Congeneric Incorporation of CsPbBr3

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Letter 3

Congeneric Incorporation of CsPbBr Nanocrystals in Hybrid Perovskite Heterojunction for Photovoltaic Efficiency Enhancement Huachao Zai, Cheng Zhu, Haipeng Xie, Yizhou Zhao, Congbo Shi, Zhenxin Chen, Xiaoxing Ke, Manling Sui, Changfeng Chen, Jin-Song Hu, Qingshan Zhang, Yongli Gao, Huanping Zhou, Yujing Li, and Qi Chen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00925 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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ACS Energy Letters

Congeneric Incorporation of CsPbBr3 Nanocrystals in Hybrid Perovskite Heterojunction for Photovoltaic Efficiency Enhancement Huachao Zai,a,b,+ Cheng Zhu,a,+ Haipeng Xie,c Yizhou Zhao,a,b Congbo Shi,a Zhenxin Chen,d Xiaoxing Ke,d Manling Sui,d Changfeng Chen,b Jinsong Hu,e Qingshan Zhang,a Yongli Gao,f Huanping Zhou,g Yujing Li,*,a and Qi Chen*,a a. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China b. Department of Materials Science and Engineering, College of Science, China University of Petroleum, Beijing 102249, China c. Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China d. Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, China e. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, China

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f. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA g. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

ABSTRACT: Organic-inorganic hybrid perovskite materials have received remarkable success in photovoltaics due to their superior optoelectronic properties and compositional abundance. Most advances focus on the improvement of the heterojunction, in which non-perovskite materials are employed at the pertaining interfaces. Herein we demonstrate the modification of perovskite absorber by incorporation of CsPbBr3 nanocrystals, which is congeneric to the absorber in terms of crystal structure and stoichiometry. It led to the significant enhancement in the photovoltaic performance in the corresponding devices, which was mainly attributed to the improved carrier dynamics over the resultant heterojunction. Therefore, a different strategy is suggested for further improvement of the perovskite heterojunction by using congeneric materials.

SYNOPSIS


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Organic-inorganic hybrid perovskite material has attracted much attention recently. Its unique properties and compositional versatility have provided limitless space to envisage and exploit both in academia and industry.1-4 To date, the hybrid perovskite material has found various applications in optoelectronics, especially photovoltaics (PV). Capitalized on c-Si PV and other emerging PV technologies, the perovskite solar cells are readily soaring on the way to commercialization,2, 5-14 whereas most PV technologies have spent several decades. In order to improve the power conversion efficiency (PCE) and stability of perovskite solar cells, most strategies are trying to improve the heterojunction with the emphasis on interfaces,10,

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defects,18-20 and perovskite compositions from atomic to crystalline regime.1, 21-24 In the context of interface engineering, major efforts have been extracted to 1) improve the carrier transport materials in carrier mobility, concentration, and the band alignment; and 2) adjust the electronic


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band edge or optical field at the perovskite surface,20,

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during which various functional

materials have been exploited. Interestingly, these materials are mostly in forms of molecules or simple salts, which are radically distinguished from the perovskite absorber in nature. It thus casts serious challenges when developing compatible approaches to robust and efficient devices. The nanostructures of halide perovskite inherit the superior optoelectronic properties of its bulk counterpart, sharing the congeneric nature in terms of stoichiometry and crystal structure. Although they are promising to integrate into the perovskite heterojunction with extra processing compatibility, it has yet been less exploited. Recently, MAPbBr3-xIx nanocrystals (NCs) were embedded at the interfaces between perovskite and hole transport layer (HTL) to adjust the energy band alignment.30 In addition, CsPbBr3 nanowires were applied on top of the perovskite absorber to achieve localized gratings for improved fluorescence effect.31 However, these incorporation approaches were designed to take effect on the macroscopic level, wherein nanostructures were simply deposited on the perovskite surfaces. Their congeneric nature has not been fully recognized and utilized to modify the perovskite absorber in a microscopic level for enhancement of device performance. Herein, we demonstrate the congeneric incorporation of CsPbBr3 NCs into the perovskite heterojunction based on the absorber of FA0.85MA0.15Pb(I0.85Br0.15)3, wherein the NCs in the coherent stoichiometry effectively tailor the optoelectronic properties of the modified heterojunction. The photovoltaic efficiency of the resultant device enhanced from 17.95% to 19.45%, which was attributed to the efficient carrier transport behavior originated from the synergetic improvements at both the absorber bulk and its adjacent interface. In addition, the incorporation of CsPbBr3 NCs also improved the thermal stability of the corresponding devices. Therefore, our contribution suggests a universal strategy to modify the perovskite heterojunction


