Diffusion interface layer controlling the acceptor phase of bilayer near-infrared polymer phototransistors with ultrahigh photosensitivity

Abstract

The narrow bandgap of near-infrared (NIR) polymers is a major barrier to improving the performance of NIR phototransistors. The existing technique for overcoming this barrier is to construct a bilayer device (channel layer/bulk heterojunction (BHJ) layer). However, acceptor phases of the BHJ dissolve into the channel layer and are randomly distributed by the spin-coating method, resulting in turn-on voltages (V_o) and off-state dark currents remaining at a high level. In this work, a diffusion interface layer is formed between the channel layer and BHJ layer after treating the film transfer method (FTM)-based NIR phototransistors with solvent vapor annealing (SVA). The newly formed diffusion interface layer makes it possible to control the acceptor phase distribution. The performance of the FTM-based device improves after SVA. V_o decreases from 26 V to zero, and the dark currents decrease by one order of magnitude. The photosensitivity (I_ph/I_dark) increases from 22 to 1.7 × 10⁷.

Fig. 1. Device structure, molecular structures and fabrication process of the FTM-based films.

a Device structure with SVA treatment and the molecular structures of PDPP3T, PC₆₁BM and OTS. b Schematic diagram for the fabrication of the FTM-based films.

Fig. 2. Basic parameters of different films.

a TEM. b AFM (scanning area is 2 × 2 μm²). c UV–vis absorption spectra (0-0 absorption peak at 850 nm, 0-1 absorption peak at 766 nm, 1-0 absorption peak at 334 nm). d Grazing-incidence X-ray diffraction (GIXRD) patterns. e Ultraviolet photoelectron spectroscopy (UPS) spectra of the valence band region. f UPS spectra of the secondary electron cutoff region.

Fig. 3. The in situ Raman and UV–Vis absorption spectra of the FTM-based films.

a In situ Raman spectra of the PDPP3T/PDPP3T:PC₆₁BM bilayers with SVA at different temperatures (PDPP3T-25 °C: the dark line represents the PDPP3T film at 25 °C; PDPP3T:PC₆₁BM-25 °C: the red line represents the PDPP3T:PC₆₁BM film at 25 °C; PDPP3T/PDPP3T:PC₆₁BM bilayers: the blue line represents 25 °C, the brown line represents 70 °C, the orange line represents 80 °C, the magenta line represents 90 °C, and the deep green line represents 100 °C). b Dependence of the normalized PC₆₁BM Raman peak in the PDPP3T/PDPP3T:PC₆₁BM layer on annealing temperature (data from Fig. 3a, ♣ represents the PDPP3T:PC₆₁BM film, ♦ represents the PDPP3T film). c Absorption spectra of the transferred films after removing the films above the PEDOT:PSS layer with different SVA times (0-0 absorption peak at 850 nm, 0-1 absorption peak at 766 nm, 1-0 absorption peak at 334 nm). d Relationship between the normalized PC₆₁BM absorption peak and annealing time (data from Fig. 3c).

Fig. 4. Schematic diagram and electrical properties of the Si/SiO₂/OTS/PDPP3T/Au/PC₆₁BM structured devices via different fabrication methods.

a Schematic diagram of the PC₆₁BM solution dissolved in chloroform solvent for the spin-coating bilayer device (^#1 S-PDPP3T/PC₆₁BM-CHCl₃). b Schematic diagram of the PC₆₁BM solution dissolved in THF solvent for the spin-coating bilayer device (^#2 S-PDPP3T/PC₆₁BM-THF). c Schematic diagram of bilayer device via FTM (^#3 T-PDPP3T/PC₆₁BM). d (^#1 S-PDPP3T/PC₆₁BM-CHCl₃), e (^#2 S-PDPP3T/PC₆₁BM-THF), f (^#3 T-PDPP3T/PC₆₁BM) Film surface photographs. g (^#1 S-PDPP3T/PC₆₁BM-CHCl₃), h (^#2 S-PDPP3T/PC₆₁BM-THF), i (^#3 T-PDPP3T/PC₆₁BM) Cross-sectional SEM images of the films (scale bar is 100 nm). j (transfer curve), k (dependence of photocurrent (∆I_ph) on the source-drain voltage (V_d)), l (V_o and off-state dark current) Electrical properties of the devices prepared by different methods. The transfer curves of the devices were measured at a constant V_d = −30 V.

Fig. 5. Electrical properties of devices with SVA (^#6) and without SVA (^#5) treatment.

a Transfer curve. b Photocurrent (∆I_ph) at various gate voltages. c Dependence of R and I_ph/I_dark on gate voltage. d Dependence of ∆I_ph and R on light intensity (850 nm). e EQE spectrum at −30 V gate voltage under illumination of 0.04 mW/cm². The light intensity is shown in Fig. 5a–c, and Fig. 5e is 0.04 mW/cm² @ 850 nm. The orange lines with both unfilled and filled circles represent Device ^#5, and the blue lines with both unfilled and filled squares represent Device ^#6. The transfer curves of the devices were measured at a constant V_d = −30 V.

Fig. 6. Time responses and variation in electrical properties under different SVA treatment conditions.

a, b Time responses via the photoelectric dual-control measurement (orange line filled with circles for Device ^#5, blue line with filled circles for Device ^#6, tr for rise time, tf for fall time). c Variation in ∆I_ph (red line with filled circles) and V_o (blue line with filled circles) with increasing SVA temperature (light purple area represents the device without SVA, light blue area represents the devices with SVA (20 min)). d Variation in ∆I_ph (red line with filled circles) and V_o (blue line with filled circles) with increasing SVA time (light purple area represents the device without SVA, light blue area represents the devices with SVA (100 °C)). The light intensity is 0.04 mW/cm² @ 850 nm.

Fig. 7. Energy level diagram of the different devices.

a Device without SVA in the dark state. b Device without SVA under illumination. c Device with SVA in the dark state. d Device with SVA under illumination.