Reuben T. Collins

Band-Edge Potentials of n-Type and p-Type GaN

The band-edge potentials of p-GaN in aqueous solutions were examined with photocurrent measurements, and those of n-GaN were examined with both photocurrent measurements and impedance spectroscopy. The measured band-edge potentials were different for both the different materials and the different measurement techniques. These differences are attributed to differences in the interface charging due to slow charge-transfer kinetics at the interface between the semiconductor and the solution. Using photocurrent measurements, the conduction band-edge potential was Φ c , s = (-1.092 - 0.063 X pH) V vs. a standard calomel electrode (SCE) for p-GaN and Φ c , s = (-0.538 - 0.046 x pH) V SCE for n-GaN. Using impedance spectroscopy, the conduction band-edge potential for n-GaN was Φ c , s = (-0.816 - 0.047 × pH) V SCE.

Near-field scanning optical nanolithography using amorphous silicon photoresists

Near-field scanning optical microscopy (NSOM) patterning of hydrogenated amorphous silicon (a-Si:H) has been explored. Our sample preparation technique produces films that are stable over several days. The etching process used is highly selective, allowing the unexposed a-Si:H to be completely removed while patterns with line heights equal to the original film thickness remain in exposed regions. We are able to generate patterns with and without the use of light. We have found that the probe dither amplitude greatly affects the linewidth and height of patterns generated without light. We also find that the exposure required for the NSOM to optically generate patterns agrees with threshold dosages determined by far-field exposure studies. Feature sizes of approximately 100 nm, comparable to the probe diameter, were obtained.

Highly efficient charge transfer in nanocrystalline Si:H solar cells

We demonstrate that in nanostructured films of nanocrystalline silicon imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are transported out of this region and therefore do not recombine in the amorphous phase. Electron paramagnetic resonance (EPR) and photoluminescence (PL) measurements show that the EPR and PL from the amorphous phase are rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 vol. %. We propose the use of similar structures to dramatically increase the open circuit voltages in solar cell devices.

The structural, optical, and electrical properties of vacuum evaporated Cu-doped ZnTe polycrystalline thin films

We have studied the structural, optical, and electrical properties of thermally evaporated, Cu-doped, ZnTe thin films as a function of Cu concentration and post-deposition annealing temperature. X-ray diffraction measurements showed that the ZnTe films evaporated on room temperature substrates were characterized by an average grain size of 300Å with a (111) preferred orientation. Optical absorption measurements yielded a bandgap of 2.21 eV for undoped ZnTe. A bandgap shrinkage was observed for the Cu-doped films. The dark resistivity of the as-deposited ZnTe decreased by more than three orders of magnitude as the Cu concentration was increased from 4 to 8 at.% and decreased to less than 1 ohm-cm after annealing at 260°C. For films doped with 6–7 at.% Cu, an increase of resistivity was also observed during annealing at 150–200°C. The activation energy of the dark conductivity was measured as a function of Cu concentration and annealing temperature. Hall measurements yielded hole mobility values in the range between 0.1 and 1 cm2/V·s for both as-deposited and annealed films. Solar cells with a CdS/CdTe/ZnTe/metal structure were fabricated using Cudoped ZnTe as a back contact layer on electrodeposited CdTe. Fill factors approaching 0.75 and energy conversion efficiencies as high as 12.1% were obtained.

A DLTS study of deep levels in n‐type CdTe

We report the results of a DLTS study on the majority carrier deep level structure of three samples of n-type CdTe and the effects on the deep level structure of indium doped CdTe due to H2 annealing. H2 annealing did not qualitatively change the deep level structure of the annealed sample. It did cause the shallow level concentration to decrease with a proportional decrease in the deep level concentrations as a result of indium out-diffusion and compensation by native defects. Levels present in all of the materials studied have been characterized and attributed to either native defects or innate chemical impurities. Other levels present in indium doped material require above band gap illumination of the sample before they are observed. A possible model proposes that these levels arise from defect complexes.

