Insights on the burn-in phase of organic photovoltaic device operation from in-situ transient optoelectronic analysis.

Neil Greenham^a, Elsa Couderc^a, Andrew Pearson^a, Paul Hopkinson^b, Konrad Domanski^c

^aCavendish Laboratory, University of Cambridge - UK, JJ Thomson Avenue, 9, Cambridge, United Kingdom

^bCenter for Advanced Materials, Ruprecht-Karls-University Heidelberg, Im Neuenheimer Feld, 225, Heidelberg, Germany

^cLaboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Station 6, CH-1015 Lausanne, Lausanne, Switzerland

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, Andrew Pearson, 048
Publication date: 5th February 2015

The performance of organic PV devices has seen a sustained period of improvement in the past decade, with recent reports of power conversion efficiencies around 10% in both single- and tandem-junction configurations. Of equal importance to the anticipated commercialisation of this technology is the demonstration of organic PV that exhibits both high performance and long life-times. Achieving this long sought-after goal is highly non-trivial due to the complexity of degradation in organic PV systems; a multifaceted parameter space exists that not only includes the role of environmental factors but also the interdependence between different material combinations (both semiconductors and electrodes) on the stability of a working cell. The technical challenge considered in this work is the minimisation of detrimental effects that arise during the burn-in phase of device operation, this period describing the relatively fast and often severe reduction in initial device performance. Although a large number of comparative studies exist that improve OPV cell stability via materials substitution, relatively few have considered the burn-in phase in detail to improve our physical understanding which can subsequently provide the necessary feedback for rational materials design and device construction. To this end we undertake an in-depth study of the organic PV system PFDTTT-EFT:PC71BM, this blend producing some of the highest literature values of photoconversion efficiency [1]. By measuring our solar cells under nitrogen the limiting factors in device stability are identified. One of the unique aspects of our setup is the ability to monitor in-situ the electrical properties of a solar cell using current-voltage (IV), transient photocurrent (TPC) and transient photovoltage (TPV), providing deeper insight into the evolution of charge carrier transport, trapping and recombination during device operation than can otherwise be obtained through analysis of the primary solar cell device metrics alone. We find that for both inverted and normal architecture solar cells, a severe drop in performance of at least 50% is measured after only 24 hrs illumination. This degradation is found to correlate with the onset of trap-assisted charge-carrier recombination that acts to reduce the maximum attainable photocurrent. The timescales for evolution in TPC response are commensurate with the ‘burn-in’ phase of device operation. Furthermore, transient photovoltage measurements indicate an apparent increase in charge carrier lifetime with aging at 1 sun conditions, also in agreement with the burn-in phase kinetics. Ex-situ transient absorption spectroscopy measurements on degraded cells are performed to provide further insight on the initial steps of photocurrent generation, and a critical analysis of the device processing protocol is undertaken to help identify the underlying cause for device instability.

[1] Side Chain Selection for Designing Highly Efficient Photovoltaic Polymers with 2D-Conjugated Structure. Shaoqing Zhang et al., Macromolecules, 47, pp 4653 - 4659 (2014)

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