- Email: [email protected]
II) porphyrin hole-transport material with enhanced performance
Inorganic Chemistry Communications 112 (2020) 107701
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Perovskite solar cells employing copper (Ⅰ/II) porphyrin hole-transport material with enhanced performance
T
Chang-Dai Sia, , Xu-Dong Lvb, Shi-Jia Longa, ⁎
a b
⁎
College of Chemical Engineering and Technology, Tianshui Normal University, Tianshui 741000, People’s Republic of China College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China
GRAPHICAL ABSTRACT
An acylhydrazone-based copper porphyrin derivative was utilized as HTM to prepare PSC. Due to the redox shuttle of copper (I/II) for efficient charge transport by copper center metal together with the efficient electronic passivation by acylhydrazone groups, the best efficiency was readily improved up to 18.2%. The corresponding PSC device also revealed improved thermal stability.
ARTICLE INFO
ABSTRACT
Keywords: Copper porphyrin Hole transport material Perovskite solar cells
Perovskite solar cells, the commonly used hole transport materials (HTMs), Spiro-OMeTAD, are considered to be responsible for its long-term instability at high temperature. Herein, it is imperative to design an efficient and stable acylhydrazone-based copper porphyrins CuP as HTMs. Benefiting from the redox shuttle process of copper ion(I/II) in CuP, the performances of corresponding PSCs devices based on CuP as HTM were almost consistent with those of PSCs with Spiro-OMeTAD as HTM. More importantly, compared with the common Spiro-OMeTADbased devices, the thermal stability of CuP-based devices was improved greatly. This work indicates that the employment of CuP as HTM can achieve the fabrication of efficient and stable PSCs.
1. Introduction Compared with the conventional solar cells, perovskite solar cells (PSCs) have exhibited huge superiority owing to their properties of facile fabrication routes with respect to low cost materials and high power conversion efficiencies [1–6]. Additionally, the certified power
⁎
conversion efficiency over 24% have been reported [7]. The typical constructure of PSCs is sandwichtype, meanwhile, hole-transporting material (HTM) is the indispensable component for achieving high PCEs. The HTMs not only play an important role in transporting the holes after exciton dissociate from the perovskite layer, but also suppressing charge recombination and maintaining the stability, including
Corresponding authors. E-mail addresses: [email protected] (C.-D. Si), [email protected] (S.-J. Long).
https://doi.org/10.1016/j.inoche.2019.107701 Received 20 August 2019; Received in revised form 23 November 2019; Accepted 27 November 2019 Available online 16 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107701
C.-D. Si, et al.
to avoid the invasion of moisture/oxygen and electrode penetration of the perovskite layer [8]. The state of the art of organic HTM 2,2́,7,7́tetrakis-(N,N-di-p-methoxyphenyl-amine)9,9́-spiro bifluoren (Spiro-
property simultaneously. Significantly, the investigation on the relationship of structural and properties of porphyrin HTMs should have a clear benefit over the development of porphyrin HTMs.
