Vikram Kuppa School of Energy, Environmental, Biological and Medical - - PowerPoint PPT Presentation

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 i Andrew Mulderig Greg Beaucage Greg Beaucage

― Renewable ― Potential for High coverage ― Low emission

1 US DOE Energy Information Administration (2012), Annual Energy Review 2011

• U. 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 coverage versatile 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 interface

LUMODonor LUMO 0 3eV EDonor HOMODonor

h

+

EAcceptor LUMOAcceptor

e-

0.3eV HOMOAcceptor

D-A interface facilitates exciton dissociation Electron transfer from donor(semiconducting polymer) to acceptor Exciton dissociation is energetically favorable Exciton diffusion length(~10 nm) polymer) to acceptor

7

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

Typical OPVs

P3HT PCBM

+ +

P3HT F8TBT

+

P3HT F8BT

+

McNeill & Greenham, Adv. Mater. 2009 21, 3840 Kim 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 persist

g 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=50nm

lateral~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, 3840 Yu & 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 spin Active layer with graphene by spin coating LiF and Aluminum ― LiF and Aluminum

Cathode

― Fabricated and annealed in N2

Solar Cell Parameters

14 Deibel 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 0.10 1.1 graphene concentration (mg/ml)

Cell Performance

Short circuit current ‐ Jsc

0.6 0.5 0.4

mA/cm2)

0.3

Jsc (m

0 1 0.2 0.1 0.05 0.1 0.15 0.2

graphene concentration (mg/ml)

Cell Performance

Fill factor ‐ FF

0.21 0.20 F 0.19 FF 0 18 0.00 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

fficiency (

0.08

ef

0 04 0.06 0.04 0.05 0.1 0.15 0.2

graphene concentration (mg/ml)

Cell Performance

External quantum efficiency ‐ EQE

PF10 PF10 G0 025 4 PF10 G0.025 PF10 G0.05 PF10 G0.1 PF10 G0.2 3 %) 2 EQE (% 1 300 350 400 450 500 550 600 650 700 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=0.66 alpha=0.62

Jsc ~ Iα

• 0.8

0.6 G 0 ent density(m p alpha=0.90

• 1.2
• 1.0

G 0 G 0.025 G 0.05 G 0.1 G 0 2 Log (Curre 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 2.6 Log (Intensity(mW/cm

2))

(a)

α = 1 for geminate α = 0.5 for bimolecular

graphene dependence of α

1.00 part 1 alpha part 2 alpha

Jsc ~ Iα

0.90 0.95 part 2 alpha 0.80 0.85 Alpha 0.70 0.75 A 0.00 0.05 0.10 0.15 0.20 0.60 0.65 Graphene concentration (mg/ml)

(b)

Role of Graphene

I i i E i i Intrinsic

Charge transport

Extrinsic

Morphology of blend Mobility Morphology of blend Structure of P3HT & F8BT Recombination Crystallization & 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

Absorption

Nucleating agent ?

Normalized 350 400 450 500 550 600 650 700 Wavelength (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 10 0.03 0.1 0.05

[P3HT+F8BT] (mg/ml)

6 0.025

[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

efficiency 0.09-0.12 0.06-0.09 0.03-0.06

0.1 0.2 0.03 0.06

e 0.03 0.06

0.025 0.05 0.1

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 viable j 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 & concentration Complex 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, crystallization Other post-production treatments: DC voltage during annealing for ordered structure *

Morphology Performance

28

Pictures 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 features BHJ features

Polymer:Fullerene BHJ device Polymer:Fullerene BHJ device High interfacial area for exciton dissociation Bicontinuous network for charge transport g p 50:50 w/w P3HT:PCBM for optimum performance Increase P3HT ratio to capture more solar energy p gy

P3HT PCBM

F t W k Future Work

Better dispersed and oriented graphene via morphological control morphological control Increase FF by reducing interfacial roughness Increase FF by reducing interfacial roughness St bilit d d i l ti Stability and device encapsulation FY and VKK thank UC and the URC for funding and support