Mechanism and New Applications of Large and Persistent Photoconductivity

Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photo-generated minority carriers at crystal defects. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photo-excitation [1-2]. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-U model of LPPC [3]. We also show that LPPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to 10,000 seconds, by controlling the activities of Cd²⁺ and S^2- ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, non-equilibrium, photo-excited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polarons and organic dyes.
We then apply our knowledge of defect physics in CdS to design selectors for resistive processing units, and aqueous chemical sensors. We make two-terminal selector devices using hole injection layers to control the charge state of sulfur vacancies, and thereby modify the film conductivity. We make chemical sensors that take advantage of the sensitive dependence of photoconductivity on electron-hole recombination at interfaces. The resulting photoconductivity chemical sensors offer a path to low-cost chemical sensing that doesn’t rely on electrochemical charge transfer.
[1] S. B. Zhang, S.-H. Wei & A. Zunger, Phys. Rev. B 63, 075205 (2001).
[2] S. Lany & A. Zuner, Phys. Rev. B 72, 035215 (2005).
[3] H. Yin, A. Akey & R. Jaramillo, arXiv 1806.01894 (2018).
Bio: Rafael Jaramillo is an assistant professor in the Department of Materials Science and Engineering at MIT. His research sits in the big, fun space between materials science, solid state physics, and opto-electronic technologies. His current interests can be characterized as defect and phase engineering of chalcogenide semiconductors, with emphasis on developing processing methods to control sulfide and selenide thin films. Previously he worked as a postdoc at Harvard and at MIT on topics in oxide electronic materials and chalcogenide thin film solar cells. He earned his PhD from The University of Chicago for work on quantum phase transitions in antiferromagnets. His thesis work included the discovery of the pressure-driven magnetic quantum phase transition in antiferromagnetic chromium. Dr. Jaramillo is the recipient of numerous awards including the Rosalind Franklin Young Investigator Award from the Advanced Photon Source at Argonne National Laboratory, the Department of Energy SunShot Potdoctoral Fellowship, and the National Science Foundation Faculty Early Career Development Award (CAREER). He lives in Cambridge, MA with his wife and two young kids.