Tuning Non-Langevin Recombination in an Organic Photovoltaic

Article pubs.acs.org/JPCC

Tuning Non-Langevin Recombination in an Organic Photovoltaic Blend Using a Processing Additive Tracey M. Clarke,*,† Christoph Lungenschmied,‡ Jeff Peet,‡ Nicolas Drolet,‡ and Attila J. Mozer*,† †

ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, North Wollongong, NSW 2500, Australia ‡ Konarka Technologies, 116 John St., Suite 12, Lowell, Massachusetts 01852, United States S Supporting Information *

ABSTRACT: The effect of altering the acceptor and exchanging a key atom in the polymer structure on the extent of non-Langevin (suppressed) recombination has been examined using the polymer/fullerene photovoltaic blend PDTSiTTz:PC60BM. Time-of-flight data show that changing the acceptor from PC60BM to PC70BM maintains the non-Langevin recombination. In contrast, altering the donor polymer by exchanging the silicon bridging atom for a carbon considerably reduces the non-Langevin behavior. Importantly, the addition of a processing additive, diiodooctane (DIO), allows a partial recovery of this non-Langevin recombination. The addition of DIO also decreases the ionization potential of the polymer, which not only explains the drop in open circuit voltage but may also contribute to the partial recovery of non-Langevin behavior observed. It is proposed that localized, more crystalline areas of lower ionization potential (or higher electron affinity) within a mixed/amorphous phase may act as energy sinks for the holes (electrons), thus potentially inhibiting bimolecular recombination. Such a phenomenon could contribute to non-Langevin behavior in organic photovoltaic blends.

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PDTSiTTz:PCBM,12 a second non-Langevin system, offers a unique opportunity to study the factors contributing to this rare but highly desirable behavior. In this paper, the control PDTSiTTz:PC60BM blend device will be altered in two “macroscopic” ways: by altering the acceptor to PC70BM and by changing the donor polymer to PCPDTTTz, the carbonbased analogue of PDTSiTTz (where a C atom replaces the bridging Si atom; see structure in Figure 1). Although the LUMO levels of PC60BM and PC70BM are identical (3.91 eV13), PC60BM is known to have a much weaker absorption in the visible region. It has been proposed that, in the case of P3HT, the lamellar spacing of the polymer crystalline domains in P3HT:PC70BM is greater than that in

INTRODUCTION The field of organic photovoltaics has produced steadily increasing power conversion efficiencies, reaching over 9% for a polymer-based single-junction device.1,2 The commercialization of organic photovoltaics (OPV) relies on a simple industrialscale fabrication method such as roll-to-roll printing. However, this method is only successful when the active layers to be printed are thick (200−300 nm). Laboratory-scale OPV devices, in contrast, are typically fabricated with an active layer thickness of less than 100 nm. This is due to Langevintype bimolecular recombination, which poses a significant problem in organic solar cells. If the device is too thick, then bimolecular recombination of charge carriers occurs before extraction can take place. This limitation is most evident from a sharp decrease in fill factorand thus efficiencyas the active layer thickness is increased. This disconnect between laboratory-scale research and applicability in an industrial setting creates a technology bottleneck, limiting both the efficiency and commercialization of OPV devices. A few systems, such as annealed P3HT:PCBM,3 have nonLangevin (reduced) bimolecular recombination. Several theories to explain this non-Langevin behavior have been proposed,4−10 including the formation of an “ideal” morphology with two-dimensional lamellae that restrict the probability of charge carriers meeting.11 Non-Langevin systems provide substantial advantages: due to the much longer-lived chargeseparated state, they are capable of producing high fill factor (>60%) solar cells with active layers greater than 300 nm thick, ideal for industrial-scale fabrication. The recent discovery of © 2015 American Chemical Society

Figure 1. Structure of the polymers investigated here, where PDTSiTTz is when A = silicon and PCPDTTTz is when A = carbon. Received: December 29, 2014 Revised: March 11, 2015 Published: March 11, 2015 7016

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The Journal of Physical Chemistry C

