Heterojunction Solar Cell TCO Layering for Lower Contact Resistance
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Solution Overview
Problem
Heterojunction solar cells face a trade-off between optical and electrical properties of transparent conductive oxide (TCO) layers, leading to increased contact resistance and reduced efficiency due to the formation of parasitic Schottky barriers at the TCO-semiconductor interface.
Innovation Solution
A transparent-conductive region with a layered structure is introduced, where a second layer with a higher work function is interposed between the first layer and the semiconductor layer to reduce parasitic potential barriers, and a first layer with a lower work function is used to increase reflectance of unabsorbed photons, enhancing charge carrier extraction and conductivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a TCO layer is configured with a low work function to increase conductivity, then electrical conductivity is improved, but contact resistance with the semiconductor layer increases due to formation of parasitic Schottky barriers
Solution Approach 1:
The TCO layer is segmented into multiple sub-layers with different work functions. The first sub-layer (closest to semiconductor) has a higher work function to reduce Schottky barrier formation, while the second sub-layer (closest to electrode) has a lower work function to enhance conductivity and carrier extraction. This segmentation allows each sub-layer to optimize for its specific function.
Solution Approach 2:
Different regions of the TCO structure are assigned different work function properties. The interface region with the semiconductor layer uses a higher work function material to minimize contact resistance, while the bulk region closer to the electrode uses a lower work function material to maximize conductivity and carrier collection efficiency.
2Object-affected harmful factors
If a TCO layer is configured with a high work function to reduce contact resistance, then contact resistance is reduced, but electrical conductivity decreases
Solution Approach 1:
The TCO layer is divided into multiple sub-layers where the first sub-layer (at the semiconductor interface) has a higher work function to reduce contact resistance, while the second sub-layer (toward the electrode) has a lower work function to maintain high conductivity for efficient carrier transport.
Solution Approach 2:
The TCO structure implements spatially varying work function properties: the region adjacent to the semiconductor layer uses higher work function material to minimize interface resistance, while the region adjacent to the electrode uses lower work function material to maximize electrical conductivity and carrier extraction.
3Device complexity
If a single-layer TCO structure is used, then device complexity is low, but it cannot simultaneously optimize both optical and electrical properties
Solution Approach 1:
The TCO layer is segmented into multiple sub-layers, each with tailored work functions. The first sub-layer optimizes for interface contact resistance reduction, while the second sub-layer optimizes for bulk conductivity and carrier extraction, enabling simultaneous optimization of electrical and optical properties.
Solution Approach 2:
The TCO structure uses a composite of different transparent conductive oxide materials with different work functions. This composite structure allows the first layer to provide low contact resistance at the semiconductor interface while the second layer provides high conductivity for carrier transport, achieving properties that neither material could provide alone.
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 configuration improves the fill factor and efficiency of the solar cell by reducing contact resistance and increasing the extraction of photo-generated carriers, while maintaining high electrical conductivity.
Implementation Method 1
a second layer having a second work function and being interposed between the first layer and the semiconductor layer. The second work function of the second layer is greater than the first work function of the first layer
Implementation Method 2
The increased contact resistance results from the formation of a potential barrier (e.g. a parasitic Schottky barrier) at the interface between the TCO and the semiconductor layer
Implementation Method 3
the first layer with a lower work function... leads to increased reflectance of unabsorbed photons back towards the photo-active layers of the solar cell
Implementation Method 4
The TCO layers are arranged to extract charge carriers from the active layers (e.g. the surface field and emitter layers) of the solar cell and transport them to the respective electrodes
Implementation Method 5
The p-n junction facilitates the generation of an electric current in response to light incident on the solar cell
Data Source
AI summary
A solar cell comprising a crystalline silicon substrate, a semiconductor layer arranged on a back surface of the substrate which is configured not to face a radiative source, when the solar cell is in use, and a transparent-conductive region arranged on a surface of the semiconductor layer, wherein the transparent conductive region comprises; a first layer having a first work function; and a second layer having a second work function and being interposed between the first layer and the semiconductor layer; wherein the second work function of the second layer is greater than the first work function of the first layer.