Charge-Transport Mechanisms in CuInSe xS2- x Quantum-Dot Films

Hyeong Jin Yun; Jaehoon Lim; Addis S Fuhr; Nikolay S Makarov; Sam Keene; Matt Law; Jeffrey M Pietryga; Victor I Klimov

doi:10.1021/acsnano.8b07179

Charge-Transport Mechanisms in CuInSe _xS_{2- x} Quantum-Dot Films

ACS Nano. 2018 Dec 26;12(12):12587-12596. doi: 10.1021/acsnano.8b07179. Epub 2018 Dec 4.

Authors

Hyeong Jin Yun¹, Jaehoon Lim^{1

2}, Addis S Fuhr^{1

3}, Nikolay S Makarov^{1

4}, Sam Keene⁵, Matt Law⁵, Jeffrey M Pietryga¹, Victor I Klimov¹

Affiliations

¹ Chemistry Division , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States.
² Department of Chemical Engineering and Department of Energy System Research , Ajou University , Suwon 16499 , Republic of Korea.
³ Department of Chemical and Biomolecular Engineering , University of California , Los Angeles , California 90095 , United States.
⁴ UbiQD, Inc. , 134 East Gate Drive , Los Alamos , New Mexico 87544 , United States.
⁵ Department of Chemistry and Department of Chemical Engineering and Materials Science , University of California , Irvine , California 92697 , United States.

PMID: 30495927
DOI: 10.1021/acsnano.8b07179

Abstract

Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSe _xS_{2- x} or CISeS) QDs are particularly attractive as an environmentally benign alternative to the much more extensively studied QDs containing toxic metals such as Cd and Pb. Carrier transport properties of CISeS-QD films, however, are still poorly understood. Here, we aim to elucidate the factors that control charge conductance in CISeS QD solids and, based on this knowledge, develop practical approaches for controlling the polarity of charge transport and carrier mobilities. To this end, we incorporate CISeS QDs into field-effect transistors (FETs) and perform detailed characterization of these devices as a function of the Se/(Se+S) ratio, surface treatment, thermal annealing, and the identity of source and drain electrodes. We observe that as-synthesized CuInSe _xS_{2- x} QDs exhibit degenerate p-type transport, likely due to metal vacancies and Cu_In^'' anti-site defects (Cu¹⁺ on an In³⁺ site) that act as acceptor states. Moderate-temperature annealing of the films in the presence of indium source and drain electrodes leads to switching of the transport polarity to nondegenerate n-type, which can be attributed to the formation of In-related defects such as In_Cu^•• (an In³⁺ cation on a Cu¹⁺ site) or In_i^••• (interstitial In³⁺) acting as donors. We observe that the carrier mobilities increase dramatically (by 3 orders of magnitude) with increasing Se/(Se+S) ratio in both n- and p-type devices. To explain this observation, we propose a two-state conductance model, which invokes a high-mobility intrinsic band-edge state and a low-mobility defect-related intragap state. These states are thermally coupled, and their relative occupancies depend on both QD composition and temperature. Our observations suggest that the increase in the relative fraction of Se moves conduction- and valence band edges closer to low-mobility intragap levels. This results in increased relative occupancy of the intrinsic band-edge states and a corresponding growth of the measured mobility. Further improvement in charge-transport characteristics of the CISeS QD samples as well as their stability is obtained by infilling the QD films with amorphous Al₂O₃ using atomic layer deposition.

Keywords: CuInSexS2−x quantum dots; atomic layer deposition; charge-carrier mobility; charge-carrier transport; field-effect transistor; n- and p-type.