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Paul Sullivan, Amelie Duraud, lan Hancox, Nicola Beaumont, Giorgio Mirri, James H.R. Tucker, Ross A. Hatton,* Michael Shipman,* and Tim S. Jones*

Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics

The fi eld of organic photovoltaics (OPVs) is attracting enor-mous interest due to its promise of providing a low-cost, easily processable technology for energy generation. Whilst the short-circuit current density ( J SC ) of OPV devices can be optimized through morphology control, [ 1 ] intermixing, [ 2 , 3 ] nanostruc-turing [ 4 ] or templating [ 5 ] amongst other techniques, the open-circuit voltage ( V OC ) is critically dependent on the choice of photoactive materials. The relative abundance of suitable com-mercially available materials has led to a wealth of reports on optimization of the donor layer; [ 6 , 7 ] however the same is not true for the acceptor layer, with most work focusing on the ubiqui-tous fullerenes. Whilst fullerenes are undoubtedly effi cient electron transport materials, this class of acceptor material has inherent issues with long-term stability [ 8 , 9 ] and relatively low bandgaps which limit the maximum obtainable V OC in single heterojunction systems. [ 10 ] Overcoming these issues by use of alternative acceptor materials, whilst maintaining or improving overall effi ciencies, is an important challenge in the OPV fi eld.

Initially, the V OC in OPVs was believed to be primarily lim-ited by the work function difference between the two electrodes, however, it has since become clear that the difference in energy between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A), i.e. the interface gap ( I G ), is the primary determi-nant of the maximum V OC obtained. [ 6 , 11 ] Importantly, the frontier orbital energies of organic materials can be readily tuned through introduction of electron-donating or electron-withdrawing sub-stituents. For example, halogenation of conjugated molecules has been shown to lower the HOMO and LUMO levels with respect to the vacuum level whilst having only a small effect on other electronic properties such as the bandgap, or the position of the Fermi level within the bandgap. [ 12 , 13 ] As an example of the extent of modifi cation possible by this method, complete fl uorination of copper phthalocyanine (CuPc) to its hexadecafl uoro derivative (F 16 CuPc) results in a shift of the HOMO level by over 1 eV. [ 14 ]

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Dr. P. Sullivan , Dr. A. Duraud , I. Hancox , N. Beaumont , Dr. R. A. Hatton , Prof. M. Shipman , Prof. T. S. Jones Department of Chemistry University of Warwick Coventry CV4 7AL, U.K. E-mail: Ross.Hatton@warwick.ac.uk; M.Shipman@warwick.ac.uk; T.S.Jones@warwick.ac.uk G. Mirri , Dr. J. H. R. Tucker School of Chemistry University of Birmingham Edgbaston, Birmingham, B15 2TT, U.K.

DOI: 10.1002/aenm.201100036

The D/A heterojunction system of boron subphthalocya-nine chloride (SubPc)/fullerene (C 60 ) is amongst the current fi eld-leaders for single small molecule heterojunction devices both in terms of V OC ( ∼ 1.1 V) and overall power conversion effi ciency ( PCE ∼ 3%). [ 15 , 16 ] However, consideration of the inter-facial energetics of this system reveals that C 60 is far from optimal as the acceptor (vide infra). In this communication, we report the synthesis of several new selectively chlorinated and fl uorinated SubPcs, and demonstrate that effi cient OPV devices can be constructed using these materials as electron acceptors. Moreover, it is shown that peripheral halogenation can be used to maximize the interface gap, and hence the V OC obtained. As well as a remarkably high V OC , device degradation studies show that OPVs utilizing Cl 6 -SubPc in place of C 60 as the electron acceptor have markedly improved stability under continuous illumination.

Three selectively halogenated SubPc derivatives were made by cyclotrimerization of the corresponding substituted phthalo-nitriles ( Figure 1 ) . [ 17 ] Treatment of 4,5-dichlorophthalonitrile in p -xylene at 140 ° C in the presence of BCl 3 produced Cl 6 -SubPc in 56% yield after column chromatography. Similarly, F 6 -SubPc and F 3 -SubPc were prepared from 4,5-difl uorophthalonitrile and 4-fl uoro-phthalonitrile respectively. F 3 -SubPc was produced as a 3:1 mixture of C 1 and C 3 -isomers, [ 17 ] and used as such in subsequent studies. Full details are provided in the Supporting Information.

