Advanced Functional Materials and Devices (AFMD) Group

Abstract:

Losses of charge carriers, due to the interfacial charge recombination processes, in small molecule organic solar cells (SMOSCs) have been investigated under operating conditions. The devices consist of zinc phthalocyanine (ZnPc) as electron donor material and C60 as electron acceptor. The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have been compared to the measurements carried out with impedance spectroscopy (IS) and show good agreement. Significantly, much difference is observed in either the charge density distribution versus the device voltage or the charge carriers lifetime when comparing bulk heterojunction versus bilayer-type ZnPc:C 60 devices. The implications of the faster charge carrier recombination with the device fill factor (FF) and the open circuit voltage (V OC) are discussed. © 2012 American Chemical Society.

Abstract:

We study the mechanism of molecular doping of the organic small molecule N,N,N ′,N ′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) doped with the fluorinated fullerene C 60F 36 or the acceptor molecule 2,2 ′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ). Varying the doping concentration, photoemission spectroscopy measurements show a comparable Fermi level shift for both dopants. The doping efficiency, defined as the ratio of free charge carriers (holes) to acceptors, is estimated from the depletion layer thickness in metal/intrinsic/p-doped structures. For low concentrations, we observe rather high doping efficiencies of up to 36% for C 60F 36, whereas for both dopants the doping efficiency strongly decreases with increasing doping concentration down to less than 10%. By numerically solving the charge neutrality equation using a classical semiconductor physics approach and comparing the results to the ultraviolet photoelectron spectroscopy data, we show that for very low concentrations doping is hindered by deep intragap states. In particular, the calculations can statistically explain the strong decrease of the doping efficiency for high doping concentrations. © 2012 American Physical Society.