Ionic liquid-assisted perovskite crystal film growth for high performance planar heterojunction perovskite solar cells

Abstract

We demonstrate an efficient approach by using a new type of ionic liquid, 1-ethylpyridinium chloride (1-EC) with a relative low melting point of 100 °C, to control the morphological growth of CH₃NH₃PbI₃ during the one-step deposition method for preparing efficient planar heterojunction perovskite solar cells, leading to a continuous and dense morphology without large voids and pinhole. With the optimized concentration, 11.8% power conversion efficiency was obtained. Our work paves the way to optimize the morphology of perovskite by adopting a new kind of ionic liquid as additive. This method has great benefits for exploiting new technologies for large-scale perovskite solar cells on the flexible substrate.

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Introduction

In the past few years, perovskite solar cells have evoked extensive attention due to their controllable band gap, suitable energy levels, a broad absorption spectrum, long diffusion lengths of electron and hole, high carrier mobility.^1–3 To date, a certified efficiency of 22.1% has been reported.⁴ The performance of perovskite solar cells is critically dependent on both the microstructures of the perovskite films and also the interfacial properties of the devices.⁵ The perovskite film morphology are generally determined by their thermodynamics and growth kinetics, which can be tuned by varying the fabrication process, polar of the solvent, the annealing time and temperature, the levels of environmental humidity and processing additives.^6–8 There are several methods reported controlling the morphology and crystal film growth of perovskite including thermal evaporation deposition, sequential solution deposition, vapor-assisted solution process (VASP) and one-step solution deposition.^8–10 Among them, one-step solution deposition is a relatively simple method. Moreover, it is a severe challenge for controlling the morphology and crystallinity of perovskite film in the inverted planar heterojunction perovskite solar cells with a structure of ITO/PEDOT:PSS/perovskite/electron transporting layer/cathode. Introduction of additives in solution deposition process is an effective approach to modulate the growth of perovskite films, and to improve the film coverage and crystallinity. Several additives, such as NH₄Cl,¹¹ CH₃NH₃Cl,¹² 1-chloronaphthalene¹³ and 1,8-diiodooctane (DIO),¹⁴ have been used to deposit continuous and uniform films, which significantly enhance the device performance. Snaith et al. reported a mixed halide precursor solution that could effectively tune the nucleation and growth kinetics of the perovskite films.¹⁵ Jen and coworkers found that bidentate halogenated additives can temporarily chelate with Pb²⁺, which encourages homogenous nucleation and modifies interfacial energy, ultimately altering the kinetics of crystal growth.¹⁴

Ionic liquid with low vapour pressure, high ionic conductivity, and excellent electrochemical and high thermal stability is a promising candidate in the area of perovskite solar cells. Moore reported the direct crystallization of CH₃NH₃PbI₃ films from an ionic liquid, methylammonium formate (MAFa).¹⁶ They found that highly oriented films with excellent coverage, uniformity, and large crystal domains can be obtained by using MAFa as solvent. Subsequently, Cao group evaluated a series of ionic liquid as processing additives and interfacial modifiers on the morphology of the CH₃NH₃PbI_3−xCl_x films.¹⁷ They found that tetra-phenylphosphonium iodide (TPPI, melting point: 333–343 °C) and chloride (TPPCl, melting point: 278 °C) were proved to be the most effective in enhancing the crystallinity and coverage of the perovskite films. Taima et al. incorporated a small amount (1.0 wt%) of ionic liquid, 1-hexyl-3-methylimidazolium chloride (HMImCl, melting point: 210 °C), in the spin-coating solution as process additive to control morphology and to fabricate uniformly CH₃NH₃PbI₃ films.¹⁸ However, their preliminary results showed it is difficult to complete removal of the residual ionic liquid contents by using a non-destructive solvent assisted washing procedure from the CH₃NH₃PbI₃ film after establishing the optimized condition. Therefore, the efficiency of the resulting solar cells is poor. Generally, ionic liquid is selected according to its suitable solubility in solvent for processing and solvent for removing it after spin coating. It would be much simpler if the ionic liquid can be removed much easier from the optimized morphology.

In this work, we have demonstrated an efficient approach by using a new type of ionic liquid, 1-ethylpyridinium chloride (1-EC) with a relative low melting point of 100 °C, to control the morphological growth of CH₃NH₃PbI₃ during the one-step deposition method for preparing efficient perovskite solar cells. The effect of the additive on the morphology, structure, optical and electrical properties of perovskite film has been investigated by UV-vis, SEM, XRD, AFM and photoluminescence spectra. The results showed that the addition of 1-EC is beneficial to regulate the crystallization transformation kinetics of perovskite to form high quality crystal films. Unlike previously used additives, especially for amine group chlorides such as NH₄Cl and CH₃NH₃Cl, 1-EC has the advantages with better solubility in organic solvent and stability. Our study provides an efficient way of developing highly efficient CH₃NH₃PbI₃-based perovskite solar cells, which is well suitable for large scale production.

Results and discussion

The chemical structure of 1-EC, the solar cell structure and the energy level diagrams of each layer are shown in Fig. 1a. The solar cell structure and the energy level diagrams of each layer are also shown in Fig. 1b and c, respectively.

Fig. 1 (a) Chemical structure of 1-EC. (b) Device structure. (c) Schematic energy level diagrams of each layer.

The current density–voltage (J–V) characteristics and IPCE curve of the optimized devices fabricated from precursor solutions doped with 1 wt% content of 1-EC are shown in Fig. 2 and the corresponding photovoltaic parameter including open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF) are summarized in Table 1.

Fig. 2 The current density–voltage (J–V) characteristics of the best device without and with 1 wt% 1-EC additive (a), IPCE curve of the best device with 1-EC additive (b), J–V curves of the optimal perovskite device measured with different sweep directions (c).

Table 1 Summary of device performance for the perovskite solar cells with different amount of 1-EC additive

Concentration (wt%)	Jsc (mA cm^−2)	Voc (V)	FF (%)	PCE (%)
0	11.56 ± 0.15	0.85 ± 0.01	51.10 ± 0.01	5.01 ± 0.05
0.25	12.33 ± 0.89	0.87 ± 0.01	56.61 ± 0.51	6.10 ± 0.05
0.50	12.67 ± 1.43	0.88 ± 0.01	65.82 ± 0.63	7.31 ± 0.81
0.75	15.26 ± 0.89	0.90 ± 0.02	71.70 ± 0.90	9.91 ± 0.75
1.00	17.34 ± 0.18	0.91 ± 0.01	75.24 ± 0.58	11.80 ± 0.24
1.25	13.24 ± 0.77	0.88 ± 0.01	67.70 ± 0.91	7.89 ± 0.44

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The reference device without additive achieved a PCE value of (5.01 ± 0.05)%, with V_oc of (0.85 ± 0.01) V, J_sc of (11.56 ± 0.15) mA cm⁻² and FF of (51.10 ± 0.01)%. In contrast, devices doped with 1-EC additive show a positive effect on the device performance. As showed in Table 1, the best efficiency is obtained for the device with the addition of 1.0 wt% additive. The addition of 1.0 wt% additive increases the V_oc to (0.91 ± 0.01) V, the J_sc to (17.34 ± 0.18) mA cm⁻² and the FF to (75.24 ± 0.58)%, and led to a PCE of (11.80 ± 0.24)%. This efficiency improvement can be attributed to the simultaneous increase of the FF and J_sc. The J_sc obtained by integration of the incident photon-to-electron conversion efficiency (IPCE) spectra agrees well with the J_sc obtained from the J–V measurement in Fig. 2b. The reason for the performance enhancement may be ascribed to the better coverage of the perovskite film and interfacial morphology. The photocurrent–voltage hysteresis is a major criteria in the evaluation of device performance and film quality in PSCs,¹⁹ we measured the J–V curves for the best cells with 1-EC as additive by changing sweep directions with the step widths of 10 mV (Fig. 2c). When measured by scanning from a positive to a negative direction and from a negative to a positive direction, a slightly larger J–V hysteresis was observed. The origin of the scan-direction dependent J–V hysteresis is likely resulting from unbalanced charge accumulation at the perovskite/PCBM interfaces.²⁰

Fig. 3a and b shows the AFM images of film without and with 1 wt% 1-EC additive. The pristine CH₃NH₃PbI₃ exhibits R_a = 11.1 nm, R_q = 14.2 nm over an area of 2 μm × 2 μm. The R_a and R_q roughness for the film prepared using 1.0 wt% additive is even small for 5.54 nm and 7.18 nm, respectively. Apparently, the film prepared using 1-EC as additive shows much smoother than the film prepared without additive. Fig. 3c shows a non-continuous film morphology with large void and pin-hole that is detrimental to the device performance for the pristine film. A rough surface of perovskite film may cause a problem for the surface coverage of PCBM electron transporting layer on the top perovskite and current leaking. This may be one of the reasons for the low performance of the control device. The perovskite films using 1-EC as additive show a dense, continuous and pinhole-free surface for forming a high-quality active layer, which was supported by SEM images in Fig. 3d. Addition of 1-EC additive to the spin-coating precursor solutions facilitated the formation of homogenous nucleation sites and prevented rapid crystal formation of CH₃NH₃PbI₃. Therefore, the presence of an ionic liquid additive resulted in uniform thin films with good morphology. As a result, enhanced performance of the CH₃NH₃PbI₃/PCBM planar heterojunction device with higher photocurrent density and FF is acquired. Here, the role of the 1-EC can be described as follows. First, it may act as an in situ scaffold additive which occupies space during the film growth. During spin-coating and solvent (DMF) evaporation process, owing to its polar nature, 1-EC can quickly turn into small nano-sized aggregates which are uniformly distributed in the precursor solutions. These aggregates, similar to the metal oxide scaffold, may provide numerous heterogeneous nucleation sites for the nearby precursor species to nucleate and crystallize. As a result, more uniform and homogenous perovskite film morphology can be obtained. It is generally accepted that the existence of trap states (un-coordinated Pb atoms) may lead charge recombination in the perovskite layer, resulting in a low current density and fill factor. 1-EC additive contains electron-rich pyridine ring, which can provide excess electron to neutralize the trap states existed at the perovskite surface and crystal boundaries. Thereby, 1-EC additive can effectively passivate the surface defects.

