Improved open-circuit voltage and ambient stability of CsPbI2Br perovskite solar cells by incorporating CH3NH3Cl
来源期刊:Rare Metals2020年第2期
论文作者:Chuan-Liang Chen Sha-Sha Zhang Tian-Lun Liu Shao-Hang Wu Zhi-Chun Yang Wei-Tao Chen Rui Chen Wei Chen
文章页码:131 - 138
摘 要:Inorganic cesium metal halide perovskites have gained research interest as absorbers in perovskite solar cells due to their superior thermal stability.Among these,CsPbI2Br,with a narrower band gap than CsPbBr3 and a better phase stability than CsPbI3,has received tremendous interest of the researchers.However,CsPbI2 Br takes adverse phase transfer easily with an exposure to the water vapor in ambient air which not only brings inconvenience for researches but also puts forward very high requirement for encapsulation.Herein,a dense and uniform film is obtained by incorporating hydrophobic CH3NH3Cl(MACl)into the precursor solution.Being attributed to a good passivation effect,the defect density is decreased from3.12×1016 to 1.49×1016 cm-3 and the average photoluminescence lifetime is increased from 8.84 to 20.6 ns.The photovoltaic device achieves a high open-circuit voltage of 1.22 V based on optimized MACl-doped film and accordingly a higher power conversion efficiency(PCE) of 12.9% which is 21.7% higher than the pristine CsPbI2Br device with PCE of 10.6%.In addition,the ambient stability of MACl-doped device has been enhanced,which is greatly attributed to the hydrophobic properties of MACl.This work provides a clue to improve ambient stability of inorganic perovskite solar cells and inspires toward further development of this material.
稀有金属(英文版) 2020,39(02),131-138
Chuan-Liang Chen Sha-Sha Zhang Tian-Lun Liu Shao-Hang Wu Zhi-Chun Yang Wei-Tao Chen Rui Chen Wei Chen
Wuhan National Laboratory for Optoelectronics,Huazhong University of Science and Technology
China-EU Institute for Clean and Renewable Energy,Huazhong University of Science and Technology
Shenzhen Key Laboratory of Nanobiomechanics,Shenzhen Institutes of Advanced Technology,Chinese Academy of Sciences
作者简介:*Wei Chen,e-mail:wnlochenwei@mail.hust.edu.cn;
收稿日期:20 February 2019
基金:financially supported by the National Natural Science Foundation of China (Nos.51672094, 51861145404 and 51822203);the China Postdoctoral Science Foundation (No.2016M602286);the Self-determined and Innovative Research Funds of HUST (No.2016JCTD111);Shenzhen Science and Technology Innovation Committee (No.JCYJ2017030716590 5513);the Natural Science Foundation of Guangdong Province (No.2017A030313342);
Chuan-Liang Chen Sha-Sha Zhang Tian-Lun Liu Shao-Hang Wu Zhi-Chun Yang Wei-Tao Chen Rui Chen Wei Chen
Wuhan National Laboratory for Optoelectronics,Huazhong University of Science and Technology
China-EU Institute for Clean and Renewable Energy,Huazhong University of Science and Technology
Shenzhen Key Laboratory of Nanobiomechanics,Shenzhen Institutes of Advanced Technology,Chinese Academy of Sciences
Abstract:
Inorganic cesium metal halide perovskites have gained research interest as absorbers in perovskite solar cells due to their superior thermal stability.Among these,CsPbI2Br,with a narrower band gap than CsPbBr3 and a better phase stability than CsPbI3,has received tremendous interest of the researchers.However,CsPbI2 Br takes adverse phase transfer easily with an exposure to the water vapor in ambient air which not only brings inconvenience for researches but also puts forward very high requirement for encapsulation.Herein,a dense and uniform film is obtained by incorporating hydrophobic CH3NH3Cl(MACl)into the precursor solution.Being attributed to a good passivation effect,the defect density is decreased from3.12×1016 to 1.49×1016 cm-3 and the average photoluminescence lifetime is increased from 8.84 to 20.6 ns.The photovoltaic device achieves a high open-circuit voltage of 1.22 V based on optimized MACl-doped film and accordingly a higher power conversion efficiency(PCE) of 12.9% which is 21.7% higher than the pristine CsPbI2Br device with PCE of 10.6%.In addition,the ambient stability of MACl-doped device has been enhanced,which is greatly attributed to the hydrophobic properties of MACl.This work provides a clue to improve ambient stability of inorganic perovskite solar cells and inspires toward further development of this material.
