Investigation of the effect of rare earth doping on the electrical and photovoltaic properties of chromotrope 2R thin film devices

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Praseodymium (Pr) doped and Samarium (Sm) doped chromotrope 2R (CHR) were used for the fabrication of Schottky devices, by the spin coating technique. The diode in which doped CHR behaves as an n-type organic semiconductor exhibits rectification behaviour in the dark. Doping with rare earths imparts an accelerated improvement in the n-type conductivity as well as in the rectification effect. The observed rectification effects are explained by n-type semiconductivity of the doped CHR thin films. The formation of a blocking contact (Schottky barrier) indium tin oxide with (ITO) electrode and an ohmic contact with Al or ln, also confirms its n-type behaviour. The position of the Fermi level shifts toward the conduction band edge on rare earth doping. Additionally, the concentration of free carriers and mobility of electrons also increase upon doping, with the simultaneous decrease in trap concentration. Various electrical parameters such as barrier height (φb), density of ionized donor (Ns) and depletion layer width (W) were calculated from the detail capacitance–voltage analysis of (C–V) characteristics. The photo-action spectra of the devices and absorption spectra of doped CHR layer reveal the formation of a Schottky barrier at the ITO-doped CHR interface and an Ohmic contact at the Al-doped CHR interface. Photovoltaic measurements of these devices provide parameters such as short circuit photocurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (η). The effect of rare earth doping on the electrical and photovoltaic parameters are discussed in detail.

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