Ternary Organic Solar Cells with Small Nonradiative Recombination

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Letter

Ternary Organic Solar Cells with Small Non-Radiative Recombination Loss Yuanpeng Xie, Tengfei Li, Jing Guo, Peng-Qing Bi, Xiaonan Xue, Hwa Sook Ryu, Yunhao Cai, Jie Min, Lijun Huo, Xiaotao Hao, Han Young Woo, Xiaowei Zhan, and Yanming Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00681 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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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 Energy Letters

Ternary

Organic

Solar

Cells

with

Small

Non-Radiative

Recombination Loss

Yuanpeng Xie†, Tengfei Li‡, Jing Guo§, Pengqing Biǁ, Xiaonan Xue†, Hwa Sook Ryu#, Yunhao Cai†, Jie Min*§, Lijun Huo†, Xiaotao Haoǁ, Han Young Woo*#, Xiaowei Zhan‡, and Yanming Sun*† †

School of Chemistry, Beihang University, Beijing 100191, China

‡

Department of Materials Science and Engineering, College of Engineering, Peking

University, Beijing 100871, China §

The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China

ǁ

School of Physics State Key Laboratory of Crystal Materials, Shandong University,

Jinan 250100, P. R. China #

Department of Chemistry, College of Science, Korea University, Seoul 136-713,

Republic of Korea *

E-mail: [email protected] (Y. S.)

*

E-mail: [email protected] (J. M.)

*

E-mail: [email protected] (H.Y. W.)

1

ACS Paragon Plus Environment

ACS Energy Letters 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 Non-radiative recombination loss ( q∆Vnon-rad ), as a large component of energy loss oc (Eloss), has become an important factor that limits the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, high-performance ternary OSCs based on a polymer donor PTB7-Th, a polymer donor PBDTm-T1and a non-fullerene acceptor FOIC were reported. When blended with FOIC, the PBDTm-T1-based device yielded a smallest q∆Vnon-rad of 0.197 eV, but with a moderate PCE of 3.3%. In contrast, PTB7oc of 0.329 eV, however, with a Th:FOIC device exhibited a relatively higher q∆Vnon-rad oc high PCE of 11.9%. This trade-off relationship has been resolved using a ternary blend. By incorporating 20% PBDTm-T1 into the PTB7-Th:FOIC blend, a small q∆Vnon-rad oc value of 0.271 eV and a significantly high PCE of 13.8% were simultaneously obtained. The results demonstrate that the non-radiative recombination loss can be effectively reduced by using a ternary strategy.

2


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ACS Energy Letters

Table of Content (TOC) Graphic

Eloss

S

S O

S S

O S

S

S

n

S

PBDTm-T1 NC

F O

qVoc

NC

CN

S S

S

S

CN

S S

S

S

O

F

F O

S

S S S

O

PTB7-Th

FOIC

3


n


Solution-processed organic solar cells (OSCs) have attracted considerable attention in the past decades because of their appealing advantages, such as easy fabrication, low cost, and flexibility.1-3 A combination of novel material synthesis, morphology control, interface engineering, and device optimization led to state-of-theart power conversion efficiencies (PCEs) over 14% for OSCs.4-10 However, it should be noted that the PCEs of OSCs are much lower than silicon and perovskite solar cells. The large energy loss (Eloss) by considering the bandgap (Egap) of photoactive materials and the open-circuit voltage (Voc) of OSCs remains a critical issue that limits their PCE. In general, the large Eloss during the exciton separation and charge recombination processes consists of three parts. The first part of voltage loss SQ (Egap - qV SQ oc , V oc

is from Shockly-Queisser (SQ) limit) is due to the radiative

recombination originating from the absorption above the bandgap. The SQ limit suggests that the radiative recombination is an unavoidable recombination mechanism in all types of solar cells and provides an upper limit for Voc. The values of Egap - qVSQ oc gap are typically in the 0.25-0.30 eV range.11 The second part (∆E2 = q∆Vrad,below ) is oc

attributed to the additional radiative recombination loss from the absorption below the banggap.12-12 The typically achieved large q∆Vrad oc in OSCs is ascribed to the energetic offset (∆Eoffset) between donor and acceptor molecular states.13, 14 The third part is nonradiative recombination loss (q∆Vnon-rad = -kTln(EQEEL), which is closely related to oc the radiative quantum effciency (EQEEL). In the past decades, fullerene-based OSCs often suffered from a large ∆Eoffset (>0.3 eV) for exciton separation with a high Eloss (0.7-1.0 eV).15, 16 Even for the most 4


