Fully Printable Organic and Perovskite Solar Cells with Transfer

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Fully printable organic and perovskite solar cells with transfer-printed flexible electrodes Xianqiang Li, Xiaohong Tang, Tao Ye, Dan Wu, Hong Wang, and Xizu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

Fully printable organic and perovskite solar cells with transfer-printed flexible electrodes Xianqiang Li1,2, Xiaohong Tang*2, Tao Ye1,3, Dan Wu2,Hong Wang2, Xizu Wang*1 1

Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and

Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634, Singapore 2

School of Electrical and Electronics Engineering, Nanyang Technological University, 50

Nanyang Avenue, Singapore 639798, Singapore 3

Department of Mechanical Engineering, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 119077, Singapore

Email: [email protected], [email protected]

Abstract The perovskite solar cells (PSCs) and organic solar cells (OSCs) with high performance were fabricated with transfer printed top metal electrodes. We have demonstrated that PSCs and OSCs with the top Au electrodes fabricated by using the transfer printing method have comparable or better performance than the devices with the top Au electrodes fabricated by using conventional thermal evaporation method. The highest PCE of the PSCs and OSCs with the top electrodes fabricated using the transfer printing method achieved 13.72% and 2.35%, respectively. It has been investigated that fewer defects between the organic thin films and Au electrodes exist by using the transfer printing method which improved the device stability. After stored the PSCs and OSCs with the transfer-printed electrodes in a nitrogen environment for 97 days and 103 days without encapsulation, the PSCs and OSCs still retained 71% and 91% of their original PCEs, respectively.

Keywords: perovskite solar cells, organic solar cells, transfer printing, stability, interfacial defects 1

ACS Paragon Plus Environment


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1. Introduction In recent years, solution processed organic-inorganic lead halide perovskite solar cells (PSCs) and organic solar cells (OSCs) have attracted lots of attentions as a new generation of photovoltaic technologies. They are attractive because of their low-cost fabrication process, mechanical flexibility, and solution processability, etc. The power conversion efficiency (PCE) of the PSCs and OSCs achieves above 20%

1-4

and 10%

5-6

, respectively. Flexible PSCs and

OSCs with high power-per-weight and promising efficiency have been reported in previous studies

7-10

, demonstrating their capability to be used for flexible devices. To further reduce

the fabrication costs and enable large scale production, various printing techniques have been developed, such as screen printing

11-15

, doctor blading

16-17

, transfer printing

18-24

and ink jet

printing 25-26, which are compatible with the fabrication of flexible PSCs and OSCs. In developing the PSCs and OSCs, most research focused on developing and optimizing solution or printable process for fabrication of active layers and buffer layers while less attention was paid to electrodes which are also important components in PSCs and OSCs. The top electrodes of PSCs and OSCs are normally fabricated by depositing a gold layer on top using thermal or E-beam evaporator in high vacuum environment. The commonly used hole transporting layers of PSCs, such as Spiro-OMeTAD

1, 27

and PTAA

28-31

, in PSCs are very

thin ~200 nm, due to their limited conductivity and charge carrier mobility. The optimal thickness of the photoactive layer in the state-of-art OSCs is around 100 nm because of a trade-off between photons absorption and photogenerated charge carriers collection efficiency 32

. To prevent excess metal atoms penetrating into the soft and thin organic films, the initial

metal deposition rate must be very low and precisely controlled during the device fabrication 33-34

. The excess metal penetration into the organic films will lead to increasing the leakage

current

35-36

and poor device stability

37-40

. Besides, the vacuum evaporation process is not

compatible with the roll-to-roll production and will significantly increase the fabrication costs. To achieve fully printable PSCs and OSCs fabrication, alternative methods of depositing highly conductive top electrodes of the devices that are compatible with roll-to-roll fabrication process and will not induce excess metal penetration must be developed. Transfer printing method is a potential candidate for replacing the traditional method by using E-beam or thermal evaporator for the devices’ top electrode fabrication because it is fast, robust and 2


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applicable to various materials and substrates