Cyclic voltammetry (CV) and UV electronic absorption spec-troscopy were used to estimate the thin fi lm frontier orbital energy levels for the SubPc and halogenated-SubPc derivatives in the condensed phase (see Supporting information). [ 18 ] The resulting zero-fi eld energy level diagram at the D/A interface for the reference SubPc/C 60 system and the SubPc/halogenated-SubPc systems are shown in Figure 2 , with the levels for C 60 taken from the literature. [ 19 ] Whilst known for providing effi -cient OPV devices, it appears from the diagram that the low lying C 60 LUMO may be limiting the obtainable V OC . A com-promise always exists between providing a suffi cient HOMO and LUMO interface offset for effi cient exciton dissociation and maximizing the I G of the heterojunction. In the case of SubPc/C 60 , the estimated LUMO offset is ∼ 0.8 eV which is signifi cantly greater than the 0.3–0.4 eV required for effi cient exciton disso-ciation [ 20 , 21 ] and hence the I G seems non-optimal.

In the halogenated-SubPc systems, the electronegativity of the halogen atoms has the effect of lowering the energy levels compared to unsubstituted SubPc, generating a HOMO and LUMO interface offset, Δ HOMO and Δ LUMO , and a consequential decrease in I G in combination with the SubPc donor. By varying the type and number of halogen substituents this shift can be

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Figure 1 . Boron subphthalocyanine chloride (SubPc) and the selectively halogenated derivatives used in this study.

Figure 3 . J–V curves under 1 sun AM1.5G simulated illumination for the series of heterojunction devices A-D. Respective dark current curves are shown as dotted lines.

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tuned in order to achieve the minimum Δ LUMO required for effi -cient exciton dissociation, with a view to maximizing I G and the achievable V OC . An increase in the number of halogen atoms results in an increase in the magnitude of this shift. [ 12 ] When the number of halogen atoms is kept constant, Cl substituents tend to provide a larger shift than may be expected from their electronegativity alone, possibly due to the ability of the Cl to accept electrons from the conjugated π -system into empty 3d orbitals, resulting in shifts similar to F substituents. [ 13 ] Coup led with a slight increase in the SubPc bandgap when using F substituents, the estimated Δ LUMO values therefore follow the trend of F 3 < F 6 < Cl 6 , with the Cl 6 derivative showing a Δ HOMO and Δ LUMO of at least 0.3 eV. F 12 -SubPc and F 13 -SubPc (central halogen also replaced) have previously been studied as acceptor materials, however these fully fl uorinated derivatives resulted in an excessive shift in the frontier orbital energies and thus a sub-optimal V OC of 0.94 V. [ 22 ]

These halogenated SubPc derivatives were used to make a series of SubPc/halogenated-SubPc OPV devices. The J–V curves for these alongside a reference SubPc/C 60 device are pre-sented in Figure 3 . All devices had the standard structure ITO/MoO x (5 nm)/SubPc (10 nm)/Acceptor ( d A nm)/BCP (8 nm)/Al. Table 1 summarizes d A and the key device parameters for each cell. The MoO x layer is used to facilitate hole extraction at the ITO electrode, [ 23 , 24 ] whilst BCP is used as an exciton blocking layer at the Al electrode. The reference SubPc/C 60 device (A) shows typical performance with V OC = 1.10 V, J SC = 5.03 mA cm − 2 , and Fill Factor ( FF ) = 0.53 leading to a PCE of 2.97%. Replacing C 60 with 10 nm of F 3 -SubPc (device B) leads to poor device performance with a PCE of just 0.19%, largely

© 2011 WILEY-VCH Verlag GmAdv. Energy Mater. 2011, 1, 352–355

Figure 2 . Frontier orbital energy level diagrams at the interface of a typical SubPc/C 60 heterojunction and the series of SubPc/halogenated-SubPc heterojunctions showing the resulting interface gap energy, I G .

IG = 1.3eV

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due to the low J SC of just 0.83 mA cm − 2 and the poor FF of 0.22. Whilst in this case the I G is large, the poor device performance is attributed to the small Δ HOMO and Δ LUMO which are below the empirical threshold of ≥ 0.3 eV for effi cient exciton dissociation at an organic heterojunction. The onset of dark current at posi-tive bias occurs at a signifi cantly higher applied voltage than the other halogenated-SubPc acceptors studied, suggesting the V OC realized is severely restricted by the poor charge generation. Using 10 nm of F 6 -SubPc as the acceptor material (device C), the increased Δ HOMO and Δ LUMO result in a slight improvement in exciton dissociation effi ciency and J SC , although the V OC is still restricted to 1.22 V, and the resulting PCE is only 0.8%. However, by using 27 nm of Cl 6 -SubPc as an acceptor (device D), a signifi cant improvement in J SC to 3.53 mA cm − 2 and FF to 0.58 are observed, suggesting effi cient exciton dissociation. Despite the onset of dark current occurring at a lower positive bias than the other derivatives (consistent with the estimated trend), the high J SC allows for a V OC value of 1.31 V to be achieved, an increase of over 0.2 V from the reference SubPc/C 60 device (A), and a resultant PCE of 2.68%.

