SCN-doped CsPbI3 for Improving Stability and Photodetection Performance of Colloidal Quantum Dots

Chao Zheng,Aqiang Liu,Chenghao Bi,Jianjun Tian

Institute for Advanced Materials and Technology,University of Science and Technology Beijing,Beijing 100083,China.

Abstract:Inorganic halide CsPbI3 perovskite colloidal quantum dots (QDs)possess remarkable potential in photovoltaics and light-emitting devices owing to their excellent optoelectronic performance.However,the poor stability of CsPbI3 limits its practical applications.The ionic radius of SCN−(217 pm)is comparable to that of I− (220 pm),whereas it is marginally larger than that of Br− (196 pm),which increases the Goldschmidt tolerance factor of CsPbI3 and improves its structural stability.Recent studies have shown that adding SCN− in the precursor solution can enhance the crystallinity and moisture resistance of perovskite film solar cells; however,the photoelectric properties of the material post SCN− doping remain unconfirmed.To date,it has not been clarified whether SCN− doping occurs solely on the perovskite surfaces,or if it advances within their structures.In this study,we synthesized inorganic perovskite CsPbI3 QDs via a hot-injection method.Pb(SCN)2 was added to the precursor for obtaining SCN−-doped CsPbI3 (SCN-CsPbI3).X-ray diffraction (XRD),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy (XPS)were conducted to demonstrate the doping of SCN− ions within the perovskite structures.XRD and TEM indicated a lattice expansion within the perovskite,stemming from the large steric hindrance of the SCN− ions,along with an enhancement in the lattice stability due to the strong bonding forces between SCN− and Pb2+.Through XPS,we confirmed the existence of the S peak,and further affirmed that the bonding energy between Pb2+ and SCN− was stronger than that between Pb2+ and I−.The space charge limited current and time-resolved photoluminescence results demonstrated a decrease in the trap density of the perovskite after being doped with SCN−;therefore,the doping process mitigated the defects of QDs,thereby increasing their optical performance,and further enhanced the bonding energy of Pb－X and crystal quality of QDs,thereby improving the stability of perovskite structure.Therefore,the photoluminescence quantum yield (PLQY)of the SCN-CsPbI3 QDs exceeded 90%,which was significantly higher than that of pristine QDs (68%).The high PLQY indicates low trap density of QDs,which is attributed to a decrease in the defects.Furthermore,the SCN-CsPbI3 QDs exhibited remarkable water-resistance performance,while maintaining 85% of their initial photoluminescence intensity under water for 4 h,whereas the undoped samples suffered complete fluorescence loss due to the phase transformations caused by water molecules.The SCN-CsPbI3 QDs photodetector measurements demonstrated a broad band range of 400–700 nm,along with a responsivity of 90 mA·W−1 and detectivity exceeding 1011 Jones,which were considerably higher than the corresponding values of the control device (responsivity:60 mA·W−1 and detectivity:1010 Jones).Finally,extending the doping of SCN− into CsPbCl3 and CsPbBr3 QDs further enhanced their optical properties on a significant scale.

Key Words:Inorganic halide perovskite; Quantum dot; Doping; Stability; Photodetector

1 Introduction

Herein,we synthesized inorganic perovskite CsPbI3quantum dots (QDs)by hot-injection method.Pb(SCN)2was added into the precursor to obtain SCN−doped CsPbI3(SCN-CsPbI3)QDs.X-ray diffraction (XRD),transmission electron microscopy(TEM)and X-ray photoelectron spectra (XPS)results had demonstrated SCN−was doped into the lattice structure of perovskite.SCN-CsPbI3QD solution achieved more than 90% of photoluminescence quantum yield (PLQY)without exchanging ligands,great higher than that of pristine sample.This is attributed to the less trap defects.In addition,the SCN−doped sample had extremely high humidity stability and storage stability,and retained more than 85% of its initial photoluminescence intensity in water for 4 h.The photodetectors based SCN-CsPbI3QDs achieved detectivities over 1011Jones,which is much higher than that of the undoped counter (1010Jones).The full-spectrum CsPbX3(X = Cl,Br and I)QDs with doping of SCN−were also obtained by adjusting the combination of halide and SCN−,the spectra can be tuned over the entire visible range.

