Enhancing the Performance of Inverted Perovskite Solar Cells via

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Enhancing the Performance of Inverted Perovskite Solar Cells via Grain Boundary Passivation with Carbon Quantum Dots Yuhui Ma, Heyi Zhang, Yewei Zhang, Ruiyuan Hu, Mao Jiang, Rui Zhang, Hao Lv, Jingjing Tian, Liang Chu, Jian Zhang, Qifan Xue, Hin-Lap Yip, Ruidong Xia, Xing'ao Li, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18867 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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ACS Applied Materials & Interfaces

Enhancing the Performance of Inverted Perovskite Solar Cells via Grain Boundary Passivation with Carbon Quantum Dots

Yuhui Ma1†, Heyi Zhang1†, Yewei Zhang1, Ruiyuan Hu1, Mao Jiang1, Rui Zhang1, Hao Lv1, Jingjing Tian3, Liang Chu2, Jian Zhang2, Qifan Xue3, Hin-Lap Yip3, Ruidong Xia1*, Xing’ao Li1, 2*, Wei Huang1, 4*

1Key

Laboratory for Organic Electronics and Information Displays (KLOEID),

Synergetic Innovation Center for Organic Electronics and Information Displays (SICOEID), Institute of Advanced Materials (IAM), School of Materials Science and Engineering (SMSE), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, P.R. China 2New

Energy Technology Engineering Laboratory of Jiangsu Provence & School of

Science, Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, P.R. China 3Institute

of Polymer Optoelectronic Materials and Devices State Key Laboratory of

Luminescent Materials and Devices South China University of Technology Guangzhou 510640, P. R. China 4Shanxi

Institute of Flexible Electronics (SIFE), Northwestern Polytechnical

University (NPU), Xi'an 710072, P.R. China

Keywords: Carbon quantum dots; Perovskite solar cell; Non-radiative recombination;

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Grain boundary passivation; MAPbI3。 Abstract The non-radiative recombination, the main energy loss channel for open circuit voltage (Voc), is one of the crucial problems for achieving high power conversion efficiency (PCE) in inverted perovskite solar cells (PSCs). Usually, grain boundary passivation is considered as an effective way to reduce non-radiative recombination because the defects (uncoordinated ions) on grain boundaries are passivated. We added the hydroxyl and carbonyl functional groups contained carbon quantum dots (CQDs) into perovskite precursor solution to passivate the uncoordinated lead ions on grain boundaries. Higher photoluminescence intensity and longer carrier lifetime were demonstrated in the perovskite film with CQDs additive. This confirmed that the addition of CQDs can reduce non-radiative recombination by grain boundary passivation. Additionally, the introduction of CQDs could increase the thickness of perovskite film. Consequently, we achieved a champion device with PCE of 18.24%. The device with CQDs remained 73.4% of its initial PCE after aged for 48 hours under 80% humidity in the dark at room temperature. Our findings reveal the mechanisms of how CQDs passivate the grain boundaries of perovskite which can improve the efficiency and stability of perovskite solar cells. 1. Introduction Organic-inorganic hybrid perovskite has caused a worldwide upsurge of research as a competitive photovoltaic material for its remarkable advantages, such as strong optical absorption ability, long charge carrier diffusion length, high charge carrier


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mobility, especially the outstanding performance and facile fabrication process of perovskite solar cells (PSCs). The power conversion efficiency (PCE) of PSC has exceeded 22% so far1-8. While many significant improvements have been achieved in PSCs, several challenges still remain. One crucial issue is how to reduce the charge carrier recombination at the grain boundaries (GBs) of polycrystalline perovskite9-12. Generally, the defects (dangling bonds) at the grain boundaries induce carrier recombination and further influence the performance of solar cells devices13-15. Therefore, reducing the density of defects at the grain boundaries, a process technically known as ‘grain boundary passivation’16, is necessary in PSCs development. Additive engineering is the most effective way to passivate the grain boundaries. In fact, many studies about additive engineering in perovskite layer have aimed at grain boundary passivation or improving the crystallinity and optical properties of the perovskite layer by the control of morphology18-24. Typically, we can divide the additives into seven categories according to functionalities17: ① Polymer18 ② Fullerene19 ③Metal halide salts20 ④Inorganic acids21 ⑤Solvent22 ⑥Organic halide salts23 ⑦ Nanoparticle24. Among them, nanoparticle additives is one of the most promising strategies to achieve high efficiency PSCs. Nanoparticle can passivate the grain boundaries and acts as the nucleation sites to enhance the crystallinity of the perovskite. For examples, Hagfeldt et al. added N-doped graphene into the perovskite precursor solution to reduce the grain boundaries and improve light harvesting properties. They achieved an increase in PCE from 17.3% to 18.7%24. Liao and



co-workers proposed a novel method to passivate the perovskite film by incorporating graphitic carbon nitride (g-C3N4) into the perovskite layer. The defect density of the perovskite layer with g-C3N4 was only half of the pristine perovskite layer and the PCE increase from 16.53% to 19.49%25. Although significant amount of research has been done on additives in the perovskite layer, the efficiency improvement mechanism of the additives is not clear up to now. As a new member of the carbon material family, carbon quantum dots drew a lot of attention because of their dramatic optical properties. Many researchers put their effort on the applications of CQDs, such as the active components of the LEDs26 and sensitizers in solar cells27. Herein, we introduce CQDs passivating grain boundaries of perovskite as a method for high performance perovskite solar cells. On the one hand, the carbonyl functional groups on CQDs can slow down the perovskite crystal growth, resulting to larger perovskite grains. On the other hand, the CQDs can passivate the uncoordinated lead ions in the grain boundaries of perovskite by hydroxyl and carbonyl functional groups to reduce non-radiative recombination. The combination of these two effects leads to a better light absorption and fewer intrinsic defects of perovskite layer. Finally, we achieved a maximum PCE of 18.24% with a Voc of 1.07 V, a Jsc of 21.68 mA cm-2 and a fill factor of 0.78. The uncapsulated device with CQDs remained 73.4% of its initial PCE after aged for 48 hours under 80% humidity in the dark at room temperature. 2. Experimental section 2.1 The synthesis and purification of Carbon Quantum Dots (CQDs):


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8g NaOH was mixed with 50 mL acetone under vigorous magnetic stirring for 1 hr. Then, the mixture was placed at ambient condition for 120 hr. Subsequently, a diluted HCl solution (1 M) was added drop by drop into the mixture to neutralize pH28. To purify the CQDs, the mixture was evaporated at 80 °C and then the product was dissolved in ethanol. This process was repeated three times. 2.2 Preparation of NiOx nanoparticles. First, 12.885 g of NiCl2·6H2O was dissolved in 100 mL of deionized water under magnetic stirring. Then, 10 M NaOH solution was added into the solution drop by drop until the pH value reached 10. A turbid green solution was obtained and centrifuged. After being washed twice with deionized water, the obtained precipitation was dried at 80 °C overnight and then annealed at 270 °C for 2 hr. 2.3 Perovskite precursor solution Perovskite (MAPbI3) precursor solution was prepared by mixing the MAI powder, PbI2 and Pb(Ac)2 at mole ratio of 2.2:0.4:0.6 in N, N-dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich) under magnetic stirring for 6 hr. MAI, PCB61M were purchased from Borun New Material Technology Corp., and Pb(Ac)2, PbI2 from Xi’an Polymer Light Technology Corp. The perovskite precursor solutions with different contents of CQDs additive were prepared by mixing the powders in CQDs-DMF dispersion solutions. The concentrations of CQDs were 0.1 mg/mL, 0.15 mg/mL and 0.2 mg/mL (0.1CQDs, 0.15CQDs, 0.2CQDs) respectively. 2.4 Device fabrication



The conductive ITO (Tin Oxide) glass substrates (7 Ω, RS-1) were cleaned in detergent, deionized water, acetone and anhydrous ethanol in an ultrasonic bath for 15 min each. Subsequently, the substrates were plasma-cleaned for 4 min before being coated with a 30 nm-thick NiOx layer. NiOx (20 mg/ml in deionized water) was spin-coated onto pre-cleaned substrate at 4000 rpm for 30 s and baked at 135 oC for 10 min. Following this, the perovskite precursor solution was spin-coated at 4000 rpm for 30s in a N2-filled glovebox. Then, the sample was heated on a hot plate at 100 oC for 20 min. After that, ETL of PCB61M solution (20 mg/ml) in chlorobenzene (99.5%, Aladding) was spin-coated on top of the perovskite layer at 1200 rpm for 30 s. Finally, 10nm-thick BCP (Borun New Material Technology Co., Ltd.) and 100 nm-thick Ag were evaporated as the interface layer and top metal electrode under a base pressure of 9×10-5 Pa. 2.5 Characterization High resolution transmission electron microscope (HRTEM) images were acquired on a Talos F200X transmission electron microscope (FEI, USA) operating at an acceleration voltage of 200 kV. CQDs dispersion solution were dropped onto Formvar-graphite-coated copper grids (300 mesh; Beijing Zhongjingkeyi Technology Co., Ltd. ) and air-dried for HRTEM imaging. Fourier Transform Infrared Spectro-scopy (FT-IR) measurements were carried out on Fourier Transform Infrared spectrometer (Model: IRPrestige-21, range 4000 to 1000 cm-1). Optical absorption (UV-Vis) measurements were carried out on a Lambda 35 spectrophotometer (Perkin–Elmer). The transientstate photoluminescences (PL) was measured by a


