The importance of fullerene percolation in the mixed regions of polymer-fullerene bulk heterojunction solar cells
AuthorsBartelt, Jonathan A.
Beiley, Zach M.
Hoke, Eric T.
Mateker, William R.
Douglas, Jessica D.
Collins, Brian A.
Tumbleston, John R.
Ade, Harald W.
Toney, Michael F.
McGehee, Michael D.
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
KAUST Solar Center (KSC)
Chemical Science Program
Materials Science and Engineering Program
Organic Electronics and Photovoltaics Group
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AbstractMost optimized donor-acceptor (D-A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than - 80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrifi cing internal quantum effi ciency, the device power conversion effi ciency (PCE) could be signifi cantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2- ethylhexyloxy)benzo[1,2- b :4,5- b ' ]dithiophene- co -octylthieno[3,4- c ]pyrrole-4,6- dione) (PBDTTPD) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) with 7.3% PCE and fi nd that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge-carrier recombination and limit device quantum effi ciency. These fi ndings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum effi ciency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells. © 2013 WILEY-VCH Verlag GmbH and Co. © 2013 WILEY-VCH Verlag GmbH & Co.
SponsorsThe authors acknowledge Nichole Cates Miller, Sean Sweetnam, Kristin Schmidt, and Christopher Tassone for helpful discussions and thank Plextronics for supplying the CA-1914. This publication was based on work supported by the Center for Advanced Molecular Photovoltaics (CAMP) (Award No KUS-C1-015-21), made by King Abdullah University of Science and Technology (KAUST). J.A.B. and Z.M.B. acknowledge government support under and awarded by the DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. E. T. H acknowledges funding from the Fannie and John Hertz Foundation. The NEXAFS miscibility study by NCSU was supported by DOE contract DE-FG02-98ER45737. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource user facility, operated by Stanford University on behalf of the U. S. Department of Energy, Office of Basic Energy Sciences and at the Advanced Light Source, Berkeley, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.
JournalAdvanced Energy Materials