Band Gap Renormalization in Resonant Raman Spectra of Multilayer Systems

In this work we show that resonant inelastic light scattering of short periods planar doped GaAs systems can probe the 

interaction between holes and the Fermi sea. Comparison between experimental and theoretical results fo samples submitted to various incident laser energies show that a good agreement with the Raman spectral lines is achieved only when a correction to the gap is taken in account. It is shown that such a correction depends on the period of the structures as well as to the free electronic density. Additionally, we demonstrate that the increase of theincident laser energies, departing from a regime of extreme resonance with the split-off gap of the GaAs, will result in spectra varying from one of pure Raman character to ones basically composed of hot luminescence.

 
 
 

Charge Transport in Porous Nanocrystalline Titanium Dioxide

Anuradha M. Biswas, Guillaume Maire, Ian M. Ballard and Jenny Nelson

Dye sensitised solar cells are a novel class of photovoltaic device.They are based on a thin (~10 micron thick) film of porous, nanocrystalline, anatase TiO2.This is sensitised by a dye absorbing in the visible.Absoption of a photon by the dye results in ultra-fast electron injection into the conduction band of the TiO2.The electron moves through the TiO2 layer to the external electrode while the dye is regenerated by a liquid electrolyte.

The TiO2 layer is a highly disordered material system which to date, is poorly understood. The electrical properties are influenced by the porous geometry and huge surface area as well as by the presence of native defects.

Recent studies have shown that charge transport in the TiO2 layer is dominated by trapping and is the rate limiting step for recombination in the solar cell. The aim of this work is to establish the nature and distribution of the trap states in the TiO2 layer and to determine the extent to whichtransport is dependent on the intrinsic properties of the material, its geometry and its chemical environment.
 
 

Photocurrent transients were measured on dye sensitised and unsensensitised cells.This enabled a comparison to be made between dye cations and holes.The transients in both cases have the same kinetics indicating that holes have no effect on thekinetics.Further, the transients are slow with a multiphasic decay consisitent with the presence of traps.The transient decays tend to a power law at long times.These results were modelled to extract an exponential density of tail states. 

Conductivity and photoconductivity measurements were made on dry TiO2 films.The observed behaviour is extremely sensitive to the ambient conditions.On illumination with UV light, no photocurrent is observed in air, but in vacuum a large photocurrent, up to 10^6 times larger than the dark current, is measured.

The photocurrent observed in vacuum has extremely long rise and decay times of the order of hours. The shape of the transient is intensity dependent, and the saturation photocurrent rises superlinearly with intensity.These features indicate the presence of deep intra band gap trap states.Analysis of the results suggests that not only do intra band gap states trap electrons, but they also mediaite recombination with holes in the valence band.The presence of such states is confirmed by measurements of the photocurrent due to sub band gap illumination.The conclusions are consistent with steady state current voltage characteristics, which indicate space charge limited conduction modified by the presence of traps, and with the temperature dependent dark current. 

The effect of ambient conditions on photoconduction confirms that the electrical properties of the material are dominated by its surface.The absence of a photoconductive effect in air may be explained by the adsorption of species such as water and oxygen from the ambient air onto the TiO2 surface, where they act as recombination centres. Transient absorption spectroscopy measurements confirm that charge recombination in this case is fast (~10 microseconds), but highly non-linear, again indicating the role of trap states. Evidence from transient absorption and other studies show that inter-particle transport is not rate limiting. The implications of our findings for the performance of the solar cell are discussed.

 

 

LIMITING EFFICIENCY FOR MULTI-BAND SOLAR CELLS CONTAINING THREE AND FOUR BANDS

 

 

Andrew S. Brown, Martin A. Green, Richard P. Corkish
 
 

Multi-band solar cells provide a possible approach to obtaining efficiencies that are greater than the single transition limit of 40.7% under maximally concentrated sunlight modelled as 6000K blackbody radiation. Previous work has shown that introducing a third band as an impurity band of zero width offers a higher efficiency (63.2%) than the normal two band, single transition solar cell. The enhancedperformance is a result of optical excitations via the intermediate band which harness sub-bandgap photons. This paper applies the detailed balance principle to the study of the limiting efficiency obtainable by the three band solar cell when the bandwidth of the intermediate band is varied. The optimum bandwidth was found to be 15 meV although the improvement in performance over the zero bandwidth case was not large.
 
 

The four band solar cell is also considered, as if two impurity bands were introduced or other schemes such as those based on multiple quantum dots were used, resulting in a calculated limiting efficiency of 71.6%. For these devices the assumption is made (without specifying how it could be achieved) that each photon is used for the highest possible energy excitation process. As one practical means of ensuring this photon selectivity, the valence and conduction bands of these devices were constrained to finite bandwidths. Limiting efficiencies of 58.9% and 59.0% respectively were calculated. Apart from their potential to create the intermediate bands, low dimensional structures offer the possibility of engineering the bandgap of the primary solar cell.

 

 
 
 
 

LIMITING EFFICIENCY FOR THE SERIES CONSTRAINED TWO TERMINAL TANDEM SOLAR CELLS

Andrew S. Brown, Martin A. Green

Tandem stacks of solar cells have clearly shown their ability to increase the efficiency of solar energy conversion. Low dimensional structures may offer advantages for tandem cells, for example by using superlattices of different periodicity to engineer the bandgap while maintaining constant lattice spacing. In such a way, a generic approach to the design of tandem cells may be developed that would make it much simpler to increase the number of cells in a tandem stack then at present. This paper investigates the detailed balance efficiency limit of the series constrained two terminal tandem solar cells which such approaches will make more attractive for the future.

The efficiency limit under maximally concentrated blackbody radiation for the normal unconstrained two terminal device which contains a large number of cells has previously been calculated to be 86.8%. This represents the upper limit for conversion of sun energy to electricity. Surprisingly the same limit has been shown to apply even when the cells are constrained by series connecting into a 2 terminal configuration. 

Efficiencies of 40.7, 55.5, 63.2, 67.9, 71.1 and73.4% for 1 to 6 cells respectively are calculated for the series constrained two terminal tandem solar cell under maximally concentrated blackbody radiation.These are slightly less than the corresponding efficiencies calculated for the unconstrained tandem (40.7, 55.8, 63.8, 68.7, 72.0 and 74.4%)

Under AM1.5 normalised global radiation efficiencies of 33.0, 45.3, 51.2, 54.9, 57.2 and59.2% for 1 to 6 cells respectively are calculated for the series constrained two terminal tandem solar cell. Again this is slightly less than the efficiencies calculated for the unconstrained tandem (33.0, 45.7, 51.2, 55.6, 58.0 and 59.6%).

The reduction in efficiencies for the two terminal tandem solar cells is due to the constraint imposed by matching the current flowing through the device. However, a simple proof has been developed that shows that the corresponding loss approaches zero as the number of cells in the stack becomes large. Hence a two terminal solar cell can, in principle approach the limiting efficiency for the conversion of sunlight to electricity.
 
 
 

 

Porous Titania Networks Fabricated using Polymer Templates

Rachel A. Caruso and Jan H. Schattka

Sol-gel nanocoating of organic templates has resulted in the formation of porous titania materials. This simple technique makes use of readily available templates, such as cellulose acetate membranes that contain pores in the micron scale. The membranes are soaked in the titanium dioxide precursor solutions followed by aqueous/alcohol solutions (for hydrolysis) or these solutions are filtered through the membrane. This results in an amorphous titania coating of the cellulose acetate walls. Annealing the sample to temperatures above 400 °C crystallizes the titania and removes the organic template. The titania network contains pore structure similar to the initial membrane and is composed of anatase crystalline nanoparticles (when annealed at 450 °C). Electron microscopy reveals the structure on the micron as well as the nanometer scale. It is believed that films of such highly porous titania will be applicable in the area of photovoltaics as well as catalysis.

