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All-Inorganic CsPbI2Br Perovskite Solar Cells with High Efficiency Exceeding 13% Chong Liu,†,§ Wenzhe Li,†,§ Cuiling Zhang,‡ Yunping Ma,‡ Jiandong Fan,*,† and Yaohua Mai*,†,‡ †

Institute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou, 510632, China Institute of Photovoltaics, College of Physics Science and Technology, Hebei University, Baoding, 071002, China

‡

S Supporting Information *

Another widely used method was to introduce hydroiodic acid (HI) into the perovskite precursor solution, aiming to improve the solubility of solute species and realize a low-temperature phase-transition process from nonperovskite Cs4PbI6 to stable α-CsPbI3.13−15 Additionally, substitution ions with a relatively smaller radius, e.g., bismuth (Bi3+, 1.03 Å)16 and strontium (Sr2+, 1.18 Å)17 relative to lead (Pb2+, 1.19 Å), as well as bromine (Br−, 1.96 Å)7,17−22 relative to iodine (I−, 2.2 Å), were employed to increase the Goldschmidt tolerance factor (t) of black-phase CsPbI3 (t = 0.81) with inferior stability.23 Among them, CsPbI2Br, with a 1.92 eV band gap, was assumed to be a promising candidate serving as an absorber layer after comprehensively balancing the trade-off between the band gap and phase stability.7 Aside from the perovskite absorbers, the inorganic CTLs were assumed to be more stable than the organic CTLs. Recently, stabilized efficiencies exceeding 20% and retaining >95% of their initial efficiency after aging 1000 h at 60 °C was achieved while using CuSCN as the inorganic hole transporting layer (HTL).24 Thus, all-inorganic PSCs, completely composed of an inorganic perovskite absorber layer and inorganic CTLs, are likely a promising way to solve the thermal instability problem of the whole PSC device. Actually, there have been some reports on the all-inorganic PSCs with the traditional structure and carbon-based structure.16,25−27 Particularly, a high PCE of 13.21% was achieved with the structure of FTO/c-TiO2/CsPb0.96Bi0.04I3/CuI/Au, but further thermal stability of the whole device was not yet verified.16 In our previous report, we proposed an inverted all-inorganic PSC structure (FTO/NiOx/CsPbIBr2/MoOx/Au), which displayed excellent thermal stability at elevated temperature up to 160 °C.21 Unfortunately, the PCE was still limited by the wide band gap of the CsPbIBr2 absorber layer (2.08 eV) and the lack of truly effective electron transfer layer (ETL). In this work, we introduced the ZnO@C60 bilayer as ETLs to fabricate all-inorganic CsPbI2Br PSCs with a novel inverted structure (FTO/NiOx/CsPbI2Br/ZnO@C60/Ag). The ZnO@ C60 bilayer exhibits superior electron-extracting performance compared to either one of them alone. The as-optimized allinorganic PSC gained a PCE of up to 13.3% with a remarkably stabilized power output (SPO) at ∼12% within 1000 s. Importantly, the devices without encapsulation exhibit highly long-term thermal stability with only 20% PCE quenched after being heated at 85 °C for 360 h.

ABSTRACT: All-inorganic perovskite solar cells provide a promising solution to tackle the thermal instability problem of organic−inorganic perovskite solar cells (PSCs). Herein, we designed an all-inorganic perovskite solar cell with novel structure (FTO/NiOx/CsPbI2Br/ ZnO@C60/Ag), in which ZnO@C60 bilayer was utilized as the electron-transporting layers that demonstrated high carrier extraction efficiency and low leakage loss. Consequently, the as-fabricated all-inorganic CsPbI2Br perovskite solar cell yielded a power conversion efficiency (PCE) as high as 13.3% with a Voc of 1.14 V, Jsc of 15.2 mA·cm−2, and FF of 0.77. The corresponding stabilized power output (SPO) of the device was demonstrated to be ∼12% and remarkably stable within 1000 s. Importantly, the obtained all-inorganic PSCs without encapsulation exhibited only 20% PCE loss with thermal treatment at 85 °C for 360 h, which largely outperformed the organicspecies-containing PSCs. The present study demonstrates potential in overcoming the intractable issue concerning the thermal instability of perovskite solar cells.

