J. van de Lagemaat

Spatial location of transport-limiting traps in TiO2 nanoparticle films in dye-sensitized solar cells

The dependence of the electron diffusion coefficient and photoinduced electron density on the internal surface area of TiO2 nanoparticle films in dye-sensitized solar cells was investigated by photocurrent transient measurements. The internal surface area was varied by altering the average particle size of the films. The density of electron traps in the films is found to change in direct proportion with the internal surface area, indicating that the traps are located predominately at the surface of TiO2 particles instead of in the bulk of the particles or at interparticle grain boundaries. The observed scaling of the electron diffusion coefficient with the internal surface area suggests that surface traps limit transport in TiO2 nanoparticle films. These results address a long-standing issue in the understanding of electron transport in dye-sensitized TiO2 solar cells.

Single-wall carbon nanotube networks as a transparent back contact in CdTe solar cells

Single-wall carbon nanotube (SWCNT) networks form a highly transparent and electrically conductive thin film that can be used to replace traditional transparent conducting oxides (TCOs) in a variety of applications. Here, the authors demonstrate their use as a transparent back contact in a near-infrared (NIR) transparent CdTe solar cell. SWCNT networks are hole-selective conductors and have a significantly greater NIR transparency than TCOs—qualities which could both make them very useful in tandem thin-film solar cells. SWCNT networks can be incorporated into single-junction CdTe devices and in CdTe top cells for mechanically stacked thin-film tandem devices, as described here. The best device efficiency using SWCNTs in the back contact was 12.4%, with 40%–50% transmission between 800 and 1500nm.

Determining the locus for photocarrier recombination in dye-sensitized solar cells

We present intensity-modulated photocurrent and infrared transmittance measurements on dye-sensitized solar cells based on a mesoporous titania (TiO2) matrix immersed in an iodine-based electrolyte. Under short-circuit conditions, we show that an elementary analysis accurately relates the two measurements. Under open-circuit conditions, infrared transmittance, and photovoltage measurements yield information on the characteristic depth at which electrons recombine with ions (the “locus of recombination”). For one particular series of samples recombination occurred near the substrate supporting the titania film, as opposed to homogeneously throughout the film.

Enhancement of the light‐to‐current conversion efficiency in an n‐SiC/solution diode by porous etching

Scanning electron microscopy micrographs of n‐type silicon carbide (SiC) anodized in HF solution showed a highly porous layer having structures with dimensions of about 50 nm. The capacitance of the porous electrodes revealed a huge surface area. The photocurrent quantum yield of a porous SiC/electrolyte diode is spectacularly enhanced with respect to that of a flat diode for light absorbed in the indirect‐bandgap and for sub‐bandgap light.

Electrochemistry of homoepitaxial CVD diamond: energetics and electrode kinetics in aqueous electrolytes

Abstract The electrochemical properties of highly doped p -type single crystalline diamond electrodes (100 and 110 oriented) in aqueous electrolytes were investigated. The interfacial capacitance obeys the Mott–Schottky relationship in a considerable potential range and can be assigned to a depletion layer in the diamond. The energetic position of the valence band edge is about 4 and 2 V versus SHE for (100) and (110) oriented diamond respectively. Oxygen and hydrogen evolution occur at large overpotentials (1 V) in agreement with previous results reported for polycrystalline diamond. Interestingly, with reversible redox systems, metal-type redox kinetics around the equilibrium potential are observed. The mechanism of electron exchange between the valence band of diamond and simple redox systems was investigated in detail using electrochemical impedance spectroscopy. A quantitative model is proposed, that assumes that electron exchange is mediated by bandgap states.

Photoelectrochemical characterization of 6H–SiC

Photoelectrochemical methods were used to characterize n-type 6H–SiC. The double layer capacitance obeyed the Mott–Schottky relationship over a large potential range (>6 V band bending). The flat-band potential was found to depend on pH with a displacement of about 40 mV per unit pH. The minority carrier diffusion length determined from the potential dependence of the photocurrent was 30 nm. From the dependence of the photocurrent on the photon energy, the absorption coefficient α(hν) was determined using the Gartner model. The results are in excellent agreement with spectra reported in literature. Sub-band-gap photocurrent with photons of energy down to 1.96 eV (≈1 eV below the band gap) was also observed.

Single-Wall Carbon Nanotubes as Transparent Electrodes for Photovoltaics

Transparent and electrically conductive coatings and films have a variety of uses in the fast-growing field of optoelectronic applications. Transparent electrodes typically include semiconductive metal oxides such as indium tin oxide (ITO), and conducting polymers such as poly(3,4-ethylenedioxythiophene), doped and stabilized with poly(styrenesulfonate) (PEDOT/PSS). In recent years, Eikos, Inc. has conceived and developed technologies to deliver novel alternatives using single-wall carbon nanotubes (SWNT). These technologies offer products having a broad range of conductivity, excellent transparency, neutral color tone, good adhesion, abrasion resistance as well as mechanical robustness. Additional benefits include ease of ambient processing and patterning capability. This paper reports our recent findings on achieving 2.6% and 1.4% efficiencies on nonoptimized organic photovoltaic cells employing SWNT as a transparent electrode

Observation and explanation of quantum efficiencies exceeding unity in amorphous silicon solar cells

Abstract The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs has to be taken into consideration also in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied Intensity Modulated Photocurrent Spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the DC quantum yield and the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can occur due to the presence of dangling bond defect centers in the band gap, which have an amphoteric character and can enhance the photocurrent due to trapping and thermal emission processes of photogenerated carriers.

Quantum efficiencies exceeding unity in amorphous silicon solar cells

The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs also has to be taken into consideration in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied intensity modulated photocurrent spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can be explained by considering trapping and thermal emission of photogenerated carriers at intragap amphoteric dangling bond defect centers.

Field-dependent charge carrier dynamics in GaN: excitonic effects

The electric-field dependence of the charge-carrier dynamics in GaN was studied by measuring excitation spectra of the sub-band-gap (yellow) luminescence as a function of bias using a Schottky junction formed at the interface between the semiconductor and an electrolyte solution. At large bias, the contribution of free electrons and holes to the photoluminescence is significantly reduced due to the dead-layer effect. As a result, striking features are revealed in the spectra close to the fundamental absorption. These features are attributed to exciton decay via yellow luminescence centers.