Interface modification effects using a halide-free lead source for perovskite solar cells

Abstract

The remnant PbI₂ at the mesoporous-TiO₂ (m-TiO₂)/CH₃NH₃PbI₃ interface can act as an interface modification layer for high-efficiency perovskite solar cells (PSCs). In this study, we adopted Pb(CH₃COO)₂·3H₂O (PbAc₂) to control the amount of remnant PbI₂ at the m-TiO₂/CH₃NH₃PbI₃ interface. The characterization results demonstrate that the amount of remnant PbI₂ could be controlled by the concentration of PbAc₂ solution without influencing the crystallographic textures of CH₃NH₃PbI₃ films. By adjusting the PbAc₂ solution concentration, the optimized PSC can achieve a 15% enhancement of PCE in the case of using a two-step method for CH₃NH₃PbI₃ preparation. Under the circumstances of using an anti-solution method, it still displayed a 10% enhancement of the average PCE for 10 individual PSCs with PbAc₂ (13.6% to 15.0%). The steady-state photoluminescence (PL), time-resolved photoluminescence (TR-PL) and open-circuit voltage decay results demonstrated that the remnant PbI₂ could reduce carrier recombination at the m-TiO₂/CH₃NH₃PbI₃ interface. Meanwhile, the enhancement of the open-circuit voltage was achieved for PSCs with PbAc₂.

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Introduction

Organic–inorganic lead halide perovskites have attracted tremendous attention as excellent materials for photovoltaic applications.^1,2 These materials exhibit good absorption coefficients,³ tunable band gaps,⁴ long electron–hole diffusion lengths and high carrier mobility.^5–7 Recently, perovskite solar cells (PSCs) have been of primary interest because of their super performance and power conversion efficiency (PCE) of 22.1%.⁸ Guaranteeing the effective separation of most photo-generated carriers before recombination occurs in PSCs remains one of the key factors.^9,10 In this regard, to improve device performance, it is essential to reduce the carrier recombination.^11–13

Interface modification at the TiO₂/perovskite interface and solvent engineering aiming to improve perovskite quality are major approaches to settle the carrier recombination issue by an improved carrier extraction rate.^14–16 An appropriate amount of PbI₂ species in the CH₃NH₃PbI₃ film is reported to have a positive effect in improving the photovoltaic performance of PSCs.¹⁷ It has been demonstrated that the PbI₂ could passivate the surface defects and reduce charge recombination effectively.¹⁸ Through a one-step solution process, Anders Hagfeldt et al. obtained PSCs with the efficiency of 20.8% by guaranteeing an excess of PbI₂ in the solution for perovskite fabrication.¹⁹ In spite of the method for perovskite film preparation, retaining the excess PbI₂ is an effective strategy to achieve good performance in PSCs by the vapor and solution process.^17,19–21 The Yang Yang group indicated that the conduction band minimum (CBM) and valence band maximum (VBM) for CH₃NH₃PbI₃ are noted to be 3.93 eV and 5.43 eV, respectively, while the CBM and VBM for PbI₂ are 3.45 eV and 5.75 eV, respectively.¹⁷ This type of band alignment between PbI₂ and CH₃NH₃PbI₃ allows for the formation of energy barriers, which could prevent excitons from recombining through non-radiative channels at trap states and surface defects at the interface.^17,22 However, a low amount of the remnant PbI₂ might not play an important role in avoiding the recombination of carriers. On the other hand, the remnant PbI₂ might retard the transfer of not only holes but also electrons under the conditions of a higher amount of PbI₂.²⁰ Therefore, it is necessary to perform a systematic investigation about the influence of the amount of remnant PbI₂ at the TiO₂/CH₃NH₃PbI₃ interface on the performance of PSCs.

