Influence of the Position of the Side Chain on Crystallization and Solar

Article pubs.acs.org/cm

Influence of the Position of the Side Chain on Crystallization and Solar Cell Performance of DPP-Based Small Molecules Veronique S. Gevaerts,† Eva M. Herzig,‡ Mindaugas Kirkus,† Koen H. Hendriks,† Martijn M. Wienk,† Jan Perlich,§ Peter Müller-Buschbaum,‡ and René A. J. Janssen*,† †

Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany § DESY, Notkestrasse 85, 22607 Hamburg, Germany ‡

S Supporting Information *

ABSTRACT: Three isomeric π-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C70 fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells. KEYWORDS: small molecules, solar cells, side chain engineering, morphology, crystallinity Compared to π-conjugated polymers, the synthesis of small molecules is more reproducible and much more amenable to rigorous purification, which gives the opportunity to reproducibly investigate well-defined systems. Like for the polymers, side chains are needed on these πconjugated molecules to make them solution processable. For polymers it is well-established that the number, length, and branching of the side chains can influence the solar cell performance.18 Also for small molecules the position and stereoisomerism19 of the side chains can have significant influence on optical, crystal, and electronic properties. A classical example is that of oligothiophenes, where alkyl chains on the terminal α,ω-positions result in significantly different optical absorption spectra compared to substitution on the β,β′ positions20 as a consequence of different intermolecular orientation.21 The effect of side chain position on solar cell performance has been largely unexplored,22 but does possibly provide a subtle way of controlling performance and will provide more detailed insights into structure−property relationships.

1. INTRODUCTION Solution-processed organic photovoltaic cells are considered to be a viable option in meeting the future global energy demand when high efficiencies can be combined with fast printing production techniques. Efficient solution-processed organic solar cells are generally obtained by blending a suitable p-type π-conjugated polymer with a n-type fullerene derivative in a bulk heterojunction photoactive layer.1,2 The power conversion efficiency (PCE) obtained does not only depend on the specific chemical structures of the polymer and fullerene derivatives, but also on the morphology of the blend.3 It has been established that the morphology and performance strongly depend on the molecular weight,4−6 regioregularity,7 and end groups of the polymer.8−10 With existing polymerization techniques to synthesize π-conjugated polymers, it is challenging to minimize batch-to-batch variations in molecular weight, comonomer composition, purity, and hence the resulting opto-electronic properties. To overcome these drawbacks, solution-processable small molecules have recently been suggested for bulk heterojunction solar cells.11 Several examples of rather efficient solar cells containing π-conjugated small molecules and fullerenes processed from solution have now been reported.12−15 PCEs exceeding 8.0% have been achieved,16 virtually identical to the highest PCE of 9.2% published to date for polymer solar cells.17 © 2013 American Chemical Society

Received: July 12, 2013 Revised: December 12, 2013 Published: December 12, 2013 916

dx.doi.org/10.1021/cm4034484 | Chem. Mater. 2014, 26, 916−926

Chemistry of Materials

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Figure 1. Molecular structure of DPP2T-3, DPP2T-4, and DPP2T-5.

Figure 2. Absorption spectra of DPP2T-3, DPP2T-4, and DPP2T-5. (a) In chloroform solution. (b) Thermally annealed thin film on glass.

Figure 3. (a) Fluorescence spectra and (b) time-resolved fluorescence intensity recorded at 840 nm for thin films of DPP2T-3, DPP2T-4, and DPP2T-5 that were annealed at 100 °C for 1 min. The fitted lifetimes are shown in the legend of panel b.

significantly affected by the positions of the side chains on DPP2T molecule. The position influences the molecular packing and the crystallization and, hence, the morphology and efficiency of the photoactive layer in the corresponding solar cells.