2 Experimental

2.1 Synthesis and purification of CsPbI3 and SCNCsPbI3 QDs

Cs2CO3(0.1 g)and OA (2 mL)were dried for 1 h at 120 °C,the mixed solution was kept at 100 °C under the atmosphere of N2before injection.PbI2(0.17 g)and Pb(SCN)2(0,2.5,5 and 7.5 mg corresponding to 0%,0.16%,3.3% and 5% SCN-doped QDs,respectively)were dissolve in 10 mL of octadecene (ODE)and dried under vacuum in a 100 mL 3-neck flask for 1.5 h at 120 °C,and then 1.6 mL of oleylamine (OAm)and 1.6 mL of oleic acid (OA)were injected into the flask under a N2atmosphere.The solution was heated to 170 °C when the PbI2completely dissolved,1 mL cesium precursor solution was injected into the PbI2precursor quickly.The resulting solution was then immediately cooled using ice-water.The CsPbI3and SCN-CsPbI3QDs were separated by the addition of methyl acetate (at a solution/methyl acetate volume ratio of 1 :3).After centrifuged for 5 min at 8000 r·min−1,the precipitate was dispersed in octane.

2.2 Fabrication of photodetectors

The fluorine-doped tin oxide (FTO)coated glass substrate was sequentially cleansed using deionized water,acetone and ethanol.The TiO2films were deposited by spraying and then annealing at 450 °C for 30 min.The CsPbI3and SCN doped CsPbI3QDs films were deposited by spin-coating using a speed of 2000 r·min−1for 20 s without annealing.The hole injection layer was produced via spin-coating spiro-OMeTAD using a speed of 3000 r·min−1for 30 s.The Ag electrode was deposited using a thermal evaporation system under a vacuum of 5 × 10−4Pa.The active area of detector was 10 mm2.

2.3 Characterizations

The patterns of XRD were acquired using a MXP21VAHF X-ray diffractometer operating with Cu Kαradiation (λ = 0.15418 nm).The absorption spectra were measured with a Beituo DUV-18S2 and a Shimadzu UV-3600 plus spectrophotometer.Photoluminescence (PL)spectroscopy performance was tested using a Gangdong F-280 fluorescence spectrophotometer.The time-resolved PL decay lifetime was tested using a HORIBA Fluorolog phosphorescence lifetime system coupled with a 375 nm,45 ps pulse laser.TEM and high-resolution TEM (HRTEM)were undertaken on a JEOL JEM-2010 microscope operated at 200 kV.The XPS was performed using PHI 5000VersaProbe III.XPS samples were prepared by dropping CsPbI3QDs onto Si wafers.The current-voltage curves measurements of the photodetectors were conducted on a digital source meter (2400,Keithley Instruments Inc.)under 3A grade S-3 AM 1.5G simulated sunlight.The responsivity was measured in direct current (DC)mode using a custom measurement system consisting of a digital source meter (2400,Keithley Instruments Inc.),a 150 W xenon lamp (7ILX150A,7 Star Optical Instruments Co.)and a monochromator (7ISW30,7 Star Optical Instruments Co.).

3 Results and discussion

The absorption spectra and PL spectra of CsPbI3QDs with different doping amounts of SCN−ions are shown in Fig.1a,b.It can be seen that the doped samples (1.6% SCN-CsPbI3,3.3% SCN-CsPbI3,5% SCN-CsPbI3)exhibit a blue shift trend in both UV and PL spectra.This can be explained by the smaller effective ionic radius of SCN−than that of I−.The PL intensity of the SCN-CsPbI3QDs has a significant improvement.3.3% SCN-CsPbI3QD displays the highest PL intensity and more than 90% of PLQY.This is remarkable high PLQY value for CsPbI3QDs obtained using OA and OAm as ligands25.In contrast,the comparative sample (CsPbI3-pristine)presents PLQY of 68%.The high PLQY suggests low trap density of QDs with greatly reducing of the non-radiative recombination26.In addition,we also doped SCN−into CsPbCl3and CsPbBr3QDs.The PL and absorption spectra are shown in Fig.S1 (Supporting Information).The results also demonstrate that the appropriate doping amount of SCN−ions (3.3%)has enhanced the PL intensity of the QDs.

