TY - JOUR
T1 - Charge-Transport Mechanisms in CuInSe x S 2-x Quantum-Dot Films
AU - Yun, Hyeong Jin
AU - Lim, Jaehoon
AU - Fuhr, Addis S.
AU - Makarov, Nikolay S.
AU - Keene, Sam
AU - Law, Matt
AU - Pietryga, Jeffrey M.
AU - Klimov, Victor I.
N1 - Funding Information:
J.M.P, M.L., S.K., N.S.M., and J.L. were supported by the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science (OS), Basic Energy Sciences (BES). V.I.K. acknowledges support by the Solar Photochemistry Program of the Chemical Sciences, Biosciences and Geosciences Division, BES, OS, DOE. H.J.Y. was supported by the UC Office of the President under the UC Laboratory Fees Research Program Collaborative Research and Training Award LFR-17-477148. A.F. acknowledges support by the African American Partnership Program at Los Alamos National Laboratory.
PY - 2018/12/26
Y1 - 2018/12/26
N2 - Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSe x S 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 x S 2-x QDs exhibit degenerate p-type transport, likely due to metal vacancies and Cu In '' anti-site defects (Cu 1+ on an In 3+ 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 In 3+ cation on a Cu 1+ site) or In i • • • (interstitial In 3+ ) 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 Al 2 O 3 using atomic layer deposition.
AB - Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSe x S 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 x S 2-x QDs exhibit degenerate p-type transport, likely due to metal vacancies and Cu In '' anti-site defects (Cu 1+ on an In 3+ 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 In 3+ cation on a Cu 1+ site) or In i • • • (interstitial In 3+ ) 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 Al 2 O 3 using atomic layer deposition.
KW - CuInSe S quantum dots
KW - atomic layer deposition
KW - charge-carrier mobility
KW - charge-carrier transport
KW - field-effect transistor
KW - n- and p-type
UR - http://www.scopus.com/inward/record.url?scp=85059529023&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85059529023&partnerID=8YFLogxK
U2 - 10.1021/acsnano.8b07179
DO - 10.1021/acsnano.8b07179
M3 - Article
C2 - 30495927
AN - SCOPUS:85059529023
VL - 12
SP - 12587
EP - 12596
JO - ACS Nano
JF - ACS Nano
SN - 1936-0851
IS - 12
ER -