Effect of Energy Alignment, Electron Mobility, and Film Morphology of

Research Article www.acsami.org

Effect of Energy Alignment, Electron Mobility, and Film Morphology of Perylene Diimide Based Polymers as Electron Transport Layer on the Performance of Perovskite Solar Cells Qiang Guo,† Yingxue Xu,† Bo Xiao,‡ Bing Zhang,† Erjun Zhou,*,‡ Fuzhi Wang,† Yiming Bai,† Tasawar Hayat,*,§ Ahmed Alsaedi,§ and Zhan’ao Tan*,† †

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power University, Beijing 102206, China ‡ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China § NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: For organic−inorganic perovskite solar cells (PerSCs), the electron transport layer (ETL) plays a crucial role in efficient electron extraction and transport for high performance PerSCs. Fullerene and its derivatives are commonly used as ETL for p−i−n structured PerSCs. However, these spherical small molecules are easy to aggregate with high annealing temperature and thus induce morphology stability problems. N-type conjugated polymers are promising candidates to overcome these problems due to the tunable energy levels, controllable aggregation behaviors, and good film formation abilities. Herein, a series of perylene diimide (PDI) based polymers (PX-PDIs), which contain different copolymeried units (X), including vinylene (V), thiophene (T), selenophene (Se), dibenzosilole (DBS), and cyclopentadithiophene (CPDT), are introduced as ETL for p−i−n structured PerSCs. The effect of energy alignment, electron mobility, and film morphology of these ETLs on the photovoltaic performance of the PerSCs are fully investigated. Among the PX-PDIs, PV-PDI demonstrates the deeper LUMO energy level, the highly delocalized LUMO electron density, and a better planar structure, making it the best electron transport material for PerSCs. The planar heterojunction PerSC with PV-PDI as ETL achieves a power conversion efficiency (PCE) of 10.14%, among the best values for non-fullerene based PerSCs. KEYWORDS: perovskite solar cell, perylene diimide, electron mobility, energy alignment, film morphology

1. INTRODUCTION

alignment with the active layer for high performance PerSCs, and this has been proven by previously reported ETL free devices with inferior photovoltaic performance.6,7 Currently, the n-type metal oxides, such as TiO2 and ZnO, have been widely employed as ETLs in n−i−p structured PerSCs.8−10 Besides the application of TiO2 and ZnO, there are several reports of using ZrO2 as ETL for PerSC.11,12 However, the performance of ZrO2 based devices is unsatisfactory for the reason that the conduction band of ZrO2 is much higher than the lowest unoccupied molecular orbital (LUMO) of perovskite, which hinders the electron injection from perovskite to ETL. Furthermore, SnO2 has also been utilized as ETL in PerSCs recently.13,14 Because SnO2 exhibits more suitable band alignment with perovskite, the SnO2 based PerSC achieves high

Organic−inorganic hybrid perovskite material have been proven to be an ideal photovoltaic material with high optical absorption coefficient, high charge mobility, and suitable band gap.1 With the fervent efforts from scientists in physics, materials, and chemistry, the power conversion efficiency (PCE) of PerSC has reached 22.1%,2 comparable to the commercially available silicon (Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe) inorganic counterparts. PerSCs originated from dye-sensitized solar cells, where Miyasaka et al. creatively utilized CH3NH3PbI3 as absorber to sensitize the TiO2 electron transport layer (ETL).3 In PerSCs, the function of ETL is selectively extracting electrons from perovskite photoactive layer and then transporting the electrons to the cathode,4 thus enhancing the fill factor (FF) and open circuit voltage (Voc) of the devices.5 Actually, the ETL is an essential component to facilitate electron extraction and band © 2017 American Chemical Society

Received: January 18, 2017 Accepted: March 9, 2017 Published: March 9, 2017 10983

DOI: 10.1021/acsami.7b00902 ACS Appl. Mater. Interfaces 2017, 9, 10983−10991

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ACS Applied Materials & Interfaces

Figure 1. Molecular structures of PX-PDIs.

