Semiconductor:Insulator Blends for Speed Enhancement in Organic Photodiodes

Organic photodiodes (OPDs) have been studied intensively for a number of years, as they enable low-cost fabrication of sensor systems for imaging, medical, or industrial applications. While the device performance in terms of responsivity and detectivity is continuously improving, detection speed still lags behind. One of the major limiting factors is the large transit times of the charges. In this work, an approach of blending insulating polymers into a poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction system, which has already shown beneficial effects on the transport properties in organic field effect transistors or organic solar cells, is followed. Starting from a basic morphological study using poly(methyl methacrylate) (PMMA) with different molecular weights, both the steady-state and dynamic characteristics of the OPDs are examined. Transient photocurrent measurements show that the inclusion of PMMA results in an increase of the -3 dB cut-off frequency without adversely affecting the device steady-state performance under illumination or in the dark. This increase is attributed to a reduced transit time in the active layer when PMMA is present.