Randy J. Ellingson

Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells

Here, we will first briefly summarize the general principles of QD synthesis using our previous work on InP as an example. Then we will focus on QDs of the IV-VI Pb chalcogenides (PbSe, PbS, and PbTe) and Si QDs because these were among the first QDs that were reported to produce multiple excitons upon absorbing single photons of appropriate energy (a process we call multiple exciton generation (MEG)). We note that in addition to Si and the Pb-VI QDs, two other semiconductor systems (III-V InP QDs(56) and II-VI core-shell CdTe/CdSe QDs(57)) were very recently reported to also produce MEG. Then we will discuss photogenerated carrier dynamics in QDs, including the issues and controversies related to the cooling of hot carriers and the magnitude and significance of MEG in QDs. Finally, we will discuss applications of QDs and QD arrays in novel quantum dot PV cells, where multiple exciton generation from single photons could yield significantly higher PV conversion efficiencies.

Schottky Solar Cells Based on Colloidal Nanocrystal Films

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.

Quantum Dot Size Dependent J - V Characteristics in Heterojunction ZnO/PbS Quantum Dot Solar Cells

The current-voltage (J-V) characteristics of ZnO/PbS quantum dot (QD) solar cells show a QD size-dependent behavior resulting from a Schottky junction that forms at the back metal electrode opposing the desirable diode formed between the ZnO and PbS QD layers. We study a QD size-dependent roll-over effect that refers to the saturation of photocurrent in forward bias and crossover effect which occurs when the light and dark J-V curves intersect. We model the J-V characteristics with a main diode formed between the n-type ZnO nanocrystal (NC) layer and p-type PbS QD layer in series with a leaky Schottky-diode formed between PbS QD layer and metal contact. We show how the characteristics of the two diodes depend on QD size, metal work function, and PbS QD layer thickness, and we discuss how the presence of the back diode complicates finding an optimal layer thickness. Finally, we present Kelvin probe measurements to determine the Fermi level of the QD layers and discuss band alignment, Fermi-level pinning, and the V(oc) within these devices.

n-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer.

Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 (+/-0.2) at a photon energy of approximately 4E(g) for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (+/-0.1) at hnu approximately 5E(g) for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.

Fabrication of Efficient Low-Bandgap Perovskite Solar Cells by Combining Formamidinium Tin Iodide with Methylammonium Lead Iodide

Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn-Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). The best-performing cell fabricated using a (FASnI3)0.6(MAPbI3)0.4 absorber with an absorption edge of ∼1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799) V, a short-circuit current density of 26.86(26.82) mA/cm(2), and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. The average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility.

Recombination lifetime of In0.53Ga0.47As as a function of doping density

We have fabricated devices with the structure InP/In0.53Ga0.47As/InP, with a InGaAs doping range varying from 2×1014 to 2×1019 cm−3. These isotype double heterostructures were doped both n and p type and were used to measure the minority-carrier lifetime of InGaAs over this doping range. At the low doping end of the series, recombination is dominated by the Shockley–Read–Hall effect. At the intermediate doping levels, radiative recombination is dominant. At the highest doping levels, Auger recombination dominates as the lifetime varies with the inverse square of the doping concentration. From fitting these data, the radiative- and Auger-recombination coefficients are deduced.

High-power, high-repetition-rate femtosecond pulses tunable in the visible

We demonstrate a Ti:sapphire-pumped intracavity-doubled optical parametric oscillator (OPO) that generates a total of up to 240 mW of sub-100-fs pulses tunable in the visible. The OPO consists of a 1.5-mm-thick KTiPO(4) (KTP) crystal configured in a ring cavity that is synchronously pumped by a self-mode-locked Ti:sapphire laser operating at an 81-MHz repetition rate and 2.1-W average power, producing 115-fs pulses at lambda = 790 nm. Intracavity doubling of the OPO is accomplished by inserting a 47-microm-thick beta-BaB(2)O(4) crystal into an additional focus in the OPO cavity. We demonstrate continuous tuning of the second-harmonic output from 580 to 657 nm. The potential tuning range of this intracavity-doubled KTP OPO is approximately 500 to 800 nm.

Recent advances of the Ti:sapphire-pumped high-repetition-rate femtosecond optical parametric oscillator

The details concerning the resonator configuration, crystal parameters, and operating characteristics of high-repetition-rate and high-average-power broadly tunable femtosecond optical parametric oscillators are reviewed and discussed in some detail. We also report new results on an intracavity-doubled optical parametric oscillator with tunability from 580 to 657 nm in the visible and the first, to our knowledge, high-repetition-rate femtosecond optical parametric oscillator with the new nonlinear-optical crystal In:KTiOAsO4, which can potentially tune to 5.3 μm.

Improving the Performance of Formamidinium and Cesium Lead Triiodide Perovskite Solar Cells using Lead Thiocyanate Additives

Formamidinium lead triiodide (FAPbI3 ) is considered as an alternative to methylammonium lead triiodide (MAPbI3 ) because of its lower band gap and better thermal stability. However, owing to the large size of FA cations, it is difficult to synthesize high-quality FAPbI3 thin films without the formation of an undesirable yellow phase. Smaller sized cations, such as MA and Cs, have been successfully used to suppress the formation of the yellow phase. Whereas FA and MA lead triiodide perovskite solar cells (PVSCs) have achieved power conversion efficiencies (PCEs) higher than 20 %, the PCEs of formamidinium and cesium lead triiodide (FA1-x Csx PbI3 ) PVSCs have been only approximately 16.5 %. Herein, we report our examination of the main factors limiting the PCEs of (FA1-x Csx PbI3 ) PVSCs. We find that one of the main limiting factors could be the small grain sizes (≈120 nm), which leads to relatively short carrier lifetimes. We further find that adding a small amount of lead thiocyanate [Pb(SCN)2 ] to the precursors can enlarge the grain size of (FA1-x Csx PbI3 ) perovskite thin films and significantly increase carrier lifetimes. As a result, we are able to fabricate (FA1-x Csx PbI3 ) PVSCs with significantly improved open-circuit voltages and fill factors and, therefore, enhanced PCEs. With an optimal 0.5 mol % Pb(SCN)2 additive, the average PCE is increased from 16.18±0.50 (13.45±0.78) % to 18.16±0.54 (16.86±0.63) % for planar FA0.8 Cs0.2 PbI3 PVSCs if measured under reverse (forward) voltage scans. The champion cell registers a PCE of 19.57 (18.12) % if measured under a reverse (forward) voltage scan, which is comparable to that of the best-performing MA-containing planar FA-based lead halide PVSCs.