PCBM Disperse-Red Ester with Strong Visible-Light Absorption

ARTICLE pubs.acs.org/JPCC

PCBM Disperse-Red Ester with Strong Visible-Light Absorption: Implication of Molecular Design and Morphological Control for Organic Solar Cells Mingfeng Wang, Eneida Chesnut, Yanming Sun, Minghong Tong, Michele Guide, Yuan Zhang, Neil D. Treat, Alessandro Varotto, Andy Mayer, Michael L. Chabinyc, Thuc-Quyen Nguyen, and Fred Wudl* Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids and Materials Research Laboratory, University of California, Santa Barbara, California 93106-6105, United States

bS Supporting Information ABSTRACT: A new dyad of fullerene/disperse-red, denoted as PCBDR, strongly absorbs visible light in the range of 400
600 nm. PCBDR showed advantages over PCBM in several aspects such as enhanced visible-light absorption, improved solubility, and the possibility to facilitate cascaded electron transfer. P3HT:PCBDR bulk heterojunction (BHJ) solar cells, nevertheless, so far have not outperformed P3HT:PCBM BHJ solar cells under similar conditions. Among factors that affect the efficiency of P3HT:PCBDR BHJ solar cells, the suppression of the interchain interaction of P3HT in the P3HT:PCBDR blend played a major role, presumably due to better interfacial miscibility between P3HT and PCBDR than that in blends of P3HT:PCBM. In contrast, benzoporphyrin (BP), due to its unique crystallinity, morphology, and nonsolubility, afforded a better control of the morphology and the interface of the p/n junctions. As a consequence, the performance of solar cells with BP/PCBDR as the active layer was comparable to that of BP/PCBM solar cells. These results suggest that a synergistic approach of synthetic design and morphological control in devices is critical to develop new electron acceptors for highly efficient organic/polymer solar cells.

1. INTRODUCTION Polymer/fullerene bulk heterojunction (BHJ) solar cells are a promising clean and renewable energy source due to their advantages such as mechanical flexibility, lightweight, solutionprocessability for large-area roll-by-roll fabrication, potentially low cost, and synthetic variety.1
4 The active layer of polymer/ fullerene solar cells typically consists of a π-conjugated polymer as the electron donor and a fullerene derivative as the electron acceptor. This active layer plays a critical role in many photophysical processes including light absorption, exciton generation, interfacial charge dissociation, and subsequent charge transport to electrodes. Among fullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), with its strong electron accepting capability, high electron mobility, and appropriate miscibility with conjugated polymers, has been widely used as an electron acceptor for BHJ solar cell devices.4 PC61BM, however, absorbs light primarily in the UV range, with little absorption of light in the visible range. As a result, another member of the fullerene family, PC71BM, which absorbs more light than PC61BM in the visible range, has resulted in relatively higher efficiency in BHJ solar cells. To enhance the light absorption of the solar spectrum by the active layer in polymer/fullerene BHJ solar cell devices, low-band gap polymers that absorb broadly in the visible and near IR range have been synthesized recently.5 Some of these polymers have given BHJs with power conversion efficiencies up to 7
8% but usually require the use of PC71BM to absorb r 2011 American Chemical Society

efficiently below ∼500 nm.6
11 The absorption band of PC71BM in the visible range is rather narrow (mainly confined to the blue region of the solar spectrum) and relatively weak. As a consequence, nonfullerene materials including electron-withdrawing small molecules and polymers12
20 and inorganic nanocrystals (quantum dots)21,22 have been explored as electron acceptors for polymer solar cells. Although these acceptors absorb more light in the visible range than PCBM, none of them have outperformed PCBM to reach high efficiency in BHJ solar cells. Recently, Mikroyannidis and co-workers reported a modified PCBM derivative (called F) in which the methyl group of PCBM was replaced by 4-nitro-40 -hydroxy-α-cyanostilbene.23 By using F as the acceptor and P3HT as the donor, higher performance of solar cells was reported in comparison to the control device with PCBM as the acceptor. This fullerene derivative (F), however, showed little enhanced absorption in the visible range compared to PCBM. Moreover, the 0.20 eV higher LUMO level of F compared to that of PCBM is surprising, which can not be rationalized by the strong electron withdrawing nitro and cyano groups in F. More recently, Wang et al. reported a porphyrinfullerene dyad, which did show enhanced absorption of visible light.24 However, the large number of dodecyl chains on the porphyrin moiety compromised the charge transport properties Received: October 11, 2011 Revised: December 2, 2011 Published: December 06, 2011 1313

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Figure 2. UV
vis absorption spectra of PCBDR in CH2Cl2 in comparison with disperse red 1, P3HT, and PCBM in the same solvent.

