The impact of Pd on the light harvesting in hybrid organic ...personales.us.es/norge/papers/NanoEnergy_34_141_2017.pdf · (PBE) [36] functionals were used that provide geometrical

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The impact of Pd on the light harvesting in hybrid organic-inorganicperovskite for solar cells

Javier Navasa,⁎, Antonio Sánchez-Coronillab,⁎, Juan Jesús Gallardoa, Jose Carlos Piñeroc,Desireé De los Santosa, Elisa I. Martínd, Norge C. Hernándeze, Rodrigo Alcántaraa,Concha Fernández-Lorenzoa, Joaquín Martín-Callejaa

a Departamento de Química Física, Facultad de Ciencias, Universidad de Cádiz, E-11510 Puerto Real, Cádiz, Spainb Departamento de Química Física, Facultad de Farmacia, Universidad de Sevilla, E-41012 Sevilla, Spainc Departamento de Ciencias de los Materiales, Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, E-11510 PuertoReal, Cádiz, Spaind Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, E-41012 Sevilla, Spaine Departamento de Física Aplicada I, Escuela Técnica Superior de Ingeniería Informática, Universidad de Sevilla, E-41012 Sevilla, Spain

A R T I C L E I N F O

Keywords:Hybrid perovskiteLight harvestingSolar cellsDFT calculationsDensity of states

A B S T R A C T

This study presents the effect of the incorporation of Pd in organic-inorganic hybrid perovskite, CH3NH3PbI3. Adecrease in the band gap energy of over 20% was observed in the samples with Pd, due to the presence of energylevels related with the Pd-I interaction. This feature can improve the light harvesting of this kind of material,which could have significant good consequences in the efficiency performance of photovoltaic devices. Thesynthesis was performed with a nominal ratio x=Pd/(Pd+Pb) of 0; 0.25; 0.5; 0.75; 1.0. No single perovskitephase was found; the samples synthesized were composed of a mixture of phases, hybrid Pb-Pd tetragonalperovksite, β-PdI2, and PbI2 and CH3NH3I. Perovskite structures were formed until x=0.75, but CH3NH3PdI3was not found. Analysis with TEM and EELS showed that the Pd and Pb were uniformly distributed. Also,periodic-DFT calculations were performed to understand the structural and electronic effects of incorporatingPd in the perovskite structure. An analysis based on the non-covalent interaction (NCI) index is presented as atheoretical complement to the study of the octahedral and Goldsmith factors for the perovskite-type structures,something that, to our knowledge, has not been performed previously. The ELF showed the role played by thePd-I interaction in the structural reorganisation of this hybrid perovskite and was confirmed by the PDOS. TheDOS showed that the theoretical band gap values were lower in the structures with Pd in accordance with theexperimental results. The effect of the Pd-I interaction in the band structure was shown. This makes it possibleto control the positions of the VB and CB depending on the elements that this kind of perovskite is formed of.

1. Introduction

Perovskite compounds have many interesting properties, such assuperconductivity, high thermoelectric power or optical properties [1].For halide perovskites, the crystal structure of the ABX3 compoundsconsists of a three-dimensional lattice of corner-sharing BX6 octahe-dra, where B is a bivalent metal cation, A is a monovalent organiccation that is small enough to fit into the structure, and X is a halogen[2]. Organic-inorganic hybrid perovskite of the composition ABX3,where usually A=[CH3NH3]

+, B=Pb2+, and X=I-, has recently attractedstrong research interest because of its photovoltaic properties [3].Perovskite solar cells (PSCs) have achieved a power conversionefficiency (PCE) record of 20.8% [4]. The different components of the

PSCs affect their PCE. One is the nature of the hole transportingmaterial (HTM), the use of spiro[fluorene-9,9′-xanthene] having beeninvestigated as an alternative to the usual Spiro-OMeTAD, with PCEresults of 19.8% [5]. Also, the effect of using mixtures of ions in thethree positions of the perovskite (A, B, X) has been analysed previously[6,7]. For example, the substitution of I- with Cl- or Br- has been widelystudied [8–14]; and doping with inorganic cations such as Sn2+, Sr2+,or Ca2+ in Pb2+ sites has been analysed to understand its effect onoptical and electronic properties [15–18].

Thus, this article presents the introduction of Pd2+ ions into theposition of the Pb2+ ions. Perovskites were synthesized with differentcompositions of Pd and Pb, responding to the formula MAPb1−xPdxI3(MA=[CH3NH3]

+), where x=0; 0.25; 0.50; 0.75; 1.00. The samples

http://dx.doi.org/10.1016/j.nanoen.2017.02.035Received 25 October 2016; Received in revised form 13 February 2017; Accepted 19 February 2017

⁎ Corresponding authors.E-mail addresses: [email protected] (J. Navas), [email protected] (A. Sánchez-Coronilla).

Nano Energy 34 (2017) 141–154

Available online 20 February 20172211-2855/ © 2017 Elsevier Ltd. All rights reserved.

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prepared were characterized using: x-ray diffraction to determine thepresence of the perovskite structure; transmission electronic micro-scopy (TEM) and electron energy loss spectroscopy (EELS) to analysethe distribution of Pd in the samples; x-ray photoelectron spectroscopy(XPS) to determine the chemical state bonding of the elements in thesamples; and UV–Vis spectroscopy to determine the optical propertiesand the band gap energy. The elemental composition was performed bymeans of x-ray fluorescence (XRF) and the CHNS technique.Consequently, the presence of Pd was seen to affect the formation ofthe perovskite structure because distortions were produced in theoctahedron that the structure is composed of. Furthermore, the Pd wasobserved to be uniformly distributed when the perovskite structure wasformed and the incorporation of Pd significantly decreased the bandgap with regards to the CH3NH3PbI3 structure. In turn, periodicdensity functional theory (DFT-periodic) calculations were performedto rationalize the experimental information on this topic. The struc-tures (MA)4Pb3PdI12, (MA)4Pb2Pd2I12 (MA)4Pb1Pd3I12 and(MA)4Pd4I12 were optimized to explore the electronic and structuraleffects of increasing the concentration of Pd. The non-covalent inter-action (NCI) [19] plots focussed on the MI6 octahedron (with M=Pb,Pd) of the structures shed light on the bonding interactions establishedwithin these hybrid systems. The analysis of both the electronlocalization function (ELF) [20–24] and projected density of state(PDOS) highlight the important role of the Pd-I interaction in thestructural reorganisation of this kind of hybrid structures. The densityof state (DOS) and band structure results, taking into account the spin-orbit coupling (SOC) [25–27] effects, were in agreement with thetendency forecast experimentally; that is, a decrease in the optical bandgap energy for the structures with Pd as compared with the MAPbI3perovskite.

2. Methods

2.1. Experimental

2.1.1. ReagentsAll reagents were from commercial sources and used without

further purification. Hydriodic acid (HI, 57 wt% in water) was fromAldrich; methylamine (CH3NH2, 33 wt% in ethanol), diethyl ether(Et2O, purity ≥99.8%), led iodide (PbI2, purity 99%), PdI2 (PdI2, purity≥99.99%), and γ-butyrolactone (purity ≥99%) were from Sigma-Aldrich.

2.1.2. SynthesisThe synthesis of the perovskite phases was performed by the

reaction of CH3NH3I and nominal amounts of PbI2 and PdI2. First,the CH3NH3I was synthesized using the following procedure [9,28,29]:HI (10 mL, 0.076 mol) and CH3NH2 (11.33 mL, 0.091 mol) werestirred in an ice bath for 2 h. Next, the mixture was evaporated at50 °C for 1 h, and CH3NH3I was obtained. Then, the solid was cleanedthree times using Et2O.

For the synthesis of the perovskite phase CH3NH3I (0.395 g) wasmixed with the corresponding stoichiometric amount of PdI2 and PbI2to obtain nominal concentrations (Pd/(Pd+Pb)) of 0, 25, 50, 75 and100 at% in γ-butyrolactone (2 mL) at 60 °C for 18 h.

2.1.3. CharacterizationThe samples synthesized were characterized in several properties,

such as elemental analysis using CHNS technique and x-ray fluores-cence (XRF), structural and morphological characterization using X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) andElectron Energy Loss Spectroscopy (EELS), electronic and opticalproperties by means of Diffuse Reflectance UV–vis spectroscopy (DR-UV-vis), X-ray photoelectron spectroscopy (XPS). The details of thischaracterization are shown in the Supplementary Material.

2.2. Computational details

DFT-periodic calculations were performed using the Vienna AbInitio Simulation Package (VASP) [30–33] with the projector-augmen-ted wave (PAW) method [34,35]. The number of plane waves in VASPwas controlled by a cut-off energy, chosen according to the pseudopo-tential and set in our calculations to Ecut=500 eV to satisfactorilydescribe the system [28]. In this way, the value of the cut-off waschosen by increasing by around 30% the highest value defined for allpseudopotentials used, i.e. carbon and nitrogen. The electron exchangeand correlation were treated within the generalized gradient approx-imation (GGA) [36]. In the case of GGA, Perdew-Burke-Ernzerhof(PBE) [36] functionals were used that provide geometrical structuresand relative stabilities for hybrid perovskites in good agreement withexperimental data, as reported elsewhere [12,17,26,37]. Both the cellshape and atomic positions were optimized using a conjugate-gradientalgorithm, where the iterative relaxation of atomic positions wasstopped when the forces on the atoms were less than 0.01 eV/Å.Also, a Gaussian smearing with kBT=0.1365 eV was applied. Thedetails of the calculations are shown in the Supplementary Material.

3. Results and discussion

3.1. Elemental analysis

The elemental composition of the samples was determined using x-ray fluorescence spectroscopy (XRF) to measure I, Pb and Pd, and theCHNS technique to determine C, H and N. The results of the weightpercentages obtained are shown in Table 1, which also shows theatomic percentage calculated for each element. It is possible to observeonly small deviations with regard to the nominal composition (seeSection 2.1). To enhance understanding, in the discussion of the resultsthe samples will be identified by the nominal value of x.

3.2. X-ray diffraction

Fig. 1 shows the XRD patterns for the samples synthesized. In turn,Table 2 shows the assignation of the mains peaks found in the patterns.The reflections assigned to PbI2, β-PdI2 and CH3NH3I were analysedaccording to PDF 00-007-0235, PDF 00-048-1715, and PDF 00-030-1797 references, respectively. The diffractogram for the sample withx=0.00, without Pd, shows the typical peaks of the reflections of theplanes of a tetragonal structure (I4/mcm space group), which has beenreported for MAPbI3 perovskite [10,11,13,37,38]. In addition, thetypical peaks for this perovskite structure show the most intensereflections at approximately 14°, 28° and 31°, (peaks shown as (1),

Table 1Weight and atomic percentage values for the elements in the samples prepared, obtainedby means of XRF and CHNS techniques.

Sample, x= Pd/(Pd+Pb)

x=0.00 x=0.25 x=0.50 x=0.75 x=1.00

wt% C 1.96 1.94 2.08 2.23 2.40wt% H 0.97 1.02 1.08 1.12 1.15wt% N 2.28 2.29 2.51 2.61 2.77wt% I 61.38 64.38 66.54 70.05 73.18wt% Pb 33.41 25.52 18.85 9.28 –wt% Pd – 4.85 8.94 14.71 20.50at% C 8.4 8.0 8.1 8.3 8.6at% H 50.0 50.45 50.7 50.3 49.65at% N 8.4 8.1 8.4 8.4 8.55at% I 24.9 25.1 24.6 24.8 24.9at% Pb 8.3 6.1 4.3 2.0 –at% Pd – 2.25 3.9 6.2 8.3xreal 0.00 0.27 0.48 0.76 1.00

J. Navas et al. Nano Energy 34 (2017) 141–154

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(6) y (8)) in Fig. 1), these being two double peaks. Fig. 1 shows that thistypical structure is maintained in the patterns for the samples withx=0.25, 0.50, 0.75. The assignment of reflections for these peaks isshown in Table 2. The peak marked (3) is assigned to the reflectionplane (211) of the tetragonal perovskite structure mentioned and is themain difference with the cubic perovskite structure (Pm3m spacegroup) that is also found in this kind of hybrid perovskites [37].Thus, the presence of perovskite cubic phase is negligible in oursamples.

Thus, for x=0.00 the typical tetragonal perovskite pattern can beseen, although there are also signs of residual PbI2, possibly due tosmall differences in the nominal amounts of the reactants added whensynthesizing the compounds. In turn, for x=0.25, the diffractogramprimarily shows the peaks associated with the reflections of thetetragonal perovskite structure, which confirms the formation of amixed perovskite structure with the presence of Pb and Pd. In the caseof x=0.50, a tetragonal perovskite pattern is also clearly observed

although some small peaks appear (marked as (16) in Fig. 1) associatedwith the β-PdI2 structure (P21/c space group), according to the PDF00-048-1715 reference. Also, for x=0.75, peaks associated with thetetragonal perovskite can still be seen with sufficient intensity, althoughcompared with the previous cases more intense peaks are observed thatare associated with β-PdI2 (marked as (16–22) in Fig. 1), PbI2(according to PDF 00-007-0235 reference) and CH3NH3I. This sug-gests that the significant increase in the amount of Pd in the samplesinhibits the formation of the perovskite structure, although not totallyas this structure can still be observed in the diffractogram. Finally, forx=1.00, peaks are observed associated with the β-PdI2 phase andsignals associated with CH3NH3I (see Fig. 1 and Table 2). In turn, peak(5) is indexed to PbI2 (see Fig. 1 and Table 2) and mainly appears inMAPbI3, as has been described previously. This peak does not appearfor x=0.25 and x=0.50, while in the case of x=0.75 an even moreintense peak is observed in the same position, which suggests that thereis more PbI2 that has not reacted and the perovskite has not beenformed, but this is not the case because this reflection can also beobserved in the diffractogram for the sample with x=1.00, so it can notbe assigned to PbI2. This peak is assigned to the CH3NH3I structure,according to the PDF 00-033-0239 reference. Thus, the presence ofPb2+ ions are seen to play an important role in the formation of thetetragonal perovskite structure in this kind of hybrid compounds.Finally, a residual presence of PdO is found in the patterns withx=0.25; 0.5 and 0.75, that is in the samples with Pd and with perovskitestructure. A peak at about 34.8° is observed in these samples which canbe assigned to PdO tetragonal structure (I4/mcm space group), whichis the same space group and structure of the tetragonal perovskitefound in the samples with Pb and Pd. The PdO is probably generatedon the surface of the tetragonal perovskite formed, as is confirmed byEELS measurements. Also, some Pd ions can be located in interstitialsites, but it is well-known that the perovskite is a close-packedstructure, lacking sufficient space [39,40]. Typically, the initial stageof the perovskite doping is realized by the substitution of ions in thelattice sites, as reported previously in the literature [41]. So, theincorporation of Pd ions in the Pb lattice sites is produced, but also alow proportion of Pd ions can be placed in interstitial sites.

If Pb replacement is produced, that is Pd is placed in Pb sites intothe tetragonal perovskite structure, as discussed above, the peaks in thediffraction pattern of the tetragonal perovskite structure should show2θ shifts. For this reason, peaks (6) and (8) in Fig. 1 have been analysedand a slight shift is observed, as is shown in Table 3. This shifts inreflections positions are small. But, previous work on an analogoussystem, CsPb1–xSnxI3, also showed no variation in lattice parametersand it was concluded that this was because the larger size of the I ionsultimately dictated the size of the unit cell [42]. This is evidence of thePb being substituted by Pd in perovskite structure. Also, this issupported by the TEM and EELS results that are discussed below.The substitution is not complete, some residual amount of PdI2 beingfound in the samples since the formation of perovskite phase with Pd isless favoured than for Pb, as is discussed below.

Fig. 1. XRD patterns of the samples synthesized.

Table 2Assignation of the main peaks observed in the XRD patterns of the samples synthesized.

Peak Assignation Peak Assignation Peak Assignation Peak Assignation

(1) Te(110)(002) (7) Te(213) (13) CH3NH3I* (19) β-PdI2(121)

(2) Te(112) (8) Te(114) (14) CH3NH3I* (20) β-PdI2(210)

Te(222)(310)CH3NH3I

* (residual) CH3NH3I*MAI*

(3) Te(211) (9) Te* (15) CH3NH3I* (21) β-PdI2

*

(4) Te(200) (10) Te* (16) β-PdI2(011) (22) β-PdI2*

MAI* (residual)(5) PbI2(002) (residual) (11) Te(330) (17) β-PdI2(111) (23) PdO (Te)(6) Te(004)(220) (12) CH3NH3I

* (18) β-PdI2(200)

* Planes not assigned in the references.


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The formation of the perovskite structure is estimated on the basisof two factors: Goldschmidt's tolerance factor (t) and the ‘octahedralfactor’ (µ). The details of this calculation are shown in theSupplementary Material. For halide-based perovskites, the usual tvalues are in the 0.813–1.107 range, while the ‘octahedral factor’ isgreater than 0.442 in these cases [43]. These two factors were used toanalyse the structures of MAPbI3 and MAPdI3. Thus, the values for tand µ are 0.822 and 0.541 for MAPbI3, and 0.910 and 0.391 forMAPdI3. Thus, the MAPbI3 system fulfills both conditions, which is inaccordance with the formation of tetragonal perovskite structureobserved in the XRD patterns for this sample. On the other hand,the MAPdI3 system shows a suitable Goldschmidt's factor for theformation of perovskite structure, while the octahedral factor is lowerthan the usual limit value for halide-based perovskites. This is in linewith the results obtained using XRD, which show how the formation ofperovskite structure decreases when the proportion of Pd in thesamples increases.

3.3. Transmission electron microscopy and electron energy lossspectroscopy

To evaluate the distribution of the Pb and Pd atoms in thenanostructure, TEM micrographs combined with EELS measurements(acquired in STEM mode) are presented here. Fig. 2a shows the Pd,PdO and Pb EELS spectra as available in the literature [https://eelsdb.eu/] [44]. Fig. 2a also shows the PdI2 spectrum, as experimentallyacquired from a palladium iodide sample supplied by Sigma-Aldrich.The main peaks are highlighted, labelled and summarized in Table 4for easy tracking.

