Nano Energy 27 (2016) 17–26
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Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Controlled growth of textured perovskite films towards high performance solar cells Chengbin Fei a, Lixue Guo a, Bo Li b, Rong Zhang a, Haoyu Fu a, Jianjun Tian b,n, Guozhong Cao a,c,n a Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, China b Advanced Material and Technology Institute, University of Science and Technology, Beijing 100083, China c Department of Materials and Engineering, University of Washington, Seattle, WA 98195-2120, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 26 May 2016 Received in revised form 20 June 2016 Accepted 22 June 2016 Available online 23 June 2016
Antisolvent precipitation method has been one of the favored strategies to fabricate compact, smooth and uniform perovskite films for high efficiency solar cells due to its dramatically accelerated crystallization process. However, the excessively fast crystallization restricted the further improvement of the photovoltaic performance. In this work, we introduced CH3NH3Cl into the pristine CH3NH3PbI3 precursor for antisolvent precipitation at low temperature and fabricated high quality perovskite films with desired morphology, crystallinity and optical properties. The X-ray diffractometry and ultraviolent-visible spectroscopy provided ample evidence that CH3NH3Cl exerted significant impacts on the perovskite crystallization process by controlling the delivery speed of PbI2 from the intermediate phase CH3NH3PbI2Cl. The possible reactions in the perovskite formation process were first elaborated. The resultant solar cells demonstrated an average power conversion efficiency around 16.63% and a best efficiency at 17.22% under the standard light illumination condition. In addition, the ion migration was first detected in the perovskite solar cells by an ordinary impedance measurement. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Crystal growth Antisolvent precipitation Crystallinity CH3NH3Cl Perovskite solar cells
1. Introduction Owing to the suitable direct band gap (around 1.5 eV) [1], large molar extinction coefficient [2] and long electron-hole diffusion length ( 4175 mm in single crystal) [3], methylammonium lead halide (MAPbX3, X¼ Cl, Br, I) based solar cell became a very promising device to achieve excellent performance of converting solar energy into electricity [4]. In recent four years, many researches focused on the synthesis of well crystallized perovskite for light harvesting photovoltaic device, since there was a broad understanding that the properties of perovskite solar cells (PSCs) had high correlation with the morphology and crystal structure of MAPbX3 [5–7]. Generally, perovskite MAPbX3 can be produced by mixing the PbX2 and MAX precursors in organic solution or vapor phase, followed by thermal annealing to remove redundant ingredient and improve the crystallinity [8]. By applying vapor phase co-evaporation film deposition process, we could fabricate perovskite films with large scale and high repeatability, but the time n
Corresponding authors. E-mail addresses:
[email protected] (J. Tian),
[email protected] (G. Cao). http://dx.doi.org/10.1016/j.nanoen.2016.06.041 2211-2855/& 2016 Elsevier Ltd. All rights reserved.
consumptions and physical defect in perovskite films were still unsolved issues.[9,10] As for the solution-processing method, due to the slow crystallization procedure, the perovskite films were non-uniform, rough and incompletely covered. The high roughness increased the light diffuse reflection and further decreased the light harvesting efficiency at the range of long-wavelength (550–800 nm), while the incomplete coverage (especially pinhole) created more recombination between electron selective layer and hole transfer material [11]. To form dense and smooth perovskite films, a rapid nucleation process followed by a slow crystal growth process were requested. Therefore, antisolvent precipitation process could be a potential method to fabricate high quality perovskite film, since the crystallization process was dramatically speed up when contrasted with previous one-step solution process [12]. Antisolvent precipitation is a method for synthesizing uniform nanosized material [13]. When a solvent to which solute had none or a very low solubility was added into the initial solution, the initial solvent would be partially extracted by the new added solvent, so the concentration of solute in the initial solution raised to supersaturation in an extremely short time, thus promoted the precipitation and crystallization of solute. After the first application in the synthesis of perovskite for PSCs by Cheng et al. [12] the
