Small-Molecule Organic Photovoltaic Modules Fabricated via Halogen

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Small Molecule Organic Photovoltaic Modules Fabricated via Halogenfree Solvent System with Roll-to-roll Compatible Scalable Printing Method Youn-Jung Heo, Yen-Sook Jung, Kyeongil Hwang, Jueng-Eun Kim, JunSeok Yeo, Sehyun Lee, Ye-jin Jeon, Donmin Lee, and Dong-Yu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12420 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Applied Materials & Interfaces

Small

Molecule

Organic

Photovoltaic

Modules

Fabricated via Halogen-free Solvent System with Rollto-roll Compatible Scalable Printing Method

Youn-Jung Heo, Yen-Sook Jung, Kyeongil Hwang, Jueng-Eun Kim, Jun-Seok Yeo, Sehyun Lee, Ye-Jin Jeon, Donmin Lee and Dong-Yu Kim*

Research Institute for Solar and Sustainable Energies (RISE) Heeger Center for Advanced Materials (HCAM) School of Materials Science and Engineering (MSE) Gwangju Institute of Science and Technology (GIST) 123 Cheomdan-gwagiro Buk-gu, Gwangju 61005, Republic of Korea E-mail: [email protected]

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ACS Paragon Plus Environment


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Keywords Small molecule solar cell, Organic photovoltaic modules, Slot-die printing method, Halogen-free solvent, Halogen-free additives

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Abstract For the first time, the photovoltaic modules composed of small molecule were successfully fabricated by using roll-to-roll compatible printing techniques. In this study, blend films of small molecules, BTR and PC71BM were slot-die coated using a halogen-free solvent system. As a result, high efficiencies of 7.46% and 6.56% were achieved from time-consuming solvent vapor annealing (SVA) treatment and roll-to-roll compatible solvent additive approaches, respectively. After successful verification of our roll-to-roll compatible method on small-area devices, we further fabricated large-area photovoltaic modules with a total active area of 10 cm2, achieving a power conversion efficiency (PCE) of 4.83%. This demonstration of large-area photovoltaic modules through roll-to-roll compatible printing methods, even based on a halogen-free solvent, suggests the great potential for the industrial-scale production of organic solar cells (OSCs).

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Introduction Bulk heterojunction organic solar cells (OSCs), composed of electron donor and acceptor materials, have been considered the next generation photovoltaic technology due to their great potentials for high-throughput, low-cost mass production as well as flexibility and light weight.1,2 In particular, small molecules, one of the organic conjugated molecules, have recently gained significant attention from researchers, accomplishing the state-of-the-art developments. As a result, power conversion efficiencies (PCEs) exceeding 10% for small-molecule based organic solar cells have now been achieved,3–5 reaching the threshold values for practical commercialization. Contrary to their polymer counterparts, small molecules have distinct advantages in terms of well-defined molecular structure, high purity, and less batch-to-batch variation, making them a more suitable candidate for industrial application.6 However, in spite of their promising properties and the rapid progress in PCE, several areas still remain unexplored compared to those of polymers, particularly the fabrication of printed photovoltaic devices. So far, very few studies have focused on the fabrication of small-molecule solar cells using the roll-to-roll compatible printing technology.7–9 However, now, along with accelerated growth of small-molecule solar cells, achievements in terms of printed large-area devices have to be accomplished in this filed to close the research gap between the small molecules and polymer materials. To realize the practical commercialization of OSCs, the successful demonstration of photovoltaic devices via printing methods is essential to apply them into the roll-to-roll process.2,10–13 Additionally, given that roll-to-roll process is continuously proceeding procedures, the optimal morphology of blend films should be achieved without the time-consuming thermal annealing or solvent vapor annealing (SVA) processes. Unfortunately, many high-performance small-molecule solar cells have been shown to require these time-consuming post

