Heterojunction model

In the examples addressed below, for a two-band phonon distribution, the HEP scheme willl be shown to yield essential insight into the interplay between high-frequency (C=C stretch) and low-frequency (ring-torsional) modes of the polymer heterojunction model of Eq. (7). 

Fig. 5.10. Fully NEA GaAs/(CsO) according to the heterojunction model. For the optimum (CsO) thickness, the work function of (CsO) is 1.08 eV, which gives a bulk GaAs electron affinity of-0.34 eV. The conduction band discontinuity, common in most coated IIl-V surfaces, is hidden for the 111 B face of GaAs. Note the slight (0.1 eV) band bending, indicating that Ess differs from its value on pure Cs-coated material. The Fermi level at the surface varies also with the choice of activated crystal face [5.46]...

Fig. 5.11. Summary of factors limiting photo yield in the heterojunction model. The optical reflectivity is R transport factors in the band-bending region are described by with similar factors Tbs at the Semiconductor surface Barrier, Tabs for electron ABSorption in the (CsO) layer, and Tbv for loss at the (usually unimportant) Vacuum Barrier [5.82]...

There are many published examples in which the coupling of two different materials leads to an increase in the photocatalytic activity. Many of them concern coupling and junctions between different nanopartides, considering also different topologies, like coupled and capped systems [72]. Tentative explanations based on possible heterojunction band profiles are given. However, in-depth analysis of the hetero junction band alignment, the physical structure of the junction, the role of (possible) interfadal traps and of spedfic catalytic properties of the material is still lacking. Some recently published models and concepts based on (nano)junction between different materials are briefly reviewed here. 

Cuiffi J, Benanti T, Nam WJ, Fonash S (2010) Modeling of bulk and bilayer organic heterojunction solar cells. Appl Phys Lett 96 143307... 

Fig. 2 Molecular structure of a model F8BT(top) TFB(bottom) polymer heterojunction in the eclipsed stacking configuration, see also Fig. 3. In the actual polymer, the residues are R = C8Hi2 in the calculations reported here, R = H was used (Adapted from Ref. [43]).

In Refs. [50-53], two levels of analysis were successively addressed (i) a two-state XT-CT model which is able to capture the basic features of the phonon-mediated exciton dissociation process (ii) a three-state XT-IS-CT model which also comprises an intermediate state (IS), i.e., an additional charge transfer state whose presence can have a significant influence on the dynamics, see Fig. 6. In the latter case, comparative calculations for several interface configurations were carried out, leading to a realistic, molecular-level picture of the photophysical events at the heterojunction. In the following, we start with a summary of the findings reported in Refs. [50,51], where the two-state model was explored (Sec. 5.1). Following this, we address in more detail the analysis of Refs. [52,53] for the three-state model (Sec. 5.2). 

We now briefly summarize the key results of the analysis of Refs. [50,51] for a reduced XT-CT model of the TFB F8BT heterojunction, using explicit quantum dynamical (MCTDH) calculations for a two-state model parametrized for 20-30 phonon modes. At this level of analysis, an ultrafast ( 200 fs) XT state decay is predicted, followed by coherent oscillations, see Fig. 8 (trace exact in panel (a)). Further analysis in terms of an effective-mode model and the associated HEP decomposition (see Sec. 4.2) highlights several aspects ... 

Further, the model allows us to estimate electrical losses in the device. Figures 5.18c and d show the local variations in the energy levels and the carrier densities for the bulk heterojunction solar cell for different mobilities. In Fig. 5.18c, balanced mobilities for electrons and holes are assumed, while Fig. 5.18d describes the situation for the case where the electron mobility is higher than the hole mobility. In the latter case recombination is enhanced as seen from the carrier densities, and the performance of the device (Jsc) is significantly lowered. 

---