Solar Cells --- frontiers in materials and devices

April 14, 2005EE 666 Advanced Semiconductor Devices

Solar Cells ---frontiers in materials and devices

Ning Su


OutlineIntroductionMarket & technology comparison Low cost solar cells thin film solar cells (TFSC)

High efficiency solar cells Advanced Si solar cells Tandem cells Thermophotovoltaic other strategies

Conclusions


Introduction

Why PV ? Average power incident upon continental United states is ~ 500 times of national energy consumption ( total, not just electricity)

Environmentally-friendly renewable energy source

Quiet Reliable

Applications Residential Cost-effective way to provide power to remote area

Space applications satellite, space stations


Photovoltaic Cells, Modules and SystemsSolar cell is the basic building blocks of solar PV Cells are connected together in series and encapsulated into models Modules can be used singly, or connected in parallel and series into an array with a larger current & voltage output PV arrays integrated in systems with components for charge regulation and storage

Cell module array system


Market for Solar PV

PV market grows at fast rate especially in recent years Cumulatively, about 2GW of solar cells are being used in a variety of applications


Comparison of PV Technology

main technologies available: single & multicystalline Si, a-Si, CuInSe2, CdTe…. Bulk cystalline Si remains dominant Different technology comparison in efficiency & cost

World PV module production in 2003


Low cost High efficiency

Thin film Organic SC tandem

Terrestrial Space

TPV

Light weight

Radiation resistance

High efficiency

Applications:

Demands:

Technology:

Materials: Multicystalline Si III-V

a-Si ; CIS; CdTe

Single crystalline Si

Low Cost vs. High Efficiency SC


Thin Film Solar Cells

“thin film” refers more to solar cell technologies with mass-production possibilities Rather than the film thickness.

requirement for suitable materials: low cost, high absorption, doping, transport, robust and stable leading materials for TFSC: CdTe, CuInSe2, (CIS) ,a-SI…

advantages: -- low material requirement -- variety of processing methods -- light weight modules

disadvantages:

-- low achieved efficiency


CIS & CdTe TFSC CIS, direct band gap with Eg~ 1eV, α>105 cm-1

high cell efficiency (19.2 %), model efficiency (13.4%) comparatively long lifetime

Current complicated and capital intensive fabrication

CdTe, direct band gap with Eg~ 1.45eV, α>105 cm-1-- ideal suited for PV applications

Record cell efficiency 16.5 % (NREL)

Numerous promising processing techniques


Effect of bandgap on efficiency

GaAs, InP have Eg close to the optimum, favored for high η cells Si less favorable Eg but cheap & abundant

Solar Cell Efficiency

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Ideal cell efficiency

Effect of spectrum on efficiency improving η by concentrating light 100 suns or more illumination

Parabolic reflector Fresnel lens


Minimize Losses in Real SCOptical loss

Electrical loss

Concentration of light

Trapping of light:

AR coatings Mirrors ( metallization rear surface or growing active layers on top of a Bragg stack) textured surface

Photon recycling reabsorption of photons emitted by radiative recombination inside the cell

Rear metal reflector

Double path length in metallized cell

Surface passivationResistive loss ……


Advanced Si Solar cells

•Martin A. Green etc.,” Very high efficiency silicon solar cells-science and technology,” IEEE Trans. Electron Devices,vol. ED-46,pp1940-47,1999.

PERL cell

Burried contact sc

large improvement in the last 15 years 1) textured surface & AR coating 2) Improved surface passivation

PERL cell ( 24% in 1994 )

Buried contact cell commercialized by BP Solarex advantage: fine grid– reduced shading–Jsc

reduced contact recombination – Voc

series resistance – concentrator sc

Crystalline Si efficiency


Tandem Cells – beyond efficiency limit

Concept

Intrinsic efficiency limit using single semiconductor material is 31%

Stack different band gap junctions in series larger band gap topmost

efficiency of 86.8% calculated for an infinite stack of independently operated cells *

* A. Marti, G. L. Araujo, Sol. Energy Mater. Sol. Cells 43 (1996) 203.


Advantages : high efficiency

Practical approaches

Cover wider range of solar spectrum

reduce thermerlisation loss (absorbed photon with energy just little higher than Eg)

individual cells grown separately and mechanically stacked

monolithically grown with a tunnel-junction interconnect

Tandem Cells -- Practical approaches


GaInP/GaAs/Ge Dual- and triple-junction SC

Dual-junction (DJ)

* N. H. Karam etc. Solar Energy Materials & Siolar cells 66 (2001) 453-466.**N. H. Karam etc. Trans. Electron Dev. 46 (10) 1999 pp.2116.

GaInP/GaAs cells on Ge (average AM0 η 21.4 %) * small-area lab cells large-scale manufacturing approach megawatt level **

Triple-junction (TJ) efficiency of 27.0% under AM0 illumination at 28 0C *


Multiple Junction CellsFour-junction cells under development

addition of 1-eV GaInNAs subcells under GaAs to form 4 junctions

InGaN – potential material for MJ cells

Direct energy gap of InGaN cover most of the solar spectrum*

MJ solar cells based on this single ternary could be very efficient

* LBNL/Conell work: J. Wu et al. APL 80, 3967 (2002).


Thermophotovoltaic (TPV)

TPV solar energy conversion

Photovoltaic conversion with the addition of an intermediate thermalabsorber/emitter is known as thermophotovoltaic (TPV) energy conversion.

Solar radiation is used to heat absorber/emitter to temperature of 1200-2500 K emitter radiates photons PV cell converts the energy of radiationinto electrical power.

Advantage

By matching the spectrum of the emitter to the PV cells, efficiency improved.


All TPV systems include: 1) heat source 2) radiator 3) PV converter 4) means of recovering unusable photons

TPV Configuration

Components of a TPV system

Selective emitter matched to PV cells


Other Strategies – for high efficiency

Intermediate band solar cells

A.Luque and A. Marti,”Increasing the effiency of ideal solar cells by photon Induced transitions at intermediate levels”, Phys. Rev. Lett. 78, 5014 (1997)

Low-dimentional strucutrues, QWs, QDs

Impact ionization solar cells

P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).

Hot carrier solar cells

P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).

……


Conclusions

Remarkable progress made in synthesis, processing and characterization leads to major improvement in PV efficiency and reduction in cost

Silicon continues to dominate the PV industry

Thin film and organic solar cells offer promising options for substantially reducing the cost, competitive for terrestrial applications

Very high efficiency achieved in multiple junction III-V semiconductors presently commercialized for space applications

New device concept for high efficiency facing challenges and prospects

Documents

Solar Cells --- frontiers in materials and devices