Enhanced perovskite electronic properties via a modified lead(II) chloride Lewis acid–base adduct and their effect in high-efficiency perovskite solar cells

Abstract

Methylammonium lead triiodide (MAPbI₃) perovskite solar cells have gained significant attention with an impressive certified power conversion efficiency of 22.1%. Suppression of recombination at the interface and grain boundaries is critical to achieve high performance perovskite solar cells (PSCs). Here, we report a simple method to improve the performance of PSCs by incorporating a lead chloride (PbCl₂) material into the MAPbI₃ perovskite precursor through a Lewis acid–base adduct. The optimal concentration of PbCl₂ that helps increase the grain size of MAPbI₃ with introduction of the ideal amount secondary phases (lead iodide and methylammonium lead tri-chloride) is 2.5% (molar ratio, relative to lead iodide). Examination by steady-state photoluminescence and time-resolved photoluminescence has shown that devices based on MAPbI₃-2.5% of PbCl₂ facilitated longer charge carrier lifetime and electron–hole collection efficiency which is ascribed to reduced defects and concurrent improved material crystallinity. Electrochemical impedance spectra (EIS) of the corresponding PSCs have revealed that, compared to the pristine MAPbI₃ perovskite film, the 2.5% PbCl₂-additive increased the recombination resistance of the PSCs by 2.4-fold. Meanwhile, measurement of the surface potential of the perovskite films has indicated that the PbCl₂-additive modifies the electronic properties of the film, shifting the fermi-level of the MAPbI₃ film by 90 meV, leading to a more favourable energetic band matching for charge transfer. As a result, the performance of PSCs is enhanced from an average efficiency of 16.5% to an average efficiency of 18.1% with maximum efficiency reaching 19% due to the significantly improved fill-factor (from 0.69 to 0.76), while the hysteresis effect is also suppressed with the PbCl₂-additive.

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Introduction

Perovskites based on organic–inorganic lead halides with a formula of APbX₃ (A = methylammonium (MA), formamidinium (FA); X = I, Br, and Cl) have attracted significant research interest due to their excellent optoelectronic properties and low-cost fabrication process.¹ Perovskite solar cells (PSCs) which originally adopted the structure of a traditional dye-sensitized solar cell using an iodide/triiodide based liquid electrolyte and MAPbI₃ nanoparticles as a light absorber were reported by Kojima et al. in 2009.¹ Following this seminal report, a significant increase in device performance has been achieved through use of solid state hole transport materials for example, 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene more commonly referred to in the literature as Spiro-OMeTAD. The power conversion efficiency (PCE) of solid-state PSCs has soared from 9.7% in 2012 to a validated PCE of 22.1% in early 2016.^3,4 Recent reports have suggested that the presence of a small amount of secondary phases such as lead iodide (PbI₂) and/or methylammonium lead tri-chloride (MAPbCl₃) in final perovskite films leads to improved performance of devices.^2–4 Furthermore, these studies have shown that un-reacted PbI₂ is mainly located at the grain boundaries (GBs) of the MAPbI₃ film potentially passivating GBs and reducing recombination in the perovskite film,² while MAPbCl₃ acts as a template for the crystallization of MAPbI₃ perovskite.⁵

The solar cell devices that use chloride-based (Cl-based) perovskite layers have demonstrated superior PCE compared to solar cells made with Cl-free, MAPbI₃ counter-parts.^6–9 Stranks et al. pointed out that the electron and hole diffusion length of perovskite films made from the Cl-based precursor can be enhanced from about 100 nm to over 1 μm,¹⁰ indicating a reduced recombination in the bulk perovskite layer or at interfaces between the perovskite layer and selective contacts. Lead chloride (PbCl₂) has been used as a Cl-source for preparing Cl-based perovskite films.^11,12 The use of lead chloride (PbCl₂) as an alternative to PbI₂ in the perovskite precursor was reported to improve the uniformity and surface coverage of MAPbI₃, controlling its nucleation.⁹ Higher concentrations of PbCl₂ in the perovskite precursor induced increased numbers of perovskite crystallites, thus leading to improved film morphology. Furthermore, Huang et al. added additional PbCl₂ (3 at% extra) to form a non-stoichiometric perovskite precursor with PbCl₂ : MAI = 1.03 : 3 (molar ratio) and found that the excess PbCl₂ facilitated the solubility of MAI in the perovskite precursor.¹³ A PCE up to 16.1% was achieved with an average PCE of 13.82% based on PbCl₂-assisted heterogeneous nucleation crystallization. Nevertheless, the effect of excess PbCl₂ on the nucleation process of MAPbI₃ is still unclear. For instance, if PbCl₂ acts as a heterogeneous nucleation site, then the small increment in the content of PbCl₂ in the original precursor solution (from 1 to 1.03 molar ratio with respect to 3 MAI) actually did not significantly increase the number of seeds (no more than 3% with respect to the original number of seeds) for crystal growth.