OMeTAD) has achieved the highest PCE in PSCs, however, it is regrettable that the pristine Spiro-OMeTAD suffers from low hole mobility and electrical conductivity[9,10]. Therefore, the additional dopants, such as 4-tert-butylpyridine (TBP) and bis(trifluoromethane) sulfon- imide lithium salt (Li-TFSI), are indispensable for overcoming such limitations and enhancing the performance of pristine SpiroOMeTAD [11]. Unfortunately, lithium salt is prone to absorb moisture, which is detrimental to the perovskite layer, this phenomenon could aggravate the degradation of device performance. Moreover, the price of Spiro-MeOTAD itself is higher than that of gold and platinum. The more additional dopant and the doping process, the more cost [12], which is not benefit for the large scale application. Therefore, there is a good motivation to explore an alternative HTM that exhibits excellent efficiency together with low cost to further improve the stability of PSCs. Nowadays, various approaches have been employed to develop the candidates of the Spiro-OMeTAD HTM, in terms of designing materials for the HTM, organic molecular [8,13–15], inorganic material [16], conjugated polymers [17], and coordination compounds [2,18–20] have been developed to replace the Spiro-OMeTAD. However, the new HTMs that can really replace Spiro-OMeTAD with high device efficiency are scarcely, exploring new classs of HTMs is still urgent to optimize the properties of HTMs further. Inspired by the natural efficient electron-transfer mediators of blue copper proteins [21], which active in various electronic transmission system. The copper complexes of bis (2,9-dimethyl-1,10-phenanthroline) copper(I/II) [CuI/II(dmp)2], as redox mediators in dye sensitized solar cells have attained promising PCE[21,22]. Moreover, it has the high self-exchange rate as the redox shuttle between the copper(I) and copper(II) complex for electron transfer, which is propitious to utilize as a hole transport material in the solid-state dye-sensitized solar [23,24]. Porphyrin and their derivatives are highly electron rich and strong π-π molecular stacking characteristics, showing unique photophysical and electrochemical properties simultaneously [25], the performance of their ability to charge conversion has been proved in many dye-sensitized solar cells and shown promising power conversion efficiencies (PCEs) [26–28]. Therefore, porphyrins have recently emerged as an efficient HTM candidate for PSCs [29–33]. For instance, the tetra-alkoxytriphenylamine-substituted copper porphyrin as HTMs in PSCs and the effects of different metal ions in the center of the porphyrin core were reported by Zhu′s group, copper porphyrins have been successfully developed as dopant-free HTMs for PSCs showed PCEs of over 17% [29]. Their results displayed that the performance of the Copper porphyrin is slightly lower mainly owing to the weaker solubility in chlorobenzene, which can be modified by molecular construction. The previous study has proved the efficient electronic passivation of undercoordinated Pb atoms in perovskite by the lone pair electrons from N atoms, thus increasing the electronic cloud density [31,34], and the acylhydrazone group has also shown the excellent solubility in common solvents. Meanwhile, considering the high self-exchange rate between the copper (I) and copper (II) complex for electron transfer, the Copper porphyrin HTMs could perform the impressive charge transfer since the adoption of copper as the metal center. Furthermore, the cooperation of acylhydrazone group and copper metal center would decrease the interfacial charge recombination and further generate an improved cell performance. In this study, we adopted a facile synthetic route and relatively cheap starting materials to synthesize an acylhydrazone group substituted copper porphyrin (coded as CuP), which was utilized as dopantfree HTM for PSCs (Fig. 1a). Comparing to the PSC with Spiro-OMeTAD HTM, the obtained efficiency was 18.21%. Meanwhile, the thermal stability of PSCs based on CuP exhibited an observably enhancement by comparison of Spiro-OMeTAD. Hence, the dopant-free CuP HTM could improve thermostability and possess good photoelectric
