All‐Inorganic Micrometric CsPbBr3:Yb3+ Powder as a Multifunctional Material for Photovoltaics and Optical Thermometry: Structural and Optical Characterization

Halide perovskites have been studied very intensively by researchers during the last decade. Development of these materials has improved their unique optoelectrical properties reaching even higher standards, making them promising candidates for photovoltaic applications. It should be noted that most inorganic halide perovskites obtained to date have been synthesized using organic solvents in the form of nanosized colloids. Here, a low‐temperature synthesis protocol for the preparation of microcrystalline CsPbBr3 perovskite powder doped with Yb3+ ions is proposed. The structural and photoluminescence features of the studied material are thoroughly investigated and described. It turns out that the excitation of the CsPbBr3:Yb3+ perovskite with a 375 nm wavelength leads to spontaneous luminescence of excitons and Yb3+ ions. Hence, the use of CsPbBr3:Yb3+ as a luminescent thermometer or an additional absorbing layer on a solar cell surface is possible. The latter application may result in an increase in the conversion efficiency of the cell. In order to verify this, such a layer is prepared and installed on a commercial silicon solar cell. Its photovoltaic properties are investigated by measurements of the current–voltage characteristics with 1‐sun illumination and the spectral characteristics of the external quantum efficiency.


Introduction
The field of nanocrystalline optoelectronic material is in need of novel inorganic materials characterized by simple synthesis route DOI: 10.1002/adom.202301672 and efficient luminescence.Recently, the attention was focused on the perovskite halides with general formula APbX 3 (A = alkaline or organic cation, X = halide anion), which besides the above-mentioned advantages exhibits high thermal stability compared to similar compounds and high charge carrier diffusion length and mobility. [1,2][5] Ybdoped materials have emerged as promising candidates for optoelectronic and photovoltaic applications.In particular, they have shown significant potential for use in nextgeneration solar cells due to their high efficiencies in spectral conversion.
Perovskite lead halides exhibit emission bands in the visible 400-750 nm range.[8][9] Yb-doped CsPbCl 3 perovskite powders were recently reported with important insights into the potential applications in optoelectronic devices.The introduction of Yb 3+ ions in the mentioned material influenced the properties of the host lattice, resulting in a deterioration of the optical properties at higher concentration.The study also included thermoluminescence analysis of CsPbCl 3 :Yb 3+ .CsPbBr 3 has already been studied for application in photovoltaics. [10]Compared to their organic analogs, they exhibit superior long-term stability, while maintaining the same charge transport properties. [11]CsPbBr 3 nanocrystals have been studied as host for dopants such as Sn 2+ , Cd 2+ , and Zn 2+ . [12]ased on the promising results on chloride perovskite doped with Yb 3+ , it is reasonable to expect CsPbBr 3 to also be a good host for lanthanide ions.
Yb-doped CsPbBr 3 was studied as a thin film solar cell, but the dopant was used along with other lanthanides as a structural modifier to optimize the morphology of the obtained layer and enhance the carrier lifetimes, resulting in better photoelectric conversion. [13]CsPbBr 3 :Yb 3+ was obtained also in form of colloidal nanocrystals [14] and spin-coated nanofilm. [15]The studies focused on stabilizing the material for application in near-infrared photodetectors and other optoelectronic devices, [14] as well as the efficient solar power and green power technology. [15]The thin film samples exhibited the downconversion (quantum cutting) process. [15]We propose that the spectroscopic properties of this compound can be further optimized by the introduction of solid-state synthesis method, which yields larger and more crystallized grains.
As demonstrated by the related studies, the efficient luminescence of Yb in CsPbBr 3 could be extremely beneficial to the field of photovoltaics.Yb-doped perovskite bromides could be combined with the thin films of the same compound realizing both the down-shifting of the absorbed spectrum, as well as the photoelectric conversion.
In this work, we study Yb-doped CsPbBr 3 obtained by the solidstate method.The morphology of the microcrystalline powders is assessed.The material is evaluated on its spectroscopic properties including host emission and emission from Yb 3+ .Moreover, the influence of a CsPbBr 3 :Yb 3+ thin layer deposited on the surface of a commercial silicon solar cell on its efficiency has been investigated.

