Synergistic Effects of Electric-Field-Assisted Annealing and Thermal

Oct 12, 2016 - We propose an optimum low-temperature-based annealing procedure for semicrystalline donor–fullerene solar cells that is well-suited f...
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Synergistic Effects of Electric-Field-Assisted Annealing and Thermal Annealing in Bulk-Heterojunction Solar Cells Raaghesh Vijayan, K. Swathi, and K. S. Narayan* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru 560064, Karnataka India S Supporting Information *

ABSTRACT: We propose an optimum low-temperature-based annealing procedure for semicrystalline donor−fullerene solar cells that is well-suited for plastic and flexible substrates. This proposed alternate strategy utilizes an external electric field (EF) across the bulk heterojunction (BHJ) film during processing at a desired temperature. This processing technique is studied for different molecular weights of the donor in the BHJ blend films. The films indicate an increase in interchain interactions of the semicrystalline polymer chains and an enhancement in hole mobility with EF-assisted annealing treatment. Besides being a controlled method, this processing technique is capable of yielding solar cell devices with performance equivalent to or better than those obtained using plain thermal procedures. KEYWORDS: bulk-heterojunction solar cells, conjugated polymers, fullerenes, electric-field-assisted annealing, charge transport, MIS-CELIV

1. INTRODUCTION The realization of bulk-heterojunction (BHJ) polymer solar cells (PSCs) with efficiencies in the range of 10%1,2 and novel device architectures3,4 has opened up vast opportunities and options in low-cost photovoltaic devices. Precise control over the nanomorphology of the BHJ is critical in tapping the full potential of suitable donor and acceptor molecules. A key attractive feature of these solution-processed solar cells is the possibility of external processing treatments to control the organization of the blend components to form an underlying suitable morphology for charge generation and transport. In this work, we demonstrate that by controlling the initial state of the BHJ film formation process using an electric field (EF) as an external parameter, we can arrive at a suitable morphology with enhanced features in a facile, controlled way. These results demonstrate an alternate processing technique conducive and well-suited to flexible electronics and photovoltaics. Binary mixture films of poly(3-hexylthiophene):phenyl-C71butyric acid methyl ester (P3HT:PCBM) have been extensively studied as a model BHJ system where the different aspects of microstructure and performance are clearly established and have been well-reported in the literature.5−7 The features © XXXX American Chemical Society

defining the optical and electrical properties have been correlated to microstructure aspects in these films.8,9 Solution-processed P3HT-based BHJ films have been found to comprise a three-phase morphology with relatively purephase regions of donor and acceptor domains and a third phase of intricately intermixed amorphous regions of both components.10−12 Though the amorphous regions tend provide larger interface sites for carrier generation, the charge transport is limited by the nongeminate recombination processes. Purephase domains of optimal size are crucial for charge separation and charge extraction. The distribution and amounts of the three phases in the BHJ film play a critical role in device performance and are found to be dependent on parameters such as the molecular weight (MW) of P3HT and the processing conditions. Ordered domains of the donors are favored for chains with MW extending to 40 kDa,13 beyond which significant increases in the chain entanglement and Special Issue: Focus on India Received: July 30, 2016 Accepted: October 4, 2016

A

DOI: 10.1021/acsami.6b09480 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces amorphous content dominate.14 Thermal annealing procedures are usually conceived to increase and optimize the fraction and size of ordered regions in semicrystalline donor BHJ films. The temperature and duration of the annealing required for optimum PSC device performance are found to strongly depend on material parameters such as the solvent15 and the molecular weight of the donor.16 There are different views on the origin of the driving factors for the phase segregation of the relatively pure-phase domains. Reports suggest that during thermal annealing, the ordering of the semicrystalline donor polymer molecules leads to increased π−π stacking initially and allows PCBM to crystallize over longer time scales.17 The crystallinity of the P3HT domains observed through XRD studies has been directly correlated to the solar cell device efficiency.8 Other reports advocate that the PCBM aggregation kinetics competes with P3HT crystallization.18 Prolonged thermal annealing at higher temperatures beyond the glass transition in general shifts the BHJ film to a larger phaseseparated regime, diminishing the advantage of the BHJ for charge generation processes. Processing with high-boiling-point solvents in the presence of additives/plasticizers has been found to provide further control of the growth kinetics required for a desired morphology.19,20 For large-scale processing, thermal annealing poses a limitation since higher temperatures and longer annealing times are detrimental for plastics/flexible substrates in general. For PET/ITO substrates, thermal treatments exceeding 150 °C were found to introduce significant increments in sheet resistance and physical deformation.21 Lowering the processing temperature is an often underlooked but pivotal necessity for scaling up of PSCs. Other strategies such as solvent annealing have solvent toxicity drawbacks and restrictions during implementation at large scale. In this context, we seek to understand the evolution of BHJ morphology upon electric-field-assisted annealing treatment (EF treatment). External EF effects in polymer thin films occur at various length scales and depend on dipolar characteristics of the molecular components, crystallinity, and the macroscopic dielectric constant. Earlier studies of EF-induced hydrodynamic instability of elastic and viscoelastic thin-film surfaces have revealed defined patterns at characteristic wavelengths.22−25 In the case of polymer semiconductor blend films, reports of electric-field-assisted thermal annealing of BHJ films have shown that this technique improves the hole mobility and crystallinity of the donor domains, subsequently improving the solar cell device performance.26−28 Applying an EF over BHJ films during solvent annealing has shown similar results and an increment in photoconversion efficiency (PCE).29 The underlying factors behind the enhancement of BHJ systems that occurs over multiple length scales ranging from the molecular level to macro domain sizes needs to be investigated. The interface ordering and tendency of the PCBM acceptor to diffuse and form aggregates are also expected to be influenced by EF. We seek to resolve these different effects and emphasize the similarity/differences of the EF treatment with that of conventional thermal annealing procedures. We highlight the utility of this approach and generalize its applicability by studying different BHJ blends. Varying the molecular weight of the donor polymer allows us to control the initial state of the film formed. BHJ films with higher-MW (HMW) donors are expected to have larger amorphous contents than BHJ films with medium-MW (MMW) donors. Inertial forces and conformational constraints

in HMW polymer systems can dominate and minimize the effect of standard thermal procedures, which may be the reason for the observed low-performance levels. The EF approach offers a viable processing route to overcome these issues of HMW donor systems. The macroscopic effect of the EF strategy in the film reorganization becomes evident in these MW-dependent studies. Additionally, the HMW donors are expected to form more stable films with better aging properties.30 A stable morphology preserved over prolonged exposure of sunlight and usage is highly desirable. We present results on EF treatment of BHJ films (at temperatures T in the range of 30−130 °C), resulting in films with characteristic microstructure. These structural observations appear to be equivalent to those obtained from conventional thermal annealing (T ≈ 140−160 °C). This processing technique yields comparable solar cell device performances at lower processing temperatures for a variety of BHJ systems.

2. EXPERIMENTAL SECTION 2.1. Materials. HMW P3HT (88 kDa, 99% regioregular (RR)) was obtained from Sycon Polymers India Pvt Ltd, and MMW P3HT (47 kDa, 95% RR) was procured from Luminescent Technologies (Taiwan). Phenyl-C71-butyric acid methyl ester (PC70BM) was procured from American Dye Source Inc. (Baie D’Urfé, QC, Canada). 2.2. Solar Cell Device Fabrication. Precleaned and patterned indium tin oxide (ITO) substrates were spin-coated with a zinc oxide (ZnO) nanoparticle dispersion (purchased from Sigma-Aldrich) in ethanol. Thermal annealing of the ZnO layer was done at 120 °C for half an hour. Typically, polymer at 14 mg/mL in a 1:1 ratio with acceptor was stirred in dichlorobenzene at 55 °C overnight. The active layer was then spin-coated on the ZnO-coated ITO substrates inside a nitrogen-rich glovebox. This was followed by thermal annealing or EF treatment as described below. Standard device fabrication involved thermal annealing of the BHJ layer at 130 °C for 10 min and gradually cooling thereafter under inert conditions. EF treatment was carried out by maintaining a constant direct-current (DC) voltage across the BHJ/ZnO-coated bottom ITO electrode (negative bias) and a suspended top ITO electrode coated glass plate (positive bias) during thermal annealing. An air gap (∼200 μm) between the ITO substrates was created using a Teflon spacer (the experimental setup is shown in Figure 1). The BHJ spin-coated

Figure 1. Experimental setup for electric-field-assisted thermal annealing treatment. substrate was transferred within 2 min onto the sample stage for EF treatment. EF was applied during the treatment and was maintained across the electrodes from the initial stage of the films with residual solvent and right through the annealing process in the glovebox environment. Conventional thermal annealing treatment of the films was also carried out using a similar arrangement with a top electrode separated from the BHJ-coated substrate by a spacer without the EF. This was done to compensate for the effect of confining the solvent vapor during both the treatments. Thermally annealed and untreated B

