Conventional models of planar and bulk heterojunction organic solar cells have been extended by introducing doping in the active layer. We have studied the performance of organic solar cells as a function of dopant concentration. For bulk heterojunction cells, the modeling shows that for the most studied material pair (poly-3-hexylthiophene, P3HT, and phenyl-C 61 -butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (J SC ), fill factor (FF) and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities or non-ohmic contacts, the efficiency can be increased by doping. For planar heterojunction cells, the modeling shows that if the acceptor layer is n doped, and the donor layer is p doped, the open-circuit voltage, J SC , FF and hence the efficiency can be increased by doping. Inversely, when the acceptor is p doped, and the donor is n doped; FF decreases rapidly with increasing dopant concentrations so that the current-voltage curve becomes S shaped. We also show that the detrimental effect of nonohmic contacts on the performance of the planar heterojunction cell can be strongly weakened by doping.
Phys. Rev. B. 84, 205318 (2011)PACS number(s): 72.20.Jv, 72.40.+w, 72.80.Le, 73.50.Pz
Organic
optoelectronics requires materials combining bright luminescence
and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers
(TPCOs) are promising highly emissive materials with decent charge-carrier
mobility; however, they typically show poor electron injection in
devices, which is usually assigned to high energies of their lowest
unoccupied molecular orbitals (LUMOs). A widely used approach to lower
the frontier orbitals energy levels of a conjugated molecule is its
fluorination. In this study, we synthesized three new fluorinated
derivatives of one of the most popular TPCOs, 2,2′-(1,4-phenylene)bis[5-phenylthiophene]
(PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence,
and Raman spectroscopies. The obtained data reveal a positive effect
of fluorination on the optoelectronic properties of PTPTP: LUMO levels
are finely tuned, and photoluminescence quantum yield and absorbance
are increased. We then grew crystals from fluorinated PTPTPs, resolved
their structures, and showed that fluorination dramatically affects
the packing motif and facilitates π-stacking. Finally, we fabricated
thin-film organic field-effect transistors (OFETs) and demonstrated
a strong impact of fluorination on charge injection/transport for
both types of charge carriers, namely, electrons and holes. Specifically,
balanced ambipolar charge transport and electroluminescence were observed
only in the OFET active channel based on the partially fluorinated
PTPTP. The obtained results can be extended to other families of conjugated
oligomers and highlight the efficiency of fluorination for rational
design of organic semiconductors for optoelectronic devices.
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