Nanowire Solar Cells

Research Project

EE663 (Optoelectronic Devices) Fall 2013

Topic:

Nanowire Solar Cells

By:

Hojjatollah Sarvari

Department of electrical & Computer Engineering

School of Engineering

University of Kentucky, Lexington, KY

Dec 20, 2013

Outline

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I. Introduction

II. Theory

III. Fabrication Procedure

IV.Existing Devices (Design & Performance)

V. Challenges and Suggestions

VI.References

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• Nanowires are nanostructures with diameter in range of nanometer. Ratio

of the length to width greater than 20.

• Many different types of nanowires: Metallic (Ni, Pt, Au) Semiconducting

(Si, InP, GaN) and Insulating (SiO2, TiO2)

• They are referred to as one-dimensional (1-D) materials. This is because

electrons in nanowires are quantum confined laterally and thus occupy

energy levels that are different from the traditional continuum of energy

levels.

I. Introduction

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(a & c ) Measured (b & d) Simulated

I. IntroductionNanowire Solar Cells

Erik C. Garnett et al., Annual. Rev. Mater. Res. 2011. 41:269–95

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There are two types of NW solar cells such as single NW solar

cells and array of NW solar cells.

In 2007, a first radial NW Solar Cell was fabricated from an

individual silicon nanowire and integrated on-chip to drive the

nanowire sensor.

It is obvious that an array of NWs can absorb sunlight more

efficient than a single NW due to more space area provided by

arrays of NW.

I. Introduction

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Axial junctions lose the radial charge separation benefit

Substrate junctions lack the radial charge separation benefit

I. IntroductionNanowire Solar Cells


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II. Theory

Boundary conditions for structure

Current density J.

The whole device model is calculated in a self-consistent way by using the finite-

element method (FEM).

Limiting efficiencies of tandem solar cells consisting of III-V nanowire arrays on silicon

Ningfeng Huang et al., Journal Of Applied Physics 112, 064321 (2012)

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Position-dependent Absorption where FDTD for electric field

Current Continuity equations

Poisson’s equation

Position-dependent carrier generation rate

I is solar irradiation AM1.5D

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III. Fabrication Procedure and Challenges

Nanowire solar cell fabrication consist of three steps:

Nanowire synthesis

Junction formation

Contacting

Nanowires are commonly grown by vapor-liquid-solid (VLS) process,

electrochemical deposition into nanoporous templates and solution growth.

Two techniques most commonly used in fabrication of nanowire solar cells:

Chemical vapor deposition (CVD)

Patterned chemical etching

Challenge: Large-scale production of the nanowires into highly controlled arrays

with high uniformity.

Nanowire Solar Cells


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IV. Existing Devices (Design & Performance)

Performance of a solar cell is often measured by factors of Jsc, Voc, and Isc.

In order to improve the performance of NW solar cells a number of new

geometries cross-sections have been recommended for NW SCs.

Circular cross-section

Rectangular cross-section

Triangular cross-section

Hexagonal cross-section

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Rectangular cross-sectional gallium arsenide (GaAs) NW solar cell

Enhancing light absorption and conversion by improving Qabs

W : Width of the photoactive rectangle

Ds : Thickness of the shell

(a) without nanoshell (b) with dielectric nanoshell.

Enhanced external quantum efficiency in rectangular single nanowire solar Cells

Xiaofeng Li et al., Applied Physics Letters 102, 021101 (2013)

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(c) and (d) Qabs vs. W

Qabs spectra with two W values [(e) and (f) where 125nm for and 250nm for ]

(c) and (e) for TE incidence (d) and (f) for TM incidence

Enhanced external quantum efficiency in rectangular single nanowire solar Cells

Xiaofeng Li et al., Applied Physics Letters 102, 021101 (2013)

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IV. Existing Devices (Design & Performance)Optical and electrical characteristics of asymmetric nanowire solar cells

Myung-Dong Ko et al., Journal Of Applied Physics 111, 073102 (2012)

Radial NW (R-NW) solar cell Asymmetric Radial NW (AS-NW) solar cell

p-type (boron) core

n-type (phosphorus) shell

The core diameter (D)

The shell thickness (TS)

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A larger diameter leads to higher absorption in the low photon energy regime due

to a decrease in transmission

In the high photon regime, a small diameter leads to high absorption due to lower

reflectance

Because the effect of the low photon energy is dominant for calculating the

overall absorption, the larger incident region gives high light absorption.

Optical and electrical characteristics of asymmetric nanowire solar cells


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Friday Dec 20, 2013Radial NW SC

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IV. Existing Devices (Design & Performance)Limiting efficiencies of tandem solar cells consisting of III-V nanowire arrays on silicon


The nanowires form a vertically aligned hexagonal array.

Silicon Substrate

It is assumed that each subcell (nanowire array) contains a p-n junction, and

that the two subcells are connected in series.

