Widely Controllable Electronic Energy Structure of ZnSe–AgInSe₂ Solid Solution Nanocrystals for Quantum-Dot-Sensitized Solar Cells

Abstract

I−III−VI 2 -semiconductor-based nanocrystals of ZnSe−AgInSe 2 solid solution ((AgIn) x Zn 2(1-x) Se 2 , ZAISe) with average sizes of 3.5−6.2 nm were successfully synthesized through thermal reaction of corresponding metal acetates and selenourea in a hot oleylamine solution. The optical property of ZAISe solid solution nanocrystals was tunable in a broad wavelength region from visible to near-infrared light by changing the composition of solid solution, where the energy gap of ZAISe nanocrystals was enlarged from 1.44 to 3.00 eV with an increase in the fraction of ZnSe in ZAISe, that is, with a decrease in x from 1.0 to 0. Both levels of conduction band and valence band edges, determined by photoelectron spectroscopy in air, were monotonously shifted to higher levels with an increase in the fraction of ZnSe. Quantum-dot-sensitized solar cells were fabricated with porous TiO 2 film electrodes immobilized with ZAISe nanocrystals using 3-mercaptopropionic acid as a cross-linking agent. The light conversion efficiency of the thus-obtained cells was enhanced by covering ZAISe nanocrystals with a CdS thin layer by the SILAR method. The photocurrent action spectra agreed well with absorption spectra of ZAISe nanocrystals immobilized on TiO 2 electrodes. Maximum energy conversion efficiency of 1.9% was obtained for the cell fabricated with ZAISe nanocrystals with x = 0.5 as a sensitizer under irradiation with simulated solar light of AM 1.5G. ■ INTRODUCTIONPhotovoltaic devices are one of the potential applications for colloidal semiconductor nanocrystals (NCs) because of their controllable physicochemical properties depending on their size. 1−6 Extending the optical absorption range of semiconductor NCs [quantum dots (QDs)] has been an important research subject to effectively utilize solar light, which contains light from the UV−vis to near-IR wavelength region. The size control of NCs has been a useful method to modify optical properties for semiconductor nanocrystals, such as CdSe, CdTe, and PbS, with the energy gap (E g ) of nanocrystals being enlarged due to the quantum size effect by decreasing particle size to less than ca. 10 nm. 7−9 Preparation of solid solution NCs has been another way to tune optical properties regardless of their particle size. 10−14 For example, solid solution NCs of ZnSe−CdSe (Zn x Cd 1-x Se) have been reported to have a tunable absorption onset from ca. 620 nm to ca. 500 nm by changing the chemical composition of x from 0 to 0.67, even though the average diameter of the solid solution NCs was enlarged with a decrease in the x value. 10 Colloidally synthesized nanocrystals have been useful light absorbers for the fabrication of quantum-dot-sensitized solar cells (QDSSCs). One possible strategy to improve the conversion efficiency of the device is to facilitate the charge separation of electron−hole pairs photogenerated in semiconductor NCs by appropriately adjusting the energy difference of conduction band edges (ΔE CB ) between semiconductor NCs and porous metal oxides as collecting electr...

Independent Composition and Size Control for Highly Luminescent Indium-Rich Silver Indium Selenide Nanocrystals

“…[21][22][23] Despite this progress, the synthesis of AISe NCs remains underdeveloped, frequently including silver-rich or even binary Ag 2 Se intermediates. 7, 18, 22 Furthermore, many protocols require growth times of 10 min to few hours to reach target compositions.…”

“…[19][20] Finally, increasing the reactivity of Se precursor by employing weaklycoordinating oleylamine-Se complexes, selenourea, or bis(trimethylsilyl)selenide has been shown to improve composition control over AISe NCs. [21][22][23] Despite this progress, the synthesis of AISe NCs remains underdeveloped, frequently including silver-rich or even binary Ag 2 Se intermediates. 7,18,22 Furthermore, many protocols require growth times of 10 min to few hours to reach target compositions.…”

Nanostructure Engineering of Size-Quantized Semiconductor Particles for Photoelectrochemical Applications

“…Similar optical tunability was observed for other kinds of Ag-based solid solution nanoparticles, Ag-In-Ga-S 51,52 and ZnCu-Ag-In-S particles. 53 Figure 9 shows E CB and E VB of multinary semiconductor nanoparticles as a function of E g , 50,54,55 determined by photoemission yield spectroscopy in air or by photoelectrochemical measurement. The electronic energy structure of ZAIS particles was significantly dependent on both their chemical composition and particle size.…”

“…54,[60][61][62][63] By appropriately selecting constituent elements, the absorption property of multinary nanoparticles can be extended to near-IR region. Nanoparticles of ZnSe-AgInSe 2 solid solution (AgIn) x Zn 2(1¹x) Se 2 , ZAISe), the average sizes of which were 3.5-6.2 nm, were prepared by reacting corresponding metal acetates with selenourea in an OLA solution at 250°C.…”

“…Nanoparticles of ZnSe-AgInSe 2 solid solution (AgIn) x Zn 2(1¹x) Se 2 , ZAISe), the average sizes of which were 3.5-6.2 nm, were prepared by reacting corresponding metal acetates with selenourea in an OLA solution at 250°C. 54 The E g of ZAISe particles was controllable from 1.44 to 3.00 eV by changing the x value of the chemical composition from 1.0 to 0. Photoanodes were prepared by immobilization of ZAISe nanoparticles on porous Fig.…”