Toward Efficient Thick Active PTB7 Photovoltaic ... - ACS Publications

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Towards efficient thick active PTB7 photovoltaic layers using diphenyl ether as a solvent additive Yifan Zheng, Tenghooi Goh, Pu Fan, Wei Shi, Junsheng Yu, and Andre D. Taylor ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03453 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces 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.

Page 1 of 27

ACS Applied Materials & Interfaces

1 2 4

3

Towards efficient thick active PTB7 photovoltaic 5 6

layers using diphenyl ether as a solvent additive 8

7

Yifan Zheng†, Tenghooi Goh‡, Pu Fan†, Wei Shi†, Junsheng Yu*†, and AndréD. 10

9

Taylor*‡ 12

1

†

13 15

14

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic

Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R.

17

16

China.

19

18

‡

21

20

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut,

06511, United States

2 23 25

24

Abstract: 26 28

27

The development of thick organic photovoltaics (OPV) could increase absorption in the 30

29

active layer and ease manufacturing constraints in large-scale solar panel production. 32

31

However, the efficiencies of most low-bandgap OPVs decrease substantially when the 34

3

active layers exceed ~ 100 nm (due to low crystallinity and short exciton diffusion 36

35

length). Herein, we report the use of solvent additive diphenyl ether (DPE) that 38

37

facilitates the fabrication of thick (180 nm) active layers and triples the power 40

39

conversion efficiency (PCE) of conventional thienothiophene-co-benzodithiophene 42

41

polymer (PTB7)-based OPVs from 1.75% to 6.19%. These results demonstrate a 20% 4

43

higher PCE than conventional (PTB7)-based OPV devices using 1,8-diiodooctane 46

45

(DIO). Morphology studies reveal that DPE promotes the formation of nano-fibrilla 48

47

networks and ordered packing of PTB7 in the active layer that facilitate charge transport 50

49

over longer distances. We further demonstrate that DPE improves the fill factor and 52

51

photocurrent collection by enhancing the overall optical absorption, reducing the series 54

53

resistance, and suppressing bimolecular recombination. 5 56 57 58 59 60

1

ACS Paragon Plus Environment


1 2 3

Keywords：Diphenyl ether (DPE), Solvent additive, Organic photovoltaic (OPV), 4 5

Thick active layer, Low-bandgap polymer 6 7 9

8

*† E-mail: [email protected]. Tel.: +86-28-83207157, Fax: +86-28-83206123. 10 1

*‡ E-mail: [email protected]. Tel.: (203) 432-2217, Fax: (203) 432-4387. 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

2


Page 2 of 27

Page 3 of 27


1 2 3

1. Introduction 4 6

5

Photovoltaic technology offers an environmentally friendly and sustainable 7 8

electricity 9

source

to

surmount

the

global

energy

crisis.1-2

In

particular,

1

10

solution-processed bulk heterojunction (BHJ) organic photovoltaics (OPVs) are scalable, 12 14

13

lightweight, and mechanically flexible, which are all attractive features for the 15 16

proliferation of solar panels.3-4 Recent notable OPV breakthroughs include the synthesis 17 19

18

of high carrier mobility small-bandgap polymers,5 the development of new device 20 2

21

architectures that enlarge the donors/accepters (D/A) interface,6 and active layer 23 25

24

nano-scale

morphology

improvements

by

post-treatment

methods.7

These

27

26

improvements have allowed researchers to demonstrate single junction OPVs with 28 30

29

power conversion efficiencies (PCE) as high as 11.7%.8 31 3

32

In both polymer and small-molecule based BHJ OPVs, the morphology of D/A 35

34

blends plays a vital role in dictating the device PCE.9 In order to operate efficiently, the 36 38

37

photo-generated excitons in OPVs must diffuse promptly to the D/A interface where 39 41

40

they can separate into holes and electrons before they recombine and dissipate as 43

42

heat.10-12 In addition, high OPV photocurrents can only be obtained when the 4 46

45

dissociated free charge carriers are able to arrive at the corresponding electrodes 47 49

48

through interconnected D and A phase networks that span several tens to hundreds of 51

50

nanometers.13-14 One option to suppress electron-hole recombination and promote high 52 54

53

carrier mobility is to use highly-ordered and purified D and A organics. Through 5 57

56

introducing these kind of D and A materials, we can facilitate the formation of purified 60

59

58

domain area that deduce the trap density.

15-16

But this route may incur high

3



Page 4 of 27

1 2 4

3

manufacturing costs. On the other hand, control and enhancement of the D/A 5 6

morphology can also be achieved by employing cheaper processing techniques such as 7 9

8

thermal treatment, solvent annealing, or solvent addition.17-19 10 12

1

Among these techniques, solvent addition is particularly interesting as it is 13 14

compatible with extensive polymer systems and does not require extra processing steps. 15 17

16

Previously 18

solvent

addition

was

reported

to

be

effective

in

20

19

Poly(3-hexylthiophene-2,5-diyl) (P3HT)/ (6,6)-phenyl-C61/C71-butyric acid methyl 21 23

2

ester (PCBM)-based devices and more recently has been extended to low-bandgap 25

24

polymer based cells.20-21 Some of the most common additives are 1,8-octanedithiol, 26 28

27

1,8-diiodooctane (DIO), 1-chloronaphthalene, and N-methyl-2-pyrrolidone.19, 29

21-23

31

30

Among these, DIO yields the best results for thin BHJ devices consisting of 3

32

benchmarked low-bandgap polymers such as thieno [3,4-b] thiophene/benzodithiophene 34 36

35

(PTB7).24-25 The addition of DIO into chlorobenzene (CB) effectively dissolves the 37 39

38

PCBM aggregation and promotes the formation of smaller acceptor domains (e.g. 41

40

greater D/A interface) within the active layer, resulting in a significant improvement in 42 4

43

the device fill factor (FF) and short-current density (JSC).26 Unfortunately, DIO is an 45 47

