Perfect Complementary in Absorption Spectra with Fullerene

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Perfect Complementary in Absorption Spectra with Fullerene, Non-fullerene Acceptors and Medium Bandgap Donor for High Performance Ternary Polymer Solar Cells Hao Liu, Jinyan Li, Lixing Xia, Yiming Bai, Siqian Hu, Jiyan Liu, Lin Liu, Tasawar Hayat, Ahmed Alsaedi, and Zhan'ao Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07993 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Perfect Complementary in Absorption Spectra with Fullerene, Non-fullerene Acceptors and Medium Bandgap Donor for High Performance Ternary Polymer Solar Cells Hao Liu,a Jinyan Li,a Lixing Xia,a Yiming Bai,a Siqian Hu,b Jiyan Liu,b Lin Liu,a Tasawar Hayat,c Ahmed Alsaedi c and Zhan’ao Tan*a a

State Key Laboratory of Alternate Electrical Power System with Renewable Energy

Sources, North China Electric Power University, Beijing 102206, China. b

Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of

Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China c

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia. E-mail: [email protected] (Z. A. Tan)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 Due to the mismatching between the solar irradiance spectra and the photoactive layer absorption spectra, only a part of sunlight can be utilized, thus fundamentally restricts the power conversion efficiency (PCE) of the polymer solar cells (PSCs). Ternary blends PSCs have become an effective approach to extend the absorption spectra and increase the mobility of the charge carriers by additional third component. Herein, we select middle bandgap PBDTBDD as electron donor, narrow bandgap ITIC and wide bandgap PC60BM as electron acceptors to construct ternary blends for simultaneously enhancing the absorption intensity and expanding the absorption band. The optical properties, morphologies and the charge/energy transfer behaviors of the ternary blends are investigated. By attentively adjusting the ratio of third component ITIC, the ternary PSCs demonstrate expanded light-response region and greatly enhanced JSC, giving improved overall PCE of 10.36%, much higher than that of binary counterparts based on PBDTBDD:PC60BM (6.63%) and PBDTBDD:ITIC (9.44%). These findings indicate that properly select donor and acceptor to construct absorption spectra complementary ternary blend photoactive layer is an effective way to achieve high performance of PSCs.

Keywords: Ternary polymer solar cells; complementary absorption; fullerene acceptor; non-fullerene acceptor; middle bandgap polymer


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1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been attracted wide interest since their easy fabrication, large area, light weight and rich raw materials.1-4 The photoactive layer for general PSC is binary blend of the conjugated polymer donor material and fullerene derivative acceptor material. Due to the mismatching between the solar irradiance spectrum and the photoactive layer absorption spectrum, only a certain of visible light can be effectively absorbed, thus fundamentally limits the efficiency of PSCs.5-7 To improve the ability of light-harvesting, two strategies are commonly used in PSCs. One is improving the absorption intensity, such as closing the molecule stacking by solvent induced self- assembly to enhance the absorption coefficient, application of surface plasmonic

resonance effect to enhance the local

electromagnetic field intensity, designing all kinds of light-trapping structures to extend the optical path length.8-10 The other approach is extending the absorption band of the photoactive layer, especially developing low bandgap electron donor and acceptor materials.11-15 Unfortunately, the full width at half maximum (FWHM) of these low bandgap materials is only 150 nm, making underutilization of the sunlight. For enhancing the absorption intensity and expanding the absorption band simultaneously, tandem polymer solar cells with spectra complementary bottom and top subcells connected by intermediate layer are designed, where photons with different energy are harvested by the top and the bottom subcell respectively with reduced energy loss, thus making more of the of light energy convert into electrical



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energy.16-17 However, the tandem solar cells have complex device structure, the fabrication processes are time-consuming and high-cost, not easy to transfer to commercial or industrial product line. 18 Ternary blends PSCs with two donors and one acceptor or one donor and two acceptors have become a hot spot to extend the absorption spectra and increase the mobility of the charge carriers by additional third component. Early studies in ternary PSCs mainly focused on two donors with different absorption bands blended with fullerene derivatives (PC60BM or PC70BM). Due to the limition of absorbtion range and big energy loss between fullerene acceptors with narrow band gap materials, non-fullerene acceptor materials with broaden absorption spectral range and higher carrier mobility are developed and used in ternary PSCs. 19 20 The synergistic effect of small molecules and polymer is the main factors for enhancing the performance in ternary PSCs. Benefited from the synergistic effect between the three components, ternary solar cells demonstrated enhanced PCE of over 10%.

