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Enhancing the perovskite solar cell performance by the treatment with mixed anti-solvent
Journal of Power Sources 404 (2018) 64–72
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhancing the perovskite solar cell performance by the treatment with mixed anti-solvent
T
Shao Jina,c, Yuelin Weia,c,∗, Feiyue Huanga,c, Xiaomin Yanga,c, Dan Luob,c, Yu Fanga,c, Yuezhu Zhaob,c, Qiyao Guoa,c, Yunfang Huangc,a,∗∗, Jihuai Wua,c a
College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China College of Chemical Engineering, Huaqiao University, Xiamen, 361021, China c Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Huaqiao University, Xiamen, 361021, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
A simplified anti-solvent treatment is • developed for high-quality perovskite films.
high-quality film improves the • The charge separation and transport efficiency.
conversion efficiency of the • Power solar cell is enhanced from 17.4% to 19.18%.
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Phthalocyanine nickel Mixed anti-solvent
At present, the surface modification of perovskites by solvent treatment has become the most effective way to improve the performance of planar perovskite solar cells. Here, phthalocyanine nickel as an additive was introduced into the anti-solvent chlorobenzene to form a mixed anti-solvent. Prior to completion of the perovskite crystals, a mixed anti-solvent was infiltrated into the perovskite surface to improve the interfacial contact of the perovskite with the hole-transporting layer. This method improved the crystal quality of the perovskite film, thus enhanced the charge transfer efficiency, and suppressed the recombination of carriers effectively. The perovskite solar cell constructed with this perovskite films treated by the optimal concentration of nickel phthalocyanine in the anti-solvent solution yielded a power conversion efficiency of 19.18% and the filling factor of 74.38% under 100 mW cm−2 illuminations.
1. Introduction In recent years, organic-inorganic hybrid perovskite materials have triggered a wave of research in the field of solar cells. Since Miyasaka's team first started using CH3NH3PbI3 perovskite in solar cells in 2009,
the energy conversion efficiency of the perovskite solar cells (PSCs) have increased from 3.8% to 22.1% [1–7]. Halogenated perovskite materials have become the “dream material” for photovoltaic applications due to their high optical absorption coefficient, high carrier mobility and long carrier diffusion length.
∗
Corresponding author. College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China. Corresponding author. Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Huaqiao University, Xiamen, 361021, China. E-mail addresses: [email protected] (Y. Wei), [email protected] (Y. Huang). ∗∗
https://doi.org/10.1016/j.jpowsour.2018.10.008 Received 9 July 2018; Received in revised form 26 September 2018; Accepted 5 October 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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At present, the most successful organic-inorganic perovskite material has an ABX3 structure comprised of an organic cation, A = (methylammonium CH3NH3+; formamidinium CH3(NH2)2+); Cs+, a divalent metal, B = (Pb2+; Sn2+; Ge2+), and an anion X = (Cl−; Br−; I−; SCN−) [8–10]. The perovskite solar cell structure has a classical mesoporous n-i-p junction and a mesoporous “planar” structure [11]. Planar heterojunction PSCs have been extensively used as one of the most efficient configurations of PSCs. The improvement of the properties of PSCs mainly depends on the crystal quality, morphology and interfacial contact of perovskite films. Currently, perovskite films are prepared by one-step spin-coating [12], dip coating [13], two-step interdiffusion [14], and vacuum assisted evaporation [15]. Among them, the one-step spin-coating method is an effective process to regulate the morphology, the grain size and the crystal quality of the perovskite film by dropping the anti-solvent (chlorobenzene, toluene, ethyl acetate) during the spin coating. Generally, interfacial engineering is an efficient approach to solve the problems of defects and the energy-barrier mismatch, thus improving the photovoltaic performance of PSCs [16–21]. For example, further annealing in dimethylfomamide steam increased the size of the perovskite grains [22]. Removal of the remaining chlorobenzene in the perovskite film with isopropyl alcohol solution reduced the grain boundary density of the perovskite film, rusting in a smooth surface topography for the large-grained perovskite [23]. The improvement of interface engineering improved the crystal quality of perovskite films, enhanced the charge transport, and effectively suppressed the carrier recombination [24–26]. Esmaiel Nouri et al. used soluble tetrabutyl copper phthalocyanine as a hole transporting material, and graphene oxide as a blocking split channel to promote the transfer of holes between the perovskite and the hole transporting layer [27]. Phthalocyanine is a p-type semiconductor, which is widely used in thin film field effect transistors and photovoltaic devices because of its high hole mobility, thermal stability and chemical stability [28–30]. As a hole transport layer (HTL) with long-term durability, its derivatives have been widely used in PSCs [31,32]. In this work, we prepared the mixed anti-solvent by doping phthalocyanine nickel (NiPc) with anti-solvent chlorobenzene (CB) and fabricated the perovskite thin films in a glove box by one-step spincoating to study the influence of different concentration of NiPc on the photoelectric properties of perovskite solar cells. It made the perovskite phase film closely contact with the HTL and reduced the defects of the perovskite interface, which was favorable for extracting charge carriers and improving the photovoltaic performance. The PSC devices based on the NiPc yielded very competitive power conversion efficiencie (PCE) up to 19% under 100 mW cm−2 illuminations.
Fig. 1. The cross-sectional FE-SEM image of a complete perovskite solar cell.
Fig. 2. XRD patterns of the CB and NiPc/CB-treated perovskite films.
2.2. Preparation of TiO2 photoanode Fluorine-doped tin oxide (FTO)-glass substrates etch into the desired device with a laser etcher prior to cleaning and then cleaned by ultrasonication in Hellmanex (2%, deionized water) for 30 min, rinsed thoroughly with acetone, isopropanol, de-ionized water and ethanol (each for 10 min), consecutively. Afterwards, the FTO glasses were treated with UV ozone for 30 min and plasma cleaning for 5 min respectively. To prepare a dense TiO2 electron transport layer, the cleaned FTO glass was coated with a TiO2 solution by a two-step spincoating method with commercial titanium diisopropoxide bis(acetylacetonate) (75% in 2-propanol, Sigma-Aldrich) diluted in 1-butanol solution (3:40, volume ratio), the first step was 700 rpm for 5 s with an acceleration of 1000 rpm s−1, the second step was 2500 for 15 s with a ramp-up of 2000 rpm s−1. The compact TiO2 coated FTO substrates were dried at 120 °C for 15 min and annealed at 550 °C for 30 min and then allowed to cool down to room temperature slowly. The thickness of the obtained TiO2 electron transport layer (ETL) is about 50 nm.
2. Experimental 2.1. Materials Anhydrous alcohol (99.7%) was from Sinopharm Chemical Reagent Co, Ltd. Acetonitrile (99.8%), 1-Butanol (99.8%), N, NDimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.8%), chlorobenzene (99.8%), tert-butyl pyridine (TBP, 96%), Nickel phthalocyanine (85%), bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99.95%) and titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol) were purchased from Sigma-Aldrich. Cesium Iodide (CsI), formamidinium iodide (FAI) and methylammonium bromide (MABr) were purchased from Xi'an Polymer Light Technology Crop, China. Lead iodide (PbI2, 99.9985%) were purchased from Tokyo Chemical Industry. The Spiro-OMeTAD as HTL was purchased from Luminescence Technology Corp, Taiwan, China. All materials were used without further purification unless otherwise stated. All steps were performed under ambient conditions, but the perovskite and holetransport material solutions were mixed inside an Argon glove box.