In the present investigation, we focus on the effect of the side chain position on the optical and crystallization properties for three isomeric DPP2T molecules that consist of a central diketopyrrolopyrrole (DPP) unit substituted by two bithiophenes (2T) of which the terminal thiophenes carry a hexyl chain in the 3, 4, or 5 position (Figure 1). The DPP unit is a polar bicyclic electron deficient conjugated unit that provides strong π−π stacking leading to materials with small optical band gaps and high charge carrier mobilities for electrons and holes when extended with suitable conjugated heterocyclic oligomers.23 In recent years, the DPP unit has been frequently used to make molecules13 and polymers24 for solutionprocessed organic solar cells with PCEs up to 5.8%13t and 8.0%,24o respectively. In fact, the DPP2T-4 isomer studied here has recently been incorporated in organic solar cells providing PCE = 1.6%.25 By using spectroscopy, microscopy, and diffraction methods in combination with solar cell characterization, we find that the optical, morphological, and device properties of the DPP2T isomers and their blends with [6,6]phenyl-C 71 -butyric acid methyl ester ([70]PCBM) are

2. RESULTS AND DISCUSSION 2.1. Optical Properties. Figure 2 shows the normalized UV−vis absorption spectra of the three DPP2T molecules in chloroform solution and in thin films. The films were spin-cast from chloroform and thermally annealed at 100 °C for 1 min. Only small differences were observed before and after annealing. The position of the hexyl chain has little influence on the optical band gap of the DPP2Ts in solution (Figure 2a). The absorption spectra of DPP2T-4 and DPP2T-5 overlap perfectly, whereas a small blue shift can be seen for DPP2T-3. This shift might be a result of a minor difference in planarity caused by steric hindrance of the side chain pointing toward the DPP unit. The higher planarity of DPP2T-4 and DPP2T-5 compared to DPP2T-3 is also reflected in the somewhat better 917



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Figure 4. J−V curves of the optimized solar cells of DPP2T-3, DPP2T-4, and DPP2T-5 blended with [70]PCBM in a 2:1 weight ratio, as-cast and after 1 min annealing at 100 °C (open symbols in dark and closed symbols under illumination).

2.2. Photovoltaic Properties. The active layers of the solar cells were made by spin-casting a 2:1 (w/w) mixture of the DPP2T molecules with [70]PCBM. When [6,6]-phenylC61-butyric acid methyl ester ([60]PCBM) was used, very similar results were obtained. Here only the results obtained with [70]PCBM are described. The active layers were deposited from chloroform and thermally annealed for 1 min at 100 °C after evaporation of the back contact. The use of different donor−acceptor ratios, solvents, and solvent additives was investigated, but did not lead to improved solar cell efficiencies. Longer annealing times or higher temperatures did not improve the overall device efficiencies. After the initial annealing for 1 min at 100 °C, the device performance did not change significantly when the devices were further annealed at 100 °C for up to 5 min or when they were stored at room temperature under N2 for at least 1 day. The J−V curves of DPP2T:[70]PCBM solar cells made are shown in Figure 4 before and after thermal annealing. The device characteristics are collected in Table 1. The standard

resolved vibronic structure near the wavelength of maximum absorption. The molar absorption coefficients are identical within experimental error. The oxidation and reduction potentials measured by cyclic voltammetry in a liquid electrolyte (see the Supporting Information, Figure S1) are virtually identical for the three isomers and the energy difference between the onset of the waves (∼1.78 eV) corresponds to the optical band gap in solution (∼1.86 eV). In thin films, interactions between the DPP2T molecules induce changes in the optical absorption that are quite different for the three isomers (Figure 2b). Compared to the spectrum in solution, the maximum of absorption for DPP2T-4 has shifted significantly to the blue, with a concomitant less intense peak at higher wavelengths. These changes are characteristic for H-type (cofacial) stacking. In contrast, DPP2T-3 shows a clear red shift of the absorption maximum when going from solution to film. This suggests a J-type (slipped) stacking. For DPP2T-5, the changes are in between those of DPP2T-3 and DPP2T-4. The formation of different types of stacking, induced by the different side chain positions in thin films, is supported by the fluorescence spectra and lifetimes (Figure 3). The fluorescence maximum is found at longer wavelength for DPP2T-4 and the fluorescence lifetime is much longer compared to DPP2T-3 and DPP2T-5. The fluorescence lifetimes at 840 nm (Figure 3b) and at 725 nm (see Figure S2 in the Supporting Information) were identical for each of the three DPP2Ts. This indicates that emission at both wavelengths originates from the same species. The fluorescence intensity of DPP2T-3 is a factor 10 higher than of DPP2T-4 and DPP2T-5, and its fluorescence spectrum has a fairly small Stokes’ shift. The longer lifetime of DPP2T-4 is consistent with the fact that emission from the low-energy exciton-coupled state in an H-type configuration has a reduced probability and shows that the reduced fluorescence intensity is not due to extrinsic quenching. Likewise, the higher fluorescence intensity and shorter lifetime of DPP2T-3 is consistent with a dipole-allowed low-energy transition, characteristic for a J-type configuration. In J-type aggregates, exciton diffusion is faster than in H-type aggregates, because the lowest energy excitation is stronger, and hence the Förster transfer is enhanced.26 The situation for DPP2T-5 seems to be more complex. Compared to DPP2T-3 the fluorescence intensity is less and the lifetime is shorter, but also the Stokes’ shift is much larger. It cannot be excluded that the reduced intensity and shorter lifetime of DPP2T-5 is due to quenching by an impurity.