Voc exceeding 1.19 V.15 In addition to the band alignment, enhancing the electron mobility of ETL is another approach to improve device performance. By doping with Li,16 Mg,17 Al,18 and Nb ions,19 the electronic properties of the ETL can be greatly improved. Contrary to n−i−p type PerSCs, for p−i−n structured devices, organic semiconductors, like n-type [6,6]-phenyl-C61butyric acid methyl ester (PCBM) and fullerene derivatives, are commonly used as ETL.20 Compared to inorganic metal oxides, the organic ETLs do not need high annealing temperature and complex deposit processes, making it possible to large-scale fabricate flexible devices with roll-to-roll methods. Unfortunately, fullerene and its derivatives are small organic molecules and the precursor solution of them has low viscosity. It is still a big challenge to obtain pinhole free ultrathin film of fullerene and fullerene derivatives. Furthermore, fullerene and its derivatives are easy to aggregate with high annealing temperature and thus induce the morphology stability problems. Ntype conjugated polymers are promising candidates to overcome these problems due to the tunable energy levels, controllable aggregation behaviors, and good film formation abilities. There are several publications that have reported the non-fullerene polymer as ETL materials for PerSCs, and the performance of the devices based on non-fullerene polymer ETL is comparable or superior to PCBM base counterpart.21,22 According to the reported papers, an ideal polymeric electron transport material for PerSCs application should have these features: solution processability, suitable highest occupied molecular orbital (HOMO) and LUMO energy levels, sufficient electron mobility, and good aggregation properties. Perylene diimides (PDIs), with fused aromatic rings, high electron affinity, and a rigid planar core, have been regarded as practicable building blocks to n-type organic semiconductors for photovoltaic (PV) and other applications. For PDI-based polymers, the physical, optical, electronic and aggregation properties can be easily modulated by functionalizing PDI or copolymerized building blocks. 23 The PDI-based small molecules and polymers have been successfully employed in organic solar cells.24−27 In PerSCs, some groups have reported the application of PDI-based molecules as interfacial layer and ETL materials.28−30 However, the PDI-based polymers have never been used as ETL in PerSC. In this study, we introduce a series of PDI copolymers (PXPDIs) as ETL materials for PerSCs and investigate the effect of energy alignment, electron mobility, and film morphology of

perylene diimide based polymers as ETL on device performance. PX-PDIs are designed and synthesized by copolymerized PDI with different conjugated units (X), including vinylene (V), thiophene (T), selenophene (Se), dibenzosilole (DBS), and cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT).25−27 Since the electron-donating groups tend to upshift the LUMO level while the electron-withdrawing tend to down-shift the LUMO level,31 PX-PDIs demonstrate different energy levels. The molecular structures of these PX-PDIs are shown in Figure 1.

2. RESULTS AND DISCUSSION The geometries of the five molecules of PX-PDI are optimized at the DFT level using the B3LYP global hybrid functional and the 6-31G* basis set. The front orbital energies are calculated on the B3LYP/6-311G** level. All the computations are carried out with the Gaussian 09 program.32 Figure 2 displays

Figure 2. Theoretical calculated HOMOs and LUMOs of one repeat unit of PX-PDIs. 10984


