Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques M.T. Neukom a,b,⇑ , S. Züfle a , B. Ruhstaller a,b a Institute of Computational Physics, ZHAW, Zurich University of Applied Sciences, Technikumstr. 9, 8401 Winterthur, Switzerland b Fluxim AG, Dorfstrasse 7, 8835 Feusisberg, Switzerland article info Article history: Received 20 July 2012 Received in revised form 30 August 2012 Accepted 2 September 2012 Available online 28 September 2012 Keywords: Numerical simulation Parameter extraction CELIV Characterization Charge carrier mobility Parameter correlation abstract A method to extract reliable material and device parameters of organic solar cells is pre- sented. We employ a comprehensive numerical device model to simulate the solar cell operation in transient and steady-state condition. Parameter extraction with numerical simulation is error-prone because model parameters are often correlated, their unique determination is very difficult and extracted parameters are likely to be inaccurate. We combine the current–voltage characteristics, the photo-CELIV currents (charge extraction with linearly increasing voltage) and the photocurrent response to a light pulse to reduce parameter correlation and increase accuracy and reliability of the extracted parameters. With a correlation matrix analysis it is shown that parameter correlation is significantly reduced when combining several experimental data sets compared with the analysis of current–voltage curves only. We find a set of parameters to reproduce the complete series of measurements with the numerical simulation. The full electrical behavior can be described using a basic drift–diffusion model with constant mobilities and direct pho- ton-to-charge conversion. With this model we extract charge carrier mobilities in the order of 10 4 cm 2 /V s, a Langevin recombination prefactor of 0.08, charge injection barriers equal at both sides in the range of 0.25 eV and further device parameters for a BHJ cell with PT5DPP as donor and PCBM (C70) as acceptor. The solar cell is simulated with the extracted parameters and internal distribution of electrons, holes and the electric field are visualized. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Improving the efficiency of organic solar cells requires the understanding of the physical processes and determi- nation of material parameters for light absorption and charge transport. In order to extract material and device parameters from the experimental data either simple ana- lytical or complex numerical approaches are used. Analytical models are based on simplified assumptions and do not consider all physical effects that are relevant. The extracted parameters therefore often lack accuracy [1]. These techniques may only be used to determine the order of magnitude of a parameter or to compare one de- vice with another. To account for more physical effects numerical simula- tion is required since analytical solutions do not exist in general. Because the models are usually complex the num- ber of unknown parameters can be large. With a large number of unknown parameters accurate parameter extraction gets very difficult because parameters are often correlated and have a similar influence on the result. There have been many efforts in the past to develop and employ numerical device models to describe charge trans- port in organic solar cells. Barker et al. [2] simulated cur- rent–voltage characteristics of a bilayer organic solar cells with a drift–diffusion model, Koster et al. [3] devel- oped a model to simulate bulk-heterojunction solar cells. Häusermann and co-workers [4] studied the influence of parameters for charge-transfer excitons by simulation of current–voltage characteristics and light pulse transients. Hwang et al. [5,6] used a time-dependent drift–diffusion 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.09.008 ⇑ Corresponding author. Tel.: +41 (0)58 934 74 88. E-mail address: martin.neukom@fluxim.com (M.T. Neukom). Organic Electronics 13 (2012) 2910–2916 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel