Abstract
In this paper, characterisation of exciton generation is carried out in three bulk-heterojunction organic solar cells (BHJ OSCs)—OSC1: an inverted non-fullerene (NF) BHJ OSC; OSC2: a conventional NF BHJ OSC; and OSC3: a conventional fullerene BHJ OSC. It is found that the overlap of the regions of strong constructive interference of incident and reflected electric fields of electromagnetic waves and those of high photon absorption within the active layer depends on the active layer thickness. An optimal thickness of the active layer can thus be obtained at which this overlap is maximum. We have simulated the rates of total exciton generation and position dependent exciton generation within the active layer as a function of the thicknesses of all the layers in all three OSCs and optimised their structures. Based on our simulated results, the inverted NF BHJ OSC1 is found to have better short circuit current density which may lead to better photovoltaic performance than the other two. It is expected that the results of this paper may provide guidance in fabricating highly efficient and cost effective BHJ OSCs.
Highlights
High efficiency, stability, low cost and short energy pay back times are the key issues to be considered in the development and commercialization of any type of solar cell to convert solar energy into electricity, which is currently dominated by the silicon solar cells [1,2,3]
In view of the above, in this paper, we studied the interference of the incident and reflected electric fields of the electromagnetic radiation and exciton generation rate in the active layer of a conventional fullerene acceptor based bulk heterojunction (BHJ) organic solar cells (OSCs), and two NF acceptor based BHJ OSCs, of inverted and conventional structures, as a function of the thickness of the active layer
Gj Figure structures of OSC1, OSC2, and OSC3 are of x=0 their structures are as follows: OSC1: an inverted BHJ OSC with a non-fullerene acceptor of where
Summary
Stability, low cost and short energy pay back times are the key issues to be considered in the development and commercialization of any type of solar cell to convert solar energy into electricity, which is currently dominated by the silicon solar cells [1,2,3]. Likewise, when a Frenkel exciton excited in the acceptor reaches a D-A interface, the hole gets transferred from acceptor highest occupied molecular orbital (HOMO) to donor HOMO, being at a lower energy, and forms a CT exciton in the form of molecular vibrations [13] If this excess energy is equal or larger than the exciton binding energy and impacts back on the CT exciton, the latter gets dissociated into a free electron and hole which are transported to their respective electrodes by the built-in electric field due to work function difference between the electrodes. Assuming equal mobility for electrons and holes, and analysing the positions of CIPs within the active layer, they found that for the efficient extraction of charge carriers, excitons should be generated halfway within the active layer; about equal distance from the two electrodes This is important in the design of organic solar cells to minimize the creation of space charge in the device.
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