K. Xerxes Steirer

National Renewable Energy Laboratory | NREL · Materials Science Center

Metal halide perovskite materials (MHPs) are a family of next-generation semiconductors that are enabling low-cost, high-performance solar cells and optoelectronic devices. The most-used halogen in MHPs, iodine, can supplement its octet by covalent bonding resulting in atomic charges intermediate to I− and I0. Here, we examine theoretically stabili...

Using time-resolved in situ X-ray photoelectron spectroscopy, we identify and suppress rapid degradation mechanisms for cesium-stabilized formamidinium lead iodide perovskite materials used in state-of-the-art photovoltaics. Accelerated degradation under high light intensity and heating reveals a photocatalytic reaction pathway responsible for rapi...

Solid-state electrolytes such as Li 2 S-P 2 S 5 compounds are promising materials that could enable Li metal anodes. However, many solid-state electrolytes are unstable against metallic lithium, and little is known about the chemical evolution of these interfaces during cycling, hindering the rational design of these materials. In this work, operan...

Achieving solar-to-hydrogen efficiencies above 15% is key for the commercial success of photoelectrochemical water-splitting devices. While tandem cells can reach those efficiencies, increasing the catalytic activity and long-term stability remains a significant challenge. Here we show that annealing a bilayer of amorphous titanium dioxide (TiO x)...

Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water oxidation and proton reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak inte...

Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part due to predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that this enabling material property is present in the halide-lead perovskite, CH3NH3PbI3 (MAPbI3), consistent with theo...

This work focuses on the role of humidity in the formation of ZnO thin films from a reactive diethylzinc precursor solution for use as the electron contact layer (ECL) in organic photovoltaic (OPV) devices. This method is well suited for flexible devices because the films are annealed at 120 °C, making the process compatible with polymer substrates...

Ex situ catalytic fast pyrolysis (CFP) is a promising route for producing fungible biofuels; however, this process requires bifunctional catalysts that favor C-O bond cleavage, activate hydrogen at near atmospheric pressure and high temperature (350-500 °C), and are stable under high-steam, low hydrogen-to-carbon environments. Recently, early trans...

Zinc Oxysulfide (ZnOS) has demonstrated potential in the last decade to replace CdS as a buffer layer material since it is a wide-band-gap semiconductor with performance advantages over CdS (E-g = 2.4 eV) in the near UV-range for solar energy conversion. However, questions remain on the growth mechanisms of chemical bath deposited ZnOS. In this stu...

Producing hydrogen through solar water splitting requires the coverage of large land areas. Abundant metal-based molecular catalysts offer scalability, but only if they match noble metal activities. We report on a highly active p-GaInP2 photocathode protected through a 35-nm TiO2 layer functionalized by a cobaloxime molecular catalyst (GaInP2-TiO2-...

We report on the remarkable stability of unmodified, epitaxially grown GaAs photocathodes during hydrogen evolution at-15 mA cm-2 in 3 M sulfuric acid electrolyte. Contrary to the perception regarding instability of III-V photoelectrodes, results here show virtually no performance degradation and minimal etching after 120 hours.

The p-type semiconductor GaInP2 has a nearly ideal bandgap (~1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting, but is unable to drive this reaction due to misalignment of the semiconductor band edges with the water redox half-reactions. Here we show that attachment of an appropriate conjugated phosphonic acid to the GaI...

A co-solvent, dimethylsulfoxide (DMSO), is added to the aqueous chemical deposition (CBD) process used to grow ZnOS buffer layers for thin film Cu2ZnSnSe4 (CZTSe) solar cells. Device performance improves markedly as fill factors increase from 0.17 to 0.51 upon the co-solvent addition. X-ray photoelectron spectroscopy (XPS) analyses are presented fo...

An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/oxidation chemistries during film formation and...

Co-evaporated Cu2ZnSnSe4 (CZTSe) is used to examine sensitivities to the device performance that originate from variations in Zn content very near the surface. While integral Zn content of the film is held approximately constant, the surface composition is manipulated via changes to the Zn flux at the end of the deposition. Surface composition, dev...

Chemical bath deposition (CBD) Zn(O,S) buffer layers grown on Cu(In1-xGax)Se2 (CIGS) thin films have recently surpassed CdS in high efficiency cells (20.9%). A critical component of a CIGS device is the buffer layer - the layer that is found between the absorber CIGS layer and the ZnO window layer. Although CBD CdS is an effective buffer layer and...

