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Effect of perovskite precursor ratios and solvents volume on the efficiency of MAPbI3-xClx mixed halide perovskite solar cells
Materials Science in Semiconductor Processing 109 (2020) 104915
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Effect of perovskite precursor ratios and solvents volume on the efficiency of MAPbI3-xClx mixed halide perovskite solar cells Hanadi Mehdi *, Asya Mhamdi, Abdelaziz Bouazizi � � Equipe Dispositifs Electroniques Organiques et Photovoltaïque Mol�eculaire, LMCN, Facult�e des Sciences de Monastir, Universit�e de Monastir, 5019, Monastir, Tunisia
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite material MAPbI3-xClx Optical properties Photovoltaic performance Chlorine content Solvents volume
The triiodide lead methylammonium MAPbI3 perovskite solar cells have attracted big attention especially when they were manufactured with different precursors mainly that based on lead chloride. In this report, the effects of the incorporation of PbCl2 into precursor solutions of the perovskite MAPbI3 on the microstructures, optical and photovoltaic properties were examined. The perovskite solar cells elaborated with the MAPbI3-xClx perovskite films treated by the optimal concentration of (MAI: PbCl2) has showed a preferred crystalline orientation and an improvement of the performances. Furthermore, the effect of different solvent volumes of (DMF: DMSO) on photovoltaic performance was investigated. The highest performance was obtained for the cell made from precursor solution dissolved in DMF resulting from the good interaction of the materials with the perovskite material.
1. Introduction The photovoltaic field has been disrupted during these last years by the development of a new technology: it is the solar cells based on the hybrid perovskite, which has exceeded a power conversion efficiency of 25.4% [1]. They have attracted significant interest due to their excellent optoelectronic properties including high absorption coefficient, high charge carrier mobilities, long charge carrier lifetimes, low exciton binding energy and strong photoluminescence efficiency [2–7]. Opti mizing the structure of devices and perovskite film deposition method ologies are the two important factors for device preparation procedure. In this context, many technologies and methods have been developed on different components of PSCs in order to improve the photovoltaic performance. Including solvent engineering [8,9], deposition methods [10–13], thermal annealing [14–17], optimization of electron transport layer [18–20], doping of the PbI2 inorganic precursor [21–23], optimi zation of hole transport layer [24–28] and the use of different archi tectures [29–32]. Specially, it has been observed that the efficiency of perovskite solar cells is strongly related to the crystalline structure of the perovskite absorber materials, which is mainly affected by the film growth pa rameters, such as the annealing treatment, the molar ratio and the concentration of precursor solutions. Different types of methyl ammonuim lead halide perovkite have been studied due to their
excellent electronic properties, such as MAPbI3 [33], MAPbBr3 [34], MAPbI3-xClx [35] and MAPbI3-xBrx [36]. The methylammonium lead iodide (MAPbI3) is the most used material in the photovoltaic field. Furthermore, the first notable performances in this domain were real ized by using this perovskite formula [37]. Unfortunately, the MAPbI3 suffers from structural instability that can affect the long-term stability of the device and limits its practical application [38,39]. To overcome this problem several efforts have been developed. In particular, it was found that the addition of chlorine in the formulation of MAPbI3 has demonstrated a higher PCE than based on MAPbI3 [40,41]. Indeed, organic solvent is one of the factors that can influence the microstructure properties of perovskite. In fact, the enhancement of the crystallization is mainly related also to the capacity of the interaction of the perovskite materials with the solvents [42–44]. Herein, we reported the elaboration of MAPbI3-xClx films by a simple one-step spin coating process. The effects of the incorporation of the PbCl2 by using a mixture solution of perovskite compounds on the mi crostructures and photovoltaic properties were investigated. As a first objective, four different perovskite precursors compositions of (MAI: PbCl2): (4:1), (3:1), (2:1), and (1:1) were used and the structural prop erties, optical properties and photovoltaic performances of the obtained perovskite films were studied. Secondly, another essential element of our research was the examination of the change of the organic solvent volumes (DMF: DMSO). For this study, we have used four organic
* Corresponding author. E-mail address: [email protected] (H. Mehdi). https://doi.org/10.1016/j.mssp.2020.104915 Received 19 October 2019; Received in revised form 2 January 2020; Accepted 3 January 2020 Available online 9 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) XRD patterns of MAPbI3-xClx films with different (MAI: PbCl2) ratio, (b) Zoom-in XRD patterns showing (220) lattice plane. (c UV–vis absorption spectra, (d) Steady-state photoluminescence (PL) and (e) MAPbI3-xClx samples with different (MAI: PbCl2) ratio.
solvent volumes of (DMF%: DMSO%): (100: 0), (80: 20), (50:50) and (0: 100).
