Sulfur Position in Pyrene-Based PTTIs Plays a Key role to

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Sulfur Position in Pyrene-Based PTTIs Plays a Key role to Determine the Performance of Perovskite Solar Cells When PTTIs Were Employed as Electron Transport Layers Wangqiao Chen, Ahmed Ali Said, Zongrui Wang, Yu Zhou, Wenbo Liu, WEIBO GAO, Ming Liu, and Qichun Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00857 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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ACS Applied Energy Materials

Sulfur Position in Pyrene-Based PTTIs Plays a Key Role to Determine the Performance of Perovskite Solar Cells When PTTIs Were Employed as Electron Transport Layers Wangqiao Chen,† § # Ahmed Ali Said,† # Zongrui Wang,† Yu Zhou,‡ Wenbo Liu,† Wei-Bo Gao,‡ Ming Liu§* and Qichun Zhang†*

†

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, 639798, Singapore Email: [email protected]

‡

Division of Physics and Applied Physics, School of Physical and Mathematical

Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore

§ Research

Techno Plaza, BorderX Block, Nanyang Technological University, 50

Nanyang Drive, 637553, Singapore, Singapore Email: [email protected]

ACS Paragon Plus Environment

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ACS Applied Energy Materials 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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KEYWORDS: Perovskite Solar Cells, Electron Transport Layer, Pyrene, Sulfur Position, Organic Electronics

ABSTRACT: In this study, two novel small organic molecules (PTTI-1 and PTTI-2, the difference between them is the position of sulfur atom in thieno[3,4-b]thiophene(TT)) are designed and synthesized through introducing the pyrene unit as the central building block and TT as the conjugated linking units. The as-prepared compounds have been demonstrated as electron transport layers (ETL) for perovskite solar cells (PSCs) and PTTI-1 shows a better power conversation efficiency (PCE) value of 15.37%, higher than that of PTTI-2 (11.07%), which may be due to the suitable energy level, strong passivation behavior, and higher electron mobility of PTTI-1. Our study clearly indicates that the sulfur position in this type of electron-transport materials plays an important role in influencing the performance of PSCs. More importantly, our devices show decent stability, where PTTI-1-based devices retain about 83 % of its initial stability after 10 days of testing.

INTRODUCTION


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In past decade, low-temperature solution-processed organic-inorganic hybrid perovskite solar cells (PSCs) attract many scientists’ interests due to their promising efficiency and easy fabrication.1-3 Among all reported PSCs, inverted PSCs (also known as p-i-n devices) with a hole transport layer (HTL) below and an electron transport layer (ETL) above the perovskite layer, are investigated extensively due to the advantages of avoiding the high temperature sintering procedure.4-6 To push up the power conversation efficiency (PCE) in inverted PSCs, discovering appropriate organic HTL and ETL materials plays a vital role.7 There has been vast research focusing on exploring novel HTL materials.8-10 As the counterpart, the progress on ETL materials is slower. To ensure that n-type materials have better performance in PSCs, they should have good solubility, high electron mobility, and suitable energy levels aligned with perovskite materials.11 Currently, most of the present electron-transport materials are based on n-type organic semiconductors, especially those with large planar π-conjugated structures. Some of the representative small molecules include imine compounds (hexaazatrinaphtho[2,3-c][1,2,5]thiadiazole (HATNT)12 and hexaazatrinaphthylene (HATNA)13), which can exhibit best PCE result higher than 18%, and imide compounds (e.g. perylenediimide (PDI)14-16, naphthalene imide (NDI)17-21 and even coronene diimide (CDIN)22), which can also displayed promising photovoltaic performance up to 20%. The rigid planar structures of the above-mentioned materials as well as the low-lying LUMO energy level were proven to be favourable for electron transport. Recently, non-fullerene acceptors in organic solar cells (OSCs) constructed from indacenodithiophene (IDT), indacenodithieno[3,2b]thiophene (IDTT) and their analogues have attracted wide attention due to their tuneable energy levels and good charge transport, resulting from the rigid multi-fused ring. With these intrinsic advantages, they were also utilized as ETL materials for PSCs and excellent PCE results up to


3


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19% were obtained, which encouraged researchers to discover more novel efficient ETL materials. [11, 23-26]

With a perfect planar structure and ten possible functional positions, pyrene has been widely used as a building core to prepare various active components for organic electronics including organic light-emitting diodes (OLEDs),27-28 organic field-effect transistors (OFETs),29-31 organic solar cells (OSCs)32 and even batteries33. In addition, pyrene is also employed as an important building block to construct the polycyclic aromatic hydrocarbons (PAH) in our groups.34-39 Nevertheless, despite of possessing outstanding photoelectrochemical properties,27 to the best of our knowledge, pyrene was rarely introduced as a donor unit to construct n-type materials as ETLs for PSCs. In this work, for the first time, we replaced the central building block of the former ETL molecules based on the indacenodithiophene (IDT) and its analogues with pyrene as the donor building block and

synthesized

two

novel

small

molecules

2,2'-((2Z,2'Z)-(((5,5,11,11-tetrakis(4-

(octyloxy)phenyl)pyreno[4,5-d:9,10-d']bis([1,3]dioxole)-2,8-diyl)bis(2-(2-ethylhexyl carboxylate)thieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3dihydro-1H-indene-2,1-diylidene))dimalononitrile (PTTI-1) and 2,2'-((2Z,2'Z)-(((5,5,11,11tetrakis(4-(octyloxy)phenyl)pyreno[4,5-d:9,10-d']bis([1,3]dioxole)-2,8-diyl)bis(2-(2-ethylhexyl carboxylate)thieno[3,4-b]thiophene-4,6-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3dihydro-1H-indene-2,1-diylidene))dimalononitrile (PTTI-2) as ETL materials. It is worth to note that a large number of previous researches have demonstrated that the perpendicular tetraphenyl substituents on the IDT and their analogue units could effectively increase the steric hindrance, reduce the intermolecular interaction, and thus prevent the over self-aggregation and large phase separation in the film, which will facilitate the transport and the separation of electrons.40 Therefore, in this work, to increase the solubility, enhance the electron density of the donor pyrene part and


4

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facilitate the molecular packing of these two ETL compounds, four alkoxy groups, which are perpendicular to the pyrene plane, were intentionally introduced on the 4,5,9,10- positions through one step diazo reaction with pyrene-4,5,9,10-tetraone. Meanwhile, there have been reported that the sulfur atom could form interfacial S-I or S-Pb interaction to passivate the crystal surfaces of the perovskite and was beneficial for enhancing the performance of PSCs.[6,

18]

Hence, the

thieno[3,4-b]thiophene unit was brought into as a bridge in the two novel molecules and the influence of the different position of sulfur atoms on the photovoltaic performance were also systematically investigated. The performance of these two molecules were evaluated and a maximum PCE of 15.37% was obtained for PTTI-1, which was higher than that of PTTI-2 (11.07%). The reasons accounting for their different PSCs performances were also studied.

RESULTS and DISCUSSION


5


C8H17O

C8H17O

OC8H17 O

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Br

C8H17O

OC8H17

THF, 0 degree

Br

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Br

N2

O

OC8H17

B B

O

ice water O

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O

O

Pd(pddf)2Cl2

O

KOAc, dioxane

O

O O

B

B O O

O

O

2

1

C8H17O

OC8H17

C8H17O OHC

NC Br

OC8H17

OHC

K2CO3, dioxane/H2O O

B

B

C8H17O

C 2H 5

EH:

O

O

O

O

HEOOC

C8H17O

OC8H17

O

CN O

7

PTTI-1

F

OC8H17

C 4H 9 C8H17O

O OHC OC8H17

4

NC

S S

F O

C8H17O

COOEH

O

S

chloroform, pyridine O

O

S CN

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F

CHO

S HEOOC

O

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Pd(PPh3)4

OC8H17

C8H17O

NC

S

OC8H17

CN

COOEH

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5

HEOOC

O

4 F

F

S

S C8H17O

C8H17O

OC8H17

3

S

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Br S HEOOC

OC8H17

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K2CO3, dioxane/H2O

NC

S

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CHO

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chloroform, pyridine

O

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6

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NC

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CN O F

C8H17O

OC8H17

8

C8H17O

OC8H17

F

PTTI-2

Scheme 1. Synthetic route to PTTI-1 and PTTI-2

Synthesis. As shown in Scheme 1, compounds 141-42 and 243-44 were synthesized from the commercially-available chemicals pyrene and 4,4'-dihydroxybenzophenone according to the reported literatures through a two-step reaction, respectively. Following a similar procedure,45 compound 3 was obtained by injecting the solution of 2 into the anhydrous THF solution of 1 at 0 degree under argon protection, and further reacted with bis(pinacolato)diboron to form the key intermediate 4 containing boron ester group in high yield (92%).46 Afterwards, compound 4 reacted with the 2-ethylhexyl 6-bromo-4-formylthieno[3,4-b]thiophene-2-carboxylate to form the aldehyde intermediate, which could be further transformed into the designed target molecule via reacting with the excessive building block 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile.47 The combined yield for these two-step reaction is around 50% for PTTI-