Table 1. Solar Cell Characteristics of the Optimized Solar Cells of DPP2T-3, DPP2T-4, and DPP2T-5 Blended with [70]PCBM in a 2:1 Weight Ratio active layer

annealed

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

DPP2T-3:[70]PCBM DPP2T-3:[70]PCBM DPP2T-4:[70]PCBM DPP2T-4:[70]PCBM DPP2T-5:[70]PCBM DPP2T-5:[70]PCBM

no yes no yes no yes

2.24 5.89 4.58 7.47 6.02 5.30

0.90 0.85 0.88 0.84 0.81 0.79

0.32 0.50 0.33 0.53 0.34 0.45

0.65 2.48 1.35 3.30 1.66 1.90

deviation in the PCE is less than 0.15% for these devices, except for the as-cast devices of DPP2T-4 where the standard deviation was 0.20%. The higher value for this combination is most likely due to the observed changes in morphology and crystallinity that occur at room temperature over time (see Sections 2.3 and 2.4). The main contribution to the standard deviation is a variation in short-circuit current, the fill factor and especially voltage are more constant. For all solar cells the fill factor (FF) and current density (J) through the active layer in forward bias increase upon annealing. Both observations indicate improved charge transport through the active layers. In the Supporting Information (Figure S3), a semilogarithmic plot of the J−V data shows that the increase in vertical charge transport (both in the dark and 918



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Figure 5. EQE curves of the best solar cells of DPP2T-3, DPP2T-4, and DPP2T-5 blended with [70]PCBM in a 2:1 weight ratio, as-cast (open symbols), and after 1 min annealing at 100 °C (closed symbols).

Figure 6. Absorption spectra of DPP2T:[70]PCBM blend films in a 2:1 weight ratio, as-cast, after 24 h at room temperature, and after 1 min annealing at 100 °C.

measured for mixed films of the DPP2Ts with [70]PCBM on a PEDOT:PSS covered glass slide (Figures 6 and 7). The layer of PEDOT:PSS was approximately 30 nm thick and was used to ensure identical film formation as for the solar cells.

under illumination) is the largest for blends of [70]PCBM with DPP2T-3, a little less for DPP2T-4, and fairly small for DPP2T5. These relative changes match with the changes observed in the optical absorption spectra upon annealing, which are the largest for DPP2T-3, less for DPP2T-4, and small for DPP2T-5 (see Sections 2.3 and 2.4). As the changes optical spectra can be ascribed to changes in the intermolecular packing of the molecules, we attribute the increased charge transport after annealing to changes in the intermolecular packing. The change in the short-circuit current (Jsc) after annealing, however, varies for the different photoactive layers. It increases for the [70]PCBM blends with DPP2T-3 and DPP2T-4, but decreases somewhat for DPP2T-5. In all cases, annealing results in small reduction of the open circuit voltage (Voc). The external quantum efficiency (EQE) of the cells (Figure 5) shows several characteristics that can directly be related to the optical absorption of the DPP2T molecules in thin films (Figure 2b). The EQEs change mainly in magnitude (at least for DPP2T-3 and DPP2T-4) with annealing but only slightly in shape (Figure 5). This suggests that the DPP2Ts are already in a crystalline state in the as-cast films and that upon annealing the morphology changes mainly with a growth of the crystallites as will be shown in Section 2.4. 2.3. Optical Properties of the Blend Films. To investigate aggregation, crystallization, and morphology evolution in the blend films, we used absorption and photoluminescence measurements, complemented with atomic force microscopy (AFM), transmission electron microscopy (TEM), and diffraction. Absorption and fluorescence spectra were

Figure 7. Fluorescence spectra of the DPP2T:[70]PCBM layers ascast (lines) and after thermal annealing (open symbols).