2. Results and discussion UV–vis absorption spectroscopy has obtained in solution to reveal the difference in competitive light-harvesting with perovskite for CuP. Two major absorption bands were observed at 420 nm and 544, 586 nm, which were characterized as Sort band and Q band of CuP, respectively. The absorption region of Spiro-OMeTAD as HTMs can be considered to exist only in the UV region. In fact, the perovskite possess a saturated absorption in visible region, only a negligible amount of visible light approaches the hole-transporting layer [30]. Therefore, CuP HTM has limited effect in light absorption competition with perovskite. In addition, to further confirm the obtained perovskite film constructed finely, the UV–vis absorption spectroscopy and X-ray diffraction (XRD) spectra were performed (Fig. 2). Firstly, the UV–vis absorption spectroscopy of obtained perovskite film clearly displayed the higher-intensity platform absorption before 485 nm, after then, the absorption was gradually decreased until 800 nm, which matched well with the standard MAPbI3 perovskite. As shown in Fig. 2a and 2b, the two dominant diffraction peaks at 14.1° and 28.4° are ascribed to the (1 1 1) and (2 0 2) crystallographic phases of 3D perovskite, respectively. Indeed, the above results can further indicate that the target perovskite structure was formed. In PSCs, the well matched Highest Occupied Molecular Orbital (HOMO) level of HTMs and the valence-band level of perovskite layer are critical factors to accomplish the excellent charge transfer from perovskite to HTM. This further affected the photovoltaic performance of the whole device. In order to characterize the redox energies of the HTMs, the electronic properties of CuP were measured by cyclic voltammetry in DMF solution containing 0.1 mM tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte in a three-electrode system (Fig. S1). The energy level of the HOMOs can be calculated according to the literature [35] (assuming that the energy level of Fc+/ Fc = −5.1 eV, calculated on the basis of the following equation: EHOMO = –5.1 –(Eox – E1/2(Fc/Fc+))), the HOMO energy level of CuP was estimated to be −5.12 eV. The HOMO energy level of CuP lies above that of perovskite (−5.43 eV), which should ensure a sufficient driving force for the hole injection from perovskite into the counter electrode. To verify whether the fabricated devices adopted the porphyrin HTM CuP could perform efficient photon-to-electron, their photovoltaic characteristics of as obtained PSCs devices were measured under the illumination of AM 1.5, 100 mW·cm−2, the obtained PSCs devices were fabricated adopting with the mesoporous configuration of FTO/TiO2/ MAPbI3/HTM/Au (Fig. 3a). For comparison, the PSCs devices with well-known doped Spiro-OMeTAD as HTM were also fabricated and tested as reference group. The photovoltaic performances of these PSCs were shown in Figs. 3a and 4b, the relevant photovoltaic parameters were listed on the Table S1. The CuP based PSCs provided a final PCE of 18.21%, with a short-circuit current density (JSC) of 22.57 mA cm−2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 73.34%. Whereas the same structure except the centre metal porphyrin HTMs ZnPy based PSCs in previous report showed a slightly lower PCE of 17.82 % [30], with the corresponding values of 22.29 mA cm−2 (JSC), 1.09 V(VOC), and 73.12% (FF). This results could ascribed to the high self-exchange rate as the redox shuttle between the copper(I) and copper(II) complex for electron transfer, which makes it suited for using as a hole transport material than ZnPy. The integrated photocurrent from incident photon-to-current conversion efficiency spectra (IPCE) of the cells was matched well with the value measured from J-V curves (Fig. 3b and Fig. 4a). To compare the charge transfer and mobility of CuP and Spiro-OMeTAD, we have prepared the Electrical impedance spectroscopy (EIS) measurement. In the Nyquist plots, for the two arcs 2
Inorganic Chemistry Communications 112 (2020) 107701
C.-D. Si, et al.
Fig. 1. (a) The molecular structure of CuP; (b) UV-vis of CuP in solution.
Fig. 2. (a) UV-vis spectra; (b) XRD patterns of perovskite film.