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treatments.4,14–17 However, in order to meet the requirements for the fabrication of large-area devices and further industrial production, the aforementioned methods must be replaced with more appropriate methods that can simplify the production steps (e.g., introduction of additives to solutions.)18,19 Thus, for the future up-scaling of OSCs, suitable processing methods as well as printing technology should be also considered. Another issue regarding the commercial viability is the toxicity of commonly used organic solvents. To date, most OSCs are being processed from organic solvents, such as chloroform, chlorobenzene (CB) and dichlorobenzene (DCB), containing halogen atoms. It is well known that OSCs based on halogenated solvents are favorable for attaining superior performances due to their suitable solubility compared to those based on non-halogenated solvents.7,20 However, the hazardous nature, expensive synthetic and removal steps of halogenated solvent serve as barriers from the perspective of manufacturing.21–23 In this regard, some efforts have been recently devoted to replacing the halogenated solvents with more environmentally friendly halogen-free solvents for spin-coated small-molecule solar cells.24–27 Considering the abovementioned

issues,

we fabricated

large-area small-molecule

photovoltaic modules via scalable method, slot-die coating based on a halogen-free solvent. Herein, blend films composed of benzodithiophene terthiophene rhodanine (BTR) and [6.6]phenyl-C71-butyric acid methyl ester (PC71BM) were used as active layers. Consequently, the slot-die printed small-molecule solar cells with the time-consuming SVA treatment and roll-toroll compatible approaches resulted in PCEs of 7.46% and 6.56%, respectively. We finally extended the roll-to-roll compatible method to the fabrication of large-area photovoltaic modules comprised of 4 stripes with 10 cm2 total active area, demonstrating a PCE of 4.83%. To the best our knowledge, this is the first report on printed photovoltaic modules composed of organic small-molecule materials.

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Results and discussion

The donor and acceptor material structures of the blend films are given in Figure 1a. The BTR, one of the commercially available small molecules, was previously reported to have a high efficiency of up to 9.3% from the spin-coating method in an N2 atmosphere, together with the SVA process.28 In our present work, however, we demonstrated small-molecule solar cells by the slot-die coating method under ambient air, as described in Figure 1b. At first, the device was fabricated under identical conditions to those in the pioneering study, with the exception of the coating methods (Figure S1). However, to further apply it into more desirable large-area photovoltaic modules, we adopted the promising halogen-free solvents, toluene, instead of halogenated solvents. In addition, more importantly, an alternative method was desired to replace the time consuming SVA post-treatment with a more roll-to-roll compatible process, for example, introduction of solvent additive. In this study, an environmentally friendly nonhalogenated molecule was also employed as an additive to give rise to morphological control similar to that found with SVA. Figure 1c displays the molecular structures of the halogen-free solvent and additive used in this study. As a result, slot-die printed small-molecule solar cells could be processed using the roll-to-roll compatible approach based on the halogen-free systems. To verify the effect of the roll-to-roll compatible approach, we fabricated the slot-die printed devices with a normal structure of ITO/PEDOT:PSS/BTR:PC71BM/Ca/Al. For comparison, three types of devices were prepared; as-cast films without any treatment, blend films following the SVA treatment and blend films processed with a diphenyl ether (DPE) additive. The current density-voltage characteristics of these devices are shown in Figure 2a, and the corresponding photovoltaic parameters are summarized in Table 1. As expected, the as-cast films without any treatment exhibited the low PCE of 3.81%. However, consistent with an earlier report,28 the

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performance of the devices was remarkably enhanced to a maximum PCE of 7.46% through the SVA treatment by tetrahydrofuran (THF) solution. Interestingly, the devices processed by the DPE additive also exhibited a similar trend, resulting in the improvement of performance due to a significant increase in fill factor (FF) and short-circuit current (Jsc), even though there were slight drops in open-circuit voltage (Voc). As a result, the highest PCE of 6.56% with a Voc of 0.90 V, a Jsc of 11.2 mA cm-2, and an FF of 69.6% could be achieved from devices processed with the non-halogenated DPE additive. These results suggest that the method involving DPE, which is a more roll-to-roll compatible approach, could be ideally translated to the fabrication of large-area photovoltaic modules. In order to explore the reason for the improvement in Jsc after SVA treatment and the introduction of the DPE additive, external quantum efficiency (EQE) was measured. As seen in Figure 2b, all devices processed under the various conditions exhibited broad photocurrent responses in the wavelength ranges from 350 nm to 700 nm. However, relative to the as-cast film, both of SVA and DPE-additive films showed more enhanced EQE intensities at longer wavelength regions. The maximum EQE peaks for as-cast, SVA, and DPE-additive films were determined as 53%, 68%, and 61%, respectively. Thus, the enhanced Jsc values, which are consistent with the integrated current densities from EQE spectra within 5% error, could have resulted from the efficient conversion of incident light to photocurrent.26,29,30 The figure 2c shows the ultraviolet-visible (UV-vis) absorption spectra of thin films. Remarkably, the films fabricated under different processing conditions exhibited similar absorption spectra tendencies with EQE spectra. However, from the UV-vis absorption spectra, we could more clearly observe the appearance of shoulder peaks (around 615 nm) for the SVA and DPE-additive films. As a result, we could suggest the intensive intermolecular interaction of donor phase for SVA and DPE-additive films.31,32 Furthermore, these variations of optical properties could be easily

