Bifunctional Stabilization of All-Inorganic α-CsPbI3 Perovskite for 17

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Bifunctional Stabilization of All-Inorganic α‑CsPbI3 Perovskite for 17% Efficiency Photovoltaics Yong Wang, Taiyang Zhang, Miao Kan, and Yixin Zhao* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

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S Supporting Information *

organic PTA cation surface passivation. The gradient Br doping could induce the crystal grain growth and enhance the phase stability with little impact on the absorbance region (less than 5 nm blue shift). Meanwhile, the hydrophobic organic PTA cation from PTABr could significantly enhance the moisture resistance. Finally, the PTABr treated CsPbI3 based PSCs showed highly reproducible champion efficiency up to 17.06%. The CsPbI3 perovskite was facilely one step deposited using CsI + HPbI3+x (x = 0.1−0.15) precursor to prepare the high quality α-CsPbI3 film followed by 180 °C annealing for 15 min in a drybox.21,26 The absorbance edge of the obtained CsPbI3 film is located ∼720 nm (Figure 1a), which is consistent with

ABSTRACT: The all-inorganic α-CsPbI3 perovskite with the most suitable band gap faces serious challenges of low phase stability and high moisture sensitivity. We discover that a simple phenyltrimethylammonium bromide (PTABr) post-treatment could achieve a bifunctional stabilization including both gradient Br doping (or alloying) and surface passivation. The PTABr treatment on CsPbI3 only induces less than 5 nm blue shift in UV− vis absorbance but significantly stabilize the perovskite phase with much better stability. Finally, the highly stable PTABr treated CsPbI3 based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion efficiency up to 17.06% and stable output of 16.3%. Therefore, this one-step bifunctional stabilization of perovskite through gradient halide doping and surface organic cation passivation presents a novel and promising strategy to design stable and high performance allinorganic lead halide.

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n the past decade, organic−inorganic hybrid lead halide perovskite solar cells (PSCs) have had unprecedentedly progress from an unstable 3.8% efficiency to certified 23.3% power conversation efficiency (PCE).1−5 The all-inorganic halide perovskite without volatile components has also become a promising alternative candidate to overcome the potential volatile cation related long-term stability issues.6−11 Among them, the Cs based lead halide perovskites, CsPbX3 (X = I, Br, Cl), show their charge carrier mobilities are similar to those reported for the hybrid perovskites, but their PV performances are still slugged.7,12−14 The main challenge to achieve high performance all-inorganic lead halide PSCs is that the CsPbI3 with the most suitable band gap could be easily degraded to nonphotoactive δ-phase (yellow phase) under ambient conditions.15−17 The low phase stability of α-CsPbI3 should be ascribed to its unideal tolerance factor. Although several approaches such as introduction of quantum dots effect, 2D/ 3D configuration and surface passivation engineering has been adopted to stabilize the perovskite phase of CsPbI3,18−25 these stabilized all inorganic CsPbI3 perovskite based PSCs devices’ performance and even stability are still much lower than those organic−inorganic perovskites with similar band gap. Therefore, it is of great and fundamental challenge to stabilize the allinorganic CsPbI3 perovskite and boost its PV performance. In this Communication, we discover that an effective PTABr post-treatment on the α-CsPbI3 thin films could enhance its phase stability through simultaneous gradient Br doping and © XXXX American Chemical Society

Figure 1. (a) UV−vis spectra and (b) XRD patterns of the CsPbI3 and PTABr-CsPbI3 thin films. Top-surface SEM images of (c) CsPbI3 and (d) PTABr-CsPbI3 thin films.

previously reported phase pure α-CsPbI3.27 The deposited CsPbI3 films show characteristic XRD pattern of cubic αCsPbI3 perovskite (Figure 1b). In order to improve the humidity resistance to prevent the attack of H2O molecule, the commercially available PTABr was selected in the present work. The phase pure α-CsPbI3 thin films were then posttreated by spin-coating with different concentration PTABr isopropyl alcohol solution (0.5−2 mg/mL). After such treatment, these CsPbI3 thin films’ UV−vis spectra exhibit PTABr concentration dependent blue shift Received: July 26, 2018