41-46

. The transfer printing method has been

widely used in thin film transistors (TFTs) 41, 47-49, light emitting diodes (LEDs) 50-51 and solar cells

18-19, 21-23, 52-54

. PEDOT:PSS, graphene and silver nanowires have been successfully

transfer printed as top electrodes for fabrication of PSCs and OSCs

18, 21, 23, 55-56

. But the

reports on transfer printing metal thin films as top electrodes of PSCs and OSCs are rare. In this study, we have developed and investigated the method of transfer-printing the Au electrodes of the PSCs and OSCs. The Au top electrodes of PSCs were deposited by a transfer printing method that Au electrodes were transferred from polydimethylsiloxane (PDMS) onto the top of the Spiro-OMeTAD layer due to the adhesion difference between Au/PDMS and Au/Spiro-OMeTAD. The highest PCE of the PSCs with the transfer-printed Au electrodes was measured 13.72% which is higher than that of the control device with thermally evaporated Au electrodes. The interface between the Spiro-OMeTAD and the transfer-printed Au electrode had less defects and better organic/metal interface than the one between Spiro-OMeTAD and thermally evaporated Au electrode as indicated by the space charge limited current (SCLC) analysis. Through X-ray photoelectron spectroscopy (XPS) measurement, we found that less Au atoms diffused into Spiro-OMeTAD layer by using the transfer printing method. Benefiting from the suppressed interfacial defects and less diffused Au atoms, the PSCs with the transfer-printed Au electrodes showed excellent stability with retaining 71% of the initial PCE after storing in N2 environment for 97 days without encapsulation. We also demonstrated the universality of this transfer printing method by successfully fabricating the P3HT based OSCs with good performance and stability. Through this study, we proposed a low-cost electrode transfer printing method that has the potential low cost and easy scale up electrode technique in fabrication of highly efficient and stable PSCs and OSCs.

2. Results and discussion Figure 1 shows the procedure of transfer printing Au from PDMS onto the top of Spiro-OMeTAD to form the Au top electrode of a PSC. An 80 nm Au layer was deposited on the top of PDMS strips by E-beam or thermal evaporator. The PDMS/Au sample was attached to a glass slide. Details of preparation of device with a structure of FTO/TiO2 compact 3



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layer/TiO2 mesoporous layer /perovskite layer/spiro-OMeTAD can be found in the Experimental Section. The PDMS/Au sample was placed on the top of Spiro-OMeTAD, then, the two parts were firmly laminated together by two long tail clips. To facilitate the Au layer to be transferred from PDMS to the Spiro-OMeTAD, the whole sample was annealed at 90 for 2 h. The annealing step can accelerate the surface hydrophobic recovery in the PDMS, thus to reduce the PDMS substrate’s surface energy and adhesion between PDMS and Au interface 44. After the annealing process, the PDMS strips were slowly peeled off and the Au film was left on the top of Spiro-OMeTAD.

Figure 1. The fabrication procedure of transfer-printing the Au top electrodes of the PSCs. (1) Put PDMS/Au on the top of FTO/TiO2 compact layer/TiO2 mesoporous layer/perovskite layer/spiro-OMeTAD. Two long tail clips were used to laminate them tightly and uniformly by clipping on two opposite sides of the sample. The device was annealed on a hot plate at 90 for 2 hours. (2) Slowly peel off PDMS substrates and the Au film was left on the surface of Spiro-OMeTAD.

To compare the performance of PSCs with Au electrodes fabricated by different methods, PSCs with the transfer-printed and thermally evaporated Au electrodes were fabricated in the same batch. All the fabrication processes of the PSCs were the same except the method of depositing the Au top electrodes. Figure 2 and Table 1 show the photovoltaic performance of the PSCs. As shown in Figure 2 (a) and Table 1, the highest PCE of the PSCs with the transfer-printed Au electrodes is 13.72% which is higher than that of the PSCs with the thermally evaporated Au electrodes, 13.02%. The short circuit current density, Jsc, calculated from the measured EQE spectrum (Figure 2 (b)) is 19.17 mA/cm2 for the transfer-printed PSCs and 19.29 mA/cm2 for thermally evaporated PSCs, which are in good agreement with 4