The largest variation is in the fill factor, which decreases from 0.62 to 0.57, leading to a decline in the overall power conversion efficiency from 4.6% for PDTSiTTz:PC60BM to 4.1% for PDTSiTTz:PC70BM. There is little evidence of a large change in charge carrier mobility or recombination between PDTSiTTz:PC60BM and PDTSiTTz:PC70BM (vide inf ra). As such, the slightly smaller fill factor in the case of PDTSiTTz:PC70BM possibly has contributions from charge carrier trapping22 or early time recombination and/or mobility variations that are not probed by TOF and photo-CELIV (charge extraction with linearly increasing voltage; see Supporting Information, Figure S1). The J−V curves of the PCPDTTTz:PCBM devices, showing the effect of the additive DIO, have already been reported18 and are shown in Figure 2 for comparison. Despite a 70 mV decrease in VOC for PCPDTTTz:PCBM when the DIO is employed, the considerable increases in JSC and FF compensate for this, such that the overall result is an increase in power conversion efficiency from 0.95% to 2.5%. The PCPDTTTz:PCBM devices, even with DIO, are more inefficient than the PDTSiTTz-based devices. TOF. Bulk photogeneration time-of-flight (TOF) enables the determination of the bimolecular recombination coefficient β with respect to that expected under Langevin conditions, βL. This technique involves the application of a constant dc bias to the device, thereby extracting charges photogenerated by a laser pulse. When bimolecularor even higher order23recombination is present, the extracted charge saturates at high laser light intensities. The ratio of β to the Langevin recombination coefficient, βL, can be determined from eq 1:3,19

P3HT:PC60BM.14 This may potentially indicate a larger interfacial area between PC70BM and P3HT, enabling more efficient charge separation and, potentially, recombination. D i ff e r e n c e s in r e c o m b i n a t i o n b e h a v io r b e t w e e n P3HT:PC60BM and P3HT:PC70BM films have previously been examined using transient absorption spectroscopy,15 where it was found that while the charge photogeneration yields were similar, the P3HT:PC70BM sample did indeed reveal slightly faster polaron decay dynamics. The effects of replacing the polymer donor’s bridging C atom with a Si atom are expected to be greater. In the case of the more well-known PCPDTBT/Si-PCPDTBT comparison,16,17 Si-PCPDTBT:PCBM outperforms PCPDTBT:PCBM in terms of device characteristics, without the requirement for a processing additive, which PCPDTBT:PCBM needs to attain a decent efficiency. This was attributed to the enhanced overall bulk crystallinity and π-stacking of Si-PCPDTBT compared to PCPDTBT, leading to a higher charge carrier mobility. Initial studies on PCPDTTTz:PC60BM and PDTSiTTz:PC60BM have revealed a higher charge photogeneration yield for the latter system.18 Since PCPDTTTz:PCBM devices are known to be relatively inefficient in the absence of a processing additive, PCPDTTTz:PCBM fabricated with the additive diiodooctane (DIO) will be assessed as well. The four devices were tested in order to ascertain the extent of non-Langevin behavior, using time-of-flight. This technique has been used in the past to assess recombination behavior in various polymer:fullerene systems, such as P3HT:PCBM,3,19 MDMO-PPV:PCBM,20 and PDTSiTTz:PCBM. 12,21 The charge extraction results show that while changing the acceptor from PC60BM to PC70BM does not adversely affect the nonLangevin recombination, changing the donor polymer by exchanging the silicon bridging atom for a carbon almost completely removes the non-Langevin behavior. However, the addition of DIO allows a partial recovery of the non-Langevin behavior.

CU0 t tr β = βL Q e te

(1)

where C is the geometric capacitance, Qe is the extracted charge, U0 is the applied voltage, ttr is the transit time (ttr = d2/ μU0, where μ is the charge carrier mobility), and te is the extraction time (defined as the difference between the t1/2 values at low and high excitation densities; the t1/2 is the time at which the photocurrent falls to half its initial value). The Langevin bimolecular recombination coefficient is given by βL = e(μp + μe)/εε0, where e is the electron charge, μp (μn) is the hole (electron) mobility, ε is the relative dielectric permittivity, and ε0 is the absolute dielectric permittivity. Bulk generation TOF is therefore capable of clearly showing whether the system in question exhibits Langevin or nonLangevin recombination. Non-Langevin systems have a very distinct extraction time (and thus a strong dependence of t1/2 on light intensity). Furthermore, a substantially higher quantity of charge compared to CU0 can be extracted due to the accumulation of charge within the device. Together, these two attributes lead to a clear gauge of the recombination behavior: the shape of the saturated photocurrent decay over time. The charge carrier reservoir in non-Langevin systems causes a photocurrent plateau region prior to the main current decay. Conversely, a decay approaching exponential behavior is observed for purely Langevin systems, such as that seen previously for systems such as MDMO-PPV:PCBM19 and PCDTBT:PCBM.24 Since only charge equal to CU0 can be extracted per transit time, the remainder of the charges recombine prior to extraction, and no reservoir/plateau is observed.

