Nov 4, 2015 - School of Engineering and Information Technology, Murdoch University, 90 South Street, ... free carrier generation in fully operational devices.
Article pubs.acs.org/JPCC
Charge Transport without Recombination in Organic Solar Cells and Photodiodes Martin Stolterfoht,† Bronson Philippa,‡ Safa Shoaee,† Hui Jin,† Wei Jiang,† Ronald D. White,‡ Paul L. Burn,† Paul Meredith,*,† and Almantas Pivrikas*,§ †
Centre For Organic Photonics & Electronics (COPE), School of Chemistry and Molecular Biosciences and School of Mathematics and Physics, The University of Queensland, Brisbane 4072, Australia ‡ College of Science, Technology and Engineering, James Cook University, Townsville 4811, Australia § School of Engineering and Information Technology, Murdoch University, 90 South Street, Perth 6150, Australia S Supporting Information *
ABSTRACT: Decoupling charge generation and extraction is critical to understanding loss mechanisms in polymer:fullerene organic solar cells and photodiodes but has thus far proven to be a challenging task. Using steady-state and time-resolved light intensity dependent photocurrent (iPC) measurements in combination with transient photovoltage, we estimate the total charge inside a typical device during steady-state photoconduction, which is defined by the trapped, doping-induced, and mobile charge populations. Our results show that nongeminate recombination of any order can be avoided as long as this charge is much less than that capable of being stored on the electrodesa criterion that is typically met in the linear iPC regime in donor:fullerene systems even with low, imbalanced mobilities. Knowing the conditions under which nongeminate recombination is essentially absent is an important device and materials design consideration. Our work also demonstrates that the technique of iPC is not only useful to assess the charge extraction efficiency but can also be used to estimate the efficiency of free carrier generation in fully operational devices.
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(TRMC),12 or with the relatively new time-delayed collection field (TDCF) technique.11,13 TAS allows the assessment and quantification of the initial population of exciton and charge transfer states prior to recombination of free carriers, and TRMC measures the product of the generation yield and the sum of the “local” charge carrier mobilities. TDCF essentially uses the (bias dependent) extracted charge after a short laser excitation as a measure of the charge generation yield by excluding nongeminate recombination. However, these techniques are typically not applied to operational solar cells and/or under relevant conditions. For example, TAS requires often orders of magnitude higher illumination irradiances than delivered under 1 Sun conditions, TRMC is a very local probe of nanometer-scale generation and transport, and TDCF is a transient experiment that does not allow examination of steady-state charge carrier populations. That is not to say that these techniques have not delivered valuable insights; however, one must always consider these results in the context of the experimental conditions under which they were obtained. Electro-optical measurements of the external and internal quantum efficiency (EQE/IQE) are used to quantify the
INTRODUCTION The past decade has seen significant progress in improving the power conversion efficiencies of organic solar cells. This progress has been underpinned by the development of numerous donor and acceptor organic semiconductors, both polymeric and nonpolymeric molecules.1 The so-called thin film bulk heterojunction (BHJ) architecture has emerged as the preferred device platform, and blends of semiconducting donor polymers in combination with fullerene acceptors are the main materials used in solution-processed organic solar cells.2 However, among the myriad of material systems and combinations reported in the literature, there are relatively few that yield a high performance of order 10% in single junctions.3−5 There are many reasons for this relative scarcity of efficient systems, not least of which is an incomplete understanding of the fundamental processes that define charge generation and extractionand in particular the underlying loss mechanisms.6−8 Most previous studies in this regard have focused on either charge generation or transport, but rarely simultaneously nor in devices under relevant operational conditions.9−11 This limitation can be attributed to an absence of appropriate experimental techniques that can clearly disentangle the two phenomena. Charge generation is often studied using transient absorption spectroscopy (TAS),10 transient microwave conductivity © 2015 American Chemical Society
Received: September 17, 2015 Revised: November 4, 2015 Published: November 4, 2015 26866
DOI: 10.1021/acs.jpcc.5b09058 J. Phys. Chem. C 2015, 119, 26866−26874
Article
The Journal of Physical Chemistry C
PDI = 5.4) and PC70BM (American Dye Source, Inc., Canada, Mw = 1032 g/mol) was prepared by using a 1:4 blend ratio by weight in 1,2-dichlorobenzene (DCB). This blend ratio has previously been determined to be optimum.33 An active layer thickness of 75 nm was obtained by using a total concentration of 25 mg/cm3 and spin-coating at 2000 rpm for 90 s. PCDTBT:PC70BM solar cells operate most effectively for junction thicknesses