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ACS Energy Letters

in different aspects by the employment of congeneric materials in various building blocks, which ultimately paves the way for the practical use of hybrid perovskite materials. Constructing the heterojunction with NC modification CsPbBr3 NCs (Figure 1) were synthesized by the hot injection method (Supporting Information).32-33 The presence of oleic acid and oleylamine in octadecene dissolves PbBr2 and stabilize the NCs colloid. It can be clearly seen that CsPbBr3 NCs are monodisperse with an average size of 12 nm (Figure 1a), and the characteristic interplanar distance of the CsPbBr3 is 0.58 nm (high resolution transmission electron microscopy, HRTEM, the inset in Figure 1a), corresponding to the spacing of (100) plane. The X-ray diffraction (XRD) pattern of assynthesized CsPbBr3 NCs sample (Figure S1) is in line with the HRTEM measurement. To analyze their optical properties, the colloidal CsPbBr3 NCs were examined by UV-vis absorption and photoluminescence (PL) emission spectra (Figure 1b). The absorption onset of CsPbBr3 NCs is around 510 nm, reveling that the calculated bandgap (Eg) between the conduction band (CB) edge and the valence band (VB) edge is ~2.36 eV for CsPbBr3 NCs (Figure S2). The PL emission spectra shows a peak position at 512 nm (2.42 eV) with excitation at 450 nm. Remarkably bright PL of CsPbBr3 NCs was observed with high quantum yield of 78% and narrow emission line width of 20 nm.34 The CsPbBr3 NCs dissolved in chlorobenzene was used as a dripping solvent to further fabricate the heterojunction of FA0.85MA0.15Pb(I0.85Br0.15)3 perovskite by one-step deposition process (Figure 1c).35 As the dripping solvent extracting the precursor solvent (N, N-dimethylformamide and dimethylsulfoxide), the carried CsPbBr3 NCs penetrated into the precursor mixture during film growth and located at a certain depth in the asprepared film.


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It is known that FA0.85MA0.15Pb(I0.85Br0.15)3 and CsPbBr3 possess the same crystal structure (cubic ABX3), wherein the distances between two six-coordinated Pb atoms are 0.631 nm (FA0.85MA0.15Pb(I0.85Br0.15)3) and 0.583 nm (CsPbBr3), exhibiting a lattice mismatch of only 7.6%.36 In the context of crystal structure, the CsPbBr3 lattice matches well with the bulk perovskite, which endows atomic-level coherence when the inorganic CsPbBr3 NCs are incorporated with organic/inorganic hybrid perovskite during film formation. In the perspective of stoichiometry, the CsPbBr3 NCs are in coherence with the perovskite film. As a result, we expect the introduction of NCs will avoid disturbance in perovskite crystal growth and extra phase impurity and/or defects to cripple the optoelectronic properties in the resulted film, as discussed in the following. We first checked the XRD patterns of the CsPbBr3 modified perovskite films with different loading of NCs. The resultant films exhibited almost the same XRD patterns (Figure S3), which indicated the modification via CsPbBr3 NCs did not disturb the phase and crystal structure. But the diffraction intensity for the NC involved film is higher than that without NCs. The reduced peak widths (cf. the fitted peak widths in Figure S3b) suggested that introducing CsPbBr3 NCs, which could modify the nucleation and growth during perovskite crystallization, substantially improved the crystallinity of the resulting perovskite films. It is possibly because the introduction of CsPbBr3 NCs provides extra seed-like nucleation sites and/or localized precursor concentration, which promotes the formation of perovskite lattice structures.2, 37 We further attempt to locate the CsPbBr3 in the resultant film. The presence of cesium leads to the shrinkage of cubo-octahedral volume, which is verified by the shrinkage of lattice parameter observed in XRD measurement (Figure S3). Since the tiny size NCs were hardly observed by SEM or TEM, we employed angle-resolved X-ray photoelectron spectroscopy (AR-XPS) to


