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C: Energy Conversion and Storage; Energy and Charge Transport

In Depth Consideration of Vertically 3D Micro-structured Bulk Heterojunction Layers via Solvent Vapor Annealing in DR3TSBDT:PC71BM Solar Cells Yu Jin Kim, and Chan Eon Park J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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The Journal of Physical Chemistry

In Depth Consideration of Vertically 3D Micro-structured Bulk Heterojunction

Layers

via

Solvent

Vapor

Annealing

in

DR3TSBDT:PC71BM Solar Cells

Yu Jin Kim1,2 and Chan Eon Park1*

1

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang

University of Science and Technology, Pohang 790-784, Republic of Korea 2

Current address: Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass

Ave. Lemont, IL 60439, USA.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The physical nature of solvent vapor annealing (SVA) treatment is quite straightforward, and its application is ideal in small molecule-based bulk heterojunction solar cells. It has been suggested to rapidly achieve high-performance small molecule photovoltaics by alternating the blends to ideally connect the crystallite morphology. However, most previous reports on SVA have shown only influences of the degree of donor/acceptor phase separation within part of active textures. Here, we investigated solution-processed small molecule solar cells consisting of the previously developed DR3TSBDT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) after SVA in terms of the vertical gradient crystalline phase in three-dimensional (3D) active layers. These systematic studies of the vertically phase-separated morphology for 3D heterojunction structures clarified in more detail varied active blend morphology underlying SVA and showed a clear structure-property relationship in related device performance. This not only provided a clear understanding of precise effect of SVA treatments on varied 3D morphological structures, but also a path towards the rational optimization of device performance.


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INTRODUCTION Over recent years, research on organic solar cells (OSCs) using small molecules has attracted great attention, fueled by the potential for a next-generation renewable energy source offering dynamic advantages of well-defined molecular structures and high reproducibility, as well as low-cost solution-processing methods..1-4 Vigorous research endeavors have been successful in increasing the power conversion efficiencies (PCE) of this type of device to > 10%.5-6 However, current high-performance small-molecule OSCs are made predominantly according to a smart molecular design, but additional device development and optimization of microstructure morphological engineering are key strategies to further improve device performance. In device optimization, one of the major challenges is controlling the active layer morphology. In particular, vertical phase segregation and crystalline domain connectivity of active blends in a device sandwich are important. Generally, however, it is quite difficult to modify the bulk heterojunction (BHJ) morphology of donor (D):acceptor (A) blend composites, providing a large D/A interface area for effectively splitting excited states as well as a proper crystal packing structure of each phase for efficient free hole and electron transport toward the electrodes. Several methods to modify the nanoscale morphology have been devised, including solvent additives7-9, thermal annealing10-11, solvent vapor annealing (SVA)12-13, and post-additive soaking.14 These morphological modifications directly impact the optical and electronic properties of the BHJ layer and also influence the competition between carrier extraction and recombination.15-16 One of the most versatile and effectively employed technique is the SVA treatment, which allows dramatic re-organization of active compounds to well-ordered crystallite domains during



the timescale of film formation. Brabec, Jones, and Chen have shown enormous variation in device performances by BHJ textures controlled via SVA.6,17-18 However, despite huge gains in efficiency for some of the most efficient small molecule OSCs using SVA methods, there is still few understanding as to how these processing techniques can be varied to affect the three-dimensional (3D) morphological structures. In particular, most research groups that have studied SVA treatments have focused only on the changed molecular crystallinity of part of the bulk layers. As a result, detailed structural morphology, particularly with respect to the vertically phase-separated morphology of 3D heterojunction structures, and their effects on photovoltaic performance, remain elusive. Given this situation, in the current study, we examined in-depth the 3D morphology responsible for the enhancement in DR3TSBDT:PC71BM photovoltaic cell efficiency by SVA (DR3TSBDT is used as a small molecule donor and PC71BM is used as an acceptor compound4; see Figure 1a). Various solvents were used, and thus two questions were addressed: how different solvent qualities affected the vertical microstructures; and what is the 3D morphology-performance relationship. We used time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and twodimensional grazing incidence wide angle X-ray scattering (2D-GIWAXS) to systematically monitor in-depth structural changes in vertically segregated micro-scaled morphologies. This report describes efficient DR3TSBDT:PC71BM BHJ solar cells with PCE near 11%, and to the best of our knowledge, no previous report has categorized 3D active microstructures in this system.

