Tetraphenylphosphonium Bromide as a Cathode Buffer Layer Material

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Tetraphenylphosphonium Bromide as Cathode Buffer Layer Material for Highly Efficient Polymer Solar Cells Monika Gupta, Dong Yan, Jianzhong Xu, Jiannian Yao, and Chuanlang Zhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17870 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Materials & Interfaces

Tetraphenylphosphonium Bromide as Cathode Buffer Layer Material for Highly Efficient Polymer Solar Cells Monika Guptaa, Dong Yana, Jianzhong Xub*, Jiannian Yaoa and Chuanlang Zhana* iD a

Beijing National Laboratory for Molecular Sciences, CAS key Laboratory of Photochemistry,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 P.R. China b

College of Chemistry and Environmental Science, Hebei University, Baoding, Baoding 071002,

Hebei province, P.R. China. Key words: Bulk-heterojunction, Solar Cell, Interfacial engineering, Solution-processible, Alcohol Soluble, Tetraphenylphosphonium Bromide

ABSTRACT Here, we introduced the role of small organic molecule tetraphenylphosphonium bromide (QPhPBr) as Electron Transporting Layer (ETL) material for fabricating high-efficiency bulk heterojunction (BHJ) polymer solar cells (PSCs). Their significantly influenced higher performance of power conversion efficiency (PCE) in well-known active layer devices (PTB7Th:PC71BM, PBDTTT-CT:PC71BM and P3HT:PC71BM) were observed compared to bare Al cathode. As well as use of N719 as ETL was also demonstrated. Observed data reveals that QPhPBr-based devices exhibit high PCEs up to 9.18%, 8.42% and 4.81% from PTB7-Th, PBDTTT-CT and P3HT, respectively. For comparisons, the bare Al devices show PCEs of

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5.37%, 4.75% and 3.01%, respectively. Moreover further enhancement of PSCs efficiency (9.83%, 8.69% and 5.35%) is achieved from mixed binary solution of N719:QPhPBr due to modulated adjustment of work function of Al electrode. Our results indicate the excellent function of tetraphenylphosphonium bromide and its binary blend as effective small-molecule organic materials to regulate the metal surface properties and the potential used as excellent cathode buffer layer materials for realizing high-efficiency PSCs.

INTRODUCTION Nowadays bulk heterojunction (BHJ) organic solar cells (OSCs) are very popular and demanding processed technology in the field of renewable energy generation.1-4 The choice of OSCs is more convenient than others due to their several attractive properties such as mechanical-flexibility, light-weight, low-cost, and semi-transparency plus the achievements in the large-area production, for example, via the roll-to-roll (R2R) printing technique which allows for using very little amount of material for the fabrication into large-scale flexible devices with potentials for future commercial market.5-10 Power Conversion Efficiency (PCE) with the solution-processed technique now raises 10-12%11-23 for single junction devices with concern of new active layer materials development. However, in addition to the development of new photovoltaic materials,24-26 comprehensive understanding of interface engineering process is also required.27-29 It is generally believed that the active-cathode interface requires excellent ohmic contact to eliminate the space charge accumulation. In this regard the role of cathode buffer layer plays a dominant role to ensure both the proper adjustment of work function (φ) of metal cathode and large electron conductivity to offer excellent active-cathode ohmic contact to achieve both efficient electron extraction and