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distinguished by a color change of the thin films, as revealed in photographic images (inset of Figure 2c). In this case, the DPE-additive introduced films seem to be almost identical to the SVA-treated films, while the as-cast films exhibited a quite different color. The surface and bulk morphologies of blend films were investigated using atomic force microscope (AFM) and transmission electron microscope (TEM) measurements. As shown in Figure 3a, the SVA and DPE-additive films exhibited coarsened domains, with increased rootmean-square (RMS) roughness of 0.74 nm and 0.77 nm, in contrast to the as-cast film, with an RMS values of 0.26 nm. Furthermore, from the TEM images, it can be seen that both the SVAtreated and DPE-additive films had appropriately phase separated large domains compared to the as-cast film. As is well known, the evolution of larger domains, which contain relatively low grain boundaries, facilitates high exciton dissociation and excellent charge transport owing to the reduced charge carrier-recombination centers.4,29,33 Therefore, an improved Jsc and FF could be provided for both films. These similar morphological features of both blend films indicated that the roll-to-roll compatible approaches involving the introduction of DPE surely provided morphological development comparable to that of the time-consuming SVA treatment. Notably, among the various halogen-free additives, only the DPE additive could provide the favorable morphological development, as shown in Figure S2. The improvement of Jsc and FF in SVA and DPE-additive conditions was further supported by the space-charge limited current (SCLC) model (Figure 4 and Table S3). For SVA and DPEadditive films, more enhanced charge carrier mobilities were calculated from current densityvoltage curves, rather than as-cast film. The higher hole and electron mobilities (µh and µe) of both films correlates well with their improved Jsc.34,35 In addition, more balanced electron and hole mobilities could be obtained in both the SVA and DPE-additive films. For example, the electron and hole mobilities ratio (µe/µh) of the SVA and DPE-additive films were decreased to

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2.41, 3.76, respectively, from that of as-cast film (5.75). This better charge balance between electrons and holes is a clear evidence of the high FF values of ca. 70 %.5,36,37 Therefore, the enhancement of Jsc and FF in the SVA and DPE-additive conditions could also be explained by the improved and balanced charge carrier transport in combination with the formation of continuous percolation pathways, confirmed by the AFM and TEM images. The charge carrier recombination behaviors were elucidated by measuring the Voc as a function of incident light intensity (Figure S3). All films exhibited strong Voc dependence on light intensity, indicating the presence of a non-germinate recombination process. However, especially, the as-cast film yielded larger slope values compared to the other two films, mainly at low light intensity (below 10 mW/cm2). Therefore, we concluded that SVA and DPE-additive films have relatively reduced trap-assisted recombination losses compared to the as cast film.38,39 To further investigate the molecular orientation of slot-die printed films, we performed grazing incidence wide angle X-ray scattering (GIWAXS) measurements on pristine and blend films. The GIWAXS image of the neat small molecule, BTR was displayed in Figure S4, suggesting the clear edge-on orientation with lamellar (h00) reflections up to three orders in the out-of-plane direction together with a pronounce π-π stacking (010) peak along the in-plane direction.3,40,41 These features in the GIWAXS images are attributed to the highly crystalline properties of BTR. In contrast, the film of neat PC71BM exhibited the ring-shaped patterns indicative of an isotropic and disordered structure. After being blended together, the texture of the as-cast film became to resemble the mixture of two neat films, as revealed in Figure 5a. In other words, the as-cast film maintained the edge-on orientation with negligible changes in arrangement. However, after the SVA treatment, the noticeable π-π stacking (010) peak was additionally observed at q = 1.68 Å-1 in out-of-plane direction, implying an occurrence of the

