The mixed alloyed chemical composition of chloro-(chloro)nboron

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The mixed alloyed chemical composition of chloro-(chloro)boron subnaphthalocyanines dictates their performance as electron donating and hole transporting materials in organic photovoltaics. Richard K Garner, Minh Trung Dang, Jeremy Duc Dang, and Timothy P Bender ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00180 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Energy Materials

The Mixed Alloyed Chemical Composition of Chloro(chloro)nboron Subnaphthalocyanines Dictates Their Performance as Electron Donating and Hole Transporting Materials in Organic Photovoltaics. Richard K. Garner,† Minh Trung Dang,† Jeremy D. Dang,† and Timothy P. Bender†‡§*

†Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada ‡Department of Materials Science Engineering, University of Toronto, 180 College Street, Toronto, Ontario, M5S 3E4, Canada §Department of Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada *Corresponding Author, email: [email protected]

Keywords: boron, subnaphthalocyanine, OPV, PHJ, BHJ, donor

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Abstract Chloro-(chloro)n-boron subnaphthalocyanine (Cl-ClnBsubNc) from a commercial source and two synthetic routes were each tested as electron donating and hole-transporting materials in planar as well as bulk heterojunction (PHJ and BHJ) organic photovoltaic (OPV) devices. We have previously reported that each Cl-ClnBsubNc sample is a mixed alloyed composition, wherein each has a varying degree of bay position chlorination. We have determined that increasing bay chlorination has a beneficial effect on the fill factor of PHJs. Comparison between this new and our past OPV data sets which utilized the same set of Cl-ClnBsubNcs as electron acceptors and transporters, reveals that the increase of fill factor and performance is likely due to improved exciton transport and higher levels of bay-position chlorination. While we identify two possible mechanisms for this, further studies will be required to determine whether the phenomenon is driven by decreased radiative relaxation or due to enhanced thermal hopping from a narrower density of states. We conclude that the usage of Cl-ClnBsubNc with higher levels of bay position chlorination, achieved through the “nitrobenzene process,” is likely to result in higherperformance OPVs.

Introduction In the global search for a scalable and sustainable energy source, organic photovoltaic devices (OPVs) have attracted significant research attention. They have the potential for both faster manufacturing and reduced material costs relative to traditional inorganic photovoltaics,1-2 while still leveraging the benefits that differentiate solar power generation from other renewable and non-renewable energy sources. A significant fraction of OPVs are fabricated using smallmolecule based active layers, with a particularly promising family of materials being the boron Page 2 of 27 ACS Paragon Plus Environment

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subphthalocyanines (BsubPcs). These molecules have been shown to provide high power conversion efficiencies and offer tuneable properties through a variety of molecular substitutions and varations.3-6 One variant in this family, chloro boron subnaphthalocyanine (Cl-BsubNc), was paired with chloro boron subphthalocyanine (Cl-BsubPc) as a bifunctional acceptor/donor layer in a device achieving a power conversion efficiency (PCE) of 8.4%.7 This stands as a record literature-reported PCE in the field of planar heterojunction (PHJ) OPVs. Additionally, recent work by Chandran et al. has shown that Cl-BsubPc and Cl-BsubNc are particularly proficient at direct generation of free charge carriers, meaning they can be used in single-layer devices, or singularly added to other device stacks to improve charge generation.8 Findings such as these demonstrate the importance of continued investigation of these versatile materials. Surprisingly, the true chemical composition and molecular structure of Cl-BsubNc was not fully understood until quite recently. Kahn et al. found evidence that commercial Cl-BsubNc samples contained higher levels of chorine than pure Cl-BsubNc would have, and that the Cl energy level was different than that of a B-Cl axial moeity.9 Concurrently, the work of Dang et al. in our laboratory determined that several synthetic procedures for Cl-BsubNc production yielded alloyed mixtures of Cl-BsubNcs which have at least one, and often more, chlorine atoms substituted in the bay position of the Cl-BsubNc chromophore,10 as shown in Figure 1. Positioning of the chlorine atoms was confirmed by x-ray crystallography. Dang et al. therefore proposed that the synthesized mixtures instead be referred to as chloro-(chloro)n-boron subnaphthalocyanine (Cl-ClnBsubNc). Each molecular mixture could not be separated according to the number of bay chlorines, and all behaved as mixed alloyed compositions in OPV devices.



Using x-ray photoelectron spectroscopy (XPS), our laboratory also quantified the level of bay position chlorination. Our results were in line with the results of Kahn et al., who determined a value of ~1.5 bay chlorines per molecule in commercially purchased Cl-BsubNc.9 We also quantified other variants: the “literature route” refers to the synthetic procedure of Torres et al.,11 “commercial supplier” refers to material purchased from Lumtec, and “nitrobenzene route” refers to our synthetic route variant taking place in nitrobenzene solvent. It was found that the amount of chlorination of literature-Cl-ClnBsubNc was similar to that of the commercial-ClClnBsubNc sample, whereas the nitrobenzene-Cl-ClnBsubNc sample(s) had significantly higher levels of chlorination.

Figure 1. Structures of the PC70BM and C70 molecules, as well as the Cl-ClnBsubNc molecule, where n ranges from 0 to 2 bay chlorines for each of the three isoindoline subgroups.

Through UPS measurements, the level of chlorination was also shown to alter the electronic energy levels of Cl-ClnBsubNcs (Table 1). Optical absorption and photoluminescence was also shown to vary for Cl-ClnBsubNcs, As a result of chlorination’s impact on these parameters, the performance of the various Cl-ClnBsubNc mixed alloy compositions within OPVs also differed. Page 4 of 27 ACS Paragon Plus Environment

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Our laboratory used OPVs with a Cl-ClnBsubNc electron acceptor layer and an α-sexithiophene (α-6T) electron donor layer to illustrate this.10 The resulting devices showed a trend wherein the VOC decreased with increasing chlorination, while the fill factor increased with increasing chlorination. The nitrobenzene-Cl-ClnBsubNc devices achieved the highest PCE with 4.3 ± 0.1 %, despite exhibiting the lowest VOC of the study as a result of changes to their highest occupied molecular orbital (HOMO) energy level. They measured at 0.930 ± 0.002 V, rather than the 0.972 ± 0.018 V or 0.976 ± 0.005 V measured on literature- and commercial-Cl-ClnBsubNc OPVs respectively. This was due to a substantially higher fill factor: 0.53 ± 0.01, rather than 0.45 ± 0.01 or 0.46 ± 0.01 for literature- and commercial-Cl-ClnBsubNc respectively. This type of observation is, however, not entirely unique to Cl-ClnBsubNc. Fleetham et al. found that using different synthetic procedures to produce zinc phthalocyanines (ZnPcs) could lead to peripheral chlorination of the phthalocyanine ligand, which benefitted the open circuit voltage of ZnPc/C60 devices.12-13



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Table 1. Material Properties from Various Synthetic Routes.a Cl-

Bay

ClnBsubNc

Chlorines via Absorption,

Energy via in

Source

XPS

UPS (eV)

Peak

Ionization

UV-Vis

Stokes Shift ΦPL Solid (Photoluminescen ce quantum yield,

Films (nm)

[Kahn et al]10 (nm) literature route commercial supplier nitrobenzene route

%)

1.21

656

5.32

58

27

1.61 [1.5]

656

5.31

47

24

4.17

664

5.42

52

21

a) All data in Table 1 is from our laboratory’s prior work by Dang et al., except that by Kahn et al.10 which is shown in square brackets.

In this follow-up study, we explore the various Cl-ClnBsubNcs as electron donors/holetransporting materials. They are paired with C70 (Figure 1) as an electron acceptor/transporter in a planar heterojunction OPV device, and with PC70BM (Figure 1) in a bulk heterojunction (BHJ) OPV configuration. Three Cl-ClnBsubNc mixtures were used as outlined above; from the literature synthetic route, the nitrobenzene synthetic route, and a sample from a commercial supplier. The energy levels of the mixtures were determined by UPS measurements of the HOMO and UV-Vis measurements of the optical gap using the onset of absorption, with the measurements taken in our prior work. This yielded HOMO and LUMO energy levels of 5.32 and 3.62, 5.31 and 3.64, and 5.42 and 3.76 electron volts for literature-, commercial-, and nitrobenzene-Cl-ClnBsubNc respectively. Page 6 of 27 ACS Paragon Plus Environment

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Figure 2. Device structures of a) PHJ and b) BHJ devices. Diagrams not to scale.