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ACS Applied Materials & Interfaces films are denoted as zero-EF and control films, respectively. In the case of EF treatment, for voltages of 8, 40, and 200 V maintained with an air gap of ∼200 μm, the EF strengths translate to 4 × 102, 2 × 103, and 1 × 10 4 V/cm, respectively, and the treated samples are correspondingly denoted as low-EF, med-EF, and high-EF. The devices (electrode area ≈ 7 mm2) were completed by thermal evaporation of an 8 nm layer of molybdenum oxide and a 100 nm layer of silver. A large number of devices (>100) of each type were fabricated for each measurement. The data presented in the Results and Discussion represent the typical trend that devices respond favorably to the external EF. The sizable statistics obtained from the studies ensured that the trend levels are significant despite the inherent variability present in the devices and affirm that the BHJ films respond to the external EF. Studies involving EF treatment of BHJ films at lower temperatures were carried out by heating the films under EF to the desired temperature and cooling with the EF. Results from this type of procedure for devices heated to T ≈ 70 °C are presented later in the paper. 2.3. Solar Cell Device Characterization. Light measurements were done at 1 sun illumination (AM 1.5 Global) provided by a Newport class AAA solar simulator. A source meter (Keithley 2420) interfaced with data acquisition software (Oriel Instruments I−V test station) was used to obtain solar cell characteristics. For external quantum efficiency (EQE) measurements, a light source (Zolix LSH T150 tungsten halogen lamp) coupled with a monochromator (SPEX 500) was used to illuminate the device area. The EQE was measured under short-circuit conditions using an electrometer (Keithley EM 6514). The light intensity was calibrated using a silicon detector (UDT Instruments). 2.4. Device Fabrication for MIS-CELIV Mobility Measurements. Precleaned and patterned ITO substrates were spin-coated with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) at 4500 rpm and annealed at 120 °C for 1 h. BHJ layer spin-coating followed the same procedure as mentioned above for the solar cell fabrication. The devices (electrode area ≈ 9.62 mm2) were completed by thermal evaporation of 100 nm layers of LiF and Al at a base pressure of 10−6 mbar. 2.5. MIS-CELIV Mobility Measurement. For metal−insulator− semiconductor charge extraction by linearly increasing voltage (MISCELIV) measurements, a Tektronix 1022 arbitrary function generator was programmed to generate a CELIV ramp pulse with adjustable slope and offset. A LeCroy Waverunner A6100 digital storage oscilloscope was used to record the response signal. The measurements were done under dark conditions. The experimental setup for MIS-CELIV is shown in Figure S6. For absorption, atomic force microscopy (AFM), and X-ray diffraction (XRD) studies, the BHJ blend was spin-coated on precleaned ITO. EF and thermal annealing treatments were carried out as explained earlier. Absorption spectra of thin films were obtained using a PerkinElmer Lambda 750 UV/vis/NIR spectrometer. For surface imaging, a JPK Instruments Nanowizard 3 atomic force microscope was used. The AFM head was mounted on an inverted microscope (Carl Zeiss). Feedback was controlled using a fourquadrant position detector measuring the deflection of the 810 nm laser from the AFM cantilever as the tip was scanned over the surface. Out-of-plane grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns were obtained using a Rigaku SmartLab X-ray diffractometer system.

crystallinity and domain sizes. The mobility measurements of carriers in the bulk along the applied EF direction indicate the extent of (dis)order, and the AFM studies reveal the EFtreatment-induced surface features. All of these observations are finally manifested in the solar cell performance. 3.1.1. Absorption Studies. The analysis procedure to elicit the structural reorganization upon thermal annealing was followed for BHJ films subjected to EF treatment. This analysis of absorption spectra of BHJ films provides insight into the degree of orientation and crystallinity in P3HT.31,32 Absorption spectra of the BHJ blend thin films formed using different processing conditions for both donor polymer molecular weights are shown in Figure 2a,b. It should be noted that even though the absolute level changes are small in magnitude, the subtle changes in the films introduced by the processing techniques are consistently reproducible. The control experiments with thermal annealing result in features that are similar to the extensively reported results.31,32 The following are the key features of the BHJ film absorption spectra: (i) Untreated films of HMW P3HT:PCBM BHJ (Figure S1) have higher amorphous content than those of MMW P3HT:PCBM BHJ, as is evident from the more blueshifted peak for the former and less intense vibronic peaks around 600 and 550 nm, (ii) A red shift in the absorption spectra is clearly evident in all of the BHJ films (with differentMW donors) formed after processing under electric field and/ or thermal treatment compared with the untreated (control/ unannealed) films. (iii) Upon comparison of the low-energy features in the UV−vis spectra of the treated thin films, it is clear that EF treatment increases the crystallinity of the P3HT chains, as is evident from the increase in the vibronic peaks. (iv) The vibronic peak at around 600 nm, attributed to the interchain interaction among P3HT chains, increases with the magnitude of the EF used during processing. According to the weakly coupled H-aggregate model developed by Spano33 for P3HT chains, the exciton bandwidth W can be found from the expression W ⎛ A 0 − 0 ⎜ 1 − 0.24 hω0 ≈ A 0 − 1 ⎜ 1 + 0.073 W ⎝ hω0

