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Exploring Exciton and Polaron Dominated Photophysical Phenomena in Ruddlesden–Popper Phases of Ban+1Zrns3n+1 (N = 1–3) From

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Exploring Exciton and Polaron Dominated Photophysical Phenomena in Ruddlesden–Popper Phases of Ban+1Zrns3n+1 (N = 1–3) From pubs.acs.org/JPCL Letter Exploring Exciton and Polaron Dominated Photophysical − Phenomena in Ruddlesden Popper Phases of Ban+1ZrnS3n+1 (n =1−3) from Many Body Perturbation Theory Deepika Gill,* Arunima Singh, Manjari Jain, and Saswata Bhattacharya* Cite This: J. Phys. Chem. Lett. 2021, 12, 6698−6706 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information − ABSTRACT: Ruddlesden Popper (RP) phases of Ban+1ZrnS3n+1 are an evolving class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic, and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect using several first- principles based methodologies under the framework of Many Body Perturbation Theory. Unlike its bulk counterpart, the optical and excitonic anisotropy are observed in Ban+1ZrnS3n+1 (n =1−3) RP phases. As per the Wannier−Mott approach, the ionic contribution to the dielectric constant is important, but it gets decreased on increasing n in Ban+1ZrnS3n+1. The exciton binding energy is found to be dependent on the presence of large electron−phonon coupling. We further observed maximum charge carrier mobility in the Ba2ZrS4 phase. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, − leading to a decrease in polaron mobility on increasing n in Ban+1ZrnS3n+1 (n =1 3). − erovskites with the general chemical formula ABX3 have research toward its new phases named as Ruddlesden Popper P found great attention in dielectric, optoelectronic, and (RP) phases.24 Tremendous efforts have been invested to tune solar cell applications due to their superb ferroelectric, the electrical and optical properties.18 The RP phases as an − piezoelectric, superconductive, and photovoltaic properties.1 3 imitation of the perovskite structure are evolving as a During the past decade, in the field of solar cell applications, semiconductor for optoelectronic applications.25,26 Their hybrid lead-halide perovskites, namely, CH3NH3PbX3 and general formula is An+1BnX3n+1, where perovskite structure CH(NH2)2PbX3 (X = Cl, Br, and I) have achieved great blocks of unit cell thickness “n” are separated by rock salt layer success owing to their small band gap, long carrier mobility, AX along the [001] direction. Alternate perovskite blocks are low manufacturing cost, and high power conversion displaced in the in-plane direction by half of the unit cell. The 4−10 efficiency. However, due to the presence of organic RP phases are included in the broad category of “2D molecules, the stability of perovskites is affected by heat, perovskites” owing to their layered structural arrangement light, and moisture, thereby degrading their efficiency with Downloaded via INDIAN INST OF TECH DELHI- IIT on July 15, 2021 at 04:37:19 (UTC). 11 (see Figure 1). time in the practical world. Moreover, the presence of toxic Note that several studies assigned to the layered perovskites See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. lead in these materials makes them hazardous for the “ ” 12 as 2D perovskites exist in the literature, where periodic environment. These shortcomings have hindered their stacking of perovskite layers results in a bulk structure. Their practical applications. material properties can be tuned either by substitution or In search of alternative perovskites that can alleviate the 27−29 fi − dimensional reduction. Due to quantum con nement limitations of lead halide perovskites, chalcogenide perov- effects,30 considerable change in bulk physical properties (such skites with S or Se anion have been proposed for photovoltaic as bulk modulus, elastic modulus, charge carrier properties, and applications.13 Several prototypical chalcogenide perovskites 14 15−17 optical properties) can be seen on reducing the dimension of (viz., SrHfS3 and AZrS3 (A = Sr, Ca, Ba), along with the material.31,32 Research in this field is highly evolv- − their related phases) have been synthesized successfully. ing.26,33 35 Among them, BaZrS3 consists of earth-abundant elements and has a moderate band gap (∼1.82 eV18) ideal for photovoltaics. Moreover, it also exhibits ambipolar doping19 Received: May 9, 2021 and is stable against different environmental conditions.20 In Accepted: July 10, 2021 order to optimize the solar cell absorption, doping at Ba/Zr sites has been attempted in this material.17,21,22 However, such doped/alloyed configurations seem to lack stability.23 Thin fi lms of BaZrS3 are also reported, which has directed the © XXXX American Chemical Society https://doi.org/10.1021/acs.jpclett.1c01486 6698 J. Phys. Chem. Lett. 2021, 12, 6698−6706 The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter − Figure 