11, 21

A well-designed

ternary systems can provide a variety of benefits effects such as broadening the absorption range of the spectra, modifying the morphology of the film and changing the energy level of the active layer to more closely match the other functional layers, thereby increasing the performance of the device. With many benefits, but the only drawback is that the voltage value of the ternary blend structure is always lower than the highest value of the binary structure which limits the device efficiency of the OSCs.


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Our previous studies indicated that PSCs based on middle bandgap PBDTBDD and PC60BM binary blends demonstrated high open-circuit voltage (Voc) and high fill factor (FF). Unfortunately, due to the optical absorption edge of PBDTBDD:PC60BM blends active layer is cutoff at about 700nm, only part of the visible light can be utilized, resulting in limited photo-generated current, thus the PCE of the device is limited to around 7%. Recently reported narrow bandgap non-fullerene acceptor ITIC bares light absorption mainly in the 650-800nm, just form a spectral complement with PBDTBDD, expanded the absorption spectra of the original binary PSCs. Therefore, in this work, we select middle bandgap PBDTBDD as electron donor, narrow bandgap ITIC and wide bandgap PC60BM as electron acceptors to construct ternary blends for enhancing the absorption intensity and expanding the absorption band simultaneously. Through careful adjustment ratio of ITIC in active layer, the ternary PSCs demonstrate expanded light-response region and greatly enhanced JSC and VOC, giving improved overall PCE of 10.36%, significantly higher than that of binary counterparts based on PBDTBDD:PC60BM (6.63%) and PBDTBDD:ITIC (9.44%).

2. Result and discussion Figure 1a displays absorption spectra of pure PC60BM, ITIC, and PBDTBDD active layers, and the structures of these molecules are shown in the insets. The wide bandgap PC60BM acceptor can utilize the shorter wavelength from 300 nm to 500 nm, the middle bandgap PBDTBDD donor can absorb middle region from 500 nm to 700 nm, and the narrow bandgap ITIC acceptor can further extend the absorption from



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700 nm to 850 nm. Obviously, these three materials demonstrate perfect complementary in absorption spectra within the whole visible region.22 The absorption spectra of the binary blends of PBDTBDD:PC60BM and PBDTBDD:ITIC and the ternary blends of PBDTBDD:PC60BM:ITIC with different mass ratios can be seen in Figure 1b. Compared with the absorption spectra of binary blends of PBDTBDD:PC60BM,

the

absorption

spectra

of

ternary

film

PBDTBDD:PC60BM:ITIC extend to 850 nm. Furthermore, by varying the ratios of PC60BM and ITIC, the absorption intensity of the blend film at shorter and longer wavelength can be effectively tuned. With the doping ratio of 1:0.6:0.4, most balanced absorption at the shorter and longer wavelength has been achieved, and this will benefit the light-harvesting in the whole wavelength range, which could induce the maximum JSC for the devices.


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Figure 1. (a) The absorption spectra of pure ITIC, PC60BM and PBDTBDD films; (b) absorption spectra of the blend films with different proportion of ITIC and PC60BM.

For bulk heterojunction PSCs, the surface morphologies of blended photoactive layers play an important role for charge transport and exciton dissociation. An ideal photoactive layer with bi-continuous interpenetrating networks can decrease the charge recombination and improve the charge transport.

11, 23

That is why the

morphology of the active mixture layer was measured by using a tapping mode atomic force microscope (AFM). Figure 2 gives the phase and topographic AFM