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ACS Energy Letters

characterize the depth profile of CsPbBr3 in the perovskite layer. Unfortunately, no Cs 3d signals were detected from the NCs modified perovskite films due to its inadequate content. By the inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement, the content of Cs in the film was determined to be 0.13 wt%, which was well below the XPS detection limit. Although the Cs could not be directly observed by AR-XPS, the distribution of CsPbBr3 can be sought out by examining Br content. As shown in Figure 1e, the Br/Pb atomic ratio of perovskite/CsPbBr3 sample increases as the take-off angle increases from 0 to 40°, but I/Pb ratio followed the opposite pattern. The measurement geometry of AR-XPS (Figure 1e inset) tells that the larger take-off angle means the smaller distance to the surface.38 It thus indicates that the CsPbBr3 depth profile in the CsPbBr3 modified perovskite film exhibited a gradual decrease. For direct and convincing evidence, we employed time-of-flight secondary-ion mass spectrometry (TOF-SIMS) to characterize the depth profile of Cs in the perovskite film. As shown in Figure S4, the Cs depth profile of the dripping-induced perovskite/CsPbBr3 sample exhibited a continuous decrease, verifying the formation of a gradient distribution. This result is in accordance with the TEM-EDX elemental line scans of the cross section, where perovskite/CsPbBr3 shows a compositional gradient of Cs-rich to Cs-lean across the perovskite film (as shown in Figure S5).


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Figure 1. Fabrication and characterization of the Perovskite/CsPbBr3 heterojunction. (a) TEM image of CsPbBr3 NCs, with an inset showing high-resolution TEM of CsPbBr3 NCs. (b) UV-vis absorption spectra (red line) and PL spectra (blue line, λexc = 450 nm) of CsPbBr3 NCs, with an inset showing colloidal solution in CB under UV lamp. (c) A schematic diagram of the


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deposition method for the perovskite film with CsPbBr3 NCs; the green dots represent CsPbBr3 NCs. (d) Cross-sectional TEM image of the dripping-induced device. (e) Atomic ratio of Br/Pb and I/Pb in the perovskite/CsPbBr3 film as a function of the take-off angle. The inset is the measurement geometry of AR-XPS, where λ is the mean diffusion length of electron in perovskite film.

To check the optoelectronic properties of the perovskite absorber, the above-mentioned FA0.85MA0.15Pb(I0.85Br0.15)3 perovskites with the modification of different concentration of CsPbBr3 NCs were further employed to fabricate solar cells, whose performance parameters were summarized in Table 1 and Figure S6. The device employed the architecture of ITO/SnO2/Perovskite/CsPbBr3/Spiro-OMeTAD/Au, consisting a SnO2 layer (40 nm) as electron transport layer (ETL) on indium doped tin oxide (ITO) glass substrate, a 550nm thick perovskite layer, a spiro-OMeTAD layer (250 nm) as HTL, and an evaporated Au layer (100 nm thick) as back electrode, as shown in the cross-sectional TEM image (Figure 1d). The concentration of CsPbBr3 NCs increased from 0 to 3 mg/mL, the power conversion efficiency (PCE) of the devices increased first and then decreased. When the NCs concentration was too high (>2 mg/ml), too many particles accumulated on the surface of the perovskite film which affected the uniformity and continuity of perovskite film (Figure S7). Apparently, the NCs were able to affect the film formation when incorporated in this process. Reduced the device performance was observed when further increasing the concentration of NCs, which was likely attributed to the poor film morphology originated from the incompatibility of the deposition method (spincoating) used here.


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Table 1. Average photovoltaic parameters of various solar cells fabricated with different concentration of CsPbBr3 NCs in the dripping solvent. Device