Although the benefits of Cl on electronic and morphological properties of MAPbI₃ films are excellent, the exact location and the amount of Cl as well as the formation of Cl-based products in the final perovskite films remain unclear. Colella et al. have suggested that Cl is preferentially located near the perovskite/TiO₂ interface, which in turns induces band banding and improves charge collection efficiency of TiO₂.⁶ Song et al. proposed that the perovskite film prepared from a mixture of PbCl₂ and MAI consists of MAPbI₃ and secondary phases, e.g., PbI₂ and MAPbCl₃.⁴ The amount of these secondary phases strongly influences the PCE of PSCs. For instance, above the “optimal amount” of secondary phases, solar cells showed lower photovoltage while below that optimized value solar cells show lower photocurrent and thus lower PCE.⁴ In mixed halide perovskite films prepared from stoichiometric MAI : PbCl₂ (3 : 1, molar ratio), the amount of the secondary phases is often controlled by tuning annealing temperature and time (100 °C, 90 minutes) which is somehow unreliable due to the intrinsic thermal instability of MAPbI₃ perovskite.¹⁴

It has been established that due to the dewetting or agglomeration process of the as-deposited perovskite films prepared from a simple one-step spin-coating method, upon annealing, pin-holes and voids are often observed in the perovskite film even with inclusion of PbCl₂ in the precursor and careful solution preparation.¹² Several methods have been developed to control the crystallization processes of perovskites including using functional additives.^3,15–19 Among them, the method based on antisolvent-dripping which takes place by sudden inducement of super-saturation in the film is found to be very effective,^3,15,16 resulting in the formation of a smooth perovskite film with full surface coverage. Recently, Ahn et al. reported a modified antisolvent-dripping method which can effectively retard the crystal growth process of the perovskite material by forming a Lewis acid–base (MAI-PbI₂-dimethyl sulfoxide (DMSO)) adduct and make the MAPbI₃ film with high quality and full surface coverage.¹⁶ To date, as reported, this is an effective approach to produce high performance MAPbI₃ solar cells. However, current–voltage hysteresis is still observed in the reported MAPbI₃ perovskite solar cell probably due to an inherent unbalanced electron–hole diffusion length (∼130 nm for electrons, ∼90 nm for holes).²⁰ Here we introduce a PbCl₂ additive in the Lewis acid–base adduct method to fabricate a hysteresis-less high PCE solar cell, which has not been previously reported. Since the impact of PbCl₂ on improving the morphology of the perovskite film using the Lewis acid–base adduct method is marginal, this allows us to uncover the underlying reasons for the change of device performance by focusing on the modification of electronic properties of the perovskite material. We have found that this simple method successfully establishes effective secondary phases (PbI₂ and MAPbCl₃) in the perovskite material while minimizing the adverse effect of annealing. These secondary phases and the incorporation of chloride into the perovskite lattice enable high PCE solar cells with less hysteresis and an improved fill factor.

Experimental

Materials preparation

All materials were purchased from Sigma-Aldrich and used as received without further purification unless otherwise stated. MAPbI₃ perovskite precursor solution was prepared in an argon-filled glovebox by dissolving 461 mg of PbI₂, 159 mg of methylammonium iodide (MAI) (Dyesol), and 78 mg of dimethyl sulfoxide (DMSO) in 700 mg of dimethyl formamide (DMF) at room temperature, under rigorous stirring for one hour. Mixed halide perovskite MAPbI₃–PbCl₂ additive precursor solution was prepared by adding different amounts of PbCl₂ (1%, 2.5%, 5%, 7.5%, and 10% (molar ratio with respect to PbI₂)) to the prepared MAPbI₃ perovskite precursor. The solution was then magnetically stirred for an additional hour. The prepared perovskite solution was filtered by using a syringe filter (pore size: 0.22 μm) prior to use for deposition of the film. A solution for the hole transporting material was prepared by addition of 72.3 mg of 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-MeOTAD) (Borun New Material), 28.8 μL of 4-tert-butylpyridine, and 17.5 μL of bis(trifluoromethane)sulfonimide lithium (Li-TFSI) solution (520 mg of Li-TFSI in acetonitrile) into 1 mL of chlorobenzene.