Results All OPVs fabricated in this work follow the design shown in Figure 2, with a structure consisting of a glass substrate; an indium tin oxide (ITO) cathode; a PEDOT:PSS hole transport layer; light-absorbing layers comprising Cl-ClnBsubNc/C70 in PHJs and ClnBsubNc:PC70BM in BHJs ; a bathocuproine (BCP) electron transport layer; and a silver anode. For PHJs, the



absorbing layers consisted of a Cl-ClnBsubNc donor layer and a C70 acceptor layer; in BHJs the absorbing layer was a Cl-ClnBsubNc:PC70BM BHJ bulk heterojunction.

Planar Heterojunction Devices Literature, commercial and nitrobenzene Cl-ClnBsubNc were first tested as electron donating/hole-transporting layers in PHJ devices with the following specific configuration: ITO/PEDOT:PSS/Cl-ClnBsubNcs (7 nm)/C70 (30 nm)/BCP (7 nm)/Ag. This structure was arrived at after a precursory optimization process detailed in Figure S1 of the supplementary information. Figure 3a and Figure 3b show current-voltage (J-V) curves and EQE measurements of each set of devices, with shaded regions indicating 95% confidence intervals about the current flux.

Figure 3. a) J-V and b) EQE plots of Cl-ClnBsubNc:C70 PHJ OPVs based on each Cl-ClnBsubNc source. Shaded regions show 95% confidence intervals about the y-axes.

Immediately apparent is the clear separation of open-circuit voltages (VOC) and short-circuit currents (JSC) with variations in the Cl-ClnBsubNc mixed alloyed composition. The Page 8 of 27 ACS Paragon Plus Environment

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nitrobenzene-Cl-ClnBsubNc devices achieve the highest VOC, followed by the commercial- and literature- devices. This ordering, and furthermore the magnitude of VOC differences between synthesis routes, directly follows the trend in degree of bay chlorination we previously identified, which is shown in Table 2. Commercial- and literature-Cl-ClnBsubNc devices demonstrated approximately equal JSCs while nitrobenzene-Cl-ClnBsubNc devices displayed modestly less, roughly following the trend in bay chlorination but in the direction of decreasing performance.

Table 2. PHJ J-V metrics for the various Cl-ClnBsubNcs. VOC: open circuit voltage, JSC: short circuit current flux, FF: fill factor, PCE: power conversion efficiency. Bracketed values in device metric columns display standard deviations.a Device Type

Bay Chlorines VOC (V)

JSC

via

(mA/cm2)

XPS

FF

PCE (%)

[Kahn et al]10 literature route

1.21

0.81 (0.01)

-5.98 (0.14)

0.41 (0.05) 1.99 (0.22)

commercial supplier

1.61 [1.5]

0.83 (0.01)

-5.99 (0.34)

0.43 (0.02) 2.13 (0.21)

nitrobenzene route

4.17

0.88 (0.01)

-5.39 (0.24)

0.48 (0.02) 2.28 (0.13)

a) All data in Table 2 is from our laboratory’s prior work by Dang et al., except that by Kahn et al.10 which is shown in square brackets. Another trend is apparent in the fill factor. Fill factor appears to rise with increasing levels of chlorination of Cl-ClnBsubNc, though the fill factor of the PHJ OPVs based on literature- ClClnBsubNc devices (0.41 ± 0.05) has too high a standard deviation to conclusively declare it



lower than that of the commercial- devices (0.43 ± 0.02). Overall the best PCE was obtained from the PHJ using nitrobenzene-Cl-ClnBsubNc, which was a result of increased VOC and FF. The clearly definable peaks in Figure 3b of both Cl-ClnBsubNc and C70, and a wide range of absorption, indicate that both active layer materials contributed to photogeneration. Generally, the even nature of the EQEs indicates an optically balanced device. Due to the wide area of the solar energy spectrum which is accessed by an absorption range of 330-730nm, further optimization and configuration considerations could yield very well-performing PHJ OPVs.

Bulk Heterojunction Devices Yang et al has previously reported a bulk heterojunction (BHJ) OPV device configuration based on Cl-ClnBsubNc and [6,6]-phenyl-C-71-butyric acid methyl ester (PC70BM),14 using the same Cl-ClnBsubNc supplier as us. They obtained a PCE of 2.3 ± 0.2 % when using the same device architecture as was employed in our efforts. In an effort to replicate the results of Yang et al., BHJ OPVs were prepared by pairing PC70BM with each Cl-ClnBsubNc, using the procedures outlined in the Experimental Methods section of the supplementary information. The J–V characteristics and EQE spectra of BHJ OPVs under AM1.5G illumination are illustrated in Figure 4. Devices incorporating commercial-ClClnBsubNc yielded an efficiency of 1.71%. The nitrobenzene-Cl-ClnBsubNc device, which was the best performing, showed a JSC of 7.07 ± 0.16 mA/cm2, a VOC of 862 ± 32 V, a fill factor of 0.347 ± 0.010 and a PCE of 2.12 ± 0.17 %. These values are modestly lower than Yang et al., who carried out extensive active-layer thickness optimizations. In our case, we simply sought a direct point of comparison amongst our Cl-ClnBsubNc samples. The pronounced peaks in the EQE at 462 nm and 690 nm indicate that both active layer compounds are contributing to Page 10 of 27 ACS Paragon Plus Environment

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photogeneration in a well-balanced manner. These results confirm those of Yang et al., that ClClnBsubNcs can act as electron donors in BHJ OPV devices.

Figure 4. a) J-V and b) EQE plots of Cl-ClnBsubNc:PC70BM BHJ OPVs based on each ClClnBsubNc source. Shaded regions show 95% confidence intervals about the y-axes.

All the BHJ devices showed similar current density, resulting in roughly equivalent EQE’s, regardless of the synthetic route used.

While lower overall, the VOC followed the same

increasing trend in the BHJ device sets as in the PHJ sets. More interestingly, the trend in fill factor, which was observed in the PHJ Cl-ClnBsubNc donors above and the PHJ Cl-ClnBsubNc acceptors that we produced previously, is also no longer distinguishable in these BHJ devices. Our views of this are discussed in the following section.



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Table 3. BHJ J-V metrics for the various Cl-ClnBsubNcs. VOC: open circuit voltage, JSC: short circuit current flux, FF: fill factor, PCE: power conversion efficiency. Bracketed values in device metric columns display standard deviations.a Device Type

Bay Chlorines VOC (V)

JSC

via

(mA/cm2)

XPS

FF

PCE (%)

[Kahn et al]10 literature route

1.21

0.77 (0.02) -7.05 (0.52)

0.35 (0.01) 1.93 (0.18)

commercial supplier

1.61 [1.5]

0.74 (0.02) -6.93 (0.05)

0.33 (0.01) 1.71 (0.17)

nitrobenzene route

4.17

0.86 (0.03) -7.07 (0.16)

0.35 (0.01) 2.12 (0.17)

a) All data in Table 3 is from our laboratory’s prior work by Dang et al., except that by Kahn et al.10 which is shown in square brackets.

Modelling In addition to our empirical results, we have modelled the energy levels of the full range of possible Cl-ClnBsubNc molecules (see Figure S2 for chemical structures), with n ranging from zero to the maximum of six, in all possible configurations. There are multiple data points for n = 2 through 5 given the variety of possible isomers. We have utilized the RM1 and PM3 models, using the updated parameter sets implemented and verified by Morse et al. for various phenoxy boron subphthalocyanine derivatives.15 The RM1 and PM3 parameter sets are available for download at pub.acs.org, with hyperlinks present in the HTML text version of the Morse et al paper.15 This modelling effort, plotted in Figure 5, helps to illustrate the effect of varying chlorination on the molecules’ electronic properties. With an understanding of the electronic characteristics of the molecules making up each mixed alloyed composition, and the knowledge Page 12 of 27 ACS Paragon Plus Environment

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that the overall crystal ordering is unaffected by changes in chlorination, some insight can be gained regarding the electronic characteristics of the bulk material.

Figure 5. HOMO and LUMO energy levels modelled by PM3 and RM1 methods. In both models, a trend can be identified in which variations in the degree of chlorination affect energy levels more strongly for less-chlorinated molecules than for more-chlorinated molecules. Phrased differently, the first bay chlorine added to a Cl-ClnBsubNc molecule has more of an impact on its energy levels than any subsequent chlorines added, with a diminishing effect caused by each successive chlorine added. The RM1 model shows this trend more strongly than the PM3 results, but it can still be noted in the PM3 HOMO energy levels. Page 13 of 27 ACS Paragon Plus Environment


Discussion The results above enable a broad comparison of the various Cl-ClnBsubNc mixed alloy compositions, tested as electron donors/hole transporters in PHJ and BHJ OPVs in this work, and as electron acceptors and transporters in our previous Cl-ClnBsubNc study. A revealing comparison can be made through these multiple lenses regarding the electronic nature of ClClnBsubNcs, their resultant mixed alloyed compositions, and their influence on the relevant properties. We have organized the following discussion by the relevant OPV device characteristics.

