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Organic Photovoltaics
Organic Photovoltaics
Organic photovoltaics (OPV) have attracted considerable interests due to the low-cost and simple manufacturing processes involved in the production of the photovoltaic devices. Among them, poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) organic solar cells are among the most studied. One of the main factors that affect the device performance is the morphology of the BHJs between donor (P3HT) and acceptor (PCBM) species in the active layer. An optimal distribution of both phases leads to an increased interfacial area between the active species with the formation of small domains that not exceed the exciton diffusion length (EDL) allowing for the charge separation process. A useful approach toward the control of the active layer morphology is the introduction of small quantities of additives able to improve the compatibility between the donor and acceptor species.
In this scenario, three copolymers obtained by an easy and inexpensive oxidative polymerization of 3,4'-dihexyl-2,2'-bithiophene and a new fulleropyrrolidine derivative mediated by FeCl3 have been synthesized by our Research group and successfully tested by Pignataro group as compatibilizers in P3HT:PCBM BHJs solar cells on plastic substrates (Figure 1). The prepared copolymers differ for the oligothiophene moieties length and this feature along with the effect of incorporation of different amounts of copolymers in the active layer on the solar cell performance have been studied. Indeed, during the thermal annealing of the device, the presence of these copolymers has resulted in an optimized nanophase morphology of the active layer that allowed to obtain the outstanding power conversion efficiency (PCE) (4.46%) and short current density (JSC) (16.15 mA cm–2) values that represent the highest among the P3HT:PCBM plastic organic solar cells.
Figure 1. Schematic representation of the flexible BHJ photovoltaic device.
As said above the inexpensive and simple manufacturing processes related with OPV are the reasons why organic solar cells are getting more and more popular, especially for some specific applications in which lightweight and low-cost of the device is required. The production of cheaper organic solar cells also depends by the cost of production of the acceptor species. Therefore, an accurate optimization of the synthetic procedures of these acceptors is of pivotal importance for a further abatement of the final cost of the device. With this in mind, we have optimized two different processes used for the functionalization of fullerenes, namely the Bamford–Stevens and [4 + 2] Diels Alder reactions, under microwave irradiation. In this manner, all the main C60- and C70-based acceptor derivatives for organic solar cells such as PCBM, DPM, BHN and ICBA, have been prepared in higher yields and shorter reaction times with respect to the reported data.
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