Incorporating an Electrode Modification Layer with a Vertical Phase

Nov 22, 2017 - Department of Mathematics, Quiad-I-Azam University, Islamabad 44000, Pakistan. ABSTRACT: For bulk heterojunction polymer solar cells...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43871−43879

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Incorporating an Electrode Modification Layer with a 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 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 a phase separated photoactive layer and a titanium chelate electrode modification layer are demonstrated. Water contact angle (WCA), scanning kelvin probe microscopy (SKPM), 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 a titanium (diisopropoxide)-bis-(2,4-pentanedionate) (TIPD) layer increases from 9.67% to 11.69% benefiting from the declined exciton recombination and fairly enhanced charge transportation. Furthermore, the nonencapsulated inverted device with a TIPD layer demonstrates the best long-term stability, 85% of initial PCE remaining and an 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 a vertical phase separated photoactive layer for producing high-efficiency and high-stability inverted nonfullerene PSCs. KEYWORDS: inverted nonfullerene polymer solar cells, vertical phase separation, long-term stability, electrode modification layer, titanium (diisopropoxide)-bis-(2,4-pentanedionate)

1. INTRODUCTION

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 photoactive layer like [6,6]-phenylC61(or C71)-butyric acid methyl ester (PCBM).13,14 Unfortunately, PCBM suffers a narrow and weak absorption in the visible spectra, limiting the photocurrent of devices. In order to solve these problems, various nonfullerene π-conjugated electron acceptors have been developed over the past few years.15−17 Nonfullerene acceptors have been endowed with many superior properties such as good electron transport ability

Polymer solar cells (PSCs) have gained broad focus all over the world since the concept was proposed in the 1990s.1,2 Because the photoactive layer of PSCs comprises organic conjugated semiconductors, PSCs are endowed with many unique characteristics including cost-savings, lightweight quality, and printable manufacture over inorganic solar cells. 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 lag far behind their inorganic counterparts. Therefore, to accelerate the commercialization process of the PSCs, great efforts are still needed to further enhance the PCE and lifetime. © 2017 American Chemical Society

Received: September 6, 2017 Accepted: November 22, 2017 Published: November 22, 2017 43871

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Figure 1. (a) Molecular structures of TIPD, ITIC-M, and PBDTBDD; (b) device structure of inverted PSCs; (c) energy level diagram of inverted PSCs.

and miscibility with polymer donors.17−22 Among the fullerenefree acceptors, ITIC-M (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-(5-or 6-methyl)-indanone)-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) behaves with wide and intense absorption in the visible range.10,23 Because of the many advantages, recently rapid progress based on nonfullerene acceptors has been made, enabling the 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 moisture and oxygen, resulting in 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 PSCs, ITO is modified by an 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 the top contact, rendering high ambient stability of the PSCs. Therefore for inverted PSCs, low temperature and a solution processable electron collection layer are in great demand for inexpensive and large area manufacturing30,31 However, only a few reports are available dealing with inverted PSCs 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 films can be obtained by solution processes followed by low temperature treatment, exhibiting good optical property with an average transparency over 90% in the UV−visible light range and an electron mobility of 3 × 10−3 cm2V−1s−1.35 Herein, we adopt TIPD as an electron transport layer for constructing nonfullerene inverted PSCs based on PBDTBDD and ITIC-M. ITIC-M is an excellent electron acceptor with a broad absorption range extended to near 800

nm and shows good electron transport ability and miscibility with polymer donors. The inverted nonfullerene PSCs with a TIPD layer demonstrate synchronous improvement in shortcircuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) compared with those of conventional control devices featuring Ca/Al and ITO/PEDOT:PSS electrodes, resulting in greatly improved overall PCE from 9.67% to 11.69%.