images of the photoactive layers with different proportion of acceptors spin-coated on the Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) sputtered ITO substrate. In Figure 2a, it suggested that the surface of PBDTBDD:PC60BM (1:1) photoactive film is very smooth and the root-mean-square (RMS) roughness is only 3.32 nm. As seen from the phase images, the different color parts can be identified as different components, so two binary structures both show ordered structure in the Figure 2d and 2l. When doping a small portion of ITIC (1:0.8:0.2), the surface morphology of the blends film changes greatly and the RMS roughness increases to 6.30 nm (Figure. 2b). Compare with the phase diagram below, Figure 2a exhibits more ordered molecular aggregation than Figure 2b and the change in aggregation state is evidence that incorporating another acceptor into binary structure significantly affects the morphology and roughness of the active layer.24-25 When further increasing the ITIC ratio to 1:0.6:0.4, the RMS roughness of the ternary blends reduces to 3.93 nm, it can be seen in Figure 2c. The change of RMS maybe attributed to the aggregation state at 1: 0.6: 0.4 doping concentration is closer to the binary structure than 1: 0.8: 0.2 from Figure 2e-f. With continuing increases the ratio of ITIC to 1:0.4:0.6 (Figure 2g), the RMS roughness of the ternary blend layers can even decrease to 2.90 nm, which is smaller than the binary structure (3.32 nm and 4.11 nm, Figure 2h-i), also the state of aggregation has also been drastically changed in comparison with the binary structure. The RMS roughness of 1:0.2:0.8 (Figure 2h) based film is 2.98 nm, being same to 1:0.4:0.6 based film (Figure 2g). But we can see from Figure 2k-l, when the proportion is 1: 0.2: 0.8 has similar molecular aggregation


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with binary structure in some area; this can be summarized as aggregation states vary from one binary structure to another as the doping concentration varies. Due to it has the most like binary state of aggregation and roughness, the fine features of phase separation can be clearly observed from the ternary blend films when the proportion is 1:0.6:0.4.



Figure 2. AFM (a-c, g-i) surface topographic and (d-f, j-l) phase images of PBDTBDD:PC60BM:ITIC ternary blends with different mass ratio from 1:1:0 to 1:0:1.

To further investigate the phases and the crystalline packing structure of ternary film, grazing incidence wide-angle X-ray scattering (GIWAX) tests were performed on the two binary films and the optimized ternary film as shown in Figure 3. From Figure 3d, the binary blends PBDTBDD: PC60BM film has the diffraction signals appeared at 0.30, 0.60 and 0.90 Å-1 (in plane), suggesting that it has the same long-range ordered laminar packing,. And the isotropic broad scattering peak of PC60BM can be observed at qxy=1.4 Å-1, indicating random aggregation in the PC60BM domains. The diffraction peak at qxy=1.75 Å-1 shows π-π stacking between the molecules in the film (d=3.59 Å). As the mass ratio of ITIC increases, the diffraction peaks of 0.30 Å-1 decrease in-plane and out-of-plane. The change in the distance and position of the characteristic peaks shows a significant change in the order of molecular packing and the crystallinity of the molecules. The in plane diffraction signals of binary blends PBDTBDD:ITIC film appeared at 0.30, 0.60, 0.90, 1.60 and 1.75 Å-1. It has the same in plane direction diffraction peaks as PBDTBDD: PC60BM at 0.30 0.60 and 0.90 Å-1, arising from the long-range ordered laminar packing with a d-spacing of 20.9Å. And the peak intensity is very high at 0.6 Å-1，it exhibits low para-crystallinity or high overall degree of crystallinity。The diffraction peak at qxy=1.60 Å-1 shows good π-π stacking between the molecules in the film (d=3.92 Å), and it exhibits relatively homogeneous morphology. Comparing Figure


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3d-f, the peak position and intensity in the out of plane are basically unchanged under the three conditions. The changes after the ternary blending occur in the in plane, and the characteristic peaks of both acceptors are preserved. This crystallinity of the molecules with the order of molecular packing changes ensure efficient electronic transmission.

26-28

These results reveal that judicious selection of acceptor pairs in

ternary blend can further improve the performance of OSCs through complementary absorption of sunlight. In addition, the active layer morphology of the ternary blend can be tuned by varying the ratio of acceptors.

Figure 3. (a-c) 2D-GIWAXS patterns and (d-f) 2D-GIWAXS extraction profiles along the qz and qxy axis for PBDTBDD:PC60BM, PBDTBDD:PC60BM:ITIC and PBDTBDD:ITIC films, respectively.

To investigate the changes of additional ITIC on the energy transfer of the ternary

film,

steady-state

photoluminescence

(PL)

and

time-resolved

photoluminescence (TRPL) tests were performed on binary and ternary blends.29 As shown in Figure 4a, by exciting with a wavelength of 500 nm light, neat ITIC film



shows two PL emission peaks at 655 and 780 nm, respectively, due to the different emission peaks can be excited from different functional groups in the organic species. The emission intensity of the long wavelength peak (780 nm) is significantly higher than that of the short wavelength peak (655 nm), but there is no obvious PL peak for pure PC60BM layer under excitation at 420 nm. The PL emission of ITIC is greatly quenched by doping PC60BM, and blend film PL curve is in the middle of neat PC60BM film and neat ITIC film, indicating either charge transfer or energy transfer exists between PC60BM and ITIC according to the parallel-linkage model. The pure donor PBDTBDD film demonstrates a broad PL peak located at 680 nm. The PL intensity of PBDTBDD is greatly quenched by doping with acceptor PC60BM in binary film PBDTBDD:PC60BM, but there still has a small PL peak at 645 nm. Similarly, the ITIC PL peak at 780 nm is not completely quenched in binary film of PBDTBDD:ITIC. Interestingly, when adding the third component ITIC in PBDTBDD:PC60BM blend film, the PL peak at 645 nm is completely quenched and the PL peak at 780 nm drop sharply, indicating that both ITIC and PC60BM can coordinately transfer the electron/energy form donor to acceptor. To further clarify the energy transfer mechanism in PBDTBDD:PC60BM:ITIC ternary blend, the TRPL spectra of PC60BM:ITIC blend films are investigated by monitoring 800 nm emission under 660 nm excitation and 560 nm emission under 485 nm light excitation, respectively. As shown in Figure 4b, the fluorescence lifetime of pure PC60BM and pure ITIC is 3.5 ns and 3.2 ns, respectively. After the incorporating of ITIC, the fluorescence lifetime of the blend film is slightly decreased to 3.1 ns and


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3.0 ns for blends with mass ratio (PC60BM:ITIC) of 3:2 and 2:3, respectively, indicating there is weak energy transfer process between the two acceptors. The energy transfer efficiency between the two acceptors can be calculated by using the following formula:

ߟ = 1−

ఛ್೗೐೙೏ ఛ

(1)

Where τ is the fluorescence lifetime of pure acceptor film, τblend is the fluorescence lifetime of the two acceptors (PC60BM:ITIC) blend film, and η is the energy transfer efficiency.25 Compared with the pure PC60BM film, the energy transfer efficiency η is calculated to be 12% and 15% for PC60BM:ITIC blends with mass ratio of 3:2 and 2:3, respectively. While for the same blends, in comparison with pure ITIC film, the energy transfer efficiency η is only 3% and 6%, respectively, indicating weak energy transfer between the two acceptors. To identify whether there is charge transfer between the two acceptors, PSCs based on PC60BM, ITIC and PC60BM:ITIC (1:1) were fabricated with structures of ITO/PEDOT:PSS/active layer/PFNBr/Al. However, from Figure 4c, it can be seen the photocurrent of organic solar cells (OSCs) based on PC60BM: ITIC (1: 1) is only 0.17 mA•cm-2, and this is no great improvement compared to that of PC60BM (0.17 mA•cm-2) and ITIC (0.23 mA•cm-2) based OSCs, indicating that charge transfer between PC60BM and ITIC molecules is almost non-existent. Combination the results of TRPL and photovoltaic performance, between the two acceptors, we can conclude that there is no apparent charge transfer, and the energy transfer is very weak.



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Figure 4. (a) PL spectra of pure acceptor PC60BM, ITIC, blend acceptors ITIC:PC60BM,

pure

donor

PBDTBDD,

binary

blends

PBDTBDD:ITIC,

PBDTBDD:PC60BM and ternary blend PBDTBDD:PC60BM:ITIC; (b)TRPL spectra


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of pure PC60BM, pure ITIC, acceptor blends PC60BM:ITIC with mass ratio of 2:3 and 3:2. (c) The current density-voltage (J-V) under the illumination of AM 1.5 G, 100 mW•cm-2 of pure PC60BM, ITIC and ITIC:PC60BM=1:1 as the active layer of OSC.