Open-Circuit Voltage (Voc) When the various Cl-ClnBsubNc compounds were applied as electron acceptors, increasing chlorination led to a drop in VOC,10 yet when applied as a donor the reverse was true for both PHJ and BHJ devices. In PHJ devices, VOCs were 0.81 ± 0.01 V, 0.83 ± 0.01 V, and 0.88 ± 0.01 V, and in BHJ devices, VOCs were 0.77 ± 0.02 V, 0.74 ± 0.02 V, and 0.86 ± 0.03, for literature-, commercial- and nitrobenzene- Cl-ClnBsubNc devices respectively. This effect is likely due to the shift in energy levels, wherein the highest occupied molecular orbital (HOMO) is pushed farther from the vacuum energy level with increasing chlorination. This is evident in the ultraviolet photoelectron spectroscopy measurements shown in Table 1, and is consistent with the modeling results in Figure 5. This trend would therefore be expected, and is consistent with the idea that VOC is, to a first approximation, determined by the electronic gap between an electron donating material and an electron accepting material.16-17 However, this is not the only item to consider. The difference in energy levels between PC70BM and C70 does not explain the degree of VOC shift between the PHJ and BHJ donor devices. Page 14 of 27 ACS Paragon Plus Environment

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Because PC70BM has a shallower LUMO level, a greater electronic gap should be present. An example of the expected behaviour, where greater electronic gap correlates with higher VOCs, can be observed in the work of Gasparini et al., who compared C70 and PC70BM BHJs and found superior VOCs from the PC70BM devices.18 We believe our unexpected results are due to differences in molecular orientation, which have been shown to cause VOC shifts when molecules such as poly(3-hexylthiophene) and α-sexithiophene are used as donors with fullerene acceptors.19-20 It is possible that a similar effect is responsible for the lower BHJ VOCs measured here, where the high interfacial density of a BHJ device would disrupt Cl-ClnBsubNc’s crystal structure and local packing through abundant PC70BM:Cl-ClnBsubNc interactions. For example, BsubPcs are known to associate with C60 through direct interactions of the concave side with the surface of the spherical C60.

Short-circuit Current (Jsc) For PHJ devices, nitrobenzene-Cl-ClnBsubNc had the lowest JSC, with commercial- and literature-Cl-ClnBsubNc JSC values being indistinguishable. This effect is most likely caused by the same phenomena that drives the VOC trend described above: as bay chlorination increases, the HOMO energy level of the donor deepens, but the HOMO-HOMO and LUMO-LUMO differences between the donor and acceptor are simultaneously reduced. This decreases the energy loss when dissociating an exciton, which reduces the thermodynamic favourability of dissociation taking place. As a result, the JSC of the device is reduced, though the current flux of the nitrobenzene-Cl-ClnBsubNcs OPVs at their maximum power point is very close to those of the less-chlorinated Cl-ClnBsubNcs. Current fluxes at the maximum power point for the nitrobenzene-, commercial-, and literature-Cl-ClnBsubNcs respectively were -3.78 ± 0.22 mA/cm2, 3.91 ± 0.41, and 3.92 ± 0.46, which are within error of each other. Page 15 of 27 ACS Paragon Plus Environment


There is a notable difference between the EQE’s of the BHJ devices and those of the PHJ devices, which is a small peak at roughly 345 nm. This is due to a peak in the absorption spectra of PC70BM that is not present in that of C7021 which also contributes to Jsc.

