nanomaterials Article
Modified Sequential Deposition Route through Localized-Liquid-Liquid-Diffusion for Improved Perovskite Multi-Crystalline Thin Films with Micrometer-Scaled Grains for Solar Cells Tao Ling, Xiaoping Zou *, Jin Cheng, Xiao Bai, Haiyan Ren and Dan Chen Research Center for Sensor Technology, Beijing Key Laboratory for Sensor, Ministry of Education Key Laboratory for Modern Measurement and Control Technology, School of Applied Sciences, Beijing Information Science and Technology University, Jianxiangqiao Campus, Beijing 100101, China;
[email protected] (T.L.);
[email protected] (J.C.);
[email protected] (X.B.);
[email protected] (H.R.);
[email protected] (D.C.); * Correspondence:
[email protected]; Tel.: +86-136-4105-6404 Received: 13 May 2018; Accepted: 6 June 2018; Published: 9 June 2018
Abstract: High-class perovskite film with beautiful surface morphology (such as large-size grain, low defect density, good continuity and flatness) is normally believed to be a very important factor for high-efficiency perovskite solar cells (PSCs). Here, we report a modified sequential deposition route through localized-liquid-liquid-diffusion (LLLD) for qualified perovskite multi-crystalline thin films with micrometer-scaled grains for solar cells. We adopted a contact-type drop method to drop Methylammonium iodide (MAI) solution and have successfully used high-concentration MAI solution (73 mg/mL) to transform PbI2 film into high-class perovskite film via our route. A high efficiency of 10.7% was achieved for the device with spongy carbon film deposited on a separated FTO-substrate as a counter electrode under one sun illumination, which is the highest efficiency (as 2.5 times as previous efficiency) ever recorded in perovskite solar cells with a such spongy carbon/FTO composite counter electrode. The preparation techniques of high-class perovskite thin films under ambient conditions and the cheap spongy carbon/FTO composite counter electrode are beneficial for large-scale applications and commercialization. Keywords: localized-liquid-liquid-diffusion; modified sequential deposition route; perovskite thin film; solar cells
1. Introduction In recent years, perovskite solar cells (PSCs) have been recognized as the most promising alternative to conventional photovoltaic devices due to their high efficiency and simple process [1–3]. High-class perovskite thin film, with beautiful surface morphology (such as large-size grain, low defect density, good continuity, and flatness), is normally believed to be a very important factor for high-efficiency perovskite solar cells [4–8]. M. Grätzel et al. firstly developed the sequential deposition method (so-called two-step method) to prepare CH3 NH3 PbI3 perovskite thin films [9]. Since then, a lot of effort has been paid to developing the two-step method, similar to the two-step spin-coating process [10–12], and Xu et al. have demonstrated that the two-step spin coating method enables perovskite layer morphology to control and easily fabricate products [13]. Previous researchers prepared perovskite films via the conventional two-step coating method with a necessary step of PbI2 layer annealing [9–13]. In fact, we found the properties of the perovskite thin film are very sensitive to the annealing time of PbI2 layer especially under the dry ambient conditions used in our experiments.
Nanomaterials 2018, 8, 416; doi:10.3390/nano8060416
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film are very sensitive to the annealing time of PbI2 layer especially under the dry ambient conditions used in our experiments. Xu et study on the of humidity on the crystallization of two-stepofspin-coated Xu etal.’s al.’sprevious previous study on effect the effect of humidity on the crystallization two-step spin-coated suggested that appropriate moisture can facilitate reactionPbI between PbI2 perovskites perovskites suggested that appropriate moisture can facilitate the reactionthe between and MAX 2 (X = MAX I, Cl), (X whereas thewhereas PbI2 filmthe reacts slowly underslowly dry inert atmosphere [14]. H2 O helps and = I, Cl), PbI2with filmMAX reacts with aMAX a under dry inert atmosphere MAIHto2Openetrate into the thick into PbI2the to thick form PbI thick purewith MAPbI phase and produce [14]. helps MAI to penetrate 2 tofilm formwith thickafilm a pure MAPbI 3 phase and 3 produce bigger gains bydown slowing the perovskite crystallization rate [15].the However, the dry bigger gains by slowing the down perovskite crystallization rate [15]. However, dry atmosphere atmosphere (~10% relativewas humidity) was our experimental (~10% relative humidity) our experimental condition. condition. In ordertoto explore the annealing time of film PbI2onthin film on properties of In order explore the the effecteffect of theof annealing time of PbI properties of corresponding 2 thin ◦ perovskite thinperovskite film, we annealed PbI at 70 PbI C for 0 s,at1070s,°C and found that corresponding thin film, we annealed 2 film for200 s, s, respectively. 10 s, and 20 s,We respectively. 2 film We that the s annealing 2 film was more conducive to the formation offilms high-class the 0found s annealing PbI02 film was morePbI conducive to the formation of high-class perovskite in our perovskite in means our experiment, which means that the sequential routeconducive through LLLD experiment,films which that the sequential deposition route throughdeposition LLLD is more to the is more conducive to the formation of qualified perovskite multi-crystalline films. In addition,drop we formation of qualified perovskite multi-crystalline films. In addition, we adopted a contact-type adopted a contact-type drop method MAI to reduce of MAI method to drop MAI solution in ordertotodrop reduce thesolution damage in of order MAI solution to the PbI2damage thin liquid film solution to PbI 2 thin liquid (illustrated in Figure 1). film (illustrated in Figure 1).