⎞2 ⎟ ⎟ ⎠

(1)

with the assumption that the Huang−Rhys parameter is unity. A0−0 and A0−1 are the intensities of the absorption peaks close to 2 and 2.25 eV, respectively, and hω0 = 180 meV is the effective energy of the main intermolecular vibrational modes coupled to the electronic transitions. W varies inversely with conjugation length in P3HT chains, and a lower W signifies enhanced wave function overlap in P3HT chains. Studies of W as a function of solvent34 and different thermal annealing conditions35 have been reported, where BHJ PSCs with lower W are correlated to enhanced PCEs. In the present case, as shown in Figure 2c, the W values obtained for different annealing conditions reveal a clear trend of an inverse relation with EF strength. For MMW P3HT:PCBM blend films, W is ∼125 meV for plain thermally annealed (zero-EF) samples, progressively decreases as the EF maintained on the BHJ is increased, and plateaus at a constant value at higher EF values. For HMW P3HT:PCBM blends, W ≈ 124 meV is obtained for the zero-EF BHJ blend films. In this case, unlike the MMW P3HT BHJ counterparts, the W value keeps on decreasing as the EF during EF treatment is increased for the entire range of field values studied. With EF treatment at 1 × 104 V/cm (high-

3. RESULTS AND DISCUSSION 3.1. Characterization. A set of measurements were carried out on the BHJ blend films with different-MW donors. The chosen combination of experiments highlights the EF-treatment-induced features at different length scales. Absorption studies of the EF-treated films bring out the dipolar interaction of the P3HT monomer with the EF during the drying stage and its consequences on the organization of the film. X-ray studies highlight the EF-treatment-induced marginal increase in C

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3.1.2. X-ray Diffraction Analysis. There have been extensive X-ray analysis studies of P3HT-based BHJs where the correlation of the degree of crystallinity of the P3HT domains with the PCE has been well-established.8,36−39 GIWAXS studies of BHJ thin films to study the effect of EF treatment on a molecular scale were carried out to probe the orientation effects in the film. The bulk of the thin-film sample was investigated by having an angle of incidence (αi = 0.5°) that was higher than the critical angle (αc ≈ 0.12°). The out-of-plane GIWAXS data show distinct (100), (200), and (300) peaks of P3HT orientation around 2θ values of 5.4°, 10.7°, and 16°, respectively (Figure 3), which are in good agreement with

Figure 3. XRD spectra of (a) MMW P3HT:PCBM and (b) HMW P3HT:PCBM BHJ thin films that were thermally annealed (zero-EF) or EF-treated at 2 × 103 V/cm (med-EF) or 1 × 104 V/cm (high-EF) during thermal annealing. The insets in both graphs show the (100) peak of P3HT chains.

Figure 2. (a, b) Absorption spectra of BHJ thin films of (a) MMW P3HT:PCBM and (b) HMW P3HT:PCBM. Different fabrication parameters in the figure represented as zero-EF, low-EF, med-EF, and high-EF indicate devices treated with EF at 130 °C for 10 min at EF strengths of 0, 4 × 102, 2 × 103, and 1 × 104 V/cm, respectively. For control samples, EF = 0 V/cm at room temperature. (c) Exciton bandwidths (W) estimated from eq 1 as functions of EF for the two systems.

the literature. From Voigt fitting of the (100) peak of P3HT, domain sizes were calculated from Scherrer’s equation. The crystalline domain size increased from 12 to 14 nm for the high-EF treated HMW P3HT:PCBM BHJ films compared with the zero-EF treated films. For MMW P3HT:PCBM BHJ films, zero-EF, med-EF, and high-EF treatment yielded domain sizes of 14, 15, and 16 nm, respectively. The edge-on orientation of the P3HT chains favored during thermal annealing8,17 is further enhanced with the EF treatment. In the out-of-plane GIWAXS spectra of BHJ thin films, a crystalline PCBM peak around 2θ = 18° was observed. This feature is close to observations of PCBM aggregates centered around 2θ = 19°,17 as well as reports suggesting crystalline PCBM domains.40,41 The

EF films), W as low as 108 meV is observed. EF treatment effectively increases the interchain interaction in the dense HMW P3HT BHJ films. The continuous decrease in W with HMW P3HT reveals the possibility of overcoming the disorder by EF, suggesting an optimum methodology for fabricating efficient devices with HMW P3HT polymers. Similar trends for different annealing conditions (70 and 30 °C) were observed (albeit with different durations of EF treatment). The presence of additional EF clearly increases the effective conjugation length at various processing temperatures. D

