4.8 Article

Charge-Transport Mechanisms in CuInSexS2-x Quantum-Dot Films

Journal

ACS NANO
Volume 12, Issue 12, Pages 12587-12596

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acsnano.8b07179

Keywords

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

Funding

  1. Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center - U.S. Department of Energy (DOE), Office of Science (OS), Basic Energy Sciences (BES)
  2. Solar Photochemistry Program of the Chemical Sciences, Biosciences and Geosciences Division, BES, OS, DOE
  3. UC Office of the President under the UC Laboratory Fees Research Program Collaborative Research and Training Award [LFR-17-477148]
  4. African American Partnership Program at Los Alamos National Laboratory

Ask authors/readers for more resources

Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSexS2-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 CuInSexS2-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-Cn (an In3+ cation on a Cu1+ site) or In-i center dot center dot center dot (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.

Authors

I am an author on this paper
Click your name to claim this paper and add it to your profile.

Reviews

Primary Rating

4.8
Not enough ratings

Secondary Ratings

Novelty
-
Significance
-
Scientific rigor
-
Rate this paper

Recommended

No Data Available
No Data Available