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High performance perovskite solar cells fabricated from porous PbI2-xBrx prepared with mixture solvent pore generation treatment
Electrochimica Acta 292 (2018) 399e406
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High performance perovskite solar cells fabricated from porous PbI2-xBrx prepared with mixture solvent pore generation treatment Chih-Wen Chang, Zai-Wen Kwang, Tsung-Yu Hsieh, Tzu-Chien Wei, Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2017 Received in revised form 22 August 2018 Accepted 24 September 2018 Available online 27 September 2018
Power conversion efficiencies of perovskite solar cells depend heavily on the uniformity, coverage, grain size, and thickness of the perovskite layer. In this study, hybrid perovskite CH3NH3PbI3-xBrx thin films were fabricated from porous PbI2-xBrx layers prepared with a mixture solvent pore generation treatment process. The mixture solvent is composed of two anti-solvents of PbI2-xBrx to promote its crystallization, one with a low boiling point to accelerate the evaporative removal of the solvents of PbI2-xBrx, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and the other with a high boiling point to modulate the evaporative removal of DMF and DMSO to achieve a suitably configured porous structure for the PbI2-xBrx layer. With this fast pore generation treatment, uniform porous PbI2-xBrx layers were obtained within minutes, far more efficient than previously reported pore generation processes. The uniform porous PbI2-xBrx layer enabled formation of hybrid perovskite CH3NH3PbI3-xBrx thin films of low PbI2-xBrx residue, good crystallinity, low defect concentrations, and long charge carrier life times. The champion cell, assembled from the optimal hybrid perovskite thin films fabricated from optimized pore generation treatments, exhibited significantly superior power conversion performances of 19.26 mA/cm2 for the short circuit current density, 1.01 V for the open circuit voltage, 0.68 for the fill factor, and 13.16% for the power conversion efficiency, as compared to the average power conversion efficiency of 10.03% for non-porous PbI2-xBrx layer based cells. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Perovskite solar cell Porous PbX2 Mixture solvent Pore generation
1. Introduction Perovskite solar cells receive tremendous research attention in the past few years mainly because of the rapidly improving power conversion efficiency, from 3.8% of 2009 to current 22.1% [1e6]. The success of the perovskite solar cell comes mainly from the unique and advantageous properties of the perovskite layer (CH3NH3PbX3, X ¼ Cl, Br, I), including high optical absorption [7], excellent ambipolar charge transport [8], high carrier mobility [9], long charge diffusion length [9], and small direct bandgap [10]. It has been generally recognized that the power conversion efficiency of the perovskite solar cell depends heavily on the morphology and crystalline quality of the perovskite layer. It is thus critical to fabricate high quality perovskite layers of complete surface coverage, uniform morphology, and large well-crystalized grains for high performance perovskite solar cells [11e13]. Consequently, a wide variety of processes has been developed for formation of the
* Corresponding author. E-mail address: [email protected] (S.-Y. Lu). https://doi.org/10.1016/j.electacta.2018.09.161 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
perovskite layer, including one-step solution method [14], two-step solution method [15,16], vapor-assisted solution process [17], and dual source vapor deposition [18]. In the one-step solution method, PbX2 and CH3NH3X, the precursors of perovskite, of suitable mixing ratios are dissolved in a polar solvent and spin-coated onto a substrate for formation of the perovskite layer upon annealing. During the spin-coating process, the rapid solvent evaporation induces fast crystallization, and the interplay between solvent evaporation and perovskite crystallization is often complicated and highly dynamic. It is thus difficult to control the morphology and crystalline quality of the resulting perovskite layer, often with low crystal quality and low surface coverage leading to low power conversion efficiencies of the cells [19,20]. To improve on this situation, Jeon et al. developed a solvent engineering approach, in which an anti-solvent was introduced at the spin-coating step to achieve formation of uniform and dense perovskite layers [21]. Similarly, Xiao et al. developed a fast crystallization deposition method, in which an anti-solvent was again added at the spin-coating step to accelerate the nucleation and crystal growth of perovskite to obtain dense perovskite layers of large grains [22].
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As for the two-step solution method, PbI2 is first spin-coated on the substrate, and then the PbI2 layer is submerged in a CH3NH3I solution (MethylAmmonium Iodide, MAI dissolved in isopropyl alcohol, IPA) for formation of the perovskite layer. MAI molecules diffuse and intercalate into the crystalline lattice of PbI2 to form CH3NH3PbI3. The intercalation of MAI molecules however expands the crystalline lattice of PbI2 and causes significant volume expansion at the formation of perovskite. A compact perovskite layer often forms on the surface of the PbI2 layer and retards the further diffusion of MAI to reach inner PbI2, resulting in incomplete formation of perovskite and excessive amount of PbI2 residue [23e35]. The excessive amount of PbI2 residue hammers not only the photoconversion efficiency but also the reproducibility of the cell, because of wide variations in the amount of PbI2 residue [24]. Zhang et al. developed a porous PbI2 approach to solve the PbI2 residue problem. With addition of 4-tert-butylpyridine (TBP), porous PbI2 layers were formed through a self-assembly mechanism, in which the contact area between PbI2 and MAI increased to enhance the rate and completeness of the perovskite formation reaction [26]. Liu et al. developed a time dependent growth method for creation of porous PbI2 layers through manipulation of the nucleation of PbI2 [27]. Henawey et al. treated PbI2 layer with vapors of an organic solvent such as toluene and chlorobenzene to generate porous PbI2 layers [28]. Li et al. created porous PbI2 layers with a two-step thermal annealing process [29]. Zhang et al. developed a solvent coordination and anti-solvent extraction strategy for preparation of porous PbI2 films for bilayered mesostructured perovskite solar cells [30]. Zhang et al. used a multi-step annealing method to deposit porous PbI2 films and improved the quality and uniformity of perovskite films [31]. Jia et al. created porous PbI2 films by using polystyrene pore-forming reagents [32]. Chen et al. achieved porous PbI2 surfaces of smaller grains through dimethyl sulfoxide vapor treatment, which promoted the migration and reaction rate between CH3NH3I vapor and PbI2 layer [33]. Kim et al. developed a two-step mediator extraction treatment method to produce porous PbI2 films [34]. All the above mentioned development generate porous PbI2 layers to enhance the completeness of the perovskite formation, leading to significantly improved power conversion efficiencies. Although successful, these methods often require long processing time and involve complicated operations, thus increasing the manufacturing cost of the cells. In this study, a simple and fast mixture solvent pore generation treatment was developed to create porous PbI2-xBrx layers, from which good quality hybrid perovskite layers of minimum PbI2-xBrx residue were obtained for high performance perovskite solar cells. For one-step solution methods, solvent engineering is a common approach to obtain high quality perovskite layers [21,22,35]. Popular solvents include ether, chlorobenzene, and toluene. The idea is to accelerate nucleation of the perovskite crystal to achieve uniform and dense perovskite layers. Here, in this study, we introduce the solvent engineering concept of one-step solution methods to twostep solution methods, with an extension of using mixture solvents. The mixture solvent is composed of two anti-solvents of PbI2-xBrx, ether and toluene or ether and ethanol, with one of them being of low boiling point (ether) and the other of high boiling point (toluene or ethanol). Submergence of the spin-coated PbI2-xBrx layer into the mixture solvent triggers the following solventsolvent interactions for fast pore generation. The low boiling point anti-solvent takes up the solvents of PbI2-xBrx, DMF and DMSO, to accelerate not only the nucleation and crystal growth of PbI2-xBrx but also the evaporative removal of DMF and DMSO, whereas the high boiling point anti-solvent balances the fast evaporative removal of DMF and DMSO to acquire a suitably configured pore structure for the PbI2-xBrx layer. With this simple and fast pore generation treatment, uniform porous PbI2-xBrx layers