For each DPP2T:[70]PCBM blend two identical films were prepared. For the first film, the optical absorption was measured directly after casting (as-cast) and again after annealing at 100 °C. The other film was kept at room temperature in dark and measured several times to investigate the kinetics of crystallization of the DPP2Ts when blended with [70]PCBM. Figure 6 shows the UV−vis absorption of the films 919



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Figure 8. AFM height images for (a−c) as-cast and (d−f) annealed blends of (a, d) DPP2T-3:[70]PCBM, (b, e) DPP2T-4:[70]PCBM, and (c, f) DPP2T-5:[70]PCBM. Size of the images is 1 × 1 μm2, height scales are (a, b, c, e, f) 10 and (d) 30 nm.

as-cast, after annealing at 100 °C, and after 24 h aging at room temperature for the three blends. The evolution of the absorption spectra of the blend films in the 1−72 h range are collected in the Supporting Information (Figure S5). In the blend of DPP2T-5:[70]PCBM only minor changes are observed in absorption spectra with time, indicating that DPP2T-5 readily crystallizes during film formation and the process only changes slightly upon aging or heating of the film. For the DPP2T-4:[70]PCBM blend film, the changes in the absorption spectra are larger, but some crystallization is already observed in the as-cast samples. This is in sharp contrast to the DPP2T-3:[70]PCBM blend, which seems to be in an amorphous state directly after casting and changes are significant upon annealing. The kinetics at room temperature are very slow for DPP2T-3; the absorption only shows significant changes after 24 h. After 3 days, the absorption of DPP2T-3:[70]PCBM blend was still not the same as the annealed film, whereas after 7 h of room temperature aging in dark the absorption of the DPP2T-4:[70]PCBM film shows little difference to the 24 h aged film (see Figure S5 in the Supporting Information). From these absorption measurements, it can be concluded that the stacking of the DPP2Ts in the presence of [70]PCBM has different kinetics depending on the position of the side chain. It should be noted that for the DPP2T-3:[70]PCBM solar cells the EQE of the as-cast sample already shows crystallization of the DPP2T unit (Figure 5), this is due to the fact that there is some time in between the spincasting of the sample and the measurements of the finished solar cells. Some crystallization can be caused by warming-up of the device during metal evaporation, by the illumination, or the current going through the active layer during J−V measurements. As mentioned, the current density through the active layers measured in forward bias increases upon annealing (see Supporting Information Figure S3). After annealing, the forward current density through the layers are about equal, but when looking more carefully Figure S3 reveals that the change is smallest for DPP2T-5, larger for DPP2T-4, and the

largest difference is found for DPP2T-3. This directly correlates to the initial crystallization of the DPPs in the blend films, which is observed in the absorption measurements (as-cast samples in Figure 6). Crystallization of the DPP2Ts was also investigated using near-infrared fluorescence for the as-cast and annealed blend films (Figure 7). For the as-cast films, most of the fluorescence which was observed for the pristine films was quenched. When thermally annealed, the fluorescence increases significantly for all blends. The shape of the fluorescence spectrum after annealing is identical to that of the pure films of the DPP2Ts (Figure 3a). This indicates that the initial morphology is very finely mixed and phase separation into larger, more pure crystalline domains occurs upon thermal annealing. When the size of the domains becomes equal or larger than the exciton diffusion length, not all photoexcitations can lead to charge transfer at the DPP2T-[70]PCBM interface. Instead, they decay intrinsically by, among others, fluorescence. 2.4. Crystallization in the Blend Films. To investigate the phase separation, we recorded the surface topology using atomic force microscopy (AFM) (Figure 8). The as-cast films are relatively smooth and show little surface features. For all samples, the surface roughness increases upon thermal annealing and, especially for DPP2T-3:[70]PCBM and DPP2T-5:[70]PCBM blends, formation of crystallites on the surface is clearly visible after annealing. Similar behavior has also been observed in pristine films of DPP-based oligophenylenethiophenes.27 The surface of the DPP2T-4:[70]PCBM blend shows similar features as-cast and after annealing, which is actually due to changes in the as-cast sample during the AFM measurement, confirmed by a color change of the film during the measurement. Again, this indicates that the kinetics of crystallization of DPP2T-4 in the blend with [70]PCBM is fast compared to the other DPP2Ts. To further investigate the molecular ordering of the DPP2Ts and [70]PCBM upon aging and annealing, we measured electron diffraction patterns for pure DPP2T films and for blend films with [70]PCBM (Figure 9). The diffractograms 920



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Figure 9. Radially integrated electron diffractograms vs scattering vector q for pristine DPP2T-3, DPP2T-4, and DPP2T-5 films and their blends with [70]PCBM, as-cast and after 1 min annealing at 100 °C. The asterisk indicates diffraction of the [70]PCBM.

