p3ht f8bt blends nanosolar africa 2013beaucag/classes/solarpowerfor... · university of cincinnati...

Vikram Kuppa

School of Energy, Environmental, Biological and Medical Engineering

College of Engineering and Applied Science

University of Cincinnati

vikram.kuppa@uc.edu

Fei Yu

Yan Jin

d ld iAndrew Mulderig

Greg BeaucageGreg Beaucage

― Renewable― Potential for High coverage― Low emission

1 US DOE Energy Information Administration (2012), Annual Energy Review 2011U. Mich., Center for Sustainable Systems. 2012. “U.S. Renewable Energy Factsheet”. Pub No. CSS03‐12

Organic Photovoltaics (OPVs)

Solar power offers unique advantages

― no mech. parts

― flexible & customizable

― relatively expensive

― storage

IIIa generation ‐ Si & Ge cells are efficient but expensive

OPVs are of IIIb type: low to moderate efficiency yp y

― processed at lower T

― versatile manufacturing

― low efficiency

― inadequate spectral coverageversatile manufacturing

― distinctive mechanical & optical properties

― tunability

― inadequate spectral coverage

― poor mobility of charges

y

― cheap

Functioning of OPVs

hν

−+)

(− +)

( )(− (− +)

Incident radiation produces e‐h pairs (excitons)

Exciton motion length & time scales  ~ 100 ps, 5‐20 nm

Morphology of active +

material is KEY−

OPV Materials

― Blend of polymer(s) and/or additive – bulk heterojunction (BHJ)

― Traditional BHJs have about 50% of polymer, and 50% PCBM

(fullerene derivative)(fullerene derivative)

― PCBM only for charge conduction and exciton dissociation

Critical Issues― Critical Issues

― Increase fraction of conjugated polymer 

― Helps capture more sunlight

― Improves efficiency

― Improve charge transport

Importance of interfaces in OPV devices

D-A interfaceLUMODonor

LUMO 0 3eVEDonor

HOMODonorh+

EAcceptor

LUMOAcceptor

e-

0.3eV

HOMOAcceptor

D-A interface facilitates exciton dissociationElectron transfer from donor(semiconducting polymer) to acceptorExciton dissociation is energetically favorableExciton diffusion length(~10 nm)

polymer) to acceptor

7

g ( )D-A interfacial area is determined by morphology

Typical OPVs

P3HT PCBM

+

+

P3HT F8TBT

+

P3HTF8BT

+

McNeill & Greenham, Adv. Mater. 2009 21, 3840Kim et al., Chem. Mater 2004 16(23), 4813

Polymer Blend OPVs

― Mix of semiconducting polymers

― Both components active & capture sunlight

― Morphology control is again key

― Critical Issues

― Poor charge mobilities persistg p

― Greater recombination losses

Crystallization of polymers and blend miscibility ?― Crystallization of polymers and blend miscibility ?

― Free charge formation and transport

V l― Voltage 

Solution ? 

Pristine graphene

― Excellent conductivity and high aspect ratio

― Percolation paths at very low concentration

― OPVs with chemically modified graphenes were reported*

F ti li d h t h l t i b h i― Functionalized sheets show poor electronic behavior

TEM image of pristine graphene flake

t=0.35 nm

Scale bar=50nmlateral~200-500nm

*Liu, Z. et al., Adv. Mater., 2008 20(20), 3924 Yu, D. et al., ACS Nano, 2010 4(10), 5633;  Yu, D. et al., J. Phys. Chem. Lett., 2011 2(10), 1113 

Graphene‐based OPVs

+

−

P3HT(~90.99%)PCBM(~9%)PCBM(~9%)Graphene(~0.01%)

― Three‐fold enhancement in efficiency― Increase in current – better mobility

Novel device physics

Yu, Bahner & Kuppa, Key Engr. Mater. 2012 21, 3840Yu & Kuppa,  Mater. Lett. 2013 99, 72

― Novel device physics

Current focus ‐ ternary blends

+

−

P3HT (59.9%)F8BT (39.9%)Graphene (~0.2%)

Device Fabrication

Anode― Patterned ITO as bottom electrode

PEDOT PSS b i ti

Anode

― PEDOT:PSS by spin coating

― Active layer with graphene by spinActive layer with graphene by spin coating

LiF and Aluminum― LiF and Aluminum

Cathode― Fabricated and annealed in N2 

Solar Cell Parameters

14Deibel and Dyakonov, Rep. Prog. Phys., 2010 73(9), 096401

Cell Performance

Open circuit voltage ‐ Voc

1.5

1.4

V) 1.3

Voc

(V

1.2

0.00 0.02 0.04 0.06 0.08 0.101.1

graphene concentration (mg/ml)

Cell PerformanceShort circuit current ‐ Jsc

0.6

0.5

0.4

mA

/cm

2 )

