Photocurrent Yield Research Articles

Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer:fullerene solar cells

We investigate the nature of ultrafast exciton dissociation and carrier generation in acceptor-doped conjugated polymers. Using a combination of two-pulse femtosecond spectroscopy with photocurrent detection, we compare the exciton dissociation and geminate charge recombination dynamics in blends of two conjugated polymers, MeLPPP [methyl-substituted ladder-type poly($p$-phenylene)] and MDMO-PPV [poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], with the electron accepting fullerene derivative PCBM [1-(3-methoxycarbonyl)-propyl-1-phenyl-$(6,6){\mathrm{C}}_{61}$]. This technique allows us to distinguish between free charge carriers and Coulombically bound polaron pairs. Our results highlight the importance of geminate pair recombination in photovoltaic devices, which limits the device performance. The comparison of different materials allows us to address the dependence of geminate recombination on the film morphology directly at the polymer:fullerene interface. We find that in the MeLPPP:PCBM blend exciton dissociation generates Coulombically bound geminate polaron pairs with a high probability for recombination, which explains the low photocurrent yield found in these samples. In contrast, in the highly efficient MDMO-PPV:PCBM blend the electron transfer leads to the formation of free carriers. The anisotropy dynamics of electronic transitions from neutral and charged states indicate that polarons in MDMO-PPV relax to delocalized states in ordered domains within $500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The results suggest that this relaxation enlarges the distance of carrier separation within the geminate pair, lowering its binding energy and favoring full dissociation. The difference in geminate pair recombination concurs with distinct dissociation dynamics. The electron transfer is preceded by exciton migration towards the PCBM sites. In MeLPPP:PCBM the exciton migration time decays smoothly with increasing PCBM concentration, indicating a trap-free exciton hopping. In MDMO-PPV:PCBM, however, the exciton migration time is found to show a threshold-like dependence on the PCBM concentration. This observation is explained by efficient interchain energy transfer in ordered MDMO-PPV domains in conjunction with exciton trapping.

Solvent Effects on Interfacial Electron Transfer from Ru(4,4‘-dicarboxylic acid-2,2‘-bipyridine)2(NCS)2 to Nanoparticulate TiO2: Spectroscopy and Solar Photoconversion

Resonance Raman spectra are reported for Ru(4,4'-dicarboxylic acid-2,2'-bipyridine)2(NCS)2 (commonly called "N3") in ethanol solution and adsorbed on nanoparticulate colloidal TiO2 in ethanol (EtOH) and in acetonitrile (ACN), at wavelengths within the visible absorption band of the dye. Raman cross sections of free N3 in EtOH are found to be similar to those of N3 adsorbed on colloidal TiO2 in EtOH, and are generally lower than those of N3 on TiO2 in ACN. Strong electronic coupling mediated by surface states results in red-shifted absorption spectra and enhanced Raman signals for N3 adsorbed on nanocolloidal TiO2 in ACN compared to EtOH. In contrast, the absorption spectrum of N3 on nanocrystalline TiO2 in contact with solvent is similar for ACN and EtOH. Wavelength-dependent depolarization ratios for N3 Raman bands of both free and adsorbed N3 reveal resonance enhancement via two or more excited electronic states. Luminescence spectra of N3 adsorbed on nanocrystalline films of TiO2 and ZrO2 in contact with solvent reveal that the quantum yield of electron injection phi(ET) into TiO2 decreases in the order ACN > EtOH > DMSO. Dye-sensitized solar cells were fabricated with N3 adsorbed on nanocrystalline films of TiO2 in contact with ACN, EtOH, and DMSO solutions containing LiI/LiI3 electrolyte. Photoconversion efficiencies eta were found to be 2.6% in ACN, 1.3% in DMSO, and 0.84% in EtOH. Higher short circuit currents are found in cells using ACN, while the maximum voltage is found to be largest in DMSO. It is concluded that the increased photocurrent and quantum yield of interfacial electron transfer in acetonitrile as compared to ethanol and DMSO is primarily the result of faster electron injection of N3 when adsorbed on TiO2 in the presence of ACN as opposed to EtOH or DMSO.

Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface

We have fabricated thick, nanostructured composite films of titania (aerogel)–silica (aerogel) on transparent electrodes for use as composite photoelectrodes. Aerogels are excellent candidate materials for photovoltaic electrodes because the high specific surface area of titania aerogel (∼150m2/g) significantly amplifies photoactive area, while the continuous pore network permits rapid mass transport of diffusing electron donors through the film. The high surface-to-volume ratio of titania aerogel films can also be a cause for concern because the amplified surface character begets a large number of surface defects that can act as energetically deep traps and hinder percolative transport of electrons through the film. Surface hydroxyls (Ti–OH, titanols) act as charge traps, whereby electrons localize at the semiconductor surface and promote recombination of electrons with solution-phase acceptors, resulting in diminished measured photocurrents. Titania (aerogel)–silica (aerogel) composite films can be modified by electrochemically generated organic radical anions, which graft to the titanols at the electrode surface. This organic modification improves photocurrent yields under bandgap-excitation by a factor of >2 relative to unmodified films, and shifts the onset of photoanodic current negatively by ∼45mV. The improved photocurrent yields are likely due to more facile electronic transport through the modified mesoporous network, caused by changing the chemical nature of the surface species on the titania electrode. The negative shift of the onset potential for the flow of cathodic current is further evidence that the enhanced photocurrent arises from a change in electron mobility, as opposed to a lowering of the conduction band edge, which would improve photoelectron injection efficiency, but would involve a positive shift in the cathodic onset potential.

Stability improvement at high emission densities for gold thin film photocathodes used in advanced electron beam lithography

Multi-electron beam lithography has been proposed as a promising approach to achieve high throughput for mask writing and direct wafer writing. Laser driven photocathodes represent an attractive candidate for multiple beam, high brightness sources. Thin film gold photocathodes that can be handled in air are of particular interest because of their potential for practical sources. In this article we present a study of the degradation mechanisms that change photocurrent yield for thin film gold photocathodes. Two general degradation mechanisms were studied: microstructural changes of the gold thin film and surface reactions. Observed microstructural changes included loss of gold coverage of the sapphire substrate, gold grain growth and an increase in surface roughness. A titanium adhesion layer was shown to stabilize coverage and proved stable to 700 °C by in situ transmission electron microscopy (TEM) experiments on planar sections of the Au/Ti/sapphire thin film stack photocathode. Reactions at the surface included physisorption and chemisorption of species and the subsequent reactions that occur when they diffuse to the laser illumination/electron emission site. X-ray photoelectron spectroscopy detected multiple monolayers of hydrocarbons on the surface of the Au following deposition and exposure to air. A product layer at the illumination sites as thick as 28 nm was detected using scanning electron macroscopy, atomic force microscopy and TEM following long term (1000–1200 min.) photoyield stability tests. Photoyield versus time plots for all the Au/Ti/sapphire cathodes were similar with an initial high photoyield, a drop to a minimum, then a rise to a second maximum followed by a slow 2%/h decay. Desorption of weakly bound physisorbed and chemisorbed species and growth of a reaction product layer within the electron emission zone are proposed as mechanisms to explain the photoyield behavior. Similar photoyield stability behavior was observed when Pt/Ti/sapphire photocathodes were tested. The photoyield degradation mechanisms observed in these tests are likely to be active on all thin film transmissive photocathodes subjected to high intensity illumination and will add to changes which occur when more reactive thin film photocathodes are used such as cesium telluride. Stability was improved to less than 1%/h for 16 h within the region of slow decay following the initial transient by illuminating the emission site on the cathode with high intensity UV light with a high O2 partial pressure. Emission current densities in excess of 6.0 A/cm2 were measured with a Faraday cup. Given these results, this work suggests that higher current densities and stability can be achieved.

Enhancement of photocurrent generation of two novel stilbazolium dye dimers in LB monolayer films

Novel amphiphilic stilbazolium dye dimers, bis{[1-(N-methylpyridinium-4-yl)-2-(3-methoxy-4-octadecyloxyphenyl)]ethenyl}methane diiodide (BPOM) and bis{[1-(N-methylpyridinium-4-yl)-2-(4-octadecyloxyphenyl)]ethenyl}methane diiodide (BPO) were synthesized, and they were successfully transferred onto semiconducting transparent indium–tin oxide (ITO) as H-aggregates by Langmuir–Blodgett (LB) techniques. The photocurrent generation properties of monolayer films of these dyes were investigated in a traditional three-electrode cell. A steady cathodic photocurrent was obtained upon excitation of the dye LB monolayer films deposited on an ITO electrode. The observed photocurrent generation quantum yield strongly depended on the applied electrode potential, concentration of the redox couples in the electrolyte solution, and the chemical structure of the dye congeners. The dye monomers, (E)-N-methyl-4-[2-(3-methoxy-4-octadecyloxyphenyl)ethenyl]pyridinium iodide (POM) and (E)-N-methyl-4-[2-(4-octadecyloxyphenyl)ethenyl]pyridinium iodide (PO) as the standards for comparison were also studied. Data show that the photocurrent generation quantum yields are 0.35% and 0.26% for BPOM and BPO, respectively, while those for POM and PO are 0.19% and 0.11%, respectively. From the photoelectrochemical measurement, the enhanced photocurrent generation properties in the dimer compared with those in the corresponding monomer were observed.

