Investigating the Trade-Off between Device Performance and Energy

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Organic Electronic Devices

Investigating the Trade-off between Device Performance and Energy Loss in Non-Fullerene Organic Solar Cells Ling Hong, Huifeng Yao, Runnan Yu, Ye Xu, Bowei Gao, Ziyi Ge, and Jianhui Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10243 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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ACS Applied Materials & Interfaces

Investigating the Trade-off between Device Performance and Energy Loss in Non-Fullerene Organic Solar Cells Ling Hong,1,3 Huifeng Yao,2,3* Runnan Yu,2,3 Ye Xu,2,3 Bowei Gao,2,3 Ziyi Ge1,3*, and Jianhui Hou2,3* AUTHOR ADDRESS 1

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China 2

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences (BNLMS), Beijing 100190, P. R. China 3 University

of Chinese Academy of Sciences, Beijing 100049, P. R. China

KEYWORDS: organic solar cells, energy loss, device performance, non-fullerene acceptor, charge recombination

ABSTRACT: The large energy loss (Eloss) in organic solar cells (OSCs) relative to those of silicon or inorganic/organic hybrid perovskite solar cells is one of the major factors limiting the power conversion efficiency (PCE) of OSCs. Recently, OSCs based on non-fullerene acceptors (NFAs) have achieved high PCEs at decreased Eloss values. Therefore, the present study investigates the

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relationship between Eloss and the device performance of NFA-based OSCs. Here, we select two polymer donors (PBDB-T and its fluorinated derivative PBDB-TF), and blend each polymer donor with each of three NFAs (indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (IEIC) and its respective fluorinated and chlorinated derivatives IE-4F and IE-4Cl), which provide varied energy level alignments. The six blends exhibit similar morphologies and charge transport properties, but varied Eloss values in OSCs. The results indicate that the charge generation and PCE of the OSCs increase with increasing Eloss. Accordingly, the PBDB-T:IE-4Cl-based device yields the highest PCE of 11.1% with an Eloss of 0.64 eV, while the PBDB-TF:IEIC-based device provides a significantly decreased PCE of 3.8% with a diminished Eloss of 0.52 eV. These results demonstrate the great importance of finely tuning the energy level alignments of these types of donor:acceptor systems to achieve the best device performance.

1. Introduction Bulk-heterojunction (BHJ) organic solar cells (OSCs) represent a promising clean-energy technology owing to their advantages of light weight, flexibility, and low cost.1-4 Over the past two decades, OSCs have made many significant advances due to the development of new materials, device structure engineering, and an enhanced understanding of device physics.5-13 However, OSC suffers from an intrinsically large energy loss in radiative/non-radiative recombination, resulting in low output voltage relative to its optical gap. Thus, the power conversion efficiencies (PCEs) obtained from the state-of-the-art OSCs are considerably less than those of conventional inorganic silicon-based solar cells and newly emerging inorganic/organic hybrid perovskite solar cells.14 Presently, the most highly efficient OSCs typically have energy loss (Eloss) values higher than 0.6


2

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eV. In contrast, the Eloss values of silicon and hybrid perovskite solar cells are approximately 0.4 – 0.5 eV.15 This indicates that reducing Eloss for OSCs will contribute toward increasing their PCE values to a more competitive level.16-30 In OSCs, the charge generation is a complicated process including several steps, 1) the formation of photo-induced exciton, 2) diffusion of exciton to the donor:acceptor interface, 3) charge transfer to form CT exciton, and 4) dissociation of CT exciton to free charges. Empirically, in most highly efficient OSCs employing fullerene derivatives as acceptors, the electron affinity (EA) and ionization potential (IP) of the donor material should be about 0.3 eV higher than those of fullerene derivatives for providing the driving force to separate bound electron-hole pairs. It should be noted that there are exceptions against this rule.31, 32 Under these conditions, the observed charge transfer state has an energy (ECT) that is less than the bandgap (Eg) of the materials, and it is one of the main sources of radiative Eloss.29 For example, the offset between Eg and ECT is about 0.2 eV for PTB7-Th:PC71BM-based OSCs, leading to a large energy loss.15 However, reducing the value of Eloss below 0.6 eV for many fullerene-based OSC systems, the external quantum efficiency (EQE) of the devices sharply decreases.33 These results imply that obtaining further improvements in the PCEs of state-of-the-art fullerene-based OSCs is subject to significant challenges. Over the past few years, the application of non-fullerene acceptors (NFAs) in OSCs has achieved rapid progress with the development of many highly efficient photoactive materials.34-38 Of particular interest is that NFA-based OSCs have been found to function very well with negligible driving force, which contributes to a decreased Eloss, and thereby promises a higher PCEs.15,

31

For example, PCEs higher than 10% have been demonstrated in some NFA-based

systems with Eloss values less than 0.6 eV.39, 40 However, the underlying working mechanism by


3


which excitons dissociate efficiently without driving force remains ambiguous.8,

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41

Therefore,

studying the relationship between Eloss, charge generation, and device performance in NFA-based OSCs is of great interest and importance. To provide more device results related to this issue, the present study investigates the impact of Eloss on the device performance of NFA-based OSCs. We selected two polymer donors (poly[(2,6(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’di-2-thienyl-5’,7’bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDBT) and its fluorinated derivative PBDB-TF42), and blended each polymer donor with each of three small molecular NFAs (indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (IEIC)43 and its respective fluorinated and chlorinated derivatives IE4F and IE-4Cl44-46) to fabricate the OSCs illustrated in Figure 1a. The respective donors and acceptors have equivalent backbone structures. Therefore, the six blends have similar morphologies and charge transport properties. However, the materials possess different molecular energy level alignments owing to fluorination and chlorination effects. We fabricated OSCs based on the six donor:acceptor combinations. The results indicate that, with decreasing Eloss from around 0.6 to 0.5 eV, the devices exhibited significantly reduced EQE and PCE. As a result, the best device based on PBDB-T:IE-4Cl achieved a PCE of 11.1% with an Eloss of 0.64 eV, while the PBDBTF:IEIC-based device achieved a PCE of only 3.8% with a decreased Eloss of 0.52 eV. These results demonstrate the great importance of finely tuning the value of Eloss for these types donor:acceptor combinations to achieve the best OSC device performance. 2. Results and Discussion 2.1.