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Figure 3a shows the UV−vis absorption spectra of PX-PDIs with broad absorption in the visible range. As we know, the wide absorption spectra of electron acceptors contribute to strong light harvesting for organic solar cells. However, the broad and strong absorption of ETLs would lead to the decrease in short-circuit current density (Jsc) in that more light is absorbed by ETL rather than perovskite active layer. Fortunately, the competitive absorption between the ETL and the perovskite is not so serious for p−i−n structured PerSCs, where the ETL is deposited behind the perovskite layer and the incident light reaches the perovskite layer first, so most of the light is absorbed by the perovskite layer. As illustrated in Figure 3b, the UV−vis absorption spectra of CH3NH3PbI3/PXPDI do not display obvious difference compared with that of the CH3NH3PbI3 film, implying the PX-PDI ETLs are more suitable for p−i−n structured PerSCs. So planar heterojunction PerSCs with the p−i−n structure of FTO/PEDOT:PSS/ CH3NH3PbI3/PX-PDI/Al are designed as shown in Figure 3c. In this planar heterojunction PerSC, CH3NH3PbI3 absorbs photons to generate electrons and holes. The PEDOT:PSS will collect and transport the holes to the FTO positive electrode; meanwhile PX-PDI will collect and transport electrons to Al negative electrode. Because the combined electron donor units with PDI are different, the lowest unoccupied molecular orbitals (LUMOs) of PX-PDI are various. Figure 3d gives the energy level diagram of the PerSCs with different PX-PDI ETLs. The LUMO of PV-PDI, PT-PDI, PSe-PDI, PDBS-PDI, and PCPDT-PDI is −4.05, −3.94, −3.97, −3.71, and −3.84 eV, respectively.25−27 Because the electron-donating groups tend to upshift LUMO level while the electron-withdrawing tend to down-shift LUMO level,31 PX-PDIs exhibit various LUMO levels because of the different electron-donating and electron-

the DFT calculated HOMOs and LUMOs of PX-PDIs. The calculated energy levels as well as the dihedral angles between X and PDI units are as listed in Table 1. As shown in Figure 2, the Table 1. Front Orbital Energies and Dihedral Angle of PXPDIsa polymer

HOMO (eV)

LUMOcal (eV)

LUMOexp (eV)

θ (deg)

PV-PDI PT-PDI PSe-PDI PDBS-PDI PCPDT-PDI

−5.91 −5.85 −5.82 −5.73 −5.33

−3.41 −3.40 −3.40 −3.33 −3.36

−4.05 −3.94 −3.97 −3.71 −3.84

36.27 56.34 55.30 56.34 49.52

a

LUMOcal: the LUMO energies calculated by Gaussian 09. LUMOexp: the LUMO energies derived from cyclic voltammograms.

LUMO of PV-PDI is delocalized in the whole molecule, while that of the other candidates is mainly localized in the PDI moieties. Moreover, the reduced dihedral angle between the PDI and X units of PV-PDI shows better planarity and, thereby, enhances the delocalization of both HOMO and LUMO along the backbone of the polymer. The calculated HOMO and LUMO levels are in good consistency with the cyclic voltammograms data. As shown in Table 1, the ethylene containing polymer PV-PDI shows the deepest LUMO level (−3.41 eV) compared to the others, which could be attributed to the relatively strong electron donating ability of the ethylene group. The deeper LUMO energy level, the highly delocalized LUMO electron density, and a better planar structure make PV-PDI the best electron transport material in the PX-PDI group.

Figure 3. (a) UV−vis absorption spectra of PX-PDI films. (b) UV−vis absorption spectra of pristine CH3NH3PbI3 film and PX-PDIs deposited on CH3NH3PbI3 films (quartz substrate). (c) Device structure of planar heterojunction PerSCs with the p−i−n structure of FTO/PEDOT:PSS/ CH3NH3PbI3/PX-PDI/Al. (d) Energy level diagram of the PerSCs. 10985


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Figure 4. AFM (5 μm × 5 μm) topography image of perovskite showing (a) top view and (b) 3D view and profile of height at one cross-section. AFM (5 μm × 5 μm) topography image of (c, d) PV-PDI, (e, f) PT-PDI, (g, h) PDBS-PDI, (i, j) PSe-PDI, (k, l) PCPDT-PDI ETL deposited on perovskite with mixed (CF/CB = 1:1) solvent and pure (CF) solvent.