2.3. Preparation of perovskite A perovskite precursor solution was obtained through using onestep spin-coating procedure. The Cs0.05(MA0.17FA0.83)0.95Pb (I0.83Br0.17)3 precursor solution was prepared containing PbI2 (1.1M), 65
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Fig. 3. (A–F) Top-view SEM images of the perovskite films treated with 0, 0.05, 0.25, 0.5, 0.75 and 1 mg/mL of NiPc/CB, respectively, (G) EDS graph and (H) Ni element maps.
started. The second step, 10 s before the end of the procedure, 120 μL of anti-solvent were quickly added to the rotating substrate. The substrate was then immediately transferred on a hotplate and heated at 100 °C for 60 min. The mixed antisolvent was prepared as follows: Nickel phthalocyanine was added to the pure CB and stirred for 2 h to prepare different concentrations of mixed anti-solvent (NiPc/CB) (0, 0.05, 0.25, 0.5, 0.75 and 1 mg mL−1). For concise, the above samples were denoted NPC0, NPC0.05, NPC0.25, NPC0.05, NPC0.75 and NPC1, respectively. Prior to use, the mixed anti-solvent was sonicated in an ultrasonic bath
FAI (1.0M), PbBr2 (0.2M) and MABr (0.2M) indimethylsulphoxide/ anhydrous dimethylformamide (1:4 (v:v)). Then, CsI, (5% volume, 1.5 M, pre-dissolved in DMSO as a stock solution) was added to the precursor solution to achieve the desired triple cation composition. The sintered TiO2 substrate was cleaned in a plasma cleaner for 3 min before the perovskite precursor was spin-coated. The mixed precursor solution was spin-coated onto the compact TiO2 films in a two-step program at 1000 rpm for 10 s and 6000 rpm for 20 s. In the first step, 20 μL perovskite precursor was evenly coated on FTO, and the program was 66
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Fig. 4. AFM topography images of perovskite films prepared with NiPc/CB concentration of (a) 0 mg mL−1, (b) 0.05 mg mL−1, (c) 0.25 mg mL−1, (d) 0.5 mg mL−1, (e) 0.75 mg mL−1, (f) 1 mg mL−1.
the battery. Intelligent mode atomic force microscopy (AFM) was carried out using a Bruker MM8 instrument to measure the coarseness of the perovskite film surface. The current-voltage (J-V) characteristics of the perovskite devices were measured using a Keithley 2420 sourcemeasure units under AM 1.5G illumination at 100 mW cm−2 provided by an Oriel Sol 3A solar simulator in an ambient environment. The effective area of the cell was defined as 0.125 cm2 using a shadow mask. Incident-photon-to-current conversion efficiency (IPCE) curves were measured as a function of wavelength from 300 to 800 nm using the Newport IPCE system (Newport, USA). Optical absorption characteristics of the perovskite film were measured in a Lambda 950 UV/Vis spectrophotometer. Photoluminescence (PL) was used to assess the carrier recombination lifetime. Electrochemical impedance spectroscopy (EIS) testing was conducted on a CIMPS-4 system (Zahner, ZOYPE).
for 5–10 min to form a uniform green liquid. 2.4. Preparation of hole transport layer The hole-transport layer was subsequently deposited on the top of perovskite film using a spiro-OMeTAD solution (70 mM in chlorobenzene). Spiro-OMeTAD was prepared using 17.5 μL of lithium bis (triuoromethanesulfonyl) imide (Li-TSFI) solution (520 mg of Li-TSFI in 1 mL of acetonitrile), 28.8 μL 4-tert-butyl pyridine and 72.3 mg spiroOMeTAD in 1 mL chlorobenzene. The HTL was spin-coated at 4000 rpm for 30 s with 20 μL of spiro-OMeTAD solution. Finally, Au electrode (approximately 100 nm thick) was deposited on the top of HTL by thermal evaporation under high vacuum. 2.5. Solar cell characterizations The crystallinity and structure of perovskite films were characterized using X-ray diffraction (XRD, Bruker AXS, D8 propulsion) with 2 Theta range from 5° to 80° at a scan rate of 10°/min. The field emission scanning electron microscope (Hitachi S-8000, Japan) was utilized to observe the morphology of perovskite and the thickness of each layer of
3. Results and discussion The cross-sectional field-emission scanning electron microscope image (FE-SEM) of the complete device is shown in Fig. 1. A conventional planar heterojunction PSC was used to investigate the effects of 67
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Fig. 5. (a) PL spectra of different concentrations of NiPc/CB on perovskite films; (b) Time-resolved PL spectra of the samples treated with different concentrations of NiPc/CB; (c) UV–Vis absorption spectra of perovskite films; (d) Nyquist plots of PSCs.
pinhole free perovskite film with a thickness of about 500 nm was clearly depicted. Then, the spiro-OMeTAD was spin-coated on the perovskite. Finally, gold back electrode with a thickness of around 100 nm was emerged coated on the HTL. Each layer in structure of the perovskite solar cell was clearly defined as a stack of glass/fluorinedoped tin oxide (FTO)/TiO2/perovskite/spiro-OMeTAD/gold. Fig. 2 shows the X-ray diffraction (XRD) data of perovskite films with different phthalocyanine nickel spiked in anti-solvent chlorobenzene. Sharp and strong reflective diffraction peaks agreed well with the peaks reported in the literature [11], indicating the good crystallinity of the as-prepared perovskite film on the FTO surfaces. The typical two major diffractions peaks at about 14.4° and 28.8° could be indexed as (110) and (220) crystal plane of the perovskite film respectively. It was worth noting that the diffraction peaks at 12.8° could be assigned to the (110) lattice plane of PbI2, which would be beneficial to the device performance of PSCS due to the passivation of surface defects [33]. It was clearly seen that the (110) peak intensity of NPC0.5 (110) was higher than that of other samples. This difference might be due to the nucleation and crystallization of perovskite were influenced by the different concentrations of mixed anti-solvent (NiPc/CB). As shown in Fig. 3, we investigated the morphological changes of perovskite films with different concentrations of mixed anti-solvent, top-view scanning electron microscopy (SEM) images of perovskite
Table 1 Fitting parameters of PL curves of perovskite samples treated with different concentrations of NiPc/CB. Samples
A1a [%]
τ1b (ns)
A2a [%]
τ2c (ns)
Average (ns)
NPC0 NPC0.05 NPC0.25 NPC0.5 NPC0.75 NPC1
19.40 18.54 19.56 31.89 27.69 26.21
10.09 9.88 9.95 7.08 8.85 9.43
80.60 81.46 80.44 68.11 72.31 73.79
80.54 77.45 75.68 45.18 58.42 52.06
66.88 64.93 62.82 33.03 44.69 40.89
a The relative proportions of the two carrier behavior decay processes, respectively. b Fast decay lifetime. c Slow decay lifetime.
introducing phthalocyanine into anti-solvent chlorobenzene on the crystal quality of perovskite films and the photovoltaic performance of device. TiO2 thin layer with a thickness of about 50 nm on the top of FTO substrate film (about 500 nm thick) was presented as electron transport and hole blocking layer. The mixed halide perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) treated by mixed anti-solvent (NiPc/CB) acted as light absorber, was deposited on the TiO2 thin film by a two-step solution spin-coating procedure. A dense, uniform, 68
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Table 3 Photovoltaic parameters of devices treated with the pure CB solvent and the best concentration of NiPc/CB.