at low forward bias, the higher frequency one is related to the charge transfer and the arc at lower frequency to the charge recombination at the interfaces of perovskite/HTM configured devices. As shown in Fig. S2a, an increased interfacial recombination resistance (Rrec) and a similar transfer resistance (Rtr) for the CuP deposited devices were observed, compared with those of devices with Spiro-OMeTAD. In order to investigate the charge dynamics of hole extraction and transport for the HTM CuP and Spiro-OMeTAD HTM, we carried out the steady state photoluminescence (PL) measurements of the perovskites in HTM/ perovskite/glass devices (Fig. S2b), compared with the bare perovskite film, the PL intensity of CuP and Spiro-OMeTAD HTMs coated films were significant quenched within the 750–800 nm. The drastic decreased PL intensity indicated the efficient hole extraction and transfer from the perovskite layer to the HTM layers. Inspiringly, the CuP shows the similar quenching property as the Spiro-OMeTAD when deposited in the perovskite film. This phenomenon indicating a comparable ability of charge separation at the perovskite interface for CuP HTM. To
characterize the passivation effect induced by the utilization of CuP molecules. space-charge-limited current (SCLC) analysis was introduced to quantitatively evaluate charge trap-state densities for characterize the passivation effect induced by the utilization of CuP molecules. see Fig. S3 (VTFL(Spiro OMe TAD) = 0.023 V, VTFL(CuP) = 0.017 V). The corresponding trap densities were calculated to be Ntrap (Spiro-OMe TAD) = 1.11 × 1015 cm−3 and Ntrap 15 cm−3 by employing the relationship between the (CuP) = 0.82 × 10 trap density (Ntrap) and VTFL: VTFL = q Ntrap d2/ 2 ε ε0, where q is the electronic charge, d is the thickness of the perovskite film, ε0 is the vacuum permittivity, and ε is the static dielectric constant of MAPbI3 (~70). The results indicated that the CuP molecules could effectively passivate the defects compared with Spiro-OMe TAD. To confirm the reproducibility of the CuP based devices performances, 30 fabricated devices based on CuP and Spiro-OMeTAD HTMs were measured. The histograms of PCEs and the statistical average of parameters are shown in Fig. 4b, the results further demonstrate the
Fig. 3. (a) The Cross-sectional SEM image of a mesoporous PSC based on CuP as HTM; (b) IPCE and corresponding integrated JSC of the corresponding devices. 3
Inorganic Chemistry Communications 112 (2020) 107701
C.-D. Si, et al.
Fig. 4. (a) Best J-V data; (b) Histograms of cell efficiencies among 30 cells on mesoporous PSCs with spiro-OMeTAD and CuP as HTMs.
Fig. 5. (a) Stability measured at room temperature with RH of ~65 %; (b) 85 °C in N2 environment; (c) 1 sun illumination on mesoporous PSCs with Spiro-OMeTAD and CuP as HTMs.
good reproducibility of the porhyrin HTMs CuP. Besides, the average PCEs of the devices based on CuP affords an average efficiency of 17.27%. In comparison, the best fabricated device based on SpiroOMeTAD has been achieved a power-conversion efficiency of 18.61% with an average efficiency at 17.64% for 30 PSCs devices. Moreover, the high reproducibility also indicates that the porhyrin HTMs have a great potential for the future industrialization. To evaluate the stability of fabricated devices based on porhyrin HTMs CuP, the stability test of the PSCs employing CuP and benchmark Spiro-OMeTAD as HTM were carried out under room temperature with RH of ~65%, 85 °C in N2 environment and AM 1.5G illumination on mesoporous PSCs(Fig. 5). For the thermal stability, after 100 h, the final PCE of the CuP-based device retains around 90% of the initial PCE, whereas the final PCE of the Spiro-OMeTAD-based devices retains only 10% of the initial PCE under the same conditions. Meanwhile, the CuP based devices shown much better moisture stability and illumination stability. Thus, the CuP based devices displays a much better stability than the Spiro-OMeTAD based devices, which is good for commercialization.
