The Effect of Solvent Additive on the Charge Generation and Photovoltaic Performance of a Solution-Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell

The photovoltaic performance and charge generation dynamics in thin film bulk heterojunction organic photovoltaic (BHJ OPV) devices comprising the small molecule donor 7,7′-(4,4-bis­(2-ethylhexyl)-4H-silolo­[3,2-b:4,5-b′]­dithiophene-2,6-diyl)­bis­(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)­benzo­[c]­[1,2,5]­thiadiazole) (p-DTS­(FBTTh₂)₂) and a perylene diimide (PDI) electron acceptor are investigated with and without the processing additive 1,8-diiodooctane (DIO). UV–vis absorption spectroscopy indicates that the use of DIO during processing increases the structural order of both p-DTS­(FBTTh₂)₂ and PDI compared to films cast from chlorobenzene alone. Excitation intensity dependent broadband vis–NIR transient absorption pump–probe experiments over a dynamic range from 100 fs to 100 μs reveal that, in blends processed without DIO, essentially none of the interfacial charge transfer states generated after exciton dissociation at the donor–acceptor interface split into spatially separated charge carriers. In contrast, in blends processed with 0.4 vol% DIO, geminate recombination is significantly reduced, and spatially separated charge carriers are generated. It appears that the drastic increase in the power conversion efficiency in p-DTS­(FBTTh₂)₂:PDI BHJ OPV devices upon the use of DIO, from 0.13% to 3.1%, is a consequence of the increased solid state order of both p-DTS­(FBTTh₂)₂ and PDI, which leads to a significant improvement of the exciton dissociation efficiency and makes this system among the most efficient non-fullerene BHJ organic solar cells to date

Evolution of Iodoplumbate Complexes in Methylammonium Lead Iodide Perovskite Precursor Solutions

Here we investigate the local structure present in single-step precursor solutions of methylammonium lead iodide (MAPbI₃) perovskite as a function of organic and inorganic precursor ratio, as well as with hydriodic acid (HI), using X-ray absorption spectroscopy. An excess of organic precursor as well as the use of HI as a processing additive has been shown to lead to the formation of smooth, continuous, pinhole free MAPbI₃ films, whereas films produced from precursor solutions containing molar equivalents of methylammonium iodide (MAI) and PbI₂ lead to the formation of a discontinuous, needlelike morphology. We now show that as the amount of excess MAI in the precursor solution is increased, the iodide coordination of iodoplumbate complexes present in solution increases. The use of HI results in a similar increase in iodide coordination. We therefore offer insight into how solution chemistry can be used to control MAPbI₃ thin film morphology by revealing a strong correlation between the lead coordination chemistry in precursor solutions and the surface coverage and morphology of the resulting MAPbI₃ film