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Table 1. Parameters for the series of devices (A-D). In all cases the device structure is ITO/MoO x (5 nm)/SubPc (10 nm)/Acceptor ( d A nm)/BCP (8 nm)/Al.

Device (D/A)

d A [nm]

V OC [V]

J SC [mAcm − 2 ]

FF PCE [%]

A: SubPc/C 60 32.5 1.10 5.03 0.53 2.97

B: SubPc/

F 3 -SubPc

10 1.10 0.83 0.22 0.19

C: SubPc/

F 6 -SubPc

10 1.22 1.56 0.43 0.80

D: SubPc/

Cl 6 -SubPc

27 1.31 3.53 0.58 2.68

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Figure 4 . Evolution of (top) the key device parameters ( J SC , V OC , FF and PCE ) of device D and (bottom) the PCE of devices A and D expressed as a percentage decrease from the initial value. All data recorded over a period of 24 h under constant illumination.

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Two important factors must be considered when interpreting the device performance parameters. Firstly, whilst halogenation provides a signifi cant shift in the frontier orbital energies, the bandgap only shows slight variation compared to the parent molecule. Electronic absorption spectra of the SubPc deriva-tives show similarity in shape, with only slight shifts in the absorption maxima, λ max , of < 30 nm from that of SubPc ( λ max = 588 nm). The similarity in absorption profi le of the D and A layers limits the region of the solar spectrum over which the device can harvest light which, combined with the loss of C 60 which contributes signifi cantly to the photocurrent in the high power 400–500 nm region, explains the reduction in J SC in even the best SubPc/halogenated-SubPc device. However, the ability of a single heterojunction to perform to a PCE of ∼ 2.7% using such a narrow wavelength range lends itself well to use in tandem architectures where both the D and A materials in the second sub-cell could be selected such that they show com-plementary absorption profi les. Secondly, the d A values were optimized for each device, with increasing d A thicknesses above these optimum values resulting in decreased FF , J SC and hence PCE . In the cases of the F 3 and F 6 derivatives, the optimum d A thickness of 10 nm was identical to that of the donor SubPc thickness resulting in extremely thin active layers of just 20 nm. These small optimized thicknesses are typical of amor-phous organic semiconductors where exciton diffusion lengths and charge carrier mobilities are low and restrict the absorption of the device. This poor absorption may contribute to the rela-tively poor J SC seen in the SubPc/F 6 -SubPc device, despite the suggestion of improved exciton dissociation. In the case of Cl 6 -SubPc however, a much larger optimized thickness of 27 nm is found which allows for more complete absorption in the active wavelength region, and a resultant higher J SC . This result sug-gests improved molecular interactions and charge mobility in the Cl 6 -SubPc layer. The increased evaporation temperature of the Cl 6 -SubPc material ( ∼ 100 ° C greater than the other substi-tuted and unsubstituted derivatives) cannot be explained simply by the increased molecular weight, and points towards stronger interactions between molecules within the material. For related copper phthalocyanines, there is evidence for increased charge mobility in Cl 16 CuPc compared to F 16 CuPc; [ 13 , 25 ] however the inconsistency in literature reports prevents any fi rm conclu-sions being drawn. [ 26 ]

The long term stability of OPV devices is an extremely important practical consideration for ultimate commercializa-tion. Whilst MoO x hole collection layers can provide a marked improvement in stability, fullerenes are known to degrade by photooxidation mechanisms even at relatively low oxygen concentrations, resulting in charge trap sites which reduce the photo conductivity of the layer. [ 8 , 9 , 27 ] Replacement of the fullerene acceptor with a stable phthalocyanine derivative might be expected to lead to improvements in device stability. To test this hypothesis, the performance of the SubPc/Cl 6 -SubPc device was evaluated over a period of 24 h under constant illumination in an N 2 atmosphere ( Figure 4 , top). It is worth noting the abso-lute parameters differ signifi cantly from those in Table 1 due to the use of a different light source for this part of the study which was not tuned to the AM1.5G spectrum. The J SC shows a decrease of ∼ 5% within the fi rst 2 h, followed by a slow decay over the following 22 h to a total loss of ∼ 12%. The V OC shows