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rganic−inorganic perovskite solar cells (PSCs) have realized significant development in the past eight years, as their PCEs rapidly increased up to 22.7%.1−5 Unfortunately, the long-term thermal stability, including either the organic− inorganic perovskite absorber layer and/or the organic chargetransporting layer (CTL), still could not satisfy the requirements toward its large-scale industrialization. Comparably, inorganic perovskites composed of cesium (Cs+) were demonstrated to result in dramatically improved thermal stability.6 For instance, the CsPbI3 and CsPbI2Br thin films can be stable up to 400 °C,7 which are excellent compared to the methylammonium (MA) and/or formamidinium (FA) containing perovskite thin films that rapidly deteriorated over 200 °C.8,9 In this scenario, the inorganic perovskites-based solar cells have been developing rapidly recently, which not only gave rise to an inspiring record PCE over 13.43% but also exploited the application of smart windows.10,11 Among these studies, dealing with the issue of phase transition and obtaining stabilized αperovskites played critical roles in obtaining high performance solar cells. One of the promising methods was to increase the surface/volume ratio by synthesizing inorganic perovskites quantum dots, so that the total Gibbs energy can be dominated by the surface energy and stable in the desired phase.10,12 © 2018 American Chemical Society

Received: December 14, 2017 Published: March 8, 2018 3825

DOI: 10.1021/jacs.7b13229 J. Am. Chem. Soc. 2018, 140, 3825−3828

Communication

Journal of the American Chemical Society It is necessary to note some details in fabricating the CsPbI2Br thin film with high iodine content compared with the CsPbIBr2 thin film via a similar two-step temperature-control method in this work: (i) Considering the better solubility of I− than Br− in dimethyl sulfoxide (DMSO) solution, we assume that higher energy is needed to facilitate the process of solvent evaporation and CsPbI2Br crystallization. The spin-coated CsPbI2Br thin film was actually heated at 42 °C (slightly higher than 30 °C in the case of CsPbIBr2 thin film) to obtain the transition film during the first-step low-temperature stage. (ii) Taking into account the CsPbI2Br perovskites with high iodine content prefer to form δ-perovskite (nonperovskite) at low temperature (Figure 1a2),22 we commenced the second-

Figure 2. (a) Cross-sectional SEM image of the ZnO@C60 bilayerbased PSC. (b) The corresponding EDS line scanning of the selected area marked within a red rectangle. (c) Schematic view of different ETL with band gap alignment. (d) TRPL spectra of CsPbI2Br thin films based on different ETLs.

From the EDS line scanning of the cross section (Figure 2b), we can confirm the morphology and elemental distribution of the C60 layer and ZnO layer, where the ZnO@C60 bilayer ETL displays a uniform layer-by-layer morphology. The schematic view of electron extraction based on a single ZnO nanoparticle layer, single C60 layer, and ZnO@C60 bilayer is shown in Figure 2c. The conduction band minimum (CBM) of CsPbI2Br material is 4.16 eV that was deduced from the Ultraviolet Photoelectron Spectroscopy (UPS) and band gap (Eg) results (Figures S3 and S5). The CBM values of ZnO (4.2 eV) and C60 (4.5 eV) can be found elsewhere.28,29 The driving force (ΔG) is the free energy difference, defined by the formula30

Figure 1. (a) XRD patterns of thin film with structural evolution. (b) Schematic view of two-step temperature-control process to fabricate CsPbI2Br thin film. (c) Top−down SEM image of as-prepared CsPbI2Br thin film.

