Conventional polymer solar cells

Interfacial Materials for Conventional Polymer Solar Cells... 

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. 

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)]. 

FIGURE 9.1 Flexible solar cells based on different kinds of polymer substrates. (A) Photograph of a flexible DSSC based on ITO-coated PET substrate wrapped on a pen. (B) Schematic illustration of the layer structure of a solid-state DSSC based on PEDOT on Goretex film as a counter electrode. (C) Schematic illustration (left) and photograph (right) of the PSC based on ITO-coated PET substrate. (D) Comparison of PSCs fabricated on conventional FTO/glass and flexible PEDOT PSS/PET substrate. (E) Schematic illustration of the flexible solar cell based on Ag-grid/PET substrate. 

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