Charge-Transport Mechanisms in CuInSe_xS_2–x Quantum-Dot Films

Journal Article · Thu Nov 29 00:00:00 EST 2018 · ACS Nano

DOI:https://doi.org/10.1021/acsnano.8b07179· OSTI ID:1489972

Yun, Hyeong Jin ^[1]; Lim, Jaehoon  ^[2]; Fuhr, Addis S. ^[3]; Makarov, Nikolay S. ^[4]; Keene, Sam ^[5]; Law, Matt ^[5]; Pietryga, Jeffrey M.  ^[1]; Klimov, Victor I.  ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Ajou University, Suwon (Korea)
3. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Univ. of California, Los Angeles, CA (United States)
4. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); UbiQD, Inc., Los Alamos, NM (United States)
5. Univ. of California, Irvine, CA (United States)

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 CuIn’’ 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. Here, 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.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Center for Advanced Solar Photophysics (CASP); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: 89233218CNA000001
• OSTI ID: 1489972
• Report Number(s): LA-UR-18-31559
• Journal Information: ACS Nano, Vol. 12, Issue 12; ISSN 1936-0851
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY