Structure-property relationship of anilino-squaraines in organic solar cells.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 1067

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Structure–property relationship of anilino-squaraines in organic solar cells† a a b b a c d S. Bru ¨ ck, C. Krause, R. Turrisi, L. Beverina, S. Wilken, W. Saak, A. Lu ¨ tzen, a a a H. Borchert, M. Schiek* and J. Parisi

Soluble molecular semiconductors are a promising alternative to semiconducting polymers in the field of organic photovoltaics. Here, three custom-made symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxy-anilino)squaraines containing systematic variations in their molecular structures are compared regarding their applicability as donor materials in bulk-heterojunction solar cells. The terminal substitution pattern of the squaraines is varied from cyclic over linear to branched including a stereogenic center. Single crystal structures are determined, and, in the case of chiral squaraine, unusual formation of stereoisomer Received 2nd October 2013, Accepted 7th November 2013

co-crystals is revealed. The thin film absorbance spectra show characteristic signatures of H- and J-bands

DOI: 10.1039/c3cp54163k

applications is studied by light-induced electron spin resonance spectroscopy. The impact of the different molecular substitution patterns on aggregation behavior and, consequently, their optoelectronic solid state

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properties including charge carrier mobility and finally the solar cell performance are investigated.

or hint at the formation of tautomers. The general feasibility of these model compounds for photovoltaic

1. Introduction Common organic solar cells are solution-processed bulkheterojunction (BHJ) devices consisting of a randomly interpenetrating network of a polymeric donor material and a fullerene acceptor.1 Mostly, this originates from the entropy-driven spontaneous nanoscale phase separation of molecular materials mixed with polymeric ones. Unfortunately, the performance of polymeric materials depends on, e.g., their regio-regularity and polydispersity, making their synthesis and purification difficult and scarcely reproducible. In contrast, small molecules are intrinsically monodisperse, straightforward to synthesize and to purify. Moreover, the vast number of structural variations around a common parent small

a

Institute of Physics, Energy and Semiconductor Research Laboratory, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany. E-mail: [email protected]; Fax: +49 441 798 3326; Tel: +49 441 798 3988 b Department of Materials Chemistry and INSTM, University of Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy c Institute of Pure and Applied Chemistry, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany d Kekule´-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, Rheinische Friedrich-Wilhelms-University of Bonn, D-53121 Bonn, Germany † Electronic supplementary information (ESI) available: XRD powder pattern of SQ1, absorbance and fluorescence maxima of SQs in solution, infrared absorption spectra of the tautomeric SQ1 compound, UV-Vis absorbance spectra of SQ:PC70BM thin films, list of photovoltaic key parameters for various mixing ratios and annealing temperatures, hole-only devices and determination of charge carrier mobility. CCDC 962720 and 947470. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cp54163k

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molecule scaffold enables the formulation of structure–property relationships, not depending on the particular batch of available material. Consequently, they are well on their way to outperform polymers in photovoltaic applications.2 Amongst the various classes of derivatives proposed for that purpose, squaraine dyes play an important role as they are, in general, environmentally stable and non-toxic small molecule organic semiconductors offering, apart from photovoltaics, a broad application potential in organic electronics and photonics.3 They show sharp and intense absorption as well as fluorescence in solutions within the deep-red visible to near-infrared range. Upon aggregation, the emission is efficiently quenched and the absorption broadened as a consequence of strong charge-transfer interactions of the resonance stabilized zwitterionic molecules. They have a pronounced tendency to form H- and J-aggregates, evidenced by blue and red-shifted absorption bands, respectively, relative to the monomer absorption.4 Originally being considered in Schottky junction solar cells, they have been shown to function as donor materials in solution processed BHJ solar cells.5 Especially, the 1,3-bis(N,N-substituted2,6-dihydroxy-anilino)squaraines are versatile because they are eligible for solution processing6 as well as vapour deposition, that way scoring efficiencies of above 6%.7 The 1,3-bis(N,N-di-isobutyl2,6-dihydroxy-anilino)squaraine, which we refer to as SQIB later, is the most investigated representative so far, and is already commercially available. These symmetric 1,3 target compounds are accessed conveniently by di-condensation of activated electron-rich aromatics with squaric acid in azeotropic solvent mixtures. The activated N,N-substituted 3,5-dihydroxyanilines are obtained by condensation of the respective secondary

Phys. Chem. Chem. Phys., 2014, 16, 1067--1077 | 1067

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Fig. 1 (a) Structural formulae of SQ1, SQ2, and SQ3. (b) BHJ device architecture.

amines with 1,3,5-trihydroxybenzene.8 Hence they are dyes with a schematic donor–acceptor–donor (D–A–D) type structure. Against this background, the aim of the present work was to study the impact of different terminal functionalization of squaraines on the establishment of molecular order and, related to the structural properties, on the suitability for their optoelectronic application in solar cells. Therefore, three different symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxyanilino)squaraines were synthesized, only differing in their N,N-alkylation. Their substitution pattern was varied from cyclic: 1,3-bis[4-(piperidyl)2,6-dihydroxyphenyl]squaraine (SQ1) over linear: 1,3-bis[4-(N,N-din-hexylamino)-2,6-dihydroxyphenyl]-squaraine (SQ2) to branched including a stereogenic center: 1,3-bis[4-(N-sec-butyl-N-n-propylamino)-2,6-dihydroxyphenyl] squaraine (SQ3) (Fig. 1a). Here, chirality adds another directing element for assembly and, therefore, tuning solid state optoelectronic properties.9 Structural properties of the different SQs were investigated by a combination of X-ray diffraction (XRD) experiments and UV-Vis absorption spectroscopy, the latter being able to study solid state molecular interactions, e.g., H- and J-type aggregation and formation of tautomers. Basic material properties related to photovoltaic applications were studied by charge carrier mobility measurements in hole-only devices and light-induced electron spin resonance (L-ESR). This method allows for studying charge separation in donor–acceptor blends upon illumination by detection of charge carriers having unpaired spins.10 Therefore, it can be used to evaluate the suitability of the SQs as donor materials in combination with fullerene acceptors independent of device architecture. Finally, for the preparation of solar cells, spin-coated squaraine:fullerene blends were sandwiched between a poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) coated indium-tin-oxide (ITO) anode and a thermally deposited aluminum cathode, Fig. 1b. Two different commercial fullerenes, namely, [6,6]-phenyl-C61butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), were used for variation.