Structural Features
Prior to conducting optical measurements, the phase structure and morphology of the as-synthesized CsPbBr 3 :Yb 3+ were characterized.Therefore, the diffractogram of the studied sample was collected and depicted in Figure 1a.Comparison of the experimental data with the standard confirmed the formation of CsPbBr 3 :Yb 3+ powder perovskite with orthorhombic structure and Pbnm space group (No. 62).It can be seen that the introduction of as much as 10% lanthanide ions does not negatively affect the quality of the structure, which is manifested by the absence of additional reflections.Figure 1b illustrates the visualization of the structure of the analyzed material.It can be seen that CsPbBr 3 :Yb 3+ contains octahedra where the corners are occupied by bromine atoms, while the central positions are taken by lead atoms.The cesium atom is located in the middle of the 3D framework cavity.It is worth noting that the geometry of the structure is not fully symmetrical and the positions of some PbBr 6 octahe-dra are slightly shifted relative to each other, which is a hallmark of the orthorhombic structure.In order to improve the spectroscopic properties, the Yb 3+ ion was introduced into the structure.Due to the similarity of the ionic radii, it can be assumed that the dopant ion partially substitutes the lead position. [13,16]As shown earlier, heterovalent doping does not affect the structural purity, however, it promotes the formation of defects that compensate the energy mismatch. [17,18]Figure S1 (Supporting Information) presents a powder diffraction pattern of pure CsPbBr 3 used as a reference material.The obtained result is identical to that of the doped sample.
It is well known that morphology has a significant influence on the physical properties of materials. [19,20]The preparation of the investigated CsPbBr 3 by the solid-state reaction method, which is very rarely used for this purpose, additionally encourages its verification.Therefore, scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) mapping were performed and are shown in Figure 2 and Figure S2 (Supporting Information), respectively.It can be seen that the obtained  crystallites have irregular, cubic-like shapes with a non-porous surface.In the presented image, crystallites with micrometric size dominate, which is consistent with predictions due to the use of dry chemistry technique during synthesis procedure.In addition, no agglomeration of the objects was observed.The EDS maps confirmed good distribution of the individual components.The intense color in Figure S2e (Supporting Information) is due to insufficient grinding of the sample.Figure S3 (Supporting Information) reveals the SEM image of the reference sample, which is not very different from the doped sample, indicating that the incorporation of 10% Yb 3+ into the CsPbBr 3 matrix has no major effect on the morphology of the material.
To identify symmetry changes induced by temperature and the sequence of phase transitions, we conducted thermal analyzes on perovskite powders, both for pure CsPbBr 3 and CsPbBr 3 :Yb 3+ doped samples.The differential scanning calorimetry (DSC) thermograms of the examined samples revealed two reversible phase transitions as a function of temperature (refer to Figure S4, Supporting Information).Anomalous specific heat parts displayed a distinct thermal anomaly during cooling at ≈402 K and during heating at ≈405 K (see Figure 3).The shape of this anomaly, coupled with the presence of a minor yet discernible thermal hysteresis of ≈3 K, signifies a first-order phase transition.Moreover, an observable decline in the magnitude of the anomalous specific heat values at phase transition temperature is evident in the doped samples.[23][24] A second reversible anomaly is observed at ≈362 K.23][24] However, this phase transition exhibits an unusual temperature dependence, with a broad and asymmetric specific heat anomaly (Figure 3), deviating from a single second-order phase transition.This behavior was previously observed in thermal analysis of pure CsPbBr 3 single crystals and was proposed to be a result of the superposition of two separate phase transitions, although this phase separation has not been identified until now. [23]In conclusion, it is evident that the ytterbium-doped CsPbBr 3 sam-ple's sequence of phase transitions aligns with that of the pure CsPbBr 3 compound.Consequently, within the investigated temperature range, the symmetry remains consistent with that of the undoped sample.
Figure 4a presents the temperature-dependent Raman spectra of CsPbBr 3 :Yb 3+ perovskite powder measured in the 80-500 K range.At the lowest temperature, nine sharp peaks are observed, and they correspond well to the orthorhombic Pbnm phase (O).[27] Upon heating, bands quickly broaden and shift due to increased thermal vibration of the lattice.The phonon lifetimes become longer, and bands quickly merge into wide contours.Since the cut-off of the applied edge filter eliminates the spectrum below 10 cm −1 , it is difficult to fit Lorentz curves to such broad bands; therefore, our fitting procedure was limited to plotting band maxima as a function of temperature for the two most intense bands labeled as a square and circle in Figure 4a and Figure S5a (Supporting Information).The results presented in Figure 4b indicate that phase transitions to tetragonal P4/mbm phase (T) at 362 K and to cubic Pm3m phase (C) at 405 K [27][28][29] causes very weak but clearly detectable changes in the slopes of band positions as a function of temperature.The changes observed for undoped CsPbBr 3 are slightly sharper (see Figure S5b, Supporting Information).Both phase transitions are governed by changes in the tilt pattern of the octahedral leadbromide 3D network. [28]In addition, small changes in the intensity of the bands presented in Figure 4c and Figure S5c (Supporting Information) are also associated with the phase transformations taking place.Since similar results were obtained for pure CsPbBr 3 , [28] the presented results are evidence that the crystal dynamics of CsPbBr 3 doped with 10 mol% of Yb 3+ ions are not strongly affected by the aliovalent doping.

Spectroscopic Properties
The optical properties of the CsPbBr 3 :Yb 3+ powder were investigated by recording the diffuse reflectance and excitation spectra, as well as photoluminescence spectrum at 300 K presented in Figure 5.It turned out that studied perovskite exhibits strong absorption of ultraviolet and visible radiation up to 520 nm (Figure 5a).[32] It is noteworthy that reflectance measurements did not directly show the presence of an exciton band expected at the absorption edge in contrast to the pure sample (see Figure S6, Supporting Information).It can be seen that the band edge is asymmetric, which may indicate the presence of a hidden exciton band.However, to definitively establish the position of the exciton absorption maximum the excitation spectrum monitored at 990 nm was measured and superimposed on the reflectance spectrum, as plotted in Figure 5a.In addition, in the near-infrared range, easily noticeable Yb 3+ transitions but showing much lower intensity compared to the host absorption were identified.Recalculation of the reflectance spectra according to the Kubelka-Munk formula [33] allowed to specify the size of the energy bandgap of powdered CsPbBr 3 :Yb 3+ and CsPbBr 3 to be 2.17 eV (Figure S7a, Supporting Information) and 2.19 eV (Figure S7b, Supporting Information), respectively.[36][37] A similar trend was observed previously for the powdered chloride analog. [38][41] The ability to record the exciton excitation spectrum by monitoring the Yb 3+ band suggests that energy transfer between host and dopant occurs.This was confirmed by measuring the photoluminescence spectrum by irradiating the sample with a 375 nm laser diode.In response, a narrow green emission band corresponding to exciton recombination with a maximum at 530 nm, as well as an Yb 3+ band localized at 992 nm, are recorded and depicted in Figure 5b. [42]To confirm that the transition at 530 nm is indeed an exciton luminescence, power dependence measurements were made with 375 nm line excitation (Figure S8, Supporting Information).As a result, a linear increase in emission intensity with a slope of 1.1 was obtained, proving the excitonic nature of this band. [43]The asymmetry of the band in the visible range indicates that it has two components, which have been attributed to free (FE) and bound (BE) excitons. [44,45]The luminescence decay curves of exciton and Yb 3+ emission were collected at 300 K (Figure S9a,b, Supporting Information).It turned out that both profiles show a non-exponential pattern that indicates the presence of non-radiative processes. [46]oth decays were fitted with a double-exponential function and the average lifetimes were determined to be ≈400 ps and ≈50 μm for exciton and lanthanide, respectively.It was found that the values obtained for exciton for the sample synthesized as solid-state polycrystalline powders were shorter than for their counterparts prepared as thin films [13] or colloids. [47]This might be a result of the increased probability of structural defects as a result of a dry chemistry method causing higher rate of non-radiative processes. [48]The obtained results were compared with pure CsPbBr 3 (Figure S9c, Supporting Information).It was found that the recorded exciton emission decay has the same characteristics, but it is longer, reaching 1.58 ns due to only one luminescent center and no need to share energy between two centers as in the case of the doped sample.It is noteworthy that both bands were recorded at room temperature that gives the possibility to use the studied CsPbBr 3 :Yb 3+ in photovoltaics as an additional absorbing layer covering commercial solar cells.
Figure 6a shows emission spectra recorded as a function of temperature at 375 nm excitation in the 80-650 K range.It turned out that at low temperatures the separation of the exciton bands into FE and BE is definitely more pronounced (Figure S10, Supporting Information).The increase in temperature causes that the boundary between these bands is blurred, forming a single, asymmetric band.Furthermore, the green luminescence intensity is gradually reduced (Figure 6b) due to temperature quenching.However, at 150 K, a gentle increase in intensity is observed, reported earlier in the literature, [45] ascribed to quenching of nonradiative defects [49] or carriers delocalization. [50,51]Moreover, an increase in temperature leads to a move the exciton band toward shorter wavelengths that is probably related to the electron-phonon renormalization and thermal expansion of the host lattice during heating. [52,53]However, other literature reports indicate that this effect may be associated with the gradual disappearance of low-frequency excitons (BE) followed by the disappearance of high-frequency excitons (FE) with increasing temperature due to enhanced thermal kinetic energy.It can gradually break the binding energy of excitons where the exciton state with low binding energy is first influenced. [54]It is worth noting that on the temperature-dependent emission spectra of pure CsPbBr 3 , symmetrical exciton bands were observed (Figure S11a, Supporting Information), which indicates that the existence of BE (defects) is determined by the presence of a lanthanide in the matrix.The course of emission intensity in the entire measurable temperature range was similar to CsPbBr 3 :Yb 3+ (Figure S11b, Supporting Information).The case of lanthanide bands is interesting because its emission is thermally activated.The most intense luminescence was achieved at 210 K, but after exceeding this value a gradual decrease of its intensity was observed due to temperature quenching.Nevertheless, above 350 K an increase in luminescence intensity appears that may be related to the phase transition evidenced in Figure S4 (Supporting Information).
The thermal kinetics of exciton and lanthanide emission observed from CsPbBr 3 :Yb 3+ perovskite were determined starting from the equation for the decay rate of the excited level: [55] Γ (T) = Γ  + Γ 0 e −ΔE∕kT (1)   where Γ is the total transition rate from the excited level, Γ v stands for radiative decay rate, Γ 0 represents attempt rate for thermal quenching and ΔE corresponds to the activation energy for thermal quenching, T is the temperature of the system, and k is the Boltzmann constant.Knowing that the emission intensity is dependent on the value of radiative transitions in relation to the overall transition rate I(T) = Γ  ∕Γ(T) and assuming that at T = 0 the transitions are purely radiative Γ(T) = Γ  , we can use the following expression: [55] where I means the intensity and I 0 is the initial intensity at low temperature.In order to calculate the activation energy of the investigated material, the above formula has been modified to: [56] − ln where ΔE represents the slope of − ln[(I 0 ∕I) − 1] in the function of 1/kT and ln(Γ 0 ∕Γ v ) is a constant.The activation energies of FE and BE, as well as lanthanide emissions observed from CsPbBr 3 :Yb 3+ perovskite were determined to be 67, 47, and 326 meV, respectively, while for the undoped sample, the activation energy for the exciton was equal to 52 meV (Figure S12, Supporting Information).The calculation of the radiative and attempt rates was presented in the Supporting information.It was found that the mechanism behind the optical phenomena observed from microcrystalline CsPbBr 3 :Yb 3+ powder differs from that previously suggested by us earlier for the chloride analog. [38]The main difference is that in CsPbCl 3 :Yb 3+ simultaneous emission of exciton and dopant was observed, while in this case spontaneous emission occurs.Here, the process begins with absorption of 375 nm radiation, which leads to the population of the valence band of the host.This is followed by a non-radiative electron transfer to the free electron state and further to the defect states, resulting in energy depopulation from both of them in a radiative manner as FE and BE emission at 80 K.A further increase in temperature may gradually break the binding energy of the exciton states, leading to a gradual decrease in the exciton emission intensity. [54]At the same time, detrapped electrons in the conduction band transfer non-radiatively to the 2 F 5/2 levels of Yb 3+ ions, resulting in spontaneous radiative depopulation in the infrared range.The identical course of exciton integral intensity in the doped (Figure 6b) and pure (Figure S11b, Supporting Information) sample shows that exciton and dopant emission are independent of each other.Schematic diagram of energy transfer mechanism for investigated material can be found in Figure 7.

