Highly efficient and stable perovskite solar cell prepared from an in situ pre-wetted PbI₂ nano-sheet array film

Abstract

Due to its good pore-filling and morphology control, a two-step deposition technique has been widely used in perovskite film preparation. However, the initial PbI₂ film used for the two-step deposition method is usually dense, crystalline and layered, leading to a poor quality perovskite film with a certain amount of PbI₂ residue, thus resulting in an inferior stability and low power conversion efficiency (PCE) of the device. In this work, we developed an in situ pre-wetted PbI₂ nano-sheet array film to achieve high-quality CH₃NH₃PbI₃ (MAPbI₃) perovskite films via a two-step deposition method. By introducing a small amount of hydrogen chloride isopropanol solution (IPA HCl) into the PbI₂ precursor solution, an in situ pre-wetted PbI₂ nano-sheet array film was obtained, and then a smooth, continuous, uniform, dense, and PbI₂-free perovskite film was formed through a reaction with CH₃NH₃I. After optimization, the device prepared using the in situ pre-wetted PbI₂ nano-sheet array film achieved a promising PCE of 19.19%. Moreover, it also had good reproducibility and long term stability, as the devices based on the in situ pre-wetted PbI₂ nano-sheet array film retained over 80% of the initial PCE after exposing to ambient air for two months.

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1. Introduction

Since the first groundbreaking report by Miyasaka in 2009,¹ perovskite solar cells (PSCs) have attracted great scientific and technological interest due to their excellent performance and simple production.^2–5 Over the past 7 years, the certified power conversion efficiency (PCE) of PSCs has increased from 3.8% to 22.1%.^1,6–12 The impressive photovoltaic performance can be attributed to the material’s intriguing optoelectronic properties, such as a relative suitable bandgap, high charge carrier mobility, long carrier diffusion length, and high absorption coefficient and lifetime.^13–17

As one of the most classic organic–inorganic hybrid perovskite materials, CH₃NH₃PbI₃ (MAPbI₃) has been frequently used as a light absorber in the PSCs. The film quality of the MAPbI₃ absorber is very crucial to the photovoltaic performance of the PSCs.¹⁸ Currently, various processes have been developed to improve the crystallinity, uniformity, coverage, and morphology of the MAPbI₃ layer, such as a one-step approach,^19–21 two-step approach,^22–24 vapor approach^8,25 and vapor-assisted solution approach.^26,27 The two-step approach for MAPbI₃ perovskite preparation was initially adapted by Michael Grätzel and his colleagues to fabricate mesoscopic PSCs,²² in which a PbI₂ film was first coated on the substrate and then reacted with CH₃NH₃I (MAI). However, PbI₂ from the initial dimethyl formamide (DMF) solution would form a dense crystalline and layered film on the substrate,^28,29 which makes the conversion of PbI₂ harder and results in a poor morphology of the MAPbI₃ film, leading to two main problems.^30,31 Firstly, an uncontrollable surface morphology and crystal size of MAPbI₃ are unconducive to the improvement of the PCE and reproducibility of the PSCs. Secondly, the residual PbI₂ in the MAPbI₃ films also has a negative impact on the reproducibility and stability of the devices. Various methods have been developed to improve the quality of the PbI₂ precursor films, such as adding additives (4-tert-butylpyridine, CH₃NH₃I, H₂O or hydrochloric acid) into the PbI₂ precursor solution,^30,32–34 careful thermal or gas-quenching treatment,^35,36 synthesizing nanoporous PbI₂,³⁷ and using a mixed solvent or mixed PbI₂ and PbCl₂ precursor solution.^29,38 By means of all these methods, the crystallinity of the PbI₂ layer was decreased and an adjustable morphology was obtained, which could ensure better conversion from PbI₂ to MAPbI₃, and precise control of the final morphology of the MAPbI₃ films, so the PCE and other performances, such as stability and consistency, were enhanced.

