Inverted thermal annealing of perovskite films: a method for enhancing photovoltaic device efficiency

Abstract

Thermal annealing (TA) plays a vital role in obtaining pinhole-free CH₃NH₃PbI₃ perovskite films with high crystallinity and morphology, which is crucial for achieving high performance for perovskite solar cells (PSCs). In this study, a novel approach named as inverted thermal annealing (ITA) was adopted to dispose the perovskite films. We studied and compared the morphology, surface roughness, and optical and electrical properties of the perovskite films treated by routine TA and ITA, respectively. The perovskite films treated by ITA exhibited higher crystallinity and more favorable morphology and showed stronger of UV light absorption and photoluminescence emission. Therefore, the corresponding PSCs also showed higher average power conversation efficiency (PCE) of 13.49% ± 0.61% than the devices treated by TA (11.37% ± 1.15%), and the PCE of the best performing PSCs is as high as 14.10% with an ITA treatment at 100 °C. These findings suggest that the ITA treatment is an effective and facile method for preparing high-quality perovskite films and can be used widely for obtaining high performance PSCs.

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Introduction

Since the first report of perovskite solar cells (PSCs) in 2009,¹ PSCs have been becoming increasingly a competitive and promising solar to electricity technology under intensity attention in the worldwide photovoltaic community. The power conversion efficiency (PCE) of PSCs has increased sharply from 3.8% to 22.1% in recent years,^1,2 which is due to the excellent optical and electronic properties of organic/inorganic halide perovskite materials, such as high absorption coefficient, long exciton diffusion length, and tunable band gap.^3–5 Furthermore, highly efficient PSCs can also be realized by a solution procedure, which is a low cost and convenient method such as one-step precursor spin-coating deposition (OSPSD)⁶ and two step sequential spin-coating deposition (TSSSD).⁴ However, for solution processed PSCs, it is extremely crucial for the perovskite precursor films to be annealed in a rational condition to accomplish crystallization of the perovskite.^7–10 The traditional thermal annealing (TA) is conducted in near 100 °C for several minutes on a hot plate, which is typically characterized as hot plate/FTO/blocking layer/perovskite films. In this process, the perovskite crystals are induced through the self-assembly of the precursor materials with the residual solvent evaporation and CH₃NH₃I sublimation. Owing to the quick evaporation of the residuals and fast growth of the crystal nucleus,^11,12 it is difficult to obtain an uniform and pinhole-free perovskite film together with high crystallinity and perfect morphology. In fact, there are also some residual precursor materials (such as PbI₂ and MAI) in or above the perovskite films, which are detrimental to device properties of PSCs.¹³

In view of the above deficiencies of a TA treatment for the perovskite films, many efforts have been put forward to develop effective annealing approaches to tune and optimize its crystallinity and morphology, such as solvent-vapor/solvent/mixed-solvents-vapor annealing,^11,14,15 two-step thermal annealing,¹⁶ vacuum-assisted thermal annealing,¹⁷ thermal gradient annealing,¹⁸ and additive,¹⁹ etc. These approaches can effectively enhance the performance of PSCs due to the improved crystallinity and optimized morphology. Nevertheless, the residuals (dimethylformamide (DMF), isopropanol (IPA) and CH₃NH₃I) in or above the perovskite precursor films cannot be effectively utilized during the annealing procedure, considering the directly evaporation and sublimation. In addition, excessive solvent during solvent annealing can break the perovskite films. In order to solve these problems, a novel annealing method needs to be developed urgently.

Herein, we initially introduced inverted thermal annealing (ITA) as the post-annealing approach for perovskite films deposited by two step sequential spin-coating deposition (TSSSD), as shown in Fig. 1. In comparison to conventional TA, it is much easier for an ITA treatment to obtain pinhole-free perovskite films with a superior morphology and high crystallinity. In addition, the intensity of UV-Vis light absorption and photoluminescence emission of the prepared perovskite films are also enhanced after the ITA treatment. As a consequence, the devices treated by ITA at 100 °C exhibit a higher average PCE of 13.49% ± 0.61%, while the devices treated with routine TA show a relative lower PCE value of 11.37% ± 1.15%, the PCE of the best performing device treated by ITA at 100 °C can reach up to 14.10%. Furthermore, an ITA treatment can improve the performance of the perovskite solar cells at random temperature. Compared to traditional TA, ITA is a more effective and facile technique for preparing high-quality perovskite films, which can be used widely for preparing high-performance perovskite photovoltaic devices.

