Quantifying the Efficiency of Free Charge Generation and Extraction in Polymer-based Bulk Heterojunction Devices
Prof. Dieter Neher
Institute of Physics and Astronomy, University of Potsdam, Germany
17:00 - 18:00 Tuesday 07 October 2014 KFU HS 5.01 The photon-to-charge conversion in organic solar cells comprises several elementary steps, including the formation of interfacial charge-transfer states (CTS) via exciton dissociation at the DA heterojunction, the split-up of these states into free charge carriers, and free charge extraction to the electrodes. Understanding of the efficiency and dynamics of these processes is essential for a comprehensive understanding of these devices.
By applying time-delayed collection field (TDCF) and bias-amplified charge extraction (BACE) we are able to quantify the efficiency of free charge formation and extraction under application-relevant illumination conditions [1]. TDCF measurements with variable photon energy reveals that the charge generation involves thermalized CT states, meaning that the field-dependence and efficiency of free carrier formation is dictated by the energy of low lying CTS with respect to the energy of the spatially-separated charge pair [2-3]. Consequently, the mutual Coulombic binding of the electron-hole pair forming the CTS must be compensated by forces originating from the specific energetics at the mesoscale. We find that for the very same donor-acceptor combination, a phase-separated structure with rather pure domains results in a less field-dependent generation, pointing to the importance of blend morphology for efficient separation of the CTS into free charge [4]. For these optimized systems, the device performance becomes limited by inefficient charge extraction. We show how BACE can be used to measure the mean mobility of extraction under solar cell working conditions and how small changes in the chemical structure of the donor component result in largely different extraction efficiencies.
[1] J. Kniepert, I. Lange, N.J. van der Kaap, L.J.A. Koster, D. Neher, Adv. Energ. Mater. 2014, published online DOI: 10.1002/aenm.201301401.
[2] K. Vandewal, S. Albrecht, E. T. Hoke, K. R. Graham, J. Widmer, J. D. Douglas, M. Schubert, W. R. Mateker, J. T. Bloking, G. F. Burkhard, A. Sellinger, J. M. J. Fr�chet, A. Amassian, M. K. Riede, M. D. McGehee, D. Neher, A. Salleo, Nature Materials 2014, 13, 63
[3] S. Albrecht, K. Vandewal, J. R. Tumbleston, F. S. U. Fischer, J. D. Douglas, J. M. J. Fr�chet, S. Ludwigs, H. Ade, A. Salleo, D. Neher, Adv. Mater, in press.
[4] S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, D. Neher, JACS 2012, 134, 14932.
|
