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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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09058 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 11, 2015
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The Journal of Physical Chemistry
Charge Transport without Recombination in Organic Solar Cells and Photodiodes Martin Stolterfoht,1 Bronson Philippa,2 Safa Shoaee,1 Hui Jin,1 Wei Jiang,1 Ronald D White,2 Paul L. Burn,1 Paul Meredith,1,*& Almantas Pivrikas,3,* 1
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. 2
College of Science, Technology and Engineering, James Cook University, Townsville 4811, Australia 3
School of Engineering and Information Technology, Murdoch University, Perth 6150, Australia
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 non-geminate recombination of any order can be avoided as long as this charge is much less than capable of being stored on the electrodes – a criterion that is typically met in the linear iPC regime in donor:fullerene systems even with low, imbalanced mobilities. Knowing the conditions under which non-geminate 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.
Author Information * Paul Meredith (
[email protected]) Centre for Organic Photonics School of Mathematics and Physics University of Queensland, Brisbane, Queensland Australia 4072 Tel: +61 733697050 * Dr. Almantas Pivrikas (
[email protected]) School of Engineering and Information Technology Murdoch University 90 South Street, Murdoch Australia 6150 Tel: +61 466 965 314
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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 non-polymeric molecules1. 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, amongst the myriad of material systems and combinations reported in the literature, there are relatively few that yield truly 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 which define charge generation and extraction – and 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 and 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 (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 non-geminate 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 nm-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 combined efficiencies of carrier photogeneration and extraction.14—17 However, charge generation and charge extraction losses cannot be differentiated through such an approach. There are several reasons for this, notably the impact of pseudo-first order non-geminate recombination (the process by which an electron and hole not originating from the same photoexcitation recombine) on the EQE. First order processes are often termed, sometimes incorrectly, monomolecular and are linearly dependent on the input light intensity. Various studies have concluded that first order, nongeminate recombination strongly determines the overall solar cell performance in BHJ systems,18—21 and substantial work has been conducted to identify the importance of trap states in the bulk and at the electrode contacts on the prevailing recombination order.21,22 Conversely, others have apparently demonstrated that bimolecular photocarrier recombination (which is non-linear with respect to the incident light intensity) is the efficiency limiting processs.6,23—26 The order and nature of the dominant recombination and its impact upon device performance is therefore a matter of active scientific interest. Recently, we employed steady-state intensity dependent photocurrent (iPC) measurements to quantify and understand the non-linear recombination losses in organic solar cells under relevant operational conditions.6 In this current work, and motivated by the need to clarify the first order recombination losses, we study the linear iPC regime with similar steady state measurements in two donor:fullerene blend systems in a full BHJ architecture. Further, we extend the methodology by measuring the iPC in a time-resolved mode (similar to previous works20,27—30) in combination with transient 2 ACS Paragon Plus Environment
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photovoltage.31,32 In so doing, we are able to estimate the amount of charge present during steady state photoconduction at short-circuit conditions which allows us to arrive at consistent conclusions regarding the origin of non-geminate recombination. We find that if the amount of photogenerated charge is significantly less than that capable of being stored on the electrodes (where C is the capacitance and U the effective voltage which equals to the built-in voltage at short-circuit conditions) then the transport of already-dissociated charge carriers is apparently free of non-geminate recombination. This is even the case in situations where the hole mobility is low and the electron-hole mobilities are imbalanced, i.e., where we would normally expect significant hole trapping even at low light intensities. Understanding the conditions under which non-geminate recombination is minimised or even eliminated is the key to disentangle the charge generation and extraction efficiencies for solar cells under normal operating conditions.
Methods Device preparation The substrates were cleaned by sonicating in sequence with Alconox, deionized water, acetone, and 2propanol for 5 min, respectively. Subsequently, 15 nm of MoO3 was deposited onto the cleaned indium tin oxide (ITO) substrates by thermal evaporation for the WJ104:PC70BM devices, while the substrates used for the PCDTBT:PC70BM devices were coated with 30 nm poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS; Baytron P VPAl4083). The active layer (junction) solution of PCDTBT (SJPC, Canada, Mw = 122 200 g/mol, PDI=5.4) and PC70BM 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
