ChemComm
Communication
V
OC and PCE than devices made with F8IDT. In fact, the PCE of electronic coupling and not just the absolute energy levels is an
.4% observed for a P3HT:FEHIDT device is amongst the highest extremely important parameter in the design of non-fullerene
reported PCEs for a BHJ OPV with a non-fullerene derivative. electron acceptors for BHJ solar cells. These results, along with
2
IPCE spectra for the devices (see ESI†) are consistent with the observation that the FxIDT molecules have relatively deep
FxIDT having largely coincident absorption with P3HT. How- LUMO energies and extremely low electron mobilities, challenge
ever, they do show some contribution from FxIDT, particularly rigid rules that state that high performing electron acceptors must
6d,g
FEHIDT. This is a promising result with regards to the potential have low-lying LUMOs and high mobilities. The basic chemical
for these materials to be used in combination with low bandgap design implication of our findings is that, as articulated by Anthony
13
donors such that charge generation could be achieved across a et al., the design of new electron accepting materials should not
wider range of wavelengths. focus solely on good p-stacking. Rather, the degree of electronic
In an attempt to measure the electron mobility of the FxIDT coupling between molecules must be considered. This work further
compounds, pristine films and blends with P3HT were analysed enhances the prospects for the design of other, non-spherical
using the photo-CELIV technique (see ESI†). No charge mobility electron acceptors that will help realise significant improvements
was observed for either of the pristine FxIDT materials. However, in BHJ solar cell device performance.
the charge mobility of the blends with P3HT was only slightly
This research was funded through the Flexible Electronics
reduced from that seen for pristine P3HT. This suggests that the Theme of the CSIRO Future Manufacturing Flagship and was
charge mobility in P3HT is not overly disrupted by the presence also supported by the Victorian Organic Solar Cell Consortium
of the FxIDT, a conclusion that is also consistent with the thin (Victorian Department of Primary Industries, Victorian Depart-
film X-ray diffraction data.
ment of Business and Innovation and the Australian Renewable
The isomeric compounds F8IDT and FEHIDT show, in general, Energy Agency (ARENA)).
very similar properties. However, devices based on blends of them
with P3HT show significant differences, see Table 1. The optimised Notes and references
PCE for a device based on FEHIDT (2.4%) is around 30% higher
1
C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia and
than that obtained for F8IDT. The key reason for this is the very
large difference in the measured open circuit voltage of devices.
Recently, a number of groups have shown that an analysis of the
S. P. Williams, Adv. Mater., 2010, 22, 3839–3856.
2 (a) Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4089–4380;
b) A. Mishra and P. B ¨a uerle, Angew. Chem., Int. Ed., 2012, 51,
020–2067.
(
2
dark J–V curves can provide insight into factors other than just
3
(a) A. Facchetti, Chem. Mater., 2011, 23, 733–758; (b) L. Bian, E. Zhu,
J. Tang, W. Tang and F. Zhang, Prog. Polym. Sci., 2012, 37,
9
the HOMO–LUMO gaps that play a role in determining the VOC
.
1
2
292–1331; (c) H. Zhou, L. Yang and W. You, Macromolecules,
012, 45, 607–632.
The generalised Shockley equation for solar cells includes a para-
meter, JS0, that can be directly related to the strength of the
intermolecular interactions in the active layer of organic solar cells.
Specifically, a smaller JS0 is found for devices where there is
less electronic coupling between molecules. This leads to reduced
4
5
J. T. Bloking, X. Han, A. T. Higgs, J. P. Krastrop, L. Pandey,
J. E. Norton, C. Risko, C. E. Chen, J.-L. Bredas, M. D. McGehee
and A. Sellinger, Chem. Mater., 2011, 23, 5484–5490.
(a) J. E. Anthony, Chem. Mater., 2011, 23, 583–590; (b) P. Sonar,
J. P. F. Lim and K. L. Chan, Energy Environ. Sci., 2011, 4, 1558–1574.
1
0a
recombination in devices and increases the VOC. Kippelen et al.
and Thompson et al.