Figure 1. Esterification between PCBA and disperse red 1 leads to PCBM disperse-red ester, denoted as PCBDR.

of the dyad, which resulted in lower efficiency in solar cells compared to the control device with PCBM. Herein we report a new fullerene-chromophore dyad that strongly absorbs light in the range of 400
600 nm, which is complementary to the light absorption of fullerene in the UV range. Our synthetic design is to maintain the major structural characteristics of PCBM while introducing an extra light absorbing moiety that is not so bulky as to affect the electron transport of fullerene. As an example here, disperse red 1 (denoted as DR), a commercially available and low-cost dye that shows an intense absorption peak at 500 nm, has been covalently linked to 1-(3carboxypropyl)-1-phenyl[6,6]C61 (PCBA) through an esterification reaction. The molecular structure of this fullerene/DR dyad, denoted as PCBDR, is shown in Figure 1. In the following sections, we first describe molecular characterization of PCBDR by a series of techniques including UV
vis spectroscopy, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and cyclic voltammetry (CV). This is followed by a study of photoinduced electron transfer in pristine PCBDR as well as in a blend of P3HT:PCBDR. Finally, we present results of solar cell devices using PCBDR as the electron acceptor and either a semiconducting polymer, P3HT, or a small-molecule semiconductor, benzoporphyrin (BP), as the electron donor. We also discuss how the morphology of the organic heterojunctions determines the performance of the solar cells.

2. RESULTS AND DISCUSSION 2.1. Molecular Characterization of PCBDR. Details of the synthesis and molecular characterization of PCBDR are described in the Experimental Section (section 4) as well as in the Supporting Information. PCBDR, a dark red solid, shows good solubility in common nonpolar solvents such as chloroform, dichloromethane, toluene, and chlorobenzene. The 1H NMR spectrum (Supporting Information, Figure S1) of PCBDR in comparison with that of DR shows characteristic peaks from both PCB and DR, while the proton signal of the
COOCH2
group is located at 4.3 ppm. The molecular structure of PCBDR is further proved by 13C NMR (Supporting Information, Figure S2) and mass spectrometry. DSC analysis (Supporting Information, Figure S5) of DR shows a crystallization peak at 139 °C. No crystallization

peak exists in PCBDR when heated up to 250 °C, suggesting that the material is likely to be less crystalline. The TGA measurements (Supporting Information, Figure S4) show a degradation temperature of PCBDR at 290 °C, which is 10 °C higher than that of DR itself. PCBDR shows absorption features of both C60 and DR in solution (Figure 2). A broad absorption band of PCBDR centered at 488 nm, with the onset at 570 nm, was observed in the absorption spectrum in dichloromethane. This absorption band appears identical to that of DR. In contrast, there is a 10 nm blue shift for the absorption of fullerene in PCBDR in dichloromethane compared to the absorption peak of PCBM at 338 nm. It is not clear whether this blue shift is a result of the ground-state interaction between the fullerene and the DR moieties in PCBDR because the additional absorption of DR at 300
350 nm may also contribute to this blue shift. A further quantitative measurement of the absorption at 486 nm gives a molar extinction coefficient of 3.24  104 L mol
1 cm
1 for PCBDR in dichloromethane and 3.69  104 L mol
1 cm
1 for DR in the same solvent. Here, the reduced extinction coefficient of PCBDR compared to that of DR, together with the change of the LUMO level of PCBDR vs PCBM, as discussed below, implies that electronic communication between the fullerene and DR is possible. Similar results have been reported by Martin and Guldi in fullerene-heptamethine conjugates.25,26 In solid films, the absorption of PCBDR is red-shifted, (Supporting Information, Figure S3) with the onset extended to 630 nm. This red shift is an implication of the π
π stacking of DR in the solid state. DR itself crystallizes upon solution casting with large domains preventing the formation of a uniform film. In contrast, PCBDR is able to form visually smooth films that are likely amorphous. The energy levels of PCBDR, in comparison to DR and PCBM, were measured by CV from welldissolved solutions. The cyclic voltammograms are presented in Figures S6
S7 of the Supporting Information. The measured values of LUMO and HOMO of DR vs ferrocene are
3.5 and
5.3 eV, respectively, while the measured LUMO level of PCBM under similar conditions is
3.8 eV. Interestingly, the LUMO level of PCBDR decreased to
3.9 eV, which is 0.1 eV deeper than that of PCBM. In addition, the peak located at
1.5 eV, corresponding to the further reduction of the DR
anion, remains visible in the cyclic voltammogram of PCBDR. A relatively weak but reversible oxidation peak was also observed in PCBDR, which gives a HOMO value of
5.1 eV assigned to the DR moiety in this dyad. Thus, because of the nonconjugated linker between DR and the fullerene and the discrete peaks in the CV, we believe that there is no substantial mixing between the frontier orbitals of the two units in PCBDR. 1314