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those obtained from the measured J-V characteristics of the devices. The PSCs with either transfer-printed or thermally evaporated Au electrodes exhibit hysteresis effects as shown in Figure 2 (c) and Table 1. Since the hysteresis effects exist in all the PSCs, it should be caused by other factors rather than using the transfer printing process. To evaluate the reproducibility of the transfer printing method, we fabricated at least 24 devices for each Au deposition method and the devices’ performance was summarized in Figure 2(d). It is obvious that the PSCs by the transfer printing method have better performance with an average PCE of 13.18%, while the PSCs with the thermally evaporated Au have an average PCE of 12.18%. The better performance of PSCs by transfer printing method mainly comes from higher Voc which might be a result of better interface properties between Au and Spiro-OMeTAD. Besides, we have observed that the shunt resistance, , of the thermally evaporated devices is smaller than that of the transfer printed devices. The Rsh was calculated from the reciprocal value of the J-V curves’ slope near V=0. After analyzing the J-V curves of all the 24 cells, the average Rsh of the thermally evaporated and transfer printed devices are received 1838.33 ±

274.54 Ωcm and 2634.45 ± 564.22 Ωcm , respectively. In agreement with the smaller Rsh, the PSCs with thermally evaporated Au also have smaller average FF. The average FF of the PSCs with thermally evaporated Au is 0.61 which is lower than that of the PSCs with transfer printed Au, 0.62. The smaller Rsh and FF of the thermally evaporated devices indicate that the thermal evaporation process might introduce excess metal penetration into the Spiro-OMeTAD layer and increased leakage current of the PSCs. These results suggest that good PSCs can be fabricated by simple transfer printing method without using vacuum process.

5



Current Density (mA/cm2)

(a)

(b)

0

100 80

-5 -10

Transfer-printed Thermally evaporated

60

EQE (%)


40

-15 20 -20 0.0

(c)

0.2

0.4

0.6 Voltage (V)

0.8

0 300

1.0

400

500

(d)

0

700

800

15 Transfer-printed Thermally evaporated

14

-5 Voltage Sweep Direction forward scan reverse scan

-10

600

Wavelength (nm)

13 PCE (%)

Current Density (mA/cm2)

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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-15

12 11

-20 0.0

10 0.2

0.4

0.6

0.8

1.0

Transfer-printed

Thermally evaporated

Voltage (V)

Figure 2. Performance of PSCs with transfer-printed or thermally evaporated Au electrodes. (a) J-V characteristics under reverse voltage seep direction and (b) EQE spectrum of PSCs. (c) J-V characteristics of PSCs with transfer-printed Au electrodes under forward and reverse voltage sweep direction. (d) PCE variation range of PSCs obtained from at least 24 devices.

Table 1. Photovoltaic parameters of the PSCs with transfer-printed and thermally evaporated Au electrodes Voltage Jsc Sample

sweep

Voc (V)

(mA/cm2)

PCE (%)

PCE (%)

Best

Average*

FF

direction Forward

1.03

20.96

0.53

11.44

11.01±0.52

Reverse

1.05

20.74

0.63

13.72

13.18±0.48

Forward

0.99

20.32

0.53

10.66

10.26±0.77

Reverse

1.02

20.26

0.63

13.02

12.18±0.63

Transfer-printed

Thermally evaporated *The average PCE is obtained from at least 24 cells

To investigate the charge carrier transport between the Spiro-OMeTAD and Au electrode, we 6


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prepared the hole-only devices with a structure of ITO/PEDOT:PSS/Spiro-OMeTAD/Au. The Au electrodes were deposited by either transfer printing or thermal evaporating method. A PEDOT:PSS layer was inserted between the ITO and Spiro-OMeTAD to block the electron injection from ITO. The space-charge-limited-current (SCLC) model, Eq. (1), was used to calculate the hole mobility in this hole-only device at low voltage 57-60:

J =

(1)

where J is the measured current density, is the permittivity of the component, is the charge carrier mobility,

is the thickness of Spito-OMeTAD layer. As shown in Figure 3,

the hole-only device with the transfer-printed Au electrode has higher hole mobility (μ" = 2.05 × 10%&cm V %( s%( ). The hole mobility of the hole-only device with the thermally evaporated Au electrode is one order lower (μ" = 2.49 × 10%+cm V %( s%(). These results indicate that the interface between the Spiro-OMeTAD and transfer-printed Au electrode is better for facilitating hole transport. It has been reported that the thermally evaporated electrode metals could penetrate into the organic layer and introduce defects and shunt paths 35-36

. So the increased hole-mobility in the hole-only device with transfer-printed Au

electrodes should be because of less defects between the Spiro-OMeTAD and Au electrode interface 61.

7



Transfer-printed Thermally evaporated Current Density (mA/cm2)

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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10

1

0.1

0.01

0.1

1 V-Vbi (V)

Figure 3. J-V characteristics of the hole-only devices (FTO/PEDOT:PSS/SpiroOMeTAD/Au) with transfer-printed and thermally evaporated Au electrodes in dark condition

XPS measurement was conducted to compare the amount of diffused Au atoms in the Spiro-OMeTAD layer in the thermal evaporating and transfer printing process. Samples of the Spiro-OMeTAD thin films with thermally evaporated or transfer-printed Au electrodes were prepared under the same conditions. Then, the Au electrodes were peeled off by using Kapton tape to expose the Spiro-OMeTAD surface. The residues of Au atoms in the Spiro-OMeTAD layer were detected by XPS measurement as shown in Figure 4 (a) and (c). The Au 4f spectra indicate that Au atoms exist in the two Spiro-OMeTAD samples even after peeling off the deposited Au electrodes. These Au residues should be introduced by the Au atoms diffusion during the thermally evaporating or transfer printing process. Compare the Figure 4 (a) and (c), we can see that the sample prepared by thermal evaporator has pronounced Au 4f peaks with relatively high intensity and the Au 4f 5/2 and 7/2 peaks can be clearly distinguished. On the other hand, the Au 4f peak intensity of the sample prepared by the transfer printing method is lower and the Au 4f 5/2 and 7/2 peaks are less distinguishable. As shown in the high-resolution Au XPS spectra (Figure 4(a), (c)), the binding energy of Au 4f 5/2 and 7/2 photoelectrons for thermal evaporating method was shifted toward a lager value compared with that of transfer printing method. This shift to high 8


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binding energy implied that the less effective core hole screening as the size of the metal clusters decreases

62-64

during Au thermally evaporated on organic layer. In the thermal evaporating

process, Au atoms have higher kinetic energy thus penetrating into the Spiro-OMeTAD film deeply and dispersing quickly, so these Au atoms are less possible to aggregate and have small cluster size. In contrast, the Au atoms with lower penetration in Spiro-OMeTAD are likely to aggregate at the surface of Spiro-OMeTAD thus forming larger Au cluster in the transfer printing process. Besides, we have noticed that there is a small peak at the binding energy of 91 eV in the Figure 4 (a), but this peak is not observed in Figure 4 (b). This peak could be attributed to Au 4f 5/2 in the formula of [(C4H9)4N][AuBr4]. The iodine migration from the perovskite layer to the interface between the Spiro-OMeTAD and Au electrodes in the perovskite solar cells has been reported recently

65

. So, we suspect that the Au and Br compounds might be caused by bromine

migrating to the interface between Spiro-OMeTAD and Au electrodes. Some of the Br atoms might react with the Au on the surface of the Spiro-OMeTAD. Due to the Au diffusion caused by the thermally evaporation process, some of the Au and Br compounds should locate inside the Spiro-OMeTAD layer and cannot be completely removed by the Kapton tape. So the residues of Au and Br compounds were detected in the XPS measurement. In the transfer printing process, the Au diffusion is suppressed. The Au atoms or the compounds of Au and Br should only exist at the surface of Spiro-OMeTAD. So the Au and Br compounds can be completely removed by the Kapton tape. As a result, there is no peak at the binding energy of 91 eV in Figure 4 (c). This finding can support that the transfer printing method introduced less diffused Au atoms into the Spiro-OMeTAD layer than the thermally evaporating method. To compare the amount of Au residues in the samples, O 1s spectra was measured and shown in Figure 4 (b) and (d) for calculation of atomic ratio of Au and O. The atomic ratio of Au and O is 0.11:99.89 for the Spiro-OMeTAD with thermally evaporated Au electrodes, while the atomic ratio of Au and O is 0.09:99.91 for the Spiro-OMeTAD with transfer-printed Au electrodes. Less Au atoms diffused into the Spiro-OMeTAD with the transfer printing method. The XPS measurement results indicate that the interfacial defects between Spiro-OMeTAD and transfer-printed Au electrodes induced by diffused Au atoms is suppressed. We also studied the depth profiling of diffused Au atoms inside the Spiro-OMeTAD layer by using secondary ion mass spectroscopy measurement (see Figure S1). In the Figure S1 (a), the depth in Spiro-OMeTAD layer is indicated by the ion counts of bromine. 9



Significant signal of Au atoms was detected in the sample by thermally evaporated method. For the sample by transfer printed method, weak or no signal was of Au atoms was detected. These results can be used to explain the better photovoltaic performance and higher hole mobility of devices fabricated by transfer printing method.

(b)

80 75

Experimental data Curve fitting Au 4f 5/2 Au 4f 7/2 Background

Au 4f

Intensity (cps)

70 65 60 55

6000

Experimental data Curve fitting Background

O 1s

5000 Intensity (cps)

(a)

50

4000 3000 2000 1000

45 82

84

86

88

90

0 530

92

531

Binding Energy (eV)

(d)

80 75

Experimental data Curve fitting Au 4f 5/2 Au 4f 7/2 Background

Au 4f

70 65 60 55

533

534

535

536

7000 6000

Experimental data Curve fitting Background

O 1s

5000 4000 3000 2000

50

1000

45 40

532

Binding Energy (eV)

Intensity (cps)

(c)

Intensity (cps)

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

Page 10 of 21

82

84

86

88

Binding Energy (eV)

90

92

0 530

531

532

533

534

535

536

Binding Energy (eV)

Figure 4. Narrow scan XPS spectra of Spiro-OMeTAD film with Au residue. Au was deposited by thermal evaporating (a,b) or transfer printing (c,d) methods then peeled off by Kapton tape.

According to the SCLC analysis and XPS measurement, thermal evaporating method introduced more diffused Au atoms into the Spiro-OMeTAD films than transfer printing method. As mentioned before, thermally evaporated metal electrodes might penetrate into organic thin films and lead to poor device stability 37-38. So with less defects, the devices with transfer-printed electrodes might have better stability than the devices with thermally evaporated electrodes. To study the stability of the PSCs with different Au electrode deposition method, we stored the fabricated PSCs with the transfer-printed and thermally evaporated Au electrodes in a N2 filled glovebox in dark condition without encapsulation. Figure 5 shows the normalized PCE of the PSCs as a function of storing days. After storing for 97 days, the PCE of the PSCs with the transfer-printed Au electrode and thermally 10


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evaporated electrode decreased to 71% and 48% of their initial PCEs, respectively. Since the only difference between the two PSCs is the deposition method of the top Au electrodes, the slower degradation of the PSCs with the transfer-printed Au electrodes should result from less interfacial defects. The superior stability of the PSCs with transfer-printed Au electrodes indicates that the transfer printing method can be applied into fabrication of highly stable PSCs.

Transfer printed Thermally evaporated

1.0 0.8 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


0.6 0.4 0.2 0.0

0

20

40

60

80

100

Time (day) Figure 5. Normalized PCE of PSCs with transfer printed or thermally evaporated Au electrodes measured in a period of 97 days.

