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In this work, the photostability of certain organic photovoltaic (OPV) active layers was demonstrated to improve by as much as a factor of five under white light illumination in air with the use of 1,7-bis-trifluoromethylfullerene (C60(CF3)2) as the acceptor in place of PC60BM. However, the results were highly dependent on the structure and functionality within the donor material. Twelve combinations of active layer blends were studied, comprised of six different high-performance donor polymers (two fluorinated and four non-fluorinated donors) and two fullerene acceptors (PC60BM and C60(CF3)2). The relative rates of irreversible photobleaching of the active layer blends were found to correlate well with the electron affinity of the fullerene when the polymer and fullerene were well blended, but a full rationalization of the photobleaching data requires consideration of both the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene components in the blend. Miscibility of those components was probed using a combination of time-resolved photoluminescence (TRPL) measurements and scanning tunneling microscopy (STM) imaging. The presence of fluorinated aromatic units in the donor materials tend to promote more intimate mixing with C60(CF3)2 as compared to PC60BM. The full results of these photobleaching studies and measurements of donor–acceptor miscibility, considered alongside additional photoconductance measurements and preliminary device work, provide new molecular optimization insights for improving the long-term stability of OPV active layers.

Wide bandgap perovskites (>1.7 eV) find applications in multi-junction solar cells and semi-transparent absorbers for building integrated photo-voltaics. Here we present the improvement in morphology and power conversion efficiency (PCE) of two mixed halide perovskite compositions: MAPbIBr 2 and MAPbBr 3 by methyl amine (MA) vapor treatment. MA treatment improves the morphology and surface coverage leading to increase in open-circuit voltage (V oc ) and short-circuit current (J sc ). Further two polymers are used as hole-transporting materials which have better HOMO matching with perovskite than Spiro-OMeTAD and results in V oc of 1.12 V and 1.17 V for MAPbIBr 2 and MAPbBr 3 respectively compared to 0.95 V and 1.10 V of the corresponding devices with Spiro-OMeTAD as HTL.

Two organic photovoltaic (OPV) donor materials (one polymer and one small molecule) are synthesized from the same constituent building blocks, namely thiophene units, cyclopentathiophene dione (CTD), and cyclopentadithiophene (CPDT). Photobleaching dynamics of these donor materials are then studied under white light illumination in air with blends of PC70BM and the bis-trifluoromethylfullerene 1,7-C60(CF3)2. For both the polymer and small molecule blends, C60(CF3)2 stabilizes the initial rate of photobleaching by a factor of 15 relative to PC70BM. However, once the small molecule:C60(CF3)2 blend bleaches to ∼80% of its initial optical density, the rate of photobleaching dramatically accelerates, which is not observed in the analogous polymer blend. We probe that phenomenon using time-resolved photoluminescence (TRPL) to measure PL quenching efficiencies at defined intervals during the photobleaching experiments. The data indicates the small molecule donor and C60(CF3)2 acceptor significantly de-mix with time, after which the blend begins to bleach at approximately the same rate as the neat donor sample. The work suggests that perfluoroalkylfullerenes have great potential to stabilize certain OPV active layers toward photodegradation, provided their morphology is stable.

A series of low band gap, planar conjugated polymers, P1 (PFDTBT), P2 (PFDTDFBT) and P3 (PFDTTBT), based on fluorene and benzothiadiazole, was synthesized. The effect of fluorine substitution and fused aromatic spacers on the optoelectronic and photovoltaic performance was studied. The polymer, derived from dithienylated benzothiodiazole and fluorene, P1, exhibited a highest occupied molecular orbital (HOMO) energy level at −5.48 eV. Density functional theory (DFT) studies as well as experimental measurements suggested that upon substitution of the acceptor with fluorine, both the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the resulting polymer, P2, were lowered, leading to a higher open circuit voltage and short circuit current with an overall improvement of more than 110% for the photovoltaic devices. Moreover, a decrease in the torsion angle between the units was also observed for the fluorinated polymer P2 due to the enhanced electrostatic interaction between the fluorine substituents and sulfur atoms, leading to a high hole mobility. The use of a fused π-bridge in polymer P3 for the enhancement of the planarity as compared to the P1 backbone was also studied. This enhanced planarity led to the highest observed mobility among the reported three polymers as well as to an improvement in the device efficiency by more than 40% for P3.

A-D-A architectured quinoxaline and bezodithiophene based small molecule exhibiting low band gap was designed and synthesized for bulk heterojunction solar cell application. Two dimension conjugation to the molecule was imparted by using alkylated aromatic units. This effective extension of conjugation broadens the absorption spectra. Optical and electrochemical properties suggest that the synthesized molecule has a low band-gap and well matching frontier molecular orbital energy levels with that of PCBM. A photovoltaic device using this A-D-A molecule as the active layer was fabricated to evaluate the optoelectronics properties.

The photobleaching dynamics of a series of three organic photovoltaic (OPV) donor polymer blends with five different fullerenes are presented. The fullerenes studied include PC60BM and four perfluoroalkylfullerenes with relatively large electron affinities, namely C-60(CF3)(2), C-60(i-C3F7)(2), C-60(CF3)4(,) and C-60(CF3)(8). The donor polymers were all based on cyclopentadithiophene (CPDT) and thienopyrrolodione (TPD), but the TPD side chains were designed to include alkyl, partially fluorinated alkyl, and fluorinated phenyl groups to improve miscibility of the active layer components. Exciton harvesting was probed with photoluminescence quenching measurements. Accelerated photodegradation studies of polymer:fullerene blends were then carried out under white light illumination at similar to 1.2 suns in air. A strong correlation was observed between the polymer donor photobleaching rate and the electron affinity of the fullerene. The most dramatic effect was observed for a blend of C-60(CF3)(8) with the donor containing fluorinated phenyl groups: the blend required 150 times the dosing of photons to bleach to 80% of its initial optical density than an analagous blend of PC60BM and non-fluorinated donor polymer. These results ultimately suggest that appropriate fluorination strategies applied to both the donor and acceptor can be a viable route toward a new paradigm of intrinsically photo- and phase-stable OPV active layers.

The open-circuit voltage (VOC) in methylammonium lead iodide (CH 3 NH 3 PbI 3 ) perovskite devices should depend on the band-alignment at the hole-transport-layer (HTL)/perovskite interface. However, in practical devices this is not observed. Irrespective of HTL, typical perovskite device show Voc of 1.05-1.07 V. Here we report two polymers that combined with spiro-OMeTAD, shows VOC that is 60 mV higher than comparable spiro-only devices. The highest occupied molecular orbital (HOMO) of these polymers is -5.6 eV, almost perfectly matched with the valence band edge of perovskite, leading to more efficient hole-extraction. The lowest unoccupied molecular orbital (LUMO) of the polymers is low, -3.75-3.9 eV, which is not ideal for blocking electron recombination. However, this negative can be overcome by using a bilayer HTL, with both polymer and spiro-OMeTAD. Perovskite solar cells with polymer/spiro bilayer HTL show a power conversion efficiency (PCE) of 13.91% and VOC of 1.09 V. In addition, the polymer HTL is hydrophobic, which leads to more stable solar cells whose PCE drops from 13.9 % to 10.7% after 240 hours of exposure in ambient.