Jianyuan Sun

Morphological Evolution and Its Impacts on Performance of Polymer Solar Cells

In this paper, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b] dithiophene-alt-5,6-bis(dodecyloxy)-4,7bis(thiophen-2-yl)-benzo[c] [1,2,5]-thiadiazole} (PBDT-ABT-1) blend films was investigated. The morphology of blend films with different Phenyl C-60-butyric acid methyl ester (PCBM) mixing ratios and solvent additives was studied using atomic force microscopy (AFM) and energy-filtered transmission electron microscopy (EFTEM). AFM and EFTEM images showed difference in the intermixing of polymer with fullerene between 1:1, 1:2, and 1:3 weight ratios. Polymer/PCBM intermixed domain size increases with higher PCBM weight ratios. X-ray diffraction measurements on the pristine polymer and blend films cast without additives did not show any peaks, suggesting an amorphous nature of PBDT-ABT-1. EFTEM images from the donor/acceptor composite showed intermixed polymer-PCBM domains separated by the polymer boundary. Furthermore, EFTEM images for di-iodooctane (DIO) additive cast film revealed purer polymer domain. Photo-charge extraction by linearly increasing voltage measurement exhibited that charge extraction is highest in the nanomorphology sample with a weight ratio of 1:2, corresponding to the lowest bimolecular recombination and the highest charge carrier mobility.

Ring-protected small molecules for organic photovoltaics

Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state. However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid Haggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail selfassemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a property desirable for photovoltaics and some other optoelectronic applications.

An oligothiophene chromophore with a macrocyclic side chain: synthesis, morphology, charge transport, and photovoltaic performance

An oligothiophene chromophore RingBDT(T3A)2 has been synthesized, where BDT is benzo[1,2-b:4,5-b′]dithiophene, Ring is a 1,12-dodecylenedioxy cyclic side chain on the benzene of BDT, T3 is 2,2′:5′,2′′-terthiophene, and A is an electron acceptor. In single crystals, the immediate precursor of RingBDT(T3A)2 formed π-dimers and the ring prevented further π-stacking of the dimers. A differential scanning calorimetry study showed that BDT(T3A)2, the ringless analog with two 2-ethylhexyloxy side chains on BDT, crystallized quickly from its melt upon cooling, while crystallization of RingBDT(T3A)2 melt upon cooling was slow and incomplete. Interestingly, RingBDT(T3A)2 solid crystallized fast at ∼110 °C upon heating, but its thin films (200 nm) remained amorphous after annealing at 80 °C. Despite the amorphous nature, the hole mobility of RingBDT(T3A)2 films (1.52 × 10−3 cm2 V−1 s−1) was 144% higher than that of the highly crystalline BDT(T3A)2 films (200–80 nm). Solar cells were fabricated from blends of the chromophores and phenyl-C61-butyric acid methyl ester (PC60BM). Thermal annealing at 100 °C for 10 minutes enhanced chromophore π–π interaction, and improved device fill factor and efficiency for the RingBDT(T3A)2 blend solar cells, while retaining the amorphous nature of blend. In stark contrast, thermal annealing under the same conditions caused the efficiency of BDT(T3A)2 cell efficiency to drop by 82%. This study demonstrates the effectiveness of using a macrocyclic side chain as a strategy for developing amorphous molecular semiconducting materials with improved mobility and morphological stability.

Synthesis and characterization of ring-protected electro-optic chromophores

Antiparallel interaction among dipolar chromophores is the dominant force in the solid state of conventional EO chromophores (long and flat). This interaction is responsible for the formation of acentric aggregates and prevents electro-optic coefficient from scaling with chromophore concentration. Antiparallel interaction can be selectively attenuated by attaching bulky groups to the middle part of chromophore. However, it is synthetically challenging to provide sufficient steric protection without causing severe reduction of chromophore concentration. In this paper, we will present the synthesis and characterization of atom-economic ring protection of chromophores against H-aggregation and show the effect of ring protection on optical properties of the chromophores in solution and film.