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The Journal of Physical Chemistry C 2.2. Photoinduced Electron Transfer in Pristine PCBDR As Well As in a Blend of P3HT:PCBDR. The offset between the

LUMO level of DR and the LUMO level of the fullerene moiety (PCB) in PCBDR suggests that electron transfer from DR to PCB is possible. To prove this hypothesis, we examined the photoinduced charge transfer in pristine films of PCBDR as well as in the blend film of P3HT and PCBDR. Figure 3 shows the photoluminescence (PL) spectra of P3HT, P3HT:PCBDR (1:1, wt.) blend, and P3HT:PCBM (1:1, wt.) blend in films. One can see that PCBDR quenches the PL of P3HT as effectively as does PCBM. On the basis of our values for the orbital levels (Figure 5A) of P3HT, DR, and PCB, there are three possible factors that contribute to the PL quenching of P3HT: (1) electron transfer from P3HT to DR; (2) electron transfer from P3HT directly to PCB; and (3) cascaded electron transfer from P3HT to DR and then to PCB in PCBDR (Figure 5A). The kinetics of the second factor have been well studied for a variety of fullerenes and are known to be extremely fast.27
30 The kinetics of the other processes are unknown, but it is likely that electron transfer from DR to PCB is as fast as that from P3HT to PCB. To investigate the possibility of photoinduced electron transfer from P3HT to DR, the PL of P3HT:DR blend (1:1, wt.) in a film, in comparison to that of pristine P3HT under similar conditions, was examined. The results are shown in Figure S8 of the Supporting Information. While the UV
vis absorption of the P3HT:DR blend showed the addition of both components, the PL intensity of P3HT in the blend was quenched by 38%

Figure 3. Photoluminescence spectra of pristine P3HT, the blend of P3HT/PCBM (1:1, wt), and the blend of P3HT:PCBDR (1:1, wt) in films.

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compared to that of pristine P3HT. We note that DR itself is nonfluorescent. Moreover, it has little absorption in the range of 600
800 nm where the PL of P3HT is mainly located. Thus, the contribution of F€orster resonance energy transfer to the PL quenching of P3HT is minimal. Therefore, we conclude that the PL quenching of P3HT in the blend with DR is dominated by photoinduced electron transfer from P3HT to DR. In addition, the extent (62%) of this PL quenching by DR is obviously lower than that of quenching by either PCBM or PCBDR (87%, in either case) but is still efficient (Figure 3). These results imply that, in the blend of P3HT and PCBDR, both DR and PCB can contribute to the PL quenching of P3HT; however, the relative extent to which each contributes requires more detailed studies. To gain a deeper understanding of the photoinduced electron transfer, we carried out photoconductivity measurements for both pristine PCBDR and P3HT:PCBDR blend films. Compared to the photoconductivity curve of PCBM in neat films, the photoconductivity of PCBDR shows relative enhancement in the range of 400
600 nm, where DR strongly absorbs light (Figure 4). Here, the absolute value of the photoconductivity is not comparable because of the difference in film thickness and light intensity. Nevertheless, the difference in the shape of the photoconductivity curves shows that the light absorption of DR contributes to the photocurrent. This contribution of DR absorption to the photoconductivity provides direct evidence for the photoinduced electron transfer from DR to PCB in the dyad of PCBDR. In the range of 350
600 nm, the photoconductivity of the blend film of P3HT:PCBDR (1:1, wt.) heated at 70 °C for 10 min appeared higher (14% enhancement at 480 nm) than that of P3HT:PCBM (1:1, wt.) blend under the same conditions (Supporting Information, Figure S9A). The photoconductivity of both films increased after being heated at 110 °C for 10 min. Interestingly, after this further heating treatment, the photoconductivity of the P3HT:PCBM blend appeared higher (20% enhancement at 480 nm) than that of P3HT:PCBDR blend (Supporting Information, Figure S9B). These results imply that, compared to the blend of P3HT:PCBM, the blend of P3HT: PCBDR may undergo different pathways of phase separation and crystallinity change under thermal annealing, which is further discussed in section 2.3. On the basis of the orbital levels of P3HT as well as DR and PCB in the dyad of PCBDR (Figure 5A), the results of photoconductivity and PL quenching experiments described above suggest that cascaded electron transfer in the blend of P3HT and PCBDR (factor 3 from above) is possible. We note that the strategy of cascaded electron transfer has been utilized in both

Figure 4. Steady-state photoconductivity spectra of (A) PCBDR and (B) PCBM in films. 1315

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Figure 5. (A) Energy-level diagram of P3HT, disperse red (DR), and PCB in a device architecture of ITO/PEDOT:PSS/P3HT:PCBDR/TiOx/Al. (B) J
V curves of the BHJ solar cell devices of P3HT/PCBM (1:1, wt), P3HT/PCBDR (1:1, wt), and P3HT/PCBDR (1:2, wt), under AM 1.5 G illumination. One (wt)% of 1,8-octanedithiol (OT) was added to each of these blends as the active layer of the devices. (C) External quantum efficiency (EQE) spectra of the solar cell devices as shown in panel B. The elliptic circle highlights the difference of EQE in the region where DR absorbs light. (D) UV
vis absorption spectra of P3HT/PCBM (1:1, wt), P3HT/PCBDR (1:1, wt), and P3HT/PCBDR (1:2, wt) in films.

Table 1. Solar Cell Performance Parameters of P3HT/ PCBM (1:1, wt), P3HT/PCBDR (1:1, wt), and P3HT/ PCBDR (1:2, wt) Blends fill

Jsc components

(mA cm
2) Voc (V) factor

power conversion efficiency (%)

P3HT/PCBM (1:1, wt)

7.4

0.48

0.59

2.1

P3HT/PCBDR (1:1, wt) P3HT/PCBDR (1:2, wt)

4.3 3.2

0.46 0.40

0.46 0.45

0.90 0.58

natural photosynthetic systems31
33 and synthetic molecular tetrads or pentads34,35 to achieve long-lived charge separated states and reduced charge recombination. It has been rarely utilized, however, in organic BHJ solar cells.36 While DR is able to pass the electron from P3HT to PCB in the redox relays, it is likely that the electron transfer from P3HT directly to PCB may be much faster than the electron relay process. In BHJs with P3HT and PCBDR, we expect that all pathways for the generation of charge are operable. 2.3. PCBDR As an Electron Acceptor in BHJ Solar Cells with P3HT As the Donor. We fabricated BHJ solar cell devices using a blend of P3HT:PCBDR (1:1, wt.) in a device architecture of ITO/PEDOT:PSS/P3HT:PCBDR/TiOx/Al. In the absence of any additive in the active layer, the device showed disappointing efficiency (0.07%), giving a short circuit current (Jsc) of 1.3 mA cm
2, the open circuit voltage (Voc) of 0.25 V, and a fill factor (FF) of 0.20. In contrast, with the addition of 1 (wt)% of 1,8-octanedithiol (OT) in the blend of P3HT:PCBDR (1:1, wt) in ortho-dichlorobenzene (ODCB), the efficiency of the solar cell device increased to 0.90%, with Jsc of 4.3 mA cm
2, Voc of 0.46 V, and a FF of 0.46. J
V curves of the films under AM 1.5 G illumination are shown in Figure 5B. The efficiency decreased to

