This PDF file contains the front matter associated with SPIE Proceedings Volume 11366, including the title page, copyright information, table of contents, and author and conference committee lists.

Perovskite silicon tandem solar cells can exceed the efficiency limit of 29.4% of single junction silicon solar cells, with comparably low additional costs for depositing the layers for the perovskite solar cell. Hence, they are an attractive option to further decrease the costs of photovoltaic electricity generation. We present, how by optimizing perovskite absorber composition, choosing adequate carrier selective contact layers, introducing surface passivation and optimizing the individual layer thicknesses solar cell efficiencies above 25% can be realized experimentally. Furthermore, we discuss different options for reducing front surface reflection by anti-reflection coatings, structured foils and deposition on textured silicon wafers and their impact both on solar cell efficiency as well as on the yearly energy yield. Deposition on textured silicon wafers promises highest energy yield. Hence, we show how perovskite absorbers can be deposited on such substrates by either co-evaporation or hybrid processing combining evaporation and subsequent wet-chemical processing. By bottom up cost calculations we finally show how and under which conditions perovskite silicon tandem solar cells can yield an economic advantage.

Currently, perovskites/silicon tandem solar cells and bifacial solar cells are amongst the most-discussed trends within the photovoltaic community. Accurate numerical simulations of large PV systems consisting of bifacial solar modules are vital their optimization. In this work we present a detailed illumination model for solar modules in a big PV field. This model takes direct and diffuse illumination from the sky and from the ground into account, accounts for shadowing of the modules onto each other and the ground, and allows to calculate the annual energy yield as a function of distance and tilt of the modules. Using realistic spectral weather data and the spectral reflectivity of the ground, we can perform detailed optimizations for the different tandem solar cell configurations. In general, four-terminal cells yield a higher electricity generation because in contrast to two-terminal (2T) cells no performance losses caused by current mismatch occur, but the balance-of-system costs for 2T cells are lower. By increasing the perovskite thickness and/or decreasing the perovskite bandgap, the top-cell current density can be increased leading to a higher overall current density under bifacial operation. We will compare the optimized energy yield for 2T cells with the energy yield of comparable 4T cells. For these simulations, we use a detailed-balance model with realistic absorption data of perovskite and silicon layers. Finally, we present minimizations of the LCOE for mono- and bifacial modules as a function of the land cost. We see that increasing the land cost leads to a lower optimal module distance, where the optimal tilt is lower than for larger module distance in order to compensate for more shadowing. With respect to the LCOE corresponding to a module distance determined by a rule-of-thumb, the minimized LCOE can differ significantly especially for higher land cost.

Single-junction Si solar cell efficiencies are intrinsically limited to 29.4%. One common strategy to overcome this fundamental issue is to combine multiple semiconductor materials with different bandgaps in a multi-junction configuration so that light is effectively absorbed over a broad range in the solar spectrum. In particular, the combination of a III-V top cell (GaInP/GaAs) that is wafer-bonded to a planar Si bottom cell led recently to an overall record efficiency of 34.1%. The efficiency of this tandem design could be further improved if absorption in the Si cell near the bandgap (1000-1200 nm) is enhanced. Here, we present a nanostructured metallodielectric back reflector placed at the rear of the Si cell that selectively steers incoming light to angles outside the escape cone of the tandem cell. The design is composed of a hexagonal array of Ag nanodisks embedded in a PMMA layer at the rear of the Si cell. Using finite-difference time-domain simulations we optimize pitch, radius, and height of the individual Ag scatterer such that we evenly distribute scattered power over the different diffraction orders. We analyze the scattering behavior in terms of plasmon scattering by the Ag disks and Mie scattering in the dielectric PMMA inclusions. To fully optimize light trapping inside the cell, we choose the geometry such that both 0th-order reflection and plasmonic losses in the Ag nanodisks are minimized. We experimentally demonstrate photonic light trapping by fabricating large scale (2.5×2.5 cm) nanopatterns on untextured Si solar cells. Large-area patterning is performed via Substrate Conformal Imprint Lithography (SCIL) using silica sol-gel as a mask to etch patterns in PMMA, followed by thermal evaporation of Ag. Cross-section SEM shows excellent conformal deposition of Ag inside the patterned nanoholes. Light scattering spectroscopy shows a clearly reduced reflection of the Si cell in the desired wavelength range (1000-1200 nm) due to light trapping, in agreement with simulations. Experimental data of the full nanopatterned III-V/Si tandem geometry will be shown.