Synthesis and Computational Study


4

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As shown in Figure 1b, the synthesis reactions for IE-4F and IE-4Cl were performed between the indaceno[1,2-b:5,6-b0]dithiophene (IDT) core unit of IEIC and the fluorinated or chlorinated end-groups via Knoevenagel Condensation. The compounds IE-4F and IE-4Cl were characterized by elemental analysis, 1H nuclear magnetic resonance (NMR), 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The compounds IE-4F and IE-4Cl have good solubility in commonly used solvents such as dichloromethane, chloroform, and chlorobenzene.

a

R

X

S

R

S

O

O

S

C6H13

R

S

NC

S S

S

X

X C6H13

X X=H, PBDB-T X=F, PBDB-TF

b

C6H13

C6H13

X

O S

S NC

CN

R C6H13 X=H, IEIC X=F, IE-4F X=Cl, IE-4Cl R=2-Ethylhexyl

C6H13

R OHC S

S

O

S R

S

n

C6H13 X

R

CN

S

S

S R C6H13 IE-CHO

CHO

+

CN

pyridine chloroform CN

O

o

65 C X

X=F, IE-4F X=Cl, IE-4Cl

X

X=F, Compound 1 X=Cl, Compound 2

Figure 1. (a) Molecular structures of donors and acceptors. (b) The synthetic route of IE-4F and IE-4Cl. Thermogravimetric analysis (TGA) curves for IEIC, IE-4F, and IE-4Cl are depicted in Figure S1. Accordingly, the decomposition temperatures (5% weight loss) of IEIC, IE-4F, and IE-4Cl in a nitrogen atmosphere are 328, 333, and 334°C, respectively, which indicates that these NFAs


5


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have good thermal stability. We investigated the crystallinity of the three NFAs by X-ray diffraction (XRD). As shown in Figure S2, the three NFAs show clear (010) diffraction peaks at approximately 2θ = 26°, and the corresponding π-π stacking distances are about 3.4 Å. We performed quantum chemistry calculations using density functional theory (DFT) at the B3LYP/6-31G (d, p) level to investigate the fundamental electronic properties of the newly designed NFAs. However, the flexible alkyls were replaced by methyl in the calculations for simplification.47 The corresponding data are summarized in Table 1. The compounds IE-4F and IE-4Cl present similar electron density distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) that differ from those of IEIC (Figure S3). Relative to the HOMO/LUMO levels of IEIC (−5.22/−3.26 eV), the HOMO/LUMO levels of IE-4F and IE-4Cl are reduced to −5.33/−3.41 and −5.38/−3.49 eV, respectively. Thus, the calculated values of EgDFT = LUMO − HOMO for IE-4F (1.92 eV) and IE-4Cl (1.89 eV) are less than that of IEIC (1.96 eV), implying that introducing F and Cl atoms can reduce Eg effectively. This could be ascribed possibly to an enhanced intramolecular charge transfer (ICT) effect. 2.2.

Absorption Spectra, Molecular Energy Levels, and Photoluminescence Measurements

Figure 2a presents the ultraviolet-visible (UV-Vis) absorption spectra of the NFAs in solutions and as thin solid films. The corresponding data are summarized in Table 1. In chloroform solution, IEIC has a maximum absorption peak located at 700 nm. In comparison, IE-4F and IE-4Cl have maximum absorption peaks that are redshifted by 15 and 30 nm, respectively. These results are also indicative of enhanced ICT effects.48 The extinction coefficients of IE-4F and IE-4Cl are also higher than that of IEIC (Figure S4). In comparison with the maximum absorption peaks obtained


6

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in chloroform solutions, the maximum absorption peaks of the thin film materials are clearly redshifted by 10, 45, and 45 nm for IEIC, IE-4F, and IE-4Cl, respectively. According to past studies,49 we can estimate the values of Eg for the NFAs via the intersections between absorption and emission spectra (Figure S5). The optical bandgaps (Egopt) are thereby calculated as 1.63, 1.52, and 1.50 eV for IEIC, IE-4F, and IE-4Cl, respectively. Ultraviolet photoelectron spectroscopy (UPS) were also conducted to investigate the energy levels of the three NFAs in parallel. As shown in Figure S6 and Table 1, the IP and EA of IEIC was 5.47 and 3.90 eV, respectively. However, after introducing F or Cl atoms, both the EA and IP levels increased due to the electron-withdrawing effect of halogen atoms, which agrees well with the theoretical calculations. In addition, we note that the chlorinated IE-4Cl has greater EA and IP values than the fluorinated IE-4F because the carbon-chlorine bond has a greater dipole moment than the carbon-fluorine bond. To investigate the impact of Eloss on OSC device performance, we first conducted photoluminescence (PL) measurements to determine the photo-induced charge transfer in the blends. Accordingly, PL measurements for the donors, acceptors, and donor:acceptor blend thin films are shown in Figures 2c and d. The results indicate that the fluorescence quenching of the acceptor in the PBDB-T:IE-4Cl blend is quite efficient, while that in PBDB-T:IEIC and PBDBT:IE-4F films show small peaks in the range of 730 – 900 nm. However, PL quenching is not as efficient for the PBDB-TF-based films as that of the PBDB-T-based films, particularly for the PBDB-TF:IEIC film. The less quenching of the PL spectra implies an inefficient transition from exciton to CT exciton and may be associated with the lower energy difference between Eg and ECT. 50-52.

We also used measured the PL spectra of the blend films using the short-wavelength light,

where donor materials were mainly excited (Figure S7). These observations are quite similar to


7


the recent reports and imply that the photo-induced charge transfer may be a problem in the resulting devices.53-55

100 80 60

EA -3.48

-5.28

0 500

600

700

800

d

PBDB-T IE-4F IE-4Cl

0.6

PBDB-T:IEIC PBDB-T:IE-4F

0.4

PBDB-T:IE-4Cl

0.2 0.0

700

800

900

PL intensity (a.u.)

