Incorporating an Electrode Modification Layer with a Vertical Phase

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Incorporating Electrode Modification Layer with Vertical Phase Separated Photoactive Layer for Efficient and Stable Inverted Nonfullerene Polymer Solar Cells Zhenzhen Shi, Hao Liu, Yaping Wang, Jinyan Li, Yiming Bai, Fuzhi Wang, Xingming Bian, Tasawar Hayat, Ahmed Alsaedi, and Zhan'ao Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13494 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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ACS Applied Materials & Interfaces

Incorporating Electrode Modification Layer with Vertical Phase Separated Photoactive Layer for Efficient and Stable Inverted Nonfullerene Polymer Solar Cells Zhenzhen Shi,† Hao Liu,† Yaping Wang,† Jinyan Li,† Yiming Bai,† Fuzhi Wang,† Xingming, Bian,† Tasawar Hayat,‡§ Ahmed Alsaedi,§ and Zhan’ao Tan†*

†

State Key Laboratory of Alternate Electrical Power System with Renewable Energy

Sources, North China Electric Power University, Beijing 102206, China ‡

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589,

Saudi Arabia §

Department of Mathematics, Quiad-I-Azam University, Islamabad 44000, Pakistan

KEYWORDS: inverted nonfullerene polymer solar cells; vertical phase separation; long-term stability; electrode modification layer; titanium (diisopropoxide) bis (2,4-pentanedionate)

ABSTRACT: For bulk heterojunction polymer solar cells (PSCs), the donors and acceptors featuring specific phase separation and concentration distribution within the electron donor:acceptor blends crucially affect the exciton dissociation and charge transportation. Herein, efficient and stable nonfullerene inverted PSCs incorporating phase separated photoactive layer and titanium chelate electrode modification layer are demonstrated. Water contact angle (WCA), scanning kelvin probe microscopy (SKPM),

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and atomic force microscopy (AFM) techniques are implemented to characterize the morphology of photoactive layers. Compared with the control conventional device, the short-circuit current density (Jsc) is enhanced from 14.74 to 17.45 mAcm-2. The power conversion efficiency (PCE) for the inverted PSCs with TIPD layer increases from 9.67% to 11.69% benifiting from the declined exciton recombination and fairly enhanced charge transportation. Furthermore, the non-encapsulated inverted device with TIPD layer demonstrates the best long-term stability, remaining 85% of initial PCE and almost undecayed open-circuit voltage (Voc ) after 1440 h. Our results reveal that the titanium chelate is an excellent electrode modification layer to incorporate with vertical phase separated photoactive layer for producing high-efficiency and high-stability inverted nonfullerene PSCs. 1. Introduction

Polymer solar cells (PSCs) have gained broad focus all over the world since the concept has been proposed in 1990s.1,

2

Due to the photoactive layer of PSCs are

comprised of organic conjugated semiconductors, many unique characteristics including cost-saving, light weight, printable manufacture over inorganic solar cells are endowed with PSCs. The power conversion efficiency (PCE) of the PSCs has been rapidly boosted via synthesizing novel light-harvesting photoactive materials, adopting unique interface materials and designing new device structures,3-8 and now the PCE of the best device is beyond 13%.9-12 However, the long-term operation lifetime and the PCE of the PSCs still


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lag far behind the inorganic counterparts. Therefore, to accelerate the commercialization process of the PSCs, great efforts are still needed to further enhance the PCE and lifetime.