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face-on orientations. As a result, the SVA films could accomplish both the edge-on and face-on orientations, enabling effective three-dimensional (3D) charge transport.42,43 Similarly, the DPEadditive films also showed the π-π stacking (010) peak in the out-of-plane direction, while the peak intensity was weaker than those of SVA films. Therefore, from these results, it was found that introduction of a halogen-free DPE additive further leads to beneficial coexisting orientations, as well as the formation of optimal morphologies. Figure 5b shows the related out-of-plane line-cut profiles of the GIWAXS images. Herein, all of line-cut profile were normalized based on the PC71BM peak intensities located at q = 1.35 Å-1. Accordingly, for SVA and DPE-additive films, we observed much sharper donor peaks with increased intensity, in good agreement with UV-vis absorption spectra. These (100) peak intensities for each condition have excellent correlations with Jsc and FF, as shown in Figure 5c. In addition, both the SVA and DPE-additive films revealed more reduced lamellar (100) distances of 17.9 Å, relative to the as cast film (19.0 Å) (Table S4). Therefore, SVA and DPE additive introduced approaches produced more compactly stacked lamellar structures with enhanced crystallinity.44,45 To verify our object on this study, we demonstrated the slot-die printed large-area photovoltaic modules composed of the small molecule, BTR via the roll-to-roll compatible DPE-additive approach. As described above, we confirmed that the DPE-additive method is sufficiently suitable for up-scaling fabrication, from our optimized small-area single-cell devices. Herein, the photovoltaic modules, consisted of 4 series-connected stripes with each 2.5 cm2 area, were used for providing the total active area of 10 cm2. Figure 6 shows the current density-voltage curves of the best-performing printed photovoltaic module and its photographic images. Based on this configuration, we could obtain a maximum PCE of 4.83% (Voc of 3.76 V, Jsc of 1.84 mA cm-2 and FF of 66.0%) and an average PCE of 4.40%. Interestingly, this is the

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first report demonstrating the small-molecule photovoltaic modules fabricated by a printing method. Furthermore, we also employed the halogen-free solvent and additive systems in this work, suggesting more desirable processing condition.

Conclusion In summary, halogen-free small-molecule solar cells were successfully fabricated by using a scalable slot-die coating method. The OSCs based on the BTR:PC71BM active layers achieved a PCE of 7.46% by SVA treatment. Additionally, we introduced the halogen-free DPE molecule as an additive with aim to further apply it into large-area photovoltaic modules. The photovoltaic results in small active-area devices showed that the DPE additive approach can be effectively used for the up-scaling of small-molecule solar cells. As a result, we demonstrated smallmolecule photovoltaic modules with an active area of 10 cm2 exhibiting a PCE of 4.83%. To the best our knowledge, this is the first report regarding printed large-area photovoltaic modules consisting of small molecules. Therefore, our work is very meaningful from the perspective of commercialization, providing important steps toward the further improvement of smallmolecule solar cells.

Experimental methods

Small area solar cell fabrication. The patterned ITO/glass substrates were treated under UV/O3 for 15min. A PEDOT:PSS (Clevios P VP Al 4083, Heraeus) layer was spin-coated at 5000 rpm for 40 s and then thermally annealed at 150 ℃ for 10 minutes. The active layer was slot-die coated from a toluene solution of BTR:PC71BM with a ratio of 1:1 wt% (15 mg ml-1) in ambient conditions at room temperature. Coating was conducted by using a previously developed 3D

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printer-based slot-die coater46 at a coating speed of 12mm/s with a gap of 100 µm between the meniscus and substrate. Furthermore, an additional N2 blower system was introduced to produce uniform and defect-free films, following an earlier report.47 For solvent vapor annealing (SVA) films, a THF (1ml) solution was introduced into a 30 mm glass petri dish and then held on for 1 min. Afterward, the slot-die coated films were attached to the lid of petri dish and then placed for 25 s. On the other hand, the additive-introduced films were prepared from the DPE (0.5 vol%) additive-incorporated solution. In this case, additional time-consuming drying or annealing processes are unessential. Finally, all films were completed by thermal evaporation of Ca (20 nm)/Al (80 nm) under a vacuum pressure of 10-7 torr defining an active area of 10 mm2. Large-area modules fabrication. Large-area ITO/glass substrates (8 x 3.3 cm2) composed of patterned stripes (width of 13 nm and interval of 2 nm) were used in this study. For module fabrication, only the roll-to-roll compatible DPE-additive approach was implemented. The coating of all layers was carried out following the aforementioned methods (e.g., preparation of solutions). The active area of modules was determined from the overlapped regions of ITO stripes and active layers. As a result, the total active area of modules was calculated as 10 cm2, which is the sum of the area (2.5 cm2) of each of the 4 stripes. In particular, the 3D printer-based slot-die coater was suitable for the patterning of active layers. Therefore, we did not use any additional etching processes. Characterization. The current density-voltage (J-V) characteristic were measured by using a Keithley 2400/Oriel solar simulator (Class AAA) under AM 1.5G illumination at an intensity of 100 mW cm-2. This measurement was calibrated from standard Si solar cell, certified by the International System of Units (SI) (SRC 1000 TC KG5 N, VLSI Standards, Inc,). The external quantum efficiency (EQE) spectrum was obtained by using a QEX-7 PV Measurements Inc. spectral response system. The ultraviolet-visible (UV-vis) spectrometry was performed by using a Perkin Elmer Lambda 750. The surface morphology of

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the devices was analyzed by using the transmission electron microscopy (TEM) (Technai G2 STwin 3600 KeV electron microscopy) and atomic force microscopy (AFM) (Nanoscope, Veeco Instruments, Inc) in the tapping-mode. The hole and electron mobilities were calculated from the space-charge limited current (SCLC) model. In this case, the hole-only and electron-only diodes

were

prepared

from

devices

with

a

structure

of

ITO/PEDOT:PSS/BTR:PC71BM/MoO3/Ag and ITO/ZnO/BTR:PC71BM/Ca/Al, respectively. The hole and electron mobilities were calculated by the Mott-gurney’s law J = (9/8)ε0εrµ(V2/L3) Where ε0 is the free-space permittivity, εr is the dielectric constant of the blend film, µ is the hole and electron mobilities, V is the voltage drop across the device, and L is the thickness of blend film. The two-dimensional (2D) grazing incident wide angle X-ray scattering (2D GIWAXS) was performed by using the Pohang Accelerator Laboratory’s 9A U-SAXS beamline.

Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government (NRF-2015R1A2A1A10054466) and the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20153010012110 and 20163030013900). We thank the Korea Basic Science Institute (KBSI) for AFM measurement.

ASSOCIATED CONTENT Supporting Information Avaliable 13



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Figure 1 (a) Active layer components, BTR and PC71BM. (b) Schematic illustration of slot-die coating. (c) Chemical structures of non-halogenated solvent and additives used in this work

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Figure 2 (a) Current density-voltage curves of slot-die coated OSCs processed under various conditions. (b) Corresponding EQE and (c) UV-vis absorption spectra. The inset shows a photographic image of organic thin films.

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Table 1 Photovoltaic parameters of OSCs fabricated from slot-die coating method

Figure 3 (a) AFM and (b) TEM images of slot-die coated blend films.

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Figure 4 Current density-voltage plots for (a) hole-only and (b) electron-only devices.

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Figure 5 (a) 2D GIXRD images of slot-die coated blend films and (b) corresponding out-of-plane line-cut profiles. (c) Relationship between donor (100) peak intensity and photovoltaic parameters, Jsc and FF.

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Figure 6 (a) Photographic image of slot-die coated large area photovoltaic modules during printing process and (b) corresponding current density-voltage curves of champion cell.

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Small-Molecule Organic Photovoltaic Modules Fabricated via Halogen

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