Device fabrication

Solar cells were fabricated using fluorine-doped tin oxide (FTO) coated glass (Nippon Electric Glass, 15 Ω □⁻¹) as the substrate which was firstly patterned through partial removal of FTO via etching using 35.5 wt% HCl and zinc powder. The substrates were then cleaned in sequence in 5% Decon-90 detergent, and a mixture of acetone, isopropanol and ethanol for 20 min each in an ultrasonic bath. Prior to use, the substrates were treated with ultraviolet ozone for 30 min to fully remove organic solvent residuals. An electron transporting layer based on the TiO₂ film (∼40 nm) was deposited in air via spin-coating a 0.15 M solution of titanium diisopropoxide bis(acetylacetonate) in 1-butanol at 2000 rpm for 20 s. The film was then dried at 125 °C for 5 min and annealed at 450 °C for 30 min. A mesoporous TiO₂ (mp-TiO₂) layer (∼200 nm) was spin-coated onto the compact TiO₂ film using a diluted TiO₂ paste (0.12 g TiO₂ paste (Dyesol) in 1 mL of absolute ethanol) at 2000 rpm for 20 s, followed by sintering at 450 °C for 30 min. After cooling to room temperature, the film was treated with 20 mM TiCl₄ aqueous solution at 90 °C for 10 min. The TiCl₄-treated film was cleaned with distilled water and annealed again at 450 °C for 30 min. After this, the TiO₂ coated film was treated in a UV-zone for 20 min before being transferred to an Ar-filled glovebox. Perovskite layers (∼400 nm) with and without PbCl₂ in the perovskite precursor solution were deposited onto the prepared TiO₂ layer at 4000 rpm for 20 s. During spin-coating, 0.5 mL of diethyl ether was dropped on the center of the spinning substrate before it turned turbid. The perovskite layer was then dried at 65 °C for 1 min, and annealed at 100 °C for 2 min.¹⁶ The hole-transport layer (∼200 nm) was deposited from the prepared Spiro-OMeTAD solution onto the as-prepared perovskite layer at 4000 rpm for 25 s. The device fabrication was finished by deposition of a 100 nm layer of gold film for back contact on the prepared sample via an e-beam evaporation process under 10⁻⁶ torr pressure.

Characterization

The top-view and cross-sectional scanning electron microscopy (SEM) images of the samples were taken using a field emission scanning electron microscope (FSEM JOEL 7001F) at an acceleration voltage of 5 kV. The UV-vis absorbance spectrum was measured with a UV-visible spectrometer (Cary 50). The crystal structure of the perovskite film as-deposited on FTO/compact TiO₂/mp-TiO₂ was determined by X-ray diffraction (Rigaku SmartLab) with monochromatic CuKα (λ = 0.154 nm) as an excitation source. A scan rate of 1.5° per minute and a step size of 0.015° were used in the XRD measurement. The performance of perovskite devices was measured under irradiation of 100 mW cm⁻² (AM1.5) provided by a solar simulator (Oriel Sol3A, Newport) equipped with a 450 W Xenon lamp. IPCE measurement was conducted by using a quantum efficiency system (IQE 200B, Newport) in AC mode. Electrochemical impedance spectroscopy (EIS) of the PSCs was performed in a frequency range from 1 MHz to 100 mHz using an electrochemical workstation (VSP BioLogic Science Instruments) at a forward bias of 0.5 V in darkness. An AC voltage with a perturbation amplitude of 10 mV was applied in the EIS measurement. X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra) using mono Al Kα (1486.6 eV) X-rays was used to detect the elements in the perovskite film. For XPS depth profiling, a 4 keV Ar⁺ ion was used for the charge-up effect. The photoluminescence (PL) spectrum was measured with a fluorescence spectrometer (Edinburgh Instruments Ltd) at room temperature. The film was photo-excited by using a laser (474 nm) with a pulse wavelength of 82.4 ps. Scanning Kelvin Probe Force Microscopy (KPFM) (Oxford instrument, Asylum Research) was performed on the prepared perovskite film under ambient conditions using a NSG-03 Pt coated cantilever at room temperature. The work function of the cantilever was measured using a HOPG standard sample. The chloride analysis was performed on a Dionex RFIC ICS-2100 Ion Chromatography (IC) system (Thermo Scientific) with an ASDV auto sampler system. A Dionex-IonPac AS18 4 mm and a Dionex EGC-KOH II Cartridge were used as the column and eluent, respectively. The suppressor (ASRS 300, 4 mm) operated at 82 mA. The calibration standards were diluted from an IC standard (containing 1000 μg mL⁻¹ of Cl) purchased from Choice Analytical. Perovskite/FTO samples (2 × 1.5 cm) were immersed into 2 mL of Milli-Q water in a clean beaker until the perovskite layer was fully removed. The amount of dissolved perovskite material in water was used for further calculation. 1.2 mL of the resulting solution was pipetted out and further diluted five fold for IC measurements.