This templating process can also be applied in steps and hence sequential deposition of materials can be achieved. For example, the incorporation of metal nanoparticles, which may enhance the semiconducting properties of the titania, will be demonstrated.
 
 
 
 

Thin Semiconducting Layers and Superlattices as Active and Passive Emitters for Thermophotonics and Thermophotovoltaics

K. Catchpole, A. Lin, M.A. Green, A.G. Aberle, R. Corkish, J. Zhao and A. Wang, 

Thermophotovoltaics involves the photovoltaic conversion by a receiver cell of radiation from a emitter which could be heated by various sources including sunlight.A prime difference from normal solar photovoltaics is that emitted energy unable to be used by the receiver, in principle, can be recycled allowing high conversion efficiency.Thermophotonics is a recent development of this concept where the emitter is ?active?, namely a heated diode, increasing the rate of energy transfer for a given emitter temperature and concentrating emission in an energy range more suited for conversion by the receiver.

Present experimental work is concentrating on the silicon and GaAs based III-V alloy systems, given the maturity of the associated technology.One realisation has been that, even as ?passive selective emitters?, thin semiconducting layers may provide an alternative offering increased design flexibility to standard rare earth doped ceramics.Calculations show that devices need to be thin to decrease sub-bandgap emission associated with radiative free carrier emission (reverse of free carrier absorption processes) and, if of similar material to the receiver, to match emission wavelengths to the receiver, due to decreased emitter bandgap at high temperature.Low dimensional structures offer additional scope for such bandgap control and for controlling free carrier absorption using the mini-band structure.
 
 
 
 

Electrical and photoelectrical processes at electrolyte - ZnIn₂S₄ interface

A. Cojocaru, A. Simashkevich

The electric and photoelectric properties of the electrolyte-semiconductor interface in the chains formed by metal auxiliary electrode-electrolyte-multinary layered semiconductors have been investigated. The dependencies of flatband potential and contactpotential at the ZnIn₂S₄ - electrolyte interface on the concentration of S ions and the value of pH of the electrolyte were studied. The values of the photopotential up to 0,6V and the short circuit current up to 1 mA/cm2 have been obtained. It was shown that the photoelectrode corrosion is practically absent. The energetic diagram for the investigated system was proposed.
 
 
 
 

Electrical Properties of Polymer-Fullerene Solar Cells

V. Dyakonov, I. Riedel, J. Parisi

Fundamental understanding of the processes of the charge generation, recombination and transport in conjugated polymer/fullerene solar cellsis decisive for their further improvement. To study the electronic transport properties, the temperature and illumination intensity dependent current-voltage analysis has been performed. The behaviour of the cell parameters, such as short-circuit current, open-circuit voltage, fill factor, power efficiency temperature coefficient, is found to be more complex compared to that for inorganic solar cell. Studies by admittance spectroscopy revealed frequency dependent contributions to the device capacitance which are relatedto the thermal activation of conductivity of the semiconductor material. 
 
 
 
 

Charge recombination processes in liquid electrolyte / solid state dye sensitised nanocrystalline semiconductor solar cells

Saif A. Haque¹ Jenny Nelson¹, Taiho Park², Andrew Holmes², Rob Potter³ and James Durrant¹

¹Centre for Electronic Materials and Devices, Departments of Chemistry and Physics, Imperial College London, SW7 

²AX, 2Melville Laboratory, Cambridge, 

³Johnson Mathey Technology Centre, Blouts court, Sonning Common, Reading RG4 9NH.

Highly porous nanocrystalline titanium dioxide films can be sensitised to visible light by the attachment of molecular dyes to their surfaces. Such films have generated considerable interest, which mainly stems from their utilisation in solar light to electrical energy conversion devices. In spite of the considerable progress that has been made in developing dye sensitised nanocrystalline solar cells the most efficient devices to date employ the iodide/ tri-iodide redox couple. In this paper we employ transient absorption spectroscopy to study the recombination reaction between the electrons in the titanium dioxide and iodine/tri-iodide ions in the liquid electrolyte and conclude that trap states in the titanium dioxide play an important role in controlling the kinetics of this reaction. 

In the second part of this paper we consider the extension of the above studies where the iodide/tri-iodide redox couple has been replaced by a solid hole conducting material. In particular we employ transient absorption spectroscopy to study the recombination reactions at the TiO2/dye/solid HTM interface.
 
 
 

Electron and electrolyte transport in the dye sensitized nanocrystalline cell
 
 
 

A Kambili, A B Walker, F Qiu, A C Fisher, A D Savin and L M Peter
 
 

Dye sensitised nanocrystalline solar cells (Grätzel cells) have achieved solar-to-electrical energy conversion efficiencie of 12% in diffuse daylight [1]. The cell is based on a thin film of dye-sensitised nanocrystalline TiO2 interpenetrated by a redox electrolyte. The high surface area of the TiO2 and the spectral characteristics of the dye allow the device to harvest 46% of the solar energy flux [1].
 
 

One of the puzzling features of dye-sensitised nano-crystalline solar cells is the slow electron transport in the titanium dioxide phase. The available experimental evidence as well as theoretical considerations suggest that the driving force for electron collection at the substrate contact arises primarily from the concentration gradient, ie the contribution of drift is negligible. The transport of electrons has been characterised by small amplitude pulse or intensity modulated illumination [2].
 
 

Here, we show how the transport of electrons in the dye-sensitised cell can be described quantitatively using trap distributions obtained from a novel charge extraction method [3] with a one-dimensional model based on solving the continuity equation for the electron density. We will present extensions of the model to establish the influence of electrolyte transport and reaction processes. To look at how the complex geometry arising from the grainy nature of the TiO2 phase influences the electron transport, We will also present results where we have solved the continuity equation for the electron in two dimensions with a boundary element method.
 
 

1. B O'Regan & M Grätzel Nature vol 353, 737 (1991)

2. A C Fisher, L M Peter, E A Ponomarev, A B Walker & K G U Wijayantha J Phys Chem B vol 104, 949 (2000)

3. N W Duffy, L M Peter, R M G Rajapkse & K G U Wijayantha Electrochem Comm vol 2, 658 (2000)
 
 
 
 

Nanostructures for solar cells with extremely thin absorbers

R. Könenkamp

Nanostructures have had a strong impact on many electronic and photonic semiconductor devices, such as transistors, LEDs, lasers, switches etc. Over the last years we have developed strategies for implementing nanostructures also large area applications, particularly solar cells. The talk will address the potential advantages of nano- and micro-structured photovoltaic devices, and report first results on solar cells with extremely thin devices.
 
 
 
 

Evidence for high negative charge densities in AlF₃ coatings of silicon-based solar cells: A promising source for large drift fields

D. König, G. Ebest

Professur Elektronische Bauelemente, Technische Universität Chemnitz, D-09107 Chemnitz, Germany

R. Scholz, S. Köstlmeier, T. Kampen, D.R.T. Zahn

Institut für Physik, Technische Universität Chemnitz, D-09107 Chemnitz, Germany

As a counterpart for the well known materials generating a large positive interface charge, we present a novel aluminium fluoride surface coating allowing for a massive localization of negative charges. From C-V measurements, we find negative sheet densities of up to -4x10¹² cm^-2, resulting in drift fields of about -0.6 MV cm^-1 in the Si substrate.

