Phase polymer heterojunctions

For proper operation of a bulk heterojunction photovoltaic cell, a special alignment of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions, as depicted in Scheme 5.8. If an exciton is formed in the polymer phase, then the electron is transferred to the NC phase and reaches the aluminum electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, that is, the formation of an exciton in the NCs phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminum electrode through the NCs phase. 

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.

Hoppe H, Drees M, Schwinger W, Schaffler F, Sariciftci NS (2005) Nano-crystalline fullerene phases in polymer/fullerene bulk-heterojunction solar cells a transmission electron microscopy study. Synth Met 152 117... 

It is now also clear what the polymer film equivalent of the concentration of a solution has to be. A highly concentrated solution results in a high frequency of collisions. In a polymer blend film excitons encounter heterojunctions more frequently when the hetero junction density is high. Hence, a high concentration of a solution corresponds to a well-mixed polymer blend with small-scale phase separation. The degree of mixing of the components in a polymer blend (i.e. the... 

Exciplex states only exist at the interface between the two dissimilar polymers in the blend. Reducing the density of these interfaces in the polymer blend is expected to reduce the amount of exciplex observed. For example, annealing mobilizes the polymers and causes the film to move closer to thermodynamic equilibrium, i.e. the two polymers phase separate and the density of heterojunction sites decreases. Indeed, we observe that the amount of exciplex emission is reduced by the annealing treatment [30]. 

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

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