High Work Function TCO for Solar Cell Contact Resistance

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Solution Overview

Problem

The challenge in solar cell technology is forming a low-resistance ohmic contact between conventional transparent conducting oxide (TCO) materials and heavily doped amorphous Si emitter layers due to their large band gap and high work function, which hinders the achievement of high energy-conversion efficiency and open-circuit voltage.

Innovation Solution

The use of high work function TCO materials such as GaInO, GaInSnO, ZnInO, and ZnInSnO, with work functions between 4.9 eV and 6.1 eV, is introduced to align with the p-type amorphous Si emitter layer, reducing band bending and contact resistance, and a bi-layer structure incorporating low resistivity TCO materials like ITO is employed to minimize overall resistance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional TCO materials (such as ITO) are used to form contact with heavily doped amorphous Si emitter layer, then the TCO layer provides transparency and basic conductivity, but high contact resistance occurs due to large band gap and high work function mismatch

Engineering Contradiction:

Improvecontact resistanceVSAvoidmaterial compatibility

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent changes the work function parameter of the TCO material by selecting materials with work functions between 4.9-6.1 eV (such as GaInO, ZnInO, GaInSnO, ZnInSnO) to match the high work function of heavily doped p-type amorphous Si emitter layer. This parameter alignment reduces band bending at the interface and achieves low contact resistance, directly resolving the contradiction between achieving low contact resistance and material compatibility.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs a bi-layer TCO structure combining high work function TCO (for interface compatibility with emitter) and low resistivity TCO (for overall conductivity). This composite approach allows the first TCO layer to provide good interface contact while the second layer minimizes overall resistance, simultaneously achieving low contact resistance and excellent electrical performance.

Inventive Principle:

Principle #40Composite materials

2Reliability

If high work function TCO materials are used to align with p-type amorphous Si emitter layer, then band bending and contact resistance are reduced, but the complexity of material selection and layer structure increases

Engineering Contradiction:

Improveopen-circuit voltageVSAvoidTCO layer structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent segments the TCO contact structure into two functional layers: a first TCO layer with high work function (4.9-6.1 eV) optimized for interface contact with the emitter, and a second TCO layer with low resistivity optimized for overall conductivity. This segmentation allows each layer to specialize in one function, achieving high open-circuit voltage through proper interface alignment while managing complexity through functional division.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent applies local quality by giving different work function characteristics to different parts of the TCO structure. The first TCO layer at the emitter interface has high work function (4.9-6.1 eV) to match the emitter and reduce interface resistance, while the second layer has lower resistivity for bulk conductivity. This local differentiation optimizes performance at each interface and region.

Inventive Principle:

Principle #3Local quality

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 lower contact resistance, higher open-circuit voltage, and improved solar cell efficiency by facilitating better charge migration and reducing surface recombination, while maintaining lower resistivity through the combination of high and low work function TCO layers.

Implementation Method 1

align with the p-type amorphous Si emitter layer, reducing band bending and contact resistance

Methodology Applied

Scientific EffectBand bending reduction:

Implementation Method 2

facilitating better charge migration and reducing surface recombination

Methodology Applied

Scientific EffectCharge migration: Conduction (electrical)

Implementation Method 3

reducing surface recombination

Methodology Applied

Scientific EffectSurface passivation:

Data Source

PatentUS10084107B2Transparent conducting oxide for photovoltaic devices

Publication Date: 2018.09.25 TESLA INC

AI summary

One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated above the Si base layer, a layer of heavily doped amorphous Si (a-Si) situated above the passivation layer, a first transparent-conducting-oxide (TCO) layer situated above the heavily doped a-Si layer, a back-side electrode situated below the Si base layer, and a front-side electrode situated above the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO.