A Comparison of Experimental Techniques to Measure Charge Carrier Lifetime in Polymer/Fullerene Solar Cells

Attila Mozer^a, Tracey Clarke^a

^aUniversity of Wollongong, Innovation Campus, AIIM Facility, Australia

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

Oral, Attila Mozer, presentation 063
Publication date: 5th February 2015

Non-geminate recombination of photo-generated charge carriers in conjugated polymer / fullerene solar cells is one of the most important topics limiting device efficiency. Langevin-type, diffusion controlled recombination rate is often responsible for short charge carrier lifetimes reducing the charge extraction efficiency and consequently, limits the active layer thickness. In this presentation, six complementary experimental techniques, namely time-of-flight photoconductivity (ToF), time resolved charge extraction (TRCE), charge extraction by linearly increasing voltage pulse (CELIV), transient absorption spectroscopy (TAS), transient photovoltage (TPV) and electrochemical impedance spectroscopy (EIS) are used to measure charge carrier lifetime versus charge density, photovoltage and delay time in conjugated polymer : PCBM blends. The polymer studied is poly [(4,4’- bis (2-ethylhexyl) dithieno [3,2-b:2’,3’-d] silole) -2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl thiazolo 5,4-d thiazole)-2,5diyl] (PDTSiTTz), which was shown to exhibit significantly reduced non-geminate recombination compared to the Langevin rate, when blended with the fullerene PCBM. The comparison of six different techniques shows that all of the above techniques give very similar charge carrier lifetime values at sufficiently high (>10¹⁶ cm^-3) charge carrier densities or when the open circuit voltage is > 0.6 V. At lower charge carrier densities or open circuit voltages, an apparent increase in the reaction order, an alteration in the slope of charge density versus open circuit voltage, and a shift from a chemical to geometric capacitance is observed using charge extraction, which is attributed to surface recombination at the contacts, rather than recombination in the bulk. It is suggested that for detailed analysis of recombination kinetics, a combination of TAS and TRCE is the most suitable among the methods surveyed. Since the optical technique TAS is not limited by the RC constraints of charge extraction, much faster time resolution into early recombination kinetics is feasible. TRCE is useful across a wide range of charge carrier densities, including low charge densities and low open circuit voltages.

Tracey M. Clarke, Christoph Lungenschmied, Jeff Peet, Nicolas Drolet andAttila J. Mozer, Adv. En. Mater., DOI: 10.1002/aenm.201401345, in press.

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