Physics and engineering of organic solar cells: Electrical p-type doping with phosphomolybdic acid
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Variations to a solution-based technique to electrically p-type dope conjugated polymer films using 12-molybdophosphoric acid (also referred to as phosphomolybdic acid or PMA) are reported and applied to simplify the fabrication of organic photovoltaic devices. From an engineering perspective, solution-based electrical p-type doping using PMA was improved by making the doping solution stable in air while keeping its ability to produce electrical p-type doping profiles down to a limited depth from the surface of organic semiconducting films. Furthermore, degradation studies reveal a superior stability of organic photovoltaic devices doped using the updated method, compared to devices fabricated using the original technique. Last, the scope of applications of the method was extended by making it compatible with state-of-the-art conjugated polymer films. Engineering advances on the application of PMA doping to organic photovoltaics were supported by basic science studies that contributed to improve the understanding of the doping process. In particular, the apparent solvent-dependent nature of the original doping technique was investigated. To this end, the free volume of conjugated polymer films and its role in solution-based electrical p-type doping using PMA were studied. It was found that the effectiveness of PMA doping critically depends on a charge transfer process, as evidenced in previous work, but also on the available free volume of the conjugated polymer film, as reported here. No fundamental limit was found pertaining the application of this method to emerging conjugated polymers and bulk heterojunctions of state-of-the-art organic photovoltaic devices.