Applied Physics Letters | 2021
Recombination and localization: Unfolding the pathways behind conductivity losses in Cs2AgBiBr6 thin films
Abstract
Cs2AgBiBr6 (CABB) has been proposed as a promising non-toxic alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this work, we study the optoelectronic properties of CABB thin films using steady state and transient absorption and reflectance spectroscopy. We find that optical measurements on such thin films are distorted as a consequence of multiple reflections within the film. Moreover, we discuss the pathways behind conductivity loss in these thin films, using a combination of microsecond transient absorption and time-resolved microwave conductivity spectroscopy. We demonstrate that a combined effect of carrier loss and localization results in the conductivity loss in CABB thin films. Moreover, we find that the charge carrier diffusion length and sample thickness are of the same order. This suggests that the material’s surface is an important contributor to charge-carrier loss. Lead-halide perovskites exhibit excellent light absorption and emission properties and micrometer-long charge carrier diffusion length, making them interesting for optoelectronic applications.1–3 However, the presence of lead and iodide ions raises toxicity concerns.4 In addition, many of the lead-based perovskites have poor stability in aqueous environments, limiting their application in e.g. photocatalysis.5 Cesium silver bismuth bromide (Cs2AgBiBr6, CABB) has been suggested as a less toxic and more stable alternative for the remarkably performing lead-halide perovskites in optoelectronic application.6–10 Microsecond-long carrier lifetimes, increased stability in water and improved photostability with respect to high performing lead-containing analogues, have triggered an extensive research effort into CABB.8,9,11–14 Its applicability in solar conversion or lighting applications is still limited due to the weak absorption and emission caused by the indirect bandgap.15,16 On the other hand, the material performs well as an X-ray detector.17,18 This can be understood from the presence of bismuth, causing efficient X-ray attenuation, and the long carrier lifetimes, which are desirable for charge extraction. More recently, CABB was also used for photocatalytic reactions such as lightdriven CO2 conversion and H2 generation from hydrobromic acid.5,19 However, in all of these applications, the charge extraction efficiency remains limited by the short diffusion length of the minority carrier. The limited extraction has been ascribed to the high trap density in CABB single crystals compared to lead-containing perovskite single crystals.20 In a recent study, Wright et al. identified charge carrier self-localization to a small polaronic state with a localization rate of ca. 1 ps-1 to be intrinsic to CABB.21 Such fast localization rates would be detrimental to its application in optoelectronic devices. However, temperature-activated delocalization results in appreciable carrier mobilities at room temperature.21 Moreover, mobile carriers were observed for microseconds after photoexcitation at elevated temperatures timescale,20 highlighting CABB as a potential alternative for lead-halide analogues. In this work, we study the charge-carrier dynamics in CABB thin films on nanosecond-tomicrosecond timescales using a combination of transient absorption (TA) and time-resolved microwave conductivity (TRMC) spectroscopy. TA experiments present long-lived carriers ranging over several microseconds, while TRMC measurements on the same thin films show that all charge carriers are immobilized within 200 nanoseconds. In the first tens of nanoseconds the charge carrier mobility remains almost constant, indicating that the intensity drop of the TRMC trace on this timescale is the result of carrier loss, after which it drops due to localization of Figure 1: Optical properties of CABB thin films. a) The fraction of transmitted (T, blue line), reflected (R, green line) and absorbed (A, red line) light, as well as the photoluminescence intensity (PL, black line, λexc = 440 nm) as a function of photon energy for a 100-nm-thick CABB film on glass. b) Schematic representation of the band structure of CABB, showing the direct and indirect absorption processes. c) The absorption length as a function of photon energy calculated based on ellipsometry data. The dotted and dashed line in panel a and c represent the indirect bandgap and excitonic transition in CABB. remaining carriers. So that we conclude that the conductivity loss in CABB thin films is the result of both carrier loss and localization. A comparison of the bleach recovery dynamics of the direct and indirect absorptions shows that photogenerated electrons are lost more rapidly than holes. Carrier diffusion lengths of the same order of magnitude as the sample thickness, suggest a dominant role of the material’s surface in charge-carrier loss. We prepared CABB thin films of ca. 100 nm thick, following the method by Li et al..22 In short, a mixture of CsBr, AgBr and BiBr3 in dimethyl sulfoxide was spin coated on an optical substrate, allowed to dry, and annealed. Experimental details and crystallographic data (Fig. S1) are provided in the Supporting Information. Atomic force microscopy shows a homogeneous film coverage with a root-mean-square roughness of 7.3 nm (Fig. S2). The steady-state optical properties were determined using an UV-vis spectrometer with an integrating sphere (see experimental methods). The transmittance (T) spectrum, i.e. fraction of light transmitted as a function of photon energy, shows a characteristic dip around 2.8 eV (blue line in Fig. 1a). This energy is significantly larger than the energy of the bandgap of CABB, which is indirect and therefore does not cause a distinct dip in the transmittance spectrum. The feature at 2.8 eV is instead often attributed to the excitonic transition between conduction and valence band at the Γ point in the dispersion diagram (Fig. 1b).23 We attribute the slow decrease in the transmittance, starting at 2.2 eV and increasing towards higher energies, to indirect absorption. Indeed, the indirect-bandgap energy of CABB single crystals,11,24 thin films8,9 and nanocrystals25,26 is reported ranging from 1.95 to 2.3 eV. The photoluminescence (PL) (black line, Figure 1a) is considerably red-shifted compared to the indirect-bandgap absorption and relatively broad (centered at 1.9 eV, FWHM of 515 meV). The red-shift is known to be due to strong exciton phonon coupling.8,11,21,27 Recent work suggests that the charge carrier recombination pathway in CABB proceeds via color centers.21,24 We observe oscillations in the reflectance (R; fraction of light reflected) spectrum (green line in Fig. 1a). These must be due to a combination of absorption resonances of CABB and the film’s dielectric properties, which determine interference effects. The real part of a material’s dielectric function peaks at frequencies just below an absorption resonance, while it shows a minimum at frequencies just above the resonance. As the reflectance of a bulk material scales with the dielectric contrast with air, it should peak just below a strong absorption resonance and dip at energies above the resonance. Indeed, R of our CABB film shows such a wiggle feature around the excitonic resonance at 2.8 eV (Fig. 1a). A similar feature is not obvious around the indirect bandgap transition at 2.2 eV, which is likely the result of interference between multiple reflections within the film distorting the reflectance spectrum. Interference becomes an especially important factor determining the film’s reflectivity in the spectral range below 2.8 eV, where the absorption of CABB is weak compared to the film thickness of 100 nm (Fig. 1c; calculated based on ellipsometry data Fig. S3) so multiple reflections of the light are possible. Next, we study the change in optical properties using pump-probe TA spectroscopy experiments in transmission mode on a microsecond timescale (for a schematic of the TA setup see Fig. S4 in the Supporting Information).28 The transient transmittance (ΔT/T) shows a distinct bleach at 2.8 eV, which is present for excitation at energies above as well as below the direct transition (compare blue and green lines in Fig. 2a). The bleach must therefore be due to holes near the top of the valence band, which can be photoexcited with either direct or indirect excitation. For excitation at 355 nm an additional weak and broad bleach feature between 1.8 and 2.2 eV is present. This energy range comprises the reported values for the indirect bandgap of CABB. We propose that this signal is the result of a combination of absorption and reflection effects, which we will discuss below. The broad bleach signal is not as evident for excitation at 532 nm because of the low signal to noise ratio. In reflection mode we study the change in reflectance of our CABB thin films, again for excitation at 355 and 532 nm (Fig. 2b). The ΔR/R spectrum displays much stronger features in the indirectabsorption region (<2.7 eV) than the ΔT/T spectrum, highlighting the potential to extract information on indirect-absorption transitions form reflectivity measurements. However, the interpretation of the spectra is not straightforward. For both excitation wavelengths we observe an inflection point in ΔR/R at 2.8 eV: decreased reflectance just below and increased reflectance just above the absorption resonance. This results in a minimum in ΔR/R at 2.75 eV followed by a maximum at 2.9 eV. This spectral shape is a consequence of the bleach of the absorption transition. Bleaching the absorption flattens the real part of the material’s dielectric function around the Figure 2: Transient transmittance and reflectance spectral slices. a) Normalized spectral slice of ΔT/T for excitation at 355 nm (blue) and 532 nm (green) after a delay time of 100 ns. In the grey-shaded area the data is magnifi