Keyword:
CsPbI2Br perovskite; Passivation; Ambient stability; MACl;
Received: 20 February 2019
1 Introduction
Hybrid organic-inorganic halide perovskite solar cells have been considered as a promising photovoltaic technology with a rapid rise in power conversion efficiency(PCE) from 3.8%to 22.7%and a low cost for fabrication by solution process
In this paper,we incorporated a small amount of hydrophobic material of CH3NH3Cl (MACl) to CsPbI2Br precursor solution.The influences of the MACl content on the morphology and defect density of the perovskite film as well as the final device performance were studied carefully.Ultraviolet-visible (UV-Vis) spectra and X-ray photoelectron spectroscopy (XPS) characterization were also conducted to analyze the band gap and composition changes.Furthermore,the stability of CsPbI2Br solar cells based on MACl additive was tested,which is much better than that of the pristine device.This work provides useful information about perovskite solar cells and is an advance of practical applications to cater the current energy need of the world.
2 Experimental
2.1 Device fabrication
Glass substrates (TEC-15,NSG Pilkington) with the etched fluorine-doped tin oxide (FTO) coating were first ultrasonically cleaned with detergent solution,Milli-Q water,ethyl alcohol and acetone in sequence.After drying with clean dry air,a p-type NiMgLiO film serving as hole extraction layer was deposited onto FTO glass by spray pyrolysis at 550℃according to our previous work
2.2 Characterization
Scanning electron microscopy (SEM) images were obtained via a Nova Nano 450 SEM (FEI Co.,the Netherlands) at a 5 kV accelerating voltage.X-ray diffraction (XRD) characterization was performed on a Philips X-ray diffractometer with Cu Kαradiation.X-ray photoelectron spectroscopy (XPS) measurements were carried out on an AXIS-ULTRA DLD-600W Ultra spectrometer (Kratos Co.,Japan).The ultraviolet-visible (UV-Vis) spectra were obtained from a Lambda 950 spectrophotometer (PerkinElmer Co.,USA).The PL spectra were performed on an Edinburgh FLS920 fluorescence spectrometer (Edinburgh Co.,UK).The current densityvoltage (J-V) curves were measured via a Keithley 2400source meter.A solar simulator (Oriel,model 9119) with AM 1.5G filter (Oriel,model 91192) was used to provide an irradiance of 100 mW·cm-2,and the light intensity of the simulated solar light was precisely calibrated with a standard Si photodiode detector.The effective area of the solar cell was defined to be 0.09 cm2 with a black metal mask.The incident photo-to-electron conversion efficiency(IPCE) was measured on a Newport IPCE system (Newport,USA).The Mo tt-Schottky plots were obtained via an electrochemical workstation (Zahner Zennium,Germany).
3 Results and discussion
Based on anti-solvent method,CsPbI2Br films with varying MACl contents were fabricated.To investigate the effect of different MACl additions on the surface morphology of perovskite films,top-view SEM images were processed and the results are shown in Fig.1.It is found that the changes are not manifest among the perovskite films when the MACl content varies from 0 to 0.03,while if the MACl content is further increased to more than 0.05,the CsPbI2Br film becomes inhomogeneous and lots of small grains emerge (Fig.1d,e).Such results indicate that moderate MACl additive is beneficial for the interface passivation and assists in achieving high-quality inorganic perovskite films.
Fig.1 Surface SEM images of CsPbI2Br(MACl)x films:a x=0,b x=0.01,c x=0.03,d x=0.05 and e x=0.08
XRD measurements are provided to quantify the effect of MACl additive on the crystallinity of CsPbI2Br film,and the results are depicted in Fig.2.It is noted that all the MACl-doped films show a typical perovskite phase with the dominant peaks at 14.6°and 29.5°,assigned to (100)and (200) planes,respectively,which is well consistent with previous studies
Fig.2 XRD patterns of CsPbl2Br(MACl)x films with varying MACl contents
For the purpose of understanding the effect of MACl additive on the elemental composition at the surface of inorganic perovskite films,XPS measurements were carried out on the CsPbI2Br and CsPbI2Br(MACl)0.03 films.The results are shown in Fig.S1.Obvious peak of Cl element centered at 198.2 eV is observed for the CsPbI2Br(MACl)0.03 film.Furthermore,there is a little shift for Pd 4f peaks toward lower binding energy,indicating that Pb-Cl bonds may be formed in the lattice.Atomic ratios are listed in Table S1.As seen,the Cl/Pb atomic ratio is about 3.2%,close to the molar content of MACl additive.Meanwhile,the reduced I/Pb atomic ratios are estimated to be 221%for the CsPbI2Br(MACl)0.03 film,far less than 247%for the CsPbI2Br film,which means that partial I-has been substituted by Cl-.Based on XPS results within the typical XPS detection depth of 10 nm,it is noted that the content of C increases sharply after incorporating 0.03 mol%MACl into CsPbI2Br film,which means that a great deal of MA+stays at the surface of inorganic film as a passivator.The results indicate that MA+is enriched at MACl-doped film surfaces,which can passivate defect states at the surface,quite similar to our previous study on Ca2+-doped perovskite films
Optical and PL measurements were also conducted on the CsPbI2Br(MACl)x films,and the results are presented in Fig.3.Figure 3a demonstrates that with the increase in MACl content from 0 to 0.03,the corresponding UV-Vis spectroscopy shows an apparent blueshift and their absorption onsets shift gradually from 656 nm (~1.9 eV)to 636 nm (~1.95 eV).In contrast,if the MACl content further increases to 0.08,the corresponding absorption peak positions keep almost constant,suggesting that it has been saturated for MACl doping just with a little content of0.03.At the same time,with the MACl content further increasing,the absorption of the inorganic film slightly degrades which may be attributed to large number of small grains demonstrated by SEM.
