Martha Symko-Davies, Robert McConnell and Fan

National Center for Photovoltaics

National Renewable Energy Laborato

FUTURE GENERATION TECH

STATUS WITHIN THE HI

PERFORMANCE PV PROJ

2006 IEEE 4th World Conference

May 11, 2006

NREL/PR-520-39867Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WC

held May 7-12, 2006 in Waikoloa, Hawaii.
Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36Department of Energy (the “DOE”). The United States Government (the “Government”) retains athe work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrepublish or reproduce the published form of this work, or allow others to do so, for Government puNeither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employeeor implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulnesproduct, or process disclosed, or represents that its use would not infringe any privately owned ri

specific commercial product, process, or service by trade name, trademark, manufacturer, or othimply its endorsement, recommendation, or favoring by the Government or any agency thereof. authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the agency thereof. 
Explore the ultimate performance limits of PV te

doubling sunlight-to-electricity conversion efficicostsBring thin-film tandem cells and modules toward efficiencies, respectively. Develop multijunction concentrators able to conthird of the sun’s energy to electricity (i.e., 33% eIncrease baseline efficiency of 3rd generation PV 

39% baseline to 50%) and increase efficiency of organic solar cells (from 11% baseline for the dy

Enable progress of high efficiency technologies prototype products

High Performance P

Project Description
Efficiency (%)

University

of Maine

Boeing

Boeing

Boeing

ARCO Boeing

Boeing

Euro-CIS

1975 1980 1985 1990 1995 

NREL

NREL

Japa

Energ

Spire

No. Carolina

State University

Multijunction Concentrators

Three-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic)

Crystalline Si Cells

Single crystalMulticrystallineThin Si

Thin Film Technologies

Cu(In,Ga)Se2

CdTeAmorphous Si:H (stabilized)Nano-, micro-, poly- Si 

Multijunction polycrystalline

Emerging PV

Dye cells Organic cells(various technologies)

Varian

RCA

Solarex

UNSW

ARCO

UNSW

UNSW

Spire Stanford

Westinghouse

Geor

Georgia Tech Sharp

Masushita

Monosolar Kodak

Kodak

AMETEK Photon Energy

University

So. Florida 

United Solar

RCA

RCA

RCA

RCA RCA

RCA

Solarex

12

8

4

0

16

20

24

283236

EPFL

40

Best Research-Cell Efficien
Objectives of Future Gen

• Increase baseline efficiency of 3rd generation PV 

39% baseline to 50%) and increase efficiency ocost organic solar cells (from 11% baseline for th20%).

• Support the DOE Solar Energy Technologies 

Year Technical Plan (MYTP)

– Organic Solar Cells

• Initiate theoretical studies for doping organic mater• Determine operational characteristics of excitonic s

biomimetic, organic, and nanotechnology concepts

• Assess efficiency potential, stability, and reliability o

small-molecule solar cells

• Identify commercialization pathways for promising 

university/industrial partnerships

– Dye-sensitized Solar Cells

• Assess efficiency potential, stability, and reliability o

solar cells

• Assess dye-sensitized solar cell options involving s
Objectives (continued)

• Support the MYTP

– Nanostructure Solar Cells

• Determine operational characteristics of excito

using nanotechnology concepts---including biomimicking a solar biological process

– Third-Generation Technologies

• Demonstrate feasibility of third-generation PV 

hot-carrier and impact-ionization concepts

• Assess potential of nanotechnologies for achie

generation goals of very high efficiency and ve
• Future Generation--Subcontract

– Organic Solar Cells

• Princeton University (Steven Forrest)

– Project Title: Approaching 10% Conversion Efficie

Organic Photovoltaic Cells with Enhanced Optica

– Collaborator: Global Photonic Energy Corp.

• Northwestern University (Thomas Mason)

– Project Title: Interface and Electrode Engineering 

Organic Photovoltaic Cells

– Collaborators: BP Solar, Konarka, Northwestern De

Physics and Astronomy, and Materials Science and 

– Dye-Sensitized Solar Cells

• University of Colorado (Josef Michl)

– Project Title: Ultra High Efficiency Excitonic Solar – Collaborators: University of Nevada-Reno, Universit

Northwestern University, NREL (Art Nozik).