Direct wafer-bonding after argon-beam surface activation is a low temperature process, which allows for the monolithic integration of various materials including Si, Ge, III-V compound semiconductors, SiC or Al2O3 etc. The process requires smooth wafer surfaces with RMS roughnesses < 1 nm and minimal particle contaminations, which is usually achieved by chemical-mechanical polishing. These wafers are sputtered with Ar in ultra-high vacuum (< 3 x 10-6 Pa) to remove few nanometers of oxides and contaminants. The process results in a thin amorphous surface layer with dangling bonds. Subsequently, the wafers are pressed together so that covalent bonds are formed, permanently joining the materials. As no intermediate layers are applied, the approach enables a high optical transparency together with mechanical stability as well as highest electrical and thermal conductivity. The process parameters are optimized for various material to obtain electrical bond resistances < 5 mΩcm2. Even in multi-junction cells operated at a few hundred suns with current densities of ~5 A/cm2, these resistances do not significantly limit the cell efficiencies. These unique characteristics of the resulting wafer-bonds make the technique promising for a wide range of innovations in photonics or power electronics. We apply direct wafer-bonding in the fabrication of various concepts for III-V based multi-junction solar cells reaching highest efficiencies. Examples are a wafer-bonded GaInP/GaAs//GaInAsP/GaInAs solar cell that exhibits an efficiency of 46.1 % at 312 suns as well as a GaInP/GaAs/GaInAs//GaSb solar cell with 43.8 % efficiency at 796 suns. Further, the process enables the monolithic integration of III-V materials on Si, at which a record efficiency of 34.1 % at 1 sun could be recently achieved with a GaInP/AlGaAs//Si solar cell.

Energy yield is a key metric for evaluating the performance of photovoltaic systems. It describes the total amount of energy generated by a photovoltaic (PV) installation over a given period, typically a year, and depends on physical properties of the solar cell like efficiency, band gap and temperature coefficient, as well as the operating conditions in a given location. Because the response of a solar cell to these conditions varies, two photovoltaic technologies may have a different energy yield, even if their lab efficiency is identical. Predicting energy yield accurately is important to system operators and installers to estimate the technical and economic performance of a PV installation. In this paper, we summarize our findings about satellite based energy yield predictions of solar cells with various technologies.

Introducing thin, light-weight and high efficiency photovoltaics will make solar cells more suitable to be integrated in urban landscapes or even small gadgets and would largely contribute to solving the global warming threat that we are facing today. Stacking of solar cells with different characteristic bandgaps is the most common strategy to surpass the Shockley-Queisser efficiency limit, but such tandem devices are typically heavy weight, rigid and costly. Thinning down of absorber materials is a good strategy to overcome these restrictions. However, nano- and micro-meter thicknesses come down to the expense of light absorption. An effective approach to tackle the absorption problem in thin materials is nanopatterning the absorbing layer. In this work we introduce hyperuniform designs as an effective way to control scattered light into particular range of angles (revealed as a ring in k-space of the reflected/transmitted light), with the aim to efficiently trap light in μm-thick Silicon (Si) cells. We first consider the –theoretical and experimental- case of a single Si solar cell, and thanks to an optimization algorithm, we show the highest light absorption in 1 μm-thick Si film to date. We also compare different designs for best anti-reflection effect on top of light trapping and characterize the increased absorption in photoelectrochemical devices. Second, we incorporate a similar light trapping strategy in a tandem solar cell, by using a periodic GaAs nanowire array as a top cell. We introduce two waveguiding effects in GaAs NW-Si thin film architectures to explain the 4-fold light absorption in the Si ultrathin bottom cell for tailored geometries of the NW array. These results represent significant light trapping scheme that is obtained “for free” when using a nanostructured top cell.