Table 2 Photovoltaic performance of PSCs based on different concentrations of NiPc/ CB. Voc (V)
Jsc (mA· cm−2)
FF(%)
PCE(%)
NPC0 NPC0.05 NPC0.25 NPC0.5 NPC0.75 NPC1
1.08 1.04 1.04 1.10 1.07 1.06
22.28 21.84 22.23 23.39 22.50 22.59
71.5 66.68 71.40 74.78 73.34 68.48
17.24 15.16 16.47 19.18 17.69 16.36
Scanning mode
Voc (V)
Jsc (mA·cm−2)
FF (%)
PCE (%)
NPC0 NPC0 NPC0.5 NPC0.5
Reverse Forward Reverse Forward
1.08 1.04 1.10 1.07
22.28 21.76 23.39 22.38
71.50 57.56 74.78 73.07
17.24 12.98 19.18 17.54
1 mg mL−1, non-uniform and small particle size of perovskite crystal film was emerged, which might be due to the higher concentration of the reverse solution reduce the film forming property of the precursor. Compared with perovskite treated by pure CB, perovskite grains treated by NiPc/CB were covered with many similar small round brilliant sequins. And, the density of sequins was increasing with the concentration of NiPc/CB. To confirm the chemical composition of brilliant sequins, we have analyzed these samples by energy-dispersive spectrometry (EDS). Fig. 3G and H shows the EDS spectrum of the perovskite film treated by mixed anti-solution (NiPc/CB) and Ni element maps (red color dots), respectively. To further investigate the effect of NiPc addition on the surface roughness of perovskite films, we performed an atomic force microscopy (AFM), as shown in Fig. 4. The height image obtained by AFM phase images indicated the clustered-domain and rough surface morphology of the perovskite thin films spin-coated on the FTO substrates with different concentration of NiPc/CB pre-treatment. The root mean square of surface roughness (Ra) of perovskites film treated with NiPc/ CB concentrations of 0, 0.05, 0.25, 0.5, 0.75 and 1 mg mL−1 are 24.8 nm, 15.2 nm, 19.1 nm, 20.3 nm, 20.7 nm, 28.3 nm, respectively. The surface roughness of perovskite films increased gradually with the increasing of NiPc/CB concentration. Despite, the surface roughness of perovskite films treated with concentration of 0.05–0.75 mg mL−1 was still smaller than that of sample treated by pure CB. It could be inferred that the addition of a small amount of NiPc in the CB could slightly reduce the defects on the perovskite surface, which was consistent well with SEM analysis. To further elucidate the photophysics of those perovskite layers pretreatment by different NiPc/CB concentration on the same electron transport layer, steady-state photoluminescence (PL) and time resolved photoluminescence (TRPL) spectra were employed and displayed in Fig. 5. As we known the PL can provide the insight into the process of photo-generated electron hole separation and injection, due to the recombination of electron hole pairs will release energy in the form of PL emission. As shown in Fig. 5a, the NPC0 perovskite film showed the highest PL intensity, indicating the serious photo-generate charge carries recombination in the perovskite film. While the PL intensity of NPC0.5 perovskite film was lowest than that of other samples, clearly demonstrating the highest separation efficiency of charge carriers with the fastest migration process or the longest lifetime. It could be concluded that the addition of NiPc dopants improved the extraction and transfer of electrons from the perovskite absorber before recombination at the interface because of the incorporation of NiPc improving the quality of the perovskite thin film and reducing the surface defects. Holes in the perovskite valence band were rapidly injected into the HTL, which effectively suppressed the interface carrier recombination. Furthermore, the TRPL spectra was performed to examine the carrier lifetime for the perovskite film pretreated by the different concentrations NiPc/CB additive on the photo-conversion process at the interface between the electron transport layer and the perovskite layer. As shown in Fig. 5b, the TRPL curves were fitted with a bi-exponential decay function containing a fast decay and a slow decay process [34]. The corresponding lifetimes were calculated and listed in Table 1. The NPC0 perovskite layer showed fastest PL decay process due to the photo-excited charge trapping by the defect states of the perovskite
Fig. 6. (a) Photovoltaic characteristics of the PSCs with different concentrations of NiPc/CB; (b) Current density-voltage (J–V) characteristics of the pure CB solvent and the best concentration of NiPc/CB.
Samples
Samples
films pre-treated with different concentration of mixed anti-solution NiPc/CB, respectively. Compared with perovskite film without pretreated by anti-solution, few changes were observed on the NPC0.05. However, with the concentration of mixed anti-solution (NiPc/CB) increased from 0.25 to 0.75 mg mL−1, the average grain size of the perovskite increased, especial for the sample NPC0.5. The increase in grain size leads to a decrease in grain boundary energy and facilitates charge transport. When the concentration of mixed anti-solution increased to 69
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Fig. 7. (a)–(d) Comparison of the photovoltaic performance of PSCs with the pure CB solvent and the best concentration of NiPc/CB NPC0.5.
