Achieving High Doping Concentration by Dopant Vapor Deposition in

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Achieving High Doping Concentration by Dopant Vapor Deposition in Organic Solar Cells Han Yan, Yabing Tang, Xiangyi Meng, Tong Xiao, Guanghao Lu, and Wei Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16162 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Achieving High Doping Concentration by Dopant Vapor Deposition in Organic Solar Cells Han Yan,*,a Yabing Tang,a Xiangyi Meng,a Tong Xiao,b Guanghao Lu,b and Wei Ma*,a a

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an,

710049, P. R. China. b

Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, 710054, P. R.

China. KEYWORDS: Lewis acid doping, sequential doping, doped morphology, Organic solar cell, high doping concentration, vapor doping

ABSTRACT: Comparing with the interfacial doping, molecular doping in bulk heterojunction (BHJ) is a more direct but challenging approach to optimize the photovoltaic performance in organic solar cells (OSCs). One of the main obstacles for its success is the low doping concentration due to the morphological damage. Starting from the phase diagram analysis, we discover that the un-preferred good miscibility between p-type dopant and the acceptor leads to incorrect dopant dispersion which reduces the achievable doping content. To overcome this, we use sequential doping by vapor annealing instead of blend solution doping, and we achieve the high doping concentration without sacrificing the blend film morphology. Benefiting from the un-damaged film, we fulfill improved photovoltaic performance. Our positive results reveal the

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feasibility of high level doping in complex organic BHJ films. It is believed that doping at high concentration potentially enlarges the extent of tunable range on electronic properties in OSCs, and indicates greater improvement for device performance.

Introduction Being the key concept for the versatility of inorganic semiconductors, doping is nowadays commonly used in state-of-the-art organic electronic devices.1-3 The physics of doping in organic devices is complex, neutral molecular dopants are understood to interact with organic semiconductors by two mechanisms: ion pair (IP) formation and charge-transfer complex (CTC) formation.4,5 The former fulfils doping by integer charge transfer, while the latter achieves it by frontier orbital hybridization. Various spectroscopic techniques are used to ascertain the effect of doping in organic semiconductors.6,7 Ever since He et al. reported in 2011 the interfacial doping approach8,9 and later Tunc et al. reported the BHJ doping approach,10 scientists have been purchasing for high-efficiency doped OSCs. Comparing with the interfacial doping, the BHJ doping is more challenging and preferred, since it is the way that directly aims at optimizing the active layer materials. Besides the well-known improved charge transport and trap-filling, other potential benefits of the BHJ doped OSCs are emphasized. Firstly, doping enhances the electric field at the BHJ interface, thus facilitates charge separation, which is particularly important for the material combination with subtle energy offset.11,12 Secondly, doping decreases the depleted width through the active layer, and correspondingly increases the electric field at the electrodes for charge extraction.13,14 These advantages encourage us for future promotion of the BHJ doped OSCs. Although the BHJ doping has been proved to be beneficial for all three factors in photovoltaic performances,15-18 the device improvement is quite limited due to the low doping efficiency. In


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the last few years, fundamental studies have revealed that the doping efficiency is directly determined by the doping concentration. A model by Arkhipov et al. predicted that the carrier mobility and conductivity improvement occurred at doping concentration above 1%.19,20 Tietze et al. explained this as the doping at low concentrations primarily resulted in trap-filling, free charges were produced at high doping content.21,22 The concentration-dependent free charge generation was further supported by Mityashin et al. They revealed that the efficient IP dissociation occurred at doping concentration above 1%.23 More recently, Pingel and Nuzzo et al. separately clarified that the doping efficiency of donor-acceptor copolymers was lower than the homo-polymers, suggesting higher doping content is required for the novel organic semiconductors.24,25 Based on these unique properties, achieving high doping concentration is urgent for the improvement of BHJ doped OSCs. The major obstacle for this vision is the morphological damage, including pre-aggregation in solution,6,26 formation of isolated dopant cluster,27 and enlarged phase segregation when combined with the acceptor component.28 It seems that avoiding all these problems would take us to the success of the high doping concentration in OSCs. Surprisingly, we recently found that even the dopant which worked as the morphology modifier at low content failed to enhance the tolerance of doping concentration.29 This un-expected result makes us to re-examine the reason for the doping induced morphological damage. In an ideal picture, the p-type dopants combine with the donor, rather than the acceptor; the mismatch will weaken the doping effect and inversely strengthen the charge traps within the active layer. Thus the miscibility between the dopant and the objective component is proposed as a long-time neglected factor that affects significantly on the doped morphology and the overall performance in OSCs.


