Molecular-Shape-Induced Efficiency Enhancement in PC61

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Molecular-Shape-Induced Efficiency Enhancement in PC BM and PC BM Based Ternary Blend Organic Solar Cells 61

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Shashi B. Srivastava, Sanjay K. Srivastava, and Samarendra P. Singh J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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The Journal of Physical Chemistry

Molecular-Shape-Induced Efficiency Enhancement in PC61BM and PC71BM Based Ternary Blend Organic Solar Cells Shashi B. Srivastava,a Sanjay K. Srivastava,b and Samarendra P. Singh*a

a

Department of Physics, School of Natural Sciences, Shiv Nadar University, NH-91, Tehsil

Dadri, Gautam Buddha Nagar, Uttar Pradesh, 201314, India b

CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110012

ACS Paragon Plus Environment

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ABSTRACT

Evolution of a self-organized molecular packing at the microscopic scale in bulk heterojunction active layer is critical for improving the performance of organic solar cells. We demonstrate that the molecular-shape-induced effects improve the morphology of the ternary thin films as PC61BM molecules were added to the host binary blend, PTB7-Th:PC71BM. Compared to the binary PTB7-Th:PC71BM devices, the ternary devices with 20% PC61BM content (relative to PC71BM) exhibited an enhanced efficiency, from 6.4% to 8.5%. In particular, we find that the spherical PC61BM molecules with better precipitation kinetics than ellipsoidal PC71BM alter the morphology of ternary thin film and modulates interfaces to expedite charge transport and collection by reducing various recombination losses.


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1. INTRODUCTION Bulk heterojunction organic solar cells (OSCs) are advanced significantly in recent years and have demonstrated a power conversion efficiency (PCE) of over 10%.1-4 Though much effort has been focused on developing efficient light harvesting organic materials, optimizing the morphology of active layers along with improvising interfaces to enhance charge generation and collection, the PCE seems to be limited to 10-12%.5-7 A ternary bulk heterojunction (BHJ) based OSCs has emerged as a promising approach to achieve higher efficiency. A wide range of options as the additional component (donor (D)/acceptor (A) or dye molecules) in ternary BHJ devices enhances the opportunities to increase the ultimate PCE of organic solar cells.8-17 An enhancement of PCE (η = J sc .Voc .FF / Pin ) can be effectively achieved by optimizing the product of the short circuit current (Jsc), open circuit voltage (Voc) and the fill-factor (FF) for the intensity of the incident light (Pin). The additional third component of ternary blend BHJ OSCs leads to an enhancement in Jsc by improving the light absorption band.7,18-20 In few reports, an improvement in Jsc and Voc is reported at the cost of FF.19-21 Khlyabich et.al. has reported an improvement in Voc maintaining high FF as the fraction of indene-C60 bisadduct (ICBA) increases in P3HT:PC61BM although Jsc decreases.18 However, Cheng and co-workers have reported a PTB7:PC71BM:ICBA based ternary OSCs with a PCE of 8.2%.22 Recently, Lu et al. has reported a highly efficient ternary organic solar cell with a PCE of 9.2%.23 Additionally, Lindqvist et al. has reported an improved thermal stability in ternary BHJ solar cells based on C60/C70 mixed acceptors.24 The improved performances of these ternary blend OSCs as compared to their counterpart binary blend devices are attributed to higher light absorption, charge/energy cascade to facilitate charge transport and collection, and improved nanomorphology of BHJ active layer.


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A careful selection of the additional component in ternary OSCs may ensure the synergistic effects leading to high performing OSCs.23,25 Inspired by this report we aimed to develop a priori designed high performing ternary BHJ. A desired architecture for a high performing BHJ OSC is proposed to have a nanostructured intermixed layer, bi-continuous interpenetrating network of donor-acceptor (D-A) blend, connected to the hole and electron collecting electrodes through a thin fully percolated layer of donor and acceptor molecules, respectively. Wunsch et al. and Oversteegen et al. have shown how different molecular size and shape affect materials miscibility.26-27 Since fullerene based acceptors are largely used in BHJ OSCs, we chose to an ellipsoidal PC71BM, and a spherical PC61BM as acceptors to study the influence of molecular-shape-induced effects on ternary blend morphology and the device performance.28 Here, using a ternary blend system involving one donor (PTB7-Th) with two structurally different fullerenes (acceptors) PC71BM and PC61BM, we report an enhancement in PCE as compared to the same of individual binary BHJ devices, PTB7Th:PC71BM and PTB7-Th:PC61BM. Voc of these ternary BHJ OSCs in inverted geometry remains similar with the addition of PC61BM as LUMO levels of PC61BM and PC71BM are comparable. Our ternary BHJ devices with 20% content (relative to PC71BM) showed the highest PCE of 8.5% (average ~ 8.0%) which is higher than that ~ 6.43% and ~ 6.47% of PTB7Th:PC71BM and PTB7-Th:PC61BM binary devices, respectively. A confocal Raman spectroscopy, and atomic force microscopy (AFM) study on these ternary and binary blend devices reveal that the molecular-shape-induced effects improve the morphology of mixed BHJ layer and modulates BHJ/ZnO interface. The impedance analysis of these OSCs confirms an interface modulation at the electron transport layer leading to better charge transport and charge collection in the ternary blend devices. To confirm the role of PC61BM as an interface modulator


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we fabricated and characterized the PTB7-Th:PC71BM blend devices having a thin PC61BM layer on ZnO. These OSCs exhibit similar PCE as demonstrate by the ternary blend devices with 20% content of PC61BM.

2. EXPERIMENTAL SECTION Materials. PTB7-Th, PC71BM, PC61BM, and ITO (Indium-doped tin oxide) substrate were purchased from Luminescence Technology Corporation, Taiwan. All other chemicals and solvents were bought from Sigma-Aldrich. These chemicals were used as received. Device Fabrication. Bulk heterojunction OSCs using PTB7-Th as a donor and the fullerene derivatives PC71BM and PC61BM as acceptors were fabricated in an inverted architecture of ITO/ZnO/PTB7-Th:PC71BM:PC61BM/MoOx/Ag. ITO-coated glass substrates were cleaned using a four step process comprising of the sonication in soap solution (2% micro-90 in >18 MΩ resistive de-ionized water) at 50 0C, DI water, acetone, and isopropyl alcohol, respectively. Thoroughly cleaned substrates were kept in the UV-Ozone treatment system for 25 minutes in order to increase their hydrophilicity. Electron selective contact at ITO interface, a sol-gel processed ZnO thin film, was prepared as previously reported.29 The resulting thickness of ZnO thin film was about 40 nm measured by Dektek surface profiler. On top of ZnO thin film, a thin layer of binary/ternary blend was deposited from its solution in ODCB:DIO (97:3, v/v).30 Two blends, PTB7-Th:PC71BM (0% PC61BM) and PTB7-Th:PC61BM (100% PC61BM), in the 1:1.5 (w/w) ratio were for making binary OSCs for the reference. To make ternary blends, additional PC61BM (fraction of PC71BM) has been used as the third component in various blend ratios. Due to the addition of PC61BM, D-A ratio was changed in the blend. The ternary blend solution concentration was kept constant by reducing donor (PTB7-Th) component, thereby resulting in


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optimized active layer thickness. Immediately after the active layer deposition, the substrates were transferred to the vacuum chamber of a thermal evaporator and finally, MoOx (~10 nm) and Ag (~100 nm) were coated sequentially at 2 × 10-6 mbar pressure to complete the devices. The devices were encapsulated with microscopic slide using Delo glue. The active area of devices was 0.09 cm2. Thin film characterization. The binary and ternary blend films were characterized for surface Raman spectra using 532 nm wavelength laser on WITec Alpha 300 confocal Raman spectrometer. The thin film surface topography and the UV-Vis absorption measurements were done for each the active layers using Atomic Force Microscope (Model # Park XE7) and Shimadtzu UV-Vis-NIR spectrometer, respectively. Scanning Kelvin Probe Microscopy (SKPM) was done to measure the work function of ZnO, PC61BM passivated ZnO thin film using Park XE7 system. Solar cell characterization. The current density-voltage (J-V) characteristics of OSCs were measured using Keithley 4200 SCS parameter analyzer under an AM1.5G illumination source (1000 Wm-2) using a Photo Emission Solar Simulator (Model # SS50AAA). The light intensity was adjusted with an NREL calibrated Si solar cell. The structure of ITO/ZnO/active layers/Al was used for electron only devices and ITO/ active layers/MoOx/Ag was used for hole only devices. Then electron and hole mobilities were calculated by Mott-Gurney equation for binary and ternary blends.

The devices were measured for the EQE using a system from M/s

Bunkoukeiki (Model: CEP-25HS-50 SR). The impedance analyzer (Autolab PGSTAT-302N) was used for C-V, and impedance measurements. Impedance spectra were recorded by applying a small voltage perturbation (10 mV RMS) at frequencies ranging from 1 Hz to 1 MHz.


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3. RESULTS AND DISCUSSION Effect of spherical PC61BM on blend thin film morphology. The molecular structures of PTB7-Th, PC71BM and PC61BM are shown in Figure 1. The differences in the geometrical shape and size of fullerenes, ellipsoidal (PC71BM) and spherical (PC61BM), affects molecular packing and miscibility.28 In order to understand the impact of the structural differences of PC71BM, and PC61BM on thin film morphology, we mixed them in various blend ratios (0:100, 20:80, 50:50 and 100:0, w/w) in o-dichlorobenzene (ODCB) and spin coated on a zinc oxide coated ITO/Glass substrate. As grown thin films were characterized using Raman spectroscopy (excitation at 532 nm) and atomic force microscopy. A surface Raman spectra of PC71BM, PC61BM and blend PC71BM:PC61BM thin films are shown in Figure 2. Inspite of having similar backbone, the two fullerenes were distinctive in several vibrational bands.31 The Raman spectrograph show the distinct vibration peaks for PC61BM (~1467 cm-1) and PC71BM (~1340 cm-1 and 1574.1 cm-1). In PC71BM:PC61BM blend samples, the characteristic peak of PC61BM at (~1467 cm-1) was found missing and the signature peaks of PC71BM has been prominently observed at the surface. These features of Raman spectra indicate PC71BM rich top surface in PC71BM:PC61BM blend thin film which can be attributed to the percolation of PC61BM molecules down in the blend thin film due to their lower miscibility and better precipitation kinetics as reported by Kronholm et al.32


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Figure 1. Molecular structures of donor (PTB7-Th) and acceptors (PC71BM and PC61BM) used in ternary blend devices.

Figure 2. Confocal Raman spectrograph of PC61BM/PC71BM blend thin films for various blend ratios (0:100, 20:80, 50:50 and 100:0, w/w) in o-dichlorobenzene (ODCB). Figure 3 exhibits the AFM topography scan images of PC71BM, PC61BM and blend PC71BM:PC61BM thin films. Out of the two fullerenes, the PC61BM thin film shows a smoother surface with root-mean-square (RMS) roughness of 0.18 nm relative to the surface of the PC71BM thin film (RMS roughness ≈0.26 nm). Larger degree of crystallinity of PC71BM molecules was observed which can be attributed to the preferential orientation of PC71BM molecules.26 Further, we reveal that the addition of PC61BM molecules in PC71BM smoothens the surface of blend thin films by lowering the RMS roughness from 0.18 nm (20% PC61BM) to


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0.14 nm (50% PC61BM). The percolation of spherical PC61BM molecules through PC71BM aggregates alters the morphology of PC71BM:PC61BM blend thin films. Such changes in thin film morphology have potential to modulate the interfaces which could be beneficial to collect charge carriers (holes and electrons) at the respective electrodes.

Figure 3. AFM images of thin film morphology of PC61BM/PC71BM blend thin films. In order to follow the corresponding changes in ternary blends, the fullerenes, PC71BM and PC61BM, were mixed with a low band gap donor polymer, PTB7-Th (Poly{4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophene-4,6-diyl}).

The

PTB7-Th:PC71BM

(1:1.5,

w/w)

bulk

heterojunction based binary organic OSCs have shown optimized PCE ~ 7.3 %.33 We prepared the ternary BHJ films by adding PC61BM (as a fraction of PC71BM) in PTB7-Th:PC71BM (1:1.5, w/w) binary blends, keeping the overall concentration of 25 mg/ml. We term two binary compositions, blends of PTB7-Th:PC71BM (1:1.5, w/w with 0% PC61BM) and PTB7Th:PC61BM (1:1.5, w/w with 0% PC71BM) as reference layers. The thickness of active layers for all devices was ensured to be 85-90 nm, confirmed by Dektek thickness profiler.


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The confocal Raman measurements were carried out on binary and ternary thin films with 0%, 20%, 50%, and 100% PC61BM content. Figure 4 shows the Raman spectra of these films along with that for neat PC61BM, neat PC71BM, and neat PTB7-Th thin films. PTB7-Th exhibits a distinct Raman spectra with characteristic peaks at 1470 cm-1, 1490 cm-1, and 1535 cm-1. We attribute these major peaks to the C=C stretching mode of the Thiophene, BDT and fluorinatedThiophene units of the conjugated backbone of PTB7-Th.34 The thin films of neat PC71BM and PC61BM show characteristic Raman peaks at 1340 cm-1 and 1574.1 cm-1, and 1467 cm-1, respectively. The Raman spectra of ternary thin films exhibit characteristic peaks of PTB7-Th and PC71BM. However, the typical Raman peak of PC61BM (~1467 cm-1) was found missing irrespective of 20% and 50% content (relative to PC71BM) of PC61BM. These observations confirm that the PC61BM molecules have a tendency to percolate towards the bottom of the BHJ layer in the ternary system.

Figure 4. Confocal Raman spectrograph of isolated PTB7-Th, PC61BM, and PC71BM thin films, PTB7-Th:PC71BM, PTB7-Th:PC61BM binary, and PTB7-Th:PC71BM:PC61BM ternary thin films.


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Moreover, to validate the percolation of PC61BM downward in the ternary BHJ thin films, we lifted off the ternary blend thin film using scotch tape. The lift off gave us an access to the underneath buried layers at ~60 nm below the surface of the ternary BHJ thin film, and we probed the chemical composition there using Raman spectroscopy. The Raman spectrographs, shown in Figure S1, exhibit an enhanced intensity of the signature peak of PC61BM at 1467 cm-1 for the lifted off thin films as compared to as-grown thin films of the ternary blend having 20% and 50% PC61BM content. This observation confirms the downward percolation of PC61BM molecules in the ternary BHJ thin films. (a)

(b)

(c)

(d)

(e)

(f)

Figure 5. (a) Binary (PTB7-Th:PC71BM) thin film morphology, (b) Phase image of Binary (PTB7-Th:PC71BM) thin film, (c) 20% PC61BM ternary thin film morphology, (d) Phase image of 20% PC61BM ternary thin film, (e) 50% PC61BM ternary thin film morphology, and (f) Phase image of 50% PC61BM ternary thin film.


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Figure 5 shows the height and phase image of PTB7-Th:PC71BM:PC61BM ternary thin films with different content of PC61BM. The Figure 5a and 5c show the thin film morphology of PTB7-Th:PC71BM binary, and PTB7-Th:PC71BM:PC61BM ternary system containing 20% PC61BM, respectively. The cluster structure with aggregated domains, and the RMS roughness of 1.13 nm (for binary) and 1.94 nm (for the ternary system with 20% PC61BM content) are observed on the surface of thin films. The film becomes smoother with an increase in weight percentage of PC61BM in the ternary mixture and roughness has reduced to 0.86 for 50% (shown in Figure 5e and 5f). The AFM phase image at the surface exhibit that the aggregated domains for 20% ternary thin film, shown in Figure 5d, are at the larger scale with a better uniformity than the aggregated domains in case of PTB7-Th:PC71BM binary. The homogeneous structure in 20% ternary blend may be evolved due to the differential precipitation kinetics of two fullerenes in the blend. The thin films with higher content of PC61BM had even smaller domain sizes which will be suitable for the charge separation in OSC devices. Although, it may not be good for charge transport which requires continuous domains.35 The absorption spectra of the binary and ternary thin films with different blend ratios of fullerenes (PC71BM and PC61BM) were studied using ultraviolet-visible-near infrared (UV-VisNIR) spectroscopy and are presented in Figure 6. The PTB7-Th:PC71BM binary thin film displays maximum absorbance between 450 nm to 750 nm wavelengths. While incorporation of PC61BM gradually decreased the absorption strength of the active layer in the same wavelength range. The peak intensity of PTB7-Th at 700 nm is also reduced due to variation in donoracceptor ratio by the addition of PC61BM as third component, thereby reduction in number of photons absorbed in ternary films. Also, PC71BM shows better absorption strength than PC61BM in 400-550 nm wavelength range. Hence, the overall output current reduction is expected with


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increase of PC61BM content. The observed absorption spectra averted the possibility of broad solar spectrum utilization for the enhancement of external quantum efficiency. The observations made from Raman spectroscopy, AFM and UV-Vis-NIR inspired us to investigate the performance of OSCs based on PTB7-Th:PC71BM and PTB7-Th:PC61BM binary,

Figure 6. UV-vis absorption spectra of binary and ternary blend thin films. and PTB7-Th:PC71BM:PC61BM ternary BHJ active layers having different weight content of PC61BM molecules. The percolation of PC61BM molecules in ternary blend system could be beneficial to achieve the desired BHJ profile having a nanostructured intermixed layer connected to the electron and the hole collecting electrodes through the acceptor-rich and the donor-rich regions in the active layer, respectively. Since the bottom of the ternary BHJ layer is apparently rich with acceptor molecules we decided to study the influence of structurally different fullerenes on the performance of OSCs in inverted architecture. Solar cell characterization.


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After the feasibility study of PTB7-Th:PC71BM:PC61BM ternary BHJ active layers, we fabricated the OSCs with an inverted architecture of ITO/ZnO/Active layer/ MoOx/Ag in ambient conditions. Here, the active layers used in our devices were reference binary BHJ, PTB7-Th:PC71BM (1:1.5) and PTB7-Th:PC61BM (1:1.5), and

ternary BHJ, PTB7-

Th:PC71BM:PC61BM, with 10%-85% weight content of PC61BM molecules relative to the content of PC71BM maintaining blend concentration of 25 mg/ml. The current density-voltage (JV) characteristics of OSCs were measured under the standard conditions. The representative J-V characteristics for the reference binary devices and ternary blend devices with 20% and 50% PC61BM content are shown in Figure 7. The average values of device parameters, collected from twelve devices for each composition fabricated in six different batches, short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), series resistance (Rs), shunt resistance (Rsh), and PCE are summarized in Table S1. Our reference binary PTB7-Th:PC71BM devices have shown the average PCE of 6.4% with a Jsc at 13.2 mA cm-2, Voc at 0.85 V and FF at 56.9%. The addition of 20% PC61BM into the host binary PTB7-Th:PC71BM blend, lead to the best PCE of 8.5% with a Jsc at 14.2 mA cm-2, a Voc at 0.84 V and FF at 72%. There is more than 32% (average 29%) improvement in PCE compared with the reference binary OSC and it could be the highest improvement in PCE of ternary devices reported so far. We could note from the data shown in Table 1 that in ternary OSCs with increasing content of PC61BM, Voc remains almost constant. The FF improves with increasing PC61BM content, reaches to 72% (for 20% ternary device) and decreases with further addition of PC61BM. Importantly, we note that both the binary blend devices, PTB7-Th:PC71BM and PTB7-Th:PC61BM exhibit Jsc at 13.2 mA cm-2 and 13.8 mA cm-2, respectively. The best performing ternary OSC had shown lowest series resistance (Rs) and highest shunt resistance (Rsh) of 6.5 Ω-cm2 and 781.0 Ω-cm2, respectively.


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Figure 7. J-V characteristics of binary and ternary blend devices. Table1. The electrical parameters of binary and ternary OSCs at different PC61BM content under 1000 W m-2 illumination at AM1.5G in ambient condition. % PC61BM

Jsc [mA cm- Voc [V] 2 ]

FF [%] Ƞ [%] [ave.]

Rs [Ω-cm2]

0

13.2

0.85

56.9

6.4 [6.2]

9.3

449

20

14.2

0.84

72.0

8.5 [8.0]

6.5

781

50

14.6

0.84

60.5

7.5 [7.0]

7.9

685

100

13.8

0.84

55.9

6.4 [6.1]

8.0

703

Rsh [Ω-cm2]

The absorption spectra of the ternary thin films show a decrease in the number of photons absorbed in 450 nm-750 nm range and thereby lesser current density was expected. Although the highest photovoltaic performance was observed in the ternary devices with 20% PC61BM


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content. In an attempt to understand the observed inconsistency between the optical absorption and the photovoltaic performances, external quantum efficiency (EQE) spectra were measured and presented in Figure S2. The maximum EQE was found for the ternary devices with 20% content of PC61BM, supporting the higher Jsc value for these devices. However, this enhancement can’t be attributed to optical absorption because the addition of PC61BM did not improve the photon absorption in whole visible wavelength spectrum. We verified the accuracy of the photovoltaic performances by calculating the Jsc from an integration of the EQE spectra. A good consistency in calculated and experimentally measured Jsc was observed. For example, the calculated Jsc for 20% PC61BM ternary device from EQE spectra was found to be 14.8 mA cm-2, which is close to 15.0 mA cm-2, experimentally measure from J-V characteristics. Charge transport and charge collection efficiency. For more insight on the composition dependent OPV performances of PTB7Th:PC71BM:PC61BM devices, we measured the capacitance-voltage (C-V) and impedance characteristics. The devices were connected to two electrodes of C-V analyzer under zero dc bias and scanned for the bias voltage between -1 to 1 V under dark condition. During the measurement, the AC amplitude was kept 10 mV (RMS) in order to maintain the linearity of the response, and measuring frequency was fixed at 5 kHz. The C-2-V plot for binary BHJ PTB7Th:PC71BM and PTB7-Th:PC61BM devices, and the ternary blend devices with 20% and 50% content of PC61BM, are shown in Figure 8. Classical Mott-Schottky (MS) analysis was applied to calculate the built-in-voltage (Vbi) and total acceptor charge carrier concentration (NA). The measured Vbi and NA for binary and ternary devices are reported in Table 2. The Vbi reduced to 0.81 V for 20% PC61BM loaded ternary OSCs relative to 0.96 V for the binary host device. The Vbi further increased to 0.83 V for the ternary devices with 50% PC61BM content. On the other


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hand, NA decreased from 1.2 × 1017 cm-3 (for the binary host device) to 5.2 × 1016 cm-3 and 5.1 × 1016 cm-3 for the ternary devices having 20% and 50% PC61BM content, respectively. A composition dependent Vbi in PC61BM loaded ternary OSCs indicate the changes in energy barrier at the interfaces. The Scanning Kelvin Probe Microscopy (SKPM) on ZnO and PC61BM passivated ZnO thin films on top of ITO coated glass substrate exhibit the effective work function of 4.32 eV and 4.21 eV, respectively. Thereafter, an increasing thickness of PC61BM on ZnO surface increases the work function. A similar modulation of the work function is expected at the electron extracting electrode side due to the downward percolation of PC61BM in the ternary devices and it explains the observed trend in the built-in voltage (Vbi). In this case, the modification in surface states could be possible due to the presence of PC61BM molecules at the BHJ/ZnO interface which is well supported by lowered value of acceptor carrier concentration at this interface. This observation indicates the interface modulation due to the

Figure 8. C-2 plot with bias voltage for binary and ternary blend devices under dark at 5 kHz frequency.


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percolation of spherical shaped PC61BM towards ZnO interface because spherical shaped PC61BM molecules are smaller in the size compared to elliptical shaped PC71BM molecules.28 Also, our analysis of Raman spectrograph for the binary (PTB7-Th:PC71BM) and the ternary active layers (PTB7-Th:PC71BM:PC61BM) support such observation.

Table 2. Built-in potential and total acceptor carrier concentration for binary and ternary blend devices from Mott-Schottky analysis. Parameters 0% PC61BM

20% PC61BM

50% PC61BM

100% PC61BM

Vbi [V]

0.96

0.81

0.83

0.89

NA [# cm-3]

1.2 × 1017

5.2 × 1016

5.1 × 1016

2.6 × 1016

Further, we used impedance spectroscopy (IS) to investigate charge transfer dynamics and recombination of charge carriers at the interfaces. Multiple non-radiative recombination of charge carriers and charge transport in the devices can be distinguished according to their response to the externally applied alternating current (AC) signal. Hence, the binary and ternary devices were probed in response to an AC perturbation of 10 mV (Vrms) varied over a frequency range of 1 Hz to 1 MHz at zero bias. The Nyquist plot of the devices measured under dark is shown in the Figure 9a. Impedance response of the binary and ternary devices are the semicircle in the complex plane with the vertical axis as an imaginary impedance value (-ImZ) and the lateral axis as real impedance value (ReZ). Figure S3a shows the high-frequency regions of Nyquist plot and their intercept with ReZ axis represents the series resistance of the devices.23 The series resistance for the host binary device (with 0% PC61BM) was 74.8 Ω and it dropped to 38.7 Ω for the ternary devices with 20% content of PC61BM. However, the series resistance of