Application Potential
As shown above, the microcrystalline CsPbBr 3 :Yb 3+ perovskite exhibits spontaneous visible and infrared emission up to room temperature.This makes the studied material a potential candidate for application as a non-contact luminescent thermometer.To verify this, the fluorescence intensity ratio (FIR) parameter, that is the ratio of the integral intensity of the exciton bands to the Yb 3+ bands (Δ) recorded in the 80-200 K range, was determined and plotted as a function of temperature (T) in Figure S13a (Supporting Information).Subsequently, using Equation ( 4), another important parameter, namely relative sensitivity (S r ), was calculated (Figure S13b, Supporting Information).It was found that a luminescent thermometer based on CsPbBr 3 :Yb 3+ microcrystalline powder achieves its maximum sensitivity at 100 K equal 6.77% K −1 .The result is not only two times higher than the one we obtained for the chloride analog, [38] but also quite high compared to others reported in the literature (0.75% K, −1 [57] ≈3.5% K, −1 [58] ≈3% K, −1 [59] ≈1.5% K −1 -all results at 100 K).To further evaluate the performance of the luminescent thermometer, the temperature uncertainty (T), which indicates the smallest detectable value change during the experiment, was estimated using Equation ( 5).The temperature dependence of T values for CsPbBr 3 :Yb 3+ microcrystalline powder is presented in Figure S13c (Supporting Information).It turned out that temperature uncertainty for studied material at 100 K is equal to 0.037 K, which is an excellent results compared to other dual-emitting luminescent thermometers. [60]r = 1 Δ In order to investigate photovoltaic properties of the solar cell with CsPbBr 3 :Yb 3+ perovskite layer, its current-voltage (I-V) characteristics have been measured at 25 °C, in dark ("dark" I-V curve) and under AM1.5G illumination ("light" I-V curve).The results of these measurements, together with a schematic illustration of the studied solar cell, are shown in Figure 8a,b.Analyzing the I-V plots depicted in Figure 8a one can notice that under AM1.5G illumination of the sample (with zero external applied bias), the short-circuit current (I SC ) and the open-circuit voltage are produced (V OC ).Thus, it means that the solar cell exhibits light-electricity conversion capabilities, with the light to dark current ratio of ≈3 × 10 2 .This demonstrates the photovoltaic effect, where photons from sunlight are absorbed by the device, exciting electrons and creating electron-hole pairs.To study the role of CsPbBr 3 :Yb 3+ perovskite material in the photovoltaic effect additional "light" I-V measurements were performed for a commercial monocrystalline Si solar cell, and a device with pure CsPbBr 3 layer.
Fundamental parameters of the solar cell with CsPbBr 3 :Yb 3+ perovskite layer were determined from the fitting of its "light" I-V characteristics with a well-known double-diode model (DDM). [61]For comparison, two reference samples were also analyzed: one-without the perovskite layer, which was a commercial monocrystalline Si solar cell, and the other-a device with pure CsPbBr 3 layer.The "light" I-V curves fitted with DDM for all the aforementioned solar cells are shown in Figure 9.The photovoltaic performance parameters of the three types of solar cells are collected in tables in the insets of Figure 9. Comparing the photovoltaic parameters of the studied solar cells, it can be seen that the cell with CsPbBr 3 :Yb 3+ layer exhibits the highest values of all of them.On the one hand, its efficiency is only 0.8 percentage points higher in comparison to the commercial monocrystalline Si solar cell, however, it indicates an enhancement in the overall efficiency of the solar cell.One can also notice, that the efficiency of the solar cell with CsPbBr 3 :Yb 3+ perovskite layer is improved by ≈4.9% with respect to the efficiency of the commercial monocrystalline Si solar cell without the additional cover.Similar result has been obtained for Cu(In,Ga)Se 2 (CIGS) solar cell with the CsPbBr 3 perovskite nanocrystal layer. [62]The authors of that paper reported that the efficiency of the aforementioned cell was of 0.5 percentage points higher compared with those of the conventional CIGS solar cell without the CsPbBr 3 perovskite layer.Thus, it was improved by 4.5%. [62]In our case, the efficiency of the solar cell with pure CsPbBr 3 layer is lower than the efficiency of the device with CsPbBr 3 :Yb 3+ cover, however, it is higher in contrast to efficiency of the Si solar cell (see Figure 9c).Despite slight differences in the I SC and V OC values of the analyzed devices, a significant change in the fill factor (FF) of individual cells is noticeable.It should be emphasized that the value of FF depends not only on I SC and V OC , but also on the values of I m and V m (the maximum current and voltage), which are not shown here.In turn, the greater the FF value, the greater the efficiency (Eff) value.
In order to verify the improvement in the efficiency of the Si solar cell with the CsPbBr 3 :Yb 3+ perovskite layer, its spectral response as well as external quantum efficiency (EQE) have been measured and compared with these of the commercial monocrystalline Si solar cell and the device with pure CsPbBr 3 layer.The results of the aforementioned measurements for three types of Si solar cells are shown in Figure 10.Analyzing the plots presented in Figure 10a, it can be clearly seen that the spectral response of the cell with the CsPbBr 3 :Yb 3+ perovskite layer is higher within the wavelength range of 800-1000 nm compared with these of the commercial Si solar cell and the device with pure CsPbBr 3 layer.The signal reaches maximum at about 920 nm.The EQE of all the analyzed solar cells is the same in the wavelength range between 530 and 760 nm.However, for the cell with CsPbBr 3 :Yb 3+ perovskite it is highly improved in the wavelength range between 800 and 1000 nm, what is in agreement with its spectral response within the same wavelength range.The reason for the observed phenomena is probably the presence of Yb 3+ ions, which have strong emission in the NIR spectral range.The graphs of spectral response and EQE of the device with pure CsPbBr 3 layer do not exhibit any improvement in the analyzed wavelength range.They are nearly the same as for the commercial Si solar cell.Thus, based on the EQE measurements, it can be concluded that the CsPbBr 3 :Yb 3+ perovskite powder enhances the photovoltaic responsivity of Si solar cell.