Fig. 3 The AFM topography (a and b) and SEM images (c and d) of the surface morphology of pristine CH₃NH₃PbI₃ and with 1 wt% 1-EC as additive (inserted photos show the photographs of surface morphology of low-magnification and the scale bar represents 5 μm).

Fig. 4 displays the absorption and photoluminescence (PL) spectra of the perovskite films prepared with and without the 1-EC additives. The films incorporating the 1-EC additive exhibit increased light absorbance, due to their relatively greater surface coverage and uniformity, which were supported by SEM images of the films. It has been reported that the trap states could led to an obvious band gap lower than that of the bulk with a red-shifted PL peak. As showed in Fig. 4b, the PL peak of pristine perovskite is located at 759 nm, which is smaller than previously reported.²¹ As expected, the PL peak of films obtained using 1 wt% 1-EC as additive is blue-shifted from 759 nm to 757 nm, suggests the fewer surface defects close to the top surface of perovskite film. Perovskite films contained fewer trap centers and defects will absorb more light and generate more excitons and therefore, produce a higher PL emission. This result is also in agreement with the photovoltaic performance of the resultant devices. In highly crystalline perovskite films, the larger carrier diffusion lengths will help to reduce the possibility of radiative recombination, and to improve the charge transport. It can also be inferred that an appropriate concentration of additives can benefit to create ascendant film contained fewer traps and defects (Fig. 5).

Fig. 4 The UV-vis absorption (a) and PL spectra (b) for pristine CH₃NH₃PbI₃ film and with 1 wt% 1-EC as additive.

Fig. 5 The XRD patterns of CH₃NH₃PbI₃ films formed on PEDOT:PSS without and with 1 wt% 1-EC as additive.

To get more in-depth understanding of the role of 1-EC during perovskite crystallization, we compare the X-ray diffraction (XRD) patterns of the pristine CH₃NH₃PbI₃ and films with 1.0 wt% 1-EC as additive. CH₃NH₃PbI₃ peaks located at 14.2° and 28.5° are assigned to the (110) and (220) crystal planes of the orthorhombic lattice of perovskite. The Scherrer equation was used to compare the crystallinities (average crystallite sizes).

D = Kλ/β cos θ

where D is the average crystallite size, λ is the wavelength of the X-ray irradiation (0.154 nm), and β is the line width at half maximum (in radians). The average crystallite size estimated from the full-width at half maximum (FWHM) of the (110) peak at 2θ = 14.2° is approximately 28 nm and 40 nm for the as-deposited films and the films with 1 wt% 1-EC as additive, respectively. This may suggest an improvement of the degree of crystallinity, or a change in the preferred orientation of perovskites with additive, which is presumably caused by the slower crystallization process when ionic liquid participates in crystallinity. It should be noted that there was no PbI₂ peak at 12.65°, which indicates the ionic liquid can play an important role in suppressing of decomposition of perovskite.

Conclusions

In conclusion, we have demonstrated an efficient approach to control the morphological growth of CH₃NH₃PbI₃ perovskite solar cells during the one-step deposition method by incorporated 1-EC into perovskite precursor solution to modulate thin film formation, leading to a continuous and dense morphology without large voids and pinhole. The addition of 1.0 wt% additive increases the V_oc to (0.91 ± 0.01) V, the J_sc to (17.34 ± 0.18) mA cm⁻² and the FF to (75.24 ± 0.58)%, and led to a PCE of (11.80 ± 0.24)%. Ionic liquid plays an important role in encouraging homogenous nucleation and modifying surface defects, ultimately altering the kinetics of crystal growth. The improvement in device efficiency is mainly attributed to the good crystal structures, more homogenous film morphology, and also fewer trap centers and defects in the films. Meanwhile, this method has great benefits for exploiting new technologies for large-scale perovskite solar cells on the flexible substrate.

Acknowledgements

The authors are grateful to the Tianjin Natural Science Council (Grant No. 13JCYBJC18900, 13JCZDJC26700), National High Technology Research and Development Program of China (863 Program) (Grant No. 2013AA014201), National Key Scientific Instrument and Equipment Development Project (2014YQ120351) and the Tianjin Key Discipline of Material Physics and Chemistry for the support of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24616h

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