ARTICLE pubs.acs.org/JPCC
Transient Optoelectronic Analysis of Charge Carrier Losses in a Selenophene/Fullerene Blend Solar Cell Andrea Maurano,† Chris G. Shuttle,† Rick Hamilton,† Amy M. Ballantyne,‡ Jenny Nelson,‡ Weimin Zhang,† Martin Heeney,† and James R. Durrant*,† Centre for Plastic Electronics, †Department of Chemistry and ‡Department of Physics, Imperial College London, South Kensington SW7 2AZ, United Kingdom
bS Supporting Information ABSTRACT: In this paper, we employ transient photovoltage, transient photocurrent, charge extraction, and transient absorption measurements to analyze the current/voltage response of bulk heterojunction solar cells employing a poly(3hexylselenophene) (P3HS)/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) blend photoactive layer. These techniques are employed to determine the charge carrier densities and lifetimes observed in devices held at open circuit as a function of light intensity. Excellent agreement is obtained between charge densities and lifetimes determined by the different techniques, supporting the validity of these analyses. These analyses are employed to calculate the nongeminate recombination flux at open circuit as a function of light intensity, and therefore open circuit voltage. This nongeminate recombination flux is found to be approximately equal and opposite to the short circuit current density measured at the same light intensity, indicating that the dominating charge carrier loss pathway determining the device open circuit voltage is nongeminate recombination. This analysis is extended across the device current/voltage curve by using charge extraction to determine the average charge density in the device as a function of applied light intensity and bias voltage. Using this analysis, and assuming that the nongeminate recombination flux depends only upon this average charge density, we demonstrate that we are able to obtain a reasonable reproduction of the device current/voltage behavior both in the dark and for light intensities up to ∼1 sun without the use of any fitting parameters. We thus conclude that a simple device model based upon a light intensity dependent charge photogeneration term and a charge density dependent nongeminate recombination flux is capable of describing the dominating factors determining the fill factor and open circuit voltage of these devices.
’ INTRODUCTION Polymer/fullerene bulk heterojunction solar cells are currently attracting extensive attention as a potentially lower cost alternative to inorganic semiconductor solar cells. The function of such devices is based upon charge photogeneration in the polymer/fullerene blend photoactive layer of the device. This charge photogeneration can result in an increase in the density of charge carriers in the photoactive layer of the device during operation. A key consideration for evaluating the limitations on device efficiency is the extent to which this increase in charge density results in the acceleration of charge carrier loss pathways. Such loss pathways, including in particular nongeminate recombination, may limit the collection of photogenerated charges by the device electrodes. Herein we employ a suite of transient optoelectronic methodologies to undertake a detailed study of charge carrier densities in organic solar cells based upon a selenophene/fullerene blend photoactive layer and quantify the impact of the increase in charge density upon device efficiency. We show that the J-V curve can be predicted from the empirical determination of the rate law for nongeminate recombination occurring between charges in the photoactive layer. r 2011 American Chemical Society
Several studies have previously considered charge carrier densities and loss processes in different types of polymer/fullerene solar cells using a range of experimental techniques.1 Such studies have employed a range of techniques including integral mode time-of-flight (Q-TOF),2 charge extraction by linear increasing voltage (CELIV),3,4 double injection currents (DoI),5 extraction plasma,6 transient absorption spectra (TAS),7-9 transient photovoltage and photocurrent (TPV and TPC) and charge extraction (CE).10-13 All the mentioned techniques have been used to investigate charge carrier densities and decay dynamics to better understand their role in determining the power conversion efficiency of devices.11 We note that among the techniques mentioned above, only TPV/TPC and CE are performed under standard device PV operating conditions in terms of light intensity and applied voltage.11 As such, findings derived from these techniques can be most easily directly related to device J-V performance. Received: October 9, 2010 Revised: February 6, 2011 Published: March 09, 2011 5947
dx.doi.org/10.1021/jp109697w | J. Phys. Chem. C 2011, 115, 5947–5957
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Illustration of the model used in the paper to analyze the J-V of bulk heterojunction solar cells, where the overall device current under illumination (J) can be decomposed into a current due to the charge photogeneration (Jgen) and a counterpoised current due to the charges that are lost (Jloss).