IEIC 0.8

-4.07 -4.11

-5.45 -5.41 -5.51 -5.54 IP

900

Wavelength (nm) 1.0

-3.84

IE-4Cl

20

400

-3.65

IE-4F

PL intensity (a.u.)

eV

IEIC

40

300

c

b

IEIC solution IEIC film IE-4F solution IE-4F film IE-4Cl solution IE-4Cl film

PBDB-TF

Normalized absorption (a.u.)

a

PBDB-T

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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1000 1100 1200 1300

PBDB-TF IEIC IE-4F IE-4Cl PBDB-TF:IEIC PBDB-TF:IE-4F PBDB-TF:IE-4Cl

1.0 0.8 0.6 0.4 0.2 0.0

Wavelength (nm)

700

800

900 1000 1100 1200 1300 Wavelength (nm)

Figure 2. (a) Normalized UV-Vis absorption spectra of IEIC, IE-4F, and IE-4Cl in chloroform solution and as thin films. (b) Energy level alignments of the donor and acceptor materials. Photoluminescence spectra of (c) pure PBDB-T, IEIC, IE-4F, and IE-4Cl thin films and the PBDBT:IEIC, PBDB-T:IE-4F, and PBDB-T:IE-4Cl blended films, respectively; and (d) pure PBDB-TF, IEIC, IE-4F, and IE-4Cl thin films and PBDB-TF:IEIC, PBDB-TF:IE-4F, and PBDB-TF:IE-4Cl blended films, respectively. Table 1. Optical and electrochemical data of IEIC, IE-4F and IE-4Cl. Acceptor

λmaxa

λmaxb

λonsetb

Egopt

HOMOc

LUMOc

IPd

EAd

(nm)

(nm)

(nm)

(eV)

(eV)

(eV)

(eV)

(eV)


8

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a

IEIC

700

710

790

1.57

−5.22

−3.26

5.41

3.84

IE-4F

715

760

860

1.44

−5.33

−3.41

5.51

4.07

IE-4Cl

730

775

867

1.43

−5.38

−3.49

5.54

4.11

Chloroform solution. b Thin film spin-coated from chloroform solution. c Calculated by DFT at

the B3LYP/6-31G(d, p) level. d IP was evaluated by UPS measurements and EA was calculated by the formula: EA = − (Egopt – IP). 2.3.

Blend Morphology

Since the blend morphology has a significant influence on the OSC device performance, we conducted atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses of the blend films. As shown in Figures 3a – f, all six of the blend films exhibit uniform and smooth surfaces. The root-mean-square roughness (Rq) values of the blend films are in the range of 1.36 – 2.67 nm. In addition, the AFM results and the TEM images given in Figures S8 indicate that the blend films show similar bi-continuous interpenetrating networks, which favor charge generation and transport in the blend. The AFM images show that the surface roughness of IEIC-, IE-4F- and IE-4Cl-containing films is gradually decreased, while it is hard to see obvious differences in the TEM patterns.

b

c

PBDB-T:IEIC Rq = 2.14nm

PBDB-TF:IEIC Rq = 2.67nm

PBDB-T:IE-4F Rq = 1.69nm

d

e

f

PBDB-TF:IE-4F Rq = 1.96nm

PBDB-T:IE-4Cl Rq = 1.36nm

a

PBDB-TF:IE-4Cl Rq = 1.65nm

Figure 3. AFM height images of the (a) PBDB-TF:IEIC, (b) PBDB-T:IEIC, (c) PBDB-TF:IE-4F, (d) PBDB-T:IE-4F, (e) PBDB-TF:IE-4Cl, and (f) PBDB-T:IE-4Cl blend films.


9


2.4.

Page 10 of 27

Photovoltaic Performance

The EA/IP of the PBDB-T and PBDB-TF polymer donors are 3.48/5.28 and 3.65/5.45 eV, respectively. Both of the polymer donors have absorption bands in the range of 500 – 680 nm, which complement the absorption bands of the three NFAs very well. We fabricated the OSC devices using blends of PBDB-T and PBDB-TF with IEIC, IE-4F, or IE-4Cl as the photoactive layer within a conventional layered structure, that included indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/photoactive layer/poly[(9,9-bis(3′((N,N-dimethyl)-N-ethyl-ammonium)-propyl)2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)]dibromide (PFN-Br)/Al. Here, the PEDOT:PSS and PFN-Br56 were employed as the hole transport layer and the electron transport layer, respectively. The device fabrication conditions, including the donor/acceptor (D/A) weight ratio, solvent additives, and thermal annealing were carefully considered to obtain the optimal device performance. Table 2 summarizes the detailed photovoltaic parameters of optimal devices under AM1.5G standard solar spectrum illumination (100 mW·cm−2), and the corresponding current density versus voltage (J−V) curves are shown in Figure 4a. Since the open-circuit voltage (VOC) is empirically proportional to the gap between the IP of the donor and the EA of the acceptor,11 the OSC device employing PBDB-TF:IEIC as the photoactive layer provided the maximum VOC of 1.11 V, while the PBDB-T:IE-4Cl-based device provided the minimum VOC of 0.86 V, with corresponding Eloss values of 0.52 and 0.64 eV, respectively. The other OSC devices provided VOC values between those of these two devices. The PBDB-T:IE-4Cl-based device yielded the best PCE of 11.1% with a short-circuit current density (JSC) of 21.49 mA·cm−2 and a fill factor (FF) of 0.60. The devices with Eloss values larger than 0.6 eV provided PCE values of around 7% – 10%.


10

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However, the PBDB-TF:IEIC-based device provided an Eloss of 0.52 eV, and the PCE decreased substantially to 3.8% with a moderate JSC of 7.65 mA·cm−2 and a poor FF of 0.45. Table 2. Photovoltaic parameters for OSC devices employing PBDB-TF:IEIC, PBDB-T:IEIC, PBDB-TF:IE-4F, PBDB-T:IE-4F, PBDB-TF:IE-4Cl, or PBDB-T:IE-4Cl photoactive layers. Device

VOC (V)

JSC (mA·cm−2)

FF

PCEa (%)

PBDB-TF:IEIC

1.11

7.65

0.45

3.8 (3.7 ± 0.1)

PBDB-T:IEIC

1.02

15.05

0.48

7.3 (7.1 ± 0.2)

PBDB-TF:IE-4F

0.91

18.23

0.56

9.3 (9.1 ± 0.1)

PBDB-T:IE-4F

0.87

21.35

0.58

10.8 (10.6 ± 0.1)

PBDB-TF:IE-4Cl

0.89

17.82

0.61

9.7 (9.5 ± 0.2)

PBDB-T:IE-4Cl 0.86 21.49 0.60 11.1 (10.9 ± 0.2) average values are based on measurements involving 20 devices fabricated with equivalent

a The

parameters. The results of EQE measurements for the six OSC devices are shown in Figure 4b. The maximum EQE values of the devices based on PBDB-T:IE-4F and PBDB-T:IE-4Cl are higher than 0.7 in their photo-response ranges, which corresponds to their high JSC values. However, the EQE values are slightly decreased for the devices with Eloss values of around 0.6 eV (i.e., those based on PBDB-T:IEIC, PBDB-TF:IE-4F, or PBDB-TF:IE-4Cl). Moreover, the EQE of the PBDB-TF:IEIC-based device with an Eloss of 0.52 eV decreased significantly to approximately 0.3, which corresponds with its low performance. The integral current densities of the devices from the EQE spectra are consistent with the corresponding JSC values obtained from the J–V curves. 2.5.

