Peter T. Erslev

Mapping out the distribution of electronic states in the mobility gap of amorphous zinc tin oxide

Amorphous zinc tin oxide (ZTO) is a wide-band-gap (transparent) semiconductor which exhibits high electron mobilities irrespective of its disordered nature. Transient photocapacitance (TPC), drive level capacitance profiling (DLCP), and modulated photocurrent spectroscopy (MPC) were used to determine the electronic state distribution within the mobility gap of ZTO. Conduction band-tail and valence band-tail Urbach energies near 10 and 110 meV were obtained by MPC and TPC, respectively. DLCP indicated free carrier densities in the mid-1015 cm−3 range plus a 0.2eV wide band of defects 0.4eV from the conduction band. The MPC spectra for ZTO also disclosed a defect band near the conduction band-tail.

Improvement in carrier transport properties by mild thermal annealing of PbS quantum dot solar cells

We studied the effect of post-deposition thermal annealing in the preparation of PbS quantum dot (QD) solar cells. We find an optimal annealing temperature that improves the power conversion efficiency by a factor of 1.5 for different sized QDs with bandgaps of 1.65 and 1.27 eV. We examined the onset of the photocurrent response and correlated that with domain grain growth and find that annealing the PbS QD array at 120 °C causes little change in the PbS QD size, bandgap, and open-circuit voltage and yet leads to an increase in the carrier transport as realized by an improved current response. We also find a decrease in the activation energy of a shallow trap, which also likely contributes to the improvement in the solar cell efficiency.

Perturbation of the electron transport mechanism by proton intercalation in nanoporous TiO2 films.

This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties of nanoporous TiO(2) films exposed to an ethanol electrolyte containing different percentages of water (0-10%). These measurements revealed that increasing the water content in the electrolyte led to increased proton intercalation into the TiO(2) films, slower transport, and a dramatic change in the dependence of the thermal activation energy (E(a)) of the electron diffusion coefficient on the photogenerated electron density in the films. Random walk simulations based on a microscopic model incorporating exponential conduction band tail (CBT) trap states combined with a proton-induced shallow trap level with a long residence time accounted for the observed effects of proton intercalation on E(a). Application of this model to the experimental results explains the conditions under which E(a) dependence on the photoelectron density is consistent with multiple trapping in exponential CBT states and under which it appears at variance with this model.

Surface Chemistry Exchange of Alloyed Germanium Nanocrystals: A Pathway Toward Conductive Group IV Nanocrystal Films.

We present an expansion of the mixed-valence iodide reduction method for the synthesis of Ge nanocrystals (NCs) to incorporate low levels (∼1 mol %) of groups III, IV, and V elements to yield main-group element-alloyed Ge NCs (Ge1-xEx NCs). Nearly every main-group element (E) that surrounds Ge on the periodic table (Al, P, Ga, As, In, Sn, and Sb) may be incorporated into Ge1-xEx NCs with remarkably high E incorporation into the product (>45% of E added to the reaction). Importantly, surface chemistry modification via ligand exchange allowed conductive films of Ge1-xEx NCs to be prepared, which exhibit conductivities over large distances (25 μm) relevant to optoelectronic device development of group IV NC thin films.

Metastable properties of Cu(In1―xGax)Se2 with and without sodium

We compare the electronic properties of Cu(In1−xGax)Se2 (CIGS, x=0.3) companion films with standard and nearly absent sodium levels. The films were examined over a wide range of metastable states produced by light-soaking. Admittance spectroscopy revealed that the activation energy of the dominant deep defect (hole trap) decreased monotonically from 300 to 60 meV with light-soaking time for samples with normal sodium, but remained nearly fixed (∼350 meV) for samples without sodium. Drive-level capacitance profiling indicated that the deep defect densities increased under light-soaking by roughly a factor of 20 for both samples and annealed at identical rates; however, the relative increases between the defect and hole carrier densities were dramatically different.

