Photoconductivity and Relaxation Dynamics in Sonochemically Synthesized Assemblies of AgBiS₂ Quantum Dots

Abstract

The transport properties of nonequilibrium (photoexcited) charge carriers in sonochemically synthesized threedimensional (3D) assemblies of AgBiS 2 quantum dots (QDs) deposited as thin films were studied. To characterize the photoconduction of quantum-confined nanocrystals close packed in thin film form, both stationary and time-resolved experiments were performed. Besides by interband electronic transitions in the bulklike part of the nanocrystals, the photoresponse of nanocrystalline films was found to be also affected to a greater extent by the crystal boundary barrier height modulation upon illumination. The surface and bulk recombination velocities were found to be comparable. Good agreement was obtained between the band gap energy determined by analysis of the photoconductivity data measured by the constant field and the constant photocurrent method (∼1.18 eV). This value is in agreement with the optical spectroscopy data. It is higher than the optical band gap of a bulk specimen of this semiconductor, due to 3D confinement effects on the charge carrier motions within individual QDs. The nonequilibrium conductivity was found to relax exponentially with a time constant of 1.67 ms, which corresponds to average lifetime of minority charge carriers (holes).

Solution-processed solar cells based on environmentally friendly AgBiS2 nanocrystals

“…1a). As previously reported [19][20][21][22][23][24] , the material crystallizes into the cubic rock salt structure (Fig. 1b, widening and small deviations in peak positions are due to strain 25 ) with nanocrystal diameters of 4.6 ± 1 nm (Fig.…”

“…AgBiS 2 , however, which belongs to the family of I-III-VI 2 compounds, comprises environmentally friendly restriction of hazardous substances (RoHS)-compliant elements. In its nanocrystalline form, AgBiS 2 exhibits photoconductivity and favourable thermoelectric properties and has been used as a sensitizer or counter-electrode in sensitized solar cells with modest efficiencies [19][20][21][22][23][24] . Here, we present colloidal AgBiS 2 nanocrystals that serve both as the photo-absorbing and charge-transporting medium in high-performance solid-state, solution-processed solar cells.…”

Matildite versus schapbachite: First-principles investigation of the origin of photoactivity inAgBiS2

“…However, more recent experiments report a reduced value of 1.3 eV even for smaller nanoparticles of 4.62 ± 0.97 nm size [1]. Other bulk E g measurements report a value 1.2 eV [5], which would limit the extend of quantum confinement effects; indeed some authors report E g values of 1.32 eV for ~16 nm diameter samples [7], and values of 1.11 eV for AgBiS 2 thin films [13], in accordance to optical measurement of 1.10 eV [14]. The above discussion does not limit to E g .…”

“…Moreover, its ultralow thermal conductivity allows for its use in thermoelectric power generation [2,3], and it has also been explored as sensitizer and/or counter-electrode in sensitized solar cells [4,5]. The renewed interest on AgBiS 2 goes hand by hand with other compounds of the I-V-VI 2 family -where I = Cu, Ag, or an alkali metal; V = Sb or Bi, and VI = S, Se, or Te-studied in the recent times in thermoelectrics [2,6], solar cells [1,7], and phase-change memory devices [8,9].…”

Beyond methylammonium lead iodide: prospects for the emergent field of ns2containing solar absorbers

“…403,404 While the bulk has been recorded to have an optical band gap of 0.9 eV, 405 strong quantum confinement effects have meant that quantum dot thin-films have band gaps reported at 1.0-1.3 eV, although with one study reporting 2.67 eV; that study also reports a very strong absorption coefficient above 10 5 for low frequencies. [405][406][407] Despite this, initial AgBiS 2 QD-sensitized TiO 2 cells reported low efficiencies of 0.53% and 0.95%, both of which suffered from low fill factors and low current densities, 402,408 while AgBiS 2 QDs have also been seen to improve on Pt as a contact material. 409 Recent work by Bernechea et al however, using tetramethylammonium iodide-treated AgBiS 2 nanocrystals in a p-i-n junction between PTB7 and ZnO, has recently given a record cell of 6.3% efficiency, equaling many of the record antimony and bismuth-based cells, with a high J sc of 22 mA cm À2 and FF of 0.63.…”