2. Results and discussion 2.1.

Synthesis

The three target compounds 1,3-bis[4-(piperidyl)-2,6-dihydroxyphenyl]squaraine (SQ1), 1,3-bis[4-(N,N-di-n-hexylamino)-2,6dihydroxyphenyl]-squaraine (SQ2), and 1,3-bis[4-(N-sec-butyl-N-

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Fig. 2

Schematic synthetic procedure for SQ1, SQ2, and SQ3.

n-propylamino)-2,6-dihydroxyphenyl] squaraine (SQ3) feature different substitution patterns at the nitrogen atoms. In particular, we considered the influence of a cyclic (SQ1) versus a linear (SQ2) chain. We also compared the effect on solid state properties of a linear versus a branched (SQ3) chain. We chose a branched chain possessing a stereogenic center here, in order to also explore possible consequences of chirality on the solid state packing. All squaraines were prepared by adapting literature protocols.8 A general reaction scheme is shown in Fig. 2. Nucleophilic substitution of 1,3,5-trihydroxybenzene with secondary amines in an azeotropic solvent mixture gave the corresponding activated aniline intermediates 1, 2, and 3, respectively. In the case of squaraines SQ1 and SQ2, the intermediates 1 and 2 were not isolated, but squaric acid was added directly to the reaction mixture to give the desired symmetric 1,3-bis(N,N-alkylated-2,6dihydroxyanilino)squaraines. In the case of SQ3, however, we adopted a stepwise protocol.8b Hence, intermediate (rac)-3 was isolated and purified and then condensed with squaric acid in an azeotropic solvent system. In contrast to the literature procedure, however, we used a lower boiling solvent system (toluene with n-propanol instead of n-butanol) in order to avoid the formation of the 1,2-condensation product. In this way, SQ3 was obtained as a mixture of three stereoisomers – the racemic pair of (S,S)- and (R,R)-configurated enantiomers and the (R,S)configurated meso-diastereomer – in a yield of 37% after crystallization from methanol. 2.2.

Crystallography of SQ2 and SQ3

SQ2 had a strong tendency to crystallize forming basically fiberlike crystals, which were so soft and flexible that they bent or coiled up during handling. Single crystals suitable for X-ray diffraction analysis could be obtained from a chlorobenzene solution by slow evaporation of the solvent. They were blue-green with a metallic golden luster. The crystal system is triclinic with the space group P1% . The unit cell parameters are: a = 5.097(2) Å, b = 10.746(5) Å, c = 16.604(7) Å, a = 96.374(10)1, b = 94.825(11)1, and g = 97.872(11)1. Fig. 3a shows the packing motif of SQ2. The molecular backbone is planarized by intramolecular hydrogen bonds from the hydroxy groups of the phenyl rings to the oxygen


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measured signal from the powder and the thin films was shifted towards smaller scattering angles by 0.51 compared to the 2y value of the single crystal, which can be calculated to be 5.41. The d-spacing of the (001)-planes is increased by about 10% from 16.6 Å to 18.1 Å. This hints at elongation of the unit cell in the c-dimension for powder and thin films due to disorder of the hexyl chains. The inset in Fig. 3b shows the preferential orientation of the SQ2 molecules assuming single crystal structure with respect to the substrate, as evident by the occurrence of only the (001) Bragg reflection. Crystals of SQ3 were grown from a chloroform solution by slow evaporation of the solvent. They were grass-green with a metallic golden luster. The crystal system is monoclinic with the space group P2(1)/c. The unit cell parameters are: a = 5.6706(2) Å, b = 16.8421(7) Å, c = 14.3536(6) Å, and b = 97.943(2)1. Interestingly, all three stereoisomers of SQ3 were found to co-crystallize within these single crystals. This resulted in a displacement of the aniline-nitrogen from co-planarity with the phenyl ring. The reason for this remarkable behavior might be a small difference in the alkyl groups (ethyl vs. methyl) at the stereogenic centers. This difference is obviously small enough to tolerate all stereoisomers within the same crystal packing, and does not enforce stereoseparation of the diastereoisomers or the enantiomers. Fig. 4a shows the packing motif of SQ3.