Steady-state photoluminescence (PL) spectra for CsPbI2Br(MACl)x films on glass substrates are given in Fig.3b.They exhibit apparent blueshift with MACl content increasing from 0 to 0.08,attributed to insertion of Clto the lattice of CsPbI2Br perovskite,which is in good agreement with the UV-Vis results.At the same time,their peak intensity increases first and then decreases with the increase in MACl content and achieves the maximum intensity for the 0.03 MACl additive.These results suggest that 0.03 MACl-doped film has the least defect states,which is consistent with their improved morphology and abrupt slope of UV-Vis absorption.
The improvement in the steady PL intensity usually relates to prolonged carrier lifetime,which can be verified by time-resolved PL spectra depicted in Fig.3c.Herein,the time-resolved PL decay curve can be fitted with following bi-exponential decay function
where F is the normalized intensity;t is the time;τ1,τ2 are lifetimes related to two kinds of recombination;and A1 and A2 are the related weight contents.The average decay time(mean PL average lifetime) can be obtained by the following equation:
whereτavg denotes the average decay time.With variation in the content of MACl additive,the fitting parameters (τ1,τ2,A1,A2) and PL average lifetime are summarized in Table S2.Obviously,the PL lifetime for MACl 0.03additive film is 20.6 ns,much larger than that for pristine CsPbI2Br film (8.84 ns) and MACl 0.08 additive film(1.11 ns).This result indicates that moderate MACl doping may be useful for improving optoelectronic properties of inorganic CsPbI2Br perovskite film.
Based on MACl-doped films,perovskite solar cells(PSCs) with inverted architecture of"FTO/NiMgLiO/CsPbI2Br(MACl)x/PCBM/BCP/Ag"(Fig.4a) have been fabricated.Their J-V characteristic curves of the champion samples under AM 1.5 G irradiation at 100 mW·cm-2 are demonstrated in Fig.4b,and the resultant performance parameters are listed in Table S3.At the same time,the statistic distribution of performance parameters depending on MACl additive content is presented in Fig.4c-f.In this study,the champion cell based on 0.03 MACl additive exhibits the highest power conversion efficiency (PCE) of12.9%,with an open-circuit voltage (VOC) of 1.22 V,a short-circuit current density (JSC) of 13.8 mA·cm-2 and a fill factor (FF) of 0.76.However,the CsPbI2Br-based cell only obtains a PCE of 10.6%,with VOC of 1.08 V,JSCof14.4 mA·cm-2 and FF of 0.68.Obviously,the main enhancement in PCE of 0.03 MACl device comes from enhanced VOc and FF due to better morphology and wider band gap.It is noteworthy that if the MACl content further increases from 0.03 to 0.08,VOC,JsC and FF decrease significantly to 0.95 V,12.6 mA·cm-2 and 0.62,respectively,resulting in a much degraded PCE of 7.4%.Such result may be attributed to poor morphology with excessive MACl additive.Furthermore,it is found that the device based on 0.03 MACl content exhibits little hysteresis compared to pristine CsPbI2Br-based device (Fig.S2),which is attributed to the excellent carrier extraction capability of surface and trap passivation by MACl.
