Study of Electronic Defects in CdSe Quantum Dots and Their Involvement in Quantum Dot Solar Cells Ruben Loef,* ,† Arjan J. Houtepen, † Elise Talgorn, † Joop Schoonman, ‡ and Albert Goossens † Opto-Electronic Materials and Materials for Energy ConVersion and Storage, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received December 11, 2008; Revised Manuscript Received January 15, 2009 ABSTRACT To enhance efficiencies of quantum dot CdSe/TiO 2 based solar cells, understanding of the space charge at the CdSe/TiO 2 interface is crucial. In this paper, the presence of a shallow acceptor in the CdSe quantum dots is found by means of a detailed impedance and Mott-Schottky (C -2 -O) study. Furthermore, it is clearly shown that this acceptor density decreases strongly with increasing quantum dot size. The presence of these defect states may give rise to Auger recombination in small quantum dots and therewith decrease the efficiency of quantum-dot- sensitized solar cells. One of the main reasons for the growing interest in quantum dots is their use in cheap solar cells, which have the possibility to increase the thermodynamic conversion ef- ficiency above the Shockley-Queisser limit. 1 Of significant practical interest are solar cells based on sensitization of nanostructured TiO 2 , ZnO, and SnO 2 with quantum dots, 2-9 although other configurations are suggested as well. 1,10-13 Efficiencies up to 1.7% are reported for TiO 2 nanoparticles sensitized with CdSe quantum dots. 14 This efficiency should clearly be enhanced, but such improvement is hampered by the lack of understanding of the space charge at the quantum dot/metal oxide interface. To date, little is known about the space charge formation at quantum dot CdSe/metal-oxide junctions. Impedance spectroscopy (IS) has been applied on quantum dot CdSe/ Au heterojunctions to determine the nature of the electrical bistability of the system. 15 Furthermore, quantum dot CdS/ Au heterojunctions have been studied by capacitance-voltage (C-φ) characterization. For n-type CdS quantum dots, donor densities of (0.35-7.7) × 10 14 cm -3 for 3-5 nm particles have been reported. 16 Both studies suggest the presence of surface/interface states. In the present paper we study quantum dot CdSe/TiO 2 heterojunctions, by means of current-voltage, IS, and C-φ measurements at different temperatures. We unambiguously find that CdSe quantum dots show p-type behavior, due to the presence of shallow electron acceptor states. Furthermore, we are able to conclude that the acceptor density decreases with increasing quantum dot diameter, which plays a role in the decreasing efficiences of quantum-dot-sensitized solar cells with decreasing particle size. 2 CdSe quantum dots are prepared following a “Greener” synthesis as described by Mekis et al. 17 Samples at different time intervals are taken during crystal growth to obtain quantum dots with different sizes. Quantum dot diameters, D QD , are determined from the maximum of the first peak of the absorption spectra. These peak values are compared with absorption peak maxima reported earlier for particles with known diameter. 18-24 The results are summarized in Table 1. The quantum dot density, N QD , in the quantum dot film is calculated according to N QD ) number of quantum dots film volume (1) In close-packed quantum dot films, either face-centered cubic (fcc) and hexagonal close-packed (hcp), it is known * Corresponding author, r.loef@tudelft.nl. † Opto-Electronic Materials. ‡ Materials for Energy Conversion and Storage. Table 1. Overview of the Samples a absorption peak (nm) absorption peak (eV) D QD (nm) N QD,max (cm -3 ) 497 2.49 2.4 8.9 × 10 18 508 2.44 2.6 8.1 × 10 18 514 2.41 2.7 7.7 × 10 18 516 2.40 2.7 7.6 × 10 18 534 2.32 3.1 6.4 × 10 18 554 2.23 3.5 5.1 × 10 18 a The quantum dot diameter (without TOPO/HDA capping), D QD , is determined from the maximum of the first peak of the absorption spectra. The maximum quantum dot density, N QD , max , is calculated for a close- packed quantum dot layer according to eq 2. NANO LETTERS 2009 Vol. 9, No. 2 856-859 10.1021/nl803738q CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009