Fig. 2b shows experimentally acquired EELS spectra, as measuredin the samples with nominal x=0.00, 0.25, 0.50, 0.75, 1.00 and x=0.5.Red dashed lines are used to highlight the energy loss correspondingwith the location of the P4 and P5 peaks. In Fig. 2b, three structuralstages are revealed: first, for x=0, Pb-related P3, P4 and Pb-O4,5 peaksare revealed; second, for x=0.25 and 0.50, EELS peaks are observed at7.1–8.8, 16.0, 23.6, 32.3 and 53.7 eV. The peak observed at 16.0 eVcorresponds with an overlap of the P4 and P5 peaks, thus revealing aPb-Pd mixture. The P1 and P2 peaks are overlapped in a single, widerpeak. Finally, for x≥0.75, the EELS spectrum corresponds with anoverlap of Pd, PdI2 and PdO spectra. No peaks characteristic of Pb areobserved. It is noteworthy that the EELS spectra are acquired in alocalized area of the samples, so it is possible that Pb related peaks arenot revealed in the sample with x=0.75. On the other hand, it isnoteworthy that the peaks assigned to PdI2 are slightly different tothose peaks assigned to Pd in PdO or metallic Pd. Also, the peaksassigned to Pd in a perovskite structure are similar to those found inPdI2 because the chemical state bonding of Pd in perovskite tetragonalstructures and in β-PdI2 are similar.

Moreover, the P4 and P5 peaks can be used as markers for mappingPd and Pb distributions in EELS. Please note that this procedurecannot be applied in samples with x≥0.75, because no Pb-characteristicpeaks are evidenced. So, Fig. 3a shows a TEM micrograph for thesample with nominal x=0.25, where nanoparticles of ≈20 nm areobserved. An EELS temperature colour map of the Pd-related P5 peakintensity is superimposed in the region where the measurement was

carried out. EELS intensity maps of the Pd-related P5 peak, Pb-relatedP4 peak and PdO related P2 peak are presented in Fig. 3b. It isimportant to note that the maximum/minimum of the relative dis-tributions of Pd and Pb are coincident. The latter demonstrates that theconcentration of Pd follows the shape of the Pb distribution (even whenthe relative intensity of Pd is lower than the Pb distribution). Fig. 4shows a TEMmicrograph of sample with x=0.5. Again, nanoparticles of≈20 nm are revealed. The dotted black line is used to highlight theregion were the EELS map was acquired. Energy-filtered EELS mapswere obtained at EELS peak energies labelled as P4, P5 and P2. Theinset in Fig. 4a (temperature colour) shows an energy-filtered EELSmap (intensity of the related EELS peaks) of the Pd-N2,3 peak. Energy-filtered EELS maps are shown in Fig. 4b. These EELS maps, filtered atP5 and P4 energies, can be used to compare the distribution of Pd andPb respectively. The latter reveals the similarity in the distribution ofPb and Pd: the Pb and Pd concentrations are homogeneous in thenanoparticles. That is, for samples x=0.25 and 0.50, when the Pbconcentration is high, so is the Pd concentration, which is evidence ofthe formation of perovskite structure with Pb and Pd, because highconcentrations of Pb and Pd are found in the same zone of the samples.Conversely, if the Pb concentration were high, Pd concentration wouldbe low, but this does not occur in the samples synthesized. Finally,Figs. 3b and 4b also show an energy-filtered EELS map of the PdO-related peak distribution. Some oxidation on the border of the structurecan be appreciated (both in x=0.25 and x=0.5) due to adsorbed speciesonto the surface, as is shown using XPS (see Fig. S1 in theSupplementary Material).

Therefore, the results obtained from XRD, considering the shift ofthe perovskite reflections in the samples with Pd) and EELS showenough evidence to make us to think that hybrid Pb-Pd perovksite isformed. However, we must conclude that in the samples there is nosingle phase perovskite, but rather a mixture of tetragonal perovskite,PbI2, MAI and PdI2 is obtained.

3.4. X-ray photoelectron spectroscopy

X-ray photoelectron spectra were acquired to analyse the oxidationstates and the bonding states of the elements in the samples. Thesamples with perovskite structures were studied. Fig. S2 in theSupplementary Material shows the general spectra of the samples witha perovskite structure and the basic assignment of the peaks found.Different zones in the spectra were recorded in detail for analysis.Fig. 5a shows the Pb 4 f spectra of the samples. The binding energy(BE) for the Pb 4f7/2 was around 138.0 eV, which is coherent withresults reported previously [45,46]. Also, the spectra show well-separated spin-orbit components. The separation between the compo-nents was around 4.9 eV, which is typical for Pb(II). Fig. 5b shows thePd 3d spectra for the samples with Pd. The Pd 3d5/2 signals appear at aBE of about 336.3 eV, which is coherent with values reported for Pd(II)interacting with I- [47]. In turn, the Pd 3d spectra show well-separatedspin-orbit components with a separation of 5.25 eV, which is coherentfor Pd(II). Also, the Pd 3d peaks are observed to be asymmetric, whichis coherent for ionic species of Pd, as the signal for atomic Pd is usuallysymmetric.

Fig. 5c shows the I 3d3/2 and I 3d5/2 spectra. Again, the spectrashow well-separated spin-orbit components with a separation of11.5 eV, which is coherent with the results reported for I- [46].Moreover, a slight shift towards lower BE for I 3d signals are observedfor the samples with Pd. This suggests slight changes in the oxidationstate of I due to the different interactions between the Pb2+ and Pd2+

cations, as we can observe from the theoretical calculations below. Inturn, in the case of the interactions of the cations with the [CH3NH3]

+

groups, small modifications can be observed in the state of the chemicalbonds, as the signal obtained for the N 1 s shows(Fig. 5d). The BE of N1 s was around 401.5–402.5 eV. As for the I 3d signal, small modifica-tions are observed depending on the nature of the B cations. These

Table 3Shifts of the main peaks assigned to tetragonal perovskite structure.

Sample, x=Pd/(Pd+Pb) 2θ

Te (004) (220) Te (114) (222) (310)

x=0.00 28.15 28.42 31.60 31.85x=0.25 28.17 28.44 31.62 31.87x=0.50 28.17 28.44 31.65 31.90x=0.75 28.19 28.47 31.65 31.90


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modifications are studied using theoretical calculations.Finally, an analysis of the Pb 4f and Pd 3d signals has been

developed to determine the impurities in the samples. Fig. 5e shows thedeconvolution of the Pd 3d for the sample with nominal x=0.25. Theresults of the deconvolution for this sample are representative of all thesamples. The deconvolution of the Pd 3d5/2 shows a peak at a BE ofabout 337.3 eV. This peak is not assigned to Pd(IV), which usuallyappears at a higher BE [46]. But, the peak assigned to Pd 3d5/2 for PdOappears in the range of 335.9–337.3 eV [46]. As reported previously,the higher values for Pd 3d5/2 in surface PdO, as in our samples,suggest strong interactions between surface PdO and the bulk of thematerial, resulting in electron deficient Pd2+ species [48]. Moreover,the Pb 4f signal was also analysed. Fig. 5f shows the deconvolution ofthe Pb 4f signal for the samples with nominal x=0.25, which isrepresentative of the results obtained for all the samples. From thedeconvolution it is possible to see two contributions at about 137.8 eV

(the predominant contribution) and 138.5 eV. According to the litera-ture, the signal at about 138.5 eV is assigned to the Pb 4f7/2 signal forPbI2 [46]. The contribution observed at about 137.8 eV must beassigned to Pb 4f7/2 in perovskite structure because this structure ispredominant in the sample analysed.

3.5. DR-UV-Vis spectroscopy

UV–Vis spectroscopy, in diffuse reflectance mode, was used todetermine the optical band gap of the samples synthesized. Fig. 6ashows the spectra recorded. These clearly show how the samples withPd have an absorption band at shorter wavelengths than the samplewith x=0.00, that is MAPbI3. In turn, in the x=1.00 sample, with no Pband in which perovskite phase is not formed, an absorption band isobserved at approximately 600 nm. This band can be assigned to thepresence of CH3NH3I due to perovskite phase not being formed. Thespectrum for the CH3NH3I synthesized is shown in Fig. S3 in theSupplementary Material. A similar shape is observed in all the sampleswith Pd; for example, in the region between 800 and 900 nm and in theregion of the increase in the diffuse reflectance, which corresponds withthe band gap of the samples. This may be due to the interactionbetween Pd and I as it is also observed in the spectrum of the samplewith x=1, in which the perovskite structure is not formed, and in thespectrum of PdI2 (see Fig. S4 in the Supplementary Material). Thespectra seem to indicate that the band gap is determined by the Pd-Iinteraction in the samples in which perovskite is formed, and also inthose in which it is not (x=1 and PdI2). This will be confirmed below bytheoretical calculations.

The optical band gap of the samples can be estimated from thespectra registered from Kubelka-Munk formulism and Tauc plots. Thedetails of this calculation are shown in the Supplementary Material.

Fig. 2. (a) Low-loss EELS spectra of Pb, Pd and PdO as available in literature [https://eelsdb.eu/] [44]. PdI2 spectrum was experimentally acquired from commercial PdI2 sample. (b)Experimental low-loss EELS spectra acquired for nominal x=0 to 1.

Table 4Nature and EELS position of the main experimentally observed peaks of Fig. 2. Note thatPd-characteristic P6 and P7 peak position changes for PdO and PdI2.

Peak E / eV Description Peak E / eV Description

P1 7.1 PdI2-relatedtransition

Pb-O4,5 23.2 Pb-relatedtransition

P2 8.8 PdO-relatedtransition

P6 25.4 Pd-relatedtransition

P3 9.2 Pb-relatedtransition

P7 32.6 Pd-relatedtransition

P4 15.8 Pb-relatedtransition

Pd-N2,3 56.3 PdO-relatedtransition

P5 17.1 PdI2-relatedtransition


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Fig. 6b shows the Tauc plots for the samples synthesized. Also, Table 5shows the values of the optical band gap estimated. Clearly, thepresence of Pd, both in perovskite form and as PdI2 (for the samplewith x=1.00), is responsible for this decrease in the band gap energy,which means that the reduction is due to the appearance of Pd-Iinteractions in the valence or conduction band, which cause one orboth of these to be displaced. This will be analysed from a theoreticalperspective below. Also, the band gap estimated for the MAPbI3perovskite is similar with the values reported for thin films. For thesamples with Pd, we estimated a band gap value lower than forMAPbI3, so we expected a decrease of the band gap of the perovskitewith Pd in thin films too. This feature can improve the light harvestingof this kind of material, which could have significant good conse-quences in the efficiency performance of photovoltaic devices. Also, thedecrease in the band gap is practically independent of the Pd content,as is usual. But, similar behaviour has been found in the literature forinternal doping [28,49,50].

3.6. Structure and local geometry analysis

Based on the results of the experimental characterization, weconsider it to be of interest to perform a theoretical study to analysethe effect of increasing the concentration of Pd on the structural andelectronic properties of MAPb1−xPdxI3 perovskites. Thus, a study wasperformed of the tetragonal structures of these perovskites with

x=0.00, 0.25, 0.50, 0.75 and 1.00.Our first-principle results show that the inclusion of Pd2+ in the

perovskite structure causes important structural changes that have notbe appreciated before when other metals are included in the sameproportions [28,51]. Thus, the local geometry for the most stableconfiguration of the optimized tetragonal structures is shown in Fig. 7.It shows a diagonal cross-section of the optimized structures and adotted square has been added to distinguish the central M-I coordina-tion shell (with M=Pb, Pd) from the exterior ones, which will bediscussed below. To clarify the discussion, the MA+ groups have beeneliminated.

Focussing on the central coordination shell shown by the dottedsquare (Fig. 7), it shows that the Pd-I distance is generally smaller thanthose of the Pb-I of the MAPbI3 perovskite structure and that itdecreases as the concentration of Pd2+ increases. For theMAPb0.75Pd0.25I3 structure, the Pd-I distance is c.a. 5–8% smallerthan the Pb-I distance of the MAPbI3 perovskite structure. For theMAPb0.50Pd0.50I3, MAPb0.25Pd0.75I3 and MAPdI3 structures, the Pd-Idistance decreases slightly (c.a. 2–3%) in each case when comparedwith the previous structure with a lower concentration. This means thatmodifications in this distance of up to 16% are produced. The reductionin the Pd-I distances suggests a structural distortion around the centralregion (dotted square in Fig. 7). As an example, this distortion is clearlyreflected in the variation in the α, β, γ and δ angles shown in Fig. 7when compared with the MAPbI3 perovskite structure. Table S1 in the

Fig. 3. (a) TEM micrograph of sample nominal x=0.25. EELS mapped area (temperature scale) is superimposed onto the TEM image showing the intensity of the P5 peak. (b) EELSmaps, filtered at P4, P5 and P2 energies.

Fig. 4. (a) TEM micrograph of sample nominal x=0.50. EELS mapped area (temperature scale) is superimposed to the TEM image showing the intensity of the Pd-N2,3 peak. (b) EELSmaps, filtered at P4, P5 and P2 energies.


