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intense panchromatic absorption [4] and high charge carrier mobilities. [5] In addition, their band gap can be easily tuned by simple variation of their ionic composition. [6] For example, a mixed-ion perovskite obtained as a mixture of formamidinium (FA) lead iodide and methy lammonium (MA) lead bromide, [FAPbI 3 ] 0.85 [MAPbBr 3 ] 0.15 , delivered the best to date energy conversion efficiencies of over 21% and revealed promising stability and reproducibility. [7-9] All the above mentioned, together with the ambipolar charge transport characteristics of perovskites, [10] are exceptional properties behind the high efficiency reported in literature. However, highly efficient PSCs are mostly achieved when sandwiching the perovskite film between an electron-transport layer (ETM, n-type) and a hole-transport layer (HTM, p-type), to enhance the selective blocking and extraction of charges into the external circuit. In this connection, using mesoporous or planar metal oxide ETM, such as TiO 2 , delivered the best results. [11] In terms of HTM, small molecules and polymers have shown impressive performances, being so far 2,2′,7,7′-tetrakis(N,N-dip -methoxyphenylamin e)-9,9′-spirobifluorene (Spiro-OMeTAD) [12] and poly(triaryl-amine) (PTAA) [13]

Black dye (BD), isomer 1 ([Ru(II)(H3-tctpy)(NCS)3](-1), where H3-tctpy = 4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) is known to be an excellent sensitizer for dye-sensitized solar cells and exhibits a very good near-IR photo response, compared to other ruthenium dyes. Because isothiocyanate is a linear ambidentate ligand, BD has three other linkage isomers, [Ru(H3-tctpy)(NCS)2(SCN)](-1), isomer 2 and 2', and [Ru(H3-tctpy))(SCN)3](-1), isomer 3. In this study, we have calculated the geometry of BD and its isomers by DFT. Further, we have analyzed the bonding in these isomers using NBO methods. TDDFT calculations combined with scalar relativistic zero-order regular approximations (SR-ZORA) have been carried out to simulate the absorption spectra. Calculations have been performed for the isomers both in vacuo and in solvent (ethanol). The inclusion of the solvent is found to be important to obtain spectra in good agreement with the experiment. The first absorption bands are dominated by the metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT).

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