Optical and Electronic Contributions in Double‐Heterojunction Organic Thin‐Film Solar Cells

Abstract

evaporation was approximately 12 cm 25 cm. The defect layer was created by the designed placement of a mask on the film during evaporation.The As 2 Se 3 coated PES film was then rolled onto the PES core and the resulting preform,~2 cm in diameter, was covered with Teflon tape and consolidated at 260 C (± 5 C) under vacuum for 20 min. The pressure was decreased quickly below 5 torr within the first 5 min. The preform was then quenched in a ±20 C freezer for about 2 min to prevent crystallization and annealed initially at 150 C and then cooled slowly for~2 h. The inner glass tube was then dissolved with HF by immersing the entire preform. After complete etching, the preform was rinsed with distilled water and isopropanol and baked under vacuum at 150 C for 5 h.Fiber Drawing: Our draw tower is composed of a three-zone vertical tube furnace (Thermcraft), a motorized capstan (Heathway) to control the speed of the drawing, and laser diameter monitors (Beta LaserMike) to monitor the outer diameter of the drawn fiber. The temperature of the upper, central, and lower zones of the oven were typically around 240 C, 295 C, and 150 C, respectively, for speeds ranging between~1±10 m s ±1 . SEM: SEM was performed on small sections of fibers embedded in epoxy resin, cut by ultramicrotome (Ultracut E, Reichert-Jung) with a diamond knife to obtain a smooth surface, and gold coated using a (Denton Vacuum) sputtercoater (~10 nm thick gold coating). We used a JEOL 6320 FEGSEM with backscattered detector at 10 keV to obtain a good contrast between the PES and As 2 Se 3 layers.Ellipsometry Measurements: The refractive indices (n,k) of As 2 Se 3 and PES were measured with a broadband spectroscopic ellipsometer (Sopra GES-5) utilizing three different detectors: photomultiplier from 250 nm to 880 nm, InGaAs from 880 nm to 2 lm, and MCT-A from 2 lm to 20 lm. Corrections were used to account for the nonlinearities and polarization dependence of the detectors. The samples consisted of a film deposited on silicon substrate. As 2 Se 3 films were deposited by thermal evaporation (similarly to the fiber fabrication process) and annealed for 1 h at 180 C, whereas PES films were spin coated (Laurell Tech. Corp. WS-400 A-6 NPP/LITE) at 1500 rpm (rpm = revolutions per minute) from a polymer solution obtained by dissolving a 25 lm PES film (Goodfellow) in N,N-Dimethylformamide. For each sample, the ellipsometric parameters tanW and cosD parameters were fitted to extract the thickness of the deposited film and to calculate n and k versus wavelength using the Levenberg±Marquardt regression method with a Cauchy-like dispersion relation and additional Lorentzian absorption peaks for PES.Fourier-Transform Infrared Spectrometer (FTIR) Measurements: Reflectivity measurements were performed using a infrared microscope (Nicolet SpectraTech NicPlan) and a FTIR (Magna 860). All backgrounds were taken using a flat gold mirror. Reflectivity spectra were normalized by a factor of~1.1 to correct assumed differences between the fiber and flat background mirror geome...

Solar cells utilizing small molecular weight organic semiconductors

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