The energetic positions of the valence bands are investigated using Ultraviolet Photoemission Spectroscopy (UPS) and compared with density functional calculations of one of the aluminium fluoride modifications. The observed high energy tails of the valence density of states (DOS) are interpreted in terms of the most strongly charged fluorine (F) ions in regions with sub-stoechiometric fluorine content. From these results and further evidence for F-deficiency obtained from inclined angle Rutherford Back-Scattering (RBS), we conclude that the material contains a large fraction of anion vacancies. These can act as deep electron traps, resulting in the observed localization of negative charges. 

Even though the negative sheet density corresponds to a large volume density oftrapped electrons of -4x10¹⁸ cm^-3, their DOS remains to small to be observable in UPS. However, from a detailed analysis of the inflection points in C-V measurements, we deduce a position of about 0.85 eV above the valence band, so that the trapped electrons remain localized under typical operating conditions of a solar cell.
 
 
 
 

Effect of Hybridization of Nano-sized Metal Oxides on the Performance of Dye-sensitized Photoelectrochemical Cells

A. Konno, G.R.R.A. Kumara, and K. Tennakone

The dye-sensitized solar cell developed by Grätzel et al is well known and the optimized system is reported have practically viable energy efficiencies.The only other reported dye-sensitized solar cell having a comparable efficiency is the device based on SnO2/ZnO composite films which the author devised at Prof. Tennakone's laboratory in Sri Lanka (the reported efficiency ~ 8 %).It was found that the recombination in dye-sensitized photoelectrochemical cells could be suppressed by coating an insulating shell on the SnO2 and as a consequence increase of the efficiencies was noted.One of the most important inferences is that the photo excited dye molecules on a thin insulating film on a semiconductor could inject electrons to the conduction band of the semiconductor via tunneling.The insulating shell effectively suppresses recombinations of the geminate pair (ie., e-, D+) and also recombinations of injected electrons with the acceptors in the electrolyte.
 
 
 
 

The effect of chemisorbed dyes on tunnelling characteristics of nanoporous TiO₂ observed in Scanning Tunnelling Spectroscopy (STS) performed in UHV

A.R. Kumarasinghe and W.R.Flavell

Currently, mesoporous (nanocrystalline) metal oxide semiconductors films, notably anatase TiO₂, are the subjects of intense discussion because of their potential applications in number of charge-separating devices such as dye-sensitised liquid state and solid state solar photvoltaics, which are considered to be the possible alternatives for the Si-based solar cells. Solid state and liquid state photovoltaics are fabricated from a colloidal suspension of TiO₂ with subsequent surface modification by a suitable metal-based charge transfer dye such as ruthenium. Dye sensitised nanocrystalline TiO₂ films have shown strikingly high efficiencies. Here we report, UHV STM/STS studies on nanoporous TiO₂ films in the presence and absence of the dye sensitisers on the surface of TiO₂. The work is aimed at investigating the surface electronic and topographical features of nanocrystalline metal oxide films. STM I/V curves are employed to measure the variation in tunnelling characteristics laterally across the sample and differing characteristics are observed in on and off grain profiles. An interesting change in tunnelling characteristics of TiO₂ is observed when Ru bipyridyl sensitiser is present on the surface. An apparent narrowing of the nominal gap is observed which may be consistent with the excitation from the ground state of the dye to the conduction band of TiO₂. Further it can be observed that the extent of the narrowing of the nominal gap depends on the type of dye on the surface of TiO₂, which suggests strongly that the observed change in I/V characteristics of the surface of nanoporous TiO₂ film mainly arises due to the adsorbed dye.
 
 
 

Multi-interface novel devices

Z.T. Kuznicki, M. Ley

Recently, new innovative concepts have been proposed to increase the photovoltaic (PV) conversion efficiency of monocrystalline Si solar cells [1-2]. The number and originality of the approaches hearlds the beginning of a new era, the third solar cell generation, which is based in large part on new mechanisms. 

One of the most promising possibilities is the MIND (multi-interface novel device) cell, which exploits nanoscale modifications of Si PV material [3]. For example, a planar nanostructure inserted by P ion implantation within the monocrystalline Si bulk can transform its optical yield. Post-implantation point defects can be controlled thanks to L-H type interfaces which modify recombination mechanisms at the edge of the amorphized material (particularly sharp a-Si/c-Si heterointerfaces). The doping and amorphising ion implantation follows an annealing kinetics which modifies the up to now well-known optoelectronics performances. 

The structural analysis obtained for multi-interface solar cells is compared with the results from Motooka et al [4]. Using electron microscopy images, we show that a-Si/c-Si interfaces are abrupt and similar in the two following cases : high energy ion implantation (5 MeV) and medium-energy ion implantation (180 keV) followed by an adequate thermal treatment. The transition zones between the two Si phases are only a few atomic layers thick. These results are in good agreement with the theoretical model of a-Si/c-Si interface obtained by molecular dynamics simulations.

Among other observations, we have seen an important and reproducible internal quantum efficiency (IQE) improvement (more than 25 %) in the near IR (above 900 nm) [4]. The IQE modification is due, on the one hand, to increased IR absorption and, on the other, to photogeneration via an extrinsic energy band (direct-like bandgap) in the presence of thermal phonons. This energy band is related to the post-implantation defects produced during the nanostructure formation within the transition zones between the two Si phases.

[1] Z.T. Kuznicki, F. Capot, S. de Unamuno, Photovoltaic conversion with multiplication: thermodynamic limits for any impact energy, 2nd World Conference on Photovoltaic Energy Conversion (WCPEC), Vienna, Austria, 6-10 July1998, 180-183.

[2] M. A. Green, Third generation photovoltaics : ultra-high conversion efficiency at low cost, Prog. Photovolt. : Res. Appl., 9, 123-135, 2001.

[3] Z.T. Kuznicki, Multi-interface solar cells Part I. Limits, modeling and design; Part II. Elements of realization, E-MRS First Polish-Ukrainian Symposium ?New Photovoltaic Materials for Solar Cells?, Cracow-Przegorzaly, Poland, October 21-22, 1996, Proceedings, pp. 58-98.

[4] T. Motooka, S. Harada, and M. Ishimaru,, Phys. Rev. Lett., 78,1997, 2980-2982.

[5] Z.T. Kuznicki, M. Ley, Thermal modifications of internal quantum efficiencies in the near IR,28thIEEE Photovoltaic Specialists Conference, September 15-22, 2000, Anchorage, Alaska, USA.
 
 
 
 

Surface Photovoltage Spectroscopy: A handy Tool for the Detection of Charge Carrier Injection Processes in Extremely Thin Absorber (ETA) solar cells ?
 
 

Frank Lenzmann, Brian O?Regan, Jeannette Wienke, Carolien Huisman, Liesbeth Reijnen, Albert Goossens
 
 

The concept of ETA solar cells is closely related to the concept of dye-sensitized solar cells. In dye-sensitized solar cells the light absorption is achieved by a monolayer of a molecular dye (organic/metalorganic), adsorbed at the surface of a very high surface area, mesoporous n-type semiconductor film (typically TiO₂). In ETA solar cells on the other hand the molecular dye is replaced by an extremely thin (approx. 2-5 nm) layer of an inorganic pigment, such as Cu₂S or CuInS₂ for instance. The photovoltaic effect in this type of solar cells is due to charge carrier injection from the absorber layer into the contacting n- and p-type layers respectively upon excitation by incident photons. These injection processes can be considered the heart of such type of injection solar cells. The detection of these processes for a given interface of interest is therefore a key step in its investigation. 
 
