Numerical modeling and simulation of bulk heterojunction organic solar cell considering electric field dependent carrier mobility

Abstract

Organic solar cell is one of the leading contender of the future of low cost photovoltaic cell. A great amount of experimental research has been done on this eld. In this thesis, a new model for bulk heterojunction organic solar cell considering electric eld dependent carrier mobility is presented for the advancement of the organic solar cell. At rst, photon absorption rate, hence exciton generation rate in the active layer of organic solar cell is calculated using transfer matrix theory by taking the complex refractive index data of the organic material. AM 1.5G irradiance is taken into consideration for the calculation of photon absorption. An oscillating pattern is observed in the generation pro le of the organic solar cell which can be attributed to the interference and re ectance of the layers. Generated excitons move to charge transfer state and they either dissociate or decay at the donor-acceptor interface. Exciton dissociation, decay, charge carrier generation, recombination and transport; all are incorporated in this model. As mobility is not constant for all applied voltages and electric eld, so considering electric eld dependent mobility is necessary to be incorporated. Variation in active layer thickness changes the electric eld. Applying this model current-voltage characteristics for active layer variation from 40nm to 140 nm of a bulk heterojunction P3HT:PCBM solar cell is obtained. Open circuit voltage remains almost constant for this region, whereas both short circuit current density and e ciency follow oscillating pattern and exhibit maximum value at 60 nm. E ciency of this bulk heterojunction organic solar cell with a 60 nm active layer is found to be 2.2%. For the bilayer structure, maximum e ciency is found to be 0.7% at 50 nm acceptor thickness. For the same structure, at lower donor thickness, e ciency is found to be higher. With the addition of a high mobility solvent carbon nano tube, 40% increase in e ciency is observed for bulk heeterojunction organic solar cell. For TAPC:C60 bulk heterojunction solar cell, at 50% donor concentration e ciency is 1.05% due to reduction in dielectric constant and electron mobility. At lower concentration, e ciency and other parameter increases. For high e ective density of states both open circuit voltage and short circuit current falls due to band bending near contacts. For high initial separation distance dissociation probability of exciton is high and it is observed for a variation of it.