Herein, we developed an in situ pre-wetted PbI₂ nano-sheet array film (HP-PbI₂) to achieve high-quality CH₃NH₃PbI₃ (MAPbI₃) perovskite films via a two-step deposition method. By introducing a small amount of hydrogen chloride isopropanol solution (IPA HCl) into the PbI₂ precursor solution, an in situ pre-wetted PbI₂ nano-sheet array film was obtained. This novel porous PbI₂ nano-sheet array film could accelerate the reaction between PbI₂ and MAI, so then a smooth, continuous, uniform, dense and PbI₂-free MAPbI₃ film could be prepared. Compared to the devices prepared from the conventional PbI₂ film, the PCE was enhanced from 12.89% to 19.19% under an optimum addition amount of IPA HCl. Furthermore, the stability and consistency of the devices were also improved significantly.

2. Experimental

2.1 Materials and reagents

Fluorine-doped SnO₂-coated transparent conducting glass (FTO, 15 Ω per square) was obtained from Pilkington TEC. CH₃NH₃I (MAI, 99%) and 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) were purchased from Xi’an Polymer Light Technology Corp. Titanium isopropoxide (98%), lithium bis(trifluoromethylsulphonyl) imide (Li-TFSI, 99.99%) and 4-tert-butylpyridine (TBP, 96%) were bought from Aldrich. Bis(acetylacetonate) (99%), isopropanol (IPA, 99.7%), N,N-dimethylformamide (DMF, 99.9%) and hydrogen chloride isopropanol solution (IPA HCl, for GC, ∼1.25 M (T)) were purchased from J&K Chemicals. TiO₂ (particle size: about 30 nm, crystalline phase: anatase) was obtained from Dysol. Lead(II) iodide (PbI₂, 99.999%) was purchased from TCI. All chemicals were used as received without further purification.

2.2 Device fabrication

A 50 nm-thick TiO₂ compact blocking layer (c-TiO₂) was deposited on the clean and etched FTO substrate by aerosol spray pyrolysis at 450 °C, using a precursor solution of 0.4 mL of bis(acetylacetonate) and 0.6 mL of titanium diisopropoxide in 7 mL of isopropanol. A mesoporous TiO₂ layer (100–150 nm in thickness) was then deposited on the c-TiO₂/FTO substrate by spin coating at 5000 rpm for 30 s. After drying at 100 °C, the mesoporous TiO₂ film was annealed at 510 °C for 20 min to remove the organic residue. For depositing the MAPbI₃ perovskite layer, first a layer of PbI₂ was spin-coated on top of the mesoporous TiO₂ layer from a 1.0 M PbI₂ DMF precursor solution (with or without an IPA HCl additive) at 2500 rpm for 30 s, then a 45 mg mL⁻¹ MAI IPA solution was spin coated on the PbI₂ film at 2500 rpm for 30 s immediately. The deposited film was then annealed at 105 °C for 60 min to form the perovskite layer. The HTM layer was deposited onto the perovskite layer by spin-coating at 2500 rpm for 20 s, using a solution of spiro-MeOTAD, 4-tert-butylpyridine, and lithium bis(trifluoromethylsulphonyl) imide and the Co(III)-complex. Finally, a 60–80 nm-thick Au counterelectrode was deposited by thermal evaporation on the top of the device.