Figure Figure 1. Schematic Schematic illustrations illustrations for for drop drop method method and and picture picture of of perovskite films: (a) (a) Schematic Schematic illustrations contact-type method methodand andnon-contact-type non-contact-typemethod methodtotodrop dropMAI MAI solution; Picture illustrations for contact-type solution; (b)(b) Picture of of perovskite films dropped contact-type method and non-contact-type method. perovskite films dropped by by contact-type method and non-contact-type method.
Previously, if MAI MAI concentration concentration is is too too high high ((≥70 mg/mL), most Previously, if ≥70 mg/mL), most of of the the PbI PbI22 precursor precursor gets gets 2− dissolved and exists as PbI 4 2−in the solution, and only very little can reprecipitate to form MAPbI3 dissolved and exists as PbI4 in the solution, and only very little can reprecipitate to form MAPbI3 nanostructures MAPbI3 cuboids increases with decreasing nanostructures [16]. [16]. Im Im et et al. al. found found that that the the size size of of the the MAPbI 3 cuboids increases with decreasing CH 3NH3I concentration [17]. However, the MAI concentration in our experiment was 73 mg/mL and CH3 NH3 I concentration [17]. However, the MAI concentration in our experiment was 73 mg/mL and the correspondingMAI MAIsolution solutionalso alsosuccessfully successfully transformed 2 film into high-class perovskite the corresponding transformed PbIPbI 2 film into high-class perovskite film film via LLLD. A high-class perovskite film with micrometer-scaled grain, density, low defect density, via LLLD. A high-class perovskite film with micrometer-scaled grain, low defect densification, densification, and flatness was obtained by our process. and flatness was obtained by our process. The The tune tune of of the the formation formation process process and and composition composition of of the the light-absorber light-absorber layer layer in in PSCs PSCs has has contributed to obtaining certified power conversion efficiencies (PCE) > 20% [18,19]. These contributed to obtaining certified power conversion efficiencies (PCE) > 20% [18,19]. These PCEsPCEs have have been obtained while perovskite solar cells were prepared using the single-step method (in been obtained while perovskite solar cells were prepared using the single-step method (in combination combination with solvent engineering) or sequential deposition method or vapor deposition with an with solvent engineering) or sequential deposition method or vapor deposition with an important important step of device fabrication being the thermal evaporation of metals to form the counter step of device fabrication being the thermal evaporation of metals to form the counter electrode electrode (CE). However, costmetallic of these metallic CEs is prohibitively high forapplications large-scale (CE). However, the cost ofthe these CEs is prohibitively high for large-scale applications and commercialization. and commercialization. Up to now, some researchers have developed carbon CEs to cut the costs of perovskite devices Up to now, some researchers have developed carbon CEs to cut the costs of perovskite [20–23]. Yang et al. prepared a 2.6%-efficient perovskite solar cell with candle soot film deposited on devices [20–23]. Yang et al. prepared a 2.6%-efficient perovskite solar cell with candle soot film adeposited separated substrate as the carbon/FTO composite counter electrode [22]. In our research group, on a separated substrate as the carbon/FTO composite counter electrode [22]. In our research previously, a spongy carbon/FTO composite structure was adopted as a counter electrode and the group, previously, a spongy carbon/FTO composite structure was adopted as a counter electrode corresponding cell achieved 4.24% power conversion efficiency (PCE) [23]. The perovskite solar cell and the corresponding cell achieved 4.24% power conversion efficiency (PCE) [23]. The perovskite was prepared via a modified sequential deposition route through LLLD under dry ambient solar cell was prepared via a modified sequential deposition route through LLLD under dry ambient conditions and got a 10.7% PCE in the size of 0.2 cm22, which is the highest efficiency ever recorded in conditions and got a 10.7% PCE in the size of 0.2 cm , which is the highest efficiency ever recorded in perovskite solar cells with such a spongy carbon/FTO composite counter electrode. perovskite solar cells with such a spongy carbon/FTO composite counter electrode.