Figure 10. Bright-field TEM images and diffraction patterns of blends consisting of DPP2T-5 and [70]PCBM in a 2:1 weight ratio, as cast (left) and after 1 min annealing at 100 °C (right).

reveal that annealing increases the crystallinity of the DPP2Ts in the blends. Again we see that DPP2T-3 initially is not crystalline when cast with [70]PCBM, whereas for DPP2T-4 and DPP2T-5, some weak reflections can be seen. In the annealed blends, the main diffraction peaks correspond to those of the pure crystalline DPP2Ts. The [70]PCBM reflections overlap with reflections of the DPP2Ts, making it difficult to distinguish unambiguously between the contributions of [70]PCBM and DPP2T. The relatively broad peaks arise because of the aggregation of locally unoriented [70]PCBM molecules with short-range order also sometimes referred to as nanocrystals.28−32 For the DPP2T-5:[70]PCBM blend films, lattice fringes were successfully imaged in bright-field TEM that correspond to a lamellar d-spacing of about 1.9 nm (Figure 10). These large dspacings in crystallites are found in the small q-range in electron diffraction. Figure 11 shows the electron diffraction in the small q-range for all pure DPP2T samples. The diffraction signal that corresponds to the observed crystal fringes is located at q ≈ 3.1 nm−1, such that d = 2π/q = 2.0 nm, close to the value determined from bright-field TEM. The diffraction patterns in Figure 11 show that the corresponding scattering vectors are larger for DPP2T-3 (q = 4.1 nm−1) and DPP2T-4 (q = 4.3 nm−1) than for DPP2T-5. The smaller distance and the fact that these samples are very sensitive to the electron beam33,34 precluded the clear imaging of lattice fringes for DPP2T-3 and DPP2T-4 samples, although they were briefly visible. Direct microscopic evidence for increased phase separation upon

Figure 11. Radially integrated electron diffractograms vs scattering vector of pristine DPP2T-3, DPP2T-4, and DPP2T-5 films in the small q-range.

annealing is found in the TEM images in Figure 10, which show small crystallites of DPP2T-5 in the as-cast DPP2T-5: [70]PCBM blend film, that grow significantly when the film is annealed. In addition to the TEM and electron diffraction measurements also grazing incidence wide-angle X-ray scattering (GIWAXS) was carried out on pristine and annealed DPP2T: [70]PCBM blend films (Figure 12).35 The recorded 2D GIWAXS patterns yield information on molecular spacings as well as packing and orientation of crystals with respect to the substrate.36 921



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Figure 12. GIWAXS for (a−c) pristine films of DPP2T-3, DPP2T-4, and DPP2T-5 and (d−f) blend films with [70]PCBM after annealing, respectively. High count scaling for panels d−f is also given in (g−i) low count scaling to emphasize on weak peaks. Special features are marked.

In the GIWAXS data of all pristine films, well-pronounced diffraction spots occur (see Figure 12a−c), indicating wellordered crystals throughout the corresponding film. A comparison of the diffraction spot patterns of the pristine DPP2T films shows a clear difference in the arrangement between DPP2T-3 in comparison to DPP2T-4 and 5, indicating different types of crystalline arrangements supporting the observations from the absorption and fluorescence measurements. The crystal structure, however, cannot be uniquely determined without single-crystal diffraction data and modeling. To interpret the available information in more detail, we simulated the GIWAXS pattern using the simDiffraction package by Breiby et al.37 using energetically minimized molecules based on the known DPP-unit38 (see Figures S6 and S7 in the Supporting Information). The diffraction rings in the GIWAXS patterns of the pristine films arising at low qvalues with a strong peak in the qz direction indicate a dominant vertical orientation with some orientational disorder. The q-value of the diffraction ring changes abruptly from 4.4 (DPP2T-3) and 4.5 (DPP2T-4) to 3.4 nm−1 (DPP2T-5), originating from real space separations of 1.4, 1.4, and 1.8 nm, respectively, and in accordance with the electron diffraction results. The latter value corresponds very well with the length (1.9 nm38) of a DPP2T molecule suggesting that the stacking direction changes from across the backbone to along the backbone if the side chain is at position 5. This change of crystal orientation also explains why the positions of the vertical reflection strings change significantly between DPP2T-4 and 5. The two most dominant vertical strings of refraction spots for DPP2T-5 are positioned at qxy = 6.2 nm−1 (d = 1.0 nm) and qxy = 9.9 nm−1 (d = 0.64 nm), corresponding well to the length of the alkyl side chains to the central axis of the backbone (0.93 nm) and the length of the double side chain at the nitrogen