46

acceptor-selective additive that does not enhance the crystallinity of the low-bandgap 49

48

polymer. This drawback leads to a decreased PCE in thicker DIO-added BHJ devices.2 50 52

51

In fact, residual DIO in the active layer could form trap centers that inhibit charge 53 54

transport.27-28 Moreover, it has been suggested that excess DIO above 3% can lead to the 57

56

5

reaggregation of PCBM and poor device reproducibility, which can impede further 58 60

59

material system designs and device physics studies.28-29 Despite these challenges, the 4


Page 5 of 27


1 2 4

3

development of thicker active layer devices could overcome many of the issues that 5 6

prevent scalable nanomanufacturing (i.e. roll-to-roll or spray coating processing).30 7 9

8

Recent work by Hwang groups and Heeger groups have opened a new route for 10 12

1

realizing high PCEs with the thick (300 nm) active layers based on different low 13 14

bandgap polymers by using diphenyl ether (DPE).30-31 However, to date, only a few 15 17

16

studies have been performed to characterize the origin of the structure improvements 18 20

19

resulting from the use of DPE.30 In addition, the impact of DPE in PTB7-based devices, 21 23

2

arguably the epitome of low-bandgap polymer OPVs, is yet unknown. 25

24

In this contribution, we demonstrate that the incorporation of DPE in the low-band 26 28

27

gap PTB7 : PC71BM BHJ system is a facile and effective strategy towards enhancing 29 31

30

photovoltaic PCE. We correlate the device performance to a charge extraction model as 3

32

characterized by impedance spectroscopy. We reveal using optical microscopy, 34 36

35

grazing-incidence X-ray diffraction (GI-XRD), and atomic force microscopy (AFM) 37 39

38

imaging tools that the DPE molecule enhances the organic crystallinity and induces 41

40

nano-fibrillar morphology. As a scalable demonstration, we show we can double the 42 4

43

thickness of the active layer and retain 90% of the PCE (compared to conventional thin 45 47

46

films) due to the high hole mobility measured using the space-charge-limited current 49

48

(SCLC) method. 50 51 52 54

53

2. Experimental 56

5

We show the architecture of the device and chemical structure of materials used in 57

60

59

58

the active layer (Figure 1). For the inverted OPV structure, the device configuration is indium tin oxide/ zinc oxide (ITO/ZnO) (40 nm)/PTB7 : PC71BM (80~180 nm)/MoO3 5



1 2 4

3

(15 nm)/Ag (100 nm). ITO-coated glass substrates with a sheet resistance of 10 /sq 5 6

were consecutively cleaned in ultrasonic bath containing detergent, acetone, deionized 7 9

8

water and ethanol for 10 min each step, then dried by nitrogen. Prior to film deposition, 10 12

1

the substrate was treated by UV light for 10 min. The ZnO precursor was prepared by 13 14

dissolving zinc acetate dihydrate (Zn(CH3COO)2·2H2O, Aldrich, 99.9%, 1 g) and 15 17

16

ethanolamine (NH2CH2CH2OH, Aldrich 99.5%, 0.28 g in 2-methoxyethanol 18 20

19

(CH3OCH2CH2OH, Aldrich, 99.8%, 10 ml) under vigorous stirring for 12 hr for the 21 23

2

hydrolysis reaction in air. 40 nm ZnO ETL was spin-casted from the precursor solution 25

24

on top of the clean ITO-glass substrate, and annealed at 200 °C for 1 hr in air. For the 26 28

27

active layer, the mixture of PTB7 : PC71BM at a weight ratio of 1 : 1.5 with a 29 31

30

concentration of 25 mg/ml was dissolved in a mixed solvent of CB/DPE. DPE was 3

32

added to the CB at various concentrations of 1~5 vol. %. The solution was then spin 34 36

35

coated at a rate of 1000 rpm for 60 s in a nitrogen glove box. Subsequently, 15 nm 37 38

MoO3 and 100 nm Ag were finally deposited at a pressure of 310-3 Pa in vacuum. The 39 41

40

anode area is 0.02 cm2 for all devices in this work. 42 4

43

The morphology of active layer was characterized by AFM (MFP-3D-BIO, Asylum 45 47

46

Research) and GI-XRD (RIGAKU, D/MAX-RC). Current density-voltage (J-V) curves 49

48

under illumination were measured with a Keithley 4200 programmable voltage-current 50 52

51

source. A light source integrated with a Xe lamp (CHF-XM35, Beijing Trusttech Co. 53 54

Ltd.) with a power illumination of 100 mW/cm2 was used as a solar simulator. An 57

56

5

Agilent 4294A Precision Impedance Analyzer was employed for Impedance 58 60

59

Spectroscopy (IS) measurements. The range of measured frequency was 40 Hz to 1 6


Page 6 of 27

Page 7 of 27


1 2 4

3

MHz. 50 mV of modulation voltage without DC bias was used to extract DC bias 5 6

dependent AC signal. The thicknesses of films obtained from solution process were 7 9

8

measured with a Dektak 150 stylus profiler. All the measurements were carried out 10 12

1

under ambient conditions without any encapsulation from air. 13 14 16

15

3. Results and discussion 17 19

18

3.1 Performance of OPVs with DPE as solvent additive 20 21 2

To elucidate the effects of DPE on standard PTB7:PC71BM BHJ devices,32-33 we 23 25

24

first limit the film thickness to about 110 nm (110 nm is optimal thickness of 26 28

27

PTB7:PC71BM BHJ with 3% DIO, which has been widely proved) and use the plain, 29 31

30

additive-free sample as the control. The DPE concentration varies from 1 to 5 vol. % 3

32

relative to the CB solvent. We show the J-V characteristics of these OPVs under 1 sun 34 36

35

irradiation (Figure 2 (a), Table 1, and Figure S1). The device without DPE exhibits not 37 38

only a low JSC of 8.0 mA/cm2 but also a poor FF of 33.7%, resulting in a PCE of 1.75%. 39 41