 

The Kelvin probe technique and Surface Photovoltage Spectroscopy (SPVS) are traditionally used for the characterization of semiconductors, yielding information about electrical properties such as work function, bandgap, semiconductor type and defects.
 
 

Recently we have shown that these techniques can also be used for the detection of electron injection processes in dye-sensitized solar cells and this proved to be useful for the elucidation of mechanistic details of electron injection in these devices.

The extreme sensitivity of the photovoltage signal to very low light intensities makes this technique a powerful analytical tool. Therefore SPVS may in some cases bear advantages over techniques, which are based on the detection of current rather than of voltage signals such as, e.g., incident photon conversion efficiency measurements.

We performed Kelvin probe measurements in order to determine the work functions of different ETA-pigment materials (PbS, In₂S₃, Cu₂-xS). Furthermore we investigated the use of SPVS for the detection of charge carrier injection processes in ETA solar cells. Despite conceptual similarities between ETA solar cells and dye sensitized solar cells it was a question whether charge carrier injection could indeed be observed by means of SPVS in ETA cells, too. A comparative SPVS study of various interfaces, such as TiO₂/PbS, ZrO₂/PbS, CuSCN/PbS was therefore undertaken in order to clarify this question.
 
 
 

A new ZnO/CdTe columnar composite film for eta-solar cells

First results of CdTe deposition on novel ZnO films consisting of free standing single crystalline columns of several micrometers height and ~100 nm diameter will be presented. The ZnO films obtained by electrochemical deposition on conductive glass have considerable potential for use in photoelectric thin film devices [1]. Morphology, electronic parameters, and basic optical behavior, such as transmission, reflectance and light trapping will be presented. The cadmium telluride deposited using an especially designed low temperature evaporation technique is lining the ZnO columns as a continuous smooth thin film with conformal coverage. The nanostructured columnar ZnO/CdTe composite films have been characterized as function of the CdTe layer thickness using scanning electron microscopy equipped with a EDS analyzer and X-ray diffrac!

!

tometry. The optical properties (transmission and reflectivity) of the (F:SnO2)-conductive-glass/ZnO/CdTe composite films have been analyzed. Some possibilities of using these composite films in devices such as extremely thin absorber solar cells (eta-solar cells) will be discussed.

[1]. R. Könenkamp, K. Boedecker, M. C. Lux-Steiner, M. Poschenrieder, F. Zenia, C. Lévy-Clément, S. Wagner, Appl. Phys. Lett. 77, 2575 (2000).
 
 
 

Near IR improvement of Si photovoltaic conversion by a nanoscale modification

Several new concepts to increase the conversion efficiency of solar cells have been presented over the last few years. One possibility could be the MIND (multi-interface novel device) solar cell with a highly doped amorphized and nanostructured layer inserted in the emitter [1]. This active substructure can be created by P ion implantation followed by an adequate thermal treatment necessary to form two sharp a-Si/c-Si heterointerfaces and the corresponding transition zones.

We report a large near IR internal quantum efficiency (IR-IQE) improvement in the near IR ( > 900 nm) for a series of MIND solar cells in comparison with the excellent classical solar cell (h = 20 %) [2]. This effect is reproducible and can be explained by the presence in the band-gap of an extrinsic band energy that allows direct optical transitions originating from post implantation defects. The IQE improvement appears even in non-optimized devices (not annealed enough after implantation).

The improved IR generation is possible because of phonons aiding transitions from extrinsic centers to the Si indirect conduction band (the improved IR absorption is then transformed into a PV conversion). The effect depends on device temperature : the higher the temperature, the better IR conversion efficiency, which is in total contradiction with classical cells.

The deconvolution of the IQE difference between a MIND and a classical cell gives four characteristic energies which are related to post-implantation defects in the recrystallized c-Si zones. No IQE perturbation due to the LH- type potential barrier studied in reference 4 is observed in the near-IR.

[1] Z.T. Kuznicki, Multi-interface solar cells Part I. Limits, modeling and design; Part II. Elements of realization, E-MRS First Polish-Ukrainian Symposium ?New Photovoltaic Materials for Solar Cells?, Cracow-Przegorzaly, Poland, October 21-22, 1996, Proceedings, pp. 58-98.

[2] Z.T. Kuznicki, M. Ley, Thermal modifications of internal quantum efficiencies in the near IR,28thIEEE Photovoltaic Specialists Conference,28th IEEE Photovoltaic Specialists Conference, September 15-22, 2000, Anchorage, Alaska, USA.

[3] M. Ley, Z.T. Kuznicki, Nouvelle barrière de potentiel dans une cellule solaire photovoltaïque, Journées Nationales du Réseau Doctoral de Microélectronique, April 24-25, 2001, Strasbourg, France, Proceedings pp. 104.

[4]M. Ley, Z.T. Kuznicki, Experimental and theoretical investigations of a new potential barrirer due to sharp a-Si/c-Si heterointerfaces buried in the solar cell emitter, E-MRS Spring Meeting, June 5-8, 2001, Strasbourg, France.
 
 
 
 

Electron transfer and Carrier relaxation dynamics in photovoltaic nanomaterials

John B. Asbury, Neil A. Anderson, Encai Hao, Xin Ai, Wanhee Goh, and Tianquan Lian

Solar cells based on dye-sensitized nanocrystalline TiO₂ thin films and nanoparticle/polymer composite materials are promising technology of future generation photovoltaics. We have undergone a series of studies to investigate both forward and backward electron transfer and carrier relaxation dynamics in a variety of dye-sensitized nanocrystalline thin film electrodes and polymer-nanoparticle composites. We have also compared dynamics in thin films with those in colloidal nanoparticles. In this talk, I will summarize the results from these studies. I will discuss the dependence of charge transfer and relaxation dynamics on the property of the semiconductor, sensitizer (dyes and conjugated polymers), and their interaction.
 
 
 
 

Quantum wires and nanoclusters in porous-Si based structures as novel object for enhancement of PV device sensitivity

V.G.Litovchenko, N.I.Klyui, V.P.Kostylyov, V.A.Skryshevsky, A.V.Sarikov, Yu.V.Rassamakin

Porous silicon samples prepared by chemically or electrochemically were studied. In order to modify the material properties deposition of diamond-like carbon (DLC) or silicon carbide films were carried out by CVD or PVD methods. The structures subjected to rapid thermal annealing (RTA) were also investigated. Optical properties of such nanostructures were studied by time resolved photoluminescence and surface photo-voltage techniques.

It has been shown that such nanostructures may be used as antireflection layers for silicon based solar cells. In this case, it is useful to protect surface porous-Si layer by DLC or SiC films and, hence, improve degradation stability of the solar cells.

Besides, it has been established that RTA of the nanostructures with DLC or SiC films results in creation of carbon or silicon carbide nanoclusters onto silicon wires. In its turn, it leads to increasing of PL intensity in spectral range where Si-based solar cell is sensitive. As a result, Si solar cell sensitivity increases that was confirmed by surface photo-voltage measurements.

The mechanisms of the effects observed are also discussed.
 