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Results and Discussion To evaluate the miscibility, we calculated the liquid ternary phase diagram (organic semiconductor, dopant, and solvent) using Flory-Huggins theory (Figure 1a and b). We use the benchmark

poly[4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-

diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]]

(PCE10):

[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend (Scheme 1) as a model system. The Lewis acid molecule-tris(pentafluorophenyl)-borane (BCF) is chosen as p-type dopant (Scheme S1) and chlorobenzene (CB) is picked according to the device fabrication condition. All input parameters for the Flory-Huggins model are listed in Table S1 and S2, and the detailed calculation methods are described elsewhere.30,31 The miscibility is indicated by the binodal curve: above which a mixed phase exists; in the region below it, a driving force for demixing is present. Comparing by this, BCF has better miscibility with PC71BM than with PCE10 (Figure 1a and b). This is further confirmed by the order of surface energy from the contact angle measurement, where it is 38.7 mN/m for PCE10, 59.2 mN/m for PC71BM, and 72.5 mN/m for BCF (Figure S1-S3). We deduce the BCF distribution from the binodal shape. PCE10 and BCF produce the asymmetry in the binodal, indicating that the BCF excludes from PCE10. With respect to PC71BM and BCF, the binodal curve remotes from the axes of the composition domain. For this blend the phase diagram predicts formation of mixed phases. We discover that due to the un-preferred good miscibility between BCF and PC71BM, the p-type dopants tend to distribute in the acceptor phase rather than the donor. It seems inevitable by co-depositing from the blend solution.


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Recently, sequential doping through vapor phase deposition is developed to achieve high degree of doping without sacrificing the film quality in single component films.32 This is ascribed to the greater activation energy for the dopants to penetrate into crystalline domains, and instead the dopants diffuse more easily into the amorphous domains.4,32-34 Inspired by this, we deposited the p-type dopants by placing the blend films underneath the lid of a sealed petri dish containing a few milligrams of BCF as schemed in the inset of Figure 1c. The bottom of the petri dish was heated to 80℃, and the samples were exposed to the BCF vapor for 5 min. The successful doping is confirmed by the appearance of CTC peak in the infrared region (Figure 1c), the CTC peak was not observed in the sample fabricated from the 1 wt% dopant contained solution.29 To estimate the doping level, we compare the vapor doped samples with those casting from the dopant contained blend solution. We normalized the PC71BM absorption peak at 476 nm to calibrate the intensity. We observe that the CTC peak from BCF vapor treatment overlaps with the reference casting from the 10 wt% BCF contained blend solution. The effective doping is also confirmed by ultraviolet photoelectron spectroscopy (UPS) (Figure 1 d and e). According to the UPS spectra, the highest occupied molecular orbital (EHOMO) moves from -4.91 eV to -5.18 eV after BCF vapor doping; and the corresponding Fermi level (EF) shifts downward by 0.35 eV from -4.07 eV to -4.42 eV. The obvious EHOMO and EF offsets by vapor doping are in sharp contrast to those doped by 0.05 wt% BCF solution (best device condition in our previous report29), where the EHOMO and EF are separately -4.90 eV and -4.10 eV (Figure S4). Tietze et al. have explained this phenomenon and stated that doping at low concentration mainly filled the traps without producing extra free charges to make energy level movement.22 The UPS data support our observation from the absorption spectrum, and we achieve high doping concentration by dopant vapor treatment. The corresponding doping depth is estimated by the depth resolved