Solid-state photoelectrochemical cell using a polythiophene derivative as photoactive electrode

In this work, we investigated the photoelectrochemical behavior of poly(4,4′-dipentoxy-2,2′-bithiophene), poly(ET2), as a single electrode in a liquid electrolyte and also its use in a solid-state photoelectrochemical device using a polymeric electrolyte. The results showed a strong dependence of the photocurrent density on the type and concentration of the redox couple. The solid-state device exhibited a short-circuit current density of 4 μA cm −2 and an open-circuit voltage of 9 mV. The maximum quantum yield of photocurrent in the visible range was 0.17%, high when compared to other photoelectrochemical devices based on organic materials; however, it is low in comparison to solid-state devices using inorganic materials. This paper shows the viability of using organic polymeric electrolytes in the construction of photoelectrochemical devices.

X-ray-induced radiation damage in CsI, Gadox, Y 2O 2S and Y 2O 3 thin films

The stability of CsI, CsI(Tl), Gd 2O 2S(Tb), Gd 2O 2S(Eu), Y 2O 2S(Eu) and Y 2O 3(Eu) thin films under bombardment by 9–18 keV X-rays is described. Both external photocurrent and scintillation light yield were measured as functions of accumulated dose at radiation fluxes of 10 6–10 7 photons s −1 mm −2 on Beamline 2.2 of the Daresbury Synchrotron Radiation Source (SRS). All of the samples studied showed changes of several percent (both reductions and increases) in photocurrent and scintillation light yield of several percent for accumulated doses of up to 5×10 11 photons mm −2. No significant dependence of the film response on the angle of X-ray incidence was observed for angles up to 45° from the normal. It was found that the accumulated dose is not the only parameter determining the degradation of photoconverter performance; the flux rate has also to be taken into account. Scanning Electron Microscope studies of the irradiated samples did not reveal any significant surface modification.

Photocurrent and Photoelectron Yield Spectroscopies of Defect States in CVD Diamond Films

Photocurrent (PC) and photoelectron yield (PY) measurements have been used to investigate defect states in CVD diamond. Diamond films exhibit a non-negligible sub-band-gap photocurrent with a well defined energy onset related to the photoionization of acceptor-like defects located in the range 1.2–1.3 eV. The PC energy onset is shifted to lower energy by chemical treatment in sulphochromic acid, which leads to the appearance of a photoionization band in the photocurrent spectrum. On the other hand, PY spectra closely follow a power law with an energy threshold in the range of 4.3–4.4 eV, which also shift to higher energies after the same chemical treatment. The resulting defect level scheme consistent with both PC and PY results is discussed.

High-temperature performances of diamond-based UV-photodetectors

Abstract Self-supported CVD diamond films having coplanar contacts spaced down to 100 μm have been characterized in the temperature range 300–600 K by electrical measurements in the dark and under illumination at λ=220 nm. Up to 600 K the dark conductivity values are smaller than 10−9 S cm−1 and spectrally resolved photocurrent yield measurements in the range 200–800 nm exhibit more than four orders of magnitude discrimination between intrinsic (below 230 nm) and extrinsic (above 230 nm) photoresponses. A thermal activation of the UV photoresponse is also observed, giving an average temperature coefficient for intrinsic photoresponses around 1%/°C.

Shallow levels and photoconductivity in K1-xLixTaO3

We present the results of experiments performed on K1-xLixTaO3 as well as on Nb-doped and nominally pure KTaO3 single crystals, in which we compared observations of thermally stimulated currents and photoconductivity. In the Li-doped compounds, the large enhancement of conductivity caused by ultraviolet excitation at low temperature was found to correlate with the filling of shallow trapping centres, giving intense charge release below 30-40 K. No sign of a corresponding release was shown by pure or Nb-doped KTaO3, consistently with a very low yield of photocurrent which one observes in these cases. The depth of the trapping levels in K1-xLixTaO3 crystals was found to be between 50 and 70 meV. On the basis of past models and recent calculations, these levels can be identified with hole traps, originating from the perturbation of O2- states at the top of the valence band. They are a plausible source of enhanced photoconductivity, via the quenching of electron-hole recombination.