2.2. Solar cells fabrication Planar heterojunction perovskite solar cells were prepared by using solution processing (spin coating), with the configuration of ITO/SnO2/ MAPbI3-xClx/P3HT/Au. ITO conducting glasses were cleaned by continuous ultrasonic treatment in acetone, isopropanol and desionized water for 10 min, respectively. Then, the substrates were treated with ultraviolet ozone for 30 min before use to fully remove the organic solvent residues. After that, an electron-transporting layer based on SnO2 was deposited by spin coating at 4000 rpm for 40 s. The films were then annealed at 200 � C for 20 mi. The prepared perovskite solutions were spin-coated on the top of the SnO2 layer at 4000 rpm for 40s. The transformation of the perovskite layer was completed by an annealing treatment at 100 � C for 30 min. After that, the hole-transport layer was deposited from the prepared P3HT solution onto the as-prepared perovskite layer at 2000 rpm for 30s. To finish, a gold electrode of 100 nm thickness was deposited as electrode through thermally evap oration under high vacuum.
2. Exprimental section 2.1. Materials and perovskite precursor solutions All materials were purchased from Sigma-Aldrich and were used as received without further purification: indium doped tin oxide (ITO) coated glass, methylamonuim iodide MAI (99.9% purity degree), lead chloride PbCl2 (99.9%, purity degree), dimethylformamide (DMF, anhydrous, 99%), dimethyl sulfoxide (DMSO, anhydrous, 99%), tin(IV) oxide nanoparticle ink(SnO2) and poly(3-hexylthiophene-2,5-diyl) (P3HT). The MAPbI3-xClx perovskite precursor solution was prepared with 0.9 M of PbCl2 and 4.8 M of MAI in different (DMF%: DMSO%) solvent volume and defined molar ratios of methylammonium iodine and lead chloride. The solutions were then stirred for overnight at room tem perature before used to ensure a complete dissolution. 2
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2.3. Device characterization The crystallinity of the perovskite film was measured by X-ray diffraction (XRD) using Co Κα X-ray radiation (λ ¼ 1.79 Å, 40 kV, 20 mA, Aeris PANAlytical, Benchtop X-ray diffractometer). The UV–vis absor bance spectrum was measured with a UV–visible spectrometer (PERKIN ELMER Lambda 35 spectrophotometer). Photoluminescence spectra have been performed with a “JOBIN YVON-SPEX Spectrum One” CCD detector, cooled at liquid nitrogen temperature. The photocurrentvoltage analyses were performed under AM 1.5 (100 mW/cm2) simu lated light radiation by Xe Oriel solar simulator with a Keithley 6430 source. 3. Results and discussion 3.1. Effect of incorporation of PbCl2 In this work, a one-step manufacturing protocol in an ambient at mosphere is used for the casting of perovskite films from different ratios of (MAI: PbCl2). We have subsequently studied the influence of the chlorine incorporation on the composition and the microstructure of perovskite.
Fig. 2. J-V characteristic of photovoltaic cells elaborated with different (MAI: PbCl2) ratio.
optical transition has been identified. The first is located around 765nm (�1.62 eV) was associated to the direct optical bandgap transition of the MAPbI3 [49]. The second transition localized around 490nm (�2.5 eV) is associated to the direct optical transition of the lead-iodine octahedra [50]. As it can be seen, the molar ratio (3: 1) presents the optimal ratio which has the high absorption intensity resulting from the few defects in the film and an improved of the crystallinity film [51,52]. Contrarily, when more chlorine was employed, the absorbance has decreased, which might due to a reducing in the film phase purity caused by the increase of the number of the defect. A previous work did by Yunlong Li has showed that the variation of the optical gap is related to the inter calation of the chlorine in the MAPbI3 material. They have showed that the MAPbI3-xClx optical band gap has been red-shifted with the increase of the chlorine level [47]. Unfortunately, in this work the evolution of the MAPbI3 optical band gap with the incorporation of the chlorine does not appear clearly with the UV–visible absorption spectroscopy, so in order to obtain a more adequate evolution of the band gap we have followed it with the photoluminescence spectroscopy. Fig. 1d shows the steady-state PL emission spectra of the perovskite films with different molar ratio (1: 1), (2: 1), (3: 1) and the (4: 1) be tween 600 nm and 900 nm. A blue shift was observed with the increase of the chlorine incorporation, which is consistent with previous report [53,54]. This can be explained by the fact that the incorporation of the chlorine can affect the valence and the conduction band position by reducing the lattice symmetry and thus increasing the band gap of the material [48]. The PL intensity has been increased and then decreased with the increase of the PbCl2 content, which indicate a variation of the films crystallinity. The high peak emission intensity of the MAPbI3-xClx perovskite films with the (3:1) molar ratio is a proof of the good crys tallinity of the perovskite film. Moreover, this can be attributed to the few numbers of crystal defects, which lead to the diminution of the non-radiative recombination pathways.