2.3 Measurement and characterization

Photocurrent and voltage curves (J–V) were measured with a solar simulator equipped with a 450 W xenon lamp (AAA simulator, Oriel USA) and a Keithley 2400 source meter. The light intensity was adjusted with an NREL-calibrated Si solar cell with a KG-2 filter for approximating 1 sun light intensity (100 mW cm⁻²). While measuring the current and voltage, the cell was covered with a black mask to define the active area of the device, and in this case it was 0.09 cm². The incident monochromatic photon-to-current conversion efficiency (IPCE) was measured using an IPCE measuring system (Newport Corporation, CA) equipped with a Xe lamp as the light source. Time-resolved photoluminescence (TR-PL) spectra were collected using a transient state spectrophotometer (F900, Edinburgh Instruments). The samples were excited with a 660 nm pulsed diode laser with a repetition rate of 2.5 MHz and an excitation intensity of about 14 nJ cm⁻². Steady-state fluorescence (PL) spectra were recorded by exciting the perovskite films without or with a TiO₂ layer at 473 nm with a standard 450 W xenon CW lamp. The signals were recorded using a spectrofluorometer (Photon Technology International) and analyzed using the software Fluorescence. The morphologies of the PbI₂ and CH₃NH₃PbI₃ (MAPbI₃) films were characterized using a field-emission scanning electron microscope (FE-SEM, ZEISS ΣIGMA) and atomic force microscopy (AFM, using a MultiMode V (Veeco) viewer and analyzer). The X-ray diffraction spectra of the PbI₂ and MAPbI₃ films were recorded on an X’pert PRO X-ray diffractometer. The data were collected at room temperature in the 2θ range 10–60°. Ultraviolet-visible (UV-vis) absorption spectra of all samples were measured using an UV-vis spectrophotometer (U-3900H, HITACHI, Japan) at room temperature.

3. Results and discussion

Scheme 1 shows the preparation process of the perovskite films from (a) the conventional PbI₂ film (C-PbI₂) and (b) the PbI₂ film fabricated using IPA HCl as the additive in the precursor PbI₂ DMF solution (HP-PbI₂). For the conventional PbI₂ film preparation, generally, PbI₂ is dissolved in pure DMF, and this precursor solution is deposited on the substrate to fabricate the PbI₂ film by spin-coating. Although DMF has a good solubility for PbI₂, it is highly volatile, resulting in an inhomogeneous, layered, fast crystallized, and dense PbI₂ film. Then a nonuniform perovskite film with an amount of PbI₂ residue will be obtained because of the limited diffusivity of MAI and the PbI₂ film cannot be transformed completely in a short time. By introducing a suitable amount of IPA HCl into the PbI₂ DMF precursor solution, the quality of the initial PbI₂ film was significantly improved. IPA is uniformly distributed in the PbI₂ film, and could provide reaction channels for the reaction of PbI₂ and MAI. Meanwhile, the existence of HCl broke the continuous PbI₂ crystals in the PbI₂ film into PbI₂ nano-sheet arrays, leading to a complete and homogeneous reaction between PbI₂ and MAI. The optimized amount of IPA HCl for PbI₂ film preparation is 100 μL of IPA HCl in 1 mL of PbI₂ DMF precursor solution.

Scheme 1 Schematic diagrams of the deposition of the perovskite films for (a) C-PbI₂ and (b) HP-PbI₂.

We studied the surface morphologies of C-PbI₂ and HP-PbI₂ using scanning electron microscopy (SEM). As shown in Fig. 1a and b, the conventional PbI₂ DMF precursor solution formed a dense, inhomogeneous, layered PbI₂ film (C-PbI₂). Due to the addition of IPA HCl, HP-PbI₂ was homogeneous and porous, and the continuous PbI₂ crystals were broken into PbI₂ nano-sheet arrays (Fig. 1c and d). This may be due to the existence of HCl in the PbI₂ precursor solution, which led to the formation of the PbI₂·HCl coordination complexes. Fig. 1e shows the AFM images of HP-PbI₂, with a very rough surface being observed. Owing to the formation of the PbI₂ nano-sheet arrays, the HP-PbI₂ film had a high surface roughness, which is in accordance with the SEM data. The XRD pattern of HP-PbI₂ exhibited a very strong unknown XRD peak at around 11.6° as shown in Fig. 2a, with a clear shift from the main peak of PbI₂ for C-PbI₂. This result also suggested that the addition of IPA HCl may have led to the formation of a novel PbI₂·xHCl precursor. The UV-vis absorption spectra of C-PbI₂ and HP-PbI₂ are shown in Fig. 2b. Compared with C-PbI₂, the absorption of HP-PbI₂ was a little weaker. As the continuous PbI₂ crystals in HP-PbI₂ were broken into PbI₂ nano-sheet arrays, this led to HP-PbI₂ being more porous than C-PbI₂. Then less light would be absorbed by HP-PbI₂, so the absorption was weaker.

