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Fundamentally, organic solar cells (OSCs) with a bulk-heterojunction active layer are made of at least two electronically dissimilar molecules, in which photoabsorption in one (donor) generates Frenkel excitons. The formation of free charge carriers emerge after exciton dissociation at the donor:acceptor interface. In the past decade, most of the progress in enhanced device performance has been steered by the rapid development of novel donor and acceptor materials and on device engineering. Among these donor materials, regioregular poly(3-hexylthiophene) (P3HT) produced better performance despite the mismatch of its absorption coefficient with the solar emission spectrum. Comparatively the donor PBDB-T exhibits an outstanding absorption coefficient with a deeper-lying highest occupied molecular orbital (HOMO) level. Previously most of the efficient acceptors were based on fullerene molecules characterized by limited photoabsorption and stability. In contrast, the recently developed non-fullerene OSCs have a tunable absorption spectrum and exhibit improved stability. In this work, we explore the fundamental sources of the differences in the device performance for different blend compositions made of fullerene derivative (PC71BM) and non-fullerene (ITIC-Th) when paired with the polymer donors P3HT and PBDB-T. The characteristic changes of the optical properties of these blends and their roles in device performance are also investigated. We also studied charge generation where PBDB-T:PC71BM showed the highest maximum exciton generation rate (Gmax) of 3.22 × 1028 s–1 while P3HT: ITIC-Th gave the lowest (0.96 × 1028 s–1). Also noted, PC71BM based counterparts gave better charge transfer capabilities as seen from the lower PL quenching and higher charge carrier dissociation plus collection probability P(E,T) derived from a plot of Jph/Jsat ratio under short-circuit conditions against the effective voltages.

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Surface Brillouin scattering has been used to study the variation of the velocities of the Rayleigh surface acoustic wave (RSAW) and the high frequency pseudo-SAW (HFPSAW) with azimuthal angle on the (110) surface of iron pyrite (FeS 2 ) over the temperature range 293 to 573 K. The elastic constants of this cubic compound, namely C 11 , C 12 and C 44 were extracted by the simultaneous fitting of the velocity data to the results of calculations using surface elastodynamic Green's functions and decrease gradually with increasing temperature. An interatomic potential model has been developed for iron pyrite and has been used to calculate the values of the elastic constants over the same range of temperature as the experimental results. There is good agreement between experimental and the computational results. 1 Introduction Pyrite (FeS 2 ) is the most common metal sulphide mineral and is a semiconductor with cubic structure with T h 6 (Pa-3) symmetry being based on NaCl in which the anions are replaced by S 2 dimers oriented along the four {111} cube directions. The electronic structure of pyrite and the nature and strength of the interatomic forces remain subjects of considerable interest [1,2]. Controversy arose from an early study using static bending and twisting experiments of oriented plates which reported a negative value of C 12 , and hence a negative Poisson's ratio [3]. However, more recent ultrasonics studies yielded a positive value of C 12 [1]. The atomistic simulation of metal sulphides has been limited owing to the complex nature of the bonding [4] and also from a lack of experimental data for fitting interatomic potentials. Recently derived interatomic potentials have, however, been successful in describing the structure, elastic properties and the pressure variation of the elastic constants [5]. The need to further explore the agreement between experiment and simulations has stimulated the present work on the experimental measurement of the temperature dependence of the elastic constants of pyrite. The method of surface Brillouin scattering (SBS) has been used for this purpose following its successful application to other opaque and near opaque materials at high temperatures [6][7][8].In SBS, two light scattering processes resulting from thermally-induced excitations need to be considered [8][9][10]. Surface ripple scattering is dominant for near-opaque materials and results from the dy-