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ACS Energy Letters

efficient fullerene-based OSCs (PCE ≈ 11%), the Eloss value was found to be 0.88 eV.17 With the emergence of novel non-fullerene acceptors,18,

19

the second part

gap q∆Vrad,below is nearly negligible because the ∆Eoffset of less than 0.1 eV can ensure oc

efficient exciton dissociation in non-fullerene solar cells.11, 20, 21 However, q∆Vnon-rad oc is still larger than 0.30 eV because of the low EQEEL of semiconductor materials.22, 23 Therefore, q∆Vnon-rad is superfluous and should be curtailed to further enhance the oc PCEs of OSCs.24-26 The ternary blend is an effective method to improve the performance of OSCs.2740

However, less efforts have been devoted to studying Eloss of ternary OSCs. Herein,

ternary OSCs comprising a medium-bandgap polymer donor PTB7-Th, a wide-bandgap polymer PBDTm-T1, and an ultra-narrow bandgap non-fullerene acceptor FOIC have been fabricated.41, 42 An extremely small q∆Vnon-rad of less than 0.30 eV has been oc successfully achieved in ternary OSCs. The morphology study shows that the PTB7Th:FOIC binary film exhibits a fibrous-network formation with strong aggregation and π-π interactions. It leads to a PCE of 11.9% with a large q∆Vnon-rad of 0.329 eV. oc Incorporating 20% PBDTm-T1 in the host PTB7-Th:FOIC blend led to a reduction of charge recombinations and more balanced charge mobilities without noticeable morphological changes in ternary devices. As a result, an improved PCE of 13.8% with a significantly decreased q∆Vnon-rad of 0.271 eV in ternary OSCs was achieved. This oc

5


ACS Energy Letters

work shows the crucial role of the ternary blend strategy in the reduction of energy loss and improvement of the device performance in OSCs.

S

CN

CN

S S

S

F

O

FOIC

0.8 0.6 0.4 0.2

-5.48 eV

0.0

(d) S F O

S

S

Normalized PL

S n

S S

O

PTB7-Th

400

500

600

700

800

900

1000

Wavelength (nm)

0.8

O

O

S S

S

S

S

PBDTm-T1

S

n

FOIC 915 nm (τ=79 ps) PTB7-Th 760 nm (τ=281 ps) PBDTm-T1 680 nm (τ=342 ps)

1.0

0.6 0.4 0.2

0.8 0.6 0.4 0.2 0.0

700

1.0

800

900

Wavelength (nm)

1000

1100

1.5

(g)

PTB7-Th:PBDTm-T1 680 nm (τ =77 ps) PTB7-Th:PBDTm-T1 760 nm (τ =381ps) PTB7-Th:FOIC 760 nm (τ =28 ps) PTB7-Th:FOIC 915 nm (τ =36 ps)

0.8 0.6

1.0

2.0

Decay Time (ns)

2.5

3.0

PTB7-Th:PBDTm-T1:FOIC 915 nm (τ = 27 ps) PTB7-Th:PBDTm-T1:FOIC 760 nm (τ = 23 ps) PTB7-Th:PBDTm-T1:FOIC 680 nm (τ = 23 ps)

Normalized Counts

(f)

-5.56 eV

(e) FOIC PTB7-Th PBDTm-T1

1.0

600

S

-5.28 eV

1100

0.0

S

-3.50 eV

PBDTm-T1

S

PTB7-Th

NC

S

-3.50 eV -4.02 eV

FOIC

O

(c) FOIC PTBT-Th PBDTm-T1

1.0

Normalized Counts

NC

F

Normalized Absorbance

(b)

(a)

Normalized Counts

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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0.8 0.6 0.4

0.4

0.2

0.2

0.0

0.0 1.5

2.0

2.5

Decay Time (ns)

3.0

1.5

2.0

Decay Time (ns)