Typically in PSCs, organic molecules like fullerene derivatives serve as electron acceptors, while conjugated polymers serve as electron donors in blend films. The predominant acceptor materials in PSCs are fullerene derivatives endowed with phase separation and superior electron mobility in the active layer like [6,6]-phenyl-C61(or C71)-butyric acid methyl ester (PCBM).13, 14 Unfortunately, PCBM suffers a narrow and weak absorption in the visible spectra, limiting photocurrent of devices. In order to solve these problems, various fullerene-free π-conjugated electron acceptors are developed over the past few years.15-17 Many superior properties have been endowed for non-fullerene acceptors with good electron transport ability and miscibility with polymer donors.17-22 Among the fullerene-free acceptors, ITIC-M behaves wide and intense absorption in visible range.10,

23

Due to so many advantages, recently rapid progresses based on

fullerene-free acceptor have been made, enabling PCE of single solar cells beyond 13%.12

As

we

know,

indium

tin

oxide

(ITO)

glass

modified

by

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is commonly employed as the positive electrode and the low-work-function Ca/Al is routinely utilized as the negative electrode in the conventional BHJ PSCs. However, the ITO electrode can be etched by the acidic PEDOT:PSS layer and the low-work-function metal is sensitive to



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moisture and oxygen, resulting inferior long-term stability of the conventional PSCs. It is widely cognized that an inverted structure is helpful to improve the stability as well as the PCE. In inverted solar cells, ITO is modified by air stable electron transporting layer, typically ZnO, Cs2CO3 and TiO2,24-27 which eliminates the erosion by acidic PEDOT:PSS.28, 29 On the other hand, high work function metal oxides (MoO3 or V2O5) combined with Au or Ag electrodes are used to replace Ca/Al as top contact, rendering high ambient stability of the PSCs. Therefore, for inverted PSCs, low temperature and solution processible electron collection layer is in great demand for inexpensive and large area manufacturing30, 31 However, only a few reports dealing with inverted polymer solar cells adopting nonfullerene acceptor.15, 32-34 Our previous studies indicate that titanium (diisopropoxide) bis (2,4-pentanedionate) (TIPD) works quite well for fullerene based inverted PSCs.35 TIPD film can be obtained by solution processes followed by low temperature treatment, exhibiting good optical property with an average transparency over 90% in UV-visible light range and electron mobility of 3×10-3 cm2V-1s-1.35 Herein, we adopt TIPD as electron transport layer for constructing nonfullerene inverted PSCs based on PBDTBDD and ITIC-M. ITIC-M is an excellent electron acceptor with broad absorption range extended to near 800nm and shows good electron transport ability and miscibility with polymer donors. The inverted nonfullerene PSCs with TIPD layer demonstrate synchronous improvement in short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) compared with conventional control devices featuring Ca/Al and ITO/PEDOT:PSS electrodes, resulting in greatly improved overall PCE from 9.67% to 11.69%.


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2. RESULTS AND DISCUSSION

The molecular structures of donor PBDTBDD, acceptor ITIC-M, and electron transport material TIPD are given in Figure 1a. The inverted device structure is illustrated in Figure 1b. The energy levels of the materials coupled in the inverted device are exhibited in Figure 1c, where the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy level of PBDTBDD, ITIC-M, TIPD and MoO3 were cited from the references.23, 35, 36 Both electron donor PBDTBDD and electron acceptor ITIC-M will harvest photons to generate excitons when light shining through transparent ITO substrate into the photoactive layer of the PSCs. The photo-generated excitons will drift towards and separate at the PBDTBDD:ITIC-M interfaces. The electrons generated in PBDTBDD will transfer to the LUMO of the acceptor ITIC-M and be collected by ITO through TIPD layer. Since the LUMO of TIPD is -3.91 eV, a little higher than ITIC-M (-3.98 eV), thus TIPD can effectively extract electrons from ITIC-M. On the other hand, the holes generated in ITIC-M will transfer to the HOMO of the donor PBDTBDD and be collected by Al electrode through MoO3 layer. Since the work-function of MoO3 (-5.30 eV) is quite similar to the HOMO level of PBDTBDD (-5.33 eV), which facilitates the hole transfer processes. Furthermore, the HOMO level of TIPD is -6.0 eV, which is much deeper than that of PBDTBDD (-5.33 eV), thus it prevents the hole transport from PBDTBDD back to ITO electrode. Therefore, from the viewpoint of energy levels, the device could extract and transport holes or electrons effectively on the corresponding electrode.