To explore the universality of the transfer printing method, we used the transfer printing method to deposit the Au electrodes of the OSCs with a structure of ITO/ZnO/P3HT:PC60BM/PEDOT: PSS/Au. Figure 6 (a) shows the J-V characteristics the OSCs with the transfer-printed and thermally evaporated Au electrodes fabricated in the same batch. The detailed photovoltaic parameters of the OSCs are shown in Table S1 (supporting information). The OSCs with transfer-printed Au electrodes have the highest PCE of 2.35% which is comparable with the highest PCE of the OSCs with the thermally evaporated Au electrodes, 2.49%. The Jsc of the OSCs was calibrated and in agreement with the EQE measurement as shown in Fig S3 (a). Similar to our observation in PSCs fabrication, the OSCs with transfer-printed Au electrodes also have a slightly 11



higher Voc, 0.60 V than the Voc of OSCs with thermally evaporated electrodes, 0.59 V. Through SCLC

analysis

we

found

that

the

hole

(ITO/PEDOT:PSS/P3HT:PC60BM/PEDOT:PSS/Au)

with

mobility

of

transfer

hole-only

printed

and

device thermally

evaporated Au electrodes is 6.97 × 10%& cm V %( s%( and 5.18 × 10%& cmV %( s%( respectively (see Figure S2). The hole mobility of the hole-only device with transfer printed Au is slightly

higher and can be considered comparable with the hole mobility of the hole-only device with thermally evaporated Au. The small difference between the mobility of the hole-only device with transfer printed and thermally evaporated Au might be due to the uncertainty from the film’s thickness measurement

66

. The OSCs by transfer printing method showed excellent

stability: after storing in N2 filled glovebox in dark conditions without encapsulation for 103 days, the OSCs still remained 91% of the original PCE as shown in Fig 6 (b) and Table S2. These results indicate that the transfer printing method can be applied not only in PSCs on Spiro-OMeTAD but also in OSCs on PEDOT:PSS. Both PSCs and OSCs fabricated by transfer printing method have good performance and stability demonstrating the universality of the transfer printing method.

(b)

20

10

0

-10 -1.0

-0.5

0.0

0.5

20

As prepared After 103 days

2

2


Current Density (mA/cm )

(a) Current Density (mA/cm )

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

Page 12 of 21

1.0

10

0

-10 -1.0

-0.5

Voltage (V)

0.0

0.5

1.0

Voltage (V)

Figure 6. (a) J-V characteristics of OSCs with transfer-printed and thermally evaporated Au top electrodes. (b) J-V characteristics of OSCs with transfer-printed Au top electrodes measured immediately after fabrication and 103 days stored in N2 filled glovebox.

3. Conclusion In summary, transfer-printed Au electrodes of the PSCs and OSCs have been developed and investigated. PSCs with the transfer-printed Au electrodes exhibited a highest PCE of 13.72% which was higher than the highest PCE of the control devices with the thermally evaporated 12


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Au electrodes. From the SCLC analysis, we found that the hole mobility of the hole-only device with the transfer-printed Au electrodes is higher than that of the hole-only device with the thermally evaporated Au electrodes. According to XPS measurement, less Au atoms diffused into the Spiro-OMeTAD films with the transfer printing Au electrode process. As a result, the PSCs with the transfer-printed Au electrodes had better stability. It was measured that the transfer-printed Au electrodes PSCs retained 71% of the original PCE after storing the devices in glovebox in dark condition without encapsulation for 97 days. While the PCE of the PSCs with thermally evaporated Au electrodes decreased to only 48% of their original values. We also applied the transfer printing method into fabrication of OSCs based on P3HT and PC60BM. The OSCs with transfer-printed Au electrodes have a comparable PCE (2.35%) with that of the OSCs with thermally evaporated Au electrodes (2.49%). The OSCs fabricated by transfer printing method also have excellent stability retaining 91% of the original PCE after storing for 103 days. Clearly, with the introducing more stable interface, transfer printing method is more suitable for depositing metal electrodes on organic thin films and fabricating highly stable PSCs and OSCs. Our findings demonstrate that the electrode transfer printing method is a potential candidate for future manufacturing of PSCs and OSCs due to its high performance, simple processibility in different surfaces. Further work will focus on applying our technique with transfer electrode on flexible devices or the higher performance solar cells.

