Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
We analyze the aging mechanism of an organic bulk heterojunction solar cell by various time-dependent, frequency-dependent and steady-state measurement techniques. We confirm the strong influence of unintentional doping [Kirchartz et al] on device performance and make use of a numerical drift-diffusion model with electron trap states that effectively dope the active layer with holes .
As experimental techniques we use transient photocurrent as response to a light pulse, double light pulse, charge extraction by linearly increasing voltage (CELIV) and photo-CELIV, impedance spectroscopy, capacitance-voltage and current-voltage curves.
In recent publications we have established a drift-diffusion device model for organic solar cells and presented a method to accurately extract material and device parameters by combining the techniques mentioned above [2-4].
In this contribution we make use of our all-in-one characterization platform [5] for transient and harmonic excitation combined with our simulation software [6] in order to study the impact of photo-oxidation aging on the performance of bulk hetero-junction solar cells [7]. Comparing electrical characterizations with numerical simulations, we have found that aged devices exhibit a higher doping density than fresh devices. This observation was confirmed by three different characterization techniques: impedance spectroscopy [8], dark CELIV and J-V measurements under light illumination. Indeed impedance spectroscopy is known to be a good technique to extract the doping concentration by the help of Mott-Schottky plots [9], however this technique should be employed carefully as it was derived for infinitely thick devices and/or heavily doped semiconductors [10]. In order to circumvent this limitation, we have combined this technique with dark-CELIV (figure 1) measurements which is known to be suitable to evaluate the number of charges present at equilibrium in the device [11] and thus the dopant concentration.
Based on the comparison to the simulation results and depending on the level of ageing, a doping concentration in the range of 1022 m-3 to 1023 m-3 was extracted which is consistent for these three electrical characterization techniques. Finally numerical simulations are able to provide an explanation of the reduced photo-current in aged devices. Indeed we show, as mentioned in [1], that hole dopants lead to a decrease of the electric field inside the device. Free charge carriers that are photo-generated in the low field region cannot contribute to the photo-current as they are subject to diffusion and recombination.
Figure 1: Measured (black lines) and simulated (red lines) dark-CELIV characteristics for fresh and aged devices.
[1] G. F. A. Dibb, M-A. Muth, T. Kirchartz, S. Engmann, H. Hoppe, G. Gobsch, M. Thelakkat, N. Blouin, S. Tierney, M. Carrasco-Orazco, J. R. Durrant, J. Nelson, ‘Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells’, Scientific Reports 3:3335 (2012). [2] R. Häusermann, E. Knapp, M. Moos, N.A. Reinke, T. Flatz, B. Ruhstaller, ‚‘ Coupled optoelectronic simulation of organic bulk-heterojunction solar cells: Parameter extraction and sensitivity analysis’ J. Appl. Phys. 106, 104507 (2009); SETFOS software by Fluxim, www.fluxim.com, Switzerland. [3] M. T. Neukom, N. A. Reinke, B. Ruhstaller, ‘Charge extraction with linearly increasing voltage: A numerical model for parameter extraction ’, Solar Energy 85, 1250-1256 (2011). [4] M. T. Neukom, S. Züfle, B. Ruhstaller, ‘Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques’, Organic Electronics 13 (2012). [5] PAIOS characterization platform by Fluxim, www.fluxim.com, Switzerland. [6] SETFOS: semiconducting emissive thin film optics simulation software by Fluxim, www.fluxim.com, Switzerland. [7] A. Seemann, T. Sauermann, C. Lungenschmied, O. Armbruster, S. Bauer, H.-J. Egelhaaf, J. Hauch, ‘Reversible and irreversible degradation of organic solar cell performance by oxygen’, Solar Energy 85, 6, 1238 (2010) [8] E. Knapp, B. Ruhstaller, ‘The role of shallow traps in dynamic characterization of organic semiconductor devices’, J. of Appl. Phys. 112, 2, (2012). [9] S. M. Sze, ‘Physics of semiconductor devices’, 2nd Edition, John Wiley&Sons, New York (1981) [10] T. Kirchartz, W. Gong, S. A. Hawks, T. Agostinelli, R. C. I. MacKenzie, Y. Yang, J. Nelson, ‘Sensitivity of the Mott-Schottky Analysis in Organic Solar Cells’, J. Phys. Chem. C 116, 7672-7680 (2012). [11] A. Armin, M. Velusamy, P. L. Burn, P. Meredith, A. Pivrikas, ‘Injected Charge extraction by linearly increasing voltage for bimolecular recombination studies in organic solar cells’, App. Phys. Lett. 101, 083306 (2012).
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