Radiative Recombination Changes under Light-Soaking in CsPbBr3

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Radiative Recombination Changes under Light-Soaking in CsPbBr3 Films on TiO2 and Insulating Glass Contacts: Interface versus Bulk Effects Jiangang Hu, Laxman Gouda, Adi Kama, Shay Tirosh, and Ronen Gottesman*,† Department of Chemistry, Institute for Nanotechnology & Advanced Materials, Bar-Ilan University, Ramat Gan 5290000, Israel

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S Supporting Information *

ABSTRACT: The steady-state photoluminescence (PL) of CsPbBr3 films with varying thicknesses was studied under light-soaking on semiconductive and insulating contacts, showing reversible changes in PL, entirely dependent on the nature of the contact and film thicknesses. The PL at 50−100 nm CsPbBr3 on TiO2 increased, and decreased in thicker layers, with no thickness-dependent PL in CsPbBr3 on glass/ Al2O3. These observations are described using a spatial charge distribution model which interprets the migration of Br− and VBr+ under light-soaking as suppressing electron injection into the TiO2 due to upward band bending, enhancing PL at the interface and nonradiative recombination further away from it. KEYWORDS: CsPbBr3 perovskites, light-soaking effect, photoluminescence, thin films, solar cells films can shed light on the nature of the recombination processes in this prototypical inorganic perovskite, which demonstrated superior robustness and stability under operating conditions and at elevated temperatures in perovskite solar cells.6 Furthermore, it contains a single halide, canceling out any concern for photoinduced phase segregation upon lightsoaking (LS).7 In the pursuit of a better understanding of the mechanisms of perovskite solar cells, we report the steady-state photoluminescence (PL) of CsPbBr3 films with different thicknesses under prolonged LS (1 sun intensity), on semiconductive and insulating contacts (TiO2, glass, and Al2O3), to examine the photoinduced radiative recombination changes in the PCI vs the perovskite bulk. Although the thickness of the mesoporous scaffold is important, it was shown that the photovoltage behavior of perovskite solar cells (PSCs) with Al 2 O 3 mesopores under light-soaking showed photoinduced interfacial changes similar to those of PSCs with TiO2 mesopores, validating the critical role of the compact TiO2 over the thickness of the mesoporous scaffold (TiO2 or Al2O3).8 Note that full experimental data are in the Supporting Information. Figure 1 shows the change in PL intensity (%) with time, of CsPbBr3 films with two thicknesses on mesoporous (mp)TiO2/compact layer (CL)-TiO2 and mp-Al2O3/glass. Under LS, in the semiconductive contact system (Figure 1a), the PL behavior showed an explicit dependence on the film’s

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ecently, there has been great interest in lead halide perovskite solar cells, particularly with regard to their high efficiencies and unique optoelectronic properties.1 However, several issues regarding the nature of charge transfer and recombination processes at the perovskite−contact interfaces (PCI) under operating conditions have not been fully resolved.2 As these are mixed ionic−electronic conductors in nature, redistribution of ions in the perovskite layer under solar cell operating conditions can significantly influence two characteristics.3 First, there is a built-in potential that can be modified by ion drift, redistributing the electric field throughout the full perovskite layer thickness, which modifies the charge collection properties due to the changes in concentration and recombination rates of the photogenerated electronic carriers. Second, there is an accumulation of ions at the PCI, commonly observed in slow (seconds → minutes) charging/discharging and capacitive responses. The type of contact at the PCI tremendously influences the recombination processes. In pristine conditions, a semiconductive contact (e.g., TiO2, which has a lower conduction band than that of CsPbBr3) will permit electron injection, quenching radiative recombination in comparison to an insulating contact (e.g., Al2O3 which has a considerably higher conduction band than that of CsPbBr3), which would not permit electron injection and would increase radiative recombination. Irrespective of the recombination processes being dependent on the semiconductive contact material at the PCI,4 they are also very sensitive to either illumination or applied electrical field pretreatments.5 Studying the effect of pretreatment and contact type on PL in freestanding CsPbBr3 © XXXX American Chemical Society

Received: February 17, 2019 Accepted: April 16, 2019

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DOI: 10.1021/acsaem.9b00335 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. PL intensity behavior of CsPbBr3 films deposited on semiconductive and insulating substrates: (a) mp-TiO2/CL-TiO2 and (b) mpAl2O3/glass.

Figure 2. (a, b) PL intensity behavior of CsPbBr3 films deposited on mp-Al2O3/CL-TiO2 with 300 and 50 nm film thicknesses. (c) The energy diagram at the perovskite−TiO2 interface in a pristine sample at open-circuit without accumulated charges at the PCI, and (d) electrons initially injected into the TiO2 and accumulated, under prolonged LS, with accumulation of Vbr+ leading to an upward band bending which suppresses electron transfer to the TiO2, leading to enhanced PL at the PCI (green arrow). The red arrows indicate nonradiative recombination centers, located farther away from the PCI.

was virtually indifferent to the perovskite thickness (50 and 300 nm); second, no intensity recovery occurred throughout the entire LS period. This further strengthens the notion that the photoinduced increase in PL intensity seen in Figure 1a originates from the perovskite interaction with the semiconductive TiO2 contact. Slow charge accumulations at the perovskite interface with the electron-selective contact (e.g., TiO2, SnO2, PCBM) are reported to induce an upward band bending, creating an energetic barrier for electron injection to the contact.2,3,8,9 Thus, such a barrier can increase radiative recombination at the interface, a critical premise but not a straightforward mechanism to measure experimentally, as it is challenging to distinguish between PL from the interface and the bulk in thin films. To determine if photoinduced PL changes depend on the film’s thickness and the contact’s nature, we deposited CsPbBr3 films on insulating mp-Al2O3 on top of semiconductive CL-

thickness. The PL of a 100 nm thick layer decreased slightly at the beginning, followed by a sharp increase throughout the LS period, reaching an intensity of ∼50% higher than the initial value. Alternatively, the thicker film showed a faster intensity decrease which recovered at a slower rate throughout the LS until it nearly reached the pristine value. This undeniably shows a difference between the radiative ↔ nonradiative recombination processes in thin vs thick films on TiO2 as electron-selective contacts during prolonged illumination. The data clearly show a deconvolution of two recombination processes: one is dominant in thinner films, and the other is more dominant in thicker films. It is reasonable that the former originates at the PCI, and the latter from the bulk (explained further below using our proposed model). Both processes appear as transitions between radiative ↔ nonradiative recombination. In the insulating contacts’ system (Figure 1b), the PL behavior under LS was entirely different and is summarized as two distinct features. First, an intensity decay B

DOI: 10.1021/acsaem.9b00335 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials TiO2, which enabled depositing CsPbBr3 films with varying thicknesses (Figure 2a,b). If the film’s thickness and the contact’s type influence the photoinduced radiative ↔ nonradiative recombination processes, a change in the PL behavior with different film thicknesses of the insulating mpAl2O3 scaffold is expected. The PL behavior profile under LS changed significantly between the 50 and 300 nm thick samples, showing that when the contact is semiconductive (CL-TiO2), and permits electron injection across the PCI, the overall ratio between radiative ↔ nonradiative recombination changes as a function of film’s thickness. The PL intensity of the 300 nm film (Figure 2a) showed a similar behavior as observed in the 300 nm film deposited on mp-Al2O3/glass (black curve, Figure 1b), though with a faster decay. The photoinduced PL behavior in the 50 nm film (Figure 2b) resembled the behavior seen in the 100 nm film deposited mp-TiO2/CL-TiO2 (red curve, Figure 1b). For more comparison, PL data of a 100 nm sample appear in Figure S3. As there is no shift in the peaks’ positions or any spectrum broadening during the LS period (Figures S4−S6), we rule out emissive trap states as a reason for changes in the PL intensity.10 In contrast, the interaction between Br ionic defects and electronic carriers was suggested to be responsible for the PL intensity change in CH3NH3PbBr3 films under LS,11,12 with similar time scales as our data. Thus, we propose that the photoinduced PL intensity changes in CsPbBr3 are due to the photoactivation (under an electrical field) of mobile Br ions or vacancies that act as nonradiative recombination centers, as was reported in methylammonium lead halide perovskites.13 We suggest the following spatial charge distribution model to interpret our results: under illumination, electrons are initially injected into the TiO2 and accumulate, similar to open-circuit voltage conditions in PSCs. As the illumination time increases, an interfacial accumulation of VBr+ leads to a realignment of energy levels at the PCI, generating the two following features. First, an upward band bending at the perovskite, followed by accumulation of holes at the interface which increases the interfacial recombination rate, is seen as a transition from Figure 2c to Figure 2d. This process is confined to the PCI. As the LS time is increasing, suppression of electron injection into the TiO2 becomes more significant, as well as a higher accumulation of charges at the interface. This accumulation will form positive polarization at the PCI, attracting photoexcited electrons toward the interface from deep within the perovskite layer (the “bulk”). Thus, band bending at the interface will enhance radiative recombination (“interfacial recombination”), seen as the green arrows. In thinner films, enhanced interfacial recombination is more apparent, as the redistributed electric field influences most of the layer. Thus, the effective thickness with enhanced radiative recombination is higher than ∼10 nm and goes much further away from the PCI, toward thicknesses of ∼50−100 nm. Further away from the PCI, nonradiative recombination centers (Br− or VBr+), created due to the ionic migration, will decrease emission (“bulk recombination”). The recovery time scales toward initial PL values are very slow, most probably due to the slow diffusion of ions,14 as was also observed in the photovoltage behavior under LS.8 In the case of insulating glass/Al2O3 contacts, an injection across the PCI followed by an electron accumulation in the contact would not occur.

In summary, the steady-state photoluminescence of CsPbBr3 films with different thicknesses was studied under prolonged illumination on both semiconductive and insulating contacts, to examine the photoinduced radiative recombination changes in the perovskite−contact interface (PCI) vs perovskite’s bulk. By controlling the nature of the contact’s conductivity and the perovskite film’s thickness, a deconvolution of two separate, reversible photoinduced changes was observed, one dominant in thinner films, originating at the perovskite−semiconductive interface, and the other more dominant in thicker films, originating within the bulk of CsPbBr3 perovskite thin films, both seen as a transition between radiative ↔ nonradiative recombination. Our findings demonstrate how the inorganic perovskite−contact−interface is slowly changing during illumination and recovers even more slowly. As already reported, perovskites undergo photoinduced physical changes which may be the reason for their superior optoelectronic properties,15 and studying the nature of the recombination processes and charge transfer mechanisms is of vital importance. However, as most PSCs and perovskite-based neighboring research fields are composed of very thin perovskite layers, changes as we report here have to be further studied16 and taken into consideration in the design of future high-performance and reliable perovskite-based technologies.

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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/acsaem.9b00335. Full experimental details of the samples’ synthesis and characterization, steady-state photoluminescence measurements under prolonged light-soaking, SEM crosssection images, additional PL data, and an IV curve of a PSC device (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Laxman Gouda: 0000-0001-9022-0151 Ronen Gottesman: 0000-0001-5223-478X Present Address

† R.G: Helmholtz-Zentrum Berlin für Materialien and Energie GmbH, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.

Funding

J.H. and L.G. received funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement 316494. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Arie Zaban for his guidance and support in this study. REFERENCES

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DOI: 10.1021/acsaem.9b00335 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX