Three-Phase Morphology Evolution in Sequentially Solution-Processed Polymer Photodetector: Toward Low Dark Current and High Photodetectivity

Sequentially solution-processed polymer photodetectors (SSP PPDs) based on poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) are fabricated by depositing the top layers of PC₇₁BM from an appropriate cosolvent of 2-chlorophenol (2-CP)/o-dichlorobenzene (ODCB) onto the predeposited bottom layers of P3HT. By adjusting the ratio of 2-CP/ODCB in the top PC₇₁BM layers, the resulting SSP PPD shows a decreased dark current and an increased photocurrent, leading to a maximum detectivity of 1.23 × 10¹² Jones at a wavelength of 550 nm. This value is 5.3-fold higher than that of the conventional bulk heterojunction PPD. Morphology studies reveal that the PC₇₁BM partially penetrates the predeposited P3HT layer during the spin-coating process, resulting in an optimal three-phase morphology with one well-mixed interdiffusion P3HT/PC₇₁BM phase in the middle of the bulk and two pure phases of P3HT and PC₇₁BM at the two electrode sides. We show that the pure phases form high Schottky barriers (>2.0 eV) at the active layer/electrodes interface and efficiently block unfavorable reverse charge carrier injection by significantly decreasing the dark current. The interdiffussion phase enlarges the donor-acceptor interfacial area leading to a large photocurrent. We also reveal that the improved performance of SSP PPDs is also due to the enhanced optical absorption, improved P3HT crystallinity, increased charge carrier mobilities, and suppressed bimolecular recombination.