Fei Qi(齐飞) Bo Wu(吴波) Junyuan Xu(徐俊源) Qian Chen(陈潜)Haiquan Shan(单海权) Jiaju Xu(许家驹) and Zong-Xiang Xu(许宗祥)

1Department of Chemistry,Southern University of Science and Technology,Shenzhen 518000,China

2Guangdong–Hong Kong–Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices,Southern University of Science and Technology,Shenzhen 518000,China

Keywords: perovskite solar cells,metal phthalocyanines,hole transport layers

1. Introduction

Hybrid organicinorganic perovskites solar cells (PSCs)have drawn consideration attention due to the potential of achieving even higher efficiencies and low fabrication cost.[1–3]In the last decade,the power conversion efficiencies(PCEs) of the PSCs increased rapidly from 3.8% in 2009[4]to a certified 25.5%in 2020,[5]making this new photovoltaic technology more and more commercially attractive. As a necessary part of PSCs, hole transport layers (HTLs) play a vital role in enhancing hole extraction and transport, reducing charge carrier recombination, and protecting the perovskite layer from the moisture invasion, which is of great importance to obtain a PSC with high efficiency and good stability.[1,6–9]Current hole transport materials (HTMs) can be generally classified into three categories, including inorganic, polymeric, and small-molecule HTMs.[8]Among these, small-molecule HTMs are regarded as one of the most promising candidates owing to low-cost synthesis and easy chemical modification,[8]which is beneficial for mass manufacture. So far, spiro-OMeTAD (2,2’,7,7’-Tetrakis[N,Ndi(4-methoxyphenyl)amino]-9,9’-spirobifluorene) is one of most widely used small-molecule HTMs, and the highest PCE of 23.32% has been achieved.[10]However, the expensive preparation limits its commercialization. Further, due to the inherently low conductivity of the pristine spiro-OMeTAD, hydrophilic additives such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 4-tert–butyl pyridine(TBP),and so on,are usually necessary in order to improve the HTL conductivity, which induces poor device stability in air.[9,11,12]

As one class of small-molecule HTMs, organometallic compounds that contain a delocalized 18π-electron conjugated system with a metal ion located at the central cavity,have also attracted considerable research attentions and efforts in recent years due to their excellent hole extraction and transport ability when employed as HTMs in PSCs.[8,9,11–14]So far, metal phthalocyanines (MPcs) are one of the most widely studied organometallic HTMs in PSCs.[15]This class of HTMs could be an ideal alternative to the expensive spiro-OMeTAD due to simple synthesis and easy purification,[8,9,16]as well as superior semiconducting nature originating from the interaction between organic ligand and central metal.[17]Also, the easy chemical modificationsviavarying central metal ions or introducing suitable functional groups to theπconjugated cores may help to systematically adjust the chemical, physical, electronic, optical, and electronic properties of the compounds, therefore affording a suitable energy alignment with different perovskite active layers,facilitating charge extraction, and transfer, and allowing cost-effective solutionprocessed film preparation.[8,16]Moreover,MPcs can be used for fabricating HTLs without any dopant, which may help to lower fabrication cost and improve device stability.[1,8]It has been reported that the PCE as high as 19.7%was obtained by using an arylamine-substituted copper phthalocyanine as nondopant HTM.[16]

Fig.1. (a)Chemical structures of the NiEt2Pc and NiPr2Pc, (b)the device configuration,and(c)energy level diagrams of PSCs studied in this work.

The incorporation of substituents to the primary MPc skeleton is well known to be an effective strategy for MPcs’chemical modification.[1,15,18–21]The attached substituents bring their own special chemical and electronic characteristics to the MPc chromophores,thus significantly affecting the properties of the MPcs.[22]Particularly, MPcs functionalized with alkyl substituents on phenyl rings are of intensive interest during the last decade,[1,9,15,21,23–25]which not only increases the MPcs’solubility in common organic solvents,but also leads to higher charge carrier mobility due to shorter intermolecularπ–πdistance arising from the driving force of alkyl chains.[9,19]Also,alkyl groups increase hydrophobic nature of thin films, which would be more beneficial for the impedance of moisture ingress.[9,15]On the other hand, substitution positions at MPc ring may also play an important role in the properties that the MPc compound possess[22,26]Since a typical MPc molecule is composed of four isoindole moieties linked by a ring of nitrogen atoms[27]there are two positions available at the phenyl rings, to which substituents can be incorporated, namely peripheral position (2,3, 9, 10, 16, 17, 23, 24) and non-peripheral position (1, 4,8, 11, 15, 18, 22, 25).[28]The substitutions at different positions have been found to significantly affect the intrinsic properties of the MPc compounds, including optical, electronic and chemical properties, as well as aggregation behavior in solid state.[28]So far,the strategy of alkyl modification on MPcs for use as HTMs in PSCs mainly focuses on peripheral substitutions,[1,9,15,29,30]while non-peripheral substitutions are still rarely studied. With this in mind,we herein introduce two non-peripherally alky-substituted nickel phthalocyanines (NiEt2Pc and NiPr2Pc, Fig. 1(a)) for use as nondopant HTMs in PSCs. The solar cells based on these two MPcs have been fabricated and characterized, and the influence of alkyl chains on the device performances was then examined.