Conclusion
A low-temperature protocol for obtaining CsPbBr 3 :Yb 3+ has been developed.Structural measurements revealed that the investigated sample is phase pure and exhibits micrometric crystallite sizes.Thermal analysis showed reversible phase transitions at 362 and 405 K also evidenced in temperature-dependent Raman spectra.Spectroscopic measurements have shown that the intense green emission of the exciton and Yb 3+ ions occurs spontaneously over a wide range of temperatures.The energy transfer mechanism is explained.The application potential of the investigated material was verified in terms of its use for non-contact temperature sensing.It turns out that the maximum S r value and T at 100 K were equal to 6.77% K −1 and 0.037 K, respectively.Moreover, the studied compound was deposited in the form of a thin layer on the surface of a commercial Si solar cell.It was found that the efficiency of the Si solar cell with the CsPbBr 3 :Yb 3+ perovskite layer is improved by ≈4.9% with respect to that of the reference samples that were the Si solar cell and a solar cell with CsPbBr 3 layer.A significant change has been noticed in the EQE that is highly improved by the CsPbBr 3 :Yb 3+ perovskite in the infrared range between 800 and 1000 nm.This observation has been assigned to the presence of Yb 3+ ions, which have strong emission in the NIR spectral range.The EQE of the device with pure CsPbBr 3 layer does not exhibit any improvement in the aforementioned spectral range.The results of photovoltaic measurements proved that the use of CsPbBr 3 :Yb 3+ as an absorbing layer in a solar cell enhances its conversion efficiency.

