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Non-Fullerene/Fullerene Acceptor Blend with Tunable Energy State for High-Performance Ternary Organic Solar Cells Min Kim, Jaewon Lee, Dong Hun Sin, Hansol Lee, Han Young Woo, and Kilwon Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06445 • Publication Date (Web): 07 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Materials & Interfaces

1

Non-Fullerene/Fullerene Acceptor Blend with Tunable Energy State for High-Performance

2

Ternary Organic Solar Cells

3 4

Min Kim1, Jaewon Lee1, Dong Hun Sin1, Hansol Lee1, Han Young Woo2, Kilwon Cho1*

5 6 7 8 9

1

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang

37673, Korea 2

Department of Chemistry, Korea University, Seoul 02841, Republic of Korea

*E-mail: [email protected]

10 11

Keywords: ternary organic solar cells, acceptor blends, non-fullerene, indacenodithiophene, fullerene

12 13

Abstract

14

Ternary blending is an effective strategy for broadening the absorption range of the active layer

15

in bulk heterojunction polymer solar cells and for constructing an efficient cascade energy

16

landscape at the donor/acceptor interface to achieve high efficiencies. In this study, we report

17

efficient ternary blend solar cells containing an acceptor alloy consisting of the

18

indacenodithiophene-based non-fullerene material, IDT2BR, and the fullerene material,

19

PC71BM. The IDT2BR materials mix fully with PC71BM materials, and the energy state of this

20

phase can be tuned by varying the blending ratio. We performed PL and EQE studies and found

21

that the ternary charge cascade structure efficiently transfers the photogenerated charges from the

22

polymer to IDT2BR and finally to PC71BM materials. Ternary blend devices containing the

23

IDT2BR:PC71BM acceptor blend and various types of donor polymers were found to exhibit

24

power conversion efficiencies (PCEs) improved by more than 10% over the PCEs of the binary

25

blend devices.

1

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

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1. Introduction

2

In the past decades, the organic solar cell (OSC) has attracted considerable attention as a

3

promising next-generation photovoltaic technology because of the possibility of mass production

4

and because it can be light in weight, low cost, and flexible.1-3 The light absorbing layer of an

5

OSC consists of a donor material and an acceptor material structured as a bulk-heterojunction

6

with a finely phase-separated morphology and a bi-continuous charge transport networks.4-9 One

7

effective strategy for enhancing the power conversion efficiencies of OSCs is the preparation of

8

the ternary-blend OSCs, which has been widely tested because of the potential to extend the

9

scope of active layer light absorption and to enhance the short-circuit current densities (Jsc) and

10

power conversion efficiencies (PCEs) in single-junction devices.10-13 This strategy integrates

11

both enhanced photon harvesting, through the incorporation of multiple organic materials in

12

tandem solar cells, and the simplicity of the fabrication process for single-junction solar cells.

13

The third component of ternary blend OSCs can be either a second electron donor material or a

14

second acceptor material. These third components have versatile functions, including

15

complementary light harvesting, the facilitation of exciton dissociation, the enhancement of

16

charge and energy transfer, and the optimization of the film morphology.10-11 Ternary blends

17

containing secondary donor material have been studied extensively, but it has been found that the

18

secondary donor material can dilute the light absorption of the host donor material.14-17

19

Moreover, depending on the compatibility of the two donor materials, the overall highest

20

occupied molecular orbital (HOMO) energy level of the donor material blend can be pinned to

21

the lower energy level of one component, which results in energy losses from the charge transfer

22

complexes.13,

23

components.19, 21-24 A representative example is that of ICBA:PCBM, which forms a well-mixed

18-20

Secondary acceptor materials have also been investigated as ternary

2


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1

blend phase and can be used to finely adjust the lowest unoccupied molecular orbital (LUMO)

2

energy level of the acceptor alloy phase.25-26 However, fullerene acceptors exhibit low absorption

3

and only limited energy level tuning.