2. RESULTS AND DISCUSSION The molecular structures of donor PBDTBDD, acceptor ITICM, 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 is shone through a transparent ITO substrate into the photoactive layer of the PSCs. The photogenerated excitons will drift toward 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 the TIPD layer. Since the LUMO of TIPD is −3.91 eV, a little higher than ITIC-M (−3.98 eV), 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 the MoO3 layer. The work-function of MoO3 (−5.30 eV) is quite similar to the HOMO level of PBDTBDD (−5.33 eV), which facilitates the hole transport 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 the ITO electrode. Therefore, from the viewpoint of energy levels, the device 43872

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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 AFM. 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 being

could extract and transport holes or electrons effectively on the corresponding electrode. 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

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

inset shows the transmittance and absorption spectra of a TIPD film deopsited on quartz glass. PEDOT:PSS modified ITO glass exhibits a higher transmittance in short wavelength from 300 to 480 nm while showing lower transmittance in 480 to 800 nm in comparison with the bare ITO glass, which should be ascribed to the sharply contrasting optical constants of PEDOT:PSS with ITO, and this is consistent with previous work.37 Obviously, the TIPD film is highly transparent in the visible region with negligible absorption, showing almost the same transmittance to bare ITO, which is even higher than that of PEDOT:PSS modified ITO. The average transmittance of TIPD modified ITO is 90 ± 2%, and only a slight absorption is observed in the short wavelength range. Such a good transmittance and low absorption makes TIPD an ideal electron collection layer for inverted PSCs. 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. For bulk heterojunction PSCs, the morphology and the phase of the bicontinuous networks between donors and acceptors critically influence exciton dissociation and charge

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.

modified by the TIPD layer (Figure 3d), the RMS slightly increases to 3.84 nm, but it is still smooth and uniform as observed from the phase image (Figure 3e). Figure 3g and j displays the surface morphology of the electron acceptor ITICM 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 43873

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ACS Applied Materials & Interfaces aggregation behaviors from the phase images (Figure 3h and k). There is no obvious aggregation for the 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 the 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 the phase image of a PBDTBDD:ITIC-M film, as shown in Figure 3n. Figure 3p and q give the morphology and phase images of the PBDTBDD:ITIC-M blend film spin-coated on the bare ITO substrate and show a slightly decreased RMS of 2.32 nm and a little larger domain size in comparison with the film deposited on TIPD coated ITO glass, indicating that the interfacial layer also influences the morphology of the photoactive layer grown on it. Figure 3s and t give the morphology and phase image of a PBDTBDD:ITIC-M blend film deposited on PEDOT:PSS coated ITO substrates, and a slightly increased RMS of 2.61 nm and a little smaller domain size are observed in comparison with those of 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 are also influenced by the electrode modification layer. 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 favorable 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 SKPM 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 a 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 a PBDTBDD:ITIC-M blend film is more close to 0.2164 V for the pure PBDTBDD film rather than 0.2335 V for the pure ITIC-M film, demonstrating that PBDTBDD should be dominant in the upper part of the PBDTBDD:ITIC-M blend film, while ITIC-M may distribute at the bottom of the blend film and that there is vertical phase separation between PBDTBDD and ITIC-M within the blend film. Furthermore, the SP for a PBDTBDD:ITIC-M blend film deposited on PEDOT:PSS modified ITO glass is 0.2076 V, much lower than that (0.2187 V) of the film deposited on TIPD coated ITO glass, indicating the electrode modification layer can greatly influence the aggregation behaviors of the blend film. To further confirm the vertical phase separation between PBDTBDD and ITIC-M within the blend film, water contact angles (WCA) of the electrode modified layer, pure ITIC-M, pure PBDTBDD, and PBDTBDD:ITIC-M films on PEDOT:PSS/ITO and TIPD/ITO substrates were examined (Figure 4). The WCA for PEDOT:PSS (Figure 4a), pure ITIC-

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, and (h) PBDTBDD:ITIC-M blend film on TIPD/ITO substrate.

M (Figure 4b), and pure PBDTBDD (Figure 4c) films are 18.24°, 92.62°, and 98.15°, respectively, and the WCA of the PBDTBDD:ITIC-M blend film (Figure 4d) is 98.85°, close to that of pure PBDTBDD film, which confirms PBDTBDD dominating the top surface of the 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 the PBDTBDD:ITIC-M blend film (Figure 4h) is 100.99° close to the value of a 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, and 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 the above-mentioned AFM and SKFM tests. Such a vertical distribution of acceptor and donor would benefit the charge transportation and further decrease the carrier recombination rate, resulting in higher Jsc for inverted devices. 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 CN− and 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 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 increasing depth and that meanwhile the PBDTBDD gathers at the top of the blend film and decreases with increasing depth. To utilize the vertical phase separation of a mixed film, inverted PSCs with a configuration of ITO/TIPD/ PBDTBDD:ITIC-M/MoO3/Al were designed and fabricated 43874

