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Light-induced metastabilities of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been extensively researched for many years. It is commonly observed that junction capacitance and p-doping level in CIGS absorbers increase considerably after light soaking (LS). In this work, we focus on the LS behaviors of cells with different minority carrier lifetimes (τn). Experiments show that high efficiency cells with long τn lose open circuit voltage (Voc) and fill factor (FF) upon LS, whereas low efficiency cells with short τn lose less or even gain Voc and FF. The sodium content measured with glow discharge optical emission spectroscopy (GD-OES) increases in the region close to the CdS/CIGS interface with LS and may contribute to the observed LS behaviors. The change in electrical parameters is explained with simulations, which relate the Voc and FF changes to a reduced recombination rate in the space charge region due to the light-induced doping increase. The simulations also suggest that cells with higher n-doping in the CdS are less sensitive to changes in interface recombination rate and doping of CIGS, which agrees with the hypothesis that the CdS buffer deposition is important for the LS behavior.

The significant gain in efficiency by a KF post deposition treatment (PDT) on small cells, observed by several groups, can be scaled to full size modules. This is shown here by a significant gain for full size modules with KF PDT and a module reaching 17.2% (aperture area efficiency, externally confirmed). Another important topic is reducing the cell-to-module losses. Two different methods are presented and compared, a metal grid on top of the TCO and an improved TCO (ZnO:B). A sub-module (300×275 mm2) reaching 17.6% efficiency and a mini-module (50×50 mm2) reaching 18.7% efficiency is presented and confirms the reduction of cell-to-module losses.

In this paper we present results of efficiency optimizations in conjunction with improved CIGS interface control. During large area thin film growth, understanding and control of the interfaces is an important key to enable a stable production. Our results indicate that efficiency, and weak light performance of CIGS modules can be increased and stabilized if wait time management and the related surface processes were better understood and controlled. The experiments indicate that oxide formation impacts the Na diffusion and has an impact on the module voltage. On the other hand defect formation before the absorber is observed to generate shunts between back contact and transparent conducting oxide with a significant drawback on the fill factor. Furthermore we show that internal sodium control helps to decouple wait times from module efficiency.

Dispersed fluorescence spectroscopy and photoelectron spectroscopy have been employed to study vibronic coupling in excited neutral and ionic states of carbon dioxide. The dispersed fluorescence excitation spectra of the CO 2 + B ˜ ( 0 0 1 ) 2 Σ g + → X ˜ ( 0 0 1 ) 2 Π u and the B ˜ ( 0 0 2 ) 2 Σ u + → X ˜ ( 0 0 2 ) 2 Π g transitions have revealed new structure. Some of this structure can be assigned to n sσ g or n d Rydberg series converging onto the CO 2 + C ˜ ( 0 0 1 ) 2 Σ u + threshold or to an n p Rydberg series converging onto the CO 2 + C ˜ ( 0 0 2 ) 2 Σ g + threshold. Autoionisation from Rydberg states belonging to these series results in the formation of ions with one or two quanta of the asymmetric stretching mode being excited. The HeI excited photoelectron spectra of the CO 2 + B ˜ 2 Σ u + and C ˜ 2 Σ g + states reveal extensive and complex vibrational structure. Transitions involving the non-totally symmetric bending and asymmetric stretching modes have been observed. The occurrence of these normally forbidden transitions is attributed to vibronic interactions.

Insertion of H+, Li+, Na+ and K+ into thin films prepared from silicotungstic acid - a photoelectron spectroscopy study

The electronic properties of TiO2 and electrochemically Li-inserted TiO2 have been studied using synchrotron radiation photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) in conjunction with resonant photoelectron spectroscopy. Core level (Ti 2p) and valence level spectra show the presence of Ti3+ states in LixTiO2. The x values determined from core level peak intensities were found to be directly correlated to the inserted amount of Li+ determined electrochemically. The x-dependent width of the Ti 2p peaks is consistent with a two-phase regime at intermediate x values. Resonant photoelectron spectroscopy at the Ti 2p edge was performed for TiO2 and Li0.5TiO2 to delineate the Ti4+ and Ti3+ contributions to the XAS spectrum.

