Bipolar Charge Transport in Fullerene Molecules in a Bilayer and Blend of Polyfluorene Copolymer and Fullerene (p NA)

Bipolar Charge Transport in Fullerene Molecules in a Bilayer and Blend of...

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Bipolar Charge Transport in Fullerene Molecules in a Bilayer and Blend of Polyfluorene Copolymer and Fullerene

By Abay Gadisa,* Kristofer Tvingstedt, Koen Vandewal, Fengling Zhang, Jean V. Manca, and Olle Inganas

Efficient polymer solar cells comprise high amounts of acceptor molecules, typically [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), where the polymer and the PCBM are blended together to form bulk heterojunction (BHJ) phase-separated nanostructures. Optimized solar cells comprise at least 50wt% of PCBM[1–4] to promote dissociation of excited states (polymer exciton) and transport of free charges. Prior to dissociation, initial excitons transfer to the polymer/PCBM interface charge transfer (CT) states, and ultimately separate into free holes and electrons.[5,6] The CT state has a ground-state absorption located in the long-wavelength spectrum, where neither the polymer nor the fullerene absorbs.[5] Concurrently, the CT state emits light as observed for BHJ films.[7] Moreover, the spectral position of the interband CT state is linearly correlated to the photovoltage of polymer/PCBM BHJ solar cells.[8] PCBM also induces enhanced hole conduction in various BHJ films,[9–13] including the model poly(3-hexylthiophene) (P3HT)/PCBM.[14] The source of this hole mobility enhancement in BHJ films has not yet been explained. However,someauthorssuggestthisphenomenontobecausedby the stretching of polymer chains in the presence of PCBM.[9] However, the strong hole mobility enhancement in P3HT/PCBM at considerably high PCBM concentration,[14] cannot be explained by the stretching of P3HT chains, since P3HT can easily crystallize regardless of PCBM content. New and fast hole percolation paths via PCBM molecules have also been suggested.[1] In general, the performance of BHJ solar cells is highly influenced by the interaction of the polymer–PCBM species within a blend, resulting in varying morphological, optical, and electrical properties.

Here we present the electroluminescence (EL) emission of polyfluorene copolymer/PCBM BHJ films as a function of PCBM weight concentration, including devices with only PCBM. The polymer studied is poly[2,7-(9,9-dioctylfluoren)-alt-5,5-(50,80- di-2-thienyl-(20,30-bis-(300-octyloxyphenyl)-quinoxaline))](APFO-15), which is known for its balanced bipolar transport, good photovoltaic performance, and red EL emission.[3] We discuss the effect of PCBM concentration on the EL emission spectra of the BHJ films. The EL emission observed in the blends is quite weak since dissociation of the excited states into free carriers is a dominant effect in polymer/acceptor BHJ blends. However, the results reported here give a clear picture of the radiative recombination processes occurring within the BHJ blend. It is observed that 20wt% of PCBM results into an EL emission primarily dominated by emission of CTstates. On the other hand, increasing PCBM concentration results in an emergence of a new peak that coincides with emission from devices with only PCBM.

The later peak is also observed in APFO-15/C60 bilayer diodes. These results demonstrate that the PCBM molecules of the BHJ films contribute to both hole and electron transport. Based on these results, we discuss the origin of the bipolar transport of the PCBM molecules of the BHJ blend and its relevance to hole transport in the blend films of BHJ diodes.

APFO-15 is characterized by two absorption peaks and one EL emission peak (at 645nm) as reported previously.[3] Bias-voltage-dependent EL emission of the polymer, PCBM and polymer/PCBM blend devices are depicted in Figure 1 (left).

The emission onset voltage, Von, (see Fig. 1, right) varies with

PCBM content. For the pure polymer, Vonis about 1.6Vand drops to 1V for high PCBM concentration in the blend, as well as for pure PCBM devices. The shift of Von with PCBM concentration is evident even with 20wt% of PCBM in the blend. This indicates the modification of electron or hole injection levels in the presence of PCBM. The lowest unoccupied molecular orbital (LUMO) of PCBM, reported to be between 3.7 and 4.3eV,[7,15] is lower than that of APFO-15, which is 3.6eV.[3] That means most of the electrons injected into the blend film at low voltage have higher probability of populating the PCBM LUMO, giving rise to EL emission in the CT state or in PCBM. Increasing injected charge density will allow emission of photons with higher energy, for example, from the polymer phase.

The EL emission spectra of the devices are dominated by CT emission at all PCBM concentrations (see Fig. 2, left). The peak emission wavelengths red-shift as the amount of PCBM in the BHJ blend increases (Fig. 2, right). As compared to the pure polymer emission peak, addition of 20wt% PCBM resulted in the CT emission peak red-shifting by 187nm.