40

The low performance of the control cells are attributed to the unsuitable packing of the 42 4

43

polymer blend and large aggregation of the PC71BM.33 With the addition of DPE, we 45 46

observe a significant improvement in JSC from 8.0 to 13.8 mA/cm2, yielding an 49

48

47

enhancement of PCE from 1.75 to 3.45%.33 Compared to the control, both the JSC and 50 52

51

the FF are enhanced as DPE increases from 1 to 4 vol. %. We obtain the best device 53 5

54

PCE of 5.92% at the optimal DPE loading (4 vol. %), with an open circuit voltage (VOC) 57

56

of 0.69V, JSC of 14.5 mA/cm2 and FF of 47.6%. An enhancement of about 300% in 58 60

59

device performance is enough to demonstrate the efficacy of DPE in PTB7 : PC71BM 7



1 2 4

3

BHJ.2, 33 We note that at an active layer thickness of 110 nm (Figure S1), the best OPV 5 6

PCE with 4% DPE (5.92%) is still 4% lower than the devices with DIO additives 7 9

8

(6.19%) with latter measurement being consistent with the known literature. As far as 10 12

1

we know, PCE will get a dramatically drop when increase the thickness of active layer, 13 14

especially in low bandgap polymer BHJ system with only DIO additive, e.g. 15 17

16

PTB7:PCBM. However, DPE has been reported to be more efficient in the thick active 18 20

19

layer rather than in thin film. In this point, we want to investigate the effect of DPE in 21 23

2

thick active layer through detailed characterization of electric and optical parameters, 25

24

which will be discussed in Section 3.4. Before that, the mechanism of DPE in general 26 28

27

thickness of PTB7:PC71BM film should be figured out at first. 29 31

30

To understand the low series resistance (RS), we fit the data using the single diode 3

32

lumped circuit model.34 We show that the addition of DPE to the active layer (1-4%), 34 36

35

decreases the RS from 3.48 to 1.26 Ωcm2 and corresponds to an increase in both the FF 37 39

38

and the JSC (Table 1). We note that raising the concentration of DPE to 5% results in an 41

40

increased in the RS which lowers the JSC and FF, corresponding to a PCE of 4.62%. This 42 4

43

may due to the poor phase separation in the D/A interface that reduces the percolation 45 46

pathways for transport charge carriers to the relative electrode.2, 35 We show a decrease 49

48

47

in the device photocurrent density (Jph) when the DPE amount increases to 5 vol. % 50 52

51

(Table 1 and Supporting Information). Reduction of the Jph signifies a decrease in the 53 5

54

charge collection efficiency and leads to the dominance of bimolecular recombination, 57

56

corresponding to a lower FF.36-37 58 60

59

We illustrate UV-Vis absorption spectra of the PTB7 : PC71BM blend added with 8


Page 8 of 27

Page 9 of 27


1 2 4

3

DPE (Figure 2 (b)). A domain absorption peak with a shoulder for PTB7 is observed 5 6

between the wavelength range of 600 nm and 750 nm while the absorption from 7 9

8

350-500 nm is mainly attributed to the presence of PC71BM.33, 35, 38 The intensity peak 10 12

1

around 600 nm corresponds to S0-S3 π-π* transition and suggests ordered packing of the 13 14

PTB7 (Figure S2(a)).35 In addition, the stronger absorption intensity at 478 nm indicates 15 17

16

the aggregation of PC71BM, which is different from the conventional DIO, modified 18 20

19

active layer (Figure S2(b)).19, 38 The effect of DPE on light harvesting sheds light on 21 23

2

results obtained from external quantum efficiency (EQE) measurements (Figure 2(c) 25

24

and Figure S3). As expected, the control device without DPE exhibits a low EQE across 26 28

27

the entire visible spectrum. For 1 vol. % addition of DPE, the average EQE enhances 29 31

30

dramatically from 18.2% to 56.8%. OPVs with 4% DPE show the highest average EQE 3

32

of 66.4% around 500-750 nm, resulting in the best JSC of 16.7 mA/cm2 and is consistent 34 36

35

with the results of Jph. 37 39

38

3.2 Morphology characterization of the active layer 40 41 43

42

In general, solvent addition impacts the device performance by altering the morphology 45

4

of active layer. To understand the influence of DPE on the microstructure of the 46 48

47

PTB7:PC71BM films, we characterize the films optical microscopy. We reveal that 50

49

without DPE, large numbers of PC71BM aggregates (about 1 m) are formed in the 53

52

51

devices (Figure S4) . These high density, large PC71BM aggregates not only reduce the 54 56

5

D/A interface area but also lead to the recommendation of charge carriers. 8 With 1% 57

60

59

58

DPE addition, we observe that the density of PC71BM aggregate domain decrease and also the decreased domain size to about 500 nm. As the DPE concentration increases 9



1 2 4

3

from 2 to 4%, small aggregations of PC71BM reappear again as indicated by the 5 6

emergence of darker color spots (Figure S4 (c) to (e)). This implies that the DPE 7 9

8

facilitates the formation of ordered packing of the PTB7 rather than just dispersing the 10 12

1

PC71BM aggregates into smaller units as with DIO.29 This is also consistent with the 13 14

increased absorption peak at 450 nm (Figure 2(b)). However, those dots seems 15 17

16

disappear at high concentration of DPE at 5% loading (Figure S4 (f)). This may due to 18 20

19

the formation of ordered packing of PTB7 on the surface that “press” the PCBM 21 23

2

towards the bottom, so that these PCBM dots is hard to detect in this situation. 25

24

To probe the nanostructure features of the BHJ films, we employ AFM (Figure 3). The 26 28

27

active layer without DPE exhibits a more coarse and spiky appearance, with root mean 29 31

30

square (RMS) roughness of 2.72 nm (Figure 3 (a) and (c)). However, when loaded with 3

