Homogeneous Junction Layer for Photovoltaic Back Surface Field
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Solution Overview
Problem
Conventional heterojunction solar cells with intrinsic thin layers face limitations in backside light collection due to absorption in doped and intrinsic a-Si:H layers, especially in bifacial modes, leading to reduced solar cell efficiency and performance.
Innovation Solution
A photovoltaic device with a back-surface-field junction featuring a homogeneous junction layer of higher active doping density and thickness less than the minority carrier diffusion length, replacing the conventional intrinsic layer, and a passivation layer, either doped or undoped, to enhance light collection and passivation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a doped layer of a-Si:H is used to establish an electric field to repel minority carriers from the back surface, then carrier repulsion is improved, but light absorption increases reducing backside collection efficiency
Solution Approach 1:
The back surface field contact is segmented into multiple functional layers: a thin intrinsic a-Si:H layer for passivation, a thin doped a-Si:H layer for electric field formation, and a novel crystalline silicon layer for enhanced carrier repulsion with reduced light absorption. This segmentation allows each layer to optimize its specific function while minimizing overall drawbacks.
Solution Approach 2:
The invention changes the material phase parameter from amorphous to crystalline in the doped layer, and optimizes the doping concentration and thickness parameters. The crystalline silicon layer with specific doping levels (10^19 to 10^21 cm^-3) and thickness (1-25 nm) provides superior electrical properties while maintaining optical transparency compared to conventional amorphous layers.
2Reliability
If an intrinsic a-Si:H layer is used for back surface passivation, then passivation is improved, but light absorption increases reducing bifacial performance
Solution Approach 1:
The invention extracts and eliminates the thick intrinsic a-Si:H layer that causes excessive light absorption, replacing it with a thin crystalline silicon layer that provides equivalent or superior passivation with minimal optical loss. This extraction of the problematic layer while retaining its essential function resolves the contradiction.
Solution Approach 2:
The back surface field contact uses a composite structure combining amorphous a-Si:H layers with a crystalline silicon layer. This composite material approach leverages the advantages of both material types: the amorphous layers provide good interface passivation while the crystalline layer provides efficient carrier repulsion with reduced light absorption.
3Ease of manufacture
If conventional heterojunction structure is used, then manufacturing is simplified, but backside light collection is limited
Solution Approach 1:
The invention adds a new dimensional aspect to the conventional heterojunction structure by introducing a crystalline silicon layer with specific orientation and doping characteristics. This additional structural dimension enables enhanced carrier repulsion and improved backside light collection without fundamentally complicating the manufacturing process.
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 short circuit current, fill-factor, and open circuit voltage by reducing absorption losses and band offsets, resulting in increased solar cell efficiency and better passivation of the back surface.
Implementation Method 1
a doped layer of a-Si:H having the same conductivity type as that of the c-Si substrate to establish an electric field to repel minority carriers
Implementation Method 2
backside collection is limited by absorption in the doped layer and intrinsic a-Si:H layer
Implementation Method 3
a thin intrinsic (i) hydrogenated amorphous silicon (a-Si:H) layer to passivate the back surface of a crystalline Si (c-Si) substrate
Data Source
AI summary
A photovoltaic device and method include a crystalline substrate and an emitter contact portion formed in contact with the substrate. A back-surface-field junction includes a homogeneous junction layer formed in contact with the crystalline substrate and having a same conductivity type and a higher active doping density than that of the substrate. The homogeneous junction layer includes a thickness less than a diffusion length of minority carriers in the homogeneous junction layer. A passivation layer is formed in contact with the homogeneous junction layer opposite the substrate, which is either undoped or has the same conductivity type as that of the substrate.