Schottky Solar Cells Based on Colloidal Nanocrystal Films

Abstract

We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport. Figure 1 shows the structure, current-voltage performance, EQE spectrum, and proposed band diagram of our device. Device fabrication consists of depositing a 60-300 nm-thick film of monodisperse, spheroidal PbSe NCs onto patterned indium tin oxide (ITO) coated glass using a layer-by-layer dip coating method, followed by evaporation of a top metal contact. In this LbL method, 1 a layer of NCs is deposited onto the ITO surface by dip coating from a hexane solution and then washed in 0.01 M 1,2-ethanedithiol (EDT) in acetonitrile to remove the electrically insulating oleate ligands that originally solubilize the NCs (see Supporting Information). Large-area, crack-free and mildly conductive (σ ) 5 × 10 -5 S cm -1 ) NC films result. The NCs pack randomly in the films, are partially coated in adsorbed ethanedithiolate, and show p-type conductivity under illumination. 1 X-ray diffraction and optical absorption spectroscopy established that the NCs neither ripen nor sinter in response to EDT exposure. We have found that using methylamine instead of EDT yields similar device performance (Supporting Information, Figure 1). 2 We have also fabricated working devices from PbS and CdSe NCs (Supporting Information, Figures 2 and 3), which indicates that the approach adopted here is not restricted to EDT-treated PbSe NCs and that it should be possible to improve cell efficiency by engineering the surface of the NCs to attain longer carrier diffusion lengths and higher photovoltages through surface state passivation and prevention of Fermi level pinning.When tested in nitrogen ambient under simulated 1-sun test conditions (100 ( 5 mW cm -2 ELH white light illumination), EDT-treated PbSe devices exhibit large shortcircuit photocurrent densities (J SC ) and modest open-circuit voltages (V OC ) and fill factors (FF), with one of the most efficient devices yielding J SC ) 24.5 mA cm -2 , V OC ) 239 mV, FF ) 0.41 and a mismatch-corrected 3 AM1.5G efficiency of 2.1% (Figure 1a; see Supporting Information regarding spectral mismatch). The mismatch-corrected J SC values of these devices are reproducibly larger than those of other nanostructured solar cells, including the best organic 4 and dye-sensitized devices, 5 which is remarkable considering the unsintered, glassy microstructure of our NC films and the fact that the NCs retain quantum confinement...

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

“…This is in agreement with the previously observed p-type doping of EDT-treated lead chalcogenide QDs. 28,29 Here we focus on another way to measure the energy level of IGS, KPFM, 26,30 which probes the change of surface potential as a function of gate bias in a QD monolayer FET. Since the occupation of states is electrostatically tuned by the gate bias in the FET, we can use KPFM to extract the density of states.…”

Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines

“…After synthesis, they can be exchanged by others, and this has proven to be a powerful method to functionalize colloidal NPs, with the aim of, for example, raising their photoluminescence quantum yield, 1 rendering them biocompatible and bioselective 2,3 or enhancing their conductivity for applications in electronics 4,5 and photovoltaics. 6 In line with the increasing importance of NP ligands, a number of studies have addressed the interaction between ligands and NPs, and at the same time, a number of experimental techniques like nuclear magnetic resonance spectroscopy (NMR), 7-10 infrared spectroscopy, 11 or photoluminescence spectroscopy 1,12-14 were developed to investigate ligands bound to colloidal NPs.An interesting model system in this respect is CdSe quantum dots (Q-CdSe) stabilized by alkylamines. It has been shown by a number of authors that alkylamines strongly enhance the Q-CdSe photoluminescence (PL).…”

The investigation of the influence of back contact metal and ligand exchanging on the air stability of ambient processed schottky Photovoltaic Cells Based on PbSe CQDs

“…13 Among the CQD materials, lead sulfide (PbS) and lead selenide (PbSe) CQDs have been the focus of research efforts, because of their optimal band gap and excellent photosensitivity. [14][15][16][17] Also, because of the low energy band gaps, they are able to absorb larger portions of solar power in the infrared region. [18][19] Different solar cell structures including schottkies and p-n junctions have been fabricated and tested.…”

“…so that, butylamine capped PbS CQDs degraded within minutes of exposing to air. 21 Later, Luther et al 17 described a schottky barrier photovoltaic cell consisting of oleic acid capped PbSe nanocrystals (NCs) that produced an exceptionally large shortcircuit (I sc ) photocurrent (>21 mA/Cm 2 ) reaching 2.1% of PCE. They treated nanocrystalline film with ethanedithiol (EDT) to remove the electrically insulating oleate ligands.…”