Fig. 1 Top-view SEM images of (a and b) C-PbI₂ and (c and d) HP-PbI₂, and (e) AFM image (10 μm × 10 μm) of HP-PbI₂.

Fig. 2 (a) XRD patterns and (b) UV-vis absorption spectra of C-PbI₂ and HP-PbI₂.

As can been seen from Fig. 3a and b, there was almost no obvious difference in the colour between the PbI₂ DMF solution with IPA HCl additive and without IPA HCl additive. After the PbI₂ films (HP-PbI₂ and C-PbI₂) had been prepared from these solutions, the colour of HP-PbI₂ was a little lighter than the colour of C-PbI₂. As HP-PbI₂ was more porous than C-PbI₂, less light would be absorbed, so the colour of HP-PbI₂ was lighter. This result was in agreement with the absorption spectra of HP-PbI₂ and C-PbI₂ (Fig. 2b). After spin-coating the MAI IPA solution on HP-PbI₂ and C-PbI₂, the HP-PbI₂/MAI film changed to dark brown immediately, while the C-PbI₂/MAI film only turned to pale brown (Fig. 3e and f), indicating that the existence of IPA in the PbI₂ film can accelerate the reaction between the PbI₂ layer and MAI layer. By annealing at 105 °C for 60 min, the films of HP-PbI₂/MAI and HP-PbI₂/MAI converted into perovskite films and the HCl in HP-PbI₂ was released to the air.^34,39 The real images of the perovskite films prepared from HP-PbI₂ and C-PbI₂ are shown in Fig. 3g and h. From the real images, we can see that the perovskite film prepared from HP-PbI₂ was more uniform and smoother than the film prepared from C-PbI₂.

Fig. 3 The real images of (a) PbI₂ DMF solution with IPA HCl additive, (b) pure PbI₂ DMF solution, (c) HP-PbI₂, (d) C-PbI₂, (e) HP-PbI₂ after spin-coating the MAI film on the top, (f) C-PbI₂ after spin-coating the MAI film on the top, (g) MAPbI₃ perovskite film based on HP-PbI₂ and (h) MAPbI₃ perovskite film based on C-PbI₂.

Fig. 4 shows the top SEM images of the perovskite films prepared from C-PbI₂ and HP-PbI₂. Compared to the MAPbI₃ perovskite film prepared from C-PbI₂, the top morphology of the MAPbI₃ perovskite film prepared from HP-PbI₂ is markedly changed. From Fig. 4a and b, for the surface morphology of the MAPbI₃ perovskite film fabricated by C-PbI₂, huge amounts of uneven grains are observed. Moreover, the whole MAPbI₃ film was also very nonuniform with many voids and pinholes. For the MAPbI₃ perovskite film prepared by HP-PbI₂, a uniform, dense and homogeneous film was obtained (Fig. 4c and d). This change can be ascribed to the following reason. The MAI droplet will react with the PbI₂ grains quickly when it comes into contact with the surface of the PbI₂ film. For C-PbI₂, it was dense and crystallized quickly, so the perovskite ripening process only occurs on the surface of the PbI₂ film and at the periphery of the PbI₂ grains. Moreover, during the perovskite ripening process, great volume expansion occurred for the material because the density of the MAPbI₃ film was only 4.29 g cm⁻³ while that of the PbI₂ film was 6.16 g cm⁻³.^40,41 As C-PbI₂ was too dense, there was no space for horizontal volume expansion during the perovskite ripening process. Therefore, the volume expansion mostly occurred in the vertical direction of the PbI₂ film. Hence, there were huge amounts of uneven grains on the MAPbI₃ film prepared from C-PbI₂. In contrast, for HP-PbI₂, it was homogeneous, porous and in situ pre-wetted by IPA, so could be infiltrated by MAI sufficiently. After annealing, a high quality MAPbI₃ perovskite film was formed. For the mesoscopic PSCs, a poor quality perovskite film results in poor light absorption and increases the charge recombination, so the MAPbI₃ film prepared from HP-PbI₂ is more suitable for the fabrication of high performance PSCs.

