Random Walk Numerical Simulation of Disordered Semiconductor Heterojunctions
G. Oskam a, H. J. Mandujano-Ramírez a, J. P. González-Vázquez b, J. A. Anta b
a Departamento de Física Aplicada, CINVESTAV-IPN, Mérida, 97310
b Área de Química Física, Universidad Pablo de Olavide, Sevilla, 41013
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Ecublens, Switzerland, 2014 May 11th - 14th
Organizers: Michael Graetzel and Mohammad Nazeeruddin
Poster, H. J. Mandujano-Ramírez, 229
Publication date: 1st March 2014

A variety of new generation solar cells are governed by charge carrier diffusion and recombination mechanisms that, although different in many of the details, show interesting similarities. We have used Random Walk Numerical Simulation (RWNS)to model general disordered semiconductor heterojunctions. Calculations are performed by simultaneously moving electrons and holes in two semiconductors in contact, according to Miller-Abrahams hopping rates. Recombination is taken into account by implementing a tunnelling distance-dependent recombination mechanism across the interface. This way, charge separation for specific heterostructures with energy distributions of surface traps in the two semiconductors involved can be adequately simulated (see Figure 1).

We show that several fundamental features of bulk heterojunction (BHJ) and quantum dot (QD) sensitised solar cells can be accounted for by this model. The dependence of the open-circuit voltage on the HOMO energy of the donor material in BHJ solar cells is accurately predicted assuming short recombination distances and a diffusion mechanism of transport. In addition, the variation of the voltage with temperature is adequately reproduced. The linearity and recombination order in electron density for both types of solar cell can be extracted in two ways: (1) the behaviour of the open-circuit voltage as a function of the illumination intensity, and (2) from the variation of the recombination current with respect to the open-circuit photovoltage. This allows for the interpretation of impedance spectroscopy measurements [1]. 


Illustration of the disordered semiconductor heterojunction model studied in this work. The disordered heterojunction is modeled by means of band-offset energy distributions of localized states for both electrons and holes. In the left picture (a) the heterojunction is represented on the energy scale. In the right picture (b), the geometrical features of the random walk model are shown. A hopping transport model is used for detrapping times (or rates).
1. Mandujano-Ramírez, H. J.; González-Vázquez, J. P.; Oskam, G.; Dittrich, T.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J.; Anta, J. A. Charge separation at disordered semiconductor heterojunctions from random walk numerical simulations. Phys. Chem. Chem. Phys., 2014, doi. 10.1039/C3CP54237H, in press.
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