Solar Conversion Efficiency Performance of a High Temperature Alloy over a Low Temperature One: Comprehending Interfaces through Excitonics Study

Abstract

To take account of the interface in the nanocrystal (NC) materials, we have synthesized high quantum yield gradient CdZnSSe alloy NC having minimal involvement of interface (G-300) through high temperature (300 °C) pyrolysis and investigated the charge carrier dynamics. The performance was unraveled through femtosecond transient absorption studies. A gradient alloy of CdZnSSe (G-250) at low alloying temperature (250 °C) was also synthesized where several interfaces were present in the form CdSe/CdS/ZnSe/ZnS within the alloy material along with other deep traps as well as surface defects. The successful formulation of minimal involvement of interface in G-300 alloy has been envisaged through its blue-shifted optical absorption spectrum as compared to the G-250 alloy due to interionic diffusion of less reactive Zn and S toward the core of the material at elevated reaction temperatures that widen the band gap. Unlike the G-250 analogue, no charge transfer (CT) state was observed in the G-300 alloy, which also suggests the nonexistence of a CdSe/CdS gradient type structure otherwise present for the G-250 analogue. The slow electron cooling time of 4 ps in the G-250 alloy is found to be absent in the G-300 alloy, which can be attributed to minimal involvement of gradient structure otherwise, where electron–hole decoupling leads to slower electron cooling. It has been observed that although the absorption cross-section of G-300 alloy is lower in the solar spectrum as compared to the G-250 analogue, photocurrent conversion efficiency (PCE) measurements of G-300 show promising 4.5% PCE due to smooth electron transfer to TiO2 through the interface free NCs whereas the G-250 analogue shows only 3.5% PCE. Our investigation suggests that engineering with alloys having less gradient structure and without any boundary restrictions can lead us to new perceptions regarding the design and development of higher efficient quantum dot sensitized solar cell (QDSSC).

Improving the Power‐Conversion Efficiency through Alloying in Common Anion CdZnX (X=S, Se) Nanocrystal Sensitized Solar Cells

“…Formation of the alloy structure removes the defect states within the NCs limiting the charge carrier trapping process. [28,31,39] In this work, with the help of ultrafast spectroscopy and photovoltaic measurements on homogeneous alloyed CdZnX NCs we have further shown here that they are better candidates for solar cell application compared to CdX NCs as they do not suffer from interfacial traps. These observations have been summarized in Scheme 1 below.…”

“…[28,29,[43][44][45] As the growth of the CdZnX NCs were carried out~300°C the inter-diffusion of cations will lead to a homogeneous structure where the cations will be evenly distributed throughout the NC. The alloy NCs can be synthesized either in graded or homogeneous structure depending on the reaction temperature employed.…”

“…The alloy NCs can be synthesized either in graded or homogeneous structure depending on the reaction temperature employed. [28] The UV-vis absorption and emission spectra as depicted in Figure 1 (A) shows CdZnS NC has the first excitonic (1 S) absorption maximã 480 nm with photoluminescence (PL) maxima~490 nm, whereas the CdZnSe NC has the 1 S band~550 nm with PL maxima~570 nm. On the other hand, the homogeneous alloy NCs, with uniform composition, are free from interfaces.…”

“…[52][53][54] Once at the band-edge the carriers can recombine or get trapped. [22,27,28] The effect of slower carrier recombination on the photo-voltaic performance of the NCs will now be discussed in detail. absorption spectra due to state filling of the 1S e state.…”

“…[16][17][18][19] However, the core/shell NCs suffers from trap state formation at the core/shell interface predominantly due to the lattice mismatch between the core and shell materials. [25][26][27][28][29] Although for II-VI semiconductors common cation alloy NCs have been investigated for solar cell assemblies in great detail such as CdSeS or CdSeTe NCs due to their absorbtion in the VIS to near-IR region, common anion alloy NCs such as CdZnX is relatively unexplored in terms of photovoltaic applications as incorporation of Zn shifts the absorption spectra to the higher energy side. These limitations can be overcome in alloy structure where the potential changes smoothly within the NCs thus providing better extraction of the charge carriers.…”