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RESULTS J−V Curves. The J−V curves of the PDTSiTTz-based devices, blended with PC60BM or PC70BM, are displayed in Figure 2. These show only small variations in short circuit current JSC and open circuit voltage VOC, with both parameters decreasing slightly from 11.8 to 11.4 mA cm−2 and 0.64 to 0.62 V, respectively, when the PC60BM is replaced with PC70BM.

Figure 2. J−V curves of encapsulated PDTSiTTz devices blended with PC60BM and PC70BM and PCPDTTTz:PC60BM devices with and without the processing additive DIO. All devices are 1:2 by weight. 7017

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extraction time, te. The increase in Qe/CU0 leads to a small drop in β/βL from 0.063 to 0.047 when the DIO is utilized. In clear contrast to altering the donor, changing the acceptor in the polymer/fullerene blend appears to have little effect on the extent of non-Langevin behavior (Figure 3b and Table 1). Both photocurrent curves exhibit the clear plateau indicative of non-Langevin recombination, producing a β/βL value of approximately 0.02 for both systems.

This difference is clearly observable when the saturated TOF traces of PCPDTTTz:PC60BM and PDTSiTTz:PC60BM are compared (Figure 3a and Table 1). The non-Langevin behavior

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DISCUSSION Although it has been suggested that it is simply the higher overall bulk crystallinity of polymers such as PDTSiTTz that causes its non-Langevin bimolecular recombination,25 the observation that both PCPDTBT:PCBM and Si-PCPDTBT:PCBM possess Langevin recombinationdespite the substantial enhancement in overall crystallinity for SiPCPDTBTimplies that non-Langevin recombination has a more complex origin. For example, it is possible that interfacial structural features beyond the resolution of current instrumentation exist. Jamieson et al.26 have proposed that the increased electron affinity of crystalline PCBM compared to that of PCBM intermixed within a polymer/PCBM phase provides a driving force for spatial charge separationessentially an energy sink for charge carriers. Expanding on this idea, Bartelt et al.27 suggested that holes can also migrate from the well-intermixed polymer/PCBM regions to polymer aggregates with lower ionization potential,28 thus improving the efficiency of charge extraction. These phenomena may also produce an energetic barrier to bimolecular recombination at the interface, as seen by the retardation of recombination kinetics observed by Jamieson et al. 26 PDTSiTTz:PC60BM has an overall moderate crystallinity, and the presence of both crystalline and amorphous/mixed domains has been suggested for this system.12,22 As such, PDTSiTTz:PC60BM may be capable of this type of energetic barrier to bimolecular recombination, and this may have a contribution to its non-Langevin behavior. The replacement of PC60BM with PC70BM, which has the same ionization potential and electron affinity,29 thus has no effect on the non-Langevin behavior under such a model. Conversely, replacement of the Si atom in PDTSiTTz with a carbon atom to form PCPDTTTz causes a substantial loss in crystallinity.18 Under the Jamieson/Bartelt model, if this loss of crystallinity also corresponds to a reduction in the interfacial area between amorphous/mixed domains and any remaining crystalline regions, then this may imply the presence of fewer energy sinks. As such, there may be fewer spatial areas within the blend film that represent an energetic barrier to recombination, thereby promoting Langevin recombination. Morphology studies o f the effect of D IO in PCPDTTTz:PCBM films have shown that while the changes in order and crystallinity are considerable smaller compared to other systems (such as PCPDTBT:PCBM16,30), there is a small

Figure 3. (a) A comparison of the TOF current transients, in the saturated regime, for the PDTSiTTz:PC60BM and PCPDTTTz:PC60BM (with and without DIO) devices using 532 nm 100 μJ cm−2 excitation with a circuit resistance of 50 Ω and an applied voltage of 3 V. (b) A comparison of the TOF saturated current transients for the PDTSiTTz:PC60BM and PDTSiTTz:PC70BM photovoltaic devices using the same excitation and resistance conditions but with an applied voltage of 4 V.

