Anisotropic charge trapping in phototransistors unlocks ultrasensitive polarimetry for bionic navigation

The direct linear dichroism photodetection without external optics is promising for simplified polarimetry. Here we exploit anisotropic charge trapping in phototransistors to boost polarization sensitivity towards an on-chip polarizer-free celestial compass for bionic navigation.
Anisotropic charge trapping in phototransistors unlocks ultrasensitive polarimetry for bionic navigation

Being able to probe light polarization states is crucial for both civilian and military applications. Currently, the successful commercialization of state-of-the-art linear polarimeters relies on bulky and complicated optics composed spatially separated polarizers and cameras. Instead, on-chip polarization-sensitive photodetectors offer unique opportunities for next-generation ultra-compact polarimeters [Science 362, 750-751 (2018)]. Thus far, the implementation of such devices has been realized by harnessing anisotropic photoactive semiconductors. However, limited by the inherent anisotropy of these materials, the obtained dichroic ratios (DRs) are at a low level of typically smaller than 10, which is insufficient for practical uses. Though a few works have incorporated photoactive crystals into heterostructures, ferroelectrics, and an external amplification circuitry for enhanced DRs of ~102, an effective and general strategy for polarization sensitivity amplification in single-component photodetectors remains elusive thus far, posing fundamental constraints to the promotion of simplified polarimetry for practical applications.

Recently, our research team led by Prof. Jiansheng Jie, Prof. Xiujuan Zhang, and Prof. Xiaohong Zhang in the Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University have proposed an anisotropic photocurrent amplification strategy to overcome the restriction of inherent anisotropy of photoactive crystals, realizing more than 2,000-fold enhanced polarization sensitivity in phototransistors. Specifically, we unites the advantages of light sensitivity enhancement in phototransistors and anisotropic light absorption of photoactive crystals. It is anticipated that if abundant trap sites exist in a phototransistor composed of a photoactive crystal, the number of trapped photogenerated charge carriers should be polarization-dependent (Fig. 1a). These trapped charge carriers can induce an additional polarization-dependent localized electric field (photo-induced gate bias), causing polarization-dependent onset/threshold voltage shift in phototransistors and amplifying the photocurrent to different extent (Fig. 1b). Theoretical estimations unveil the striking enhancement of DRs by several orders of magnitude, which far exceeds the records of 2D material-based polarization-sensitive photodetectors and reaches over those of commercial polarizers (Fig. 1c).

Figure 1. a, Schematic illustration of a bottom-gate top-contact phototransistor based on an anisotropic photoactive crystal. b, Schematic illustrations of the anisotropic charge trapping effect. c, Predicted values of intrinsic anisotropic ratio (ain)-related DR enhanced by the anisotropic charge trapping effect.

Using small-molecule organic semiconductor as an example. We fabricated organic phototransistors (OPTs) based on well-aligned C8-BTBT crystal array (Fig. 2a-d). Thanks to the highly anisotropic nature of C8-BTBT crystals and the considerable charge trapping capability of SiO2 dielectrics, the OPT exhibited remarkable transfer curve drift under polarized ultraviolet (UV) light with a maximum DR of > 104 (Fig. 2e-g), more than 2 orders of magnitude higher than the largest value ever reported in 2D material-based heterojunction photodiodes. Notably, our strategy enables a facile fabrication technique and is also applicable to different organic material systems and low-power consumption devices to realize ultrahigh polarization sensitivity, thus providing a robust, general, and scalable solution for developing ultrasensitive polarimetry with simple device structure. Looking forward, we will be very excited if our proposed strategy could be implemented in chiral organic semiconductors for the ultrasensitive detection of circularly polarized light, or further be extended to inorganic material systems in a series of follow-up works.

Figure 2. a, Angle-resolved POM images of the C8-BTBT crystal array. b, SEM image of the C8-BTBT crystal array. The right inset shows the structure of a C8-BTBT molecule, and the left inset is an AFM image of a C8-BTBT crystal inside the SiO2 channel. c, Schematic illustration of the bottom-gate top-contact OPT. d, Colored cross-sectional SEM image of the OPT. e, Polarization-dependent transfer curves of the OPT under the illumination of 365 nm polarized light with a fixed intensity of 110 μW cm-2 (VDS = -40 V). f, Periodic variation of ΔVth with the change of polarization angles. g, Smoothed contour plot of polarization-dependent IDS versus VG (VDS = -40 V).

Intriguingly, we note that the as-fabricated OPTs based on C8-BTBT crystal array are endowed with the visible-blind UV photoresponse nature, which are especially applicable in bio-inspired polarization navigation. Because UV polarized light maintains most reliable under complex weather conditions compared with that in visible region, desert ant Cataglyphis uses two sets of orthogonally aligned UV photoreceptors in its ommatidia to sense the e-vector (E) of skylight at zenith for navigation (Fig. 3a,b). Conventional bionic polarization navigation sensors consist of bulky and spatially separated UV light filter, polarizer, and polarization-insensitive photodiode (Fig. 3c), while our OPT enables a filterless, polarizer-free, and miniaturized route towards polarization navigation (Fig. 3d). Based on the robust skylight polarization mode where the polarization direction of scattered sunlight is symmetrical about the solar meridian (SM, Fig. 3e,f), we demonstrated a novel on-chip celestial compass (Fig. 3g), which can truly reflect the skylight polarization direction in different weather conditions (Fig. 3h-m).

Figure 3. a, Schematic illustration of the polarization of skylight. b, Cross-sectional sketch of the dorsal rim ommatidia of Cataglyphis. c, Schematic illustration of a conventional polarization navigation sensor, which is composed of spatially separated light filter, polarizer, and photodiode. d, Schematic illustration of the ordered molecular packing of C8-BTBT crystal, which combines visible-blind UV detection with ultrahigh polarization sensitivity in a single component. e, Schematic illustration of the ideal skylight polarization mode with the direction of E marked by black arrows. f, 2D projection of the skylight polarization mode, where E at zenith is always perpendicular to the SM. g, Photograph of the C8-BTBT crystal array-based polarization navigation sensor after encapsulation. h, Real-time photograph of the clear sky at 3:42-4:17 p.m. (23/09/2021). i, Smoothed contour plot of angle-resolved IDS measured under clear sky. j, Polar coordinate plot of normalized IDS measured under clear sky. k, Real-time photograph of the cloudy sky at 3:27-4:07 p.m. (24/09/2021). l, Smoothed contour plot of angle resolved IDS measured under cloudy sky. m, Polar coordinate plot of angle-resolved IDS measured under cloudy sky.

We believe that these findings not only lay the foundation for the design of next-generation ultrasensitive polarimeters, but also provide fresh perspectives for the realization of highly compact optoelectronic systems for real applications.

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