Results and discussion

Depicted in Fig. 1(a)–(e) are the top-view SEM images of perovskite layers with various concentrations of PbCl₂ in the precursor solution (from 0% to 10% with respect to the content of PbI₂). The MAPbI₃ film consists of grains with the size of 100–300 nm which are connected closely, resulting in a pinhole-free and highly compact film (Fig. 1a), which is desirable for PSCs. When only 1% PbCl₂ was added into the perovskite precursor, there is no observable change in the film morphology in terms of grain size and film compactness (Fig. 1b) compared to the pure MAPbI₃ film. This indicates that a negligible amount of PbCl₂ in the perovskite precursor does not dramatically change the surface morphology of the perovskite film in our experiments. This phenomenon is different from what has been reported previously where a small amount of PbCl₂ (∼1%) significantly improved surface coverage of the perovskite film.¹³ It is ascribed to the effectiveness of the Lewis acid–base adduct method which results in high quality MAPbI₃ perovskite films.^16,21 However, when the PbCl₂-additive increased to 2.5%, larger grains with the crystallite size in the range of 300–500 nm are observed in the perovskite film (Fig. 1c). This effect could be attributed to a better crystallization of the perovskite layer, which leads to larger perovskite crystals in the presence of PbCl₂. Probably MAPbCl₃ is formed first and then acts as a template to aid grain growth.⁵ To gain more insight into the influence of PbCl₂ on the morphological properties of the perovskite film, the concentration of PbCl₂ in the MAPbI₃ precursor was further increased to 5%, 7.5%, and 10%, respectively. As shown in Fig. 1(d)–(f) under the same deposition and annealing conditions, there is no further increase in grain sizes of the perovskite material. Nevertheless, small voids/pinholes can be seen clearly at the GBs on the surface of perovskite films with precursor solutions in excess of 5% PbCl₂. The number of voids/pinholes increases with the increase of PbCl₂ concentration. It is known that voids at the GBs can increase recombination in the device due to poor contact/necking between the light absorbing particles, thus impacting device performance. Formation of voids in films with over 5% PbCl₂ in the perovskite precursor might be a consequence of the different nucleation rates and competitive growth of nuclei MAPbI₃, MAPbCl₃ and PbI₂, which somehow affects the surface coverage of the film.⁹ Furthermore, irregular grain sizes are also observed in the perovskite film with 10% PbCl₂ in the precursor solution (Fig. 1f), which could be due to the coexistence of MAPbI₃ and methylammonium lead chloride (MAPbCl₃) as confirmed by XRD (see below). The above SEM results indicate that 2.5% PbCl₂ provides the optimal quality of surface morphology of the perovskite film.

Fig. 1 Top-view SEM images of perovskite films. (a) MAPbI₃, (b) MAPbI₃ + 1% PbCl₂, (c) MAPbI₃ + 2.5% PbCl₂, (d) MAPbI₃ + 5% PbCl₂, (e) MAPbI₃ + 7.5% PbCl₂, and (f) MAPbI₃ + 10% PbCl₂.

Since the optimal light absorption and energy bandgap are very critical for the performance of a solar cell, the light absorption properties of the MAPbI₃ film made from different contents of PbCl₂ in the precursor were investigated by ultraviolet-visible (UV-vis) spectroscopy (Fig. S1†). However, negligible change is found in the onset light absorbing wavelength and the intensity of light absorption of the perovskite layers with the content of PbCl₂ below 5%. A slight blue-shift in the UV-vis spectrum is observed with the film made from a higher content of the PbCl₂ additive (above 5%), which could be related to incorporation of chloride into the MAPbI₃ crystal lattice, which reduces the lattice symmetry and thus increases the material bandgap²² and the coexistence of a small amount of MAPbCl₃. The reduction in intensity of light absorption of the film with a higher content of PbCl₂ is associated with the existence of more voids as observed in Fig. 1(d)–(f) and possible secondary impurities such as PbI₂ and MAPbCl₃ in the film as indicated in the XRD (below).

To gain insight into the effects of the PbCl₂-additive on the crystal structure of the perovskite material, X-ray diffraction (XRD) data were collected for the diffraction patterns of the films (Fig. 2). From the data, it is found that when the concentration of PbCl₂ is less than 5%, all XRD peaks can be well indexed to the reflection of MAPbI₃ without any impurities detected within the instrumental limit of the XRD facility. However, a small peak at 12.8° which is ascribed to the (001) plane of PbI₂ is observed when the amount of PbCl₂-additive exceeds 5%. For the film with 10% PbCl₂, a diffraction peak at 15.6° which belongs to the reflection of the (110) plane of methylammonium lead tri-chloride (MAPbCl₃) is also observed. It is worth noting that no peak from PbCl₂ is observed in all the XRD spectra, indicating that the added PbCl₂ is completely consumed in the reaction with MAI to either form PbI₂ or MAPbCl₃. This is in good agreement with a previous study.¹⁶ A moderate excess of PbI₂ in the MAPbI₃ perovskite film which mainly exists at GBs is beneficial because it forms an energy barrier that hinders leakage of both electrons and holes from MAPbI₃ for recombination.²³ However, a large amount of PbI₂ in the perovskite film is detrimental for light harvesting due to its much larger bandgap (2.41 eV for PbI₂) and low light absorption, which in turns reduces the current density of devices.² Furthermore, a small shift of the XRD patterns towards the smaller angle of MAPbI₃ was observed with the presence of PbCl₂ in the precursor (Fig. 2(b)–(d)). For example, by comparison of the XRD patterns, it is found that peaks at 14.14° ((110) plane, Fig. 2b), 28.47° ((220) plane, Fig. 2c) and 31.9° ((310) plane, Fig. 2d) for the pristine MAPbI₃ shift to 14.16° ((110) plane), 28.50° ((220) plane) and 31.93° ((310) plane) for the MAPbI₃-2.5% PbCl₂ additive. The shifts is slightly larger than the XRD measurement step (0.015°), suggesting the effect of inclusion of Cl on the crystal lattice of MAPbI₃. Furthermore, the trend illustrating that the more PbCl₂ in the MAPbI₃ precursor leads to the more shift of the XRD patterns towards the smaller angle of MAPbI₃ is observed, confirming that a certain amount of Cl might be incorporated into the crystal lattice of MAPbI₃.