 
 

Thermodynamics of Solar Energy Conversion in Novel Structures

Thermodynamics provides a deep understanding of solar energy conversion. It also allows the identification of what are the irreversible processes that when suppressed can lead to the maximum efficiency conversion. Novel devices have been proposed recently in the field of photovoltaics being worthwhile to study what are the irreversible processes taking place in them. These devices comprise compromise, for example, the intermediate band solar cell, the hot electron solar cells and the thermophotonic converters.
 
 
 

Design Constrains of the Quantum Dot Intermediate Band Solar Cell

The Quantum Dot Intermediate Band Solar Cell is a solar cell being investigated to implement in practice the Intermediate Band Solar Cell, a cell with the potential of achieving a 63.2 % of photovoltaic energy conversion under concentrated sunlight. The physical principle behind this operation is the absorption of two below bandgap energy photons to create one electron-hole pair. These photons are absorbed thanks to the existence of an electron band structure within in what in ordinary semiconductors constitutes the gap.
 
 
 
 

Competing electron transfer pathways in dye-sensitised TiO₂ electrodes.

Ivan Montanari*, Jenny Nelson and James R. Durrant

Dye-sensitisation of wide bandgap semiconductors has been succesfully utilised in photoelectrochemical cells for a number of years¹. A detailed understanding of the kinetics involved in the functioning of such cells is both scientifically interesting per se and important in technological development of such devices. Previous studies have established the ultrafast nature of electron injection² and looked at the strongly bias dependentrecombination kinetics³. In this poster we extend these studies to the events following ultrafast injection, namely kinetic competition between charge recombination from the semiconductor film and electron transfer from I- ions in solution to rereduce the dye cation. The kinetics of the rereduction reaction have inparticular received little attention to date.

Transient absorption spectroscopy was employed to monitor the decay of absorption of the dye cation as its concentration is depleted by the two competing processes. Data were collected as a function of iodide concentration and applied electrical bias. The iodide reaction was found to be strongly dependent upon iodide concentration but independent of the bias applied. The effect of externally applied bias on this branching ratio has been assessed. The kinetic curves are modelled by Monte Carlo simulation of electron dynamics and cation re-reduction following excitation by the laser pulse. This approach leads to a good agreement with the experimantal data. The implications for solar cell function are discussed.

References

1) O'Reagan & Grätzel Nature (1991), 353, 737-739

2) Tachibana et al. J. Phys. Chem. (1996), 100, 20056-20062

3) Haque et al. J. Phys. Chem. B (1998), 102, 1745-1749
 
 
 

Theoretical considerations quantum well solar: coupled wells vs. superlattice

The carrier escape mechanisms from GasAs/AlGaAs multiple quantum wells had been studied in an old paper by Fox et al. (Appl. Phys. Lett. 63,2917(1993). In the photovoltaic regime, a system of coupled wells performed very differently from a regular superlattice. We review the results and discuss theoretical implications.
 
 
 
 

Charge transport in electrodes consisting of metal oxide nano-particles filled with electrolyte solution

S. Nakade, S. Kambe, M. Matsuda, S. Ito, T. Kitamura, Y. Wada, S. Yanagida

Charge transport property of nano-porous metal oxide films filled with electrolyte solution is studied by pulsed laser induced photocurrent transient measurements.It is found that the property depends on diffusion coefficient, concentration, and species of cation in the solution, in addition to the annealing temperature and crystallinity of metal oxide nano-particles.The transport property is interpreted by ambipolar diffusion and trapping model.In order to obtain the fast rate of charge transfer in the films, high cation concentration in an electrolyte and high crystallinity of metal oxide particles are preferred.However, for the crystallinity, small portion of amorphous state in the particles seems necessary for effective neck growth between particles by annealing.
 
 
 
 

Charge Transport vs. Recapture, a Critical Competition in Low-Temperature and Solid- State Sensitized Photovoltaic Cells

Brian O'Regan, Joop van Deelen, Jan Kroon

Energy research Centre of the Netherlands

Westerduinweg 3, 1755 ZG Petten, The Netherlands

Hans van't Spijker and Albert Goosens

Laboratory for Inorganic Chemistry, Delft University of Technology

Julianalann 136, 2628 BL Delft, The Netherlands

Successful performance of any nano-scale sensitized photovoltaic cell (NSP) depends on the outcome of three kinetic competitions: charge injection vs. excited state decay, sensitizer regeneration vs. recombination, and charge transport vs. capture.It is in the latter two competitions that there is the most scope for improvement in today's liquid junction cells.For two future directions of NSP cells: solid-state cells, and low temperature, flexible substrate cells, the latter competition becomes the most difficult to manage.This is due to slower charge transport in low temperature films and/or increased recapture rates of solid state hole conductors.These assertions will be discussed in terms of the following new results:

1. We will discuss photocurrent transients in dye sensitized liquid junction cells having standard colloidal TiO₂ films, colloidal ZnO films, or electrodeposited ZnO films.Electron mobility is much higher in single crystal ZnO than in TiO₂, but up to now most colloidal ZnO films have not shown strongly improved charge transport.We have synthesized colloidal ZnO films where the charge transport is considerably faster (~3x) than in TiO2 films.In electrodeposited ZnO films, in which the grains are crystallographically aligned, we have found that photocurrent transients are >10 times faster than in colloidal ZnO films.

2. We have measured, for the first time, the transient spectra of the mobile hole in the p-type semiconductor CuSCN.W can now measure the dye regeneration (hole injection), and electron recapture rates for solid-state NSP cells using CuSCN.Using current transients from the same cells, it is possible to show that the transport and recapture are in much closer competition in these cells compared to cells using iodine/tri-iodide.This is why literature reports show successful CuSCN cells only from "accelerated transport" nano-porous electron conductors such as "super-adherent TiO₂", and electrodeposited ZnO.

3. We will also compare liquid junction cells made with standard (colloidal) TiO₂, low temperature TiO₂, high porosity TiO₂, and TiO₂ films compacted by high pressure, as well as "standard treatment" (400 °C) and low heat (150 °C) electrodeposited ZnO.These results also point to a controlling role for electron transport in the cell performance.
 
 
 
 

HISTONS, NEW QUASI-PARTICLES, AND ELECTRON TRANSFER IN BACTERIAL PHOTOSYNTHESIS

V. S. Pavlovich

It is shown in terms of the histons, new quasi-particles, that the collective dipole librations in the antenna systems cause a high thermal broadening of the B800 and B850 absorption spectra. Obtained equation for the full half-width provides an excellent fit to the Rb. sphaeroides and Rps. acidophila B800 known data at 4.2-270 K with an average histon frequency of 63 and 50 cm-1, respectively. Taking into account this fact we developed and applied a simple theory to be useful for experimental analyses and forconceptual understanding of the electron transfer (ET) in bacterial photosynthesis. We report the quantitative examination of the effect of histons (collective librations) on photoinducedET in isolated reaction centre complexes of Rps. Viridis and Rb. sphaeroides as a function of temperature.
 
 
 
 
 
 
 

Growth studies of Ge islands for enhanced performance of Si thin film solar cells

We have studied the self organized growth of Ge-islands on Si-substrates in the Stranski-Krastanow growth mode with Si MBE for the use in thin film solar cells. For samples with high Ge concentrations and high crystal quality the increased infrared absorption of the SiGe-islands incorporated in the base material of a Si solar cell should overcome the reduced open circuit voltage due to the lower band gap. A series with growth temperatures from 500-700'C has been grown to optimize Ge island/dome formation and crystal quality of single and multiple island layers. Photoluminescence measurements, atomic force and transmission electron microscopy exhibit optimized three dimensional growth with moderate intermixing of Si into Ge-islands and wetting layers at temperatures around 650'C. The onset of 3d-island growth and transition of island shape and density with increasing Ge-coverage was studied in the range between 2-8 monolayers of Ge. Photoluminescence and atomic force measurements clearly indicate a temperature dependent growth of Ge-islands starting at a wetting layer thickness of 4,5 ML. With increasing Ge coverage the dot shape changes from small islands to dome like clusters. Preliminary measurements of Si solar cells with Ge islands deposited in the active base material exhibit open circuit voltages up to 504meV at AM1.5 under non optimized conditions (no AR-coating, weak passivation).
 