32

1% DPE, the roughness of the active layer significantly decreases to 0.94 nm (Figure 34 36

35

S5). With the addition of DPE, the morphology changes from rough, spiky features to a 37 39

38

substantially smoother surface. The changes in morphology are less drastic between the 41

40

films with 1 to 5 vol. % DPE concentration; the RMS roughness values are 0.94, 1.00, 42 4

43

1.09, 1.09 and 1.18 nm, respectively, (Figure S5). We speculate that in comparison to 45 47

46

DIO, the DPE blended films present a nano-fibrilla like pattern, indicative of PTB7 49

48

lamellar formation (Figure 3 (b) and (d)). In general, DPE induces planar and 50 52

51

intermixed polymer-fullerene network that has been associated to efficient charge 53 54

transport, enabling the high PCEs.39-41 57

56

5

To investigate the orientation changes in the bulk, we utilize GI-XRD to further 58 60

59

characterize the packing condition in PTB7:PC71BM BHJ (Figure 4). The BHJ of 10


Page 10 of 27

Page 11 of 27


1 2 4

3

PTB7:PC71BM with or without DIO shares the same peak in 2=22o, with an intensity 5 6

of 350. This result agrees with conclusions in earlier investigations, that DIO does not 7 9

8

change the crystallinity of PTB7.22, 24, 42 However, the BHJ of PTB7:PC71BM with 4% 10 12

1

DPE shows a peak intensity increase of about 450 in the 2=19.8o, implying that the 13 14

DPE indeed facilitates ordered packing of the PTB7. These results correlate well with 15 17

16

the formation of a cross-link D/A network, resulting in the improvement of Jsc and 18 20

19

FF.43-44 To further confirm the formation of cross-link D/A network, we use CB to wash 21 23

2

the film. The blended films with 1 to 2 vol. % of DPE can be easily washed out, 25

24

indicating a weak interaction between the donor and acceptor (Figure S6 (a) to (c)). 26 28

27

However, as the DPE concentration goes above 2%, the blended film develops into and 29 31

30

becomes darker in color and more difficult to wash (Figure S6 (d) to (f)). This 3

32

observation suggests that the formation of a cross-link D/A network can provide a good 34 36

35

pathway for charge transport and suppresses large PC71BM aggregation.7 37 39

38

3.3. Equivalent circuit model and impedance analysis 40 41 43

42

In order to examine the carrier dynamics in our devices, we show the impedance 45

4

spectroscopy results with equivalent circuit modeled (Figure 5 (a)). We illustrate 46 48

47

Cole-Cole plots, or the characteristic retarded AC electrical response through the device 49 51

50

in the frequency range of 40 Hz to 1 MHz (Figure 5 (b)). We show two distinct regions: 53

52

a large semicircle at low frequencies associated to the carrier recombination process, 54 56

5

and a quasi straight line in the high frequency region containing information about the 57

60

59

58

diffusion mechanism.45 Real impedances Z′ of OPVs decrease significantly with the addition of DPE, which is related to the increased JSC.46 In this circuit model, the 11



Page 12 of 27

1 2 4

3

constant phase element (CPE) suggests a non-ideal behavior of the capacitor. CPE is 5 6

often used to represent a capacitance-like element to compensate for inhomogeneity in 7 9

8

the interface. CPE is defined by two values, CPE-T and CPE-P. CPE-T is capacitance 10 12

1

and CPE-P is a nonhomogeneity constant. If CPE-P equals 1, then the CPE is identical 13 14

to an ideal capacitor without defects and/or grain boundary.47 With the increase of DPE 15 17

16

concentration from 2% to 4%, CPE1-P increases from 0.87 to 0.99 (Table S1), which 18 20

19

indicates the interface capacitance between PTB7 and PCBM is electrically ideal and in 21 2

a more homogeneous manner. Simultaneously, CPE1-T increases from 2.110-9 to 23 25

24

2.410-8, which indicates that the large active layer surface provides more potential 26 28

27

pathways for charge transportation. However, as the DPE concentration increases to 5 29 31

30

vol. %, both the CPE1-T and CPE1-P begin to decrease, which means there is an 3

32

increase in the formation of grain boundary defects that lead to charge recombination. 34 36

35

R1 in Figure 5 (a) corresponds to the active layer resistance, and the shunt pair R2 and C2 37 39

38

in Figure 5 (a) is associated to the two electrical contacts of the interfaces between the 41

40

active layer and electrodes.41 A high device R2 with DPE 1% suggests that the interface 42 4

43

between the active layer and the buffer layer is not efficient for charge transport due to 45 47

46

the amorphous polymer ordering. For devices with 4% DPE, we observe that the R2 49

48

decreases to 3.0105 cm2, revealing that the loss due to interfacial resistance between 50 52

51

the active layer and the MoO3 is minimized at optimal DPE loading and thus contributes 54

53

to better charge transport performance. The average of the carrier transition time (avg), 57

56

5

in the active layer is defined by Equation (Eq.) 1: 58 60

59

avg = R1∙CPE-T 12


(1)

Page 13 of 27


1 2 3

The avg are estimated to be 4.5, 13.5, 37.1, and 27.2 s respectively for devices with 0, 6

5

4

2, 4, and 5% vol. DPE (Table S1). Longer avg is associated to a lower recombination 7 9

8

rate and hence higher possibility for carriers to reach the electrode.46 Our results suggest 10 12

1

that the addition of DPE suppresses the charge recombination significantly in the bulk 14

13

film and prolong the carrier lifetime by about nine-fold, from 4.5 s (devices without 15 17

16

DPE) to 37.1 s (DPE 4%). 18 19 21

20

3.4 Potential of DPE in industrial fabrication of thick OPVs 2 24

23

In the following section, we study the efficacy of DPE in thick OPVs (greater than 100 25 27

26

nm). Since there is ample evidence showing that 4% DPE is the optimal loading, we 29