In most previous studies, PbI₂ was generally used as the lead source.²³ Snaith et al. changed lead halide (PbCl₂ and PbI₂) to a non-halide lead source, PbAc₂, via a simple one-step solution-coating technique.²⁴ Their results demonstrated that PbAc₂ could accelerate the CH₃NH₃PbI₃ crystal growth. The facile removal of CH₃NH₃Ac could enhance both the smoothness and surface coverage of the perovskite films. Henk J. Bolink noted that smooth and pinhole-free perovskite layers over a large area could be obtained with a lead acetate precursor.²⁵ It has been proven that PbAc₂ and CH₃NH₃I react to produce PbI₄²⁻ in the first step, subsequently producing PbI₂. Ammonium acetate is significantly easier to volatilize, and can avoid the additional introduction of a halogen element.^26,27 In these research studies, the remnant of PbI₂ is controlled by employing higher PbI₂ concentration or the decomposition of CH₃NH₃PbI₃. Actually, the amount of the remnant PbI₂ is hard to regulate and an excess of the halogen element might be introduced into the reaction system.

In this study, PbAc₂ is used to control the amount of remnant PbI₂, and furthermore improve the performance of PSCs. The results demonstrate that adjusting the concentration of PbAc₂ solution is an effective approach for controlling the amount of remnant PbI₂ at the TiO₂/CH₃NH₃PbI₃ interface. The adoption of PbAc₂ could avoid the uncertainty amount of remnant PbI₂ by higher PbI₂ concentration or decomposition of CH₃NH₃PbI₃. When PbAc₂ is used to adjust the PbI₂ amount at the m-TiO₂/perovskite interface, smoother and more uniform CH₃NH₃PbI₃ films are obtained within a two-step method. Upon changing the concentration of the PbAc₂ solution, the amount of remnant PbI₂ could be easily adjusted. The interface modification effect of remnant PbI₂ on the PSCs is investigated using steady-state PL and TR-PL. The optimized PSC exhibits a PCE of 11.7%, a 15% enhancement compared to that of the PSC without PbAc₂. Subsequently, by an anti-solvent method, an increase of the average PCE of PSCs of 10% (from 13.6% to 15.0%) with a 70 mV enhancement on V_oc is observed. By understanding and utilizing the generation of remnant PbI₂ ascribed to the introduction of PbAc₂, it is believed that the synthesis technique provides an effective and efficient approach to improve the properties of the interface and increase the performance of PSCs.

Experimental section

Materials

The TiO₂ paste with 18 nm particles (Dyesol) was diluted with ethanol with a 1 : 3.5 weight ratio. Lead acetate (PbAc₂, 98%) was purchased from Aladdin. CH₃NH₃I and spiro-OMeTAD(2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) powders were purchased from Ningbo Borun New Material Technology Co., Ltd. PbI₂ (99%) was purchased from Acros. The N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) and chlorobenzene solvents were purchased from Alfa Aesar. Isopropanol was obtained from J&K Chemicals. FK209 (tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)) bis(trifluoromethylsulfonyl)imide was purchased from Shanghai MaterWin New Materials Co., Ltd, and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) (99.95%) was purchased from Aldrich.

Device fabrication

Fluorine-doped tin oxide (FTO) glass (15 Ω □⁻¹) was first etched from one side by HCl and Zn powder. The etched substrates were sequentially cleaned in alkaline liquor, soapy water, deionized water, and ethanol in an ultrasonic bath. Then, the substrates were calcined at 500 °C in air. A compact TiO₂ (c-TiO₂) blocking layer was formed on FTO by spin-coating tetrabutyltitanate (20 mM in isopropanol) at 3000 rpm for 30 s, which was followed by annealing at 500 °C for 30 minutes in air. The diluted TiO₂ paste was then spin-coated on top at 5000 rpm for 30 s and calcined at 500 °C for 30 minutes to form a mesoporous-TiO₂ (m-TiO₂) layer. The FTO/c-TiO₂/m-TiO₂ anode used in this study was noted as A. Then PbAc₂ solution in a mixed solvent of glycerol and DMF (volume ratio, 1 : 3) was spin-coated. Several PbAc₂ concentrations, including without PbAc₂ (PA0), 50 mg mL⁻¹ (PA50), 100 mg mL⁻¹ (PA100), and 150 mg mL⁻¹ (PA150), were adopted under a PbI₂ layer to prepare PSCs.