2

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transport processes.30-31 Moreover the use of appropriate cathode buffer layer is not only helpful to carry out the energy level alignments between the LUMO of acceptor and conduction band of metal cathode, and even also possess a significantly deep HOMO for blocking the mobile holes. To achieve this goal, modifying the active-cathode interface is a very much important step for fabricating high-efficiency OSCs. Earlier thermally deposition of LiF32-33 or Ca6, 34-35 before Al cathode was commonly used to improve PCE performance. However their high vacuum deposition process and to achieve very thin film6 are very difficult for large-scale processing technique. Also the high sensitivity towards air and moisture of the Ca layer largely limits the device stability.5 So far, several types of cathode buffer layer materials have been developed, including inorganic metal oxides (e.g., ZnO and TiOx)36-40 and water/alcohol soluble organic/polymeric materials41-59 which have been used to fabricate cathode buffer layer for highperformance single-junction22, 60-61 or tandem62-63 OSCs. Nevertheless, many of these materials require relatively complicated synthetic processes which inevitably lead to the cost increase. While, the future commercial demand requires the use of low-cost but high-performance cathode interlayer materials. In this contribution we introduce a new small organic molecule to act as cathode interfacial layer, e.g., electron transporting layer (ETL) material named QPhPBr (tetraphenylphosphonium bromide) with molecular weight of 419. The small molecule consists of four benzene rings, covalently bonded with phosphate atom and has bromide anion in the vicinity. In addition, we also utilized another well-known dye sensitizer N719 as well as their binary mixture as the ETL. It has been observed that binary mixture with N71964 as ETL has more probability to form more complexed ion exchange to increase electron extraction from interface which become helpful to improve charge extraction and transport to cathode. The main advantage to select QPhPBr


3


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molecule as the ETL material is its high solubility in methanol due to its ionic nature. Such solution of ETL material allows for the fabrication of ETL and active layer with orthogonal solvents, which is greatly helpful to avoid damage of surface of already spin coated organic active layer. Triarylphosphine is the most widely used phosphorous compound and it was used as ETL material in 2008.65 However, due to the presence of the lone pair of electrons,66 the phosphorous atom can be oxidized. Upon oxidization, the chemical stability of the compound is improved, and importantly, the lower-lying lowest unoccupied molecular orbit (LUMO) level becomes more suitable for the electron injection. As consequences, many of phosphine oxides have been synthesized and used as ETL materials in organic light-emitting diode (OLEDs)67

68

with one of them used in OSCs in 2014.46 Herein, we for the first time use tetraarylphosphonium halide as ETL material. Despite of its ionic nature and excellent solution-processibility using orthogonal solvent, long duration stability, easy to synthesize and purify in bulk, cheap in cost as well as excellent film-forming ability made it suitable substrate to utilize as ETL material in PSCs study.

RESULTS AND DISCUSSION To examine the properties of three cathode interfacial layer materialss such as QPhPBr, N719 and their binary mixture, three different well known polymer donor PTB7-Th, PBDTTT-CT and P3HT with fullerene acceptor PC71BM as blend active layer were chosen in this study. Figure 1 gives the chemical structures of all polymer donors, PCBM acceptor and ETL materials and their energy levels diagram with the energies of PTB7-Th, PBDTTT-CT, P3HT, PC71BM and N719 taken from the reported values.57-58, 69. Optimized weight ratio was used for combined mixed solution of N719 and QPhPBr. All the photovoltaic measurements were demonstrated using conventional device structure as ITO/PEDOT:PSS/active layer/ETL/Al. Two control device


4

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structures without ETL ITO/PEDOT:PSS/active layer/Al and with the active layer surface treated by pure methanol ITO/PEDOT:PSS/active layer/methanol/Al were also performed for comparative studies. ETL

a) PTB7-Th

C4H9

C4H9

C2H5

O

c) C6H13

C2H5

C8H17

O

O

C6H13

P3HT

PBDTTT-C-T

C4H9

*

C2H5

S

*

-2.58

S S

-2.94

F

O

-4.94 -5.17 -5.11 S

Al -4.3

-5.34

-5.87

S C6H13

QPPBr QPhPBr

S

S

S

-3.91 PC71BM

* S

O

N719

*

*

PTB7-Th

PC71BM

S *

P3HT

S S

-3.01

-3.25 -3.59

S

PBDTTT-CT

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


C4H9

-6.77

C2H5

C8H17

b)