– Third-Generation Technologies

• University of Delaware (Christiana Honsberg)

– Project Title: Theoretical and Experimental Investi

>50% Efficient Solar Cells

– Collaborators: Imperial College, Purdue University, U

Wales.
Summary of Activitie

• Methods and Expected Outcomes--Sub

– Organic Solar Cells

• Princeton University (Stephen Forrest)

– Develop “toolbox” of concepts for optimiz

cells targeting 10% efficiency

• Northwestern University (Thomas Mason)

– Characterize stability of n-type TCOs for 

solar cells

– Dye-Sensitized Solar Cells

• University of Colorado (Josef Michl)

– Synthesize multiple-exciton generation s

solar cell targeting 20% efficiency 

– Third-Generation Technologies 

• University of Delaware (Christiana Honsbe

– Develop design rules for non-convention

processes leading to >50% efficient sola
In-house Activities

• NREL Basic Sciences Center (High Effici

PV)

– Third Generation Technologies

• Third Generation PV: Quantum Dot Solar Cells (Art 

– Dye-Sensitized Solar Cells

• Dye-Sensitized Solar Cell Research (Art Frank)

– Computational Modeling, Analysis and Design of P

• TCO, III-V, Polycrystalline thin films and Si (Suhuai W

– Solid State Spectroscopy of PV Materials

• Enhance designs for high efficiency monolithic multi-

cells. Modulated Reflectance (MR) measurements, ameasurements(Angelo Mascarenhas)

– Solid State Theory of PV Materials and Devices

• Polycrystalline thin films, new materials for wide-ban

enhance efficiencies of III-V tandem cells for high pethem particles creating Intermediate Gap absorption 
Exciton Solar Cells

•Electrons and holes are still created 

simultaneously

•But they are bound together and 

move as bound pairs to the nearest material boundary

•Separation of electrons and holes at 

a material interface is followed by diffusion or electrochemical drivers of the charge carriers

•Primarily associated with organic 

materials......plastics

•Potentially low costs for materials 

and processing

Dye Sol
Multiple Electron-Ho

Pairs-per-Photon Solar 

• Electron and hole created simultaneously• Hot electron and hole create multiple (m) 

electron-hole pairs through Auger emission and absorption instead of thermalizing

• Need materials (e.g., quantum dots) 

susceptible to creation of multiple Auger 

electron-hole pairs

• Recently observed by NREL and Los 

Alamos scientists...3 carriers for 1 photon

• Can be added to a multijunction solar cell 

stack to achieve higher efficiency

H. Queisser et al., Max Planck Inst.,Solar Energy Materials and Solar Cells, 41/42, 1996
Intermediate-Band Solar 

• Involves a narrow energy level, 

instead of stacks of energy bands as in multijunction cells

• Theoretical efficiency 

advantages over equivalent 

multijunction cells

• Can simplify and augment 

multijunction solar cell efficiencies

A. Luque et al., U. Politecnica de MadridPhys. Rev. Lett., 78, No. 26, 1997 Band di

with an 
Demo organic PV cells with high 

efficiency by using new structures based on vapor deposited small molecular weight organic thin films.

1. Demonstrate new materials systems 

and structures to enhance efficiency.

• CuPc/C60 demonstrated• Double heterstructure cell invented

2. Use tandem structures with metal 

nanoparticles for plasmon optical field enhancements

Currently working with Global Photonic Energy Corp. to find partners for production and commercialization of our 5+% efficiency cells on low cost plastic (flexible) substrates.

• In a hybrid cell consistin

(material system first de

record efficiencies of 

illumination observed

• First demonstration of o

based on small molecul

• Research Objectives:• Approach:• Industry Impact:

• Significant Results:

• Formation of a bulk 

thermal annealing omolecules: 

Peumans, et al., Natu

PI: Stephen Forrest, Princeton 
Tcell

Organic Vapor 

Different strain a

⇒ differ

Controlled Growth of a Bulk Hete

Ideal Bulk HJ PV Cell Ideal Bulk HJ PV Cell

Donor Layer: Highly Folded

Acceptor Layer: Planarizes Cell

C

Bulk HJ C
• Scalability

– Cells thin to maximize absorption and 

resistance/exciton bottleneck limits ⇒ 

avoid shorts 

– ITO very resistive

• Low Voc ⇒ new materials & conce

• Organics typically do not absorb in• Reliability ⇒ packaging, new mate• Large area, low cost manufacturin