were obtained within minutes, far more efficient than previously reported pore generation processes. The uniform porous PbI2-xBrx layer enabled formation of hybrid perovskite layers of low PbI2-xBrx residue, good crystallinity, low defect concentrations, and long charge carrier life times. The champion cell assembled from the optimal hybrid perovskite layers fabricated from an optimized pore generation treatment, exhibited significantly higher power conversion efficiency of 13.16% than the average power conversion efficiency of 10.03% for non-porous PbI2-xBrx layer based cells. This mixture solvent pore generation treatment process is unique, simple, fast, and effective in creation of porous PbI2-xBrx layers, leading to significant improvements, more than 30%, in power conversion efficiencies of the cell. 2. Materials and methods 2.1. Synthesis of CH3NH3I CH3NH3I was synthesized by mixing 24 mL of methylamine (33 wt% in absolute ethanol, Sigma Aldrich) and 10 mL of hydroiodic acid (57 wt% in water, Sigma Aldrich) in an ice bath for 2 h under stirring. The solvents, ethanol and water, were removed with a rotary evaporator operated at 50 C. Recrystallization purification was conducted three times with ethanol and ether as the solvents. White solid products were collected and dried at 60 C in a vacuum oven for 24 h for later use. 2.2. Fabrication of perovskite solar cells Fluorine-doped tin oxide glass substrates (FTO, 10 U/sq) were patterned with a laser etcher (LMF-020F, Taiwan), and cleaned sequentially by a neutral detergent, deionized water, acetone, and ethanol in an ultrasonic bath. After drying in air, the FTO substrate was further cleaned in a UV-ozone cleaner for 15 min. A 30 nm thick TiO2 compact layer (c-TiO2) was spin-coated onto the FTO substrate using a coating solution of 0.15 M titanium dissopropoxide bis(acetylacetonate) (75 wt % in 2-propanol, Sigma Aldrich) at 3000 rpm for 30 s. The film was then dried with a hotplate set at 125 C for 5 min followed by calcination at 500 C for 30 min. A 150 nm thick mesoporous TiO2 layer (ms-TiO2) (Dysol, 30NR, diluted with ethanol at a ratio of 1:6, w/w) was deposited through spin coating at 5000 rpm for 30 s onto the c-TiO2/FTO substrate. The film was dried with a hotplate set at 125 C for 5 min followed by calcination at 500 C for 30 min. Solutions of PbI2 (99%, Sigma Aldrich) and PbBr2 (99%, Sigma Aldrich) of 1.2 M were prepared using DMF and DMSO, respectively as the solvents through constant stirring at 70 C for 30 min. The PbI2 and PbBr2 solutions were mixed at desired molar ratios and spin-coated onto the ms-TiO2/c-TiO2/FTO substrate at 3000 rpm for 30 s. The wet PbI2-xBrx film was dipped into a mixture solvent, containing ether and toluene or ether and ethanol at the volume ratio of 1:2, for 10 s. The resulting film was annealed with a hotplate set at 70 C (or other desired temperatures) for 10 min. After cooling to room temperature, the film was exposed to an MAI solution of 8 mg/ml in 2-isopropyl alcohol for 90 s followed by first spinning at 4000 rpm for 10 s and annealing at 100 C for 10 min. An HTM solution, composed of 75 mM 2,20 ,7,70 -Tetrakis(N,N-di-4methoxyphenylamino)-9,90 -spirobifluorene (spiro-OMeTAD, > 99%, Lumtec, Taiwan), 25 mM lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI, 99.95%, Sigma-Aldrich), and 120 mM tertbutylpyridine (TBP, > 96%, Sigma-Aldrich) in chlorobenzene (99.8%, Sigma-Aldrich), was spin-coated onto the hybrid perovskite/ms-TiO2/c-TiO2/FTO substrate at 4000 rpm for 30 s. Li-TFSI was pre-dissolved in acetonitrile (99.5%, Merck) at a
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concentration of 340 mg/ml. Finally, a 100 nm thick gold electrode was thermally evaporated on top of the spiro-OMeTAD layer to complete the cell assembly. 2.3. Characterizations Field-emission scanning electron microscopy (FESEM) images were obtained with a Hitachi SU-8010 operated at an accelerating voltage of 10 kV. The X-ray diffraction (XRD) patterns were recorded with a Rigaku Ultima IV diffractometer using Cu (Ka, l ¼ 1.5418 Å) irradiation. The UVevisible spectra were collected with an HP-8453 spectrophotometer. Current densityvoltage (JV) curves of the cell were recorded under irradiation of a standard AM1.5G solar simulator (PEC-L15, Peccell, Japan) of 100 mW/ cm2. The light intensity was calibrated with a reference monocrystalline silicon photodiode (91150 V, Newport). All measurements were conducted under application of a non-reflective metal mask of a 0.1 cm2 aperture area to precisely control the active area of the cell. JV curves were recorded using both forward (Jsc to Voc) and backward (Voc to Jsc) scans. The scan step was controlled at 10 mV and the delay time was 50 ms. No light or voltage biasing was applied prior to the measurement. Incident photon-to-current efficiency (IPCE) spectra were measured with PEC-S20 (Peccell Technology, Japan) set at action spectrum measurement setup in DC mode. The excitation beam coming from a 150 W Xe lamp was focused through a monochromator (modified asymmetric CzernyTurner design) with irradiation area of 3 mm in diameter. The data were calibrated with a Si photodiode S1337-1010BQ. 3. Results and discussion Fig. S1 shows the photographs of PbI2-xBrx/ms-TiO2/c-TiO2/FTO samples treated with single anti-solvent of ether, mixture antisolvent of ether and toluene, and mixture anti-solvent of ether and ethanol. The photograph for the control case of no pore generation treatment is also presented for comparison. Evidently, the sample treated with ether is with a non-uniform appearance. It is caused by the fast evaporative removal of the solvents of PbI2-xBrx, DMF and DMSO, because of the high volatility of ether (bp: 34.6 C), leading to fast but non-uniform nucleation and crystal growth of PbI2-xBrx. Such non-uniformity is detrimental to the cell performance. To improve on the situation, a co-anti-solvent of relatively high boiling point was introduced to balance and modulate the evaporative solvent removal and subsequent crystal growth of PbI2 xBrx. Here, both toluene (bp: 110.6 C) and ethanol (bp: 78.4 C) were used. It is evident from Fig. S1 that the films thus formed with treatment by the two mixture anti-solvents, ether/toluene and ether/ethanol, are uniform in appearance. The detailed morphology of the resulting PbI2-xBrx layers were observed with SEM as shown in Fig. 1. Fig. 1a shows the top view SEM image of the control sample, termed c-PbX2. It is a dense film with a thickness of about 150 nm, as estimated from the corresponding cross section SEM image of Fig. 1g. It has been proposed that [PbX6]4- complex forms first at the spin-coating step of PbX2 solutions, and the complex transforms to a dense film at the annealing step because of the supply of thermal energy triggering the further removal of the solvent and increase in ionic mobility of PbX2 [36]. Fig. 1c and e show the top view SEM images of the PbI2xBrx films treated with mixture solvents ether/toluene and ether/ ethanol, termed ethter/toluene-PbX2 and ether/ethanol-PbX2, respectively. Evidently, the two films are porous, confirming the success of the mixture solvent treatment in creating pores. The pores of the film can also be clearly observed from the cross section SEM image of sample ether/toluene-PbX2, given in Fig. 1h, in contrast to the dense morphology of sample c-PbX2. Because of the