Experimental Section
Synthesis Procedure: The synthesis protocol for powdered micrometric CsPbBr 3 :10%Yb 3+ included weighing and carefully grinding stoichiometric amounts of cesium (CsBr, 99.9%) and lead (PbBr 2 , ≥98%) bromides bought from Sigma-Aldrich, as well as a hydrate of ytterbium bromide (YbBr 3 •xH 2 O, 99.9%) from Alfa Aesar.The grinding process was carried out in an ethanol environment to homogenize the starting materials in an agate mortar until the alcohol completely evaporated.As a result, a fine orange powder was obtained, which was heated in a multi-stage thermal treatment (550 °C/3 h, 480 °C/48 h, and 300 °C/24 h) in nitrogen flow.A reference sample of pure CsPbBr 3 was prepared according to the same protocol.
Device Fabrication: The absorbing layer was prepared by suspending CsPbBr 3 :Yb 3+ in poly(methyl methacrylate) (PMMA) polymer.For this purpose, 0.6 g of PMMA (ABCR GmbH) was dissolved in 10 mL of toluene (AR, POCH) and stirred vigorously on a magnetic stirrer at 50 °C for 15 min.Meanwhile, 0.05 g of CsPbBr 3 :Yb 3+ in 0.3 mL of toluene was flooded and ultrasonicated for 15 min.0.6 mL of PMMA dissolved in toluene was added to the mixture and again ultrasonicated for 15 min.To enable performance of electrical measurements, the prepared solution was deposited directly onto a commercial monocrystalline Si solar cell of 2 cm × 2 cm area, using a spin-coater in two cycles (1000 rpm 30 s −1 for one cycle) applying 0.3 mL of liquid each time.A reference device containing a layer of pure CsPbBr 3 was prepared according to the same protocol.
Equipment: X-ray diffraction (XRD) pattern was measured using a X'Pert PRO powder diffractometer (PANalytical, The Netherlands) equipped with a linear PIXcel detector and using Cu-K radiation ( = 1.54056Å).The chemical composition and general morphology of the samples were checked using an FE-SEM microscope (FEI Nova NanoSEM 230) equipped with EDS analyzer (EDAX Genesis XM4).Heat capacity was measured using Mettler Toledo DSC-1 calorimeter with a high resolution of 0.4 μW.Nitrogen was used as a purging gas and the heating and cooling rate was 5 K min −1 .The excess heat capacity associated with the phase transitions was evaluated by subtraction from the data the baseline representing variation in the absence of the phase transitions.The temperature-dependent Raman spectra in the 10-200 cm −1 spectral range were measured using a Renishaw inVia Raman spectrometer operating at 830 nm and coupled to a confocal microscope.The temperature was controlled using a Linkam THMS600 heating/cooling cell.The absorption spectra were measured in the back scattering mode using a Agilent Cary 5000 spectrophotometer.The excitation spectrum and the luminescence decay profiles of the Yb 3+ emission intensity were measured using a FLS1000 Fluorescence Spectrometer from Edinburgh Instruments.The luminescent decay profile of exciton was recorded using a femtosecond laser (Coherent Model Libra).Temperature-dependent emission spectra were recorded with the Hamamatsu photonic multichannel analyzer PMA-12 equipped with a BT-CCD linear image sensor.The laser diode operating under 375 nm excitation line was applied as the excitation source.The temperature of the samples during emission measurements was controlled by Linkam THMS600 Heating/Freezing Stage.The CsPbBr 3 :Yb 3+ thin layer was prepared using a spin-coater model: HO-TH-05 from Holmarc.Current-voltage (I-V) characteristics of the studied solar cells were measured at 25 °C using the I-V curve tracer equipped with the PET Solar Simulator (#SS100AAA) operating at light intensity of 1000 Wm −2 (i.e., 1-sun illumination, AM1.5G).The measured, illuminated I-V curves were fitted by the IV Curve Fitter v1.8Copyright '2002 software [63] that uses double-diode model [61] to extract fundamental parameters of solar cells, such as: open circuit voltage, shortcircuit current, fill factor, efficiency, etc.The external quantum efficiency (EQE) and spectral response of the solar cells were measured with a Bentham PVE300 system calibrated by a certified reference Si photodetector, using a xenon-quartz tungsten halogen dual lamp.The spectral characteristics were registered at room temperature, in the wavelength range of 300-1100 nm.