(0.67 nm). Altogether the data imply a vertically oriented backbone with randomly oriented in plane π−π stacking. Simulating such a molecular arrangement reproduces the dominant scattering features very well as shown in Figure S6 in the Supporting Information. DPP2T-5 is also the sample with least orientational order and smallest crystal sizes or largest paracrystallinity as seen from the weak powder rings between the diffraction spots and the width of the peaks, respectively. For DPP2T-4, the GIWAXS data (Figure 12b) and simulations (see Figure S7 in the Supporting Information) suggest that the backbone lies within the substrate plane. The two most dominant vertical strings of refraction spots are found at qxy = 3.4 nm−1 (d = 1.8 nm) and qxy = 15.6 nm−1 (d = 0.40 nm), showing that the DPP2T-length and a second very short length occur parallel to the substrate in agreement with backbones parallel to the substrate. Qiao et al.38 observed that the DPP core is planar but the short double side chain forms an angle of 93° to that plane. In the present investigation, a different tilt angle of the short double side chains is necessary for the reduced stacking distance of 0.4 nm, and implies a π−πstacking within the substrate plane like for DPP2T-5 but with a reduced π−π-stacking distance beneficial for charge transport. When adding [70]PCBM comparably little changes are found for the GIWAXS pattern of the DPP2T-3:[70]PCBM blend (Figure 12d, g), which shows virtually the same diffraction peaks as the pure DPP2T-3 film (Figure 12a). This indicates that [70]PCBM only influences the system slightly, and does not interdigitate into the DPP2T-3 crystals. However, most reflections of DPP2T-3 are less well-defined in the blend with [70]PCBM, suggesting either smaller crystal sizes or larger paracrystallinity. The semicircles at q ≈ 4.4 and 16 nm−1 have disappeared, and the peak at q ≈ 4.4 nm−1 is 922



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degree of ordering for DPP2T-3. Although the shorter π−πstacking distance in the DPP2T-4 blend together with a certain freedom of orientation also allowing charge transport in the vertical direction can explain the high Jsc and fill factor for this system. At the same time, the DPP2T-5 sample results in a comparatively lower charge transport because of the longer π−π-stacking separation.