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significance of this process have already been demonstrated by many research groups, [6,14] but there were only few works aimed to solve the problem caused by the excessively fast crystal growth, including the time dependence of film formation (known as “the sixth second”) and the undesirable grain size and crystallinity of the resulting perovskite films. Among the few works, the standard precursor (MAI and PbI2 in equimolar mixture) with the addition of DMSO has achieved successful synthesis of perovskite films with time dependence, but the amelioration of crystallinity was not noticed [15]. In contrast to the mixed chlorine- and iodinecontaining salt precursors in planner structure solar cells, the formation of CH3NH3PbI3 was slowed down, improving the film morphology and extending the carrier's lifetime [16–18]. Therefore, Cl-containing perovskite precursor was desirable to further improve and perfect the antisolvent precipitation process. Herein, we present a new antisolvent precipitation process to fabricate perovskite film with a decelerated crystal growth process. By introducing MACl to pristine CH3NH3PbI3 precursor in equivalent molar, the films' qualities were significantly improved, including relatively large grain, pin-hole-free and smooth perovskite film. With the assistant of X-ray diffractometer and ultraviolent-visible spectroscopy, the possible reaction route with the addition of MACl were studied. During the reaction process, MACl was believed to have slowed down the crystal growth process by controlling the delivery speed of PbI2, leading to much reduced nucleation rate and near thermodynamic equilibrium subsequent growth and, thus, resulting in a large crystallites with good crystallinity. With this new method, high stability and repeatability photovoltaic devices were readily fabricated, and the solar cells obtained a 16.63% 70.49% average efficiency and an impressive 17.22% efficiency for the champion cell. Besides, the development on photo-to-current efficiency was proved to originate from the increased light harvesting and charge separation. Finally, an obviously ion migration behavior was first detected by a simple impedance spectrum.
2. Experimental procedures 2.1. Device fabrication SnO2 electron selective layer was prepared by spin-coating precursor solutions of SnCl4 5 H2O in ethanol on clean FTO substrates. The SnO2 thin films were finally heated in air at 180 °C for 1 h. Afterwards, 0.02 M TiCl4 aqueous solution was used to treat the surface of SnO2 as the previously reported method [19]. The perovskite CH3NH3PbI3 films were prepared through the following method: firstly, 159 mg CH3NH3I and 461 mg PbI2 were dissolved in 1 mL dimethylformamide (DMF) at the ambient temperature, then 69 mg CH3NH3Cl was added into the solution to form the precursor. 30 μl perovskite precursor solution was dropped onto the prepared electron selective layer. After the substrate was spun at 4000 r/min for 4–15 s, ethyl acetate (100 μl, three big drops) was dropped on the center of the substrate continuously to induce quick crystallization of the perovskite. The pristine film was prepared according to the literature [12]. The obtained films were heated in air at 100 °C for 1.5 h. The hole transfer layer was spincoated by the chlorobenzene solution including 72.3 mg/mL SpiroMeOTAD, 28.8 μl/mL tertbutylpyridine and 17.5 μL/mL li-TFSI solution at 4000 r/min for 30 s. Finally, an Au counter electrode was formed by thermal evaporation. The active area was 0.07 cm2. 2.2. Characterization The morphologies of the perovskite films and completed devices were imaged by a high-resolution field emission SEM (SU-
8020, Hitachi). The absorption and transmission spectra were characterized by an ultraviolet–visible (UV–vis) spectrophotometer (UV-3600, Shimadzu) at ambient temperature. The X-ray diffraction pattern was recorded on X-ray diffractometer (PANalytical, Netherlands) using Cu Kα radiation. The photovoltaic characteristics and the electrochemical impedance spectroscopy (EIS) of the devices were performed using an electrochemical workstation (Zahner, Zennium) under simulated standard one sun illumination (AM 1.5, 100 mW/cm2) provided by a solar simulator (SOL02 series, Crowntech). The intensity-modulated photovoltage/photocurrent spectroscopy (IMVS/IMPS) were also characterized on electrochemical workstation with the assistant from light source Zahner (PP211). The photoluminescence (PL) spectra and fluorescence decay curve were taken out with combined steady state and time resolved fluorescence spectrometer (FLS980, Endinbergh). The incident photon-to-current conversion efficiency (IPCE) spectra was measured from an accessory kit of the electrochemical workstation.