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2. Materials and Methods 2.1. Materials Fluorine-doped SnO2 (FTO) substrates were obtained from Dalian HeptaChroma Solar Technology Development Corp (Dalian, China). N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Sa’en Chemical Technology Corp (Shanghai, China). PbI2 , 18NR-T TiO2 paste (mp-TiO2 ), acidic titanium dioxide solution (bl-TiO2 ), Methylammonium iodide (MAI), and isopropyl alcohol (IPA) were purchased from Shanghai MaterWin New Materials Corp, (Shanghai, China). Spiro-OMeTAD solution was purchased from Xi’an Polymer Light Technology Corp (Xi’an, China). 2.2. Device Fabrication Fluorine-doped SnO2 (FTO) substrates were ultrasonically cleaned with mixed solution (detergent and deionized water), glass water (acetone:deionized water:2-propanol = 1:1:1), and alcohol, respectively. Before using, the FTO was cleaned by Ultraviolet Ozone (UVO) for 90 min. Compact TiO2 layer (bl-TiO2 ) was deposited on FTO substrate by spin-coating an acidic titanium dioxide solution at 2000 rpm for 60 s. Then the substrate was annealed on a hotplate at 100 ◦ C for 10 min, and sintered in a muffle furnace at 500 ◦ C for 30 min. Mesoporous TiO2 film (mp-TiO2 ) was coated on bl-TiO2 thin film by spin coating using a commercial 18NR-T TiO2 paste (4%, solid content) at 2000 rpm for 30 s. Subsequently, the substrate was dried on a hotplate at 100 ◦ C for 10 min, and then sintered in a muffle furnace at 500 ◦ C for 60 min. After that, 599.3 mg of PbI2 was dissolved in a component solvent (DMF:DMSO = 9.5:0.5) and then spin-coated onto a TiO2 (bl-TiO2 and mp-TiO2 ) layer at 1500 rpm for 30 s, and then annealed on a hotplate at 70 ◦ C for 0 s, 10 s, and 20 s, respectively. After the PbI2 film had cooled down to room temperature, the MAI solution (73 mg in 1 mL IPA) was spin coated onto the PbI2 layer at 1500 rpm for 30 s (without loading time), and then a thermal annealing of 150 ◦ C for 15 min under low humidity air condition (~10% humidity) was processed. The hole transporting layer was deposited on top of the perovskite layer by 3000 rpm for 30 s using a commercial 2,20 ,7,70 -tetrakis(N,N-dip-methoxyphenylamine)-9,90 -spirobifluorene (Spiro-OMeTAD) solution. Finally, some cleaned FTO glasses were used as substrates to obtain the soot of a burning candle as spongy carbon CEs. The spongy carbon films on FTO glasses were then pressed on the as-prepared uncompleted devices. 2.3. Characterization An X-ray diffractometer (XRD) (Broker, D8 Focus, Beijing, China) was used to obtain XRD spectra from samples of perovskite films deposited on mp-TiO2 /bl-TiO2 /FTO substrates. The morphologies of these perovskite films were watched by a scanning electron microscope (SEM) (Zeiss SIGMA). The as-prepared devices were measured under full sun illumination (AM 1.5, 100 mW/cm2 ). 3. Results and Discussion In order to reduce the possible damage of the MAI solution to PbI2 thin liquid film, we adopted a contact-type drop method to drop MAI solution (illustrated in Figure 1a), and the resulting perovskite films are shown in Figure 1b. In contrast to the non-contact-type drop method, this drop method reduced the damage of the MAI solution to PbI2 thin film according to Figure 1b. The perovskite films were prepared by sequential deposition route with PbI2 0 s, 10 s, and 20 s annealing, which are named as sample A, sample B, and sample C, respectively. Scanning electron microscopy (SEM) images and image of perovskite layers prepared by two-step spin-coating with different PbI2 annealing times are shown in Figure 2. We found that the surfaces of the perovskite layers gradually became a white color with the increase of PbI2 annealing time, according to the image (Figure 2a) of the perovskite layers. From the surface SEM image (Figure 2b–d), it can be found that sample A showed obvious densification, sample B and C have double layers, and the bottom layer is dense and the top layer is loose. PbI2 solution film reacts with MAI solution to form a high-class
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bottom layer is dense and the top layer is loose. PbI2 solution film reacts with MAI solution to form a perovskite perovskite thin film with surface morphology (such as large-size low defect density, high-class thinbeautiful film with beautiful surface morphology (such grain, as large-size grain, low good continuity, and continuity, flatness) viaand LLLD, whereas, PbI2 solid film reacts with MAI solution to form defect density, good flatness) via LLLD, whereas, PbI2 solid film reacts with MAI loose perovskite via localized-solid-liquid-diffusion (LSLD). The bottom layers The of sample solution to form thin loosefilm perovskite thin film via localized-solid-liquid-diffusion (LSLD). bottomB and C are observed contain but thelarge-size top layer grain, of sample shows some of small square layers of sample B to and C arelarge-size observedgrain, to contain but Bthe top layer sample B holes and the top layer of sample C shows massive square unordered holes. shows some small square holes and the top layer of sample C shows massive square unordered holes.
