Transparent Tunnel Junction Back Contact for Bifacial Solar Cells
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Solution Overview
Problem
Conventional thin film solar cells face challenges in achieving high conversion efficiency due to difficulties in creating satisfactory back contacts for ohmic contact and charge carrier transport, as well as issues with back surface hole-electron recombination and inefficient collection of light, particularly infrared photons.
Innovation Solution
The implementation of a bifacial photovoltaic device with an n-type transparent back contact layer, incorporating a tunnel junction, an optical reflector, and an electron reflector layer to enhance light collection and charge carrier transport, thereby improving conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional back contact structures are used, then manufacturing is simpler, but charge carrier transport and ohmic contact are insufficient
Solution Approach 1:
The back contact is segmented into multiple functional layers: a first back contact layer for ohmic contact with the absorber layer, a second back contact layer for enhanced charge carrier transport, and a third back contact layer for additional transport improvement. This segmentation allows each layer to specialize in specific functions, resolving the contradiction between reliability and complexity.
Solution Approach 2:
The patent employs composite back contact structures combining different materials with complementary properties. The multi-layer configuration uses materials selected for their specific electrical and optical characteristics, creating a composite structure that achieves superior charge carrier transport and ohmic contact while managing the increased structural complexity.
2Quantity of substance
If absorber layer thickness is increased for IR absorption, then more infrared photons are captured, but charge carrier creation outside depletion region increases
Solution Approach 1:
The patent creates local quality variations through the multi-layer back contact structure, where each layer is optimized for specific functions. The first layer provides ohmic contact, while subsequent layers enhance charge carrier transport from deeper regions. This localized optimization allows thick absorber layers to effectively collect charges from IR-generated carriers throughout the entire thickness, resolving the contradiction between photon capture and recombination loss.
3Quantity of substance
If conventional single-sided light collection is used, then device structure is simpler, but available light from reflection and diffusion is uncollected
Solution Approach 1:
The transparent back contact structure serves multiple functions: it acts as an electrical contact for charge carrier collection, allows transmitted light to reach the absorber layer, and enables potential bifacial operation. This multi-functionality increases light collection capability while managing structural complexity through integrated design.
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 described configuration enables improved collection of both visible and infrared light, reducing recombination losses and increasing the overall conversion efficiency of the photovoltaic device.
Implementation Method 1
A photovoltaic device generates electrical power by converting light into direct current electricity using semiconductor materials that exhibit the photovoltaic effect
Implementation Method 2
The photovoltaic device further includes an optical reflector disposed on the transparent contact layer
Implementation Method 3
incorporating a tunnel junction
Data Source
AI summary
A photovoltaic device includes a substrate, a semiconductor stack and a transparent tunnel junction. The semiconductor stack includes an n-type layer selected from a first transparent conductive oxide layer, or a window layer, or both; and a p-type absorber layer disposed on the n-type layer, wherein the absorber layer consists essentially of CdSexTe(1-x), wherein x is from 1 to about 40 at. %. The transparent tunnel junction comprises a transparent interface layer of CdyZn(1-y)Te doped to be p+type, and a transparent contact layer doped to be n+type, and the interface layer is disposed between the p-type absorber layer and the transparent contact layer. In bifacial embodiments, the tunnel junction forms a transparent back contact and electrode; and in multi-junction embodiments, the tunnel junction forms a diode-like connector between top and bottom cells. The transparent contact layer may comprise tin oxide or zinc oxide doped with aluminum, fluorine or indium. The photovoltaic device may also include an electron reflector layer and/or an optical reflector layer.