d) O

- +

O

NBu4

N719

OH

QPhPBr

Al ETL

e-

O NCS

P Br

Ru

e+

NCS HO

PEDOT:PSS ITO

O

O

O NBu4

Figure 1. Chemical Structures (a and b) and Energy Levels (c) of Donor: PTB7-Th, PBDTTTCT, P3HT, Acceptor: PC71BM (a) and Electron Transporting Layer Material: N719 and QPhPBr (b). (d) A Schematic Device Architecture Model Used in This Study. UV-Vis Absorption Study Absorption spectrum of ETL materials QPhPBr and N719 on film were measured and shown in Figure 2a. QPhPBr have no absorption in visible region. Peak maxima occurs at 297nm which is below 300nm, quantifying them as perfect material for ETL due to no interference of absorption with active layer during light illumination. On the other hand dye sensitizer N719


5


shows good absorption in the range of 300-700nm with peak maxima at 557, 406, 317nm, indicating that the small portion of that material has possibility to take part into illuminated light absorption with active layer. QPhPBr film displayed an optical band gap (Egopt) of 4.19eV. Its HOMO/LUMO energy was calculated from UPS data (Supporting Information Table S2) and observed as −6.77/−2.58 (eV), respectively (Figure 2b, Table 1). Whereas HOMO/LUMO energy and Egopt of the well-known dye N719 was reported69 as −5.34/−3.01 and 2.33eV, respectively, shown in Table 1. The deep HOMO of QPhPBr is helpful for blocking the mobile holes accessible towards the metal cathode, which makes QPhPBr as an excellent ETL material acquired in good agreement with desirable properties.

b)

a) N719 QPhPBr

5000000

QPhPBr

Counts/s

4000000

0.2

3000000

F

0.3 Absorption

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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2000000

0.1 1000000

0.0

300

400

500

600

700

0

800

0

16000 14000 12000 10000 8000 6000 4000 2000 0 -2000 17

5

Wavelength (nm)

18 19 Kinetic energy/eV

20

10 15 Kinetic energy/eV

20

Figure 2. (a) UV-Vis Absorption Spectrum of QPhPBr and N719 in Film and (b) UPS Spectra of The Onset (Inset) and The Secondary Edge Region of QPhPBr Table 1. Summarized Data of HOMO and LUMO Energy levels and Egopt of ETL maetrials N719 and QPhPBr ETL

λonset (nm)

Egopt (eV)

HOMO (eV)

LUMO (eV)

N719a

536

2.33

−5.34

−3.01

QPhPBr

298

4.19

−6.77

−2.58

a Taken from reported data69


6

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Photovoltaic Properties Study The solar cells were fabricated with QPhPBr, N719 or their binary mixture as the ETL, and again, without ETL for control. The control device with PTB7-Th: PC71BM based blend active layer showed a Voc of 0.710 V, a Jsc of 14.66 mA/cm2, an FF of 0.51, and a PCE of 5.37%. After insertion of a QPhPBr layer atop of the active layer, characteristic improvement in device parameters was achieved with Voc, Jsc, FF and PCE up to 0.815 V, 16.78 mA/cm2, 0.67, and 9.18%. Totally, the PCE is increased by 70.9%. There is no further increment in Voc on changing the ETL from QPhPBr to N719 or binary mixture N719:QPhPBr. On the other hand fabrication with binary solution of N719:QPhPBr as ETL, device performance further increased with Jsc, FF and PCE going to 17.26 mA/cm2, 0.70 and 9.83% and the PCE value showed a total increase of 83%. Whereas in case of N719 as the ETL showed lower results of Jsc and FF (16.34 mA/cm2 and 0.65) in comparison to others (QPhPBr and N719:QPhPBr) with a PCE 8.58% (59.7%). Since it was observed earlier that by simply applying methanol solvent70

