ABSTRACT
Microdisks are excellent whispering-gallery mode (WGM) optical resonators, but their emissions are invariably in-plane isotropic due to their circularities and thus difficult to be extracted efficiently. In this work, a waveguide with a width of 0.16 µm directly coupled to a microdisk with a diameter of 10 µm is fabricated on a 0.77 µm thick GaN thin film containing InGaN/GaN multi-quantum wells. This eliminates the need for precision patterning required by evanescent coupling schemes in which coupling gaps of the order of tens of nanometers must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of WGMs to the waveguide from the microdisk is successfully demonstrated.
ABSTRACT
The effect of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) properties on the optical properties, charge transport and photovoltaic performance of PTB7:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films is investigated. We found that the variations in the molecular weight (Mw) and polydispersity index (PDI) of PTB7 mainly affect the phase separation and charge transport (hole mobility) in the blend films. The optical properties are also affected, but the increase in the extinction coefficient does not necessarily imply increased power conversion efficiency. The obtained power conversion efficiency for optimized thickness varied from 4.8% to 7.8% depending on the properties of PTB7. The wide range of obtained power conversion efficiencies illustrates the importance of optimizing the Mw and PDI to optimize bulk heterojunction morphology and achieve high performance solar cells.
ABSTRACT
Annealing is a common method to improve the efficiency of polymer photovoltaic cells. Annealing changes the microphase separation in a polymer blend film and typically also results in a change in its optical properties. We investigated the optical properties of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) before and after thermal annealing using spectroscopic ellipsometry and transmission measurements, with simultaneous fitting of samples with different thicknesses to ensure reliability of extracted index of refraction values. We found that, after annealing, it is necessary to consider an anisotropic model to describe the properties of P3HT:PCBM blend films, which reflects the increased order of P3HT chains as a result of annealing. Different fitting models (simple anisotropic layer, graded isotropic, graded anisotropic model, generalized oscillator, and oscillator model including Huang-Rhys vibronic envelope) have been compared and discussed. The effect of the number of samples used for fitting and surface roughness corrections is also discussed.