2.5

3.0

Figure 1. (a) Chemical structures of FOIC, PTB7-Th and PBDTm-T1. (b) Normalized UV-vis absorbance spectra of FOIC, PTB7-Th, and PBDTm-T1 neat films at 840 nm, 705 nm and 573 nm, respectively. (c) Energy levels of FOIC, PTB7-Th, and PBDTm-T1 materials. (d) Normalized steady-state PL of FOIC, PTB7-Th and PBDTm-T1 neat films at 915 nm, 760 nm and 685nm respectively. (e) TRPL spectra of FOIC, PTB7-Th, and PBDTm-T1 neat films. (f) TRPL spectra of PTB7-Th:PBDTm-T1 and PTB7-Th:FOIC binary films. (g) TRPL spectra of PTB7-Th:PBDTmT1:FOIC (0.8:0.2:1.5) ternary film (excite wavelength of 500 nm and monitor wavelength of 685 nm, 760 nm, and 915 nm). The maximum signature of TRPL spectra was normalized.

The chemical structures and normalized UV-visible absorbance spectra of FOIC, PTB7-Th, and PBDTm-T1 are presented in Figure 1. The FOIC film has strong absorption from 600 nm to 1000 nm, and the main absorption of PTB7-Th is located in the range of 400-800 nm. The third component, PBDTm-T1, mainly shows the visible absorption in the 400-700 nm range. Owing to the complementary absorption of these three components, ternary blends possess a broader light absorption that spans the 400 6


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ACS Energy Letters

nm to 1000 nm wavelength range (Supporting Information, Figure S1). Frontier molecular orbitals (FMOs) energy levels of FOIC, PTB7-Th, and PBDTm-T1 were investigated through the electrochemical cyclic voltammetry. The energy level diagram is illustrated in Figure 1c and summarized in Table S1. The lowest unoccupied molecular orbital (LUMO) energy levels and the highest occupied molecular orbital (HOMO) of FOIC and PTB7-Th were determined to be -4.02 eV/-5.48 eV and-3.50 eV/-5.28 eV, respectively. PBDTm-T1 showed a deeper HOMO value of -5.56 eV than that of PTB7-Th, which is helpful to achieve a higher Voc in ternary OSCs. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra of FOIC, PTB7-Th, PBDTm-T1, and their blend films were measured. As displayed in Figure 1d-e, the FOIC pure film shows an emission spectrum at 915 nm with a lifetime of 79 ps, while PTB7-Th and PBDTm-T1 neat films present lifetimes of 281 ps at 760 nm and 342 ps at 685 nm, respectively. After blending PTB7-Th with PBDTm-T1, the PL spectrum of PBDTm-T1 completely diminished with a decreased PL lifetime of 77 ps at 685 nm. Moreover, OSCs based on PTB7-Th:PBDTm-T1 blend exhibited a Jsc of 0.24 mA cm-2, which is lower than that (0.50 mA cm-2) of PTB7-Thbased device. These results reveal that no charge transfer between PBDTm-T1 and PTB7-Th. In contrast, it was observed that the emission of PBDTm-T1 strongly overlapped the absorption spectrum of PTB7-Th (Figure S2). These results suggest the existence of energy transfer between PBDTm-T1 and PTB7-Th. The efficiency of energy transfer is calculated by 1−

τ𝐷𝐷𝐷𝐷 τ𝐷𝐷

7


ACS Energy Letters

where τD and τDA are the PL lifetimes of the energy donor (D) without and with energy acceptor (A), respectively. The energy transfer efficiency was estimated to be ~77% from a shorter PL decay time of 77 ps of PBDTm-T1 in the PBT7-Th:PBDTm-T1 blend. As displayed in Figure 1f, the PTB7-Th:FOIC blend shows short lifetimes of 28 ps at 760 nm and 36 ps at 915 nm, indicating fast and efficient charge transfer in PTB7Th:FOIC blend. More efficient charge transfer in PTB7-Th:PBDTm-T1:FOIC ternary blend was verified from shorter lifetimes of 23 ps at 760 nm and 27 ps at 915 nm (Figure 1g). The efficient energy transfer between PBDTm-T1 and PTB7-Th and efficient charge transfer in ternary blends are expected to play an important role in improving device performance. 1:0:1.5 0.8:0.2:1.5 0.5:0.5:1.5