Fill Factor The fill factor of the PHJ donor devices improved with increased bay chlorination, in addition to the VOC increase, with values of 0.41 ± 0.05, 0.43 ± 0.02, and 0.48 ± 0.02 for the literature-, commercial- and nitrobenzene-Cl-ClnBsubNc sources respectively. While the difference between the similarly-chlorinated literature- and commercial-Cl-ClnBsubNc (1.21 and 1.61 bay chlorines respectively) are not statistically significant (P = 0.064 rather than a desired P < 0.050 in a twotailed t-test), the differences between these samples and the more-chlorinated nitrobenzene-ClClnBsubNc (4.17 bay chlorines) are pronounced (P < 0.001). This is despite the expectation that increasing the electronic gap in the device would typically decrease the energy potential associated with dissociation of the exciton, rendering it less thermodynamically favourable. This same trend of increasing fill factor was also observed when Cl-ClnBsubNcs were applied as acceptors.10 While the field’s wisdom in the past has been that all impurities form trap states,22-23 work carried out by Street et al. on fullerene derivatives,24-25 our own study on mixed alloy compositions of μ-oxo-(BsubPc)2 dimers and Cl-BsubPc,26 results from Fleetham et al. on zinc phthalocyanines,12-13 and indeed the prior efforts by Dang et al.,10 are contributing to a consensus that mixed alloy compositions can have beneficial effects on OPV performance. In particular, we have shown that chlorination of Cl-ClnBsubNc molecules in the bay position impacts the electronic properties, but does not alter molecular shape enough to impact crystalline ordering.10


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In contrast, the fill factor trend was not present in the BHJ devices, whose mean values fell in the low range of 0.33-0.35. We can infer meaning from this by comparing the fundamental properties of BHJ and PHJ devices. Speaking generally, in a PHJ device, excitons and dissociated charges must both travel similar distances – the excitons must reach the interface to dissociate, and the separated charges must travel from the interface to the electrodes. In a BHJ device, excitons are formed much closer to dissociating interfaces, while dissociated charges follow a tortuous path to escape the active region. Seeing an improving trend with chlorination in a device type with longer exciton diffusion pathways and shorter charge transport pathways, and no such trend in a device type with short exciton and long charge transport paths, implies that exciton transport has been affected. Identifying the cause of this requires a deeper analysis of the differences between the materials. According to the Smoluchowski-Einstein theory of random walks, the diffusion length of an exciton in a solid is equal to:27

= =

√

, where

=

!"

!" #!$"

where ΦPL is the photoluminescence quantum yield, κ is the orientation factor, n is the spectrally weighted refractive index, τf/τ0 is the lifetime ratio and J is the degree to which the solid’s absorption and emission spectra overlap. κ and n are not expected to have changed due to the negligible effect of chlorination on crystal structure.10 J is also unlikely to be responsible: the degrees of Stokes shifts shown in Table 1, smaller values of which correspond to greater spectral overlap and therefore greater diffusion length, do not form a trend which would explain the device results. The trend of increasing ΦPL, which is the fraction of radiative exciton relaxation Page 17 of 27 ACS Paragon Plus Environment


over all forms of exciton relaxation, runs directly opposite to the fill factor trend in this study, as well as that of our prior work by Dang et al.10 An increasing ΦPL suggests that the rate of radiative relaxation is decreased, or that the rate of nonradiative relaxation is increased. Given our positive results, the former is likely dominant. Thus, a possible explanation is that a decreased rate of fluorescent emission from more-chlorinated Cl-ClnBsubNc compounds is responsible for the fill factor trend, leading to an increase in exciton lifetime. A gap in support for this hypothesis is that ΦPL only describes the ratio of fluorescence to nonradiative relaxation, rather than the actual rate, so further studies such as transient absorption spectroscopy have been initiated in order to determine the true rate and compare it to exciton lifetimes. A more probable explanation extends from the assumption that the difference in energy levels between variously individual chlorinated Cl-ClnBsubNcs is less substantial at high levels of chlorination than it is at lower levels. While no study has been directly conducted on this to our knowledge, it is consistent with our modeling results illustrated above. A systematic consideration of chlorination of Cl-ClnBsubNcs is also underway, through the development of synthetic pathways to defined Cl-ClnBsubNcs. This theory implies that a more chlorinated mixture like the nitrobenzene-Cl-ClnBsubNc would have a narrower density of states, since the various molecular energy levels present would be more similar. A narrower density of states can be associated with superior exciton transport, since more states are accessible to the exciton as it follows a thermally-induced hopping transport mode. This theory is also supported by our prior work, where we found that commercial- and literature-Cl-ClnBsubNc mixed alloyed compositions both contained a fraction of Cl-ClnBsubNc compounds with no bay chlorines (i.e. n=0), while nitrobenzene-Cl-ClnBsubNc Page 18 of 27 ACS Paragon Plus Environment