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prevailing in the HMW systems as observed in the absorption studies. These results indicate that the larger amorphous content of the HMW P3HT BHJ film evident from structural characterization offers a higher barrier for charge transport in the BHJ. Upon EF treatment, we observe that μh increases for both BHJ blend devices by an order of magnitude, to ∼1.44 × 10−4 and ∼5.5 × 10−5 cm2 V−1 s−1 for MMW P3HT:PCBM and HMW P3HT:PCBM devices, respectively. This mobility trend further provides validation of the enhanced wave function overlap and conjugation length for charge carriers upon EF treatment of BHJ films, as seen from the absorption studies. 3.1.4. Atomic Force Microscopy (AFM) Imaging. AFM imaging of the BHJ film surface revealed significant changes induced by the EF treatment for both the MMW and HMW P3HT BHJ blends. For the MMW P3HT blend film surface, topographic images (Figure 4a,b) show an increase in surface roughness due to EF treatment. The destabilization of the surface by the electric field can be explained by different physical viewpoints. The origin of roughness values can be traced to the presence of electric dipoles along the P3HT backbone chains. It is expected that the different configurations of P3HT (edge-on and face-on) respond differently to the EF to enable maximizing the interfacial area, leading to roughening.45 In general, the controlled increase in surface roughness of the BHJ films has been reported to lead to better charge extraction of the separated charges as a result of the larger interface area with the electrode.32,46 The phase images (Figure 4e,f) show that EF treatment leads to larger domains for the MMW P3HT:PCBM BHJ films in comparison with the plain thermally annealed case. The HMW P3HT:PCBM BHJ films do not show a sizable variation in the surface roughness for the two treatments, but the phase images (Figure 4g,h) reveal a subtle increase in domain size for the EF-treated samples. 3.1.5. Solar Cell Characterization. The solar cell device characteristics obtained from a set of 10 devices in each case are summarized in Table 2, and representative current density− voltage (J−V) curves are depicted in Figure 5. From Table 2 we observe that for thermally annealed (zero-EF) samples, the average PCEs for large sets of MMW and HMW P3HT:PCBM devices are 3.09% and 2.69%, respectively, under identical conditions. The lower PCE observed for the HMW P3HT:PCBM devices is expected since HMW P3HT:PCBM films were not optimized; better devices can be obtained at higher thermal annealing temperatures or longer annealing times, but this poses constraints on the substrates and the fabrication process. It is clearly seen that for both the types of P3HT:PCBM solar cells, among the entire set of device parameters, the shortcircuit current density (Jsc) is found to vary strongly with EF treatment. For the MMW P3HT:PCBM blends at 1 sun illumination, Jsc increases by ∼8% (from ∼9.5 to ∼10.25 mA/ cm2) for med-EF-treated devices. However, when treated with higher EF strengths (high-EF treatment), the devices show a reduction in Jsc. The fill factor (FF) increases by 12% (from 0.57 to 0.64) for the EF-treated devices. Though the opencircuit voltage (Voc) remains almost independent of the processing conditions, the improvement in Jsc and FF is reflected in an improvement of the PCE from 3.09% to 3.78%. In the case of HMW P3HT:PCBM solar cells, Jsc exhibits a substantial increase from 8.44 to 10.12 mA/cm2 upon EF treatment. The other parameters (FF and Voc) remain unchanged with the EF treatment, and the PCE increases from 2.69% to 3.32%. With EF treatment, PCEs as high as

intensity of the PCBM peak decreases for the EF-treated samples, and the degree of reduction scales with the EF magnitude. Interestingly, this feature corresponding to PCBM depends on the donor MW. In the case of MMW P3HT:PCBM, the reduction with EF is gradual and the PCBM cluster signature is present at high EF, while in the case of the HMW P3HT:PCBM BHJ thin film this PCBM cluster signature is suppressed significantly. The PCBM domains were estimated to be ∼21 nm in size at low EF in both systems. Reports suggest that large-scale PCBM crystallization is detrimental to BHJ solar cell performance since electron percolation is compromised and further the formed crystals act as nucleation sites for further crystallization, which can deplete PCBM in the vicinity.9 This interdiffusion of the PCBM does not usually require high temperatures and has been shown to happen even at slightly above room temperature.11,12,42 The depletion of PCBM in the mixed phase of the BHJ would lead to a scenario where the percolation threshold for electron transport through the BHJ film would not be achieved.43 The EF treatment appears to counter the PCBM crystallization in the BHJ systems, as evidenced in these GIWAXS studies. The mechanism of restricting PCBM crystallization in HMW P3HT:PCBM films and appreciably limiting it in MMW P3HT:PCBM films needs to be studied with high-resolution structural probes. We speculate that the EF-induced molecular reorientation of the P3HT chains permits PCBM to diffuse and intercalate in the chain network, thereby increasing the interface area between the donors and acceptors. 3.1.3. MIS-CELIV Hole Mobility Measurements. It is also instructive to follow the effects of EF treatment on the hole mobility (μh) prior to examining the solar cell performance. The charge carrier mobility was studied using the dark MISCELIV technique44 systematically as a function of processing conditions. The MIS-CELIV technique extracts the unipolar charge carrier mobility in a metal−insulator−semiconductor structure and is more accurate than conventional techniques such as space-charge-limited current (SCLC) and time-of-flight (TOF) mobility studies and utilizes films of the same thickness as used for solar cell studies. Details of this technique are described in the Supporting Information. MIS-CELIV mobility measurements were carried out for devices fabricated with conventional thermal and EF treatments with annealing at T ≈ 130 °C. A sufficient number of devices (more than five for each case) made via different procedures were measured to establish the magnitude and trend of the hole mobility observed with the additional processing treatment, and the results are summarized in Table 1. In the case of the thermally annealed devices (zero-EF), these studies point to higher μh for MMW-P3HT-based blend films (∼3.2 × 10−5 cm2 V−1 s−1) compared with the HMW P3HT case (∼5.4 × 10−6 cm2 V−1 s−1) in spite of the higher effective conjugation length Table 1. Hole Mobilities Extracted from Dark MIS-CELIV Experiments on Zero-, Low-, Med-, and High-EF-Treated Devices under Thermal Annealing at 130 °C for 10 min hole mobility (cm2 V−1 s−1) EF treatment zero-EF low-EF med-EF high-EF