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FLSP920 spectrometer (Edinburgh Instruments Ltd) and the excitation wavelength were 390 nm~470 nm for CQDs dispersion solution, 550 nm for the perovskite films. The time-resolved decay transients (TR-PL) was measured by a FLS980E fluorometer (Edinburgh Instruments Ltd) with the excitation wavelength of 765 nm. The morphologies of the different perovskite films were obtained by a field emission scanning electron microscopy (SEM, Hitachi S4800 microscope). The X-ray diffraction (XRD) patterns of the films were conducted by a Bruker D8 ADVANCE X-ray diffractometer under operation condition of 40 kV and 40 mA. The fluorescence spectra were obtained by confocal laser scanning fluorescence microscopy (CLSM, FV1000MPE, Olympus). The excitation wavelength was 458 nm and the emission collection range was between 550 nm and 600 nm. External quantum efficiency (EQE) measurements were carried out with QE-R3011 (Enlitech, Taiwan). The electrical impedance spectroscopy (EIS) of devices were conducted by a Wayne Kerr 6500B analyzer, the spectra were fitted using software ZView 3.10. The current density−voltage (J-V) curves of devices were measured by a Keithley 2400 Source Meter under an illumination of 1 sun (100 mW/cm2 AM 1.5G, generated by a solar simulator Oriel Sol3A, Newport Corp.), which was calibrated with a standard Si photodiode. The active area was 0.096 cm2. 3. Results and discussion The CQDs was synthesized via an effective one–step NaOH-assisted treatment of acetone28. Under strong alkaline conditions, oligomer chains can be formed because of the aldol reaction and polymerization reaction of acetone. Then the oligomer chains



curl and intertwine to produce CQDs with the assistance of NaOH. Fig. 1a shows the transmission electron microscope (TEM) image of CQDs. It is obvious that CQDs can be well dispersed in N,N-Dimethylformamide (DMF) with diameters in the range of 1.62-7.36 nm. The distribution of the nanoparticle size is shown in Fig. 1b. It is noteworthy that CQDs can be easily dispersed in DMF, but not in an aqueous system, suggesting hydrophobic behavior of the CQDs (Fig. S1) (Supporting Information). The photograph of CQDs powder is shown in Fig. S1 and the XRD pattern is shown in Fig. S2. Fourier transform infrared spectroscopy (FT-IR) (Fig. 1c) was studied to identify the functional groups on the surface of CQDs. The broad peak around 3443 cm-1 corresponds to the stretching vibrations of C-OH. The stretching vibrations of C-H (2963 cm-1), C=O (1709 cm-1) and C=C (1664 cm-1) were observed which suggested the existence of alkyl functional groups. The peaks in the range of 1100-1460 cm-1 are due to –CH2– stretching vibration deformation. The high ratio of hydrophobic functional groups on the surface of CQDs makes it hydrophobic, which is consistent with other reported hydrophobic CQDs29. Fig. S3 shows the UV-vis absorption spectra of CQDs. A wide peak range from 255 nm to 275 nm represented the typical n-p* transition of the carbonyl functional groups30. The steady state photoluminescence (PL) of CQDs in DMF solution (1 mg/ml) was investigated as shown in Fig. 1d, which exhibits a dependence on the excitation wavelength. As the excitation wavelength increases from 390 to 470 nm, the emission peak of the CQDs is red shifted from 510 to 550 nm. The excitation-dependent PL behavior which is typical of CQDs can be ascribed as the multiple discrete electronic states and slow


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solvent relaxation31.

Fig. 1. a) HRTEM image of CQDs dispersed in DMF, b) The distribution of the CQD nanoparticle size, c) FT-IR spectra and d) PL spectra of CQDs in different excitation wavelengths. It is well known that the nanoparticle additives can enhance the crystallinity of perovskite32. Fig. 2a and 2b illustrate the scanning electron microscope (SEM) images of surface morphology of pristine and 0.15CQDs films, respectively. The 0.15CQDs film expresses nearly full surface coverage with larger grains with the pristine film by comparison. The mean grain size was approximated to be ~174 nm for the pristine perovskite and ~271 nm for the 0.15CQDs. We ascribe the larger grain size to the carbonyl groups on CQDs which forms an intermediate adduct with Pb2+. As a result, the growth rate of perovskite grains was retard as claimed by Bi and his



co-workers33. In contrast, for the pristine perovskite layer, excess amounts of MAI provide too many nucleation sites for perovskite to grow which restrict the grain size, therefore, the thickness of the films34. The cross-section SEM images of the PSC devices further evidence the rise in grain sizes of perovskite with CQDs (0.15 mg/mL) additive as shown in Fig. 2c and 2d, wherein the compact NiOx film is used as the hole transport layer (green), the [6,6]-phenyl-C61-butyric acid methyl ester (PCB61M) as the electron transport layer (red), and the perovskite as the absorber (blue). ITO and silver was chosen as the bottom and top electrode respectively. The average thickness of pristine perovskite layer is only 209 nm. But, with 0.15CQDs additive, the average thickness of light absorption layer is 267 nm. The film thickness of perovskite layer is only related to the accelerated speed, the rotation speed in spin-coating process and the concentration of perovskite precursor because the perovskite film was made without dripping anti-solvent. However, all these parameters are the same between the pristine perovskite film and CQDs modified perovskite film. So we supposed that the addition of CQDs would lead to the increase in viscosity of perovskite precursor which will make the precursor film thicker. Therefore, the CQDs modified perovskite layer is thicker than the pristine perovskite film. To identify correlation between CQDs concentration and grain size, the SEM measurements were repeated with 0.1-0.2 mg/mL CQDs concentration. The results are presented in Fig. S4 and S5. The distribution statistics of the grain size under different CQDs concentration in Fig. S6. The SEM study shows that modest concentration of CQDs increases grain size and assist heterogeneous crystallization. However, a higher concentration (over 0.2


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mg/mL) of CQDs would lead to a decrease in grain size. So we speculate the nucleation of CQDs modified perovskite as three points: Ⅰ. the hydroxyl and carbonyl functional groups on CQDs surface interact with Pb2+ ions which forms an adduct in perovskite precursor (proved by CQDs’ photoluminescence quenching in perovskite precursor in Fig. S7). Ⅱ. the CQDs-PbI2 intermediate adduct template as the heterogeneous nucleation sites for MAPbI3. Ⅲ. The MAI reacted with the CQDs-PbI2 intermediate adduct template which substituted CQDs during the spin-coating and annealing process. Hence, a too high concentration of CQDs would lead to an increase in the number of nucleation sites. So a higher concentration (over 0.15 mg/mL) of CQDs would lead to a decrease in grain size. It is already established that the high crystallinity of perovskite can significantly improve the photovoltaic performance32. The effect of CQDs on the crystallization of perovskite layer was investigated by the X-ray diffractometer (XRD) (Fig. 2e). The XRD patterns of all samples revealed that the peaks correspond to the reflections from (110), (112), (211), (220), (104), (114) and (312) planes of orthorhombic space group (I4/mcm)35. This means the CQDs additive doesn’t affect the crystal structure and crystalline orientation of MAPbI3. The increase in diffraction peak intensities of (110) and (220) planes implied that the CQDs cause the perovskite a better crystallinity. The UV-Vis absorption spectra (Fig. 2f) illustrate a same trend with XRD data. The 0.15CQDs film had a slight enhancement in the visible light spectral range compared with the pristine film. We believe the slight enhancement in absorbance in the visible light spectral was caused by the higher crystallinity and thicker light absorption layer.



Nevertheless, carbon quantum dot can absorb sun light competing with the perovskite. In this work, the amount of CQDs is only 0.15mg/mL in perovskite precursor which is too low to absorb sun light against with perovskite (729.2 mg/mL) and the CQDs mainly absorb the near ultraviolet light (Figure. S3) while the enhancement in absorbance is in whole visible light spectral range. Hence, the CQDs in perovskite layer didn’t affect the absorbance of perovskite layer much.

