P-doped organic semiconductor: potential replacement for PEDOT:PSS in organic photodetectors

In this work we present an alternative to the use of PEDOT:PSS as hole transport and electron blocking layer in organic photodetectors processed by solution. As PEDOT:PSS is known to be sensitive to humidity, oxygen and UV, removing this layer is essential for lifetime improvements. As a first step to achieving this goal, we need to find an alternative layer that fulfills the same role in order to obtain a working diode with similar or better performance. As a replacement, a layer of Poly((4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno(3,4-b]thiophene-)-2-6-diyl)) (PBDTTT-c) p-doped with the dopant tris-(1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd-COCF3)3) is used. This p-doped layer effectively lowers the hole injection barrier, and the low electron affinity of the polymer prevents the injection of electrons in the active layer. We show similar device performance under light and the improvements of detection performance with the doped layer in comparison with PEDOT:PSS, leading to a detectivity of 1.9x1E13 cm(Hz)1/2(W)-1, competitive with silicon diodes used in imaging applications. Moreover, contrary to PEDOT:PSS, no localization of the p-doped layer is needed, leading to a diode active area defined by the patterned electrodes.