Optical susceptibilities of supported indium tin oxide thin films

The third-order nonlinear optical susceptibility of indium tin oxide (ITO) thin films on glass substrates, χ(3)ITO, was determined in the near-IR spectral region using degenerate four wave mixing (DFWM)spectroscopy with 100fs laser pulses. A DFWM method for measuring thin films on thick substrates was refined for the characterization of films less than 100nm thick and applied to ∼25nm thick ITO films. It was found that χ(3)ITO is purely electronic at 900–1300nm (11000–7700cm−1) and has a value of (2.16±0.18)×10−18m2V−2. Theχ(3)ITO value reaches (3.36±0.28)×10−18m2V−2 at 1500nm (6700cm−1) due to two-photon absorption by free carriers (electrons). Ultrafast electron relaxation was also observed. The ∼100fs lifetime of this process could reflect electron scattering in the conduction band

Electron Transfer Dynamics in Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Ruthenium or Osmium Polypyridyl Complexes

The electron transfer dynamics in solar cells that utilize sensitized nanocrystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale. The ruthenium and osmium bipyridyl complexes Ru(H_2L‘)_2(CN)_2, Os(H_2L‘)_2(CN)_2, Ru(H_2L‘)_2(NCS)_2, and Os(H_2L‘)_2(NCS)_2, where H_2L‘ is 4,4‘-dicarboxylic acid 2,2‘-bipyridine, inject electrons into the semiconductor with a rate constant >10^8 s^(-1). The effects of excitation intensity, temperature, and applied potential on the recombination reaction were analyzed using a second-order kinetics model. The rates of charge recombination decrease with increasing driving force to the oxidized sensitizer, indicating that charge recombination occurs in the Marcus inverted region. The electronic coupling factors between the oxidized sensitizer and the injected electrons in TiO_2 and the reorganization energies for the recombination reaction vary significantly for the different metal complexes. The charge recombination rates are well described by semiclassical electron transfer theory with reorganization energies of 0.55−1.18 eV. Solar cells sensitized with Ru(H_2L‘)_2(CN)_2, Os(H_2L‘)_2(CN)_2, and Ru(H_2L‘)_2(NCS)_2 have favorable photoelectrochemical characteristics, and iodide is oxidized efficiently. In contrast, iodide oxidation limits the efficiency of cells based on sensitization of TiO_2 with Os(H_2L‘)_2(NCS)_2. The observation that charge recombination occurs in the Marcus inverted region has important implications for the design of molecular sensitizers in nanocrystalline solar cells operated under our experimental conditions

Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se-2 absorber materials

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se-2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se-2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (V-Se-V-Cu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at E-v+0.98eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V-Se-V-Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V-Se-V-Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.Peer reviewe

Effects of internal luminescence and internal optics on V-oc and J(sc) of III-V solar cells

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. Internally radiated photons can be directly emitted from the cell, but if confined by good internal reflectors at the front and back of the cell they can also be re-absorbed with a significant probability. This so-called photon recycling leads to an increase in the equilibrium minority carrier concentration and therefore the open-circuit voltage, Voc. In multijunction cells, the internal luminescence from a particular junction can also be coupled into a lower bandgap junction where it generates photocurrent in addition to the externally generated photocurrent, and affects the overall performance of the tandem. We demonstrate and discuss the implications of a detailed model that we have developed for real, non-idealized solar cells that calculates the external luminescent efficiency, accounting for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell and isotropic internal emission. The calculation leads to Voc, and we show data on high quality GaAs cells that agree with the trends in the model as the optics are systematically varied. For multijunction cells the calculation also leads to the luminescent coupling efficiency, and we show data on GaInP/GaAs tandems where the trends also agree as the coupling is systematically varied. In both cases, the effects of the optics are most prominent in cells with good material quality. The model is applicable to any solar cell for which the optical properties of each layer are well-characterized, and can be used to explore a wide phase space of design for single junction and multijunction solar cells

Sputtered aluminum oxide and p+ amorphous silicon back-contact for improved hole extraction in polycrystalline CdSexTe1-x and CdTe photovoltaics

A thin layer of Al2O3 at the back of CdSexTe1-x/CdTe devices is shown to passivate the back interface and drastically improve surface recombination lifetimes and photoluminescent response. Despite this, such devices do not show an improvement in open-circuit voltage (VOC.) Adding a p + amorphous silicon layer behind the Al2O3 bends the conduction band upward, reducing the barrier to hole extraction and improving collection. Further optimization of the Al2O3, amorphous silicon (a-Si), and indiumdoped tin oxide (ITO) layers, as well as their interaction with the CdCl2 passivation process, are necessary to translate these electrooptical improvements into gains in voltag