MATERIALS AND METHODS


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Materials: DR3TSBDT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were obtained from 1-Material Inc. (St-Laurent, Quebec, Canada) and Nano-C Co., Ltd., Westwood, MA,USA) respectively, and used without further purification. The chloroform (CF), tetrahydrofuran (THF), ethanol, and dichloromethane (DCM) used as processing solvents of SVA were from SigmaAldrich (St. Louis, MO, USA).

Solvent vapor annealing treatment: SVA was conducted in ambient conditions at room temperature. The solvent (3 mL) was injected into a 50-mm glass Petri dish. The Petri dish was closed for 10 min to let the vapor saturate the treatment chamber. Then, as-cast active films were attached to the backside of a Petri dish lid, which was swapped with the lid covering the solventcontaining Petri dish. The film was about 1.5 cm above the solvent level during the SVA. There is no direct contact between solvent and sample. However, the solvent evaporated and the vapor could act on the organic material. After 1 min, the film was removed from the treatment chamber.

Film characterization: Optical absorption measurements were carried out with a Cary 5000 UVVis-NIR double-beam spectrophotometer in two-beam transmission mode. The surface morphology of active films was examined using a tapping-mode scanning probe microscope system (atomic force microscopy (AFM), Multimode IIIa; Digital Instruments, Santa Barbara, CA, USA). The bulk morphology for blend films was acquired on a JEM-2100F high-resolution transmission electron microscopy (TEM) at 200 kV acclerating voltage. The vertical phase separation of the SVA-treated films was analyzed by TOF-SIMS at the National Nano Fab Center in KAIST. For TOF-SIMS analysis, a solid sample surface is bombarded with a pulsed



primary ion beam. While the first beam is sputtering a crater, the second beam is progressively analyzing the bottom of the crater. The mass is measured using the ToF to the detector. For depth profiling, two ion beams (Cs, O2) and an Ar gas cluster source were operated in dual-beam mode. 2D-GIWAXS experiments were performed at the 3C beam line (X-ray wavelength = 0.1213 nm, incidence angle = 0.07–0.14°) at the Pohang Accelerator Laboratory (PAL) in Korea. The X-ray beams are focused both vertically (60) and horizontally (450) (in full-width half maximum at sample position) using K-B type mirrors. The scattering signal was recorded on a 2D charge-coupled detector (CCD; Roper Scientific, Trenton, NJ, USA) with a pixel size of 182 µm, and the detector was located at a distance of 227.3 mm from the sample center. The 2DGIWAXS sample stage is equipped with a 7-axis motorized stage for the fine alignment of sample. Thin film samples were prepared on silicon wafers using a spin-coating method. The data were processed and analyzed using the GISAS Plot software package. TPV measurements were performed using HuaMing equipment (model 201501). Devices were connected to a high input (1 MΩ) impedance oscilloscope (Agilent Infiniium 1 GHz; Agilent Technologies, Santa Clara, CA, USA) and a white light bias on the device was generated from an array of diodes. The voltage dynamics were recorded on a PC-interfaced Keithley 2602A source meter (Keithley Instruments, Cleveland, OH, USA) with a 100 µs response time.

Device fabrication: All the fabrication and characterization processes were carried out in air. Photovoltaic DR3TSBDT:PC71BM devices were fabricated with a general structure of patternedITO/MoO3/active layers/LiF/Al. Patterned ITO (sheet resistance, 10 Ω/cm2) on a glass substrate was first rinsed with detergent, deionized water, acetone, and isopropyl alcohol by ultrasonication, and then dried with nitrogen gas. Then, the cleaned substrates were treated with a UV


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ozone cleaner for 20 min and a 9 nm-thick MoO3 layer was deposited via a thermal evaporation method. A solution comprising 11.1 mg DR3TSBDT and 8.89 mg PC71BM in 1 mL of CF was spin coated at 1,000 rpm on top of the MoO3-deposited ITO substrate. The active films were solvent vapor-annealed with the respective solvent for 1 min. Then, 0.8 nm LiF and 100 nm Al were deposited through a shadow mask (active area was 0.04 cm2) using thermal evaporation at a base pressure of 2 × 10-6 Torr. The space-charge limited current (SCLC) devices were fabricated with active blend solutions onto the bottom electrodes. For hole-only devices, a 9-nm MoO3 layer was thermally evaporated as the interlayer between ITO and the active blend layer, followed by 110 nm Au, evaporated as the top electrode. For the electron-only devices, two Al layers were evaporated for the bottom and top electrodes.

