Donor/acceptor interface

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

The interface dipole in organic-organic junctions is negligible with the exception of strong donor-acceptor interfaces where a barrier of 0.2-0.3 eV may exist due to the charge transfer process. 

This exciton diffuses to the donor/acceptor interface via an energy-transfer mechanism (i.e., no net transport of mass or charge occurs). (3) Charge-transfer quenching of the exciton at the D/A interface produces a charge- transfer (CT) state, in the form of a coulombically interacting donor/acceptor complex (D A ). The nomenclature used to describe this species has been relatively imprecise, and has... 

Penmans P, Forrest SR (2004) Separation of geminate charge-pairs at donor-acceptor interfaces in disordered solids. Chem Phys Lett 398 27... 

Fig. 5.13. Schematic variation of Voc with acceptor strength (solid double headed arrow, Voci) or/and electrode work function (dotted arrow, V0c2)> m a donor/acceptor BHJ solar cell. The electron transfer, occurring at the donor/acceptor interface after light excitation, is indicated by the bent arrow [134].

This limitation was finally overcome by the concept of the bulk heterojunction, where the donor and acceptor materials are intimately blended throughout the bulk [28-30]. In this way, excitons do not need to travel long distances to reach the donor/acceptor interface, and charge separation can take place throughout the whole depth of the photoactive layer. Thus the active zone extends throughout the volume, as illustrated in Fig. 11. Conse-... 

FlG. 11.2. The number of the dissociated pairs (n2) as a function of the total number of excitations (m + n.2) at the donor-acceptor interface (according to the simplified model of eqn (11.20) and in units consistent with the numerical simulations discussed below. Reprinted with permission from Agranovich et al. (14). Copyright Elsevier (1993). 

To numerically simulate the time evolution of CTEs distributed over a two-dimensional donor-acceptor interface the D-A sites were arranged in a square lattice. It was assumed that the D-A interface is uniformly irradiated with a time independent source of intensity I. Only one CTE can be generated at any site, so every D-A site can be either occupied or not. The CT exciton generated at a given lattice site will stay there and it cannot move to another D-A site because of self-trapping. 

FlG. 11.3. The steady state number of CTEs (ni) occupying the donor-acceptor interface and the number of dissociated pairs (52) as a function of the pumping intensity. The pumping intensity S is equal to the number of charge-transfer exci-tons produced at the interface during a CTE lifetime in the absence of dissociation processes. The results are from the numerical simulations of the CTE system described in the text. Reprinted with permission from Kiselev et al. (20). Copyright Elsevier (1998). 

Nonlinear optical response of charge-transfer excitons at donor—acceptor interface... 

The number of the dissociated pairs as a function of the total number of excitations at the donor-acceptor interface... 

Abstract. Copper phthalocyanine (CuPc)-fullerene (C60) photovoltaic cells are produced by organic vapour phase deposition reaching efficiencies of 3%. The electronic transport properties of the devices are investigated as a function of the CuPc C60 absorber blend layer composition and its preparation temperature. The analysis of the transport properties of the devices employs the one-diode model. It is shown that the dominant recombination process takes place at the donor-acceptor interfaces of the CuPc and C60 absorber domains. The activation energy of recombination is related to the effective band gap of the blend layer. 

Devices in molecular electronics typically have a multilayered structure. An understanding of processes at the interfaces between different layers is imperative to achieve high efficiency of the devices. It is often necessary to know how electronic levels of different organic layers are located with respect to each other. In the simple Shottky-Mott model, two different organic layers share the common vacuum level. However, in a number of experimental studies this picture has been shown to be not correct [1]. Usually, an additional potential is present at the interface, shifting the vacuum level (VL) of one material with respect to the other. This additional potential at the donor/acceptor interface is caused by an interfacial dipole layer [1]. 

First, the IDL might stem from the admixture of a charge transfer (CT) in the ground state at the donor/acceptor interface. At the second order of perturbation theory, the wavefunction of the complex is represented in the following way ... 

On illumination, the excitons generated within the exciton diffusion length from the donor-acceptor interface can reach the interface within their respective exciton lifetime, where charge separation is favorable when the following condition is fiilfilled ... 

To ensure that all photogenerated excitons reach a donor-acceptor interface, the heterojunction formed between the two materials has to be scaled down to the nanometer level to form an architecture that is referred to as bulk heterojunction [42]. As such, the bulk heterojunction can be regarded as an ensemble of nanoscale heterojunctions distributed all over the volume forming a bicontinous network. 

---