Documento PDF - UniCA Eprints - Università degli studi di Cagliari.
Documento PDF - UniCA Eprints - Università degli studi di Cagliari.
Documento PDF - UniCA Eprints - Università degli studi di Cagliari.
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1.1 hybrid interfaces for photovoltaics 3ficient electron-hole separation, the junction between thetwo materials must be of the type-II (staggered), for whichthe HOMO and LUMO positions decrease in energy whenmoving from the donor to the acceptor (see Figure 1.1).Excitons can recombine efficiently unless they <strong>di</strong>ffuse andseparate at the interface within their lifetime. In order toachieve high performance bilayers, trasport must be efficientin comparison to recombination mechanisms, such asluminescence or non-ra<strong>di</strong>ative recombination. For the majorityof molecules, the exciton lifetime is in the order ofnanoseconds while the <strong>di</strong>stance that an exciton can crossis limited to about 10 nm [11]. This means that only theexcitons formed within this <strong>di</strong>stance from the interface cancontribute to charge separation.Figure 1.1.: Working principle of an organic bilayer solar cell.The simple bilayer can be replaced by a more complexbulk heterojunction architecture. In this kind of solar cellthe donor and acceptor components interpenetrate one another,giving an interface not planar but spatially <strong>di</strong>stributed[13]. This feature makes possible to partially overcome thelimitation due to the <strong>di</strong>ffusion length of excitons since thelarge surface-to-volume ratio makes possible to collect atthe interface a larger fraction of excitons.Unfortunately, the <strong>di</strong>sadvantages are represented by the<strong>di</strong>fficult separation of the charges due to the increased <strong>di</strong>s-