3.1.1. XRD diffraction To better understand the effects of the addition of PbCl2 on the crystalline structure of the perovskite material, X-ray diffraction mea surement (XRD) were done. The XRD patterns of MAPbI3-xClx thin films deposited on the ITO/SnO2 substrates, with different molar ratio of (MAI: PbCl2) are shown in Fig. 1a. These results in demonstrate the obvious presence of two crystalline phases of perovskites: the main one is expected to the tetragonal phase of MAPbI3 and the second one to the cubic phase of MAPbCl3. The existence of these two phases is not sur prising if we consider the nature of the precursors used. The main characteristic peaks located at 17.11� , 33.98� and 37.96� are assigned to the (110), (220) and (310) planes corresponding to a tetragonal perovskite structure [45]. We reveal that the presence of this additional chlorinated phase is specific to the N layer (SnO2) used, since the pres ence of this phase has not been observed on a structure using an N layer of mesoporous TiO2 [46]. We can reveal from fig.1b that the presence of this second chlorinated perovskite is highly depending on the molar ratio used (MAI: PbCl2) during the manufacture of the precursor solu tion. The significant increase of the chlorine phase for the ratio (3: 1) may suggest the idea that the chlorine has enriched the phase contrib uting to the improvement of crystallinity [47]. A small shift it can be seen with the increase of the chlorine content (Fig. 1b), which is in good agreement with a previous study [48]. Furthermore, more the precursor of PbCl2 increase more the XRD diffraction diagrams shifted to a smaller angle of MAPbI3, which suggest the incorporation of a certain amount of Cl into the MAPbI3 crystal lattice. The enlargement of the X-ray diffraction patterns at 2ϴ of 33.98� presented in Fig. 1b shows that in the presence of a small percentage of chlorine, the MAPbI3-xClx crystals have a strong preferred orientation on the tetragonal T (220) and T (004) facets. For a large doping of chlorine, the two tetragonal facets are gradually fused into a single peak corre sponding to the C (200) facet in the cubic phase. We can also noticed the presence of the diffraction peak of PbCl2 at 18.89� and 40.70� in the XRD patterns of the (4:1) and (1:1) ratios, which indicate the incomplete reaction of PbCl2 with the methylammonium cation.
3.1.3. Photovoltaic performance To analyze the effect of the chlorine inclusion on the charge transport and the PSCs efficiency, a J-V measure was made. Fig. 2 shows the Table 1 Device parameters of the MAPbI3-xClx perovskite solar cells with various con tents of PbCl2 under AM 1.5G illumination.
3.1.2. UV–vis spectra and photoluminescence properties To report the effect of the doping ratio of chlorine on the optical properties, a series of MAPbI3-xClx perovskites were synthesized with different (MAI: PbCl2) ratio (1:1), (2:1), (3:1) and (4:1). Fig. 1c shows the UV–Vis absorption spectra of the perovskite films, which each 3
Ratio (MAI: PbCl2)
Voc(mV)
Jsc(mA/cm2)
FF(%)
PCE(%)
(1:1) (2:1) (3:1) (4:1)
0.78 0.61 0.82 0.85
5.95 15.24 18.98 14.20
41.49 34.80 52.98 49.08
2.94 4.26 9.30 6.94
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Fig. 3. (a) XRD patterns of CH3NH3PbI3-X ClX perovskite films prepared using different volume solvents; (*) pic attributed to ITO, (b) UV–vis spectra of MAPbI3-xClx films and (c) Photoluminescence spectra.
photocurrent density–voltage (J–V) curves of the as-prepared PSCs, elaborated from different (MAI: PbCl2) molar ratio. All the solar cells parameters are summarized in Table 1. The results demonstrate that there is an optimal concentration of chlorine incorporation in the MAPbI3 perovskite phase. Furthermore, the presence of chlorine in the MAPbI3 phase is beneficial, since it allows for preferential crystalline growth, which results in the longer mobilities and longer life of the charge carriers [55]. The highest efficiency was obtained for MAP bI3-xClx photovoltaic devices with (3:1) molar ratio with a PCE of 9.30%, a fill factor (FF) of 51.98%, a short-circuit current density (JSC) of 18.98 mA/cm2, and an open-circuit voltage (VOC) of 0.82V. The increase of the VOC and FF is due to the improvement of the crystallinity of the perov skite film and the passivation of PbI2 [54,56], that can help to extract the charges carriers from the perovskite. The Enhancement of JSC is attrib uted to the reduced recombination in perovskite film [57]. All the characterizations have demonstrated the presence of an optimal level of chlorine incorporation in the MAPbI3 formulation. We sought then to improve the performance of the devices treated with the molar ratio (3: 1) of the precursors by varying the solvent volume. Four organic solvent volumes of (DMF%: DMSO%): (100: 0), (80: 20), (50: 50) and (0: 100) have been investigated.
presented in Fig. 3a. All the films exhibit the typical diffraction peaks of the tetragonal perovskite phase with diffraction peaks at 17.08� , 18.83� , 33.98� and 37.65� , assigned to the (110), (100), (220), and (310) planes, respectively [45,46]. It is clearly demonstrated that the change of the solvent volume between the DMF and the DMSO have caused the change of the crystallinity of the films. We note the disappearance of some diffraction peak with the increase of the DMSO volume which indicates a decrease in the film crystallinity. The disappearance of the peak of PbI2 and MAPbCl3 peaks is due to the weak interaction of PbI2 and MAPbCl3 with DMSO. The higher peak intensity of the film elaborated with pure DMF may be attributed to a complete crystallization of the perovskite film. 3.2.2. Optical band gap and photoluminescence To further elucidate the absorption capacity of those perovskite layers pretreatment by different (DMF: DMSO) solvents volumes, UV–visible absorption spectra were employed and displayed in Fig. 3b. The absorption coefficient of the film prepared by using DMF as a sol vent is higher than the other samples, due to the better interaction of the perovskite material with the solvent and the high crystallinity of the film which is confirmed previously by the XRD measurements. It can be also explained by the smallest viscosity of the DMF compared to the largest viscosity of the DMSO which led to unsuccessful infiltration of perov skite with the DMSO solvent [42]. We can also notice that the peak around 490 related to the PbI2 have been decreased with the increase of the volume of DMSO, this behavior can be explained by the low inter action of the PbI2 with the DMSO.
