Introduction

The dramatic emergence of the hybrid inorganic-organic perovskites as a photovoltaic material resulted in a remarkable impact on the field of photovoltaics1,2,3. Such impressive progress in the development of the perovskite-based photovoltaic cells is attributed to the desirable physical properties for this class of materials such as their broadly tunable bandgaps4, high absorption coefficients over a wide range of visible light spectrum5, extremely long carrier diffusion lengths6,40,41,42. We have performed I-V measurements on the spiro-MeOTAD film using the Transfer Length Method (TLM). Resistances across the electrodes with different separations are shown in Fig. 5. The results clearly indicate significant reduction in the resistance of the spiro-MeOTAD layer annealed in O2 ambient. This is indicative of improved do** level for the annealed film. Similar observations of reduction in the cell efficiency for solid-state dye-sensitized solar cells fabricated in N2 had also been reported previously42. This situation differs from the fabrication of organic devices, such as organic light emitting devices and organic solar cells, for which exposure to O2 should be avoided as it can lead to severe device degradation43,44. Considering the fact that moisture is highly devastating to the performance of perovskite-based devices while O2 is necessary for obtaining high efficiency devices, we deposited all the different layers of the device in N2-filled glove box to prevent the exposure to moisture, after that the prepared samples were annealed in dry O2 (high purity grade, >99.9%) for 12 hours at different annealing temperatures.

Figure 5
figure 5

The resistance of the film of spiro-MeOTAD across the electrodes with and without O2 post-deposition treatments.

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The photovoltaic parameters of CH3NH3PbI3-based devices with different post-deposition treatments are summarized in Table 1 while the I-V curves of the representative devices, which are close to the average performance for each post-deposition condition, are presented in Fig. 6a. The experimental data demonstrates substantial enhancement in the PCE (from 6.0% to 8.5%) for the ART-devices compared to the control devices (NRT-devices). The device performances were further improved when the samples were annealed in O2 at an elevated temperature. Under the optimized temperature (65°C), the JSC and fill factor of the A65-devices are significantly enhanced, yielding an average PCE of 12.0%. Since there is no obvious change in surface morphology and absorption as observed from SEM (Fig. S1) and absorption measurement (Fig. 1b) before and after O2 annealing process, the improvement in device performance is believed to originate from the lowering of the defect densities of the perovskite films. It is believed that the thermal annealing process may facilitate the diffusion of O2 to the perovskite film as well as the material interfaces to passivate the under-coordinated cations of the perovskite layer leading to the reduction in the density of the trap states and thereby reducing the recombination rate, which is reflected by the trend of obtained JSC shown in Fig. 6a. It is found that the JSC of the devices strongly depends on the post-deposition treatments, which is observed to increase from an average of 11.7 mA/cm2 for NRT-devices to 21.0 mA/cm2 for A65-devices, suggesting that charges can be extracted more efficiently, likely due to the inhibition of charge recombination at the trap states. Similar work for passivation of halide anions resulting in improved device performance has also been reported previously45. However, further increasing the thermal annealing temperature to 85°C resulted in a drop in the JSC, fill factor and accordingly PCE, which is possibly due to thermally induced degradation of the materials. Besides, a decrease in the magnitude of the JSC for the NRT-devices for V < 0.3 V is observed, which could be attributed to the low conductivity of spiro-MeOTAD and high density of interface states in the absence of O2 annealing, resulting in poor charge collection efficiency. It should be noted that hysteresis effect is generally observed in the I-V characteristics of all our devices, which basically remains unchanged even after the O2 annealing process. This suggests that the O2 annealing process is not able to eliminate the processes/defects responsible for the observed hysteresis effect. The I-V curves shown in the figures were obtained from the reverse scan at the scan rate of 0.01 V/s from 1.2 V to −0.2 V.

Table 1 The average values of photovoltaic parameters obtained from I-V measurements for planar CH3NH3PbI3 based solar cells with different post-deposition treatments and integrated photocurrent from the EQE spectra are indicated in the bracket
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Figure 6
figure 6

(a) The I-V curves and (b) EQE for representative devices with different post-deposition treatments.

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The EQE spectra of CH3NH3PbI3-based devices with different post-deposition treatments are shown in Fig. 6b, which demonstrate wide spectral response from 400 nm to 800 nm in good agreement with the absorption spectra shown in Fig. 1b. The trend of the EQE spectra is also consistent with the I-V performance of the devices. There is a significant enhancement in EQE for the ART and A45 devices compared to the NRT devices. The EQE is further enhanced for the samples annealed in dry O2 at 65 °C while further increasing the annealing temperature to 85°C in the presence of O2 results in a reduction in the EQE. It is interesting to point out that a drop in the EQE results in the short wavelength range is typically attributed to high concentration of defect states in the front heterojunction. Detailed comparison between the EQE data for the control device (NRT) and the annealed device under optimal conditions (A65) we observe significant improvement in the EQE results at the short wavelength range. In general, the reduction in the EQE of solar cells can be attributed to charge recombination and short charge carrier diffusion length46, unequal enhancement in EQE along the spectrum is likely to occur when the condition of post-deposition treatment has not been completely optimized and considerable amount of traps still exist. It appears that O2 annealing may lead to an improvement in the carrier collection efficiency across the TiO2/perovskite interface47and the grain boundaries within the perovskite layer.

Detailed characterization of the time-resolved photoluminescence signal has been performed for both the control sample and the perovskite film annealed in O2 ambient at 65°C. The films were deposited directly on quartz substrates in order to pinpoint the effects of the O2 annealing process on the perovskite film. The experimental data are shown in Fig. 7. It is clear from the data that two separate lifetimes exist in the annihilation of the PL signal indicative of the presence of two recombination pathways48. Substantial improvements in the carrier lifetimes are observed in the O2 annealed sample indicating significant reduction in the density of the localized states in the perovskite film, which is consistent with the results of device performance. It is likely that most of these localized states reside at the grain boundaries however based on the data alone it is not possible to identify the specific location of the trap states. Further work needs to be done to specifically identify the mechanism underlying the observed phenomenon.

Figure 7
figure 7

The time-resolved photoluminescence of the bare perovskite film with or without O2 post-deposition treatment.

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By adopting the strategy of thermally assisted O2 annealing of the deposited films, a champion device with a PCE as high as 15.4% and a high JSC of 23 mA/cm2 can be achieved and the corresponding I-V curves are shown in Fig. 8. It is noted that the reproducibility of the perovskite-based solar cells can be significantly improved by post-deposition treatments, which is reflected by the lower values in the standard deviations in the photovoltaic parameters indicated in Table 1. The proposed fabrication strategy is straightforward and controllable for assembling highly efficient and good reproducible perovskite-based solar cells.

Figure 8
figure 8

(a) The I-V curve of the best device measured at 100 mW/cm2 and in dark.

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Discussion

CH3NH3PbI3-based devices have been fabricated under well controlled fabrication procedures. Our work demonstrated that post-deposition O2 treatments are critical, which not only enhance the device performance substantially, but also improve the device reproducibility. We found that placing the solution-processed films in dry O2 at 65°C prior to electrode deposition is an essential process for device fabrication, yielding an averaged enhanced PCE of 12%. Our results suggest that the improvement in device performance by O2 annealing process is due to the enhancement in the conductivity of spiro-MeOTAD as well as the reduction in the defect density of the perovskite film.

Methods

Materials

Patterned FTO coated glass substrates with a sheet resistance of 7-10 Ω/□ were supplied from KINTEC company. Lead (II) iodide (purity 99%, PbI2), titanium (IV) isopropoxide (TTIP), lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI), 4-tert-butylpyridine (tBP), N,N-dimethylformamide (DMF) and MoO3 were purchased from Sigma-Aldrich while spiro-MeOTAD (purity> 99.5%) was purchased from Luminescence Technology Corp. Methylammonium iodide (CH3NH3I) was purchased from Dyenamo. All chemicals were used as received.

Device fabrication

Patterned FTO on glass substrates were cleaned sequentially by ultrasonication in toluene, acetone, ethanol and deionized water. The substrates were dried by the nitrogen flow and then exposed to UV-ozone for 20 min prior to the spin coating step. TiO2 compact layer was prepared by spin coating a mildly acidic diluted TTIP solution (1.25 ml) in ethanol (25 ml) at 3000 rpm followed by sintering at 450°C for 2 hours. CH3NH3PbI3 was synthesized by a two-step technique14 with optimized solution concentrations in N2 filled glove box. PbI2 films were prepared by spin coating a solution of PbI2 (1500 rpm) dissolved in DMF with a concentration of 462 mg/ml which were then annealed on a hotplate at 70°C for 1 hour prior to the reaction with CH3NH3I (30 mg/ml in isopropanol). The samples were then annealed at 90°C to ensure complete reaction between PbI2 and CH3NH3I. The hole transport layer (HTL) was prepared by spin coating a solution of spiro-MeOTAD (80 mg/ml) dissolved in chlorobenzene with the additives of Li-TFSI (17.5 µL from a stock solution of 520 mg/ml in acetonitrile) and 29 µL of tBP at 4500 rpm. The prepared samples were then stored in N2 ambient at room temperature or in O2 (high purity grade >99.9%) ambient at a flow rate of 2 lit/min at room temperature, 45°C, 65°C or 85°C for 12 hours. Electrodes (MoO3 (15 nm)/Al (120 nm)) were then deposited by thermal evaporation through a shadow mask and the device area was 0.1 cm2.

Device and sample characterizations

The I-V characteristics of the devices were measured using a B1500 A semiconductor parameter analyzer under the calibrated ABET Technologies SUN 2000 solar simulator equipped with an AM 1.5 filter at 100 mW/cm2. The I-V curves are obtained from the reverse scan at the scan rate of 0.01 Vs−1 from 1.2 V to −0.2 V. External quantum efficiency (EQE) was determined by a QE system from Enli Technology Co. Ltd. The surface morphologies of the films were characterized by atomic force microscopy (AFM) in the tap** mode using a Bruker NanoScope 8. UV-visible spectroscopy was performed by using a UV-2550 Shimadzu UV-VIS spectrophotometer for the perovskite film deposited on quartz. X-ray diffraction (XRD) patterns was determined by using a Rigaku SmartLab X-ray diffractometer in a step of 0.01° for 2θ from 10° to 70°. Scanning electron microscopy (SEM) was performed by using Hitachi S-4800 field emission scanning electron microscope. Time-resolved photoluminescence signals of perovskite film were monitored at 775 nm and recorded by using Edinburgh FLSP920 spectrophotometer equipped with the excitation source of 485 nm picosecond pulsed diode laser.