Introduction

Organic photovoltaic cells (OPVs) offer to be a promising source of electrical energy owing to their low fabrication cost and ease of processing on flexible substrates.1, 2, 3, 4, 5, 6, 7, 8 The most promising OPV is that based on regioregular poly(3-hexylthiophene) (P3HT) as a donor material blended with the soluble fullerene, (6,6)-phenyl C61 butyric acid methyl ester (PC60BM), as an acceptor material to form a bulk-heterojunction (BHJ) layer.9, 10 Many researchers have investigated the influence of weight average molecular weight (Mw) on the performance of P3HT:PC60BM-based BHJ OPVs to obtain high performance.11, 12, 13 For example, Schilinsky et al.14 characterized BHJ OPVs using P3HTs with a Mw ranging from 3100 to 25 700. They found that high power-conversion efficiencies (PCEs) of >2.5% were obtained for P3HTs with an Mw of >20 400. Hiorns et al.15 characterized BHJ OPVs using P3HTs with an Mw ranging from 5400 to 308 000 and found that an optimized annealing temperature was required for increased performance due to the high Mw. Overall, these previous works have demonstrated a strong correlation between photovoltaic performance, the Mw of a π-conjugated polymer and the aggregation state of the polymer in PC60BM-blended films.

In the present work, we focused on P3HT with a high Mw, which has a high viscosity in solution. We demonstrated that the amount of both donor and acceptor materials in the fabrication of BHJ OPVs could be strongly reduced using a high-viscosity P3HT solution with an Mw of 680 900. In addition, only slight fluctuations were observed in PCEs of around 3.8% for all OPVs based on P3HT for Mw ranging from 46 600 to 680 900. In this way, we found that a higher Mw possessed advantages over low Mw P3HT compounds in the OPV fabrication process.

Experimental procedure

P3HT batches with different Mw were supplied by Soken Chemical & Engineering Co., Ltd (Sayama, Japan). Synthesis of these P3HTs will be reported in due course. P3HT batches in this study had the following properties, where Mn corresponds to number average molecular weight determined by gel permeation chromatography (GPC) using polystyrene standards (HLC-8320GPC EcoSEC, Tosoh Co., Tokyo, Japan), PD is polydispersity and RR is regioregularity: Mw/Mn=46 600/23 000 with PD=2.03 and RR=98%, Mw/Mn=341 400/140 400 with PD=2.43 and RR=99%, and Mw/Mn=680 900/208 600 with PD=3.26 and RR=99%, labeled low (L-Mw), middle (M-Mw), and high (H-Mw) molecular weight samples, respectively. As a reference material, commercially available P3HT (synthesized by Rieke method)16 with Mw/Mn=48 100/24 400, PD=2.00 and RR=95%, labeled R-Mw, was purchased from Rieke Metals Inc., Lincoln, NE, USA. These P3HT parameters are summarized in Table 1. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, CLEVIOS P VP AI 4083) was purchased from Heraeus (Leverkusen, Germany). PC60BM (purity 99%) was purchased from Solenne (Groningen, Netherlands).

Table 1 Characteristics of the four P3HT samples used in this study
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BHJ OPVs were fabricated in the following configuration, where ITO corresponds to indium tin oxide: ITO/PEDOT:PSS/BHJ layer/LiF/Al. The patterned ITO (10 Ω/square conductivity) glass was pre-cleaned in an ultrasonic bath of acetone and ethanol and then treated in an ultraviolet/ozone chamber. A thin layer (40 nm) of PEDOT:PSS was spin coated at 3000 r.p.m. and subsequently dried at 110 °C for 10 min on a hot plate in air. Substrates were transferred to an N2 glove box and then dried again at 110 °C for 10 min on a hot plate. An o-dichlorobenzene (o-DCB) and P3HT:PCBM blend (1:1 ratio by weight) was subsequently spin coated onto the PEDOT:PSS surface, forming a BHJ layer. The solution concentration was changed to maintain the same thickness for all BHJ layers under constant spin coating conditions (600 r.p.m. for 60 s). The substrates with the BHJ layers were dried for 10 min at 110 °C for the film spin coated from o-DCB in the N2 glove box. Finally, LiF (1 nm) and Al (80 nm) were deposited onto the BHJ layer using a conventional thermal evaporation at a chamber pressure lower than 5 × 10−4 Pa. In this way, devices with an active area of 2 × 2 mm2 were obtained.