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absorption spectra (Figure 1f). The initially weaker spectrum of vapor doped sample overlaps with the reference after etching for 105 s. Therefore, the BCF diffusion depth is deduced from the etched film thickness, and it’s about 45 nm in the 80 nm thick film. The influence of vapor doping on the active layer morphology is first characterized by tapping-mode atomic force microscopy (TM-AFM). No obvious change is observed after 5 min BCF deposition (the TM-AFM image of the 10 wt% BCF solution doped sample is listed in Figure S5). Thus we state that most of BCF molecules efficiently diffuse into the blend film instead of forming an interlayer (Figure 2a and b). Grazing incident wide angle X-ray scattering (GIWAXS) was performed to compare the molecular order after doping at high concentration through two pathways.35 No peaks assigned to BCF are observed from GIWAXS, demonstrating BCF molecules are well mixed with PCE10, PC71BM, or both. Here, we focus on the PC71BM because it’s more vulnerable according to the phase diagram analysis. As shown in the scattering profiles in Figure 2c and Figure S6, the coherence length (CL) of the peak at 1.32 Å-1 is fitted to estimate the PC71BM molecular order. The control sample owns the largest CL of 3.02 nm. Contrastively, the CL value reduces to 2.83 nm in the solution doped film. Doping by BCF vapor, the CL recovers to 3.01 nm. The nanoscale phase separation was studied using resonant soft X-ray scattering (R-SoXS) at 284.2 eV (Figure 2d and Figure S7). For all films, a single strong scattering at 0.22nm-1 is observed without forming isolated BCF aggregates. The R-SoXS intensity is determined by the difference of scattering contrast between PCE10-rich and PC71BM phases.36-38 At 284.2 eV, the absorption intensity of PC71BM is the highest, while the absorption of BCF is the lowest (Figure S8). Comparing the doped blends, the vapor approach shows higher R-SoXS scattering intensity than the solution approach. The higher scattering intensity indicates that less BCF penetrating to the PC71BM phase. Judging from the X-ray results, two


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conclusions are made apparently on the doped morphology: firstly, PC71BM crystallization is disrupted by the BCF inclusion using solution doping; secondly, vapor deposition effectively reduces the BCF content in PC71BM with maintained optimum morphology. Due to the correction of BCF dispersion in the blend films, we deduce that the total content of BCF in vapor deposited films is less than those casting from doped solution. The control and doped BHJ models are summarized in Figure 2e-g. We propose that BCF distribute in the amorphous and mixed regions of PCE10 by vapor doping (Figure 2f); however, BCF molecules disperse more in PC71BM phase by solution doping (Figure 2g). Altogether, sequential doping by BCF vapor avoids the thermodynamic morphology damage from those casted by the doped solution. To lend more support to our high-quality films from vapor doping, the semiconducting properties are examined. The diode performance including the reverse saturated current density (Js) and the ideality factor (n) is estimated from the dark J-V curves according to the Shockley equation.39,40 The vapor doping results both of lower Js and n. The Js reduces from 8.54×10-7 mA/cm2 to 1.69×10-8 mA/cm2, and n decreases from 1.82 to 1.51 in the vapor doped sample (Figure 3a). Notably, casting from the 10 wt% dopant contained solution negatively affects the diode performance with largely increased Js and n. Under illumination the charge transport requires considerations of the photocurrent density (Jph) at both high and low effective voltages. We extract the saturated photocurrent density (Jsat) at 2 V for the control and vapor doped samples, while no saturated region is observed in the doped solution casting film (Figure 3b). The maximal exciton generation rates (Gmax, given by Jsat=qGmaxL where q is the elemental charge, and L is the thickness of the active layer) are consistent for the control (1.32×1028 m-3/s) and vapor doped sample (1.35×1028 m-3/s). The differences of the photo J-V curves are dominated by the diffusion-controlled photocurrent in the low effective voltage range. According


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to the Einstein relation (D=kTμ/q, where D is the diffusion coefficient, k is the Boltzmann’s constant, T is the temperature, and μ is the mobility), the charge diffusion is determined by the charge mobility. We then investigated the hole and electron mobility from the transporting curves (Figure 3c and d). Both pathways of BCF doping increase the hole mobility to similar extent. However, only vapor doping optimizes the electron mobility (from (2.18±0.99)×10-5 cm2 V-1 s-1 to (4.60±1.94)×10-5 cm2 V-1 s-1). The harm for electron transporting in the 10 wt% BCF solution doped sample is notably observed as the steep trap-filling region (Figure 3d). The observed large amount of electron traps is due to the un-preferred BCF distribution in PC71BM. The transporting tests once again support the idea that the vapor doping improves the electronic properties without sacrificing the film morphology by reducing the annoying large content of BCF in PC71BM clusters. Since the vapor doped sample achieves high-level doping with optimized film quality, we then fabricate photovoltaic devices to test whether doping at high concentration could lead to improved performances. Photovoltaic devices are fabricated in inverted structure and characterized in terms of short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) (Figure 4a and Table 1). The vapor doped sample results in better PCE value than the control device, the PCE increases from 8.6% to 9.4%. The Jsc slightly increases from 16.38 mA/cm2 to 17.29 mA/cm2, which is quite consistent with the integrated current density from the external quantum efficiency (EQE) curves (Figure 4b). The improvement also comes from the FF, which increases from 65.6% to 69.0%. As reference, the best device condition of 0.05 wt% solution doping makes a PCE value of 9.2% (Figure S9).29 In spite of similar PCE increase by both pathways, we point out that the underlying optimizing mechanisms are different. The PCE improvements at low doping concentration are ascribed to