2. Experimental details

2.1. Materials

All the chemicals were purchased from the commercial sources and used without any further purification. The NiEt2Pc and NiPr2Pc utilized in this study were synthesized and purified according to similar method described in the reported literature,[26]involving a one-step synthesis from the starting materials of 36-diethylphthalonitrile and 36-dipropylphthalonitrile, respectively. The final products were further purifiedviavacuum sublimation in a tube-type heating furnace(Technol VDS-80)in order to fulfill the requirements of electronic applications. NiEt2Pc: yield, 18%; MALDITOF MSm/z(M+): calcd. for [C48H48Ni8Ni]+: 794.34;found: 794.30. UV-vis(chlorobenzene):λmax=697,628 nm(Q-band). FTIR (KBr):ν=2960, 2928, 2866, 1605, 1573,1515, 1450, 1405, 1341, 1177, 1091, 1054, 984, 916, 829,759 cm−1.NiPr2Pc:yield,16%;MALDI-TOF MSm/z(M+):calcd. for [C56H64Ni8Ni]+: 906.46; found: 906.27. UVvis(Dichloromethane):λmax=699,630 nm(Q-band). FTIR(KBr):ν=2954,2929,2870,1604,1571,1514,1456,1375,1330,1184,1089,985,929,819,761 cm−1.

2.2. Device fabrication and characterization

The n–i–p perovskites solar cells with the configuration of FTO/SnO2/perovskite/NiEt2Py(or NiPr2Pc)/Au(Fig.1(b))were fabricated and tested by the reported method[26]CH3NH3PbI3(MAPbI3) was used as perovskite layer, and vacuum-deposited NiEt2Pc and NiPr2Pc layers as non-dopant HTLs.

3. Results and discussion

Thermogravimetric analysis was performed to investigate the thermal stability of the studied materials, as illustrated in Figs.2(a)and 2(b). It can be found that both the NiEt2Pc and NiPr2Pc are thermally stable, and the decomposition temperatures, at which a 5%weight loss occurs, are estimated to be 437°C and 446°C,respectively,suggesting that the materials used this study are suitable HTMs in PSCs.

The ultraviolet-visible(UV-vis)absorption spectra of the NiEt2Pc and NiPr2Pc thin films deposition on fluorine-doped tin oxide(FTO)substrates are depicted in Figs.3(a)and 3(b).Both the samples illustrate two strong characteristic absorption bands: one in the near UV region, known as B-band absorption, and another in the range of ca. 600 nm–800 nm,known as Q-band absorption, which is the typical absorption behavior of the MPcs in solid state.[18]The B-band absorption is induced byπ →π∗transition (a2u→e∗1g) from the highest occupied molecular orbital(HOMO)to the lowest unoccupied molecular orbital(LUMO),and the Q-band absorption is due toπ →π∗transition (a1u→e∗1g) from HOMO to LUMO of the MPc ring.[1,9,30]The optical bandgaps of the NiEt2Pc and NiPr2Pc samples were determined from the absorption onsets and calculated to be 1.69 eV and 1.68 eV,respectively.

Fig.2. Thermogravimetric analysis curves of(a)NiEt2Pc and(b)NiPr2Pc.

The ionization energy measurement system (IPS-4) was employed to test the HOMO levels of the studied materials,as shown in Figs. 3(c) and 3(d). The HOMO level of the NiEt2Pc was found to be−5.30 eV, and with the length extension of alkyl chains from ethyl to propyl,a higher HOMO level is observed for the NiPr2Pc(5.11 eV).Using the optical bandgaps,the LUMO levels of the NiEt2Pc and NiPr2Pc were estimated to be−3.61 eV and−3.43 eV,respectively. The energy band diagram of the PSCs studied in this work is depicted in Fig.1(c). It can be found that both the HOMO levels of the NiEt2Pc and NiPr2Pc are higher than the valence band (VB)of the CH3NH3PbI3absorber(−5.40 eV),which may help to provide sufficient driving forces for hole injection from the perovskite layer into the HTLs.[1,31]Also, the HOMO levels of the NiEt2Pc and NiPr2Pc are lower than the work function of the Au (−5.1 eV), which is favorable for hole collection on the Au electrode.[15]Furthermore, the NiPr2Pc exhibits a higher LUMO level than that of the NiEt2Pc, which is more beneficial for blocking electron migration from the perovskite into the HTL, and thus reducing the opportunity for charge carrier recombination at the interface and within the HTL.[1]

Fig.3. (a), (b)UV-vis absorption spectra of the NiEt2Pc and NiPr2Pc thin films deposited on FTO with the thickness of 60 nm. (c),(d)The photoelectron yield spectroscopy(IPS-4)of the NiEt2Pc and the NiPr2Pc.