4. Experimental section PSCs fabrication: FTO substrates were cleaned using soap water and sonicated sequentially with de-ionized water, acetone and isopropanol for 10 min each. After drying at 60 in oven for several hours, the FTO substrates were exposed to ultraviolet light and ozone for 15 min. After being exposed to ultraviolet light and ozone, a TiO2 compact layer was spin coated from a Titanium diisopropoxide bis(acetylacetonate) solution at 6000 rpm for 30s followed by sintering at 450 for 20 min. After cooling down to room temperature, a mesoporous scaffold of TiO2 nanoparticles was deposited by spin coating a solution of TiO2 paste in ethanol (150 mg/ml) at 6000 rpm with an acceleration of 500 rpm/s for 30 s. The substrate with TiO2 mesoporous layer was sintered at 500 for 20 min and then cooled to room temperature. The perovskite precursor solution was prepared by dissolving PbI2, PbBr2, 13



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CH3NH3Br (MABr) and NH=CHNH3I (FAI) in a mixed solvent of anhydrous dimethyl formamide (DMF) and anhydrous dimethyl sulfoxide (DMSO)

67

. The molar ratio of

PbI2:PbBr2:MABr:FAI was 1.05:0.19:0.19:1. The volume ratio of DMF to DMSO was 4:1. The perovskite precursor solution was spin coated in a N2 filled glovebox: first, 2000 rpm for 10 s; second, 6000 rpm for 30 s. During the spin coating process, 110 μl chlorobenzene (CB) was dropped on the spinning substrate 20 s before the end of the procedure. The substrate was then annealed at 100 for 60 min and then cooled to room temperature followed by spin coating Spiro-OMeTAD at 3000 rpm for 30 s on the perovskite layer. The Spiro-OMeTAD solution was prepared by dissolving 72 mg Spiro-OMeTAD in 1 ml chlorobenzene (CB) doped with 17.6 μl Li–TFSI (1.8 M in acetonitrile), 29.2 μl tert-butylpyridine and 29.2 μl Co[t-BuPyPz]3[TFSI]3 (FK209) acetonitrile solution (100 mg/ml). The device fabrication was completed by depositing Au top electrodes on Spiro-OMeTAD by transfer printing method as shown in Figure 1. For the control device, an 80 nm Au layer was deposited by a thermal evaporator at 2 × 10%-./01. The initial deposition rate was controlled less than 0.5 Å/s until 10 nm Au was deposited. The device active area was defined by a shadow mask to be 0.2 cm2.

OSCs fabrication: ITO substrates were cleaned using soap water and sonicated sequentially with de-ionized water, acetone and isopropanol for 10 min each. After drying at 60 in oven for several hours, the ITO substrates were exposed to ultraviolet light and ozone for 15 min. The fabrication procedure of the OSCs followed other people’s report 55. A 30 nm ZnO layer was prepared by spin coating ZnO sol-gel precursor at 2500 rpm for 30 s on ITO substrates, which were then annealed at 150 for 1 h in the air. The ZnO sol-gel precursor was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2⋅2H2O) and ethanolamine (NH2CH2CH2OH) in 2-methoxyethanol (CH3OCH2CH2OH). The molar ratio of zinc acetate dihydrate to ethanolamine was 1:1 and the Zn concentration in 2-methoxyethanol was 0.5 M. The solution was stirred overnight in air and then passed through a 0.2 μm PTFE filter before spin coating. Then the substrates were transferred into a N2 filled glovebox. A P3HT and PC60BM (weight ratio 1:1) blend solution with a total concentration of 40 mg/ml in 1,2-dichlorobenzene (DCB) was spin coated at 800 rpm for 60 s. After annealing at 120 for 10 min, a ~200 nm P3HT and PC60BM blend film was obtained. After being filtered 14