3. Results and discussion In the new antisolvent precipitation method, dimethylformamide (DMF) was used as the initial solution to dissolve MAI, MACl and PbI2, while ethyl acetate (EA) was used as antisolvent to make the solute precipitation and crystallization from the initial solution rapidly. With a spinning-coating and low temperature heating processing, the perovskite film with TiO2 treated SnO2 compact layer and FTO substance were developed. The morphology of films produced from the precursor with and without MACl were shown as Fig. 1. The films were both compact and well covered (100% coverage) which efficiently block the charge leakage between SnO2 compact layer and Spiro-OMeTAD. With a MACl additive, the average grain size increased from 200 to 300 nm to above 500 nm, which provided a potential of promoted light harvesting in all wavelength below the absorption edge (commonly around 800 nm) [20]. Besides, the textured surface was very important in eliminating the shunting path and diffuse reflectance loss which has been proved harmful to the photon capture and electron collection in such a nanoscaled film [21]. The cross-section image was shown as Fig. 1e, the thickness of perovskite MAPbI3 layer in the planar junction structure is around 400 nm, which ensure enough light harvesting efficiency and electron collection efficiency [22]. To reveal the variation of crystal structure in perovskite films, the X-Ray diffractograms (XRD) were investigated. As shown in Fig. 2 and Table S1, the perovskite films both identical to the one obtained with solution-process method, exhibiting a preferential orientation accompanied by the appearance of two diffraction peaks at the angels 14.2° and 28.4°, corresponding to the (110) and (220) lattice planes [23]. Notably, the peak intensity of the perovskite film fabricated from MACl containing precursor was 2 orders of magnitudes higher than that from the common precursor (pristine sample), while the full width at half maxima (FWHM) of (110) plane was reduced from 0.14° to 0.08°, which directly confirmed the improvement of crystallinity [24]. This improvement was mainly due to the slow crystallization rate and increased grain size which strengthen the long-range order in the film. Besides the two prominent peaks, several weak peaks were also found in MAPbI3 diffraction pattern. However, those peaks reduced to almost invisible after the introduction of MACl, thus suggested that the presence of MACl in the precursors also impacted the crystallographic orientation in the resultant films. In this consideration, the introduction of MACl in MAPbI3 precursors played a predominant role in the lattice orientation selection and crystallinity optimization in perovskite formation process. Optical properties were always very powerful criterion for
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Fig. 1. SEM images of perovskite films fabricated in different antisolvent methods: (a) and (c) without any additive, (b) and (d) with equivalent MACl; (e) Cross-section image of the perovskite solar cells fabricated with MACl containing precursor.
Fig. 2. XRD patterns of FTO substance and perovskite films with different fabrication precursors, the intensity of the vertical axis is indexed to make the weak peak higher recognition. ■ Highlights the peaks belong to FTO substance. The inset is a unit cell of perovskite MAPbI3.
judging the quality of the photoactive material, which included absorption, transmission and emission (photoluminescence). When compared with the pristine film, the film with the
introduction of MACl shown stronger absorption at the long-wavelength range over 510 nm, and lower light harvesting at the short-wavelength ranges (Fig. 3a). According to the Lambert-Beer law, the decrease in absorption at short-wavelength ranges was probably due to the reduced film thickness, since absorption value A is defined as A ¼lg(1/T) ¼lg(I0/I) ¼K d C, where K is the molar extinction coefficient, d is the transport distance and C is the concentrate of a material [25]. As shown in Fig. S1, the thickness of the MACl-assisted film is about 400 nm while the one of the pristine sample is nearly 500 nm. The improved absorption in long-wavelength range is likely the combined result of better crystallinity and decreased reflectance. With the introduction of MACl, defect states and crystal disorder in the perovskite films would be significantly decreased, resulting in reduced defect states and better energy level distribution, which would ensure the low energy photons be captured by perovskite [26]. Moreover, the absorption edge was slight red-shifted which means narrowed bandgap and broaden absorbable region. Corresponding to the absorption, the films fabricated from new method showed weakened transmittance at the long-wavelength range and almost equal light transmittance at the short-wavelength ranges. Since the internal transmittance T has an exponential correlation with its absorption value A, the
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Fig. 3. (a) UV–vis measurement for perovskite films with and without MACl additive, the solid line and the dash line represent absorption and transmittance curve, respectively; (b) steady-state photoluminescence curves emitted at 770 nm obtained for the pristine sample and for the MACl added films upon excitation at 450 nm.