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presence of the pores, the thickness of sample ether/toluene-PbX2 is larger than that of sample c-PbX2, 180 vs. 150 nm. The areal void fractions of samples c-PbX2, ether/toluene-PbX2, and ether/ethanoPbX2 were estimated with software ImageJ to quantify the relative porousness of the samples. The results are 0.2, 13, and 14% for samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2, as shown in Fig. 1b, d, and f, respectively. For comparison, the areal void fraction of sample ether-PbX2 was determined as 4.2% as shown in Fig. S2b. The crystalline structure of samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2 was characterized with XRD as shown in Fig. S3a. The diffraction peaks located at the 2q values of 12.7, 25.5, and 38.5 came from the crystalline planes of (001), (002), and (003), respectively of PbI2 of 2H hexagonal phase [15]. The absence of detectable shifts in the major diffraction peaks of PbI2 and the absence of diffraction peaks of PbBr2 indicate the minor inclusion of Br in PbI2-xBrx as a dopant, in good agreement with the finding previously reported [37]. Br was intercalated into the lattice of PbI2 as a dopant and the doping amount was not enough to effect a detectable shift in diffraction peaks of the host material. Fig. S3b shows the UVevisible spectra of samples c-PbX2, ether/toluenePbX2, and ether/ethanol-PbX2. The onset absorption wavelengths of all three samples can be determined to be slightly above 500 nm, corresponding well to the characteristic band-gap excitation wavelength of PbI2 [24]. Interestingly, the absorption intensity of the three samples is in the order of ether/ethanol-PbX2 > ether/ toluene-PbX2 > c-PbX2, in good agreement with the areal void fraction trend of the samples, attributable to the phenomenon of increasing light scattering contribution to absorption with increasing porousness [27]. At the second step of the two-step solution method, samples cPbX2, ether-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2 reacted with MAI to form hybrid perovskite layers, termed c-PVSK, ether-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK, respectively for referring convenience. Fig. 2 shows the top view SEM images of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. The perovskites appear as irregularly shaped faceted crystals, with crystal sizes of 232 ± 48, 310 ± 53, and 355 ± 61 nm, respectively. The crystal sizes of ether/toluene-PVSK and ether/ethanolPVSK are significantly larger than that of c-PVSK, possibly due to the enhanced reaction rate enabled by the porous structure of samples ether/toluene-PbX2 and ether/ethanol-PbX2. The large crystal size is in general beneficial for the cell performance. For comparison, the top view SEM image of ether-PVSK is shown in Fig. S2c. It can be observed clear exposure of the TiO2 layer underneath the perovskite layer, which is detrimental to cell performance because of the potential short-circuiting problem. The severe TiO2 exposure problem may come from the non-uniform morphology of ether-PbX2 as shown in Fig. S1. The crystalline structure of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK was characterized with XRD as shown in Fig. 3a. The diffraction peaks located at the 2q values of 14.0, 24.6, 28.3, 32, and 43.2 are attributed to the diffraction of crystalline planes of (110), (022), (220), (310), and (330), respectively. It is evident that a pronounced diffraction peak of PbI2 at 12.7 is still present for sample c-PVSK, indicating an incomplete conversion of PbX2 to perovskite and thus a large amount of PbX2 residue. The intensity of this diffraction peak is in fact even higher than that of the main diffraction peak of the perovskite, the (110) diffraction peak. This phenomenon can be related to the non-porous nature of c-PbX2. It has been shown that a surface compact perovskite skin forms when PbI2 is exposed to and reacts with MAI. A significant volume expansion occurs when PbI2 is converted into perovskite since the density of PbI2 is 6.16 g/cm3, much larger than 4.29 g/cm3 of perovskite CH3NH3PbI3. The volume expansion leads to
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Fig. 1. (a), (c), (e) Top view SEM images of samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2, respectively, (b), (d), (f) images (a), (c), (e) processed with ImageJ for areal void fraction determination. Voids are highlighted in red. (g), (h) Cross section SEM images of samples c-PbX2 and ether/toluene-PbX2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 2. Top view SEM images of (a) c-PVSK, (b) ether/toluene-PVSK, and (c) ether/ethanol-PVSK.
Fig. 3. (a) XRD patterns and (b) UVevisible spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK.
formation of the surface compact perovskite skin, which retards the further diffusion of MAI across the compact skin to meet and react with the inner PbI2, consequently the incomplete conversion of PbI2 and a large amount of PbI2 residue [23]. On the contrary, ether/ toluene-PVSK and ether/ethanol-PVSK, derived from porous ether/ toluene-PbX2 and ether/ethanol-PbX2, respectively, exhibit very weak PbI2 diffraction at 12.7, implying very minor amounts of PbX2 residue. The porous structure of ether/toluene-PbX2 and ether/ ethanol-PbX2 not only enlarges the contact area of PbX2 with MAI, but also enables the easy access of inner PbX2 for large and complete perovskite formation. Furthermore, the diffraction peak intensities of ether/toluene-PVSK and ether/ethanol-PVSK at 14.0 , the (110) planes of the perovskite, are significantly higher than that of c-PVSK, implying again more perovskite formation and less PbX2 residue. Fig. 3b shows the UVevisible spectra recorded for c-PVSK, ether/ toluene-PVSK, and ether/ethanol-PVSK. The onset absorption wavelengths of all three samples are around 770 nm, in good agreement with the energy bandgap of perovskite, around 1.6 eV [1]. Interestingly, the absorption intensities of ether/toluene-PVSK and ether/ethanol-PVSK are significantly higher than that of cPVSK. This can be attributed to the more complete formation and larger sizes of perovskite crystals for ether/toluene-PVSK and ether/ ethanol-PVSK. The larger sizes of perovskite crystals give increased roughness and thus more intense light scattering and better light harvesting [10,38]. More interestingly, the ether/toluene-PVSK gives the highest light absorption among the three. This may be understood through the following reasoning. The ether/toluenePVSK possesses slightly smaller but more uniform crystal sizes than those of the ether/ethanol-PVSK, which is beneficial to create a more uniform porous domain to confine and retain scattered light within the porous domain for more effective light harvesting. It is worth noting that hybrid perovskites, CH3NH3PbI3-xBrx, may exhibit a very minor shift of 0.01e0.02 at around 14.0 , the major
diffraction peak of CH3NH3PbI3 [39], because of the Br incorporation. This shift is however absent from Fig. 3a, probably due to the limited amount of Br incorporation in the present product. To confirm the successful Br incorporation in the present product, ether/toluene was taken as the pore generation reagent for PbX2 films, fabricated by using increasing molar ratio of PbBr2, from 0, 10, 15, to 20%. The PbX2 of zero percent PbBr2 addition serves as a reference. Fig. S4 shows the high resolution XPS spectra of Br-3d of the product perovskite films derived from the above PbX2 films. Evidently, no characteristic binding energy peak appears between 68 and 70 eV, the characteristic binding energy range of Br-3d, for the perovskite film derived from the reference PbX2, indicating that the resulting perovskite is not Br incorporated. The characteristic binding energy peak of Br-3d however emerges at 68e70 eV with increasing PbBr2 addition, confirming the successful incorporation of Br into the hybrid perovskite. The energy bandgaps of CH3NH3PbI3 and CH3NH3PbBr3 have been determined to be around 1.6 and 2.3 eV, respectively [1]. If Br is successfully incorporated into CH3NH3PbI3, one expects increase in energy bandgap and thus blue-shift in onset absorption wavelength. Fig. S5 shows the corresponding UVevisible spectra of the perovskite samples. Evidently, the onset absorption wavelength blue shifts with increasing molar ratio of PbBr2 addition, confirming again the successful Br incorporation in the hybrid perovskite product. Solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK were characterized for their cell performances under irradiation of AM1.5G light source at 100 mW/cm2. Fig. 4a shows the typical current density vs. applied voltage curves (J-V curves) of the cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK, with the corresponding average short circuit current densities (Jsc), open circuit voltages (Voc), fill factors (FF), and power conversion efficiencies (PCE) determined and summarized in Table 1 for comparison. Evidently, significant improvements in PCE were achieved by cells fabricated from ether/
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Fig. 4. (a) J-V curves and (b) IPCE spectra of solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK.