3.2. Solvent effect 3.2.1. X-ray analyses The X-ray patterns of the perovskite films coated on the top of the SnO2 layers with different solvent volume of (DMF%: DMSO%) were 4
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Fig. 5. J-V plots of best solar cell using the with forward and reverse scan.
Fig. 4. Photovoltaic characteristics of the PSCs with different solvent volume of (DMF %: DMSO%).
Table 3 Photovoltaic performance of PSC based on MAPbI3-xClx with forward and reverse scan.
Table 2 Photovoltaic performance of PSCs based on different solvent volume (DMF%: DMSO%). Solvent volume (DMF%: DMSO%)
Voc(V)
Jsc(mA/cm2)
FF(%)
PCE(%)
(100%:0%) (80%:20%) (50%:50%) (0%:100%)
0.82 0.78 0.79 0.65
18.98 18.26 18.56 19.03
52.98 49.56 43.84 41.79
9.30 7.08 6.41 5.18
Scan
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Forward Reverse
0.82 0.80
18.98 18.90
52.98 50.90
9.30 9.10
the extracted parameters are summarized in Table 3. This cell achieves a PCE of 9.3% with a Voc of 0.82 V, a Jsc of 18.98 mA/cm2, and a FF of 52.98% under forward voltage scan. For the reverse scan a slight lower PCE has been seen to achieve 9.1% with a Voc of 0.80V, a Jsc of 18.90 mA/cm2, and an FF of 50.90%. A slight inverted hysteresis phenomenon was seen which is attributed to various physical causes such as the ionic accumulation or the nature of the interface between the perovskite and charge transport layers such as the electron transport layer (ETL) and hole transport layer (HTL) [60–62]. Indeed, in the mixed interfaces perovskite/ETL and perovskite/HTL it can be formed an energy extraction barrier due to the imbalance in their alignment of the energy level. Therefore, the photogenerated charges are piling up at the barrier. This fact reduces the charge transfer and the integrated potential, resulting in subsequent charge recombination which decreases the VOC and the JSC. Moreover, ionic accumulation by external polarization at the charge extraction interfaces may also generate barriers that lead to the accumulation of traps at the ETL/perovskite and HTL/perovskite interfaces which result in inverted hysteresis.
The change in organic solvent volume does not only affect the light absorption but also the charge transfer. Hence, we studied the impact of the organic solvent on the charge carrier transfer. Fig. 3c shows the PL emission spectra of MAPbI3-xClx with different (DMF%: DMSO %) organic solvents volume. All the PL spectra show an emission peak around 770nm. The emission intensity changes with the variation of the solvent volume. The sample elaborated from the solution prepared with DMF has the lowest emission intensity, indicating the low charge carrier recombination and the highest separation efficiency of the charge car rier with the fastest migration process. While the perovskite film with the DMSO solvent showed the highest PL intensity, indicating the high surface defects due to the poor infiltration of the perovskite leading to a high charge recombination [58]. 3.2.3. Photovoltaic performance J–V characteristics of the PSCs with the different solvent volume were investigated. Fig. 4 displays the photocurrent-voltage (J-V) curve measured in the forward voltage sweep direction under simulated AM 1.5 sunlight conditions, and the detailed J-V parameters are listed in Table .2. The performance of the PSCs based on the pure DMF exhibit the highest performance with around 9.3%, a VOC, JSC and FF of 0.82 V, 18.98 mA/cm2, 52.98%, and 9.3%, respectively. With the increase of the DMSO volume from 20% to 100%, the PCE has decreased from 7.08% to 5.18%. It can be seen that the change in the solvent volume did not cause a large change in the short-circuit current, (JSC) but rather caused a remarkable change in the open-circuit voltage (Voc) and fill factor (FF). The improvement in FF and VOC could be attributed to the enhancement of the charge carrier’s extraction due to the low interface loss and the good quality of the perovskite film which was well confirmed by the PL measurement [58,59].
4. Conclusion In summary, the effect of PbCl2 incorporation on the crystallinity and the photovoltaic properties of MAPbI3-x Clx perovskite solar cells have been investigated. We have shown that the incorporation of chloride in perovskite has a beneficial effect that allows to obtaining a high quality of MAPbI3-xClx films. Indeed, we have highlighted the presence of an optimal rate of chlorine concentration in this phase, which corresponds to the optimal PV performance. The electronic and structural properties of the perovskite film were improved by the addition of a small amount of PbCl2. This fact has induced the improvement of the perovskite solar cells efficiency. In another part, the solvent effect was investigated in this report. The influence of the organic solvent volume (DMF: DMSO) has been studied. It was seen that the use of the DMF has improved the crystallinity of the film, which results from the good dissociation of the materials and their good interaction with the solvent.
