D. Aaron R. Barkhouse

Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins of Exceptional Air- and Light-Stability

We report colloidal quantum dot (CQDs) photovoltaics having a approximately 930 nm bandgap. The devices exhibit AM1.5G power conversion efficiencies in excess of 2%. Remarkably, the devices are stable in air under many tens of hours of solar illumination without the need for encapsulation. We explore herein the origins of this orders-of-magnitude improvement in air stability compared to larger PbS dots. We find that small and large dots form dramatically different oxidation products, with small dots forming lead sulfite primarily and large dots, lead sulfate. The lead sulfite produced on small dots results in shallow electron traps that are compatible with excellent device performance; whereas the sulfates formed on large dots lead to deep traps, midgap recombination, and consequent catastrophic loss of performance. We propose and offer evidence in support of an explanation based on the high rate of oxidation of sulfur-rich surfaces preponderant in highly faceted large-diameter PbS colloidal quantum dots.

Depleted Bulk Heterojunction Colloidal Quantum Dot Photovoltaics

The first solution-processed depleted bulk heterojunction colloidal quantum dot solar cells are presented. The architecture allows high absorption with full depletion, thereby breaking the photon absorption/carrier extraction compromise inherent in planar devices. A record power conversion of 5.5% under simulated AM 1.5 illumination conditions is reported.

Thiols passivate recombination centers in colloidal quantum dots leading to enhanced photovoltaic device efficiency.

The use of thiol-terminated ligands has recently been reported to enhance 10-fold the power conversion efficiency (PCE) of colloidal quantum dot (CQD) photovoltaic (PV) devices. We find herein that, in a representative amine-capped PbS colloidal quantum dot materials system, improved mobility following thiol treatment accounts for only a 1.4-fold increase in PCE. We then proceed to investigate the origins of the remainder of the quadrupling in PCE following thiol treatment. We find through measurements of photoluminescence quantum efficiency that exposure to thiols dramatically enhances photoluminescence in colloidal quantum dot films. The same molecules increase open-circuit voltage (V(oc)) from 0.28 to 0.43 V. Combined, these findings suggest that mid-gap states, which serve as recombination centers (lowering external quantum efficiency (EQE)) and metal-semiconductor junction interface states (lowering V(oc)), are substantially passivated using thiols. Through exposure to thiols, we improve EQE from 5 to 22% and, combined with the improvement in V(oc), improve power conversion efficiency to 2.6% under 76 mW/cm(2) at 1 microm wavelength. These findings are consistent with recent reports in photoconductive PbS CQD photodetectors that thiol exposure substantially removes deep (0.3 eV) electron traps, leaving only shallow (0.1 eV) traps.

Schottky quantum dot solar cells stable in air under solar illumination.

2010 WILEY-VCH Verlag Gmb Colloidal quantum dots (CQDs) solar cells offer great potential in solar energy conversion in view of their compatibility with solution processing, enabling rapid, large-area, low-cost fabrication. Compared with organic and polymer solar cells also benefiting from solution-processing, solar cells based on PbS, PbSe, and PbSSe CQDs access a greater portion of the sun’s spectrum in the infrared range through the use of low-bandgap PbS and PbSe nanoparticles. A specific solar cell architecture—a planar film of p-type colloidal quantum dots topped by a shallow-work-function contact, producing a Schottky barrier that generates a depletion region for carrier separation—has seen rapid recent progress. Monochromatic power conversion efficiencies (MPCE) have now reached 4.2% in the infrared and AM1.5G power conversion efficiencies (AM1.5G PCE) have reached 3.3%. This otherwise promising class of photovoltaics suffers amajor limitation: every report details a lack of stability in air, though different reasons have been given. The first high-efficiency reports employed butylamine capped PbS nanoparticles and degraded in air within minutes; the butylamine was suspected of reacting with the shallow-work-function metal contact. Passivating PbSe using 1,4-benzenedithiol led to devices stable in a glovebox over weeks, and in air over a few hours, a considerable improvement. Other reports using ethanedithiol (EDT) indicated that even minutes’ removal of the devices from a glovebox produced rapid degradation. Two general areas of possible degradation may be posited:

Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

Admittance spectra and drive-level-capacitance profiles of several high performance Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with bandgap ∼1.0–1.5 eV are reported. In contrast to the case for Cu(In,Ga)(S,Se)2, the CZTSSe capacitance spectra exhibit a dielectric freeze out to the geometric capacitance plateau at moderately low frequencies and intermediate temperatures (120–200 K). These spectra reveal important information regarding the bulk properties of the CZTSSe films, such as the dielectric constant and a dominant acceptor with energy level of 0.13–0.2 eV depending on the bandgap. This deep acceptor leads to a carrier freeze out effect that quenches the CZTSSe fill factor and efficiency at low temperatures.