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Figure 5. Depth profile of CN- and S2- in the PBDTBDD:ITIC-M blend photoactive layer.

as shown in Figure 1b. For comparison, control conventional devices (ITO/PEDOT:PSS/PBDTBDD:ITIC-M/Ca/Al) and control inverted devices without a TIPD layer (ITO/ PBDTBDD:ITIC-M/MoO3/Al) were constructed likewise. Figure 6 illustrates the dark J−V (current density−voltage), one sun illuminated 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 a TIPD layer displays higher leakage current density and lower rectification ratio, which suggests an unmatched energy level between the ITO electrode without a buffer layer and the blend film (Figure 1c), resulting in decreased Voc for devices. In contrast, for devices with electrode modification layers (TIPD or PEDOT:PSS), the leakage current is greatly suppressed, resulting in 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 a 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 that of the conventional device due to such a favorable vertical phase separation in the blend layer. After modification with a 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 that of an inverted cell without TIPD, and 21% improvement compared with that of a conventional device. Therefore, higher Voc of 0.94 V for an inverted device with TIPD demonstrates that the interfacial layer plays an important role in recovering 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 a 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 the short spectral range (Figure 2a). The inverted device with TIPD shows an enhanced EQE response in 400−800 nm 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 the long wavelength range, which is consistent 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 a conventional device (ITO/PEDOT:PSS/ PBDTBDD:ITIC-M/Ca/Al), an inverted device without TIPD (ITO/PBDTBDD:ITIC-M/MoO3/Al), and an inverted device with TIPD (ITO/TIPD/PBDTBDD:ITIC-M/MoO3/Al).

To optimize the photovoltaic properties of the inverted nonfullerene PSCs, we carefully tuned the baking temperature and film thickness of the 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 to 160 °C are shown in Figure 7a, and the device performance just slightly changes. Jsc slightly increases along with enhancing the annealing temperature to 140 °C, and then Jsc slowly decreases to 16.63 mAcm−2 when the annealing temperature reaches 160 °C. Annealing temperature has no influence on the Voc, maintaining a high value of 0.94 V from 110 to 160 °C. 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 temperatures from 110 to 160 °C are shown in Figure 7b. All the devices exhibit wide photoelectric response range, and the peak EQE at 600 nm reaches 75%. Interestingly, 43875

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ACS Applied Materials & Interfaces Table 1. Performance Parameters of PSCs Based on PBDTBDD/ITIC-Ma

a

device structure

Jsc (mAcm−2)

Voc (V)

FF (%)

PCE (%)

conventional inverted without TIPD inverted with TIPD

14.74 ± 0.24 15.80 ± 0.34 17.45 ± 0.26

0.936 ± 0.01 0.720 ± 0.03 0.940 ± 0.01

70.10 ± 0.85 53.88 ± 0.61 71.24 ± 1.0

9.67 ± 0.26 6.13 ± 0.36 11.69 ± 0.30

The parameters are extracted from 20 individual cells.

Figure 7. (a) J−V and (b) EQE curves of inverted PSCs with the TIPD layer treated at different temperature from 110 to 160 °C. (c) J−V and (d) EQE curves of inverted PSCs with the TIPD layer spin-coated at different rotation speeds from 2500 to 5000 rpm.

Table 2. Parameters of Inverted PSCs with a TIPD Electron Collection Layer Baked at Various Temperatures and Spin-Coated at Different Rotation Speedsa Jsc (mAcm−2)

conditions TIPD annealing temperature (°C)

TIPD rotation speed (rpm)

a

110 120 130 140 150 160 2500 3000 3500 4000 4500 5000

16.38 16.39 17.00 17.40 16.92 16.63 16.67 16.78 17.22 17.45 17.30 16.93

± ± ± ± ± ± ± ± ± ± ± ±

0.26 0.24 0.18 0.24 0.23 0.18 0.20 0.18 0.24 0.23 0.24 0.22

Voc (V) 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94

± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

FF (%) 69.95 70.05 70.24 70.35 69.63 70.00 70.20 70.18 70.57 71.24 70.36 69.77

± ± ± ± ± ± ± ± ± ± ± ±

0.70 0.71 0.74 0.80 0.65 0.67 0.78 0.76 0.75 0.86 0.76 0.62

PCE (%) 10.77 10.79 11.22 11.50 11.07 10.94 11.00 11.32 11.42 11.69 11.44 11.10

± ± ± ± ± ± ± ± ± ± ± ±

0.20 0.21 0.26 0.32 0.24 0.22 0.24 0.28 0.27 0.30 0.26 0.24

The parameters are extracted from 20 individual cells.