Charge transfer and Recombination

We estimated the hole and electron mobilities using the space charge limited current (SCLC) method, where devices with ITO/PEDOT:PSS/photoactive layer/Au and ITO/ZnO/photoactive layer/Al structures were respectively adopted during testing.57 The hole/electron mobilities of the


11


devices are listed in Table 3 based on fitting to the curves in Figure S9. The blend films provide hole/electron mobilities of around 10–5 to 10–4 cm2·V−1·s−1. The difference in mobility may not play a determining role in affecting the device performance since many devices obtained very good efficiencies with charge mobilities at magnitudes of 10–5 to 10–4 cm2 V–1 s–1. For comparison, the IE-4Cl-containing films have slightly higher hole mobilities, which may be related to the smoother surface. We then conducted photo-induced charge-carrier extraction by linearly increasing voltage (photo-CELIV) measurements to obtain the mobility of the faster carrier component in the blend films.38, 58, 59 Here, the charge carrier mobility is calculated as: 2𝑑2

μ=

3𝐴𝑡2max [1 + 0.36

𝑖𝑓 ∆𝑗 ≤ 𝑗(0)

∆𝑗 𝑗(0)

(1)

]

where A is the rate at which the applied voltage is increased (V·s−1), d is the thickness of the sample (m), tmax is the time corresponding to the extraction peak maximum (s), and j(0) is the displacement current (A). The photo-CELIV mobilities of the devices are summarized in Table S2, where we note that the values range from 1.0 × 10−4 to 2.9 × 10−4 cm2·V−1·s−1.

-15

0.0

0.4 0.8 Voltage (V)

1.2

e

PBDB-TF:IEIC PBDB-T:IEIC PBDB-TF:IE-4F PBDB-T:IE-4F PBDB-TF:IE-4Cl PBDB-T:IE-4Cl

10

2

1

Current (mA)

d

0.1 1

10 2 Light Intensity (mW/cm )

100

0 300

400

500 600 700 Wavelength (nm)

VOC (V)

30 15

-20

1.08


45

0.6 0.4

800

0

2

4 6 Time (s)

8

0.99 0.90

1

900

0.2 0.0

PBDB-T:IEIC PBDB-T:IE-4F PBDB-T:IE-4Cl

0.81


0.8

PBDB-TF:IEIC PBDB-TF:IE-4F PBDB-TF:IE-4Cl

1.17

10

f

100

2

-10

60

EQE (%)

-5

c

75

10 2 Light Intensity (mW/cm )

100

10

Jph (A/m )

b


0

2

Current Density (mA/cm )

a

JSC (mA/cm )

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

Page 12 of 27

1

0.1

PBDB-TF:IEIC PBDB-TF:IE-4F PBDB-TF:IE-4Cl

0.1

PBDB-T:IEIC PBDB-T:IE-4F PBDB-T:IE-4Cl

1 Veff (V)


12

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Figure 4 (a) J–V and (b) EQE curves of the optimized OSC devices employing PBDB-TF:IEIC, PBDB-T:IEIC, PBDB-TF:IE-4F, PBDB-T:IE-4F, PBDB-TF:IE-4Cl, and PBDB-T:IE-4Cl photoactive layers. Dependences of (c) VOC and (d) JSC on Plight for the optimized OSC devices employing PBDB-TF:IEIC, PBDB-T:IEIC, PBDB-TF:IE-4F, PBDB-T:IE-4F, PBDB-TF:IE-4Cl, and PBDB-T:IE-4Cl photoactive layers. (e) Photo-CELIV versus light intensity and (f) Jph−Veff curves for the devices based on PBDB-TF:IEIC, PBDB-T:IEIC, PBDB-TF:IE-4F, PBDB-T:IE4F, PBDB-TF:IE-4Cl, and PBDB-T:IE-4Cl. Table 3. Photovoltaic performance parameters of OSC devices based on PBDB-T:IEIC, PBDBTF:IEIC, PBDB-TF:IE-4F, PBDB-T:IE-4F, PBDB-TF:IE-4Cl, and PBDB-T:IE-4Cl.

Device

Eloss [eV]

Pdiss

PBDB-TF:IEIC

0.52

PBDB-T:IEIC

slope

μe

μh

JSC

VOC

[cm2·V−1·s−1]

[cm2·V−1·s−1]

0.53

0.94

1.18

9.8×10−5

3.5×10−5

0.61

0.84

0.95

1.00

1.1×10−4

1.1×10−4

PBDB-TF:IE-4F

0.61

0.82

0.97

0.96

3.1×10−4

4.4×10−5

PBDB-T:IE-4F

0.65

0.92

0.96

0.93

3.0×10−4

1.4×10−4

PBDB-TF:IE-4Cl

0.61

0.81

0.95

1.01

3.6×10−4

5.3×10−5

PBDB-T:IE-4Cl

0.64

0.91

0.94

0.94

3.7×10−4

1.6×10−4

We then studied the recombination dynamics by analyzing the photovoltaic parameters of the OSCs at different illumination intensities.60-63 Figure 4c shows semi-logarithmic plots of VOC versus illumination intensity (Plight) for each of the OSC devices. The slopes of the plots were then linearly fitted to nKBT/q, where n is between 1 and 2, KB is Boltzmann’s constant, q is the elementary charge, and T is the temperature. A value of n = 1 indicates that bimolecular recombination dominates, while monomolecular Shockley-Read-Hall (SRH) recombination dominates owing to trap states when n = 2.64 All six OSCs exhibit slopes representative of n ≈ 1, indicating that SRH recombination in the photoactive layers was rather minor.