Ambient-Processed Colloidal Quantum Dot Solar Cells via Individual Pre-Encapsulation of Nanoparticles

We report colloidal quantum dot solar cells fabricated under ambient atmosphere with an active area of 2.9 mm(2) that exhibit 3.6% solar power conversion efficiency. The devices are based on PbS tuned via the quantum size effect to have a first excitonic peak at 950 nm. Because the formation of native oxides and sulfates on PbS leads to p-type doping and deep trap formation and because such dopants and traps dramatically influence device performance, prior reports of colloidal quantum dot solar cells have insisted on processing under an inert atmosphere. Here we report a novel ligand strategy in which we first encapsulate the quantum dots in the solution phase with the aid of a strongly bound N-2,4,6-trimethylphenyl-N-methyldithiocarbamate ligand. This allows us to carry out film formation and all subsequent device fabrication under an air atmosphere.

Prospects and performance limitations for Cu–Zn–Sn–S–Se photovoltaic technology

While cadmium telluride and copper–indium–gallium–sulfide–selenide (CIGSSe) solar cells have either already surpassed (for CdTe) or reached (for CIGSSe) the 1 GW yr−1 production level, highlighting the promise of these rapidly growing thin-film technologies, reliance on the heavy metal cadmium and scarce elements indium and tellurium has prompted concern about scalability towards the terawatt level. Despite recent advances in structurally related copper–zinc–tin–sulfide–selenide (CZTSSe) absorbers, in which indium from CIGSSe is replaced with more plentiful and lower cost zinc and tin, there is still a sizeable performance gap between the kesterite CZTSSe and the more mature CdTe and CIGSSe technologies. This review will discuss recent progress in the CZTSSe field, especially focusing on a direct comparison with analogous higher performing CIGSSe to probe the performance bottlenecks in Earth-abundant kesterite devices. Key limitations in the current generation of CZTSSe devices include a shortfall in open circuit voltage relative to the absorber band gap and secondarily a high series resistance, which contributes to a lower device fill factor. Understanding and addressing these performance issues should yield closer performance parity between CZTSSe and CdTe/CIGSSe absorbers and hopefully facilitate a successful launch of commercialization for the kesterite-based technology.

Cd-free buffer layer materials on Cu2ZnSn(SxSe1−x)4: Band alignments with ZnO, ZnS, and In2S3

The heterojunctions formed between Cu2ZnSn(SxSe1−x)4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In2S3/CZTSSe system. The chemical bath deposited In2S3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In2S3/CZTSSe solar cells with 7.6% power conversion efficiency.

Depleted-heterojunction colloidal quantum dot photovoltaics employing low-cost electrical contacts

With an aim to reduce the cost of depleted-heterojunction colloidal quantum dot solar cells, we describe herein a strategy that replaces costly Au with a low-cost Ni-based Ohmic contact. The resultant devices achieve 3.5% Air Mass 1.5 power conversion efficiency. Only by incorporating a 1.2-nm-thick LiF layer between the PbS quantum dot film and Ni, we were able to prevent undesired reactions and degradation at the metal-semiconductor interface.

Dead zones in colloidal quantum dot photovoltaics: evidence and implications

In order to fabricate photovoltaic (PV) cells incorporating light-trapping electrodes, flexible foil substrates, or more than one junction, illumination through the top-contact (i.e.: non-substrate) side of a photovoltaic device is desirable. We investigate the relative collection efficiency for illumination through the top vs. bottom of PbS colloidal quantum dot (CQD) PV devices. The external quantum efficiency spectra of FTO/TiO₂/PbS CQD/ITO PV devices with various PbS layer thicknesses were measured for illumination through either the top (ITO) or bottom (FTO) contacts. By comparing the relative shapes and intensities of these spectra with those calculated from an estimation of the carrier generation profile and the internal quantum efficiency as a function of distance from the TiO₂ interface in the devices, a substantial dead zone, where carrier extraction is dramatically reduced, is identified near the ITO top contact. The implications for device design, and possible means of avoiding the formation of such a dead zone, are discussed.