the EQE peak at 370 nm increases with enhancing TIPD annealing temperature from 100 to 140 °C, and these enhancements should be attributed to the decreased absorption of the TIPD film under thermal annealing to transform into the 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 the spin coat, and we set the rotation speed from 2500 to 5000 rpm

by a 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 is achieved 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%. The shelf life of nonencapsulated inverted and conventional PSCs were examined by chronologically recording the Jsc 43876

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

4. EXPERIMENTAL SECTION

(Figure 8a), Voc (Figure 8b), FF (Figure 8c), and PCE (Figure 8d) values in a N2-filled glovebox. The inverted device with a 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, the conventional device shows good Jsc and Voc retention but poor FF retention, resulting in an average PCE maintenance of 78% after 1440 h. In contrast, the inverted device without a 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 in device stability.

4.1. Materials. Etched ITO substrate (15 Ω/□) 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. 4.2. Device Fabrication. The ITO substrate was successively ultrasonically washed as depicted in our previous study.8 For inverted nonfullerene PSCs with a configuration of 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 glovebox and baking for 10 min at 100 °C. PBDTBDD:ITIC-M solution was made in chlorobenzene with a mass ratio of 1:1, the PBDTBDD density is 10 mgmL−1, and 0.5 v/v% DIO additive was added into the mixture before spin-coating. In nonfullerene 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, resulting in suppressed bimolecular charge recombination and enhanced charge transportation.10,12 Then, MoO3 (10 nm) and Al (80 nm) were deposited by vacuum thermal evaporation. For comparison, control conventional devices (ITO/PEDOT:PSS/ PBDTBDD:ITIC-M/Ca/Al) and control inverted PSCs without a TIPD layer (ITO/PBDTBDD:ITIC-M/MoO3/Al) were also fabricated. For a conventional device, the PEDOT:PSS layer was obtained by spin-coating the PEDOT:PSS aqueous solution filtered through a 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 spincoated onto the PEDOT:PSS layer, and then Ca (20 nm) and Al (80 nm) were successively thermally deposited on the photoactive film. For the control inverted device without a TIPD layer, the photoactive mixture solution was directly spin-coated onto the precleaned ITO substrate, and other steps are the same as mentioned above.

3. CONCLUSION In conclusion, we have successfully demonstrated efficient and stable nonfullerene inverted PSCs with easily processed TIPD as an electrode modification layer. The photoactive layer of a PBDTBDD:ITIC-M blend tends to form vertical phase separation as confirmed by AFM, SKPM, WCA, and TOFSIMS measurements. Such a vertical distribution of acceptor and donor greatly benefits charge transportation and further decreases the carrier recombination rate, resulting in higher Jsc of the inverted devices. By carefully tuning the thickness and annealing temperature of the TIPD layer, the inverted PSCs based on PBDTBDD:ITIC-M display promising photovoltaic property with a PCE of 11.69%, achieving 91% enhancement compared with that of the inverted device without TIPD (6.13%) and 21% higher than that of a conventional device (9.67%). These findings reveal that the TIPD layer is an excellent electrode modification layer to incorporate with vertical phase separated photoactive layers for constructing stable and efficient inverted nonfullerene PSCs. 43877

DOI: 10.1021/acsami.7b13494 ACS Appl. Mater. Interfaces 2017, 9, 43871−43879

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ACS Applied Materials & Interfaces 4.3. Device Characterization. The current density−voltage (J− V) and EQE measurements were carried out according to our previous operations.41 4.4. 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 with a type of Agilent 5500 in standard SKPM mode. The optical property of the interfacial layer were characterized by a LAMBDA 950 UV/vis/NIR spectrophotometer. The film thickness was measured by a Dektak XT (Bruker) surface profilometer. The 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 a 45° incident angle was used. A depth profiling experiment was performed using an analysis beam of Bi3+ at 30 keV with a scanned area of 100 μm by 100 μm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhan’ao Tan: 0000-0003-2700-4725 Notes

The authors declare no competing financial interest.

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

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DOI: 10.1021/acsami.7b13494 ACS Appl. Mater. Interfaces 2017, 9, 43871−43879