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We also explored the dependence of JSC on Plight for the OSC devices, and the results are plotted in Figure 4d. In general, the relationship between JSC and Plight should follow a power law relationship given as JSC ∝ Plightα.61 If the charge carriers are collected by the corresponding electrodes, α should be equal to 1, implying that bimolecular recombination is very weak. The results plotted in Figure 4d are summarized in Table 3, which indicates that the values of α are not less than 0.94 for all of the devices, demonstrating that the devices have weak bimolecular recombination. We also conducted photocurrent analysis to investigate the overall exciton dissociation, and carrier transport and collection processes in the OSCs. To this end, Figure 4f presents the photocurrent density (Jph) versus the effective voltage (Veff). Here, Jph is defined as the difference in the photocurrent density under illumination (JL) and that in the dark (JD) (i.e., Jph = JL − JD), and Veff is the difference between the applied voltage (V0) when Jph = 0 and the applied voltage Va (i.e., Veff = V0 − Va). Figure 4f reflects exciton dissociation, and charge transport and collection properties because Veff is determined by the internal electric field in the devices. In general, we find that Jph increases significantly for Veff less than 1 V, and tends to be stable at high Veff, indicating that free carriers are effectively swept out, and the devices therefore approach the saturation photocurrent density (Jsat) at high Veff. The Jph/Jsat ratios of the six devices were calculated, and these are listed in Table 3 for comparison.65-67 The data show that the OSCs based on PBDB-T:IE-4F and PBDB-T:IE-4Cl present Jph/Jsat values greater than 0.9. For the OSCs based on PBDB-T:IEIC, PBDB-TF:IE-4F, and PBDBTF:IE-4Cl with decreased Eloss values of around 0.6 eV, the Jph/Jsat ratios decrease to about 0.8. However, for the PBDB-TF:IEIC-based device with a further decreased Eloss of 0.52 eV, Jph/Jsat decreases significantly to 0.53. These results demonstrate that the performance of devices


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based on this donor:acceptor system rapidly diminishes when Eloss decreases to less than 0.6 eV. This trend is further illustrated by the plots of the exciton dissociation probability (Pdiss) and PCE versus Eloss in Figure 5 for the six OSC devices. 12

1.0

Pdiss PCE

4

0.6

0.50

0.55

0.60

0.65

PCE (%)

8

0.8

Pdiss

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


0

Energy Loss (eV)

Figure 5. Pdiss and PCE values versus Eloss for OSC devices based on PBDB-TF:IEIC, PBDBT:IEIC, PBDB-TF:IE-4F, PBDB-T:IE-4F, PBDB-TF:IE-4Cl and PBDB-T:IE-4Cl. 3. Conclusion In summary, we synthesized two new NFAs denoted as IE-4F and IE-4Cl via introducing fluorine and chlorine atoms into the IEIC structure, respectively. These three NFAs possess varied molecular energy level structures, and they were employed with the polymer donors PBDB-T and PBDB-TF to fabricate six OSC devices with different Eloss values from 0.52 to 0.65 eV. Since the donor:acceptor combinations have analogous chemical structures, they possess similar blend morphologies and charge transport properties. The devices with Elose values higher than 0.6 eV demonstrated good charge generation efficiencies, and thus provided high PCE values. However, the PBDB-TF:IEIC-based device with an Eloss below 0.6 eV demonstrated inefficient charge transfer between PBDB-TF and IEIC, resulting in low EQE and PCE. As a result, the PBDB-T:IE4Cl-based device yielded the maximum PCE of 11.1% with an Eloss of 0.64 eV, while the PBDB-


15


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TF:IEIC-based device obtained the lowest PCE of 3.8% with an Eloss of 0.52 eV. These results demonstrate that finely tuning the bandgap structures of donors and/or acceptors to obtain a suitable molecular energy alignment is of great importance for optimizing the energy loss and achieving optimal OSC device performance. Experimental Section Material synthesis: We synthesized IE-CHO and IEIC according to previously published procedures.68 IE-4F. IE-CHO (270 mg, 0.2 mmol) and fluorine-containing endgroup compound 148 (230 mg, 1 mmol) were dissolved in a mixed solvent of chloroform (30 mL) and pyridine (0.3 mL) under an argon atmosphere. The reaction mixture was stirred and refluxed at 65°C for 16 h. The crude product was purified by silica gel column chromatography using petroleum ether and dichloromethane as the eluent to obtain the target product as a black solid (238 mg, yield 67%). 1H

NMR (CDCl3, 400 MHz, δ): 8.76 (s, 2H), 8.54 (dd, 2H), 7.67 (t, 2H), 7.61 (s, 2H), 7.48 (d,

4H), 7.15 (dd, 16H), 2.79 (d, 4H), 2.65–2.51 (m, 8H), 1.75 (s, 2H), 1.64–1.55 (m, 10H), 1.36–1.25 (m, 38H), 0.94–0.76 (m, 24H). Anal. Calcd (%) for C114H114F4N4O2S4: C 77.08, H 6.47, N 3.15. Found: C 76.80, H 6.53, N 3.26. IE-4Cl. IE-CHO (270 mg, 0.2 mmol) and endgroup compound 246 (263 mg, 1 mmol) were added to a mixed solvent of chloroform (30 mL) and pyridine (1 mL). The reaction mixture was stirred and refluxed at 65°C for 20 h under an argon atmosphere. The mixture was purified by silica gel column chromatography using petroleum ether and dichloromethane as the eluent to obtain the target product (306 mg, yield 83%). 1H NMR (CDCl3, 400 MHz, δ): 8.77 (d, 4H), 7.93 (s, 2H), 7.61 (s, 2H), 7.50 (d, 4H), 7.20 (d, 8H), 7.12 (d, 8H), 2.79 (d, 4H), 2.62–2.55 (m, 8H), 1.75 (s,


16

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2H), 1.70–1.50 (m, 9H), 1.31 (dd, 38H), 0.87 (q, 24H). Anal. Calcd (%) for C114H114Cl4N4O2S4: C 74.33, H 6.24, N 3.04. Found: C 73.73, H 6.09, N 3.10. Device Fabrication: Conventional layered ITO/PEDOT:PSS/photoactive layer/PFN-Br/Al OSC devices with an effective cell area of 3.70 mm2 were fabricated by the following procedures. We first cleaned the ITO-coated glass substrates in an ultrasonic bath using a surfactant scrub, deionized water, acetone, and isopropanol for 30 min in each solution, and the ITO-coated glass substrates were further treated by UV–ozone for 20 min. An approximately 30-nm-thick layer of PEDOT:PSS was spin-coated onto the ITO surface. The assembly was annealed at 150°C for 15 min, and immediately moved into a nitrogen-filled glovebox. All of the donors and acceptors were blended at a total concentration of 20 mg/ml (weight ratio of 1:1) and dissolved in chlorobenzene solution at room temperature for more than 4 h with stirring. Subsequently, 0.5 vol% of 1,8diiodooctane was introduced over a 30-min period as a solvent additive. An approximately 100nm-thick layer of the blend was spin-coated onto the PEDOT:PSS surface, followed by thermal annealing at 110°C for 10 min. Subsequently, PFN-Br in methanol solution (0.5 mg/ml) was spincoated at 3000 rpm on the surface of the photoactive layer. Finally, an approximately 100-nmthick layer of Al was deposited on the surface of the PFN-Br layer under high vacuum.