Our work here presents a novel HTMs toward developing efficient HTMs and fabricated stable PSCs. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgment This work was supported by grants from the Natural Science Foundation of China (No. 21761030), the research projects of colleges and universities in Gansu provinc (No. 2017A–075), and Tianshui Normal University ‘QinglanTalents’ Project. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107701. References
3. Conclusion
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In summary, an acylhydrazone-based copper porphyrin was synthesized through the facile routes in this work, coding CuP, which acted as HTM in PSCs aims to improve the efficiency and stability of PSCs. Benefiting from the redox shuttle of copper ion in CuP and efficient electronic passivation for efficient charge transport, the efficiency of the devices was improved to 18.21% as compared with the efficiency of reported ZnPy (17.82%). Meanwhile, compared to Spiro-OMeTADbased devices, possessing the outstanding stability of porphyrin HTMs, the thermal stability of CuP devices was promoted greatly. 4
Inorganic Chemistry Communications 112 (2020) 107701
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Perovskite solar cells employing copper (Ⅰ/II) porphyrin hole-transport material with enhanced performance
T
Chang-Dai Sia, , Xu-Dong Lvb, Shi-Jia Longa, ⁎
a b
⁎
College of Chemical Engineering and Technology, Tianshui Normal University, Tianshui 741000, People’s Republic of China College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China
GRAPHICAL ABSTRACT
An acylhydrazone-based copper porphyrin derivative was utilized as HTM to prepare PSC. Due to the redox shuttle of copper (I/II) for efficient charge transport by copper center metal together with the efficient electronic passivation by acylhydrazone groups, the best efficiency was readily improved up to 18.2%. The corresponding PSC device also revealed improved thermal stability.
ARTICLE INFO
ABSTRACT
Keywords: Copper porphyrin Hole transport material Perovskite solar cells
Perovskite solar cells, the commonly used hole transport materials (HTMs), Spiro-OMeTAD, are considered to be responsible for its long-term instability at high temperature. Herein, it is imperative to design an efficient and stable acylhydrazone-based copper porphyrins CuP as HTMs. Benefiting from the redox shuttle process of copper ion(I/II) in CuP, the performances of corresponding PSCs devices based on CuP as HTM were almost consistent with those of PSCs with Spiro-OMeTAD as HTM. More importantly, compared with the common Spiro-OMeTADbased devices, the thermal stability of CuP-based devices was improved greatly. This work indicates that the employment of CuP as HTM can achieve the fabrication of efficient and stable PSCs.
1. Introduction Compared with the conventional solar cells, perovskite solar cells (PSCs) have exhibited huge superiority owing to their properties of facile fabrication routes with respect to low cost materials and high power conversion efficiencies [1–6]. Additionally, the certified power
⁎
conversion efficiency over 24% have been reported [7]. The typical constructure of PSCs is sandwichtype, meanwhile, hole-transporting material (HTM) is the indispensable component for achieving high PCEs. The HTMs not only play an important role in transporting the holes after exciton dissociate from the perovskite layer, but also suppressing charge recombination and maintaining the stability, including
Corresponding authors. E-mail addresses: [email protected] (C.-D. Si), [email protected] (S.-J. Long).
https://doi.org/10.1016/j.inoche.2019.107701 Received 20 August 2019; Received in revised form 23 November 2019; Accepted 27 November 2019 Available online 16 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107701
C.-D. Si, et al.
to avoid the invasion of moisture/oxygen and electrode penetration of the perovskite layer [8]. The state of the art of organic HTM 2,2́,7,7́tetrakis-(N,N-di-p-methoxyphenyl-amine)9,9́-spiro bifluoren (Spiro-
property simultaneously. Significantly, the investigation on the relationship of structural and properties of porphyrin HTMs should have a clear benefit over the development of porphyrin HTMs.