of PDTSiTTz:PC60BM is considerably reduced in the PCPDTTTz:PC60BM system, as seen by the loss of the strong photocurrent plateau. This denotes not only an appreciable loss in charge density able to be extracted from the device but also a much shorter lifetime. The shape of the PCPDTTTz:PC60BM photocurrent decay is not fully exponential; thus, purely Langevin behavior has not been reached. Application of eq 1 shows that the β/βL has increased from 0.020 for PDTSiTTz:PC60BM to 0.063 in PCPDTTTz:PC60BM, indicating a significantbut not completeloss in non-Langevin behavior. Interestingly, there is a partial recovery of the non-Langevin behavior when the DIO is added to PCPDTTTz:PCBM. Most notably, this takes place as an increase in Qe/CU0 and thus

Table 1. Bulk Generation TOF Results for Each Photovoltaic Device sample PCPDTTTz:PC60BM PCPDTTTz:PC60BM/DIO PDTSiTTz:PC60BM PDTSiTTz:PC70BM a

μa (cm2 V−1 s−1) 4 4 8 8

× × × ×

ttr (s)

10−4 10−4 10−4 10−4

1.5 2.0 9.7 1.0

× × × ×

10−7 10−7 10−8 10−7

Qe/CU0

β/βL

2.9 3.8 8.2 8.0

0.063 0.047 0.020 0.022

Measured using photo-CELIV; see Supporting Information, Figure S1. 7018

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Figure 5. Schematic showing that the presence of crystalline or ordered domains in the polymer or fullerene can decrease the ionization potential or increase the electron affinity, respectively. Either process would lead to fewer holes and electrons at the interface in a mixed polymer/fullerene amorphous region, potentially creating a barrier for recombination. This is represented pictorially in the diagram below. Charges generated in the mixed/amorphous region can recombine immediately (red arrow) or follow favorable energy gradients to the more crystalline regions (long blue solid arrows). After a time “trapped” in such regions, the charges must go back up the energy gradients (in the unfavorable direction, short blue solid arrow) before recombination can occur (dotted blue arrow).

Figure 4. Cyclic voltammograms of pristine PCPDTTTz and its blend with PC60BM, with and without DIO. All were measured as solid thin films on ITO substrates in an acetonitrile medium, using a Ag/AgNO3 reference electrode and a scan rate of 100 mV s−1.

with and without DIO. Furthermore, the measured decrease in ionization potential from 5.34 to 5.26 eV when DIO is employed is enough to account for the decrease in VOC observed in the PCPDTTTz:PCBM J−V curves. This ordered blend phase in PCPDTTTz:PCBM with DIO, with the correlated lower ionization potential, may act as an energy sink, creating a barrier to recombination and thereby partially recovering the non-Langevin behavior. The potential role of crystalline regions (polymer, fullerene, or both) adjoining amorphous/mixed regions in recombination processes is proposed in Figure 5. Although techniques to visualize the morphology of organic photovoltaic active layers have been improving substantially in recent years, it is still not possible to probe the structure of donor−acceptor and phase interfaces directly, nor to monitor movement of charges between these interfaces. At this stage it is therefore only possible to suggest mechanisms by which interfacial morphologies affect recombination. It is likely that in a system where more pure crystalline regions exist in addition to amorphous and/or mixed phases, the majority of charge carriers are generated in the mixed regions where the density of donor− acceptor interfaces are higher. From the viewpoint of the hole (Figure 5), the hole has therefore been generated in a region of high ionization potential. The hole can therefore move down an energy gradient until it is located in a region of lower ionization potential, in a crystalline polymer phase. Since the likelihood of a fullerene molecule being present is lower in this phase, the hole is essentially trapped and cannot recombine. It must be thermally activated back up the energy gradient before it can encounter a donor−acceptor interface in the mixed/amorphous region and thus be able to recombine: this is the energetic barrier to recombination. A similar argument can apply to the electron. The spatial separation from the opposing charge, in addition to the effective trapping in a crystalline phase, essentially leads to a longer lifetime for the charge carrier. This process may have a significant contribution to nonLangevin recombination. This proposal is also consistent with

the observation of two separate populations of polymer polarons in the transient absorption spectra of non-Langevin systems such as P3HT:PC60BM,33 PDTSiTTz:PCBM,22 and, to a lesser extent, PCPDTTTz:PCBM with DIO.22 Under such a model, it is therefore not the overall bulk crystallinity that leads to non-Langevin behavior, but the presence of adjoining crystalline and mixed/amorphous regions with varying ionization potentials and electron affinities. However, it is likely that numerous factors contribute to nonLangevin recombination. While the presence of domains with varying ionization potentials and/or electron affinities may have a strong influence on this behavior, it cannot be the complete answer. Si-PCPDTBT:PC60BM and PDTSiTTZ:PC60BM have been found to have very similar bulk morphologies,12 although, as previously stated, their interfacial morphology currently beyond the resolution of current techniquesmay be different. However, it has been discovered that the charge transfer state of PDTSiTTZ:PC60BM dissociates more readily compared to Si-PCPDTBT:PC60BM or PCPDTTTz:PC60BM.18 If a similar state forms during the recombination process, which can easily dissociate prior to the actual recombination event, then this may also constitute a barrier to recombination.