3.3. Hysteresis effect Fig. 5 shows the J-V plots of the optimum cell, elaborated with the molar ratio (3: 1) measured in forward and reverse scan directions and 5
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Declaration of competing interest
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6
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Effect of perovskite precursor ratios and solvents volume on the efficiency of MAPbI3-xClx mixed halide perovskite solar cells Hanadi Mehdi *, Asya Mhamdi, Abdelaziz Bouazizi � � Equipe Dispositifs Electroniques Organiques et Photovoltaïque Mol�eculaire, LMCN, Facult�e des Sciences de Monastir, Universit�e de Monastir, 5019, Monastir, Tunisia
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite material MAPbI3-xClx Optical properties Photovoltaic performance Chlorine content Solvents volume
The triiodide lead methylammonium MAPbI3 perovskite solar cells have attracted big attention especially when they were manufactured with different precursors mainly that based on lead chloride. In this report, the effects of the incorporation of PbCl2 into precursor solutions of the perovskite MAPbI3 on the microstructures, optical and photovoltaic properties were examined. The perovskite solar cells elaborated with the MAPbI3-xClx perovskite films treated by the optimal concentration of (MAI: PbCl2) has showed a preferred crystalline orientation and an improvement of the performances. Furthermore, the effect of different solvent volumes of (DMF: DMSO) on photovoltaic performance was investigated. The highest performance was obtained for the cell made from precursor solution dissolved in DMF resulting from the good interaction of the materials with the perovskite material.
1. Introduction The photovoltaic field has been disrupted during these last years by the development of a new technology: it is the solar cells based on the hybrid perovskite, which has exceeded a power conversion efficiency of 25.4% [1]. They have attracted significant interest due to their excellent optoelectronic properties including high absorption coefficient, high charge carrier mobilities, long charge carrier lifetimes, low exciton binding energy and strong photoluminescence efficiency [2–7]. Opti mizing the structure of devices and perovskite film deposition method ologies are the two important factors for device preparation procedure. In this context, many technologies and methods have been developed on different components of PSCs in order to improve the photovoltaic performance. Including solvent engineering [8,9], deposition methods [10–13], thermal annealing [14–17], optimization of electron transport layer [18–20], doping of the PbI2 inorganic precursor [21–23], optimi zation of hole transport layer [24–28] and the use of different archi tectures [29–32]. Specially, it has been observed that the efficiency of perovskite solar cells is strongly related to the crystalline structure of the perovskite absorber materials, which is mainly affected by the film growth pa rameters, such as the annealing treatment, the molar ratio and the concentration of precursor solutions. Different types of methyl ammonuim lead halide perovkite have been studied due to their
excellent electronic properties, such as MAPbI3 [33], MAPbBr3 [34], MAPbI3-xClx [35] and MAPbI3-xBrx [36]. The methylammonium lead iodide (MAPbI3) is the most used material in the photovoltaic field. Furthermore, the first notable performances in this domain were real ized by using this perovskite formula [37]. Unfortunately, the MAPbI3 suffers from structural instability that can affect the long-term stability of the device and limits its practical application [38,39]. To overcome this problem several efforts have been developed. In particular, it was found that the addition of chlorine in the formulation of MAPbI3 has demonstrated a higher PCE than based on MAPbI3 [40,41]. Indeed, organic solvent is one of the factors that can influence the microstructure properties of perovskite. In fact, the enhancement of the crystallization is mainly related also to the capacity of the interaction of the perovskite materials with the solvents [42–44]. Herein, we reported the elaboration of MAPbI3-xClx films by a simple one-step spin coating process. The effects of the incorporation of the PbCl2 by using a mixture solution of perovskite compounds on the mi crostructures and photovoltaic properties were investigated. As a first objective, four different perovskite precursors compositions of (MAI: PbCl2): (4:1), (3:1), (2:1), and (1:1) were used and the structural prop erties, optical properties and photovoltaic performances of the obtained perovskite films were studied. Secondly, another essential element of our research was the examination of the change of the organic solvent volumes (DMF: DMSO). For this study, we have used four organic
* Corresponding author. E-mail address: [email protected] (H. Mehdi). https://doi.org/10.1016/j.mssp.2020.104915 Received 19 October 2019; Received in revised form 2 January 2020; Accepted 3 January 2020 Available online 9 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) XRD patterns of MAPbI3-xClx films with different (MAI: PbCl2) ratio, (b) Zoom-in XRD patterns showing (220) lattice plane. (c UV–vis absorption spectra, (d) Steady-state photoluminescence (PL) and (e) MAPbI3-xClx samples with different (MAI: PbCl2) ratio.
solvent volumes of (DMF%: DMSO%): (100: 0), (80: 20), (50:50) and (0: 100).
2.2. Solar cells fabrication Planar heterojunction perovskite solar cells were prepared by using solution processing (spin coating), with the configuration of ITO/SnO2/ MAPbI3-xClx/P3HT/Au. ITO conducting glasses were cleaned by continuous ultrasonic treatment in acetone, isopropanol and desionized water for 10 min, respectively. Then, the substrates were treated with ultraviolet ozone for 30 min before use to fully remove the organic solvent residues. After that, an electron-transporting layer based on SnO2 was deposited by spin coating at 4000 rpm for 40 s. The films were then annealed at 200 � C for 20 mi. The prepared perovskite solutions were spin-coated on the top of the SnO2 layer at 4000 rpm for 40s. The transformation of the perovskite layer was completed by an annealing treatment at 100 � C for 30 min. After that, the hole-transport layer was deposited from the prepared P3HT solution onto the as-prepared perovskite layer at 2000 rpm for 30s. To finish, a gold electrode of 100 nm thickness was deposited as electrode through thermally evap oration under high vacuum.