MMW P3HT:PCBM (3.23 (3.94 (1.31 (1.44

± ± ± ±

0.56) 0.27) 0.33) 0.31)

× × × ×

10−5 10−5 10−4 10−4

HMW P3HT:PCBM (5.46 (1.30 (5.50 (1.40

± ± ± ±

0.65) 0.34) 0.52) 0.45)

× × × ×

10−6 10−5 10−5 10−5 E

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Figure 4. (a−d) Topographic AFM images (3 μm × 3 μm) and (e−h) corresponding phase images of (a, e) thermally annealed (zero-EF) and (b, f) EF-treated MMW P3HT:PCBM BHJ films and (c, g) thermally annealed (zero-EF) and (d, h) EF-treated HMW P3HT:PCBM BHJ films.

Table 2. Solar Cell Parameters Obtained from J−V Characteristics Measured under 1 Sun Light Illumination for Control and Zero-, Low-, Med-, and High-EF-Treated Devices under Thermal Annealing for 10 min at 130 °C treatment

Voc (V)

control zero-EF low-EF med-EF high-EF

0.60 0.57 0.56 0.58 0.59

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

control zero-EF low-EF med-EF high-EF

0.62 0.55 0.56 0.56 0.56

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

Jsc (mA/cm2)

FF (%)

MMW P3HT:PCBM 4.11 ± 0.50 42.56 9.53 ± 0.16 57.01 9.95 ± 0.39 57.83 10.25 ± 0.33 63.77 9.64 ± 0.32 61.43 HMW P3HT:PCBM 3.01 ± 0.65 43.30 8.44 ± 0.20 57.37 9.48 ± 0.28 56.05 9.65 ± 0.16 58.51 10.12 ± 0.27 58.55

efficiency (%)

± ± ± ± ±

1.94 3.37 2.45 0.60 0.94

1.05 3.09 3.25 3.78 3.49

± ± ± ± ±

0.12 0.13 0.20 0.11 0.16

± ± ± ± ±

2.01 1.43 1.76 1.22 1.91

0.81 2.69 2.98 3.19 3.32

± ± ± ± ±

0.18 0.11 0.13 0.07 0.15

4.06% and 3.68% are achieved for MMW P3HT:PCBM and HMW P3HT:PCBM solar cells, respectively. The statistics and analysis of the results in every batch of devices clearly indicate that the significant changes (on the order of 20%) in Jsc and PCE induced by the EF strategy cannot be explained by any inherent statistical variations of these organic solar cells. The sizable improvement in Jsc in all cases clearly indicates a significant change in the alignment and interconnected microstructure due to the EF treatment. The enhancement in Jsc for all of the devices is strongly correlated to the increase in charge carrier mobility. It is interesting to note that the device parameters do not show a decrease at higher EF values for HMW P3HT:PCBM devices. The EQE spectra shown in Figure 6 for the MMW and HMW P3HT:PCBM solar cell devices show two distinct peaks centered around 550 and 600 nm corresponding to absorption features of P3HT. The thermally annealed (zero-EF) devices yield EQEmax = 60% and 51% for MMW- and HMW-P3HTbased solar cell devices, respectively. MMW P3HT BHJ samples treated with EF at a strength of 2 × 103 V/cm

Figure 5. Typical J−V characteristics of (a) MMW P3HT:PCBM and (b) HMW P3HT:PCBM solar cells measured under 1 sun light illumination fabricated using different processing conditions: zero-EF, low-EF, med-EF, and high-EF indicate devices treated with EF at 130 °C for 10 min at EF strengths of 0, 4 × 102, 2 × 103, and 1 × 104 V/ cm, respectively. For control samples, EF = 0 V/cm at room temperature.