Fig 2. Top SEM image of a) Pristine and b) 0.15CQDs additive perovskite films.


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Cross-section SEM of c) Pristine and d) 0.15CQDs additive perovskite devices. e) XRD patterns of perovskite film with or without CQDs additive. f) Absorbance spectra of pristine and 0.15CQDs additive films. To further investigate the presence of CQDs in perovskite layer, we used confocal laser scanning fluorescence microscopy (CLSM) to obtain the fluorescence spectra of CQDs on perovskite surface36. To avoid the fluorescence of PbI2 (emission between 480 and 550 nm37 ), the emission collection range was set between 550 and 600 nm. The bright field imaging of 0.15CQDs additive perovskite films is shown in Fig. 3a. The perovskite grains (represented as red) can be observed obviously. The emission between 550 and 600 nm is attributed to CQDs represented as green in Fig. 3b. Fig. 3c shows the combined picture of Fig. 3a and Fig. 3b. Apparently, the emission points are mainly distributed in the surrounding of the perovskite grains with only very few in grains or on the surface of grains, confirming that the CQDs are mainly distributed in the perovskite grain boundaries. This is consistent with the grain boundary theory that grain boundaries are preferential sites for segregation of impurities. Here, the CQDs act as ‘impurities’ in perovskite grain growth. As mentioned above, there are some functional groups on the surface of CQDs ‘impurities’ such as hydroxyl and carbonyl functional groups. The interaction between hydroxyl functional groups and Pb2+ ions, and between carbonyl functional groups and lead iodide have been reported previously38. Both these two interactions can also be proved by the photoluminescence quenching in the CQDs solution with Pb2+ (Fig. S7). Therefore, the mechanism of perovskite grain boundaries passivation



by CQDs can be concluded. The schematic diagram of CQDs passivation is shown in Fig. 4. The mechanism of perovskite grain boundaries passivation by CQDs can be speculated as six steps. Firstly, the hydroxyl and carbonyl functional groups on CQDs surface interact with Pb2+ ions which forms an adduct in perovskite precursor. Secondly, the CQDs-PbI2 intermediate adduct template as the heterogeneous nucleation sites for MAPbI3. Thirdly, the MAI reacted with the CQDs-PbI2 intermediate adduct template which substituted CQDs during the spin-coating and annealing process because the bonding between the CQDs and lead iodide is weaker than MAI and lead iodide. So the perovskite crystal growth was retard due to the interaction between CQDs and lead iodide. Fourthly, the CQDs act as the ‘impurities’ and aggregate at the grain boundaries in the process of perovskite grains growth. Fifthly, the uncoordinated lead ions on grain boundaries (dangling bonds) interact with hydroxyl and carbonyl functional groups on CQDs surface. Finally, the elimination of the dangling bonds of uncoordinated lead ions could reduce the lead ions migration and the non-radiative recombination.

Fig. 3. a) Bright field imaging of 0.15CQDs additive perovskite film morphology. Perovskite grains is represented as red. b) CLSM of 0.15CQDs additive perovskite


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film wherein the emission between 550 nm and 600 nm is attributed to CQDs (green). c) Combined picture of a) and b).

Fig. 4. a) Schematic diagram of the proposed mechanism of CQDs passivation. Pb2+, C, O, H and CQDs are illustrated as orange, black, red, green and cyan, respectively. To explore the effect of the CQDs additive, we measured the photovoltaic (PV) performance of devices with various CQDs additive perovskite as the absorber under AM1.5G illumination. Table in Fig. 5a summarizes all the PV parameters which were derived from J-V curve (Fig. 5a) of champion cells with different absorption layers. Compared with PV parameters obtained from pristine perovskite device, a pronounced increase of open circuit voltage (Voc) (from 1.04 to 1.07 V) and short



circuit photocurrent density (Jsc) (from 19.71 to 21.68 mA cm-2) were observed in device with 0.15CQDs additive perovskite as the absorber. The fill factor (FF) also increased slightly from 76.82% to 78.78%. All these improvements in key parameters translated into an increase in the overall PCE from to 15.7% to 18.2%. Here, two effects are involved in the increase in PCE. One is the increased grain size of perovskite which can improve the device performance. The other is the passivation effects of grain boundaries by CQDs. To identify which one dominate the increased device performance, we made a comparison. The average perovskite grain size of pristine film (173.81 nm) and 0.2CQDs (189.82 nm) is almost the same but the PCE increased from 15.67% to 16.75% (~ 1% up). Moreover, the distinction of average perovskite grain size between 0.1CQDs film (231.74 nm) and 0.2CQDs (189.82 nm) is 41.92 nm but the PCE only increased from 16.75% to 17.11% (~ 0.4% up). So it is obvious that the appropriate modification effects of boundary effects by CQDs dominate the increased device performance.