Recent research has enabled Cu2ZnSnSe4 (CZTSe) to reach efficiencies close to 10% in photovoltaic devices with CdS as the junction partner and over 12% when the CZTSe is alloyed with sulfur. Little work, however, has been reported on the potential for wide band gap, Cd-free buffer layers in these devices. Reported here are photoelectron spectroscop...

We have demonstrated that pentafluoro phenoxy boron subphthalocyanine (F5BsubPc) can function as either an electron donor or an electron acceptor layer in planar heterojunction organic photovolatic (PHJ OPV) cell. F5BsubPc was incorporated into devices with the configurations: ITO/MoO3/F5BsubPc/C60/BCP/Al (F5BsubPc used as an electron-donor/hole-tr...

Alkyl chains are often attached to the periphery of semiconductor molecules to impart solubility and they represent a pervasive structural element in solution processable, organic photovoltaics (OPV). It is important to understand the effects of such substitutions on the morphology and performance of organic solar cells. This investigation focuses...

We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doublin...

We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P3HT/PC61BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large...

We report on the investigation of nickel cobalt oxide (NixCo3−xO4) thin films grown by pulsed laser deposition as hole-transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO2) in the range of 2–200 mTorr. We explore both bulk and surface properties of these...

Blended heterojunctions continue to improve in overall efficiency, with current power conversion efficiencies (PCE) at >;7%, with interfaces being a dominating factor in improving PCE. Charge separation is dictated by energetic alignment between the donor and acceptor. Selective interlayers are used to preferentially harvest one charge via either t...

Solution-based NiOâ outperforms PEDOT:PSS in device performance and stability when used as a hole-collection layer in bulk-heterojunction (BHJ) solar cells formed with poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and PC70BM. The origin of the enhancement is clarified by studying the interfaci...

Hybrid organic/inorganic solar cells have not lived up to their potential because of poor interface properties. Interfacial molecular layers provide a way of adjusting these devices to improve their performance. We have studied a prototypical system involving poly(3-hexylthiophene) (P3HT) on planar zinc oxide (ZnO) films that have been modified wit...

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiOx) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge en...

We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work-function of the hole collecting indium-tin oxide (ITO) contact, modified with a solution-processed nickel oxide (NiOx) hole-transport layer (HTL), is matched to the ionization potential of the donor material in a bulk-heterojunction solar cell. Ad...

Organic solar cells (OPVs) continue to see striking increases in efficiencies, with some research cells exceeding 8% power conversion efficiency, and module efficiencies approaching or exceeding ca. 4%.[1–3] These efficiencies are not yet adequate to create a PV technology which hits the DOE target of $1 per watt (peak), but there is reason for opt...

Organic solar cells require suitable anode surface modifiers in order to selectively collect positive charge carriers and improve device performance. We employ a nickel metal organic ink precursor to fabricate NiO hole transport layers on indium tin oxide anodes. This solution deposited NiO annealed at 250 °C and plasma treated, achieves similar OP...

The use of oxide materials as a hole transport layers (HTL) offers the opportunity to optimize hole collection in a bulk heterojunction organic photovoltaic (OPV) device. We discuss the use of NiO_x deposited by three different methods, pulsed laser deposition, sputtering and a solution precursor as an alternative to the standard OPV HTL....

Organic photovoltaics (OPVs) are realizing power conversion efficiencies that are of interest for commercial production. Consequently, understanding device lifetime and mitigating degradation pathways have become vital to the success of a new industry. Historically, the active organic components are considered vulnerable to photo-oxidation and repr...

Recent improvements of organic photovoltaic power conversion efficiencies have motivated development of scalable processing techniques. We compare chlorobenzene and p-xylene, as solvents with similar bulk properties, in a case study of ultrasonic spray depositions of bulk heterojunction layers in photovoltaic devices. Structure and morphology of sp...

Thin film pi-conjugated poly(3,4ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) as a hole transport layer on indium tin oxide is a key element in some of the most efficient organic photovoltaic and light emitting devices to date. Films are typically deposited by spincoating, which is not readily scalable. In this paper we investigate t...

Organic photovoltaics devices may pose one of the least expensive routes toward conversion of solar power. Two significant obstacles are low intrinsic material stabilities as well as poor interfacial charge transfer kinetics between the transparent conducting oxide and organic semiconductor. Presented is a series of investigations for several surfa...