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Considered four different III-V materials for the nanowire array, each with

different band gap energy:

InP 1.34 eV

GaAs 1.43 eV

Al0.2Ga0.8As 1.72 eV

Ga0.5In0.5 P 1.9 eV

The highest limiting efficiency (45.3%) occurs when the band gaps of the top

and bottom subcells are 1.57eV and 0.935eV, respectively.

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(a) Efficiency as a function of nanowire height

(b) Efficiency as a function of nanowire band gap energy

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(a) lattice constant (b) d/a ratio

(c) diameterFriday Dec 20, 2013

Optimal structural parameters for different materials

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Junction designs for III-V NW on silicon cells with

(a) Radial junction (b) Axial junction in the nanowire.

(c) Carrier generation rate profile in GaAs nanowire

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(a) Radial junction (b) Axial junction GaAs nanowire top cell.

(c) J-V curves for axial and radial junction geometries for varying SRV.

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Surface Recombination Velocity

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IV. Existing Devices (Design & Performance)Numerical Simulation of Light-Trapping and Photoelectric Conversion in Single Nanowire Silicon Solar Cells

Yaohui Zhan et al., IEEE Journal Of Selected Topics In Quantum Electronics, VOL. 19, NO. 5, (2013)

Cylindrical single nanowire solar cell (SNSC)

Under solar illumination with TE or TM polarization

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p-type core (diameter 110 nm)

intrinsic layer (thickness 40 nm)

n-type shell (thickness 60 nm).

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Effects of doping

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Numerical Simulation of Light-Trapping and Photoelectric Conversion in Single Nanowire Silicon Solar Cells


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Effect of i-layer thicknesses

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Numerical Simulation of Light-Trapping and Photoelectric Conversion in Single Nanowire Silicon Solar Cells


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IV. Existing Devices (Design & Performance)Optical and electrical simulations of two-junction III-V nanowires on Si solar cell

Shaojiang Bu. et al., Appl. Phys. Lett. 102, 031106 (2013)

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NWs with length of 2 micrometer Si substrate with thickness 400 nm.

Period of the square lattice P NWs diameter D

Filling ratio D/P

The NWs are composed of Ga0.35 In0.65 P with bandgap of 1.7 eV.

Two-junction III-V GaInP NWA/Si thin film solar cell

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The maximum efficiency is 34.3% with FF=0.6 and D=200 nm

Higher filling ratio (D/P) of NWA means more consumption of Ga and In

elements.

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The Jsc decreases substantially from 17.78 to 15.62mA/cm2, when SRV increases from 0 to 5*106 cm/s.

At non-ideal conditions (SRV=7*104 cm/s), the net J-V characteristic yielded Jsc=17.1 mA.cm-2,

Voc=1.837 V, FF=0.86, and g=27.1%.

The maximum efficiency is 34.3% with FF=0.6 and D=200 nm

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NWs Reduce the quantity and quality of material necessary to approach those

limits which allowing for substantial cost reductions.

NWs provide opportunities to fabricate complex single-crystalline

semiconductor devices directly on low-cost substrates and electrodes such as

aluminum foil and conductive glasses.

Many materials relevant to solar cells, including zinc oxide, germanium,

silicon, indium gallium nitride and cadmium sulfide.

Some challenges in the field of nanowire solar cells that include:

Surface and interface recombination

surface roughness

mechanical and chemical stability

doping control

nanowire array uniformity

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Silica substrates will slightly degrade the absorption efficiency of SNSCs,

while metal substrates will greatly enhance the absorption performance.

Rectangular cross section nanowire has the greatest absorption efficiency

among all the geometries

Increase with the number of NW in a parallel array for Jsc

Synthesis methods such as VLS it is possible to very accurately control

nanowire architecture including: size, morphology and material.

As a suggestion:

Multi-junction (two or three multi-junction) nanowire array solar cells with

rectangular geometry on glass or metallic substrates with fabrication methods

such as VLS.

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VI. References

1. Shaojiang Bu. et al., Optical and electrical simulations of two-junction III-V nanowires

on Si solar cell, Appl. Phys. Lett. 102, 031106 (2013)

2. Yaohui Zhan et al., Numerical Simulation of Light-Trapping and Photoelectric

Conversion in Single Nanowire Silicon Solar Cells, IEEE Journal Of Selected Topics

In Quantum Electronics, VOL. 19, NO. 5, (2013)

3. Xiaofeng Li et al., Enhanced external quantum efficiency in rectangular single

nanowire solar Cells, Applied Physics Letters 102, 021101 (2013)

4. Myung-Dong Ko et al., Optical and electrical characteristics of asymmetric nanowire

solar cells, Journal Of Applied Physics 111, 073102 (2012)

5. Ningfeng Huang et al., Limiting efficiencies of tandem solar cells consisting of III-V

nanowire arrays on silicon, Journal Of Applied Physics 112, 064321 (2012)

6. Erik C. Garnett et al., Nanowire Solar Cells, Annual. Rev. Mater. Res. 2011. 41:269–95

Friday Dec 20, 2013

31Friday Dec 20, 2013

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