4

In recent years, various types of non-fullerene acceptor materials have been reported that

5

exhibit impressive advantages, e.g., strong light absorption in the visible region, high miscibility

6

with polymers to ensure nanomorphology formation, low energy losses and high open circuit

7

voltages, and thus their associated devices exhibit excellent power conversion efficiencies.6, 27-33

8

Such non-fullerene acceptor materials can also be used as a ternary component because they do

9

not dilute the donor material, which means not only that the strong light absorbing properties of

10

the donor material are retained but also that an efficient cascade energy structure for charge

11

transfer is formed.34-35 However, non-fullerene molecules have a planar molecular structure and

12

anisotropic charge transport properties, so for applications in photovoltaic cells, they must be

13

carefully optimized to prevent strong aggregation, whereas fullerene derivatives have long been

14

used for electron acceptors in OSCs because of their complete miscibility with conjugated

15

polymers.36-40 Thus, ternary blends using both fullerene and non-fullerene acceptors have the

16

potential to exhibit improved organic photovoltaics.

17

In this work, we employed IDT2BR containing the indacenodithiophene (IDT) unit as the core

18

with 5-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazo-lidin-4-one (BR) units

19

as end-capping electron-withdrawing groups in combination with the fullerene acceptor

20

PC71BM, which forms an electron-cascade structure with electron donor polymer, PPDT2FBT.

21

The IDT2BR and PC71BM materials form an intermixed alloy phase with energy levels that are

22

tunably by varying IDT2BR:PC71BM blending ratio, which enables the effective control of the

23

open circuit voltage of the associated ternary OSCs. This alloy acceptor could also enable 3



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1

efficient cascade charge transfer from PPDT2FBT to PC71BM, which would increase the internal

2

quantum efficiency. We investigated the carrier transport and recombination dynamics of the

3

resulting PPDT2FBT:IDT2BR:PC71BM ternary OSCs. Finally, the alloy acceptor was combined

4

with various polymers (i.e., P3HT, PTB7-th, PffBT4T-2OD) and found to increase the power

5

conversion efficiencies of the associated devices. Thus the IDT2BR:PC71BM acceptor alloy can

6

be used to fabricate high-performance ternary OSCs.

7

2. Experimental Section

8

Material preparation: The PPDT2FBT polymer and the indacenodithiophene-based IDT2BR

9

were synthesized by following previously described procedures.41-42 Poly-3-hexyl-thiophene

10

(P3HT) was purchased from Rieke Metals and used as received. The polymers PTB7-th and

11

PffBT4T-2OD were purchased from 1-Material. Phenyl-C71-butyric acid methyl ester (PC71BM)

12

(99.5% pure; Nano-C) was used as received. 1,8-Diiodooctane, diphenyl ether, 1-

13

chloronaphthalene, and chlorobenzene were purchased from Sigma-Aldrich.

14

Sample preparation: The ternary blend, polymer:IDT2BR: PC71BM was dissolved with various

15

weight ratios in chlorobenzene (1:1.5 donor to acceptor weight ratio, 25 mg·mL-1 in total) and

16

stirred at 60°C overnight, and then a volume of 20 µL of a solvent additive, either diphenyl ether

17

or 1,8-diiodooctane, was added to 1 mL of each solution, which was stirred for 30 min. PTB7-th:

18

PC71BM (1:1.5 weight ratio, 25 mg·mL-1 in total) and PffBT4T-2OD: PC71BM (1:1.2 weight

19

ratio, 22 mg·mL-1 in total) were dissolved in chlorobenzene.