71

could

improve the device performance, we also fabricated active layer followed by methanol treatment and slightly improved performance than bare Al while poorer than N719 as ETL was observed (see Table 2). Figure 3a displays the current-density−voltage (J−V) curves of the PTB7Th:PC71BM based solar cells with different ETLs. Observed improvement in Voc, Jsc, FF with addition of the ETL between active layer and metal electrode is related to the formation of good active-cathode ohmic contact and the increase in the electron mobilities, as indicated vide post. However no further improvement in Voc was achieved on changing the ETL. The reason might be correlated with good absorption in visible region and lesser deep HOMO energy level of N719 causes no further increment in Voc upon binary mixture of ETL. Whereas increase of FF and PCE was noticed from replacement of N719 to QPhPBr


7


and binary mixture of N719:QPhPBr as the ETL in device. Moreover large improvement in presence of QPhPBr as ETL reveals that phosphonium bromide ion may have strong stability to become more electropositive and show better tendency to extract electron from exciton diffusion. This result proves that formation of four σ P-C bonds create symmetrical force on positive phosphorous atom as well as breaks the conjugation after electrostatic binding with Br ion makes more stable electropositive complex resultant enhancement in PCE. Further improvement in PCE upon binary mixture with N719 as ETL suggested that binary mixture creates large number of positive-negative ion exchange at interfaces is another approach to extraction, separation and collection of electrons to cathode.

a)

b)

10

J (mA/cm )

-10

2

2

0 -5

Bare Al Methanol N719/Al QPhPBr/Al N719:QPhPBr/Al

0 -5 -10

Bare Al Methanol N719/Al QPhPBr/Al N719:QPhPBr/Al

-15

-15 -20 -0.5

10 5

5

J(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

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0.0

0.5

1.0

-20 -0.5

0.0

V/V

0.5

1.0

V/V

Figure 3. Light J−V Curves With Different ETLs and Different Active Layer Blends; (a) PTB7Th:PC71BM (1:1.5, wt/wt) and (b) PBDTTT-CT:PC71BM (1:1.5, wt/wt). We again select PBDTTT-CT:PC71BM system to check the properties of QPhPBr and its binary blend as the ETL materials. The photovoltaic data are collected in Table 2 and the J−V curves are given in Figure 3b. Without the ETL, a PCE of 4.75% with Voc = 0.646 V, Jsc = 15.05mA/cm2, an FF = 0.49 was obtained. With using QPhPBr as the ETL, the device parameters PCE, Voc, Jsc and FF increased to 8.42%, 0.771 V, 16.64 mA/cm2 and 0.65. Similar


8

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trend of slight increment in device performance was observed with use of binary solution of N719:QPhPBr as ETL. The Voc remained constant as 0.770V whereas Jsc, FF, and PCE increased to 17.01 mA/cm2, 0.66, and 8.69%, respectively. In both conditions a total 77.3% and 82.9% PCE value improved were observed in comparison to bare Al cathode device performance. Table 2. Summarized Device Performance of PTB7-Th:PC71BM, PBDTTT-CT:PC71BM and P3HT:PC71BM Based PSCs With Different ETLs Donor

ETL

Voc (V)

Jsc (mA/cm2)

(Jcacla) (mA/cm2)

FF (%)

PCEbMax (%)

PCEcAve (%)