(med-EF devices) show pronounced enhancement of EQE throughout the entire visible wavelength range, reaching a maximum at ∼70%. Further increasing the magnitude of the EF (>104 V/cm) reduces the EQE to ∼60%. The trend observed F

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The MMW-P3HT-based devices exhibit higher efficiency than the HMW-P3HT-based devices using thermal annealing methods similar to previous reports in the literature.13,14 Introducing optimum EF treatment increases the solar cell device efficiencies by 20% for both the MMW P3HT and HMW P3HT BHJ systems. The absolute higher-performance PCE for the MMW P3HT system comes from the initial material characteristics in comparison with the HMW P3HT case. The higher degree of chain entanglement and inertial features of the HMW P3HT system result in a weaker response to external EF treatment, especially at low EF. However, at higher EF the response is more effective. This magnitude of EF for the HMW P3HT system to surpass the MMW P3HT system values in the present case could not be accessed because of the constraints posed by the spacer and the consequent breakdown of the dielectric layer. On the basis of these observations, it is evident that EF treatment of BHJ films is equivalent to thermal annealing treatment at higher temperatures. An upper threshold of the EF is clearly present for the MMW P3HT:PCBM solar cells beyond which the performance level decreases, and in the case of HMW P3HT:PCBM this threshold magnitude appears to be quite sizable, offering a large window to improve HMW P3HT:PCBM solar cells. 3.2. Performance Optimization. The performance of the OPV devices was further optimized to yield higher PCE. The duration of EF treatment, the magnitude of EF, and the temperature were the variable independent parameters in the fabrication process. The possibility of arriving at an optimum morphology of the BHJ films without high-temperature exposure is emphasized in this section. We chose a mildly elevated temperature of 70 °C, which appears to be sufficient to realize the benefits of the EF procedure. In these studies, the duration of the treatment and the strength of the EF (160 °C) on P3HT-based BHJ thin films,46−48 where the dominant crystallization processes lead to larger-sized domains and reduce the BHJ interface density. On the other hand, for HMW P3HT BHJ devices, the EQE across the entire wavelength range increases for high EF, reaching a magnitude as high as 66% for high-EF-treated samples. The HMW P3HT:PCBM-based BHJs clearly provide a larger window for improvement by EF treatment. The charge generation factor continues to increase with EF in spite of the decrease in μh, revealing the dominant role of EF-induced interfacial ordering in these systems.

Table 3. Solar Cell Parameters Obtained from J−V Characteristics Measured under 1 Sun Light Illumination for MMW P3HT:PCBM Devices with Zero-EF, Med-EF, or High-EF Treatment under Thermal Annealing at 70 °C for Various Times Tdur Tdur (min)

treatment

20

zero-EF med-EF high-EF zero-EF med-EF high-EF zero-EF med-EF high-EF

90

150

Jsc (mA/cm2)

Voc (V) 0.57 0.56 0.58 0.59 0.57 0.59 0.57 0.55 0.57

± ± ± ± ± ± ± ± ±

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00

7.97 8.2 9.24 9.28 10.51 9.90 9.93 9.43 9.44 G

± ± ± ± ± ± ± ± ±

0.23 0.17 0.30 0.30 0.10 0.17 0.24 0.08 0.19

FF (%) 38.22 44.68 45.85 66.86 63.26 66.58 62.50 53.06 38.26

± ± ± ± ± ± ± ± ±

1.44 1.07 0.02 0.30 0.76 0.01 0.91 1.75 1.86

efficiency (%) 1.74 2.05 2.48 3.67 3.78 3.90 3.57 2.77 2.07

± ± ± ± ± ± ± ± ±

0.11 0.01 0.08 0.12 0.08 0.07 0.11 0.08 0.15

DOI: 10.1021/acsami.6b09480 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces at 70 °C (the PCEs of the best devices were as high as 4.1%). Increasing Tdur to 150 min resulted in a reduction in PCE and Jsc upon EF treatment. This shows that the system has entered a regime where EF treatment is detrimental, similar to the case we have seen earlier for standard annealing treatment with higher EF values for MMW P3HT:PCBM devices. It is interesting that a lower annealing temperature Tann can be utilized, if it is compensated by EF treatment, albeit for longer duration. The physical processes of EF treatment at higher (130 °C) and lower (70 °C) temperatures are fundamentally different, even though the outcomes under certain conditions appear to be similar. The slow dynamics upon EF treatment at low temperature is a complex phenomenon, and the evolving morphological changes are different from those in thermal annealing procedures at higher temperatures. The EF treatment studies at 70 °C were carried out to show that the treatment is effectively possible over a wider temperature range. 3.3. Effect of EF Treatment. The dynamic nature of the EF treatment, i.e., the effect of EF duration, is emphasized using absorption spectra changes. The residual solvent in the films contributes to the inherent process of chain dynamics and is a factor in the polymer film rheology and surface deformation. An important point of note is that there is no lower threshold of the electric field for the electrohydrodynamic instability to prevail in these films because of the dissipative character of the viscous drag that opposes the destabilizing electrostatic force. Hence, the finite presence of an electric field can introduce perturbations that can grow. Upon solvent expulsion, the electric field response diminishes rapidly as the film stiffens. The need for residual solvent in the EF treatment process at low T is evident in the experiments involving the evolution of absorption spectra. These studies (see the Supporting Information) essentially point out that the dynamics due to EF treatment leads to a reduction in the parameter W within the early stage of film alteration (t < 3 min). In situ optical measurements are presently being pursued to view these structural changes induced by the additional EF treatment. The variations in W over time for EF-treated and control thin films of pristine MMW P3HT have been plotted in Figure 7. The studies were carried out by varying the thickness of the film by coating at higher spin speeds, thereby effectively removing residual solvent content in the thin film as the thickness is reduced. In the case where EF treatment was carried out at room temperature for more than 6 h on MMW P3HT:PCBM BHJ films that were spin-coated at low spin speeds to intentionally retain large solvent content, a distinct surface morphology pattern was obtained (shown in Figure S3). The spontaneous periodic pattern appears to be similar to electrohydrodynamic instability patterns observed in polymer films due to application of an electric field.22−25 These spontaneous field-induced features are not visible for films with low solvent content. The surface instability patterns for stiffer films require much higher EF thresholds and do not come into play for the EF regime used in the present work. The results from the different experimental measurements reveal the effects of EF treatment. The active BHJ layer formed under EF appears to influence each of the important processes of charge carrier generation, transport, and extraction in the organic solar cell. The morphology derived from the two components of the BHJ and defined by the different phases and domains is a function of EF treatment parameters (magnitude