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Fig. 5. a) J-V characteristics of champion cells without and with various CQDs additive perovskite as the absorber. b) EQE and integrated current intensities of all the champion devices. c) PL spectra and d) time resolved PL spectra of devices with different amount of CQDs additive perovskite as the absorber. The steady-state photocurrent and PCE output under AM1.5G illumination for continually 300 seconds for both pristine and the 0.15CQDs cells were investigated (Fig. S8). The pristine device yielded a stable PCE of 15.25% with a photocurrent density of 16.945 mA/cm2 at a bias voltage of 0.9 V. For the 0.15CQDs device, a stabilized PCE of 17.73% was achieved with a photocurrent density of 19.064 mA/cm2 at a bias voltage of 0.93 V. The statistical histograms chart of PCE of 80 individual cells for both the pristine and 0.15CQDs can be found in Fig. S9. It can be clearly seen that the PCE of pristine devices is mainly distributed in 14.5-15.7% while



the PCE of 0.15CQDs is distributed in 16.8-18.2%. The monochromatic incident photon-to-current conversion efficiency (IPCE) is an effective tool to testify the escalation in photocurrent. Fig. 5b shows the EQE and the integrated current intensities of all the champion devices (pristine and various CQDs additive). A stronger spectral response in the range below 750 nm was observed for 0.15CQDs sample compared with the pristine sample as a reference. The integrated current density (19.16 mA cm-2 and 20.82 mA cm-2 for pristine and 0.15CQDs, respectively) calculated from the IPCE data had the same trend with results from the J-V curve. The enhancement in the photocurrent density was put down to the higher absorbance of 0.15CQDs additive perovskite caused by higher crystallinity and increased thickness. To further understand the mechanisms of increase in key parameters of PSC devices caused by CQDs additive, several measurements were taken. The photoluminescence spectroscopy (PL) and time resolved photoluminescence spectroscopy (TRPL) were employed to gain insight into the effect of the CQDs additive in PSCs. Fig. 5c shows the PL peaks fixed at around 765 nm for all the samples. However, the PL intensities of the modified perovskite film are much higher than that of the reference film. It has been demonstrated that the main energy loss channel is the non-radiative recombination loss39, which could reduce the PL intensity. Hence, the significant increase of the PL intensity, i.e., the radiative recombination intensity, demonstrated by the CQDs additive films suggested less non-radiative recombination loss. We attribute the less non-radiative recombination loss to the passivation of grain boundaries caused by the CQDs additive since the grain boundary


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is the main non-radiative recombination center. We note that the PL intensities decrease with the content of CQDs increased from 0.15 mg/mL to 0.2mg/mL. This could be the case that the excess CQDs incorporated into grains as impurities, which would act as the non-radiative recombination center in perovskite grains to decrease PL intensities. The TRPL data further support our hypothesis that the CQDs additive could passivate the grain boundaries which means reduced levels of non-radiative recombination. A bi-exponential decay function was used to fit the PL decay time (τi) and the amplitudes (Ai) of the Glass/perovskite samples (Fig. 5d) because there are two different channels for PL quenching. Typically, the fast decay time τ1 is assigned as bimolecular recombination of photo-generated carriers, while the long decay time τ2 is ascribed to trap assisted recombination40. The key parameters were listed in Table S1. The pristine sample’s PL decay time are τ1 = 1.62 ns and τ2 = 2.91 ns with corresponding amplitudes 66.4% and 33.6%, respectively. With the 0.15mg/mL CQDs additive in the perovskite layer, the PL decay time τ1 and τ2 are increased to 25.09 ns and 95.62 ns. The CQDs passivation should mainly reduce traps between the grain boundaries and at the surface, so the increase in slow decay lifetime supports our proposed mechanism. For purpose of seeking the difference in carrier lifetime, the Eq. (1) was calculated41.

