Energy polymer heterojunctions

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

Fig. 54 Potential energy diagram describing the energetics and kinetics at type II polymer heterojunctions. The energetic order of A D")r = oo and A D)r = oo may be reversed for PFB F8BT vs TFB F8BT. The inset shows the band offsets at a type II heterojunction. (Reprinted with permission from [57], 2004, American Physical Society)...

Fig. 2.45 Potential energy diagram describing the energetics and kinetics attype-ll polymer heterojunctions. The energetic...

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). 

Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

The heterojunctions of the polyacetylene were realized not only with inorganic photoconductors but also with organic polymers [139]. The results obtained show good similarity with barrier and heterojunction characteristics for inorganic semiconductors. Photoelectrochemical cell for solar energy conversion with polyacetylene electrodes and Na2S, electrolyte had an efficiency of 1 % at 2.4 eV [140], The complicated phenomena take place at the electrodeelectrolyte interface. 

Fig. 1 Band offsets, i.e., relative HOMO/LUMO energies, for two representative type II polymer junctions, i.e., the TFB F8BT and PFB F8BT heterojunctions. Both are fluorene-based polymer materials [26,33]. In this chapter, we focus on the TFB F8BT junction.

Scheme 5.8 Energy level alignment of bulk heterojunction components (conjugated polymer and semiconductor nanocrystals) facilitating the dissociation of excitons and charge separation. Left panel Case describing excitons formed in the nanocrystal phase. Right panel case describing excitons formed in the polymer phase.

Fig. 5.41. Schematic overview of different strategies for spectral sensitization of bulk heterojunction solar cells utilizing a low bandgap polymer, (a) shows the scenario for an energy transfer between the dye and the low bandgap polymer, while (b) illustrates the scenario for an electron transfer between the dye and the low bandgap polymer...

The observed experimental result that Voc decreases linearly for bulk heterojunction solar cells allows us to conclude that, at least in the high temperature range (T > 200 K), these solar cells may be described by a diode model with Ip exp(E/kT). Here E is a parameter analogous to Eg for conventional semiconductors. For conjugated polymer/fullerene bulk heterojunction solar cells, E should correspond to the energy difference between the HOMO level of the donor and the LUMO level of the acceptor components of the active layer [as also suggested by the extrapolated value of V oc(0 K)]. 

This work summarizes the physics of a special area of photovoltaic energy conversion, i.e., polymer-based, bulk heterojunction solar cells. With about... 

The excellent photosensitivity and relatively high energy conversion efficiencies obtained from the bulk heterojunction materials are promising. The monochromatic power efficiencies for conjugated polymer photovoltaic devices are around... 

Fig. 17 Simple relationship of open circuit voltage Vqc for drift-current dominated bulk heterojunction polymer solar cells. The first limitation arises from the molecular energy levels (Voci) secondly, improper match with the contact work function may further reduce the achievable voltage to 002- (Reprinted with permission from [105], 2003, American Institute of Physics)...

Recently, the Konarka group achieved power conversion efficiencies of 5.2% for a low band gap polymer-fuUerene bulk heterojunction solar cell, as confirmed by NREL (National Renewable Energy Laboratory, USA). This encourages the practical use of this concept for low cost, large area production of photovoltaic devices. 

Hoppe H, Arnold N, Meissner D, Sariciftci NS (2003) Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells. Sol Energy Mater Sol Cells 80 105... 

At short times the decay from the blend is faster than that from the pure polymers. This is due to the energy transfer from the excitons to the exciplex, as well as the charge separation that excitons undergo at the heterojunction. In Section 2.3.1 we investigate these phenomena and develop a model for charge generation and exciplex formation at the heterojunction. 

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