INTERNAL ELECTRIC FIELD, ELECTRON-TRAPPING VOIDS, DEAD ELECTRONS, AND THE INCREASING OF THE EFFICIENCY OF POLYMER SOLAR CELLS WITH FLUORINATION
Abstract
We propose a model that allows an understanding of the nature of electron traps in π-onjugated polymers that are used in photovoltaic devices. It is assumed that the free-cavity voids in the polar π-conjugated polymer exhibit electron-accepting affinity and are filled with electrons, called erein as ‘dead’ electrons because they are hold by a static random internal electric field and are not driven to the cathode by external built-in voltage. As a result, the dead electrons into electron-trapping voids are unsuitable for use in the external circuit of organic solar cells. As justified here, the exciton dissociation on the surfaces of voids, the capture of drift electrons by voids, the emerging of the dead electrons and their non-geminate recombination are the main obstacles to create highly efficient polymer solar cells. The model of dead electrons allows explaining the increase in the power conversion efficiency of solar cells caused by the polymer fluorination, side-chain polymer manipulation, and dopant-controlled trap-filling. Some characteristics of hybrid P3HT:CdSe solar cells are also analyzed with the help of this model.
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