0.58% when the weight ratio of P3HT:PCBDR increased to 1:2. The highest efficiency (0.90%) obtained so far with P3HT: PCBDR blends remained lower than that (2.1%) of the P3HT: PCBM blend (1:1, wt) under similar conditions (Table 1). We expect that these values can be optimized by further exploration of processing conditions, but for the purpose of examining the details of the charge generation process, these efficiencies are adequate. What is more important for the purpose here is the relative enhancement of external quantum efficiency (EQE) (Figure 5C) in the regime of 400
600 nm, where DR absorbs light. In BHJs with PCBM, there is a dip in the EQE near 450 nm due to the decreasing absorbance of P3HT. In contrast, no such dip near 450 nm, where DR absorbs, is observed in BHJs with PCBDR implying that the light absorption from DR in PCBDR contributes to the photocurrent, consistent with the photoconductivity results as shown in Figure 4A. To investigate factors that affect the performance of P3HT: PCBDR solar cell devices, we first measured the electron mobility of PCBDR in field effect transistor (FET) devices. The results are shown in Figure 6. The electron mobility of PCBDR films at room temperature was 1.7  10
3 cm2 V
1 s
1, which gradually increased to 3.9  10
3 cm2 V
1 s
1 upon annealing at 250 °C. This mobility is comparable to that (4.0  10
3 cm2 V
1 s
1, at room temperature) of PCBM. Therefore, we do not believe that the electron mobility of PCBDR is the main factor that limits the efficiency of solar cells. We further employed UV
vis absorption spectroscopy to examine the effects of PCBDR as well as the additive, OT, on the interchain ordering of P3HT in the blend films. The results are shown in Figure S10A of the Supporting Information. The device processed from P3HT:PCBM:OT shows three features in the absorption spectrum: two absorption peaks at 520 and 550 nm, as well as one shoulder at 600 nm due to strong interchain interactions.37 1316

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Figure 6. (A) Schematic structure of bottom-gate, top-contact FET device using PCBDR as the active layer. (B) Transfer curves of PCBDR FET devices after being annealed at different temperatures. Output curves of PCBDR FET devices before annealing (C) and after being annealed (D) at 250 °C for 10 min.

Figure 7. (A) Molecular structures of CP (precursor) and BP (donor) as well as a schematic presentation of the transformation of CP to BP upon being annealed at 180 °C on PEDOT/PSS-coated ITO substrates. (B) Energy-level diagram of BP, disperse red (DR), and PCB in a device architecture of ITO/PEDOT:PSS/BP/PCBDR/Al. (C) J
V curves of the BHJ solar cell devices of BP/PCBM and BP/PCBDR, under AM 1.5 G illumination. (D) J
V curves of the BHJ solar cell devices of BP/PCBM and BP/PCBDR, in the dark and under AM 1.5 G illumination.

In contrast, in the blend film of P3HT:PCBDR (1:1 wt) in the presence of 1 (wt)% OT, the absorption at 550 and 600 nm was suppressed relative to that at 520 nm. In the absence of OT, the blend film of P3HT:PCBDR (1:1 wt) showed much lower absorption at 550 and 600 nm, respectively, corresponding to a poor performance in the solar cell devices. These results indicate that interchain interaction of P3HT is suppressed in the blend of P3HT:PCBDR compared to that of P3HT:PCBM blend.

The presence of OT promotes the phase separation in the blend of P3HT:PCBDR, which was further supported by atomic force microscopy (AFM) measurement as discussed in the following. The surface morphologies of the blend films of P3HT:PCBM: OT, P3HT:PCBDR:OT, and P3HT:PCBDR are shown in Figures S10B
D of the Supporting Information, respectively. The surfaces of the films from both P3HT:PCBM:OT and 1317