Current density–voltage (J–V) curves were measured using an ADCMT 6244 DC Voltage Current Source/Monitor under simulated AM 1.5 solar light irradiation at 100 mW cm−2 (OTENTO-SUN III, Bunkoh-Keiki Co., Ltd., Tokyo, Japan). Viscosity measurements for each P3HT solution were carried out using a sine-wave vibro viscometer (A&D Company, Tokyo, Japan). For all measurements, a 5-mg ml−1 o-DCB solution was prepared and measured at 24.5 °C under a constant frequency of 30 Hz. Film thickness was measured using an automatic microfigure-measuring instrument (Surfcorder ET200, Kosaka Laboratory Ltd., Tokyo, Japan). Surface morphology was studied using an atomic force microscope (Nanocute, SII NanoTechnology Inc., Chiba, Japan). UV–vis absorption spectra were recorded on a U-3010 spectrometer (Hitachi High-Technologies Co., Tokyo, Japan).

Results and disscussion

As listed in Table 1, the viscosity of the P3HT solution changed from 2.12 to 6.34 mPa s with increasing Mw. In general, inks are required to possess low viscosity for inkjet printing, whereas the coating solution for screen printing is required to have a relatively high viscosity.17 Thus, the P3HT solutions in this study were found to be useful in the many fabrication routes for solution-processable OPVs, as they could be prepared with various viscosities. For spin coating, the film thickness (t) is known to depend mainly on rotational speed and viscosity, which is related to Mw and solute concentration as expressed by the empirical relationship:

where ω is the angular velocity, and k and α are empirical constants. Typically, α has a value of around −0.5 and the constant k, which contains many parameters such as the initial viscosity of the solution, increases for higher viscosity solutions.17 Thus, to achieve a consistent thickness by spin coating at the same rotational speed, the solution concentration was changed with the P3HT solution viscosity over a range of Mw.

For example, as listed in Table 2, to obtain a 250-nm-thick BHJ layer, the P3HT:PC60BM ratio was 20 mg:20 mg ml−1 for L-Mw and R-Mw P3HT, 12 mg:12 mg ml−1 for M-Mw P3HT and 9 mg:9 mg ml−1 for H-Mw P3HT. In this way, we demonstrated that amounts of both donor and acceptor materials in the fabrication of BHJ OPVs can be reduced using a high-viscosity P3HT solution. This observation could lead to further reductions in the cost of OPV fabrication. Moreover, the ability to reduce the amount of acceptor material has the advantage that an acceptor material with low solubility in common organic solvents can be used for fabricating BHJ OPVs.

Table 2 Thickness, surface roughness (RMS) of BHJ layers based on P3HT:PC60BM (1:1) and performance of BHJ OPVs
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Film properties and device performance of BHJ OPVs based on H-Mw P3HT (Mw/Mn=680 900/208 600) is another major concern, as the PCE of a BHJ OPV based on P3HT with Mw/Mn=308 000/280 000 was reported to be lower than that with Mw/Mn=31 100/14 800.15 The normalized optical absorption spectra of P3HT:PC60BM (1:1) for four films are shown in Figure 1. The absorbance spectra of the L-Mw P3HT displayed red-shifted absorption peaks with vibrational fine structures, such as shoulder peaks on each side of the maximum absorbance peak. These features are known to be enhanced by the coplanarization of polymer chains due to the ease of crystallization in P3HT with a low Mw and the corresponding π-electron delocalization. A comparison of the spectra of the P3HT films with L-Mw and R-Mw reveals that the former has pronounced vibronic peaks, indicating strong interchain interactions among P3HT chains due to the higher regioregularity in L-Mw P3HT.18 The lack of red shifts and fine vibronic peaks in the absorption maxima of M-Mw and H-Mw P3HT films is due to an entanglement effect observed in H-Mw polymers, in sharp contrast to L-Mw P3HT.15