Photoelectron spectroscopy is used to measure the electronic d. of states in the band gap region of a nanostructured TiO2 film. Such nanostructured films are used in dye-sensitized solar cells, and a plain TiO2 film as well as a dye-sensitized film were examd. To simulate the electrolyte effect on the energy levels at the oxide/dye/electrolyte interface in a solar cell, the d. of states in the band gap region for an electrochem. prepd. TiO2 film inserted with Li+ was measured. The distribution and chem. nature of electron trap states in the band gap and the energy matching between the LUMO level of the adsorbed dye, cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II), and the TiO2 conduction band edge are discussed.

The charge transport in nanoporous ZnO was studied by laser flash induced photocurrent transients. The results are discussed using a diffusion model and compared with previous results on TiO2. The charge transport was highly dependent on the potential giving apparent diffusion coeffs. for the electron ranging from 1×10-4 to 1×10-6 cm2/s with an applied bias of +100 and +300 mV vs. Ag/AgCl in ethanol, resp. The electrolyte was 0.5 M LiClO4 in ethanol. The potential dependence was much more pronounced for ZnO than for TiO2. The charge transport was also dependent on the electrolyte giving a linear dependence between the cond. of the electrolyte and the apparent electronic diffusion coeff. The dependence of the light intensity was also studied. Intensity-dependent losses were obsd.

A series of Ru-polypyridine dyes has been studied with electron spectroscopy using AlKα and synchrotron radiation. Both pure complexes and complexes adsorbed on nanostructured TiO2 (anatase) surfaces have been examined and special emphasis was given to the dye complex cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis-(isothiocyanato)-ruthenium(II) [Ru(dcbpy)2(NCS)2]. The measurements provide information concerning the energy level matching between the dyes and the TiO2, which is of importance in photoinduced charge transfer reactions and in applications such as dye-sensitized solar cells. The measurements also support the general picture of bonding of carboxylated complexes to the surfaces via the carboxyl groups of a single bi-isonicotinic acid ligand, and that, for Ru(dcbpy)2(NCS)2, the NCS-ligand–TiO2 interaction is small. Corroborative support is provided via quantum chemical calculations on the ligand (bi-isonicotinic acid) adsorbed on a TiO2 anatase (101) surface.

The charge transport in dye-sensitized nanostructured TiO2 was studied by laser pulse induced photocurrent transients. The experimental curves were compared to simulations using a diffusion model with an initial electron distribution of an exponential decay. The simulations were optimized with respect to the experimental curves giving an apparent diffusion coefficient of 6 × 10-6 cm2/s for the electrons with an electrolyte of 0.1 M KI in propylene carbonate, the potential being +300 mV vs Ag/AgCl in ethanol. The charge transport was highly dependent on electrolyte composition and light intensity.

Angular resolved core electron spectroscopy has been applied to the study of solutions of a surfactant onium salt (tetrabutylammonium nitrate) in formamide. The effect of dissolving CsI into the surfactant system was studied as a function of concentration

Electrochemical properties of Li+ ion insertion in nanoporous TiO2 (anatase) electrodes were studied by voltammetry. Linear and cyclic potential scans were recorded as a function of electrolyte con ...

Electrochemical insertion of lithium in nanoporous and CVD samples of TiO2 (anatase) was studied by chronoamperometry. The currents following cathodic and anodic potential steps were monitored as a ...

The electronic structure of cis-bis(4,4'-dicarboxy-2,2'-bipyridine)bis(isothiocyanato)ruthenium(II) and of its ligand 2,2'-bipyridine-4,4'-dicarboxylic acid was studied with electron spectroscopy. Valence level spectra were studied with respect to photoelectron angular distributions and photon energy dependence. The exptl. information coupled with semi-empirical INDO/S calcns. provides a mapping of the orbital parentage of the dye complex and its ligands. The energy levels of the dye complex in the context of a sensitized semiconductor (nanostructured TiO2) surface are related to the bands of the semiconductor.

Intercalation of lithium ions in nanoporous anatase TiO2 was studied with XPS using a novel electrochem. prepn. technique. The electrolyte was 0.5 M LiClO4 in acetonitrile (not water-free). The electrochem. insertion of lithium ions creates reduced Ti ions (Ti3+), which was obsd. as a new well-resolved state in the Ti 2p core level spectrum shifted 2.1 eV to lower binding energy. The new Ti 2p state was reversible when the electrode was deintercalated. Changes in the O 1s core spectra were also obsd. and discussed in terms of hydrated lithium ions in the Helmholtz layer, or formation of Li2O/LiOH.