The most widely studied organic solar cells are those based upon bulk heterojunctions (BHJ) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM). In terms of charge carrier loss pathways, P3HT/ PC61BM solar cell research has tended to focus upon whether geminate or nongeminate recombination is the main loss processes, not only at open circuit but also at other voltages across the device operating range.12-15 Our own studies have, for example, indicated that while geminate losses are primarily responsible for limiting the short circuit photocurrent Jsc,16 the shape of the J-V curve in the light and in the dark, and thus the device fill factor (FF) and open circuit voltage (VOC) are primarily determined by increases in nongeminate recombination losses due to increases charge density in the active layer as the device operating condition moves away from short circuit.11,13,17,18 This viewpoint is illustrated in Figure 1, which indicates how the observed device current (J) can be decomposed into a forward (negative) current due to the photogenerated charges (Jgen) and a loss (positive) current due to the charges that are lost (Jloss) before collection by the device electrodes18 J ¼ Jgen þ Jloss
ð1Þ
In this simple analysis, Jgen is assumed to depend only upon light intensity, and be independent of device voltage, and therefore device electric field. The loss current Jloss is assumed to be only dependent upon the average charge carrier density (n)13 and can be calculated from the experimental observation of n and charge carrier lifetime as a function of charge density (τn) Jloss ¼
edn τn
ð2Þ
where e is the elementary charge and d the thickness of the device. By comparison of film and device data, we have previously concluded for annealed P3HT/PC61BM device that for these devices Jloss is dominated by nongeminate recombination losses (Jrec).12,17 A key advantage of this analysis is that it allows the explicit consideration of the role of intraband localized “trap” states in influencing both the total charge density present in the device and the decay dynamics of these charges.18 There is extensive evidence that such charge trapping, and the associated energetic disorder, may strongly influence the dynamics of organic semiconductor devices.18-24 The analysis detailed above rests for eq 1 on the assumption that Jgen is independent of device electric field and that for eq 2 τn is dependent only upon n.18 For annealed P3HT/PC61BM
devices, we have obtained strong evidence in support of the validity of both assumptions.12 However the validity of both of these assumptions, and the extent to which they may applicable to other organic solar cells, remains highly controversial. Other studies have focused upon the role of other factors in determining device J-V performance, including the importance of device electric fields in driving charge dissociation,24 the impact of selective versus nonselective contacts,18,25 the role of interfacial charge transfer states in determining VOC,26,27 and the extent to which the behavior of optically generated charge carriers should be analyzed separately from those injected by the device electrodes (often referred to as “corrected photocurrent” analyses).28 As such, there is currently no clear consensus over the factors determining the J-V behavior of organic solar cells. An obvious question in addressing this controversy is the extent to which the analysis illustrated in Figure 1 can be successfully applied to other organic photovoltaic devices apart from those based upon P3HT/PC61BM bulk heterojunctions. This is the primary motivation for the study reported herein, where we employ a range of transient techniques to evaluate charge carrier losses in devices employing an alternative, lower optical band gap polymer, poly(3-hexylselenophene) (P3HS), and in particular consider the extent to which the analysis illustrated in Figure 1 can be successfully employed to understand the J-V behavior of such devices. P3HS has attracted interest for organic solar cell applications as its lower optical band gap relative to P3HT improves the overlap between the polymer absorption spectrum and solar irradiance, thereby improving light harvesting by the photoactive layer.29,30 In this paper, we present a detailed analysis, employing TPV/TPC and CE techniques, complimented by the use of TAS, to measure n and τn in BHJ devices of P3HS as electron donor and PC61BM as electron acceptor. We have previously reported evidence that P3HS/PC61BM devices exhibit faster nongeminate recombination dynamics than analogous P3HT/PC61BM based devices, which is attributed to differences in the phase purity of the blend films.31 Herein, we undertake a detailed analysis of these loss dynamics as a function of n and thus quantify the impact of these faster dynamics upon the J-V performance of P3HS/PC61BM devices, focusing in particular upon VOC and FF.
’ EXPERIMENTAL METHODS Details of P3HS synthesis and optical characterization are provided in Heeney at al.29 and in Ballantyne et al.30 P3HS was synthesized by Merck Chemicals Ltd. and blended with PC61BM purchased by Aldrich in solutions prepared with chlorobenzene purchased from Aldrich at a concentration of 20 mg/mL. Devices consisted of a blend of P3HS/PC61BM (composition 1:1 in weight) and had a typical thickness ranging from 80 to 150 nm. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) (Baytron P standard grade, HC Stark) was spincoated at 2000 rpm on top of indium-tin oxide (ITO) coated glass with sheet resistance 15 Ω/sq, purchased from PsioTec Ltd., U.K. ITO glass substrates had been cleaned by sonicating them with soaped distilled water, acetone, and isopropyl alcohol and surfaced treated in an ozone oven and subsequently heated at 150 °C in air for 30 min. Blend solutions were spin coated on to the PEDOT/PSS layer at 1000 rpm. To complete the devices, they were loaded into a thermal evaporation chamber, where the aluminum top metal electrode (100 nm) (purchased from Aldrich) was thermally evaporated through a shadow mask under a vacuum