Figure 3 .
Figure 3. Change in specific heat c p related to the phase transitions in the cooling runs for CsPbBr 3 :Yb 3+ and CsPbBr 3 perovskite powders.

Figure 4 .
Figure 4. a) Temperature-dependent Raman spectra obtained for CsPbBr 3 :Yb 3+ perovskite powder, along with b,c) changes of band maxima (b) and intensity (c) as a function of temperature; the square and circle in (a) refer to the dependencies shown in (b); vertical or horizontal lines in (b) and (c) indicate temperatures of phase transitions between orthorhombic (O), tetragonal (T) and cubic (C) phases.

Figure 5 .
Figure 5. a) Diffuse reflectance and excitation spectra (monitored at 990 nm) with magnified view of the Yb 3+ band and b) photoluminescence spectrum of microcrystalline CsPbBr 3 :Yb 3+ perovskite powder recorded at room temperature.The inset in (b) presents a photograph of the glowing sample at 80 K.

Figure 6 .
Figure 6.a) Thermal evolution of CsPbBr 3 :Yb 3+ photoluminescence spectra registered at 375 nm excitation and b) integral intensities of the recorded emission bands.

Figure 7 .
Figure 7. Schematic diagram of energy transfer mechanism for the microcrystalline CsPbBr 3 :Yb 3+ perovskite powder at different temperatures.

Figure 8 .
Figure 8. a) I-V characteristics for the solar cell with CsPbBr 3 :Yb 3+ thin layer measured at 25 °C in dark and under 1-sun illumination, and b) a scheme of the studied solar cell.

Figure 9 .
Figure 9. a-c) "Light" I-V characteristics of three silicon solar cells: a) with CsPbBr 3 :Yb 3+ layer, b) without the perovskite layer, and c) with pure CsPbBr 3 layer.The insets tables compare photovoltaic parameters of the three types of solar cells, which are: the short-circuit current (I SC ), the open-circuit voltage (V OC ), fill factor (FF), and efficiency (Eff).

Figure 10 .
Figure 10.a) Spectral response measurements and b) EQE for three types of Si solar cells: with CsPbBr 3 , with CsPbBr 3 :Yb 3+ , and without the perovskite layer.