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P3HT:PCBDR:OT appeared relatively rough, with rms values of 8.5 and 9.8 nm, respectively (Supporting Information, Figures S10B
C). In contrast, the surface of the film from P3HT:PCBDR without the additive appeared rather smooth (Supporting Information, Figure S10D), with a rms value of only 0.6 nm. These results indicate that the blend of P3HT:PCBDR undergoes very different phase separation from that of P3HT:PCBM, presumably due to the better miscibility of PCBDR with P3HT. Moreover, these results imply that further optimization to tune the phase separation and ordering of P3HT may lead to higher efficiency of BHJ solar cells. We note that there is much overlap of the absorption between P3HT and PCBDR in the visible range and that PCBDR could be a more efficient acceptor than PCBM for other low-band gap polymers with both HOMO levels similar to that of P3HT and complementary absorption to that of PCBDR. Our conclusion is consistent with previous studies indicating that crystallization of fullerene molecules38
40 and/or poly(3alkylthiophenes)41 shifts the energy of charge transfer states. This shift is directly manifested in the open-circuit voltage of solar cells. For instance, in our P3HT:PCBDR BHJ solar cells, the Voc of the devices decreases with the increase of the weight ratio of P3HT: PCBDR. This trend, consistent with the amorphous nature of PCBDR, also supports the conclusion that the presence of PCBDR suppresses the crystallization of P3HT in the blend. 2.4. PCBDR As an Electron Acceptor in Solar Cells with Tetrabenzoporphyrin As the Donor. We also examined the behavior of PCBDR in bilayer solar cells using a small molecule donor with which we could compare the performance of PCBDR with that of PCBM in bilayer devices with well-defined donor
acceptor interfaces. Solution-processed solar cells with efficiency of 5.2% have been achieved with blends of tetrabenzoporphyrin (BP) and silylmethyl[60]fullerene.42 The precursor to BP, 1,4:8, 11:15, 18:22, 25-tetraethano-29H,31H-tetrabenzo [b,g,l,q]porphyrin (CP), is a solution processable from organic solvents. A CP film can be converted to BP by thermal annealing at 180 °C. Once the conversion takes place, the BP film is polycrystalline and insoluble in common organic solvent (Figure 7A). The advantage of this procedure is that it allows for facile solution processing of a subsequent layer of n-type materials such as Table 2. Solar Cell Performance Parameters of BP/PCBM and BP/PCBDR Jsc components

(mA cm
2)

Voc (V)

fill

power conversion

factor

efficiency (%)

BP/PCBM


5.36

0.56

0.65

1.95

BP/PCBDR


5.06

0.48

0.64

1.55

fullerene derivatives, while leaving the bottom layer of BP intact. Taking advantage of this behavior, very recently, the Nguyen group as well as the Hawker and Chabinyc groups have developed solution-processed solar cells with BP as the donor and PCBM or [6,6]-phenyl-C61-butyric acid n-butyl ester (PCBNB) as the acceptor.42
44 The efficiencies of these solar cells varied from 1.1% to 2.8%. We expected that the characteristics of BP described above would overcome the difficulty of morphology control that we faced in BHJ solar cells in the blends of P3HT:PCBDR (section 2.3) or PCBDR with a diketopyrrolopyrrole derivative (Supporting Information, Figure S11). Therefore, we made a bilayer solar cell device (Device 1) with BP as the donor and PCBDR as the acceptor in a device architecture of ITO/PEDOT:PSS/BP/ PCBDR/Al. The control device (Device 2) was made with PCBM as the acceptor, while other conditions were kept the same. J
V curves of these devices under AM 1.5 G illumination are shown in Figure 7C. The efficiency (1.55%) of Device 1, with Jsc of 5.06 mA cm
2, Voc of 0.48 V, and a FF of 0.64, was only slightly lower than that (1.95%) of Device 2, with Jsc of 5.36 mA cm
2, Voc of 0.56 V, and a FF of 0.65 (Table 2). The nearly 0.1 V decrease in Voc is expected as a result of the 0.1 eV difference in the LUMO energy of PCBDR (
3.9 eV) and that of PCBM (
3.8 eV). In addition, the dark current of Device 1 is clearly lower than that of Device 2 (Figure 7D). Both devices had nearly the same FF, ∼0.64, suggesting that charge extraction is similar in both devices. These results demonstrate that the presence of the DR group in PCBDR is not causing any deleterious effects on the optoelectronic performance compared to PCBM. We note that Device 1 did not show a large enhancement of absorption in the range of 400
600 nm (contributed by DR) compared to the absorption of Device 2 (Figure 8A), due to the thickness of the BP film (∼50 nm) compared to that of the PCBDR film (∼25 nm). While BP does not have an absorption peak between 500 and 600 nm, it still absorbs some light in that region; as the EQE spectrum of the BP/PCBM device shows, the signal between 500 and 600 nm is still substantial compared to the overall EQE, even without enhanced absorption from the dyad. Thus, the addition of a thin film of PCBDR is not sufficient to cause an observable increase in current density in the 500
600 nm range. Thicker PCBDR films resulted in lower device performance, most likely due to the limited exciton diffusion and charge mobility in PCBDR. The fact that a thicker layer of BP (∼50 nm) can be used without lowering device performance perhaps suggests that the exciton diffusion length and charge mobility of BP is greater than that of PCBDR.