Introduction In order to increase efficiency in flexible thin-film solar cells, it is crucial to employ advanced light trapping schemes [1]. Such light-trapping schemes must be coupled with high quality absorber layers to ensure high open-circuit voltages in multi-junction solar cells [2][3][4]. The challenge is that the introduction of textured interfaces that facilitate enhanced light trapping competes with the ability to process high quality PV materials on top of it. To cope with these limitations, modulated surface texturing (MST) approach has been employed [5]. It consists of superimposing the micro-sized craters induced on the substrate with naturally nano-texturing of a TCO [6]. The embodiment of this approach has lead very high efficiency for tandem micro-morph solar cells on glass substrates [7]. In this work, we show the development of such MST approach on temporary Al foil substrates. We first investigate and characterize different texturing techniques, then we grow nc-Si:H absorber layer to investigate its quality. Experimental details Bare Al foils are divided in two different categories; i) direct etching and ii) sacrificial layer etching. The first samples are etched directly in KOH diluted in H2O at T > 30 °C. The second category samples instead undergo a sputtering deposition of ITO/AZO as sacrificial layers. This layer is subsequently etched in HF:H2O2:H2O or KOH diluted in H2O at T > 30 °C. The samples are characterized by AFM, SEM and angular intensity diffraction spectrophotometer. Results and discussion Aluminum samples etched in KOH result in a relatively deep craters with an aspect ratio of 10% (RRMS = 322 nm, LC = 3.02 μm). Al samples etched via sacrificial layer have a similar aspect ratio to the one of direct etching (~10%). The difference between ITO and AZO sacrificial is the craters’ size and depth. The different etching solutions (HF:H2O2:H2O or KOH/H2O) have also an impact on craters’ distribution in Al textured samples. The physical mechanism of this etching is that the porous sacrificial layers deposited allow an anisotropic etching that will induce craters in the Al substrate once the etching is completed. A further etching with diluted H3PO4 leads a cleaning of solid precipitates after texturing process. All these process conditions set, it is possible to grow high quality, >2 μm-thick nc-Si absorber layers in combination with excellent light trapping for micro-morph tandem application. References [1] A. Shah et. al., Prog. Photovolt: Res. Appl. 2004; 12:113–142 (DOI: 10.1002/pip.533). [2] H. Sai et. al., Appl. Phys. Lett 101, 173901 (2012). [3] M. Kambe et al., doi: 10.1109/PVSC.2009.5411411. [4] J. Bailat et al., JAP 2003; 93: 5727–5732. [5] H. Tan, et al, Appl Phys. Lett. 103, 173905 (2013). [6] J. Müller et. al., Solar Energy, doi.org/10.1016/j.solener.2004.03.015. [7] H.Tan et. al., doi: 10.1002/pip.

On macroscopic scale, solar cell efficiency can be increased with concentrating lenses, that focus the sunlight on the cell. An equivalent effect can be achieved with nanostructures that make the light emitted by the solar cell directive towards the sun, while eliminating some of the drawbacks of macroscopic concentration. We have developed an evolutionary algorithm to design dielectric nanostructures for directional emission, leading to structures with a directivity as high as 306. In our previous work[1] the experimentally achieved value was significantly lower than the modeled directivity, due to several limiting factors. In this work we overcome these limitations, by using a newly build measurement setup and a specially designed material system. By combining a Fourier microscope with an integrating sphere, we are able to measure emission into the full 4π solid angle, which allows us to measure full directivity. The predicted directivity is calculated for a point source at the center of the nanolens. In the experiment the point source is resembled by small clusters of emitting particles, which are placed in an ordered array to facilitate the proper alignment. This is achieved with a recently developed technique of direct patterning of CsPbBr3 perovskite nanocrystals by e-beam lithography, by which the ligands of the nanocrystals are directly crosslinked upon exposure. Subsequently the dielectric nanolenses are fabricated on top of the clusters by 2-photon lithography. [1] Johlin, E.; Mann, S. A.; Kasture, S.; Koenderink, A. F.; Garnett, E. C. Broadband highly directive 3D nanophotonic lenses. Nature Communications 2018, 9, 1-7.

In this work, transient absorption (TA) spectra of lead based organic-inorganic mixed halide (CH₃NH₃PbI_3-xCl_x) perovskite thin film were investigated under excitation of femto-second pump pulse (400 nm, 60 fs) for various carrier density (10¹⁸-10¹⁹ cm^-3). The ultrafast hot carrier (HC) relaxation dynamics is examined by fitting the high energy tail (1.8-2.0 eV) of bleaching band when pumping (~2.63 eV) above the perovskite band-gap (~1.6 eV). It shows that HCs cool down via electron-LO phonon emission over ~800 fs. However cooling is slows down due to delayed relaxation of LO phonon into acoustic phonon in ~11 ps and shows “photon bottleneck” at higher carrier injection density. Further, the decay dynamics of band limited bleaching peak signal around 1.69 eV (730 nm) obtained from TA spectra is well studied to predict the rate constant of radiative recombination. The results shows radiative recombination rate of 10^-11 cm^-3s^-1 and effective diffusion length of ~10 μm in mixed halide perovskite.

The energy conversion efficiency of perovskite solar cells can be boosted by enhancing the short-circuit current density, where efficient photon management allows realizing a high short-circuit current density. The efficient photon management relies mostly on the solar cell surface, hence, the front contact of the device. The front contact must fulfil some basic requirements so that both lateral charge transport and light incoupling can be achieved efficiently by lowering the optical losses. In this study, we utilized metal oxide films as a potential charge transport material and front contact, which allows achieving efficient photon management in perovskite solar cells. In the current study, a planar perovskite solar cell was fabricated experimentally, where nickel oxide hole transport layer is prepared by using electron beam deposition at a low temperature. Necessary material characterizations were performed to ensure the high-quality films. Spectroscopic ellipsometry measurements were carried out to extract the complex refractive index of the deposited films, which is used to study the optics of perovskite solar cells. Finite-difference time-domain optical simulations were used to investigate the optics and optimize the perovskite solar cells. Simulation results give a very good agreement with experimental results. Finally, an optimized perovskite solar cell structure will be proposed which can enhance the short-circuit current density and energy conversion efficiency by 20% and >25%, respectively. The optimized device design can be further applied to the implementation of perovskite/silicon tandem solar cells. Detail discussion of the proposed structure will be provided to attain effective photon management for perovskite solar cells.

Organic semiconducting polymers, due to their tunable optical and electronic properties and ease of fabrication processes, are useful in numerous photonic applications. They have found increased interest in the field of photovoltaics due to their comparative low cost as compared to current commercial silicon solar cell modules. The introduction of plasmonic effects in these organic polymer-based solar cells leads to better performance characteristics of these cells. The plasmonic nanoparticles, which can be placed in the different layers of the organic solar cell (OSC), scatter light into the active layer thereby increasing the optical path length of the incident light leading to higher absorption and short circuit current density of the OSC. In this paper, an organic solar cell based on a low bandgap polymer blend and containing complex plasmonic metal nanoparticles has been presented. Finite difference time domain (FDTD) method has been used to simulate models to study the interaction of incident light with the OSCs containing the plasmonic nanoparticles and then compare their performance with that of the OSCs without the nanoparticles. The effect of varying nanoparticle and solar cell parameters on the absorption enhancement of the OSC was studied to determine the best configuration for fabrication. Short circuit current density enhancement of 19.3% was obtained in the OSC containing the nanoparticles. The plasmonic nanoparticles, thus obtained, were synthesized by chemical processing to be introduced in OSCs with different active layer materials for high power conversion efficiency.