PDBS-PDI is 2.3 × 10−3, 8.9 × 10−4, and 8.8 × 10−4 cm2 V−1 s−1, respectively, according to the literature.25 The electron mobility of PSe-PDI and PCPDT-PDI was measured by spacecharge-limited current methods,35 calculated to be 7.4 × 10−4 and 2.8 × 10−5 cm2 V−1 s−1, respectively. The current−voltage data of SCLC model is given in Figure S1. The electron mobility of PV-PDI is comparable to that of widely used PCBM, estimated to be 3.0 × 10−3 cm2 V−1 s−1,36,37 guaranteeing efficient electron transport to the Al electrode for application in PerSCs. The electron mobility distinction should owe to different delocalized LUMO levels and different dihedral angles between X and PDI units as shown in Figure 2. The morphology of ETL is another important factor to influence on the PCE of PerSCs. The morphology of perovskite and PX-PDI ETL deposited on perovskite is investigated by atomic force microscopy (AFM) measurements. As shown in Figure 4a and Figure 4b, the perovskite on FTO/PEDOT:PSS substrate exhibits a surface with root-mean-square roughness (RMS) of 9.34 nm, but there are undesired pinholes in the perovskite film. The pinholes may lead to the leakage current

withdrawing nature of V, T, Se, DBS, and CPDT groups. The energy levels of CH3NH3PbI3, PEDOT:PSS, FTO, and Al are taken from the literature.33,34 The LUMOs of PV-PDI, PT-PDI, and PSe-PDI are lower than that of CH3NH3PbI3 (−3.90 eV), about 0.06−0.15 eV, which will facilitate the electron transfer from perovskite to ETLs. Nonetheless, the LUMOs of PDBSPDI and PCPDT-PDI are a little higher than that of CH3NH3PbI3. This means that the electron transfer from perovskite to ETLs should be hampered due to the energy barriers induced by energy level mismatch at the interfaces of CH3NH3PbI3/PDBS-PDI and CH3NH3PbI3/PCPDT-PDI. In addition, the HOMO energy levels of PX-PDIs are sufficiently low (0.3−0.6 eV) to efficiently block holes from the perovskite layer. Therefore, from the viewpoint of energy level alignment, PerSCs based on PV-PDI, PT-PDI, and PSe-PDI should exhibit superior photovoltaic performance than the devices based on PDBS-PDI and PCPDT-PDI. Besides the band alignment, the electron mobility of these ETL materials is another impact factor on the device performance. The electron mobility of PV-PDI, PT-PDI, and 10986