Figure 8. (A) UV
vis absorption spectra of BP/PCBM and BP/PCBDR in films. (B) External quantum efficiency (EQE) spectra of the films as shown in panel A. 1318

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The Journal of Physical Chemistry C Figure 8B shows that the external quantum efficiency (EQE) of Device 1 was slightly lower than that of Device 2. In addition, the surface morphology and roughness appeared similar in these two devices as measured by AFM (Supporting Information, Figure S12). We expect that further optimization of the thickness of the PCBDR layer in Device 1 may lead to higher efficiency in solar cells.

3. CONCLUSIONS We have synthesized and characterized a fullerene/disperse-red dyad (PCBDR) to enhance the photon absorption of fullerene derivatives in the visible range. PCBDR showed advantages over PCBM in several aspects such as enhanced visible-light absorption, increased solubility, and the possibility for cascaded electron transfer. The EQE spectra of P3HT:PCBDR BHJs show that the dye-attached fullerene allows efficient charge generation at the absorption maxima of the dye. PCBDR:P3HT BHJs, nevertheless, showed lower performance than P3HT:PCBM BHJs under similar conditions. Among factors that affect the efficiency of P3HT:PCBDR BHJs, we believe that the suppression of the interchain interaction of P3HT in the P3HT:PCBDR blend played a major role, presumably due to better interfacial miscibility between P3HT and PCBDR than that in blends of P3HT: PCBM. In contrast, in bilayer solar cells formed with BP, the performance of PCBDR and PCBM was similar. The nonsolubility of BP allowed for bilayer cells with similar interfacial morphology to be compared. These results suggest that fullerene dyads are not intrinsically poor choices as acceptors in organic solar cells if the morphology of the cell can be controlled. Our study suggests that a synergistic approach is critical in the design of new electron acceptors for organic/polymer solar cells. For example, the morphology of the phase separation in bulk heterojunctions of PCBDR with polymers or small molecules, which remains too empirical to control, limits the efficiency of BHJ solar cells. Further optimization of both synthetic design and morphological control is needed to develop new electron acceptors for highly efficient organic solar cells. 4. EXPERIMENTAL SECTION 4.1. Materials and Instruments. Disperse red 1 (95%) was purchased from Aldrich and used without further purification. PCBM was synthesized and converted to 1-(3-carboxypropyl)-1phenyl[6,6]C61 (PCBA) according to a reported literature procedure.45 1,4:8,11:15,18:22,25-Tetraethano-29H,31H tetrabenzo[b,g,l,q]porphyrin (CP) was received from Mitsubishi Chemical Corporation and stored in a refrigerator and glovebox, respectively. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) (99.5%) was purchased from Nano-C and stored in a glovebox.1H and 13C NMR spectra were obtained on a Varian Unity Inova 500 MHz spectrometer and referenced to the solvent peak. Mass spectrometry was performed by the UC Santa Barbara Mass Spectrometry Laboratory. UV
vis spectra were recorded on an Agilent 8453 spectrophotometer. Photoluminescence spectra were measured using a SPEX Fluorolog spectrofluorometer (Jobin Yvon/SPEX, Edison, New Jersey). 4.2. Photoconductivity Measurement. Films used for the photoconductivity measurement were spin-cast onto alumina substrates from a 1 wt % dichlororbenzene solution, comprising pure P3HT or P3HT:PCBDR (or PCBM) at 1:1 weight ratio, resulting in a film thickness of ∼100 nm. The Auston switch

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sample geometry was used, with a 50 μm gap between the evaporated gold electrodes. Steady-state photoconductivity measurements were carried out using monochromatic light generated from a tungsten lamp source; the incident light beam was mechanically chopped at 170 Hz to enable the use of the standard lock-in amplifier modulation technique. The applied electric field was F = 4.0  104 V cm
1. Samples were kept under dynamic vacuum (