In this work, we investigate a tetracene/Si singlet-triplet down-conversion solar cell geometry in which we control the directional emission of quantum dots (QDs). In this system, photons in the visible range (450-550 nm) excite high-energy singlet-excitons in tetracene that rapidly convert into two triplet-excitons at about half the energy. The triplet-exciton energy is transferred to the QDs that subsequently emit at 1000-1100 nm. A significant loss channel is the QD emission that is directed upwards, so anisotropic downward emission into the underlying Si cell is essential. Here, we demonstrate directional light emission of CdSe/ZnS core-shell quantum dots (QDs) coupled to Si Mie resonators fabricated on a Si solar cell. By varying the shape and size of the Mie resonator, interference of the electric and magnetic multipolar modes supported by the resonator is controlled. Placing the QDs in the near-field of the resonator enables efficient coupling of the QD transition dipole with these multipolar modes. Using numerical FDTD calculations we show that the QD emission is efficiently directed into the solar cell. We fabricate nanostructures on a Si substrate using electron-beam lithography and reactive-ion etching. Using soft-stamp imprinting we selectively print CdSe/ZnS QDs (peak emission 800 nm) on top of the nanostructures. We then map the QD emission anisotropy for different Mie resonator sizes, using photoluminescence spectroscopy. Photoluminescence lifetimes show a systematic increase from 7 ns to 17 ns, for Si nanocylinder diameter from 200 to 425 nm (cylinder height 125 nm), consistent with the varying nanostructure resonances as found in FDTD simulations. The anisotropic downward emission demonstrated in this work of QDs coupled to Si nanostructures can enhance the efficiency of a tetracene/Si down-conversion system. Moreover, this work can impact a broad range of other applications in which directional emission is relevant, in solid-state lighting, integrated optics, and photovoltaics.

Thin-film silicon single- and multi-junctions are a viable option to manufacture lightweight, flexible solar modules via high-throughput roll-to-roll (R2R) processes, starting from earth-abundant, non-toxic raw materials, at very cost competitive levels and with a range of application spanning from large area solar plants to portable devices. Nevertheless, flexible thin-film silicon modules have currently lower power conversion efficiency (PCE) compared to modules fabricated on glass substrates. Here, we focus on improving the efficiency of flexible single-junction modules by changing the chemical composition and the growth conditions of the p-doped window layer. Highly efficient devices require a window layer with excellent optical and electronic properties so that incoming light photons can easily reach the absorber layer, while photogenerated holes can be promptly extracted from the device. Our baseline modules have a p-doped hydrogenated silicon carbide (p-SiC:H) window layer. In order to simultaneously reduce optical losses and improve the charge collection, we reduced the thickness of p-SiC:H by modifying the plasma enhanced chemical vapor deposition tool, and we inserted a layer of p-doped nanocrystalline silicon oxide (p-nc-SiOx:H) in between p-SiC:H and TCO. The double p-layer modules that we obtained showed a 2% increase in the open circuit voltage compared to the single p-layer modules. Fine-tuning the deposition conditions for both p-layers will further reduce optical and resistive losses and improve the PCE of the modules; additionally, the double p-layer architecture will allow for an accurate control of the light transmission through the window layer, facilitating the current matching for multi-junction modules.

We design optical metagratings that enable the control of spectrum and directivity of light in mono- and multi-junction solar cells. First, we demonstrate 1 D Si metagratings that create directional scattering creating red-colored solar cells for rooftop application. Second, we create a spectrum splitting metasurface to optimize the light absorption performance of 4-terminal perovskite-silicon solar cells. The base units in our scattering designs are Si nanobars and nanocylinders that scatter light in a well-defined spectral band. When placed on a transparent substrate light is preferentially scattered on resonance, while off-resonant light is transmitted. By choosing an appropriate shape of the scatterers in the unit cell we suppress zero-order reflection. The metagrating directivity is controlled by the diffraction channels and is tuned by changing the array pitch. We demonstrate a colored PV module glass for rooftop application that reflects a red spectral band around 650 nm under angles between 30-75 degrees. This metasurface is composed Si nanobars arranged in differently pitched 1 D gratings that together results in a Lambertian-like scattering profile between 30-75 degrees. The reflectivity is tuned to 10% around 650 nm to achieve a clear perception of the red color, while keeping optical losses minimal. We fabricate the grating metasurface on top of a Si heterojunction solar cell and show experimentally that the short-circuit current of the cell is only reduced by 7%. Next, we design a semi-transparent spectrum splitting metasurface to improve the performance of perovskite-silicon 4-terminal tandem devices. A 2D hexagonal grid of amorphous silicon cylinders is placed at the Si/perovskite interface and resonantly scatters upward the 600-800 nm spectral band that is otherwise incompletely absorbed in the perovskite. The spectrum splitting layer redirects light into the perovskite under a steep angle (50-90 degrees), resulting in total internal reflection inside the perovskite and thus enhanced light trapping. An absolute improvement of over 1% of the overall tandem efficiency may be achieved.

In this paper a photovoltaic system is proposed that achieves high energy yield by integrating bifacial silicon cells into a spectrum-splitting module. Spectrum-splitting is accomplished using volume holographic elements to spectrally divide sunlight onto an array of PV cells with different bandgap energies. Diffuse sunlight is transmitted through the holographic element and converted. Light that is reflected off the ground surface is incident upon the rear side of the module and converted by the bifacial silicon cells. A diffuse scattering surface is applied to the rear-side of the monofacial wide-bandgap cell to redirect light to the bifacial silicon and increase the light collection. The volume holographic element optimization is automated and practical system design parameters such as concentration and aspect ratio are analyzed. An example using 22.5% efficient silicon and 28.8% efficient GaAs is presented and shows that an energy conversion efficiency of 32.9% can be achieved using typical utility scale illumination parameters. An economic analysis is presented that shows the installed cost per watt can be reduced by over 30% compared to a monofacial silicon panel and can even provide benefit if the cost of the wide-bandgap cell is over 10X the cost of silicon cells.

Conversion of solar energy into electricity is crucial to meet our ever-growing energy needs. The broadband spectrum of the sunlight limits the conversion efficiency of the single- and multi-junction based solar cells. Moreover, the angle of incident radiation dramatically decreases the amount of converted energy. In fact, diffractive optical elements (DOE) designed for spectrally splitting solar light are optimized for normal incidence, and their performance drastically decreases under angled-illumination. Unfortunately, once the number of design parameters two of whose are the number of wavelengths and number of incident angles increases, computational expense for DOEs design rises. Here, we design DOEs which concentrate and split the broadband radiation under angled-illumination. In our design, we take thin, transparent and cost-effective materials into account, and we manage to disperse broadband radiation 400 nm - 1100 nm into two separate bands which are the visible band 400 nm - 700 nm and the short-IR band 701 nm - 1100 nm. Here we optimize the DOEs for angled-illumination using computationally cost-effective approaches. We observe that spectral splitting of the broadband light is less sensitive to variation of incident angle of solar radiation once DOE optimization performed for the area which is half of the output plane. As a result, 8% and 18% excess solar energy conversion can be achieved within the visible band and the short-IR band, respectively. What's interesting is that less than 0.6% deviation in output intensity can be observed when a single DOE is illuminated at angle spans from 0 to 80 degrees.

In this study, the microscopic carrier dynamics that govern the UV stability of perovskite solar cells was investigated using pump-probe spectroscopy. In conventional perovskite solar cells, the UV-active oxygen vacancy in compact TiO₂ prohibits current generation after UV degradation. On the other hand, the dominant vacancy type in 2D Ti_1-xO₂ atomic sheet transporting layer (ASTL) is a titanium vacancy, not UV-sensitive. Consequently, the carrier recombination are suppressed and further extends UV stability in perovskite solar cells with a 2D Ti_1-xO₂ ASTL. The dynamics of electron diffusion, electron injection, and hot hole transfer processes are found to be less sensitive to the UV irradiation. The ultrafast time-resolved data shown here clearly represent a close correlation between the carrier dynamics and UV aging of perovskite, thus providing insight into the origin of UV-induced degradation in perovskite solar cells.

To reduce the surface reflection loss of the incident sun light in solar cells, an anti-reflection coating (ARC) is typically used. As an alternative, metallic nanoparticles (NPs), which show plasmonic effect and scatter the sun light efficiently at resonance wavelength, have been suggested. The light reflection from NPs-based ARC (hybrid ARC) is highly influenced by shape, size, host medium, volume fraction of NPs and the underneath layers. Herein, we use an analytical model for silver NPs-based hybrid ARC on a substrate and compare it with traditional dielectric based ARC. Our calculations of weighted reflectance from the ARC at various angles of incidence (AOI) show that the hybrid ARC performs better than dielectric based ARC at non-normal AOI. Also, the 80 nm thick hybrid ARC have shown the best wide-angle improvement in ARC performance.

The article presents a driver for two-axis tracking system which tracks the apparent position of Sun on elliptical sphere. The device usues data from the GPS module, based on received information, determines current latitude and longitude. Then the time of sunrise and sunset times and day length are calculated. The next step is to accurately determine the direction of sunrise and sunset using data obtained from a magnetometer module, that measures the change in magnetic field. The final step is to determine and adjust the angle of photovoltaic panels inclination. The gyroscope is responsible for the proper positioning of photovoltaic panels, which determines their inclination angle in vertical position. The accelerometer in cooperation with magnetometer module determine the horizontal position of solar panels. In the article compares the methods used to determine the maximum power point of photovoltaic installation.

The potential utility of nanocrystals (NCs) based on cesium lead halide (CsPbX₃, X= Cl, Br, and I) for various domains like optoelectronic and solar cells have driven a lot of interest in broadening the span of their exotic, unique and useful properties by tailoring the physical and chemical nature. In this communication, we have done the synthesis and characterization of the NCs to study the effect of Mn dopant in CsPbBr₃. Structural parameters of the synthesized compounds have been characterized by using the X-ray diffraction (XRD) technique while electronic and photophysical characterization was done by using Photoluminescence (PL) spectroscopy. The structural characterization of the synthesized sample shows decrement in the lattice parameter due to the incorporation of Mn as the dopant in CsPbBr₃. The experimentally calculated values of lattice parameters for CsPbBr₃ and doped CsPbBr₃ are found as 5.723 Å and 5.695 Å respectively. These results indicate a reduction in the lattice parameter due to the introduction of the Mn dopant in CsPbBr₃. Also, XRD patterns confirm the cubic morphology of the synthesized samples. Understanding of photophysics and electron behavior has been done using PL of the synthesized samples and PL pattern shows a high-intensity peak around 520 nm and 510 nm for CsPbBr₃ and doped CsPbBr₃ respectively. PL results indicate the blue shift phenomena because of the Mn dopant in the CsPbBr₃ sample. Also, the PL pattern shows small full-width half maxima (FWHM) for both samples which clearly suggests the homogeneity and orderness of the synthesized NCs. The experimentally calculated bandgap from PL pattern for CsPbBr₃ and its doped version are found as 2.384 eV and 2.431 eV respectively. Also, the computer experiment has been done to scientifically envisage and understand the structural and electronic nature of our synthesized NCs. Plane-wave density functional theory (DFT) based computations have been performed to validate the experimentally characterized results of the synthesized NCs. Structural computations are done by optimizing the crystal structures of CsPbBr₃ and its doped version. The theoretically computed lattice parameters after geometrical optimization (before optimization) for CsPbBr₃ and doped CsPbBr₃ are 5.731 Å (5.742 Å) and 5.705 Å (5.687 Å) respectively which also confirms the reduction in the lattice parameters after introducing Mn dopant in CsPbBr₃. Computed lattice parameters also indicate the cubic crystal nature of both CsPbBr₃ and its doped version and these computationally obtained structural properties show consistency with the experimental results. Further, the electronic behavior has been studied in a framework of DFT by calculating the band structure of the CsPbBr₃ and its doped version. Band structure of CsPbBr₃ and its doped version shows direct bandgap nature with valence band maxima (VBM) and conduction band minima (CBM) at the Gamma (Γ) point. The calculated direct bandgap for the undoped and doped NCs is 2.372 eV and 2.445 eV respectively. These results show very good agreement with the PL characterized results. This work successfully investigated the effect of Mn dopant in CsPbBr₃ NCs showing fascinating nature for the applications in the area of light-emitting diodes (LEDs) and photovoltaics.

Designing of optical and solar devices based on cesium-based lead halide perovskites have revolutionized the research on direct bandgap semiconducting materials. These perovskites exhibit unique and interesting semiconducting behavior with excellent photophysical properties which suggests their promising utility for solar and optical devices. Nowadays the tailoring of the photophysical, structural and electronic behavior has been continuously done using fully inorganic cesium based mixed halide perovskites using low-cost precursors. Herein to get the physical insight behind the existence of unique and fascinating behavior of CsPb(Cl/Br)₃, density functional theory (DFT) have been exploited with experimental validation. Structural behavior has been examined by optimizing the crystal structure for the CsPb(Cl/Br)₃ nanoparticles (NPs). Geometry optimization of the CsPb(Cl/Br)₃ crystal gives the optimized lattice parameter as 5.812 Å. The cubic nature has been observed in the CsPb(Cl/Br)₃ crystal after geometry relaxation. The stability analysis of the CsPb(Cl/Br)₃, NPs has been done by calculating the total energy and the formation energy of the system. The negative value of the formation energy suggests the stability of the CsPb(Cl/Br)₃ NPs. To investigate the charge neutrality in the system Bader charge analysis has been performed on the optimized crystal geometry of the CsPb(Cl/Br)₃. Further to understand the electronic behavior of the CsPb(Cl/Br)₃ NPs, band structure has been computed and results show the existence of the occupied state and unoccupied state at the same symmetry point of the Brillouin zone. This clearly indicates the direct bandgap nature of the CsPb(Cl/Br)₃ NPs. The calculated value of the bandgap for CsPb(Cl/Br)₃ NPs from the electronic band structure is found as 2.672 eV. To support the calculated theoretical results based on DFT we have also synthesized the monodisperse colloidal CsPb(Cl/Br)₃ NPs by hot injection method. The structural properties of the as-synthesized CsPb(Cl/Br)₃ NPs have been evaluated using X-ray diffraction (XRD) technique. The observed sharp peaks with high intensity in the XRD patterns show the cubic crystalline nature of CsPb(Cl/Br)₃ NPs. The evaluated lattice parameters from the XRD pattern is found as 5.781 Å. This calculated value shows very good consistency with the theoretically calculated lattice parameters for the optimized crystal structure of CsPb(Cl/Br)₃ NPs. Further, the electronic and photoluminescent nature of the synthesized sample has been studied by Photoluminescence (PL) spectroscopy. The evaluated value of fullwidth half maxima (FWHM) from the PL plot is 16 nm which suggests the presence of homogeneity with orderliness in the cubic lattice of the synthesized NPs. PL plot with a very high-intensity peak around 466 nm shows a good luminescent property. The observed PL peak around 466 nm gives a bandgap of 2.660 eV which is consistent with the DFT calculated results. The convincing combination of electronic and structural robustness make these mixed halide NPs appealing for solar cells and light-emitting diodes (LEDs).