Smooth‐Morphology Ultrasensitive Solution‐Processed Photodetectors

Abstract

Solution-processed optoelectronic materials offer a route to low-cost photodetectors, large-area solar cells, and integrated optical sources. [1][2][3] While significant progress has been reported in organic and polymer spin-cast optoelectronics, colloidal quantum dots offer a distinct further advantage -the convenient tuning of absorption onset via the quantum size effect. [4] Electronic transport has recently been enhanced in sizeeffect-tuned colloidal quantum dot films using ligand exchange, resulting in ultrasensitive photodetectors in both visible and infrared wavelengths; however, solid-film ligand exchange generally results in rough film morphologies, incompatible with high-uniformity image sensors. [1,[5][6][7] The frequent cracking of films is attributed to the significant loss of volume during the exchange process. Here, we report a new route to visiblewavelength spin-cast PbS-nanocrystal photoconductive photodetectors with sub-1% roughness, compared to the $10% roughness obtained using previously reported approaches. The new procedure yields devices that exhibit 10 A W À1 responsivities and reveal an added significant advantage: when illumination conditions change, the photodetectors respond with a single time constant of 20 ms. This compares very favorably with the multisecond and multi-time-constant response of previously reported PbS-nanocrystal photoconductive photodetectors.To be relevant to imaging applications, a top-surface photodetector must exhibit three key traits: high sensitivity to the illumination of interest, spatial uniformity across an imaging array, and video-frame-rate speed of response. Sensitivity derives from the photoconductive gain-the number of electrical carriers collected per incident photon given at a specific light level-and the noise-equivalent power (NEP)-the lowest light level that can be distinguished from noise. Spatial uniformity better than 1% across a pixel array is sought to minimize the introduction of spatial noise, which produces an image photoresponse nonuniformity of the same order. Speed of response allowing video-capture frame rates demands sub-100 ms time constants.Large photoconductive gains, as high as thousands of electrons collected per incident photon, and large dark resistivities have recently been shown to enable record high sensitivity in PbS colloidal quantum dot photoconductive photodetectors.[7] These devices rely on enhanced chargecarrier mobilities, achieved by chemically displacing their original insulating oleate ligands with shorter ligands. The use of long aliphatic ligands in the synthesis and initial stages of processing prevent aggregation. However, films made from such materials are highly insulating. A procedure to exchange original ligands with shorter ones is needed to achieve sufficiently efficient carrier transport among the nanoparticles. Ligand removal, ligand replacement, and film crosslinking all produce the desired increase in mobility, critical to increased sensitivity; but are accompanied by a significant loss of film volum...

Enhancement of the photovoltaic performance in P3HT: PbS hybrid solar cells using small size PbS quantum dots

“…The development of colloidal QD solar cells has benefitted further from postdeposition ligand exchange by thiols, which was used in order to avoid excessive aggregation of the PbS QDs, which occurred when the ligand exchange was performed in solution. 18 Moreover, Seo et al 19 previously reported an improvement of photovoltaic performance (energy efficiency (PCE)) of a P3HT/PbS device to 0.01% employing post-deposition ligand exchange using short-length ligands as acetic acid. Post-deposition ligand exchange with bifunctional linker molecules such as ethanedithiol and 1,4-benzenedithiol have also been used recently.…”

Lead sulphide nanocrystal photodetector technologies

“…2. The Sargent group were early pioneers of photodetector devices based on PbS NCs, [20][21][22][23][24][25] though earlier to this photovoltaic devices based on PbS NCs had also been the focus of research attention. 26,27 Following early reports of ultrasensitive PbS NC based photoconductor devices 21,22 there has been a rapid development of many aspects to understand and improve performance.…”

“…Ligand exchange can also be undertaken in the solution phase prior to depositing the PbS NCs films with this procedure enabling smooth film morphologies to be obtained. 24 However, in the majority of devices the former method is utilized, referred to as solid-state ligand exchange, often inducing cracks and voids within the PbS NC films. To minimise these, PbS NCs films may be deposited via sequential layer-by-layer deposition and treatment (the same method also being employed for phototransistor and photodiode device fabrication).…”

Colloidal quantum dot photodetectors

“…[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]). Both reports in which sensitivity is given, as well as those from which it can be inferred, are included in this table.…”