Improving Solar Cell Performance Using Quantum Dot Triad Charge-Separation Engines

Abstract

We use kinetic modeling to explore the current–voltage, power–voltage, and power conversion efficiency characteristics of quantum dot dyads and triads as possible light absorption and charge separation engines in quantum dot, bulk heterojunction solar cells. The external and internal power conversion quantum efficiencies are significantly enhanced by introducing a third quantum dot between the donor and acceptor quantum dots. Given the constraint of comparable charge-recombination and charge-separation rates, open-circuit voltages for triads are predicted to be about 10%–17% larger than those for dyads, and short-circuit currents for triads are about 400% larger than those for dyads. These improvements in the efficiencies can be further enhanced by tuning the band-edge energy offset of the middle-position quantum dot from its neighbors. The band-edge energies of the middle quantum dot should be tuned so that they form a cascading band-edge energy alignment from the band-edge energies of the left CdTe QD to the right CdSe QD. To produce the most favorable solar cell performance, the middle quantum dot’s conduction (valence) band edge should be closer to the right quantum dot’s band edge when the charge recombination rates are low (high) and near the conduction (valence) band edge of the left quantum dot when the charge recombination rates are high (low). This analysis identifies important strategies to design multi-QD assemblies for solar energy harvesting and conversion.

Size-controlled electron transfer rates determine hydrogen generation efficiency in colloidal Pt-decorated CdS quantum dots

“…The electron transfer rate from the smallest CdS QDs to Pt is comparable to typical rates from QDs to various acceptors including oxides, inorganic complexes, organic compounds and carbon allotropes. [3][4][5][6][7] More relevantly, a rate constant of 1.22 × 10 9 s -1 was found from 3.6 nm CdSe QDs to Pt, 17 which is close to that measured here from 3.7 nm CdS QDs to Pt. Half-life times ( 1/2, when 50% of initial signal decays), which is another commonly used parameter in some relevant transient absorption work, follows the same trend and is also included in Table 1.…”

“…Similar phenomena have been observed for charge transfer from QDs to various acceptors including oxides, inorganic complexes, organic compounds and carbon allotropes mainly used for photovoltaics applications. [3][4][5][6][7] In order to provide more physical insights, we perform a quantitative analysis of our result. If electron transfer to Pt is dominant additional pathway for the excited-state interaction between CdS and Pt, we can evaluate the rate constant by comparing the bleaching recovery lifetimes in the presence and absence of Pt.…”

“…Particularly, recent work has focused on the importance of QD size in optimizing charge transfer from QDs to various acceptors including oxides, inorganic complexes, organic compounds and carbon allotropes, thus enabling the use of QDs in photovoltaics. [3][4][5][6][7] Charge transfer in each of the aforementioned systems has been recently studied using transient spectroscopic techniques. The dynamics of primary photo-induced processes (i.e.…”

Heterojunction Area-Controlled Inorganic Nanocrystal Solar Cells Fabricated Using Supra-Quantum Dots

“…(i) Too many charge-separation events can lead to charge-carrier accumulation, giving rise to carrier leakage and recombination. (ii) Small p-type and/or n-type domain sizes can inhibit depletion-layer (i.e., built-in potential) formation because of spatial confinement, reducing the open-circuit voltage ( V OC ). ,, Therefore, controlling p–n heterojunction areas is important for achieving high device performance. However, finding proper methods to control the heterojunction areas remains a big challenge.…”

“…In this work, we demonstrate p-n heterojunction area control in CdTe/CdSe NC solar cells by using self-assembled QDs. The solar cells have CdTe NCs as electron acceptors and CdSe NCs as electron donors, based on their cascaded band edge alignments (i.e., the conduction and valence band edge positions of CdTe are higher in energy than the corresponding ones of CdSe), , and the p–n heterojunction is formed at the CdTe/CdSe interface . We evaluate the performances of photovoltaic devices with controlled interfacial areas between the p-type CdTe domains and n-type CdSe domains, fabricated using the three-dimensional (3D) supra-QD (SQD) structures and one-dimensional nanorod (NR) structures, as a proof-of-concept study.…”