Fig. 2 (a) XRD patterns of MAPbI₃ films prepared from perovskite precursor solution with different concentrations of PbCl₂. Zoom-in XRD patterns showing at (b) (110) lattice plane; (c) (220) plane; and (d) (310) plane.

We have attempted to determine the chlorine content in the MAPbI₃ films made from the PbCl₂ additive based precursor by a conventional method such as X-ray photoelectron spectroscopy (XPS) (Fig. S2†). Nevertheless, no characteristic peak of chlorine was detected in the film even in the XPS depth profiling spectrum (Fig. S2†). The failure of XPS for detection of Cl in perovskite films could be due to the strong photon/ion energy, which destroys volatile Cl-based species. Unlike common techniques such as EDS and XPS, ion chromatography (IC) is a highly accurate non-destructive testing method, which guarantees that the measurement does not lose Cl-based species. The chloride content in the final perovskite films prepared from different concentrations of PbCl₂ in the perovskite precursor solutions was assessed by IC as demonstrated in Table 1. It reveals that Cl remains in perovskite films after a mild annealing (2 min, 100 °C) which is consistent with previous reports.^5,24 The preservation of Cl in our annealed perovskite films could be due to the high compactness of the as-deposited perovskite film which slows down the rates of diffusion of Cl to the surface and prevent its loss afterwards.²⁵ The exact forms of Cl-based products are beyond the scope of this study; we speculate that the remaining chloride could incorporate into the MAPbI₃ lattice, at the GBs, interfaces, or in MAPbCl₃ form. The induced MAPbCl₃ is not detected in the XRD pattern in low concentration PbCl₂-based perovskite films (below 10%), which is presumably because of the poor crystallinity of the MAPbCl₃-like form.^5,26 Furthermore the unreacted PbI₂ passivates GBs which generally worked as recombination centers.²

Table 1 Concentration of chloride in the final perovskite films prepared by different contents of PbCl₂ in the perovskite precursor

Content of PbCl2 in the perovskite precursor (%, molar ratio relative to MAPbI3)	Content of Cl^− ions in the perovskite precursor (%, molar ratio relative to MAPbI3)	Content of Cl^− ions in the final perovskite films (%, molar ratio relative to MAPbI3)
2.5 ± 0.2	5 ± 0.4	4.9 ± 0.2
5 ± 0.2	10 ± 0.4	10.1 ± 0.2
10 ± 0.2	20 ± 0.4	20.6 ± 0.2

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To explore the benefits of the PbCl₂-additive in the perovskite precursor on the performance of solar cells, solar cells based on different PbCl₂ contents were fabricated and the comparison of J_sc, V_oc, FF and PCE is summarized in Fig. 3(a)–(d). It is found that the PbCl₂-additive does not significantly change the V_oc and J_sc of the solar cells with the content of the PbCl₂-additive up to 2.5%. Beyond this, J_sc decreases marginally which is ascribed to reduced light absorption associated with formation of PbI₂ impurity and increased voids/pinholes. The most remarkable change is observed with respect to the FF of the solar cells. When the PbCl₂ content is increased from 0% to 2.5%, the FF is improved from 0.69 to 0.76 primarily as decreased series resistance (R_s) and increased shunt resistance (R_sh) (Table 2). The decrease in R_s is presumably attributed to the reduction of GBs as observed in Fig. 1(a)–(c), while the increase in R_sh corresponds to improved perovskite film quality and more efficient interfacial charge transfer between MAPbI₃ and TiO₂ film. The increase of PbCl₂ concentration to 5% led to an increased R_s and reduced R_sh, which could be due to the occurrence of more pinhole/voids and impurities as discussed above. With further increasing the PbCl₂ content to 7.5%, devices exhibited higher R_s, yet higher average R_sh. However, an extremely large variation was observed, which is possibly related to the non-uniformity of the film due to the distribution of pinhole/voids and a large amount of impurities (PbI₂ and MAPbCl₃) in the perovskite film. Thus the devices with the 2.5% PbCl₂ additive result in the maximum FF, which is in line with PCEs obtained. As a result, the best performance devices were obtained with the 2.5% PbCl₂-additive in the precursor solution with an average J_sc of ∼23.5 mA cm⁻², V_oc of ∼1.04 V and FF of ∼0.75, leading to an average efficiency of 18.1% (based on a batch of 10 cells), which is much higher than that of devices without the PbCl₂-additive which shows an average PCE of 16.5% (J_sc of ∼23.5 mA cm⁻², V_oc of ∼1.02 V, and FF of ∼0.69). The best performance of the 2.5% PbCl₂-additive based cell produced an efficiency of 19% with details shown in the following section.

