Tunable structural and optical properties of CuInS₂ colloidal quantum dots as photovoltaic absorbers

Abstract

We report the facile hot-injection colloidal synthesis of near-stoichiometric CuInS2 quantum dots at varying reaction times and temperatures which exhibit both optical and structural tunability with implications for enhanced photovoltaic utility.

Tunable structural and optical properties of AgxCuyInS2 colloidal quantum dots

“…Similar outcomes reported by Xie and co-workers 76 showed that a gradual increase in particle growth temperature induced transformation from primarily wurtzite to chalcopyrite phases. Therefore, as in our previous report, 13 we suggest that formation of wurtzite Ag:CIS in this case likely proceeds via a wurtzite phase Ag:Cu 2 S intermediate, Ag x Cu 2Àx S, since the soft Lewis acid ions, Ag + and Cu + would proportionally coordinate with the dodecanethiol ligand, a soft Lewis base. Subsequent decomposition of this in situ complex to Ag x Cu 2Àx S nanoparticles is followed by cationic exchange with In 3+ to form Ag x Cu y InS 2 QDs stabilized by the capping ligand.…”

“…12 In particular, CIS is of interest as its optoelectronic properties can be tuned solely by intrinsic lattice defects due to the large range of anion and cation off-stoichiometry tolerance. 13 Congruent with other reports involving the defect chemistry of CIS, 14,15 recently we reported various intrinsic defect states implicated in the broad photoluminescence emission of slightly off-stoichiometric undoped CIS QDs. 13 Since shallow energy states from some of these defects act as trapping states suppressing conductivity and overall power conversion efficiency (PCE) of CIS-based solar cells, 16 it has been shown that strategic doping and alloying are useful in tailoring their morphological, optical and electrical properties, 3,9,10 for more spectrally adaptive thin film and quantum dot solar cells.…”

“…13 Congruent with other reports involving the defect chemistry of CIS, 14,15 recently we reported various intrinsic defect states implicated in the broad photoluminescence emission of slightly off-stoichiometric undoped CIS QDs. 13 Since shallow energy states from some of these defects act as trapping states suppressing conductivity and overall power conversion efficiency (PCE) of CIS-based solar cells, 16 it has been shown that strategic doping and alloying are useful in tailoring their morphological, optical and electrical properties, 3,9,10 for more spectrally adaptive thin film and quantum dot solar cells. 17,18 Therefore, synthetic strategies must be tailored for successful doping or alloying rather than the commonly reported QD surface adsorption 19 for effective tunable optical and conductive properties.…”

“…Indeed, it has been established that the electronic band structure for CIS comprises a valence band of Cu 3d and S 3p energy states from Cu-S bonds, whilst the conduction band comprises Cu 4s, S 3p and In 5s antibonding states. 13,81 Since Laporte and spin-allowed direct bandgap transitions are influenced by nanoparticle size, composition and structure, 50,82 a decrease in Cu + content would reduce its density of electronic states, thereby lowering the valence band maximum and widening the bandgap. 50,83 However, since this contradicts the trend (Fig.…”

“…Notwithstanding this, the most interesting features of the absorption and emission are the corresponding shoulders in the 300 and 1100 nm regions, respectively, which slightly red-shift and intensify with increasing Ag + fraction. Whilst broad absorption and emission for CIS in the NIR region have been attributed to donor-acceptor pair (DAP) defect states, 13,43,53,84 the corresponding absorption and emission shoulders suggest an influence on the electronic structure by energy states associated with Ag + ions. Furthermore, the fullwidth at half maximum (FWHM) ranging between 171 and 201 nm for these broad emissions are typical for CIS nanostructures and can be attributed to recombination involving defect states, with some contribution from particle size distribution.…”