Fig. 3 (a) Crystalline packing of SQ2. (b) XRD-patterns of SQ2 powder, as well as cast films of neat SQ2 and SQ2 : PC60BM blends. The inset shows the molecular orientation relative to the substrate assuming bulk structure packing.

of the squaric core. The molecules adopt a face-to-face slipped p-stacking, so that donor-type aniline moieties interact with acceptor-type squaric cores from adjacent molecules. All molecular backbones are parallel to each other over the entire crystal, so that there is only one stacking direction making the crystal highly anisotropic. The hexyl chains are well ordered and are pointing upwards on one side and downwards on the other side. In addition to the single crystal studies, X-ray diffraction of powders and solution-cast thin films was applied. No true XRD powder pattern of SQ2 could be obtained due to its strong tendency to assemble in a preferential order even on the glass substrate used for the XRD measurements. The pattern was dominated by a peak at an angle of 2y = 4.91, Fig. 3b. The XRD patterns of both neat SQ2 and SQ2:fullerene blended films prepared by spin-coating showed a signal at the same position. The signal intensity for the neat films was found to be almost independent of annealing temperature. For the blended films, the signal was broadened compared to the neat film suggesting smaller crystalline domains. The signal intensity increased with increasing annealing temperature, which can be explained by a gain in crystallinity. The peak position is assigned as a (001)peak according to the calculated powder pattern. Note that the


Fig. 4 (a) Crystalline packing of SQ3. (b) Measured and calculated XRDpattern of SQ3 powder.


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Again, the hydroxy groups of the phenyl rings form intramolecular hydrogen bonds with the oxygen of the squaric core ensuring co-planarity of the phenyl rings and the squaric core. The face-to-face slipped p-stacking allows donor–acceptor– moiety interaction between neighboring molecules. However, the stacks are tilted towards each other in this case so that the anilino alkyl groups from one stack point towards the squaric cores from another stack. Interestingly, the unit cell and the packing motif are very similar to those of SQIB,11 so it is considered as typical for anilino SQs with a bulky and branched substitution pattern. Furthermore, the experimental powder XRD pattern of the stereoisomeric mixture of SQ3 matched the calculated pattern from the single crystal structure very well, Fig. 4b. This revealed that there was only one noticeable (micro-) crystalline phase within the powder and that the SQ3 molecules adopted the same packing in the powder as in the single crystal structure. Hence, the tendency for phase separation of the different stereoisomers was truly low. Usually, mixtures of diastereomers tend at least to separate during crystallization. In fact, this rather unusual behavior turned out to be rather beneficial in our case, since we employed the powder of the stereoisomeric mixture in our device studies (see below). The common phase separation observed for racemic mixtures upon solidification could have led to inhomogeneous films which would have hampered device performance. XRD patterns of neat SQ3 and SQ3:fullerene blended films could not be obtained. This indicated the lack of long-range crystalline order, and hinted at a good intermixing of the blended films, and it was also in accordance with the formation of homogenous and smooth films. Nevertheless, control of the stereochemistry of these dye molecules is still a promising approach to achieve tailored aggregation, and hence, solid state properties because diastereomerically or enantiomerically pure compounds can be expected to show a deviating aggregation behavior leading to a different film morphology and, consequently, device performance. However, this will be addressed in future work. No crystals could be grown from SQ1, nor could XRDpatterns of cast films be obtained. However, the powder pattern of SQ1 is shown in Fig. S1 in the ESI.† 2.3.

UV-Vis absorption spectra

All squaraines showed strong and sharp absorption as well as emission in solution. The absorbance spectra in chloroform solution are shown in Fig. 5, and the peak positions of both absorbance and fluorescence are listed in Table S1 (ESI†). All films were spin-coated on glass from the same chlorobenzene solutions as used for the solar cell preparation later. The films were dried under ambient conditions and not subjected to any thermal annealing. All UV/Vis absorption spectra are plotted in Fig. 5. For the blended films, the mixtures containing PC60BM were chosen to be depicted here because the absorbance of PC60BM does not overlap with the squaraine absorbance as in the case of PC70BM. The latter is shown in Fig. S2 (ESI†). In general, the absorption of the cast films was substantially


Fig. 5 Absorbance spectra from spin-coated films of neat SQs and their blends with PC60BM in various mixing ratios as denoted in the graphics. (a) SQ1, (b) SQ2, and (c) SQ3. The dashed lines display the respective absorbance spectra in highly diluted chloroform solution, which are rescaled for readability.

broadened due to strong molecular interactions compared to the absorbance in solutions. The absorbance of a neat SQ1 film, Fig. 5a, was very faint compared to the other derivatives, because the layer was thin and discontinuous. The SQ1 powder, however, had an intense royal blue color. A thin powder layer was coated onto a glass slide to record the spectrum, which has been rescaled in the graph. Two absorbance maxima were visible, resembling H- and J-bands similar to the SQ2 thin film spectra (see discussion below). Conversely, the cast SQ1:PC60BM blend was dominated by the absorbance of PC60BM. Such an unusual and unexpected