– Plastic substrates– OVPD

√

√√

Some Significant Issues Ad
PI: Thomas O. Mason, Northwester

1. Improved transparent electrodes 

(transparent conducting oxides, TCOs) for efficient organic photovoltaics

2. Engineered organic-transparent 

electrode interfaces for high-efficiency 

organic photovoltaics

Demonstration of prototype OPVs and technology transfer of novel transparent electrodes and engineered organic-TCO interfaces to NREL and our partners (BP Solar, Konarka Technologies)

• Interfacial science of OL• Theory-guided synthesi• Correlation of surface v

properties

• Nanostructured TCO el

• Research Objectives:• Approach:• Industry Impact:

• Significant Results:

Proposed Organic 

Fu

Light

+

Anode

-

+

+

ElectronBlocking 

Layer, EBL

Pho

L

2. Charge-blocking 

at interfaces

1. New TCO materials 

as alternative anodes

To advance the science and technology of next-generation, efficient, easily manufacturable, and durable organic photovoltaics.
50 Å

TCO

electrode

300 Å TiO2

3.2 TiO

ee

Transparent

TCO

Electrode

Singlet fission molecule

2 e’s 

produced 

instead of 

one

Device efficiencies could increase by ~ 50% with SF

PI: Josef Michl, CU Bould
To demonstrate multiple electron-hole pair 

(exciton) generation from single photons in quantum dots (QDs) and apply this effect to QD solar cells to produce greatly enhanced photocurrent and conversion efficiency

Ultra-high conversion efficiency in 3rd-generation 

solar cells could significantly reduce the cost of PV power. Incorporating quantum dot solar cells into multijunction cells already in production could provide a feasible route to 50% efficient solar cells.

• Research Objective:

•Approach: 

• Industry Impact:

• Significant Results:

• Graphics: 

300250200

Quantum Yield (%) 150

(b)

Quantum yield vs. photon energfor PbSe and PbS QDs of differdiameters).(Published in: NanoLetters, 5, 5

• Synthesize QDs that are expected to show greatly enhanced multiple exciton generation (MEG).• Measure MEG using transient absorption spectroscopy.• Develop and characterize nanocrystalline solar cells sensitized with QDs exhibiting efficient MEG and greater photocurrent.• Model performance of QD solar cells showing efficient MEG.

1. Quantum yields of up to 

per photon) were measured quantum dots at photon enbandgap.

2. The enhanced performance ansolar cells based on MEG have b

Quantum Yield

PI: Arthur J. Nozik, NREL

(Collaborative Project with Office of Science/BES/Che
Project Objectives: Develop realistic solar cell device 

reaching an efficiency of 50%

Approach:

– Analyze physical concepts that increase efficie

junction device

• Multiple energy level solar cells achieve this a

implemented with wide range of materials

– Develop practical models for quantum dot mult

solar cells:

1. Program to calculate band structure based on 

derived from Schrödinger Equation. Searchefor material that synthesize an optimum band 

2. Program to determine optimum 

band structure taking into account non-idealities including:

• Concentration• Finite intermediate band width• Spectral selectivity• Intraband transitions within the

intermediate band

PI: Christiana Honsberg, University o
Future Directions

1. Calculate and include in efficiency calcu

of a finite density of states on QD MEL sradiative transitions (absorption and reco

• Calculations based on piece-wise 

continuous parabolic density of states (DOS).

2. Refine band structure model as 

needed to include additional effects (e.g. strain) and determine optimummaterials including finite DOS calculations.

3. Experimentally implement QD MEL

solar cells with optimum materials and demonstrate validity of model and design.

EG,CEG,C

ΔIB

Energy
The Promise of

Future Generation PV Re

• Theoretical efficiencies are 

much higher than demonstrated efficiencies.

• New PV materials and devices 

may be cheaper and more plentiful.

• An English tradition says that if 

you follow a rainbow to its end, you will find a pot of gold. But to find gold, you have to be patient, supportive of a variety of approaches and capable of affording risk.

A cautionary the ancient GSophocles wyou will find iunsought wil