28

fixate this additive concentration and measure the photovoltaic performance of thick 30 32

31

films under 1sun illumination (Figure 6 (a) and Table 2). To showcase the DPE potency 3 35

34

in this scenario, we vary the thickness of active layer from 80 nm to 180 nm every 20 37

36

nm. We demonstrate that DPE outperforms the conventional DIO as an effective 38 40

39

additive for devices with an active layer thickness of 180 nm. Even though the BHJ 41 43

42

thickness is doubled, the addition of 4% DPE allows devices to maintain 90% of the 45

4

PCE. In comparison, the PCE of DIO modified OPVs drops considerably by 66% with 46 48

47

respect to the 100 nm thick control. Overall, the DPE-modified OPVs possess a similar 49 51

50

VOC and JSC when the thickness of the active layer increases from 80 to 180 nm. 53

52

However, the best FF of the device is obtained at 120~140 nm, which can be ascribed to 54 56

5

the improved ordering of PTB7. Further increases of the BHJ film thickness over 160 57

60

59

58

nm compromises the FF to 47%. Although thicker BHJ films can absorb more light for photocurrent generation, adverse effects such as an increase in bimolecular 13



Page 14 of 27

1 2 4

3

recombination induced by low charge-carrier mobility of the BHJ components reduces 5 6

the FF.40 Hence, we deduce that the recombination loss starts outweighing the benefits 7 9

8

in devices with thickness above 200 nm. 10 12

1

In order to substantiate our notion that DPE assists long distance charge transfer in a 13 14

thick BHJ, we measure the hole mobility using a space-charge limited current (SCLC) 15 17

16

model with a configuration of ITO/MoO3 (15 nm)/PTB7 : PC71BM (80~180 nm)/MoO3 18 20

19

(15 nm)/Ag (100 nm) based on Mott-Gurney law as Eq. (2) 21 2

9 V2 J   0  3 8 d 25

24

23

(2)

26

where μ is the charge carrier mobility, and ε ≈ 3 is the relative dielectric constant of 27 29

28

organic film. ε0 is the vacuum dielectric constant of 8.85 × 10-12 F/m, and d is the film 30 32

31

thickness.48 The detailed J-V characteristics of the hole-only devices are presented in 3 35

34

Figure S7. The hole mobilities of OPVs based on 4% DPE with the BHJ film thickness 37

36

from 80 to 180 nm (every 20 nm) are 1.2810-5, 1.7210-5, 2.8810-5, 7.1410-5, 38 40

39

4.2710-5 and 3.6810-5 cm2V-1s-1, respectively, (Figure 6 (b)). These results indicate 41 43

42

that the hole mobility increase leads to a more balanced charge transport, resulting in 45

4

the enhancement of the FF at 120 nm. Although 140 nm owes the highest hole 46 48

47

mobilities, the increased Rs limits the PCE. We calculate the extraction time (Tex) is 49 51

50

quantified to represent the average time carriers are extracted at d/2 and by using the 53

52

relationship Tex=d/2E, in which E is electric field and d is the thickness of the active 54 56

5

layer (Table 3).46 . We observe that the short Tex of 4.9 s in the thick films (180 nm) 57

60

59

58

reassures that charge carriers in the DPE-enhanced films overcome interfacial recombination and can thus be efficiently extracted by the electrodes.49 14


Page 15 of 27


1 2 4

3

4. Conclusion. 5 6

In summary, we demonstrate donor-selective solvent additive DPE is effectively to 7 9

8

increase the active layer thickness in low-bandgap polymer based OPVs, and a 3-fold 10 12

1

PCE enhancement is obtained (from 2% to 6%). Through detailed characterization of 13 14

the morphology and electric parameters, we reveal that DPE (i) promotes the formation 15 17

16

of nano-fibrilla structure in the active layer, (ii) induces efficient charge transport 18 20

19

pathways, and (iii) suppresses bimolecular recombination. Our results highlight the 21 23

2

tremendous potential of DPE to shift the paradigm of research and industrial focus 25

24

towards thick OPVs, and pave the way for the realization of scalable commercialization. 26 27 28 29 30

Supporting Information 3

32

31

Parameters employed for the fitting of the impedance spectra by use of an 34 36

35

equivalent circuit model; figures of J-V curves, EQE, UV-Vis absorption 37 38

for devices based on variety DPE ratio; morphology images of devices, 41

40

39

including AFM, metallurgical microscope, and optical images; J-V curves 42 4

43

of hole only devices by using SCLC method; and calculation method for 45 47

46

photogeneration current density in this work. 48 49 50 52

51

Acknowledgement 53 5

54

This research was funded by the Foundation of the National Natural Science Foundation 57

56

of China (NSFC) (Grant No. 61177032), and the Foundation for Innovation Research 58 60

59

Groups of the NSFC (Grant No. 61421002). The authors gratefully acknowledge the 15



Page 16 of 27

1 2 4

3

National 5

Science

Foundation

(DMR-1410171)

and

NSF-PECASE

award

6

(CBET-0954985), for partial support of this work. The Yale Institute for Nanoscience 7 9