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absorption spectrum could be the consequence of the formation of the tautomeric carbonyl form SQ1(t), shown in Fig. S3 (ESI†). Such a form would have a completely different and likely blue shifted absorption spectrum. The solution UV-Vis absorption as well as the solution NMR characterization of SQ1 did not show such a structure, yet, the situation in the solid state appears to be different. Indeed, Fig. S3 (ESI†) shows that the ATR-FTIR spectra of SQ1 thin films cast from chlorobenzene and chloroform solutions are different. As no chemical degradation was involved, one must assume that the solid-state structure adopted by SQ1 depends upon the solvent used in the deposition. The occurrence of such a tautomeric equilibrium has been previously postulated in the literature.12 However, this seems to be a property of the SQ1 making it unsuitable for our devices, in addition to the pronounced tendency for selfaggregation, which is discussed below. The SQ2 showed two distinct absorbance maxima (Fig. 5b) with a spectral blue- as well as a red-shift compared to the absorbance in solution, which can be assigned as H- and J-bands, respectively. They are determined by the interaction of transition dipole moments of molecular dimers, where H-bands arise from a vertical stacking motif (side-by-side) and J-bands from a head-to-tail arrangement. The coexistence of both signatures in the absorbance spectrum does not necessarily hint at two different crystal phases. In the case of an oblique arrangement of the transition dipoles, both absorption bands are allowed simultaneously.4d,13 The ratio of the H- and J-type peaks was clearly dependent on the mixing ratio of SQ2 and PC60BM. With increasing amounts of PC60BM, the J-type peak became more pronounced, while the H-type peaks diminished. In addition, the J-band showed a pronounced red-shift with increasing PC60BM content, while the H-band is almost unaffected. For higher PC60BM shares, e.g., 1 : 7, no further changes in the spectrum could be seen, thus, it is not plotted for better readability. The peak ratio H-band to J-band and the respective peak positions are listed in Table 1. Recently, the absorbance of an anilino squaraine carrying isopentyl side chains has been discussed,4d showing H- and J-aggregate signatures in the absorbance spectrum similar to the case of SQ2. Since they could not find evidence for two different crystal phases either, they used a similar explanation for the origin of the simultaneous appearance of both spectral features. Different from our spectra, however, they saw another

Table 1 Absorbance properties of SQ2 and SQ3 neat and blended film with PC60BM

SQ2

SQ3

Mixing ratio [weight%]

H-peak [nm]

J-peak [nm]

Peak intensity ratio [H/J]

J-peak [nm]

Neat 1:1 1:2 1:3 1:5 1:7

561 564 564 564 561 560

655 657 674 676 677 677

1.6 1.4 0.8 0.7 0.5 0.5

699 696 696 688 682 681


peak which they related to monomer absorption. However, for the neat SQ2 film, the J-band could also be interpreted as a monomer band, since the red-shift is very little here. Furthermore, mixing with PC60BM disrupted the formation of J-types for their squaraine, while we witnessed an opposite trend for the SQ2. For the neat and blended SQ3 films, the absorbance was dominated by J-bands, Fig. 5c, quite similar to the absorbance of SQIB.6 This could be due to increased steric repulsion of the branched terminal groups, compared to the linear terminal alkyl chains of SQ2, driving the molecules into slightly different packing motifs, which can be seen in the crystal structures. For the neat SQ3 film, a broad shoulder was visible, lying in the spectral region of a possible H-band. This shoulder diminished for increasing shares of PC60BM. The J-band peak position is also affected by the SQ3 : PC60BM mixing ratio but shows a blue-shift in contrast to the red-shift for SQ2. Peak positions are listed in Table 1. 2.4. Light induced electron spin resonance (L-ESR) spectroscopy The general feasibility of the squaraines as donor materials together with a fullerene acceptor in photovoltaic applications was evaluated by L-ESR measurements. This method gives experimental evidence for photoinduced charge separation in an active layer blend independent of the prevailing solar cell device architecture.10 ESR, in general, detects unpaired electrons, which here stem from radical anions on the fullerene acceptor. These species are generated due to splitting of photogenerated excitons at the donor acceptor interface. Note that the photogenerated singlet excitons cannot be detected. Charge transfer in 1 : 3 mixing ratio SQ:PC60BM blends was investigated by the following procedure: first, a spectrum without illumination was measured (dark spectrum), then the light excitation was switched on, and the excited spectrum was recorded. After cessation of light excitation, the decay dynamics were monitored for 2500 seconds, directly followed by the acquisition of the relaxed spectrum. Fig. 6 displays ESR spectra in the dark, under illumination, and after relaxation of all three squaraine blends at 30 K. The blends did not show any signal in the dark, proving that the sample is fully relaxed, and no persistent signal of trapped charge carriers was present. The excited spectra of all blends showed a pronounced signal corresponding to fullerene anions in the PC60BM10a upon excitation with laser light (660 nm). Such signals usually arise from the splitting of photogenerated excitons at donor–acceptor interfaces. Since the neat films of either squaraine or PC60BM did not give any signal, the PC60BM signals of the blends were a consequence of an electron transfer from the squaraine to the PC60BM. Excitons were created in the squaraine due to laser excitation followed by splitting at the SQ:PC60BM interface. For SQ2 and SQ3 blends, only the PC60BM radical anion signal was visible (Fig. 6b and c) respectively, while a distinct signal was recorded for the SQ1 blend at a lower magnetic field (Fig. 6a). This signal is probably the superposition of two signals with g-factors in the range of


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Fig. 7 SEM images from spin-coated SQ:PC60BM blends with optimum mixing ratio and annealing temperature regarding device performance. (a) SQ1:PC60BM 1 : 3 annealed at 110 1C. (b) SQ2:PC60BM 1 : 5 annealed at 50 1C. (c) SQ3:PC60BM 1 : 3 without annealing.

excitation was switched off. For all squaraine blends the decay dynamics comprised two components, a prompt decay followed by a slow decay. Similar decay dynamics have been observed for poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene] (MDMO-PPV):PCBM10a and poly(3-hexylthiophene-2,5-diyl) (P3HT):CdSe nanocrystal10c blends and were attributed to bimolecular recombination of mobile charges and recombination of trapped charges, respectively. The SQ1:PC60BM blend showed only a weak prompt decay, followed by a slow decay that saturates at 80% of the excited level, indicating that a large amount of the charges remained trapped. However, the SQ2 and SQ3 blends showed much more pronounced prompt decays and steeper slow decays, which suggest that charge trapping played a less important role in these blends. The relaxed spectrum of the SQ1 blend showed that the low field signal was not reduced after the recombination of the mobile charges, indicating that it stemmed from trapped charges. Fig. 6 Light induced electron spin resonance spectra of 1 : 3 blends of PC60BM with (a) SQ1, (b) SQ2, and (c) SQ3 at 30 K. The insets show the respective decay dynamics of the PC60BM signal normalized to the intensity under illumination. Laser excitation was turned off at t = 0 seconds.