step heating process with elevated temperature immediately once the color of the film changed from transparent (Figure 1a1) into light brown (Figure 1a2). Consequently, the desired dark-brown CsPbI2Br thin film with high orientation along the [100] crystallographic direction was obtained after being annealed at 160 °C for 10 min (Figure 1a3). The XRD peaks located at 14.6°, 20.8°, and 29.5° correspond to the (100), (110), and (200) planes of the CsPbI2Br perovskite thin film, respectively, which certifies the as-prepared CsPbI2Br perovskite possesses a cubic phase (Figure 1b). Note that the CsPbI2Br thin film prepared without the transition film is demonstrated to have poor coverage (Figure S1), whereas the as-optimized CsPbI2Br thin film using our two-step temperature-control method displays uniform and full coverage morphology (Figure 1c), which would play an important role in optimizing the performance of all-inorganic PSCs. Here, we fabricate all-inorganic PSCs with the structure of FTO/NiOx/CsPbI2Br/ETL/Ag (ETL is ZnO, C60, and ZnO@ C60 bilayer, respectively) (Figures 2 and S2). The ZnO-based cell displays relatively inferior interface contact conditions and photovoltaic performance (Figure S2a, S2c, and S2e). Although the single C60 layer can be uniformly coated and form an ideal interface contact with the perovskite layer and electrode (Figure S2b and S2d), the C60-based solar cells show a serious hysteresis effect (Figure S2e). Interestingly, the external quantum efficiency (EQE) of ZnO- and C60-based PSCs are proved to be complementary for each other (Figure S2f), which is the original motivation to fabricate the ZnO@C60 bilayer. Figure 2a displays the cross-sectional scanning electron microscope (SEM) images of the ZnO@C60 bilayer-based PSC, which show that all of the thin films are well layered.

Perovskite ETL ΔG = ECBM − ECBM

EPerovskite CBM

(1)

EETL CBM

where is the CBM of perovskite and is the CBM of ETL. The ΔG for ZnO ETL is 0.04 eV, whereas the ΔG for C60 ETL is 0.34 eV. It is obviously concluded that the C60 ETL has higher electron extraction efficiency than ZnO ETL in this work. When it comes to the ZnO@C60 bilayer, the C60 capping layer is able to fill in the ZnO pinholes, which not only reduce the interface recombination but also increase the pathway for electron extraction. Consequently, the electron extraction in ZnO@C60 bilayer is enhanced in comparison to the single ZnO layer due to the reduced trap-state density and multiple injection pathways. Figure 2d shows the time-resolved photoluminescence (TRPL) spectra of the CsPbI2Br perovskite thin films based on different ETLs. The TRPL spectra were fitted with exponentials, and the corresponding parameters were listed in Table S1, where τi is the decay time, Ai is the decay amplitude, and τave is the average decay time. The τave of a glass/perovskite sample (1.45 ns) is longer than that of glass/perovskite/ETL samples, which is associated with the charge extraction process by ETL. Likewise, the corresponding value of τave of ZnO@ C60-, ZnO-, and C60-based devices are 0.73, 1.43, and 1.04 ns, respectively, which represents the ability of carrier extraction for these three types of ETLs. Clearly, PL quenching in the C60and ZnO@C60-based devices are faster than that of the single ZnO-based device, which is consistent with our presumption 3826

DOI: 10.1021/jacs.7b13229 J. Am. Chem. Soc. 2018, 140, 3825−3828

Communication

Journal of the American Chemical Society

thermal aging process (Figure S7). Clearly, the CsPbI2Br allinorganic PSCs just display an ∼20% PCE loss while continually being heated at 85 °C for 360 h (Figure 3d); only the Jsc exhibits a slight quench by a factor of 12.5% (Figure S8), which is likely associated with the humidity-induced phase change during the characterization process. Comparably, the control organic−inorganic PSCs have already shown a PCE loss of nearly 50% after 192 h; all of the photovoltaic parameters (Jsc, Voc, FF, and PCE) tend to quench due to the thermal degeneration of the organic−inorganic perovskites and/or the organic CTL. In summary, we developed a two-step temperature-control approach to prepare a compact and uniform CsPbI2Br thin film with high iodine content. Subsequently, we innovatively designed an all-inorganic CsPbI2Br solar cell by means of employing the ZnO@C60 bilayer as the ETL. The ZnO@C60 bilayer ETL was demonstrated to be capable of effectively enhancing charge extraction, reducing leakage loss and trapstate density. The PCE of such all-inorganic PSCs was demonstrated to be 13.3%, and the SPO could remain stable at ∼12% for 1000 s at applied forward bias. Importantly, the long-term thermal stability of the all-inorganic PSCs exhibited great improvement in comparison to the organic-speciescontaining PSCs. The present work helps to tackle the intractable issue regarding the thermal instability of perovskite devices and is a step forward toward realizing highly efficient and stable perovskite solar cells.