Photoelectrochemical response and differential capacitance of poly(3-methylthiophene)

Following the theory used to study the semiconductor/electrolyte interface the differential capacitance of poly(3-methylthiophene) films has been determined from measurements with a lock-in amplifier and by electrochemical impedance spectroscopy (EIS). According to our findings, the best results were obtained by EIS because the space charge capacitance can be separated from the other capacitances. Using Mott–Schottky plots ( C −2 vs. E) we obtained the flat band potential E fb=80 mV and the carrier density N=6×10 17 cm −3 for the PMeT film in contact with the electrolyte, where dissolved O 2 played the role of the electron acceptor. The determined width of the depletion layer is 0.04 μm. We also investigated the photoelectrochemical response of the PMeT film. The plot of the square of the photocurrent vs. potential yields E fb=90 mV, in good agreement with the EIS measurement. The dependence of the photocurrent with the frequency of the incident light shows that PMeT has a long response time (order of ms), compared to an inorganic semiconductor. The band gap was also determined from the photocurrent spectra. The value obtained, for a direct transition is 1.9 eV and is coincident with the value obtained from the absorption spectra.

XANES and XPS Analyses of Silicon Irradiated by Deuterium Ions

XANES and X-ray photoelectron spectra (XPS) using synchrotron radiation as a X-ray source were applied for analysis of silicon crystalline irradiated by 5 keV deuterium ions. Si K-edge XANES spectra recorded either by a photocurrent or electron yield modes clearly showed a new absorption peak in addition to the absorption due to surface SiO2. In Si Is XPS spectra, on the contrary, no clear peak or shoulder was detected except SiO2 species. Taking into account that Si K-edge XANES reflects the electron transition from Si 1s to np (empty bound state), the new absorption peak is very likely caused by the generation of localized electronic bond between Si and D in the vicinity of the damaged region in the irradiated Si.

Photovoltaic yield from exciton dissociation in organic dye layers

Researchers working on organic photovoltaic cells are confronted with the problem of finding the optimal thickness of the organic layers. Obviously, there is a competition between the absorption length of the incident light and the exciton diffusion length, leading to optimal film thicknesses lying somewhere between these two mean path lengths (D. Wohrle, L. Kreienhoop and D. Schlettwein, PhthalocyaninesandRelatedMacrocyclesinOrganicPhotovoltaicJunctions, ed. C. C. Leznoff and A. B. P. Lever, VCH, New York, 1996). In this paper we investigate the general conditions for optimization of the thickness of the organic layer for a maximum photovoltaic yield. At normal absorption, the optimal layer thickness is typically 1.5 times the exciton diffusion length. As an application we consider the zinc phthalocyanine layer in an organic p–n cell, for which an exciton diffusion length of 30±10 nm is determined from simulations. Using this value, a good correlation between simulations and experiments is found with respect to the thickness-dependent photocurrent yield of the solar cells.

Performance of diamond-based photoconductive devices in the UV range

A detailed investigation of diamond-based UV planar photoconductors with interdigitated contacts is presented, analyzing in particular the influence of film morphology and texturing on device performance. Photoluminescence and photocurrent spectroscopies reveal a strong correlation between structural defects and detector selectivity, whereas impurities affect photocurrent yield mainly in highly oriented materials. Responsivity measurements show that diamond-based devices give the highest UV-visible discrimination in comparison to UV-enhanced Si- and GaN-based sensors. For λ ≤225 nm, an almost linear photocurrent-light intensity relationship is observed, although, in a few cases, a superlinear dependence on illumination intensity is found. A theoretical model, capable of describing the observed field dependendence of photocurrent gain is presented, giving minority carriers lifetime and mobility in the range 0.1–1 ns and 10–300 cm 2 V −1 s −1 , respectively.

Laterally-resolved study of the Au/SiN x/GaAs(100) interface

We report a microscopic study of the Au/5Å SiN x/GaAs Schottky barrier based on scanning near-field optical microscopy. Photocurrent yield microimages taken at different photon energies reveal non-topographic features. Taking into account their dependence on photon energy and bias, such features are consistent with lateral variations of the local density of states related to defects. The results, therefore, confirm that lateral variations must be considered when analyzing semiconductor interfaces and of the corresponding devices.