Fig.1 Optical properties of CsPbI3 QDs with and without doping.

The morphologies of CsPbI3SCN-CsPbI3QDs were investigated by TEM and HRTEM,as shown in Fig.2.We can see that the SCN-doping can improve the size distribution of QDs as shown in size statistical histograms (Fig.S2,Supporting Information).The doped samples have a narrow size distribution and the particle size is about 9.7 nm,while the sample without doping shows a wide size distribution and a large average particle size of about 9.9 nm.It also shows that both undoped and doped QDs possess a cubic crystal structure corresponding to the (100)plane as shown in Fig.2e–h.The lattice constants of QDs for the (100)plane can be obtained based on the HRTEM and fast Fourier transform (FFT)images.The interplanar spacing increases from 0.621 to 0.627 nm as the doping amount of SCN−ions increased from 0 to 5%.The lattice expansion can be explained by large steric hindrance caused by SCN−rod structure27.In order to further prove the lattice expansion caused by SCN−doping,we have done a doping limit experiment,in which Pb(SCN)2is completely substituted for PbI2in the precursor to synthesize CsPb(SCN)3.Its corresponding absorption spectrum and TEM image are shown in Fig.S3(Supporting Information).The sample does not show fluorescence under the ultraviolet light,and its band gap is calculated to be 4.3 eV according to its absorption edge,which is in agreement with the theoretically calculated band gap22.From the morphology,the average particle size is large (～20 nm),which is consistent with the increased steric hindrance caused by the rod-like structure of SCN−.

The crystal structures of CsPbI3and SCN-CsPbI3(3.3% SCNCsPbI3with the highest PLQY is chose as SCN-CsPbI3)QDs were investigated by XRD as shown in Fig.3a.It can be seen that the SCN-CsPbI3QD still maintains the original perovskite structure.The diffraction peak intensity of SCN-CsPbI3is much higher than that of the pristine CsPbI3,especially for crystal planes of (111),(210)and (211).This indicates that SCN−doping improves the crystallinity of CsPbI3QD28.The XRD patterns of 1.6% SCN-CsPbI3and 5% SCN-CsPbI3are shown in Fig.S4a (Supporting Information).As shown in Fig.3b,when the doping amount of SCN−is less (＜ 5%),the peak position has only a slight offset (～0.02°)to the small angle,which is also consistent with the increase of lattice fringe.When the doping amount is more (25%),the sample has a significant shift to a small angle (～0.2°)compared to the peak position of the CsPbI3,which proves that the SCN−ion occupied the CsPbI3octahedral unit cell.As schematically illustrated in Fig.3c,in the case of successful doping,SCN−ions would occupy part of the I−ion site (X site).The lattice expansion is due to the large steric hindrance of the SCN−.

Fig.3 Crystal structures of doped CsPbI3 QD.

Fig.4 XPS and TRPL spectra of QDs.

Fig.4d shows the time-resolved PL (TRPL)spectra of CsPbI3QDs.Each transient can be fitted by a triexponential decay function.The radiation recombination rate of SCN-CsPbI3is 0.031 ns−1,while CsPbI3is only 0.020 ns−1.The faster radiation recombination rate also indicates that the sample with doping of SCN−possesses lower defect density than that of the pristine sample.Fig.4e shows the Urbach energy (EU)fitting curves of QDs.EUis extracted by fitting the exponential part of the Urbach tail according to the following equation26:

The smaller EUis,the lower trap density is.EUof SCN-CsPbI3is 66 meV,which is lower than that of the CsPbI3(89 meV).These results further demonstrate that doping of SCN−can reduce the defects of the QDs.