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Supplementary Material includes the values of these angles for thestructures studied. As this Table shows, in general, angle α decreasesand the β, γ and δ angles increase in accordance with the concentrationof Pd. The MAPb0.75Pd0.25I3 structure is the one that remains the mostsimilar to MAPbI3 perovskite. However, for the MAPb0.50Pd0.50I3 andMAPb0.25Pd0.75I3, structures with the lowest percentage of Pb

2+, the βand δ angles change the most, c.a. 10–12°. Thus, the geometricalresults suggest that in the MAPb0.50Pd0.50I3 and MAPb0.25Pd0.75I3structures, formed experimentally as mentioned above, low-intensitystructural tension should appear that mean that overall the perovskite-type structure remains stable, as is shown experimentally. In this sense,the theoretical results show that the presence of an ion such as Pb2+,which has an ionic radius greater than Pd2+, must compensate for this

structural tension favouring the formation of both structures, as hasbeen reported for the case of Cd2+ ions [51]. The most striking changesare produced in the MAPdI3 structure, in which the α angle decreasesby c.a. 7.5° but the β angle increases noticeably by c.a. 16.5° comparedwith the same angle without the MAPbI3 perovskite structure.Undoubtedly, this drastic variation of c.a. 16.5° involves strongstructural tension, which explains why the MAPdI3 structure is notformed experimentally, as predicted above by the octahedral factor.The instability predicted by the octahedral factor for the MAPdI3structure can be seen with an analytical tool such as the NCI index [19].This index enables the qualitative identification and characterization ofweak interactions of various strengths as chemically intuitive isosur-faces that reveal both stabilizing (strong and attractive interactions in

Fig. 5. X-ray photoelectron spectra of (a) Pb 4f; (b) Pd 3d; (c) I 3d; and (d) N 1 s. Analysis of (e) Pd 3d and (f) Pb 4f signals for sample with x=0.25.


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blue, weak interactions in green) and destabilizing interactions (strongand repulsive interactions in red). Fig. 8 shows a comparison of theNCI plots focussed on the MI6 octahedron (with M=Pb, Pd) forMAPbI3, MAPb0.50Pd0.50I3 and MAPdI3 structures, respectively.Supplementary Material includes the NCI plots for the remainingstructures (Fig. S5). Comparing Figs. 8a and b, it is interesting thatthe experimentally formed structures (MAPbI3 and MAPb0.50Pd0.50I3)show isosurfaces in blue for the Pb-I and Pd-I interactions what impliesstabilizing interactions. However, the blue tones of the isosurfaces forPb-I and Pd-I are different. In this case, the strongest blue colour is forthe Pd-I isosurface lobes in MAPb0.50Pd0.50I3, indicating the strength ofthe interactions, which follow the order Pd-I > Pb-I forMAPb0.50Pd0.50I3 and MAPbI3, respectively. In the same way, thisdiscussion is extended to the MAPb0.75Pd0.25I3 structure (Fig. S5). Butin the MAPb0.25Pd0.75I3 structure (Fig. S5), as well as the dark bluelobes there are others with a green tone, which confirms the presence ofcertain structural tensions that are compensated for by the stabilizingPb-I interactions because this structure can be formed experimentally.The case of the MAPdI3 is different as the lobes of the Pd-I interactionare mainly green (Fig. 8c). The green interactions are weak andalthough some blue stabilizing ones appear, these are insufficient for

the perovskite-type structure to remain stable, as predicted by theoctahedral factor, and hence it is not formed experimentally. Theseresults indicate that the NCI index can be used as a theoreticalcomplement to the study of the octahedral and Goldschmith's factorsfor perovskite-type structures.

3.7. Electron localization function

The electronic properties of the structures may be rationalized bymeans of the ELF study. Fig. 9 shows the ELF plots corresponding tothe geometries represented in 2D in Fig. 7. Fig. 9 also includes the ELFplot for the PdI2 used as the precursor in the experimental synthesis.

For the discussion of the ELF in Fig. 9, the central area and itssurrounding will be distinguished, in accordance with Fig. 7. Thus,Fig. 9 shows that the contour plot for Pb is different to that of Pd. Forthe MAPbI3 structure, the outlines of the ELF for I and Pb almostoverlap by means of slight electron localization (sky blue colour) thatprovides stability to the Pb-I interaction. For the MAPb0.75Pd0.25I3structure with the lowest proportion of Pd, the central area is seen toproduce a directionality of the halos from the I towards the Pd. Thisdirectionality explains the decrease in the Pd-I distances observed inthis structure and indicates in turn that the Pd-I interaction is slightlystronger than the Pb-I one, as the NCI analysis also predicted. As anexample, this directionality is shown in the enlarged ELF image of theMAPb0.75Pd0.25I3 structure. This Pd-I directionality in the central zoneis also observed in the rest of the structures with a higher concentrationof Pd, which leads to a reorganisation of the structure, explaining thevariation in the angles shown before. However, for the MAPdI3structure with the highest concentration of Pd, the approximation of

Fig. 6. (a) UV–Vis spectra for the samples synthesized; (b) Tauc plots obtained from the spectra measured.

Table 5Optical band gap values for the samples MAPb1−xPdxI3 synthesized.

Sample, x Eg/eV Sample, x Eg/eV

0.00 1.54 0.75 1.220.25 1.21 1.00 1.240.50 1.22

Fig. 7. The local geometry in 2D for the tetragonal MAPb1−xPdxI3 perovskite with x=0.00, 0.25, 0.50, 0.75 and 1.00.


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Pd and I leads to the appearance of blue spaces in the ELF, suggestingthe absence of electron localization and certain structural instability, inaccordance with the changes seen in the angles. At the bottom of Fig. 9there is a comparison of the ELF of the I-Pd-I interaction in theMAPdI3 structure and the PdI2 structure. It shows that the contours ofthe ELF for both structures are very similar and it is possible to observean approximation of two I to each Pd. Thus, it is reasonable to thinkthat it is not favourable for the MAPdI3 structure to adopt a perovskite-type structure, and hence it is not formed experimentally.

3.8. Density of states and projected density of states analysis

We will discuss the electronic structure in terms of the density of

states (DOS), the projected density of states (PDOS) and the bandstructure analysis. The study was performed with SOC, so althoughqualitatively the band gaps follow the same experimental trend, theyare slightly lower than without SOC [25–27,51]. Likewise, the testsperformed with the functional hybrid B3LYP produce the same resultsfor the DOS and PDOS.