Fig.4. J–V characteristics of best PSCs based on(a)NiEt2Pc,(b)NiPr2Pc,and(c)spiro-OMeTAD(doped)HTMs.

The PSCs using NiEt2Pc and NiPr2Pc HTLs were fabricated with the conventional n–i–p planar device configuration of FTO/SnO2/perovskite/NiEt2Pc (or NiPr2Pc)/Au, and the standard PSC using doped spiro-OMeTAD as HTL with the same configuration was also fabricated and characterized for reference according to the method described in the literature[26]The PSCs were characterized under a xenon lamp with AM 1.5,100 mW/cm2simulated light illumination. The current–voltage (J–V) characteristics of the best devices are depicted in Fig. 4, and the relevant parameters such as the open-circuit voltage(VOC),short-circuit current density(JSC),fill factor (FF), and PCE are summarized in the inset table.For each type of PSCs,10 devices were characterized and the statistics of the photovoltaic parameters are depicted in Fig.5 and Table 1. The champion PSC based on NiEt2Pc exhibits aJSCof 18.65 mA/cm2,VOCof 0.96 V, FF of 48.32%, and PCE of 8.63%,while the best PSC based on NiPr2Pc has aJSCof 22.83 mA/cm2,VOCof 1.04 V,FF of 59.40%, and PCE of 14.07% (Fig. 4 and Table 1). Higher values inJSC,VOC, and FF can be observed for the NiPr2Pc-based PSC,giving rise to its higher PCE and exhibiting better hole extraction and transfer ability of the NiEt2Pc layer.

Interestingly,as shown in Fig.1(c),although the NiEt2Pc has a deeper HOMO level of−5.30 eV than that of the NiPr2Pc(−5.11 eV),the NiPr2Pc-based PSC has been found to exhibit a higherVOCof 1.04 V than that of the NiEt2Pcbased device (0.96 V) (Fig. 4, Table 1). It is well known that theVOCof PSCs is strongly affected by charge carrier recombination inside the perovskite layer and/or at the adjacent interfaces, and serious charge recombination typically results in significantVOCloss.[32,33]As shown in Fig. 1(c),higher LUMO level of the NiPr2Pc(−3.43 eV)compared with that of the NiEt2Pc may help to block electron injection from the perovskite layer into the HTL,and thus reduce charge recombination at the perovskite/HTL interface more effectively,which is more useful for reducing theVOCloss, and leads to higherVOCobserved for the NiPr2Pc-based device. Also, reduced charge carrier recombination at the perovskite/HTL interface and within the HTL arising from higher LUMO level of the NiPr2Pc is thought to be one main reason responsible for the enhancedJSCand FF observed for the NiPr2Pc-based device.[1,31]

Fig.5. Photovoltaic parameter statistics of the PSCs based on NiEt2Pc and NiPr2Pc: (a)JSC,(b)VOC,(c)FF,and(d)PCE.

Table 1. Photovoltaic parameters of the PSCs based on NiEt2Pc,NiPr2Pc,and spiro-OMeTAD.

Fig.6. GIXRD patterns of bare perovskite,(a)NiEt2Pc/perovskite and(b)NiPr2Pc/perovskite.

The film quality should be another factor that affects hole extraction and transfer ability of the HTLs. Thus grazing incidence x-ray diffraction (GIXRD) on two HTLs was examined to probe crystalline structures within the thin films. Figure 6 depicts the GIXRD patterns of the vacuum-deposited NiEt2Pc and NiPr2Pc thin films with the same thickness on top of the perovskite layer, and the GIXRD pattern of the bare perovskite layer was also characterized and presented for reference. As shown in Fig. 6(a), no obvious crystalline characteristic is observed for the NiEt2Pc/perovskite sample,while the NiPr2Pc/perovskite sample exhibits a diffraction signal with a 2θvalue of 5.7°(Fig. 6(b)), indicating higher thin film crystallinity as well as higher order of molecular arrangements within the thin film, which is more beneficial for hole extraction and transport, and thus leads to higherJSCand FF found for the NiPr2Pc-based PSC.[12]Atomic force microscopy(AFM)technologies were employed to study the morphologies of the HTLs, as illustrated in Fig. 7. The AFM image reveals that the bare perovskite layer exhibits the roughest surface morphology with a surface root-meansquare (RMS) roughness of 18.5 nm (Fig. 7(a)), while lower surface RMS roughness values are observed for the NiEt2Pc HTL(9.2 nm,Fig.7(b))and NiPr2Pc HTL(5.2 nm,Fig.7(c)),which may help to provide better interface contact and form a more uniform conducting path for charge transfer.[1]The much lower surface RMS roughness observed for the NiPr2Pc HTL than that of the NiEt2Pc sample,which is more favorable for charge extraction and transfer,is considered to be another reason responsible for higherJSCand FF of the device based on the NiPr2Pc HTL.