Fig. 1 Schematic of the preparation process of the perovskite films using different post-annealing methods: thermal annealing (TA) and inverted thermal annealing (ITA).

Experimental section

Materials

Unless specified otherwise, all chemicals were purchased from Sigma Aldrich or Acros Organics and used as received without purification, Spiro-OMeTAD was purchased from Toronto Research Chemicals Inc., and methylammonium iodide (CH₃NH₃I) was synthesized according to a previously reported method.²⁰

Device fabrication

The fluorine-doped tin oxide coated conducting glass substrates (FTO, 15 Ω per square, NSG, Japan), which were initially etched with 2 M HCl and Zn powder to a certain pattern, were sequentially cleaned in an ultrasonic bath with detergent, distilled water, acetone, isopropanol and then dried with a nitrogen stream and treated in oxygen plasma for 10 min.

The devices with FTO/C-TiO₂/M-TiO₂/MAPbI₃/spiro-OMeTAD/Au were prepared according to the following process. A TiO₂ compact layer (C-TiO₂), which can reduce the recombination of the electrons and holes in FTO, was prepared by spin-coating (3000 rpm, 45 s) a mildly acidic solution of titanium isopropoxide in isopropanol on the as-cleaned FTO substrates according to the reported procedure and²¹ then annealing at 500 °C for 30 min. Before using, the as-prepared C-TiO₂ substrates were immersed in a 0.02 mM TiCl₄ solution at 70 °C for 30 min and sintered at 500 °C for 30 min after rinsing with deionized water and ethanol. After cooling to room temperature, the TiO₂ mesoporous layer (M-TiO₂) was deposited on the as-prepared C-TiO₂ by spin-coating (5000 rpm, 45 s) commercial TiO₂ paste (Dyesol 18NR-T) that was diluted with anhydrous ethanol at a weight ratio of 2 : 7 and heated at 150 °C for 20 min on a hot plate and then the substrates coated with M-TiO₂ were subsequently sintered at 500 °C for 30 min in an annealing furnace in air.

The CH₃NH₃PbI₃ films were formed using two-step sequential deposition procedure (TSSSD) in a glovebox with N₂ atmosphere. A 462 mg mL⁻¹ (1 M) PbI₂ solution was obtained by dissolving PbI₂ into anhydrous DMF with continuous stirring and heating at 70 °C for 12 h in the dark. The CH₃NH₃I solution was prepared by dissolving 8 mg CH₃NH₃I in 1 mL isopropanol with stirring at room temperature. For the perovskite layer preparation, the PbI₂ layer was first deposited by spin-coating a hot PbI₂ solution on the FTO/C-TiO₂/M-TiO₂ substrates at 3000 rpm for 5 s and 5000 rpm for 5 s. For superior reproducibility of the devices, it is significant to continue heating the PbI₂ solution at 70 °C in this process. After depositing the PbI₂ layer, 70 μL CH₃NH₃I solution in isopropanol was dropped onto the as-prepared PbI₂ layer for 20 s, which was spun with a speed of 6000 rpm for 20 s. After washing with clean anhydrous isopropanol, the films were annealed with two different heating process (TA and ITA) at 80 °C, 100 °C, 130 °C, and 160 °C for 20 min onto a hot plate. Once the films completely converted into the CH₃NH₃PbI₃, 20 μL of HTM solution was dropped onto the CH₃NH₃PbI₃ perovskite layer and spin-coated at 4000 rpm for 30 s. The HTM solution was prepared by dissolving 72.3 mg 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-MeOTAD) in 1 mL anhydrous chlorobenzene, and 30 μL 4-tert-butylpyridine (tbp) and 18 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TSFI in 1 mL acetonitrile) was then added as an additive with stirring.²² The devices were then left in a dry air atmosphere with a low relative humidity in the dark for spiro-MeOTAD oxidation. Finally, 80 nm of the back contact electrodes were deposited via thermally evaporating Au on the top of the spiro-MeOTAD coated devices under a pressure of ca. 10⁻⁶ Torr.

