Substrate Effects on the Bandwidth of CdSe Quantum Dot Photodetectors

Abstract

We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc β-tetraaminophthalocyanine (Zn4APc) (Kumar et al., ACS Appl. Mater. Interfaces, 2019, 11, 48271-48280) on three different substrates, namely, silicon with 230 nm SiO 2 dielectric, glass, and polyimide. While Si/SiO 2 (230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find that the photoresponse of the CdSe quantum dot films can be described as a combination of carrier recombination and fast trapping within 2.7 ns followed by slower multiple trapping events. The latter are less pronounced on polyimide, which leads to the higher bandwidth. We show that all devices are resistance-capacitance (RC)-time limited and that improvements of photoresistance are the key to further increasing the bandwidth.

Pitfalls in Determining the Electrical Bandwidth of Nonideal Nanomaterials for Photodetection

“…After film formation over-night, excess solvent was spun-off the substrate, the film was washed with acetonitrile to get rid of unbound Zn4APc, and the film was annealed at 190 °C for 30 min. The film was inhomogeneous with up to 3.5 µm thickness, as reported elsewhere 48 . The detector was examined under vacuum.…”

“…The film was annealed at 190 °C for 30 min. The film was inhomogeneous with up to 3.5 μm thickness, as reported elsewhere . The detector was examined under vacuum.…”

Sub-nanosecond Intrinsic Response Time of PbS Nanocrystal IR-Photodetectors

“…The conversion of light into electrical signals is the basis of numerous technological applications, ranging from high-speed optical communications, video imaging, and night-vision to gas sensing and biomedical imaging. − A promising class of materials for this action are colloidal NCs, facilitating solution-processable low-cost photodetectors with large photoabsorption cross sections and a compatibility with flexible substrates. − In particular, lead sulfide (PbS) NCs are high-potential candidates due to their size-tunable absorption throughout the near-IR region of ∼600–3000 nm. ,, Significant progress has been achieved in recent years regarding the PbS-NC photodetector’s key figures of merit, the detectivity and response time. ,,, Recently, high-speed PbS-NC photodetectors with promising response times down to 7–10 ns have been presented, , while a huge range of response times up to seconds have been previously reported. ,, As those response times were determined by transient photocurrent measurements, they represent extrinsic response times. The extrinsic response time combines the intrinsic material properties with limitations due to parasitic capacitances, device geometry, and resistance-capacitance (RC) time. − Hence, the intrinsic response time of PbS-NC photodetectors, that is, their material-specific physical limit, is in fact still unknown.…”

Substrate effects on the speed limiting factor of WSe2 photodetectors

“…13 We are unaware of comparable studies for WSe 2 , in particular beyond Si/SiO 2 as the substrate, which is problematic for investigating the photodetector speed due to a convolution with the fast photoresponse of the underlying silicon. 24…”