Surface States and Photovoltaic Effects in CdSe Quantum Dot Films

Abstract

Photovoltaic effects in CdSe quantum dot (QD) films have been studied using surface photovoltage spectroscopy and complementary methods. The results show that, contrary to previous studies, nonnegligible electric fields can exist in QD films. As a result, driftlike currents must be considered, in addition to the well-known diffusionlike currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusionlike transport. The latter is highly influenced by preferential trapping of one type of charge carrier. The preferential trapping is shown to be surface localized and is strongly ambient dependent. It is shown that the photovoltaic properties of these CdSe QD films are dominated by their surface state distribution.

Surface photovoltage phenomena: theory, experiment, and applications

“…Second, etched CdSe quantum dot ®lms were found to exhibit a p-type response in a humid ambient and an n-type response in a dry ambient, as shown in Fig. 45 [380]. We note that the SPV onset in Fig.…”

“…The second knee therefore indicates the partial amorphization of the studied ®lms. More recently, Tong et al [368] and Kronik et al [380] have used SPS to monitor the well-known quantum size effect (i.e., the increase of the bandgap with decreasing crystallite size) at nanocrystalline CdS and CdSe quantum dot ®lms, respectively.…”

“…45. SPV spectra of a 4.5 nm crystallite size, CdSe quantum dot ®lm after the following sequence of treatments: (i) asdeposited, (ii) after etching, (iii) in a dry ambient, (iv) in a humid ambient, and (v) in a dry ambient ± second cycle (after Kronik et al [380]). …”

Apparent Semiconductor Type Reversal in Anatase TiO2 Nanocrystalline Films

“…A water-mediated switching between n-type and apparent p-type SPV behavior with superband gap irradiation, similar to that reported here, has been observed for etched CdSe quantum dot (QD) particle films. 30 In their nonetched state, the QDs displayed the expected n-type SPV spectra. However, upon etching the QDs exhibited p-typelike behavior under wet nitrogen ambient, but n-typelike spectra were obtained for the same particles under a dry ambient.…”

“…It is known that the carrier transport mechanism in TiO 2 particle films is influenced by particle size. [30][31][32][33][34] In large particles, there is a deep space charge (electron depletion) layer and carrier transport is driftlike due to the potential drop at the surface or at grain boundaries. However, in small particles, there is little band bending and the potential gradient experienced by the charge carriers is small.…”