µe (10-3) cm2/Vs

Rs Ωcm2

PTB7-Th

Bare Al

0.710±0.021

14.66 ±0.46

14.03

51.61±1.52

5.37

4.96±0.41

0.54±0.08

17.04±0.25

Methanol

0.780±0.016

15.36±0.22

-

59.75±1.88

7.15

6.84±0.31

-

-

N719

0.803±0.021

16.34±0.32

15.98

65.41±2.32

8.58

8.27±0.31

0.65±0.11

9.84±0.18

QPhPBr

0.815±0.015

16.78±0.44

16.23

67.12±1.46

9.18

8.66±0.52

0.96±0.15

8.28±0.20

N719:QPhPBr

0.814±0.015

17.26±0.31

16.59

69.96±1.44

9.83

9.40±0.43

2.88±0.18

7.14±0.26

Bare Al

0.646±0.011

15.05±0.41

14.56

48.89±2.40

4.75

4.23±0.52

0.74±0.20

32.58±0.14

Methanol

0.682±0.020

15.11±0.25

-

51.66±1.78

5.32

4.94±0.38

-

-

N719

0.752±0.015

15.52±0.33

15.00

65.23±1.27

7.61

7.21±0.40

1.08±0.14

11.28±0.24

QPhPBr

0.771±0.015

16.64±0.32

15.93

65.68±1.54

8.42

7.92±0.50

4.29±0.12

9.96±0.18

N719:QPhPBr

0.770±0.011

17.01±0.31

16.45

66.22±1.43

8.69

8.45±0.24

4.74±0.15

9.72±0.22

Bare Al

0.581±0.021

9.05±0.46

8.62

57.42±0.52

3.01

2.63±0.38

3.80±0.12

15.24±0.20

Methanol

0.601±0.020

9.51±0.28

-

59.49±0.66

3.40

3.17±0.23

-

-

N719

0.640±0.011

10.48±0.41

9.97

62.86±0.41

4.21

3.89±0.32

4.05±0.13

12.60±0.18

QPhPBr

0.661±0.025

11.52±0.50

10.97

63.25±0.44

4.81

4.29±0.52

4.64±0.22

12.54±0.19

N719:QPhPBr

0.662±0.011

12.47±0.38

12.14

65.01±0.36

5.35

5.00±0.35

5.57±0.23

10.62±0.16

PBDTTT-CT

P3HT

a

Calculated from EQE Spectrum,

b

Obtained From The best performing device,

c

The

statistical results were obtained from 25 cells.


9


Figure 4 are the dark J–V characteristics of the PTB7-Th:PC71BM and PBDTTT-CT:PC71BM based devices. The dark leakage current decreased in order with QPhPBr and further its binary blend as the ETL: N719:QPhPBr < QPhPBr < N719 < Bare Al only device, which suggests the good properties of QPhPBr as the ETL and the good diode property implies good electron transporting and the hole blocking properties for the QPhPBr and its binary blend based ETLs. b)

a) 10000

10000

PTB7-Th:PC71BM

PBDTTT-CT:PC71BM

100

100 2

J(mA/cm )

2

J (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

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1 0.01 1E-4 -1.5

Bare Al N719/Al QPhPBr N719:QPhPBr/Al

-1.0

-0.5

1 0.01 1E-4

0.0

0.5

1.0

-1.5

Bare Al N719/Al QPhPBr/Al N719:QPhPBr/Al

-1.0

Voltage/V

-0.5 0.0 0.5 Voltage (V)

1.0

Figure 4. Dark J–V Characteristics of the PTB7-Th:PC71BM (a) and PBDTTT-C-T:PC71BM (b) based optimal devices With Different ETLs. Table 2 also gives the series resistance (Rs) values, from which one can see that the contact resistance was reduced upon inserting the QPhPBr and further its binary blend as the ETL. Order of decreasing Rs value upon using the N719/Al, QPhPBr/Al and N719:QPhPBr/Al cathode were found as 9.84 > 8.28 > 7.14 Ω cm2 with PTB7-Th and 11.28 > 9.96 > 9.72 Ω cm2 with PBDTTT-CT, respectively, which was lower than bare Al cathode Rs value 17.04 Ω cm2 (PTB7-Th) and 32.58 Ω cm2 (PBDTTT-CT). The reduction of the contact resistance offers the formation of good ohmic contact between the metal cathode and the active layer, which leads to the increase in Jsc compared to bare Al cathode. Moreover QPhPBr showed better achievement of photovoltaic results than N719 as the ETL.