To verify the feasibility of ITIC as additional electron acceptor for extension the light-harvesting,

traditional

BHJ

PSCs

with

structures

of

ITO/PEDOT:PSS/PBDTBDD:PC60BM:ITIC/Zracac/Al are manufactured, as shown in Figure 5a. The PEDOT:PSS layer works as hole extraction layer because of its high

work

function

(5.0

eV)

and

good

hole

transport

efficiency.30

Solution-proccessable zirconium acetylacetonate (ZrAcac) is used as electron extration layer due to its suitable electronic energy levels and excellent optical transparence.31 For comparsion, control devices with the similar structure based on PBDTBDD:ITIC and PBDTBDD:PC60BM as active layers are also fabricated. The energy levels of donor and acceptors in the ternary PSC are shown in Figure 5b. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels of PBDTBDD, ITIC and PC60BM are taken from previous reports.32-34 The HOMO of PBDTBDD (-5.32 eV) is only 0.16 eV higher than that of ITIC (-5.48 eV), therefore, the holes produce by ITIC can be rapid separation and easily transfer to PBDTBDD and finally collected by anode through hole extraction layers. Furthermore, the HOMO of PC60BM (-5.90 eV) is much lower than that of PBDTBDD and ITIC, which can effectively block holes transfer back to cathode, thus greatly suppressed the leakage current and may decrease the energy



loss. On the other hand, the LUMO of acceptor ITIC (-3.83 eV) and PC60BM (-3.90 eV) has around 0.7 eV offset in compared with that of PBDTBDD (-3.18 eV), thus electrons produce by PBDTBDD can effectively transfer to ITIC or PC60BM and last collected by metal electrode through Zracac electron extraction layer.

Figure 5. (a) Device structure of polymer solar cells with ternary blend photoactive layer of PBDTBDD:PC60BM:ITIC; (b) Schematic energy level diagram of the materials involved in the ternary device. In order to further confirm the energy level change of the active layer, the ultraviolet photoelectron spectroscopy (UPS) measurement of the binary films


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(PBDTBDD:PC60BM=1:1,

PBDTBDD:ITIC=1:1)

and

ternary

film

(PBDTBDD:PC60BM:ITIC=1:0.6:0.4) were conducted as shown in Figure 6. Since the onset of the photoemission energy for PBDTBDD:PC60BM, PBDTBDD:ITIC and PBDTBDD:PC60BM:ITIC is 17.02 eV, 16.87 eV and 16.84 eV, and the cutoff of the binding energy is 0.78 eV ,0.83 eV and 0.68 eV, respectively. The energy level of the ternary film is very close to that of the active layer of PBDTBDD:ITIC, making the Voc of the ternary PSCs close to that of PBDTBDD:ITIC based binary devices.

Figure 6. (a) UPS binding energy of the secondary electron cut-off edge, and (b) Fermi edge of the PBDTBDD:PC60BM and PBDTBDD:ITIC binary films and



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PBDTBDD:PC60BM:ITIC ternary film.

Figure 7a gives the current density-voltage (J-V) curves of the PSCs with different PBDTBDD:PC60BM:ITIC mass ratio under the illumination of AM 1.5 G, 100 mW•cm-2, and the detailed device parameters of VOC, JSC, FF and PCE are listed in Table 1 Firstly, the OSC devices based on binary blends structure PBDTBDD:PC60BM (1:1) reach a PCE of 6.63% with an VOC of 0.87 V, a JSC of 10.99 mA•cm-2, a FF of 69.19%. And the other binary PBDTBDD:ITIC (1:1) exhibit a PCE of 9.44% with an VOC of 0.93V, a JSC of 16.46mA•cm-2 and a FF of 61.79%. As we look forward to as, benefiting from the expanded light-harvesting (Figure. 1) and the optimized charge separation/transportation networks (Figure. 2, 3 and 4), by doping

ITIC

as

the

additive

component,

the

ternary

structure

(PBDTBDD:PC60BM:ITIC with mass ratios of 1:0.8:0.2, 1:0.6:0.4 and 1:0.4:0.6) show enhanced JSC compare to PBDTBDD:PC60BM and PBDTBDD:ITIC binary devices. When further increasing the proportion of ITIC to 1:0.2:0.8, the JSC obviously low to 14.64 mA•cm-2 is owing to the unbalanced absorption of the short wavelength photons. The optimal ratio of PBDTBDD:PC60BM:ITIC is 1:0.6:0.4, and reaching to a highest PCE of 10.36% with a maximum JSC (17.76 mA•cm-2) and VOC (0.91 V). In general, the voltage value of the ternary blend structure device of the donor structure or the acceptor structure fluctuates in the middle of two binary structures, and there is no ternary structure that reaches or exceeds the voltage of the binary structure was reported. 20-21, 35-38 Under normal circumstances, the voltage value of PBTTBDD:ITIC