Figure Figure 2. 2. SEM SEM images images and and picture: picture: (a) (a) Surface Surface picture picture of of perovskite perovskite films films prepared prepared by by sequential sequential deposition route with different PbI 2 annealing times (0 s, 10 s and 20 s; corresponding sample deposition route with different PbI2 annealing times (0 s, 10 s and 20 s; corresponding sample A, A, sample of of sample A, A, sample B and sample C; (e) sample BB and and sample sampleC); C);(b–d) (b–d)Surface SurfaceSEM SEMimages images sample sample B and sample C; Cross-sectional SEM image of sample A. A. (e) Cross-sectional SEM image of sample
In the cross-sectional SEM image (Figure 2e) of sample A, we can see the FTO layer, bl-TiO2 layer, In the cross-sectional SEM image (Figure 2e) of sample A, we can see the FTO layer, bl-TiO2 layer, mp-TiO2 layer and perovskite layer clearly from the bottom to the top, and the longitudinal size of mp-TiO2 layer and perovskite layer clearly from the bottom to the top, and the longitudinal size of perovskite grains equal to the film thickness. This qualified perovskite film (sample A) indicates that perovskite grains equal to the film thickness. This qualified perovskite film (sample A) indicates that the the modified sequential deposition route through LLLD is beneficial to the growth of perovskite crystals modified sequential deposition route through LLLD is beneficial to the growth of perovskite crystals under dry ambient condition and enhancement of the corresponding photovoltaic performance. under dry ambient condition and enhancement of the corresponding photovoltaic performance. Figure 3 shows XRD pattern of the samples prepared by two-step spin-coating method with Figure 3 shows XRD pattern of the samples prepared by two-step spin-coating method different PbI2 annealing times. According to the Figure 3, there are no peaks of PbI2 (corresponding with different PbI2 annealing times. According to the Figure 3, there are no peaks of PbI2 to ~26°) and MAI (corresponding to ~19°), which means there are no residuals of PbI2 and MAI in the (corresponding to ~26◦ ) and MAI (corresponding to ~19◦ ), which means there are no residuals of samples, and the compositions of perovskites in the samples are very pure. As the PbI2 annealing PbI2 and MAI in the samples, and the compositions of perovskites in the samples are very pure. As time increased to 10 s, the (004) peak of the tetragonal structure for CH3NH3PbI3 emerged, which the PbI2 annealing time increased to 10 s, the (004) peak of the tetragonal structure for CH3 NH3 PbI3 indicates a phase transition from cubic to tetragonal structure [24]. This result suggest that the emerged, which indicates a phase transition from cubic to tetragonal structure [24]. This result suggest perovskite in sample A and bottom layers of sample B and C is cubic structure, and the perovskite in that the perovskite in sample A and bottom layers of sample B and C is cubic structure, and the top layers of sample B and C is tetragonal structure. In addition, the intensity ratio (~2.06) of the perovskite in top layers of sample B and C is tetragonal structure. In addition, the intensity ratio highest peak (110) of sample A to its secondary peak (220) is lightly above that of others (~1.93 and (~2.06) of the highest peak (110) of sample A to its secondary peak (220) is lightly above that of others ~2.01). The above points means the crystal structure of sample A is better than others and sequential (~1.93 and ~2.01). The above points means the crystal structure of sample A is better than others and deposition route through LLLD is more propitious to the formation of qualified perovskite multi-crystalline films, which can enhance the corresponding device efficiency.
