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43083-AEF
The Use of Size-Selective Excitation to Study Photocurrent through Junctions containing Single-Size and Multi-Size Arrays of Colloidal CdSe Quantum Dots
We measured the current produced by junctions incorporating arrays of colloidal CdSe QDs of a single size and of multiple sizes, with indium tin oxide (ITO) and eutectic Ga/In (EGaIn) electrodes, when these junctions were excited with various wavelengths of light. In the junctions containing multiple sizes of QDs, we could localize the photoexcitation in different parts of the junction (i.e., near the interface between the QDs and either PEDOT:PSS or EGaIn) by using light that was absorbed primarily by only one of the three sizes of QDs.
Our work suggests that, by constructing multiple junctions that have effectively indistinguishable absorption spectra but a different spatial arrangement of QDs, one can determine how the location of a particular QD within the array—that is, its proximity to an electrode, a complementary active material, or another QD of a different size—affects the contribution of its excited state to the observed photocurrent. Specifically, size-selective excitation in junctions incorporating multiple sizes of QDs allowed us to answer the following three questions:
(i) What is the location of the interface at which photoinduced charge separation (to create electrons and holes from excitons) occurred? We determined conclusively that, at V = 0 V, separation of charge at the interface between the QDs and PEDOT:PSS dominated the production of photocurrent. We believe that this mechanism dominated over the majority of the range of V we examined—V = 0 to +1 V—but we did not determine the value of V at which other mechanisms (hole conduction induced by separation of charge at the interface between the QDs and EGaIn, and ionization of excitons within the array of QDs) began to contribute.
(ii) Does the energy absorbed by the QDs redistribute before separation of charge? The photocurrent action (PCA) spectra of the junctions containing multiple sizes of QDs indicated that, when energetically favorable, excitons created away from the interface between the QDs and PEDOT:PSS traveled to this interface and split to create charge carriers. The inspection of the PCA spectra is a much more direct way to determine how energy is redistributed in the junction than is comparison of external quantum efficiencies (EQEs) between junctions. For instance, one would expect the EQE of LMS to be approximately three times that of SML with excitation at 565 nm, because excited states in all three layers are contributing to the photocurrent in LMS, while only excited states in the layer of S QDs is contributing to the photocurrent in SML. In fact, EQE(LMS)565 is only a factor of ~1.4 greater than EQE(SML)565, but this result may be attributed to several factors, including different rates of charge transfer across the interface between the QDs and PEDOT:PSS or between the QDs and EGaIn, or a different density of sites that trap charge in the two junctions (due to slight variation in conditions used to prepare or deposit the QDs). Both of these factors would affect the current through the device, but are independent of the tendency of energy to redistribute through the array prior to separation of charge.
(iii) How does the magnitude of the photovoltage depend on the locations of various sizes of QDs within the junction? The magnitude of the photovoltage (VOC under illumination) increased as the size of the QDs—and the gap between the energies of the LUMOs of the QDs and the valence band of PEDOT:PSS—at the interface between the QDs and PEDOT:PSS decreased. The photovoltage therefore appears to be proportional to the difference between the energy absorbed when an electron is promoted from the HOMO of the QD to the LUMO of the QD, and the energy lost when an electron is transferred from the valence band of PEDOT:PSS to the half-filled HOMO of the QD. This result supports our hypothesis that photocurrent is generated by charge transfer at the interface between the QDs and PEDOT:PSS.
The method of localizing excitation in an array of QDs of multiple sizes in order to identify the interface for separation of charge would be useful in analysis of a solar cell composed of QDs plus a p-type material (a heterojunction cell). Ideally, in a heterojunction cell, excitons migrate to the interface between the n-type and p-type materials and separate into electrons and holes. Use of localized excitation would determine whether another unfavorable process—for example, separation of charge at the interface between QDs and an electrode—were taking place, in which case a layer of a material that blocks the passage of electrons or holes might be incorporated in between the QDs and the electrodes to stop the unproductive quenching process.
