Achieving High Performance Solar Cells through Innovative Approaches

8/14/2019 Achieving High Performance Solar Cells through Innovative Approaches

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Achieving High Performance Solar

Cells through Innovative Approaches

Yang Yang ([email protected])Materials Science and Engineering andCalifornia Nano-System Institute

UCLA


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Lets start from a number:

0.08%!

2

The ratio Solar energy occupied inUS energy sources.

Why?


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The answer is:

Cost, Availability

Installation (counts for 50% of the cost)

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We need a disruptive technology ortechnologies to solve this problem


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Our Goals in PV technology

Low cost (or very low cost) and high performancesolar cells as future source of energy.

To reach this goal, solar cell technology must besimple to manufacture, robust in variousenvironments, and it must be high performance,and easy to deploy.

Solution process is the way to go


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09/10/095

Introduction- Organic materials vs Inorganics

van de Waals force

With no charge carriers

Very small amount due to traps orimpurities

Discrete energy levels (however,band structure is currently used)

Covalent bonds

With certain amount of charge carriers

1010~1018 cm-3

Continuous band structure

Remember:

Exciton binding energy~0.3 eV

* NOTE : Ba nd st ru ctu re is u sed fo r o rg an ic e le ct ro n i c s fo rs imp licit y
http://en.wikipedia.org/wiki/Image:Silicon-unit-cell-3D-balls.png


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09/10/096

Introduction-II Conjugated Polymers

conjugation

bonds from pz orbitals

Alternating single-double bonds

Delocalized -electron clouds


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Y LAB

UCLA 09/10/09Department seminar

7


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YY LABs

UCLAEconomic Viability

1.1. Can renewable energy supply our energy needs?Can renewable energy supply our energy needs? Annual energy consumption (US): 134 quadrillion btu[1]

Average solar radiation: 6 kWh/m2/day[2]

Assuming average solar cell efficiency: 10%

Require area: 70,600 sq-mile ~ 25% of Texas

2.2. Can it be done economically?Can it be done economically? Current electricity cost: $0.08-0.10/kWh[1]

Assuming module cost $75 per m2

(FYI, x-Si panel is $400/m2

)

1. US Dept of Energy - 2007 Annual Energy Outlook2. http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

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YY LABs

UCLA

12

8

4

0

16

20

24

200019951990198519801975

Efficiency(%)

Crystalline Si CellsMulticrystalline

Thin Film TechnologiesCu(In,Ga)Se2CdTe

Amorphous Si:H(stabilized)

Emerging PVOrganic cells

Best Research Cell Efficiencies

www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt9

Organic cells

Amorphous Si:H

CdTe

Cu(In,Ga)Se2Poly-silicon


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UCLA Polymer Solar Cells (~5%)

S SS S

n/4

O

OCH2

-+

Regioregular poly-(3-hexyl-thiophene)

(RR-P3HT)

PCBM

ITO

PEDOT

Ca 2.9eV

HOMO PCBM

LUMO P3HT

HOMO P3HT

4.7eV

5.2eV

6.1eV

3.7eV

4.9eV

3.0eV

LUMO PCBM

Glass ITO PEDOT:PSS

Active layer

Metal electrode

YY Lab

UCLA

Li et al., Nature Materials, 4, 865, (2005)


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Photovoltaic Effect

The production of opposite chargesin semiconductor devices underillumination and the subsequentcollection of charges at the electrodes

Steps in the process Light absorption (A)

Motion of excited species (ED)

Charge separation (CT)

Charge collection (CC) Overall Efficiency

= A ED CT CC

Operation Mechanism of OPV Device

PEDOT

ED

CT

CC

A

PCBMP3HT

2.9 eV

ITO

6.1 eV HOMO PCBM

4.9 eV HOMO P3HT

3.7 eV LUMO PCBM

3.0 eV LUMO P3HT

5.2 eV

4.7 eV

Ca

Under Short Circuit Condition


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YY LABs

UCLA

Efficiency

12

Incident PowerEfficiency () =

Vmax Jmax 100


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However, OPV is not simple device What happens in solvent drying process?

Solvent removing speed is function of: solvent, spin-speed, spin-cast time, environment

Purple phase Yellow phaseTransition

Shorten the polymer film drying time

Decrease of the Polymer Crystallinity


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200nm 200nm

Solvent annealed Fast drying

(100)

(200)

(300) (010)

qxy (-1)

0.0 0.5 1.0 1.5 2.0

(010)

qxy (-1)

0.0 0.5 1.0 1.5 2.0

PEDOT/PSS

Nanoscale Phase Separationin P3HT:PCBM


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Photovoltaic performance- strong function of cast condition

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-10

-8

-6

-4

-2

0

Jsc

(mA/cm

2)

Bias (V)

ts

20 sec

30 sec

40 sec

50 sec

52 sec

55 sec

80 sec

Fast drying film

20 minutes drying time


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External Quantum Efficiency

300 400 500 600 700 8000

10

20

30

40

50

60

70

IPCE(%)

(nm)

Spin-coating time

20 sec

50 sec

55 sec

80 sec

Changing in both

magnitude & shape

Note the loss of IPCE

@ 600 nm

In P3HT absorption,600 nm corresponding to

the interchain

interaction ( -

stacking)

PCE = ~ 4 - 5%

20 min drying time

Fast drying film


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Strategies on New Generation OPV

Basic research is needed to understand the deviceoperation mechanism: charge transport, defects,interface property, ..