6 0

0.9:0.1:1.5 0.7:0.3:1.5 0:1:1.5

80 70 60

-6

50 40

-12

30

-18

20 10

-24 -0.2

0.0

0.2

0.4

0.6

0 300

0.8

(d) 0.90

(c)

1:0:1.5 α = 0.88 0.8:0.2:1.5 α = 0.96 0:1:1.5 α = 0.84

0.85

10

Voc (V)

Jsc (mA cm-2)

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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400

500

600

700

800

900

1000

1:0:1.5 1.35kT/q 0.8:0.2:1.5 1.08kT/q 0:1:1.5 1.69kT/q

0.80

0.75

0.70

20

40

60

80

-2

Light Intensity (mW cm )

0.65 20

100

40

60

80

Light Intensity (mW cm-2)

100

Figure 2. (a) Current-voltage (J-V) measurements of ternary OSCs with different PBDTm-T1 contents (b) the corresponding EQE spectra. (c) Jsc and (d) Voc as a logarithmic function of light intensity of binary and the optimized ternary OSCs.

Inverted structure of indium tin oxide (ITO)/zinc oxide (ZnO)/active layer/molybdenum oxide (MoO3)/Ag was used to fabricate ternary OSCs. The weight ratio of polymer donors and nonfullerene acceptor was 1:1.5, and the optimal 8


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ACS Energy Letters

thicknesses of active layers were ~100 nm. The current density-voltage (J-V) curves of OSCs with varying the PBDTm-T1 contents are illustrated in Figure 2a, and the detailed device parameters are listed in Table 1. The PTB7-Th:FOIC binary OSC yielded a PCE of 11.9%, with a Voc of 0.741 V, a current density (Jsc) of 22.70 mA cm2

and a fill factor (FF) of 70.5%, comparable to the previously reported device

performance.40 The PBDTm-T1:FOIC device showed a PCE of 3.3%, with a high Voc of 0.865 V, a low Jsc of 10.5 mA cm-2 and a low FF of 36.0%,. The moderate performance of PBDTm-T1:FOIC device could be attributed to the negative HOMO offset (-0.08 eV) between PBDTm-T1 and FOIC.43 The PTB7-Th:PBDTm-T:FOIC (0.8:0.2:1.5) ternary device showed the best PCE of 13.8%, with a Voc of 0.763 V, Jsc of 24.20 mA cm-2 and FF of 74.5%, which represented one of the best results for OSCs (Figure S3). The EQE curves are shown in Figure 2b. PTB7-Th:FOIC device covered a broad spectral from 300 nm to 950 nm with a maximum EQE value of 79.0% at 580 nm. The incorporation of 20% PBDTm-T1 achieved a maximum EQE value of 81.9% and over 70% in the 550-820 nm range. The integrated current densities of PTB7Th:FOIC and PTB7-Th:PBDTm-T1:FOIC (0.8:0.2:1.5) devices were 22.50 mA cm-2 and 23.07 mA cm-2, respectively, agreeing well with Jsc measured from J-V curves. Table 1. Device parameters of ternary OSCs with different PBDTm-T1 contents. Ternary Ratio

a)

Jsc (mA

cm-2)

Jsc.cal (mA

cm-2)

Voc

FF

(V)

(%)

(%)

µh

PCEa) (cm

2V-1·s-1)

µe (cm

2·V-1·s-1)

µh/µe

1:0:1.5

22.50±0.5

21.75

0.740±0.002

70.3±0.6

11.7(11.9)

1.2×10-3

6.7×10-4

1.79

0.9:0.1:1.5

23.54±0.6

22.70

0.754±0.002

71.3±0.5

12.7(12.9)

1.1×10-3

6.6×10-4

1.67

0.8:0.2:1.5

24.0±0.4

23.07

0.762±0.003

73.5±0.6

13.4(13.8)

8.9×10-4

6.6×10-4

1.34

0.7:0.3:1.5

22.05±0.3

21.70

0.765±0.004

70.7±0.3

11.9(12.1)

8.3×10-4

6.7×10-4

1.23

0.5:0.5:1.5

21.80±0.4

21.30

0.770±0.006

61.2±0.4

10.3(10.5)

5.0×10-4

6.5×10-4

0.77

0:1:1.5

10.40±0.5

9.92

0.861±0.004

35.3±0.7

3.2(3.3)

2.2×10-4

6.3×10-4

0.35

The value obtained from 10 independent devices.