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ACS Applied Materials & Interfaces and deterioration of the device performance.38 When a thin layer of PX-PDI (dissolved in mixed solvent with chloroform (CF)/chlorobenzene (CB) = 1:1) is spin-coated on the pervskite, as shown in Figure 4c,e,g,i,k, the surfaces of all the PX-PDIs ETL are much smoother than the bare perovskite film, with RMS less than 5.3 nm. However, there are differences of morphology between these different PX-PDI ETLs. The RMS of PV-PDI ETL is 3.1 nm, which is the lowest among the PX-PDI due to the decreased dihedral angles. Moreover, the pinholes of perovskite are significantly reduced when depositing PV-PDI as ETL. A uniform and pinhole free ETL can prevent direct contact between Al cathode and perovskite layer. On the other hand, the reduction of pinholes is not obvious when depositing the rest of four PX-PDIs as ETL. This obvious difference can be due to molecular structure affecting ETL aggregation morphology. The PX-PDI aggregated to form ETL as the solvent evaporated, so the solvent chosen to dissolve PXPDIs for ETL deposition has a great impact on ETL morphology. As the ratio of CB and CF for mixed solvent changes, the morphology of PX-PDI ETLs changes. The most typical is as displayed in Figure 4d,f,h,j,l. The morphology of PX-PDIs ETL deposited with pure CF differs greatly from that deposited with the mixed solvent (CF/CB = 1:1). The PX-PDI ETLs obtained by spin-coating PX-PDI/CF solution are inhomogeneous, and even clear holes can be seen in the ETL, though their RMS values are still comparable to the mixed solvent (3.84 nm of PV-PDI, 6.87 nm of PT-PDI, 3.67 nm PDBS-PDI, 4.42 nm of PSe-PDI, 2.97 nm of PCPDT-PDI). Since the boiling points of CB and CF are distinct and CF volatizes much faster than CB at the spin-coating progress, the PX-PDI molecules are more likely rapidly stacked with the pure CF solvent compared to CF/CB mixed solvent. The different stacking of PX-PDI molecules leads to the distinct film morphology and thus affects the electronic transportation. In order to evaluate the charge transfer process between CH3NH3PbI3/PX-PDIs interfaces, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) characterizations were performed. As illustrated in Figure 5a, the CH3NH3PbI3 film shows stronger steady-state PL intensity, while the peak intensity of the CH3NH3PbI3 film quenches 79.9%, 76.6%, 75.4%, 70.6%, and 59.7%, when junctioned with PV-PDI, PT-PDI, PDBS-PDI, PSe-PDI, and PCPDT-PID film. The PL quench should contribute to charge transfer between the interfaces.39 The TRPL characterizations exhibited the same trend as shown in Figure 5b. The TRPL decay curve of pristine perovskite is fitted well with single exponential function, implying great crystalline quality of the CH3NH3PbI3 film.39 The pristine CH3NH3PbI3 film shows a PL lifetime of 25.47 ns. When depositing PX-PDIs as ETL on CH3NH3PbI3 film, the curves decayed much faster. With fitting of the curves of CH3NH3PbI3/PX-PDIs with biexponential function, the average PL lifetime of CH3NH3PbI3/PV-PDI, CH3NH3PbI3/ PT-PDI, CH3NH3PbI3/PDBS-PDI, CH3NH3PbI3/PSe-PDI, and CH3NH3PbI3/PCPDT-PDI is 3.86, 3.72, 5.56, 5.21, and 12.14 ns, respectively, much shorter than pristine CH3NH3PbI3 film. Much faster PL decay of CH3NH3PbI3/PX-PDIs samples suggests efficient charge injected from perovskite layer to ETL.40 The detail fitting parameters are listed in Table 2. The short lifetime τ1 and long lifetime τ2 could attribute to bimolecular recombination and free carriers recombination, respectively.41 The long lifetime fraction A2 for PV-PDI is 32.03%, much lower than that of 82.11% for PCPDT-PDI. Since the increased proportion of τ2 means more free charges

Figure 5. (a) PL and (b) TRPL spectra of pristine CH3NH3PbI3 film and CH3NH3PbI3/PX-PDIs films deposited on quartz substrate.

are recombined at the interface, the ETLs are less efficient to extract and transport free charge. The TRPL measurements indicate that PV-PDI should be the best ETL due to the most efficient charge transfer with CH3NH3PbI3 layer. The effects of PX-PDIs on the photovoltaic performance of PerSCs were examined by constructing p−i−n structured devices as shown in Figure 2c. The current density−voltage (J− V) curves of the devices are shown in Figure 6, and there is no obvious J−V hysteresis in forward and reverse scans. The detailed photovoltaic parameters averaged over 20 individual devices are listed in Table 3. The PerSCs devices based on different PX-PDIs exhibit varied performance. The PCE of device based on PV-PDI, PT-PDI, PDBS-PDI, PSe-PDI, and PCPDT-PDI reaches 10.14%, 5.85%, 5.73%, 5.37%, and 1.91%, respectively (reverse scan), and this difference can mainly contribute to the variation of Jsc and FF. The Voc values of the PX-PDIs based devices are over 0.90 V except for PDBS-PDI based PerSC. The low Voc of PDBS-PDI based device should attribute that the LUMO energy of PDBS-PDI (−3.7 eV) is much higher than that of perovskite (−3.9 eV), thus inducing charge transfer barriers. The average Jsc of PV-PDI, PT-PDI, PSe-PDI, and PCPDT-PDI based device with forward scan and reverse scan is 16.7, 14.2, 13.9, and 6.8 mA/cm2, respectively. The Jsc from J−V measurement is in good agreement with the integrated current from EQE test. There Jsc are consistent with electron mobility of PX-PDIs, namely, the higher electron mobility brings the higher current. One exception occurs for PDBS-PDI based device; the Jsc reaches 18.3 mA/cm2, higher than the PV-PDI based device, while the electron mobility of PDBS-PDI is lower than that of PV-PDI, and the electron extraction and transport for PDBS-PDI are not as good as PVPDI from TRPL test. The unusual Jsc of PDBS-PDI device should be related to the bad morphology of PDBS-PDI film (Figure 4), leading to high leakage current. So the Jsc of PDBS10987