The band gap of the structures with Pd, calculated by analysing thedensity of states (DOS), is smaller than that of the MAPbI3 perovskite(Fig. 10). Fig. 10a shows that when the concentration of Pd increases,the valence band (VB) shifts to the right due to the appearance of newpeaks. These peaks are associated with electron levels of the Pd, as theanalysis of the PDOS will show below. For the purpose of comparison,in Fig. 10b the Fermi level is set to 0 for all the structures and it shows

Fig. 8. NCI plots for MI6 octahedron (with M=Pb, Pd) for tetragonal structures: (a) MAPbI3; (b) MAPb0.5Pd0.5I3; (c) MAPdI3.

Fig. 9. ELF contour plots for structures: (a) MAPbI3; (b) MAPb0.75Pd0.25I3; (c) MAPb0.5Pd0.5I3; (d) MAPb0.25Pd0.75I3; (e) MAPdI3; (f) PdI2.


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that when the concentration of Pd increases the band gap remainspractically constant, in agreement with the tendency observed experi-mentally. Figs. 10a and b also include the density of states of the PdI2for comparison purposes and similar behaviour is observed on the edgeof the VB.

Fig. 11 shows the contribution of Pb, Pd, and I to the valence band(VB) and the conduction band (CB) for the MAPbI3, MAPb0.50Pd0.50I3,MAPdI3 and PdI2 structures, which were chosen as representative forthe discussion below. Fig. S6 in the Supplementary Material includesthe same contributions for the MAPb0.75Pd0.25I3 and MAPb0.25Pd0.75I3structures. The MA groups contribute more internally to the edges ofthe VB and CB, as reported previously [28,51], so they have not beenincluded in this discussion. For the MAPbI3 structure, the VB and CBare mostly composed of I p and Pb p states (see Fig. 11a), respectively.Fig. 11b and S5 for the MAPb0.75Pd0.25I3,

MAPb0.50Pd0.50I3 and MAPb0.25Pd0.75I3 structures show that thelevels that appear on the edge of the VB predominantly correspondwith the d states of the Pd and the p states of the I. The region of the VBis clearly dominated by an overlap of the d states of the Pd with the pstates of the I, which indicates the Pd-I interaction. Similar overlapshave been observed in other Pd structures [52]. That is, the d orbitals ofthe Pd in these perovskite-type structures provide empty and filledelectron levels near to the edge of the VB which makes the material

behave in a similar way to a p-type semiconductor. This can also beobserved in the PDOS of the MAPdI3 and PdI2 structures, in whichthere can only be overlapping between the Pd-d and I-p states; thisexplains the similarity of the ELF of both structures. In turn, thisinteraction between the Pd-d and I-p states on the edge of the VB for allthe structures with Pd must play an important role in the chargetransfer processes and explains why the UV–Vis spectra (Fig. 6) havethe same shape, in both the samples in which the perovskite structureis formed and in those in which it is not, for example for x=1.00, oreven for the PdI2. This means that the optical properties of theperovskite analysed in this study are determined by the presence ofPd in the samples and its interaction with I.

Fig. 12 shows the band structure for the MAPbI3, MAPb0.50Pd0.50I3,MAPdI3 and PdI2 structures. The Brillouin Zone for the tetragonal andmonoclinic lattices was chosen as described in the literature [26,53]. AsFig. 12 shows, the transition from the VB towards the CB for theMAPbI3, MAPb0.50Pd0.50I3 and PdI2 structures is direct and takes placeat point Γ. However, the transition for MAPdI3 is indirect. The valence-band-maximum (VBM) corresponds to point Μ and the conduction-band-minimum (CBM) to point Γ. For the MAPbI3 structure, a splittingof the bands away from the critical point [27] associated with the lossof inversion symmetry has been reported. This splitting in the presenceof SOC is also produced in the MAPb0.50Pd0.50I3 and MAPdI3 struc-

Fig. 10. (a) DOS of the perovskite structures simulated. (b) DOS where the Fermi level is set to 0 to compare de band gap energy with regard to the Pd concentration. The DOS for PdI2is included for comparison purposes.


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tures around the critical point Γ in the CB. On the other hand, thissplitting is not observed around the point Γ and M of the VB (Fig. 12).In fact, as seen in the analysis of the PDOS, the d states of the Pd andthe p states of the I, which overlapped in all the structures, played animportant role in the VB maximum. Thus, understandably, this kind ofsplitting is not produced in the VB of the PdI2 (Fig. 12).

4. Conclusions

In this study, organic-inorganic hybrid perovskite MAPbI3 wassynthesized incorporating Pd. A decrease in the band gap energy wasobserved in the perovskites with levels of Pd over 20%. This feature canimprove the light harvesting of this kind of material, which could havesignificant good consequences in the efficiency performance of photo-voltaic devices. The samples with Pd were composed of a mixture ofcrystalline phases; that is, hybrid Pb-Pd tetragonal perovskites, β-PdI2,and PbI2 and MAI. The formation of perovskite was observed for all theconcentrations of Pd, except for x=1, the sample without Pb, in whichcase the MAPdI3 structure was not formed. This is because thepresence of Pd greatly distorts the octahedron on which the perovskitestructure is based; distortions are produced when there is no Pb, theperovskite structure not being allowed to be formed. In turn, for theperovskite structures formed with Pd, the distribution of Pd wasobserved to be uniform with regard to the distribution of Pb, which

is evidence of the formation of hybrid Pb-Pd tetragonal perovskite.Also, the reflections assigned to tetragonal perovskite in XRD patternsshow a shift in the samples with Pd, which is evidence of thedeformation of the perovskite due to the presence of Pd in Pb sites.Furthermore, XPS revealed that the Pd found in these situations isPd2+. This leads to small modifications in the interactions between theI- ion and the corresponding B ion, which is responsible for thedistortions in the structure. On the other hand, the band gap valuesobtained theoretically were concordant with those obtained using UV–Vis spectroscopy. DFT calculations showed that new peaks appear inthe valence-band-maximum (VBM) when the concentration of Pdincreases. The analysis of the PDOS indicates that the contribution ofthe d-Pd and p-I states is predominant in these peaks, which lead tothink in the great impact of the presence of Pd in band structure in theperovskite. This result suggests that, in general, the Pd-I interactionmust be strengthened when the concentration of Pd increases. So muchso that for the MAPdI3 structure with the highest percentage of Pd, theperovskite-type structure is not stable and tends to be similar to that ofits experimental precursor, PdI2, which explains why it is not formedexperimentally. Finally, NCI plots focusing on the MI6 octahedron(with M=Pb, Pd) corroborate that for the MAPdI3 structure the Pd doesnot have the capacity to keep the perovskite-type structure stable, aspredicted by the octahedral factor, and hence it is not formedexperimentally. These results indicate that the NCI index can be used

Fig. 11. I, Pb and Pd projected states with SOC+U for tetragonal structures: (a) MAPbI3; (b) MAPb0.5Pd0.5I3; (c) MAPdI3; (d) PdI2.