EL spectra shown in Figure 2 have shoulder emission peaks that are blue-shifted with respect to the main CT emission peak and resolve well as the PCBM content increases. The shoulder

[*] Dr. A. Gadisa, K. Vandewal, Prof. J. V. Manca

Institute for Materials Research, Hasselt University Wetenschapspark 1, 3590 Diepenbeek (Belgium) E-mail:

Dr. K. Tvingstedt, Dr. F. Zhang, Prof. O. Inganas Biomolecular and Organic Electronics, Department of Physics Chemistry and Biology (IFM), Linkoping University 581 83 Linkoping (Sweden)

DOI: 10.1002/adma.200902579

Adv. Mater. 2009, 21, 1–4 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Final page numbers not assigned

peak emission wavelengths, extracted using double Gaussians and least-square fitting method, lie at 733, 735, and 734nm for a PCBM content of 67, 75, and 80wt%, respectively. These emission peaks are all close to the pure PCBM emission peak of 738nm and do not shift with bias voltage. Pure PCBM emission spectrum is also depicted in Figure 2. We attribute the shoulder emissions recorded in the blend films to PCBM molecules. The intensity of the shoulder emission peaks is influenced both by PCBM content and bias voltage as shown in Figure 3. On the other hand, the shape and location of the shoulder peak remain the same upon changing the thickness of the active films. For devices with 50wt% PCBM, the shoulder peak does not resolve well for any thickness of the active layer.

The enhanced shoulder emission upon increasing PCBM content in the blend may have several origins. First, since PCBM makes an ohmic contact with the cathode (LiF/Al), it can be easily populated by injected electrons. Secondly, increasing PCBM loading results in an improvement of electron percolation paths, thereby increasing EL yield. Moreover, the injection of holes into PCBM molecules, either through enhanced direct injection from the anode or subsequent transfer of holes from polymer to PCBM, can be considered as a third option. Hole transfer from polymer to PCBM was previously evidenced by S. Yamamoto et al.[16] who observed the formation of PCBM cations in BHJ films of poly[2-methoxy-5-(3,7- dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and PCBM. They have shown that at low PCBM concentration (<10wt%) the charge carriers in the blend are the MDMO-PPV hole polaron and PCBM radical anion alone, while at high PCBM concentration (>30wt%) additional charge carriers, namely PCBM radical cations, were detected. Most remarkably they found a nearly exponential increase of the fraction of PCBM cations with increasing PCBM concentration in the blend, exceeding 50% at low (<40wt%) PCBM concentration. This coincides with the reported monotonic increase of hole mobility in the MDMO-PPV/PCBM BHJ blend upon increasing PCBM concentration.[9]

Recently, the highest occupied molecular orbital (HOMO) and LUMO of PCBM were determined by square waveelectrochemistry to be 6.6 and 4.1eV, respectively.[17] The HOMO of APFO-15 at 6.3eV[3] is close to that of PCBM. The rather similar HOMO values of APFO-15 and PCBM may facilitate charge transfer processes at their interface. The partial electron wavefunction overlap on both materials of the polymer/PCBM blend gives rise to the formation of interfacial CT complexes.[18] This new energetic configuration can change frontier energiesat thedonor/acceptorinterface, leading to the observed charge transfer processes.We notethatCT EL has beenreported in many polymer/PCBMBHJs recently.[7,19–21]

On the other hand, the injection EL from PCBM diodes (see

Fig. 1 and 2) is proof for direct injection of both holes and electrons into PCBM. Therefore, to understand the dominant hole injecting process into the PCBM molecules in the blend devices, we constructed APFO-15/Buckminsterfullerene (C60) bilayer diodes and analyzed their EL emission (see Fig. 4). The EL spectra of the bilayer devices are characterized by two or three EL peaks depending on bias voltages. The locations of the EL peak emissions are listed in Table 1 for all bias voltages.

The most blue-shifted peak (Peak 1, Fig. 4) barely varies with voltage and is located at 639nm, which matches with the pure polymer EL emission peak (645nm) and hence originates from the polymer. The main emission peak (Peak 3, Fig. 4) changes with bias voltage, and it is attributed to CTemission located at the bilayer interface. Due to the difference in LUMO levels of PCBM and evaporated C60 molecules, the bilayer CT emission is, in general, red-shifted with respect to that of the BHJ films with

PCBM. The different dielectric media can also contribute to these

Figure 1. EL emission as a function of voltage (left), and the onset voltage for EL emission (right). The left figure shows EL emission for various polymer/PCBM blends as well as pure materials.