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DOI: 10.1021/jacs.8b07927 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

exact molar ratio, but it still can effectively represent the trend of elemental distribution. The trend of Br count decreases and I increases from surface to bottom while the Cs and Pb counts almost keep constant. These results reveal the gradient Br doping in CsPbI3-PTABr perovskite thin film. The observed gradient Br doping in the PTABr-CsPbI3 sample should be ascribed to the following mechanism: the PTABr treatment starts from the surface and more Br/I halide exchange reactions take place on the surface to form a Br rich perovskite composite. The XPS spectrum of PTABr-CsPbI3 thin film in Figure 2d and Figure S5 has shown the characteristic C and N signals related to the organic content of PTABr. The previous study has suggested that even the small size FA or alkyl ammonium cation would not alloy with CsPbI3 quantum dots but only absorb on the perovskite surface during the FAI or alkyl ammonium salt treatment.20,31 Here, the FTIR spectrum of CsPbI3 and PTABr-CsPbI3 perovskite films in Figure S6 did not show any PTA related signal, which could be ascribed to too small amount of PTA+ functional group terminated on the CsPbI3 perovskite’s surface. Furthermore, the XRD patterns of the PTABr (Figure S7) and PTABr-CsPbI3 (Figure 1b) indicate that there is no orientation found for PTABr. On the basis of the above material characterization, here we proposed that our PTABr-CsPbI3 was stabilized by both the gradient Br doping and PTA organic cation surface passivation as schematically illustrated in Figure 3a. The thermal stability

(Figure 1a and S1). Because 1 mg/mL PTABr treated CsPbI3 is the optimal one with enhanced stability and PV performance, the corresponding perovskite films are noted as PTABrCsPbI3. The PTABr-CsPbI3 thin films exhibit the similar XRD pattern as CsPbI3 samples without any impurity peak or peak shift. The absorbance blue shift of PTABr-CsPbI3 is less than 5 nm, which is similar to the CsPbI2.95Br0.05 perovskite (Figure S2). The very small blue shift in UV−vis spectra and the unchanged XRD pattern suggest the Br doping should be very low in the PTABr-CsPbI3 thin film. As shown in Figure 1c, the deposited CsPbI3 film is compact but still has some pinhole and the crystal grain sizes are about 200−800 nm with an average grain size of ∼480 nm. Although the PTABr-CsPbI3 exhibits the similar UV−vis spectrum and XRD pattern as CsPbI3 film, its morphology is different from that of CsPbI3. The PTABr-CsPbI3 turns into a pinhole free morphology, the crystal grain size increases a little as shown in Figure 1d. The grain in PTABr-CsPbI3 thin film has an average grain size ∼510 nm, as shown in Figure S3. The increase of PTABrCsPbI3 crystal grains could be ascribed to the Br/I ion exchange reaction, which has been previously observed in MAPbI3 or FAPbI3 hybrid perovskite after MABr/FABr treatment.28−30 The grain size growth in these PTABr treated samples shows some dependence on PTABr concentrations as compared in Figures S1 and S3. In contrast, the CsPb2.95Br0.05 shows the similar morphology as CsPbI3 in Figure S2. Because the PTABr post-treatment might have induced the Br/I ion exchange reaction, it is necessary to explore whether the Br-doping distribution is homogeneous or depth dependent. The EDX elemental mapping was taken to characterize the Br distribution on the top surface and the cross section of the PTABr-CsPbI3 sample. The EDX data measured on the surface show the homogeneous distribution of Cs, Pb, I, Br, N, and C as listed in Figure 2a and S4. The surface Br/I ratio of the

Figure 3. (a) Schematic illustration of gradient Br doping and PTA organic cation surface passivation on CsPbI3 perovskite thin film. XRD patterns evolution of (b) CsPbI3 and PTABr-CsPbI3 thin films heated 80 °C in a N2 glovebox for 72 h and (c) PTABr-CsPbI3 and CsPbI3 thin films after exposed to 80 ± 5% RH at ∼35 °C for 0.5 h; inset is their photographs.

of the PTABr-CsPbI3 thin film is compared with that of CsPbI3 in Figure 3b. After 80 °C annealing for 72 h, the phase pure αCsPbI3 turns into the yellow δ phase, while the PTABr-CsPbI3 thin film shows no any impurity phase peaks as shown in Figure 3b. The enhanced thermal stability in PTABr-CsPbI3 should be ascribed to both gradient Br doping and PTA organic cation protection because CsPbI2.95Br0.05 films do not exhibit the thermal stability as well as PTABr-CsPbI3 in Figure S8. Besides the enhanced thermal phase stability, the moisture resistance is another key challenge for all-inorganic perovskite. As shown in Figure 3c, the black CsPbI3 film turns to yellow within 0.5 h when exposed to 80 ± 5% relative humidity (RH)

Figure 2. (a) EDX top view element mapping of the PTABr-CsPbI3 thin film. (b) Cross section view of PTABr-CsPbI3 based device and (c) corresponding element profile. (d) C 1s and N 1s core-level spectra for the CsPbI3 and PTABr-CsPbI3 samples.

CsPbI2.95Br0.05 thin film listed in Figure S2 and Table S1 is lower than that of the PTABr-CsPbI3 (Table S2), even though they exhibit the same absorbance properties. Furthermore, the cross section element profiling of the CsPbI3-PTABr based PSC device is listed in Figure 2b,c. It is noted that the normalized element counts in Figure 2c are not equal to the B

DOI: 10.1021/jacs.8b07927 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society at 35 °C, while PTABr-CsPbI3 thin film retains the pure black phase under the same condition. The CsPbI2.95Br0.05 shows the similar moisture resistance as the CsPbI3 samples (Figure S9), thus the enhanced moisture resistance should be mainly ascribed to PTA cation organic composites passivation. All the above results suggest that the gradient Br doping and the PTA organic cation on surface could significantly enhance the thermal stability and moisture resistance of PTABr-CsPbI3. We fabricated the typical planar PSCs with configuration of FTO/c-TiO2/perovskite/Spiro-OMeTAD/Ag. The statistics of PV performance in Figure S10 show that all the PV parameters of the PTABr-CsPbI3 device are improved compared to the CsPbI3 ones, especially in Voc and FF. Figure 4a lists the current density−voltage (J−V) curves of champion

PSC retained 91% of its initial PCE after stored in a N2 glovebox with 500 h of continuous white light LED illumination as shown in Figure 4c. In contrast, the PCE of CsPbI3 based PSCs drop to 10% of initial PCE after ∼300 h of illumination and the perovskite layer also turn into yellow, as shown in Figure S16. The charge transport properties of the CsPbI3 and PTABrCsPbI3 based PSCs were further investigated by the transient photocurrent decay (TPC) and photovoltage decay (TPV) measurement. The TPC response of PTABr-CsPbI3 based PSC device is slightly quicker than that of CsPbI3-based one (Figure S17), a sign of quicker charge transport. Furthermore, the EIS result in Figure S18 also indicates the PTABr-CsPbI3 based PSC device has a smaller resistance than the CsPbI3 ones. Meanwhile, the TPV decay curve of the PTABr-CsPbI3 based PSC device shows much longer lifetime than CsPbI3based PSC device (Figure 4d), indicating significantly decrease of charge-carrier recombination.34 In summary, the metastable CsPbI3 perovskite could be stabilized through bifunctionally stabilization of gradient Br doping and PTA organic cation surface passivation. The gradient Br doping and PTA surface termination on CsPbI3 could induce the regrowth of crystal size and enhance the phase stability with little blue shift in UV−vis absorbance. The PTABr-CsPbI3 based PV devices exhibited passivated effects of higher Voc and FF. The champion PTABr-CsPbI3 PSCs exhibit PCE up to 17.06% with 16.3% stable output, a state-of-the-art efficiency for all inorganic perovskite solar cells. Thus, bifunctional gradient halide doping and organic cation surface passivation would be a promising chemical strategy to stabilize perovskite for various optoelectronic applications.

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Figure 4. (a) J−V characteristics of champion CsPbI3 and PTABrCsPbI3 based PSCs under simulated AM 1.5G illumination of 100 mW·cm−2 in reverse scan. (b) Efficiency histogram of CsPbI3 and PTABr-CsPbI3 PSCs. (c) Photostability of PTABr-CsPbI3 PSC under continuous white light LED illumination (100 mW·cm−2) in a N2 glovebox. (d) TPV of CsPbI3 and PTABr-CsPbI3 based PSCs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07927. Experimental method, material characterization, solar cell parameters (PDF)

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PSCs based on CsPbI3 and PTABr-CsPbI3 samples. Obviously, the PTABr-CsPbI3 based PSC exhibits a significantly improved PCE of 17.06% over 13.59% of CsPbI3 based PSC. Figure S11 shows the EQEs of CsPbI3 and PTABr-CsPbI3 PSCs with integrated JSC of 18.42 and 18.76 mA·cm−2, respectively. These results indicate that the improved PV performance of CsPbI3PTABr over CsPbI3 can be mainly attributed to the enhancement of the Voc and FF, in which Voc (1.051 V vs 1.104 V) and fill factor (FF) (0.685 vs 0.806). The Mott− Schottky in Figure S12 shows that the flat-band potential of PTABr-CsPbI3 devices has ∼70 mV shift compared to that of CsPbI3 ones. This is consistent with the higher Voc and could be ascribed to the Br doping and surface cations termination.32,33 The PTABr-CsPbI3 based PSCs also show an important advantage of higher reproducibility with a narrow PCE distribution as shown in Figure 4b. Furthermore, the CsPbI3-PTABr based PSCs show smaller J−V hysteresis (Figure S13 and Table S3), resulting in a stable 16.3% output (Figure S14). We also investigated the effect of PTABr concentration on the device performance, and corresponding champion J−V curves are listed in Figures S15. All of them show higher PV performances than CsPbI3 based devices. Besides the higher efficiency, the CsPbI3-PTABr based PSCs also show excellent photostability. The CsPbI3-PTABr based

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yixin Zhao: 0000-0002-8663-9993 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grant 51861145101, 21777096), Shanghai Shuguang Grant (17SG11) and the China Postdoctoral Science Foundation (2017M621466).

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DOI: 10.1021/jacs.8b07927 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX