Abstract
Energy loss due to carrier recombination is among the major factors limiting the performance of TiO2/PbS colloidal quantum dot (QD) heterojunction solar cells. In this work, enhanced photocurrent is achieved by incorporating another type of hole-transporting QDs, Zn-doped CuInS2 (Zn-CIS) QDs into the PbS QD matrix. Binary QD solar cells exhibit a reduced charge recombination associated with the spatial charge separation between these two types of QDs. A ~30% increase in short-circuit current density and a ~20% increase in power conversion efficiency are observed in binary QD solar cells compared to cells built from PbS QDs only. In agreement with the charge transfer process identified through ultrafast pump/probe spectroscopy between these two QD components, transient photovoltage characteristics of single-component and binary QDs solar cells reveal longer carrier recombination time constants associated with the incorporation of Zn-CIS QDs. This work presents a straightforward, solution-processed method based on the incorporation of another QDs in the PbS QD matrix to control the carrier dynamics in colloidal QD materials and enhance solar cell performance.
Highlights
Energy loss due to carrier recombination is among the major factors limiting the performance of TiO2/PbS colloidal quantum dot (QD) heterojunction solar cells
Different volume fractions of Zn-doped CuInS2 (Zn-CIS) QDs in the PbS QD matrix were examined: a 10% (v/v) addition of Zn-CIS QDs can lead to a ~30% increase in short circuit current density (Jsc), a ~20% increase in power conversion efficiency (PCE), and prolonged recombination time constants compared to solar cells built from PbS QDs only
In both cases ligand-exchange was confirmed by Fourier transform infrared (FT-IR) spectroscopy which reveals new absorption signatures associated with the mercaptopropionic acid (MPA) molecules (Figure S2 and S3 in the supporting information)
Summary
Energy loss due to carrier recombination is among the major factors limiting the performance of TiO2/PbS colloidal quantum dot (QD) heterojunction solar cells. Due to the large surface-to-volume ratio in colloidal QDs, there can be abundant surface states in QD materials acting as recombination centers during charge transport[18,23,24,25,26,27,28,29] Under this context, the possibility to separate electrons and holes in different areas of the active layer, for example by using a mixture of different QDs, can lead to a substantial suppression of the recombination rates. The recently introduced bulk nano-heterojunction device configuration, where p-type PbS QDs in the active layer are blended together with n-type QDs of a different composition, allows for higher device efficiencies[14,17] In these devices the energy levels of the two QD components form a “type II” alignment leading to charge separation. Different volume fractions of Zn-CIS QDs in the PbS QD matrix were examined: a 10% (v/v) addition of Zn-CIS QDs can lead to a ~30% increase in short circuit current density (Jsc), a ~20% increase in power conversion efficiency (PCE), and prolonged recombination time constants compared to solar cells built from PbS QDs only
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