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0 mg/mL

1.09 ± 0.01

22.70 ± 0.31

74.91 ± 1.18

17.95 ± 0.54

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22.90 ± 0.37

75.56 ± 1.37

18.55 ± 0.56

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1.11 ± 0.01

23.19 ± 0.38

76.93 ± 1.28

19.45 ± 0.54

3 mg/mL

1.06 ± 0.02

22.04 ± 0.55

73.82 ± 3.04

17.21 ± 0.86

The best concentration of NCs is 2 mg/mL in our case, achieving the average PCE of 19.45%, which is 8% higher than the control cells. The enhancement of PCE is attributed to the significant improvement in short-circuit photocurrent density (Jsc) and fill factor (FF) as well as the slight increase in open-circuit photovoltage (Voc). As shown in Figure 2a, a best device produced a PCE of 20.56 %, with an Voc of 1.12 V, a Jsc of 23.51 mA/cm2, and a FF of 77.96 %. The forward and reverse scanning current density-voltage (J-V) curves showed negligible hysteresis in the resultant device. By holding a bias near the maximum power output point (0.94 V), we obtained a stabilized photocurrent of 21.03 mA/cm2, corresponding to a stabilized efficiency of 19.77 % (Figure 2b). Figure 2c shows the histograms of PCEs for 40 devices (2mg/mL). The PCEs were distributed in a narrow range between 18.40% and 20.56% (average PCE over 19%), which shows the good reproducibility of the approach. Such a high reproducibility can be partly attributed to the high-quality perovskite films crystallized with the involvement of CsPbBr3 NCs. The External quantum efficiency (EQE) spectra of the device (2mg/mL) shows improved carrier harvesting than that of the reference device, along the entire absorption wavelength range of 350-750 nm (Figure 2d). Both EQE spectra follow the same pattern with the peaks at around 420 nm and 580 nm, and gradually diminish along the longer


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wavelength. The integrated photocurrent densities are 21.81 and 22.78 mA/cm2, in good agreement with the Jsc derived from the J-V measurement.

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Figure 2. Device performance of perovskite solar cells under AM 1.5 radiation at ambient condition. (a) J-V curves for the best performing device (2 mg/mL NCs) measured by forward and reverse scans. (b) Steady-state photocurrent measured at a bias voltage (0.94 V) near the maximum power point and stabilized power output. (c) Histogram of solar cell efficiencies for 40 devices fabricated without and with 2mg/mL CsPbBr3 NCs in the dripping solvent. (d) EQE spectra together with integrated Jsc for perovskite solar cell fabricated with 2mg/mL CsPbBr3 NCs in the dripping solvent.


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Defects passivation on the absorber Next, the factors that influence the photovoltaic performance of the as-prepared devices were analyzed. The photocurrent generation in the absorber may be directly related to the lightabsorption efficiency. The fluorescence of CsPbBr3 NCs may enhance the light-harvesting of the active layer, as the emitted photons by NCs can be absorbed by the perovskite film. However, this did not apply to our case, because the UV-vis absorption spectra for the perovskite films with different concentration of NCs are similar (Figure S8), and the EQE spectra were similar in shape. The fluorescence and light absorption of the CsPbBr3 NCs are negligible, which is reasonable because (i) the amount of CsPbBr3 NCs on the active layer is small (0.13 wt% Cs); (ii) most of the incident visible light is absorbed by the perovskite film considering its high absorption coefficient at 550nm. As the light-absorption of the absorbers was similar, the observed difference in performance should be associated with the heterojunction itself and/or the relevant interfaces that largely influence the carrier behavior across the device. We then investigate the influence of NC incorporation on the perovskite absorber. The PL decay lifetime of the 2 mg/mL sample and the reference were determined by time-resolved PL measurement (Figure 3a and Table S1), wherein τave = 230 ns, compared with 177 ns for the reference. Since there is no HTL in the sample, the PL decay is attributed not to carrier extraction by the HTL, but rather to radiative and nonradiative recombination. The prolonged PL decay lifetime of the NC modified perovskite film is the result of suppressed nonradiative recombination channels.18, 35 The steady-state PL spectra (Figure S9) were recorded, wherein the modified perovskite film exhibits an obvious PL increase by 220% at the same emission wavelength of 763 nm. The increased PL intensity indicates that the introduction of CsPbBr3 NCs may reduce the defect density of the perovskite film and thus decreases nonradiative