Table 1 Performance indicators of solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. Jsc (mA cm2)
Voc(V)
FF
Forward [%] (best)
Reverse [%] (best)
Avg PCE [%] (best)
c-PVSK
14.88 þ 0.32
0.98 þ 0.01
0.68 þ 0.02
ether/toluene-PVSK
17.58 þ 0.11
1.01 þ 0.01
0.69 þ 0.01
ether/ethanol-PVSK
16.73 þ 0.52
1.01 þ 0.01
0.66 þ 0.01
8.67 þ 0.21 (8.61) 12.06 þ 0.06 (12.09) 10.61 þ 0.31 (10.96)
11.52 þ 0.19 (11.66) 12.62 þ 0.27 (12.92) 11.53 þ 0.51 (12.11)
10.03 þ 0.25 (10.05) 12.32 þ 0.17 (12.51) 11.07 þ 0.40 (11.52)
toluene-PVSK and ether/ethanol-PVSK over that of cells from cPVSK. The improvement comes mainly from the boost in Jsc, attributable to the enhanced light harvesting resulting from the more complete formation and larger sizes of perovskite crystals, which are in turn related to the porous structure of the PbX2 layer. As expected, the cells assembled from the ether/toluene-PVSK gives the highest Jsc and thus the highest PCE, because of the most effective light harvesting of the ether/toluene-PVSK as shown in Fig. 3b. Fig. 4b shows the incident photon-to-current efficiency (IPCE) spectra of the cells. The results are consistent with those of UVevisible absorption spectra, J-V curves, and Jsc, with the spectrum of the c-PVSK based cell lying consistently beneath those of ether/toluene-PVSK and ether/ethanol-PVSK based cells. The spectrum of the ether/toluene-PVSK based cell lies above those of the other two cells, with the highest IPCE reaching 75% and most IPCEs above 70% within the visible light region. The cell performances were further investigated through examining the dynamics of the photo-induced charge carriers with static and time-resolved photoluminescence (TR-PL) spectra of cPVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. The PL of the
perovskite film will be quenched by trap states and defects. Fig. 5a shows the static PL spectra of the three PVSK films. The trend in PL intensity is in the order of ether/toluene-PVSK > ether/ethanolPVSK > c-PVSK, in consistence with the trends of IPCE and PCE. The non-radiative charge recombination is more effectively suppressed in ether/toluene-PVSK and ether/ethanol-PVSK than in c-PVSK. Fig. 5b shows the TR-PL spectra of the three PVSK films, with the determined charge carrier life time constants summarized in Table S1 for comparison. The TR-PL spectrum was fitted with a biexponential decay model, from which two charge carrier life time constants can be determined, with the longer one (t1) accounting for free carrier recombination and the shorter one (t2) for trap state quenching [40,41]. Evidently, the photo-induced charge carriers of the ether/toluene-PVSK film decay the slowest with a largest average charge carrier life time constant. The ether/ethanol-PVSK comes in next and the c-PVSK the worst, again consistent with the conclusion drawn from the IPCE spectra and PCE data. From the above results and discussion, one concludes that the mixture solvent pore generation treatment does improve the PCE of the resulting perovskite solar cells by 2.29%, from 10.03% for the c-
Fig. 5. (a) Static PL spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK films excited at 510 nm. (b) TR-PL spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanolPVSK films excited at 375 nm.
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Table 2 Performance indicators of solar cells assembled from optimized ether/toluene-PVSK films.
100 C
Jsc (mA cm2)
Voc(V)
FF
Forward [%] (best)
Reverse [%] (best)
Avg PCE [%] (best)
19.28 þ 0.12
1.01 þ 0.01
0.66 þ 0.01
12.58 þ 0.34 (12.89)
13.25 þ 0.24 (13.43)
12.92 þ 0.28 (13.16)
PVSK based cell to 12.32% for the ether/toluene-PVSK based cell. To gain further improvements, the processing condition for the fabrication of the PbX2 layer was optimized. It was found that the annealing temperature of the PbX2 layer played the key role in constructing the optimal PbX2 layer for the ether/toluene-PVSK film that led to cells of the best performance. With an annealing temperature of 100 C for the fabrication of the PbX2 layer, the cells thus assembled achieved the highest average Jsc of 19.28 mA/cm2 and the highest PCE of 12.92%. The relevant cell performance indicators were summarized in Table 2. The champion cell exhibited an average Jsc of 19.26 mA/cm2, Voc of 1.01 V, FF of 0.68, and PCE of 13.16%, a boost of 3.13% in PCE than 10.03% of the control cell. Its J-V curve is shown in Fig. 6 with the performance indicators from both forward and reverse scans presented to illustrate its minimum hysteresis behavior. The hysteresis phenomenon is attributed to the existence of internal capacitive currents at the layer interfaces of the cell [42]. Minimum hysteresis may imply good layer contacts. The cross section SEM image of the champion cell is shown in Fig. 7. The thickness of the perovskite capping layer is 300 nm, which is generally accepted as the optimal thickness for the perovskite capping layer [37,43]. The perovskite capping layer should be thick
enough to effectively absorb incoming light, but thin enough to shorten the charge transport path for effective collection of the photo-induced charge carriers. The perovskite capping layer can also be observed to be a dense film composed of large size perovskite crystals, advantageous for light harvesting and charge transport. 4. Conclusion A simple and fast mixture solvent pore generation treatment was successfully developed to create uniform porous PbX2 layers, with which the contact area for the perovskite formation was significantly increased and the inner PbX2 can be readily accessed for perovskite formation. The PbX2 residue problem is thus much lessened, leading to more complete formation and larger sizes of perovskite crystals, from which the light harvesting and charge carrier transport were significantly improved to give perovskite cells of much better performances. The superior quality of the perovskite film, large grains, few defects, good crystallinity, and long charge carrier life times, derived from the mixture solvent treated PbX2 layer was investigated and confirmed with SEM, XRD, UVevisible, PL, and TR-PL characterizations. The champion cell thus obtained exhibited a Jsc of 19.26 mA/cm2, Voc of 1.01 V, FF of 0.68, and an average PCE of 13.16%, a 3.13% boost in PCE as compared with 10.03% of the control cell. Acknowledgements This work was financially supported by the Ministry of Science and Technology of Taiwan, ROC under grant MOST 105-2221-E007-126-MY2. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.09.161. References
Fig. 6. J-V curves of champion cell.
Fig. 7. Cross section SEM image of champion cell.