6

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1. However, when the position of sulfur atom changed from isomer 5 to isomer 6, the yield of the coupling reaction decreased sharply to ~ 22%. In the final step, PTTI-2 could be successfully obtained by adopting the same procedure to prepare PTTI-1. All the compounds were confirmed by NMR and MS spectra as shown in Figure S1 and S2.

Figure 1. a) UV/Vis absorption (10–5 M solution) and thin film of PTTI-1 and PTTI-2; b) CV of PTTI-1 and PTTI-2; c) Structure of p-i-n PSC device in this work; d) Energy levels diagram of each material used in this work.


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Physical Properties. The UV-Vis absorption spectra of PTTI-1 and PTTI-2 were measured in CH2Cl2. As shown in Figure 1a, both PTTI-1 and PTTI-2 displayed an apparent broad absorption between 500 and 700 nm with the λmax at 638 nm and 612 nm, respectively. Meanwhile, the λonset was blue-shifted from 694 nm (for PTTI-1) to 668 nm (for PTTI-2). By using the equation of Egopt =1240 nm/ λonset, the band gaps of PTTI-1 and PTTI-2 are calculated to be 1.79 eV and 1.86 eV, respectively. For the thin films of PTTI-1 and PTTI-2, λonset was red-shifted to 756 nm and 735 nm, respectively. Thus, the band gaps of PTTI-1 and PTTI-2 decreased to 1.64 eV and 1.69 eV, accordingly. Electrochemical properties of PTTI-1 and PTTI-2 were tested through cyclic voltammetry (CV) in a dry CH2Cl2 solution containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as electrolyte and the corresponding data were provided in Figure 1b. The Eredonset of PTTI-1 and PTTI-2 were recorded to be –0.88 V and –0.98 V with ferrocene as a standard. Thus, their LUMO energy levels were calculated to be –3.92 eV and –3.82 eV respectively by using the empirical equation of LUMO= – (4.80 + Eredonset). Consequently, their HOMOs were calculated to be –5.56 eV and –5.51 eV accordingly by the equation of LUMO = HOMO + Egopt. The corresponding data are summarized in Table 1. Further DFT studies in Figure S3 indicate that both PTTI-1 and PTTI-2 exhibit much similar HOMO energy levels (–5.35 eV and –5.34 eV, respectively). However, for LUMO energy level, due to the different position of the sulfur atom in thieno[3,4-b]thiophene, the electron density of PTTI-1 delocalizes on all double bonds in thieno[3,4-b]thiophene from the outer malononitrile unit compared with that of PTTI-2, which may contribute to a much lower LUMO energy level of PTTI-1 (–3.54 eV). DFT calculation data corresponds well with the experimental data. Table 1. Physicochemical properties of PTTI-1 and PTTI-2.


8

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λonset [nm] Egopt [eV]a

Eredonset [V]

LUMO [eV]b

HOMO [eV]c

PTTI-1

756

1.64

–0.88

–3.92

–5.56

PTTI-2

735

1.69

–0.98

–3.82

–5.51

Molecule (thin film)

