LI Chang(李畅)， XUE Wei(薛唯)， ZHANG Ting(章婷), YU Zhi-nong(喻志农)， JIANG Yu-rong(蒋玉蓉)

(School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China)



Effect of dual annealing upon photovoltaic properties of polymer solar cells based on poly(3-hexylthiophene)

LI Chang(李畅)， XUE Wei(薛唯)， ZHANG Ting(章婷), YU Zhi-nong(喻志农)， JIANG Yu-rong(蒋玉蓉)

(School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China)

A dual annealing method comprised of toluene vapor treatment and post thermal annealing was employed to fabricate polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) film. It is found that the P3HT crystallinity and chain ordering can be dramatically enhanced by this annealing process as compared with the films treated merely with solvent vapor annealing, which is verified by a higher X-ray diffraction intensity peak and clearly visible fibrillar crystalline domains of P3HT. The result suggests that a favorable equilibrium condition was established by dual annealing in the morphology reorganization. Due to the morphological improvement of active layer, the dually annealed PSCs show better overall performances, with a mean power conversion efficiency of 4.06% and an increase in each electrical parameter, than any solely annealed ones.

dual annealing; solvent vapor annealing; thermal annealing; polymer solar cells

Polymer solar cells (PSCs) with a bulk heterojunctions(BHJs) active layer have attracted significant interest, owing to its unique advantages of low cost and ease of large-area processing[1-4]. Of all the polymeric systems reported in the literature, poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are widely-used electron donor and acceptor for the BHJ polymer solar cells, respectively[5-7]. The ideal BHJ morphology contains nanoscale interpenetrating networks with charge-separating heterojunctions throughout the photoactive layer[8-9]. More specifically, the molecular packing of both the acceptor and donor within their respective domains must be optimized for maximum charge-carrier mobility and exciton transport. In addition, the nanophase segregated heterojunction domain size must be less than the exciton diffusion length in order to keep efficient conversion of adsorbed photons into electrical current[10].

Since the power conversion efficiency (PCE) of solar cells based on the P3HT-fullerene system depends strongly on processing conditions[11], several strategies have been used to manipulate the morphology of polymer-fullerene BHJs, including film deposition conditions and post processing techniques, such as slow growth[12], thermal annealing[13-15]and solvent vapor annealing[16-17]. Although the protocols differ significantly, the maximum PCE values reported for the various strategies are comparably 4%-5%[18]. It is the general belief that solvent vapor annealing provides potential advantages over thermal annealing, namely selective annealing of individual components and more controlled nanoscale phase segregation[17,19-20], whereas thermal annealing is prone to lead to degradation of conjugated polymers and result in large scale phase segregation[21-22]. Previous studies have showed that a successive annealing process consisting of a solvent treatment and post thermal annealing was effective in improving the efficiency of the device, yet toluene might not be a favorable choice in the solvent annealing process due to its poor solubility to PCBM[23]. In this work, a combinative annealing process were introduced to the device fabrication procedure, defined as “dual annealing”, applying toluene to the solvent vapor annealing[24]and resulting in PCE up to 4.0%. The main objective of this study is to examine the integrated effect of dual annealing upon the morphology of slow drying-cast P3HT:PCBM active layer, by monitoring the extent of crystalline within the domains of each component.

1 Experimental

All the BHJ layers in this work were prepared either on commercially available indium-tin oxide (ITO) coated glass with a layer thickness of ca.140 nm and a sheet resistance of 15 Ω/□ or on glass microscope slides. Substrates were ultrasonicated in acetone, isopropanol and deionized water sequentially and dried in a nitrogen flow, then exposed to ozone for 10 min under UV irradiation. The blend solutions were mainly made by dissolving 10 mg of P3HT (Rieke Metals,Mw≈70 000) and 84 mg of PCBM (Solenne BV) in 1 ml of 1,2-dichlorobenzene (ODCB), and then stirred at 50 ℃ overnight before use.