 
 

ESEM imaging of polyfluorene blend cross-sections

Photovoltaic devices based on blends of conjugated polymers give improved performance due to efficient exciton dissociation at thepolymer-polymer interface. The phase separation of polyfluorene derivative blends has been extensively studied, using AFM to examine the film surface. However, it is the phase separation in depth of the film that directly affects device performance. Here we report the use of Environmental Scanning Electron Microscopy (ESEM) to examine cross-sections of 200nm thick blend films. Differences in chemical composition and electronic structure provide contrast between the phases in the blend. The results show that the large scale phase separation at the surface propagates through the film to the underlying substrate. We discuss the implications of this structural information for the emissive properties of blend films.
 
 
 

Nondispersive trap-limited electron transport in macroporous GaP

Electron transport in macroporous GaP networks permeated with electrolyte solutions has been studied under steady-state conditions, by analysis of the photocurrent response upon a small-amplitude modulation of the light intensity. It is found that electron transport is nondispersive, characterized by a single transit time that depends on the thickness of the porous layer and the background light intensity. The transit time is determined by multiple trapping in interfacial states close to the electron Fermi level. The density-of-states function in a considerable region of the band gap can be determined from the transit time, when the energy of the electron Fermi level is 

changed by the background light intensity.
 
 
 
 

InGaAs/InGaAs Strain-Compensated Quantum Well Cells for Thermophotovoltaic Applications

Carsten Rohr, Paul Abbott, James P. Connolly, Ian Ballard, Keith W.J. Barnham, Massimo Mazzer

Quantum well cells (QWCs) based on InP substrates have shown several advantages for thermophotovoltaic (TPV) applications where heat radiation is converted into electricity: (a) a better voltage performance compared to a homogenous cell made from the well material, (b) the possibility to match the spectral response to a given illuminating spectrum up to about 2 mm, including using strain-compensated QWCs, and (c) a better temperature coefficient of efficiency. There is considerable interest in longer-wavelength, rare-earth emitters (such as Holmia) because the lower temperature at which they operate can give higher overall system efficiency and also reduced pollution. However, it is not easy to find such low-bandgap binary or ternary semiconductors which are lattice-matched to practical substrates and strained systems inevitably mean higher dark currents. By strain-compensating InGaAs barriers and InGaAs quantum wells of appropriate compositions, the absorption of a QWC can be extended to about 2 mm. Due to the higher barriers the dark current remains at a low level more appropriate to lattice-matched InGaAs. Great care has to be taken in design and growth to achieve a situation that is close to strain-balance with zero stress, in order to keep the residual strain small and not to exceed the critical thickness. The spectral response of these strain-compensated QWCs are modelled and conversion efficiencies are predicted. Strain-compensated InGaAs/InGaAs QWCs show superior performance when compared with bulk InGaAs on InP monolithic interconnected modules and bulk GaSb TPV cells.
 
 
 
 

Electric field effect on the bound polaron in spherical quantum dots

H. Satori and M. Fliyou

Using a variational calculation within the effective masse approximation, we calculated the binding energy of a bound polaron to shallow donor impurity in spherical Quantum Dots (QD), where a weak electric field is applied in the z-direction. The interactions of carrier charges (electron and ion) with the longitudinal optical phonons as well as the surface optical phonons are considered. Calculations are made using a finite confinement potential. Results are obtained as a function of the dot size and the donor impurity position for different electric field strengths. It is found that the binding energy is very sensitive to the polaronic effect, the confinement

and the electric field.
 
 
 

Nanostructural effects in plasmatically prepared polysilylenes

The plasmatic polysilylenes, demonstrated on poly ( methylphenylsilylane), prepared both in radio frequency and microwave plasmas are examined as prospective materials with nanostructural features for application in both luminescence and as a host material for photovoltaics in combination with small organic molecules. 

The 1D structure is preserved in plasmatically prepared materials on nanostructural scale, what is demonstrated by excitonic photolumincescence on 360 nm. 

The main advantage of those materials are their exceptional transport and good stability, compared to their 1D counterparts, prepared in conventional way from solutions. The influence of the conditions of preparation and especially the role of hydrogen on the structural parameters are examined in detail.
 
 
 
 

HIGH EFFICIENCY ORGANIC PHOTOVOLTAICS FROM SOLUBLE DISCOTIC LIQUID CRYSTALLINE MATERIALS

L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R.H. Friend, J.D. MacKenzie

Self-organisation of phase separation discotic liquid crystalline and crystalline soluble conjugated molecular materials has been employed to create, directly from solution, segregated structures optimised for charge separation and transport in photovoltaic device structures.

High external quantum efficiencies up to 34% near 490nm have been reached with a soluble electron accepting perylene derivative and a soluble liquid crystalline discotic peri-hexabenzocoronene. The high efficiencies result form efficient photoinduced charge transfer between the materials as well as effective transport of electrons and holes to the cathode and anode through segregated perylene and discotic hexabenzocoronene p-system.
 
 

Atomic force microscopy and device characteristics suggest that the drivng force for phase separation and surface energy effects during spin coation result in a spontaneous vertical segregation of the hexabenzocoronene and the perylene normal to the plane of the spun film. The existence of a nearly ideal, self-organised structure in which vertical segregation of charge transport layers coexist with a high interfacial area between the two charge transfer components is supported by electron microscopy and photocurrent action spectra comparisons.

This work demonstrates that discotic liquid crystalline organics can be utilised for optoelectronic applications where high performance is attained through the exploitation of intermolecular and mesoscopic ordering.

CONDUCTANCE OF THE ELLIPTICALLY SHAPED QUANTUM WIRE

We have studied the effect of the curvature on the conductance of an ideal elliptically shaped quantum wire in the zeroth-order approximation in the width of the wire. It has been shown, in particular, that, due to the quantum-mechanical effective potential induced by the curvature, the dependence of the conductance G(V) on the applied bias changes drastically. Thus, the effect of the curvature can be observed by measuring the conductance of a quantum wire. On the other hand, one can change the characteristics of the quantum wire, such as conductance, setting its size, shape, or applied bias.
 
 
 
 

SYNTHESIS OF SURFACE-MODIFIED SEMICONDUCTOR NANOCRYSTALS AND INVESTIGATION OF PHOTOINDUCED CHARGE TRANSPORT IN NANOCRYSTAL-POLYMER COMPOSITES

D. V. TALAPIN, N. P. GAPONIK, A. L. ROGACH, A. EYCHMÜLLER

Institute of Physical Chemistry, University of Hamburg, 

20146 Hamburg, Germany

S. K. POZNYAK

Physico-Chemical Research Institute, Belarusian State University, 220050 Minsk, Belarus

Water soluble CdS, CdSe, CdTe, and HgTe nanocrystals were synthesized by the reaction of Cd2+ (Hg2+) ions and H2S (H2Se, H2Te) in the presence of different thiols as stabilizing agents. 