ASSOCIATED CONTENT Quantum chemistry calculation, TGA curve, XRD pattern, ultraviolet photoelectron spectroscopy, absorption coefficient, mobility plots and AFM phase images are shown in supporting information, Figures S1-S8 and Tables S1-S2.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] * E-mail: [email protected] ORCID Huifeng Yao: 0000-0003-2814-4850 Runnan Yu: 0000-0001-6482-4314 Ziyi Ge: 0000-0003-3656-6017 Jianhui Hou: 0000-0002-2105-6922 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT J. H. would like to acknowledge the financial support from the National Natural Science Foundation of China (91633301 51673201, and 21835006), the Chinese Academy of Science (XDB12030200). H. Y. thanks the financial support from the National Natural Science Foundation of China (21805287) the Youth Innovation Promotion Association CAS (No. 2018043). This work was supported by Beijing National Laboratory for Molecular Sciences (BNLMS-CXXM-201903). Z. G. gratefully acknowledge the financial support from the Ningbo Municipal Science and Technology Innovative Research Team (2015B11002 and 2016B10005).


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REFERENCES (1)

Dou, L.; Liu, Y.; Hong, Z.; Li, G.; Yang, Y. Low-Bandgap Near-IR Conjugated Polymers/Molecules for Organic Electronics. Chem. Rev. 2015, 115, 12633-12665.

(2)

Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat Photon. 2012, 6, 153.

(3)

Inganäs, O. Organic Photovoltaics over Three Decades. Adv. Mater. 2018, 30, 1800388.

(4)

Heeger, A. J. 25th anniversary article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10-27.

(5)

Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371.

(6)

Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006.

(7)

Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994.

(8)

Gao, F.; Inganas, O. Charge Generation in Polymer–Fullerene Bulk-Heterojunction Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 20291.

(9)

Cui, Y.; Yao, H.; Yang, C.; Zhang, S.; Hou, J. Organic Solar Cells with an Efficiency Approaching 15%. Acta. Polym. Sin. 2018, 2, 223.

(10)

Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868.

(11)

Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10 % Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789.

(12)

Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009.

(13)

Henson, Z. B.; Mullen, K.; Bazan, G. C. Design Strategies for Organic Semiconductors beyond the Molecular Formula. Nat. Chem. 2012, 4, 699.

(14)

Menke, S. M.; Ran, N. A.; Bazan, G. C.; Friend, R. H. Understanding Energy Loss in Organic Solar Cells: Toward a New Efficiency Regime. Joule 2018, 2, 25.

(15)

Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast Charge Separation in a


19


Page 20 of 27

Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089. (16)

Benduhn, J.; Tvingstedt, K.; Piersimoni, F.; Ullbrich, S.; Fan, Y.; Tropiano, M.; McGarry, K. A.; Zeika, O.; Riede, M. K.; Douglas, C. J.; Barlow, S.; Marder, S. R.; Neher, D.; Spoltore, D.; Vandewal, K. Intrinsic Non-Radiative Voltage Losses in Fullerene-Based Organic Solar Cells Nat. Energy 2017, 2, 17053.

(17)

Chen, X.-K.; Brédas, J.-L. Voltage Losses in Organic Solar Cells: Understanding the Contributions of Intramolecular Vibrations to Nonradiative Recombinations. Adv. Energy Mater. 2018, 8, 1702227.

(18)

Collins, S. D.; Proctor, C. M.; Ran, N. A.; Nguyen, T.-Q. Understanding Open-Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2016, 6, 1501721.

(19)

Liu, X.; Du, X.; Wang, J.; Duan, C.; Tang, X.; Heumueller, T.; Liu, G.; Li, Y.; Wang, Z.; Wang, J.; Liu, F.; Li, N.; Brabec, C. J.; Huang, F.; Cao, Y. Efficient Organic Solar Cells with Extremely High Open‐Circuit Voltages and Low Voltage Losses by Suppressing Nonradiative Recombination Losses. Adv. Energy Mater. 2018, 8, 1801699.

(20)

Nikolis, V. C.; Benduhn, J.; Holzmueller, F.; Piersimoni, F.; Lau, M.; Zeika, O.; Neher, D.; Koerner, C.; Spoltore, D.; Vandewal, K. Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies. Adv. Energy Mater. 2017, 7, 1700855.

(21)

Wang, C.; Xu, X.; Zhang, W.; Bergqvist, J.; Xia, Y.; Meng, X.; Bini, K.; Ma, W.; Yartsev, A.; Vandewal, K.; Andersson, M. R.; Inganäs, O.; Fahlman, M.; Wang, E. Low Band Gap Polymer Solar Cells With Minimal Voltage Losses. Adv. Energy Mater. 2016, 6, 1600148.

(22)

Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. The Energy of Charge-Transfer States in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells. Adv. Funct. Mater. 2009, 19, 1939.

(23)

Chen, S.; Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. Dielectric Effect on the Photovoltage Loss in Organic Photovoltaic Cells. Adv. Mater. 2014, 26, 6125.


20

Page 21 of 27 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


(24)

Shi, X.; Zuo, L.; Jo, S. B.; Gao, K.; Lin, F.; Liu, F.; Jen, A. K. Y. Design of a Highly Crystalline Low-Band Gap Fused-Ring Electron Acceptor for High-Efficiency Solar Cells with Low Energy Loss. Chem. Mater. 2017, 29, 8369.

(25)

Li, Y.; Liu, X.; Wu, F.-P.; Zhou, Y.; Jiang, Z.-Q.; Song, B.; Xia, Y.; Zhang, Z.-G.; Gao, F.; Inganas, O.; Li, Y.; Liao, L.-S. Non-Fullerene Acceptor with Low Energy Loss and High External Quantum Efficiency: towards High Performance Polymer Solar Cells. J. Mater. Chem. A 2016, 4, 5890.