10

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Further in agreement with these results another widely used active layer P3HT:PC71BM based device was also characterized and similar influence in photovoltaic performance was observed with PCE from 3.01 % in bare Al to 5.35% in presence of ETL (Table 2 and Figure S1-S4). Among three ETLs device output, use of QPhPBr and its binary blend as the ETL provided better device performance than the use of N719, which further supports the good properties of QPhPBr and its binary blend as the ETL for fabricating high-efficiency OSCs. EQE measurement study The external quantum efficiency (EQE) spectra (Figure 5) were measured to verify the J−V measurements. The integrated current-density values (Table 2) match well with the Jsc values obtained from the J−V measurements. The EQE responses with the QPhPBr and its mix binary substrate N719:QPhPBr as the ETL were higher than that of N719 as ETL and without the ETL. The improved EQE spectrum in presence of QPhPBr agrees well with the increase of Jsc. a)

b) PTB7-Th:PC71BM

PBDTTT-CT:PC71BM

75

75

60

60 EQE (%)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45 Bare Al N719/Al QPhPBr/Al N719:QPhPBr/Al

30 15 0

400

500

600

700

45 Bare Al N719/Al QPhPBr/Al N719:QPhPBr/Al

30 15

800

900

0

400

Wavelength (nm)

500

600

700

800

900

Wavelength (nm)

Figure 5. EQE Spectra of PTB7-Th:PC71BM (a) and PBDTTT-CT:PC71BM (b) Based devices With Different ETLs.


11


Page 12 of 27

Ultraviolet photoelectron spectroscopy (UPS) Study In order to study the roles of different ETLs which are spin-coated atop the active layer to improve the normal-type device performance, the effects of ETL materials on the work function of the Al electrode were measured with UPS method. As shown in Figure 6, with respect to the bare Al electrode, the ETL treatment reduces the kinetic energy in the inelastic cutoff region and the N719 treatment reduces much more than the QPhPBr and its binary blend treatments. From the cutoff energy (Ecutoff), the work function (WF) value was estimated. All three ETLs after treatments get up shifted in work function of Al electrode. The Ecutoff values of Al cathode after ETL treatment appeared at 3.16, 3.95 and 3.93eV for N719, QPhPBr and binary mixture, respectively. The Ecutoff value of the Al cathode after ETL treatment with N719, QPhPBr, N719:QPhPBr were 1.09, 0.3 and 0.32 eV lower than that of the bare Al electrode 4.25eV, respectively, demonstrating that different ETL treatment shifts the WF of the Al electrode towards the vacuum level. Among them Ecutoff values of QPhPBr and the binary ETL were in close proximity with LUMO energy level of PC71BM 3.9eV (taken from reported data).72 This reveals that the interfacial dipole resultant from the already spin-coated thin ETL bridges the energy level gap between the Al cathode and the active layer upon treatment with QPhPBr and its mixed binary substrate N719:QPhPBr as the interfacial layer more effectively as well as better ohmic contact upon inserting the ETL significantly increases the internal built-in potential across the active layer, allowing them to get larger Voc and facilitating the charge separation, transport and collection more efficiently, resulting in higher Jsc and FF both. With another P3HT:PC71BM based device (Figure S2-S4) consistently similar improvement in photovoltaic performance was observed as occurred in the devices for PTB7-Th and PBDTTTCT based PSCs (Table 2). Therefore the photovoltaic data from these three donor:acceptor


12

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systems clearly demonstrate that the QPhPBr/Al and its binary N719:QPhPBr/Al cathode were better than the N719/Al cathode and much better than the bare Al cathode for achieving overall high

photovoltaic

characteristics,

which

supporting

that

the

alcohol

soluble

tetraphenylphosphonium halide is promising small-molecule ETL material for fabricating highefficiency OSCs.

