Random Walk Numerical Simulation of Charge Transport and Recombination in Disordered Semiconductor Heterojunctions

Juan A. Anta^a, Gerko Oskam^b, Humberto Mandujano-Ramirez^b

^aPablo de Olavide University, Sevilla, Spain, Carretera de Utrera, km. 1, Montequinto, Spain

^bDepartment of Applied Physics, CINVESTAV-IPN, Ant. Carr. a Progreso km 6, Cordemex, Mérida, Yucatán, 97310, Mexico

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Gerko Oskam, 169
Publication date: 5th February 2015

A variety of new generation solar cells, including dye-sensitized solar cells, QD-sensitized solar cells, organic bulk heterojunction solar cells, etc., are governed by charge carrier transport and recombination mechanisms that show interesting similarities. We have used Random Walk Numerical Simulation (RWNS) to model solar cells based on 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. In addition, we analyze two different models for the recombination process in bulk heterojunction solar cells, where the rate is limited either by the carrier diffusion dynamics or by the recombination reaction, as a function of the electron density. We find that a cross-over can be observed from transport to reaction-limited recombination with increasing carrier density. The RWNS results are discussed in terms of experimental results from the literature, including the dependence of the open-circuit voltage on the HOMO energy of the donor material in BHJ solar cells: this dependence 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 has been extracted in two ways: (1) the behavior 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 [1]

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, 16, 4082-4091.

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