(1) The average carrier lifetime (τavg) reflects the excited-state decay and free carrier recombination dynamics in the perovskite layer. The results show that the average



carrier lifetime of pristine film is only 2.23 ns. The average carrier lifetime of the CQDs passivated film becomes much longer than the average carrier lifetime of pristine film. It increases to 79.55, 86.01 and 16.62 ns when the CQDs content increases to 0.1, 0.15 and 0.2 mg/mL. Since no charge transfer layer exists, the non-radiative recombination should be the main PL decay components. The longer lifetime suggests that the non-radiative recombination was suppressed by the CQDs passivation, making it less possible for carriers to quench. Therefore, a higher photocurrent density could be achieved in PSC device with CQDs additive perovskite as the absorber. To study the densities of defects (Ndefects) of pristine and CQDs additive perovskite films, capacitor like devices were fabricated by sandwiching the perovskite films between indium tin oxide (ITO) and silver (Ag). The space-charge-limited current (in the dark condition) was measured as a function of the different bias voltage (Fig. 6a). The density of defects can be calculated by the Eq. (2)

(2) where ε is the relative dielectric constant42, ε0 the dielectric constant of vacuum permittivity, L the perovskite film thickness, e the elementary charge, and the VTFL trap-filling limited voltage. The trap-filling limit region is a range of bias voltage where all the defects are occupied by charge carriers. There is a kink point in the current density-voltage (J - V) curve where the bias voltage reaches VTFL. The VTFL shown in Fig. 6a are 0.225 V, 0.164 V, 0.123 V and 0.175 V for the pristine (reference), 0.1CQDs, 0.15CQDs, and 0.2CQDs samples, respectively. The Ndefects


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were estimated to be 1.8×1016 cm-3, 8.13×1015 cm-3, 6.1×1015 cm-3 and 8.68×1015 cm-3 for the reference, 0.1CQDs, 0.15CQDs, and 0.2CQDs samples, respectively. The defect density in pristine device is three times of that in the 0.15CQDs device, suggesting the defects has been passivated significantly by CQDs. The reduced defect density is in agreement with the enhanced PL intensity and prolonged PL lifetime. The density of defects raised with the content of CQDs additive increased from 0.15 to 0.2 mg/mL, which is in line with the speculation that the excess amount of CQDs would act as the recombination center in perovskite grains.

Fig 6. a) Dark J-V curves of devices with ITO/perovskite/Ag used for analyzing the densities of defects b) EIS of devices with different amount of CQDs in dark condition c) Dark J-V curves of champion cells without and with CQDs d) Light



intensity dependence of the Voc of pristine and 0.15CQDs perovskite film. The light intensity increased from 1mW/cm2 to 100mW/cm2 . To quantitatively evaluate the recombination resistance and series resistance, the electrical impedance spectroscopy (EIS) analysis of all the samples was characterized. Fig. 6b gives the Nyquist plots in the dark condition at an applied voltage of 1 V. The equivalent circuit composed of series resistance (Rs), recombination resistance (Rrec) and a parallel capacitor was employed to extrapolate the value of Rs and Rrec. While Rs was assigned to the high-frequency region (>500Hz) in EIS, Rrec ( 175 um in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347 (6225), 967-970. (43). Niu, T.; Lu, J.; Munir, R.; Li, J.; Barrit, D.; Zhang, X.; Hu, H.; Yang, Z.; Amassian, A.; Zhao, K.; Liu, S. Stable High-Performance Perovskite Solar Cells via Grain Boundary Passivation. Adv. Mater. 2018, 30 (16), 1706576. (44). Qi, B.; Wang, J. Fill Factor in Organic Solar Cells. Phys. Chem. Chem. Phys. 2013, 15 (23), 8972-8982. (45). Chen, C.; Liu, D.; Zhang, B.; Bi, W.; Li, H.; Jin, J.; Chen, X.; Xu, L.; Song, H.; Dai, Q. Carrier Interfacial Engineering by Bismuth Modification for Efficient and Thermoresistant Perovskite Solar Cells. Adv. Energy Mater. 2018, 8(20), 1703659. (46). Chen, C.; Liu, D.; Wu, Y.; Bi, W.; Sun, X.; Chen, X.; Liu, W.; Xu, L.; Song, H.; Dai, Q. Dual Interfacial Modifications by Conjugated Small-Molecules and Lanthanides Doping for Full Functional Perovskite Solar Cells. Nano Energy 2018, 53, 849-862. Graphical Abstract:


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