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sequential deposition route through LLLD is more propitious to the formation of qualified perovskite 5 of 8 the corresponding device efficiency. 5 of 8
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Figure 3. X-ray diffraction (XRD) patterns of perovskite films prepared by sequential deposition Figure 3. X-ray X-ray diffraction (XRD)patterns patterns perovskite films prepared by sequential deposition Figurewith 3. diffraction (XRD) perovskite films prepared by sequential deposition route route different PbI2 annealing times. of of route with different PbI 2 annealing times. with different PbI annealing times. 2
Schematic illustrations for LLLD and LSLD for qualified perovskite multi-crystalline film are Schematic LLLD and for qualified perovskite filmMAI are shown in Figureillustrations 4a. As PbI2 for annealing timeLSLD increases, its solid part and themulti-crystalline ratio of LLLD with Schematic illustrations for LLLD and LSLD for qualified perovskite multi-crystalline film are shown in Figure 4a. As PbI 2 annealing time increases, its solid part and the ratio of LLLD with MAI solution The in the morphology of theits perovskite have happened. Due to shown indecreases. Figure 4a. Aschanges PbI2 annealing time increases, solid part andfinally the ratio of LLLD with solution decreases. The changes in the morphology of MAI the perovskite have finally happened. Due to the 0 s annealing PbI 2 film was the solution state, and could more easily diffuse to the PbI 2 thin MAI solution decreases. The changes in the morphology of the perovskite have finally happened. the 0 interior s annealing PbI 2 film was the solution state, and MAI could more easily diffuse to the PbI2 thin film LLLD and PbI2solution film into a high-class perovskite Due to the 0 svia annealing PbI2transform film was the state, and MAI cubic could structure more easily diffuse tofilm. the film interior via LLLD and transform PbI 2 film into a high-class cubic structure perovskite film. Moreover, the PbI 2 thin film had some of the solvent of DMF, which could facilitate perovskite PbI2 thin film interior via LLLD and transform PbI2 film into a high-class cubic structure perovskite Moreover, the PbI2 thin film some of the DMF, defects, which could facilitate perovskite conversion, improve the filmhad morphology, andsolvent reduceofcrystal and thus enhance charge film. Moreover, the PbI 2 thin film had some of the solvent of DMF, which could facilitate perovskite conversion, improve theDue filmtomorphology, and reduce crystal defects, and thus enhancelayers charge transfer efficiency [25]. the annealing time being relatively short, there are double in conversion, improve the film morphology, and reduce crystal defects, and thus enhance charge transfer transfer efficiency [25]. Due to the annealing time being relatively short, there are double layers in the 10 s annealing PbI 2 film and 20 s annealing PbI2 film. The top layer is solid state, as a result of efficiency [25]. Due to the annealing time being relatively short, there are double layers in the 10 s the 10 s annealing 2 film and 20 s annealing PbI2 film. The top layer is solid state, as a result of contacting with airPbI during the spin-coating. MAI is difficult to diffuse to the solid top layer of the annealing PbI 2 film and 20 s annealing PbI2 film. The top layer is solid state, as a result of contacting contacting with air during the spin-coating. MAI is difficult to diffuse towith the asolid top layer of the PbI 2 thin film interior via LSLD transform into perovskite film with air during the spin-coating.and MAI is difficult to poor diffuse to the solid top layertetragonal of the PbI2structure. thin film PbI 2 thin film interior via LSLD and transform into poor perovskite film with a tetragonal structure. The bottom layer is in transform solution state having not made with air during the spin-coating, interior via LSLD and intofor poor perovskite filmcontact with a tetragonal structure. The bottom The bottom layerto is sample in solution statecan forbe having not made contact with aircubic during the spin-coating, which A, and transformed a high-class structure layer isisinsimilar solution state for having not made contact withinto air during the spin-coating, whichperovskite is similar which is similar to sample A, and can be transformed into a high-class cubic structure perovskite film. The hydrolysis of perovskite must be reduced because it was prepared in dry air. A to sample A, and can be transformed into a high-class cubic structure perovskite film. The hydrolysis film. The hydrolysis of perovskite must be reduced because it was prepared in dry air. A combination thebeabove factors leaditto theprepared high-class perovskite films beingoffabricated by the of perovskite of must reduced because was in dry air. A combination the above factors combination of the above lead to the high-class perovskite films and beingthe fabricated by sequential routefactors through LLLD dry ambient conditions structure of the the lead to the deposition high-class perovskite films beingunder fabricated by the sequential deposition route through sequential deposition route through LLLD under dry ambient conditions and the structure of the corresponding devices with the low-cost spongy carbon/FTO composite counter electrode is shown LLLD under dry ambient conditions and the structure of the corresponding devices with the low-cost corresponding with the low-cost spongy carbon/FTO composite counter electrode is shown in Figure 4b. devices spongy carbon/FTO composite counter electrode is shown in Figure 4b. in Figure 4b.
(a) (b) (a) (b) Figure 4. Schematic devices: (a) Schematic Figure 4. Schematic illustration illustration for for diffusion diffusion mechanisms mechanisms and and structure structure of of devices: (a) Schematic Figure 4. Schematic illustrationof for diffusion mechanisms and structure of devices: (a) Schematic illustration for the mechanisms LLLD and LSLD for perovskite multi-crystalline film; (b) illustration for the mechanisms of LLLD and LSLD for perovskite multi-crystalline film; (b) Structure Structure illustration for the mechanisms LLLD and LSLD for perovskiteelectrode. multi-crystalline film; (b) Structure of devices with with low-cost spongyofcarbon/FTO carbon/FTO composite of devices low-cost spongy composite counter counter electrode. of devices with low-cost spongy carbon/FTO composite counter electrode.
