Germanium Photovoltaic Cell Back Surface Texturing
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Solution Overview
Problem
Existing multi-junction photovoltaic cells are inefficient in converting electromagnetic radiation at wavelengths between 1,550 nm to 1,800 nm due to indirect electronic transitions and high recombination rates of minority carriers in germanium cells, leading to low external quantum efficiency and reduced photocurrent.
Innovation Solution
A photoactive device with an active region comprising a semiconductor material having a bandgap of 0.60 eV to 2.10 eV, featuring a front surface with low surface roughness and a back surface with increased surface roughness to enhance optical path length and absorption of longer wavelengths, and employing epitaxial growth techniques to minimize defects and optimize minority carrier diffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If germanium cells are used to capture longer wavelengths (1,550 nm to 1,800 nm), then the spectral absorption range is extended, but the external quantum efficiency drops due to indirect electronic transitions requiring phonon assistance
Solution Approach 1:
The patent changes the physical parameters of the germanium cell by reducing its thickness to 1-5 micrometers and introducing a textured surface with specific geometric features. These parameter changes modify the optical and electrical characteristics to compensate for the indirect bandgap limitation, enabling efficient absorption and conversion of longer wavelength photons while maintaining high external quantum efficiency.
Solution Approach 2:
The patent introduces surface texturing that creates three-dimensional optical paths within the thin germanium layer. By creating a textured surface with ridges, grooves, or pyramidal structures, photons undergo multiple internal reflections and traverses through the active layer, effectively increasing the optical path length without increasing the physical thickness, thus enhancing absorption probability for indirect transitions.
2Quantity of substance
If the germanium layer thickness is increased to improve absorption of longer wavelengths, then more photons can be absorbed, but minority carrier recombination increases due to longer diffusion paths
Solution Approach 1:
The patent uses surface texturing to create multiple optical paths and internal reflections within the thin germanium layer. This dimensional transformation allows photons to traverse the absorption medium multiple times without increasing the physical thickness, thereby increasing the effective absorption path length while keeping the carrier diffusion distance short and recombination losses minimal.
Solution Approach 2:
The textured surface introduces curved and non-linear optical paths through the germanium layer via internal reflections at the textured interfaces. This curvature in the optical path compensates for the limited physical thickness by effectively increasing the absorption distance without proportionally increasing the carrier transport distance, thus reducing recombination losses.
3Quantity of substance
If a textured surface is introduced to increase optical path length, then absorption of longer wavelengths is improved, but the manufacturing complexity increases
Solution Approach 1:
The patent optimizes the texturing parameters (feature size, depth, spacing) to fall within specific ranges that can be achieved by standard semiconductor fabrication techniques such as chemical etching or plasma treatment. By controlling these parameters, the complex surface structure can be manufactured with existing industrial processes, balancing enhanced optical absorption with manufacturing feasibility.
4Reliability
If the germanium cell is designed for direct electronic transitions, then higher efficiency is achieved at shorter wavelengths, but absorption at longer wavelengths (1,550 nm to 1,800 nm) becomes inefficient
Solution Approach 1:
The patent modifies the physical parameters of the germanium structure (thickness, surface topology) to enhance the probability of indirect electronic transitions by increasing the optical interaction time and path length. This allows the cell to efficiently convert longer wavelength photons despite the indirect bandgap nature, thereby expanding the usable wavelength range while maintaining acceptable conversion efficiency.
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 solution significantly improves the absorption of photons across a broader spectral range, enhancing the conversion efficiency of electromagnetic radiation into electrical energy by increasing the probability of electron-hole pair generation and reducing recombination losses.
Implementation Method 1
Photoactive devices are semiconductor devices that employ semiconductor material to convert electromagnetic radiation into electrical energy
Implementation Method 2
indirect electronic transition between the conduction band and the valence band
Implementation Method 3
the back surface of the first active layer has a surface roughness that is greater than a surface roughness of the front surface of the first active layer
Implementation Method 4
the minority carrier (electron) diffusion length within the Ge is shorter than the actual physical thickness of the Ge layer
Implementation Method 5
Defects in the crystal lattices of the semiconductor materials at the pn junctions provide locations at which electrons and holes previously generated by absorption of radiation can recombine
Data Source
AI summary
Photoactive devices include an active region disposed between first and second electrodes and configured to absorb radiation and generate a voltage between the electrodes. The active region includes an active layer comprising a semiconductor material exhibiting a relatively low bandgap. The active layer has a front surface through which radiation enters the active layer and a relatively rougher back surface on an opposing side of the active layer. Methods of fabricating photoactive devices include the formation of such an active region and electrodes.