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Mobility-limited polyarylamine biscarbonate ester (PABC) /[6,6]-phenyl $C_{61}$ butyric acid methyl ester (PCBM) bulk heterojunction photovoltaic device

Photovoltaic (PV) devices made from blends of a polyarylamine biscarbonate ester (PABC) and [6,6]-phenyl $C_{61}$ butyric acid methyl ester (PCBM) have been fabricated and characterized. PABC is a hole transporting co-polymer prepared from reacting N,N'diphenyl-N,N'bis(3-hydroxyphenyl)1,1;biphenyl(4,4'diamine), diethylene glycol bischloroformate, and triethylemine. By varying the polymer loading in the blend, optimal power conversion efficiency (PCE) of approximately 0.45\% has been achieved for a blend consisting of 25 wt\% PABC, which is an order of magnitude higher than the PCE for a 45 wt\% blend. The optimal ratio is at about 0.44:0.56 molar ratio of the active hole transporting to electron transporting moieties. Results of mobility studies suggest that blends with higher PABC loading have efficiencies limited by 'hole' transport. Also responsible for the lower efficiency at higher PABC concentrations was optical filtering. The efficiency does not appear to be limited by deep charge trapping. Density functional theory calculations conducted pointed toward a strong localization of the frontier orbitals, which might offer an explanation for the low PABC hole mobility. Calculated frontier orbital energy levels found band gap to be consistent with the $V_{OC}$ measured.