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ACS Applied Materials & Interfaces Table 2. Fitted TRPL Decay Parameters of CH3NH3PbI3 and CH3NH3PbI3/PX-PDIs Filmsa sample

A1 (%)

τ1 (ns)

A2 (%)

τ2 (ns)

τavg (ns)

CH3NH3PbI3 CH3NH3PbI3/PV-PDI CH3NH3PbI3/PT-PDI CH3NH3PbI3/PDBS-PDI CH3NH3PbI3/PSe-PDI CH3NH3PbI3/PCPDT-PDI

100 67.97 70.49 56.24 57.09 17.89

25.47 0.438 0.754 0.765 0.463 1.23

32.03 29.51 43.76 42.91 82.11

11.108 10.908 11.728 11.531 14.512

25.47 3.856 3.750 5.562 5.212 12.136

a

τavg = ∑i Ai τi , where ∑i Ai = 1.

Figure 6. J−V curves of PerSCs with different ETL: (a) PV-PDI, (b) PT-PDI, (c) PDBS-PDI, (d) PSe-PDI, and (e) PCPDT-PDI. (f) EQE curves of the devices with PX-PDIs.

PDI device is not confirmed to EQE integrated current. Moreover, the effect of ETL morphologies on the device performance can also be confirmed from the devices with PXPDIs ETL fabricated with PX-PDIs dissolved in pure CF solvent or CF and CB mixed solvent. The device parameters of PV-PDI based device with PV-PDI dissolved in mixed solvent of CF and CB with different ratio are list in Table S1, and the best performance is achieved with a volume ratio of 1:1. The PCEs of PV-PDI based devices fabricated from pure CF are much worse than the devices made from CF/CB mixed solvent due to the bad film morphology, indicating that the

morphology of the ETL is also a critical point that should be taken into consideration for high efficiency PerSCs. For p−i−n structured PerSCs, the photovoltaic performance is also sensitive to the thickness of ETLs. In order to obtain the optimal thickness of ETL for the PerSC, we varied the PV-PDI thickness from 25 to 55 nm, and the best performance (PCE = 10.14%) is achieved with a 35 nm PV-PDI layer. Figure 7 shows the J−V and EQE curves of PerSCs based on different thickness of PV-PDI. For devices with 55 nm PV-PDI, the PCE is only 6.60%. The deteriorated performance should be ascribed to the increased internal electrical resistance of thick PV-PDI layer, 10988


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ACS Applied Materials & Interfaces Table 3. Performance Parameters of PerSC with PX-PDI ETLs ETL

solvent CB/CF = 1:1

PV-PDI CF CB/CF = 1:1 PT-PDI CF CB/CF = 1:1 PDBS-PDI CF CB/CF = 1:1 PSe-PDI CF CB/CF = 1:1 PCPDT-PDI CF

scan

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

forward reverse forward reverse forward reverse forward reverse forward reverse forward reverse forward reverse forward reverse forward reverse forward reverse

16.8 16.6 6.36 7.11 14.7 13.7 6.19 6.82 17.6 19.0 5.86 6.61 13.8 14.0 8.02 8.83 6.96 6.62 0.008 0.01

