Sujitra J. Pookpanratana

Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films

The surface composition of Cu(In,Ga)(S,Se)2 (“CIGSSe”) thin films intrinsically deviates from the corresponding bulk composition, which also modifies the electronic structure and thus the optical properties. We used a combination of photon and electron spectroscopies with different information depths to gain depth-resolved information on the band gap energy (Eg) in CIG(S)Se thin films. We find an increasing Eg with decreasing information depth, indicating the formation of a surface region with significantly higher Eg. This Eg-widened surface region extends further into the bulk of the sulfur-free CIGSe thin film compared to the CIGSSe thin film.

Impact of KCN etching on the chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers

The chemical and electronic surface structure of Cu2ZnSnS4thin-filmsolar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surfaceband gap increased from (1.53 ± 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 ± 0.15) eV.

Solid and liquid spectroscopic analysis (SALSA)--a soft x-ray spectroscopy endstation with a novel flow-through liquid cell.

We present a novel synchrotron endstation with a flow-through liquid cell designed to study the electronic structure of liquids using soft x-ray spectroscopies. In this cell, the liquid under study is separated from the vacuum by a thin window membrane, such that the sample liquid can be investigated at ambient pressure. The temperature of the probing volume can be varied in a broad range and with a fast temperature response. The optimized design of the cell significantly reduces the amount of required sample liquid and allows the use of different window membrane types necessary to cover a broad energy range. The liquid cell is integrated into the solid and liquid spectroscopic analysis (SALSA) endstation that includes a high-resolution, high-transmission x-ray spectrometer and a state-of-the-art electron analyzer. The modular design of SALSA also allows the measurement of solid-state samples. The capabilities of the liquid cell and the x-ray spectrometer are demonstrated using a resonant inelastic x-ray scattering map of a 25 wt % NaOD solution.

Native oxidation and Cu-poor surface structure of thin film Cu2ZnSnS4 solar cell absorbers

Air-exposed Cu2ZnSnS4 (“CZTS”) thin-film solar cell absorbers have been investigated by surface-sensitive x-ray photoelectron and x-ray-excited Auger electron spectroscopy, as well as by bulk-sensitive energy dispersive x-ray spectroscopy. We find a native surface oxidation of (mainly) tin, but also (to a lesser extent) of zinc and sulfur as well as evidence for a Cu-poor region at the surface of the absorber, best described by a Cu-free Zn-Sn-S surface layer.

Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyrite-based thin film solar cells

We have investigated the electronic structure of the absorber/back contact interface for S-free [Cu(In,Ga)Se2 (“CIGSe”)] and S-containing [Cu(In,Ga)(S,Se)2 (“CIGSSe”)] chalcopyrites with direct and inverse photoemission. Comparison of the electronic levels of the cleavage planes reveals a pronounced cliff in the conduction band at the CIG(S)Se∕Mo interface. For the valence band, we find a flat alignment and a small spike for the CIGSe- and CIGSSe-based structures, respectively.

Nondestructive depth-resolved spectroscopic investigation of the heavily intermixed In2S3/Cu(In,Ga)Se2 interface

The chemical structure of the interface between a nominal In2S3 buffer and a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber was investigated by soft x-ray photoelectron and emission spectroscopy. We find a heavily intermixed, complex interface structure, in which Cu diffuses into (and Na through) the buffer layer, while the CIGSe absorber surface/interface region is partially sulfurized. Based on our spectroscopic analysis, a comprehensive picture of the chemical interface structure is proposed.

Redox-Active Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications

In this work, high-performance top-gated nanowire molecular flash memory has been fabricated with redox-active molecules. Different molecules with one and two redox centers have been tested. The flash memory has clean solid/molecule and dielectric interfaces, due to the pristine molecular self-assembly and the nanowire device self-alignment fabrication process. The memory cells exhibit discrete charged states at small gate voltages. Such multi-bit memory in one cell is favorable for high-density storage. These memory devices exhibit fast speed, low power, long memory retention, and exceptionally good endurance (>10(9) cycles). The excellent characteristics are derived from the intrinsic charge-storage properties of the protected redox-active molecules. Such multi-bit molecular flash memory is very attractive for high-endurance and high-density on-chip memory applications in future portable electronics.

Effects of postdeposition treatments on surfaces of CdTe/CdS solar cells

Soft x-ray spectroscopy has been used to follow the effects of postdeposition steps (CdCl2 activation and back contact treatment) on surfaces and interfaces in CdTe-based superstrate solar cells. We find that the CdCl2 activation drives sulfur atoms from the CdS layer toward the back contact but not to its surface. Using atomic force microscopy, we find that both treatments strongly influence the morphology of the Au/Cu back contact. The spectroscopic results, in contrast, suggest that CdCl2 activation exhibits a larger impact on the surface composition and chemical structure of the interfaces involved in CdTe solar cells.

Polymorphism in the 1:1 Charge-Transfer Complex DBTTF–TCNQ and Its Effects on Optical and Electronic Properties

The organic charge-transfer (CT) complex dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) is found to crystallize in two polymorphs when grown by physical vapor transport: the known α-polymorph and a new structure, the β-polymorph. Structural and elemental analysis via selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and polarized IR spectroscopy reveal that the complexes have the same stoichiometry with a 1:1 donor:acceptor ratio, but exhibit unique unit cells. The structural variations result in significant differences in the optoelectronic properties of the crystals, as observed in our experiments and electronic-structure calculations. Raman spectroscopy shows that the α-polymorph has a degree of charge transfer of about 0.5e, while the β-polymorph is nearly neutral. Organic field-effect transistors fabricated on these crystals reveal that in the same device structure both polymorphs show ambipolar charge transport, but the α-polymorph exhibits electron-dominant transport while the β-polymorph is hole-dominant. Together, these measurements imply that the transport features result from differing donor-acceptor overlap and consequential varying in frontier molecular orbital mixing, as suggested theoretically for charge-transfer complexes.