0.3Jsc

(m

0 1

0.2

0.10 0.05 0.1 0.15 0.2

graphene concentration (mg/ml)

Cell PerformanceFill factor ‐ FF

0.21

0.20

F

0.19

FF

0 180.00 0.02 0.04 0.06 0.08 0.10

0.18

graphene concentration (mg/ml)

Cell Performance

0.16

0.14

(%)

0.1

0.12

ffic

ienc

y (

0.08

ef

0 04

0.06

0.040 0.05 0.1 0.15 0.2

graphene concentration (mg/ml)

Cell PerformanceExternal quantum efficiency ‐ EQE

PF10PF10 G0 0254 PF10 G0.025 PF10 G0.05 PF10 G0.1PF10 G0.2

3

%)

2

EQE

(%

1

300 350 400 450 500 550 600 650 7000

Wavelength (nm)

Device physics ‐ recombination

-0.2

0.0

alpha=0.67

alpha=0.73

-0 6

-0.4

mA

/cm

2 ))

alpha=0.74

alpha=0.66alpha=0.62

Jsc ~ Iα

-0.8

0.6

G 0ent d

ensi

ty(m

p

alpha=0.90

-1.2

-1.0 G 0 G 0.025 G 0.05 G 0.1G 0 2

Log

(Cur

re

alpha=0.93

alpha=0.91

alpha=0.84

1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6-1.6

-1.4 G 0.2 Linear Fit

alpha=0.99

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6Log (Intensity(mW/cm2))

(a)α = 1 for geminateα = 0.5 for bimolecular

graphene dependence of α

1.00 part 1 alphapart 2 alpha

Jsc ~ Iα0.90

0.95part 2 alpha

0.80

0.85

Alp

ha

0.70

0.75

A

0.00 0.05 0.10 0.15 0.200.60

0.65

Graphene concentration (mg/ml)(b)

Role of Graphene

I i iE i i IntrinsicCharge transport

Extrinsic

Morphology of blend

Mobility

Morphology of blend

Structure of P3HT & F8BT

RecombinationCrystallization & Aggregation

UV‐VIS of thin films

Peaks at 550‐600nm w/ Increasing [Gr]

P3HT/F8BT 6/4 P3HT/F8BT/G 6/4/0.05 P3HT/F8BT/G 6/4/0.1 P3HT/F8BT/G 6/4/0.2 Increasing [Gr] 

P3HT crystallites

Abso

rptio

n

Nucleating agent ?

Nor

mal

ized

350 400 450 500 550 600 650 700Wavelength (nm)

Concentration dependence

0.15‐0.18

0.15

0.18

0 5 0 8

0.12‐0.15

0.09‐0.12

0.12

iency (%

) 0.06‐0.09

0.03‐0.06

10

0.06

0.09

effici

8

100.03

0.10.05 [P3HT+F8BT] (mg/ml)

60.0250[graphene] (mg/ml)

Thickness dependence

0 18

0 12

0.15

0.18

(%) 0.15-0.18

0.12-0.15

0 06

0.09

0.12

effic

ienc

y 0.09-0.120.06-0.090.03-0.06

0.10.2

0.03

0.06e 0.03 0.06

00.025

0.050.1

film thickess( )

graphene concentration (mg/ml)(nm)

New paradigms in OPV BHJs− Graphene enhances charge transport ‐ high Jsc , FF and η

− Cells with majority active layer are now viablej y y− Better harnessing of solar energy− Improved mobility

− Morphology of blend is altered – enhanced crystallization

− Intrinsic and extrinsic effects are observed

− Complex influence of thickness & concentrationComplex influence of thickness & concentration

− Synergistic role of high‐aspect ratio graphene additives

Jin, Yu and Kuppa, (manuscript in preparation)

Choice of solvent: polymer chain packing

Choice of donor and acceptor materials: band gap and miscibility

Donor-acceptor ratio: domain size

Annealing conditions: reorganize polymer chains, crystallizationOther post-production treatments: DC voltage during annealing for ordered structure *

Morphology Performance

28Pictures source: Dennler, Scharber and Brabec, Adv. Mater. 2009, 21(13): p. 1323-1338. * Padinger, Rittberger and Sariciftci, Adv. Funct. Mater., 2003. 13(1): p. 85-88.

BHJ featuresBHJ featuresPolymer:Fullerene BHJ devicePolymer:Fullerene BHJ device

High interfacial area for exciton dissociationBicontinuous network for charge transportg p50:50 w/w P3HT:PCBM for optimum performanceIncrease P3HT ratio to capture more solar energyp gy

P3HT PCBM

F t W kFuture Work

Better dispersed and oriented graphene via morphological controlmorphological control

Increase FF by reducing interfacial roughnessIncrease FF by reducing interfacial roughness

St bilit d d i l tiStability and device encapsulationFY and VKK thank UC and the URC for funding and support