Fabrication and testing of organic solar cells

Inverted organic solar cells were fabricated on prepatterned ITO-coated glass. The ITO-coated glass substrates were cleaned in detergent (sodium dodecyl sulfate), successively ultrasonicated in deionized water, acetone, and isopropyl alcohol, and exposed to oxygen plasma for 3 min. PTB7:PC₇₁BM blend solutions were prepared by dissolving the components in a ratio of 1:1.5 (by weight), with a total concentration of 25 mg/mL in chlorobenzene, with 3 vol% DIO. In the case of the PTB7-Th:PC₇₁BM blend, the donor:acceptor components were weighed in a ratio of 1:1.5, with a total concentration of 25 mg/mL in ortho-dichlorobenzene solvent. For the PTB7-Th:EH-IDTBR blend system, the active layer solution was prepared by blending the donor-acceptor components in a 1:1 weight ratio, with a total concentration of 20 mg/mL in ortho-dichlorobenzene solvent. For all the inverted organic solar cells fabricated, the electron transporting layer was a thin film of amorphous ZnO (a-ZnO) with a thickness of ~25 nm and was prepared following the method used by Jagadamma et al.⁴⁴. The active layer was prepared by spin coating on glass/ITO/a-ZnO substrates inside a nitrogen-filled glove box as follows: for the PTB7:PC₇₁BM blend, the spin-coating condition was 1500 rpm for 60 s; for the PTB7-Th:PC₇₁BM layer, spin coating was performed at 1200 rpm for 60 s; and for PTB7-Th:EH-IDTBR, the spin-coating process was carried out at 900 rpm for 60 s. The samples were then transferred to a vacuum thermal evaporator (1 × 10⁻⁶ mbar base pressure) and kept under vacuum overnight before thermally evaporating the hole transporting layer of MoOx (4 nm) and anode of Ag (100 nm) using a shadow mask. The aperture area of the shadow mask used for the measurement of the OPV devices was 0.065 cm². After electrode deposition, the devices were encapsulated with a UV optical adhesive and a glass coverslip. The current–voltage characteristics were determined under an illumination intensity of 100 mW/cm² in the air using an AM 1.5 global Sciencetech (SS150 -AAA) solar simulator at a distance of 40 cm and a Keithley 2400 source-measure unit. The illumination intensity was verified with a calibrated monosilicon detector and a KG-5 filter. The EQE measurements were performed at zero bias by illuminating the device with monochromatic light supplied from a xenon arc lamp in combination with a dual-grating monochromator. The number of photons incident on the sample was calculated for each wavelength by using a silicon photodiode calibrated by the National Physical Laboratory. For the indoor measurements, the LED light source used was a Cree XML T6, and the illumination intensity (5.9 mW/cm²) was adjusted by changing the input voltage at a distance of 4 cm. A fluorescent lamp (RS components, PL 11 lamp, 11 W) was also used as an illumination source (0.7 mW/cm²) at a distance of 30 cm. The irradiance level was measured using an RK 5710 power radiometer and an Optometer P9710. The absorption spectra of the active layer blends were recorded using a CARY 300 UV-Visible Spectrophotometer.