20

Device fabrication: Glass substrates coated with indium tin oxide (ITO) were cleaned

21

sequentially with detergent, distilled water, acetone, and isopropyl alcohol. To prepare devices

22

with

23

ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS, Baytron P VP AI4083, Clevios)

the

standard

structure

(ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al)

4


poly(3,4-

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1

was spin-coated after UV-ozone treatment onto the substrates and then baked at 120°C for 30

2

min in a convection oven. The thicknesses of the PEDOT:PSS layer were measured to be ~40 nm

3

by using a surface profiler (Alpha-Step 500, Tencor). The polymer and PC71BM blend solution

4

were spin-coated onto the PEDOT-PSS-coated substrates, then soft-baked at 70 °C for 10 min.

5

To deposit the electrodes on the active layer, the samples were transferred into a vacuum

6

chamber (pressure < 1×10-6 Torr), and then LiF (0.6 nm)/Al (100 nm) was deposited sequentially

7

on top of the thin films by using thermal evaporation.

8

For the inverted structure (ITO/ZnO/polymer:PC71BM/MoO3/Ag), ZnO nanoparticle solution

9

was synthesized following a procedure described previously,40 spin-coated onto the substrates

10

after filtration through a 0.2 µm polytetrafluoroethylene filter, then thermally annealed at 200°C

11

for 1 h. The polymer and PC71BM blend solution was spin-coated onto the ZnO-coated

12

substrates. After transferring the films coated with active layers into a vacuum chamber, MoO3 (3

13

nm)/Ag (100 nm) was deposited sequentially on top of the thin films by performing thermal

14

evaporation.

15

The electrical characteristics were measured with a source/measure unit (Keithley 4200) in the

16

dark and under 100 mW·cm-2 AM1.5 solar illuminations in an N2-filled glove box. The light was

17

generated with an Oriel 1 kW solar simulator referenced by using Reference Cell PVM 132

18

calibrated at the US National Renewable Energy Laboratory.

19

Film characterization: Grazing incidence X-ray diffraction was performed at the 5A and 9C

20

beamlines at the Pohang Accelerator Laboratory (PAL). The two-dimensional GIXS images of

21

the films were analyzed according to the relationship q = 2π/d between the scattering vector q

22

and the d spacing. The GIWAXS images shown are normalized with respect to the exposure

23

time. The XPS, UPS, and NEXAFS spectra were measured at the 4D and 8A2 beamlines at PAL. 5



1

Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images were

2

obtained by using a MultiMode 8 Scanning Probe Microscope (VEECO Instruments Inc.) and a

3

JEOL JEM-2200FS (with Image Cs-corrector) respectively. UV-Vis spectra were recorded on a

4

Varian CARY-5000 UV-vis spectrophotometer. PL was measured with an FP-650 (JASCO

5

Corporation).

6

3. Results and Discussion

7

In this study, we selected three conjugated semiconductors as constituents of the ternary blend

8

system: the polymer PPDT2FBT as the electron donor, IDT2BR as the non-fullerene electron

9

acceptor, and PC71BM as the fullerene electron acceptor (Fig. 1).41-42 IDT2BR and PC71BM

10

acceptor blends with various compositions were mixed with the PPDT2FBT polymer. The

11

HOMO and LUMO energy levels of PPDT2FBT, IDT2BR, and PC71BM are compared in Figure

12

1a. The LUMO and HOMO energy levels of IDT2BR are located between those of PPDT2FBT

13

and PC71BM, which is expected to facilitate charge transfer. The LUMO offset between

14

PPDT2FBT and IDT2BR is 0.18 eV, and that between IDT2BR and PC71BM is 0.23 eV. These

15

values are sufficient to drive electron transfer from the donor to the acceptor.43-44

16

The absorption spectra of pristine PPDT2FBT, IDT2BR, and PC71BM films were recorded (Fig.

17

1b). The absorption profiles of the three films complement each other, with complete coverage

18

from 300 nm to 750 nm. The spectrum of the neat PC71BM film contains a strong absorption

19

peak at 300 nm, and a continuously decreasing profile for longer wavelengths up to 700 nm. The

20

neat PPDT2FBT film exhibits the typical absorption behavior of conjugated donor-acceptor type

21

copolymers, with two broad absorption peaks at 350 – 450 nm for band I originating from the π

22

− π* transition of the conjugated backbone and 500 – 700 nm for band II originating from 6


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1

intramolecular charge transfer. The optical band gap of the PPDT2FBT polymer was determined

2

to be 1.76 eV based on its λonset of 705 nm. The spectrum of the pristine IDT2BR film also

3

contains two distinct absorption peaks, 350 – 450 nm for band I and 500 – 750 nm for band II,

4

with a band edge at 750 nm that is slightly red-shifted with respect to that of PPDT2FBT. The

5

optical band gap estimated from the absorption edge of the IDT2BR film is 1.65 eV. As the

6

relative proportion in the ternary blend film of IDT2BR with respect to PC71BM increases, the

7

absorption edges shift to longer wavelength from 700 nm to 750 nm, and the intensities of the

8

absorption due to the PC71BM material at 300 nm decreased (Fig. 1c).

9

To investigate the photovoltaic properties of the ternary blend films, we fabricated photovoltaic

10

devices with the standard structure ITO/PEDOT:PSS/PPDT2FBT:IDT2BR:PC71BM/LiF/Al. The

11

J–V curves and the photovoltaic parameters of ternary solar cells with various weight ratios of

12

IDT2BR with respect to PC71BM (0, 10, 20, 40, 60, 80, and 100% IDT2BR) are shown and

13

summarized in Fig. 2a and Table 1. The Jsc values of the ternary OSCs are enhanced remarkably

14

by incorporation of IDT2BR and are highest at 10% IDT2BR. This increase is presumably due to

15

not only to the extension of the light absorption range to ~750 nm but also to the formation of a

16

charge cascade energy landscape. In particular, the addition of IDT2BR to the acceptor phase is

17

expected to enhance the charge collection of the blend, as is confirmed by the high external and

18

internal quantum efficiency (EQE and IQE) of the 10% IDT2BR ternary blend (the dashed lines

19

in Fig. 2b). The IQE values of the ternary blend in the absorption range from 600 to 750 nm are

20

more than 10% higher than those of the binary blend.

21

The Voc values of the ternary OSCs continuously increase from 0.78 V to 1.11 V as the

22

proportion of IDT2BR increases (Fig. 2c). The diode parameters were obtained by fitting the

23

dark J-V characteristics of the devices with the Shockley equation (Fig. S1 and Table 1).45-46 If 7



1

we assume that Rs is small and Rp is large, then Voc is strongly dependent on J0 as given by the

2

equation,47-48 ௃

‫ܸݍ‬௢௖ ≈ ݊݇ܶln ቀ ௃ೞ೎ቁ

3

బ

(1)

4

where J0 is the saturation current density, Jsc is the short circuit current density under

5

illumination, k is the Boltzmann constant, T is the temperature in Kelvin, and n is the ideality

6

factor. The Voc values calculated from J0 are in good agreement with the measured Voc with a

7

small relative error. As the proportion of IDT2BR increases, J0 decreases from 1.97 × 10-7 to 3.36

8

× 10-11 mA·cm-2 because J0 is primarily dependent on the energy level difference, ∆E, between

9

the HOMO of the donor and the LUMO of the acceptor with the equation, J0 = J00·exp(−∆E/nkT)

10

where J00 is the pre-exponential factor. These results indicate that variations in the short circuit

11

current and the resistances have only minimal effects on the measured Voc and that the

12

characteristics of the diode in the dark are useful for the analysis of the Voc values of these

13

ternary blend organic solar cells.