For device fabrication,the cleaned ITO was spin-coated with a 40-nm-thick of poly(3,4-ethylene-dioxythiophene):polystyrene (PEDOT:PSS, Clevios P VP Al 4083). The PEDOT:PSS coated samples were then heat-treated at 120 ℃ for 20 min and then transferred into a nitrogen glove box for the remainder of the device fabrication. The active layer of the solar cells were spin-coated at 800 r/min for 80 s to allow the casting films to dry slowly, where a dramatic change in the color of the film can be observed when it is transformed from the liquid (orange) to the solid (dark purple) phase[12]. For solvent annealing, the solid films were then placed in a covered glass crystallizing dish filled with saturated toluene vapor. Afterward, the samples were moved into a high vacuum chamber (ca. 5×10-4Pa), where 150 nm Al electrode were vapor-deposited through a mask leaving six solar cells with an active area of 0.09 cm2. After metal deposition, the samples were either directly measured or annealed on a hotplate at 130 ℃ for 20 min for post annealing in a glove box.

The current density-voltage (J-V) characteristics were measured using a Keithley 2612 source measurement unit under the illumination of 100 mW/cm2from a 500 W xenon lamp with AM1.5 filter. The light intensity was calibrated with a Si reference cell (Oriel). The X-ray diffraction (XRD) patterns of the P3HT:PCBM films were recorded by a Bruker D8 Advance with Cu Kα(λ=0.154 nm). The surface morphology of the blend films was examined using a multimode scanning probe microscope system (digital instruments) operated in the tapping mode. All the devices and samples were characterized under ambient conditions at room temperature.

2 Results and discussion

2.1 XRD peak analysis

The crystallinity of conjugated polymer and fullerene plays an important role in determining the optoelectrical characteristics of the devices[10]. To investigate the molecular rearrangement during annealing process, we studied the XRD patterns of P3HT:PCBM blend films annealed with different methods, as shown in Fig.1. To exclude the impact of solvent evaporation time on the resulting BHJ thin film morphology[17], all sample films were spin-cast using the “slow dried” method[18], and the XRD pattern of the pristine blend film is also plotted for comparison. The diffraction intensity peak at ca.2θ=5.4° represents first-order reflection (100) of P3HT, which is associated with the crystallographic direction along the alkyl side chains (a axis)[25-26]. Since the relative intensity of the peak is correlated with polymer crystallinity, the crystallinity within P3HT domains induced by dual annealing (DA), where toluene solvent vapor annealing (SA) employed prior to thermal annealing (PA), is superlative as compared with the solely-annealed samples. Meanwhile, both thermal annealing and solvent vapor annealing can induce the formation of crystalline P3HT. It is worth noting that there are no signs of crystalline PCBM under any treatment in the scanning scale of 3°-25° (2θ), which indicates that PCBM could not form ordered crystals in the presence of a large proportion of copolymers.

Fig.1 XRD patterns of blend films treated by different annealing processes

In addition, the diffraction intensity peak of thermal treated films shifts “blue” slightly, as compared to the solvent annealed films. Consequently the interlayer spacing for thermal treated films, regardless of solvent annealing, is determined to be 1.65 nm according to Bragg’s law, which indicates that toluene solvent annealing tends to “compress” the interlayer in a lamellar structure as shown in Fig.1. To further increase solvent annealing duration from 20 min to 1 h, the angle between the incident and scattered X-ray wavevectors is also increased from 5.37° to 5.48°, corresponding to an interlayer spacing of 1.62 nm and 1.60 nm respectively. These results suggest that in a dual annealing process, thermal annealing possibly possesses a dominant position in improving the ordering of the alkyl chains, and leads to a more extended conformation of the alkyl chains and a larger layer spacing[10]. Besides, the integrated effect of dual annealing could further optimize the morphology of blend films to a higher level. The improvement of the crystallinity and main chain order is deemed to enhance device photovoltaic characteristics[26].

2.2 Devices fabrication and characteristics

Polymer solar cells based on P3HT:PCBM were fabricated with different treatments and the measured device parameters are summarized in Tab.1. Due to the inferior performance with the PCE less than 1%, the as-product device will not be discussed here. Fig. 2 shows the statistical data for device parameters derived from six cells of each type. As expected, the maximum power conversion efficiency of 4.12% is attained for the dual annealed device, where the thermal annealing section was employed after metal electrode deposition. A nearly identical checkmark-like trend in every single parameter has been observed for the devices treated with merely post thermal annealing, solvent annealing and a dual annealing process. In comparison with the solar cell underwent sole annealing treatment, the dual annealed device demonstrated an improvement in overall photovoltaic characteristics, which agrees well with XRD spectra. To elucidate this observation, the series resistance (RS) of devices were calculated by fitting the J-V curves to the Shockley equation[27], as listed in Tab.1. TheRSof the cells with dual annealing treatment was the smallest amongst all devices. Given that a smallerRSis associated with higher charge mobility, which can be ascribed to better crystalline polymer within the blend films[28], these values are thus consistent with device photovoltaic characteristics and XRD results.