To evaluate charge generation and dissociation, the photocurrent density (Jph) versus the effective voltage (Veff) of the binary and optimal ternary OSCs were plotted. 9



Page 10 of 23

As shown in Figure S4, the PTB7-Th:FOIC device showed saturation current density (Jsat) of 23.80 mA cm-2 and charge dissociation probabilities P(E,T) of 91.3%. However, the PBDTm-T1:FOIC device could not reach the saturation value even at a high Veff of over 2 V, indicating inefficient exciton dissociation in the PBDTm-T1:FOIC blend. Compared to the binary devices, the PTB7-Th:PBDTm-T:FOIC (0.8:0.2:1.5) ternary device showed a higher Jsat of 24.81 mA cm-2 and P(E,T) of 95.6%, which indicated their increased exciton and improved charge dissociation capability. The charge transport properties were investigated by using the space-charge-limited current (SCLC) method. The electron mobilities (μe) exceeded 6.0×10-4 cm2 V-1 s-1 in all the ternary blends, suggesting that the addition of PBDTm-T1 had a negligible influence on the electron transport of FOIC. On the contrary, the hole mobilities (μh) decreased from 1.2×10-3 cm2 V-1 s-1 to 2.2×10-4 cm2 V-1s-1 as the PBDTm-T1 content vary from 0% to 100% in ternary blends. In particular, the PTB7-Th:PBDTm-T1:FOIC (0.8:0.2:1.5) ternary device showed hole and electron mobilities of 8.9×10-4 cm2 V-1 s-1 and 6.6×104

cm2 V-1 s-1, respectively, with a μh/μe value of 1.23, which was closer to 1 compared

to 1.79 and 0.35 of PTB7-Th:FOIC and PBDTm-T1:FOIC binary devices, respectively. Jsc as a logarithmic function of light intensity (I) was investigated to evaluate the charge recombination of the binary and optimal ternary OSCs (Figure 2c). In general, Jsc followed the function Jsc∝Iα, where α is the power-law exponent. The α values were 0.88, 0.84, and 0.96 for PTB7-Th:FOIC (1:1.5), PBDTm-T1:FOIC (1:1.5), and PTB7Th:PBDTm-T1:FOIC (0.8:0.2:1.5) OSCs, respectively, which revealed suppressed bimolecular

recombination

in

the

optimized

ternary device. Trap-assisted

10


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ACS Energy Letters

recombination was also detected by Voc versus light intensity curves in Figure 2d. The slope S of kT/q implies that bimolecular charge recombination is the dominant recombination channel. When S is larger than kT/q, monomolecular recombination and trap-assisted recombination are involved. The slopes were calculated to be 1.35KT/q, 1.69KT/q, and 1.08KT/q for PTB7-Th:FOIC, PBDTm-T1:FOIC and PTB7Th:PBDTm-T1:FOIC (0.8:0.2:1.5) OSCs, indicating less monomolecular and trapassisted recombinations in the ternary OSC. As a result, the reduced charge recombination afforded the increased FF and Jsc values in the optimal ternary system as

compared

with

the

11


binary

systems.


(a) 1:0:1.5

RMS = 1.78 nm

(d) 0.7:0.3:1.5

RMS =1.48 mm

(b) 0.9:0.1:1.5

RMS = 1.67 nm

(e) 0.5:0.5:1.5

RMS = 1.43 nm

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(c) 0.8:0.2:1.5

RMS = 1.56 nm

(f) 0:1:1.5

RMS = 1.33 nm

Figure 3. AFM height and phase images (size 1 µm × 1 µm) of PTB7-Th:PBDTm-T1:FOIC ternary films with ratios of (a) 1:0:1.5, (b) 0.9:0.1:1.5, (c) 0.8:0.2:1.5, (d) 0.7:0.3:1.5, (e) 0.5:0.5:1.5 and (f) 0:1:1.5.