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trap-filling.16,17 Doping at high concentration opens more pathways to improved device performance including better exciton separation,41-43 outperformed charge transport5,33 and optimized charge collection.18,44 Although the detailed device physics are beyond the scope of this work, we address the greater potential of high doping content for organic solar cells. Another important achievement is that the vapor doped device performs over those casting from the 10 wt% doped solution. The vapor doping avoids the FF loss at high doping concentration. This reveals the great potential for the high level doped BHJ OSCs. With longer BCF vapor exposed time, the PCE drops to 8.3% at 10 min. Insights into FF reveal the device physics through various doping conditions. We measured FF as a function of light intensity (Figure 4c). In an ideal device, FF keeps stable in the whole measured range. Trap-assisted recombination makes FF increase at low light intensity, while bimolecular recombination reduces FF at high light intensity. Comparing with the stable curve of the vapor doped sample, we observe a large drop of FF in the low light intensity range for the solution doped one, suggesting trap-assisted recombination within it. This is further supported by the Voc study (Figure 4d). By plotting Voc on the logarithm of light intensity, the slope increases from 1.04 kT/q to 1.41 kT/q through changing doping process from vapor to solution. The Voc slope of the vapor doped device is even lower than the control one, which has been explained by trap-filling in our previous work.16 Conclusions In summary, we start from the theoretical analysis by calculating the ternary phase diagram, and assume the incorrect miscibility between BCF and PC71BM as the major obstacle to fabricate high level BHJ doped OSCs. Taking sequential doping by exposing the sample in BCF vapor, we achieve high doping concentration without interrupting the active layer. The un-damaged BHJ


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morphology including the corrected dispersion of the dopants is confirmed by the synchrotron and electric testing experiments. Using the high-quality doped films, the vapor doped BHJ films largely perform over the solution doped samples in both semiconducting and photovoltaic performances, and even better than the optimized pristine ones. The importance of this manuscript is summarized in two aspects: firstly, this is the first successful example related to the optimization of doped BHJ morphology in organic solar cell; Secondly, the success of device at high doping concentration indicates the great potential for further performance improvement. Expecting for the continuous knowledge growth on doping strategy and device physics, we are optimistic to the wide application of the BHJ doped OSCs in the future.


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Scheme 1. Chemical structures of PCE10, PC71BM, and BCF.


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Figure 1. Ternary phase diagrams calculated from the Flory-Huggins theory: a) PCE10, BCF, and CB; b) PC71BM, BCF, and CB. Volume fractions of the three components are indicted on the axes. c) UV-vis spectra of the control and doped blend films. The inset is the scheme of vapor doping route used to achieve doped blend films. d and e) UPS spectra of control and vapor doped PCE10 films: d) low energy part; e) high energy part. f) Depth resolved absorption spectra of the control and vapor doped blend films; the red solid lines represent the vapor doped sample, and the black dotted lines represent the control sample.


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Figure 2. a, b) TM-AFM images of control and 80℃/5 min vapor doped PCE10/PC71BM films (2 μm×2 μm). c) In plane and out of plane line cuts of control, 80 ℃ / 5 min vapor doped, 10 wt% BCF solution doped PCE10/PC71BM films. d) Corresponding R-SoXS profiles. e-g) Proposed doped morphology through different doping pathways. The orange lines represent intrinsic PCE10; the red lines represent doped PCE10; the black spheres represent PC71BM; the blue spheres represent BCF.