CdSe, CdTe, InP and InAs nanocrystals were prepared via organometallic approaches either by high-temperature thermolysis of the precursors or by a dehalosylilation reaction. The nanocrystals were readily dispersible in a variety of organic solvents such as toluene, n-hexane, chloroform etc. 

In all cases successful wide range tuning of the semiconductor band gap energies was achieved by the control over the nanocrystal size. Post-preparative treatments resulting in the exchange of the capping groups with amines, thiols, pyridines, thiophenes, pyrrols etc. allowed the transfer of the nanocrystals into a variety of solvents and the modification of their optical properties (e.g. increasing the photoluminescence quantum efficiency). Furthermore, this treatment facilitated a charge exchange between the nanocrystals and their environment. The isolated nanocrystals and films of nanocrystals were characterized by SEM, HRTEM, XRD, SAXS, UV-Vis-NIR absorption and photoluminescence spectroscopies.

The combination of electron conducting nanocrystals and hole conducting conjugated polymers in a single composite may provide an effective charge transport and collection of the photogenerated carriers. Conducting polymer based nanocomposites have been produced both via the treatment of electrochemically prepared polyaniline films with CdTe aqueous colloidal solutions and via the electrochemical polymerisation of pyrrole into closely-packed layers of CdTe nanocrystals. Another kind of nanocrystal/conducting polymer composite was prepared by mixing CdTe nanocrystals with the anionic poly (3,4-ethylenedioxythiophene):poly (4-styrenesulphonate) complex (PEDOT:PSS, Baytron P). 

Both the photocurrent spectra and the photocurrent-potential curves of the composites clearly exhibited quantum confinement of the semiconductor quantum dots and indicated an efficient exchange of photogenerated charges between the nanocrystals and the polymer matrix. The as-prepared highly doped composite CdTe/PEDT:PSS films can electrochemically be reduced to a range of doping levels permitting a better match between the band edges of the nanocrystals and the polymer matrix.

This work was partially supported by the NATO Collaborative Linkage Grant CLG 976365.
 
 
 
 

OUT-DOOR PHOTOVOLTAIC CHARACTERISATION OF PLASTIC SOLAR CELLS

S.M. Tuladhar (1,2), E.A. Katz (2), D. Faiman (2), F.Padinger (3), T. Fromherz (3), C. Brabec and N.S. Sariciftci (4)

(1)Albert Katz International School for Desert Studies, Ben-Gurion University of the Negev, Sede Boqer Campus, 89990 Israel;

(2) National Solar Energy Center,J. Blaustein Institute for Desert Research,Ben-Gurion University of the Negev, Sede Boqer Campus, 89990 Israel;

(3) Quantum Solar Energy Linz (QSEL), Guberstr. 40-42, A-4020 Linz, Austria;

(4) Christian Doppler Laboratory for Plastic Solar Cells,Physical Chemistry,Johannes Kepler University of Linz, A-4040 Austria.

Bulk donor-acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing energy conversion efficiencies above 2 % [1]. The photoresponse of these plastic devices is based on the ultrafast transfer of photoinduced electrons from a conjugated polymer as a donor to a nearby fullerene molecule (or molecule of fullerene derivative) as an acceptor [2]. 

The present paper reports some measurements of the photovoltaic properties of such devices with initial efficiency of about 3 %, performed under real sun irradiation at Sede Boqer on 11 successive cloudless days, during the noon-time period, at normal incidence to the incoming solar beam radiation. The solar irradiance, measured with a thermopile pyranometer (Eppley PSP), was found to remain constant, during the test runs, to within approximately ± 0.3 % at levels that slightly exceeded 1000 W/sq.m. Moreover, under such conditions the measured sunlight spectrum at Sede Boqer is found [3] to be exceedingly close to the standard AM1.5 spectrum. Cell temperatures were controlled using a thermoelectric base plate and I-V curves were measured at various temperatures in the range 20-60 degrees C.
 
 

Open-circuit voltage, Voc, of the cells studied was found to decrease linearly with increasing temperature, with a temperature coefficient dVoc/dT = - (1.40 - 1.65) mV/K. Room temperature values of Voc (850 - 880 mV) and its temperature dependence remained almost constant after 6 hours of the real sun irradiation in air during 11 successive days of the experiment. At the same time, substantial degradation of short-circuit current, Isc, and fill-factor, FF, was in evidence. These results point to a degradation of photoconductivty and charge collection rather than photovoltage generation process.

The irradiation/air exposure was demonstrated to change particular parameters of the temperature dependencies of Isc, FF and efficiency but for all stages of the degradation, the regions with positive temperature coefficient for Isc and efficiency were observed. The temperature dependencies of the main cell parameters are in accord with the results of similar measurements under irradiation by a solar simulator [4]. 

References:

[1] S. E. Shaheen, C.J. Brabec, N.S. Sariciftci , F. Padinger, T. Fromherz and J.C. Hummelen, Appl. Phys. Lett. 78, 841 (2001).

[2] N.S. Sariciftci and A.J. Heeger, Handbook of Organic Conductive Molecules and Polymers (Willey, New York, 1997) pp. 413 - 455.

[3] D. Berman and D. Faiman, Sol. Energy Mater. & Sol. Cells 45, 401 (1997).

[4] J.M. Kroon, private communication.
 
 
 
 

Micro-Photoluminescence Studies on Polycrystalline CuInGaSe₂ Thin Films

T. Unold, K. Bothe, G.H.Bauer

Polycrystalline thin films are often materials with inhomogeneities on different length scales consisting ofcrystal grains, grain boundaries, and different phases causedby composition gradients and impurities. We have investigated the spatial variation of the recombination properties of polycrystalline CuInGaSe2 on a sub-micrometer scale using a confocal micro-photoluminescence setup. The reflection signal which is simultaneously detected with the photoluminescence signal correlates well with the surface morphology consisting of micrometer size crystallites as determined from atomic force microscopy and scanning electron microscopy.The photoluminescence signal, however, revealsthe presence of island-like structures on a larger length scale that not only differ in their signal intensity but also in their spectral characteristics. The photoluminescence mapping can be used to deduce the variation of the local quasi-Fermi level splitting in the samples.The implications for the optimization of solar cells will be discussed.
 
 
 
 

Probing the single-particle orbital energy spectrum of insulating quantum dots by resonant tunneling spectroscopy

Vanmaekelbergh, E. Bakkers, Z. Hens

Current techniques in colloid chemistry permit the synthesis of a large variety of insulating nanocrystals with a tunable size in the 1-5 nm range, hence considerably smaller than what can be reached by lithography [1]. The energy level spectrum of these nanocrystalline quantum dots (Q-dots) in relation to their chemical identity, crystal structure, size and shape is a matter of experimental and theoretical interest. This is also true for the inter-particle Coulomb interactions in these confined insulating systems. There is a growing tendency to study these systems on the level of a single nanocrystal by optical and electrical techniques. 

Resonant tunneling through a single Q-dot can be studied using a Scanning Tunneling Microscope. The colloidal crystals are anchored on a conducting substrate, the tip is positioned above a crystal and scanning and feedback controls are disabled. In such a way, a metal substrate / dot / tip Double-Barrier Tunnel Junction (DBTJ) is formed. In an asymmetrical configuration, the tunneling spectrum is related to the spectrum of discrete energy levels of the insulating nanocrystal, while the zero-conductivity gap is related to the (quasi-particle) band gap [2].