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through 0.45 μm PVDF filter and sonicated, PEDOT:PSS (Al 4083) mixed with 1 vol % Zonyl surfactant (FSO-100) was spin coated on P3HT and PC60BM blend film at 5000 rpm for 50 s followed by annealing at 110 for 10 min. The device fabrication was completed by transfer printing Au top electrodes on PEDOT:PSS layer. For the control device, an 80 nm Au electrode was deposited by a thermal evaporator at 2 × 10%- ./01. The initial deposition rate was controlled less than 0.5 Å/s until 10 nm Au was deposited. The device active area was defined by a shadow mask to be 0.09 cm2.

Hole-only device: The hole-only device with a structure of ITO/PEDOT:PSS/SpiroOMeTAD/Au was fabricated by spin coating PEDOT:PSS (Al 4083) solution filtered through 0.45μm PVDF filter on UV-ozone treated ITO substrate at 5000 rpm with an acceleration of 4000 rpm/s for 50 s followed by annealing at 140 for 10 min. The Spiro-OMeTAD solution was prepared by dissolving 72 mg Spiro-OMeTAD in 1 ml chlorobenzene (CB) doped with 17.6 μl Li–TFSI (1.8 M in acetonitrile), 29.2 μl tert-butylpyridine and 29.2 μl Co[t-BuPyPz]3[TFSI]3 (FK209) acetonitrile solution (100 mg/ml). Spiro-OMeTAD was spin coated on the top of PEDOT:PSS layer following the same procedure described in PSCs fabrication part. The thickness of Spiro-OMeTAD was around 170 nm determined by surface profiler. Hole-only device based on P3HT:PC60BM was fabricated by replacing ZnO layer in OSCs by PEDOT:PSS layer. The PEDOT:PSS layer was spin coated on ITO substrate at 5000 rpm with an acceleration of 4000 rpm/s for 50 s from a PEDOT:PSS (Al 4083) solution filtered through 0.45μm PVDF filter followed by annealing at 140 for 10 min. The other fabrication process for the hole-only device was the same as that of the OSCs. All hole-only devices were completed by deposition of Au top electrodes by using transfer printing or thermally evaporating method.

Characterization: The current density-voltage (J-V) characteristics of devices were measured by a Keithley 2400 parameter analyzer under one sun solar spectrum illumination (AM 1.5G) from a solar simulator with intensity of 100 mW/cm2 calibrated via a silicon reference cell. Incident photon-to-current efficiency (IPCE) measurements were conducted on devices under short-circuit conditions through a lock-in amplifier (SR510, Stanford Research System) at a 15



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chopping frequency of 8Hz during illumination with a monochromatic light from a Xenon arc lamp. The J-V and IPCE measurements were carried out in a nitrogen-filled glove box. A KLA Tencor P-16+ Surface Profiler was used to measure the thickness of Spiro-OMeTAD and P3HT:PC60BM blend films. X-ray photoelectron spectroscopy (XPS) spectra were obtained with a monochromatic Al Kα radiation (VGESCALAB 200i-XL).

Supporting Information Images of secondary ion mass spectroscopy results; Plots of J-V characteristics of hole-only device based on P3HT:PC60BM with transfer-printed and thermally evaporated Au electrodes in dark condition; Plots of EQE results of OSCs and EQE results of OSCs with transfer-printed Au electrodes measured after stored for 103 days; Table of photovoltaic parameters of OSCs; Table of photovoltaic parameters of OSCs with transfer-printed Au electrodes measure after 103 days.

Acknowledgements

The authors would acknowledge the Ministry of Education of Singapore for fund this work, RG97/14. We thank Dr. Nikolai Yakovlev for the secondary ion mass spectrometry measurement. They would also thank IMRE, Singapore, for supporting the experimental works of the research. One of the authors, Li Xianqiang, would thank NTU for offering him the scholarship

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Fully Printable Organic and Perovskite Solar Cells with Transfer

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