Figure 6. Normalized UPS Spectra of The Al Cathode, Before and After ETL Treatment of N719, QPhPBr and Binary Mixture of N719:QPhPBr. Space charge limited current (SCLC) measurements study To investigate the influence of different ETLs in charge extraction, SCLC method was used to measure the electron mobilities (µe) by using the electron-only devices and the related values were estimated and given in Table 2, while their dark J−V curves were shown in Figure 7 and S4. The µe values of the PTB7-Th:PC71BM blend after treatment with N719, QPhPBr and their binary substrate N719:QPhPBr were estimated to be 6.52×10-4, 9.57×10-4 and 2.87×10-3 cm2/(Vs), respectively, which were larger than the value 5.44×10−4 cm2 /(Vs) obtained from the bare Al only layer. For PBDTTT-CT:PC71BM based device, the electron mobilities in presence


13


of N719, QPhPBr and their binary N719:QPhPBr were of 1.08×10−3, 4.29×10-3 and 4.74×10-3 cm2/(V s) which was higher than the device fabricated with only bare Al (7.35×10−4 cm2 /(Vs)). The increase of the electron mobilities upon inserting the ETL is consistent well with the formation of good ohmic contact between the Al cathode and the active layer, contributing to the increase of Jsc.

2

b) 3

4

10 PTB7-Th:PC71BM

3

10

PBDTTT-CT:PC71BM

2

10 2

10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 0.1

J(A/m )

a)

J(A/m )

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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Bare Al N719/Al QPhPBr/Al N719+QPhPBr/Al

Vapp -Vbi (V)

1

1

10

0

10

-1

10

-2

10

BareAl N719/Al QPhPBr/Al N719+QPhPBr/Al

-3

10

-4

10

0.1

1 Vapp -Vbi (V)

Figure 7. Plots of dark J–V Curves Obtained From The PTB7-Th:PC71BM (a) and PBDTTTCT:PC71BM (b) Based Electron Only Devices. Similarly, the enhancement effects of these organic small molecules and their binary blend as ETLs on the P3HT:PC71BM based device performance were observed and this indicated that those rules working for other two different polymer:PC71BM systems are still valid in influencing the P3HT:PC71BM based device performance (Table 2 and Figure S1-S4). From the photovoltaic data summarized in Table 2, both QPhPBr and N719:QPhPBr as ETLs afford better device performance than N719 and the bare Al. The benefits come from the tuning on the work function of the Al cathode and the increasing in the electron mobility, as a result of the single organic component and their synergistic effects from the binary blend, leading to improvement in the charge separation, transport and collection with improved device


14

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performance. The vast potentials using organic QPhPBr as ETL and N719:QPhPBr binary mixture of ETL to enhance the PSC performance (Figure 3, Table 2) creates many future aspects for selecting or designing suitable small-molecule organic phosphonium derivatives as highperformance ETL materials.

CONCLUSION We have shown for the first time the roles and enhancement effects of a small-molecule phosphonium derivative, e.g. tetraphenylphosphonium bromide (QPhPBr), and its binary blend with N719 as the cathode buffer layer materials for improving the fullerene-based polymer solar cells’ device performance. Our data clearly indicate that the QPhPBr/Al and its binary QPhPBr:N719/Al cathodes present better photovoltaic performance than the N719/Al, methanol/Al and the bare Al cathodes in a normal-type device configuration. The rules working for the deice performance enhancement are related to the effective modifications on the metal surface work function and the achievement in the active-cathode ohmic contact eliminate the space charge accumulation at the interface, leading to the obtaining of larger electron mobilities, and consequently, the higher Voc, Jsc, FF and PCE values. These results indicate the excellent functions of phosphonium derivatives for use as ETL materials to realize highly efficient organic solar cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.???????. Experimental details including experimental details, related tables and figures (PDF)


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Page 16 of 27

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (C.Z.). [email protected] (J.X.) Notes The authors declare no competing financial interest. ORCID iD

Chuanlang Zhan: 0000-0001-5127-0973

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support Funded by the National Natural Science Foundation of China (NSFC, Nos. 91433202, 21773262, 21521062, 21276059 and 91227112) and Chinese Academy of Sciences (CAS, XDB12010200).

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Use for ToC only


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Tetraphenylphosphonium Bromide as a Cathode Buffer Layer Material

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