Atomic force microscopy (AFM) was performed to study the influence of PBDTm-T1 on the surface morphology of ternary blends. As presented in Figure 3, all the blend films exhibited a fibrous-network morphology, which favored charge 12


Page 13 of 23

transport.44 The fibrous structure was more obvious in PTB7-Th:FOIC and PTB7Th:PBDTm-T1:FOIC films than in PBDTm-T1:FOIC films, as was also evidenced by the TEM images (Figure 4). Moreover, it was found that the addition of PBDTm-T1 in ternary blends decreased the root-mean-square (RMS) roughness from 1.78 nm to 1.33 nm when the PBDTm-T1 content increased from 0% to 100%. (a) 1:0:1.5

1012

(b) 0.9:0.1:1.5

1010

108

(c) 0.8:0.2:1.5

(d) 0.7:0.3:1.5

106

Intensity (a.u.)

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

ACS Energy Letters

((g)

in-plane out-of-plane PTB7-Th

PBDTm-T1 FOIC

1:0:1.5 4

10

102

(e) 0.5:0.5:1.5

(f) 0:1:1.5 100

10-2

0.9:0.1:1.5 0.8:0.2:1.5 0.7:0.3:1.5

0.5:0.5:1.5 0:1:1.5

10-4

-1

q(Å )

1

2

Figure 4. TEM images of PTB7-Th:PBDTm-T1:FOIC ternary films with ratios of (a) 1:0:1.5, (b) 0.9:0.1:1.5, (c) 0.8:0.2:1.5, (d) 0.7:0.3:1.5, (e) 0.5:0.5:1.5, (f) 0:1:1.5 and (g) the corresponding outof-plane and in-plane line-cut profiles from 2D GIWAXS patterns.

Furthermore, grazing-incidence wide-angle X-ray scattering (GIWAXS) was introduced to evaluate the molecular packing and orientation in films. As shown in 13



Figure 4g and Figure S5, PTB7-Th and PBDTm-T1 neat films exhibited a (010) reflection at similar positions of qz ≈ 1.63 Å-1 in out-of-plane, corresponding to the ππ stacking distance of 3.9 Å. The FOIC neat film showed a sharper (010) diffraction peak at qz = 1.77 Å-1 in the out-of-plane direction. These results demonstrate that PTB7Th, PBDTm-T1, and FOIC neat films preferentially adopted the face-on orientation. In the blend films, similar scattering patterns were observed irrespective of different blend ratios, indicating that the incorporation of PBDTm-T1 into ternary blends had negligible influence on the π-π stacking and orientation of FOIC. To quantitatively analyze the crystalline packing in the ternary blends, the crystal coherence lengths (CCL) were estimated by CCL=

2π FWHM

where FWHM is full width at half maxima, which is calculated by the fitting the (010) peaks of 1D line cuts in the out-of-plane direction (see Figure S6 and Table S3). It can be seen that CCLs of the face-on (010) diffractions were estimated to be ~12.0 nm for PTB7-Th/PBDTm-T1 and ~26-29 nm for FOIC in the ternary blends. Similar scattering patterns (with favorable face-on orientation of all the components of two donors and acceptor) were observed irrespective of different blend ratios in the ternary blends, indicating that the incorporation of PBDTm-T1 into ternary blends had negligible influence on the π-π stacking and orientation of consisting components. The results agree well with the trend of SCLC mobilities: the electron mobilities were measured to be more than 6.0×10-4 cm2 V-1 s-1 in all the ternary blends and the hole mobilities decreased with increasing the PBDTm-T1 content since PTB7-Th exhibited higher hole 14


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Page 15 of 23

mobility than that of PBDTm-T1.