Measurement and characterization

Scanning electron microscopy (SEM) images were recorded by Rili SU 8000HSD Series Hitachi New Generation Cold Field Emission SEM. Atomic force microscopy (AFM) analysis of the perovskite films without depositing HTM and Au were performed using a Bruker Dimension ICON-PT with Co/Cr tips. X-ray diffraction patterns of the perovskite films were obtained using a Shimadzu XRD-6000 X-ray Diffraction instrument with Cu-Kα radiation. The UV-Vis absorption measurements were carried out on a Japan Shimadzu model UV-2250 spectrophotometer. A steady-state (Ss) and time-resolved (Tr) FLS 920 luminescence spectrometer with an excitation wavelength of 425 nm that passed through a 650 nm low-pass filter. The annealing temperature and time of all samples was 100 °C and 20 min, respectively. In particular, the samples for XRD and PL characterizing were produced by directly spin-coating the precursor solution on clean glass slides using the TSSSD procedure.

The photocurrent and voltage properties of PSCs were characterized using CHI660D electrochemical workstation by applying an external bias potential to the solar cells under simulated AM 1.5G sunlight (100 mW cm⁻²) generated from an AAA Class 150 W solar simulator (SAN-EI ELECTRIC, model XES-40S2-CE, Japan) with an AM 1.5G type filter. The light intensity was calibrated by a Newport Oriel PV standard reference cell and meter (model 91150 V); the photoactive area of the solar cells was 0.09 cm², as determined by an metal aperture and the scan rate was 20 mV s⁻¹. Electrochemical impedance spectra (EIS) were obtained on a CHI660D electrochemical workstation (Chenhua, China) at a frequency ranging from 0.1 to 10⁵ Hz with 5 mV perturbation. The impedance spectra data was fitted using the Z-SimpWin software. The External Quantum Efficiency (EQE) were recorded by IPCE measurement system (model 2931-C, Newport, USA) using a 300 W xenon lamp (model 66902, Newport, USA) with a 1/4 monochromator (model 74125 Oriel Cornerstone 260, Newport, USA), light intensity was calibrated using a silicon detector (model 71675, Newport, USA).

Results and discussion

In this study, perovskite films were prepared by two step sequential spin-coating deposition (TTSSD) procedure, as illustrated in Fig. 1. After finishing spin-coating of the MAI layer, it is extremely vital to anneal at a certain temperature for the precursor films completely converting to CH₃NH₃PbI₃. During the process of annealing, the colour of the films gradually changed to dark brown, which is the characteristics feature colour of the perovskite reported elsewhere,²³ and the surface of the films increasingly had a smooth mirror-like appearance, which indicated that the conversion of the precursors to perovskite was accomplished. The change in colour and appearance was accompanied by solvent (DMF and IPA) evaporation in the films, the unreacted MAI sublimation on the surface, and precursors (PbI₂ and MAI) self-assembly reaction. Fig. 1 shows that FTO substrates contacted with a hot-plate in conventional TA, which is from down to up characterized as hot-plate/FTO/TiO₂/MAPbI₃; thus, it is quite obvious that this process failed to make complete use of the residual solvents (DMF and IPA) and reactants (MAI) because the unreacted residues were directly evaporated or sublimated. Furthermore, it was confirmed that a trace of unreacted PbI₂ can be found in perovskite films prepared by TSSSD using TA post-annealing even in a high efficiency device, which cannot guarantee harvesting enough light for the perovskite films.¹³

However, ITA adopted in this study is from down to up characterized as hot-plate/MAPbI₃/TiO₂/FTO (shown in Fig. 1). Undoubtedly, the perovskite films directly contacted with the hot-plate. In this procedure, the vapor of the residual DMF, IPA and MAI are blocked by FTO during evaporating inside the films, which will extend the residence time of these residuals in films. The residuals will continue to react with the precursors on the substrates; thus, the amount of the unreacted PbI₂ can effectively be reduced, and the crystallinity and morphology of perovskite films can be improved.

To verify the influence by the ITA treatment, the morphology of the as-prepared perovskite films were investigated by SEM. In comparison with the films by TA (Fig. 2(a) and (b)), we can clearly observe that the perovskite films treated by TA exhibit randomly distributed different sized crystals with a large amount of pinholes and incomplete coverage on the substrates. In contrast, the films prepared by the ITA treatment are very uniform, and can completely cover the substrate surface due to the fewer grain boundaries. This suggests that the residues (DMF, IPA and MAI) play an outstanding role for re-dissolving the grain boundaries and re-bonding the adjacent grains when the residual vapor went into the voids area and pinholes of the perovskite films.^24–26 Therefore, we propose that this pinhole-free perovskite films with optimal crystallinity and morphology can effectively suppress the recombination of the photo-generated carriers that generally occur at the grain boundaries and defect sites, which can be demonstrated by measuring the dark current and EIS of the corresponding photovoltaic devices.