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the ternary device with 50% PC61BM and the PTB7-Th:PC61BM binary device was 44.0 Ω and 57.3 Ω, respectively. The modulation of the work function observed in SKPM measurements leads to the lower series resistance in the ternary system with 20% PC61BM content.36 The drop in the series resistance for ternary devices may be associated with better energy level alignment for charge extraction due to PC61BM percolation towards ZnO interface. Similarly, the intercept of Nyquist plot towards the low-frequency region on the lateral axis gives the recombination resistance37 (Rrec) and among all the devices, the highest value of 232 kΩ was noted for the ternary device with 20% PC61BM content. The IS data was modeled as an electrical circuit comprising the circuit elements representing the various physical processes occurring in the devices. The underlying electrical parameters related to the various processes were extracted by fitting the Nyquist plot using equivalent circuit proposed by Bisquert et al. as shown in Figure S3b.38-39 The resultant IS data fitting is shown by the solid lines in Figure 9a and Figure S3a. The ternary OSCs, comprising PTB7Th:PC71BM:PC61BM blend layer sandwiched between ITO/ZnO and MoOx/Ag electrodes, is modeled as a parallel combination of a resistor (R)-capacitor (C). The parallel combination of resistance (Rrec) and capacitance (Cµ) accounts for the recombination process in the bulk at the donor-acceptor interfaces. In this model, the constant phase element (CCPE) is used instead of infinite R||C circuits in series (transmission line model), and it covers the aspects of the recombination processes with different relaxation times. Here, the Cg represents a bulk capacitor associated with the depletion region inside the active layer and is equivalent to the geometrical capacitance of the device. The resistor, Rt represents the charge transport in the bulk of the active layer. The electrical resistance present at metal electrodes and electrode/contact layer/active layer interfaces is assigned to a series resistance Rs. The random characteristics of a blended active


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layer in OSC can be attributed to the distribution of relaxation times and are related to the impedance as Z = Y0−1 ( jω ) − n , where Y0 is the coefficient of the CPE and n represents an “ideality” factor characteristic of the distribution of relaxation times. n varies from 0 to 1.0, with 0 corresponding to pure resistive and 1.0 corresponding to pure capacitive behavior.40-42 The average charge carrier lifetime (τn) can be estimated using following Equation 1 and 2: 1

(Y R ) n Cµ = 0 rec , Rrec

(1)

τ n = Rrec Cµ ,

(2)

The physical parameters estimated from the fitting of Nyquist plots are in a good agreement with the experimental data. The extracted electrical parameters, Rrec, and τn for the content of PC61BM in the ternary devices are displayed in the Figure S3c. Initially the Rrec increased with the PC61BM content in the device and reached to maximum for 20% composition. Later, it started decreasing with increasing content of PC61BM and reached to the lowest value for PTB7Th:PC61BM (100%) binary devices. The effective lifetime of the electrons followed the similar trend as shown by Rrec. The ternary devices with 20% PC61BM content exhibit the highest lifetime and it can be attributed to less recombination (high recombination resistance) leading to an efficient extraction of electrons. This observation once again hints to an improved BHJ morphology and the interface modulation at BHJ/ZnO junction in the ternary devices with 20% PC61BM content. Further, using the impedance data we analyzed the device performance in terms the charge collection efficiency for the binary and ternary blend devices. The charge collection efficiency depends on the ratio of electron diffusion length (Ln) to the blend layer thickness (L), which equals (Rrec/Rt)1/2. Higher the value of Ln/L, better the charge carrier collection at zero bias.40,43-46


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Figure 9b displays the variation of (Rrec/Rt)1/2 and FF with respect to PC61BM content in the ternary OSCs. We observe that (Rrec/Rt)1/2 is maximum for 20% PC61BM ternary devices along with FF. The higher values of (Rrec/Rt)1/2 indicate an improvement in the charge collection efficiency in the ternary devices which can be attributed to the interface modulation.