transmittance are all below 1% when A is greater than 2, i.e. without considering the reflectance, the proportion of absorbed photons (absorbance or absorption factor) A′ ¼1 T is over 99%. Actually, A is commonly calculated from T, while T is directly measured from power intensity rate of the monochromatic radiant transmitted by the absorbing medium (I) and the monochromatic radiant incident on the medium (I0) in the ultraviolet visible (UV– vis) spectroscopy. Therefore, transmittance is a more intuitive parameter to identify the light-absorbance capacity of a material. Fig. 3b demonstrated the emission property of the perovskite films when excited at a given wavelength. All the substance were passivated by an insulated Al2O3 layer to avoid the influence of charge injection from perovskite to the metal oxide semiconductor. In steady-state photoluminescence (PL) curve, the intensity and FWHM of the fluorescence emission peak were mainly determined by the materials’ crystallinity and defect state [27]. Compared with the pristine sample, the PL peak intensity of the sample with the MACI addition increases while the FWHM slightly reduces; both of which are indicative of better crystallinity. Corresponding to the absorption edge, the peak position of emission was also red-shifted by 4 nm which revealed the tuning of energy level position and dispersion. From the morphology and optical characterization, it could be seen that the films fabricated with MACl additive precursor showed significantly influence on grain size, crystallinity and optical response, however, the role that MACl played in the precursor was not clear. To unveil how the MACl works in the whole reaction process, we measured the XRD patterns of perovskite films annealed with different time during the reaction process. The annealing temperature was kept at 100 °C from the very beginning, and the samples were rapidly cooled to ambient temperature after the annealing. After the sample preparation, X-ray diffractograms were immediately measured to ensure accuracy of the correlation between heating time and peak intensity. Fig. 4a demonstrated the full scan diffractograms obtained for different heating durations from 0 min to 80 min. It was clear from the data that the tetragonal perovskite phase of MAPbI3 was presented in all films. For (110) and (220) plane, the two most representative lattice planes of perovskite, increased with the time, the increased diffraction peak intensities were clearly pronounced. Besides MAPbI3, there were several other peaks that related to the intermediate phase which were helpful to insight the course of the reaction. As shown in Figs. 4b and e, the peak around 12.4° corresponded to the (001) plane of 2H PbI2 phase [28]. During the heating process, the peak intensity shown a first increase and then decrease tendency and got a maximum at the 8 min, indicated that the release and depletion of PbI2 were simultaneously existed in the
reaction process. Fig. 4c revealed the variation of two intermediate phases (CH3NH3)x þ yPbI2 þ xCly and CH3NH3PbI2Cl which were located at 15.5° and 15.9°, respectively [29]. The (CH3NH3)x þ yPbI2 þ xCly peak was substantially maintained at similar intensity until 8 min, while the position of which kept a slight right shift revealing a possible changing on the phase formula in x and y. CH3NH3PbI2Cl phase that reported as PbI2 CH3NH3Cl in literature has showed a similar variation tendency with PbI2, indicating a relationship of mutual transformation between these two phases [30]. In Fig. 4d, the three peaks at 31.9°, 32.5° and 33.7° reflected the information belong to (310) plane of MAPbI3, (CH3NH3)x þ yPbI2 þ xCly and (101) plane of FTO, respectively. The peak of FTO was commonly used as a datum of peak position. As mentioned above, MACl has a remarkable influence on the regulation of perovskite crystal orientation during the crystal grown process, so the (310) plane was almost invisible before the 16 min. From 48 min, the reaction process was basically completed, so the decrease of peak intensity revealed the trimming of lattice structure in the continuous heating treatment. On the other hand, the influence of sintering temperature on the crystallinity of the final films was very weak, since there was no significant difference between the perovskite diffraction patterns with different temperatures (Fig. S4a). With the preliminary analysis by XRD, the possible chemical reactions involved in the perovskite formation process were clear. In the order of heating times, the working principal of MACl in the chemical reaction process was almost realized as Scheme 1 and the following reactions: (1) PbI2 þCH3NH3Iþ CH3NH3Cl - (CH3NH3I) PbI2 (CH3NH3Cl)-(CH3NH3)x þ yPbI2 þ xCly - (CH3NH3)x′ þ y′PbI2 þ x′Cly′ þm CH3NH3PbI3 þn CH3NH3PbI2Cl (2) CH3NH3PbI2Cl-PbI2 þCH3NH3Cl (g)↑ (3) PbI2 þCH3NH3I-CH3NH3PbI3 Interestingly, CH3NH3Cl was insoluble in the DMF solution containing PbI2 initially, but easily dissolved in the DMF mixture of CH3NH3I and PbI2, indicating that a new precursor phase must be formed in a chemical reaction (see Fig. S2). So, in the MACl containing precursor, the three solutes MAI, MACl and PbI2 will combine into (CH3NH3I) PbI2 (CH3NH3Cl) through formation of coordinative bonds. When the precursor was coated on the substrate, (CH3NH3I) PbI2 (CH3NH3Cl) would convert to (CH3NH3)x þ yPbI2 þ xCly immediately (Reaction (1), thus numerous (CH3NH3)x þ yPbI2 þ xCly and tiny amount of PbI2 and MAPbI3 were
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Fig. 4. XRD patterns of different annealing timed perovskite films which fabricated from MACl containing precursor: (a) The panorama curve from 10° to 36°, (b)– (d) Selected ranges which containing particular characteristics; (e) XRD patterns of pure MAPbI2Cl, MACl, MAI and PbI2 phase. Note that all the intensities of the vertical axis above are indexed.
detected from the films at the very beginning of heating process. The decomposition reactions of (CH3NH3)x þ yPbI2 þ xCly took place with a thermal driving force, followed by the generation of MAPbI3, MAPbI2Cl, so as the amount of both two new generated phase increased until 8 min. The formula of that phase was meanwhile changed into (CH3NH3)x′ þ y′PbI2 þ x′Cly′. Since CH3NH3PbI2Cl was not stable at the temperature above 60 °C in our experiment, which would be degraded into PbI2 and CH3NH3Cl vapor under the Reaction (2). Then, as shown in Reaction (3), PbI2 was consumed by MAI which inserted in the framework layers. Because those reactions were carry out in series, the amount of some intermediate products were changed synchronous, especially PbI2 and CH3NH3PbI2Cl. Therefore, we believe the role of
MACl was to retard the rapid reaction between PbI2 and MAI during the evaporation of solvent in the spin-coating process. In addition to the XRD characterization, the changes on optical properties during the annealing process were also measured. As shown in Fig. 5, the sample without any heating treatment (0 min) showed a very weak light response at the wavelength above 600 nm, which mainly related to the low content of MAPbI3. Besides, on the absorption curve, there was a notably drop at around 440 nm and an iconic depression at 340 nm, corresponding to the absorption characteristic of PbI2. However, the absorption between 440 nm and 600 nm also cannot be ignored, which was most likely contributed by (CH3NH3)x þ yPbI2 þ xCly. During the heating process before 48 min, the content of MAPbI3 was always increasing, so
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Scheme 1. The reaction process in the different perovskite fabrication routes: (a) Normal antisolvent precipitation method, (b) MACl introduced antisolvent precipitation method. Intermediate phase I represent the complex of MAPbI3, MAPbI2Cl, (CH3NH3)x þ yPbI2 þ xCly and PbI2, while intermediate phase II represent the lattice distorted MAPbI3 and the residual MACl.