The PbI₂ solution was dissolved and kept at 70 °C at a concentration of 462 mg mL⁻¹. All A/PbAc₂/PbI₂ films were heated on a hot stage at 70 °C for 30 min. After cooling to room temperature, CH₃NH₃I solution (8.5 mg mL⁻¹ in isopropanol) was sprayed on top of the lead layer, and a reaction time of 30 s was guaranteed before spin-coating. The as-prepared CH₃NH₃PbI₃ films were heated at 70 °C for 30 min on a hot plate. For improvement, an anti-solution method was also used to prepare CH₃NH₃PbI₃ films onto A/PbAc₂. CH₃NH₃I and PbI₂ (molar ratio, 1 : 1) of 1.2 M in a mixture of DMSO and DMF (v/v = 8 : 2) were dissolved at 60 °C. A spin-coating process was set at 1000 rpm and 5000 rpm for 20 s and 30 s, respectively. At the last 10 s of the second spin-coating step, the substrate was treated with chlorobenzene. The substrates were heated at 105 °C for 60 min to complete crystallization of CH₃NH₃PbI₃ films.

Subsequently, 20 μL of a spiro-OMeTAD solution was spin-coated on the perovskite layer at 4000 rpm for 30 s. The solution was prepared by mixing 72.3 mg of spiro-OMeTAD, 28.8 μL of TBP (4-tert-butylpyridine), 17.5 μL of LiTFSI (520 mg mL⁻¹ in acetonitrile) and 29 μL of FK209 (300 mg mL⁻¹ in acetonitrile) in 1 mL of acetonitrile. Finally, the back contact was made by thermally evaporating a 60 nm thick gold film on top of the device. A 3 mm × 3 mm square mask was cleaved on the glass side before photovoltaic performance characterization.

Characterization and measurements

Scanning electron microscopy (SEM) images were taken with a SU8010 (Hitachi, Japan). The measurement was conducted at 5.0 kV, 10 μA. X-ray diffraction was measured by using a D8 FOCUS (Bruker, German) which was equipped with a Cu Kα X-ray (λ = 1.5406 Å) tube. Ultraviolet-visible (UV-Vis) absorbance spectra were measured with a UV2400 (Shimadzu, Japan) from 350 to 800 nm. An AC Mode III (Agilent, America) atomic force microscope (AFM) was used to measure the surface morphology of the thin films in tapping mode. Time-resolved photoluminescence (TR-PL) measurements (F900, UK) of the A/PA/CH₃NH₃PbI₃ samples were obtained by excitation with a 480 nm laser light under ambient air conditions at the Technical Institute of Physics and Chemistry of China. The steady-state PL was measured at the excitation wavelength of 510 nm with a FL4600 (Hitachi, Japan).

The photovoltaic performance was measured with a Keithley 2400 sourcemeter under AM 1.5 irradiation generated by a solar simulator (XES 300T1, SAN-EI Electric Co., Japan). The intensity of the irradiation was calibrated by a standard Si reference cell. The incident photo-to-current efficiency (IPCE) was characterized by a QE-R system (Enli Tech.) under ambient conditions. The V_oc decay within 1000 μs was recorded using an electrochemical workstation (Zahner ennium, Germany) with PSCs illuminated under LED white light of 1000 W m⁻².