Figure 2. EL emission spectra of the blend films recorded at the lowest emission voltage of 5V for the device with 20wt% PCBM content and 2V for all the other devices (left). The spectra were scaled for clarity. The arrow indicates the position of a shoulder emission peak, which is resolved well with increasing PCBM content in the BHJ blend. The pure PCBM emission spectrum is also depicted. The main peak emission is also shown as a function of PCBM content (right).

2 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–4 Final page numbers not assigned

shifts.[19] At higher bias voltages, a third EL emission peak appears (Peak 2, Fig. 4), located at a wavelength of 731nm. This emission peak is close to the emission peak of PCBM (738nm), and we attribute it to the emission of C60 molecules. Since each layer in the bilayer devices carries unipolar charge carriers, the

C60 emission is realized only under injection of holes from APFO-15 to C60. Conversely, emission of APFO-15 (Peak 1, Fig. 4) occurs only via electron injection through the C60 layer. Analogously, the PCBM EL emission recorded in the BHJ blend

therefore partly originate from annihilation of holes transferred fromAPFO-15toPCBMwithelectrons injectedintothePCBM.It is worth mentioning that analogous polymer emission in the blend is greatly suppressed by the competing fast process of electron transfer to the CT states and/or PCBM. This indicates that the APFO-15/PCBM blend is indeed greatly favorable for dissociation of photogenerated excitons in solar cells, as reported in ref. [3].

It is observed that the CT EL emission peaks shift in both device configurations, as a result of changing electric field or increasing PCBM content in the polymer/PCBM blend. The blue-shift of CTemission peak with increasing electric field both in the bilayer (Fig. 4) and blend (Fig. 2 and 3) devices can be explained based on the distribution of injected charges. At low applied voltages, recombination through the lowest lying CT state is the most dominating process. The emission blueshift occurring at high fields is attributed to an increasing population of the higher energy states including that of the pure materials, which accordingly allows for blue-shifted emissive recombination. This might equally well be true for the spectral blue-shift observed for the blend devices. In the blend, however, addition of more and more PCBM may lead to growing aggregation of PCBM which forms nanocrystals[2] and hence leads to change in the LUMO of PCBM. Similarly, the increasing fraction of PCBM in the blend may also lead to stretching of the polymer chains, which may change the HOMO of the polymer due to increase of conjugation length. These shifts in frontier energies of the electron donor and acceptor materials may narrow the CT bandgap, which can be manifested in the red-shift of the CT emission. The increased PCBM content also has a strong influence on the dielectric media of the blend,[19] which should also translate into shifting of the CT emission. The transfer of holes from polymer to PCBM observed from the EL measurement is consistent with the PCBM cations observed in MDMO-PPV/PCBM BHJ films.[16] It is also reported that PCBM conducts both electrons and holes equally well.[1,23–25] The results presented here prove that PCBM is bipolar even in the blend BHJ films. We have reported the rather well-balanced electron and hole mobility in APFO-15/PCBM, from field-effect transistor measurements.[3] The reported high hole mobility in polymer/PCBM BHJ films,[9,10] therefore, most probably occurs due to conduction of holes in the percolation path of the PCBM network. This bipolar nature of PCBM hence may contribute to efficient collection of photocurrents in BHJ solar cells. PCBM has

Figure 3. EL spectra of the blend films recorded at various bias voltages. For a given blend, the shoulder emission (731nm) grows with increasing bias voltage. This emission becomes more evident for devices with 75 and 80wt% of PCBM. The polymer/PCBM blend ratios are shown in the figures as I:X, where X is the PCBM content. The arrows indicate the position of the shoulder peak.

Table 1. EL emission peaks of APFO-15/C60 bilayer diodes, as extracted from Figure 4.

Bias voltage [V] Peak 1[nm] Peak 2[nm] Peak 3[nm]

Figure 4. Typical EL emission spectra of APFO-15/C60 bilayer device for varying bias voltages. Three emission peaks are evident, with the third shoulder emission peak (Peak 2) appearing at high bias voltages. Pure PCBM emission spectrum is also depicted for a comparison.

Adv. Mater. 2009, 21, 1–4 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3 Final page numbers not assigned remained the best acceptor material in polymer solar cells mostly due to its fast and well-balanced bipolar transport, and appreciable solubility in several organic solvents. In summary, we have measured and discussed the EL spectra of APFO-15/PCBM BHJ and APFO-15/C60 bilayer films in diodes. The EL spectra of both devices are dominated by emissions of interface CTstates, marked by a shoulder emission peak originating from the fullerene molecules. The C60 EL emission recorded in the bilayer films confirms transfer of holes from the polymer to C60 molecules, which indicates the possibility of having the same process in the blend. The bipolar charge transport nature of PCBM in the blend, as confirmed in this investigation, can enhance hole conduction in BHJ films.

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