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Figure 1. (a) Molecular structures of TIPD, ITIC-M and PBDTBDD; (b) Device structure of inverted solar cells; (c) Energy level diagram of inverted solar cells. The optical properties of the interfacial layer and the photoactive layer play an important role in PSCs. The transmittance spectra of bare ITO glass, PEDOT:PSS and TIPD modified ITO substrates are shown in Figure 2a, and the inset is transmittance and absorption spectra of TIPD film deopsited on quartz glass. PEDOT:PSS modified ITO glass exhibits a higher transmittance in short wavelength from 300 to 400 nm while lower transmittance in 400 to 800 nm in comparison with the bare ITO glass, which should ascribe to the sharply contrast optical constants of PEDOT:PSS with ITO, and this is consistent with previous work.37 Obviously, the TIPD film is highly transparent in visible region with negligible absorption, showing almost the same transmittance to bare ITO


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and which is even higher than PEDOT:PSS modified ITO. The average transmittance of TIPD modified ITO is 90±2% and only a slight absorption is observed in short wavelength range. Such a good transmittance and low absorption makes TIPD an ideal electron collection layer for inverted solar cells. Figure 2b displays the film absorption spectra of PBDTBDD, ITIC-M and PBDTBDD:ITIC-M films. The electron donor PBDTBDD film and the electron acceptor ITIC-M film exhibit an absorption cutoff at 670 and 780 nm, respectively. Therefore, a complementary absorption is formed by the donor and acceptor blend film in the whole visible range from 300 to 780 nm.

Figure 2. (a) Transmittance spectra of bare ITO, PEDOT:PSS and TIPD modified ITO substrates. The inset is transmittance and absorption spectra of TIPD film deposited on quartz glass; (b) Absorption spectra of pure PBDTBDD, pure ITIC-M and



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PBDTBDD:ITIC-M blend films deposited on quartz glass.

For bulk heterojunction PSCs, the morphology and the phase of the bi-continuous networks between donors and acceptors critically influence exciton dissociation and charge transportation. The morphology, phase, and the corresponding surface potential of the substrate, modification layer and photoactive layer deposited on the substrate are investigated using the standard SKPM mode atomic force microscopy. The bare ITO substrate is fairly uniform and smooth displaying a root mean square roughness (RMS) of 3.13 nm (Figure 3a, b), and after modified by TIPD layer (Figure 3d), the RMS slightly increases to 3.84 nm, but it is still flat and uniform observing from the phase image (Figure 3e). Figure 3g and j display the surface morphology of the electron acceptor ITIC-M and electron donor PBDTBDD deposited on TIPD coated ITO substrates with corresponding RMS of 1.46 and 2.95 nm. Interestingly, these two materials demonstrate totally different aggregation behaviors from the phase images (Figure 3h and k). There is no obvious aggregation for ITIC-M film while PBDTBDD aggregates tens of nanometer needle-like domains. As displayed in Figure 3m, the PBDTBDD:ITIC-M blend film spin-coated onto ITO/TIPD substrate shows a RMS of 2.56 nm, more close to that of pure PBDTBDD film (2.95 nm) and far from that of pure ITIC-M film (1.46 nm). These trends can be further proved in phase image of PBDTBDD:ITIC-M film as shown in Figure 3n. Figure 3p and q give the morphology and phase images of PBDTBDD:ITIC-M blend film spin-coated on the bare ITO substrate, and show slightly decreased RMS of 2.32 nm and a little larger domain size in compare with the film deposited on TIPD coated ITO glass, indicating that the interfacial layer also influence the morphology of the photoactive layer grown on it. Figure 3s and t give the