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as a theoretical complement to the study of the octahedral andGoldschmith's factors for perovskite-type structures. To our knowl-edge, this kind of analysis has not been proposed previously.

Acknowledgements

We thank the Ministerio de Economia y Competitividad (MINECO)of the Spanish Government for funding under Grant no. ENE2014-58085-R.

Calculations were done through CICA – Centro InformáticoCientífico de Andalucía (Spain).

Antonio Sánchez-Coronilla thanks the financial support from VPPI-US.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2017.02.035.

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Fig. 12. Band structure with SOC+U for: (a) MAPbI3; (b) MAPb0.5Pd0.5I3; (c) MAPdI3; (d) PdI2.


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Javier Navas is lecturer at the Science Faculty, Universityof Cádiz, Spain. He received his Chemistry degree in 1998and his Ph.D. in 2004 in the University of Cádiz. Hisresearch interest includes advanced nanomaterials for solarenergy. Currently, he is working in the synthesis anddoping of nanomaterials with applications in photovoltaicand photocatalytic applications, such as TiO2 or hybridperovskites from an experimental and theoretical perspec-tive. Also, he is working in the application of nanoparticlesin the design of nanofluids with applications inConcentrating Solar Power.

Antonio Sánchez-Coronilla is Assistant Professor atUniversity of Sevilla. He was post-doctoral researcher atthe University of Santiago de Compostela, TechnicalUniversity of Valencia-Institute of Chemical Technology(UPV-CSIC), Technical University of Lisbon and Universityof Cádiz. His research activity is focused on the theoreticalstudy of materials chemistry, with particular interest in thestudy of materials for energy applications.

Juan Jesus Gallardo earned his Degree in Chemistryand Ph.D. in the Physical Chemistry Department atUniversity of Cadiz, Spain. Currently he is developing hisresearch, from an experimental point of view, in the field ofsolar energy including nanofluids, perovskites, DSSC, etc.Expert in spectroscopic characterization of different semi-conductor materials, he has used these data to determinesome properties of the active layer, focused in the estima-tion of the thermal properties of thin layer of semiconduc-tors.

José Piñero is working as hired researcher at Departmentof Materials Science, University of Cádiz (Spain). Histechnical skills are focused on electron-microscopy basedtechniques (HAADF, HREM, CTEM) as well as a variety ofspectroscopy techniques (cathodo/photoluminescence,EELS). His research focuses on characterization and devel-opment of diamond-based devices for power electronicapplications, exciton dynamics and material characteriza-tion (NWs, nanoparticles and interfaces, among others).

Desireé de los Santos Martínez is working as research-er at Department of Physical Chemistry, University ofCádiz. Her Ph.D. was focused on doping with broad bandsemiconductor nanoparticle: Structural characterizationand photoelectrochemical evaluation. Currently, she isworking in the synthesis and doping of nanomaterials withphotocatalytic and photovoltaic applications.

Elisa I. Martín is working as Assistant Professor atDepartment of Chemical Engineering, University ofSeville. Her Ph.D. was focused on classical and ab initioMolecular Dynamics (MD) Simulations of copper com-plexes with catalytic applications and solar energy conver-sion. During her research stays at the Rutherford AppletonLaboratory, Oxford University, UK, and at the InstitutoSuperior Técnico of Lisbon, Portugal, she developed quan-tum mechanical interaction potentials and studied electrontransfer processes of metal-organic complexes by MDsimulation. Currently, she is working on DFT calculationsof semiconductors.

Norge Cruz Hernández is Associated Professor atUniversity of Sevilla. He got his Ph.D. Thesis on PhysicalChemistry, Surface Science in 2002. His research interestsare mainly focused on computational surface science, beingspecialist in molecular dynamics and periodic-DFT calcula-tions. Currently, he is working on DFT-calculations appliedto nanomaterials for magnetism and photovoltaic applica-tions.

Rodrigo Alcántara is working as Associate Professor atDepartment of Physical Chemistry, University of Cádiz(Spain). He received his Chemistry degree and Ph.D. fromthe same institution. His research work focuses on devel-opment of photoelectrochemical solar cells and nanofluidswith applications as heat transfer fluid in thermal solarenergy. He also works on UV–Vis spectroscopy, Raman andIR spectroscopy.


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Concha Fernández Lorenzo is Associate Professor atDepartment of Chemical Physics, University of Cádiz,Spain. She leads the research group Synthesis,Characterization and Evolution of Materials. Current re-search activity is focused on nanoscale devices for produc-tion and storage of clean energies, in particular perovskitesolar cells and dye-sensitized solar cells. She also works inboron-doped diamond for applications in high powerElectronics.

Joaquín Martín Calleja is Professor of Chemical PhysicsDepartment of Cádiz University. He has obtained his Ph.D.degree in 1982 working about Raman Spectroscopy. From1987–2015 he was responsible for the SCEM (Simulation,Characterization and Evolution Of Materials) researchgroup, working on spectroscopy, materials analysis andcharacterization, solar energy and scientific instrumenta-tion design. He has also been the main investigator ofseveral research projects and contracts with multiplecompanies. The main published works are focused onPhooelectrochemical Solar Cells, instrumentation designand, in the last time, on the use of nanoparticles forenhanced the properties of thermal fluids.


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The impact of Pd on the light harvesting in hybrid organic-inorganic perovskite for solar cellsIntroductionMethodsExperimentalReagentsSynthesisCharacterization

Computational details

Results and discussionElemental analysisX-ray diffractionTransmission electron microscopy and electron energy loss spectroscopyX-ray photoelectron spectroscopyDR-UV-Vis spectroscopyStructure and local geometry analysisElectron localization functionDensity of states and projected density of states analysis

ConclusionsAcknowledgementsSupporting informationReferences

Documents

The impact of Pd on the light harvesting in hybrid organic ...personales.us.es/norge/papers/NanoEnergy_34_141_2017.pdf · (PBE) [36] functionals were used that provide geometrical