Letter pubs.acs.org/NanoLett
Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics Paul H. Rekemeyer,† Chia-Hao M. Chuang,†,‡ Moungi G. Bawendi,‡ and Silvija Gradečak*,† †
Department of Materials Science and Engineering and ‡Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, Massachusetts 02141, United States S Supporting Information *
ABSTRACT: Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beaminduced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p−n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n+−n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths. KEYWORDS: Electron beam induced current, quantum dots, PbS, diffusion length, photovoltaics
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be used to increase the optical density, yet the nanowire bulk heterojunction reduces the length that carriers must be transported before being collected. The optimal design of an ordered bulk heterojunction device requires knowledge of the minority carrier diffusion length. Field effect transistor (FET) measurements18,19 can determine carrier mobility, yet the bias-stress effect20 and Fermi level pinning21 are issues known to affect the validity of FET measurement of carrier mobility in lead chalcogenide QD films. Furthermore, the high resistivity of PbS QD films makes Hall effect measurements of the mobility difficult and yields the majority carrier mobility, rather than the desired minority carrier mobility. Optical methods of determining the carrier diffusion length have been developed,22,23 but these measurements require simplified architectures that do not capture the effects of all processing steps in the highest-efficiency device architecture, which utilizes multiple ligand exchange processes. 7,24 By contrast, electron beam-induced current (EBIC)25 allows for the direct visualization of minority carrier collection, even in complex device geometries, and enables quantification of the carrier diffusion length.26−28 Using EBIC
olloidal quantum dots (QDs) are emerging active materials for a variety of optoelectronic devices, including light-emitting diodes,1−3 photodetectors,4 and lasers,5 due to their direct band gaps and facile deposition from solution. In particular, lead sulfide (PbS) QDs are promising for singlejunction6,7 and tandem8 photovoltaics (PV) applications due to a band gap that can be size-tuned from 0.41 to 1.6 eV,8,9 with devices achieving certified power conversion efficiency exceeding 11%.10 Recent advances in PbS QD PV include the use of ligand exchange treatments to form junctions between the QD layers7,11,12 and passivation strategies to reduce trap state density13 and increase the minority carrier diffusion length.14 However, even the best passivation strategies still yield QD films with carrier diffusion lengths that are much shorter ( Da, which indicates that the lower bound for the bulk hole diffusion length is Lh,bulk > 110 nm. It should also be noted that under high-level injection conditions the large number of free carriers reduces mobility due to electron−hole scattering.47,48 The injection level under 1-sun AM1.5G illumination is about 5 × 1016 cm−3, which is at least 35 times lower than the injection level under EBIC measurement conditions (see Supporting Information), so the hole diffusion length at injection levels relevant to solar cells could be even greater than 110 nm. To summarize, contrary to the conventional device model, it is shown that the PbS-TBAI/PbS-EDT junction, rather than the ZnO/PbS-TBAI junction, is the dominant rectifying junction, indicating an n+−n−p device structure, with holes as the minority carrier in PbS-TBAI films. We show that, by using low-energy electron beams (E0 ≤ 10 keV), the electron− sample interaction volume is sufficiently small such that that the nanoscale diffusion length in PbS QD films can be resolved. Monte Carlo simulations of electron trajectories within the PbS QD film indicate that the conventional analytical form for the EBIC carrier generation does not accurately describe the interaction volume, which is a critical consideration in quantitative EBIC measurements of materials systems with short diffusion lengths. By developing an axisymmetric generation model, we were able to accurately back out the carrier collection profile. By measuring the effective diffusion length at different beam energies, we were able to extrapolate the lower bound for the bulk hole diffusion length in PbS-TBAI films as Lh,bulk > 110 nm, which represents the first use of quantitative EBIC to study charge transport in PbS QD films. This quantitative result and the qualitative finding that the PbSTBAI film is n-type inform crucial design rules to improve QD solar cell efficiency. Finally, the low-energy EBIC technique and the general analytical framework developed in this work can readily be applied to study minority carrier transport in operando in other emerging thin-film PV materials with nanoscale diffusion lengths.
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Letter
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers Center. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR1419807. The authors gratefully acknowledge the Organic and Nanostructured Electronics Laboratory (ONE Lab) at MIT for the use of fabrication and testing facilities and O. Hentz for assistance with device fabrication.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02916. Experimental methods, EBIC map showing a weak junction at the ZnO/PbS-TBAI interface, more on design rules for QD PV devices, details of Monte Carlo simulations, details of quantitative EBIC analysis, estimation of injected carrier density, dark I−V curve of a device after cross-section preparation, degradation of QD film under electron beam excitation (PDF) 6226
DOI: 10.1021/acs.nanolett.7b02916 Nano Lett. 2017, 17, 6221−6227
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