Influence of thickness of poly(3-hexylthiophene-2,5-diyl) hole transport layer on electrical characteristics of lead sulfide quantum dot-based shortwave infrared photodiodes

Lead sulfide quantum dots (PbS QDs) have emerged as one of the promising photoactive materials for shortwave infrared (SWIR) photodiodes because of their uncooled processability, cost efficiency, and broadband tunability. In this study, PbS QD-based SWIR photodiodes were fabricated by using poly(3-hexylthiophene-2,5-diyl) (P3HT) and zinc oxide nanoparticles as hole and electron transporting materials, respectively. The P3HT films with different thicknesses of approximately 32, 71, and 141 nm were fabricated by using the solutions with the P3HT/chlorobenzene composition conditions of 5, 10, and 20 mg/mL, respectively. Herein, the PbS QD-based SWIR photodiodes showed an increase in current by SWIR light irradiation, and in particular, the device with a 71-nm-thick P3HT hole transport layer (HTL) exhibited the most superior photo-induced current, i.e., current amplification upon SWIR light. Since the difference (approximately 0.13 − 0.41 nm) in surface roughness among P3HT films with different thicknesses was much less than the size (4.5 − 5.0 nm) of PbS QDs, the results could be understood through the influence of the P3HT HTL thickness on the electrical characteristics of PbS QD-based SWIR photodiodes. Further analysis of the photodiode performance according to the P3HT HTL’s thickness reveals that the transport properties of photogenerated charge carriers through the hole transport layer have a crucial role in the photo-induced current and therefore decisively influence the photoresponsivity. These results suggest that P3HT HTLs affect the transport and extraction processes of photogenerated holes so that proper optimization of their thickness can contribute to enhancing the photoresponse efficiency of PbS QD-based SWIR photodiodes.