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High performance perovskite solar cells fabricated from porous PbI2-xBrx prepared with mixture solvent pore generation treatment Chih-Wen Chang, Zai-Wen Kwang, Tsung-Yu Hsieh, Tzu-Chien Wei, Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2017 Received in revised form 22 August 2018 Accepted 24 September 2018 Available online 27 September 2018
Power conversion efficiencies of perovskite solar cells depend heavily on the uniformity, coverage, grain size, and thickness of the perovskite layer. In this study, hybrid perovskite CH3NH3PbI3-xBrx thin films were fabricated from porous PbI2-xBrx layers prepared with a mixture solvent pore generation treatment process. The mixture solvent is composed of two anti-solvents of PbI2-xBrx to promote its crystallization, one with a low boiling point to accelerate the evaporative removal of the solvents of PbI2-xBrx, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and the other with a high boiling point to modulate the evaporative removal of DMF and DMSO to achieve a suitably configured porous structure for the PbI2-xBrx layer. With this fast pore generation treatment, uniform porous PbI2-xBrx layers were obtained within minutes, far more efficient than previously reported pore generation processes. The uniform porous PbI2-xBrx layer enabled formation of hybrid perovskite CH3NH3PbI3-xBrx thin films of low PbI2-xBrx residue, good crystallinity, low defect concentrations, and long charge carrier life times. The champion cell, assembled from the optimal hybrid perovskite thin films fabricated from optimized pore generation treatments, exhibited significantly superior power conversion performances of 19.26 mA/cm2 for the short circuit current density, 1.01 V for the open circuit voltage, 0.68 for the fill factor, and 13.16% for the power conversion efficiency, as compared to the average power conversion efficiency of 10.03% for non-porous PbI2-xBrx layer based cells. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Perovskite solar cell Porous PbX2 Mixture solvent Pore generation
1. Introduction Perovskite solar cells receive tremendous research attention in the past few years mainly because of the rapidly improving power conversion efficiency, from 3.8% of 2009 to current 22.1% [1e6]. The success of the perovskite solar cell comes mainly from the unique and advantageous properties of the perovskite layer (CH3NH3PbX3, X ¼ Cl, Br, I), including high optical absorption [7], excellent ambipolar charge transport [8], high carrier mobility [9], long charge diffusion length [9], and small direct bandgap [10]. It has been generally recognized that the power conversion efficiency of the perovskite solar cell depends heavily on the morphology and crystalline quality of the perovskite layer. It is thus critical to fabricate high quality perovskite layers of complete surface coverage, uniform morphology, and large well-crystalized grains for high performance perovskite solar cells [11e13]. Consequently, a wide variety of processes has been developed for formation of the
* Corresponding author. E-mail address: [email protected] (S.-Y. Lu). https://doi.org/10.1016/j.electacta.2018.09.161 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
perovskite layer, including one-step solution method [14], two-step solution method [15,16], vapor-assisted solution process [17], and dual source vapor deposition [18]. In the one-step solution method, PbX2 and CH3NH3X, the precursors of perovskite, of suitable mixing ratios are dissolved in a polar solvent and spin-coated onto a substrate for formation of the perovskite layer upon annealing. During the spin-coating process, the rapid solvent evaporation induces fast crystallization, and the interplay between solvent evaporation and perovskite crystallization is often complicated and highly dynamic. It is thus difficult to control the morphology and crystalline quality of the resulting perovskite layer, often with low crystal quality and low surface coverage leading to low power conversion efficiencies of the cells [19,20]. To improve on this situation, Jeon et al. developed a solvent engineering approach, in which an anti-solvent was introduced at the spin-coating step to achieve formation of uniform and dense perovskite layers [21]. Similarly, Xiao et al. developed a fast crystallization deposition method, in which an anti-solvent was again added at the spin-coating step to accelerate the nucleation and crystal growth of perovskite to obtain dense perovskite layers of large grains [22].
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As for the two-step solution method, PbI2 is first spin-coated on the substrate, and then the PbI2 layer is submerged in a CH3NH3I solution (MethylAmmonium Iodide, MAI dissolved in isopropyl alcohol, IPA) for formation of the perovskite layer. MAI molecules diffuse and intercalate into the crystalline lattice of PbI2 to form CH3NH3PbI3. The intercalation of MAI molecules however expands the crystalline lattice of PbI2 and causes significant volume expansion at the formation of perovskite. A compact perovskite layer often forms on the surface of the PbI2 layer and retards the further diffusion of MAI to reach inner PbI2, resulting in incomplete formation of perovskite and excessive amount of PbI2 residue [23e35]. The excessive amount of PbI2 residue hammers not only the photoconversion efficiency but also the reproducibility of the cell, because of wide variations in the amount of PbI2 residue [24]. Zhang et al. developed a porous PbI2 approach to solve the PbI2 residue problem. With addition of 4-tert-butylpyridine (TBP), porous PbI2 layers were formed through a self-assembly mechanism, in which the contact area between PbI2 and MAI increased to enhance the rate and completeness of the perovskite formation reaction [26]. Liu et al. developed a time dependent growth method for creation of porous PbI2 layers through manipulation of the nucleation of PbI2 [27]. Henawey et al. treated PbI2 layer with vapors of an organic solvent such as toluene and chlorobenzene to generate porous PbI2 layers [28]. Li et al. created porous PbI2 layers with a two-step thermal annealing process [29]. Zhang et al. developed a solvent coordination and anti-solvent extraction strategy for preparation of porous PbI2 films for bilayered mesostructured perovskite solar cells [30]. Zhang et al. used a multi-step annealing method to deposit porous PbI2 films and improved the quality and uniformity of perovskite films [31]. Jia et al. created porous PbI2 films by using polystyrene pore-forming reagents [32]. Chen et al. achieved porous PbI2 surfaces of smaller grains through dimethyl sulfoxide vapor treatment, which promoted the migration and reaction rate between CH3NH3I vapor and PbI2 layer [33]. Kim et al. developed a two-step mediator extraction treatment method to produce porous PbI2 films [34]. All the above mentioned development generate porous PbI2 layers to enhance the completeness of the perovskite formation, leading to significantly improved power conversion efficiencies. Although successful, these methods often require long processing time and involve complicated operations, thus increasing the manufacturing cost of the cells. In this study, a simple and fast mixture solvent pore generation treatment was developed to create porous PbI2-xBrx layers, from which good quality hybrid perovskite layers of minimum PbI2-xBrx residue were obtained for high performance perovskite solar cells. For one-step solution methods, solvent engineering is a common approach to obtain high quality perovskite layers [21,22,35]. Popular solvents include ether, chlorobenzene, and toluene. The idea is to accelerate nucleation of the perovskite crystal to achieve uniform and dense perovskite layers. Here, in this study, we introduce the solvent engineering concept of one-step solution methods to twostep solution methods, with an extension of using mixture solvents. The mixture solvent is composed of two anti-solvents of PbI2-xBrx, ether and toluene or ether and ethanol, with one of them being of low boiling point (ether) and the other of high boiling point (toluene or ethanol). Submergence of the spin-coated PbI2-xBrx layer into the mixture solvent triggers the following solventsolvent interactions for fast pore generation. The low boiling point anti-solvent takes up the solvents of PbI2-xBrx, DMF and DMSO, to accelerate not only the nucleation and crystal growth of PbI2-xBrx but also the evaporative removal of DMF and DMSO, whereas the high boiling point anti-solvent balances the fast evaporative removal of DMF and DMSO to acquire a suitably configured pore structure for the PbI2-xBrx layer. With this simple and fast pore generation treatment, uniform porous PbI2-xBrx layers