0.904 0.931 0.87 0.87 0.898 0.907 0.90 0.89 0.793 0.894 0.70 0.71 0.886 0.901 0.74 0.79 0.921 0.921 0.78 0.82

62.7 65.6 23.5 23.3 47.4 47.3 24.9 27.2 32.5 33.8 25.1 26.3 40.8 42.6 20.9 24.1 25.7 31.3 15.7 15.0

9.52 10.14 1.3 1.44 6.24 5.85 1.38 1.65 4.54 5.73 1.04 1.23 4.98 5.37 1.25 1.69 1.64 1.91 0.001 0.001

states.44 The optimized thickness of the ETL should be the trade-off between the surface coverage and charge transport.

3. CONCLUSION A series of perylene diimide (PDI) based polymers (PX-PDIs) by copolymerized PDI with different conjugated units (X), including vinylene (V), thiophene (T), selenophene (Se), dibenzosilole (DBS), and cyclopentadithiophene (CPDT), are introduced as ETL for p−i−n structured PerSCs. The energy alignment between the perovskite and the ETL can greatly impact the Voc of the PerSCs. The electron mobility and the film morphology of ETLs play an important role in the Jsc of the devices. Due to the deeper LUMO energy level, the highly delocalized LUMO electron density and a better planar structure, PV-PDI demonstrates the best electron transport properties among the PX-PDIs for PerSCs application. The planar heterojunction PerSC with PV-PDI as ETL achieves a PCE of 10.14%, among the best values for non-fullerene based PerSCs. Our findings pave the way for design of PDI based polymers as ETLs for highly efficient and stable PerSCs. 4. EXPERIMENTAL SECTION Materials. Fluorine doped tin oxide (FTO) glass (15 Ω sq−1) was purchased from Wuhan Geao (China). Poly(3,4-ethylenedioxythiophene) doped poly(styrene sulfonate) (PEDOT:PSS, Clevious PVP AI 4083) was bought from H. C. Stark. Methylamine solution (33 wt % in absolute ethanol), hydriodic acid (45 wt % in water), and isopropanol (99.8%, extra dry) were purchased from Acros Organics. N,N-Dimethylformamide (DMF, 99.7+%) and PbI2 (99.9985%) were purchased from Alfa Aesar. All these commercial products are used without further process. PX-PDIs are synthesized according to reported methods.25−27 CH3NH3I Synthesis. CH3NH3I was synthesized according to reported literature with some revision.34 Typically, 11 mL of 40 wt % CH3NH2 aqueous was mixed with 50 mL of ethanol in a roundbottomed flask and then stirred in ice−water bath. Then, 15 mL of 45 wt % HI aqueous was added to CH3NH2/ethanol mixed solution and stirred at 0 °C for 2 h. The reacted solution was rotary evaporated at 60 °C for 1 h. The obtained powder was washed with ethyl ether 3 times. Finally, the product was dried in a vacuum oven at 60 °C overnight.

Figure 7. (a) J−V and (b) EQE curves of devices with PV-PDI ETL thickness of 25, 35, and 55 nm.

leading to high charge recombination.42 Thinner PV-PDI layer is good for decreasing electrical resistance. However, too thin (25 nm) PV-PDI layer cannot completely cover the perovskite surface, which would induce trap state at the perovskite/ETL interface and result in the device PCE decreased to 8.15%. Actually, unlike inorganic semiconductors that have distinct hole and electron quasi Fermi level, the organic semiconductors exhibit gap tail states induced by disorder,43 and not welldistributed ETL will make electron hop between the trap 10989