a) Egopt =1240 nm/ λonset; b) LUMO = – (4.80+ Eredonset); c) HOMO = LUMO – Egopt

Device Performance. To evaluate the appropriateness of PTTI-1 and PTTI-2 as ETLs, PTTI-1 and PTTI-2 have been employed as ETLs materials in p-i-n PSC with the following structure: ITO/PEDOT:PSS/Perovskite layer/ETLs/Ag as shown in Figure 1c. PEDOT: PSS is utilized as a hole transport layer, which can block the electrons moving to anode and extracts holes from perovskite layer. The light photons can be absorbed by perovskite layer, wherein excitons originate and dissociate to electrons and holes that can be extracted by ETLs and PEDOT: PSS, respectively. Finally, Ag electrode and ITO have been used to collect the electrons and the holes, respectively. Figure 1d shows the energy levels of materials used in this work. It is noteworthy that, the LUMO of PTTI-1 matches well with the conduction band of perovskite layer, generating the driving force that enhances the electron extraction and intuitive electrons transfer from perovskite layer to Ag electrode. More importantly, its HOMO is deeper than valence band of perovskite layer, which supports the holes blocking property of PTTI-1. Such merits decrease the leakage current and enhance VOC and FF. Undesirably, PTTI-2 shows higher LUMO compared with the conduction band of perovskite layer, which produces electrons accumulation and charges recombination, leading to VOC loss and the deteriorated FF. Figure S4 shows the X-ray diffraction pattern of perovskite layer, where three characteristic peaks significantly appeared at 14.2°, 28.5°, and 31.9° that be assigned to the (110), (220), and (310)


9


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planes of the perovskite crystal. Scanning electron microscope (SEM) has been utilized to further investigate the quality and the morphology of the as-prepared perovskite layers. As shown in Figure S5a and S5b, the as-prepared perovskite layer surface showed large grains with pin holes free surface. More importantly, cross section SEM of perovskite layer indicated that the perovskite layer consisted of giant grains in vertical dimension. Figure S6a shows the UV-vis absorption spectrum of perovskite layer. The absorption onset approximately equals to 785 nm, which indicates a band gap of 1.58 eV. The value corresponds with the band gap confirmed by Tauc plot as depicted in Figure S6b.

Figure 2. a) J-V curves of PSCs with different concentrations of PTTI-1 as ETLs; b) J-V curves of PSCs with different concentrations of PTTI-2 as ETLs

To fully evaluate PTTI-1 and PTTI-2 as ETL materials, their PSCs were fabricated. The optimized thickness of each ETL was achieved by utilizing different concentration of PTTI-1 solution in dichlorobenzene (DCB) (6, 8, 10, 12 and 14 mg/ml), as well as, PTTI-2 with different concentration (6, 8, 10, 12 and 14 mg/ml). As shown in Figure 2a, the as-fabricated devices based


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on PTTI-1 showed various PCEs with different concentrations of PTTI-1 solutions as an ETL material, whereas the highest PCE of 15.37 % was obtained with 10 mg/ml of PTTI-1 solution with JSC of 21.26 mA/cm2, VOC of 1.027 V and FF of 70.4%. The lower concentrations (8 mg/ml and 6 mg/ml) showed a decreased PCE from 14.2 % to 10.56%, which resulted from the decline of JSC and FF. This deterioration was attributed to the low coverage of ETL on perovskite layer, therefore charge recombination increased but FF and JSC decreased. On the contrary, when increasing ETL concentration to 12 mg/ml, the corresponding PCE of the as-fabricated PSC achieved 13.03%. This decreased PCE resulted from the increased thickness of ETL, which resulted in the increased series resistance. Therefore, the declined FF and JSC were observed. Additional increasing to 14 mg/ml deteriorated PCE to 9.4 %. Regrettably, PTTI-2 showed lower PCEs compared with that of PTTI. The highest PCE of 11.07% was obtained for PTTI-2-based PSC with JSC of 21.15 mA/cm2, VOC of 0.915 V and FF of 57.2 % in 10 mg/ml as shown in Figure 2b. When the concentration of PTTI-2 solution increased gradually to 14 mg/ml or decreased gradually to 8 mg/ml, PCE declined due to the high series resistance and low coverage, respectively, as we mentioned above. All the corresponding photovoltaic data are summarized in Table 2. To compare the as-synthesised molecules in this research with conventional electron transporting materials, PCBM-based devices as control experiments have been fabricated and tested. Under the same fabrication condition, the control devices only gave a PCE of 15.6% as shown in Figure S7.

Table 2. Photovoltaic parameters of the as-fabricated PSCs with different concentrations of PTTI-1 and PTTI-2 as well as the control device of PCBM


11


Molecule

PTTI-1

PTTI-2

PCBM

Concentration

JSC

[mg/ml]

(mA/cm2)

6

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VOC (V)

FF [%]

PCE[%]