2. Exprimental section 2.1. Materials and perovskite precursor solutions All materials were purchased from Sigma-Aldrich and were used as received without further purification: indium doped tin oxide (ITO) coated glass, methylamonuim iodide MAI (99.9% purity degree), lead chloride PbCl2 (99.9%, purity degree), dimethylformamide (DMF, anhydrous, 99%), dimethyl sulfoxide (DMSO, anhydrous, 99%), tin(IV) oxide nanoparticle ink(SnO2) and poly(3-hexylthiophene-2,5-diyl) (P3HT). The MAPbI3-xClx perovskite precursor solution was prepared with 0.9 M of PbCl2 and 4.8 M of MAI in different (DMF%: DMSO%) solvent volume and defined molar ratios of methylammonium iodine and lead chloride. The solutions were then stirred for overnight at room tem perature before used to ensure a complete dissolution. 2
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2.3. Device characterization The crystallinity of the perovskite film was measured by X-ray diffraction (XRD) using Co Κα X-ray radiation (λ ¼ 1.79 Å, 40 kV, 20 mA, Aeris PANAlytical, Benchtop X-ray diffractometer). The UV–vis absor bance spectrum was measured with a UV–visible spectrometer (PERKIN ELMER Lambda 35 spectrophotometer). Photoluminescence spectra have been performed with a “JOBIN YVON-SPEX Spectrum One” CCD detector, cooled at liquid nitrogen temperature. The photocurrentvoltage analyses were performed under AM 1.5 (100 mW/cm2) simu lated light radiation by Xe Oriel solar simulator with a Keithley 6430 source. 3. Results and discussion 3.1. Effect of incorporation of PbCl2 In this work, a one-step manufacturing protocol in an ambient at mosphere is used for the casting of perovskite films from different ratios of (MAI: PbCl2). We have subsequently studied the influence of the chlorine incorporation on the composition and the microstructure of perovskite.
Fig. 2. J-V characteristic of photovoltaic cells elaborated with different (MAI: PbCl2) ratio.
optical transition has been identified. The first is located around 765nm (�1.62 eV) was associated to the direct optical bandgap transition of the MAPbI3 [49]. The second transition localized around 490nm (�2.5 eV) is associated to the direct optical transition of the lead-iodine octahedra [50]. As it can be seen, the molar ratio (3: 1) presents the optimal ratio which has the high absorption intensity resulting from the few defects in the film and an improved of the crystallinity film [51,52]. Contrarily, when more chlorine was employed, the absorbance has decreased, which might due to a reducing in the film phase purity caused by the increase of the number of the defect. A previous work did by Yunlong Li has showed that the variation of the optical gap is related to the inter calation of the chlorine in the MAPbI3 material. They have showed that the MAPbI3-xClx optical band gap has been red-shifted with the increase of the chlorine level [47]. Unfortunately, in this work the evolution of the MAPbI3 optical band gap with the incorporation of the chlorine does not appear clearly with the UV–visible absorption spectroscopy, so in order to obtain a more adequate evolution of the band gap we have followed it with the photoluminescence spectroscopy. Fig. 1d shows the steady-state PL emission spectra of the perovskite films with different molar ratio (1: 1), (2: 1), (3: 1) and the (4: 1) be tween 600 nm and 900 nm. A blue shift was observed with the increase of the chlorine incorporation, which is consistent with previous report [53,54]. This can be explained by the fact that the incorporation of the chlorine can affect the valence and the conduction band position by reducing the lattice symmetry and thus increasing the band gap of the material [48]. The PL intensity has been increased and then decreased with the increase of the PbCl2 content, which indicate a variation of the films crystallinity. The high peak emission intensity of the MAPbI3-xClx perovskite films with the (3:1) molar ratio is a proof of the good crys tallinity of the perovskite film. Moreover, this can be attributed to the few numbers of crystal defects, which lead to the diminution of the non-radiative recombination pathways.
