Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films Jing Zhao, Benjamin A. Nail, Michael A. Holmes, and Frank E. Osterloh* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States * S Supporting Information ABSTRACT: Surface photovoltage spectroscopy (SPS) was used to study the photochemistry of mercaptoethanol-ligated CdSe quantum dot (2.0-4.2 nm diameter) films on indium doped tin oxide (ITO) in the absence of an external bias or electrolyte. The n-type films generate negative voltages under super band gap illumination (0.1-0.5 mW cm -2 ) by majority carrier injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (-125 to -750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe-ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron-hole (polaron) pairs that are polarized by a space charge layer at the CdSe-ITO boundary. The space charge layer is 70-150 nm wide, based on thickness-dependent photovoltage measurements. The ability of SPS to directly measure built-in voltages, space charge layer thickness, sub-band gap states, and effective band gaps in drop-cast quantum dot films aids the understanding of photochemical charge transport in quantum dot solar cells. T he quantum size effect is the basis for applications of CdSe quantum dots in third-generation solar cells, 1-6 photoelectrochemical cells, 7-10 photocatalysts, 11-15 and fluo- rescent labels. The size-dependent energetics of quantum dots control photochemical charge transfer, 16-25 photovolt- age, 19,26-30 and hydrogen evolution. 31-34 Quantum dot solar cells employ nanocrystals as films sandwiched between electron- or hole-selective materials. 4,35 In hybrid solar cells, 3,36 the dots are mixed with organic polymers to aid light absorption and charge separation. Photochemical charge separation, transport, and recombination are key to the performance of these devices, but the details of these processes are often not fully understood. 6,35-37 Also, usually fully assembled devices are required for a characterization of these processes. 38 Here we employ surface photovoltage spectroscopy (SPS) to observe the intrinsic photochemistry of CdSe nanocrystals in easy to fabricate drop-cast films in the absence of an external bias or added redox reagents. SPS is a contactless technique that probes contact potential difference changes (ΔCPD) in semiconductors, molecular light absorbers, 39,40 and nanocryst- als 40-42 upon excitation with light (Figure 1). 43,44 The sensitivity of SPS is much higher than that of photo- electrochemistry, 45 thus allowing the detection of majority carrier type, 46 mid-gap states, 47,48 defects, 49 and electro- chemical reactions at interfaces. 50 Measurements are typically performed in vacuum on sample films deposited onto metallic or semiconducting substrates. Earlier SPS studies on CdSe nanocrystal films by Hodes et al. observed photovoltage influenced by the size of the dots, 29 their chemical history, and the ambient environment. 28 Photochemical charge separation in such films was attributed to “slanted bands” across the nanocrystal films, in analogy to the space charge layer in solid semiconductors. Differences in hole and electron diffusion rates were attributed to trap sites on the surface of the dots. This diffusive transport was described as a special case of a Dember effect. 51 Our measurements show that photovoltages in CdSe quantum dot (QD) films on indium tin oxide (ITO) Received: July 17, 2016 Accepted: August 9, 2016 Figure 1. (A) Schematic configuration used for SPS measurement and (B) example spectra for electron and hole injection into the FTO substrate. Letter pubs.acs.org/JPCL © XXXX American Chemical Society 3335 DOI: 10.1021/acs.jpclett.6b01569 J. Phys. Chem. Lett. 2016, 7, 3335-3340