Fig. 3 Statistical parameters of (a) V_oc, (b) J_sc, (c) FF, and (d) PCEs measured under reverse scanning (V_oc, to J_sc) for 10 cells using MAPbI₃ with varied contents of PbCl₂ in the precursor.

Table 2 Average series resistance (R_s) and shunt resistance (R_sh) of PSCs measured under reverse scanning with different contents of PbCl₂ in the perovskite precursor

Content of PbCl2 (%)	R s (Ω cm^−2)	R sh (kΩ cm^−2)
0%	46.9 ± 5.4	6.8 ± 1.7
1%	42.0 ± 6.3	9.4 ± 1.9
2.5%	31.9 ± 2.9	10.8 ± 1.8
5%	34.1 ± 3.8	8.2 ± 1.4
7.5%	38.6 ± 12.6	14.9 ± 8.6
10%	33.8 ± 6.6	8.1 ± 1.1

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The detailed cross-sectional SEM structure of a representative solar cell made from the precursor containing the 2.5% PbCl₂-additive in MAPbI₃ solution is depicted in Fig. 4a. As depicted, the perovskite layer is homogeneous with large crystals which connect closely and seem to form a monolithic layer from the mesoporous bottom layer to top, suggesting a low density of GBs.

Fig. 4 (a) Cross-sectional SEM image of the completed MAPbI₃ + 2.5% PbCl₂-additive solar cell; (b) J–V curves of perovskite solar cells using MAPbI₃ with and without 2.5% PbCl₂ in the perovskite precursor, under reverse and forward voltage scan; (c) corresponding IPCE spectra; and (d) steady-state photocurrent measured at a bias voltage (0.85 V for the MAPbI₃ device and 0.89 V for the MAPbI₃ + 2.5% PbCl₂ device) at maximum power point and stabilized power output 10 s pre-conditioned to 1 sun light illumination.

The J–V curves of the MAPbI₃ perovskite solar cells with and without 2.5% of PbCl₂ in the perovskite precursor measured under reverse (from V_oc to J_sc) and forward (from J_sc to V_oc) scanning are provided in Fig. 4b. The MAPbI₃-based solar cell produces a PCE of 17.1 (15.1)%, with a V_oc of 1.03 (0.98) V, a J_sc of 23.7 (23.8) mA cm⁻², and a FF of 70 (65)% when measured under reverse (forward) scan, respectively, revealing a hysteresis (H) of 11.7% (H = 100% × (PCE_rev − PCE_fw)/PCE_rev). In contrast, the MAPbI₃–2.5% PbCl₂-based solar cell achieved a PCE of 18.9 (17.2)%, a V_oc of 1.05 (1.03) V, a J_sc of 23.7 (23.9) mA cm⁻², and a FF of 76 (70)% when measured under reverse (forward) scan, respectively, with only 8.9% hysteresis.

The external quantum efficiency (EQE) spectra of the MAPbI₃-based and MAPbI₃–2.5% PbCl₂-based device exhibit a high EQE value between 80% and 86% across a broad wavelength range from 400 nm to 750 nm with the photon response spectral tail extending to around 800 nm as illustrated in Fig. 4c. The integrated current densities are 20.1 mA cm⁻² for the device with MAPbI₃ and 20.8 mA cm⁻² for the device with MAPbI₃–2.5% PbCl₂, which are slightly lower than those measured from J–V curves. This is ascribed to the presence of shallow trap states in the perovskite films. Shallow trap states in perovskite and/or interfaces can be easily filled under illumination during the J–V testing through very short light soaking (10 seconds in our case), which in turn facilitates charge carrier transport. However, the IPCE measurement was performed under very low intensity of monochromatic light without bias illumination. The electron generated in the IPCE measurement first needs to fill the trap states before being collected.²⁷ This leads to a minor discrepancy in integrated J_sc from the IPCE spectrum as compared to the J_sc measured by constant steady-state illumination of the solar simulator.

The steady-state performance of the two representative cells (MAPbI₃ and MAPbI₃–2.5% PbCl₂) at the maximum power point (MPP) was also measured (Fig. 4d). Both devices were pre-conditioned by illumination for 10 s prior to the measurement. The current densities at the MPP drop within a few seconds from 20.1 mA cm⁻² to 19.1 mA cm⁻² for the MAPbI₃-based solar cell and from 20.4 mA cm⁻² to 19.7 mA cm⁻² for the MAPbI₃–2.5% PbCl₂-based solar cell under a continuous measurement duration of 100 s. The applied voltage bias near the MPP is 0.85 V for the MAPbI₃-based device and 0.89 V for the MAPbI₃–2.5% PbCl₂-based device, which has a corresponding stabilized power output of 16.2% and 17.5%, respectively. This result suggests that addition of 2.5% PbCl₂ into the perovskite precursor also dramatically improves the steady-state efficiency of PSCs.