Figure 5a,b show the light current-voltage (J-V) curves from reverse scan (RS) for the devices 5a,b show the light electrode current-voltage (J-V)electrode, curves from reverseunder scan (RS) for the sunlight devices withFigure a carbon/FTO composite and gold measured simulated with a carbon/FTO composite electrode and gold electrode, measured under simulated sunlight with an intensity of 100 mW/cm2 (AM 1.5 G). According to the J-V curve, it is found that the device 2 (AM 1.5 G). According to the J-V curve, it is found that the device with an intensitytoofPbI 1002 mW/cm (corresponding 0 s annealing) via the modified sequential deposition route through LLLD (corresponding to PbI2 0 s annealing) via the modified sequential deposition route through LLLD
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Figure 5a,b show the light current-voltage (J-V) curves from reverse scan (RS) for the devices with a carbon/FTO composite electrode and gold electrode, measured under simulated sunlight 2 (AM 1.5 G). According to the J-V curve, it is found that the device Nanomaterials 2018, 8, xofFOR REVIEW 6 of 8 with an intensity 100PEER mW/cm (corresponding to PbI2 0 s annealing) via the modified sequential deposition route through LLLD showed in major major parameters parameters (such (such as as V Voc,, JJsc and PCE, and their detailed values showed the the best best performance performance in oc sc and PCE, and their detailed values can be seen in Table 1) and the others showed obvious attenuation in major parameters. sure can be seen in Table 1) and the others showed obvious attenuation in major parameters. ToTo bebe sure of of the efficacy of our new coating processes and the reliability of current density data, we measured the efficacy of our new coating processes and the reliability of current density data, we measured the the IPCE of the devices with a carbon/FTO composite electrode, and results shown in Figure IPCE of the devices with a carbon/FTO composite electrode, and thethe results areare shown in Figure 5c. 5c. Generally, the device performance is increased with the decrease of PbI 2 annealing times, Generally, the device performance is increased with the decrease of PbI2 annealing times, indicating indicating that the sequential routeLLLD through LLLD is more conducive to the formation of that the sequential depositiondeposition route through is more conducive to the formation of qualified qualified perovskite multi-crystalline films dryconditions. ambient conditions. perovskite multi-crystalline films under dryunder ambient 24
24
PbI2 0s annealing
PbI2 10s annealing
20
Current Density (mA/cm )
PbI2 20s annealing
PbI2 20s annealing
2
2
Current Density (mA/cm )
PbI2 0s annealing
PbI2 10s annealing
20
16
12
8
4
0 0.2
0.4
0.6
0.8
16
12
8
4
0 0.2
1.0
0.4
0.6
0.8
Voltage (v)
Voltage (v)
(a)
(b)
1.0
1.2
28
PbI2 0s annealing
100
PbI2 10s annealing
24
PbI2 20s annealing 20
12 40
2
16
60
Jsc（mA/cm ）
IPCE（%）
80
8 20
0
4
300
400
500
600
700
800
0
wavelengh (nm)
(c) Figure 5. J-V curves from reverse scan (RS) and monochromatic incident photon-to-electron Figure 5. J-V curves from reverse scan (RS) and monochromatic incident photon-to-electron conversion conversion efficiency (IPCE) of PSCs prepared deposition by sequential deposition routePbI with different PbI2 efficiency (IPCE) of PSCs prepared by sequential route with different 2 annealing times: annealing times: (a) J-V curves for the devices with carbon/FTO composite electrode; (b) J-V curves (a) J-V curves for the devices with carbon/FTO composite electrode; (b) J-V curves for the devices with for the devices with gold electrode; (c) IPCE of devices with carbon/FTO composite electrode. gold electrode; (c) IPCE of devices with carbon/FTO composite electrode.
According to the data of Table 1, it can be found that the photovoltaic performance of According to the data of Table 1, it can be found that the photovoltaic performance of contact-type contact-type processed PSCs is better than the non-contact-type processed PSCs. This further processed PSCs is better than the non-contact-type processed PSCs. This further validates the reduced validates the reduced damage of the MAI solution to the PbI2 thin film when dropping the MAI damage of the MAI solution to the PbI2 thin film when dropping the MAI solution via contact-type solution via contact-type method. A high efficiency of 10.7% was achieved for the device with the method. A high efficiency of 10.7% was achieved for the device with the carbon/FTO composite carbon/FTO composite electrode. Although this efficiency is not the highest in PSCs, it is the highest electrode. Although this efficiency is not the highest in PSCs, it is the highest efficiency (as 2.5 times efficiency (as 2.5 times as previous efficiency) ever recorded in PSCs with such a spongy carbon/FTO composite counter electrode. Furthermore, the preparation techniques of high-class perovskite thin films and the cheap spongy carbon/FTO composite counter electrode, under absolute ambient conditions, are beneficial for large-scale applications and commercialization.