18.16

0.981

59.3

10.56

8

20.20

1.025

68.5

14.20

10

21.26

1.027

70.4

15.37

12

19.26

1.027

65.9

13.03

14

16.02

0.995

58.8

9.40

6

18.3

0.76

33.0

4.59

8

20.11

0.905

53.8

9.80

10

21.15

0.915

57.2

11.07

12

18.55

0.880

49.0

8.00

14

14.6

0.73

31.6

3.37

20

22.7

0.994

69.1

15.6

To further investigate the effect of ETL concentrations on the performance of devices, electrochemical impedance spectroscopy (EIS) measurements have been conducted in the range of 106 Hz to 1 Hz. The results are in agreement with the illuminated J-V curves. Nyquist plots showed two significant semi-circles: 1) at low frequency which is related to charge recombination resistance, and 2) at high frequency, which is related to charge transfer resistance. For both molecules at 10 mg/ ml, the charge recombination resistance was the highest compared with other concentrations. By increasing the concentration, the charge recombination resistance decreases


12

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due to charge accumulation that leads to charge recombination. Decreasing the concentration (less than 10 mg/ ml) leads to the low coverage, which also decreases charge recombination resistance. By increasing the concentration of both molecules (more than 10 mg/ ml), the charge transfer resistance increased due to the increased thickness as shown in Figure S8.

Figure 3. a) Steady state PL of bare perovskite layer, PTTI-1/perovskite bilayer and PTTI-2/perovskite bilayer; b) TRPL of PTTI-1/perovskite bilayer and PTTI-2/perovskite bilayer with fitted curves.

As shown in Figure 3a, strong photoluminescence (PL) was obtained by the excitation of perovskite layer with 532 nm light. The maximum peak of the corresponding PL was 769.02 nm. The as-obtained PL, which was produced by the excitation of PTTI-1/perovskite bilayer and PTTI2/perovskite bilayer, quenched with a blue-shifted peak at 760.96 nm and 761.94 nm, respectively. Regarding to these results, we believe that the passivation property of surface trap states by PTTI-1 is higher than that of PTTI-2, since the surface traps and grain boundaries produce band bending,


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which decreases the band gap of the surface compared with the band gap of the bulk of perovskite layer as reported by Huang et al.48-49 𝒀 = 𝒀𝒐 +𝐀𝟏𝒆 ―𝒙/𝝉𝟏 +𝐀𝟐𝒆 ―𝒙/𝝉𝟐………………………… (1)

To further investigate the passivation properties of PTTI-1 and PTTI-2, as depicted in Figure 3b, time resolved photoluminescence (TRPL) measurements were carried out and the nascent spectra were fitted regarding to exponential decay equation (1), where A is the amplitude and τ in nanosecond (ns) is life time of charge carriers. All calculated data are summarized in Table 3. By resolving the equation (1), we discovered two transport processes involved in electron transfer processes, termed as τ1 and τ2. It is known that τ1 referred to two possible mechanisms: nonradiative recombination of the charges in perovskite layer and charges transfer to ETL from perovskite layer. The life time τ2 is attributed to charge-radiative recombination inside perovskite layer.50-52 TRPL confirmed the higher passivation property of PTTI-1 than that of PTTI-2. It is worthy to note that PTTI-1 has higher electron extraction power than that of PTTI-2, whereas τ1 of PTT1-1/perovskite bilayer is shorter than τ1 of PTTI-2/perovskite bilayer. This result is ascribed to stronger electron extraction power of PTTI-1 from perovskite layer higher than that of PTTI-2. The longer τ2 of PTTI-1/perovskite bilayer than τ2 of PTTI-2/perovskite bilayer is attributed to stronger trap states passivation of PTTI-1 than PTTI-2. These results were in agreement with these which reported by Bazan and Huang et al.53 To compare our synthesized molecules with PCBM, TRPL of PCBM/perovskite layer was investigated. It was found that τ1=3.96 ns and τ2=40.0 ns for PCBM-based devices as shown in Figure S9. Interestingly, the electron transfer from perovskite layer to PCBM is faster than that from perovskite to PTTI-1. However, PTTI-1 has stronger passivation power of electron traps than PCBM.