10

EQEFTPS

-5

10

10

φEL/φbb

10-6

10

-5

0.8

1.0

1.2

-6

10 1.4

1.6

1.8

Energy (eV)

2.0

(c) 0.8:0.2:1.5

FTPS_EQE

10

10-3

10-3 10-4

10-5

10-5

φEL/φbb

10-7 101 0

10

0.8

1.0

1.2

1.4

10-6 1.6

1.8

Energy (eV)

2.0

(e) 0.5:0.5:1.5

10

φEL/φbb

-7

10

1.0

1.2

1.4

1.6

1.8

Energy (eV)

2.0

1.8

Energy (eV)

2.0

101 100

10-2

φEL

10-3 10-4

10-3 10-4

EQEFTPS

10-5

10-1

10-5

φEL/φbb

0.8

1.0

1.2

1.4

10-6 1.6

1.8

Energy (eV)

2.0

100 10-1

EQE

10-2

10-2

φEL

10-3 10-4

10-3 10-4

EQEFTPS

10-5

10-5

φEL/φbb

10

10-6

-7

10

10-7 2.2

101

(f) 0:1:1.5

-6

10 2.2

10-7 2.2

10-1

10-6

-7

0.8

1.6

EQE

10

10

10-6

1.4

(d) 0.7:0.3:1.5

-2

-1

10-5

1.2

10

0

10-4

1.0

-1

101

10-4

10-6

10

0

10-3 EQEFTPS

0.8

101

10-3

10-5

10

10-7

10-2

φEL

10-6

-7

10-7 2.2

10

EQE

10-2

10

10

-1

10-5

φEL/φbb

-6

0

10-4 EQE FTPS 10-6

10

0

10

φEL

10-4

EQEFTPS

-5

101

-2

10-3

-4

101

10

EQE

-2

10-2

-3

10

10-7 2.2

-1

10-1

φEL

10

10

-1

10

10

FTPS_EQE

10

-4

EQE

-2

0.8

Normalized EL

10

10

10

Normalized EL

-3

100

1.0

1.2

1.4

1.6

1.8

Energy (eV)

2.0

Normalized EL

10

101

(b) 0.9:0.1:1.5

-1

FTPS_EQE

-2

-3 -4

10

FTPS_EQE

FTPS_EQE

10

10

φEL

EQE

-2

0

0

-1

10

101

101

0

10

-1

10-7

101

Normalized EL

10

(a) 1:0:1.5

Normalized EL

0

Normalized EL

101

FTPS_EQE

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

ACS Energy Letters

10-7 2.2

Figure 5. The measured quantum efficiency (EQE, blue line), Fourier transform photocurrent spectroscopy (FTPS-EQE, red lines), electroluminescence (EL, green lines) and external quantum efficiency (black lines) of the ternary OSCs with ratios of (a) 1:0:1.5, (b) 0.9:0.1:1.5, (c) 0.8:0.2:1.5, (d) 0.7:0.3:1.5, (e) 0.5:0.5:1.5 and (f) 0:1:1.5. The external quantum efficiency is determined by EL and the black-body emission (φbb). Table 2. Summary of parameters measured and calculated from FTPS-EQE and EL. Vrad oc

∆E2=q∆Vrad oc

(V)

(eV)

0.27

1.069

0.041

0.329

1.11

0.27

1.028

0.082

0.274

0.620

1.11

0.27

1.031

0.079

0.271

0.765

0.615

1.11

0.27

1.042

0.068

0.277

0.770

0.610

1.11

0.27

1.044

0.066

0.274

VSQ oc

∆E1=Egap − qV

0.640

1.11

0.754

0.626

1.38

0.762

0.7:0.3:1.5

1.38

0.5:0.5:1.5

1.38

Egap

Voc

Eloss

(eV)

(V)

(eV)

1:0:1.5

1.38

0.740

0.9:0.1:1.5

1.38

0.8:0.2:1.5

Ternary Ratio

(V)

SQ oc

(eV)

15


∆E3=q∆Vnon-rad oc (eV)