Results and discussion

The XRD results of the A/PA/CH₃NH₃PbI₃ samples with different PbAc₂ concentrations are shown in Fig. 1. As shown in Fig. 1, seven peaks marked by black pentagrams and located at 14.1°, 20.2°, 24.5°, 28.5°, 32.0°, 40.7° and 43.3° can be observed, corresponding to the (110), (112), (111), (220), (310), (224) and (314) crystal planes of the β-phase CH₃NH₃PbI₃, respectively.^28,29 The peaks marked with A are related to the diffraction patterns of FTO/c-TiO₂/m-TiO₂ anodes, which are in good agreement with previous reports.^30,31 The peak marked with an inverted triangle, located at 12.9°, is indexed to the (001) plane of PbI₂.³² With the increase of the PbAc₂ solution concentrations, the peak intensity of the remnant PbI₂ enhanced. The results indicate that the amount of remnant PbI₂ could be adjusted through controlling PbAc₂ concentration. The reaction process is given in formula (1) according to the XRD results. Such a method is easy to operate and might contribute to modifying the photovoltaic performance of PSCs.

PbAc₂ + PbI₂ + 3CH₃NH₃I → PbI₂ + CH₃NH₃PbI₃ + 2CH₃NH₃Ac↑

Fig. 1 XRD results of the A/PA/CH₃NH₃PbI₃ samples with different concentrations of PbAc₂ involved (from the bottom to the top: PA0–PA150).

The quality of CH₃NH₃PbI₃ films is of vital importance for the performance of PSCs.^33,34 The morphologies of the CH₃NH₃PbI₃ films characterized by SEM are shown in Fig. 2. Without the adoption of PbAc₂, the CH₃NH₃PbI₃ particles showed a cubic shape with an average size of 300 nm. The CH₃NH₃PbI₃ film could not fully cover the m-TiO₂ layer. From Fig. 2(a), it can be seen that some pin-holes are distributed on the surface of the CH₃NH₃PbI₃ film. The pin-holes might cause the recombination of electrons and holes.^35,36Fig. 2(b) shows the morphology of CH₃NH₃PbI₃ with the PbAc₂ solution concentration of 100 mg mL⁻¹. In contrast, through the adoption of PbAc₂ solution, pin-hole free films were obtained and the cubic CH₃NH₃PbI₃ connected each other tightly, it is encouraging that the surface coverage of the perovskite films on m-TiO₂ by a two-step solution process is greatly improved with the adoption of PbAc₂. The cross-sectional image of the A/PA/CH₃NH₃PbI₃ samples in Fig. 2 displays that the thickness of the CH₃NH₃PbI₃ layer is around 200 nm. As displayed in Fig. 2(c)–(f), all perovskite films infiltrated well into the m-TiO₂ films, which is beneficial for the interface connection between the m-TiO₂ and perovskite layers.³⁷

Fig. 2 SEM of A/PA/CH₃NH₃PbI₃ films, including the surface morphologies of PA0 (a) and PA100 (b); cross-sections of PA0 (c), PA50 (d), PA100 (e) and PA150 (f).

The absorbance spectra of the A/PA/CH₃NH₃PbI₃ samples are displayed in Fig. 3. Four samples involving different PbAc₂ concentrations (PA0, PA50, PA100 and PA150) demonstrate similar absorption edges at around 780 nm. The intensity decrease of the absorbance curves from PA0 to PA150 was caused by the increase of the remnant PbI₂ amount.²⁰ Consistent with the XRD results, it supports the point that adopting a higher concentration of PbAc₂ within a two-step method can efficiently enhance the amount of remnant PbI₂.

Fig. 3 UV-Vis curves of A/PbI₂ and A/PA/CH₃NH₃PbI₃ films with different concentrations of PbAc₂ involved.

To study the influence of the amount of remnant PbI₂ at the m-TiO₂/CH₃NH₃PbI₃ interface on the performance of PSCs, the as-prepared perovskite films were used to fabricate the FTO/c-TiO₂/m-TiO₂/CH₃NH₃PbI₃/spiro-OMeTAD/Au devices. The J–V results measured under simulated AM 1.5G (100 mW cm⁻²) were carried out to evaluate the performance of the PSCs (Fig. 4). And the photovoltaic parameters are summarized in Table 1. The PSC without introducing PbAc₂ shows a V_oc of 0.88 V, a J_sc of 18.3 mA cm⁻² and a fill factor of 63%, which give an overall PCE of 10.2%. With the concentration of PbAc₂ solution increasing from 50 to 150 mg mL⁻¹, the V_oc was slightly increased. The voltage exhibits a linear growth from 0.88 V to 1.02 V as a higher concentration of the PbAc₂ was deposited. Optimal cells were obtained when PbAc₂ solution with the concentration of 100 mg mL⁻¹ PbAc₂ was adopted, yielding a V_oc of 0.98 V, a J_sc of 18.5 mA cm⁻², a fill factor of 64.6% and an overall PCE of 11.7%. Usually, the carrier recombination easily happens when the electron transfer layer is in direct contact with the hole-transport layer.³⁸ As seen from Fig. 2(b), the improvement in the coverage of the CH₃NH₃PbI₃ film on m-TiO₂ can effectively reduce the possibility of the electron–hole recombination.^20,39 On the other hand, the higher CBM of PbI₂ compared to that of CH₃NH₃PbI₃ could prevent excitons from recombining. However, the excessively thick PbI₂ blocking layer might slow the electron transfer to the m-TiO₂ layers, which would degrade the performance of PSCs as shown by the J–V results of PA150 samples.

Fig. 4 J–V curves of the PSCs with different concentrations of PbAc₂ involved.

Table 1 Photovoltaic parameters of PSCs with different concentrations of PbAc₂ solution

Devices	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)
PA0	0.88	18.3	63.2	10.2
PA50	0.95	17.1	64.6	10.5
PA100	0.98	18.5	64.6	11.7
PA150	1.02	14.3	52.0	7.7

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Time-resolved photoluminescence (TR-PL) measurements were carried out to study the emission lifetime of the A/PA/CH₃NH₃PbI₃ samples of PA0 and PA100 (Fig. 5). All the curves are fitted with three-component exponential decay according to a previous study, the electron lifetime and the calculated percentage can be read from the insert chart. The photoluminescence decay components are attributed to the carrier recombination occurring in the perovskite bulk and surface.^40,41 The sample without PbAc₂ (PA0) possesses a fast decay of 24.63 ns at a proportion of 62.8%. Compared to PA0, slower photoluminescence decay for samples with 100 mg mL⁻¹ PbAc₂ (PA100) is observed. It possesses a fast decay of 36.04 ns at a proportion of 69.2%. As it is reported that the remnant PbI₂ would affect the carrier transfer in two ways in PSCs: one is the positive effect that the remnant PbI₂ would effectively prevent the holes in the perovskite layer from recombining with the electrons in the m-TiO₂; the second is the negative effect that the remnant PbI₂ could block the photogenerated electron transfer from the perovskite to the m-TiO₂.¹⁷ Thus, it is needed to adopt the optimal amount of PbI₂ to optimize the performance of PSCs. From the TR-PL results (Fig. 5), it can be seen that the PA100 sample showed a longer electron lifetime than the PA0 case. This result indicates a slower electron extraction rate, which is caused by the block of electron transfer of remnant PbI₂. However, a weaker photoluminescence intensity in the steady state PL spectra in Fig. S1† indicates that less electron–hole recombination happens in the sample. As we know, too much amount of the remnant PbI₂ at the m-TiO₂/perovskite interface would confine electron transfer, which leads to a longer electron lifetime and decreased J_sc in devices. On the other hand, the remnant PbI₂ could modify the m-TiO₂/perovskite interface, therefore the highest PCE could be obtained in a curtailed PbI₂ concentration, which is reported by Bi et al.²⁰ The J–V results in Fig. 4 show similar results, the J_sc tends to decrease when the concentration of PbAc₂ solution increases, which is the result of the blocking of electron transfer. However, the V_oc gradually increases as the concentration of PbAc₂ solution increases, which is the result of the modification effect of the remnant PbI₂ at the m-TiO₂/perovskite interface. In the case of PA100, the appropriate amount of remnant PbI₂ at the m-TiO₂/perovskite interface optimizes the performance of PSCs by blocking the electron–hole recombination and interface modification.

Fig. 5 Normalized TR-PL decay curves of the A/PA/CH₃NH₃PbI₃ samples excited by a laser at 485 nm. The data are shown as a scatter plot, and the fittings are in continuous lines.

To explore the reaction of PbAc₂ with CH₃NH₃I (MAI), PbAc₂ was individually spin-coated on the top of FTO/c-TiO₂/m-TiO₂ (A), and the MAI isopropanol solution with a concentration of 10 mg mL⁻¹ is added dropwise on its surface. The XRD diffraction patterns of A, A/PA and A/PA + MAI are shown in Fig. 6. The peaks marked with a black pentagram are assigned to CH₃NH₃PbI₃ for the A/PA + MAI sample. In the absence of the PbI₂ layer, there is no clue for remnant PbI₂. By further depositing spiro-OMeTAD and the Au electrode, the device displays a PCE of 4.64% with a J_sc of 11.3 mA cm⁻² and a V_oc of 0.69 V (Fig. S2†). The possible reaction mechanism according to earlier reports is given in the equation below.^23,27 To conclude from the above, the Ac⁻ ion is easily replaced by I⁻, and volatilizes from the film as CH₃NH₃Ac. In our system, MAI first reacted with PbI₂ to form CH₃NH₃PbI₃. After the PbI₂ was consumed out, MAI would react with PbAc₂, which lead to the formation of new PbI₂. Because of the lack of MAI, more CH₃NH₃PbI₃ could not be produced. And then, part of remnant PbI₂ from PbAc₂ would exist in the films.

PbAc₂ + 3CH₃NH₃I → CH₃NH₃PbI₃ + 2CH₃NH₃Ac↑

Fig. 6 XRD diffraction patterns of A, A/PA and A/PA + MAI.

Actually, the CH₃NH₃PbI₃ films prepared by the two-step method were not the ideal films in the application of PSCs, and the improvement of CH₃NH₃PbI₃ films is essential for the consideration of PbAc₂. An anti-solution method can achieve CH₃NH₃PbI₃ films with good uniformity and surface coverage according to previous studies.^42,43 PbAc₂ solution of 100 mg mL⁻¹ (PA100) was used for the one-step deposition of CH₃NH₃PbI₃ films. Compared to those of the samples without PbAc₂ (PA0), the XRD diffraction patterns of A/PA/CH₃NH₃PbI₃ show an enhancement of remnant PbI₂ (Fig. 7(a)). The result is consistent with our previous study which uses the two-step method for the formation of CH₃NH₃PbI₃ films (Fig. 2). As displayed in Fig. S3(a) and (b),† the morphologies of CH₃NH₃PbI₃ films with/without PbAc₂ are different, the root mean square roughness of the CH₃NH₃PbI₃ films demonstrates a slight reduction from 30.6 to 26.5 nm by adoption of PbAc₂.

Fig. 7 (a) XRD diffraction patterns of A/PA/CH₃NH₃PbI₃ samples in 10–17°, (b) histograms of PCEs measured for 10 individual cells with/without PbAc₂, (c) I–V curves and cross-sections of PSCs with/without PA, and (d) IPCE result of PSCs with PA100.

Subsequently, device performances were evaluated after assembling PSCs. Fig. 7(b) and S4(a)† display histograms of PCEs and J–V curves of 10 nominally identical devices with/without PbAc₂. Compared to PSCs without PA, a narrow PCE distribution of PSCs with PA100 was achieved (Fig. 7(b)). An average PCE of 15.0% is obtained for PA100 while 13.6% is obtained for PA0. The I–V curves of PSCs with a PCE of 13.4% for PA0 and 15.1% for PA100 are displayed in Fig. 7(c); the V_oc is 0.95 V, J_sc is 21.5 mA cm⁻², and FF is 65.3% and V_oc of 1.03 V, J_sc of 20.8 mA cm⁻², FF of 70.2%, respectively (Table 2). The box plots of V_oc in Fig. S4(b)† indicate that a statistical increase of V_oc of about 70 mV was demonstrated by the adoption of PbAc₂. The devices structures with/without PbAc₂ are shown in the inset in Fig. 7(c). The IPCE results of PSCs with PA100 in Fig. 7(d) demonstrate good light response within the absorption band (300–800 nm). An integrated J_sc of 20 mA cm⁻² was achieved, which is consistent with the J–V results, indicating that most photo-induced electrons are collected and become free carriers.

Table 2 Photovoltaic parameters of PSCs with/without PA

Devices	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)
PA0	0.95	21.5	65.3	13.4
PA100	1.03	20.8	70.2	15.1

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Considering the good coverage of CH₃NH₃PbI₃ films on m-TiO₂ by the anti-solution method, the increase of the average PCE improvement of 10% could be attributed to the enhancement of remnant PbI₂, which can effectively prevent the recombination between the electrons and holes leading to the V_oc.^7,44 The normalized open-circuit voltage decay curves were acquired as shown in Fig. 8(a). The PSC with PA100 exhibited a longer V_oc decay time than the PSC without PbAc₂ (PA0). Using the following equation, the electron lifetime τ can be obtained:

τ = −k_BTe⁻¹(dV_oc/dt)⁻¹ (1)

where e is the elementary charge, T is the temperature, and k_B is the Boltzmann constant. The PSC with PA100 displays a longer electron lifetime over the same voltage region (Fig. 8(b)), indicating that the device undergoes a much lower interface recombination. This could be mainly attributed to the enhancement of the amount of remnant PbI₂ at the A/CH₃NH₃PbI₃ interface, which is in good agreement with previous reports.^7,45

Fig. 8 (a) V_oc decay curves and (b) the relationship between the electron lifetime and the V_oc of PSCs with/without PA.

Conclusions

In summary, we have demonstrated that the adoption of PbAc₂ is an efficient way to control the amount of remnant PbI₂ at the m-TiO₂/CH₃NH₃PbI₃ interface. The MAI might first react with PbI₂, it is consumed and subsequently reacts with PbAc₂. Then, the PbI₂ forms due to the incomplete reaction of PbAc₂ and MAI. The remnant PbI₂ could act as an interface modification layer for PSCs in our study despite the method we use to prepare CH₃NH₃PbI₃ films. However, the introduction of PbAc₂ would not influence the crystallinity of the CH₃NH₃PbI₃ films. By adjusting the PbAc₂ solution concentration, the optimized PSC can achieve a 15% enhancement of PCE in the case of using a two-step method. Under the circumstances of using an anti-solution method, it still displayed a 10% enhancement of the average PCE for PSCs with PbAc₂ (13.6% to 15.0%). The obvious V_oc increase of about 70 mV is attributed to the remnant PbI₂. Our results suggest that PbAc₂ is effective in controlling the amount of remnant PbI₂ at the m-TiO₂/CH₃NH₃PbI₃ interface, which provides a new way to optimize the performance of PSCs.

Acknowledgements

We gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (863 program) under Grant No. 2015AA050602, the National Natural Science Foundation of China (No. 51372083), the Fundamental Research Funds for the Central Universities (No. 2014ZZD07, 2015XS57, 2012ZX14), the Science and Technology Commission of Beijing Municipality, China (No. Z141100003314003), and Jiangsu Province Science and Technology Support Program, China (No. BE2014147-4).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00200a

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