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morphology and phase image of PBDTBDD:ITIC-M blend film deposited on PEDOT:PSS coated ITO substrates, and slightly increased RMS of 2.61 nm and a little smaller domain size are observed in compare with the films deposited on bare ITO and TIPD coated ITO. From both topography and phase images of pure PBDTBDD, pure ITIC-M and PBDTBDD:ITIC-M blend films deposited on bare ITO, PEDOT:PSS coated ITO or TIPD coated ITO substrates, we can conclude that the morphologies of the PBDTBDD:ITIC-M blend films are governed by PBDTBDD and also be influenced by the electrode modification layer, and it is possible to deduce that there is vertical phase-separation between PBDTBDD and ITIC-M within the blend film. There is more PBDTBDD aggregating on the top and more ITIC-M gathering on the bottom of the blend film, which is favourable to efficient exciton dissociation at the interface and to carrier transportation for inverted devices. Such a fine morphology provides a guarantee to achieve a high PCE of the PSCs.38

To verify the formation of vertical phase-separation between PBDTBDD and ITIC-M within the blend film, the surface potential (SP) measurements on scanning kelvin probe microscopy were conducted for pure ITIC-M, pure PBDTBDD, and PBDTBDD:ITIC-M blend films deposited on ITO/TIPD substrates with a 2×2 µm2 scanning area. The SP of bare ITO substrate is 0.2435 V (Figure 3m), and when modified by TIPD the SP increases to 0.2581 V (Figure 3n). The SPs for ITIC-M (Figure 3o), PBDTBDD (Figure 3p), and PBDTBDD:ITIC-M (Figure 3q) are 0.2335, 0.2164 and 0.2187 V, respectively. Obviously, the SP of 0.2187 V for PBDTBDD:ITIC-M blend film is more close to 0.2164 V for the pure PBDTBDD film rather than 0.2335 V for pure ITIC-M film, demonstrating that PBDTBDD should be



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dominate in the upper part of PBDTBDD:ITIC-M blend film while ITIC-M may distributes in the bottom of the blend film and there is vertical phase-separation between PBDTBDD

and

ITIC-M

within

the

blend

film.

Furthermore,

the

SP

for

PBDTBDD:ITIC-M blend film deposited on PEDOT:PSS modified ITO glass is 0.2076 V, much lower than that (0.2187 V) of deposited on TIPD coated ITO glass, indicating the electrode modification layer can greatly influence the aggregation behaviours of the blend film.


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Figure 3. Topography, phase and surface potential images of (a, b, c) bare ITO, (d, e, f) ITO/TIPD, (g, h, i) ITO/TIPD/ITIC-M, (j, k, l) ITO/TIPD/PBDTBDD, (m, n, o) ITO/TIPD/PBDTBDD:ITIC-M, (p, q, r) ITO/PBDTBDD:ITIC-M, and (s, t, u) ITO/PEDOT:PSS/PBDTBDD:ITIC-M, respectively.

To further confirm the vertical phase-separation between PBDTBDD and ITIC-M within the blend film, water contact angles (WCA) of electrode modified layer, pure ITIC-M, pure PBDTBDD, and PBDTBDD:ITIC-M films on PEDOT:PSS/ITO and TIPD/ITO substrates are examined (Figure 4). The WCA for PEDOT:PSS (Figure 4a), pure ITIC-M (Figure 4b) and pure PBDTBDD (Figure 4c) films are 18.24˚, 92.62˚ and 98.15˚, respectively, and the WCA of PBDTBDD:ITIC-M blend film (Figure 4d) is 98.85˚, close to that of pure PBDTBDD film, which confirms the PBDTBDD dominating the top surface of PBDTBDD:ITIC-M blend film. The WAC of TIPD (Figure 4e), pure ITIC-M (Figure 4f) and pure PBDTBDD films (Figure 4g), are 87.34˚, 93.56˚ and 100.41˚, respectively. The WCA for PBDTBDD:ITIC-M blend film (Figure 4h) is 100.99˚ close to the value of pure PBDTBDD film, which confirms the vertical phase-separation between PBDTBDD and ITIC-M within the blend film. Interestingly, the WCA of corresponding pure and blend films on PEDOT:PSS/ITO and TIPD/ITO are distinct, it further demonstrates that the morphologies of the PBDTBDD:ITIC-M blend films are influenced by the interfacial modification layer. These observations are in good agreement with above mentioned AFM and SKFM tests. Such a vertical distribution of acceptor and donor would benefit to the charge transportation and further decrease the carrier recombination rate, resulting higher Jsc for inverted devices.