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structure can reach about 0.90V, and the highest value is 0.93V, otherwise the average value

of

the

voltage

of

the

optimized

ternary

blend

structure

(PBDTBDD:PC60BM:ITIC) is 0.9V, the maximum value can also reach the ideal situation

of

0.91V(PBDTBDD:PC60BM:ITIC

is

1:0.6:0.4),

indicating

that

PBDTBDD:PC60BM:ITIC three kinds of materials can fabricate the most ideal OSCs ternary blend active layer structure. The external quantum efficiency (EQE) curves of the PSCs with different PBDTBDD:PC60BM:ITIC mass ratio are displayed in Figure 7b. Obviously, the sensitivity of the photo-response is very consistent with the absorbance of the active layers as shown in Figure 1b, indicating that the harvested photons has been effectively converted into electrons. The PBDTBDD:PC60BM (1:1) based device shows two hump shaped EQE peaks covering 300-450 nm and 450-750 nm, attributed to fullerene absorption and PBDTBDD absorption, respectively. By adding small ratio of ITIC, the EQE curve of the ternary PSC based on PBDTBDD:PC60BM:ITIC (1:0.8:0.2) extends to 850 nm and a new peak covering 650-850 nm generated due to the contribution of ITIC. Furthermore, the relative photo-response intensity of the three peaks ascribed to PC60BM, PBDTBDD and ITIC can be simply tuned by changing the mass ratio. With the mass ratio of 1:0.6:0.4 (PBDTBDD:PC60BM:ITIC), the ternary PSC demonstrates intensive light response over the entire wavelength range between 300 nm with 850 nm due to the balanced absorption at the shorter and longer wavelength. From the EQE curves, it can be seen that although the quantum efficiency in the long-wavelength of 1:0.4:0.6 and 1:0.2:0.8 is closer to that of



1:0.6:0.4 based device, the absorption and photo-response in the short-wavelength is much weaker than that of 1:0.6:0.4 based devices. And the synergy between acceptors at 1:0.6:0.4 enhances the EQE of the entire wavelength, so the optimal mass ratio was achieved with PBDTBDD:PC60BM:ITIC=1:0.6:0.4. The integral current density values for PBDTBDD:PC60BM (1:1), PBDTBDD:PC60BM:ITIC (1:0.6:0.4) and PBDTBDD:ITIC (1:1) based devices are 10.1, 15.9 and 12.9 mA•cm-2, respectively, in good agreement with the JSC values derived from the corresponding J-V curves.


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Figure 7. (a) The current density-voltage (J-V) under the illumination of AM 1.5 G, 100 mW•cm-2 of PC60BM accounts for different proportions; (b) EQE Figure under



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the conditions of PC60BM accounts for the different proportion of the acceptor in the device; (c) The dark current curve of PC60BM accounts for different proportions

Table

1.

Photovoltaic

properties

of

the

devices

with

different

PBDTBDD:PC60BM:ITIC mass ratio. JSC

VOC

FF

PCE

Thickness

(mA•cm-2)

(V)

(%)

(%)

(nm)

1： 1： 0

10.99

0.87

69.19

6.63

120

1：0.8：0.2

13.53

0.89

66.53

8.02

125

1：0.6：0.4

17.76

0.91

63.47

10.36 130

1：0.4：0.6

15.95

0.89

63.44

9.08

125

1：0.2：0.8

14.64

0.87

65.64

8.39

130

1： 0： 1

16.46

0.92

61.79

9.44

130

PBDTBDD:PC60BM:ITIC

Conclusion In conclusion, the ternary acceptor blending device was fabricated by doping ITIC in the PBDTBDD:PC60BM blends. The three materials form the complementary spectrum and better photon harvesting ability to increase the JSC. Incorporation of ITIC in the PBDTBDD:PC60BM blend can form suitable donor/acceptor phase separation, benefiting for efficient exciton dissociation and charge transport (low


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recombination loss). Therefore, synergistic effect of two acceptors maybe provide more charge-transfer channels in ternary blends film can not only improve the JSC effectively, but also make the VOC reach the same level as the binary structure. By adjusting the different ratio of the two acceptors, the best efficiency of 10.3% can be obtained. Our findings indicate that properly select donor and acceptor to construct absorption spectra complementary ternary blend photoactive layer is an effective approach for achieving high performance PSCs.