Fig. 2 Top surfaces SEM and AFM height images of CH₃NH₃PbI₃ films with TA (a and c) and ITA (b and d) treatments at 100 °C for 20 min (the inset images of AFM for 2.0 × 2.0 μm).

We also carried out AFM measurement to assess the influence of the different annealing treatments on the morphology and roughness of the perovskite films, as shown in Fig. 2(c) and (d). This is similar to the SEM results, the ITA treatment for perovskite films can result in lower roughness thin films with a RMS (root-mean-square) of 21.9 nm in the scanning range of 8.0 × 8.0 μm compared to the TA films (30.7 nm). Moreover, it can also be observed that films using ITA have fewer pinholes than the films treated by TA, which is further evidence of the optimized morphology of the perovskite films by ITA.

Perfect perovskite films could lead to a high intensity of light absorption and X-ray diffraction. Therefore, UV-Vis absorption spectroscopy and X-ray diffraction (XRD) was performed to analyze the influence of perovskite films originating from ITA treatment. As shown in Fig. 3(a), regardless of which annealing approach was adopted, the perovskite films exhibit a similar absorption range from visible to near-infrared with the onset at ca. 800 nm, which is the characteristic feature absorption spectrum of the CH₃NH₃PbI₃ films as reported.^27,28 More importantly, we can clearly observe that there is an obvious enhancement over the entire absorption region for the films treated with ITA. Therefore, this phenomenon can be attributed to the continuous coverage on the substrates by uniform perovskite films with perfect crystals treated by ITA, which has been verified by SEM and AFM. XRD was further employed to examine of the degree of crystallinity of CH₃NH₃PbI₃. The XRD peaks (Fig. 3(b)) at 14.2°, 28.5°, and 31.9° correspond to 110, 220, 310 crystal planes of the orthorhombic lattice of CH₃NH₃PbI₃, respectively, as reported elsewhere.²⁹ However, there is also a weak characteristic signal for PbI₂ at 11.5° after the film was treated by TA, which is a common problem for the films prepared by TTSSD.¹³ In addition, the FWHM values of the highest peak at 14.2° were 0.22909 with ITA and 0.25943 with the TA treatment (Fig. S1†). Because the thicknesses of the two films were similar due to the same spin-coating process, the lower value of FWHM suggests an overall increase in the perovskite crystallinity for the films with the ITA treatment.¹⁹

Fig. 3 (a) UV-Vis absorption spectra and (b) XRD patterns of the perovskite films with different thermal annealing treatments (TA and ITA).

To further investigate the carrier behaviors of the perovskite films formed using different annealing processes (TA and ITA), steady-state photoluminescence (Ss-PL) and time-resolved photoluminescence (Tr-PL) measurements were performed for the perovskite films deposited on clean glass slides. As shown in Fig. 4(a) and (b), when the perovskite films were excited at a wavelength of 425 nm, a remarkable emission peak at 785 nm can be observed in the Ss-PL spectra. Comparing to the TA treatment, the intensity of the emission peak of the perovskite films treated by ITA was enhanced, which can be explained by the reduced recombination in the perovskite films due to the improved crystallinity and surface coverage on the substrates.^10,27 In addition, the Tr-PL measurements (Fig. 4(b)) show that the lifetime of the excitons can be prolonged, which also indicates the non-radiative recombination of the perovskite films can be suppressed due to the ITA treatment.

Fig. 4 (a) Normalized steady-state (Ss) and (b) time-resolved (Tr) photoluminescence (PL) spectra of the perovskite films by different thermal annealing treatments collected with a 650 nm low-pass filter (TA and ITA).

