Heterojunction Solar Cell Transparent Conductive Stack
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Solution Overview
Problem
Hetero-junction solar cells face low short-circuit current density due to the compromise between transparency and resistivity required in the transparent conductive material (TCM) layer, primarily attributed to the relatively low electron mobility of indium tin oxide (ITO) used in conventional TCM layers.
Innovation Solution
A front-sided transparent conductive material stack comprising at least two layers with different compositions and dopings, where one layer has higher carrier density for electrical contact and the other has higher mobility for optical transparency, formed from materials like indium oxide, zinc oxide, and cadmium oxide, with specific carrier concentrations and mobilities optimized to enhance both electrical and optical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single-layer transparent conductive material (TCM) is used, then the structure is simple and manufacturing is easy, but the short-circuit current density is low due to the compromise between transparency and resistivity
Solution Approach 1:
The patent divides the single TCM layer into multiple sub-layers with different compositions and functions. The first sub-layer (closer to the emitter) has higher carrier density for electrical contact, while the second sub-layer (closer to the TCO) has higher electron mobility for optical transparency and current extraction. This segmentation resolves the contradiction by allowing each sub-layer to be optimized for its specific function.
Solution Approach 2:
The patent applies local quality by giving different regions of the TCM stack different properties. The first sub-layer has higher doping concentration (1×10^20 to 1×10^21 cm^-3) for electrical contact, while the second sub-layer has lower doping (1×10^19 to 1×10^20 cm^-3) for transparency. This local differentiation allows simultaneous optimization of electrical and optical properties.
2Reliability
If the TCM layer has high carrier density for good electrical contact, then contact resistance decreases, but optical transparency is reduced
Solution Approach 1:
The patent segments the TCM into two sub-layers where the first sub-layer (near the emitter) has high carrier density (1×10^20 to 1×10^21 cm^-3) for low contact resistance, while the second sub-layer (near the TCO) has lower carrier density (1×10^19 to 1×10^20 cm^-3) for high optical transparency. This segmentation allows each layer to fulfill its specific electrical or optical function without compromising the other.
Solution Approach 2:
The patent implements local quality by spatially distributing different carrier densities within the TCM stack. The region closer to the emitter has higher doping for electrical contact, while the region closer to the TCO has lower doping for light transmission. This local differentiation resolves the contradiction between electrical reliability and optical performance.
3Illumination intensity
If the TCM layer has high electron mobility for optical transparency, then light transmission improves, but electrical conductivity is reduced
Solution Approach 1:
The patent segments the TCM functionality into two separate sub-layers: the second sub-layer (near TCO) has high electron mobility and lower carrier density for optimal optical transparency and current extraction, while the first sub-layer (near emitter) has high carrier density for electrical conductivity. This segmentation allows independent optimization of optical and electrical properties in different regions.
4Ease of manufacture
If conventional ITO is used as TCM, then the material is readily available and easy to manufacture, but the electron mobility is relatively low limiting current density
Solution Approach 1:
The patent uses composite TCM materials, specifically zinc oxide (ZnO) or cadmium oxide (CdO) as the second sub-layer, which have inherently higher electron mobility than conventional ITO. These materials are combined with an indium-based first sub-layer to create a composite stack that leverages the high mobility of ZnO/CdO while maintaining the good electrical contact properties of indium-based materials.
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 increases current density while reducing contact resistance with the front metallization grid, resulting in a photovoltaic device with improved efficiency by combining high transparency and conductivity in the TCM stack.
Implementation Method 1
a front-sided transparent conductive material stack... acting as a protective layer, as an antireflection coating and as a conductive film in order to extract laterally the current to the front grid metallization
Implementation Method 2
a front-sided transparent conductive material stack... acting as a protective layer, as an antireflection coating and as a conductive film
Implementation Method 3
their bandgap is wider than that of crystalline silicon (c-Si), and such films can reduce the c-Si surface state density by hydrogenation
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
Hetero-junction solar cell with front side TCO stack comprising a front-sided first transparent conductive layer (5) of a first base material with a first doping and a first non-stoichiometric composition, wherein the first base material is chosen from indium oxide, zinc oxide, tin oxide, and cadmium oxide; and a front-sided second transparent conductive layer (6, 6a, 6b) or structure of the first base material or a second base material, having the first or second doping and a second non-stoichiometric composition. The average carrier concentration of the whole front-sided transparent conductive material stack is between 1 x 1020 cm-3 and 2 x 1020 cm-3; the refractive indices (n) of the first and second layers are in the same range, wherein n > 2 at a wavelength of about 550 nm; and the carrier mobility in the font-sided first transparent conductive layer is higher than the carrier mobility in the front-sided second transparent conductive layer or structure if the layers/structure have a comparable thickness.