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compositions contained only Cl-ClnBsubNc compounds with one or more bay chlorines (i.e. n>0). 10 A comparison can also be drawn to other studies on phthalocyanines within OPVs. Fleetham et al. demonstrate a 0.20 V increase in the VOC of zinc phthalocyanines (ZnPc)-containing OPVs when switching from unchlorinated ZnPcs to mixtures with zero, one or two chlorines per molecule.12 Cnops et al. demonstrate VOC shifts of 0.07 V, 0.10 V, 0.08 V and 0.21 V, depending on the choice of donor, when switching from a Cl-Cl4BsubPc to a Cl-Cl6BsubPc acceptor layer.28 These results also suggest that each subsequent chlorine added to a phthalocyanines-type material has a diminishing effect on its energy level. Notably, Gommans et al. determined the diffusion length of an exciton in Cl-BsubPc films to be 28 nm and the time to fluorescent relaxation to be 0.3 ns.29 If Cl-ClnBsubNc has similar properties, it would be unlikely that exciton transport efficiency is to blame with our 7 nm thick films. However, during our PHJ device structure optimization, a variety of substantial tweaks to device structure were made in order to try to improve a low fill factor. Addition or removal of a hole injection layer, changing C70 layer thickness, and altering electron transport layer thickness all failed to improve the fill factor. Reducing the Cl-ClnBsubNc layer thickness from 10 nm to 7 nm improved fill factor from 0.32 to 0.44 with no sacrifices to other device parameters. These results, illustrated in Figure S1, suggests a short exciton diffusion length, which is then mitigated by increasing levels of chlorination. A full space charge limiting current study could be undertaken to investigate this further. Another possibility is that the increase in fill factor is simply due to an energy level alignment between donor and acceptor that is more thermodynamically favourable for exciton dissociation; were this the case, however, it would be Page 19 of 27 ACS Paragon Plus Environment


highly unintuitive to see a simultaneous increase in VOC, which our data shows. Lastly, imbalanced carrier mobility in a device can impact fill factor,30 but if such a phenomenon was responsible for the PHJ fill factor shift seen here, we would expect the BHJs to be more affected than the PHJs due to the tortuous path that their dissociated charges must take.

Conclusions In this work, we have demonstrated the usage of various alloyed mixtures of Cl-ClnBsubNcs as electron donating materials in PHJ and BHJ device structures. We have confirmed that the VOC of the PHJ OPVs is largely dependent on electronic gap between Cl-ClnBsubNcs and the paired electron acceptors in this work or electron donors in our previous work. The BHJ devices follow a similar trend albeit in a lower range, which is possibly due to differences in interfacial molecular orientation between Cl-ClnBsubNcs/PC70BM and Cl-ClnBsubNcs/C70. Through comparison of the device structures presented here, and our previous work on ClClnBsubNc acceptor-based devices, we have surmised that the degree of chlorination of ClClnBsubNcs exerts a substantial influence on exciton diffusion length, which in turn improves device fill factor. We propose two potential causes: first, that decreased photoluminescence is reducing radiative relaxation of excitons before dissociation, or second, that a narrower density of states for higher levels of chlorination provided by the mixed alloy compositions is more amenable to exciton transport via thermal hopping. If the latter is the cause, it suggests that mixed alloyed compositions of more energetically similar molecules will improve exciton transport. If the former is correct, it suggests that bay chlorination of BsubNcs can suppress radiative relaxation. Further investigation is needed to elucidate the precise nature of this beneficial phenomenon, which is underway. In either case, we recommend that similar effects be Page 20 of 27 ACS Paragon Plus Environment

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sought for in other small molecules applied in OPVs. Specific to the use of Cl-ClnBsubNcs in bilayer structures, we advise that these versatile molecules be applied as electron donors/transporting materials, so that the improvements to VOC and fill factor can be enjoyed simultaneously. In more complex structures such as the 8.4% efficient cell developed by Cnops et al.,7 fill factor and therefore performance could be enhanced through use of a more chlorinated Cl-ClnBsubNc interlayer.

Associated Content The Electronic Supporting Information contains the Experimental Methods ; JV plots for OPV devices fabricated during the optimization process and the chemical structures of Cl-ClnBsubNcs used to prepare the semi empirical model. This material is available free of charge via the Internet at http://pubs.acs.org.

Funding Sources This research was supported by the Canadian National Sciences and Engineering Research Council (NSERC) through a Discovery Grant (T.P.B.) and by a provincial Ontario Graduate Scholarship awarded to R.K.G..



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TOC Graphic:



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The mixed alloyed chemical composition of chloro-(chloro)nboron

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