4. Experimental Materials: PBDTBDD, ITIC and PC60BM were supplied by Solarmer Company. 1,8-diiodooctane (DIO) was purchased from Sigma Aldrich Company. Zirconium acetylacetonate was bought from Alfa Aesar. All materials are used directly without any treatment. Devices fabrication and characterization: In this work, we designed three kinds of PSCs and the device structures listed below: (1) ITO/PEDOT:PSS/PBDTBDD:PC60BM/Zracac/Al, (2) ITO/PEDOT:PSS/PBDTBDD:ITIC/Zracac/Al , (3) ITO/PEDOT:PSS/PBDTBDD:ITIC:PC60BM/Zracac/Al. Cleaning of ITO glass substrate was conducted by ultrasonic treatment in cleanser essence, water, ultrapure water, acetone, isopropyl alcohol and subsequently dried in a heating plate at 150 °C for a few minutes in air. Then put into an



ultraviolet-ozone (UVO) chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 15 min. After the temperature of substrates is cooling down, spin-coating with PEDOT:PSS solution on ITO glass substrate at 4000 rpm for 30 seconds, then heated at 165 °C for 15 minutes in air. The ITO substrate was transferred to the glove box as quickly as possible, and the other layers were spin-coated in a nitrogen atmosphere. The active layer was spin-coated by the 1,2-dichlorobenzene solution of PBDTBDD:PC60BM, PBDTBDD:ITIC, and PBDTBDD:ITIC:PC60BM with donor: acceptor weight ratio of 1:1 and kept the donor concentration at 10 mg/mL, 0.5% volume ratio of DIO additive was added into the photoactive solution before spin-coating on PEDOT:PSS coated ITO glasses at 1500 rpm for 55 s. The Zracac (Acetylacetone zirconium) solution was made by adding zirconium acetylacetonate solid into ethanol solvent with concentration of 1 mg • mL-1 to obtain a colorless solution. The Zracac solution is directly spin-coated on the active layer at 4000 rpm for 40 s to obtain a cathode buffer layer, and do not require any further post-treatment. Finally, a 100-nm-thick metal electrode Al is deposited on the buffer layer in a vacuum chamber. The current density-voltage (J-V) measurements of devices were performed inside a nitrogen-filled glovebox using Keithley 2400 Source Measure Unit (SMU) under simulated AM1.5G irradiation (100 mW/cm2) using a xenon-lamp-based solar simulator (SAN-EI, AAA grade). An AC Mode III (Agilent) atomic force microscope (AFM) operated in the tapping mode under normal atmosphere and room temperature was used to measure the surface morphologies of the active layer. The external


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quantum efficiency (EQE) measurements were conducted on QE-R systems (Enli Tech) with the standard single-crystal Si photovoltaic cell calibrated at each wavelength. The transmittance and absorbance spectra of the devices were measured by a LAMBDA 950 UV/vis/NIR spectrophotometer. Time-resolved fluorescence device by Edinburgh Instruments Ltd. F900 measured Steady/Transient State Fluorescence Spectrometer. Spectrometer. Ultraviolet photoelectron spectroscopy (UPS) measurements were conducted on a KRATOS Axis Ultra DLD spectrometer with a base pressure of 3×10-8 Torr and bias of -9 V, and He I (21.22 eV) was applied as the excitation source. Two-dimensional GIWAXS (2D-GIWAXS) specular scans were obtained in the National Center for Nanotechnology.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51573042, 51602102), the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, Jianghan University (JDGD-201801) and the Fundamental Research Funds for the Central Universities in China (2016YQ06, 2018MS032, 2016MS50, 2017MS027, 2018ZD07, 2017XS084).

Abbreviations



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PCE, power conversion efficiency; PSCs, polymer solar cells; BHJ, bulk heterojunction; FWHM, full width at half maximum; JSC, short-circuit current density; VOC, open-circuit voltage; FF, fill factor; PL, photoluminescence; TRPL, time-resolved

photoluminescence; OSCs,

organic

solar cells,

J-V,

current

density-voltage; ZrAcac, zirconium acetylacetonate; EQE, external quantum efficiency.

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TOC Graph


Perfect Complementary in Absorption Spectra with Fullerene

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