mentioned above. Moreover, the ZnO@C60 bilayer exhibits even higher electron extraction efficiency than that of the single C60 layer. Aside from the reduced trap-state density, we assumed it is likely attributed to the good gradient energy band matching between the perovskite layer and ZnO@C60 bilayer, which favors the photogenerated electron to be extracted from the perovskite layer to the metal electrode effectively. Consequently, the PSCs based on the ZnO@C60 bilayer ETL exhibit relatively higher PCE in comparison with either ZnOor C60-based cells (Figure 3a). Around 77% of the cells realized

Figure 3. (a) Statistical histogram of the efficiency of three ETLsbased PSCs, represented for 48 data points in each structure devices. (b) J−V curve and SPO as a function of time held at 0.89 V forward bias for the best performing cell. (c) EQE spectrum and integrated current density of the corresponding device. (d) Normalized PCEs of the CsPbI2Br all inorganic PSCs and Cs0.04FA0.8MA0.16PbI0.85Br0.15 organic−inorganic PSCs continuously heated at 85 °C in N2 atmosphere without encapsulation and tested in air (relative humidity 30−40%).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b13229.

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a PCE higher than 12%, and 21% of them even can gain an efficiency exceeding 13%. The champion PCE is 13.3% with a Voc of 1.14 V, Jsc of 15.2 mA·cm−2, and FF of 0.77 (Figure 3b). In order to investigate the SPO performance of the cells, we applied 0.89 V forward bias under AM 1.5 simulated sun light. Subject to the hysteresis effect (Figure S4), the SPO gradually reduces from over 13% to 12% in the initial 100 s, and finally remains stable at ∼12% within 1000 s. The EQE spectrum of the champion cell is shown in Figure 3c, where the integral current density as a function of wavelength is also presented. Almost all of the EQE values exceed 80% in the absorbance range from 350 to 650 nm, which overwhelmingly surpasses that of single ZnO and/or C60 ETL-based PSCs (Figure S2). In addition, the band gap of 1.92 eV calculated from the EQE spectrum for our CsPbI2Br thin film is consistent with other previous reports (Figure S5).7,22 We further studied the effect of the electron beam on the allinorganic and organic−inorganic hybrid PSCs, respectively (Figure S6). The results suggest that our all-inorganic perovskite solar cells process superior stability against the morphological and structural distortion induced by electron beam illumination. Meanwhile, the long-term thermal stability of FTO/NiOx/CsPbI2Br/ZnO@C60/Au PSCs was further monitored in comparison to the state-of-the-art FTO/cTiO2/m-TiO2/Cs0.04FA0.8MA0.16PbI0.85Br0.15/PTAA/Au PSCs that is a widely accepted structure with highly thermal stability. Note that we employed Au as the electrode during the longterm thermal-stability test, as it is slightly more stable in the

Experimental section, SEM, UPS, and table (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] (J.F.) *[email protected] (Y.M.) ORCID

Wenzhe Li: 0000-0002-7231-7686 Jiandong Fan: 0000-0002-8728-6333 Author Contributions §

C.L. and W.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was funded by the National Natural Science Foundation of China (No. 51672111), Advanced Talents Program of Hebei Province (No. GCC2014013), and Research project of scientific research cultivation and innovation fund of Jinan University (No. 21617341).

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DOI: 10.1021/jacs.7b13229 J. Am. Chem. Soc. 2018, 140, 3825−3828

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DOI: 10.1021/jacs.7b13229 J. Am. Chem. Soc. 2018, 140, 3825−3828