grains, more uniform and dense perovskite film, thus leading to more sunlight absorption for photon-generated carriers and effectively charges transport. For better realizing the charge transport process in the perovskite solar cells, the electrochemical impedance spectroscopy (EIS) was used to study the internal resistance of the photovoltaic device, and the equivalent circuit of this model was also shown in the inset of Fig. 5d. Obviously, two distinct semicircles related to two different transport processes were presented. The first semicircle represents the high frequency region related to the charge transport at the perovskite/spiroOMeTAD/Au electrode interface, the second semicircle represents the low frequency region associated with the charge transfer at the perovskite film/TiO2 interface. The lager circle radius reflects the higher interface resistance. R1 represents the charge transfer resistance at the interface between the carrier (electron or hole) selective layer and the photoactive layer. R2 represents the interfacial resistance of the TiO2/ perovskite/HTL interface, which is a composite resistor combination. Clearly, the NPC0.5 sample demonstrated the least interface resistance, indicating the least interface defect and lowest electron recombination rate. Hence, we could conclude that the perovskite film pre-treated by suitable concentrations of NiPc/CB anti-solution forcefully improved electron injection and reduced the transfer resistance between the FTO and the perovskite films. Typical J–V characteristics of the best PSCs with different NiPc doping level were investigated. Fig. 6a displays the photocurrent-voltage (J-V) curve measured in the reverse voltage sweep direction under simulated AM 1.5 sunlight conditions, and corresponding photovoltaic performance parameters are listed in Table 2. Encouragingly, the PSC
layer. However, the all perovskite layers pre-treated by NiPc/CB mixed anti-solution presented a prolonged PL lifetime compared with the NPC0, suggesting that the defect state concentration in the perovskite layer could be reduced efficiently by NiPc/CB mixed anti-solution pretreatment. Among the samples, the NPC0.5 perovskite thin film showed shortest decay lifetime τ2 45.18 ns (fraction A2 = 68.11%), further indicating that the carrier transport rate was fastest and most effective. As we known, the shorter value means that the charge faster transfer from the perovskite to the FTO. It further indicated a lowest defect density and hence superior electronic quality for the NPC0.5 perovskite thin film, which could be attributed to the high crystallinity or few grain boundaries and crystal defects in the perovskite material crystallized upon the FTO surface with NiPc/CB mixed anti-solution treatment. The perovskite film is the core layer of the device, not only the medium for charge transport but also the light absorption layer. Hence, we investigated the impact of NiPc/CB mixed anti-solution treatment on the light absorption of the perovskite material. Fig. 5c shows the UV–visible absorption spectra of the perovskite films fabricated by different concentrations of NiPc/CB. The all perovskite films demonstrated good visible light-harvesting capabilities in the wavelength range from 460 nm to 800 nm. As shown in the curves, the pre-treatment of NiPc/CB mixed anti-solution did not change the shape of the absorption spectrum of perovskite but obviously improved the absorption intensity, which would increase the photovoltaic performance of devices. It was noteworthy that the NPC0.5 perovskite film has highest visible light absorbance performance in the range of 460–800 nm compared with other samples. It could be attributed that the as-prepared NPC0.5 sample possessed fewer defects, larger crystal 70
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perovskite solar cells due to its intrinsic instability under illumination. On the contrary, more defects were formed in the crystal face of the perovskite film prepared by the high concentration mixed anti-solvent (NPC0.75 and NPC1), which led to the degradation in performance of devices. The current-voltage (J-V) hysteresis of perovskite solar cells is another urgent crisis to be solved. The hysteresis of PSCs with different concentrations of NiPc/CB was investigated by forward and reverse bias scanning for the best-performing solar cells (NPC0.5). The detailed performance parameters obtained from the J-V curves in Fig. 6b were listed in Table 3. It was obviously that NPC0.5 device showed lower hysteresis efficiency than that of NPC0 device treated by pure CB. For further comparison, the photovoltaic parameters (VOC, JSC, FF, and PCE) of 20 devices were plotted in box format, as shown in Fig. 7 (a)(d). The results showed that the performance of the optimal concentration device (NPC0.5) was greatly improved compared with the conventional devices. Fig. 8a shows the incident-photon-to-current conversion efficiency (IPCE) spectra with wavelengths ranging from 300 nm to 800 nm for PSCs based on 0.5 mg mL−1 NiPc/CB and pure CB, respectively. The devices showed the best performance over practically the whole visible range of 360–750 nm. It confirmed that the IPCE of NPC0.5 device was indeed higher than that of the conventional NPC0 device. Furthermore, the integrated current density obtained from the best NPC0.5 device IPCE spectrum was 21.31 mA cm−2, which was higher than that of pure CB device (20.24 mA cm−2). The response capability was in good agreement with the UV–vis absorption spectrum of perovskite and the current extracted from the J-V experiment. In order to avoid erroneous PCE estimation due to hysteresis of J-V. Fig. 8b exhibits the steady-state photocurrent density and output PCE of the devices at the maximum power points. The steady-state photocurrent measured at maximum power point (0.80 v) was close to that of value measured by J-V scan. For NPC0.5 PSC device, a stable photocurrent density of 22.6 mA cm−2 and PCE of 18.3% were obtained. 4. Conclusions In summary, a simple, effective and versatile solvent engineering technology has been developed, which used nickel phthalocyanine (NiPc) as an additive in an anticholinergic chlorobenzene to form a mixed antisolvent for the preparation of high performance PSCs. The application of perovskite films pre-treated with optimal NiPc/CB concentrations (NPC0.5) in solar cell construction acquired the excellent photovoltaic performance: a PCE of 19.18%, a VOC of 1.10 v, a JSC of 23.39 mA cm−2, a FF of 74.78%. Compared with the reference conventional cell, the enhancement of photovoltaic performance pretreated by mixed anti-solution NiPc/CB (NPC0.5) mainly stemmed from the high-quality perovskite films, achieving effective charge separation, enhancing the interfacial contact and improving the carrier transport.
Fig. 8. (a) IPCE spectra and integrated photocurrent density curves of the PSCs with the pure CB solvent and the best concentration of NiPc/CB; (b) Stabilized photocurrent density and output PCE at maximum power point as a function of time for the best performing PSC (NPC0.5).
based on 0.5 mg mL−1 NiPc/CB exhibited a promising PCE of 19.18% with a VOC of 1.096 V, a JSC of 23.39 mA cm−2, and a fill factor (FF) of 74.78%, outperforming the performance of the PSC based on pure CB (VOC, JSC, FF, and PCE were 1.082 V, 22.28 mA cm−2, 71.5%, and 17.4%, respectively.). With the NiPc/CB concentration increased from 0.05 to 1 mg mL−1, the PCE increased from 15.16% to 19.18% and then decreased to 16.36%. Therefore, the optimal NiPc/CB concentration was 0.5 mg mL−1. The enhancement of FF and VOC could be attributed the improvement quality of the perovskite film and low interface loss, which was consistent well with the PL spectrum. The enhancement of open circuit voltage (VOC) and film fill factor (FF) might be due to the deep trap introduced by the phthalocyanine molecule to block the reticular wave electrons. The presence of NiPC reduced the size of pinholes formed during film deposition and improved the quality of the organic hole transport layer [31]. Furthermore, the recombination of photo-carriers was restrained by the new heterojunction formed between the small amount of residual PbI2 and perovskite [35]. In addition, small amount of residual PbI2 not only passivated the defects at surfaces and grain boundaries, but also increased the size of perovskite crystals, thus contributing to the improved performance of the solar cells [31,36]. However, excessive PbI2 remaining in the entire perovskite film would destroy the compact structure of perovskite film, reduced the stability of films and enhanced the hysteresis effect of
Acknowledgement The project is jointly supported by the National Natural Science Foundation of China (No. 61306077), the Fundamental Research Funds for the Central Universities (JB-ZR1109, JB-ZR1212), Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-PY207), Discipline Innovation Team Project of Huaqiao University (201320), the Open Project Program of Provincial Key Laboratory of Eco-Industrial Green Technology of Wuyi University. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] J.H. Heo, S.H. Im, H.J. Kim, P.P. Boix, S.J. Lee, S. II Seok, I. Moraseró, J. Bisquert, J. Phys. Chem. C 116 (2012) 20717–20721.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhancing the perovskite solar cell performance by the treatment with mixed anti-solvent
T
Shao Jina,c, Yuelin Weia,c,∗, Feiyue Huanga,c, Xiaomin Yanga,c, Dan Luob,c, Yu Fanga,c, Yuezhu Zhaob,c, Qiyao Guoa,c, Yunfang Huangc,a,∗∗, Jihuai Wua,c a
College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China College of Chemical Engineering, Huaqiao University, Xiamen, 361021, China c Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Huaqiao University, Xiamen, 361021, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
A simplified anti-solvent treatment is • developed for high-quality perovskite films.
high-quality film improves the • The charge separation and transport efficiency.
conversion efficiency of the • Power solar cell is enhanced from 17.4% to 19.18%.
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cell Phthalocyanine nickel Mixed anti-solvent
At present, the surface modification of perovskites by solvent treatment has become the most effective way to improve the performance of planar perovskite solar cells. Here, phthalocyanine nickel as an additive was introduced into the anti-solvent chlorobenzene to form a mixed anti-solvent. Prior to completion of the perovskite crystals, a mixed anti-solvent was infiltrated into the perovskite surface to improve the interfacial contact of the perovskite with the hole-transporting layer. This method improved the crystal quality of the perovskite film, thus enhanced the charge transfer efficiency, and suppressed the recombination of carriers effectively. The perovskite solar cell constructed with this perovskite films treated by the optimal concentration of nickel phthalocyanine in the anti-solvent solution yielded a power conversion efficiency of 19.18% and the filling factor of 74.38% under 100 mW cm−2 illuminations.
1. Introduction In recent years, organic-inorganic hybrid perovskite materials have triggered a wave of research in the field of solar cells. Since Miyasaka's team first started using CH3NH3PbI3 perovskite in solar cells in 2009,
the energy conversion efficiency of the perovskite solar cells (PSCs) have increased from 3.8% to 22.1% [1–7]. Halogenated perovskite materials have become the “dream material” for photovoltaic applications due to their high optical absorption coefficient, high carrier mobility and long carrier diffusion length.
∗
Corresponding author. College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China. Corresponding author. Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Huaqiao University, Xiamen, 361021, China. E-mail addresses: [email protected] (Y. Wei), [email protected] (Y. Huang). ∗∗
https://doi.org/10.1016/j.jpowsour.2018.10.008 Received 9 July 2018; Received in revised form 26 September 2018; Accepted 5 October 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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At present, the most successful organic-inorganic perovskite material has an ABX3 structure comprised of an organic cation, A = (methylammonium CH3NH3+; formamidinium CH3(NH2)2+); Cs+, a divalent metal, B = (Pb2+; Sn2+; Ge2+), and an anion X = (Cl−; Br−; I−; SCN−) [8–10]. The perovskite solar cell structure has a classical mesoporous n-i-p junction and a mesoporous “planar” structure [11]. Planar heterojunction PSCs have been extensively used as one of the most efficient configurations of PSCs. The improvement of the properties of PSCs mainly depends on the crystal quality, morphology and interfacial contact of perovskite films. Currently, perovskite films are prepared by one-step spin-coating [12], dip coating [13], two-step interdiffusion [14], and vacuum assisted evaporation [15]. Among them, the one-step spin-coating method is an effective process to regulate the morphology, the grain size and the crystal quality of the perovskite film by dropping the anti-solvent (chlorobenzene, toluene, ethyl acetate) during the spin coating. Generally, interfacial engineering is an efficient approach to solve the problems of defects and the energy-barrier mismatch, thus improving the photovoltaic performance of PSCs [16–21]. For example, further annealing in dimethylfomamide steam increased the size of the perovskite grains [22]. Removal of the remaining chlorobenzene in the perovskite film with isopropyl alcohol solution reduced the grain boundary density of the perovskite film, rusting in a smooth surface topography for the large-grained perovskite [23]. The improvement of interface engineering improved the crystal quality of perovskite films, enhanced the charge transport, and effectively suppressed the carrier recombination [24–26]. Esmaiel Nouri et al. used soluble tetrabutyl copper phthalocyanine as a hole transporting material, and graphene oxide as a blocking split channel to promote the transfer of holes between the perovskite and the hole transporting layer [27]. Phthalocyanine is a p-type semiconductor, which is widely used in thin film field effect transistors and photovoltaic devices because of its high hole mobility, thermal stability and chemical stability [28–30]. As a hole transport layer (HTL) with long-term durability, its derivatives have been widely used in PSCs [31,32]. In this work, we prepared the mixed anti-solvent by doping phthalocyanine nickel (NiPc) with anti-solvent chlorobenzene (CB) and fabricated the perovskite thin films in a glove box by one-step spincoating to study the influence of different concentration of NiPc on the photoelectric properties of perovskite solar cells. It made the perovskite phase film closely contact with the HTL and reduced the defects of the perovskite interface, which was favorable for extracting charge carriers and improving the photovoltaic performance. The PSC devices based on the NiPc yielded very competitive power conversion efficiencie (PCE) up to 19% under 100 mW cm−2 illuminations.
Fig. 1. The cross-sectional FE-SEM image of a complete perovskite solar cell.
Fig. 2. XRD patterns of the CB and NiPc/CB-treated perovskite films.
2.2. Preparation of TiO2 photoanode Fluorine-doped tin oxide (FTO)-glass substrates etch into the desired device with a laser etcher prior to cleaning and then cleaned by ultrasonication in Hellmanex (2%, deionized water) for 30 min, rinsed thoroughly with acetone, isopropanol, de-ionized water and ethanol (each for 10 min), consecutively. Afterwards, the FTO glasses were treated with UV ozone for 30 min and plasma cleaning for 5 min respectively. To prepare a dense TiO2 electron transport layer, the cleaned FTO glass was coated with a TiO2 solution by a two-step spincoating method with commercial titanium diisopropoxide bis(acetylacetonate) (75% in 2-propanol, Sigma-Aldrich) diluted in 1-butanol solution (3:40, volume ratio), the first step was 700 rpm for 5 s with an acceleration of 1000 rpm s−1, the second step was 2500 for 15 s with a ramp-up of 2000 rpm s−1. The compact TiO2 coated FTO substrates were dried at 120 °C for 15 min and annealed at 550 °C for 30 min and then allowed to cool down to room temperature slowly. The thickness of the obtained TiO2 electron transport layer (ETL) is about 50 nm.
2. Experimental 2.1. Materials Anhydrous alcohol (99.7%) was from Sinopharm Chemical Reagent Co, Ltd. Acetonitrile (99.8%), 1-Butanol (99.8%), N, NDimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.8%), chlorobenzene (99.8%), tert-butyl pyridine (TBP, 96%), Nickel phthalocyanine (85%), bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99.95%) and titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol) were purchased from Sigma-Aldrich. Cesium Iodide (CsI), formamidinium iodide (FAI) and methylammonium bromide (MABr) were purchased from Xi'an Polymer Light Technology Crop, China. Lead iodide (PbI2, 99.9985%) were purchased from Tokyo Chemical Industry. The Spiro-OMeTAD as HTL was purchased from Luminescence Technology Corp, Taiwan, China. All materials were used without further purification unless otherwise stated. All steps were performed under ambient conditions, but the perovskite and holetransport material solutions were mixed inside an Argon glove box.