Novel materials for smaller Eg, higher carriermobility, high optical density are required.

Stability study

Hybrid-structure, combing with nano-structure, QD,and other formats of materials structures

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High Gain Photoconductivity (PC)

Device structure and chemical structure of capping ligand

Glass

ITO

P3HT:PCBMblend (with or

without CdTeNPs)

PEDOT:PSS

Cathode

Anode

SCdTe

CdTe nanoparticles(NPs) with the

designed ligand, PMDTC, are

blended into the P3HT:PCBM film to

achieve high photoconductive gain

devices.

Chen et al, Nature Nanotechnology, 3, 543, 2008.


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J-V charateristics

ITO/PEDOT:PSS/Polymer Blend(P3HT:PCBM/CdTe NPs)/Ca/Al

Device A- P3HT:PCBM blend

Device B- P3HT:PCBM blend with CdTe NPs

High photoconductive gain was

observed under reverse bias after

blending with CdTe NPs.


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EQE measurement

External quantum efficiency (EQE) ofdevice B

(with CdTe NPs) reached 8000% at 350nm under

-4.5V. Higher than 100% EQE implies there is

current injection from external circuit, since

impact ionization is unlikely to happen inorganic/polymer system.

For regular P3HT:PCBM(Device

A) solar cell, only 10% increase inEQE when applied bias to -5V.


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Phase

AFM Images

P3HT:PCBM

+ solvent annealing

P3HT:PCBM

+ 3.1% CdTe NPs

P3HT:PCBM

+ 3.1% CdTe NPs+ solvent annealing

The morphology changed significantly after adding CdTe NPs

coupled with solvent annealing, possibly due to the higher

concentration of NPs near the top surface.

No PC Gain Minor PC Gain High PC Gain


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Summary for High Gain PC

We demonstrated a high-gain photoconductivity based on nano-particleblend in P3HT:PCBM blend.

The mechanism is attributed to the interface charge trapping andsubsequently lower the hole injection barrier at low voltage.

Trap-engineering for polymer electronics is an important topic.


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Radiation Effect of

Polymer Solar Cells

Manuscript accepted by Nanotechnology


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Motivation Exploring the effect of high dose x-ray radiation

on organic solar cell Exploring the possibility of the application of

OPV in space Organic vs. Inorganic PV

Lower efficiency now, but constantly improving Light weight


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Outer space impact on OPV ProsNo O2 & H2O concernsPotentially it improves OPV Lifetime

ConsStronger Sun Light (AM0 is 1360W/m2)Much stronger high energy radiation!

We present the very first set of data onradiation effect on OPV


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Experiment Polymer solar cells fabricated, tested (one-sun

condition) & encapsulated in UCLA Devices shipped to Air Force Research

Laboratory (AFRL) Kirtland AFB, NM ARACOR 4100 X-irradiation system Electrical probing - Hewlett Packard 4142

Light source - halogen lamp (maximum 150 W)


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Device Data - UCLA

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-12

-10

-8

-6

-4

-2

0

4.16% 10mA/cm2

0.613V 68%

PCE Jsc Voc FF

J(mA/cm

2)

Bias (V)

Device 1

Device 2

Device 3

Device 4


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Response of Voc under radiation

0 50 100 150 200 250 3000.53

0.54

0.55

0.56

0.57

0.58

0.59

V

oc

(V)

Time (minutes)

Irradiation at 8.33 kRad(SiO2)/min

60 min500K Rad(SiO2)


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Response of PCE under radiation

Irradiation at 8.33 kRad(SiO2)/min

60 min500K Rad(SiO2)

0 50 100 150 200 250 3000.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Norma

lize

dPower

Conver

sion

Efficieny

Time (minutes)

4.16%

2.2%

2.9%


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Reduced Glass Transmission ~ 7%i.e. 92% to 85%

300 400 500 600 700 800 900 1000

50

60

70

80

90

100

Transm

iss

ion

(%)

(nm)

Glass

Glass/PEDOT

Glass/ITO/PEDOT

250K Rad Glass

250K Rad Glass/PEDOT

250K Rad Glass/ITO/PEDOT

Air as reference

OPV

response


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Minimal polymer absorption loss- Better than glass?

300 400 500 600 700 800 900 1000

20

40

60

80

100

Fresh Glass_PEDOT as reference

P3HT:PCBM film transmission with various radiation

T

ransmission

(%)

(nm)

No Rad

M54 50K rad

M71 100k Rad

M88 250k Rad

M89 500k Rad


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Summary Polymer solar cell efficiency drops to 54% of

original value after 500K Rad strong x-rayirradiation

Significant efficiency recovery phenomenonobserved recover to 70% after 2 hours

Take into account of radiation induced

transmission loss from glass, polymer PV cellcan recover to 76% of original efficiency


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Conclusion of Part III

Very interesting radiation damage and recoveryphenomenon effect observed

Organic solar cells could be sufficiently radiationtolerant to be useful for space applications


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Technology transfer

One startup Solarmer Energy Inc.established in 2006 to commercialize our

technology. Solarmer has licensed 7 UCLA patents

and hire 5 of our students.

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Acknowledgements:

Financial support from ONR, AFOSR, SolarmerEnergy on the nano-technology and itsapplication in polymer opto-electronic devices.

NSF & NSFC Joint project, starting Oct. 08 Students involved in the projects:Gang Li, Vishal Shrotriya, Fishier Chen, Jinsong Huang.

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Achieving High Performance Solar Cells through Innovative Approaches

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