To check the influence of the corresponding PSCs for the ITA treatment, perovskite photovoltaic devices with the structure, FTO/C-TiO₂/M-TiO₂/MAPbI₃/Spiro-OMeTAD/Au, were fabricated using an ITA and TA treatment at 100 °C. As shown in Fig. 5(a), the current–voltage curves under illumination with AM 1.5 simulated sunlight (100 mW cm⁻²) and dark is shown in Fig. 5(b). Obviously, remarkable improvement due to ITA treatment can be observed for the short-circuit current density (J_SC), open-circuit voltage (V_OC), file factor (FF) of the PSCs from 20.44 mA cm⁻², 0.93 V, and 0.57 to 21.10 mA cm⁻², 0.95 V, and 0.64, respectively. As a consequence, the PCE of the PSCs was enhanced from 10.89% to 12.88%, which is due to the pin-hole-free perovskite films with improved crystallinity, crystal morphology and continuous coverage on the substrates. In addition, we can also notice that the dark current of the PSCs characterized in the dark treated with ITA can be inhibited, which can be associated with the reduced internal recombination of the photo-generated charges (electrons and holes) because of the decreased grain boundaries after ITA treatment, as demonstrated by SEM and AFM. The external quantum efficiency (EQE) of the as-prepared PSCs was measured. As shown in Fig. 5(c), the PSCs with the ITA treatment displays higher EQE values at a wavelength range from 400 nm to 650 nm than with the TA treatment, which can be accounted for by the enhanced PCE and J_SC.

Fig. 5 (a) Schematic of perovskite solar cells prepared using the TSSSD procedure, (b) typical photocurrent–voltage curve collected under AM 1.5 simulated sunlight (100 mW cm⁻²), at a scan rate of 20 mV s⁻¹; the active area was defined as 0.09 cm² by shielding a black metal mask, (c) the corresponding EQE spectra, insets are the equivalent circuit for fitting (above) and the amplification part of the EIS spectra (underneath). (d) Nyquist plots measured at 0 V bias under simulated AM 1.5G.

Electrochemical impedance spectroscopy (EIS) is a powerful tool for investigating the charge transport and recombination behaviours of the interfaces in different photovoltaic devices.^30,31 To acquire a deeper understanding of the relationship between the performance of the photovoltaic devices and charge transportation, we further carried out EIS on the PSCs with ITA and TA treatment under illumination with a light intensity of 100 mW cm⁻² at a bias voltage of 0 V over the frequency range from 0.1 to 10⁵ Hz. The Nyquist plots are shown in Fig. 5(d), the above inset corresponds to the equivalent circuit for fitting, while the underneath inset is the amplification part of the EIS spectra. Two main features can be observed in the impedance spectra of the PSCs, one semicircle at the high frequency range, and the other transmission line (TL) in the low frequency range. The arc at high frequency was attributed to the charge transfer resistance (R_tr) at the interface of the hole transporting materials (HTM) and perovskite films. In addition, the TL at low frequency was assigned to the recombination resistance (R_rec) at the interface of the TiO₂ and perovskite films. R_s can be obtained from the intersection of the high frequency arc with the real axis.²⁹ The corresponding EIS results for the PSCs treated with TA and ITA are shown in Table S1.† By comparison, it shows that the PSCs with ITA treatment have lower R_tr value of 5.36 ohm cm² (smaller semicircle arc) and higher R_rec value of 10.71 ohm cm² (larger slope of TL), while the PSCs with TA treatment have a higher R_tr value of 5.64 ohm cm² and a lower R_rec of 10.35 ohm cm². Therefore, it can be inferred that the recombination of the photo-generated carriers in PSCs can be effectively suppressed by the ITA treatment, which is attributed indirectly to the enhanced morphology and crystallinity due to the ITA treatment. As a consequence, PSCs treated by ITA can offer a superior PCE with the enhanced J_SC, V_OC and FF.

To further investigate the influence of the properties of the PSCs with ITA treatment at different temperatures, we prepared 20 PSCs for testing and analysis at 80 °C, 100 °C, 130 °C and 160 °C. The statistical analysis results are shown in Fig. 6(a). We can clearly observe that the statistical PCE of the PSCs increased initially and then decreased with increasing annealing temperature from 80 °C to 160 °C, which can be associated with incomplete conversion of precursors to perovskite crystal at low temperature and degradation of the MAPbI₃ at high temperatures (>120 °C). The best performance of the PSCs was obtained by annealing at 100 °C. More importantly, we can clearly observe that the ITA treatment at random temperature can improve the performance of the PSCs compared to the TA treatment. The statistical photovoltaic parameters of these devices are listed in Table 1. Obviously, the devices using the ITA treatment exhibit a higher PCE of 13.49% ± 0.61% with J_SC of 20.87 ± 0.57 mA cm⁻², V_OC of 0.96 ± 0.03 V and FF of 0.69 ± 0.03, while the devices treated with routine TA show a relative lower PCE value of 11.37% ± 1.15% with J_SC of 19.25 ± 0.91 mA cm⁻², V_OC of 0.93 ± 0.04 V and FF of 0.61 ± 0.03, which is in agreement with the abovementioned I–V measurement results.