Novel photovoltaic devices based on donor-acceptor molecular and conducting polymer systems

Interest in organic photocells was significantly renewed a few years ago due to discovery of extremely fast photo-induced charge transfer (CT) from conducting polymer (CP) to fullerene C/sub 60/ molecules in CP(C/sub 60/) composites. This allowed creation of interfacial junction photocells with relatively high efficiencies. It has been recognized that those are not conventional p/n type semiconducting junction devices, but are so called donor-acceptor (D A) photocells, reminiscent of reaction centers of natural photosynthetic systems. Even more efficient photovoltaic devices have recently been fabricated with interpenetrating D-A networks which allow electron-hole separation to take place throughout the bulk of the device. In the present paper we demonstrate how these photocells can be further improved; we describe new experimental results, give general discussion of the efficiency of such devices, and analyze strategies for design of better D-A type photocells. Our recent experimental results on sensitization by inserting an excitonic layer between donor and acceptor layers multilayered structures, and by doping the D-layer by A-molecules in D(A)/A cells are briefly discussed. The organic photocell, which consists of the multilayer structure: ITO/poly(2,5-dioctyloxy-phenylenvinylene) (OOPPV)/octaethylporphine (OEP)/C/sub 60//Al, has been fabricated. Photocurrent yield spectra are interpreted by light absorption at OEP layer, exciton migration, and charge generation at both organic heterojunctions. That is, double heterojunctions of OOPPV/OEP and OEP/C/sub 60/ contribute to the charge generation by excitonic dissociation. We also report new type of "fractal network" photovoltaic devices made of two conducting polymers, particularly D and A derivatives of poly(phenylene vinylene). Conceptual theoretical work and modeling have been carried out to understand the ways for improvement of the device performance. Strategies to improve each step involving selective doping have been suggested to improve each step. Selective doping can provide low serial resistance and create internal electric fields for the collection of charges, while in the undoped parts excitons are effectively photogenerated. Ways to increase the effective use of absorbed photon energy and filling factor are also suggested.

Electron Transport Properties in Dye-Sensitized Nanoporous−Nanocrystalline TiO2 Films

Spectral response measurements have been performed on dye-sensitized nanoporous−nanocrystalline TiO2 photoelectrodes. The effects of film thickness, electron scavengers in the electrolyte, and surface treatment of the nanocrystalline film were studied by means of action spectra for front- and back-side illumination. Our results show that electron acceptors such as dioxygen and iodine strongly decrease the IPCE. Surface treatment of the electrode with pyridine induces a substantial increase of the photocurrent yields. The observations are discussed in terms of kinetics at the semiconductor−electrolyte interface. IPCE values for sandwich cells were generally much higher than those obtained from three-electrode measurements.

Modification and photosensitization of size-quantized particulate CdS film photoelectrodes with bacteriorhodopsin (BR)

Bacteriorhodopsin (BR) was used to modify the surface of a novel size-quantized particulate CdS photoelectrode fabricated by electro-deposition. The photovoltaic features and interfacial electron transfer of the electrodes were studied by surface photovoltaic spectroscopy and a photoelectrochemical method. The results show that BR-modified CdS film photoelectrodes achieve a higher photocurrent yield due to the incorporation of the proton pump BR. On the basis of the observations, the mechanisms of photosensitization and interfacial electron transfer are proposed.

Photoelectrochemical Studies on Nanocrystalline Hematite Films

Nanocrystalline hematite films supported on conducting glass have been prepared from colloidal Fe{sub 2}O{sub 3} particles, giving a roughness factor of about 30. The current-potential characteristic showed an abnormal drop in photocurrent under reverse polarization. Photocurrent yields are low over the entire potential range. In the presence of iodide the abnormal drop was removed and the photocurrent yield increased. Action spectra were determined by illumination from the electrolyte side and the conducting glass support, respectively. The incident photon-to-current conversion efficiencies (IPCE) is a factor of 100 higher in the former compared to the latter case if water is used as a hole scavenger. In the presence of iodide this ratio is diminished 4-fold due to an increase in the IPCE value measured by illumination from the electrolyte side. The poor photocurrent yields observed are due to charge carrier recombination. Recombination at surface sites can be partially intercepted by adsorbed iodide. 9 refs., 9 figs.

Synthesis and Photochemistry of Quantum‐Size Semiconductor Particles in Solution and in Modified Layers

AbstractWe describe the preparation of highly monodisperse, quantum‐size CdS particles. In these samples a band splitting into discrete exciton‐like states is observed. A mechanism for the excitonic fluorescence is presented involving shallow traps as a reservoir for electrons which finally recombine with free positive holes across the bandgap. Quantum‐size CdS and PbS particles prepared in situ on highly porous, polycrystalline TiO2 electrodes work as highly efficient sensitizers. Photocurrent yields of more than 70% are reported. In the case of PbS, it is shown that an efficient electron transfer is possible only with particles in the size of the quantization regime.