Black Silicon Solar Cell Blue Response via Nanostructure Sizing
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Solution Overview
Problem
Black silicon solar cells face efficiency limitations due to high photocarrier recombination and poor spectral response at short wavelengths, particularly blue and green light, leading to lower conversion efficiencies compared to conventional anti-reflection coatings.
Innovation Solution
The implementation of a nanostructured silicon solar cell design with controlled Auger recombination in the black silicon emitter, reduced doping, and larger feature sizes of 50 to 200 nm for the black silicon nanostructures, along with the use of silver instead of gold to minimize recombination, enhances blue response and overall efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If black silicon nanostructures with small lateral dimensions are used to reduce reflectivity, then anti-reflection performance is improved, but photocarrier recombination increases and blue response deteriorates
Solution Approach 1:
The patent changes the lateral dimension parameter of black silicon nanostructures from conventional small sizes (below 60 nm) to larger sizes (65 to 200 nm). This parameter change reduces the surface area to volume ratio, thereby reducing photocarrier recombination at the silicon surface while maintaining effective anti-reflection performance. The larger lateral dimensions allow better collection of blue photons that would otherwise be lost to recombination in high-surface-area structures.
Solution Approach 2:
The patent combines black silicon nanostructures with a specific doping profile (emitter layer) to create a composite structure that addresses both anti-reflection and carrier collection needs. The composite design integrates the optical benefits of black silicon with the electrical benefits of controlled doping, achieving both low reflectivity and high blue response efficiency.
2Reliability
If heavy doping is applied in black silicon solar cells to improve electrical properties, then conductivity is improved, but Auger recombination increases and efficiency decreases
Solution Approach 1:
The patent optimizes the doping concentration parameter in the emitter layer, using moderate doping levels rather than heavy doping. Specifically, the emitter layer is doped to achieve appropriate conductivity while maintaining dopant concentrations that avoid excessive Auger recombination. This parameter optimization balances electrical conductivity requirements with the need to minimize recombination losses in the black silicon structure.
3Manufacturing precision
If gold is used as catalyst in black silicon formation, then nanostructure formation is improved, but recombination centers are introduced and blue response is reduced
Solution Approach 1:
The patent replaces gold catalyst with silver catalyst in the black silicon formation process. Silver is less expensive than gold and, more importantly, does not introduce strong recombination centers in silicon. The silver catalyst performs the necessary function of catalyzing the etching process to form black silicon nanostructures while being more compatible with silicon's electrical properties, thereby preserving blue response efficiency.
Solution Approach 2:
The patent uses silver as an intermediary catalyst material that mediates the formation of black silicon nanostructures without creating harmful recombination centers. Silver serves as the intermediary between the etching process and the silicon substrate, providing the necessary catalytic function while being electrically benign in the silicon lattice, unlike gold which creates deep-level recombination centers.
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 a 18.2% efficient solar cell with improved internal quantum efficiency at short wavelengths, surpassing previous black silicon solar cell efficiencies and approaching those of conventional AR-coated cells, while maintaining low reflectance and cost-effectiveness.
Implementation Method 1
a density-graded surface is provided that includes structures that have low porosity near the bulk material but that gradually change to high porosity near the material-air surface (e.g., upper hundred of nanometers or few microns of the silicon material or layer of the solar cell or other silicon-based device)
Implementation Method 2
In testing, such density-graded surfaces have proven to reduce reflective losses from a silicon surface to below two percent, or even below one percent, across the useful solar spectrum
Implementation Method 3
The performance of solar cells and other optoelectronic devices is directly related to optical losses caused by high reflectivity
Implementation Method 4
The emitter layer is formed to have a dopant level such that it has a sheet resistance of at least 90 ohms/sq (e.g., 90 to 170 ohm/sq)
Data Source
AI summary
A photovoltaic (PV) device with improved blue response. The PV device includes a silicon substrate with an emitter layer on a light receiving side. The emitter layer has a low opant level such that it has sheet resistance of 90 to 170 ohm/sq. Anti-reflection in the PV device is provided solely by a nano-structured or black silicon surface on the light-receiving surface, through which the emitter is formed by diffusion. The nano structures of the black silicon are formed in a manner that does not result in gold or another high-recombination metal being left in the black silicon such as with metal-assisted etching using silver. The black silicon is further processed to widen these pores so as to provide larger nanostructures with lateral dimensions in the range of 65 to 150 nanometers so as to reduce surface area and also to etch away a highly doped portion of the emitter.