2.3. Preparation of perovskite A perovskite precursor solution was obtained through using onestep spin-coating procedure. The Cs0.05(MA0.17FA0.83)0.95Pb (I0.83Br0.17)3 precursor solution was prepared containing PbI2 (1.1M), 65
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Fig. 3. (A–F) Top-view SEM images of the perovskite films treated with 0, 0.05, 0.25, 0.5, 0.75 and 1 mg/mL of NiPc/CB, respectively, (G) EDS graph and (H) Ni element maps.
started. The second step, 10 s before the end of the procedure, 120 μL of anti-solvent were quickly added to the rotating substrate. The substrate was then immediately transferred on a hotplate and heated at 100 °C for 60 min. The mixed antisolvent was prepared as follows: Nickel phthalocyanine was added to the pure CB and stirred for 2 h to prepare different concentrations of mixed anti-solvent (NiPc/CB) (0, 0.05, 0.25, 0.5, 0.75 and 1 mg mL−1). For concise, the above samples were denoted NPC0, NPC0.05, NPC0.25, NPC0.05, NPC0.75 and NPC1, respectively. Prior to use, the mixed anti-solvent was sonicated in an ultrasonic bath
FAI (1.0M), PbBr2 (0.2M) and MABr (0.2M) indimethylsulphoxide/ anhydrous dimethylformamide (1:4 (v:v)). Then, CsI, (5% volume, 1.5 M, pre-dissolved in DMSO as a stock solution) was added to the precursor solution to achieve the desired triple cation composition. The sintered TiO2 substrate was cleaned in a plasma cleaner for 3 min before the perovskite precursor was spin-coated. The mixed precursor solution was spin-coated onto the compact TiO2 films in a two-step program at 1000 rpm for 10 s and 6000 rpm for 20 s. In the first step, 20 μL perovskite precursor was evenly coated on FTO, and the program was 66
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Fig. 4. AFM topography images of perovskite films prepared with NiPc/CB concentration of (a) 0 mg mL−1, (b) 0.05 mg mL−1, (c) 0.25 mg mL−1, (d) 0.5 mg mL−1, (e) 0.75 mg mL−1, (f) 1 mg mL−1.
the battery. Intelligent mode atomic force microscopy (AFM) was carried out using a Bruker MM8 instrument to measure the coarseness of the perovskite film surface. The current-voltage (J-V) characteristics of the perovskite devices were measured using a Keithley 2420 sourcemeasure units under AM 1.5G illumination at 100 mW cm−2 provided by an Oriel Sol 3A solar simulator in an ambient environment. The effective area of the cell was defined as 0.125 cm2 using a shadow mask. Incident-photon-to-current conversion efficiency (IPCE) curves were measured as a function of wavelength from 300 to 800 nm using the Newport IPCE system (Newport, USA). Optical absorption characteristics of the perovskite film were measured in a Lambda 950 UV/Vis spectrophotometer. Photoluminescence (PL) was used to assess the carrier recombination lifetime. Electrochemical impedance spectroscopy (EIS) testing was conducted on a CIMPS-4 system (Zahner, ZOYPE).
for 5–10 min to form a uniform green liquid. 2.4. Preparation of hole transport layer The hole-transport layer was subsequently deposited on the top of perovskite film using a spiro-OMeTAD solution (70 mM in chlorobenzene). Spiro-OMeTAD was prepared using 17.5 μL of lithium bis (triuoromethanesulfonyl) imide (Li-TSFI) solution (520 mg of Li-TSFI in 1 mL of acetonitrile), 28.8 μL 4-tert-butyl pyridine and 72.3 mg spiroOMeTAD in 1 mL chlorobenzene. The HTL was spin-coated at 4000 rpm for 30 s with 20 μL of spiro-OMeTAD solution. Finally, Au electrode (approximately 100 nm thick) was deposited on the top of HTL by thermal evaporation under high vacuum. 2.5. Solar cell characterizations The crystallinity and structure of perovskite films were characterized using X-ray diffraction (XRD, Bruker AXS, D8 propulsion) with 2 Theta range from 5° to 80° at a scan rate of 10°/min. The field emission scanning electron microscope (Hitachi S-8000, Japan) was utilized to observe the morphology of perovskite and the thickness of each layer of
3. Results and discussion The cross-sectional field-emission scanning electron microscope image (FE-SEM) of the complete device is shown in Fig. 1. A conventional planar heterojunction PSC was used to investigate the effects of 67
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Fig. 5. (a) PL spectra of different concentrations of NiPc/CB on perovskite films; (b) Time-resolved PL spectra of the samples treated with different concentrations of NiPc/CB; (c) UV–Vis absorption spectra of perovskite films; (d) Nyquist plots of PSCs.
pinhole free perovskite film with a thickness of about 500 nm was clearly depicted. Then, the spiro-OMeTAD was spin-coated on the perovskite. Finally, gold back electrode with a thickness of around 100 nm was emerged coated on the HTL. Each layer in structure of the perovskite solar cell was clearly defined as a stack of glass/fluorinedoped tin oxide (FTO)/TiO2/perovskite/spiro-OMeTAD/gold. Fig. 2 shows the X-ray diffraction (XRD) data of perovskite films with different phthalocyanine nickel spiked in anti-solvent chlorobenzene. Sharp and strong reflective diffraction peaks agreed well with the peaks reported in the literature [11], indicating the good crystallinity of the as-prepared perovskite film on the FTO surfaces. The typical two major diffractions peaks at about 14.4° and 28.8° could be indexed as (110) and (220) crystal plane of the perovskite film respectively. It was worth noting that the diffraction peaks at 12.8° could be assigned to the (110) lattice plane of PbI2, which would be beneficial to the device performance of PSCS due to the passivation of surface defects [33]. It was clearly seen that the (110) peak intensity of NPC0.5 (110) was higher than that of other samples. This difference might be due to the nucleation and crystallization of perovskite were influenced by the different concentrations of mixed anti-solvent (NiPc/CB). As shown in Fig. 3, we investigated the morphological changes of perovskite films with different concentrations of mixed anti-solvent, top-view scanning electron microscopy (SEM) images of perovskite
Table 1 Fitting parameters of PL curves of perovskite samples treated with different concentrations of NiPc/CB. Samples
A1a [%]
τ1b (ns)
A2a [%]
τ2c (ns)
Average (ns)
NPC0 NPC0.05 NPC0.25 NPC0.5 NPC0.75 NPC1
19.40 18.54 19.56 31.89 27.69 26.21
10.09 9.88 9.95 7.08 8.85 9.43
80.60 81.46 80.44 68.11 72.31 73.79
80.54 77.45 75.68 45.18 58.42 52.06
66.88 64.93 62.82 33.03 44.69 40.89
a The relative proportions of the two carrier behavior decay processes, respectively. b Fast decay lifetime. c Slow decay lifetime.
introducing phthalocyanine into anti-solvent chlorobenzene on the crystal quality of perovskite films and the photovoltaic performance of device. TiO2 thin layer with a thickness of about 50 nm on the top of FTO substrate film (about 500 nm thick) was presented as electron transport and hole blocking layer. The mixed halide perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) treated by mixed anti-solvent (NiPc/CB) acted as light absorber, was deposited on the TiO2 thin film by a two-step solution spin-coating procedure. A dense, uniform, 68
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Table 3 Photovoltaic parameters of devices treated with the pure CB solvent and the best concentration of NiPc/CB.
Table 2 Photovoltaic performance of PSCs based on different concentrations of NiPc/ CB. Voc (V)
Jsc (mA· cm−2)
FF(%)
PCE(%)
NPC0 NPC0.05 NPC0.25 NPC0.5 NPC0.75 NPC1
1.08 1.04 1.04 1.10 1.07 1.06
22.28 21.84 22.23 23.39 22.50 22.59
71.5 66.68 71.40 74.78 73.34 68.48
17.24 15.16 16.47 19.18 17.69 16.36
Scanning mode
Voc (V)
Jsc (mA·cm−2)
FF (%)
PCE (%)
NPC0 NPC0 NPC0.5 NPC0.5
Reverse Forward Reverse Forward
1.08 1.04 1.10 1.07
22.28 21.76 23.39 22.38
71.50 57.56 74.78 73.07
17.24 12.98 19.18 17.54
1 mg mL−1, non-uniform and small particle size of perovskite crystal film was emerged, which might be due to the higher concentration of the reverse solution reduce the film forming property of the precursor. Compared with perovskite treated by pure CB, perovskite grains treated by NiPc/CB were covered with many similar small round brilliant sequins. And, the density of sequins was increasing with the concentration of NiPc/CB. To confirm the chemical composition of brilliant sequins, we have analyzed these samples by energy-dispersive spectrometry (EDS). Fig. 3G and H shows the EDS spectrum of the perovskite film treated by mixed anti-solution (NiPc/CB) and Ni element maps (red color dots), respectively. To further investigate the effect of NiPc addition on the surface roughness of perovskite films, we performed an atomic force microscopy (AFM), as shown in Fig. 4. The height image obtained by AFM phase images indicated the clustered-domain and rough surface morphology of the perovskite thin films spin-coated on the FTO substrates with different concentration of NiPc/CB pre-treatment. The root mean square of surface roughness (Ra) of perovskites film treated with NiPc/ CB concentrations of 0, 0.05, 0.25, 0.5, 0.75 and 1 mg mL−1 are 24.8 nm, 15.2 nm, 19.1 nm, 20.3 nm, 20.7 nm, 28.3 nm, respectively. The surface roughness of perovskite films increased gradually with the increasing of NiPc/CB concentration. Despite, the surface roughness of perovskite films treated with concentration of 0.05–0.75 mg mL−1 was still smaller than that of sample treated by pure CB. It could be inferred that the addition of a small amount of NiPc in the CB could slightly reduce the defects on the perovskite surface, which was consistent well with SEM analysis. To further elucidate the photophysics of those perovskite layers pretreatment by different NiPc/CB concentration on the same electron transport layer, steady-state photoluminescence (PL) and time resolved photoluminescence (TRPL) spectra were employed and displayed in Fig. 5. As we known the PL can provide the insight into the process of photo-generated electron hole separation and injection, due to the recombination of electron hole pairs will release energy in the form of PL emission. As shown in Fig. 5a, the NPC0 perovskite film showed the highest PL intensity, indicating the serious photo-generate charge carries recombination in the perovskite film. While the PL intensity of NPC0.5 perovskite film was lowest than that of other samples, clearly demonstrating the highest separation efficiency of charge carriers with the fastest migration process or the longest lifetime. It could be concluded that the addition of NiPc dopants improved the extraction and transfer of electrons from the perovskite absorber before recombination at the interface because of the incorporation of NiPc improving the quality of the perovskite thin film and reducing the surface defects. Holes in the perovskite valence band were rapidly injected into the HTL, which effectively suppressed the interface carrier recombination. Furthermore, the TRPL spectra was performed to examine the carrier lifetime for the perovskite film pretreated by the different concentrations NiPc/CB additive on the photo-conversion process at the interface between the electron transport layer and the perovskite layer. As shown in Fig. 5b, the TRPL curves were fitted with a bi-exponential decay function containing a fast decay and a slow decay process [34]. The corresponding lifetimes were calculated and listed in Table 1. The NPC0 perovskite layer showed fastest PL decay process due to the photo-excited charge trapping by the defect states of the perovskite
Fig. 6. (a) Photovoltaic characteristics of the PSCs with different concentrations of NiPc/CB; (b) Current density-voltage (J–V) characteristics of the pure CB solvent and the best concentration of NiPc/CB.
Samples
Samples
films pre-treated with different concentration of mixed anti-solution NiPc/CB, respectively. Compared with perovskite film without pretreated by anti-solution, few changes were observed on the NPC0.05. However, with the concentration of mixed anti-solution (NiPc/CB) increased from 0.25 to 0.75 mg mL−1, the average grain size of the perovskite increased, especial for the sample NPC0.5. The increase in grain size leads to a decrease in grain boundary energy and facilitates charge transport. When the concentration of mixed anti-solution increased to 69
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Fig. 7. (a)–(d) Comparison of the photovoltaic performance of PSCs with the pure CB solvent and the best concentration of NiPc/CB NPC0.5.