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ACS Energy Letters

pathways. The improved perovskite film quality was further checked by measuring the Urbach energy (Eu) of the films, which probe the sharp onset of absorption at the direct band gap and deliver information of disorders in shallow energy level.39 The Urbach energies for perovskite films are calculated from the formula equation: α = α0 exp(E/Eu), where α0 is constant and Eu denotes Urbach energy. We observed a consistent decrease of Eu values for the reference and perovskite/CsPbBr3 samples, respectively, as shown in Figure S10. The smaller Eu value obtained in the perovskite sample is in good agreement with the longer carrier lifetime as observed in TRPL measurements, which indicates a high quality the perovskite film due to the NCs incorporation in terms of electronic disorder. To quantitatively access the density of defects, we fabricated capacitor-like devices by sandwiching the perovskite films between ITO and Au, and characterized the evolution of the space-charge-limited current (SCLC) for different biases (Figure 3b). The sharp rise of the current density-voltage (J-V) curve correlates to a trap-filled limit (TFL), where all the defects are occupied by charge carriers. The defect density is calculated according to the equation: Ndefects = 2εε0VTFL/eL2, where ε and ε0 are the dielectric constants of perovskite and the vacuum permittivity, respectively, L is the thickness of the obtained perovskite film, and e is the elementary charge.17,

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We estimated the defect density Ndefects to be 0.77×1016 cm-3 and

1.13×1016 cm-3 for the modified perovskite samples and the reference, respectively. The defect density substantially decreased with the modification of CsPbBr3 NCs, which was in correspondence to the PL and TRPL. The J-V characteristics were recorded under dark conditions, which was well fitted by the Mott-Gurney Law: J = (9/8)εε0µ(V2/L3).40 The dielectric constant is average in the frequency range from 100 kHz to 1 MHz (plateau region), and V is the voltage drop across the device. The mobilities (µ) of the samples with and without CsPbBr3 NCs


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were calculated to be 7.57×10-4 cm2 V-1 S-1 and 3.82×10-4 cm2 V-1 S-1, respectively. The increased mobility is possibly attributed to the reduction of defects in the perovskite absorber with NC incorporation, providing less scattering centers to inhibit carrier transport. A higher mobility observed in the perovskite sample with CsPbBr3 NCs benefits the transport of photocarriers in the thin films.

Improvement of carrier behavior at the interface It is shown that the introduction of CsPbBr3 NCs could reduce the defect density in the absorber layer. However, the reduction of defects may not serve as the only cause for the PCE enhancement. In fact, in the early stages of the studies, we tried different nanocrystals. Such as (FAPbI3)0.85(MAPbBr3)0.15 NCs, their composition is the same as the perovskite films. They can also improve the quality of the film, but the enhanced efficiency is not obvious (see Figure S11). Therefore, in the case of CsPbBr3 NCs, the enhanced film quality is one but not the foremost factor in improving PSCs performance. To further explore the influence on the interface of the heterojunction upon CsPbBr3 NCs incorporation, the devices were characterized by intensitymodulated photocurrent spectroscopy (IMPS).41-42 The IMPS was employed to estimate the charge transport lifetime (τtr) for the sample and control devices under different illumination intensity. τtr can be obtained from the expression: τtr = 1/2πftr, where ftr is the characteristic frequency at the minimum of the IMPS imaginary component.41 As shown in Figure 3c, the reference device showed a carrier lifetime on the millisecond domain. The device based on the perovskite with CsPbBr3 NCs exhibit a smaller τtr values. These results indicate that CsPbBr3 modification at the interface can enhance charge separation and transport, thus improves the FF and PCE. The improved charge transport across the device was also investigated by


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ACS Energy Letters

electrochemical impedance spectroscopy (EIS) characterization under illumination.43 Figure S12 shows the Nyquist spectra of PSCs with and without NCs under one-sun illumination along with the equivalent circuit (Figure S12 inset). Only one semicircle is resolved in the EIS spectra, which describes the charge transfer behavior at the perovskite-HTL interface.30 The intercept of the semicircle on the real axis corresponds to the series resistance (Rs), which largely depends on the ITO substrate and external wire contact. The values of Rs are similar for both sample and reference, excluding the effect of Rs on the FF. However, charge transfer resistance (Rct) decreases significantly upon NCs introduction, which indicates more efficient hole extraction at the perovskite/HTL interface. Considering the tiny amount of NCs involved, it is possible that the NCs may change the energy level alignment at the interface, which results in the improved carrier behavior as will be discussed later. In addition, transient photovoltage (TPV) and photocurrent (TPC) measurements were employed to investigate the carrier dynamics along the entire pathway in the completed cells.18, 44 TPV measurements, which correlate to the electron lifetime in the absorber, provide insight into carrier recombination rates in the cell. Based on the voltage decay time, the device with CsPbBr3 NCs showed ~76 % longer electron lifetime than that of the control cell (Figure 3d). TPC measurements indicate that carrier transport in the devices was improved by interface engineering, and the perovskite/CsPbBr3-based devices exhibited faster photocurrent generation and decay than that of reference devices (Figure S13). This result is in accordance with the IMPS measurements, where perovskite/CsPbBr3 showed a slightly smaller decay time than the reference. More efficient carrier extraction in the devices probably resulted from the improved interface between perovskite and HTL. These results are in line with the significant improvement of fill factor in the resultant device, indicating the efficient carrier pathway across the device.