Device testing: The current-voltage (J-V) characteristics of the photovoltaics were measured at room temperature using a Keithley 2400 source (Keithley Instruments) as a solar simulator with a light intensity of 100 mW cm-2. The light intensity was calibrated with a National Renewable Energy Laboratory (NREL)-certified monocrystalline silicon photodiode. The solar cells were tested under various light intensities by modulating the intensity of the light with a series of two neutral density filter wheels with six filters each, allowing for up to nine steps of intensity, from 100 to 5 mW cm-2. The intensity of the light transmitted through the filter was independently measured using a power meter. The external quantum efficiency (EQE) spectra were measured from a photo-modulation spectroscopic setup (model Merlin; Oriel Instruments, Stratford, CT, USA), a calibrated Si UV detector, and an SR570 low-noise current amplifier.



Dark hole- and electron-only J-V curves were fitted to the following equation, of the SCLC model, to determine the charge carrier mobilities: 9

= 0 ℎ/ 8

2 3

exp (0.891 )

where J represents the hole (electron) current, L represents the film thickness of the active layer, V represents the voltage, − (Vappl is the applied voltage to the device, Vbi is the built-in voltage resulting from the difference in work functions of the two electrodes). µh/e represents the zero-field mobility of the holes (electrons), ε0εr represents the dielectric permittivity of the active layer, and β is the field activation factor. The results are the average of eight devices.

RESULTS AND DISCUSSIONS Photovoltaic performance characteristics To investigate the impact of SVA on solar cell efficiency, we fabricated devices with a conventional geometry and indium tin oxide (ITO)/MoO3/active layer/lithium floride (LiF)/aluminum (Al). The same fabrication conditions were used for all devices, including the blend composition, solution concentration, and film thickness of the active layers (a detailed description of the process can be found in the Experimental Section), to maximize only the impact of SVA treatment on photovoltaic cell performance. The current density versus voltage curves of photovoltaic devices at different SVA conditions under AM 1.5 illumination (100 mW cm-2) are shown in Figure 1b and relevant photovoltaic properties are summarized in Table 1. The device without SVA showed a PCE of 7.66%, open-circuit voltage (Voc) of 0.88 V, shortcircuit current density (Jsc) of 14.5 mA cm-2, and fill factor (FF) of 60.1%. Notably, compared with an untreated (as-cast) device, the solar cells after SVA exhibited improvements in device


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performance: the device treated with DCM revealed a PCE of 9.96%. Photovoltaic cells with THF and ethanol treatment gave PCEs of 8.94% and 8.42%, respectively. The enhancement in PCE has contributions from two main factors: photo-current density (Jsc) and the FF (see Figure 1c and S1), which are strongly associated with nanostructural morphological characteristics in the blend films. The in-depth nanoscale morphological textures of the active films are discussed below. A further major improvement was achieved for CF-treated solar cells with a PCE of 10.83%, Voc of 0.91 V, Jsc of 17.8 mA cm−2, and FF of 67.5%, which is one of the best reported device performance. This result corresponds well to the findings of Chen’s group, who tested a series of different solvents for SVA on small-molecule solar cells and concluded that a good solvent with the CF solvent resulted in the best performance.18-19 To the best of our knowledge, this efficiency represents the one of the highest value reported in the literature to date for singlejunction solution-processed organic solar cells.20-21 The effect of SVA on the photovoltaic performance was also investigated by external EQE measurements. It can be seen in Figure 1d that broad wavelength improvements in EQE were recorded for SVA-treated samples. The calculated JSC values obtained from the integration of the EQE data for the devices using different treatments were close to the JSC values from the current density-voltage (J-V) measurements, with ~5% mismatch.