Device characterization of (AgCu)(InGa)Se 2 solar cells

Ag-alloying of Cu(InGa)Se<inf>2</inf> thin films presents the possibility to increase the bandgap with improved structural properties as a result of a lower melting temperature. (AgCu)(InGa)Se<inf>2</inf> films were deposited by elemental co-evaporation and the resulting solar cell behavior was characterized. While the bandgap in the highest efficiency Cu(InGa)Se<inf>2</inf> cells is ∼1.15 eV, Ag alloying allows the bandgap to be increased to 1.3 eV with an increase in V<inf>OC</inf>, no loss in device efficiency, and fill factors up to 80%. With high Ga content to increase bandgap > 1.5 eV, Ag alloying improves solar cell efficiency. Analysis of the device behavior shows that the basic mechanisms controlling (AgCu)(InGa)Se<inf>2</inf> solar cells and limiting performance with wide bandgap are comparable to those with Cu(InGa)Se<inf>2</inf>. Finally the effect of Na in (AgCu)(InGa)Se<inf>2</inf> devices is shown to be comparable to that with Cu(InGa)Se<inf>2</inf> including a decrease in V<inf>OC</inf> attributed to interface recombination with insufficient Na.

Characterizing the effects of silver alloying in chalcopyrite CIGS with junction capacitance methods

A variety of junction capacitance-based characterization methods were used to investigate alloys of Ag into Cu(In 1-x Ga x )Se 2 photovoltaic solar cells over a broad range of compositions. Alloys show encouraging trends of increasing V OC with increasing Ag content, opening the possibility of wide-gap cells for use in tandem device applications. Drive level capacitance profiling (DLCP) has shown very low free carrier concentrations for all Ag-alloyed devices, in some cases less than 10 14 cm −3 , which is roughly an order of magnitude lower than that of CIGS devices. Transient photocapacitance spectroscopy has revealed very steep Urbach edges, with energies between 10 meV and 20 meV, in the Ag-alloyed samples. This is in general lower than the Urbach edges measured for standard CIGS samples and suggests a significantly lower degree of structural disorder.

Study of the Electronic Properties of Matched Na-Containing and Reduced-Na CuInGaSe2 Samples Using Junction Capacitance Methods

Junction capacitance methods were used to examine a matched pair of CuInGaSe 2 (CIGS) thin film solar cells, one with Na incorporated into the absorber and the other with a diffusion barrier to inhibit the Na incorporation from the soda-lime glass. Typical cells showed a 50% increase in efficiency with the addition of Na. Forward biased admittance spectroscopy revealed a large defect density located near the CdS/CIGS heterojunction in the reduced Na samples not present in the higher Na samples. This may be responsible for the lower V oc , contributing to the loss in efficiency when Na is not added. Drive-level capacitance profiles revealed free carrier densities of 3×10 14 cm -3 and 1.1×10 14 cm -3 for the higher and reduced Na samples, respectively. Transient photocapacitance spectra indicated a slight improvement in absorber properties with the addition of Na, but not enough to account for the large loss in efficiency.

Characterization of the Electronic Properties of Wide Bandgap CuIn(SeS) 2 Alloys

The electronic properties of sulfur containing CIS chalcopyrite alloys have been characterized using junction capacitance methods. Two devices were examined; one containing CuIn(S,Se) 2 alloy with a 1:2 S:Se ratio and a bandgap near 1.15eV, and the other an endpoint CuInS2 alloy with a bandgap slightly above 1.5eV. Drive-level capacitance profiling measurements indicated hole carrier densities of less than 1 x 10 15 cm -3 and 1.5 x 10 16 cm -3 , respectively. Transient photocapacitance (TPC) sub-bandgap spectroscopic measurements revealed sharp bandtails plus a broad defect band within the bandgap of each alloy. The TPC spectra for the CuInS2 sample revealed a couple of unusual features, including a bandtail signal that reversed sign below 250K. This indicated poorer hole collection than electron collection in the low temperature regime. Comparing these results to TPC spectra obtained previously for Cu(InGa)Se 2 alloys indicate some similarities but also some striking differences.