8

and Quantum Engineering (YINQE) and NSF MRSEC DMR 1119826 (CRISP) 10 12

1

provided facility support. 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

16


Page 17 of 27


1 2 4

3

References

60

59

58

57

56

5

54

53

52

51

50

49

48

47

46

45

4

43

42

41

40

39

38

37

36

35

34

3

32

31

30

29

28

27

26

25

24

23

2

21

20

19

18

17

16

15

14

13

12

1

10

9

8

7

6

5

1. Yang, Y. M.; Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.; Yang, Y., High-Performance Multiple-Donor Bulk Heterojunction Solar Cells. Nature Photon. 2015, 9 (3), 190-198. 2. Yu, J.; Zheng, Y.; Huang, J., Towards High Performance Organic Photovoltaic Cells: A Review Of Recent Development In Organic Photovoltaics. Polymers 2014, 6 (9), 2473-2509. 3. Bundgaard, E.; Krebs, F. C., Low Band Gap Polymers For Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2007, 91 (11), 954-985. 4. Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L., Ternary Blend Polymer Solar Cells With Enhanced Power Conversion Efficiency. Nature Photon. 2014, 8 (9), 716-722. 5. Jung, J. W.; Jo, W. H., Low-Bandgap Small Molecules As Non-Fullerene Electron Acceptors Composed Of Benzothiadiazole And Diketopyrrolopyrrole For All Organic Solar Cells. Chem. Mater. 2015, 27 (17), 6038-6043. 6. Huang, J.-S.; Goh, T.; Li, X.; Sfeir, M. Y.; Bielinski, E. A.; Tomasulo, S.; Lee, M. L.; Hazari, N.; Taylor, A. D., Polymer Bulk Heterojunction Solar Cells Employing Forster Resonance Energy Transfer. Nature Photon. 2013, 7 (6), 479-485. 7. Zheng, Y.; Li, S.; Zheng, D.; Yu, J., Effects Of Different Polar Solvents For Solvent Vapor Annealing Treatment On The Performance Of Polymer Solar Cells. Org. Electron. 2014, 15 (11), 2647-2653. 8. Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H., Efficient Organic Solar Cells Processed From Hydrocarbon Solvents. Nature Energy 2016, 1 (2), 15027. 9. Graham, K. R.; Wieruszewski, P. M.; Stalder, R.; Hartel, M. J.; Mei, J.; So, F.; Reynolds, J. R., Improved Performance Of Molecular Bulk ‐ Heterojunction Photovoltaic Cells Through Predictable Selection Of Solvent Additives. Adv. Funct. Mater. 2012, 22 (22), 4801-4813. 10. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J., Polymer Photovoltiac Cells: Enhanced Efficiencies Via A Network Of Internal Donor-Acceptor Heterojunctions. Science 1995, 270 (5243), 1789. 11. Halls, J.; Walsh, C.; Greenham, N.; Marseglia, E.; Friend, R.; Moratti, S.; Holmes, A., Efficient Photodiodes From Interpenetrating Polymer Networks. 1995. 12. Wang, H.; Zheng, Y.; Zhang, L.; Yu, J., Effect Of Two-Step Annealing On The Performance Of Ternary Polymer Solar Cells Based On P3HT: PC71BM: SQ. Sol. Energy Mater. Sol. Cells 2014, 128, 215-220. 13. Li, G.; Zhu, R.; Yang, Y., Polymer Solar Cells. Nature Photon. 2012, 6 (3), 153-161. 14. Zheng, Y.; Wu, R.; Shi, W.; Guan, Z.; Yu, J., Effect Of In Situ Annealing On The Performance Of Spray Coated Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2013, 111, 200-205. 15. Wang, F.; Chen, Z.; Xiao, L.; Qu, B.; Gong, Q., Papery Solar Cells Based On Dielectric/Metal Hybrid Transparent Cathode. Sol. Energy Mater. Sol. Cells 2010, 94 (7), 1270-1274. 16. Zhang, L.; Xing, X.; Chen, Z.; Xiao, L.; Qu, B.; Gong, Q., Highly Efficient 17



1

60

59

58

57

56

5

54

53

52

51

50

49

48

47

46

45

4

43

42

41

40

39

38

37

36

35

34

3

32

31

30

29

28

27

26

25

24

23

2

21

20

19

18

17

16

15

14

13

12

1

10

9

8

7

6

5

4

3

2

Polymer Solar Cells By Using The Homogeneous Self‐Assembly Of A Sulphydryl‐ Capped Photoactive Polymer Covalently Bound To The Anode. Energy Technol 2013, 1 (10), 613-616. 17. Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., High-Efficiency Solution Processable Polymer Photovoltaic Cells By Self-Organization Of Polymer Blends. Nature Mater. 2005, 4 (11), 864-868. 18. Chen, H.; Hu, S.; Zang, H.; Hu, B.; Dadmun, M., Precise Structural Development And Its Correlation To Function In Conjugated Polymer: Fullerene Thin Films By Controlled Solvent Annealing. Adv. Funct. Mater. 2013, 23 (13), 1701-1710. 19. Hwang, I.-W.; Kong, J.; Yoo, H. K.; Lee, K., Improved Carrier Dynamics And High Solar Cell Performance In Postadditive-Soaked PTB7: PC71BM Bulk Heterojunction Materials. J. Phys. Chem. C 2015, 119 (23), 12896-12903. 20. Salim, T.; Wong, L. H.; Bräuer, B.; Kukreja, R.; Foo, Y. L.; Bao, Z.; Lam, Y. M., Solvent Additives And Their Effects On Blend Morphologies Of Bulk Heterojunctions. J. Mater. Chem. 2011, 21 (1), 242-250. 21. Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C., Efficiency Enhancement In Low-Bandgap Polymer Solar Cells By Processing With Alkane Dithiols. Nature Mater. 2007, 6 (7), 497-500. 22. Guo, X.; Cui, C.; Zhang, M.; Huo, L.; Huang, Y.; Hou, J.; Li, Y., High Efficiency Polymer Solar Cells Based On Poly (3-Hexylthiophene)/Indene-C 70 Bisadduct With Solvent Additive. Energy Environ. Sci. 2012, 5 (7), 7943-7949. 23. Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y., Effects Of Solvent Mixtures On The Nanoscale Phase Separation In Polymer Solar Cells. Adv. Funct. Mater. 2008, 18 (12), 1783-1789. 24. Kniepert, J.; Lange, I.; Heidbrink, J.; Kurpiers, J.; Brenner, T. J.; Koster, L. J. A.; Neher, D., Effect Of Solvent Additive On Generation, Recombination, And Extraction In PTB7: PCBM Solar Cells: A Conclusive Experimental And Numerical Simulation Study. J. Phys. Chem. C 2015, 119 (15), 8310-8320. 25. Gupta, V.; Bharti, V.; Kumar, M.; Chand, S.; Heeger, A. J., Polymer–Polymer Förster Resonance Energy Transfer Significantly Boosts The Power Conversion Efficiency Of Bulk‐Heterojunction Solar Cells. Adv. Mater. 2015, 27 (30), 4398-4404. 26. Gu, Y.; Wang, C.; Russell, T. P., Multi ‐ Length ‐ Scale Morphologies In PCPDTBT/PCBM Bulk‐Heterojunction Solar Cells. Adv. Energy Mater. 2012, 2 (6), 683-690. 27. Chang, L.; Lademann, H. W.; Bonekamp, J. B.; Meerholz, K.; Moulé, A. J., Effect Of Trace Solvent On The Morphology Of P3HT: PCBM Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2011, 21 (10), 1779-1787. 28. Ye, L.; Jing, Y.; Guo, X.; Sun, H.; Zhang, S.; Zhang, M.; Huo, L.; Hou, J., Remove The Residual Additives Toward Enhanced Efficiency With Higher Reproducibility In Polymer Solar Cells. J. Phys. Chem. C 2013, 117 (29), 14920-14928. 29. Kim, W.; Kim, J. K.; Kim, E.; Ahn, T. K.; Wang, D. H.; Park, J. H., Conflicted Effects Of A Solvent Additive On PTB7: PC71BM Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2015, 119 (11), 5954-5961. 30. Choi, H.; Ko, S. J.; Kim, T.; Morin, P. O.; Walker, B.; Lee, B. H.; Leclerc, M.; Kim, 18