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ACS Applied Materials & Interfaces Device Fabrication and Characterization. The FTO glass was patterned by Zn powder reacted with hydrochloric acid. The patterned FTO was ultrasonic cleaned with detergent, water, deionized water, and ethanol. The cleaned FTO was treated with ultraviolet ozone for 15 min. PEDOT:PSS was first filtered with 0.45 μm filter. After that, 35 nm PEDOT:PSS thin film was deposited on FTO by spin-coating at 3000 rpm for 30 s and then annealed in a 150 °C oven for 15 min. The obtained FTO/PEDOT:PSS substrate was transferred to a nitrogen-filled glovebox for perovskite layer fabrication. The perovskite layer was fabricated via two-step method. First, 35 μL of PbI2 dimethylformamide (DMF, 500 mg/mL) solution was dropped on FTO/PEDOT:PSS substrate and then spin-coated at 5000 rpm for 30 s. After 30 s, 60 μL of CH3NH3I isopropanol solution was dropped on the PbI2 film and immediately spin-coated at 5000 rpm for 30 s with a ramp of 2000 rpm/s. The obtained pristine film was put on a 100 °C hot plate for 1 h to form perovskite active layer with a thickness of about 290 nm. The PX-PDIs were dissolved in CF or the CF and CB mixed solvent (8 mg/mL) and then spin-coated on the perovsikte film at 2000, 3000, and 5000 rpm for 30 s to form ETL with thickness of 55, 35, and 25 nm, respectively. Finally, 100 nm Al electrode was deposited by thermal evaporation under a vacuum of 3 × 10−4 Pa. The current density−voltage (J−V) curves were measured by computer controlled Keithley 2400 source meter coupled with the 100 mW/cm2 (AM 1.5G) illumination of SAN-EI (AAA grade) solar simulator, which was calibrated with a Si solar cell. External quantum efficiency (EQE) was measured by QE-R systems (Enli Tech.) with light intensity adjusted by a standard single-crystal Si solar cell. The J−V performance of the devices was tested in a glovebox, and the active area is 4 mm2. The EQE test was carried out in atmosphere with relative humidity of 20−40%, and all the devices for the test are unencapsulated. Instrumentation. UV−vis absorption spectra were measured by Shimadzu UV2450 UV−vis spectrophotometer. Photoluminescence (PL) spectra were recorded by exciting with a standard 450 W xenon CW lamp and recording signal by a spectrofluorometer (Photon Technology International). Time-resolved photoluminescence (TRPL) spectra were tested by steady/transient state fluorescence spectrometer (F900, Edinburgh Instruments, U.K.) with 450 nm exciting light. Atomic force microscopy (AFM) morphology data were recorded by Agilent 5500 under tapping mode, and the probe tip model is MikroMasch (HQ:NSC15/Al BS). Electron Mobility Measurement. The electron mobility measurements of PSe-PDI and PCPDT-PDI used the space-chargelimited current (SCLC) method. The single carrier devices with structure of ITO/Al(40 nm)/PSe-PDI(64 nm)/Al(100 nm) and ITO/ Al(40 nm)/PCPDT-PDI(75 nm)/Al(100 nm) were fabricated for electron mobility testing. Electron mobility of the five PX-PDIs were calculated with the equation: J=

ORCID

Erjun Zhou: 0000-0003-1182-311X 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 (Grant 51573042), the National Natural Science Foundation of Beijing (Grant 2162045), the Chinese Academy of Sciences (Grant QYZDBSSW-SLH033), the National Key Basic Research Program of China (973 Project, Grant 2015CB932201), Science and Technology Support Program of Jiangsu Province (Grant BE2014147-4), State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University, Grant 201404), and Fundamental Research Funds for the Central Universities, China (Grants JB2015RCJ02, 2016YQ06, 2015ZZD06).

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⎛ V⎞ 9 V2 εε0μ 3 exp⎜β ⎟ 8 ⎝ L⎠ L

where ε0 is the vacuum permittivity, ε is polymer permittivity (assume ε = 3), and L is the thickness of polymer layer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00902. Current−voltage data of SCLC model; device performance with PV-PDI ETL dissolved in different solvents (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E.Z.: e-mail, [email protected]. *T.H.: e-mail, [email protected]. *Z.T.: e-mail, [email protected]. 10990


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Effect of Energy Alignment, Electron Mobility, and Film Morphology of

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