3.1.1. XRD diffraction To better understand the effects of the addition of PbCl2 on the crystalline structure of the perovskite material, X-ray diffraction mea surement (XRD) were done. The XRD patterns of MAPbI3-xClx thin films deposited on the ITO/SnO2 substrates, with different molar ratio of (MAI: PbCl2) are shown in Fig. 1a. These results in demonstrate the obvious presence of two crystalline phases of perovskites: the main one is expected to the tetragonal phase of MAPbI3 and the second one to the cubic phase of MAPbCl3. The existence of these two phases is not sur prising if we consider the nature of the precursors used. The main characteristic peaks located at 17.11� , 33.98� and 37.96� are assigned to the (110), (220) and (310) planes corresponding to a tetragonal perovskite structure [45]. We reveal that the presence of this additional chlorinated phase is specific to the N layer (SnO2) used, since the pres ence of this phase has not been observed on a structure using an N layer of mesoporous TiO2 [46]. We can reveal from fig.1b that the presence of this second chlorinated perovskite is highly depending on the molar ratio used (MAI: PbCl2) during the manufacture of the precursor solu tion. The significant increase of the chlorine phase for the ratio (3: 1) may suggest the idea that the chlorine has enriched the phase contrib uting to the improvement of crystallinity [47]. A small shift it can be seen with the increase of the chlorine content (Fig. 1b), which is in good agreement with a previous study [48]. Furthermore, more the precursor of PbCl2 increase more the XRD diffraction diagrams shifted to a smaller angle of MAPbI3, which suggest the incorporation of a certain amount of Cl into the MAPbI3 crystal lattice. The enlargement of the X-ray diffraction patterns at 2ϴ of 33.98� presented in Fig. 1b shows that in the presence of a small percentage of chlorine, the MAPbI3-xClx crystals have a strong preferred orientation on the tetragonal T (220) and T (004) facets. For a large doping of chlorine, the two tetragonal facets are gradually fused into a single peak corre sponding to the C (200) facet in the cubic phase. We can also noticed the presence of the diffraction peak of PbCl2 at 18.89� and 40.70� in the XRD patterns of the (4:1) and (1:1) ratios, which indicate the incomplete reaction of PbCl2 with the methylammonium cation.
3.1.3. Photovoltaic performance To analyze the effect of the chlorine inclusion on the charge transport and the PSCs efficiency, a J-V measure was made. Fig. 2 shows the Table 1 Device parameters of the MAPbI3-xClx perovskite solar cells with various con tents of PbCl2 under AM 1.5G illumination.
3.1.2. UV–vis spectra and photoluminescence properties To report the effect of the doping ratio of chlorine on the optical properties, a series of MAPbI3-xClx perovskites were synthesized with different (MAI: PbCl2) ratio (1:1), (2:1), (3:1) and (4:1). Fig. 1c shows the UV–Vis absorption spectra of the perovskite films, which each 3
Ratio (MAI: PbCl2)
Voc(mV)
Jsc(mA/cm2)
FF(%)
PCE(%)
(1:1) (2:1) (3:1) (4:1)
0.78 0.61 0.82 0.85
5.95 15.24 18.98 14.20
41.49 34.80 52.98 49.08
2.94 4.26 9.30 6.94
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Fig. 3. (a) XRD patterns of CH3NH3PbI3-X ClX perovskite films prepared using different volume solvents; (*) pic attributed to ITO, (b) UV–vis spectra of MAPbI3-xClx films and (c) Photoluminescence spectra.
photocurrent density–voltage (J–V) curves of the as-prepared PSCs, elaborated from different (MAI: PbCl2) molar ratio. All the solar cells parameters are summarized in Table 1. The results demonstrate that there is an optimal concentration of chlorine incorporation in the MAPbI3 perovskite phase. Furthermore, the presence of chlorine in the MAPbI3 phase is beneficial, since it allows for preferential crystalline growth, which results in the longer mobilities and longer life of the charge carriers [55]. The highest efficiency was obtained for MAP bI3-xClx photovoltaic devices with (3:1) molar ratio with a PCE of 9.30%, a fill factor (FF) of 51.98%, a short-circuit current density (JSC) of 18.98 mA/cm2, and an open-circuit voltage (VOC) of 0.82V. The increase of the VOC and FF is due to the improvement of the crystallinity of the perov skite film and the passivation of PbI2 [54,56], that can help to extract the charges carriers from the perovskite. The Enhancement of JSC is attrib uted to the reduced recombination in perovskite film [57]. All the characterizations have demonstrated the presence of an optimal level of chlorine incorporation in the MAPbI3 formulation. We sought then to improve the performance of the devices treated with the molar ratio (3: 1) of the precursors by varying the solvent volume. Four organic solvent volumes of (DMF%: DMSO%): (100: 0), (80: 20), (50: 50) and (0: 100) have been investigated.
presented in Fig. 3a. All the films exhibit the typical diffraction peaks of the tetragonal perovskite phase with diffraction peaks at 17.08� , 18.83� , 33.98� and 37.65� , assigned to the (110), (100), (220), and (310) planes, respectively [45,46]. It is clearly demonstrated that the change of the solvent volume between the DMF and the DMSO have caused the change of the crystallinity of the films. We note the disappearance of some diffraction peak with the increase of the DMSO volume which indicates a decrease in the film crystallinity. The disappearance of the peak of PbI2 and MAPbCl3 peaks is due to the weak interaction of PbI2 and MAPbCl3 with DMSO. The higher peak intensity of the film elaborated with pure DMF may be attributed to a complete crystallization of the perovskite film. 3.2.2. Optical band gap and photoluminescence To further elucidate the absorption capacity of those perovskite layers pretreatment by different (DMF: DMSO) solvents volumes, UV–visible absorption spectra were employed and displayed in Fig. 3b. The absorption coefficient of the film prepared by using DMF as a sol vent is higher than the other samples, due to the better interaction of the perovskite material with the solvent and the high crystallinity of the film which is confirmed previously by the XRD measurements. It can be also explained by the smallest viscosity of the DMF compared to the largest viscosity of the DMSO which led to unsuccessful infiltration of perov skite with the DMSO solvent [42]. We can also notice that the peak around 490 related to the PbI2 have been decreased with the increase of the volume of DMSO, this behavior can be explained by the low inter action of the PbI2 with the DMSO.