much more confined to the out-of-plane (z) direction, implying that [70]PCBM suppresses some orientational freedom. The packing of DPP2T-4 is much more influenced upon mixing with [70]PCBM (Figure 12e,h). The stacking in vertical direction is widened to a region of allowed tilt around the outof-plane direction. The dominant peaks smear out into vertical streaks, suggesting a loss of orientational order in several directions, however signatures of defined packing and unaltered packing are still clearly discernible (vertical streaks and 3 diffraction orders along qz direction) although the intensity of the signal from crystalline contributions is much lower than for the pristine film. For the DPP2T-5:[70]PCBM blend (Figure 12f, i), very little change occurs compared to the pristine DPP2T-5 film. The positions of reflections change only minimally if at all, indicating that [70]PCBM does not interdigitate within crystalline DPP2T-5 and no changes in preferential order occur. Furthermore, no smearing out of the reflections is observed; however, a slight increase in randomly oriented crystals is found as can be seen from the superposition of individual peaks on slightly stronger rings of randomly ordered DPP2T-5 crystallites in the presence of [70]PCBM. From this, we infer that the addition of [70]PCBM results in randomly oriented DPP2T-5 crystallites, coexisting with well-oriented crystals. In the GIWAXS data of the blend films, part of the isotropic intensity rings can be attributed to agglomerated [70]PCBM. The broad peaks present at q = 13.0 and 18.5 nm−1 are in good agreement with pure [70]PCBM,39 but are overlain with DPP2T signals. The peak expected at q = 6.5 nm−1 is very weak and not clearly discernible from the DPP2T. In the Supporting Information (Figure S4), we compare the optical absorption spectra of the annealed neat DPP2T films to those of the annealed blends. The spectra are virtually identical in the spectral range of 550−750 nm, although the resolution of the spectral features seems slightly more pronounced in the neat films. Because the optical spectra measure all of the molecules in the film, they confirm the conclusion inferred from the GIWAXS data that blending with [70]PCBM does not change the type of packing of the DPP2T molecules and that a defined packing remains for the entire sample. 2.5. Effect of Crystallization on Solar Cell Behavior. Combining the various techniques used to characterize the blend films some correlations between solar cell performance and crystallization can be established. For the as-cast samples, the difference in charge transport through and the photovoltaic performance of the active layers for the different DPP2Ts can be related to the crystallinity of the DPP2T molecules. The efficiency of the as-cast films decreases from DPP2T-5: [70]PCBM, via DPP2T-4:[70]PCBM, to DPP2T-3: [70]PCBM, which is in the same order as the degree of crystallinity of the donor DPP2T in these films. Upon thermal annealing of all DPP2T:[70]PCBM films, the DPP2Ts crystallize, phase separation increases, and the morphology coarsens as evidenced from the increased fluorescence in the blends because excitons need to diffuse over longer distance to the interface with the [70]PCBM phase to generate charge carriers. The enhanced crystallization also increases charge transport through the active layers as suggested by the increase in fill factor. The higher fill factors observed for the blends of DPP2T-3 and DPP2T-4, compared to DPP2T-5, suggest better transport in the vertical direction. The GIWAXS data suggest that the reason for the improved charge transport is the type of packing structure and a high

3. CONCLUSIONS Changing the position of the side chain provides a means to control the crystallization behavior and therefore the solar cell performance of π-conjugated molecules. The effects were studied using three isomeric DPP2T molecules with hexyl side chains on different positions of the outer thiophene rings. Although the side chain position has little influence on the optical absorption in solution, it changes the optical and crystallization properties of the molecules in film significantly. From optical absorption, photoluminescence, and lifetime measurements we infer that the DPP2T molecules align in an H-type configuration when the side chain is on the 4-position, in a J-type configuration for the 3-position, whereas an intermediate orientation is found for the molecule with the alkyl chain in the 5-position. This leads to significant differences in absorption spectra, but does not preclude that all three isomers do give working photovoltaic cells when mixed with [70]PCBM as acceptor. Optimized power conversion efficiencies between 1.9 and 3.3% were obtained when the DPP2Ts were blended with [70]PCBM in a 2:1 weight ratio in a bulk heterojunction solar cell. Absorption, AFM, TEM, and GIWAXS experiments show that despite a number of similarities, such as peak widths and retention of H-type to Jtype orientation, the three isomeric DPP2Ts differ significantly in the extent to which [70]PCBM influences the film morphology, and the kinetics of crystallization and crystallite orientation. These differences affect morphology and in the end efficiency and behavior of solar cells made with these DPP2T isomers. The fact that the small donor molecules tend to crystallize, sometimes in identifiable small domains, creates a new question with respect to the optimized morphology and its consequences for charge generation and charge transport in solutionprocessed small molecule−fullerene solar cells as compared to polymer−fullerene devices in which the fullerene can mix with the polymer domains.40,41 The morphology reflected in the type of self-organization and crystallization of the molecules and the effect of that on the electronic properties (H- or J-type aggregate), the introduction of grain boundaries, and the interconnectivity of crystalline domains can have profound effects on the device properties of these materials. By studying three DPP2T isomers, we find a complex, but yet unsolved, relation between molecular structure, film morphology, and device performance. Obtaining detailed insights in the relation between molecular structure, processing, morphology, and performance will be critical in the future design and optimization of new small molecule materials for organic solar cells. 4. EXPERIMENTAL SECTION DPP2T-3, DPP2T-4, and DPP2T-5 were synthesized using a Suzuki reaction from 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione and thiophen-2-yl boronic esters with hexyl side chains in the 3, 4, or 5 position. Details on 923