Fig.3 a UV-Vis absorption spectra;b steady-state PL spectra;and c normalized time-resolved PL spectra of CsPbI2Br(MACl)x films with varying MACl contents on glass substrates
Fig.4 a Device architecture of FTO/NiMgLiO/CsPbI2Br(MACl)x/PCBM/BCP/Ag;b J-V characteristics of champion devices based on CsPbI2Br(MACl)x under forward scan direction;photovoltaic parameters of perovskite devices as a function of MACl additive content:c JSC,d VOC,e FF,f PCE (16 pieces of solar cells included)
It is obviously found that the JSC value for 0.03 MACldoped device is 13.8 mA·cm-2,which is smaller than that for the pristine CsPbI2Br-based device (14.4 mA·cm-2),matching well with the integratedJSC values from the IPCE data presented in Fig.5a.This result may be ascribed to the blueshift of absorption spectra,leading to a smaller cutoff wavelength.
Besides,VOC of CsPbI2Br(MACl)0.03-based cell achieves a value of 1.22 V,much higher than that of pristine CsPbI2Br-based cell (1.08 V) and CsPbI2Br(-MACl)0.08-based cell (0.95 V).The higher Voc is partly attributed to the wider band gap of~1.95 eV,but more importantly,the much-reduced defect density and more effective charge extraction at the surface may play a great role.To elucidate the origin of enhanced VOC,MottSchottky analysis has been performed on the two photovoltaic devices based on CsPbI2Br and CsPbI2Br(-MACl)0.03.Figure 5b presents the capacitance-voltage (1/C2-V) plots for the corresponding cells,and the built-in potentials (Vbi) can be acquired with the following MottSchottky equation
where V is the applied bias and the parametersε,ε0,q,A and N represent mean relative permittivity,vacuum permittivity,elementary charge,active area and free carrier concentration,respectively.Based on the method reported in our previous literature
Fig.5 a Incident photo-to-electron conversion efficiency (IPCE) curves (solid lines) with integrated photocurrents (dashed lines);b Mott-Schottky plots for champion devices based on CsPbI2Br and CsPbI2Br(MACl)0.03 films;c bilogarithmic diagram ofⅠ-Ⅴcurves in dark for devices with architecture of FTO/perovskite/Au
Fig.6 a Normalized PCEs of unencapsulated photovoltaic devices based on CsPbI2Br and CsPbI2Br(MACl)0.03 films under continuous illumination (simulated solar light,100 mW·cm-2),placed in ambient atmosphere with a humidity of~30%;b comparison of degradation speeds from two different samples:CsPbI2Br film and CsPbI2Br(MACl)0.03 film,two films were all placed in ambient atmosphere for 20 min
To further elucidate the impact of MACl addition on perovskite trap density,the I-V responses of two samples based on CsPbI2Br and CsPbI2Br(MACl)0.03,with the architecture of FTO/perovskite/Au,have been performed
In addition to device efficiency,the ambient stability of devices based on CsPbI2Br and CsPbI2Br(MACl)0.03 films is also examined.Figure 6a shows the normalized PCEs of unencapsulated devices under continuous illumination(simulated solar light,100 mW·cm-2) in ambient atmosphere (T=~25℃and relative humidity (RH)=~30%).It is found that the CsPbI2Br(MACl)0.03 device retains 80%of its initial efficiency for~8.1 h,nearly 1.8times that for CsPbI2Br device (~4.4 h).The enhanced ambient stability may be partially attributed to interface passivation;moreover,the incorporated MACl additive as a hydrophobic material may play an important role.Herein,two different devices,namely FTO/NiMgLiO/CsPbI2Br and FTO/NiMgLiO/CsPbI2Br(MACl)0.03,were fabricated and compared under their optimal interfacial condition.As demonstrated in Fig.6b,when the two devices were placed in ambient atmosphere,the pristine CsPbI2Br perovskite layer was corroded rapidly within~3 min,while the CsPbI2Br(MACl)0.03 layer was kept nearly undamaged for~10 min.Thus,the much stable CsPbI2Br(MACl)0.03layer gives the device higher stability in ambient atmosphere.
4 Conclusion
In summary,the effects of MACl on CsPbI2Br perovskite films and the based devices were investigated.With the incorporation of a small amount of MACl (≤0.03),CsPbI2Br perovskite films demonstrate denser and uniform morphology and achieve the best state when the MACl content is equal to 0.03.Being attributed to a good passivation effect,the defect density decreases dramatically and the average PL lifetime increases to 20.6 ns.Based on such optimized MACl-doped films,the champion cell achieves a PCE of 12.9%with much higher Voc of 1.22 V compared to the pristine CsPbI2Br cells,with the Voc of 1.08 V.In addition,compared with the CsPbI2Br-based device,the CsPbI2Br(MACl)0.03-based cell reveals a superior ambient stability,which is attributed to the improved perovskite quality and better hydrophobic characteristics of CsPbI2Br(MACl)0.03 layers.This work provides a clue for improving ambient stability of inorganic perovskite cells,which is of particular importance for practical application.
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