the GaAs substrate and the GaInAs photoelectric conversion cell. This buffer layer has a composition and lattice constant that gradually transitions from matching the GaAs substrate to matching the GaInAs cell, thereby mediating the lattice mismatch and enabling the fabrication of high band gap cells that absorb broader wavelength sunlight without structural defects
Solution Approach 2:
The composition ratio of the buffer layer is carefully controlled and adjusted to achieve optimal lattice matching. By varying the indium and gallium content in the buffer layer, the lattice constant is tuned to bridge the gap between the GaAs substrate and the GaInAs cell, enabling successful epitaxial growth of high efficiency photoelectric conversion cells with reduced dislocation density
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 design significantly reduces the resistance at the tunnel junction interface, enabling improved tunnel current flow and increasing the overall conversion efficiency of the photovoltaic cell by allowing sunlight to be absorbed across a broader wavelength range.
Implementation Method 1
the tunnel junction layer includes a p-type layer disposed on the incident side and a n-type layer disposed on the back side, the p-type layer being a p+-type (Al)GaInAs layer, the n-type layer being an n+-type InP layer, an n+-type GaInP layer having a tensile strain with respect to InP, or an n+-type Ga(In)PSb layer having a tensile strain with respect to InP
Implementation Method 2
one or more first photoelectric conversion cells configured to be made of a first compound semiconductor material and to be formed on the first substrate
Data Source
AI summary
A compound photovoltaic cell includes a substrate, a first cell made of a first semiconductor material and formed on the substrate, a tunnel layer, and a second cell made of a second semiconductor material lattice mismatched with a material of the substrate, connected to the first cell via the tunnel layer, and disposed on an incident side with respect to the first cell, wherein band gaps of the first and the second cells become smaller from an incident side to a back side, and wherein the tunnel layer includes a p-type layer disposed on the incident side and a n-type layer disposed on the back side, the p-type layer being a p+-type (Al)GaInAs layer, the n-type layer being an n+-type InP layer, an n+-type GaInP layer having a tensile strain with respect to InP or n+-type Ga(In)PSb layer having a tensile strain with respect to InP.