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as previous efficiency) ever recorded in PSCs with such a spongy carbon/FTO composite counter electrode. Furthermore, the preparation techniques of high-class perovskite thin films and the cheap spongy carbon/FTO composite counter electrode, under absolute ambient conditions, are beneficial for large-scale applications and commercialization. Table 1. Parameters of contact-type or non-contact-type processed PSCs with carbon/FTO composite electrode or gold electrode prepared by sequential deposition route with different PbI2 annealing times under RS. T a (s)
0
DM b
EM c
non-contact-type
carbon/FTO
contact-type
(mA/cm−2 )
Voc e (v)
FF f
PCE g (%)
17.19
0.95
0.48
7.89
carbon/FTO
19.89
1.00
0.54
10.70
non-contact-type
gold
19.69
1.02
0.66
13.29
contact-type
gold
20.18
1.05
0.70
14.86
contact-type
carbon/FTO
17.49
0.89
0.49
7.64
contact-type
gold
19.11
1.04
0.64
12.73
contact-type
carbon/FTO
14.22
0.84
0.56
6.66
contact-type
gold
19.66
0.98
0.51
9.73
10
20
Jsc
d
a
T: PbI2 annealing times; b DM: Drop method of MAI solution; c EM: Electrode materials; d Jsc : Short-circuit photocurrent density; e Voc : Open-circuit voltage; f FF: Fill factor; g PCE: Power conversion efficiency.
4. Conclusions A modified sequential deposition route through LLLD was more conducive to the formation of qualified perovskite multi-crystalline thin films with micrometer-scaled grain, low defect density, densification, and flatness under dry ambient condition. The two diffusion mechanisms of LLLD and LSLD were suggested and preliminarily confirmed. A contact-type drop method to drop MAI solution can reduce the damage of MAI solution to PbI2 thin films, and high-concentration MAI solution can also transform PbI2 film into high-class perovskite film via our route. Moreover, a perovskite solar cell was prepared via the modified sequential deposition route through LLLD under absolute ambient conditions and got a 10.7% PCE in a size of 0.2 cm2 , which is the highest efficiency ever recorded in perovskite solar cells with such a spongy carbon/FTO composite counter electrode. Obviously, a great deal of production of high-efficiency perovskite solar cells, with a low-cost spongy carbon/FTO composite counter electrode, under an absolutely ambient atmosphere, is possible. Author Contributions: T.L. wrote the paper, designed the experiments. J.C. analyzed the data. X.B. and D.C. prepared the samples. H.R. performed all the measurements. X.Z. supervised the project. All authors commented and approved the paper. Funding: This research was funded by Project of Natural Science Foundation of China (61376057), Project of Natural Science Foundation of Beijing (Z160002), Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2016KF19) and Beijing Key Laboratory for Sensors of BISTU (KF20181077203). Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
Park, N.-G. Organometal perovskite light absorbers toward a 20% efficiency low-cost solid-state mesoscopic solar cell. J. Phys. Chem. Lett. 2013, 4, 2423–2429. [CrossRef] Yin, X.; Guo, Y.J.; Xue, Z.; Xu, P.; He, M.; Liu, B. Performance enhancement of perovskite-sensitized mesoscopic solar cells using Nb-doped TiO2 compact layer. Nano Res. 2015, 8, 1997–2003. [CrossRef] Yang, W.S.; Noh, J.H.; Jeon, J.N.; Kim, Y.C.; Ryu, S.C.; Seo, S.J.; Seok, S.I.I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234–1237. [CrossRef] [PubMed]
Nanomaterials 2018, 8, 416
4.
5.
6.
7.
8. 9.
10.
11.
12. 13.
14.
15. 16.
17. 18. 19.
20.
21.
22.