Figure 7. Exciton bandwidths (W) obtained from absorption spectra of pristine MMW P3HT thin films with different thicknessed dried under EF (104 V/cm) at room temperature along with a control sample (EF = 0). The thicknesses of the thin films were (a) 120, (b) 75, and (c) 65 nm.

of EF, duration of EF, and temperature). The size and distribution vary with these parameters. For the case of P3HT:PCBM systems, the PCBM acceptor, with its low dipole moment associated with the low symmetry and strong intermolecular interactions, is primarily inert toward the EF. However, the partial reorientation of the donor molecule provides additional room for the acceptor molecule to disperse in the BHJ film, thereby allowing control over the phase segregation kinetics. The dependence of the EF treatment on the MW of the donor shows that partial reorientation of the donor upon EF treatment is possible. Simulation models with EF factored into the formalism should provide insight into the relevant energetics and thermodynamics to arrive at particular structure in this binary component system. The influence of EF on the crystallinity and the D−A interfacial energy can be estimated. The secondary effect of PCBM diffusion through the H

DOI: 10.1021/acsami.6b09480 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS The authors acknowledge Prof. Anil Kumar and Sycon Polymers India Pvt. Ltd. for providing HMW P3HT.

crystalline and amorphous phases can be clearly seen in out-ofplane GIWAXS studies. The additional factor of EF in the thermal annealing process introduces a directional component, enabling a desired vertical gradient and distribution required for efficient OPVs. The equivalent effects of thermal and EF treatments, i.e., kBT and the electrostatic energy (1/2Cϕ2, where C is the effective capacitance and ϕ is the effective voltage drop across the air gap of thickness d and the blend film), respectively, with the magnitude and duration as parameters to obtain similar morphology poses an interesting question. The synergistic effect of the electric field treatment and the thermal energy is evident in the P3HT-based BHJ system upon examination of the performance and structure in the medium-T (130 °C) and low-T (70 °C) regimes subjected to different levels of EF treatment. The equivalent performance of the devices possibly arises because the active layer accesses similar morphologies/ microstructures. The requirement of a longer duration of EF at lower T poses a constraint on the use of this strategy. However, it should be noted that this lowering of annealing temperature even in a limited range offers additional choices of substrates. EF treatment is not restricted to P3HT:PCBM solar cells. The efficacy of this treatment on solar cells with the amorphous donor polymer PBDTTT-C-T and PCBM has also been observed. Thermal annealing of these amorphous donor systems has been shown to have minimal impact. However, maintaining EF during solvent evaporation at room temperature has shown considerable enhancement. Detailed studies of these systems and other small-molecule blends are presently underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09480. Absorption spectra of untreated BHJ blends, in situ absorption studies of BHJ blend films at room temperature, description of the MIS-CELIV mobility measurement technique, SCLC mobility studies of pristine HMW P3HT thin films, and electric-field-assisted thermal annealing on 66 kDa P3HT:PCBM BHJ solar cells (PDF)

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REFERENCES

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4. CONCLUSION EF treatment is an effective strategy in the fabrication of solution-processed BHJs for solar cells. The present studies clearly indicate that the EF annealing procedure is capable of organizing the microstructure and can be tuned to tailor specific BHJ components. Importantly, the thermal factors and issues that come into the design of substrates for lightweight, flexible, low-cost solar cells can be circumvented by this method. It is expected that this method will be an integral step in the protocol for manufacturing solution-processed PVs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was funded by support from the Indo−UK APEX and SERI Project, DST, Government of India. Notes

The authors declare no competing financial interest. I

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J

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