3.2. Solvent effect 3.2.1. X-ray analyses The X-ray patterns of the perovskite films coated on the top of the SnO2 layers with different solvent volume of (DMF%: DMSO%) were 4
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Fig. 5. J-V plots of best solar cell using the with forward and reverse scan.
Fig. 4. Photovoltaic characteristics of the PSCs with different solvent volume of (DMF %: DMSO%).
Table 3 Photovoltaic performance of PSC based on MAPbI3-xClx with forward and reverse scan.
Table 2 Photovoltaic performance of PSCs based on different solvent volume (DMF%: DMSO%). Solvent volume (DMF%: DMSO%)
Voc(V)
Jsc(mA/cm2)
FF(%)
PCE(%)
(100%:0%) (80%:20%) (50%:50%) (0%:100%)
0.82 0.78 0.79 0.65
18.98 18.26 18.56 19.03
52.98 49.56 43.84 41.79
9.30 7.08 6.41 5.18
Scan
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
Forward Reverse
0.82 0.80
18.98 18.90
52.98 50.90
9.30 9.10
the extracted parameters are summarized in Table 3. This cell achieves a PCE of 9.3% with a Voc of 0.82 V, a Jsc of 18.98 mA/cm2, and a FF of 52.98% under forward voltage scan. For the reverse scan a slight lower PCE has been seen to achieve 9.1% with a Voc of 0.80V, a Jsc of 18.90 mA/cm2, and an FF of 50.90%. A slight inverted hysteresis phenomenon was seen which is attributed to various physical causes such as the ionic accumulation or the nature of the interface between the perovskite and charge transport layers such as the electron transport layer (ETL) and hole transport layer (HTL) [60–62]. Indeed, in the mixed interfaces perovskite/ETL and perovskite/HTL it can be formed an energy extraction barrier due to the imbalance in their alignment of the energy level. Therefore, the photogenerated charges are piling up at the barrier. This fact reduces the charge transfer and the integrated potential, resulting in subsequent charge recombination which decreases the VOC and the JSC. Moreover, ionic accumulation by external polarization at the charge extraction interfaces may also generate barriers that lead to the accumulation of traps at the ETL/perovskite and HTL/perovskite interfaces which result in inverted hysteresis.
The change in organic solvent volume does not only affect the light absorption but also the charge transfer. Hence, we studied the impact of the organic solvent on the charge carrier transfer. Fig. 3c shows the PL emission spectra of MAPbI3-xClx with different (DMF%: DMSO %) organic solvents volume. All the PL spectra show an emission peak around 770nm. The emission intensity changes with the variation of the solvent volume. The sample elaborated from the solution prepared with DMF has the lowest emission intensity, indicating the low charge carrier recombination and the highest separation efficiency of the charge car rier with the fastest migration process. While the perovskite film with the DMSO solvent showed the highest PL intensity, indicating the high surface defects due to the poor infiltration of the perovskite leading to a high charge recombination [58]. 3.2.3. Photovoltaic performance J–V characteristics of the PSCs with the different solvent volume were investigated. Fig. 4 displays the photocurrent-voltage (J-V) curve measured in the forward voltage sweep direction under simulated AM 1.5 sunlight conditions, and the detailed J-V parameters are listed in Table .2. The performance of the PSCs based on the pure DMF exhibit the highest performance with around 9.3%, a VOC, JSC and FF of 0.82 V, 18.98 mA/cm2, 52.98%, and 9.3%, respectively. With the increase of the DMSO volume from 20% to 100%, the PCE has decreased from 7.08% to 5.18%. It can be seen that the change in the solvent volume did not cause a large change in the short-circuit current, (JSC) but rather caused a remarkable change in the open-circuit voltage (Voc) and fill factor (FF). The improvement in FF and VOC could be attributed to the enhancement of the charge carrier’s extraction due to the low interface loss and the good quality of the perovskite film which was well confirmed by the PL measurement [58,59].
4. Conclusion In summary, the effect of PbCl2 incorporation on the crystallinity and the photovoltaic properties of MAPbI3-x Clx perovskite solar cells have been investigated. We have shown that the incorporation of chloride in perovskite has a beneficial effect that allows to obtaining a high quality of MAPbI3-xClx films. Indeed, we have highlighted the presence of an optimal rate of chlorine concentration in this phase, which corresponds to the optimal PV performance. The electronic and structural properties of the perovskite film were improved by the addition of a small amount of PbCl2. This fact has induced the improvement of the perovskite solar cells efficiency. In another part, the solvent effect was investigated in this report. The influence of the organic solvent volume (DMF: DMSO) has been studied. It was seen that the use of the DMF has improved the crystallinity of the film, which results from the good dissociation of the materials and their good interaction with the solvent.
3.3. Hysteresis effect Fig. 5 shows the J-V plots of the optimum cell, elaborated with the molar ratio (3: 1) measured in forward and reverse scan directions and 5
H. Mehdi et al.
Materials Science in Semiconductor Processing 109 (2020) 104915
Declaration of competing interest
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