2.00 which could originate from localized charge carriers in SQ1 domains. This appears reasonable when taking into account that the films contained extended domains of aggregated SQ1 (see also Fig. 7). To get deeper insight into the charge carrier dynamics the decay of charges after cessation of laser excitation was recorded. The insets in Fig. 6 show the decay dynamics of the [PC60BM] signal of the respective squaraine blends, normalized to the maximum intensity under illumination. The magnetic field was set to the maximum of the [PC60BM] signal, and the ESR signal intensity was monitored over time. At t = 0 the laser


2.5.

Bulk heterojunction junction (BHJ) solar cells

The solar cells had the layer sequence ITO/PEDOT:PSS/SQ:PCBM/Al, which is depicted in Fig. 1b. This architecture is not optimized in a sense that for convenience standard electrode materials and interfacial layers were used. Especially, PEDOT:PSS is suspected to be not the ideal choice to achieve highest performance.6d, f,g However, this is not the scope of this paper which is rather to address the structure–property relationship of varying SQs in the active layer of a model BHJ solar cell. The active layer parameters have been optimized regarding layer thickness, annealing temperature, the choice of fullerene acceptor, i.e., PC60BM or PC70BM, and its mixing ratio with the SQs. All photovoltaic key parameters of this systematic investigation are listed in Tables S2 and S3 (ESI†). They were recorded under ambient conditions at room temperature with a simulated AM1.5G solar spectrum at 100 mW cm 2.


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SQ:fullerene blends were spin-coated from chlorobenzene (CB) in all cases. Blends could not be deposed from chloroform due to strong dewetting. The active layer thickness was 80–90 nm for the best functioning devices. Data for the optimization of the active layer thickness are given in Table S4 (ESI†). This ensured pinhole free films, and gave the best trade-off between the amount of absorbed light and charge carrier mobility. Fig. 7 displays scanning electron microscopy (SEM) images of films cast from all three SQs blended with PC60BM subjected to optimum treatment regarding performance. For SQ1:PC60BM blends, Fig. 7a, a strong phase separation was visible, which appeared to be independent of mixing ratio and annealing temperature. Consequently, the devices did not show any photovoltaic performance, not even rectifying behavior in the dark. Since the ESR measurements witnessed charge separation upon illumination, the morphology clearly hampered device performance here. The strong tendency of SQ1 to selfaggregate hindered the formation of a homogeneous nanoscaled donor–acceptor network, which represents the core of a BHJ solar cell. SQ1 possesses no long or bulky terminal groups at the amine-nitrogen atoms so the molecules interact strongly with each other forming large aggregates. Interestingly, these were apparently rather non-crystalline, since XRD performed on cast neat and blended SQ1 films showed no diffraction peaks. In the cases of SQ2 and SQ3, self-aggregation is reduced due to their more space filling terminal groups. This is also reflected in their increased solubility compared to SQ1. For SQ2 and SQ3, intermixing with the fullerene acceptor in the solid state is enabled, which allows generation of rather homogeneous films. The pattern visible in Fig. 7b suggests the formation of micro-sized domains within a SQ2:PC60BM layer, which became more pronounced at elevated annealing temperatures. SQ3:PC60BM blends, however, shown in Fig. 7c appeared to be beneficially uniform. This correlated well with the device performances: for optimized active layer parameters, the SQ3 showed a PCE of up to 1.85% while the SQ2 reached only 1.50% (Table 2). Note that PC70BM was used to achieve the highest PCE values, but the same trend regarding morphology was seen for both fullerene acceptors. For both materials, low annealing temperatures and high shares of PCBM are preferred (see also Tables S2 and S3, ESI†): 50 1C and a 1 : 5 mixing ratio by weight for SQ2,

Table 2 Comparison of the fullerene acceptors PC60BM and PC70BM. Annealing temperatures were 50 1C for SQ2 and no annealing for SQ3; AM1.5G illumination

PCXBM

Mixing ratio [weight%]

Voc [V]

Jsc [mA cm 2]

FF [%]

PCE [%]