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Figure 3. Defects reduction enhanced device performance. (a) time-resolved PL decay curves. (b) current density-voltage characteristics of devices with or without CsPbBr3 NCs. (c) The charge transport lifetime measured by IMPS and (d) TPV decay curves for PSCs with (red) or without (black) CsPbBr3 NCs.

We would like to further illustrate how interface is improved by NCs incorporation. First, the perovskite films were characterized by Kelvin probe force microscopy (KPFM) to reveal the surface potentials possibly influence the carrier dynamics across the heterojunction upon CsPbBr3 NCs modification.16 Spatial maps of local surface potential for perovskite with and


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without CsPbBr3 NCs on ITO substrates are shown in Figure 4. KPFM provides a reliable measurement of local surface potentials stemming from contact potential differences (CPD) between the tip and sample surface associated with their relative work functions. Comparing figure 4a with 4b, minimal variations in the CPD were observed between the grain boundary (GB) and grain bulk for both samples thereby indicating that CsPbBr3 NCs has a negligible effect on the energy band edge at GBs and thus carrier transport within the perovskite film. However, CsPbBr3 incorporation was seen to affect the mean values of the CPD in the perovskite film, which increased from ~420 mV to 545 mV significantly. The presence of CsPbBr3 upshifted the surface potential, indicating a deeper Fermi level. It may enlarge the build-in potential and reduce Voc loss,21 and may also help to facilitate hole transfer from the perovskite to the HTM as discussed later. The incorporation of CsPbBr3 in perovskite can affect its work function and the interfacial band alignment, which was also investigated by UV photoelectron spectroscopy (UPS) measurement. As determined by UPS (Figure 4c), the work functions are 4.1, 4.4, and 4.3 eV for perovskite, CsPbBr3 NCs, and NCs modified perovskite, respectively. It also reveals the VB edges of each sample are 1.5, 1.0 and 1.2 eV below the Fermi level. And the UV-vis absorption spectra of the samples (Figure S2 and S14) indicate bandgaps are 1.62, 2.36, and 1.62 eV for each sample. Accordingly, the schematic energy level diagram of the materials used in the device and the corresponding illustration of band bending are displayed in Figure 4d and 4e. We can clearly see that the VB/CB edge of the absorber has shifted upwards upon NCs incorporation. This upshifts in VB/CB benefit the device operation in two folds. 1) The photo-generated holes in the perovskite film may transfer to the HTM with less energy loss; and 2) the electrons are bouncing back at the interface due to the band bending, which eventually impede the carrier


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recombination process. As a consequence, Jsc, Voc and FF all are enhanced, resulting in a significant improvement in the PCE. To dig into the underlying mechanism that govern the improvement of charge extraction efficiency, we conducted X-ray photoelectron spectra (XPS) to further verify the band alignment at the interfaces. The binding energy of each element at the surface of perovskite film was calibrated with reference to amorphous carbon C 1s core level, which was justified by the alignment to O 1s (see Figure S15). Upon NC incorporation, we observed significant energy shift regarding I 3d (618.78 to 619.72 eV) and Pb 4f (138.04 to 138.93 eV), as shown in Figure 4f. This energy shift is possibly due to the change either 1) in oxidation states or 2) electronic configuration at the band edge. Due to the congeneric nature of FA0.85MA0.15Pb(I0.85Br0.15)3 and CsPbBr3 NCs, it lacks redox couple change the oxidation states in the perovskite film during NCs incorporation. Therefore, the energy shift is attributed to the redistribution of electronic configuration, which indicates that the CsPbBr3 is likely to be incorporated within the perovskite crystal lattice. The binding energy shift is in line with the UPS results wherein the VB shifted to from 1.5 to 1.2 eV below the Fermi level upon NCs incorporation. Interestingly, the N 1s didn’t follow the same pattern along the electronic structure at the band edge of the absorber. It is in coincidence with the simulation results indicating the band edge is mostly composed of Pb and I orbitals, wherein organic components compensate the charges. In addition, we calculated the core-level differences between two different elements including Pb 4f7/2, Br 3d5/2, I 3d5/2 and N 1s for samples before and after Ar+ etching (Table S2). Since the Cs incorporation reduce the perovskite crystal lattice constant, which changes the chemical bonding and thus the binding energy of each element.45 We can see that the core-level differences between two elements in modified perovskite film surface are close to those of CsPbBr3, while those in the sample bulk


