R V Patil , M R Ranganath and Blaise Lobo

1. Optical And Electrical Properties of Cobalt Chloride Doped PolyvinylalcoholPolyvinylpyrrolidone Blend R V Patil, M R Ranganath and Blaise Lobo Department of Physics, Karnatak Science College, Karnatak University, Dharwad 580001, Karnataka, India *Email:blaise.lobo@gmail.com DAE-SSPS 2013; Thapar University, Patiala.

2. OVERVIEW • Films of Cobalt Chloride (CoCl2) doped Polyvinylalcohol – Polyvinylpyrrolidone (PVA-PVP) blend were prepared by solution casting method, in the doping range 0 wt% up to 42 wt%. • XRD, UV-Visible spectrometry, FTIR, thermal analysis and electrical measurements were performed.

3. OPTICAL ANALYSIS • The optical band gap due to indirect allowed transitions (in k-space) decreases from 4.6 eV for 1.5 wt% doping level to 4.0 eV at 35 wt% doping level. • Absorption peaks were observed at 2.3 eV, 3.0 eV and 1.7 eV, which indicate that doping results in formation of allowed energy bands within the forbidden gap. • The Urbachenergy (width of band tails within the forbidden gap) is found to significantly decrease with increase in doping level.

4. ELECTRICAL MEASUREMENTS • DC electrical measurements show a good fit for 3-D Variable Range Hopping model of conductivity. • The temperature variation of electrical resistivity obeys the Arrhenius relation, from which the activation energy obtained is found to decrease from 4.1 eV for 1.5 wt% doping level to 3.1eV for 19 wt% doping level.

5. SAMPLE PREPARATION • A standard 1M doping solution of cobalt chloride in distilled water was prepared. • Different volumes of this solution were added to two grams each, of an aqueous solution of PVA and PVP which was prepared earlier, and the same was stirred magnetically, and later placed in an air cooled temperature controlled oven, maintained at 40 degree Celsius. • The films were slowly dried on flat glass petri-dishes, peeled off & stored in dessicator.

6. UV-VISIBLE SPECTROSCOPY • A Hitachi U 3310 UV-Visible spectrometer was used in the wavelength range 200-1000 nm for obtaining the Optical absorption (UV-Visible) spectra. • The spectra were analyzed to extract optical parameters like activation energy, optical band gap and Urbach energy. • It was observed that the absorption coefficient is a function of photon energy, and obeys Mott-Davis model .

7. results and discussion • It was observed (from UV-Visible spectra) that the absorption edge gives a good fit for both indirect allowed transitions and direct allowed transitions in k-space. • From the data of variation of electrical resistivity with temperature, the energy gap for charge mobility (Activation energy Ea) has been determined.

8. INDIRECT ALLOWED TRANSITIONS

9. IATs in k-space

10. Direct Allowed Transitions

11. Urbach Energy

12. UV-Visible Spectrum

13. XRD Spectrum

14. Electrical Conductivity • In CoCl2 doped PVA-PVP polymeric blend sample, there was found to be a negative temperature coefficient of electrical resistivity (ρ). In fact, there is an exponential drop in electrical resistivity with absolute temperature (T), which can be explained with the help of the Arrhenius relation. • The activation energy decreases from a high of 4.1 eV for 1.5 wt% doping level to a low value of 3.1eV for 19 wt% of CoCl2 in PVA-PVP blend.