Fig. 5. Absorption spectra of the MACl introduced perovskite films with different times during the heating process. The curves of PbI2 and CH3NH3PbI2Cl were used as references.
the absorption value at long-wavelength range over 440 nm were increased gradually. Owning to the powerful light photon trapping of PbI2 and CH3NH3PbI2Cl at the short-wavelength range, the absorption value changes slightly. As the reaction proceeds, the colors of the films changed accordingly, from light brown to emerald, and finally to dark brown (see Fig. S5). From 48 min, the drop and the depression in all the samples previously were disappeared, which reflected the depletion of all the intermediate phases. Afterwards, the increased absorption at the range below 510 nm mainly benefit from the trimming of lattice structure. With the optical test, our understanding on the reaction mechanism was further validated. Through the measurements above, the new antisolvent precipitation method with MACl containing precursor provided a potential to obtain high performance device on photoelectric application. As shown in Fig. 6a, this method was applied into solar cells while the photovoltaic parameters were measured. Due to lower light transmission loss, the solar cells fabricated from the new method performed higher short-circuit current density (JSC) which increased from 19.99 to 21.87 mA/cm2 compared with the previous method. The second important parameter in solar cells
was open-circuit voltage (VOC) which was commonly determined by the relative position of energy band and charge recombination, since the recombination between electron and hole could reduce the density of electrons in conductive band and further drop the Fermi level [31]. Benefited from the tuned band structure and reduced surface defect, the VOC was improved from 1.01 to 1.06 V. Fill factor was a parameter that used to assess the energy loss in the photoelectric conversion process, which containing the substance reflection loss, series resistance loss (RS, electrode resistance and charge transfer resistance) and shunt resistance losses (RSH, recombination and edge leakage). As the substance were totally the same, the increasing of fill factor in the new method fabricated device was mainly originated from the less recombination and large grain size which provided a fast electron transport. Within the development of the overall performance, the energy conversion efficiency was increased from 13.97% to an impressive efficiency 17.22%. In addition, the influence of precursor concentration on the device performance were also investigated (Fig. S4b). High precursor concentration generally resulted in an increased film thickness and absorption value, leading to the development of current density. Correspondingly, the electron diffusion distance was also expanded by the increased film thickness, which caused large recombination rate of electron-hole pair and then decreased the voltage. Due to the poor solubility of MACl in the DMF solution, the best performance of the solar cells were achieved with the 1 M precursor solution. By virtue of the high reproducibility and uniformity of the new method fabricated films, the average PCE of the solar cells increased from 12.78% 71.10% to 16.63% 70.49% (Fig. 6b). The narrowed peak of the simulate line indicated the improvement of film uniformity, which is desired for the actuality application of perovskite solar cells. The incident photon-to-current efficiency (IPCE) as a function of wavelength for the champion(optimized) cell was displayed in Fig. 7a. The IPCE curve of the new method fabricated solar cells showed a stable and strong spectral response in the range below 750 nm with an efficiency close to 90%, while the spectral response of the pristine sample showed a slow drop after 520 nm. The generation photocurrent started at 815 nm and the integrated current density (20.95 mA/cm2 and 19.46 mA/cm2) calculated from the IPCE data were in close agreement with those obtained
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Fig. 6. (a) Current density-voltage curve of the perovskite solar cells with different antisolvent precipitation method, all the photovoltaic parameters are listed. The canning range is 0.1 V to 1.2 V, while the scanning rate is 10 mV/s; (b) the statistics result for the power conversion efficiency of the different method fabricated solar cells. The data of “Pristine” samples were calculated from 29 solar cells, and the data of “MACl” samples were calculated from 30 solar cells.
from the J-V curve. As IPCE is defined as IPCE ¼ LHE ɳsep ɳcoll, where LHE was the light harvesting efficiency, ɳsep was the electron-hole seperation efficiency and ɳcoll was the charge collection efficiency [32], the variation of IPCE was studied from the three parts. Since LHE has a negative correlation with the transmittance and reflectance in the perovskite film, the films fabricated from the new method kept a more stable and excellent light response in all the ranges below absorption edges (Fig. S6). The charge seperation efficiency (ɳsep) was commonly used to reflect the charge
extraction properties from the interface of perovskite and the electron selective layer, which was charactered by the time-solved photoluminescence decay measurement. In Fig. 7b, the PL quenching in the FTO/MAPbI3 film was simulated by double exponential τ1 and τ2, which commonly represent the fast decay and slow decay, respectively. The fast decay (τ1) correlated with the electron injection process, indirectly providing a measure of charge separation efficiency, while the slow decay (τ2) related to the non-radiative recombination in the
Fig. 7. (a) Internal photo-to-current efficiency measurement and the calculated integral current density for the devices which fabricated from different precursors, (b) Timesolved photoluminescence decay at 770 nm obtained for the pristine sample and for the MACl added films upon excitation at 450 nm; (c) Diffusion coefficient of solar cells at different working voltage of the light source; (d) Nyquist plots of the devices after one hour light illumination under the standard light intensity, the bias voltage was 0.8 VOC and the scanning range was 1 MHz to 0.1 Hz.