grains, more uniform and dense perovskite film, thus leading to more sunlight absorption for photon-generated carriers and effectively charges transport. For better realizing the charge transport process in the perovskite solar cells, the electrochemical impedance spectroscopy (EIS) was used to study the internal resistance of the photovoltaic device, and the equivalent circuit of this model was also shown in the inset of Fig. 5d. Obviously, two distinct semicircles related to two different transport processes were presented. The first semicircle represents the high frequency region related to the charge transport at the perovskite/spiroOMeTAD/Au electrode interface, the second semicircle represents the low frequency region associated with the charge transfer at the perovskite film/TiO2 interface. The lager circle radius reflects the higher interface resistance. R1 represents the charge transfer resistance at the interface between the carrier (electron or hole) selective layer and the photoactive layer. R2 represents the interfacial resistance of the TiO2/ perovskite/HTL interface, which is a composite resistor combination. Clearly, the NPC0.5 sample demonstrated the least interface resistance, indicating the least interface defect and lowest electron recombination rate. Hence, we could conclude that the perovskite film pre-treated by suitable concentrations of NiPc/CB anti-solution forcefully improved electron injection and reduced the transfer resistance between the FTO and the perovskite films. Typical J–V characteristics of the best PSCs with different NiPc doping level were investigated. Fig. 6a displays the photocurrent-voltage (J-V) curve measured in the reverse voltage sweep direction under simulated AM 1.5 sunlight conditions, and corresponding photovoltaic performance parameters are listed in Table 2. Encouragingly, the PSC
layer. However, the all perovskite layers pre-treated by NiPc/CB mixed anti-solution presented a prolonged PL lifetime compared with the NPC0, suggesting that the defect state concentration in the perovskite layer could be reduced efficiently by NiPc/CB mixed anti-solution pretreatment. Among the samples, the NPC0.5 perovskite thin film showed shortest decay lifetime τ2 45.18 ns (fraction A2 = 68.11%), further indicating that the carrier transport rate was fastest and most effective. As we known, the shorter value means that the charge faster transfer from the perovskite to the FTO. It further indicated a lowest defect density and hence superior electronic quality for the NPC0.5 perovskite thin film, which could be attributed to the high crystallinity or few grain boundaries and crystal defects in the perovskite material crystallized upon the FTO surface with NiPc/CB mixed anti-solution treatment. The perovskite film is the core layer of the device, not only the medium for charge transport but also the light absorption layer. Hence, we investigated the impact of NiPc/CB mixed anti-solution treatment on the light absorption of the perovskite material. Fig. 5c shows the UV–visible absorption spectra of the perovskite films fabricated by different concentrations of NiPc/CB. The all perovskite films demonstrated good visible light-harvesting capabilities in the wavelength range from 460 nm to 800 nm. As shown in the curves, the pre-treatment of NiPc/CB mixed anti-solution did not change the shape of the absorption spectrum of perovskite but obviously improved the absorption intensity, which would increase the photovoltaic performance of devices. It was noteworthy that the NPC0.5 perovskite film has highest visible light absorbance performance in the range of 460–800 nm compared with other samples. It could be attributed that the as-prepared NPC0.5 sample possessed fewer defects, larger crystal 70
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perovskite solar cells due to its intrinsic instability under illumination. On the contrary, more defects were formed in the crystal face of the perovskite film prepared by the high concentration mixed anti-solvent (NPC0.75 and NPC1), which led to the degradation in performance of devices. The current-voltage (J-V) hysteresis of perovskite solar cells is another urgent crisis to be solved. The hysteresis of PSCs with different concentrations of NiPc/CB was investigated by forward and reverse bias scanning for the best-performing solar cells (NPC0.5). The detailed performance parameters obtained from the J-V curves in Fig. 6b were listed in Table 3. It was obviously that NPC0.5 device showed lower hysteresis efficiency than that of NPC0 device treated by pure CB. For further comparison, the photovoltaic parameters (VOC, JSC, FF, and PCE) of 20 devices were plotted in box format, as shown in Fig. 7 (a)(d). The results showed that the performance of the optimal concentration device (NPC0.5) was greatly improved compared with the conventional devices. Fig. 8a shows the incident-photon-to-current conversion efficiency (IPCE) spectra with wavelengths ranging from 300 nm to 800 nm for PSCs based on 0.5 mg mL−1 NiPc/CB and pure CB, respectively. The devices showed the best performance over practically the whole visible range of 360–750 nm. It confirmed that the IPCE of NPC0.5 device was indeed higher than that of the conventional NPC0 device. Furthermore, the integrated current density obtained from the best NPC0.5 device IPCE spectrum was 21.31 mA cm−2, which was higher than that of pure CB device (20.24 mA cm−2). The response capability was in good agreement with the UV–vis absorption spectrum of perovskite and the current extracted from the J-V experiment. In order to avoid erroneous PCE estimation due to hysteresis of J-V. Fig. 8b exhibits the steady-state photocurrent density and output PCE of the devices at the maximum power points. The steady-state photocurrent measured at maximum power point (0.80 v) was close to that of value measured by J-V scan. For NPC0.5 PSC device, a stable photocurrent density of 22.6 mA cm−2 and PCE of 18.3% were obtained. 4. Conclusions In summary, a simple, effective and versatile solvent engineering technology has been developed, which used nickel phthalocyanine (NiPc) as an additive in an anticholinergic chlorobenzene to form a mixed antisolvent for the preparation of high performance PSCs. The application of perovskite films pre-treated with optimal NiPc/CB concentrations (NPC0.5) in solar cell construction acquired the excellent photovoltaic performance: a PCE of 19.18%, a VOC of 1.10 v, a JSC of 23.39 mA cm−2, a FF of 74.78%. Compared with the reference conventional cell, the enhancement of photovoltaic performance pretreated by mixed anti-solution NiPc/CB (NPC0.5) mainly stemmed from the high-quality perovskite films, achieving effective charge separation, enhancing the interfacial contact and improving the carrier transport.
Fig. 8. (a) IPCE spectra and integrated photocurrent density curves of the PSCs with the pure CB solvent and the best concentration of NiPc/CB; (b) Stabilized photocurrent density and output PCE at maximum power point as a function of time for the best performing PSC (NPC0.5).
based on 0.5 mg mL−1 NiPc/CB exhibited a promising PCE of 19.18% with a VOC of 1.096 V, a JSC of 23.39 mA cm−2, and a fill factor (FF) of 74.78%, outperforming the performance of the PSC based on pure CB (VOC, JSC, FF, and PCE were 1.082 V, 22.28 mA cm−2, 71.5%, and 17.4%, respectively.). With the NiPc/CB concentration increased from 0.05 to 1 mg mL−1, the PCE increased from 15.16% to 19.18% and then decreased to 16.36%. Therefore, the optimal NiPc/CB concentration was 0.5 mg mL−1. The enhancement of FF and VOC could be attributed the improvement quality of the perovskite film and low interface loss, which was consistent well with the PL spectrum. The enhancement of open circuit voltage (VOC) and film fill factor (FF) might be due to the deep trap introduced by the phthalocyanine molecule to block the reticular wave electrons. The presence of NiPC reduced the size of pinholes formed during film deposition and improved the quality of the organic hole transport layer [31]. Furthermore, the recombination of photo-carriers was restrained by the new heterojunction formed between the small amount of residual PbI2 and perovskite [35]. In addition, small amount of residual PbI2 not only passivated the defects at surfaces and grain boundaries, but also increased the size of perovskite crystals, thus contributing to the improved performance of the solar cells [31,36]. However, excessive PbI2 remaining in the entire perovskite film would destroy the compact structure of perovskite film, reduced the stability of films and enhanced the hysteresis effect of
Acknowledgement The project is jointly supported by the National Natural Science Foundation of China (No. 61306077), the Fundamental Research Funds for the Central Universities (JB-ZR1109, JB-ZR1212), Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-PY207), Discipline Innovation Team Project of Huaqiao University (201320), the Open Project Program of Provincial Key Laboratory of Eco-Industrial Green Technology of Wuyi University. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] J.H. Heo, S.H. Im, H.J. Kim, P.P. Boix, S.J. Lee, S. II Seok, I. Moraseró, J. Bisquert, J. Phys. Chem. C 116 (2012) 20717–20721.
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