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Figure 4. WCA images of (a) PEDOT:PSS/ITO, (b) pure ITIC-M film on PEDOT:PSS/ITO substrate, (c) pure PBDTBDD film on PEDOT:PSS/ITO substrate, and (d) PBDTBDD:ITIC-M blend film on PEDOT:PSS/ITO substrate, (e) TIPD/ITO, (f) pure ITIC-M film on TIPD/ITO substrate, (g) pure PBDTBDD film on TIPD/ITO substrate, (h) PBDTBDD:ITIC-M blend film on TIPD/ITO substrate.

To further confirm the vertical phase separation of the photoactive layer, we investigated the donor and acceptor distribution within the photoactive layer by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) measurements. Since CNand S2- are characteristic species for ITIC-M and PBDTBDD, respectively (Figure 1a), the intensity variation of the signals for S2- and CN- with sputter time can directly reflect



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the change of ITIC-M and PBDTBDD distribution within the photoactive layer. As shown in Figure 5, with sputter time increasing, the CN- intensity slowly enhances and S2- intensity sharply decreases. These findings indicate that at the bottom of the blend film ITIC-M gathers and increases with depth increasing, meanwhile, the PBDTBDD gather at the top of the blend film and decreases with increasing depth.

Figure 5. Depth profile of CN- and S2- in the PBDTBDD:ITIC-M blend photoactive layer. To utilize the vertical phase separation of mixed film, inverted PSCs with configuration of ITO/TIPD/PBDTBDD:ITIC-M/MoO3/Al are designed and fabricated as shown

in

Figure

1b.

For

comparison,

control

conventional

devices

(ITO/PEDOT:PSS/PBDTBDD:ITIC-M/Ca/Al) and control inverted device without TIPD layer (ITO/PBDTBDD:ITIC-M/MoO3/Al) were constructed likewise. Figure 6 illustrates the dark J-V (current density-voltage), one sun illumnated J-V, and EQE

(external

quantum efficiency) curves of these cells. The device parameters of Voc, Jsc, FF and PCE averaged over 20 individual cells are listed in Table 1. The dark J-V characteristics of the three kinds of cells are displayed in Figure 6a. Obviously, the control inverted device without TIPD layer displays higher leakage current density and lower rectification ratio,


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which suggests unmatched energy level between ITO electrode without buffer layer and the blend film (Figure 1c), resulting decreased Voc for devices. In contrast, for devices with electrode modification layers (TIPD or PEDOT:PSS), the leakage current is greatly suppressed, resulting enhanced Voc.38-40 As shown in Figure 6b, the control conventional device gives a PCE of 9.67%, a Voc of 0.936 V, a Jsc of 14.74 mAcm-2, and an FF of 70.10%. The inverted devices without TIPD layer demonstrate inferior performance, and the Voc, FF, Jsc, and PCE are 0.72 V, 53.88%, 15.80 mAcm-2, and 6.13%, respectively, due to the poor electron collection and big energy loss at the cathode interfaces. Fortunately, the Jsc is still higher than the conventional device due to such a favourable vertical phase separation in the blend layer. After modified with TIPD layer, the inverted devices demonstrate simultaneous enhancement in Voc (0.94 V), Jsc (17.45 mAcm-2) and FF (71.24%) compared with conventional control devices with Ca/Al and ITO/PEDOT:PSS electrodes, resulting in greatly improved overall PCE from 9.67% to 11.69%, achieving 91% enhancement compared with inverted cell without TIPD, and 21% improvement compared with conventional device. Therefore higher Voc of 0.94 V for inverted device with TIPD demonstrates that the interfacial layer plays an important role to recovery the Voc. As displayed in Figure 6c, all the devices exhibit wide light response in the spectra of 300-800 nm. The conventional device in range of 300 to 420 nm shows a little higher EQE response than inverted devices, which should be attributed to the high transmittance of ITO/PEDOT:PSS in short spectral range (Figure 2a). The inverted device with TIPD shows enhanced EQE response in 400~800nm with a peak of 75% at 580 nm, implying that TIPD has changed the light distribution in the blend film and further enhanced the light response at long wavelength range, which is in consistent



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with improved Jsc in J-V curves. These results indicate that TIPD is an excellent electron collection material in nonfullerene-based inverted PSCs.

Figure 6. (a) dark J-V, (b) one sun illuminated J-V, and (c) EQE curves of conventional device (ITO/PEDOT:PSS/ PBDTBDD:ITIC-M/Ca/Al), inverted device without TIPD


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(ITO/PBDTBDD:ITIC-M/MoO3/Al),

and

inverted

device

with

TIPD

(ITO/TIPD/PBDTBDD:ITIC-M/MoO3/Al). Table 1. The performance parameters of solar cells based on PBDTBDD:ITIC-M. Device structure

Jsc (mAcm-2)

Voc (V)

Conventional

14.74±0.24

0.936±0.01

70.10±0.85 9.67±0.26

Inverted without TIPD

15.80±0.34

0.720±0.03

53.88±0.61 6.13±0.36

Inverted with TIPD

17.45±0.26

0.940±0.01

71.24±1.0 11.69±0.30

FF (%)

PCE (%)

The parameters are extracted from 20 individual cells. To optimize the photovoltaic properties of the inverted nonfullerene PSCs, we carefully tuned the baking temperature and film thickness of TIPD layer. J-V and EQE curves of the cells are depicted in Figure 7, and the averaged parameters over 20 individual cells are listed in Table 2. The J-V curves of PSCs with TIPD layers annealed at a temperature from 110˚C to 160 ˚C are shown in Figure 7a, and the device performance just slightly changes. The Jsc slightly increases along with enhancing the annealing temperature to 140 ˚C, and then Jsc slowly decreases to 16.63 mAcm-2 when annealing temperature reaches 160 ˚C. While annealing temperature has no influence on the Voc, maintaining a high value of 0.94 V from 110 ˚C to 160 ˚C. The FF reaches a maximum value of 70.35% at 140 ˚C with the best PCE of 11.50%, a Voc of 0.94 V and a Jsc of 17.40 mAcm-2. The EQE curves of nonfullerene inverted PSCs with TIPD treated at different temperature from 110 ˚C to 160 ˚C are shown in Figure 7b. All the devices exhibit wide light-electron response range and the peak EQE at 600nm reaches 75%.



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Interestingly, the EQE peak at 370 nm increases with enhancing the TIPD annealing temperature from 100 to 140 ˚C, and these enhancements should attribute to the decreased absorption of TIPD film under thermal annealing to transform into TOPD layer.35 The properties of nonfullerene inverted PSCs influenced by TIPD thickness are shown in Figure 7c-d and Table 2. The thickness is adjusted by the rotation speed of spin coat, and we set the rotation speed from 2500 to 5000 rpm by step of 500 rpm. The photovoltaic performance is independent to TIPD layer thickness in a large range from 2500 to 5000 rpm, the PCE of the device is over 11%, and the best device performance achieves when spin-coating TIPD at 4000 rpm, giving an FF of 71.24%, a Voc of 0.94 V, a Jsc of 17.45 mAcm-2, and an overall PCE of 11.69%.