Active domain structure via SVA The lateral

morphology of DR3TSBDT and DR3TSBDT:PCBM was investigated using

tapping-mode atomic force microscopy (AFM), in which a cantilever beam equipped with an AFM scans the surface of the active layer. Figure 2 shows the AFM topographic images of the active films after SVA treatment. In pure small molecule films (Figure 2a–d), the top surface



morphology was found to be significantly affected by DCM/THF/ethanol/CF vapor annealing. For the DCM-treated film, clear aggregated domains and surface textures having a root-meansquare (RMS) roughness of 6.81 nm were observed. With THF and ethanol vapor annealing, both DR3TSBDT films showed smaller surface aggregates, and notably smaller scaled domains versus DCM films. In particular, the RMS roughnesses decreased to 5.28 nm and 4.34 nm, respectively. However, considerable aggregated surface features, with quite large circular domains and a significantly greater RMS roughness of 7.19 nm, were revealed in CF vaporannealed samples. This trend in AFM data was consistent with the features of the optical properties of the pure small molecule films on SVA with the different solvents (Figure S2 a and c, Supporting Information). Likewise, in DR3TSBDT:PC71BM blends (Figure 2e–h), relatively more aggregated domain surfaces with higher RMS roughness were seen in the sequence: CF > DCM > THF > ethanol. Obivously, the transmission electron microscopy (TEM) measurement agreed well with the AFM results: larger bright domians (DR3TSBDT aggregations) appear in the order of CF, DCM, THF, and ethanol (see Figure S3). From the AFM and TEM results, we anticipated that vapor annealing using solvents with relatively higher solubility provided a stronger driving force to facilitate domain growth, resulting in more obvious surface/bulk segregation and more ordered film morphologies of the active layer at the nanoscale. Furthermore, when we investigated the solubility of the DR3TSBDT in solvents, we found that the higher solubility value appears in the order of CF, DCM, THF, and ethanol (Table S2). Thus, we could also argue that higher solubility allowed a stronger driving force for DR3TSBT rearrangement to the aggregated/crystalline structure, the morphology are thereby varied by different solvents.22-23


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Investigation of SVA effects on vertical morphology by ToF-SIMS depth profiles AFM only provides insight into the top surface morphology, which is not necessarily representative of the bulk morphology. To investigate the influence of the SVA on the gradient 3D structure in bulk morphology, we used ToF-SIMS depth profiling. The elemental composition was monitored as a function of the film depth until the silicon signals increased sharply, indicating that the Si substrate surface had been reached. Because sulfur (S-) is only present in DR3TSBDT, it can be used as an indicator of the vertical composition of the blend films. The ToF-SIMS results in the vertical direction of blend films, along with the sputter time, are shown in Figure 3, providing clear evidence of the dramatic redistribution of component materials

throughout

the films

processed

with

different

SVA

treatments.

In

the

DR3TSBDT:PC71BM film after DCM and THF SVA treatment (Figure 3a, c), both BHJ films showed a sharp S- peak in the initial period (t < 100 s), which might be indicative of DR3TSBDT segregating at the film surface.23 From 100 s up to 400 s sputter time, abrupt compositional fluctuations appeared; the S- signal decreased to a quite low intensity. After 400 s, the S- signal intensity recovered, similar to the initial period. In a comparison of two blends, the THF-treated films shows further fluctuating S- signals, having a lower intensity than those in DCM-treated films, indicating that the DR3TSBDT compound was more sparsely spread along the vertical direction.24-25 The ethanol-annealed film did not exhibit any local segregation of DR3TSBDT components (i.e., the absence of sharp ToF-SIMS peaks), and the high S- peak remained from the surface to the mid-bulk region. For CF-soaked films, an ‘ideal’ S- signal was revealed: the maximum-intensity initial S- signal, showing the surface area, was maintained continuously to the bottom of the BHJ layers except for a short sputter period.



Based on these measurements, to clearly visually understand the DR3TSBDT distribution along the vertical direction in 3D structures, 3D renderings of the negatively charged sulfur ion were analyzed (Figure 3b, d, f, and h). The 3D renderings of ToF-SIMS depth profile data are an effective way to determine if species are enriched in selected voxels or distributed uniformly across the region being analyzed.26 From the 3D images, we can clearly see that S- was more predominant in the center bulk area with the skin surface in DCM-annealed films than in THFannealed films, suggesting that DR3TSBDT materials segregated more in the bulk region, indicating why higher device performances were achieved with DCM-annealed cells.27-28 In ethanol-treated films, the S- was evenly enriched near the top area, meaning that the DR3TSBDT molecules were clearly pushed towards the top side of the film, providing opposite pathways for the holes and electrons to that in conventional solar cells. Thus, the device efficiency was the lowest in ethanol-annealed solar cells because a reverse path-region was formed for hole and electron flow.29 However, DR3TSBDT molecules in CF-treated films were distributed uniformly in heterojunction textures, likely indicative of forming more continuous charge carrier channels in the BHJ, which notably benefitted charge transport and extraction efficiency30 such that the highest (10.83%) device efficiency was achieved.