Fig.2 Device parameters derived from six cells under post annealing (PA), solvent annealing (SA), dual annealing (DA) and tri-annealing (TA), respectively

Tab.1 Cell characteristics measured under 100 mW/cm2illumination

TreatmentVOC/VJSC/(mA·cm-2)FFPCE/%RS/(Ω·cm2)Postannealing0 6310 200 593 784 8Solventannealing0 589 400 472 585 9Dualannealing0 6310 760 604 064 2SA+Prea+PA(TA)0 6211 080 553 755 1

aPre-annealing

Moreover, in order to better understand the role of thermal annealing in the dual annealing process, an extra heat-treatment was added to the fabrication procedure right after solvent annealing, referred to as “pre-annealing”. The output characteristics of these tri-annealed devices are also exhibited in Fig. 2. The PCE is reduced slightly from 4.06% to 3.75%, as compared with the dual annealed one. This value is almost equivalent to the cells treated with only post annealing, which suggests that the equilibrium condition of optimization established by dual annealing in the morphology reorganization, was not reserved anymore. Excessive introduction of thermal annealing would lead to unfavorable coarsening of the nanoscale phase segregation, resulting in device performance deterioration[10].

2.3 Surface morphology of blend films

To further investigate the morphological changes induced by different annealing processes, the mesoscale film morphology in the lateral direction of the blend films has been visualized using tapping mode atomic force microscopy (TM-AFM). The film processing conditions for AFM images were kept the same as those in device fabrication for accurate comparison. Fig. 3 shows typical TM-AFM phase images of P3HT:PCBM blend films treated with thermal, toluene vapor, and dual annealing. It has been found that the surface topography of the thermally annealed film is significantly rougher than the solvent annealed film; the former films have a root mean square roughness (Rrms) of 1.3 nm, compared to 0.6 nm for the latter ones. The rough surface can be deemed as a “signature” of polymer self-organization induced by thermal annealing[28-29]. Dual annealing results in a moderately rough surface withRrmsof 0.8 nm, however, the fibrillar crystalline domains of P3HT, with a lamellar periodicity of approximately 30 nm are clearly visible only in the dual annealed films (Fig. 3c), whereas fragmentary nanofibrillar domains are observed in the other annealed films (Fig. 3a and Fig.3b). This difference in film morphology suggests that the enhanced molecular mobility that a successive dual annealing afford enables the molecules to reach a more thermodynamically favorable morphology[18]. As P3HT self-organizes to fibrillar highly crystalline domains with periodicity close to twice the exciton diffusion length[12,30], the mobile P3HT chains force amorphous PCBM to form nano-clusters among these ordered structures during annealing process, hence a percolated network of crystalline P3HT and PCBM takes shape,without allowing for large scale disordered regions with harbor structural defects like chain ends and folds and tie segments[31](Fig. 2b). Therefore, the device performance improvement through dual annealing can be explained by film morphology rearrangement during this processing series.

Fig.3 TM-AFM phase images for blend films fabricated using different annealing processes

3 Conclusion

In summary, polymer solar cells fabricated with different annealing processes were examined. The solar cells which underwent the dual annealing comprised of solvent vapor annealing and post thermal annealing, exhibits high output characteristics with enhancedJSC,VOCand FF. The maximum power conversion efficiency of 4.12% is obtained from dual annealed device, which is higher than all of the solely annealed solar cells in this case. The improved performance were attributed to the improvement in P3HT crystallinity and chain ordering, which is resulted from the integrated effect of dual annealing, by taking XRD analysis and surface morphologies into consideration.

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(Edited by Cai Jianying)

10.15918/j.jbit1004-0579.201524.0416

O 43; O 469 Document code: A Article ID: 1004- 0579(2015)04- 0534- 06

Received 2014- 02- 18

Supported by the National Natural Science Foundation of China (10904002); the Excellent Young Scholars Research Fund of Beijing Institute of Technology (2009Y0408); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (3040036821101)

E-mail: zhangting@bit.edu.cn