0:1:1.5

1.40

0.861

0.539

1.13

0.27

Page 16 of 23

1.058

0.072

0.197

The energy loss from non-radiative recombination in OSCs (0.3-0.48 eV) is typically larger than that in inorganic solar cells (0.04-0.21 eV).45 In this work, we found that the addition of PBDTm-T1 could efficiently reduce the energy loss in PTB7Th:FOIC binary solar cells. To further analyze the Eloss value in ternary devices, we first measured the Egap values of six ternary solar cells with different PBDTm-T1 contents. All the blends showed an Egap of ~1.38 eV, which was determined by the crossing point between the emission and absorption spectra of ternary blends (Figure S7), mainly determined by the low bandgap FOIC in these blends. As summarized in Table 2, the PTB7-Th:FOIC binary cell showed a relatively high Eloss of 0.640 V, whereas the PBDTm-T1:FOIC binary cell presented a low Eloss of 0.539 eV. Importantly, we found that the energy losses followed a monotonic behavior with the increase in the third component, PBDTm-T1, in ternary solar cells. This implies that increasing the PBDTm-T1 contents can efficiently improve Voc in ternary solar cells. The detailed balance theory is a preferable way to quantitatively calculate the energy losses in organic solar cells. Following the SQ limit, the voltage losses (q∆Voc) can be rad non-rad 45 categorized into three different terms: q∆Voc = (Egap -qVSQ ) oc ) + (q∆Voc ) + ( q∆Voc

We quantified the energy and voltage losses by characterizing the Fourier-transform photocurrent

spectroscopy

external

quantum

efficiency

(FTPS-EQE)

and

electroluminescence (EL) spectra in these investigated solar cells, as presented in Figure 5. The first part, ∆E1= Egap - qVSQ oc , depends on Egap and the theoretical maximum voltage by the SQ limit. All of the devices showed the same Egap - qVSQ with a value of oc 16


Page 17 of 23 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

ACS Energy Letters

0.27 eV. This substantial loss is unavoidable for OSCs. The second part, ∆E2 = q∆Vrad oc , SQ is the difference between qVSQ and qVrad oc oc , which is reduced relative to Voc , because the

emission is redshifted further relative to the absorption edge. Here we found that the six devices showed negligible ∆E2 of less than 0.1 eV, which is attributed to the fact that the charge transfer state can lie near below the onset of strong absorption. The third loss, ∆E3 = q∆Vnon-rad , is the difference between qVrad oc oc and the measured qVoc under AM1.5G simulated solar spectrum. PBDTm-T1:FOIC solar cells gave an extremely small q∆Vnon-rad of 0.197 V, which represented the lowest q∆Vnon-rad in OSCs so oc oc far.46 In contrast, the PTB7-Th:FOIC solar cell exhibited a relatively high q∆Vnon-rad oc of 0.329 eV. After incorporating PBDTm-T1 in PTB7-Th:FOIC blends, the ternary OSC yielded a small q∆Vnon-rad of ~0.27 eV with a broad composition tolerance. The oc smaller q∆Vnon-rad value in the PTB7-Th:PBDTm-T1:FOIC ternary solar cell oc indicated that the third component, PBDTm-T1, had a significant contribution to the reduction of non-radiative recombination. In summary, high performance ternary OSCs with small non-radiative recombination losses were fabricated by employing PTB7-Th as donor, FOIC as acceptor, and the PBDTm-T1 polymer donor as the third component. The PBDTm-T1based binary OSC gave the lowest non-radiative recombination loss (q∆Vnon-rad , 0.197 oc eV) with a PCE of 3.3%, whereas the PTB7-Th-based binary OSC showed a higher PCE of 11.9% with a larger q∆Vnon-rad of 0.329 eV. This trade-off relationship was oc resolved in a ternary structure, thereby we acquired a remarkable PCE of 13.8% with a very small q∆Vnon-rad of 0.271 eV, which is among the best values for OSCs. This oc 17



work shows that this ternary strategy can be a useful approach for reducing nonradiative recombination losses and improving PCE in OSCs.

Supporting Information The Supporting Information is available: details of device fabrication, measurements and characterizations, electrochemical cyclic voltammetry (CV), steady-state PL spectra, charge mobilities and 2D GIWAXS patterns. Author Information Corresponding Authors *

E-mail: [email protected] (Y. S.)

*

E-mail: [email protected] (J. M.)

*

E-mail: [email protected] (H.Y. W.)

Notes The authors declare no competing financial interest

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 21734001, 51825301, 21674007, 21702154 and 51773157), and the Natural Science Foundation of Hubei Province (Grant No. 2017CFB118). HYW acknowledges the financial support from National Research Foundation (NRF) of Korea (2012M3A6A7055540, 2016M1A2A2940911).

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ACS Energy Letters

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Ternary Organic Solar Cells with Small Nonradiative Recombination

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