Fig. 6 (a) Statistical analysis of PCE from 20 PSCs treated by ITA and TA at 80 °C, 100 °C, 130 °C and 160 °C, (b) J–V characteristics of the best performing solar cell by the ITA treatment with two different scan directions: reverse scan (scan from higher bias to lower) and forward scan (scan from lower bias to higher) at a scan rate of 20 mV s⁻¹.

Table 1 Device parameters of the PSCs from 20 devices fabricated with ITA and TA at different temperatures

Annealing	JSC (mA cm^−2)	VOC (V)	FF (%)	PCE^a (%)
a The values are average efficiencies.
ITA 80 °C	19.02 ± 0.78	0.92 ± 0.05	0.63 ± 0.03	11.08 ± 0.96
TA 80 °C	17.35 ± 0.93	0.93 ± 0.04	0.59 ± 0.03	9.46 ± 0.67
ITA 100 °C	20.87 ± 0.57	0.96 ± 0.03	0.69 ± 0.03	13.49 ± 0.61
TA 100 °C	19.25 ± 0.91	0.93 ± 0.04	0.61 ± 0.03	11.37 ± 1.15
ITA 130 °C	19.35 ± 0.47	0.91 ± 0.03	0.65 ± 0.04	11.79 ± 0.85
TA 130 °C	17.62 ± 0.97	0.89 ± 0.03	0.62 ± 0.03	9.79 ± 0.27
ITA 160 °C	17.03 ± 0.87	0.90 ± 0.05	0.61 ± 0.02	9.50 ± 0.0.55

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Hysteresis is a common notorious phenomenon for the J–V characteristics in many photovoltaic devices, which leads to inaccurately assessing the performance of the devices. We investigated further the hysteresis of the best performance devices treated by ITA with different scan directions (forward and reverse scan), as shown in Fig. 6(b) and Table 2. Although the ITA treatment can enhance the PCE for the perovskite devices, a slight J–V hysteresis in the pattern can also be observed. A higher PCE of 14.10% was obtained by reverse scanning with J_SC of 21.25 mA cm⁻², V_OC of 0.99 V and FF of 0.67, while the device measured by forward scanning shows a lower PCE of 12.63% with a J_SC of 20.90 mA cm⁻², V_OC of 0.97 V, and FF of 0.63. Therefore, we can obtain an average PCE of 13.36%. It is well known that the reason for the hysteresis in perovskite devices can be explained by the trapping/de-trapping of charge, ferroelectricity and migration.^32,33 For our own measurement, the deviation between the PCEs acquired by forward and reverse scanning may also be attributed to the decomposition of the perovskite film, which was exposed in air and illumination during characterizing.

Table 2 Device parameters of the best performing PSCs fabricated with ITA

Scan direction	JSC (mA cm^−2)	VOC (V)	FF	PCE (%)	PCE^a (%)
a The values are the average efficiencies.
Reverse scan	21.25	0.99	0.67	14.10	13.36
Forward scan	20.90	0.97	0.63	12.63

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Conclusions

In summary, we report an effective annealing method named inverted thermal annealing (ITA) to induce optimal perovskite films for enhancing the performance of PSCs. By treating with ITA, the perovskite films exhibited higher crystallinity, superior morphology, and stronger UV-Vis light absorption and PL emission. In the ITA treatment process, the residual vapor (DMF, IPA and CH₃NH₃I) can enter the voids area and pinholes of the perovskite films, which can indirectly re-dissolve the grain boundaries and the re-bonded adjacent grains. Consequently, the PCE of the corresponding PSCs using this annealing method can be enhanced effectively at random temperature, the PCE of the best performing device can reach 14.10%, which was treated with ITA at 100 °C. In conclusion, the ITA treatment is an effective and facile technique for preparing high quality perovskite films in the high performance perovskite photovoltaic field.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant 21571042 and 21371040) and the National key Basic Research Program of China (973 Program, 2013CB632900).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07497a

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