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perovskite crystal. The MACl assisted perovskite film (τ1 ¼4.3 ns, τ2 ¼ 66.8 ns) showed smaller τ1 and larger τ2 than that in the pristine film (τ1 ¼ 8.1 ns, τ2 ¼ 58.4 ns), suggesting faster charge carriers transfer from the large grain sized MAPbI3 (the MACl sample) into the electron conductor material and longer charge diffusion length [33]. As shown in Fig. 7c, the diffusion coefficient (D) with intensity modulated photocurrent spectroscopy (IMPS) was measured. The plots of D with light source working voltage (V) indicated that the charge diffusion rate in the new method fabricated solar cell were increased by 1.26-fold, comparing with that of the pristine cell. Besides, the electron lifetime (τn) of perovskite solar cells were also measured by the intensity modulated photovoltage spectroscopy (IMVS), both of which were found to be around 10 6 s. As a result, the diffusion length (L ¼(D τn)0.5) of the perovskite with and without MACl assistance formation were both over 1 mm, which was sufficient to guarantee effective collection efficiency (ηcoll) for both the samples [34]. To investigate the charge transfer properties of the interfaces, under a standard one sun illumination (AM1.5, 100 mW/cm2). The RS consists of the internal resistance of the electrodes (RE), the charge-transfer resistance (RCT) at the interfaces between the carrier (electron or hole) selective layer and the photo-active material [35]. The main difference between the perovskite solar cells fabricated by different methods was the RCT at the interface between different grain size MAPbI3 and electron selective layer, since the resistance between Spiro-OMeTAD and perovskite was sufficient low to be ignored. Fig. 7d showed the Nyquist plots measured at the bias voltage of 0.8 VOC (0.84 V and 0.81 V for the MACl and Pristine sample, respectively) which reflected the information of maximum output power intensity point. From the simulated result, the values of RCT in the solar cells were 59 Ω with MACl and 104 Ω without MACl, respectively. The large RCT value in the pristine sample indicated inferior interphase contact which was detrimental to the electron conduction. In addition, a slash at the low frequency ranges (below 10 Hz) was found which commonly meant Warburg diffusion resistance (W) of ion in the device. Unlike electrochemical cells, liquid electrolyte was not permitted in such an all-solid structure solar cell, thus the diffusion resistance was most probably originated from the migration of MA þ or I in perovskite. As the slope of the slash has positive correlation with ion diffusion coefficient, [36] the pristine sample with a larger slope was believed to have faster ion immigration than the sample with MACl. Since the ion immigration was one possible reason that lead to the hysteresis, structure instability and several other drawbacks in perovskite solar cells, this discovery
was meaningful to provide a convenient method for insight the perovskite. Note that hysteretic behavior was an important issue in the current density–voltage characteristics of PSCs, which made it difficult to estimate the actual PCE [37]. To show the impact of the hysteresis on the solar cells which fabricated from the new method, the current density–voltage traces of PSCs were collected with a 10 mV/s scanning rate between the backward and the forward direction (Fig. 8a). From the data that summarized in the insert, the device shown a relatively low hysteresis which was better than most of reports on planar heterojunction structure with inorganic electron selective layer. In order to rule out the actual PCE of the device, the current density and power conversion efficiency as a function of time for the same cell were performed. As shown in Fig. 8b, the photocurrent density and stabilized power output of the cells remained stable within 300 s, and a 16.54% average PCE was obtained. This result suggests that the new antisolvent precipitation method with MACl containing precursor provides a possibility of preparing efficient and stable perovskite solar cells.