Vertical phase segregation analysis through 2D-GIWAXS techniques DR3TSBDT:PC71BM blends treated with different SVA treatment were further examined via 2D-GIWAXS to clarify how SVA altered the molecule crystallinity in internal active textures. In particular, to understand the varied molecular packing formations in 3D structures, we investigated the preferred crystalline domain distribution by 2D-GIWAXS analysis with two different incident angles (αi = 0.085, and 0.13°), where the X-ray with an incident angle of


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0.085° (αi αc; see Figure 4).31-32 From the 2D-GIWAXS images in diffractions of all blend films, (h00) reflections were clearly observed in the out-of-plane (qz) direction (the (100) peak at ≈ 0.33 Å-1, corresponding lamellarspacing distance ≈ 19.04 Å) due to DR3TSBDT packing molecules33, indicating that most of the DR3TSBDT crystallites adopted an edge-on orientation on the substrate in the entire film.33 A complimentary (010) (located at qxy ≈ 1.67 Å-1 – π-π stacking distance ≈ 3.76 Å) diffraction in the in-plane direction also appeared. Furthermore, a complete isotropic halo peak at qxy,z ≈ 1.31 Å-1 was observed according to the presence of PC71BM domains.34 First, distinctly scattered reflection patterns corresponding to the lamellar structure (h00) and π-π stacking (010) with high intensity were seen in the sequence: CF- > DCM- > THF- > ethanol-soaked films. This was particularly apparent in the extracted profiles (Figure S4-S5, Supporting Information). From the data, we can suggest that the formation of large and well-ordered DR3TSBDT crystalline domians occurred in the order of CF- > DCM- > THF- > ethanol-vapor annealed blends.35 To support this with a quantitative analysis, the compound crystallite size was calculated from the full-width-at-half-maximum (FWHM) of a (200) lamellar peak using modern form Scherrer’s formula36 in Eq., as follows: (!"#$%& #'() = 2)*/∆,(-./0). where K is a dimensionless shape factor equal to 0.93 (see Figure S6). The estimated DR3TSBDT crystallite size was clearly enhanced when the film was vapor-annealed with solvents having a lower boiling point and higher solubility. That is, in CF-annealed films, the crystalline domain size of DR3TSBDT molecules was the largest at 194.78 Å (153.77 Å for



DCM-treated films, 135.89 Å for THF-treated films and 112.37 Å for ethanol-treated films). Thus, it is important to note that elevated average crystal size with more oriented crystalline structures is favorable for charge transport in BHJ thin films, which can lead to enhanced device performance in solar cells with CF SVA treatments.37-39 As αi increased further to 0.13°, most diffraction patterns of blend films revealed similar stacking patterns with that of near surface. In scattering results except for CF, when the X-ray penetrates the bulk area (αi > αc), the (h00) Bragg peak is slightly weakly scattered, as well as π-π stacking reflections. These findings are acceptable given that the DR3TSBDT crystalline domains are relatively more aggregated in the skin-surface, which is consistent with the ToFSIMS results above. However, diffractions in the CF-based film still indicate strong DR3TSBDT crystalline patterns, according to the distinct scattering peak with high intensity. Thus, we can carefully argue that CF-vapor treated films have ‘ideal’ 3D crystalline structures, having the most ordered DR3TSBDT crystalline domains in the whole region from bulk to the surface area. To quantify the observed changes in the bulk film crystalline morphology, we extracted the distribution of crystallinity from the diffracted 2D-GIWAXS patterns of blend films treated with various SVA treatments. This analysis followed procedures described in previous work.33 Pole figures were obtained by plotting the (200) peak intensity as a function of the polar angle, χ, which describes the relative orientation with respect to the substrate (shown in Figure 5). To understand the distribution of molecular crystallite orientations in 3D vertical structures, we carried out pole figure analysis from 2D-GIWAXS patterns measured at two diffraction points. From both pole figure analyses, the important feature is that DR3TSBDT crystallites were relatively titled in totally edge-on stacking structures in the sequence: CF- ≥ DCM- > THF- > ethanol-vapor treated films.40 Relative titled orientation has been deemed beneficial for solar cell


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performance because it can facilitate charge transport in the vertical direction between the electrodes, which explains why device performance was higher in the order of CF > DCM > THF > ethanol SVA treatments.41 On the basis of the morphological characterization results (ToF-SIMS and 2D-GIWAXS data), we propose the following picture of the structural evolution and discuss the impact of SVA on the 3D heterojunction structures (Figure 6). Notably, among the four different systems, CF annealing allowed the most ordered DR3TSBDT crystallites with titled orientation within evenly distributed molecular arrays, whereas DCM and THF treatments revealed 3D morphologies with relatively less DR3TSBDT crystalline packing structures than CF treatments in the near-surface and center-bulk area. Ethanol SVA resulted in the most random DR3TSBDT crystalline stacking with the weakest titled orientation in the top side regions. These models of morphological evolution as a function of annealing solvents are consistent with our experimental data and allow insights into the fundamental mechanisms behind SVA.

Charge transport properties probed with the SCLC model To gather evidence on the influence of the different treatments on the charge transport properties, charge carrier mobility was measured using the SCLC method. The structures of hole-only

and

electron-only devices

were

ITO/MoO3/

DR3TSBDT:PC71BM/Au

and

Al/DR3TSBDT:PC71BM/Al, to estimate the hole (µh) and electron mobility (µe), respectively. The dark J-V characteristics of both hole-only and electron-only diodes can be well fitted to the Mott-Gurney relationship for SCLC.42 As shown in Figure 7a and Table 2, µh rises gradually in the order: ethanol-treated films (1.76 × 10-4 cm2 V-1 s-1) < THF-treated films (3.32 × 10-4 cm2 V-1 s-1) < DCM-treated films (4.01 × 10-4 cm2 V-1 s-1) < CF-treated films (6.38 × 10-4 cm2 V-1 s-1). On



the basis of this hole mobility result, we propose the impact of different SVA treatments on hole carrier transport properties and discuss the relationship between DR3TSBDT crystal domain size/orientations and hole mobility: lower boiling point and higher soluility SVA allows a higher degree of DR3TSBDT crystallinity in the mixture, leading to larger packing domains, enabling higher charge carrier movement (Figure 7c). Furthermore, the more favorable backbone orientation of DR3TSBDT clearly resulted in enhanced vertical charge transport.43-44 Consequently, this hole mobility evaluation is reflected in the morphological characteristics, as demonstrated above by the AFM, 2D-GIWAXS, and pole figure data. Similar changes were observed for electron mobilities (Figure 7b); solvents with lower boiling points can elevate the electron mobility, from 2.52 × 10-4 cm2 V-1 s-1 to 6.82 × 10-4 cm2 V-1 s-1. CF SVA-treated films showed the highest electron mobility. Moreover, we also found quite wellbalanced hole and electron mobility (charge carrier ratio, µe/µh = 1.07) for composites treated with CF SVA, but the other devices only exhibited µe/µh = 1.16 for DCM, µe/µh = 1.20 for THF, and µe/µh = 1.43 for ethanol treatments. Because the accumulation of space charges and recombination of charge carriers are processes that are closely related to the carrier mobilities of holes and electrons, and the ratio between them, we conclude that CF vapor annealing was beneficial for a higher FF and was responsible for the improvement in JSC and overall performance.45-46

Charge carrier recombination in devices treated with SVA Mobility measurements give insights into transport properties, but a comprehensive understanding of device performance requires the acquisition of both recombination dynamics and transport dynamics. We first studied the relationship between photocurrent density (Jph) and


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effective voltage (Veff) of the cells to gain information about the charge generation and exciton dissociation processes (Figure 8a). The Jph in all devices with SVA increased sharply with effective voltage, and gradually reached a saturated value (Jph,sat) at higher effective voltages (≥ 2 V in this case). At high Veff values, mobile charge carriers move rapidly towards the corresponding electrodes with minimal recombination47, and thus our results suggested that all photogenerated charge carriers were extracted by the electrodes. The exciton dissociation efficiency and charge collection efficiency can be assessed by the Jph/Jph,sat under short circuit conditions.48 Near the maximum power output point, the ratio of the device treated with CF SVA was 90.2%, while the devices with DCM, THF, and ethanol solvent annealing showed 88.4%, 85.7%, and 84.1%, respectively. This result indicates that the larger value of this ratio for the CF-treated device was attributable to the nanostructural morphology in the active layers, increased hole mobility, and improved balanced charge transport.47-48 All these parameters demonstrate that CF SVA improved the exciton dissociation efficiency, charge transport and collection capability, and reduced carrier recombination. To look further into the recombination kinetics for these four devices, we investigated the dependence of JSC and FF values as a function of light intensity (illumination intensities ranging from 100 to 5 mW cm-2), as illustrated in Figure 8b and c. In principle, it has been reported that the correlation between Jsc and illumination intensity (Plight) can be addressed by Jsc ∝ Plightα, where α should be unity when second-order recombination is negligible (maximum carrier sweep-out).49 Fitting of the data yielded α = 0.89 for the DCM device, α = 0.87 for the THF device, α = 0.78 for the ethanol device, and α = 0.95 for the CF device. A value close to unity (α = 1) in the CF vapor-annealed device illustrates that carrier sweep-out was the most efficient and fully suppressed second-order recombination.50 This result is consistent with the results above: a