Figure 7. (a) J-V and (b) EQE curves of inverted PSCs with TIPD layer treated at


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different temperature from 110 ˚C to 160 ˚C. (c) J-V and (d) EQE curves of inverted PSCs with TIPD layer spin-coated at different rotation speed from 2500 rpm to 5000 rpm.

Table 2. The parameters of inverted PSCs with TIPD electron collection layer baked at various temperatures and spin-coated at different rotation speeds. Conditions TIPD Annealing Temperature (˚C)

TIPD Rotation speed (rpm)

110

Jsc (mAcm-2) 16.38±0.26

Voc (V) 0.94±0.01

FF (%) 69.95±0.70

PCE (%) 10.77±0.20

120

16.39±0.24

0.94±0.01

70.05±0.71

10.79±0.21

130

17.00±0.18

0.94±0.01

70.24±0.74

11.22±0.26

140

17.40±0.24

0.94±0.01

70.35±0.80

11.50±0.32

150

16.92±0.23

0.94±0.01

69.63±0.65

11.07±0.24

160

16.63±0.18

0.94±0.01

70.00±0.67

10.94±0.22

2500

16.67±0.20

0.94±0.01

70.20±0.78

11.00±0.24

3000

16.78±0.18

0.94±0.01

70.18±0.76

11.32±0.28

3500

17.22±0.24

0.94±0.01

70.57±0.75

11.42±0.27

4000

17.45±0.23

0.94±0.01

71.24±0.86

11.69±0.30

4500

17.30±0.24

0.94±0.01

70.36±0.76

11.44±0.26

5000

16.93±0.22

0.94±0.01

69.77±0.62

11.10±0.24

The parameters are extracted from 20 individual cells. The shelf life of non-encapsulated inverted and conventional PSCs were examined by chronologically recording the Jsc (Figure 8a), Voc (Figure 8b), FF (Figure 8c), and PCE (Figure 8d) in a N2-filled glove-box. The inverted device with TIPD layer demonstrates the best long-term stability with almost undecayed Voc and the PCE remains 85% of the initial value after 1440 h due to the slightly decreased Jsc and FF. Similarly,



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the conventional device shows good Jsc and Voc retention but poor FF retention, resulting an average PCE maintenance of 78% after 1440 h. In contrast, the inverted device without TIPD layer displays poor stability and after 1440 h only 71% of the initial PCE keeps with fast Jsc and Voc decline. These results indicate that the electrode modification layer plays an important role for device stability.

Figure 8. Long-term stability of (a) Jsc, (b) Voc, (c) FF and (d) PCE of the conventional and inverted devices with and without TIPD layer.

3. CONCULSION

In conclusion, we have successfully demonstrated efficient and stable non-fullerene inverted PSCs with easily processed TIPD as electrode modification layer. The


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photoactive layer of PBDTBDD:ITIC-M blend trends to form vertical phase separation as confirmed by AFM, SKPM, WCA and TOF-SIMS measurements. Such a vertical distribution of acceptor and donor greatly benefits to the charge transportation and further decrease the carrier recombination rate, resulting higher Jsc of the inverted devices. By carefully tuning the thickness and annealing temperature of TIPD layer, the inverted PSCs based on PBDTBDD:ITIC-M displays promising photovoltaic property with a PCE of 11.69%, achieving 91% enhancement compared with inverted device without TIPD (6.13%), and 21% higher than that of conventional device (9.67%). These findings reveal that TIPD layer is an excellent electrode modification layer to incorporate with vertical phase separated photoactive layer for constructing stable and efficient inverted nonfullerene PSCs.

4. EXPERIMENTAL SECTION

Materials. Etched ITO substrate (15Ω/□) was bought from CSG HOLDING Co., Ltd (China). 1,8-diiodooctane (DIO) and TIPD were acquired from Alfa Aesar. PEDOT:PSS with brand Clevious PVP AI 4083 was purchased from H. C. Stark Company. PBDTBDD and ITIC-M were supplied by Solarmer Materials Inc. MoO3, isopropanol and chlorobenzene (CB) were purchased from Acros. All purchased materials were utilized without purification.

Device fabrication. ITO substrate was successively ultrasonically washed as depicted in our previous study.8 For inverted nonfullerene PSCs with configuration of



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ITO/TIPD/PBDTBDD:ITIC-M/MoO3/Al, the electron collection layer TIPD was prepared by spin-coating diluted TIPD isopropyl alcohol solution onto the cleaned ITO substrates followed by baking in air for 15 min. The photoactive layer was set as 80 nm and prepared by spin-coating PBDTBDD:ITIC-M solution in a N2-filled glove-box and baked for 10 min at 100 ˚C. PBDTBDD:ITIC-M solution was made in chlorobenzene with a mass ratio of 1:1 and the PBDTBDD density is 10 mgmL-1, and 0.5 v/v% DIO additive was added into the mixture before spin-coating. In non-fullerene based PSCs, solvent additive DIO can enhance the nanoscaled phase separation of the blend films, improving the crystallinity and the phase purity of donor and acceptor domains, and resulting in suppressed bimolecular charge recombination and enhanced charge transportation.10, thermal

12

Then MoO3 (10 nm) and Al (80 nm) was deposited by vacuum

evaporation.

For

comparison,

control

conventional

devices

(ITO/PEDOT:PSS/PBDTBDD:ITIC-M/Ca/Al) and control inverted PSCs without TIPD layer (ITO/PBDTBDD:ITIC-M/MoO3/Al) were also fabricated. For conventional device, the PEDOT:PSS layer was obtained by spin-coating the PEDOT:PSS aqueous solution filtered through 0.45 µm filter on the UV ozone treated ITO substrate, and then baked in an oven at 150 ˚C for 15 min. The PEDOT:PSS layer is approximately 30 nm thick. The photoactive layer was directly spin-coated on the PEDOT:PSS layer. Then successively thermally deposite Ca (20 nm) and Al (80 nm) on the photoactive film. For the control inverted device without TIPD layer, the photoactive mixture solution was directly


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spin-coated on the pre-cleaned ITO substrate, and other steps are the same as above mentioned.

Device Characterization. The current density-voltage (J-V) and external quantum efficiency (EQE) measurements were carried out according to our previous operations.41

Instruments.

The

surface

property

of

ITO,

ITO/TIPD,

ITO/TIPD/ITIC-M,

ITO/TIPD/PBDTBDD, ITO/TIPD/PBDTBDD:ITIC-M, ITO/PBDTBDD:ITIC-M and ITO/PEDOT:PSS/PBDTBDD:ITIC-M were analyzed under ambient atmosphere on an atomic force microscope (AFM) with a type of Agilent 5500 in standard SKPM mode. The optical property of interfacial layer were characterized by LAMBDA 950 UV/Vis/NIR Spectrophotometer. The film thickness was measured by Dektak XT (Bruker) surface profilometer. The water contact angle (WCA) measurement was carried out on a JC2000D3 contact angle instrument (Powereach). The secondary ion mass spectra were obtained using a time-of-flight secondary ion mass spectrometer TOF-SIMS 5 from ION-TOF GmbH (Münster, Germany). An Ar+ sputter beam operating at 5 keV with 45° incident angle was used. A depth profiling experiment was performed using an analysis beam of Bi3+ at 30 keV with scanned area of 100 µm by 100 µm.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Z. A. Tan)



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Notes

The authors declare no competing financial interest. ACKNOWLEDGEMENTS

This work was supported by the NSFC (51573042), and Fundamental Research Funds for the Central Universities, China (JB2015RCJ02, 2016YQ06, 2016MS50, 2016XS47).

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TOC


Incorporating an Electrode Modification Layer with a Vertical Phase

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