8 of 9
Stranks, S.D.; Eperon, G.E.; Grancini, G.; Menelaou, C.; Alcocer, M.J.P.; Leijtens, T.; Herz, L.M.; Petrozza, A.; Snaith, H.J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341–344. [CrossRef] [PubMed] Xing, G.; Mathews, N.; Sun, S.; Lim, S.S.; Lam, Y.M.; Gratzel, M.; Mhaisalkar, S.; Sum, T.C. Long-range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3 NH3 PbI3 . Science 2013, 342, 344–347. [CrossRef] [PubMed] Eperon, G.E.; Burlakov, V.M.; Docampo, P.; Goriely, A.; Snaith, H.J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151–157. [CrossRef] Chen, Z.; Li, H.; Tang, Y.; Huang, X.; Ho, D.; Lee, C.-S. Shape-Controlled Synthesis of Organolead Halide Perovskite Nanocrystals and Their Tunable Optical Absorption. Mater. Res. Express 2014, 1, 015034. [CrossRef] Yin, W.J.; Shi, T.; Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26, 4653–4658. [CrossRef] [PubMed] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M.K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316–319. [CrossRef] [PubMed] Xiao, Z.; Dong, Q.; Bi, C.; Shao, Y.; Yuan, Y.; Huang, J. Solvent Annealing of Perovskite-Induced Crystal Growth for Photovoltaic-Device Efficiency Enhancement. Adv. Mater. 2014, 26, 6503–6509. [CrossRef] [PubMed] Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J. Efficient, High Yield Perovskite Photovoltaic Devices Grown by Interdiffusion of Solution-Processed Precursor Stacking Layers. Energy Environ. Sci. 2014, 7, 2619–2623. [CrossRef] Chae, J.; Dong, Q.; Huang, J.; Centrone, A. Chloride Incorporation Process in CH3 NH3 PbI3-x Clx Perovskites via Nanoscale Bandgap Maps. Nano Lett. 2015, 15, 8114–8121. [CrossRef] [PubMed] Xu, Y.; Zhu, L.; Shi, J.; Lv, S.; Xu, X.; Xiao, J.; Dong, J.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. Efficient Hybrid Mesoscopic Solar Cells with Morphology-Controlled CH3 NH3 PbI3-x Clx Derived from Two-Step Spin Coating Method. ACS Appl. Mater. Interfaces 2015, 7, 2242. [CrossRef] [PubMed] Xu, Y.; Zhu, L.; Shi, J.; Xu, X.; Xiao, J.; Dong, J.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. The Effect of Humidity upon the Crystallization Process of Two-Step Spin-Coated Organic-Inorganic Perovskites. ChemPhysChem 2016, 17, 112–118. [CrossRef] [PubMed] Chiang, C.H.; Nazeeruddin, M.K.; Grätzel, M.; Wu, C.G. The synergistic effect of H2 O and DMF towards stable and 20% efficiency inverted perovskite solar cells. Energy Environ. Sci. 2017, 10, 808–817. [CrossRef] Fu, Y.; Meng, F.; Rowley, M.B.; Thompson, B.J.; Shearer, M.J.; Ma, D.; Hamers, R.J.; Wright, J.C.; Jin, S. Solution growth of single crystal methylammonium lead halide perovskite nanostructures for optoelectronic and photovoltaic applications. J. Am. Chem. Soc. 2015, 137, 5810–5818. [CrossRef] [PubMed] Im, J.H.; Jang, I.H.; Pellet, N.; Grätzel, M.; Park, N.G. Growth of CH3 NH3 PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. 2014, 9, 927–932. [CrossRef] [PubMed] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. [CrossRef] [PubMed] Yang, W.S.; Park, B.-W.; Jung, E.H.; Jeon, N.J.; Kim, Y.C.; Lee, D.U.; Shin, S.S.; Seo, J.; Kim, E.K.; Noh, J.H.; et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376–1379. [CrossRef] [PubMed] Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.G.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; et al. A holeconductor-free, fully printable mesoscopic perovskite solar cell with high stability. Science 2014, 345, 295–298. [CrossRef] [PubMed] Habisreutinger, S.N.; Leijtens, T.; Eperon, G.E.; Stranks, S.D.; Nicholas, R.J.; Snaith, H.J. Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells. Nano Lett. 2014, 14, 5561–5568. [CrossRef] [PubMed] Wei, Z.; Yan, K.; Chen, H.; Yi, Y.; Zhang, T.; Long, X.; Li, J.; Zhang, L.; Wang, J.; Yang, S. Cost-efficient clamping solar cells using candle soot for hole extraction from ambipolar perovskites. Energy Environ. Sci. 2014, 7, 3326–3333. [CrossRef]
Nanomaterials 2018, 8, 416
23. 24. 25.
9 of 9
Zhang, N.; Guo, Y.; Yin, X.; He, M.; Zou, X. Spongy carbon film deposited on a separated substrate as counter electrode for perovskite-based solar cell. Mater. Lett. 2016, 182, 248–252. [CrossRef] Luo, D.; Yu, L.; Wang, H.; Zou, T.; Luo, L.; Liu, Z.; Lu, Z. Cubic structure of the mixed halide perovskite CH3 NH3 PbI3-x Clx via thermal annealing. RSC Adv. 2015, 5, 85480–85485. [CrossRef] Wu, J.; Xu, X.; Zhao, Y.; Shi, J.; Xu, Y.; Luo, Y.; Li, D.; Wu, H.; Meng, Q. DMF as an additive in two-step spin coating method for 20% conversion efficiency perovskite solar cells. ACS Appl. Mater. Interfaces 2017, 9, 26937–26947. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