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high structural order typically causes high or balanced carrier transport and thus reduces secondorder recombination in devices. The decrease in FF with decreasing light intensity was lower in the sequence of CF-treated devices > DCM-treated devices > THF-treated devices > ethanoltreated devices. The smaller variation in FF values occurs with light intensity, indicating that second-order recombination losses become minor at higher current densities51-53; thus, this may be one of the reasons for the higher FF in the device with CF solvent annealing. To finally understand the recombination dynamics of the varied microstructures under the different SVA conditions, transient photovoltage (TPV) measurements were made. The TPV curves were obtained under one sunlight illumination using the same pulsed light intensity. The average lifetime of photo-generated charges could approximately be estimated by fitting a decay of the open-circuit potential transient with exp (−$

34 2 ),

where $ is the time and

2

is an average

time constant before recombination.52 As seen in Figure 8d, the ethanol SVA device showed a relatively short carrier lifetime of 0.72 µs. The THF and DCM solvent vapor-annealed devices had slightly increased lifetimes of 1.23 µs and 1.41 µs, respectively. A longer lifetime of 2.45 µs was found for the devices under CF vapor treatment, indicating that the recombination channel was shut down and, consequently, the lifetime of photo-generated charges was prolonged.54-55

CONCULSION In summary, we systematically studied the effect of SVA treatments in solution-processed small-molecule thin films on markedly varied nanoscale morphologies and showed significantly enhanced device performance. Above all, we report here the first delineation of the relationships between the 3D vertically aggregated crystalline phase and solar cell performance with various solvents in SVA-treated DR3TSBDT small molecule BHJ systems. When solvents with lower


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boiling points and stronger solubility penetrate into the active textures, dramatically increased DR3TSBDT structural order and a more ideal phase distribution can occur, leading to high and balanced hole and electron mobilities, which, in turn, significantly improve Jsc and FF. Consequently, the relative device parameters and charge transport properties were all found to peak with CF SVA treatment. Films treated with DCM vapor showed DR3TSBDT segregated domains located relatively in the near surface and center-bulk regions. For THF-treated films, DR3TSBDT molecules were widely distributed in the bulk region and exhibited less crystalline packing structures than DCM-treated films. In the ethanol vapor annealing case, its permeation into BHJ thin films resulted in top side-located DR3TSBDT phases with the strongest random molecule packing. In contrast, CF annealing resulted in a more ideal molecule distribution in the whole film as well as the largest crystals of DR3TSBDT. Also, as in the pole figure analysis extracted from 2D-GIWAXS patterns, the relative tilt orientation of DR3TSBDT crystallites appeared in the order: CF- ≥ DCM- > THF- > ethanol-soaked films. These were confirmed by ToF-SIMS and 2D-GIWAXS results. Furthermore, light intensity-dependent J-V analysis and TPV measurement indicated that defect-assisted and second-order recombination were reduced upon increasing structural order and more titled orientation to the substrate. As a result, CF vapor-annealed devices showed the highest performance with a PCE of 10.83%. To the best of our knowledge, this is the first demonstration of vertically analyzed 3D heterojunction structures in SVA treated-active films in one of the highest performance solar cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:.



A variation of Voc values of devices, UV-vis absorption spectra, TEM images and 1D XRD profiles of DR3TSBDT:PC71BM blend films, and detailed PCE results for 12 cells depending on the solvent type are shown in Figure S1-S7 and Table S1-S2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was supported by a grant from the Center for Advanced Soft Electronics (2013M3A6A5073175) under the Global Frontier Research Program of the Ministry of Education, Science, and Technology, Korea. This research was also supported by a New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government through the Ministry of Knowledge Economy (20123010010140).