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CONCLUSIONS Time-of-flight experiments demonstrate that altering the acceptor in a PDTSiTTz blend film from PC60BM to PC70BM does not have a detrimental effect on the nonLangevin recombination present. However, changing the donor polymer by exchanging the silicon bridging atom for a carbon largely eliminates the non-Langevin behavior. Importantly, the addition of DIO enables a partial recovery of the non-Langevin 7019

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behavior. Cyclic voltammetry was used to show that this addition of DIO to PCPDTTTz:PC60BM also causes a decrease in the ionization potential of the polymer: this accounts for the drop in VOC observed. The extent of non-Langevin behavior can therefore be manipulated using processing additives in the case of PCPDTTTz:PC60BM, and this can be directly correlated with measured changes in the ionization potential of the polymer. The decreased ionization potential upon addition of DIO appears to also be correlated with an increased presence of an ordered blend phase. Combined, these factors may contribute to the partial recovery of non-Langevin behavior in PCPDTTTz:PC60BM. It is proposed that localized areas of lower ionization potential (or higher electron affinity) may act as energy sinks for the holes (electrons), thereby impeding bimolecular recombination. Such a phenomenon could be a significant contributor to non-Langevin recombination in organic photovoltaics.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: photo-CELIV curves of each device, used to determine charge carrier mobility. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.M.C.). *E-mail [email protected] (A.J.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Linkage Projects funding scheme and by Konarka Technologies. A.J.M. acknowledges the ARC for providing equipment support through LIEF as well as supporting A.J.M. and T.M.C. with an Australian Research Fellowship and DECRA fellowship, respectively.

EXPERIMENTAL SECTION

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Device Fabrication. Devices of PCPDTTz:PC60BM, PCPDTTz:PC60BM with DIO, PDTSiTTz:PC60BM, and PDTSiTTz:PC70BM (all 1:2 by weight) were fabricated using the same method as Peet et al.31 with an inverted Ag/ hole-injecting layer (HIL)/active layer/electron-injecting layer (EIL)/ITO structure, where the HIL and EIL are Konarka proprietary materials. The PC60BM and PC70BM (99.5% and >99% purity, respectively) were sourced from Solenne, and the two polymers were provided by Konarka Technologies Limited, synthesized using the procedure in ref 34. No thermal annealing was performed. For the device with 1,8-diiodooctane (Aldrich, 98% purity), a 2 vol % quantity was added to the odichlorobenzene (Aldrich, anhydrous, 99% purity) solution prior to stirring overnight at 120 °C. The active layer thickness was ∼170 nm with an active area of ∼5 mm2, which was doctor blade-coated using solutions at 70 °C. The device with DIO was dried in a low vacuum for 1 h prior to coating layers on top of the active layer. Devices were fabricated in air and then transferred to a glovebox for epoxy encapsulation with a glass cover layer. Device efficiencies were measured with a Newport−Oriel AAA certified solar simulator operating at 100 mW cm−2. Solar simulator illumination intensity was calibrated using a standard silicon photovoltaic with a protective KG5 filter calibrated at the National Renewable Energy Laboratory. Time-of-Flight. The devices were illuminated by a laser pulse (6 ns, 532 nm, repetition rate 10 Hz) from a Nd:YAG laser (Spectra-Physics, INDI-40-10), while electrically biased by applying −3 or −4 V using a function generator. The transient photocurrents were recorded using an oscilloscope (50 Ω input impedance). Photo-CELIV Measurements. The same samples were illuminated by a laser pulse as above. After an adjustable delay time controlled by a digital delay generator (Stanford Research DG535), photogenerated charges were extracted using a linearly increasing voltage pulse applied by a function generator. The current transients were recorded using an oscilloscope (50 ohm input impedance). The time resolution of the setup is around 5 ns.

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DOI: 10.1021/jp5129707 J. Phys. Chem. C 2015, 119, 7016−7021

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DOI: 10.1021/jp5129707 J. Phys. Chem. C 2015, 119, 7016−7021