SQ2

60 70 60 70

1:3 1:3 1:5 1:5

0.82 0.85 0.83 0.88

3.24 4.54 3.74 5.48

32 36 30 31

0.84 1.39 0.92 1.50

SQ3

60 70 60 60

1:3 1:3 1:5 1:5

0.78 0.76 0.73 0.66

5.35 7.75 6.23 7.84

38 32 32 29

1.59 1.85 1.44 1.47


and 30 1C or no annealing on 1 : 3 blends for SQ3. With increasing annealing temperatures all photovoltaic key parameters decreased. At low annealing temperature crystallization is unlikely, which often is accompanied by excessive phase separation. But evaporation of residual high boiling point solvent such as chlorobenzene in our case is promoted. All SQ2 based devices gave higher open circuit voltages than SQ3 based ones. Nevertheless, the efficiency was generally higher for SQ3 based cells than for SQ2, which was due to higher photocurrents. In general, the photocurrent could be increased by exchanging PC60BM with PC70BM, which is possibly due to the extended absorption of PC70BM within the visible range. Absorption spectra of SQ:PC70BM blends are presented in Fig. S2 (ESI†). The trends for voltage and photocurrent can also be correlated to the UV/Vis absorbance spectra, i.e. with the spectral signatures of H- and J-type bands. Although H-aggregates have a relatively long exciton diffusion length,14 photoinduced intermolecular charge transfer is favorably enhanced for J-aggregates.4f,h,14 In our case, the predominant J-type aggregation for SQ3 based blends correlated with increased photocurrent compared to the mixed type aggregation behavior of SQ2 blends. For the latter, photocurrent was highest for a mixing ratio giving the greatest share of J-type aggregation. On the other hand H-type aggregates exhibit a deeper lying HOMO level relative to the J-type aggregates, which affects the open circuit voltage.15 Indeed, the SQ2 based devices showed a higher open circuit voltage (see Table 2), which was presumably promoted by the H-type aggregates. Typical J–V curves are plotted in Fig. 8. The low fill factor dominates their shape, which was actually the weak spot of the SQ solar cells. It was averaging only around 30–40%. Standard polymeric BHJ solar cells employing, e.g., poly(3-hexylthiophene2,5-diyl) (P3HT) and PC60BM, usually have a fill factor value ranging between 60 and 70%.1 Low fill factors are typical for solar cells with a space charge limited photocurrent, which arises from unbalanced charge carrier mobility of donor and acceptor material. It has been reported, that in case the charge carrier mobility of donor and acceptor deviates by two or more orders of magnitude, this leads to a fundamental electrostatic limit resulting in the build up of space charges. Theoretical considerations by Mihailetchi et al.16 predicted that the fill factor is limited to a maximum of 42% for space-charge-limited photocurrents that follow a V0.5 dependence on the applied voltage. The effective hole mobility mh for SQ2 and SQ3 was extracted from the J–V characteristics of hole-only devices by the SCLC model.17 Experimental details and applied theory are documented in the ESI.† Note that the estimative character of the experiment rules the quality of the data more strongly than the experimental error. The effective hole mobility mh was basically the same for SQ2 and SQ3, being roughly 5 10 6 cm2 V 1 s 1 and 6 10 6 cm2 V 1 s 1, respectively. This is low compared to the standard polymeric material P3HT, which showed values of around 3 10 4 cm2 V 1 s 1.18 Electron mobility for PC60BM and PC70BM have been documented to be 3 10 3 cm2 V 1 s 1,18 and 9 10 4 cm2 V 1 s 1,6d respectively. Indeed, this gives a difference


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they all could be classified as potentially suitable donor materials in organic solar cells. In the case of SQ1, however, tautomerization and especially the tendency to agglomerate (probably supported by the formation of tautomers) was too high due to the least space-filling cyclic substitution, so no layers suitable for thin film devices could be obtained. SQ2 and SQ3 both showed sizeable photovoltaic performance, considering non-optimized electrodes and interfacial layers, reaching PCE values of 1.5% and 1.85%, respectively. In general, the device characteristics were dominated by the low charge carrier mobility of the SQs, which resulted in space-charge-limited photocurrents appearing as low fill factors. The packing motifs of the materials and the corresponding optical absorbance could be correlated with photocurrent and open circuit voltage of the solar cells. SQ3 could be identified as the ‘‘high current’’ material in the present study due to the prevailing appearance of J-band spectral features. The additional H-type contribution in the case of SQ2 made this compound the ‘‘high voltage’’ material. This demonstrates not only the versatility of squaraines in photovoltaic applications, but shows also the possibility of controlling the device performance by simple structural tuning of the molecular building blocks. SQ3 also demonstrated that stereogenic centers, stereoisomerism, and chirality are additional directing parameters for tailored aggregation that can be utilized as further fine-tuning parameters for customized performance, which will be studied in due course. Fig. 8 J–V-plots in the dark and under 1 sun illumination of devices from (a) SQ2 blended with PC60BM and PC70BM, mixing ratio 1 : 5, annealed at 50 1C and (b) SQ3 blended with PC60BM and PC70BM, mixing ratio 1 : 3, without annealing.

4. Experimental details 4.1.

in charge carrier mobility for our donor–acceptor system clearly exceeding two orders of magnitude, and the fill factor values of our devices are in good agreement with this theory. Hence, the low charge carrier mobility is concluded to be one of the major limiting factors for the solar cell performance.

3. Conclusions We successfully synthesized symmetric anilino-squaraines SQ1, SQ2, and SQ3 with varying terminal N-alkylation patterns in reasonable yields. The aggregation behavior of these SQs is determined by their terminal substitution pattern and, thus, governs the solar cell performance. Single crystal structures of SQ2 and SQ3 could be obtained revealing a slightly different packing motif. In the case of SQ3, the unusual formation of co-crystals containing all three stereoisomers was found to be the dominant crystallization type. Optical absorption measurements reflected the distinct packing behavior of SQ2 and SQ3 with pronounced H- and J-type spectral signatures. The long-chain substitution of SQ2 allowed the appearance of both H- and J-band absorbance simultaneously. For the bulky branched substitution of SQ3, the expression of an H-band was suppressed in favor of a J-band. Since light induced charge separation could be witnessed for all three SQs blended with a fullerene acceptor by L-ESR measurements,