4. Conclusions A new antisolvent precipitation method was developed to fabricate high efficiency perovskite solar cells with controllable decelerating crystallization process. With a MACl-containing precursor, the fabrication process showed no difficult on time dependence, while the production film displayed excellent morphology with textured surface, large grain size, high uniformity and 100% coverage. When compared to the pristine antisolvent precipitation method, the MACl-containing precursor fabricated perovskite film exhibit much purer crystalline phase with strong (110) preferred orientation. Such high crystallinity benefits from a decelerated crystallization involving controlled slow release of PbI2 from the CH3NH3PbI2Cl intermediate phase, thus lead to excellent light response including high absorption and low transmission at the whole spectrum scale. Besides, the possible chemical reactions with the presence of MACl during the annealing process were revealed, which were confirmed by means of X-ray diffraction and absorption spectrum. The high quality perovskite films were integrated in a typical planar solar cell structure with a TiO2 passivated SnO2 electron selective layer and yield an impressive average efficiency of 16.63% 70.49% and champion efficiency of 17.22% under standard one sun illumination. In the whole photo convert to electron process, the light harvesting efficiency
Fig. 8. (a) Current density–voltage curve of the MACl assistant perovskite solar cells with different scanning direction. The canning range is 0.1 V to 1.2 V, while the scanning rate is 10 mV/s. All the photovoltaic parameters of the different scanning direction curves are listed. (b) Steady power conversion efficiency curve of the new method fabricated solar cells (light on at 5 s).
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was significantly increased at the long-wavelength ranges while the electron separation efficiency was developed. Through the impedance spectrometry, fast electron transport and low ion immigration were found in the perovskite solar cells with MACl containing precursors, which was the first time to detect ion immigration with such a simple characterization. Collectively, this new antisolvent precipitation method were believed to open new possibilities to fabricate perovskite with high performance on stability, repeatability and efficiency.
Acknowledgments This work was supported by the “thousands talents” program for pioneer researcher and his innovation team, China. This work was also supported by the National Natural Science Foundation of China (51374029 and 91433102), Program for New Century Excellent Talents in University (NCET-13-0668), Fundamental Research Funds for the Central Universities (FRF-TP-14-008C1) and China Postdoctoral Science Foundation (2014M550675).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.2016.06. 041.
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Chengbin Fei received his B.S. degree from Xi'an Jiaotong University. He is now working for Ph.D. degree in Beijing Institute of Nanoenergy and Nanosystems under the supervision of Prof. Guozhong Cao. His current researches focus on the interfacial modification of nanostructured metal oxide semiconductor and the synthesis of high crystallinity perovskite for perovskite solar cells.
Lixue Guo is currently a master candidate in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. Her research interest focuses on the application of Plasmon effect in perovskite solar cells.
Bo Li was born in Inner Mongolia, China, in 1990. He received his BS (2014) and currently is a Ph.D. candidate in University of Science and Technology Beijing. His research interest includes design of materials for stretchable electronics, perovskite solar cells and organic-inorganic hybrid nanomaterial.
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C. Fei et al. / Nano Energy 27 (2016) 17–26 Rong Zhang received his bachelor degree from Dalian University of Technology, China in June 2014. Now he is a master degree candidate in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His current research focuses on the fabrication of perovskite solar cells.
Jianjun Tian is a professor in Advanced Material and Technology Institute, University of Science and Technology Beijing. He has worked as a visiting scholar in University of Washington in 2011. His current research is focused on the fabrication of high quality quantum dot sensitized solar cells and perovskite solar cells.
Haoyu Fu received his bachelor degree from Huazhong University of Science and Technology (2011) and master degree from Beijing University of Chemical Technology (2014). He is currently a Ph.D. candidate in Prof. Guozhong Cao's group. His research interest is the developments of materials for the energy storage device.
Guozhong Cao is a Boeing-Steiner Professor of Materials Science and Engineering at the University of Washington, and a senior professor at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. He has published over 500 papers, 8 books and 4 proceedings. His recent research is focused mainly on solar cells, lithium-ion batteries, super capacitors, and hydrogen storage.