(a

(b)


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Figure 9. a) Impedance response of binary and ternary devices under dark at zero external dc bias. Dots represent experimental data and solid lines are the fit, and b) (Rrec/Rt)1/2 and FF as a function of PC61BM content in the binary and ternary devices. The electron and hole mobility of PTB7-Th:PC71BM:PC61BM blend thin films with the different weight content of PC61BM were measured by the space charge limited current (SCLC) method. Figure S4 shows a composition dependent electron mobility with increasing weight content of PC61BM in ternary blend films. The electron mobility of the ternary blend thin films was higher than that of the binary devices and 20% PC61BM blend ternary film exhibit the highest electron mobility of 2.2 × 10-4 cm2 V-1 s-1. The hole mobility also increases with increasing PC61BM content in ternary blend films and 100% PC61BM binary device exhibit the hole mobility of 1.02 × 10-4 cm2 V-1 s-1. The lower electron mobility observed in the ternary blend devices with higher PC61BM content are due to uniform distribution of smaller D-A domains in blend thin films. The energetics for inverted ternary OSC, shown in Figure S5, affirms a constant Voc as observed due to a similar LUMO energy levels of PC71BM and PC61BM. Here, we also infer that such energetics in our devices do not facilitate the charge cascading. In our ternary devices, a high value of Jsc in spite of reduced light absorption suggests a significant improvement in the charge transport and charge collection at the respective electrodes. The reduced Rs and increased Rsh, thereby resulting in an enhanced FF in these devices, also supports our inference. The enhanced charge transport and charge collection efficiency are attributed to a downward percolation on PC61BM in the ternary blend thin films due to the molecular-shape-induced effects of an ellipsoidal PC71BM and a spherical PC61BM.


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4. CONCLUSION In summary, we have designed an efficient ternary BHJ active layer involving one donor (PTB7-Th) and two structurally different fullerene acceptors, an ellipsoidal PC71BM and a spherical PC61BM, through a prior investigation of Raman spectroscopy and thin film morphology. The addition of PC61BM to host binary system, PTB7-Th:PC71BM shows a percolation of PC61BM molecules towards the bottom in ternary thin films and have potential to alter morphology to facilitate charge transport and collection. The inverted OSCs based on the ternary active layer show a composition dependent efficiency and fill factor. The ternary blend devices with 20% PC61BM content exhibited the efficiency of 8.5% which is ~32% enhancement compared with the efficiency of the host PTB7-Th:PC71BM blend devices. It could be amongst the highest efficiency enhancement (relative to the host binary devices) in the ternary devices reported so far. A detailed C-V, impedance analysis and SCLC measurements of the binary and ternary devices confirm an improvement in charge transport and charge collection by reducing built-in potential and defect states which is apparently happening due to a downward percolation of spherical PC61BM molecules in ternary active layers towards BHJ/ZnO interface. Our study demonstrates that the molecular shape and size aspects of the third component in a fullerene based ternary blend active layers could be an important factor to realize high performing OSCs.

SUPPORTING INFORMATION. Confocal Raman spectrograph before and after active layer lift-off, EQE spectra of binary and ternary blend devices, high frequency Nyquist plot, equivalent circuit model, recombination resistance and lifetime variation with PC61BM content in ternary blend, SCLC electron mobility, energetics of ternary solar cell, and table containing device performance for other compositions.


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ACKNOWLEDGMENTS This work was supported by the Shiv Nadar Foundation (SNF). Authors would like to thank National Physical Laboratory, New Delhi and AIRF facility, Jawaharlal Nehru University, New Delhi for facilitating EQE measurement and confocal Raman spectra respectively. REFERENCES 1. Li, G.; Zhu, R.; Yang, Y. Polymer solar cells. Nat. Photonics 2012, 6, 153-161. 2. He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591-595. 3. Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nat. Commun. 2014, 5, 5293. 4. You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.C.; Gao, J.; Li, G. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446. 5. Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.; Shultz, G. V.; Ng C. F.; Mangubat, L. B. Polymer-based solar cells: state-of-the-art principles for the design of active layer components. Green 2011, 1, 29. 6. Kotlarski J. D.; Blom, P. W. M. Ultimate performance of polymer: fullerene bulk heterojunction tandem solar cells. Appl. Phys. Lett. 2011, 98, 053301.


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BRIEFS Molecular shape dependent percolation study of fullerene molecules towards modulation of interface and morphology of bulk heterojunction active layer resulting in an efficient ternary blend organic solar cell


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Molecular-Shape-Induced Efficiency Enhancement in PC61

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