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Figure 1. (a) Chemical structures of DR3TSBDT donor compound and PC71BM acceptor material, (b) photo current density-voltage (J-V) curves of DR3TSBDT:PC71BM devices, (c) photovoltaic performance parameters Jsc, FF, and best PCE of the devices with different treatments, and (d) the external quantum efficiency spectra.



Figure 2. Atomic force microscopy (AFM) topography images of pure small-molecule films (first row) and small molecule:PC71BM blend films (second row) prepared with dichloromethane (DCM) (a and e), tetrahydrofuran (THF) (b and f), ethanol (c and g), and chloroform (CF) (d and h).


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Figure 3. (a) Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) depth profiles (negative ion depth profiles) showing elemental distribution for DR3TSBDT:PC71BM absorber layers on silicon (Si) substrates with different solvent vapor annealing (SVA) treatments (left column) and (b) three-dimensional (3D) renderings of the negative polarity data: 3D plots for S- amounts (yellow parts: maximum / black parts: minimum) (right column). (a, b: DCM, c, d: THF, e, f: ethanol, and g, h: CF).



Figure 4. 2D-GIWAXS scattering patterns of the active blend film with different SVA treatments (first vertical line: DCM, second vertical line: THF, third vertical line: ethanol, and forth vertical line: CF). The incident angle involves two points (αi: incident angle and αc: critical angle): αi>αc (bulk region), and αi≪αc (near surface).


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Figure 5. Pole figures extracted from the lamellar (200) diffraction at two incident angles (αi αc) for DR3TSBDT:PC71BM blend films with various SVA treatments.



Figure 6. Schematic representations of the model used to fit ToF-SIMS and 2D-GIWAXS data, showing typical DR3TSBDT (red boxes) and PCBM (yellow balls) distributions after solvent vapor annealing with DCM, THF, ethanol, and CF.


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Figure 7. Dark J-V characteristics of the hole-only (a) and electron-only (b) devices including DR3TSBDT:PC71BM blend films with different SVA solvents. (c) Correlation between DR3TSBDT crystal size calculated from (200) scattering peak and hole mobility. (d) Relationship of balanced mobilities and best FF values in the DR3TSBDT:PC71BM device treated with different SVA treatments.



Figure 8. (a) Photocurrent density versus effective voltage (Jph-Veff) characteristics for the devices under constant incident light intensity (AM 1.5 G, 100 mW cm-2) with different SVA treatments. Measured (b) JSC and (c) FF of devices with various SVA solvents plotted against light intensity. (d) Transient photovoltage curves of the DR3TSBDT:PC71BM solar cells under various SVA treatments.


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Table 1. Photovoltaic performance parameters of DR3TSBDT:PC71BM bulk heterojunction solar cells fabricated and tested under different conditions. Active layer

DR3TSBDT :PC71BM

Treatment (boiling point, °C)

Voc (V)

Jsc (mA / cm2)

FF (%)

PCEbest (%)

PCEavg,a (%)

As-cast (non SVA)

0.88±0.02

14.5±1.21

60.1±0.44

7.66

7.21

DCM (40)

0.91±0.01

17.1±0.70

64.0±0.62

9.96

9.38

8.94

8.42

8.42

8.09

10.83

10.14

THF (66) ethanol (78) CF (62)

0.89±0.02 0.90±0.03 0.91±0.02

16.2±1.05 15.4±1.42 17.8±0.63

62.2±0.71 60.3±0.60 67.5±0.52

a

Average PCE values were obtained from 12 devices-presented in Supporting Information. CF: chloroform, THF: tetrahydrofuran, and DCM: dichloromethane.

Table 2. Charge carrier mobilities of DR3TSBDT:PC71BM with various SVA treatments, determined from space charge limited current (SCLC) measurements. Hole mobility (µh) (cm2 /V s)

Electron mobility (µe) (cm2 /V s)

DCM

4.01×10-4 ± 3.01×10-5

4.65×10-4 ± 2.79×10-5

1.16 ± 0.040

THF

3.32×10-4 ± 3.62×10-5

3.98×10-4 ± 3.11×10-5

1.20 ± 0.056

6.38×10-4 ± 2.41×10-5

6.82×10-4 ± 2.12×10-5

1.07 ± 0.030

Treatment

ethanol CF

1.76×10-4 ± 4.26×10-5

2.52×10-4 ± 3.81×10-5


µe/µh

1.43 ± 0.058


Table of Content (TOC) Graphic


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