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are close to those in the reference. It is in accordance with the AR-XPS results, indicating the depth dependent distribution of CsPbBr3 in the modified perovskite film.

c

CsPbBr3 Perovskite/CsPbBr3 Perovskite 17.1 eV EF

1.5 eV

16.9 eV

1.2 eV 1.0 eV 16.8 eV

18

17

16

15

3

2

1

0

-1

Binding Energy(eV)

f

138.93 eV Perovskite 138.04 eV Perovskite/CsPbBr3 Pb 4f

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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146

144

142

140 138 136 619.72 eV 618.78 eV

I 3d

632

628

624

620

616

Binding Energy (eV)


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Figure 4. Energy band alignments for the GHJ-structured PSC. (a)-(b) KPFM surface potential spectra of perovskite film with (a) or without (b) CsPbBr3 NCs. (c) UPS of perovskite film, perovskite film with CsPbBr3 NCs and pure CsPbBr3 NCs. (d) Energy-level diagram of the materials used in the PSC, with energy levels given in eV. (e) Schematic diagram of band shift of perovskite and CsPbBr3 NCs in a graded heterojunction structure with a mixed and graded interlayer. (f) XPS of Pb 4f and I 3d peaks for perovskite and perovskite/CsPbBr3 NCs.

Moreover, we examined the long-term stability of devices based on modified perovskite stored in dark as well as under nitrogen conditions. The devices with modification of CsPbBr3 NCs showed substantially enhanced stability relative to those without CsPbBr3 NCs in dark (Figure S16). The NCs modified Perovskite based devices maintained 87.60% of their initial PCE after storage in the dark over 60 days, whereas control devices only retained 74.63% of their initial efficiency. In summary, we have developed a simple and feasible approach to modify the perovskite absorber by utilizing congeneric lead halide perovskite NCs. The coherence in structure and stoichiometry and compatibility in processing upon the incorporation of NCs, significantly improves the as-resulted heterojunction without hampering its optoelectronic properties. Appropriate band alignment is achieved to facilitate the hole extraction and to reduce recombination loss. It eventually enhances the photovoltaic performance and stability of perovskite solar cells with the substantial improvement at the relevant interface. By employing this strategy, we have achieved a best PCE of 20.56 % in the resultant device. This work thus suggests a general strategy to tailor the optoelectronic properties of perovskite semiconductors by employing congeneric materials for higher performance in various applications.


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ACS Energy Letters

ASSOCIATED CONTENT Supporting Information. XRD patterns, TOF-SIMS profiles, SEM images, TEM-EDX elemental line scanning, PV metrics, UV-vis spectra, PL and Time-resolved PL data, EIS curves, TPC decay curves, XPS spectra, Urbach energy analysis, bandgap analysis and Long-term device stability. AUTHOR INFORMATION Corresponding Author *E-mail (Y. Li): [email protected]. *E-mail (Q. Chen): [email protected]. Author Contributions + H. Zai and C. Zhu contributed equally to this work. ACKNOWLEDGMENT The authors acknowledge funding support from National Key Research and Development Program of China Grant No. 2016YFB0700700, National Natural Science Foundation of China (51672008 and 51673025), and the Young Talent Thousand Program. Acknowledge Mr Zhang Deliang, Mr Liu Lang, Dr Shi Jiangjian and Ms Ma jingyuan for their experimental support and/or discussions. REFERENCES


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Congeneric Incorporation of CsPbBr3

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