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• Optical properties of monoclinic SbI3 are investigated using DFT calculations and experiment. • The calculated absorption spectra of six (hkl) planes show highly anisotropic nature. • 2.66 eV or 2.7 eV are possible indirect band gaps from band structure compared to measured 2.8 eV. • One exciton appears at 3.56 eV for (hkl) oriented film and two excitons at 2.95 eV and 3.30 eV for (1 3 0) • The experimental and theoretical optical properties match reasonably well. Structurally characterized antimony triiodide films grown by vacuum thermal evaporation exhibit anisotropic exciton formation at room temperature for different (hkl) oriented growth. Band structure of monoclinic SbI 3 was calculated using Density functional Theory with appropriate functional along with the absorption spectra of different (hkl) planes for comparing with the experimental results. The indirect band gap of 2.8 eV determined by measured absorption spectra matches closely with the calculated 2.66 eV. All (hkl) oriented films show one exciton at 3.56 eV which can be interpreted as direct type along D symmetry from the band structure. Similarly, the two excitons at 2.95 eV and 3.30 eV appearing for (1 3 0) oriented films can also be direct either at D or B symmetry. All three excitonic transitions are found to be from Iodine p states to Antimony p states. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fluorescence Resonance Energy Transfer from P3HT to CuInSe 2. • Analytical study of the absorption and emission spectra of P3HT:CuInSe 2 composites. • Förster distance of critical energy transfer is estimated as 3.61 nm. • The average distance between donor–acceptor is found to be 4.71 nm. • Analysis of resonance charge transfer by estimation of HOMO-LUMO energy of donor–acceptor. This letter reports the paramount fluorescence resonance energy transfer mechanism for photo induced charge transfer from P3HT to solvothermally derived CuInSe 2. The HOMO (-4.85 eV) and LUMO (-3.38 eV) energy states of CuInSe 2 (electrical conductivity = 1.1x 10-7 Scm−1) are determined from cyclic voltammetry and optical study. This HOMO-LUMO position agrees to select P3HT polymer as possible donor of excitons. Steady-state luminescence study of composite (P3HT:CuInSe 2) demonstrates possibility of successful charge transfer. Stern–Volmer analysis of absorption and emission spectroscopy ensures static energy quenching phenomena. The Förster distance (R 0) of critical energy transfer is estimated as 3.61 nm. The average distance between donor–acceptor (r avg = 4.71 nm) is<8 nm and within the range 0.5R 0 < r < 1.5R 0 (1.81 nm < r < 5.42 nm), which ensures energy transfer from P3HT to CuInSe 2. [ABSTRACT FROM AUTHOR]