C O M M U N I C A T I O N S
constitutes the highest reported efficiency to date for a solution
processed all-polymer OPV.11
electronic and structural properties for application to OPVs. The
AM 1.5 efficiency of 2% achieved with POPT/CNPPV is the highest
reported to date for an all-polymer based device. POPT outper-
formed P3HT in all-polymer devices due to a doubling of the Jsc.
At individually optimized bilayer thicknesses, the superior perfor-
mance of POPT vs P3HT in the devices with CNPPV is counter to
expectations based on absorption, charge mobility, and energy level
comparisons. This emphasizes the importance of understanding
charge separation processes in OPV devices, particularly the effects
of Coulombic attraction and lattice polarization energy. Addition-
ally, the synthetic simplicity and tunability of the phenylthiophene
class of polymers makes POPT and other 3-phenyl derivatives
attractive materials for further exploration of structure-property
relationships in the field of polymer-based solar cells.
Significantly, similar all-polymer devices optimized from GRIM
P3HT yielded a max efficiency of 0.93% (Figure 2A) with an
average of 0.75%. This lower efficiency in P3HT devices is due to
a reduction in the short circuit current (Jsc). The increased Jsc
exhibited by the POPT/CNPPV devices does not derive from
increased absorption, as illustrated by the absorption spectra in
Figure 2C. Under optimized conditions, the POPT/CNPPV bilayer
absorbs ∼75% of the light but exhibits approximately twice the
photocurrent of the P3HT/CNPPV bilayers with improved photo-
current across the entire absorption spectrum of the device (Figure
2D). Additionally, all-polymer POPT:CNPPV blend devices were
fabricated but did not perform as well as the bilayer devices, making
a POPT/P3HT comparison hard to evaluate in that architecture.
As neither light absorption nor hole mobility can explain this
striking difference in photocurrent, we examined the electronic
driving forces behind charge separation.
Acknowledgment. This work was supported by the U.S.
Department of Energy under Contract No. DE-AC02-05CH11231
and in part by the King Abdullah University of Science and
Technology (KAUST) Center for Advanced Molecular Photovol-
taics (Award No. KUS-C1-015-21). T.W.H. and C.H.W. thank the
NSF for graduate research fellowships. We also thank Jill E.
Millstone and Alejandro L. Brisen˜o for helpful discussions.
Considering that OPVs require a donor/acceptor interface to
separate excitons and generate free charges, understanding charge
separation is critical for advancing the field of OPVs.12 Recent
rel
literature has attempted to relate ∆GCS (the relative free energy
of charge separation) to the excited state energy (Es) and the relative
band offsets in the abbreviated Weller equation ∆GCS ) ES -
Supporting Information Available: Synthetic and device fabrica-
tion procedures, NMR, SEC, CV, mobility, and TEM data. This material
rel
(HOMOdonor - LUMOacceptor).12a Values for ∆GCSrel calculated from
this equation correlate well with the observed short circuit currents
for several polymer:PCBM devices.12a However, in our case this
equation predicts a larger driving force for charge separation in
the P3HT/CNPPV device, as ∆Grel is 0 eV for POPT/CNPPV but
is 0.3 eV for P3HT/CNPPV (Figure 2B). The large difference in
Jsc between these polythiophene devices indicates that charges are
either extracted or generated more efficiently from the POPT device,
contrary to measured hole mobilities, light absorption, and predicted
∆Grel. Notably, the abbreviated Weller equation does not include
the lattice polarization energy or Coulombic attraction between
bound electron-hole pairs. We believe these neglected terms are
important in explaining the increased Jsc in POPT/CNPPV devices.
References
(1) (a) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.;
Yang, Y. Nat. Mater. 2005, 4, 864–868. (b) Thompson, B. C.; Fre´chet,
J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77.
(2) (a) Andersson, M. R.; Selse, D.; Berggren, M.; Jaervinen, H.; Hjertberg,
T.; Inganas, O.; Wennerstroem, O.; Oesterholm, J. E. Macromolecules 1994,
27, 6503–6. (b) Pei, Q.; Jarvinen, H.; Osterholm, J. E.; Inganas, O.; Laakso,
J. Macromolecules 1992, 25, 4297–4301.
(3) Johansson, T.; Mammo, W.; Svensson, M.; Andersson, M. R.; Inganas, O.
J. Mater. Chem. 2003, 13, 1316–1323.
(4) Gadisa, A.; Svensson, M.; Andersson, M. R.; Inganas, O. Appl. Phys. Lett.
2004, 84, 1609–1611.
(5) (a) deLeeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F.
Synth. Met. 1997, 87, 53–59.
(6) (a) Aasmundtveit, K. E.; Samuelsen, E. J.; Mammo, W.; Svensson, M.;
Andersson, M. R.; Pettersson, L. A. A.; Inganas, O. Macromolecules 2000,
33, 5481–5489. (b) Andersson, M. R.; Berggren, M.; Inganas, O.;
Gustafsson, G.; Gustafssoncarlberg, J. C.; Selse, D.; Hjertberg, T.;
Wennerstrom, O. Macromolecules 1995, 28, 7525–7529. (c) Theander, M.;
Inganas, O.; Mammo, W.; Olinga, T.; Svensson, M.; Andersson, M. R. J.
Phys. Chem. B 1999, 103, 7771–7780.
(7) Granstrom, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.;
Friend, R. H. Nature 1998, 395, 257–260.
(8) (a) Chen, L. C.; Godovsky, D.; Inganas, O.; Hummelen, J. C.; Janssens,
R. A. J.; Svensson, M.; Andersson, M. R. AdV. Mater. 2000, 12, 1367–
1370. (b) Brabec, C. J.; Winder, C.; Scharber, M. C.; Sariciftci, N. S.;
Hummelen, J. C.; Svensson, M.; Andersson, M. R. J. Chem. Phys. 2001,
115, 7235–7244. (c) Roman, L. S.; Arias, A. C.; Theander, M.; Andersson,
M. R.; Inganas, O. Braz. J. Phys. 2003, 33, 376–381.
(9) (a) Wen, L.; Duck, B.; Dastoor, P. C.; Rasmussen, S. C. Macromolecules
2008, 41, 4576–4578. (b) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney,
M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2008, 48, 16324–16329. (c)
McCullough, R. D.; Tristramnagle, S.; Williams, S. P.; Lowe, R. D.;
Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910–4911.
(10) (a) McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992,
70–72. (b) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem.
Soc. 2005, 127, 17542–17547.
(11) (a) Kietzke, T; Ho¨rhold, H; Neher, D. Chem. Mater. 2005, 17, 6532–6537.
(b) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4647–4656. (c)
Jenekhe, S. A.; Yi, S. Appl. Phys. Lett. 2000, 77, 2635–2637.
(12) (a) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.;
Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R.
J. Am. Chem. Soc. 2008, 130, 3030–3042. (b) Huang, Y. S.; Westenhoff,
S.; Avilov, I.; Sreearunothai, P.; Hodgkiss, J. M.; Deleener, C.; Friend,
R. H.; Beljonne, D. Nat. Mater. 2008, 7, 483–489. (c) Mihailetchi, V. D.;
Koster, L. J. A.; Hummelen, J. C.; Blom, P. W. M. Phy. ReV. Lett. 2004,
93, 216601. (d) Veldman, D.; Ipek, O.; Meskers, S. C. J.; Sweelssen, J.;
Koetse, M. M.; Veenstra, S. C.; Kroon, J. M.; van Bavel, S. S.; Loos, J.;
Janssen, R. A. J. J. Am. Chem. Soc. 2008, 130, 7721–7735.
Figure 2. (A) J-V curves for POPT and P3HT devices under AM 1.5
100 mW/cm2 illumination. (B) Material energy band levels. (C) Absorption
spectra of bilayers at optimized thicknesses for devices. (D) EQE plots of
optimized devices.
In conclusion, it is clear that the more controlled GRIM method
we have used to prepare POPT affords a polymer with desirable
JA9059359
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14161