Synthesis

1,3-bis[4-(piperidyl)-2,6-dihydroxyphenyl]squaraine (SQ1) and 1,3-bis[4-(N,N-di-n-hexylamino)-2,6-dihydroxyphenyl]-squaraine (SQ2): Derivatives SQ1 and SQ2 were prepared according to the following standard procedure: a solution of phloroglucinol dihydrate (1 equivalent) and the appropriate secondary amine (1.1 equivalent) was refluxed using a Dean–Stark trap in a 1 : 3 toluene:n-butanol mixture (2 mL for every mmol of phloroglucinol) for 12 h, and the reaction was monitored by TLC (SiO2 AcOEt). When all of the water was azeotropically distilled, nBuOH was added until the solvent mixture reached a 1 : 1 ratio, squaric acid was added (0.5 equivalents), and reflux was continued for an additional 12 h. During that time, the color turned deep blue. Then all volatiles were removed and the residue was taken up with EtOH to give a blue precipitate. The precipitate was filtered off and extracted using a Soxhlet apparatus with CHCl3. Removal of the solvent then afforded the title compounds as a powder. SQ1. 37% yield, blue powder. 1H-NMR (500 MHz, CDCl3, d): 10.97 (s, 4H), 5.93 (s, 4H), 3.55–3.50 (m, 8H), 1.72–1.70 (m, 4H), 1.67–1.64 (m, 8H); anal. calcd for C26H27N2O6: C 67.37, H 5.87, N 6.04; found: C 67.48, H 5.85, N 5.87. 13C NMR was not recorded due to the very low solubility of the compound. SQ2. 18% yield, golden green powder. 1H-NMR (500 MHz, CDCl3, d): 10.98 (s, 4H), 5.77 (s, 4H), 3.32 (t, J = 7.75 Hz, 8H),


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1.62 (quin broad, 8H), 1.32–1.30 (m, 24H), 0.90 (t, J = 6.83 Hz, 12H); 13C NMR (125.7 MHz, CDCl3, d): 182.2, 163.7, 161.9, 158.7, 103.2, 94.5, 52.5, 32.5, 28.8, 28.3, 23.5, 15.1. Anal. calcd for C40H60N2O6: C 72.25, H 9.10, N 4.21; found: C, 72.35; H 8.88, N 4.18. 5-N-sec-Butyl-N-n-propylamino-1,3-dihydroxybenzene (3). 1,3,5-Trihydroxybenzene (2 g, 15.9 mmol), N-sec-butyl-npropylamine (3.62 g, 31.8 mmol), 40 mL of toluene and 10 mL of 1-butanol were refluxed for 16 h using a Dean–Stark apparatus. The solvents were evaporated under reduced pressure. The resulting brown liquid was purified by column chromatography on silica gel with n-hexane/acetone (volume ratio 1 : 1) as the eluents to give a brown viscous liquid of 5-N-sec-butyl-N-n-propylamino-1,3-dihydroxybenzene (3), 1.3 g (37%). 1H-NMR (400 MHz, methanol-d4, d): 5.77 (s, 2H), 5.65–5.64 (m, 1H), 3.70–3.60 (m, 1H), 3.06–2.93 (m, 2H), 1.68–1.33 (m, 4H), 1.12 (d, J = 6.7 Hz, 3H), 0.91 (t, J = 7.4 Hz, 3H), 0.90 (t, J = 7.4 Hz, 3H). 1,3-bis[4-(N-sec-Butyl-N-n-propylamino)-2,6-dihydroxyphenyl]squaraine (SQ3). The aniline intermediate 3 (1 g, 4.5 mmol), squaric acid (0.26 g, 2.3 mmol), 24 mL of toluene and 6 mL of 1-propanol were refluxed for 16 h using a Dean–Stark apparatus. The solvents were evaporated under reduced pressure, and the crude product was recrystallized from methanol, to give a shining green powder (SQ3), 0.435 g (37%). 1H-NMR (400 MHz, CDCl3, d): 10.94 (s, 4H), 5.86 (s, 4H), 4.00–3.88 (m, 2H), 3.26–3.10 (m, 4H), 1.73–1.51 (m, 8H), 1.23 (d, J = 6.6 Hz, 3H), 0.95 (t, J = 7.4 Hz, 6H), 0.89 (t, J = 7.4 Hz, 6H); 13C NMR (101 MHz, CDCl3, d): 181.5, 162.9, 161.3, 158.9, 102.7, 94.4, 56.0, 46.5, 28.1, 22.7, 18.7, 11.6, 11.4; MS (ESI [0.8 eV]) m/z (Int.): 525.3 (0.5 104) [M + H]+, 547.3 (0.9 104) [M + Na]+, 1071 (0.4 104) [M + 2Na]+. HRMS (ESI) m/z calcd for [C30H40N2O6Na]+: 547.2779; found 547.2772. 4.2.

Crystallography

The single crystal structures of SQ2 and SQ3 were determined using a Bruker AXS Kappa Apex II diffractometer at 120 (2) K using graphite-monochromated Mo Ka radiation (l = 0.71073 Å). The structures were solved with SHELXS-97 and then refined with the SHELXL-97 program system.19 Crystal structures were edited using Diamond 3.0. SQ2. Crystal dimensions 0.28 0.07 0.07 mm, green with golden reflectance, C40H60N2O6, M = 664.90, triclinic, space group P1% , a = 5.097 (2), b = 10.746(5), c = 16.604(7) Å, a = 96.374(10)1, b = 94.825(11)1, g = 97.872(11)1, V = 890.6(7) Å3, Z = 1, r = 1.240 g cm 3, m = 0.082 mm 1, F(000) = 362, 24 735 reflections (2ymax = 27.101) measured (3942 unique, Rint = 0.0986, completeness = 99.9%), R (I > 2s(I)) = 0.0583, wR2 (all data) = 0.1162. GOF = 1.007 for 215 parameters and 6 restraints, largest diff. peak and hole 0.288/–0.275 e Å3. CCDC 962720 contains the supplementary crystallographic data for this compound. SQ3. Crystal dimensions 0.30 0.16 0.10 mm, green with golden reflectance, C30H40N2O6, M = 524.64, monoclinic, space group P2(1)/c, a = 5.6706 (2), b = 16.8421 (7), c = 14.3536(6) Å, a = 90, b = 97.943 (2), g = 901, V = 1357.69 (9) Å3, Z = 2, r = 1.283 g cm 3,


