An alternative approach to simulate the power conversion efficiency (η PCE) of bulk heterojunction organic solar cells (BHJ OSCs), as a product of efficiencies of absorption (η abs), dissociation (η dis), and extraction (η ext), is presented. Although η abs and η dis do not directly contribute to the simulation of η PCE , the approach enables us in understanding their roles in optimizing the power conversion efficiency of BHJ OSCs. This method is applied to simulate η PCE as a function of the thickness of active layer for three different BHJ OSC structures, one with a fullerene acceptor and two with two different nonfullerene acceptors. The results are found to be in good agreement with those from the previous simulation and experimental works and are expected to be useful in optimizing the thickness of the active layer.

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.

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Thermal stability, closely associated with the operating temperature, is one of the desired properties for practical applications of organic solar cells (OSCs). In this paper, an OSC of the structure of ITO/PEDOT:PSS/P3HT:PCBM/ZnO/Ag was fabricated, and its current-voltage (J-V) characteristics and operating temperature were measured. The operating temperature of the same OSC was simulated using an analytical model, taking into consideration the heat transfer, charge carrier drift-diffusion and different thermal generation processes. The simulated results agreed well with the experimental ones. It was found that the thermalization of charge carriers above the band gap had the highest influence on the operating temperature of the OSCs. The energy off-set at the donor/acceptor interface in the bulk heterojunction (BHJ) was shown to have a negligible impact on the thermal stability of the OSCs. However, the energy off-sets at the electrode/charge-transporting layer and BHJ/charge-transporting layer interfaces had greater impacts on the operating temperature of OSCs at the short circuit current and maximum power point conditions. Our results revealed that a variation over the energy off-set range from 0.1 to 0.9 eV would induce an almost 10-time increase in the corresponding thermal power generation, e.g., from 0.001 to 0.01 W, in the cells operated at the short circuit current condition, contributing to about 16.7% of the total solar power absorbed in the OSC.

Using the optical transfer matrix method, we optimized the layered structure of a conventional and an inverted BHJ OSC with the active layer made of blended PTB7-Th:PNDI-T10 by maximizing the optical absorption and, hence, the JSC. The maximum JSC thus obtained from the optimised structure of the inverted OSC was 139 Am−2 and that of the conventional OSC was 135 Am−2. Simulation of the electric field distribution in both inverted and conventional OSCs showed that the formation of a single CIP was obtained in the active layer of thickness 105 nm in both OSCs. As the light incidents from the ITO side, it was found that excitons were generated more closely to ITO electrode, which favors the efficient charge transport and collection at the opposite electrodes in the inverted OSC, which produces higher JSC.