Due to poor stability,CsPbI3is easily degraded under humidity condition32,33.In order to prove that the SCN-CsPbI3has good stability,we have tested the water-resistance ability.As shown in Fig.5a,water and QDs were mixed in a cuvette in a volume ratio (1 :2),and the PL intensity was measured at intervals (Fig.5b).After 4 h,the mixture solution became transparent,indicating degradation of all of CsPbI3QDs.However,the SCN-CsPbI3still retained its initial PL intensity of 85% after contact with water for 4 h,indicating that the SCNCsPbI3possesses extremely strong water-resistance ability,which is also attributed to the strong bonding energy of Pb-SCN.In addition,we also tested the stability of the film,exposed the QDs film to a room with a relative humidity of 60% as shown in Fig.S6 (Supporting Information).The SCN-CsPbI3retained its cubic phase after 8 days of exposure in the environment,while the CsPbI3turned into yellow phase and lost fluorescence.Therefore,the doping of SCN−not only improves the crystallinity of the perovskite,but also increases the PLQY and the stability of both the perovskite QDs solution and the film.

Fig.5 Stability of QDs in water.

We carried out space charge limited current (SCLC)measurements to test the charge transfer performance of QD films.The structure of the electron-only device with FTO/TiO2/QDs/PCBM/Ag was designed as shown in Fig.S7a(Supporting Information).Three featured regions can be observed in SCLC curves.At low voltages,the I–V response shows linear (I ∝ V),called ohmic conduction.At high voltages,the current shows a quadratic voltage dependence (I∝ V2),called Child’s regime.The intermediate region is the trap-filled limit (TFL)area (I ∝ Vn,n ＞ 2),indicating that the available trap states are filled by the injected carriers.34−36The onset voltage of the TFL (VTFL)is proportional to the density of trap states ntrap.The SCN-CsPbI3film has a VTFLof only 0.08 V,which is much smaller than that of the CsPbI3film (0.22 V),which further confirms that the QDs film doped with SCN has a lower defect density.Photodetectors based on CsPbI3and SCNCsPbI3QDs have been fabricated,with the device configuration and energy band alignment shown in Fig.6a,b.The photodetector based on SCN-CsPbI3QDs exhibits a higher photocurrent density than that of the control device as shown in Fig.6c.Furthermore,the dark current of the SCN-CsPbI3QD-based photodetector is lower than that of the CsPbI3QD-based device as shown in Fig.6d,indicating that the less defects within the SCN-CsPbI3QD film.As shown in Fig.6e,both devices exhibit broad responses from 400 to 700 nm.The SCN-CsPbI3QD based photodetector shows almost twice the responsivity(more than 90 mA·W−1)than that of the CsPbI3QD based device(55 mA·W−1).Specific detectivities (D*)of two photodetectors are shown in Fig.6f.D* of the device based on the SCN-CsPbI3QDs is 1.5 × 1011Jones at 680 nm while the device based on the CsPbI3QD is only 9 × 1010Jones.These are due to SCN-doped CsPbI3QDs and their films possessing low defect density as already described.

Fig.6 Device structure and photodetection performance of photodetectors based on QDs.

4 Conclusions

SCN−doped CsPbI3QDs was successfully obtained by hotinjection synthesis process.The doping of SCN−greatly reduced the defect density and enhanced the crystal quality of the QDs.The PLQY of the doped sample exceeded 90%,great higher than that of the pristine (68%).The SCLC test showed that the VTFL(0.08 V)of the SCN-CsPbI3QD was significantly lower than that of the undoped CsPbI3(0.22 V),indicating low trap density for doped samples.At the same time,the doping of SCN−also greatly enhanced the water-resistance ability of CsPbI3.The doped sample maintained its initial PL intensity above 85% in water for 4 h.The photodetectors based on SCN-CsPbI3QDs showed broad band detection from ultraviolet (400 nm)to near infrared (700 nm)The detectivities of the detector based doped QD at 700 nm is as high as 1011Jones,which is much higher than that of the control photodetector (1010Jones).In addition,extending the doping of SCN−into CsPbCl3and CsPbBr3QDs also gave a significant increase in optical properties.

Supporting Information:available free of charge via the internet at http://www.whxb.pku.edu.cn.