Transcript Slide 1

Tuning the Energy Level Offset between Donor and Acceptor with Ferroelectric Dipole Layers for
Increased Efficiency in Bilayer Organic Photovoltaic Cells
B. Yang, Y. Yuan, P. Sharma, S. Poddar, R. Korlacki, S. Ducharme, A. Gruverman, R. Saraf, and J. Huang*
Department of Mechanical and Materials Engineering
Nebraska Center for Materials and Nanoscience
University of Nebraska-Lincoln, Nebraska, United States
E-mail: [email protected]
Website: www.huanggroup.unl.edu
Research motivation
Proposed mechanism
-
Konarka large scale OPV
Low cost (cent/kWh)
Eight19 flexible OPV
Donor
Donor
Donor
Acceptor
LUMO
Donor
LUMO
LUMO
Donor
Light
Cathode
-
HOMO
50%
Cathode
LUMO
Acceptor
Voc2
Voc1
Recombination
loss
40%
Anode HOMO
+
Anode
Transparent Substrate
+
Acceptor
C
H
--
-
-
--
+
+
+
--
--
+
-
P3HT
PEDOT:PSS
ITO/Glass
Langmuir-Blodgett deposition
Real device
 In this work, a polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, P(VDF-TrFE), was used as the
tunable dipole layer, which was coated by Langmuir-Blodgett (LB) deposition technique.
20%
10%
0%
HOMO
Typical device structure
-
-
-
 The P(VDF-TrFE) layer was sandwiched between P3HT and PCBM bilayers which can precisely grow the
ferroelectric layer thickness by monolayer (ML) and generates P(VDF-TrFE) with excellent crystallinity.
HOMO
30%
Acceptor
Donor
-
- - - - -- -PCBM
F
- - - - - - - -- -- +
Small
offset
+
-
Ca/Al
+
Insert Dipole Layer
Large
offset
 Organic photovoltaic (OPV) technology is one of the most attractive candidates for solving future energy
shortage problem due to its advantage of light weight, flexibility, low cost and large scale production.
-
+
Acceptor
Acceptor
-
Device fabrication and maximum attainable Voc
Device working principle
PCDTBT
1
PTB7
2
 The Voc of OPV devices is determined by the difference of LUMO (or conduction band) of the acceptor material and
HOMO (or valence band) of the donor material. A small LUMO offset between the donor and the acceptor is preferred
for a high Voc.
DTG-TPD
PBnDT-FTAZNDT(TDPP)2
3
4
5
6PBDT
TT
Voc / Bandgap of polymer
 Power conversion efficiency (PCE) of OPV devices already exceeds 10%, further increasing the PCE to over
15% is needed for OPV to compete with silicon solar cells and other thin film photovoltaic technologies.
 In polymer solar cells with PCE over 6%, none of them has open circuit voltage (Voc) reaching half of the
optical bandgap of the semiconducting polymers, caused by the too large lowest unoccupied molecular orbital
(LUMO) offsets between the donors and the acceptors.
 A tunable dipole layer, consisting of an ultrathin ferroelectric polymer film, was inserted between the donor and the
acceptor semiconductor layers, which shifts the relative energy levels of the donor and the acceptor. Therefore, the Voc
could be increased by tuning the LUMO offset between the donor and the acceptor without changing their chemical
structures. It is thus a general method to increase the efficiency of OPV devices consisting of any active materials.
 According to above equation, a thickness as thin as 0.6 nm P(VDF-TrFE), about one monomolecular layer, is
needed to induce an energy level shift of 0.8 eV.
 The P3HT/PCBM system, however, has a Voc output of only about 0.6 V, which is significantly lower than the
optical bandgap (~2.0 eV) of P3HT. This discrepancy is caused by a very large LUMO offset close to 1.0 eV.
 The maximum attainable Voc of 1.5 V which assumes LUMO offset 0.2 V loss for the efficient charge transfer
and a Voc loss of 0.3 V due to the non-ideal diode in P3HT/PCBM OPV devices.
Experimental results and discussion
Piezoelectric force microscopy measurement
0.0
0.1
0.2
0.3
0.4
0.5
0.6
2
0.4
-4
0.6
0.2
400 nm
0.4
645
660
675
(b)
-8
0.0
0.0
630
400 nm
6
300 0.0
615
-6
0.2
0.2
0.4
0.6
0.8
1.0
0.0
µm
(c)
Topography
200
4
690
200
0.2
0.4
0.6
0.8
i
1.0
µm
(d)
150
+4V
100
0
150
0
http://www.brukerafmprobes.com
1.0
(c)
0.5
-1.5
(b)
-2.0 -1.5 -1.0 -0.5 0.0
Voltage /V
0
-2.4 -1.8 -1.2 -0.6
0.0
0.6
Voltage /V
1.2
1.8
2.4
Under dark
0
(e)50
250
Phase
100
150
nm
200
250
initial
150
-6
300
300
250
-2
200
-8
-4
-4
-2
2
0
4
6
8
Bias /V
(f)
Phase
after
200
-4
150
-6
100
-8
50
V
-1.0
AFM
300
0
 As shown in Figure a-b, only approximately 20% of
the P3HT surface was covered by the ferroelectric
P(VDF-TrFE) nanoislands. It could explain why the
obtained Voc is far less than the maximum attainable
-2
Voc of 1.5 V since most of the P3HT polymers directly
contact with PCBM molecules.