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an initial rapid decrease from 1.27 V to 1.22 V over 30 min before stabilizing for the remainder of the experiment. The V OC was found to recover to its initial value when illumination was stopped for a few minutes, suggesting this loss is unavoidable and due to a combination of the increased temperature and the equilibration of accumulated charges within the device under constant illumination. The FF was surprisingly stable at ∼ 0.6 for the duration of the experiment which suggests little degra-dation in the D/A interface properties or charge collection path-ways. Due to the stability of the FF and V OC , the PCE follows a similar trend as J SC after the initial period of V OC decrease, with a total degradation of ∼ 15%. A comparison of the PCE of this device with the C 60 containing device (A) expressed as a percentage loss is shown in Figure 4 (bottom). With C 60 used as the acceptor material the initial degradation in the fi rst 2 h is similar, however it then proceeds to degrade at a signifi cantly faster rate for the remainder of the 24 h, resulting in a total deg-radation of ∼ 36%. This decrease in performance is likely to be a result of gradual uptake of the low levels of oxygen still present in the sealed analysis chamber resulting in photooxidation of the fullerene. Replacement of the C 60 with a halogenated-SubPc clearly improves the stability of the device due to the lower pro-pensity for photooxidation.

In conclusion, we have demonstrated the use of selective halogenation as a method to tune the energy levels of SubPc in order to maximize V OC when used as an acceptor material in an organic heterojunction device with an unsubstituted SubPc donor. Cl 6 -SubPc was found to provide suffi cient interfacial HOMO and LUMO offsets for effi cient exciton dissociation,

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whilst maximizing the interface gap. This system delivers excel-lent performance characteristics including one of the highest V OC values reported to date in a small molecule, single het-erojunction OPV system. Combined with its ease of synthesis and improved stability in devices, Cl 6 -SubPc is a serious con-tender as a replacement for C 60 in small molecule, photovoltaic devices.

Experimental Section Experimental details relating to the synthesis and characterization of Cl 6 -SubPc, F 6 -SubPc, and F 3 -SubPc can be found in the Supporting information alongside NMR spectra. Details of the cyclic voltammetry measurements, UV electronic absorption spectra and estimated bandgaps for these compounds are also provided.

Device growth: Devices were grown on 15 Ω/sq. ITO substrates (Thin Film Devices Inc.) after initial solvent cleaning and exposure to a UV/Ozone atmosphere. Layers were grown by vacuum evaporation in a Kurt J. Lesker Spectros system with a base pressure of ∼ 5 × 10 − 8 mbar. The commercially available materials C 60 (Nano-C Inc.) and SubPc (Sigma Aldrich) were each purifi ed by thermal gradient sublimation prior to use. Molybdenum oxide (MoO x , Sigma Aldrich) and bathocuproine (BCP, Sigma Aldrich) were used as received. Al electrodes were evaporated through a shadow mask to a device active area of 0.16 cm 2 . The halogenated SubPc derivatives were purifi ed by gradient sublimation purifi cation prior to use.

Device Testing: J–V curves were measured using a Keithley 2400 sourcemeter interfaced to a custom Labview data collection program. Two light sources were used for illumination. For absolute performance characteristics a Newport Oriel solar simulator was used, set to 100 mW cm − 2 AM1.5G simulated radiation by way of neutral density fi lters and a Fraunhofer calibrated silicon photodiode (PV Measurements Inc.). For stability measurements light of ∼ 100 mW cm − 2 was produced by a quartz halogen lamp and a series of 450 nm high power LEDs to approximately match the main features of the solar spectrum. All J–V measurements were recorded after sealing the device in an airtight sample holder inside a nitrogen fi lled glovebox with oxygen content of < 5ppm.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge the fi nancial support of the Engineering and Physical Sciences Research Council (EPSRC), UK, through the Supergen Excitonic Solar Cell consortium. RAH is grateful to the Royal Academy of Engineering/EPSRC for the award of a Fellowship.

Received: January 28, 2011 Revised: February 24, 2011

Published online: March 30, 2011

© 2011 WILEY-VCH Verlag GmAdv. Energy Mater. 2011, 1, 352–355

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