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We measured the OFET properties for these two molecules and found out that the electron mobility of PTTI-1 (~ 9.0 × 10-4 m2/V.s) is much higher than that of PTTI-2 (~1.5 × 10-4 m2/V.s). Hence, the higher PCE of PTTI-1, compared with PCE of PTTI-2-based devices, may be partially ascribed to the high performance of PTTI-1based devices. Furthermore, the well-matched LUMO and HOMO of PTTI-1 to the energy levels of perovskite layer plays a crucial role in enhancing photovoltaic parameters of PTTI-1-based PSCs. Moreover, the strong passivation power of PTTI-1, which is confirmed by steady state and TRPL, also contributes to its better performance. We investigated the surface morphology of PTTI-1 and PTTI-2 on perovskite layer as shown in Figure S10, where the roughness of PTTI-1 (11.2 nm) was lower than roughness of PTTI-2 (12.38 nm). It is believed that the lower roughness of ETL declines the leakage current of the as-fabricate devices. Finally, the I-V curves under dark condition of PTTI-1-based and PTTI-2-based PSCs are in consistent with all previous measurements. Figure S11 shows that, the leakage current of PTTI1-based PSCs is lower than that of PTTI-2-based device. To evaluate the reliability of our devices, we prepared 27 devices with the EFL concentration of 10 mg/ml. The distribution of photovoltaic parameters has been shown in Figure S12.

Table 3. TRPL calculated amplitudes A1, A2 and lifetimes τ1, τ2 from Figure 3b.

A1

τ1(ns)

A2

τ2 (ns)

PTTI-1/Perovskite 1.64379

7.66

0.47534

57.63

PTTI-2/Perovskite 1.28667

15.43

0.33323

46.80


15


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PTTI-1 based PSCs showed neglectable hysteresis as shown in Figure S13, whereas, the PCE obtained by scan in reverse direction was 15.37 % and in forward direction was 14.5 %. All photovoltaic parameters are summarized in Table S1. The surface trap passivation by PTTI-1 could be attributed to the decline of hysteresis of PTTI-1-based PSCs. Stability is another important factor, which can hinder PSCs technology toward their commercialization. Therefore, we investigated the stability of PTTI-1-based PSCs and compared with that of PCBM-based devices. PTTI-1-based device retained 83% of its initial stability after 10 days while PCBM-based devices lost 27 % of its initial stability after 10 days of testing. PTTI-2based devices showed the worst case. Since the devices lost 33 % of its initial efficiency as depicted in Figure 4. We attributed the high stability of PTTI-1 based devices to the strong electron traps passivation power of PTTI-1, which decreases the electron recombination and increases operational stability. Our assumption is in agreement with the research done by Naveen et. al.54 Also we ascribed the reason behind the decent stability of PTTI-1-based PSCs to the hydrophobicity nature of PTTI-1 (compared with PCBM based devices), whereas PTTI-1 showed the larger contact angle (100.4o) while PCBM only has 90 o as exposed in Figure S14. All devices were kept in air with the controlled humidity environment (30%) during the period of stability measurement.


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1.00

PTTI-1-based device PTTI-2-based device PCBM-based device

0.95

Normalized PCE

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


0.90 0.85 0.80 0.75 0.70 0.65 0.60 0

2

4

6

8

10

Days

1

Figure 4. Stability test diagram of PTTI-1-based device, PTTI-2-based device and PCBMbased device.

Conclusion. We have designed and synthesized two new organic compounds (PTTI-1 and PTTI-2) as ETL materials in PSCs by firstly introducing four-alkoxy-substituted pyrene as the donor unit. The best PCE result is 15.37% for PTTI-1, higher than that of PTTI-2 (11.07%), due to the different position of sulfur atom in thieno[3,4-b]thiophene(TT) which may account for a different LUMO energy level, surface passivation and electron mobility behaviour between these two molecules. Our study clearly indicates that the sulfur position in the ETLs materials plays an important role in influencing the performance of the as-fabricated PSCs. Moreover, much stronger donating groups might be necessary to introduce when adopting pyrene as the donor unit in the future.


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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section details, NMR and MS, DFT, XRD, SEM, UV-vis absorption, J-V curve of PCBM, Nyquist plot, TRPL of PCBM/perovskite bilayer, AFM, I–V curves under dark conditions, distributions of photovoltaic parameters of PTTI-1and PTTI-2-based devices, J–V hysteresis curves and contact angle test. AUTHOR INFORMATION

Corresponding Author E-mail: [email protected]. (Q. Z.);

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

Author Contributions # W. Chen and A. A. Said contributed equally to this work.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT

Q.Z. acknowledges financial support from AcRF Tier 1 (RG 111/17, RG 2/17, RG 114/16, RG 113/18) and Tier 2 (MOE 2017-T2-1-021 and MOE 2018-T2-1-070), Singapore. QZ also thanks the support (sklssm2019036) from State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, P. R. China.

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Sulfur Position in Pyrene-Based PTTIs Plays a Key role to

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