14

To investigate the photon harvesting and exciton dissociation process in the active layers, the

15

photocurrent density (Jph) versus effective voltage (Veff) curves were determined for the ternary

16

OSCs (Fig. 2d). Here, Jph is defined by the equation Jph = Jl – Jd, where Jl and Jd are the current

17

density under 100 mW·cm-2 illumination and in the dark respectively. Veff is defined by the

18

equation Veff = V0 – Va, where V0 is the voltage when Jph = 0 and Va is the applied bias. It is

19

apparent that the Jph values of the PPDT2FBT:PC71BM-based binary solar cells and of the

20

ternary solar cells with an IDT2BR blending ratio of 10 wt% rapidly reach the saturation state at

21

Veff = ~0.2 V, which indicates that efficient exciton dissociation and charge carrier collection

22

occur in the corresponding cells. IDT2BR blending ratios higher than 20 wt% produce an 8


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1

unsaturated state even at Veff = 1 V, which is correlated with less efficient exciton dissociation

2

and charge collection. The Gmax values were determined from the equation, Jsat = qLGmax where q

3

is the elementary charge, L is the thickness of the active layers, and Jsat is the saturation current

4

density at Veff = 2 V, and compared. The Gmax values of the PPDT2FBT:PC71BM-based and the

5

optimized PPDT2FBT:IDT2BR:PC71BM-based solar cells were found to be 0.809 × 1028 m-3·s-1

6

and 0.932 × 1028 m-3·s-1 respectively, which suggests that the ternary active layers with an

7

IDT2BR blending ratio of 10 wt% exhibit increased overall exciton generation that is

8

presumably due to the increased light absorption of the ternary blend layer.

9

To clarify the role of IDT2BR in the internal charge transfer, external quantum efficiencies

10

(EQEs) and photoluminescences (PLs) of the blend films were measured (Fig. 3). The EQE

11

spectra of the ternary PV devices with various IDT2BR blending ratios are compared in Fig. 3a.

12

The EQE values of the ternary PV devices continuously decrease as the blending ratio of

13

IDT2BR rises above 20%. To distinguish charge transfer involving IDT2BR from that of other

14

materials, we focused on the EQE values in the wavelength range from 700 to 750 nm, which is

15

the dominant absorption range only for IDT2BR among the blend components. In this absorption

16

region, the EQE values increase as the blending ratio of IDT2BR increases up to 40%, and then

17

decrease for higher IDT2BR proportion. The EQE values at the selected wavelengths of 720 and

18

750 nm were plotted in Fig. 3b to illustrate this trend. These results indicate that a PC71BM-

19

abundant acceptor alloy (up to 40% IDT2BR) can efficiently transfer the photogenerated charge

20

carriers through the IDT2BR materials, whereas an IDT2BR-abundant acceptor alloy (higher

21

than 40% IDT2BR) transfers the photogenerated charge carriers inefficiently to the charge-

22

collecting electrodes.

23

To gain deeper insights into these charge transfer characteristics, the PL emissions of the 9



1

IDT2BR:PC71BM blend films with various compositions but identical thicknesses were recorded

2

(Fig. 3c). The neat IDT2BR film exhibits strong PL emission with a sharp emission peak at

3

approximately 730 nm. The neat PPDT2FBT film produces a rather weak PL emission composed

4

of two broad emission peaks at ~725 nm and ~760 nm. The PL spectra of those two films

5

partially overlap with slightly different peak maxima. For the PPDT2FBT:IDT2BR (1:1) blend

6

film, the intensity of the PL emission is quenched more than 60% compared to that of the neat

7

IDT2BR film, but the PL spectrum of the blend film contains the PL peak shape of the neat

8

IDT2BR film, which indicates that hole transfer from IDT2BR to PPDT2FBT occurs

9

incompletely. In contrast, the IDT2BR:PC71BM (1:1) blend film exhibits complete PL quenching

10

with an efficiency of 96%, which indicates that the charge transfer from IDT2BR to PC71BM is

11

efficient.

12

To further investigate the charge transfer properties of the ternary blend films, the PL emission

13

under a selective excitation wavelength for the IDT2BR material (λex = 750 nm) was measured

14

(Fig. 3d). The PL spectra of neat IDT2BR and the IDT2BR:PDPT2FBT (1:1) blend film have

15

similar intensities whereas the IDT2BR:PC71BM (1:1) blend film exhibits effective quenching.