(26)

Chen, S.; Liu, Y.; Zhang, L.; Chow, Z. P. C. Y.; Wang, Z.; Zhang, G.; Ma, W.; Yan, H. A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene Organic Solar Cells with a Small Voltage Loss. J. Am. Chem. Soc. 2017, 139, 6289

(27)

Gao, K.; Li, L.; Lai, T.; Xiao, L.; Huang, Y.; Huang, F.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P.; Janssen, R. A. J.; Peng, X. Deep Absorbing Porphyrin Small Molecule for High-Performance Organic Solar Cells with Very Low Energy Losses. J. Am. Chem. Soc. 2015, 137, 7282.

(28)

Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. High-Efficiency Polymer Solar Cells with Small Photon Energy Loss. Nat. Commun. 2015, 6, 10085.

(29)

Qian, D.; Zheng, Z.; Yao, H.; Tress, W.; Hopper, T. R.; Chen, S.; Li, S.; Liu, J.; Chen, S.; Zhang, J.; Liu, X.-K.; Gao, B.; Ouyang, L.; Jin, Y.; Pozina, G.; Buyanova, I. A.; Chen, W. M.; Inganäs, O.; Coropceanu, V.; Bredas, J.-L.; Yan, H.; Hou, J.; Zhang, F.; Bakulin, A. A.; Gao, F. Design Rules for Minimizing Voltage Losses in High-Efficiency Organic Solar Cells. Nat. Mater. 2018, 17, 703.

(30)

Yao, J.; Kirchartz, T.; Vezie, M. S.; Faist, M. A.; Gong, W.; He, Z.; Wu, H.; Troughton, J.; Watson, T.; Bryant, D.; Nelson, J. Quantifying Losses in Open-Circuit Voltage in Solution-Processable Solar Cells. Phys. Rev. Appl. 2015, 4, 014020.

(31)

Bin, H.; Gao, L.; Zhang, Z. G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency Non-Fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2D-Conjugated Polymer as Donor. Nat. Commun. 2016, 7, 13651.

(32)

Hendriks, K. H.; Wijpkema, A. S. G.; Franeker, J. J. Van.; Wienk, M. M.; Janssen, R. A. J. Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells. J. Am. Chem. Soc. 2016, 138, 10026.


21


(33)

Page 22 of 27

Li, W.; Hendriks, K. H.; Furlan, A. M.; Wienk, M.; Janssen, R. A. J. High Quantum Efficiencies in Polymer Solar Cells at Energy Losses Below 0.6 eV. J. Am. Chem. Soc. 2015, 137, 2231.

(34)

Zhang, J.; Tan, H. S.; Guo, X.; Facchetti, A.; Yan, H. Material Insights and Challenges for Non-Fullerene Organic Solar Cells Based on Small Molecular Acceptors. Nat. Energy 2018, 3, 720.

(35)

Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.; Gasparini, N.; Rohr, J. A.; Holliday, S.; Wadsworth, A.; Lockett, S.; Neophytou, M.; Emmott, C. J.; Nelson, J.; Brabec, C. J.; Amassian, A.; Salleo, A.; Kirchartz, T.; Durrant, J. R.; McCulloch, I. Reducing the Efficiency-Stability-Cost Gap of Organic Photovoltaics with Highly Efficient and Stable Small Molecule Acceptor Ternary Solar Cells. Nat. Mater. 2016, 16, 363.

(36)

Hou, J.; Inganäs, O.; Friend, R. H.; Gao, F. Organic Solar Cells Based on Non-Fullerene Acceptors. Nat. Mater. 2018, 17, 119.

(37)

Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X. NonFullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003.

(38)

Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C. H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J. R.; McCulloch, I. High-Efficiency and Air-Stable P3HT-Based Polymer Solar Cells with A New Non-Fullerene Acceptor. Nat. Commun. 2016, 7, 11585.

(39)

Cheng, P.; Zhang, M.; Lau, T.-K.; Wu, Y.; Jia, B.; Wang, J.; Yan, C.; Qin, M.; Lu, X.; Zhan, X. Realizing small energy loss of 0.55 eV, High Open-Circuit Voltage >1 V and High Efficiency >10% in Fullerene-Free Polymer Solar Cells via Energy Driver. Adv. Mater. 2017, 29, 1605216.

(40)

Kan, B.; Feng, H.; Yao, H.; Chang, M.; Wan, X.; Li, C.; Hou, J.; Chen, Y. A Chlorinated Low-Bandgap Small-Molecule Acceptor for Organic Solar Cells with 14.1% Efficiency and Low Energy Loss. Sci. Chi. Chem. 2018, DOI: 10.1007/s11426-018-9334-9.

(41)

Yao, H.; Qian, D.; Zhang, H.; Qin, Y.; Xu, B.; Cui, Y.; Yu, R.; Gao, F.; Hou, J. Critical Role of Molecular Electrostatic Potential on Charge Generation in Organic Solar Cells. Chin. J. Chem. 2018, 36, 491.


22

Page 23 of 27 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


(42)

Fan, Q.; Su, W.; Wang, Y.; Guo, B.; Jiang, Y.; Guo, X.; Liu, F.; Russell, T. P.; Zhang, M.; Li, Y. Synergistic Effect of Fluorination on Both Donor and Acceptor Materials for High Performance Non-Fullerene Polymer Solar Cells with 13.5% Efficiency. Sci. Chi. Chem. 2018, 61, 531.

(43)

Yan, C.; Wu, Y.; Wang, J.; Li, R.; Cheng, P.; Bai, H.; Zhan, Z.; Ma, W.; Zhan, X. Enhancing Performance of Non-Fullerene Organic Solar Cells via Side Chain Engineering of Fused-Ring Electron Acceptors. Dyes Pigm. 2017, 139, 627.

(44)

Zhang, Q.; Kelly, M. A.; Bauer, N.; You, W. The Curious Case of Fluorination of Conjugated Polymers for Solar Cells. Acc. Chem. Res. 2017, 50, 2401.

(45)

Mo, D.; Wang, H.; Chen, H.; Qu, S.; Chao, P.; Yang, Z.; Tian, L.; Su, Y.-A.; Gao, Y.; Yang, B.; Chen, W.; He, F. Chlorination of Low-Band-Gap Polymers: Toward HighPerformance Polymer Solar Cells. Chem. Mater. 2017, 29, 2819.