Laser flash induced photocurrent transient measurements have been used to investigate the electron transport in nanostructured TiO2 (anatase) thin film electrodes in contact with an electrolyte. The shape and the time domain for the current transients were found to be dependent on film thickness and electrolyte conductivity. The experimental results are discussed using a diffusion model. If the experimental results are interpreted as diffusion, a chemical diffusion coefficient for the electrons in the nanostructured system is determined to 1.5 × 10-5 cm2/s using 700 mM LiClO4 in ethanol as electrolyte.

Photoelectrochemical properties of nanostructured ZnO thin film electrodes have been investigated in the UV and visible regions. For films consisting of 15 nm large undoped crystallites a maximum monochromatic current conversion efficiency of 58% was obtained in the visible using a ruthenium-based dye as a sensitizer. The overall solar energy conversion efficiency for this film was 2%. In comparison, sensitized films consisting of 150 nm large Al-doped crystallites yield a monochromatic current conversion efficiency of 31% and an overall solar energy conversion efficiency of 0.5%. The study also shows that nanostructured ZnO may give high efficiencies in the UV region, approaching unity for the Al-doped films.

The photoelectrochemical properties of nanostructured highly porous TiO2 electrodes with different thicknesses (2 to 38 mu m) were investigated. The effects of electron accepters, such as oxygen and iodine, in the electrolyte were studied by action spectr

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.

The effect of oxygen in nanocryst. colloidal TiO2 film photoelectrodes were studied by optically induced photocurrent transient measurements. O2 played an important role in the recombination of electrons and holes. Some effects esp. assocd. with the nanoporous morphol. of these photoelectrodes are discussed.

Verification of High Efficiencies for the Gratzel Cell : A 7% Efficient Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films.

A review. Recent work on the charge transport in mesoscopic nanostructured film electrodes is summarized. Results from intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) studies are given, and the attempts to model the charge transport through the dye-sensitized semiconductor (SC) film are discussed. The charge transport is mainly diffusional and the electrons are driven by a 1D concn. gradient of the electrons normal to the conducting substrate. The SC and the electrolyte are regarded as one effective medium. The charge sepn. and kinetics at the nanostructured-semiconductor, electrode/electrolyte interface (NSEI) is strongly affected by the redistribution of ions and dipoles between the soln. in the pores and the surface of the nanostructured SC. Trapping and detrapping of electrons is regarded as the main cause of the slow transport.

Photoemission Spectroscopy (PES) is a powerful method to investigate electronic structure of materials. However, conventional vacuum ultraviolet (VUV) and soft x‐ray (SX) PES is surface sensitive because of the short inelastic‐mean‐free‐paths of photoelectrons. This requires us to prepare clean surface and sometimes obscures intrinsic bulk states. In order to realize bulk‐sensitive or surface‐insensitive PES, we have developed hard x‐ray (HX) PES using high‐brilliance synchrotron radiation at BL29XU in SPring‐8. Large probing depth of high energy photoelectrons enables us to probe intrinsic bulk states almost free from surface condition. A combination of x‐ray optics and an electron energy analyzer dedicated for HX‐PES achieved the total instrumental energy resolution of 63 and 55 meV (FWHM) at 5.95 and 7.94 keV, respectively. A special arrangement of an analyzer and a sample was employed and increased photoelectron intensity drastically. We describe present performance of our apparatus and characterize HX‐PES by showing typical spectra.

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells1,2,3,4, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies4. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds5,6,7,8,9 for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution10,11 and intramolecular thermalization of excited states12,13,14 occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution15,16, to show that electron transfer from an aromatic adsorbate to a TiO2 semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes17.

Photoelectrochem. measurements have been performed on film electrodes consisting of linked nano-sized TiO2 colloids. The film thickness ranged from 1-40 μm. The film network was attached to a thin transparent conducting layer of SnO2 allowing for photogenerated electrons to be collected in an outer elec. circuit. By illuminating electrodes of different thicknesses with monochromatic light from either sides, it was possible to induce charge sepn. in different regions of the film network. In this way, it was proved that electrons have different probabilities of reaching the back contact depending on the location in the film where they are created. The results also illustrate the importance of the redox species in the cavities of these porous electrodes. By adding acceptors to the electrolyte it was possible to alter the conditions for charge transfer in the nanocryst. film. It was shown that electron acceptors such as oxygen or iodine in the soln. strongly affects the rate of charge transfer at the particle-electrolyte interface and the transport of electrons throughout the TiO2 film-network. Modification of the semiconductor-electrolyte interface with surface adsorbed pyridine, induced major changes in the charge transfer events at the interface. The photocurrent yields was greatly improved by this surface treatment. The effect of pH in soln. was also investigated. The rate of charge transfer at the particle-electrolyte interface was changed at high surface d. of OH--ions. This was explained due to the change of the surface energy causing different driving forces for redox reactions, but also due to the more neg. electrostatic surface potential of the particles preventing the encounter of neg. charged redox species with the colloid surface. Phototransient measurements indicated a depletion of redox species in the pores of the film. Thereby it was pointed out that the dynamics of the redox species in the confined cavities of the film is a limiting factor for the charge sepn. efficiency in nanocryst. film. The photovoltage in anaerobic solns. sustained for very long periods, indicating that the linked particles may work as reservoirs for photoexcited electrons if the access to electron acceptors in the soln. is choked. It was concluded that surface processes are favored at the small semiconductor particles used in this study.

Dye-sensitized micro -or, to be more precise, nanoporous - Ti02 film electrodes for solar cells1' 2 are currently under investigation in our laboratory35. We have earlier presented methods to gain information about fundamental properties of semiconducting thin film electrodes from front-side (EE) and backside (SE) illumination of Ti02 electrodes by analysis of the action spectra3' 6 (For the experimental set up and the definition of SE and EE, see Fig. 1.) We are here extending our experimental work into the visible region and give preliminary result from an investigation of the action spectra of dye-sensitized Ti02-film electrodes.

The photoelectrochem. properties of colloidal film electrodes are compared with solid transparent thin films of TiO2. In particular the information that can be derived from anal. of the action spectra for front and back side illumination of the films is given. The mechanism of charge sepn. in the colloidal film is qual. discussed in terms of the kinetics at the electrode-electrolyte interface, and some effects of the presence of O2 in the electrolyte are demonstrated. The solar energy efficiency from measurements on dye-sensitized TiO2 film electrodes in solar cells is given.

Proton insertion in polycryst. tungsten oxide has been studied with photoelectron spectroscopy and semi-empirical calcns. The W 4f spectra show growth of W5+ and W4+ oxidn. states as a function of inserted proton content in the crystal lattice. A concomitant growth of bandgap structure displays two sep. peaks. Semi-empirical calcns. using the periodic large unit cell (LUC) method suggest the growth of W4+ states based on the total energy variation with respect to the no. of protons/unit cell. At this stage the general picture of insertion is that of a multiphase system contg. regions of different x values.

Nanostructured anatase TiO2 electrodes surface modified with inorg. metal oxide clusters, i.e., phosphotungstic acid (PWA), was studied by XPS and spectroelectrochem. The surface modifiers were electroactive and could be addressed reversibly by changing the applied bias. The formation of new states in the electronic structure of TiO2 and the PWA was monitored by XPS using a novel prepn. technique. The coloration efficiency of the deposit was about 20 cm2/C. Prolonged electrochem. cycling degraded the elec. contact between the PWA and the TiO2 surface.

The present paper describes a new method for manufacturing a nanostructured porous layer of a semiconductor material on a conducting plastic substrate for use in an electrochemical or photoelectrochemical cell. The method involves the deposition of a layer of semiconductor particles on conducting plastic and the compression of the particle layer to form a mechanically stable, electrically conducting, porous nanostructured film at room temperature. Photoelectrochemical characteristics of the resulting nanostructured films are presented showing, for example, overall solar to electric conversion efficiencies of up to 4.9% (0.1 sun). The potential use of the new manufacturing method in future applications of nanostructured electrodes is discussed.

N 1s x-ray absorption spectra of bi-isonicotinic acid (2,2′-bipyridine–4,4′-dicarboxylic acid) on rutile TiO2(110) have been studied experimentally and quantum chemically. Differences between multilayer and monolayer spectra are explained by the adsorbate bonding to the substrate. A connection to the electronic coupling in dye-sensitized electrochemical devices is made.