6 (a) F. G. Brunetti, X. Gong, M. Tong, A. J. Heeger and F. Wudl, Angew.
Chem., Int. Ed., 2010, 49, 532–536; (b) P. E. Schwenn, K. Gui,
A. M. Nardes, K. B. Krueger, K. H. Lee, K. Mutkins, H. Rubinstein-
Dunlop, P. E. Shaw, N. Kopidakis, P. L. Burn and P. Meredith, Adv.
Energy Mater., 2011, 1, 73–81; (c) G. Ren, E. Ahmed and
S. A. Jenekhe, Adv. Energy Mater., 2011, 1, 946–953; (d) T. Zhou,
T. Jia, B. Kang, F. Li, M. Fahlman and Y. Wang, Adv. Energy Mater.,
10b
demonstrated this type of analysis for
bi-layer devices while You et al. have performed similar analyses on
11
BHJ devices. In particular, it has been shown that side chains on
11
electron donors can have a significant influence on the VOC
.
Ito
et al. have also used an analysis of the JS0 values to demonstrate
2011, 1, 431–439; (e) Y. Zhou, L. Ding, K. Shi, Y.-Z. Dai, N. Ai, J. Wang
12
that substituents on fullerenes have a similar effect. Dark current
analyses of J–V curves from devices based on P3HT:FxIDT blends
reveal JS0 values for F8IDT that are 3–4 orders of magnitude higher
than for FEHIDT (see ESI†). Furthermore, the use of these JS0 values
and the measured energy levels for the materials gave calculated
VOC values that are a close match to the measured values. In
combination, these data are consistent with a conclusion that the
degree of electronic coupling (and therefore the rate of recombina-
tion) in P3HT:FEHIDT devices is significantly lower than in the
F8IDT blends. The presence of branched alkyl chains and the use
of an isomeric mixture are both possible explanations for the
reduced electronic coupling in FEHIDT. Further study of these
factors remains as future work.
and J. Pei, Adv. Mater., 2012, 24, 957–961; ( f ) T. V. Pho, F. M. Toma,
M. L. Chabinyc and F. Wudl, Angew. Chem., Int. Ed., 2013, 52,
1
446–1451; (g) Y. Lin, Y. Li and X. Zhan, Adv. Energy Mater., 2013,
DOI: 10.1002/aenm.201200911.
7 H. B u¨ rckst u¨ mmer, E. V. Tulyakova, M. Deppisch, M. R. Lenze,
N. M. Kronenberg, M. Gs ¨a nger, M. Stolte, K. Meerholz and
F. W u¨ rthner, Angew. Chem., Int. Ed., 2011, 50, 11628–11632.
8 F. H. Scholes, T. Ehlig, M. James, K. H. Lee, N. Duffy, A. D. Scully,
Th. B. Singh, K. N. Winzenberg, P. Kemppinen and S. E. Watkins,
Adv. Funct. Mater., 2013, DOI: 10.1002/adfm.201300726.
9
B. Qi and J. Wang, J. Mater. Chem., 2012, 22, 24315–24325.
1
0 (a) W. J. Potscavage, Jr, S. Yoo and B. Kippelen, Appl. Phys. Lett.,
2008, 93, 193308; (b) M. D. Perez, C. Borek, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2009, 131, 9281–9286.
11 (a) L. Yang, H. Zhou and W. You, J. Phys. Chem. C, 2010, 114,
16793–16800; (b) L. Yang, J. R. Tumbleston, H. Zhou, H. Ade and
W. You, Energy Environ. Sci., 2013, 6, 316–326.
2 S. Yamamoto, A. Orimo, H. Ohkita, H. Benten and S. Ito, Adv. Energy
Mater., 2012, 2, 229–237.
3 J. E. Anthony, A. Facchetti, M. Heeney, S. R. Marder and X. Zhan,
Adv. Mater., 2010, 22, 3876–3892.
1
1
In summary, the FxIDT molecules described here represent
an important new class of electron acceptors for BHJ solar
cells. In particular, our findings confirm, for the first time, that
This journal is c The Royal Society of Chemistry 2013
Chem. Commun.