The hole transport properties of the NiEt2Pc and NiPr2Pc were investigatedviabottom-contact field-effect transistor (FET) technology. The FETs using vacuum-deposited NiEt2Pc and NiPr2Pc thin films as active layers were fabricated according to the method described in the literature,[15]and the output and transfer characteristics of as-fabricated FETs are depicted in Fig. 8. Both the devices exhibits exhibit a p-type FET characteristic, with varying hole mobility values. The NiEt2Pc-based FET exhibits a mobility of 1.33×10−5cm2/V·s and a threshold voltage of−14 V(Figs.8(a)and 8(b)),which values are 3.64×10−4cm2/V·s and−4 V for the NiPr2Pc-based device (Figs. 8(c) and 8(d)). The mobility of the NiPr2Pc-based FET is one order of magnitude higher than that of the NiEt2Pc-based one,clearly revealing that the length extension of alkyl chains from ethyl to propyl tunes the MPcs’hole transport properties effectively,which leads to better hole extraction of the NiPr2Pc HTL, and thus partly contributes to higherJSCand FF values observed for the NiPr2Pc-based PSC.[1,15]

Fig.7. AFM images of(a)FTO/SnO2/perovskite,(b)FTO/SnO2/perovskite/NiEt2Pc,and(c)FTO/SnO2/perovskite/NiPr2Pc.

Fig.8. (a)Output and(b)transfer curves of bottom-contact FET based on NiEt2Pc,and(c)output and(d)transfer curves of bottom-contact FET based on NiPr2Pc(L=10µm,W =1500µm).

The hole extraction ability of the NiEt2Pc and NiPr2Pc HTLs was investigatedviasteady-state photoluminescence(PL) spectra. The PL spectra of the perovskite layers coated with or without HTLs were characterized, as shown in Fig. 9(a). A strong PL signal appearing at～774 nm is found for the bare perovskite layer, which is induced by the recombination between the photo-generated charge carriers within the perovskite layer.[1,12]The PL spectra was significantly quenched when the perovskite absorber was covered with NiEt2Pc and NiPr2Pc layers indicative of efficient separation of photo-induced excitons, as well as efficient hole transport from the perovskite layer to the HTLs.[9,12,15,31]A higher quenching efficiency of 96.5% is observed for the NiPr2Pc/perovskite sample compared with that of the NiEt2Pc/perovskite one (95.5%), indicative of better hole extraction ability of the NiPr2Pc HTL. To further explore the quenching process, the measurement of time-resolved photoluminescence (TRPL) spectra was performed, as illustrated in Fig. 9(b). The PL decay lifetimes extracted from the TRPL spectra were found to be 71.647, 33.550, and 8.017 ns for the bare perovskite the NiEt2Pc/perovskite and NiPr2Pc/perovskite samples, respectively. Shorter PL decay lifetime observed for the NiPr2Pc/perovskite sample further confirms the superior hole extraction ability of the NiPr2Pc HTL,[16,31,34]and the results are in good agreement with theJSCvalues observed for the studied PSCs(Fig.4,Table 1).

Fig. 9. (a) Steady-state and (b) time resolved PL spectra of bare perovskite, NiEt2Pc/perovskite, and NiPr2Pc/perovskite. The inset in panel(a)is the enlarged steady-state PL spectra of NiEt2Pc/perovskite and NiPr2Pc/perovskite.

4. Conclusions

In summary, two non-peripherally octaethyl- and octapropyl-substituted NiPcs,i.e.,NiEt2Pc and NiPr2Pc,were successfully synthesized and characterized. Both the NiPcs exhibited efficient hole extraction and transport properties when used as dopant-free HTMs in PSCs. The length extension of the alkyl chains from ethyl to propyl significantly increased the NiPcs’ LUMO level, which effectively reduced charge carrier recombination at the perovskite/HTL interface,leading to reducedVOCloss and enhancedJSCand FF for the NiPr2Pc-based device. Also,the alkyl chain extension significantly increased hole carrier mobility and affected the crystallinity and surface roughness of thin film, resulting in enhanced hole extraction and transport ability of the NiPr2Pc HTL.As a result,the NiPr2Pc-based PSC exhibited better device performances than those of the NiEt2Pc-based device.