Fig. 4 Top-view SEM images of (a and b) the perovskite film prepared from C-PbI₂ and (c and d) the perovskite film prepared from HP-PbI₂.

Fig. 5a shows the XRD patterns of the perovskite films prepared from C-PbI₂ and HP-PbI₂. A strong diffraction peak of PbI₂ around 12.6° is observed in the XRD pattern of the MAPbI₃ film prepared from C-PbI₂, indicating incomplete conversion of PbI₂ in MAPbI₃ preparation, while for the MAPbI₃ film prepared from HP-PbI₂, the signal of PbI₂ almost disappears. Since C-PbI₂ was too dense and crystallized quickly, it was hard for MAI to permeate to the interior and bottom of the PbI₂ film. Therefore, the perovskite ripening process only occurs on the top of the PbI₂ film and periphery of the PbI₂ grains. Hence, a certain amount of PbI₂ residue existed in the MAPbI₃ film based on C-PbI₂. For HP-PbI₂, two great improvements can help to eliminate the PbI₂ residue effectively. Because IPA has a good affinity to PbI₂, it can infiltrate into the PbI₂ film sufficiently and homogeneously. After spin-coating the MAI IPA solution, a concentration gradient of MAI between the MAI layer and PbI₂ layer was formed. Through the IPA diffusion channels, MAI could diffuse to the interior and bottom of the PbI₂ film rapidly. Good permeation of MAI in the PbI₂ film was propitious for the reaction of PbI₂ and MAI. Moreover, the ordered PbI₂ nano-sheet arrays also can react with MAI efficiently. Therefore, compared to C-PbI₂, HP-PbI₂ can eliminate the PbI₂ residue effectively in the MAPbI₃ perovskite preparation. The UV-vis absorption spectra of the perovskite films based on C-PbI₂ and HP-PbI₂ are shown in Fig. 5b. There was a very notable increase in the absorption of the perovskite film prepared from HP-PbI₂ between 500–780 nm, while the perovskite film prepared from C-PbI₂ had a relatively poor absorption. These results also demonstrated that HP-PbI₂ was beneficial for the full conversion of PbI₂ to MAPbI₃ perovskite.

Fig. 5 (a) XRD patterns and (b) UV-vis absorption spectra of the perovskite films prepared from C-PbI₂ and HP-PbI₂.

Using the perovskite films based on C-PbI₂ and HP-PbI₂ as photoactive layers, we fabricated mesoscopic PSCs with a structure of FTO/compact TiO₂/porous TiO₂/MAPbI₃/spiro-MeOTAD/Au. The current–voltage (J–V) curves of typical PSCs based on C-PbI₂ and HP-PbI₂ are presented in Fig. 6a and the corresponding detailed parameters are summarized in Table 1. The typical device based on C-PbI₂ exhibited a PCE of 12.89%, open-circuit voltage (V_oc) of 1.00 V, short-circuit current (J_sc) of 18.79 mA cm⁻², and fill factor (FF) of 0.68. When the perovskite layer was prepared from HP-PbI₂ during the device fabrication, the resulting device exhibited better performance, with V_oc increasing from 1.00 to 1.12 V, J_sc increasing from 18.89 to 22.52 mA cm⁻², and the FF increasing from 0.68 to 0.77. Consequently, the PCE improved from 12.89% to 19.19%. The enhanced V_oc and FF can be explained by the reduced charge carrier recombination which is due to the high quality of the MAPbI₃ film with fewer defects and trap sites when prepared from HP-PbI₂. Moreover, as the MAPbI₃ film based on HP-PbI₂ was of high quality, the hysteresis of the device would be suppressed, which agreed with the J–V curves of the typical PSC based on HP-PbI₂ measured in different scan directions (Fig. 6c). Owing to the full conversion of PbI₂ to CH₃NH₃PbI₃ and as the MAIPbI₃ film was highly crystalline, more visible light would be absorbed and converted, so the J_sc of the PSC based on HP-PbI₂ increased obviously. The incident photon-to-current conversion efficiency (IPCE) spectra for the typical PSCs based on C-PbI₂ and HP-PbI₂ are shown in Fig. 6b. The PSC based on HP-PbI₂ showed a much higher IPCE than the PSC based on C-PbI₂, especially at long wavelengths. The integrated currents of the PSC based on HP-PbI₂ and C-PbI₂ are 19.95 and 16.37 mA cm⁻², which are close to the measured values from the J–V curves. Statistical PCE distributions of 20 samples of the PSCs based on C-PbI₂ and HP-PbI₂ fabricated in one experiment are shown in Fig. 6d. The PSCs based on HP-PbI₂ showed a narrower PCE distribution than the PSCs based on C-PbI₂, which meant a higher consistency was obtained by the PSCs based on HP-PbI₂.