The steady-state photoluminescence (PL) emission and time-resolved photoluminescence (TRPL) were carried out to elucidate the charge carrier dynamics of MAPbI₃ and MAPbI₃–2.5% PbCl₂ films with and without the presence of charge transport layers (TiO₂ and Spiro-OMeTAD) (Fig. 5(a)–(d)). Compared to the pristine MAPbI₃ perovskite film, the intensity of the PL peak of the MAPbI₃–2.5% PbCl₂ film increases while the full width half maximum (FWHM) remains constant (Fig. 5a), indicating that more electron–hole pairs in the film lead to stronger radiative recombination. This indicates improved crystallinity of the as-deposited film.²⁸ Moreover, the steady-state PL spectrum of the MAPbI₃–2.5% PbCl₂ film in Fig. 5a depicts a blue-shift by 2 nm (from 769 nm to 767 nm) which is consistent with the blue-shift observed in UV-vis absorption spectra acquired for the films, further suggesting the incorporation of chloride into the MAPbI₃ lattice. On contacting with charge transport layers, a considerable decrease in PL intensity was observed. Compared to MAPbI₃, the drop of PL peaks of MAPbI₃–2.5% PbCl₂ is more significant when in contact with the TiO₂ film, suggesting a more efficient interfacial charge transfer. This is in good agreement with the enhanced performance of MAPbI₃–2.5% PbCl₂-based PSCs compared to pristine MAPbI₃ devices.

Fig. 5 (a) Steady-state photoluminescence (PL) and normalized time-resolved photoluminescence (TRPL); (b) perovskite; (c) perovskite/TiO₂; and (d) Spiro-OMeTAD/perovskite.

The TRPL spectra of samples are fitted using a bi-exponential function of time (t) (F(t) = ∑a_ie^−t/τ_i, i = 1, 2), which contains a fast decay time component and a slow decay time component.²⁹ The fast decay component is associated with recombination behavior at the surface, while the slow decay component corresponds to recombination in the bulk of the perovskite film.³⁰ The result indicates that, compared to pristine MAPbI₃, addition of PbCl₂ into the perovskite precursor results in films with a marginal decrease in the fast decay component (from 2.1 ns to 1.44 ns) but longer in the slow component of lifetime (from 80 ns to 85 ns) (Fig. 5b). Therefore the overall average PL lifetime of the perovskite layer is increased from 75 ns to 80 ns.

Time-resolved photoluminescence measurements of the MAPbI₃ show a decrease in the PL lifetime of the perovskite layer from 75 ns to 56.7 ns in the presence of TiO₂ (40 nm compact TiO₂/200 nm mesoporous TiO₂). When in contact with Spiro-OMeTAD, the lifetime = 45 ns is obtained. Whereas the PL lifetime of the MAPbI₃–2.5% PbCl₂ layer is reduced more significant from 80 ns to only 49.5 ns and 26.3 ns, respectively, when in contact with the TiO₂ film and with Spiro-OMeTAD layer, indicating a faster and more effective electron and hole transport. The more effective hole transport is crucial for MAPbI₃ perovskite due to the unbalanced electron–hole diffusion length (∼130 nm for electrons, ∼90 nm for holes) according to a previous report.²⁰ These results indicate improved charge collection efficiency of MAPbI₃–2.5% PbCl₂-based PSCs compared to that of pristine MAPbI₃, which is consistent with the improved FF of devices fabricated in this study.

To further explore the possible origin of the superior optoelectronic properties of the MAPbI₃–2.5% PbCl₂ based solar cell to the pristine MAPbI₃ based solar cell, electrochemical impedance spectroscopy (EIS) measurements were conducted and the corresponding Nyquist plots are depicted in Fig. 6. The intercept point at the real part of the Nyquist plots is associated with series resistance of the device (R_s), which is mainly contributed to the sheet resistance of FTO glass. The small semicircle at high frequency corresponds to the charge transfer process between perovskite and selective contacts while the large semicircle at low frequency demonstrates the recombination of charge carriers in the perovskite layer (according to previous reports).^31,32 Fitting the EIS of the MAPbI₃ and MAPbI₃–2.5% PbCl₂ solar cells using an equivalent circuit depicted in the inset in Fig. 6 indicates that both devices have comparable interfacial charge transport behavior. Nevertheless, the MAPbI₃–2.5% PbCl₂ based solar cell has nearly 2.4-times higher resistance for recombination (1.68 × 10⁵ Ω) than the pristine MAPbI₃ solar cell (6.93 × 10⁴ Ω), whereas both devices exhibit similar capacitance (0.789 μF for MAPbI₃versus 0.769 μF for MAPbI₃–2.5% PbCl₂). As a result, the electron lifetime, a product of resistance and capacitance (τ = R × C), of the solar cell with the 2.5% PbCl₂-additive in the precursor (τ_r = 0.12 s) is over 2-fold higher than the device made from pure MAPbI₃ (τ_r = 0.055 s). The increased carrier lifetime is one of the factors that are responsible for the higher performance of the solar cell.

Fig. 6 Nyquist plots of PSCs based on pristine MAPbI₃ perovskite (square black) and fitting (green), and MAPbI₃–2.5% PbCl₂ perovskite (dot red) and fitting (blue) in the dark at 0.5 V forward bias. The inset depicts the corresponding equivalent circuit.

Kelvin probe force microscopy (KPFM) is a powerful technique not only to examine surface topography but also the work function of the surface. The KPFM topography of both pristine MAPbI₃ and MAPbI₃–2.5% PbCl₂ is shown in Fig. 7a and c. It appears that the surface of both perovskite films is uniform over a large area (5 × 5 μm²). However, the film made from the 2.5% PbCl₂ based precursor consists of larger grains. The root mean square as an indicator of roughness of the film is 15.75 nm for the PbCl₂-additive film and 14.27 nm for the pristine MAPbI₃ film. The slightly rougher surface is attributed to its larger grain size. Measurement of the surface potential spectra of MAPbI₃ and MAPbI₃–2.5% PbCl₂ films (Fig. 7b and d) reveals an increase of mean values of contact potential difference (CPD) by ∼90 mV in the MAPbI₃–2.5% PbCl₂ film compared to the MAPbI₃ film. It is known that the conduction band of MAPbI₃ perovskite is ∼80 meV more negative than that of n-type TiO₂, which in principle forms an energy barrier for electron injection from MAPbI₃ to n-TiO₂ during the charge transport process.³³ Therefore, the increase of the electron quasi-Fermi level near the conduction band edge of MAPbI₃–2.5% PbCl₂ favors improved energy level alignment with adjacent TiO₂ for electron transfer at the MAPbI₃–2.5% PbCl₂/n-TiO₂ interface. This work provides new insight into the mechanism that governs the impact of PbCl₂ on both morphology and electronic properties of the MAPbI₃ film for high energy conversion efficiency.

Fig. 7 KPFM topography of the (a) MAPbI₃/mp-TiO₂/compact TiO₂/FTO-glass film and (c) MAPbI₃–2.5% PbCl₂/mp-TiO₂/compact TiO₂/FTO-glass film; (b and d) corresponding contact potential difference (CPD) images.

The stability of un-encapsulated perovskite solar cells based on MAPbI₃ with and without 2.5% PbCl₂ was tested by aging the device at the maximum power point (MPP) under constant illumination (100 mW cm⁻²) at room temperature in ambient air (RH% = 50–60%). It is found that both devices retained ∼65% power conversion efficiency after 80 minutes stability testing (Fig. 8). The above results suggest that the impact of the PbCl₂ additive on the stability of the perovskite solar cells is negligible in this work.

Fig. 8 Relative performance of perovskite solar cells based on MAPbI₃ with and without the 2.5% PbCl₂ additive under continuous illumination for 80 minutes in ambient air with relative humidity: 50–60%.

Conclusions

We have demonstrated the effect of PbCl₂ on the morphology, crystallinity and recombination of the MAPbI₃ perovskite film that was prepared via a one-step Lewis acid–base adduct method. Although addition of a low content of PbCl₂ (less than 2.5%) into the MAPbI₃ precursor did not pose a significant change in the crystal structure and optical properties, the resulting films show a larger grain size. More importantly, the PbCl₂ additive improved the electronic properties and dynamics of charge carriers in the MAPbI₃ film. The electronic properties of the perovskite film were modified by the PbCl₂ additive with a significant downward shift of the surface work function by 90 meV, which facilitates electron injection from perovskite to TiO₂. Analysis of TRPL indicated that the PbCl₂-additive in the precursor led to perovskite films with a more balanced charge (electron and hole) collection and longer carrier lifetime. The EIS results of the corresponding PSCs also confirm that devices with the 2.5% PbCl₂ additive have a 2.4-fold higher resistance for recombination compared to that in the pristine MAPbI₃-based solar cell, indicating reduced trap density, resulting in a lower observed current–voltage hysteresis of solar cells. As a result, the performance of PSCs was increased from an average performance of 16.5% with pristine MAPbI₃ to 18.1% with MAPbI₃–2.5% PbCl₂. The nearly 9% enhancement in device performance is mainly due to the improvement of the FF from 0.69 to 0.76.

Acknowledgements

This work was supported by the Australian Research Council Future Fellowship (FT120100674) and Queensland-Chinese Academy of Science (Q-CAS) collaborative research fund. N. D. P. thanks Queensland University of Technology (QUT) postgraduate scholarship and CSIRO top-up scholarship. G. J. W. acknowledges funding support from CSIRO Research Office Julius Career Award and support of the CSIRO top-up scholarship for N. D. P. The data of XRD, SEM, and Ion Chromatography reported in this paper were obtained at the Central Analytical Research facility (CARF), QUT. Access to CARF was supported by the generous funding from Science and Engineering faculty, QUT. Dr Sunny Hu and her team at CARF are acknowledged for the Ion Chromatography measurement. In addition, we acknowledge the measurement of KPMS by Dr Kinnari Shelat at the Queensland node of the Australian national Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta11139d

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