High-Performance Fused Ring Electron Acceptor-Perovskite Hybrid

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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High-Performance Fused Ring Electron Acceptor−Perovskite Hybrid Mingyu Zhang,† Shuixing Dai,† Sreelakshmi Chandrabose,‡ Kai Chen,‡ Kuan Liu,† Minchao Qin,§ Xinhui Lu,§ Justin M. Hodgkiss,‡ Huanping Zhou,*,† and Xiaowei Zhan*,† †

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Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology and School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6010, New Zealand § Department of Physics, The Chinese University of Hong Kong, New Territories 999077, Hong Kong, China S Supporting Information *

ABSTRACT: We report the fused ring electron acceptor (FREA)− perovskite hybrid as a promising platform to fabricate organic−inorganic hybrid solar cells with simple preparation, high efficiency, and good stability. The FREA−perovskite hybrid films exhibit larger grain sizes and stronger crystallinity than the pristine perovskite films. Moreover, the FREA molecules can form coordination bonding with undercoordinated Pb atoms and passivate the trap states in the perovskite films. Time-resolved photoluminescence and transient absorption measurements reveal that FREA facilitates efficient electron extraction and collection. Transient photocurrent and photovoltage measurements suggest faster charge transfer and reduced charge recombination in solar cells based on FREA− perovskite hybrid films. Consequently, solar cells based on FREA− perovskite hybrid films yield a champion efficiency of 21.7% with enhanced stability, which is higher than that of the control devices based on pristine perovskite films (19.6%).

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transport.40−44 However, only a few works used n-type organic semiconductors as additives in perovskite films to realize grain boundary passivation45 or long-term stability of the perovskite precursor solution,46 leading to PCEs of ca. 19%. Recently, we proposed the concept of a fused ring electron acceptor (FREA), which consists of a strong electron-donating planar fused ring core flanked by two strong electron-accepting terminal groups, and now the FREA is emerging as a class of best-performing nonfullerene acceptors in organic solar cells (OSCs).47−52 For example, INIC2 is a FREA molecule based on 6,6,12,12-tetrakis(4-hexylphenyl)indacenobis(dithieno[3,2b;2′,3′-d]thiophene) end-capped by 3-(1,1-dicyanomethylene)-5/6-fluoro-1-indanone terminal groups and exhibits strong absorption in the visible and near-infrared region, high electron mobility, and high efficiency in OSCs.50 In this work, we fabricated FREA−perovskite hybrid films for organic−inorganic hybrid solar cells with high efficiency and good stability. We chose INIC2 based on the following considerations. First, the fluorinated electron-withdrawing endgroups promote the compatibility of INIC2 and DMF solvent. The existence of INIC2 facilitates the nucleation of PbI2 and thus controls the morphology of the perovskite films. Therefore, we obtained optimal perovskite films with larger grain sizes and stronger crystallinity. Second, the INIC2

INTRODUCTION Since the organic−inorganic hybrid perovskite was first used as a light-absorbing material in solar cells in 2009,1 power conversion efficiency (PCE) of perovskite solar cells (PSCs) has achieved an astonishingly rapid increase from 3.8% to over 22%.2−7 The high-quality absorbing layer plays the most important role in the high-performance PSCs. Since pinholes, grain boundaries, crystal defects, and trap states in perovskite films would induce nonradiative charge recombination, reduce carrier lifetime, and decrease device performance, larger grain sizes, stronger crystallinity, and less defects of perovskite films are desired.8−14 Numerous strategies, such as solvent selection for perovskite precursor, thermal annealing, solvent annealing, and additive engineering, have been used to improve perovskite film quality.15−20 In particular, various additives, such as hydroiodic acid (HI),21,22 ionic liquid,23 Lewis base,18,24,25 alkali metal cations (e.g., Li+, Na+, K+, Al3+),26−28 polymers,10,29−35 and fullerene derivatives,11,12,36−39 were incorporated into the precursor solution to manipulate the nucleation and growth process of perovskite films and passivate the trap states caused by undercoordinated Pb2+. n-Type organic semiconductors have been widely used as an electron-transporting layer and interfacial modifying layer in PSCs since they have diverse functional groups with lone pair electrons that can effectively passivate the trap states of the perovskite film and high electron affinity and electron mobility that can promote the electron extraction and electron © XXXX American Chemical Society

Received: August 29, 2018 Published: October 15, 2018 A

DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

Figure 1. Device structure of PSCs and chemical structure of INIC2.

