Suppressing the Dark Current in Quantum Dot Infrared Photodetectors by Controlling Carrier Statistics

Abstract

Lead sulfide colloidal quantum dot photodiodes (PbS QDPDs) exhibit a high energy conversion efficiency for infrared detection. Despite the high photoinduced current, the performance of PbS QDPDs is limited by the high dark current which is rarely investigated. Understanding the dark current in PbS QDPDs is critical to improving the detectivity of PbS QDPDs. Herein, it is demonstrated that minority carriers of I‐passivated PbS films and trap sites of EDT‐passivated PbS films are related to the dark current of PbS QDPD. Utilizing annealing and low‐temperature ligand exchange processes, the dark current density can be decreased almost tenfold by suppressing the minority carrier diffusion in the PN junction and trap‐assisted charge injection from the electrode. PN junction simulation, space charge limited current measurements, as well as structural, optical, and chemical characterizations are conducted to elucidate the origins of the dark current suppression. The authors achieve the lowest dark current density of 2.9 × 10−5 mA cm−2 at −1 V among PbS‐based QDPDs and a high detectivity of 6.7 × 1012 Jones at 980 nm. It is believed that this work provides fundamental understanding of carrier statistics in nanomaterials and device performance as well as a technological basis for realizing low‐cost high‐performance optoelectronic devices.

Elucidating the Gain Mechanism in PbS Colloidal Quantum Dot Visible–Near-Infrared Photodiodes

“…It should be noted that there are two values of k within the incident power range, suggesting the two different surface recombination effects. The exponent k = 0.96 (close to 1) at high light intensities indicates that the generation-limited photogenerated current where monomolecular recombination is dominant and behaves like a common diode based on the photovoltaic effect . At low light intensities, k = 0.62 (close to 0.5) indicates that bimolecular recombination mode is dominant, accompanied by trap-mediated capture of photogenerated carriers .…”

“…The exponent k = 0.96 (close to 1) at high light intensities indicates that the generation-limited photogenerated current where monomolecular recombination is dominant and behaves like a common diode based on the photovoltaic effect. 19 At low light intensities, k = 0.62 (close to 0.5) indicates that bimolecular recombination mode is dominant, accompanied by trapmediated capture of photogenerated carriers. 39 Intensitydependent open-circuit voltage (V OC ) measurement gives additional insight into the charge generation and recombination mechanisms in the device.…”

“…The device structure of CQD photodetectors normally used is either a photoconductive detector (photoconductor or phototransistor) or a photovoltaic detector (photodiode). , The photoconductive devices show a large external quantum efficiency (EQE, ratio of the number of photogenerated carriers collected by the electrode to the number of incident photons) and a high responsivity ( R (λ), ratio of the photogenerated current to the incident optical power at a given wavelength λ). − This is because gain ( G ) that the photon-induced carriers can circulate several times in the whole channel before recombination exists in this kind of photodetector . Unfortunately, the large dark current and slow response speed are inevitable in these devices due to the limitation of structure. − Compared with photoconductive detectors, photodiodes exhibit the merits of low dark current, fast response, and CMOS-compatible charge processing ability. − Although CQD photodiodes with ultralow dark current density (nA cm –2 ) or ultrafast response (ns) have been successfully reported, , their EQE/ R (λ) values are still unsatisfactory. The relatively low responsivity and large dark current cannot meet the expectation of practical application, especially in the detection of weak light, such as forewarning for the immune system of humans, ambient light monitoring in smart buildings, and medical imaging .…”

“…Otherwise, the photogenerated holes are captured by the trap sites formed in PbS-EDT. These trap states originated from the oxidation of PbS-EDT films (acceptor doping) fabricated under an ambient environment and making air exposure before electrical measurements. ,, Oxygen is expected to create acceptor states on the PbS CQD films, , and the negative electricity center is formed by thermal ionization at room temperature or under a high electric field in the fully depleted junction (Figure S5). Accumulation of trapped holes in the PbS-EDT layer builds up a high electric field at the PbS-EDT/Au interface, causing a large band bending. , The light capacitance–voltage ( C – V ) measurement verifies the existence of such accumulated charges (Figure S6).…”

“…13−15 Compared with photoconductive detectors, photodiodes exhibit the merits of low dark current, fast response, and CMOS-compatible charge processing ability. 16−18 Although CQD photodiodes with ultralow dark current density (nA cm −2 ) or ultrafast response (ns) have been successfully reported, 19,20 their EQE/R(λ) values are still unsatisfactory. The relatively low responsivity and large dark current cannot meet the expectation of practical application, 21 especially in the detection of weak light, such as forewarning for the immune system of humans, 22 ambient light monitoring in smart buildings, 23 and medical imaging.…”