16

These results confirm that the excited charge carrier in IDT2BR is effectively transferred towards

17

PC71BM, but less effectively towards PPDT2FBT. This conclusion is in agreement with the

18

observation that the IDT2BR-loaded ternary blend exhibits an enhanced EQE in the 700-750 nm

19

region only when it is blended with less than 40% IDT2BR. We suggest that an

20

IDT2BR:PC71BM acceptor alloy with a low IDT2BR proportion will form an efficient charge

21

cascade energy landscape exhibiting charge flow from PPDT2FBT to IDT2BR and PC71BM. We

22

confirmed that the binary blends PPDT2FBT:IDT2BR and IDT2BR:PC71BM exhibit

23

photovoltaic behaviors with high Voc of 1.1 and 0.86 V respectively, although the 10


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1

IDT2BR:PC71BM device exhibits low Jsc and FF due to its unoptimized and overly intermixed

2

nanomorphology (Fig. S2).

3

When IDT2BR is introduced into the ternary blend, the resulting Voc increases with the weight

4

ratio in the blend of IDT2BR with respect to PC71BM. This variation in Voc can be correlated

5

with the LUMO energy level of the acceptor alloy phase. The HOMO and LUMO energy levels

6

of the acceptor alloys with various IDT2BR:PC71BM weight ratios were measured in an

7

ultrahigh vacuum (base pressure 2×10-10 mbar) by using ultraviolet photoelectron spectroscopy

8

and near edge X-ray absorption fine structure (NEXAFS) spectroscopy (Fig. 4a, Fig. S3),49-51

9

which showed that there is a linear increase in the LUMO and HOMO energy levels of the

10

acceptor alloy with increases in IDT2BR:PC71BM blend ratio. The details of the calculation of

11

energy levels are explained in the supporting information. These trends imply that the two

12

components exhibit strong and uniform electronic interactions and that the delocalization of

13

states is extensive. We confirmed that the two acceptor materials are well-intermixed with the

14

molecular scale by using AFM and XRD measurements (Fig. S5 and S6); hence the electron

15

wavefunction is expected to be substantially delocalized, which leads to the dependence of the

16

electron energy on the average composition of the acceptor alloy.

17

Figure 4b shows that the Voc values of the ternary systems that have a quasilinear relationship of

18

the mass fraction of IDT2BR in the acceptor blend. When two semiconductor materials form a

19

well-mixed alloy, they have electronic properties determined by their relative composition;19, 21

20

for example, the energy level of the blend is expected to vary linearly with its composition and

21

thus can be correlated with Voc. The Gaussian distribution theory and this linear energy equation

22

for an alloy with two different acceptors can be combined to derive a quasilinear equation for Voc

23

as follows:52-54 11



1

ܸை஼,்௘௥௡௔௥௬ =

୤భ ∙୒೐భ ∙୚ೀ಴,ಳ೔೙ೌೝ೤భ ା୤మ ∙୒೐మ ∙୚ೀ಴,ಳ೔೙ೌೝ೤మ ୤భ ∙୒೐భ ା୤మ ∙୒೐మ

Page 12 of 35

(2)

2

where f1 and f2 are the weight composition ratios of Acceptor 1 and Acceptor 2 with respect to

3

the total acceptor concentration respectively, the Voc,Ternary is the Voc of ternary solar cell, and

4

Voc,Binary1, and Voc,Binary2 are the Voc values of binary solar cells with donor:Acceptor 1 and

5

donor:Acceptor 2 active layers respectively. Here, Acceptor 1 is PC71BM, and Acceptor 2 is

6

IDT2BR. Ne is the total electron DOS of unit mass. Ne of PC71BM and IDT2BR can be

7

calculated by using the equation Ne = nl, where n is the molecular number of unit mass and l is

8

the number of quasi-degenerate LUMOs (

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