Figure 2. (a) XRD patterns of PbI2 films without and with INIC2 (0.3 mg mL−1); (b) XRD patterns of perovskite films without and with INIC2 (0.3 mg mL−1); GIWAXS patterns of perovskite films without INIC2 (c) and with INIC2 (0.3 mg mL−1) (d).

solution and perovskite film. Top-view scanning electron microscopy (SEM) images of PbI2 films without and with INIC2 are shown in Figure S2. More and smaller PbI2 grains with INIC2 were observed, indicating more nucleation sites and lower nucleation free energy of PbI2. The top-view SEM images of perovskite films (Figure S2) show that the grain size of perovskite films is affected by INIC2 concentration. The grain size of the perovskite film increases with increasing INIC2 concentration from 0 to 0.3 mg mL−1. However, the grain size of perovskite film decreases at an INIC2 concentration of 0.5 mg mL−1. The addition of INIC2 into the PbI2 solution provides heterogeneous nucleation sites to reduce the nucleation free energy of PbI2 and facilitate PbI2 diffusion and reaction with organic cations, leading to the enlarged grain size of perovskite. The crystal structures of PbI2 and perovskite films were studied by X-ray diffraction (XRD). The main diffraction peak intensity of PbI2 decreases (Figure 2a), and its full width at half-maximum (fwhm) increases (Figure S3a) with INIC2. The existence of INIC2 increases the nucleation sites of PbI2

molecule has lone pair electrons, which can passivate the defects caused by undercoordinated Pb atoms and reduce charge recombination. Third, INIC2 promotes efficient charge transport in PSCs due to suitable energy levels and high mobility. Benefitting from the above advantages, the planar heterojunction PSCs based on INIC2−perovskite hybrid films (Figure 1) achieved a champion PCE of 21.7%, which was higher than that of the control devices without INIC2 (19.6%). Moreover, the ambient stability of the devices based on INIC2−perovskite hybrid films was dramatically enhanced relative to the devices based on pristine perovskite films.

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RESULTS AND DISCUSSION Morphology. FREA−perovskite hybrid films were prepared using a simple method; for details see the Supporting Information. The energy-dispersive X-ray spectrometric microanalysis (EDX) of Pb and F elements in PbI2:INIC2 and perovskite:INIC2 films (Figure S1) indicates that both elements show excellent homogeneity and dispersivity, suggesting that INIC2 has good compatibility in the PbI2 B

DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 3. (a) Steady-state PL and (b) TRPL spectra of perovskite without and with INIC2 (0.3 mg mL−1) on a glass/SnO2 substrate.

the inset, indicating the existence of interaction between PbI2 and INIC2.46 Ultraviolet photoelectron spectroscopy (UPS) was conducted to measure the energy levels of perovskite films without and with INIC2 (Figure S5a), and there is no obvious change induced by INIC2, perhaps due to the trace amount of INIC2. The energy level diagram of PSCs is shown in Figure S5b. In order to further investigate the interaction between INIC2 and perovskite, X-ray photoelectron spectroscopy (XPS) was used to measure the surface chemical states of Pb atoms in perovskite films. Figure S6a shows the binding energies of valence electrons for two 4f orbitals of Pb (4f7/2 and 4f5/2). Compared with the pristine perovskite film, the binding energies of C 1s valence electrons show no shift (Figure S6b), while the binding energies of Pb valence electrons in the INIC2−perovskite hybrid film exhibit a slight shift toward lower values. Some atoms or atomic groups with lone pair electrons in INIC2, such as S,46 CO, and CN,40 can form coordination bonds with undercoordinated Pb atoms. The binding energy of Pb 4f orbitals decreases while acquiring electrons. The chemical interaction between Pb atoms and INIC2 was investigated by Fourier transform infrared (FTIR) spectroscopy (Figure S6c). The stretching vibration of carbonyl (CO) in INIC2 shifts to a lower wavenumber, indicating the existence of interaction between PbI2 and INIC2.58 Meanwhile, the trap density of states (tDOS) of the entire devices was measured by thermal admittance spectroscopy (TAS) (Figure S6d). The tDOS with an energy depth of 0.50−0.60 eV is decreased from 1.7 × 1018 to 1.1 × 1018 cm−3 eV−1 after incorporating with INIC2, indicating that INIC2 plays an important role in defect passivation. Photo and Device Physics. To investigate the charge extraction, the steady-state photoluminescence (PL) and timeresolved PL (TRPL) spectra of glass/SnO2/perovskite and glass/SnO2/perovskite:INIC2 were measured. The PL intensity of the perovskite film incorporated with INIC2 becomes weaker than that of the pristine perovskite film on the glass/ SnO2 substrate, indicating more efficient charge transfer at the interface between perovskite and SnO2 (Figure 3a). The TRPL decay was measured with an excitation at 470 nm and fitted the data with a two-component exponential function (Figure 3b). The fast (τ1), slow (τ2), and average lifetime (τav) are calculated and listed in Table S1. The τ av of the perovskite:INIC2 film is shorter than that of the pristine perovskite film on a glass/SnO2 substrate, suggesting a faster electron extraction from the INIC2−perovskite hybrid film to SnO2. We also measured the PL and TRPL spectra of glass/ perovskite (without and with INIC2) and glass/perovskite (without and with INIC2)/HTL, where the hole-transporting

and reduces the energy barrier of nucleation, leading to a faster nucleation and decreased intensity of the main peak.53,54 The XRD patterns of the perovskite films (Figure 2b) have typical perovskite peaks at 13.98° and 28.2°, assigned to the (001) and (002) planes of cubic phase perovskite, respectively. No impurity phases were observed in the INIC2−perovskite hybrid films. The INIC2−perovskite hybrid films exhibit stronger intensity of the (001) diffraction peak with a smaller fwhm (Figure S3b). The PbI2 grains with weaker crystallinity have stronger reactivity with organic cations, leading to formation of high-quality perovskite films with stronger diffraction peaks.55,56 The XRD patterns of perovskite films with different INIC2 concentrations are shown in Figure S3c. The intensity of the (001) diffraction peak of the perovskite film is affected by the INIC2 concentration. The intensity of the (001) diffraction peak enhances with increasing INIC2 concentration from 0 to 0.3 mg mL−1. However, the intensity of the (001) diffraction peak decreases at an INIC2 concentration of 0.5 mg mL−1. These XRD results are consistent with the SEM images. To further investigate the effect of INIC2 on the crystallinity of perovskite films, grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed. The two-dimensional GIWAXS patterns of the perovskite films without and with INIC2 are shown in Figure 2c and d. No diffraction peaks from INIC2 can be identified due to the tiny amount added and the relatively strong crystallinity of perovskite. Both GIWAXS patterns exhibit a strong (001) peak at q = 1 Å−1. The diffraction peak of PbI2 is greatly suppressed after incorporating with INIC2, implying that INIC2 can promote the conversion of PbI2 to the perovskite phase. The similar polar intensity profiles along the ring of 0.98−1.02 Å−1 indicate that the orientation of perovskite is barely changed (Figure S3d). Figure S4a exhibits the UV−vis absorption spectra of perovskite films without and with INIC2. Clearly, compared with the pristine perovskite films, the absorption intensity of the INIC2−perovskite hybrid films is enhanced due to the larger grain size and higher crystallinity of perovskite.57 The transmission and reflection spectra of the perovskite films without and with INIC2 (Figure S4b and c) indicate that the perovskite films with INIC2 exhibit slightly weaker transmittance and reflectance, further proving its stronger absorption. However, the absorption edge is unchanged, indicating the bandgap of the INIC2−perovskite hybrid films is the same as that of the pristine perovskite film. We also measured the absorption spectra of a PbI2 solution without and with INIC2 (Figure S4d); new peaks at 430−500 and 570− 740 nm were observed for the PbI2 solution with INIC2 from C

DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 4. Transient absorption kinetic traces of (a) perovskite (PVSK) and PVSK:INIC2 on glass substrates probed at 750 nm after laser excitation at 400 nm at a pump fluence of 1.8 μJ cm−2 with the corresponding spectra at 10 ps in the inset; (b) SnO2/PVSK and SnO2/PVSK:INIC2 on glass substrates probed at 750 nm after laser excitation at 400 nm at a pump fluence of 0.9 μJ cm−2, which are fitted to a double-exponential function (blue dashes). The extracted effective lifetimes are given in parentheses.

transport in the perovskite:INIC2 samples with larger crystal domain size and fewer defects, in agreement with the TRPL experiment. To further explore the charge transfer and recombination dynamics in devices, transient photovoltage (TPV) and photocurrent (TPC) measurements were carried out. The TPV and TPC measurements were performed in dark conditions, excited by a 532 nm pulse laser with certain intensity, and the data are fitted by a biexponential decay function. As shown in Figure S8a, the devices with INIC2 exhibit a photovoltage decay time of 0.44 ms, which is longer than that of the control devices without INIC2 (0.39 ms). The prolonged lifetime suggests that less carrier recombination takes place in the devices with INIC2. From the TPC measurement (Figure S8b), the devices with INIC2 yield a larger photocurrent with a shorter charge transfer time (1.68 μs) compared with the control devices (2.58 μs), indicating improved electron transport and charge collection. Photovoltaics. The photovoltaic parameters of the PSCs with INIC2 in different concentrations are summarized in Table S2. The optimal concentration of INIC2 is 0.3 mg mL−1. The optimized devices with INIC2 exhibit a peak PCE of 21.7% with a short-circuit current density (JSC) of 23.8 mA cm−2, an open-circuit voltage (VOC) of 1.13 V, and a fill factor (FF) of 80.3%, which is higher than the control devices without INIC2: a maximum PCE of 19.6%, JSC of 23.6 mA cm−2, VOC of 1.08 V, and FF of 77.2% (Figure 5a, Table 1). The increase in VOC and FF for PSCs with INIC2 benefits from more effective charge transport and less carrier recombination. The hysteresis is similar in devices without and with INIC2 (Figure S9). The external quantum efficiency (EQE) of the devices with INIC2 is higher than that of the control devices without INIC2 (Figure 5b), and the calculated JSC from integration of EQE spectra of PSCs with INIC2 is 23.3 mA cm−2, which is higher than that of the control devices (22.5 mA cm−2). The device with INIC2 exhibits a stabilized efficiency of 20.5% over 60 s (Figure 5c), whereas the control device presents a stabilized PCE of 18.3%. The performance of PSCs with INIC2 exhibits good reproducibility (Figure S9c). We also studied the stability of the unencapsulated PSCs without and with INIC2 under ambient conditions (humidity about 30% and temperature about 25 °C) in the dark (Figure 5d). The PCE of the control device without INIC2 retained 20% of its initial value over 60 days, while the optimized device with INIC2 retained about 60% of its initial value. Apparently, PSCs with INIC2 exhibited significantly improved stability due to the higher quality perovskite film with larger grain sizes and better crystallinity to resist moisture penetration.63

the evolution of TA probed at ∼750 nm of perovskite and perovskite:INIC2 on a glass substrate. The similarity of TA kinetics in both samples suggests that INIC2 molecules do not form recombination centers in perovskite, consistent with the high device performance. In Figure 4b, we also investigated the samples of perovskite and perovskite:INIC2 with a SnO2 electron extraction layer. Comparing Figure 4b with Figure 4a, it clearly shows the shorter carrier lifetime due to the carrier recombination at the SnO2/perovskite interface; similar dynamic behavior has been reported in perovskite on an electron extraction layer.61,62 By analyzing sub-nanosecond TA decays, we obtained effective lifetimes 439 and 335 ps of perovskite and perovskite:INIC2 on SnO2, respectively. The slightly faster decay is ascribed to the more efficient carrier