Fig. 6 (a) Current–voltage (J–V) curves and (b) IPCE spectra of the PSCs based on C-PbI₂ and HP-PbI₂. (c) Current–voltage (J–V) curves of the PSC based on HP-PbI₂ in different scan directions and (d) PCE histogram of 20 samples of the PSCs based on C-PbI₂ and HP-PbI₂.

Table 1 Photovoltaic performance summary of the PSCs based on C-PbI₂ and HP-PbI₂

Samples	V oc (V)	J sc (mA cm^−2)	FF	PCE (%)
PSC based on C-PbI2	1.00	18.79	0.68	12.89
PSC based on HP-PbI2	1.12	22.52	0.77	19.19

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Generally, for high efficiency devices, effective charge transport, low charge recombination and long carrier lifetime are needed. A high quality perovskite film may play a key role in achieving these factors for the PSCs. To further study the quality of the perovskite films based on C-PbI₂ and HP-PbI₂ and the electron transport process in the devices, we investigated the steady-state photoluminescence (PL) spectra for the two kinds of perovskite films on different substrates under the excitation wavelength of 473 nm. The PL spectra of glass/MAPbI₃(Bare), glass/MAPbI₃(IPA HCl), glass/TiO₂/MAPbI₃(Bare) and glass/TiO₂/MAPbI₃(IPA HCl) films are shown in Fig. 7a. The MAPbI₃(Bare) and MAPbI₃(IPA HCl) films represent the perovskite films that were prepared from C-PbI₂ and HP-PbI₂, respectively. In comparison to the sample of glass/MAPbI₃(Bare), glass/MAPbI₃(IPA HCl) presented a higher PL intensity. After adding the TiO₂ layer between the perovskite layer and glass, great quenching effects occurred for both of the samples glass/TiO₂/MAPbI₃(Bare) and glass/TiO₂/MAPbI₃(IPA HCl). Moreover, glass/TiO₂/MAPbI₃(IPA HCl) had a lower intensity. These results suggested that the electrons in the MAPbI₃ films prepared from HP-PbI₂ could enter into the TiO₂ electron transport layer easily. In other words, the MAPbI₃ films prepared from HP-PbI₂ were of high quality with fewer defects and trap sites, which were conducive to electron diffusion to the electron transport layer. Time-resolved PL decay (TR-PL) curves of the perovskite films based on C-PbI₂ and HP-PbI₂ are shown in Fig. 7b. The MAPbI₃ perovskite films were prepared on glass. From the TR-PL curves, the charge transfer and carrier recombination behavior in the perovskite films were estimated. From Fig. 7b, the carrier lifetime of the MAPbI₃ film prepared from HP-PbI₂ was 3.8 ns, which was much longer than that of the MAPbI₃ film prepared from C-PbI₂ (1.1 ns). A high quality MAPbI₃ perovskite film with fewer defects and trap sites was obtained from HP-PbI₂, so the carrier lifetime was increased. This result was in accordance with the PL spectra. Hence, the efficiency of the PSCs based on HP-PbI₂ was enhanced.

Fig. 7 (a) Steady-state PL spectra for the perovskite films on different substrates which were prepared from C-PbI₂ and HP-PbI₂. (b) Time resolved photoluminescence spectroscopy (TR-PL) results at 765 nm of the perovskite films prepared from C-PbI₂ and HP-PbI₂.

In addition, the stability of the PSCs based on C-PbI₂ and HP-PbI₂ was investigated. All cells were unsealed and tested in ambient air at room temperature and with a humidity of 30–75%. The V_oc, J_sc, FF, and PCE stability curves of the PSCs based on C-PbI₂ and HP-PbI₂ are shown in Fig. 8. After espousing in ambient air for two months, over 80% of the initial PCE was retained by the PSC based on HP-PbI₂, while the PCE of the PSC based on C-PbI₂ dropped to 40%. Obviously, the PSC based on HP-PbI₂ showed very good long-term stability. According to the literature,^30,42 the unreacted PbI₂ in the MAPbI₃ perovskite film plays a negative role in the long-term stability of the devices. In comparison with the perovskite film prepared from C-PbI₂ which had a large amount of PbI₂ residue, the perovskite film prepared from HP-PbI₂ was PbI₂-free. Moreover, as the perovskite film prepared from HP-PbI₂ was denser and had a higher crystallinity, the harmful substances in the air for perovskites, such as moisture, were less able to penetrate into the internal perovskite layer, so the perovskite film did not decompose easily. Therefore, the PSC based on HP-PbI₂ was more stable than the PSC based on C-PbI₂ in the ambient air.

Fig. 8 Device stability of the PSCs based on C-PbI₂ and HP-PbI₂ with time.

4. Conclusion

In summary, we have developed an in situ pre-wetted PbI₂ nano-sheet array film to achieve high-quality CH₃NH₃PbI₃ (MAPbI₃) perovskite films via a two-step deposition method. By introducing a small amount of IPA HCl into the PbI₂ precursor solutions, an in situ pre-wetted PbI₂ nano-sheet array film was obtained. Both the HCl and IPA in the IPA HCl solution have a positive impact on the formation of the perovskite film. Through the IPA diffusion channels in the PbI₂ film, MAI could diffuse to the interior and bottom of the PbI₂ film rapidly. Meanwhile, the existence of HCl broke the continuous PbI₂ crystals in the PbI₂ film into PbI₂ nano-sheet arrays. The breaking of the ordered PbI₂ crystal structure and good permeation of MAI in the PbI₂ film can make the reaction between PbI₂ and MAI proceed completely and homogeneously. After reacting with MAI, a smooth, continuous, uniform, dense, and PbI₂-free perovskite film was formed. The influence of the addition of IPA HCl on the formation rate, morphology, chemical composition, crystallization, absorbance, and carrier transportation properties of the perovskite films was systemically investigated. With the optimized amount of IPA HCl, which was 100 μL of IPA HCl added in 1 mL of PbI₂ DMF precursor solution, a promising PCE of 19.19% was achieved with the photovoltaic performance giving a V_oc of 1.12 V, J_sc of 22.52 mA cm⁻² and FF of 0.77, which was much higher than the devices prepared from the conventional PbI₂ film. Additionally, the devices fabricated using in situ pre-wetted PbI₂ nano-sheet array films showed very good long-term stability in ambient air due to their PbI₂-free and high quality perovskite film. It is expected that our work provides a simple and rapid way to prepare stable and high efficiency PSCs.

Acknowledgements

This work was financially supported by the National High Technology Research and Development Program of China under Grant No. 2015AA050602, the National Natural Science Foundation of China under Grant No. 21273242, and State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources under Grant No. LAPS14012. This work was also supported by the Science and Technology Support Program of Jiangsu Province under Grant No. BE2014147-4 and the External Cooperation Program of BIC, Chinese Academy of Sciences under Grant No. GJHZ1607.

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