INORGANIC, ORGANIC/INORGANIC HYBRID MATERIALS FOR PHOTOVOLTAIC APPLICATION

Abstract

Abstract of first chapter Here, in this chapter we will discuss two cutting-edge areas- firstly, the chapter details the strategies to overcome the room temperature 3MC→S0 non-emissive deactivation pathway in Ruthenium(II) polyterpyridyl complexes via 3MLCT -3MC energy-gap engineering. Also, give the brief account of interfacial charge transfer studies of Ruthenium(II) polypyridyl dyes anchored on the semiconducting TiO2 nanoparticle. Secondly, the chapter details another class of very promising photovoltaic material organicinorganic hybrid halide perovskite ABX3 having extraordinary optoelectronic properties and also explained the influence of different components of hybrid organic-inorganic perovskite on electronic structure. In addition, the chapter account the varies components of perovskite solar cell, its charge transport mechanism, versatility in fabrication technique employed for perovskite fabrication and the factor affecting the stability of perovskite solar cell. Finally, the chapter ended with discussing some current state of the art to overcome Pb associated toxicity by substituting with other non-toxic metal cation. Abstract of second chapter We unveiled the chemical synthesis and characterisation of Mg/Pb binary metal mixed halide perovskite with chemical formula CH3NH3PbxMg1-xI3-yCly using MgCl2 as a compositional gradient with nominal value of x from 0.1 to 0.9. The FESEM images of compositions correspond to higher proportion of Cl􀀃 (or MgCl2) demonstrate improved particle (or grain) size of ~8-12 μm. There is a close resemblance of stoichiometric ratio of Pb:Mg obtained from EDX analysis with that of incorporated stoichiometric ratio of Pb:Mg. Further, Mg and Cl incorporation is strengthened by the characteristic spectral peak for core level electron of Mg(2p) and Cl(2p) other than Pb(4f) and I(3d) in the survey spectrum of composition x = 0.5. These binary Mg/Pb Perovskite with bandgap in the range 1.57 eV -1.59 eV can behave as comparatively less toxic potential candidate for the single junction module as well as for the top cell in tandem architecture. The IR studies at room temperature shows an observable shift in peak position on comparing two extreme compositions i.e, x = 0.1 and x = 1.0. It is noticeable that both standard CH3NH3PbI3 and binary Mg/Pb Perovskite with nominal value x = 0.7 have comparable thermal stability, however, the composition x = 0.1 have lower thermal stability than x = 0.7. Abstract of third chapter We report synthetic steps towards a lead free manganese based perovskite MAPbxMn1– xI1+2xCl2–2x (nominal x = 0.1–1.0), photovoltaic material via a solid state reaction. Further application as an excellent panchromatic light harvester involves the fabrication of a cell with an inverted planar architecture at low processing temperature. This novel perovskite material offers an outstanding Voc of 1.19 V with ff of 87.9%. Abstract of fourth chapter In this chapter, we have synthesized a heteroleptic complex [(L2)Ru(L1)Ru(L4)]4PF6 and [(L3)Ru(L1)Ru(L4)]4PF6 named complex 1 and 2 based on D-P-A architecture [where 2-(4-(4- (2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(pyridin-2-yl)pyridin-2-yl) pyridine (L1), 4'-([1,1'- biphenyl]-4-yl)-2,2':6',2″-terpyridine (L2), 4'-(3,4-dimethoxy phenyl)-2,2':6',2″-terpyridine (L3), 4'-(4-Carboxyphenyl)-2,2'6':2''-terpyridine (L4), Photosensitizer (P), Acceptor (A), and Donor (D)]. Both complexes posses prolonged excited state, efficient for interfacial electron injection and low electron-hole recombination (LUMO→HOMO of complex). The complex 1 and 2 shows the average excited state lifetimes (τ􀀆􀀇􀀈) of 25 ns and 12.67 ns respectively compared to 0.25 ns for [Ru(tpy)2]2PF6. The τ􀀆􀀇􀀈 of such an order is sufficient for performing the interfacial electron transfer into the conduction band (CB) of TiO2 and redox chemistry of excited state. The electrochemical studies exhibit the oxidation potential (􀀋􀀌􀀍) of 1.23 V and 1.27 V vs SCE with respective excited-state redox potentials (􀀋􀀌􀀍∗ = -0.95 V and -0.85 V and 􀀋􀀏􀀐􀀑∗ = 1.0 V and 0.88 V for complex 1 and 2 respectively) show evidence for stronger reductant and oxidant behaviour in 3MLCT excited state. Subsequent interfacial study with TiO2 nanocrystal is in good agreement with photo induced electron injection from LUMO of complex to CB of TiO2 as LUMO level (-3.22 eV and 3.16 eV for complex 1 and 2 respectively) of complexes is above the CB of TiO2 (-3.65 eV). The experimental photophysical results of both the complexes are well supported by time dependent density functional theory (TDDFT). Abstract of fifth chapter The necessity of improving the excited state properties for efficient interfacial charge transfer to semiconducting TiO2 material drag us to design complex 2 [(L2)Ru(L4)]2PF6 and complex 3 [(L2)Ru(L1)Ru(L3)]4PF6 based on D-P-A functionalisation [where 2-(4-(4-(2,6-di(pyridin-2- yl)pyridin-4-yl) phenyl)-6-(pyridin-2-yl)pyridin-2-yl) pyridine (L1) 4'-(4 N,N-dimethyl phenyl)-2,2':6'2''-terpyridine (L2), 4'-(4-Carboxyphenyl)-2,2':6'2''-terpyridine (L3), 4'-(3,4- dicarboxyphenyl)-2,2':6'2''-terpyridine (L4)]. The study involves a comparative account of complexes 1-3, where our previously reported complex 1 [(L2)Ru(L3)]2PF6 considered as a basic motif. We have improved the excited state properties of complex 1 by substituting with an additional electron withdrawing group and extended π-conjugation in mononuclear complex 2 and binuclear complex 3 respectively. Compared to the average excited state lifetime (τ􀀆􀀇􀀈) of 5.54 ns for complex 1, both complexes 2 and 3 show improved excited state lifetime of 10.24 ns and 22.06 ns respectively. Further, the anchoring of complexes on high bandgap TiO2 semiconductor surface to perform interfacial charge transfer has been shown via Time- Correlated Single Photon Counting (TCSPC) studies which displayed quenched decay pattern and decreased excited state lifetime in the complex 2/3-TiO2 system compared to their bare complex solution. Therefore, the obtained average excited state lifetime in complex 2 and 3 is sufficient to perform interfacial charge transfer study in the semiconducting TiO2 nanoparticle. This interfacial charge transfer further strengthened by the femtosecond transient absorption studies in colloidal TiO2 nanoparticle which give the evidence for the presence cation radical and injected conduction band electron (e-CB) through their characteristic absorption. The Cyclic voltammetry experiment exhibit first reduction potential (􀀋􀀏􀀐􀀑 􀀒) for complex 2 and 3 at -1.22 V and -1.17 V respectively compared to -1.27 V for complex 1. The higher value of 􀀋􀀏􀀐􀀑 􀀒 in complex 2 and 3 indicate the higher electron affinity of the ligand for reduction due to an additional electron withdrawing group and extended π- conjugation respectively. Also, the TDDFT studies using B3LYP exchange correlations functional and LANL2DZ basis set along with solvation effect in Gaussian 09 programs give an overview of transitions involved in 1MLCT and are in support with the experimental UV-Vis absorption spectra of complexes. The trend of LUMO energy level for complexes obtained from electrochemical studies is also in agreement with the LUMO energy level distribution obtained from DFT analysis.