OMeTAD) has achieved the highest PCE in PSCs, however, it is regrettable that the pristine Spiro-OMeTAD suffers from low hole mobility and electrical conductivity[9,10]. Therefore, the additional dopants, such as 4-tert-butylpyridine (TBP) and bis(trifluoromethane) sulfon- imide lithium salt (Li-TFSI), are indispensable for overcoming such limitations and enhancing the performance of pristine SpiroOMeTAD [11]. Unfortunately, lithium salt is prone to absorb moisture, which is detrimental to the perovskite layer, this phenomenon could aggravate the degradation of device performance. Moreover, the price of Spiro-MeOTAD itself is higher than that of gold and platinum. The more additional dopant and the doping process, the more cost [12], which is not benefit for the large scale application. Therefore, there is a good motivation to explore an alternative HTM that exhibits excellent efficiency together with low cost to further improve the stability of PSCs. Nowadays, various approaches have been employed to develop the candidates of the Spiro-OMeTAD HTM, in terms of designing materials for the HTM, organic molecular [8,13–15], inorganic material [16], conjugated polymers [17], and coordination compounds [2,18–20] have been developed to replace the Spiro-OMeTAD. However, the new HTMs that can really replace Spiro-OMeTAD with high device efficiency are scarcely, exploring new classs of HTMs is still urgent to optimize the properties of HTMs further. Inspired by the natural efficient electron-transfer mediators of blue copper proteins [21], which active in various electronic transmission system. The copper complexes of bis (2,9-dimethyl-1,10-phenanthroline) copper(I/II) [CuI/II(dmp)2], as redox mediators in dye sensitized solar cells have attained promising PCE[21,22]. Moreover, it has the high self-exchange rate as the redox shuttle between the copper(I) and copper(II) complex for electron transfer, which is propitious to utilize as a hole transport material in the solid-state dye-sensitized solar [23,24]. Porphyrin and their derivatives are highly electron rich and strong π-π molecular stacking characteristics, showing unique photophysical and electrochemical properties simultaneously [25], the performance of their ability to charge conversion has been proved in many dye-sensitized solar cells and shown promising power conversion efficiencies (PCEs) [26–28]. Therefore, porphyrins have recently emerged as an efficient HTM candidate for PSCs [29–33]. For instance, the tetra-alkoxytriphenylamine-substituted copper porphyrin as HTMs in PSCs and the effects of different metal ions in the center of the porphyrin core were reported by Zhu′s group, copper porphyrins have been successfully developed as dopant-free HTMs for PSCs showed PCEs of over 17% [29]. Their results displayed that the performance of the Copper porphyrin is slightly lower mainly owing to the weaker solubility in chlorobenzene, which can be modified by molecular construction. The previous study has proved the efficient electronic passivation of undercoordinated Pb atoms in perovskite by the lone pair electrons from N atoms, thus increasing the electronic cloud density [31,34], and the acylhydrazone group has also shown the excellent solubility in common solvents. Meanwhile, considering the high self-exchange rate between the copper (I) and copper (II) complex for electron transfer, the Copper porphyrin HTMs could perform the impressive charge transfer since the adoption of copper as the metal center. Furthermore, the cooperation of acylhydrazone group and copper metal center would decrease the interfacial charge recombination and further generate an improved cell performance. In this study, we adopted a facile synthetic route and relatively cheap starting materials to synthesize an acylhydrazone group substituted copper porphyrin (coded as CuP), which was utilized as dopantfree HTM for PSCs (Fig. 1a). Comparing to the PSC with Spiro-OMeTAD HTM, the obtained efficiency was 18.21%. Meanwhile, the thermal stability of PSCs based on CuP exhibited an observably enhancement by comparison of Spiro-OMeTAD. Hence, the dopant-free CuP HTM could improve thermostability and possess good photoelectric