Figure 1
figure 1

Normalized optical absorption spectra of P3HT:PC60BM (1:1) films fabricated by spin coating from o-DCB solutions at 600 r.p.m. for 60 s and then annealed at 110 °C for 10 min. A full color version of this figure is available at Polymer Journal online.

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Atomic force microscopy images for the BHJ layers of R-Mw and L-Mw P3HT:PC60BM in Figure 2a and b show a significant roughness, indicating the presence of P3HT and PC60BM aggregates from the ease of crystallization in P3HT with a low Mw. In contrast, homogeneous surfaces were observed for the BHJ layers with M-Mw and H-Mw P3HT:PC60BM as shown in Figure 2c and d. The lack of absorption shifts and the homogeneous surfaces observed for the films based on M-Mw and H-Mw P3HT indicate that the P3HT and PC60BM are homogenously intermixed and the polymer exhibits a low electronic coupling between neighboring polymer chains.

Figure 2
figure 2

Atomic force microscopy images (5 × 5 μm2) of P3HT:PC60BM (1:1) films using (a) R-Mw P3HT, (b) L-Mw P3HT, (c) M-Mw P3HT and (d) H-Mw P3HT. A full color version of this figure is available at Polymer Journal online.

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Figure 3 shows the J–V curves of OPVs based on P3HT:PC60BM blends with various different values for Mw. The J–V curves show nearly the same shapes for all OPVs. Photovoltaic parameters for ITO/PEDOT:PSS (40 nm)/BHJ layer/LiF (1 nm)/Al (80 nm) OPVs under 100 mW cm−2 of simulated AM 1.5 solar irradiation are summarized in Table 2. The enhanced red-light absorption of the L-Mw P3HT film observed in Figure 1 might lead to higher PCEs compared with M-Mw and H-Mw P3HT. However, only slight fluctuations were observed in the PCEs for all OPVs fabricated by the procedures described in this study. Although polymer aggregates in the L-Mw P3HT film lead to enhanced red-light absorption, charge transport between the aggregates is consequently hindered. On the other hand, the long polymer chains of M-Mw and H-Mw P3HT can soften polymer aggregates and prevent charge-carrier trapping at the boundaries between polymer aggregates by creating a continuous pathway through the film.19 In this way, the slight fluctuations observed in the PCEs for the OPVs may be due to the balance of enhanced red-light absorption in the L-Mw P3HT film and enhanced carrier mobility through the M-Mw and H-Mw P3HT film.

Figure 3
figure 3

J–V curves of OPVs based on P3HT:PC60BM (1:1) films fabricated by spin coating from o-DCB solutions at 600 r.p.m. for 60 s and then annealed at 110 °C for 10 min. A full color version of this figure is available at Polymer Journal online.

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Conclusion

In summary, we demonstrated that the amounts of both P3HT and PC60BM materials in the fabrication process of BHJ layers can be much reduced by using a high-viscosity solution of P3HT. The P3HT solution used in this study had the highest Mw (680 900) used to date, to our knowledge. To obtain 250-nm-thick BHJ layers under identical fabrication conditions, the required concentrations of P3HT:PC60BM were 20 mg:20 mg ml−1 for L-Mw P3HT and 9 mg:9 mg ml−1 for H-Mw P3HT. Finally, only slight fluctuations were observed in PCEs of around 3.8% for all OPVs based on P3HT with Mw ranging from 46 600 to 680 900.