were obtained within minutes, far more efficient than previously reported pore generation processes. The uniform porous PbI2-xBrx layer enabled formation of hybrid perovskite layers of low PbI2-xBrx residue, good crystallinity, low defect concentrations, and long charge carrier life times. The champion cell assembled from the optimal hybrid perovskite layers fabricated from an optimized pore generation treatment, exhibited significantly higher power conversion efficiency of 13.16% than the average power conversion efficiency of 10.03% for non-porous PbI2-xBrx layer based cells. This mixture solvent pore generation treatment process is unique, simple, fast, and effective in creation of porous PbI2-xBrx layers, leading to significant improvements, more than 30%, in power conversion efficiencies of the cell. 2. Materials and methods 2.1. Synthesis of CH3NH3I CH3NH3I was synthesized by mixing 24 mL of methylamine (33 wt% in absolute ethanol, Sigma Aldrich) and 10 mL of hydroiodic acid (57 wt% in water, Sigma Aldrich) in an ice bath for 2 h under stirring. The solvents, ethanol and water, were removed with a rotary evaporator operated at 50 C. Recrystallization purification was conducted three times with ethanol and ether as the solvents. White solid products were collected and dried at 60 C in a vacuum oven for 24 h for later use. 2.2. Fabrication of perovskite solar cells Fluorine-doped tin oxide glass substrates (FTO, 10 U/sq) were patterned with a laser etcher (LMF-020F, Taiwan), and cleaned sequentially by a neutral detergent, deionized water, acetone, and ethanol in an ultrasonic bath. After drying in air, the FTO substrate was further cleaned in a UV-ozone cleaner for 15 min. A 30 nm thick TiO2 compact layer (c-TiO2) was spin-coated onto the FTO substrate using a coating solution of 0.15 M titanium dissopropoxide bis(acetylacetonate) (75 wt % in 2-propanol, Sigma Aldrich) at 3000 rpm for 30 s. The film was then dried with a hotplate set at 125 C for 5 min followed by calcination at 500 C for 30 min. A 150 nm thick mesoporous TiO2 layer (ms-TiO2) (Dysol, 30NR, diluted with ethanol at a ratio of 1:6, w/w) was deposited through spin coating at 5000 rpm for 30 s onto the c-TiO2/FTO substrate. The film was dried with a hotplate set at 125 C for 5 min followed by calcination at 500 C for 30 min. Solutions of PbI2 (99%, Sigma Aldrich) and PbBr2 (99%, Sigma Aldrich) of 1.2 M were prepared using DMF and DMSO, respectively as the solvents through constant stirring at 70 C for 30 min. The PbI2 and PbBr2 solutions were mixed at desired molar ratios and spin-coated onto the ms-TiO2/c-TiO2/FTO substrate at 3000 rpm for 30 s. The wet PbI2-xBrx film was dipped into a mixture solvent, containing ether and toluene or ether and ethanol at the volume ratio of 1:2, for 10 s. The resulting film was annealed with a hotplate set at 70 C (or other desired temperatures) for 10 min. After cooling to room temperature, the film was exposed to an MAI solution of 8 mg/ml in 2-isopropyl alcohol for 90 s followed by first spinning at 4000 rpm for 10 s and annealing at 100 C for 10 min. An HTM solution, composed of 75 mM 2,20 ,7,70 -Tetrakis(N,N-di-4methoxyphenylamino)-9,90 -spirobifluorene (spiro-OMeTAD, > 99%, Lumtec, Taiwan), 25 mM lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI, 99.95%, Sigma-Aldrich), and 120 mM tertbutylpyridine (TBP, > 96%, Sigma-Aldrich) in chlorobenzene (99.8%, Sigma-Aldrich), was spin-coated onto the hybrid perovskite/ms-TiO2/c-TiO2/FTO substrate at 4000 rpm for 30 s. Li-TFSI was pre-dissolved in acetonitrile (99.5%, Merck) at a
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concentration of 340 mg/ml. Finally, a 100 nm thick gold electrode was thermally evaporated on top of the spiro-OMeTAD layer to complete the cell assembly. 2.3. Characterizations Field-emission scanning electron microscopy (FESEM) images were obtained with a Hitachi SU-8010 operated at an accelerating voltage of 10 kV. The X-ray diffraction (XRD) patterns were recorded with a Rigaku Ultima IV diffractometer using Cu (Ka, l ¼ 1.5418 Å) irradiation. The UVevisible spectra were collected with an HP-8453 spectrophotometer. Current densityvoltage (JV) curves of the cell were recorded under irradiation of a standard AM1.5G solar simulator (PEC-L15, Peccell, Japan) of 100 mW/ cm2. The light intensity was calibrated with a reference monocrystalline silicon photodiode (91150 V, Newport). All measurements were conducted under application of a non-reflective metal mask of a 0.1 cm2 aperture area to precisely control the active area of the cell. JV curves were recorded using both forward (Jsc to Voc) and backward (Voc to Jsc) scans. The scan step was controlled at 10 mV and the delay time was 50 ms. No light or voltage biasing was applied prior to the measurement. Incident photon-to-current efficiency (IPCE) spectra were measured with PEC-S20 (Peccell Technology, Japan) set at action spectrum measurement setup in DC mode. The excitation beam coming from a 150 W Xe lamp was focused through a monochromator (modified asymmetric CzernyTurner design) with irradiation area of 3 mm in diameter. The data were calibrated with a Si photodiode S1337-1010BQ. 3. Results and discussion Fig. S1 shows the photographs of PbI2-xBrx/ms-TiO2/c-TiO2/FTO samples treated with single anti-solvent of ether, mixture antisolvent of ether and toluene, and mixture anti-solvent of ether and ethanol. The photograph for the control case of no pore generation treatment is also presented for comparison. Evidently, the sample treated with ether is with a non-uniform appearance. It is caused by the fast evaporative removal of the solvents of PbI2-xBrx, DMF and DMSO, because of the high volatility of ether (bp: 34.6 C), leading to fast but non-uniform nucleation and crystal growth of PbI2-xBrx. Such non-uniformity is detrimental to the cell performance. To improve on the situation, a co-anti-solvent of relatively high boiling point was introduced to balance and modulate the evaporative solvent removal and subsequent crystal growth of PbI2 xBrx. Here, both toluene (bp: 110.6 C) and ethanol (bp: 78.4 C) were used. It is evident from Fig. S1 that the films thus formed with treatment by the two mixture anti-solvents, ether/toluene and ether/ethanol, are uniform in appearance. The detailed morphology of the resulting PbI2-xBrx layers were observed with SEM as shown in Fig. 1. Fig. 1a shows the top view SEM image of the control sample, termed c-PbX2. It is a dense film with a thickness of about 150 nm, as estimated from the corresponding cross section SEM image of Fig. 1g. It has been proposed that [PbX6]4- complex forms first at the spin-coating step of PbX2 solutions, and the complex transforms to a dense film at the annealing step because of the supply of thermal energy triggering the further removal of the solvent and increase in ionic mobility of PbX2 [36]. Fig. 1c and e show the top view SEM images of the PbI2xBrx films treated with mixture solvents ether/toluene and ether/ ethanol, termed ethter/toluene-PbX2 and ether/ethanol-PbX2, respectively. Evidently, the two films are porous, confirming the success of the mixture solvent treatment in creating pores. The pores of the film can also be clearly observed from the cross section SEM image of sample ether/toluene-PbX2, given in Fig. 1h, in contrast to the dense morphology of sample c-PbX2. Because of the
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presence of the pores, the thickness of sample ether/toluene-PbX2 is larger than that of sample c-PbX2, 180 vs. 150 nm. The areal void fractions of samples c-PbX2, ether/toluene-PbX2, and ether/ethanoPbX2 were estimated with software ImageJ to quantify the relative porousness of the samples. The results are 0.2, 13, and 14% for samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2, as shown in Fig. 1b, d, and f, respectively. For comparison, the areal void fraction of sample ether-PbX2 was determined as 4.2% as shown in Fig. S2b. The crystalline structure of samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2 was characterized with XRD as shown in Fig. S3a. The diffraction peaks located at the 2q values of 12.7, 25.5, and 38.5 came from the crystalline planes of (001), (002), and (003), respectively of PbI2 of 2H hexagonal phase [15]. The absence of detectable shifts in the major diffraction peaks of PbI2 and the absence of diffraction peaks of PbBr2 indicate the minor inclusion of Br in PbI2-xBrx as a dopant, in good agreement with the finding previously reported [37]. Br was intercalated into the lattice of PbI2 as a dopant and the doping amount was not enough to effect a detectable shift in diffraction peaks of the host material. Fig. S3b shows the UVevisible spectra of samples c-PbX2, ether/toluenePbX2, and ether/ethanol-PbX2. The onset absorption wavelengths of all three samples can be determined to be slightly above 500 nm, corresponding well to the characteristic band-gap excitation wavelength of PbI2 [24]. Interestingly, the absorption intensity of the three samples is in the order of ether/ethanol-PbX2 > ether/ toluene-PbX2 > c-PbX2, in good agreement with the areal void fraction trend of the samples, attributable to the phenomenon of increasing light scattering contribution to absorption with increasing porousness [27]. At the second step of the two-step solution method, samples cPbX2, ether-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2 reacted with MAI to form hybrid perovskite layers, termed c-PVSK, ether-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK, respectively for referring convenience. Fig. 2 shows the top view SEM images of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. The perovskites appear as irregularly shaped faceted crystals, with crystal sizes of 232 ± 48, 310 ± 53, and 355 ± 61 nm, respectively. The crystal sizes of ether/toluene-PVSK and ether/ethanolPVSK are significantly larger than that of c-PVSK, possibly due to the enhanced reaction rate enabled by the porous structure of samples ether/toluene-PbX2 and ether/ethanol-PbX2. The large crystal size is in general beneficial for the cell performance. For comparison, the top view SEM image of ether-PVSK is shown in Fig. S2c. It can be observed clear exposure of the TiO2 layer underneath the perovskite layer, which is detrimental to cell performance because of the potential short-circuiting problem. The severe TiO2 exposure problem may come from the non-uniform morphology of ether-PbX2 as shown in Fig. S1. The crystalline structure of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK was characterized with XRD as shown in Fig. 3a. The diffraction peaks located at the 2q values of 14.0, 24.6, 28.3, 32, and 43.2 are attributed to the diffraction of crystalline planes of (110), (022), (220), (310), and (330), respectively. It is evident that a pronounced diffraction peak of PbI2 at 12.7 is still present for sample c-PVSK, indicating an incomplete conversion of PbX2 to perovskite and thus a large amount of PbX2 residue. The intensity of this diffraction peak is in fact even higher than that of the main diffraction peak of the perovskite, the (110) diffraction peak. This phenomenon can be related to the non-porous nature of c-PbX2. It has been shown that a surface compact perovskite skin forms when PbI2 is exposed to and reacts with MAI. A significant volume expansion occurs when PbI2 is converted into perovskite since the density of PbI2 is 6.16 g/cm3, much larger than 4.29 g/cm3 of perovskite CH3NH3PbI3. The volume expansion leads to
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Fig. 1. (a), (c), (e) Top view SEM images of samples c-PbX2, ether/toluene-PbX2, and ether/ethanol-PbX2, respectively, (b), (d), (f) images (a), (c), (e) processed with ImageJ for areal void fraction determination. Voids are highlighted in red. (g), (h) Cross section SEM images of samples c-PbX2 and ether/toluene-PbX2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 2. Top view SEM images of (a) c-PVSK, (b) ether/toluene-PVSK, and (c) ether/ethanol-PVSK.
Fig. 3. (a) XRD patterns and (b) UVevisible spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK.
formation of the surface compact perovskite skin, which retards the further diffusion of MAI across the compact skin to meet and react with the inner PbI2, consequently the incomplete conversion of PbI2 and a large amount of PbI2 residue [23]. On the contrary, ether/ toluene-PVSK and ether/ethanol-PVSK, derived from porous ether/ toluene-PbX2 and ether/ethanol-PbX2, respectively, exhibit very weak PbI2 diffraction at 12.7, implying very minor amounts of PbX2 residue. The porous structure of ether/toluene-PbX2 and ether/ ethanol-PbX2 not only enlarges the contact area of PbX2 with MAI, but also enables the easy access of inner PbX2 for large and complete perovskite formation. Furthermore, the diffraction peak intensities of ether/toluene-PVSK and ether/ethanol-PVSK at 14.0 , the (110) planes of the perovskite, are significantly higher than that of c-PVSK, implying again more perovskite formation and less PbX2 residue. Fig. 3b shows the UVevisible spectra recorded for c-PVSK, ether/ toluene-PVSK, and ether/ethanol-PVSK. The onset absorption wavelengths of all three samples are around 770 nm, in good agreement with the energy bandgap of perovskite, around 1.6 eV [1]. Interestingly, the absorption intensities of ether/toluene-PVSK and ether/ethanol-PVSK are significantly higher than that of cPVSK. This can be attributed to the more complete formation and larger sizes of perovskite crystals for ether/toluene-PVSK and ether/ ethanol-PVSK. The larger sizes of perovskite crystals give increased roughness and thus more intense light scattering and better light harvesting [10,38]. More interestingly, the ether/toluene-PVSK gives the highest light absorption among the three. This may be understood through the following reasoning. The ether/toluenePVSK possesses slightly smaller but more uniform crystal sizes than those of the ether/ethanol-PVSK, which is beneficial to create a more uniform porous domain to confine and retain scattered light within the porous domain for more effective light harvesting. It is worth noting that hybrid perovskites, CH3NH3PbI3-xBrx, may exhibit a very minor shift of 0.01e0.02 at around 14.0 , the major
diffraction peak of CH3NH3PbI3 [39], because of the Br incorporation. This shift is however absent from Fig. 3a, probably due to the limited amount of Br incorporation in the present product. To confirm the successful Br incorporation in the present product, ether/toluene was taken as the pore generation reagent for PbX2 films, fabricated by using increasing molar ratio of PbBr2, from 0, 10, 15, to 20%. The PbX2 of zero percent PbBr2 addition serves as a reference. Fig. S4 shows the high resolution XPS spectra of Br-3d of the product perovskite films derived from the above PbX2 films. Evidently, no characteristic binding energy peak appears between 68 and 70 eV, the characteristic binding energy range of Br-3d, for the perovskite film derived from the reference PbX2, indicating that the resulting perovskite is not Br incorporated. The characteristic binding energy peak of Br-3d however emerges at 68e70 eV with increasing PbBr2 addition, confirming the successful incorporation of Br into the hybrid perovskite. The energy bandgaps of CH3NH3PbI3 and CH3NH3PbBr3 have been determined to be around 1.6 and 2.3 eV, respectively [1]. If Br is successfully incorporated into CH3NH3PbI3, one expects increase in energy bandgap and thus blue-shift in onset absorption wavelength. Fig. S5 shows the corresponding UVevisible spectra of the perovskite samples. Evidently, the onset absorption wavelength blue shifts with increasing molar ratio of PbBr2 addition, confirming again the successful Br incorporation in the hybrid perovskite product. Solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK were characterized for their cell performances under irradiation of AM1.5G light source at 100 mW/cm2. Fig. 4a shows the typical current density vs. applied voltage curves (J-V curves) of the cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK, with the corresponding average short circuit current densities (Jsc), open circuit voltages (Voc), fill factors (FF), and power conversion efficiencies (PCE) determined and summarized in Table 1 for comparison. Evidently, significant improvements in PCE were achieved by cells fabricated from ether/
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Fig. 4. (a) J-V curves and (b) IPCE spectra of solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK.
Table 1 Performance indicators of solar cells assembled from c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. Jsc (mA cm2)
Voc(V)
FF
Forward [%] (best)
Reverse [%] (best)
Avg PCE [%] (best)
c-PVSK
14.88 þ 0.32
0.98 þ 0.01
0.68 þ 0.02
ether/toluene-PVSK
17.58 þ 0.11
1.01 þ 0.01
0.69 þ 0.01
ether/ethanol-PVSK
16.73 þ 0.52
1.01 þ 0.01
0.66 þ 0.01
8.67 þ 0.21 (8.61) 12.06 þ 0.06 (12.09) 10.61 þ 0.31 (10.96)
11.52 þ 0.19 (11.66) 12.62 þ 0.27 (12.92) 11.53 þ 0.51 (12.11)
10.03 þ 0.25 (10.05) 12.32 þ 0.17 (12.51) 11.07 þ 0.40 (11.52)
toluene-PVSK and ether/ethanol-PVSK over that of cells from cPVSK. The improvement comes mainly from the boost in Jsc, attributable to the enhanced light harvesting resulting from the more complete formation and larger sizes of perovskite crystals, which are in turn related to the porous structure of the PbX2 layer. As expected, the cells assembled from the ether/toluene-PVSK gives the highest Jsc and thus the highest PCE, because of the most effective light harvesting of the ether/toluene-PVSK as shown in Fig. 3b. Fig. 4b shows the incident photon-to-current efficiency (IPCE) spectra of the cells. The results are consistent with those of UVevisible absorption spectra, J-V curves, and Jsc, with the spectrum of the c-PVSK based cell lying consistently beneath those of ether/toluene-PVSK and ether/ethanol-PVSK based cells. The spectrum of the ether/toluene-PVSK based cell lies above those of the other two cells, with the highest IPCE reaching 75% and most IPCEs above 70% within the visible light region. The cell performances were further investigated through examining the dynamics of the photo-induced charge carriers with static and time-resolved photoluminescence (TR-PL) spectra of cPVSK, ether/toluene-PVSK, and ether/ethanol-PVSK. The PL of the
perovskite film will be quenched by trap states and defects. Fig. 5a shows the static PL spectra of the three PVSK films. The trend in PL intensity is in the order of ether/toluene-PVSK > ether/ethanolPVSK > c-PVSK, in consistence with the trends of IPCE and PCE. The non-radiative charge recombination is more effectively suppressed in ether/toluene-PVSK and ether/ethanol-PVSK than in c-PVSK. Fig. 5b shows the TR-PL spectra of the three PVSK films, with the determined charge carrier life time constants summarized in Table S1 for comparison. The TR-PL spectrum was fitted with a biexponential decay model, from which two charge carrier life time constants can be determined, with the longer one (t1) accounting for free carrier recombination and the shorter one (t2) for trap state quenching [40,41]. Evidently, the photo-induced charge carriers of the ether/toluene-PVSK film decay the slowest with a largest average charge carrier life time constant. The ether/ethanol-PVSK comes in next and the c-PVSK the worst, again consistent with the conclusion drawn from the IPCE spectra and PCE data. From the above results and discussion, one concludes that the mixture solvent pore generation treatment does improve the PCE of the resulting perovskite solar cells by 2.29%, from 10.03% for the c-
Fig. 5. (a) Static PL spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanol-PVSK films excited at 510 nm. (b) TR-PL spectra of c-PVSK, ether/toluene-PVSK, and ether/ethanolPVSK films excited at 375 nm.
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Table 2 Performance indicators of solar cells assembled from optimized ether/toluene-PVSK films.
100 C
Jsc (mA cm2)
Voc(V)
FF
Forward [%] (best)
Reverse [%] (best)
Avg PCE [%] (best)
19.28 þ 0.12
1.01 þ 0.01
0.66 þ 0.01
12.58 þ 0.34 (12.89)
13.25 þ 0.24 (13.43)
12.92 þ 0.28 (13.16)
PVSK based cell to 12.32% for the ether/toluene-PVSK based cell. To gain further improvements, the processing condition for the fabrication of the PbX2 layer was optimized. It was found that the annealing temperature of the PbX2 layer played the key role in constructing the optimal PbX2 layer for the ether/toluene-PVSK film that led to cells of the best performance. With an annealing temperature of 100 C for the fabrication of the PbX2 layer, the cells thus assembled achieved the highest average Jsc of 19.28 mA/cm2 and the highest PCE of 12.92%. The relevant cell performance indicators were summarized in Table 2. The champion cell exhibited an average Jsc of 19.26 mA/cm2, Voc of 1.01 V, FF of 0.68, and PCE of 13.16%, a boost of 3.13% in PCE than 10.03% of the control cell. Its J-V curve is shown in Fig. 6 with the performance indicators from both forward and reverse scans presented to illustrate its minimum hysteresis behavior. The hysteresis phenomenon is attributed to the existence of internal capacitive currents at the layer interfaces of the cell [42]. Minimum hysteresis may imply good layer contacts. The cross section SEM image of the champion cell is shown in Fig. 7. The thickness of the perovskite capping layer is 300 nm, which is generally accepted as the optimal thickness for the perovskite capping layer [37,43]. The perovskite capping layer should be thick
enough to effectively absorb incoming light, but thin enough to shorten the charge transport path for effective collection of the photo-induced charge carriers. The perovskite capping layer can also be observed to be a dense film composed of large size perovskite crystals, advantageous for light harvesting and charge transport. 4. Conclusion A simple and fast mixture solvent pore generation treatment was successfully developed to create uniform porous PbX2 layers, with which the contact area for the perovskite formation was significantly increased and the inner PbX2 can be readily accessed for perovskite formation. The PbX2 residue problem is thus much lessened, leading to more complete formation and larger sizes of perovskite crystals, from which the light harvesting and charge carrier transport were significantly improved to give perovskite cells of much better performances. The superior quality of the perovskite film, large grains, few defects, good crystallinity, and long charge carrier life times, derived from the mixture solvent treated PbX2 layer was investigated and confirmed with SEM, XRD, UVevisible, PL, and TR-PL characterizations. The champion cell thus obtained exhibited a Jsc of 19.26 mA/cm2, Voc of 1.01 V, FF of 0.68, and an average PCE of 13.16%, a 3.13% boost in PCE as compared with 10.03% of the control cell. Acknowledgements This work was financially supported by the Ministry of Science and Technology of Taiwan, ROC under grant MOST 105-2221-E007-126-MY2. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.09.161. References
Fig. 6. J-V curves of champion cell.
Fig. 7. Cross section SEM image of champion cell.
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