A number of recent studies have shown that the non-radiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor:acceptor blends to determine the effect that energetic offset has on both radiative and non-radiative recombination of the charge transfer (CT) state. We find that for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the non-radiative voltage loss to values as low as 0.23V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low non-radiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low non-radiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states, and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low non-radiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

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High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerene Cso in the temperature range 5 K to 320 K. Solid C , adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the hightemperature face-centred-cubic phase (T > 260 K), the Cs0 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single (111) direction.In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists. 1. Introduction. c 6 0 buckminsterfullerene [l-31 is the most stable member of the whole family of closed carbon cage molecules-the fullerenes [4,5]. Its extraction and purification from arc-processed carbon [2,31 have not only enabled the original structural proposal [ll to be confirmed [2,3,61 but have also led to numerous experiments that are revealing many novel physical and chemical properties [7]. At room temperature, crystalline c& adopts a face-centred cubic crystal structure in which each of the c 6 0 molecules is orientationally disordered [8,9]. The structure may be regarded as cubic-closed-packed in which rotations and orientations of individual c 6 0 molecules are uncorrelated with their neighbours [9]. 13C NMR [lo-121 and quasi-elastic neutron scattering [13] measurements confirm this rapid isotropic reorientation at room temperature that results in time-averaging of the truncated icosahedron to spherical symmetry. Perhaps surprisingly in view of the almost spherical symmetry, this orientational disorder does not persist to low temperatures. Differential scanning calorimetry (DSC) [14] and X-ray diffraction [9] measurements established the existence of a first-order phase transition near 250 K. More recent work [9,15,16] has confirmed an ordered simple cubic crystal structure for Cw at low temperatures. The reason for the orientational order has been discussed in terms of van der Waals bonding and electrostatic repulsion that results in the facing of the

We prepared the novel fullerene derivative (a-bis-PCBM) by separating it from the as-produced bis-phenyl-C 61 -butyric acid methyl (bis-[60] PCBM) ester isomer mixture using preparative peak-recycling high performance liquid chromatography (HPLC). We employed the compound as a templating agent for the solution processing of metal halide perovskite films by the antisolvent method. Perovskite solar cells (PSCs) containing a-bis-PCBM perovskite achieve better stability, efficiency, and reproducibility compared with those employing traditional PCBM. The a-bis-PCBM can fill the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of 2 slow electron extraction. In addition, it can also resist the ingression of moisture, protect the interfaces from chemical erosion, and passivate the voids or pinholes generated in the holetransporting layer. As a result, we obtain an outstanding power conversion efficiency (PCE) of 20.8 % compared with 19.9 % by PCBM, accompanied by excellent stability under heat and simulated sunlight,. The PCE of unsealed devcies dropped by less than 10% in ambient air (40% RH) after 44 days at 65 ℃ and by 4% after 600 h under continuous full sun illumination and maximum power point tracking respectively.Hybrid organic-inorganic lead halide perovskite solar cells (PSCs) have emerged as a promising candidate for the next generation photovoltaic technology due to their low manufacturing cost and high performance. 1−6 Through judicious manipulation of perovskite morphology and improvement of interfacial properties, 2−6 PSCs have reached a certified power conversion efficiency (PCE) up to 22.1%. 7 Generally, the PSCs with the best performance employ a sandwich configuration, composed of a layer of TiO 2 electron selective contact, which is infiltrated by the intrinsic perovskite light harvester, followed by a layer of hole transport material (HTM) as p-type contact and a metal back contact. 8 Despite of these stunning advances, several challenges still remain before PSCs become a competitive commercial technology, one crucial issue being the device stability. 9−11 Uncontrolled film morphology associated with poor crystallity of the perovskites results in low efficiency and poor reproducibility of the device performance. 12 Previous studies have indicated that the degradation of PSCs is primarily governed by the ingress of atmospheric oxygen and water vapor into the film upon exposure to air, which in turn causes undesired reactions with the active materials. 13,14 Various methods have been tried to modify the morphology of perovskite films aiming to improve the stability, for example, poly(methyl methacrylate) (PMMA) was used as a template to control nucleation and crystal growth, resulting in considerable increase in both the device efficiency and stability when kept under 3 dry condition in the dark. 15 Other studies use additives like 1-methyl-3-(1H,1H,2H,2H-nonafluorohexyl)-imidazolium iodide, 16 or phenyl-C 61 -b...

The as-produced isomer mixture of the organic photovoltaic device acceptor material bis-[60]PCBM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry. A total of 18 isomers were purified from the mixture to a standard exceeding 99.5% with respect to other isomers. The HOMOs, LUMOs, and HOMO-LUMO gaps of the purified isomers vary from -5.673 to -5.444 eV, -3.901 to -3.729 eV, and 1.664 to 1.883 eV, respectively. We also find a correlation between HPLC retention time and the relative positions of the addends; in that generally the closer the addends are to each other the longer the retention time of the isomer, and vice versa.