(46)

Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.; Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene Small-Molecule Acceptors. Adv. Mater. 2018, 30, 1800613.

(47)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery J. A.; Peralta, Jr., J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09, Revision B.01 Gaussian, Inc., Wallingford, CT, USA, 2009.

(48)

Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design, Synthesis,and Photovoltaic Characterization of aSmall Molecular Acceptor with an Ultra-NarrowBand Gap. Angew. Chem. Int. Ed. 2017, 56, 3045.


23


(49)

Page 24 of 27

Vandewal, K.; Benduhn, J.; Nikolis, V. C. How to Determine Optical Gaps and Voltage Losses in Organic Photovoltaic Materials. Sustainable Energy Fuels 2018, 2, 538.

(50)

Faist, M. A.; Kirchartz, T.; Gong, W.; Ashraf, R. S.; McCulloch, I.; de Mello, J. C.; Ekins-Daukes, N. J.; Bradley, D. D.; Nelson, J., Competition between the Charge Ttransfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells. J. Am. Chem. Soc. 2012, 134 (1), 685-692.

(51)

Vandewal, K.; Ma, Z.; Bergqvist, J.; Tang, Z.; Wang, E.; Henriksson, P.; Tvingstedt, K.; Andersson, M. R.; Zhang, F.; Inganäs, O., Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage. Adv. Funct. Mater. 2012, 22 (16), 3480-3490.

(52)

Nakano, K.; Chen, Y.; Xiao, B.; Han, W.; Huang, J.; Yoshida, H.; Zhou, E.; Tajima, K., Anatomy of the Energetic Ddriving Force for Charge Generation in Organic Solar Cells. Nat. Commun. 2019, 10 (1), 2520.

(53)

Li, S.; Zhan, L.; Sun, C.; Zhu, H.; Zhou, G.; Yang, W.; Shi, M.; Li, C. Z.; Hou, J.; Li, Y.; Chen, H., Highly Efficient Fullerene-Free Organic Solar Cells Operate at Near Zero Highest Occupied Molecular Orbital Offsets. J. Am. Chem. Soc. 2019, 141 (7), 30733082.

(54)

Eisner, F. D.; Azzouzi, M.; Fei, Z.; Hou, X.; Anthopoulos, T. D.; Dennis, T. J. S.; Heeney, M.; Nelson, J., Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells. J. Am. Chem. Soc. 2019, 141 (15), 6362-6374.

(55)

Yang, C.; Zhang, J.; Liang, N.; Yao, H.; Wei, Z.; He, C.; Yuan, X.; Hou, J., Effects of Energy-Llevel Offset between a Donor and Acceptor on the Photovoltaic Performance of Non-Fullerene Organic Solar Cells. J. Mater. Chem. A 2019, DOI:10.1039/C9TA04789A.

(56)

Yang, T. B.; Wang, M.; Duan, C. H.; Hu, X. W.; Huang, L.; Peng, J. B.; Huang, F.; Gong, X. Inverted Polymer Solar Cells with 8.4% Efficiency by Conjugated Polyelectrolyte. Energy Environ. Sci. 2012, 5, 8208.

(57)

Rose, A. Space-Charge-Limited Currents in Solids. Phys. Rev. 1955, 97, 1538.


24

Page 25 of 27 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


(58)

Chen, S.; Choudhury, K. R.; Subbiah, J.; Amb, C. M.; Reynolds, J. R.; So, F. PhotoCarrier Recombination in Polymer Solar Cells Based on P3HT and Silole-Based Copolymer. Adv. Energy Mater. 2011, 1, 963.

(59)

Genevičius, K.; Österbacka, R.; Juška, G.; Arlauskas, K.; Stubb, H., Separation of Fast and Slow Transport in Regiorandom Poly(3-hexylthiophene). Synthetic. Met. 2003, 137 (1-3), 1407-1408.

(60)

Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 82, 245207.

(61)

Gupta, V.; Kyaw, A. K. K.; Wang, D. H.; Chand, S.; Bazan, G. C.; Heeger, A. J. Barium: An Efficient Cathode Layer for Bulk-Heterojunction Solar Cells. Sci. Rep. 2013, 3, 1965.

(62)

Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. HighEfficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864.

(63)

Larsen-Olsen, T. T.; Machui, F.; Lechene, B.; Berny, S.; Angmo, D.; Sondergaard, R.; Blouin, N.; Mitchell, W.; Tierney, S.; Cull, T.; Tiwana, P.; Meyer, F.; Carrasco-Orozco, M.; Scheel, A.; Lovenich, W.; de Bettignies, R.; Brabec, C. J.; Krebs, F. C. Round-Robin Studies as a Method for Testing and Validating High-Efficiency ITO-Free Polymer Solar Cells Based on Roll-to-Roll-Coated Highly Conductive and Transparent Flexible Substrates. Adv. Energy Mater. 2012, 2, 1091.

(64)

Mandoc, M. M.; Kooistra, F. B.; Hummelen, J. C.; de Boer, B.; Blom, P. W. M. Effect of Traps on the Performance of Bulk Heterojunction Organic Solar Cells. Appl. Phys. Lett. 2007, 91, 263505.

(65)

Mihailetchi, V. D.; Koster, L. J.; Hummelen, J. C.; Blom, P. W. Photocurrent Generation in Polymer-Fullerene Bulk Heterojunctions. Phys. Rev. Lett. 2004, 9, 216601.

(66)

He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y., Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636-4643.

(67)

Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L., Ternary Blend Polymer Solar Cells with Enhanced Power Conversion Efficiency. Nat Photo. 2014, 8, 716-722.


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(68)

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Lin, Y.; Zhang, Z.-G.; Bai, H.; Wang, J.; Yao, Y.; Li, Y.; Zhu, D.; Zhan, X. HighPerformance Fullerene-Free Polymer Solar Cells with 6.31% Efficiency. Energy Environ. Sci. 2015, 8, 610.


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Page 27 of 27

ToC

eV -3.48

-3.84

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IE-4Cl

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IEIC

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EA -3.65

PBDB-TF

PBDB-T

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


-5.51

-5.54

IP

F

R

R

S

S

S

R

O S

S R

NC

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S

S

n

F

C6H13

C6H13

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27

Investigating the Trade-Off between Device Performance and Energy

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