-4
 Figure c-f showed that the inserted P(VDF-TrFE)
-6
layer was in the ferroelectric state which could be
switched between two states by applying opposite
-8
bias.
0.0
V
100
-0.5
nm
200
-2
Forward bias poling
100 nm
50
nm
Reverse bias poling
300
J /A cm
-2
Current density /mA cm
-2
 After insertion 1 ML P(VDF-TrFE) LB film between the P3HT
and PCBM layers, the photoluminescence (PL) intensity only
showed little change, which demonstrated that the 1 ML
P(VDF-TrFE) LB film does not hinder photoinduced charge
transfer from the donor to the acceptor.
 The Voc was increased from 0.55 V to 0.67 V after poling
the P(VDF-TrFE) layer by applying a reverse bias on the device,
and was poled back with an opposite bias on the device.
 The dark current density was also reduced due to the
decreased charge transfer exciton recombination after poling
the P(VDF-TrFE) layer by applying a reverse bias on the device.
 Thus, the PCE was more than doubled from 1.5% to 3.3%.
100
100 nm
50
ii
ii
http:// www.asylumresearch.com
50
2
100
W/O poling
i
(d)
7
Wavelength /nm
PL change after insertion P(VDF-TrFE)
between P3HT and PCBM layers
Under light 100 mW cm-2
0
-2
0.4
250
Voltage /V
0.6
V
8
0.8
0.2
0.7
9
0.6
500 mV
(a)
-8
(a)
4
0.8
Potential /mV
-6
Amplitude
(b)
10
nm
-4
1.0
0.8
Phase /o
Forward bias poling
(c)
Amplitude /a.u.
-2
1.2
Amplitude
µm
Reverse bias poling
nm
0
P3HT
P3HT/PCBM
P3HT/P(VDF-TrFE)/PCBM
1.0
(a)
mV
µm
1.4
W/O poling
PL intensity /a. u.
-2
Current density /mA cm
1.0
Electrostatic force microscopy measurement
15 nm
J-V curve measurement
100 nm
1µm
Position /nm
 As shown in the figure a-d, the average surface
potential difference is 0.1 V, which is consistent
with tuned Voc.
 The fluctuation of surface potential is consistent
with the nonuniform distribution of the
ferroelectric P(VDF-TrFE) nanoislands on P3HT layer.
 This result indicates a large potential to further
increase the energy tuning capability by the
ferroelectric dipoles with increased coverage of the
dipole layer.
0
0
0
50
0
50
100
150
200
250
Local
piezoelectric
response
of the
P(VDF-TrFE)
grains
100
150
200
250
300
nm
nm
imbedded
under thin PCBM layer (20 nm)
300
Conclusions
References
 We demonstrated a method to increase the Voc of bilayer OPV devices by tuning the energy level offset of the donor and the acceptor with tunable ferroelectric P(VDF-TrFE)
dipole layers.
 Both Jsc and FF also increased due to the reduced recombination loss of charge transfer excitons. Thus, a double efficiency was achieved for P3HT and PCBM based bilayer OPV
devices.
 We expect that a further increase of Voc can be achieved by improving the coverage of the P(VDF-TrFE) dipole layer on P3HT.
 The Voc can be potentially improved to above 1.0 V without compromising other photovoltaic parameters such as Jsc and FF.
[1] C. M. Amb, S. Chen, K. R. Graham, J. Subbiah, C. E. Small, F. So, J. R. Reynolds, J. Am. Chem. Soc. 2011, 133, 10062.
[2] H. Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photonics 2009, 3, 649.
[3] Z. He, C. Zhong, X. Huang, W. Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 2011, 23, 4636.
[4] Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135.
[5] S. Loser, C. J. Bruns, H. Miyauchi, R. Ponce Ortiz, A. Facchetti, S. I. Stupp, T. J. Marks, J. Am. Chem. Soc. 2011, 133, 8142.
[6] S. H. Park, A. Roy, S. Beaupr, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photonics 2009, 3, 297.
[7] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, J. Am. Chem. Soc. 2011, 133, 4625.
[8] G. Li, R. Zhu, Y. Yang, Nat. Photonics 2012 DOI: 10.1038/nphoton.2012.11
[9] Y. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang, J. Huang, Nat. Mater. 2011, 10, 296.
[10] B. Yang, Y. Yuan, P. Sharma, S. Poddar, R. Korlacki, S. Ducharme, A. Gruverman, R. Saraf, J. Huang. Adv. Mater. 2012, 24, 1455.
Acknowledgements
J. Huang acknowledges partial support of this work by the Defense Threat Reduction Agency, Basic Research Award No. HDTRA1-10-1-0098 and National Science Foundation MRSEC Program Award No.
DMR-0820521, and the Nebraska Research Initiative. J. Huang and S. Ducharme acknowledge the support of Nebraska Center for Energy Sciences Research. P. Sharma and A. Gruverman acknowledge the
support of U. S. Department of Energy under Award DE-SC0004530.