1. Optimized crystal structure of Ban+1ZrnS3n+1 (n =1 3) Ruddlesden−Popper phases (RP phases). In optoelectronic materials, exciton formation greatly influences the charge separation properties and, hence, the excitonic parameter such as exciton binding energy (EB) acts as an important descriptor for optoelectronic applications. Solar cell performance depends upon the fraction of thermally dissociated excitons into electrons and holes, giving rise to the free-charge carriers. In addition, the concept of polarons has been used to explain multiple photophysical phenomena in 36 these materials. Polaronic effects have been suggested to play Figure 2. Imaginary part (Im(ε)) of the dielectric function for (a) an important role in the excitation dynamics and carrier Ba2ZrS4, (b) Ba3Zr2S7, and (c) Ba4Zr3S10 using single shot GW fl ε transport. The separation of free charge is also in uenced by (G0W0) and BSE. Im( ) of the dielectric function for (d) Ba2ZrS4, (e) ∥ ε the carrier mobility. Hence, understanding the effect of Ba3Zr2S7, and (f) Ba4Zr3S10 along the E xy direction and Im( )of electron−phonon coupling in terms of polaron mobility is dielectric function for (g) Ba2ZrS4, (h) Ba3Zr2S7, and (i) Ba4Zr3S10 ∥ important. A systematic study on the excitonic and polaronic along the E z direction, using G0W0 and BSE. Here, the colored effect in the RP phases of BaZrS is hitherto unknown. The regions indicate the energy window that lies in the visible region of 3 the electromagnetic spectra. present Letter, therefore, explores the excitonic properties ff along with polaronic e ect in RP phases of Ban+1ZrnS3n+1 (n = 1−3) under the framework of Many Body Perturbation HSE06 are overestimated in comparison to the experimental Theory. The electron−phonon coupling is also taken care of band gap, as they do not take into account the exciton binding using the Fröhlich model to compute the polaron mobility. energy. Since BSE takes into account the excitonic effect fi The exciton binding energy is de ned as the energy required (which is ignored in the G0W0 calculation), the results get to decouple the exciton into an individual electron (e) and improved when we solve the BSE to obtain the optical band hole (h) pair. Theoretically, the exciton binding energy (EB)is gap. Therefore, we have performed BSE@G0W0@PBE and ff − calculated by taking the di erence of the energy of the BSE@G0W0@HSE06 to incorporate e h interactions. − − bounded electron hole (e h) pair (i.e., BSE gap) and By performing BSE@G0W0@PBE and BSE@G0W0@ − fi unbounded e h pair (i.e., GW gap). In order to determine HSE06, we nd the optical peak positions of Ba2ZrS4 are, − 18 the optical response of Ban+1ZrnS3n+1 (n =1 3) RP phases, we respectively, 1.71 eV (1.33 eV ) (see Figure 2a) and 1.85 eV have calculated the imaginary part of the dielectric function (see Figure S4 in the SI). Since the former is more close to the (Im(ε)).Initially,wehavebenchmarkedtheexchange- experimental value, we have preferred to compute GW/BSE on ε − correlation ( xc) functional for our system. As it is already top of the PBE Kohn Sham orbitals as the starting point. known that single shot GW (G0W0) calculation strongly Similarly, we have performed G0W0@PBE and BSE@ depends on the starting point, we need to validate the suitable G0W0@PBE calculations to capture the optical and excitonic − ff starting point for G0W0 calculation. Note that the spin orbit e ect for Ba3Zr2S7 and Ba4Zr3S10, respectively. As per the coupling (SOC) effect is negligible in these systems (see previous analysis, we report the BSE peak positions for 18 Figure S3). Hence, we have excluded SOC in our calculations. Ba3Zr2S7 and Ba4Zr3S10 as 1.49 eV (1.28 eV ) and 1.43 eV, The band gaps of Ba2ZrS4,Ba3Zr2S7 and Ba4Zr3S10 are quite respectively, whereas the G0W0@PBE peak is obtained at 1.82 underestimated using PBE, and the values are 0.61 eV, 0.42 eV, and 1.69 eV, respectively (see Figure 2a−c). Note that Figure and 0.34 eV, respectively. On the other hand, the same 2a−c corresponds to the average of the optical responses in the calculations with default parameters (viz., exact exchange = x, y, and z directions. However, Figure 2d−f and Figure 2g−i 25% and screening parameter 0.2 Å−1) of HSE06 are 1.39 eV, show the directional optical response of the RP phases along 1.18 eV, and 1.08 eV, respectively. The HSE06 numbers are in the E∥xy and E∥z directions, respectively (see discussion good agreement with the experimental findings.18 Further, the later). peak position, which is underestimated by PBE, is improved by It should be noted here that these numbers are highly performing G0W0 by taking both PBE and HSE06 as a starting dependent on the k-mesh, and it is very challenging (even with point, which shifts the peak to 2.11 eV (see Figure 2a) and the fastest supercomputers) to converge the BSE calculation to 2.17 eV (see Figure S4 in the SI), respectively.
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