2. Results and discussion UV–vis absorption spectroscopy has obtained in solution to reveal the difference in competitive light-harvesting with perovskite for CuP. Two major absorption bands were observed at 420 nm and 544, 586 nm, which were characterized as Sort band and Q band of CuP, respectively. The absorption region of Spiro-OMeTAD as HTMs can be considered to exist only in the UV region. In fact, the perovskite possess a saturated absorption in visible region, only a negligible amount of visible light approaches the hole-transporting layer [30]. Therefore, CuP HTM has limited effect in light absorption competition with perovskite. In addition, to further confirm the obtained perovskite film constructed finely, the UV–vis absorption spectroscopy and X-ray diffraction (XRD) spectra were performed (Fig. 2). Firstly, the UV–vis absorption spectroscopy of obtained perovskite film clearly displayed the higher-intensity platform absorption before 485 nm, after then, the absorption was gradually decreased until 800 nm, which matched well with the standard MAPbI3 perovskite. As shown in Fig. 2a and 2b, the two dominant diffraction peaks at 14.1° and 28.4° are ascribed to the (1 1 1) and (2 0 2) crystallographic phases of 3D perovskite, respectively. Indeed, the above results can further indicate that the target perovskite structure was formed. In PSCs, the well matched Highest Occupied Molecular Orbital (HOMO) level of HTMs and the valence-band level of perovskite layer are critical factors to accomplish the excellent charge transfer from perovskite to HTM. This further affected the photovoltaic performance of the whole device. In order to characterize the redox energies of the HTMs, the electronic properties of CuP were measured by cyclic voltammetry in DMF solution containing 0.1 mM tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte in a three-electrode system (Fig. S1). The energy level of the HOMOs can be calculated according to the literature [35] (assuming that the energy level of Fc+/ Fc = −5.1 eV, calculated on the basis of the following equation: EHOMO = –5.1 –(Eox – E1/2(Fc/Fc+))), the HOMO energy level of CuP was estimated to be −5.12 eV. The HOMO energy level of CuP lies above that of perovskite (−5.43 eV), which should ensure a sufficient driving force for the hole injection from perovskite into the counter electrode. To verify whether the fabricated devices adopted the porphyrin HTM CuP could perform efficient photon-to-electron, their photovoltaic characteristics of as obtained PSCs devices were measured under the illumination of AM 1.5, 100 mW·cm−2, the obtained PSCs devices were fabricated adopting with the mesoporous configuration of FTO/TiO2/ MAPbI3/HTM/Au (Fig. 3a). For comparison, the PSCs devices with well-known doped Spiro-OMeTAD as HTM were also fabricated and tested as reference group. The photovoltaic performances of these PSCs were shown in Figs. 3a and 4b, the relevant photovoltaic parameters were listed on the Table S1. The CuP based PSCs provided a final PCE of 18.21%, with a short-circuit current density (JSC) of 22.57 mA cm−2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 73.34%. Whereas the same structure except the centre metal porphyrin HTMs ZnPy based PSCs in previous report showed a slightly lower PCE of 17.82 % [30], with the corresponding values of 22.29 mA cm−2 (JSC), 1.09 V(VOC), and 73.12% (FF). This results could ascribed to the high self-exchange rate as the redox shuttle between the copper(I) and copper(II) complex for electron transfer, which makes it suited for using as a hole transport material than ZnPy. The integrated photocurrent from incident photon-to-current conversion efficiency spectra (IPCE) of the cells was matched well with the value measured from J-V curves (Fig. 3b and Fig. 4a). To compare the charge transfer and mobility of CuP and Spiro-OMeTAD, we have prepared the Electrical impedance spectroscopy (EIS) measurement. In the Nyquist plots, for the two arcs 2
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Fig. 1. (a) The molecular structure of CuP; (b) UV-vis of CuP in solution.
Fig. 2. (a) UV-vis spectra; (b) XRD patterns of perovskite film.
at low forward bias, the higher frequency one is related to the charge transfer and the arc at lower frequency to the charge recombination at the interfaces of perovskite/HTM configured devices. As shown in Fig. S2a, an increased interfacial recombination resistance (Rrec) and a similar transfer resistance (Rtr) for the CuP deposited devices were observed, compared with those of devices with Spiro-OMeTAD. In order to investigate the charge dynamics of hole extraction and transport for the HTM CuP and Spiro-OMeTAD HTM, we carried out the steady state photoluminescence (PL) measurements of the perovskites in HTM/ perovskite/glass devices (Fig. S2b), compared with the bare perovskite film, the PL intensity of CuP and Spiro-OMeTAD HTMs coated films were significant quenched within the 750–800 nm. The drastic decreased PL intensity indicated the efficient hole extraction and transfer from the perovskite layer to the HTM layers. Inspiringly, the CuP shows the similar quenching property as the Spiro-OMeTAD when deposited in the perovskite film. This phenomenon indicating a comparable ability of charge separation at the perovskite interface for CuP HTM. To
characterize the passivation effect induced by the utilization of CuP molecules. space-charge-limited current (SCLC) analysis was introduced to quantitatively evaluate charge trap-state densities for characterize the passivation effect induced by the utilization of CuP molecules. see Fig. S3 (VTFL(Spiro OMe TAD) = 0.023 V, VTFL(CuP) = 0.017 V). The corresponding trap densities were calculated to be Ntrap (Spiro-OMe TAD) = 1.11 × 1015 cm−3 and Ntrap 15 cm−3 by employing the relationship between the (CuP) = 0.82 × 10 trap density (Ntrap) and VTFL: VTFL = q Ntrap d2/ 2 ε ε0, where q is the electronic charge, d is the thickness of the perovskite film, ε0 is the vacuum permittivity, and ε is the static dielectric constant of MAPbI3 (~70). The results indicated that the CuP molecules could effectively passivate the defects compared with Spiro-OMe TAD. To confirm the reproducibility of the CuP based devices performances, 30 fabricated devices based on CuP and Spiro-OMeTAD HTMs were measured. The histograms of PCEs and the statistical average of parameters are shown in Fig. 4b, the results further demonstrate the
Fig. 3. (a) The Cross-sectional SEM image of a mesoporous PSC based on CuP as HTM; (b) IPCE and corresponding integrated JSC of the corresponding devices. 3
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Fig. 4. (a) Best J-V data; (b) Histograms of cell efficiencies among 30 cells on mesoporous PSCs with spiro-OMeTAD and CuP as HTMs.
Fig. 5. (a) Stability measured at room temperature with RH of ~65 %; (b) 85 °C in N2 environment; (c) 1 sun illumination on mesoporous PSCs with Spiro-OMeTAD and CuP as HTMs.
good reproducibility of the porhyrin HTMs CuP. Besides, the average PCEs of the devices based on CuP affords an average efficiency of 17.27%. In comparison, the best fabricated device based on SpiroOMeTAD has been achieved a power-conversion efficiency of 18.61% with an average efficiency at 17.64% for 30 PSCs devices. Moreover, the high reproducibility also indicates that the porhyrin HTMs have a great potential for the future industrialization. To evaluate the stability of fabricated devices based on porhyrin HTMs CuP, the stability test of the PSCs employing CuP and benchmark Spiro-OMeTAD as HTM were carried out under room temperature with RH of ~65%, 85 °C in N2 environment and AM 1.5G illumination on mesoporous PSCs(Fig. 5). For the thermal stability, after 100 h, the final PCE of the CuP-based device retains around 90% of the initial PCE, whereas the final PCE of the Spiro-OMeTAD-based devices retains only 10% of the initial PCE under the same conditions. Meanwhile, the CuP based devices shown much better moisture stability and illumination stability. Thus, the CuP based devices displays a much better stability than the Spiro-OMeTAD based devices, which is good for commercialization.
Our work here presents a novel HTMs toward developing efficient HTMs and fabricated stable PSCs. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgment This work was supported by grants from the Natural Science Foundation of China (No. 21761030), the research projects of colleges and universities in Gansu provinc (No. 2017A–075), and Tianshui Normal University ‘QinglanTalents’ Project. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107701. References
3. Conclusion
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In summary, an acylhydrazone-based copper porphyrin was synthesized through the facile routes in this work, coding CuP, which acted as HTM in PSCs aims to improve the efficiency and stability of PSCs. Benefiting from the redox shuttle of copper ion in CuP and efficient electronic passivation for efficient charge transport, the efficiency of the devices was improved to 18.21% as compared with the efficiency of reported ZnPy (17.82%). Meanwhile, compared to Spiro-OMeTADbased devices, possessing the outstanding stability of porphyrin HTMs, the thermal stability of CuP devices was promoted greatly. 4
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