Conjugated Phosphonic Acid Modified Zinc Oxide Electron Transport Layers for Improved Performance in Organic Solar Cells

Phosphonic acid modification of zinc oxide (ZnO) electron transport layers in inverted P3HT:ICBA solar cells was studied to determine the effect of conjugated linkages between the aromatic and phosphonic acid attachment groups. For example, zinc oxide treated with 2,6-difluorophenylvinylphosphonic acid, having a conjugated vinyl group connecting the aromatic moiety to the phosphonic acid group, showed a 0.78 eV decrease in the effective work function versus unmodified ZnO, whereas nonconjugated 2,6-difluorophenylethylphosphonic acid resulted in a 0.57 eV decrease, as measured by Kelvin probe. This resulted in an average power conversion efficiency of 5.89% for conjugated 2,6-difluorophenyvinylphosphonic acid modified solar cells, an improvement over unmodified (5.24%) and nonconjugated phosphonic acid modified devices (5.64%), indicating the importance of the conjugated linkage.

Interdiffusion in polycrystalline thin-film CdTe/CdS solar cells

We have investigated the interdiffusion between CdTe and CdS resulting from post‐deposition annealing treatment. X‐ray diffraction (XRD) measurements reveal the presence of CdTe1−xSx in the CdTe layer and CdS1−y Tey in the CdS layer. x and y values are estimated to be 0.03 and 0.08, respectively, based on the measured lattice spacing. CdCl2 coated on the CdTe/CdS structure prior to annealing enhances significantly the degree of interdiffusion. The S diffusion profile in the CdTe layer is explored. A 10 meV decrease of CdS bandgap is observed as a result of interdiffusion.

Band gaps and lattice parameters of 0.9 μm thick InxGa1-xN films for 0≤x≤0.140

The c0 lattice parameter, band gap, and photoluminescence spectra of n-type 0.9 μm thick InxGa1−xN films with x=0, 0.045, 0.085, and 0.140 were examined. The c0 lattice parameter followed Vegard’s law using c0=0.5185 nm for GaN and c0=0.569 nm for InN. Band gap measurements by photocurrent spectroscopy fit well with data published by one other research group, with the combined set being described by the equation Eg=3.41−7.31x+14.99x2 for 0⩽x⩽0.15. Photoluminescence measurements with a pulsed nitrogen laser showed a peak well below the measured band gap, as well as significant luminescence above the measured band gap. The above-gap luminescence appears to be due to band filling by the high intensity laser pulses.

Effects of Cu in CdS∕CdTe solar cells studied with patterned doping and spatially resolved luminescence

CdS∕CdTe solar cells were nonuniformly doped at the backsurface of the CdTe with Cu evaporated through a shadow mask. Spatially resolved electroluminescence measurements showed strong correlation of emission intensity with the Cu pattern for all photon energies. Photoluminescence (PL) measurements performed on the exact same region showed no correlation with the Cu pattern when integrated over all energies. However, lower energy PL (located in a broad defect-related band) was slightly more intense in Cu-doped regions, whereas the intensity of PL from shallower states was slightly greater in undoped regions. These small differences in spectra were discernable only with the patterned doping and spatially resolved characterization used here.

Molecular Design for Tuning Work Functions of Transparent Conducting Electrodes

In this Perspective, we provide a brief background on the use of aromatic phosphonic acid modifiers for tuning work functions of transparent conducting oxides, for example, zinc oxide (ZnO) and indium tin oxide (ITO). We then introduce our preliminary results in this area using conjugated phosphonic acid molecules, having a substantially larger range of dipole moments than their unconjugated analogues, leading to the tuning of ZnO and ITO electrodes over a 2 eV range as derived from Kelvin probe measurements. We have found that these work function changes are directly correlated to the magnitude and the direction of the computationally derived molecular dipole of the conjugated phosphonic acids, leading to the predictive power of computation to drive the synthesis of new and improved phosphonic acid ligands.