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the synthesis and characterization can be found in the Supporting Information. Solar cells were prepared by spin-casting PEDOT:PSS (Clevios P, VP Al4083) on precleaned ITO patterned glass substrates (Naranjo substrates). The PEDOT:PSS layer was dried by heating at 120 °C for 20 min before deposition of the active layer. On top of the dried PEDOT:PSS the active layers were deposited by spin-casting a mixture of the DPP2T with [70]PCBM (Solenne BV) in a 2:1 ratio from chloroform. A back contact of 1 nm LiF and 100 nm Al was evaporated in vacuum. Annealing of the active layers was performed after evaporation of the back contact in a nitrogen environment. Device areas of 0.09 and 0.16 cm2 were used, which provided similar results. Layer thicknesses were measured using a Veeco Dektak 150 Surface Profiler. Current-density versus voltage curves (J−V) were measured under simulated solar light (100 mW/cm2) from a tungsten−halogen lamp filtered by a Schott GG385 UV filter and a Hoya LB120 daylight using a Keithley 2400 source meter. EQE was measured using mechanically modulated (Stanford Research, SR 540) monochromatic (Oriel, Cornerstone 130) light from a 50 W tungsten halogen lamp (Osram 64610) as probe light, in combination with continuous bias light from a solid state laser (B&W Tek Inc. 532 nm, 30 mW). The intensity of the bias light was adjusted until the short-circuit current density equivalent to 1 sun was obtained. The response was recorded as the voltage over a 50 Ω resistance, using a lock-in amplifier (Stanford Research Systems SR830). During EQE measurements the devices were contacted in a nitrogen filled box with a quartz window and illuminated through an aperture of 2 mm. Thin films for optical absorption, fluorescence, and grazing incidence wide-angle X-ray scattering measurements were prepared by spin coating on PEDOT:PSS coated glass substrates using the same conditions as for solar cell preparation. Steady state photoluminescence spectra were recorded at room temperature using an Edinburgh Instruments FLSP920 double-monochromator luminescence spectrometer equipped with a nitrogen-cooled near-IR sensitive photomultiplier (Hamamatsu). All spectra were corrected for the spectral response of the monochromators and photomultiplier. Time-resolved fluorescence measurements were performed on an Edinburgh Instruments LifeSpec-PS spectrometer using a 405 nm (3.06 eV) pulsed laser (PicoQuant PDL 800B) operated at 2.5 MHz with a pulse duration of 59 ps. For detection a Peltier-cooled Hamamatsu microchannel plate photomultiplier (R3809U-50) was used. Each intensity decay curve was fitted by a multiexponential fit by reconvolution of the instrument response function (IRF). AFM was measured using a Veeco MultiMode AFM in tapping mode with PPP-NCH-50 tips from Nanosensors. Transmission electron microscopy and electron diffraction was performed on a Tecnai G2 Sphera TEM (FEI) operated at 200 kV. Bright field TEM images were acquired under slight defocusing conditions.42 For TEM the films were processed on PEDOT:PSS. Delamination of the DPP2T:[70]PCBM blend films from the PEDOR:PSS layer was achieved in water to create a free-standing film that was transferred onto a TEM grid. Electron diffraction patterns were integrated using ImageJ (1.43u) software with the Radial Profile plug-in. GIWAXS measurements were performed at the beamline BW4 at HASYLAB, DESY in Hamburg.30 Scattering intensities are expressed as a function of the scattering vector, q = 4π/λsin θ, where θ is the half scattering angle and λ = 0.138 nm is the wavelength of the X-ray beam. The coordinate system is chosen with x-direction parallel to the X-ray beam, y-direction parallel to the sample surface and z-direction along the surface normal.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is part of the research program of the Dutch Polymer Institute (DPI project 660). The research forms part of the Solliance OPV programme and has received funding from the Ministry of Education, Culture and Science (Gravity program 024.001.035). Funding via the “Interface Science for Photovoltaics” (ISPV) project of the EuroTech Universities Green Tech Initiative is acknowledged. E.M.H. thanks the Munich School of Engineering (MSE) for funding this research work. The X-ray scattering experiment were carried out at the BW4 beamline of the light source DORIS III at DESY, a member of the Helmholtz Association (HGF).

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of the DPP2T molecules, the cyclic voltammetry, additional luminescence lifetime, and temporal evolution of the optical absorption of blend films are shown. This material is available free of charge via the Internet at http://pubs.acs.org. 924



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Influence of the Position of the Side Chain on Crystallization and Solar

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