Page 18 of 27

Page 19 of 27


1

60

59

58

57

56

5

54

53

52

51

50

49

48

47

46

45

4

43

42

41

40

39

38

37

36

35

34

3

32

31

30

29

28

27

26

25

24

23

2

21

20

19

18

17

16

15

14

13

12

1

10

9

8

7

6

5

4

3

2

J. Y.; Heeger, A. J., Small‐Bandgap Polymer Solar Cells With Unprecedented Short‐ Circuit Current Density And High Fill Factor. Adv. Mater. 2015, 27 (21), 3318-3324. 31. Nguyen, T. L.; Choi, H.; Ko, S.-J.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J.-E.; Yun, M. H.; Shin, T.; Hwang, S., Semi-Crystalline Photovoltaic Polymers With Efficiency Exceeding 9% In A 300 Nm Thick Conventional Single-Cell Device. Energy Environ. Sci. 2014, 7 (9), 3040-3051. 32. 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 (40), 4636-4643. 33. Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L., For The Bright Future—Bulk Heterojunction Polymer Solar Cells With Power Conversion Efficiency Of 7.4%. Adv. Mater. 2010, 22 (20). 34. Bouzidi, K.; Chegaar, M.; Bouhemadou, A., Solar Cells Parameters Evaluation Considering The Series And Shunt Resistance. Sol. Energy Mater. Sol. Cells 2007, 91 (18), 1647-1651. 35. Lu, L.; Yu, L., Understanding Low Bandgap Polymer PTB7 And Optimizing Polymer Solar Cells Based On It. Adv. Mater. 2014, 26 (26), 4413-4430. 36. Kim, Y. J.; Ahn, S.; Wang, D. H.; Park, C. E., A Mechanistic Understanding Of A Binary Additive System To Synergistically Boost Efficiency In All-Polymer Solar Cells. Sci. Rep. 2015, 5. 37. Proctor, C. M.; Nguyen, T.-Q., Effect Of Leakage Current And Shunt Resistance On The Light Intensity Dependence Of Organic Solar Cells. Appl. Phys. Lett. 2015, 106 (8), 083301. 38. Guo, S.; Ning, J.; Körstgens, V.; Yao, Y.; Herzig, E. M.; Roth, S. V.; Müller-Buschbaum, P., The Effect Of Fluorination In Manipulating The Nanomorphology In PTB7:PC71BM Bulk Heterojunction Systems. Adv. Energy Mater. 2015, 5 (4), n/a-n/a. 39. Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y., Single‐Junction Organic Solar Cells Based On A Novel Wide‐Bandgap Polymer With Efficiency Of 9.7%. Adv. Mater. 2015, 27 (18), 2938-2944. 40. Hendriks, K. H.; Heintges, G. H.; Gevaerts, V. S.; Wienk, M. M.; Janssen, R. A., High ‐ Molecular ‐ Weight Regular Alternating Diketopyrrolopyrrole ‐ Based Terpolymers For Efficient Organic Solar Cells. Angew. Chem. Int. Ed. 2013, 52 (32), 8341-8344. 41. Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. C.; Wienk, M. M.; Janssen, R. A., Universal Correlation Between Fibril Width And Quantum Efficiency In Diketopyrrolopyrrole-Based Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135 (50), 18942-18948. 42. Guo, S.; Herzig, E. M.; Naumann, A.; Tainter, G.; Perlich, J.; Muller-Buschbaum, P., Influence Of Solvent And Solvent Additive On The Morphology Of PTB7 Films Probed Via X-Ray Scattering. J. Phys. Chem. B 2014, 118 (1), 344-50. 43. Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H., Absolute Measurement Of Domain Composition And Nanoscale Size Distribution Explains Performance In PTB7: PC71BM Solar Cells. Adv. Energy Mater. 2013, 3 (1), 19



