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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.png8/14/2019 Achieving High Performance Solar Cells through Innovative Approaches
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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
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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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