Investigation of sensing properties of semitransparent PTB7-TH and PCBM blend for electronic applications

Abstract

PTB7-Th and PCBM are two of the most prominent materials in the area of organics and polymer solar cell applications. These materials are well investigated in the area of solar cell application. In this work sensing properties of PTB7-Th and PCBM were investigated on the basis of two types of structure, surface type and semi-surface type. In this work semitransparent sensor for temperature, humidity, ultraviolet radiations and pressure sensing were fabricated. The transparency of semi-surface type device having structure ITO/PTB7-Th:PCBM/Graphene composite was 58-60%. The dependences of the resistance, impedance and capacitance at frequencies 100 Hz, 1 kHz, 10 kHz, 100 kHz and 200 kHz and temperature in the range of 23.8-80 oC for the sensor were studied. It was observed that as the temperature increased from 23.8 oC to 80.0 oC the resistance, impedance and capacitance (at 1 kHz) of the samples decreased, on the average, by a factor of 3.51, 3.79 and 9.6, respectively. It was also noted that as frequency increased from 100 Hz to 200 kHz, the impedance of the sensor decreased by a factor of 21 and 12, at temperatures 24 oC and 58 oC, respectively. Under the same conditions the capacitance decreased by a factor of 30 and 28, respectively. The temperature resistance coefficients were measured to be -1.31%/oC, -1.30%/oC, -1.27%/oC, -0.84%/oC, -0.72 %/oC and 0.33%/oC for R, Z(100 Hz), Z(1 kHz), Z(10 kHz), Z(100 kHz) and Z(200 kHz), respectively. For capacitance measurement the temperature capacitance coefficients were measured as -1.39%/oC, -1.38 %/oC, -1.37%/oC, -1.36%/oC and -1.34%/oC, respectively. The dependences of the resistance, impedance and capacitance at 100 Hz, 1 kHz, 10 kHz, 100 kHz and 200 kHz of the ITO/PTB7-Th:PCBM/Graphene composite samples on relative humidity in the range of (50-93)% was investigated. It was observed that as humidity increased from 50% to 93% the resistance and impedances (at 1 kHz) of the samples decreased, on the average, by a factor of 7.48 and 58.75, respectively. Under the same experimental conditions (1 kHz) the capacitances of the samples increased by a factor of 42. As we increased frequency from 100 Hz to 200 kHz, the impedance decreased by a factor of 20 and 7 at relative humidity of 50% and 62%, respectively. The corresponding capacitance decreased by a factor of 33 and 178, respectively. The semitransparent combined sandwich-surface type samples ITO/PTB7-Th:PCBM/ Graphene composite were fabricated for UV effect investigation. The sensor was annealed at 60 oC for 2hrs and its measured transparency was around (29-30)%. The dependence of impedance and capacitance at frequencies 100 Hz, 1 kHz, 10 kHz, 100 kHz and 200 kHz of the samples under the effect of UV-light were investigated. It was observed that as intensity of UV-light increased from zero to 20,000 µw/cm2, the impedance of the samples decreased by a factor of 1.35(at 100 Hz), 1.24(at 1 kHz), 1.22(at 10 kHz), 1.12(at 100 kHz), and 1.09(at 200 kHz), respectively. Under same conditions the capacitance of the samples increased by a factor of 1.25(at 100 Hz), 1.24(at 1 kHz), 1.18(at 10 kHz), 1.14(at 100 kHz) and 1.08(at 200 kHz), respectively. Within frequency range 100 Hz - 200 kHz the impedance decreased by a factor of 56.8(intensity of UV=0), 50.6(UV=10000 µw/cm2), 47.0(UV=150000 µw/cm2) and 45.6(UV=200000 µw/cm2), respectively. Under the same conditions and frequency range the capacitance decreased by a factor of 19.4(intensity of UV=0), 19.5(UV=5000 µw/cm2), 19.6(UV=10000 µw/cm2), 20.1(UV=150000µw/cm2), and 21.2(UV=200000 µw/cm2), respectively. We attribute these differences due to the generation of electronhole pairs under UV-irradiation and by the increase of their concentration, by presence of the built-in capacitance and possible frequency dependence of the mobility of the charges. The semitransparent PTB7-Th and PCBM based temperature, humidity and UV sensors can be used as a teaching aid in situations where visual control of illumination and light intensity is required. We investigated surface morphology and nano-mechanicals properties of PTB7Th:PCBM thin film deposited over different substrates, before and after heat treatment at 1500C. It was observed that PCBM crystals are formed due to annealing. The nanomechanical properties show very interesting and promising behaviors, in particular, of pop-out region in the unloading curve and uniform thin film properties with best reproducibility. This pop-out vanished as the substrate changed from glass to glass/ITO and suggested that the pop-out is not only temperature dependent but also depends on substrate properties.