Role of the Metal-Organic Interfaces in Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells

Ana Belén Sancho Pareja^a, Ana Mañas Martínez^a, Pilar López Varo^a, Juan Antonio Jiménez Tejada^a, Ognian Marinov^b, Jamal Deen^b

^aUniversidad de Granada, ES, Departamento de Electrónica y Tecnología de Computadores, Facultad de Ciencias. Campus Fuentenueva, Granada, Spain

^bMcMaster University, Department of Electrical and Computer Engineering McMaster University, Hamilton, ON L8S 4K1

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, Pilar López Varo, 116
Publication date: 5th February 2015

We present a model for the current-voltage curves of bulk heterojunction organic solar cells (OSCs), operating at dark and at illumination conditions. The model relates the free charge density at the contact interface with the current density in OSC. The strength of the model lies in the use of physically-based boundary conditions for the charge and potential at the metal-organic interfaces. These boundary conditions contain information for the doping close to interfaces and for built-in voltage and band bending due to asymmetric contacts. We show that the values for the boundary conditions obtained from symmetric single-carrier diodes can be successfully used also for OSCs irrespectively of the dominant limit (space-charge, injection or diffusion) in the charge transport. In a previous study on the effects of the charge density at the interfaces of single-carrier diodes, we have proposed a model for the current density–voltage (J-V) curves, considering conduction limited by space-charge (SCLC) and injection (ILC) [1-3]. We now validate this model also for the diffusion regime of double-carrier diodes. In our model, the boundary conditions for charge concentrations at their respective injecting contacts are given in the equation (1) in the figure, where p and n are the hole and electron concentrations, respectively, and the model parameters K_1p, K_1p, mp, mn, K_,p and K_2n depend on the organic materials, energy barriers at interfaces and doping [4]. These parameters can be extracted from experimental J-V curves (symbols in Fig.1) for electron-only Yb/PPV/Ca and hole-only ITO/Ag/PPV/Ag devices [5], by fitting the experimental J-V curves with the curves obtained from the numerical solutions of the transport equations, using (1) as boundary conditions at the injection interfaces and (2) for the extracting electrodes, where, ø₁to ø₄are the barriers seen at the interfaces, and N_C is the density of sites. Subsequently, the charge boundary conditions extracted from single-carrier devices are introduced in the model of the double-carrier device. Assuming a linear distribution for the potential in the semiconductor V(x) (equation (3), in Fig. 1, where b is a band-bending parameter [6]), leads to analytical relations between the hole J_p and electron J_n current densities with the potential (equations (4) and (5), respectively). Introducing the above boundary conditions in (4)-(5), the resulting J-V model (solid lines in Fig. 1) is in an excellent agreement with published experimental results at dark conditions, both for the aforementioned single-carrier devices and for the double-carrier Ag/PPV/Ca diode.

[1] P. López Varo, J. A. Jiménez Tejada, J. A. López Villanueva, J. E. Carceller, M. J. Deen, Org. Electron., 13, 1700 (2012) [2] P. López Varo, J. A. Jiménez Tejada, J. A. López Villanueva and M. J. Deen, Org. Electron., 15, 2526 (2014). [3] P. L. Bullejos, J.A. Jiménez Tejada, S Rodríguez-Bolivar, M.J. Deen, O Marinov, J. App. Phys. 105, 084516 (2009). [4] P. López Varo, J. A. Jiménez Tejada, J. A. López Villanueva and M. J. Deen, Org. Electron., 15, 2536 (2014). [5]T. van Woudenbergh, P. W. M. Blom, and J. N. Huiberts, Appl. Phys. Lett. 82, 985 (2003). [6] P de Bruyn, A.H.P. van Rest, G.A.H. Wetzelaer, D.M. de Leeuw, and P.W.M. Blom Phys. Rev. Lett. 111, 186801 (2013).

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