m = 0.089 mm 1, F(000) = 564, 35 437 reflections (2ymax = 28.281) measured (3371 unique, Rint = 0.0423, completeness = 99.9%), R (I > 2s(I)) = 0.0614, wR2 (all data) = 0.1467. GOF = 1.054 for 215 parameters and 6 restraints, largest diff. peak and hole 0.502/–0.299 e Å3. CCDC 947470 contains the supplementary crystallographic data for this compound. 4.3.

L-ESR measurements

Light-induced electron spin resonance measurements were performed using a Bruker Elexsys E500 continuous wave X-band spectrometer with an attached liquid helium cryostat. Light excitation was done by a continuous wave laser with a wavelength of 660 nm. L-ESR samples were prepared in a nitrogen glove box by drop casting a solution of squaraines blended with PC60BM (mixing ratio 1 : 3) onto plastic foils and drying. These foils were cut into strips, transferred into a standard ESR-tube and sealed under vacuum. All spectra were recorded at 30 K. 4.4.

Device preparation

Indium-tin-oxide (ITO) coated glass (approx. 20 ohms sheet resistance) was cut into 15 15 mm2 pieces, and about one quarter of the ITO-layer was removed using nascent hydrogen generated from zinc powder and concentrated hydrochloric acid. The substrates were cleaned using a detergent and subsequent sonification in acetone and 2-propanol. They were further treated with oxygen plasma prior to spin-coating of a 35 nm thick PEDOT:PSS layer (Clevios P VP Al 4083). They were then transferred into a nitrogen-filled glove box system and annealed at 180 1C for 15 min. The active solar cell layer was spin-coated (1500 rpm for 60 s followed by 3000 rpm for 20 s) from chlorobenzene solutions containing 3 mg mL 1 squaraine (SQ1) or 4 mg mL 1 squaraine (SQ2 and SQ3) and varying amounts of fullerene to give mixing ratios by weight of 1 : 1, 1 : 2, 1 : 3, 1 : 5, and 1 : 7. Either PC60BM or PC70BM was used as received from Solenne. For the hole-only devices, neat squaraine solutions were used (3 mg mL 1 SQ1, 4 mg mL 1 SQ2 and SQ3, all in chlorobenzene). The samples were annealed for 15 minutes at temperatures varying from room temperature (RT) to 130 1C. They were then transferred directly into high vacuum and a 150 nm thick aluminum electrode was thermally evaporated through a shadow mask giving three solar cells with an area of approximately 15 mm2 for every sample. For the holeonly devices, a 150 nm thick gold layer was used instead. A strip of the active layer was removed on one side with a cotton swab dipped into toluene to uncover the ITO-anode. 4.5.

Further characterization

Characterization was done under ambient conditions apart from scanning electron microscopy imaging using an FEI Helios NanoLab 600i system operating under high vacuum. The current–voltage characteristics were recorded using a Keithley 4200 semiconductor characterization system with fourprobe sensing. The three cells on one substrate were contacted with gold spring contacts and measured simultaneously. For illumination, a Photo Emission Tech. AAA solar simulator


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was used, providing an AM1.5G spectrum. The intensity was adjusted to 100 mW cm 2 using a calibrated Si solar cell. For UV-Vis absorbance measurements thin film samples were prepared by spin casting (1000 rpm) the neat and the blended SQ chlorobenzene solutions (the same as for device preparation) onto glass substrates cut from objective slides. For measuring absorbance in solution the SQs were dissolved in chloroform and filled into a standard quartz cuvette. The spectra were then recorded using a Varian Cary 100. X-ray diffraction was performed using a PANalytical X‘PertPro MPD diffractometer operating in Bragg–Brentano geometry and using Cu-Ka radiation (l = 1.542 Å). Samples were prepared on glass substrates cut from objective slides to fit the holder for the sample spinner. Films were prepared by spincoating (the same solutions as for device preparation) and subsequent annealing or by simple spreading of the respective SQ powders. Film thicknesses were measured using a confocal laser scanning microscope Olympus LEXT OLS 4100.

Acknowledgements MS and SB thank the Hanse Institute for Advanced Studies (HWK) in Delmenhorst, Germany, for a regular and a twin fellowship grant, respectively. LB and RT gratefully acknowledge financial support from ‘‘Fondazione Cariplo’’ through grant Exphon-2011-1832. Funding by the ‘‘EWE-Nachwuchsgruppe ¨nnschicht-Photovoltaic’’ by the EWE AG Oldenburg is grateDu fully acknowledged. Dorothea Scheunemann, Matthias Macke, Ulf Mikolajczak ¨rn Kempken are cordially thanked for various practical and Bjo support. Frank Balzer and Leonid Govor are thanked for fruitful discussions. We thank OLYMPUS EUROPA SE & Co. KG, especially M. Fabich and J. Mallmann, for providing the LEXT OLS 4100 laser scanning microscope.

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