1 3

2

65-74. 44. Park, S.-J.; Lee, K.-Y.; Kim, D.-H.; Shin, M.-H.; Kim, Y.-J., Improvement Of Electrical Conductivity For High-Performance Organic Solar Cells By Multi-Temperature Solvent Annealing. Jpn. J. Appl. Phys. 2015, 54 (4S), 04DK07. 45. Arredondo, B.; Romero, B.; Del Pozo, G.; Sessler, M.; Veit, C.; Würfel, U., Impedance Spectroscopy Analysis Of Small Molecule Solution Processed Organic Solar Cell. Sol. Energy Mater. Sol. Cells 2014, 128, 351-356. 46. Yao, E.-P.; Chen, C.-C.; Gao, J.; Liu, Y.; Chen, Q.; Cai, M.; Hsu, W.-C.; Hong, Z.; Li, G.; Yang, Y., The Study Of Solvent Additive Effects In Efficient Polymer Photovoltaics Via Impedance Spectroscopy. Sol. Energy Mater. Sol. Cells 2014, 130, 20-26. 47. Hess, K., Advanced Theory Of Semiconductor Devices. Wiley-IEEE Press: 2000. 48. Mihailetchi, V.; Wildeman, J.; Blom, P., Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94 (12), 126602. 49. Gasparini, N.; Righi, S.; Tinti, F.; Savoini, A.; Cominetti, A.; Po, R.; Camaioni, N., Neat C70-Based Bulk-Heterojunction Polymer Solar Cells With Excellent Acceptor Dispersion. ACS Appl. Mater. Interfaces 2014, 6 (23), 21416-21425. 25

24

23

2

21

20

19

18

17

16

15

14

13

12

1

10

9

8

7

6

5

4

26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

20


Page 20 of 27

Page 21 of 27


1 2 4

3

Figures 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 26

25

Figure 1. Device architecture and active layer materials. 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 45

4

Figure 2. (a)J-V characteristic curves of OPVs under AM 1.5 simulated 1 sun illumination, (b) UV-Vis absorption spectra of PTB7:PC71BM BHJ film and (c) EQE spectra of devices with different ratios of solvent additives. 49

48

47

46

50 51 52 53 54 5 56 57 58 59 60

21



1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 29

28

Figure 3. AFM 3D exhibition (a) and (b), and AFM phase image (c) and (d) of PTB7:PC71BM BHJ blend films without and with DPE. 31

30 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 58

57

Figure 4. GIXRD image of active layer with or without solvent additive 59 60

22


Page 22 of 27

Page 23 of 27


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 23

2

Figure 5. (a) The equivalent circuit model of the devices (b) Cole–Cole plots of the devices with different concentrations of DPE. 25

24 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 42

41

Figure 6. (a) Photovoltaics parameters and (b) extraction time and hole mobility of OPVs based on 4% DPE with variety BHJ thickness. 4

43 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

23



Page 24 of 27

1 2 3 4

Tables 5 6 7 8 10

9

Table 1 Summary of photovoltaic performance based on PTB7 : PC71BM BHJ with 12

1

different concentrations of DPE. 13 14 15 16 17

b,c

w/o DPE DPE 1%c DPE 2%c DPE 3%c DPE 4%c DPE 5%c DIO 3%b,c 26

25

24

23

2

21

20

19

18

VOCa (V)

JSCa (mA/cm2)

FFa (%)

PCEa (%)

0.650.02 0.660.01 0.650.02 0.690.02 0.690.02 0.690.01 0.680.02

8.00.5 13.80.5 15.00.6 16.00.5 16.70.5 14.50.6 15.80.3

33.71.5 38.01.0 43.81.6 49.92.0 51.62.1 47.61.3 57.33.0

1.750.30 3.450.28 4.300.44 5.570.51 5.920.64 4.620.55 6.190.61

RS

RP 2

(cm ) 3.48 3.02 2.65 2.11 1.26 1.88 1.77

a All

2

(cm ) 111.60 90.66 93.79 125.55 127.96 107.73 178.19

JPh (mA/cm2) 7.83 12.97 14.28 15.40 16.04 14.08 15.21

the photovoltaic parameters are the average of a batch of six devices. The related low PCE compared with reference [33] is because of the quality of this batch PTB7 polymer and without the use of filter to eliminate the large PC71BM aggregation. c All the active layer were spin coated follow the same recipe, resulting in the similar thickness of ~110 nm. 27

32

31

30

29

28

b

3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

24


Page 25 of 27


1 2 3 5

4

Table 2 Summary of best photovoltaic performance with a variety of active layer 7

6

thickness. 8

Thicknessa (nm)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

12

805

0.71

15.79

48.2

5.30

(cm ) 6.35

(cm ) 165.02

13

1005

0.72

16.64

48.3

5.65

1.72

173.62

15.20

1205

0.72

15.69

53.2

6.05

0.65

205.48

14.48

1405

0.72

15.48

52.7

5.83

1.83

213.59

14.24

16010

0.72

16.32

49.2

5.76

1.47

159.78

15.07

9

Solvent additive 1

10

15

14 16

DPE 4% 18

17

RS

RP 2

2

JPh (mA/cm2) 14.33

0.71 15.79 49.0 5.46 3.71 168.17 14.64 18010 0.65 13.11 54.0 4.60 9.85 285.62 13.49 18010 a The thickness was characterized by step profiler, which exists an error of about 5 to 10 nm.

20

19

2

21

DIO 3% 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

25



Page 26 of 27

1 2 3 5

4

Table 3 Hole mobility and charge extraction time in PTB7 : PC71BM BHJ blend film 7

6

with 4% DPE at different thickness

1

10

9

8

Thickness of BHJ film (nm)

Hole mobility

80 100 120 140 160 180

1.2810-5

12 13 14 15 16 17 19

18

-1 -1

(cm2V s ) 1.7210-5 2.8810-5 7.1410-5 4.2710-5 3.6810-5

Texa (s) 6.3 5.8 4.2 1.9 3.7 4.9

values of E is approximated to be 5 × 104 V-cm−1 obtained at forward bias 0.5 V across 100 nm thick film which is close to the net potential at the maximum power point of the cells. a The

21

20 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

26


Page 27 of 27


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 28

27 Graphical Abstrate 280x174mm (150 x 150 DPI)

29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60


Toward Efficient Thick Active PTB7 Photovoltaic ... - ACS Publications

Recommend Documents