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Figure 3. a, b) Dark and photo J-V curves of PCE10/PC71BM. c, d) Hole-only and electron-only charge transport curves of PCE10/PC71BM. The insets are the histograms of statistical mobility.


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Figure 4. Photovoltaic performance of PCE10/PC71BM: a) J-V curves, the inset is the device structure; b) Corresponding EQE plots; c) Measured FF of PCE10/PC71BM solar cells plotted against light intensity. d) Measured Voc of PCE10/PC71BM solar cells plotted against light intensity.


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Table 1. Photovoltaic performances of PCE10/PC71BM. Average PCE of 10 devices fabricated under identical conditions ±1 standard deviation. Materials

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE max (%)

PCE avg (%)

Control

16.38

0.81

65.6

8.6

8.6±0.1

80 ℃ / 5 min

17.29

0.79

69.0

9.4

9.3±0.1

80 ℃ / 10 min

16.08

0.80

64.7

8.3

8.2±0.1

10wt% BCF

15.90

0.77

58.1

7.1

7.0±0.1

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Further details on experiments details including materials, instrumentation, device fabrication, and X-ray characterization are provided. (PDF) Data of phase diagram calculation, chemical structures, contact angle tests, X-ray patterns, and the index refraction of materials near carbon K-edge are also provided. (PDF) AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The work was supported by the Ministry of Science and Technology of China, National Natural Science Foundation of China, Ministry of Education of China, Shaanxi Province of China. ACKNOWLEDGMENT The authors thank for the support from Ministry of Science and Technology of China (2016YFA0200700), National Natural Science Foundation of China (51803162, 21875182, 21534003), Ministry of Education of China (1191329815), and Shaanxi Province of China (2018JM5007). UPS data were collected at Instrument Analysis Center of Xi'an Jiaotong University. X-ray data were acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which was supported by the Director, Office of Science, Office of Basic Energy Science, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3 and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition. ABBREVIATIONS BHJ, bulk heterojunction; OSCs, organic solar cells; IP, ion pair; CTC, charge-transfer complex; PCE10,

poly[4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-

alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]]; PC71BM, [6,6]phenyl-C71-butyric acid methyl ester; CB, chlorobenzene; UPS, ultraviolet photoelectron


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spectroscopy; TM-AFM, tapping-mode atomic force microscopy; GIWAXS, grazing incident wide angle X-ray scattering; CL, coherence length; R-SoXS, resonant soft X-ray scattering; Js, reverse saturated current density; n, ideality factor; Jph, photocurrent density; Jsat, saturated photocurrent density; Gmax, maximal exciton generation rates; Jsc, short-circuit current; Voc, opencircuit voltage; FF, fill factor; PCE, power conversion efficiency; EQE, external quantum efficiency. REFERENCES (1) Lüssem, B.; Riede, M.; Leo, K., Doping of Organic Semiconductors. Phys. Status Solidi A 2013, 210, 9-43. (2) Lüssem, B.; Keum, C.; Kasemann, D.; Naab, B.; Bao, Z.; Leo, K., Doped Organic Transistors. Chem. Rev. 2016, 116, 13714-13751. (3) Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. R., Organic Thermoelectric Materials for Energy Harvesting and Temperature Control. Nat. Rev. Mater. 2016, 1, 1-14. (4) Jacobs, I. E.; Moulé, A. J., Controlling Molecular Doping in Organic Semiconductors. Adv. Mater. 2017, 29, 1703063. (5) Méndez, H.; Heimel, G.; Winkler, S.; Frisch, J.; Opitz, A.; Sauer, K.; Wegner, B.; Oehzelt, M.; Röthel, C.; Duhm, S.; Többens, D.; Koch, N.; Salzmann, I., Charge-transfer crystallites as molecular electrical dopants. Nat. Commun. 2015, 6, 8560. (6) Müller, L.; Nanova, D.; Glaser, T.; Beck, S.; Pucci, A.; Kast, A. K.; Schröder, R. R.; Mankel, E.; Pingel, P.; Neher, D.; Kowalsky, W.; Lovrincic, R., Charge-Transfer-Solvent Interaction Predefines Doping Efficiency in P-Doped P3HT Films. Chem. Mater. 2016, 28, 4432-4439.


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