Effect of annealing on the optical, electrical, and photovoltaic properties of bulk hetero-junction device based on PPAT:TY blend

Polythiophenes are the promising candidates for polymer photovoltaic devices due to their stability and absorption in near-red region of visible spectrum. In this paper, the optical, electrical, and photovoltaic properties of device fabricated with the blend of poly(phenyl azo methane thiophene) (PPAT) (donor) and thiazole yellow (TY) (acceptor) (70:30 by weight ratio), sandwiched between ITO and Al electrodes have been reported. The observed strong quenching in the fluorescence of PPAT after blending it with TY indicates the rapid and effective separation of photo-induced electron and hole and subsequent transfer of electron from PPAT to TY and holes from TY to PPAT. The absorption spectrum of the blend shows a super-imposition of both components indicating that there is no charge transfer at the ground state. The thermal annealing shows significant improvement in both optical and photoelectrical properties of the blend, as can be seen from the recorded current-voltage characteristics (J-V) of the device in dark and under illumination. This change is also evidenced by the modification in absorption spectrum on annealing resulting in red shifts, due to the donor component. The appearance of red shift may by attributed to the molecular diffusion of TY out to the PPAT matrix on annealing, resulting in a rise to the TY clusters. The growth of TY clusters lead to the formation of percolation paths and therefore improves photocurrent. By annealing the device under applied external voltage, both the electrical and photovoltaic properties of the device have been improved. The enhancement in photocurrent, quantum collection efficiency (EQE), and fill factor of annealed device may be due to ail increase in carrier mobility after annealing the device. (c) 2006 Elsevier B.V. All rights reserved.