CONCLUSION In summary, we incorporated a fused ring electron acceptor (INIC2) into the PbI2 solution to prepare FREA−perovskite hybrid films. The SEM, XRD, and GIWAXS measurements indicate the perovskite films with INIC2 exhibit larger grain sizes and stronger crystallinity relative to the pristine perovskite films. FTIR, XPS spectra, and tDOS measurements suggest that the electron-rich atoms in INIC2 can form coordination bonds with undercoordinated Pb atoms and passivate the trap states in the perovskite films. Steady-state and time-resolved PL spectra as well as TA measurements reveal that INIC2 is beneficial to efficient electron extraction and collection. TPC measurement demonstrates the faster charge transfer in devices with INIC2, while TPV measurement demonstrates that

layer (HTL) is spiro-OMeTAD. For glass/perovskite (without and with INIC2), the PL intensity of the perovskite film with INIC2 becomes weaker (Figure S7a), and its TRPL decay becomes faster (Figure S7c) relative to that without INIC2, due to the electron-transport property of n-type organic semiconductor INIC2. For glass/perovskite (without and with INIC2)/HTL, the PL intensity of the perovskite with INIC2 becomes weaker (Figure S7b) and its TRPL decay becomes faster (Figure S7d) relative to that without INIC2, suggesting that the hole extraction properties also improve. Since the electrons are extracted by INIC2, the recombination of electrons and holes decreases, facilitating hole extraction. Transient absorption (TA) spectroscopy was performed to investigate ultrafast carrier dynamics in perovskite and perovskite:INIC2 films. The samples were excited at 400 nm and probed by broadband supercontinuum, and the experimental details are in the Supporting Information. Since the majority of photoexcitation species are free carriers in perovskite, we used the bleach peak at ∼750 nm as the signature of carrier population dynamics.59,60 Figure 4a shows

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DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 5. (a) J−V curves and (b) EQE spectra of PSCs without and with INIC2 (0.3 mg mL−1); (c) steady-state measurement of current for PSCs without and with INIC2 (0.3 mg mL−1); (d) stability tests of the PSCs without and with INIC2 (0.3 mg mL−1) under ambient conditions.

Table 1. Photovoltaic Parameters of the Optimized PSCs under Different Scanning Directions PCE (%) −2

−2

a

scanning direction

JSC (mA cm )

calculated JSC (mA cm )

VOC (V)

FF (%)

best

average

reverse forward reverse forward

23.6 22.9 23.8 23.7

22.5

1.08 1.06 1.13 1.11

77.2 74.9 80.3 75.4

19.6 18.1 21.7 19.9

17.9

w/o INIC2 w/INIC2

23.3

19.8

a

Calculated by integration of EQE spectra.

*[email protected]

carrier recombination is reduced due to the defect passivation by INIC2. As a result, the PSCs with INIC2 exhibit PCEs up to 21.7%, higher than that of the control devices without INIC2 (19.6%). Moreover, the devices based on INIC2− perovskite hybrid films exhibit better stability. This work demonstrates that the FREA−perovskite hybrid is a promising platform to fabricate organic−inorganic hybrid solar cells with simple preparation, high efficiency, and good stability.

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ORCID

Xinhui Lu: 0000-0002-1908-3294 Justin M. Hodgkiss: 0000-0002-9629-8213 Huanping Zhou: 0000-0003-4220-1161 Xiaowei Zhan: 0000-0002-1006-3342 Notes

The authors declare no competing financial interest.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09300. Experimental details including materials, device fabrication and measurements, the details of tDOS and TA, device data and additional characterization data, such as EDX, SEM, XRD, UV−vis, transmission, reflection, UPS, XPS, FTIR, tDOS, PL, TRPL, TPV, and TPC (PDF)

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ACKNOWLEDGMENTS

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REFERENCES

X.Z. acknowledges the funding support from the National Natural Science Foundation of China (NSFC, No. 21673011). H.Z. thanks the funding support from the NSFC (Nos. 51722201, 51672008, 91733301), National Key Research and Development Program of China (No. 2017YFA0206701), and Beijing Natural Science Foundation (No. 4182026).

AUTHOR INFORMATION

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050.

Corresponding Authors

*[email protected]

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DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b09300 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX