The ability to reverse the composition profile of active layers by delaminating and transferring P3HT:PCBM thin films has allowed us to probe directly the sole influence of interfacial segregation on solar cell device characteristics on the same device platform.
We adsorbed fluorinated-alkyl and
hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide
(ITO) to form self-assembled monolayers (SAMs). Polymer solar cells
having these treated ITOs as anodes display open-circuit voltages
(V
ocs) that are higher than those with
bare ITO as anodes. Although the work function of ITO can be significantly
tuned by SAM adsorption, the position of the Fermi level of the anode
with respect to the hole transport level in the polymer active layer
is essentially the same in all of the devices, suggesting that changes
in the work function of the anode are not responsible for the V
oc variation. Rather, the barrier for minority
carrier transport to ITO is altered through SAM adsorption. The adsorption
of fluorinated-alkyl phosphonic acid on ITO, in particular, induces
a barrier of 2.4 eV for minority carrier transport, which effectively
increases carrier selectivity at the anode and increases the V
oc in polymer solar cells comprising such treated
ITO as anodes compared to those with untreated anodes.
A combination of density functional theory and experimental measurements via ultraviolet and X-ray photoelectron spectroscopies is used to explore the nature of the interface between the stoichiometric molybdenum trioxide (MoO 3 ) or its under-stoichiometric counterpart with oxygen vacancies, and an organic hole-transport layer represented by 4,4 ′ -N , N ′ -dicarbazole-biphenyl (CBP). Upon adsorption of CBP, special attention is paid to i) the appearance of gap states and the reduction of the molybdenum oxide surface, and ii) the evolution of the work function. Very good agreement is found between theory and experiment. The near alignment of the CBP highest occupied molecular orbital with the Fermi level and the conduction band edge of molybdenum oxide points to facile hole collection or injection. Figure 1 . The chemical structure of 4,4 ′ -N , N ′ -dicarbazole-biphenyl (CBP).
Using scanning tunneling microscopy, we demonstrate that the nucleation density of Fe islands on the surface of nanoscale Pb films oscillates with the film thickness, providing a direct manifestation of the quantum size effect on surface diffusion. The Fe adatom diffusion barriers were derived to be 204+/-5 and 187+/-5 meV on a 21 and 26 monolayer (ML) Pb film, respectively, by matching the kinetic Monte Carlo simulations to the experimental island densities. The effect is further illustrated by the growth of Fe islands on wedged Pb films, where the Fe island density is consistently higher on the odd-layer films than on the even-layer films in the thickness range of 11 to 15 ML.
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