We report on a yet unexplored possibility of STS that may play a key role in the determination of the single-particle orbital energy spectrum of insulating nanocrystalline Q-dots [3]. The time-averaged electron occupation of a given (resonant) energy level of the nanocrystal is determined by the relative rates of electron transfer, into - and out of the energy level. A Scanning Tunneling Microscope offers the possibility to vary the tip-to-dot distance and, thus the time-averaged electron occupation of the resonant orbitals of the Q-dot. This means that Coulomb interactions between electrons in resonant orbitals can be turned on and off by variation of the tip-to-dot distance. 

In shell-tunneling spectroscopy, electrons tunnel one at a time. The conductance spectrum corresponds to the single-particle orbital energy spectrum of the Q-dot. We obtained the orbital energy spectrum of a 4.3 nm CdSe Q-dot at 4.2 K and found a very good agreement with the results of pseudo-potential theory [see figure]. When the tip is brought closer to the dot, the orbitals are filled accumulatively with electrons (shell-filling spectroscopy). Electron-electron Coulomb interactions break down the spin and orbital degeneracy of the electron states of the Q-dot.
 
 

[1] Distance-dependent electron transfer in Au/spacer/Q-CdSe assemblies. E.P.A.M. Bakkers, A.W. Marsman, L.W. Jenneskens and D. Vanmaekelbergh, Angew. Chem. Int. Ed. English 39, 2297 (2000).

[2] Resonant electron tunneling through semiconducting nanocrystals in a symmetrical and asymmetrical junction. E.P.A.M. Bakkers and D. Vanmaekelbergh, Phys. Rev. B 62, R7743 (2000).

[3] Single-particle orbital energy level spectrum of nanocrystalline CdSe quantum dots obtained by scanning tunneling spectroscopy. D. Vanmaekelbergh et al. Submitted.
 
 
 

Multi-junction Solar Cells and Novel Structures for Solar Cell Applications
 
 

Masafumi Yamaguchi
 
 

Present status of R&D program for super-high-efficiency III-V compound solar cells in the New Sunshine Project in Japan is presented.
 
 

Multi-junction (Tandem) solar cells have the potential for achieving conversion efficiencies of over 40% and are promising for space and terrestrial applications.The super-high efficiency solar cell R&D project has started in FY (fiscal year) 1990 as an long-term target to the early 21st century, in which challenges and efforts are made in the development of super-high efficiency solar cell technology, aiming at a dramatic increase in conversion efficiency of over 40% and developing innovational technologies.
 
 

Physics for realizing high-efficiency multi-junction cells are also presented; high quality top cell layers, high performance tunnel junction for optically and electrically low-loss interconnection of two or more cells, mechanism for preventing impurity diffusion from a highly doped tunnel junction during overgrowth of the top cell by using double hetero (DH) structure, and optical and carrier confinement.

Up to now, as a result of proposal of wide-bandgap InGaP DH (double hetero) structure tunnel junction and InGaP top cell performance, high-efficiency InGaP/GaAs 2-junction cells (9cm2) with efficiencies of 30.6% at AM1.5 were fabricated on GaAs substrates by the MOCVD method by Japan Energy Co.The mechanically stacked 3-junction cells of monolithically grown InGaP/GaAs 2-junction cells and InGaAs bottom cells (1cm2) have reached the highest (world-record) efficiency of 33.3% at 1-sun AM1.5 following joint work by Japan Energy Co., Sumitomo Electric Co. and Toyota Tech. Inst.More recently, as a result of proposal of lattice-matched InGaAs middle cell and introduction of the C-doped AlGaAs/Si-doped InGaP hetero-structure tunnel junction with AlInP barriers, high-efficiency InGaP/InGaAs/Ge 3-junction cells with efficiencies of 31.2% (25cm2) and 31.7% (1cm2) at AM1.5 have been realized on Ge substrates by the MOCVD method by Japan Energy Co.This value is the highest ever reported for the monolithically grown 3-junction cells.31.5% efficiency has also been attained with InGaP/GaAs 2-junction cells under 20-suns of AM1.5 by the authors.
 
 

III-V compound thin-film solar cells fabricated on Si substrates have great potential for high-efficiency, low-cost and light-weight space solar cells.Previously, the authors have developed high efficiency GaAs-on-Si cells with efficiencies of 18.3% at AM0 and 20% at AM1.5.Recently, 17.85% radiation damages to GaAs-on-Si single-junction cells and first space flight of them have been examined by the authors, NTT and NASDA.48 2cmx2cm GaAs-on-Si cells with an average AM0 efficiency of 16.9% have been evaluated using the Engineering Test Satellite (ETS-VI).The GaAs-on-Si cells have been demonstrated to be more radiation-resistant in space than GaAs-on-GaAs cells and 50?m, 100?m and 200?m thick Si cells.These results show that the GaAs-on-Si cells have great potential for space applications.
 
 

Unique radiation resistant properties of InGaP/GaAs multi-junction cells have been found by the authors.Even at room temperature, photoinjection-enhanced annealing of radiation damage to InGaP/GaAs tandem cells has been observed.These results suggest that InGaP-based multi-junction cells have great potential for space applications.
 
 

In order to apply super high-efficiency cells widely, it is necessary to improve their conversion efficiency and reduce their cost.Concentrator 3-junction and 4-junction solar cells have great potential for realizing super high-efficiency of over 40%.Future prospects will also be presented.
 
 
 
 

MULTISTEP PHOTOINDUCED ELECTRON TRANSFER IN SELF-ORGANISED NANOSCALE PORPHYRIN TRIADS

Zenkevich1, A M. Shulga1, U. Rempel2, A. Willert2 and C. von Borczyskowski2

1Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, Laboratory of Molecular Photonics, 70 F. Skaryna Avenue., 220072 Minsk, Belarus

2University of Technology Chemnitz, Reichenhainer Str. 70, 09107 Chemnitz, Germany

On the basis of a covalent approach and self-organisation principles we have succeeded recently, to form well-defined structurally organised porphyrin triads of a controled geometry, containing zinc-octaethylporphyrin chemical dimer, (ZnOEP)2Ph, with or without covalently linked electron acceptors (p-benzoquinone, Q), and additional dipyridyl-substituted tetrapyrrolic extra-ligand porphyrin (H2P). In this report we present experimental results on the complex charge transfer dynamics between interacting subunits obtained by the steady-state, picosecond time-resolved fluorescence (the experimental response Dt1/2 " 30 ps) and femtosecond transient absorption (D1/2 " 280 fs), spectroscopy.
 
 

The formation of self-organized triads based on (ZnOEP)2Ph-Q subunit and porphyrin extra-ligands manifests itself not only in the additional fluorescence quenching of (ZnOEP)2Ph (tS<1 ps) but in extra-ligand S1 state lifetime shortening. Non-radiative relaxation processes with participation of the dimer S1 state are attributed both to the singlet-singlet EnT (ZnOEP)2Ph(r) extra-ligand and the sequential ET ZnOEP)2Ph(r) Q at rDA=10.8 A0. The extra-ligand S1 state decay shortening (from 9-7 ns down to 950 ps in toluene at 293 K) is not affected noticeably by the temperature rise, but becomes stronger upon the solvent polarity increase. The extra-ligand tS shortening is connected with the increased "superexchange" mediated long distant (R"18-21A0) one-step ET extra-ligand(r)Q.

The preliminary studies shows that the temperature stability of the triads in thin polymeric films is higher than that in solutions. In addition, for some of these triads the photoinduced electron transfer takes place in rigid polymeric (PMMA) films at 293 K and remains still effective at 77 K. These properties make the systems under consideration to be perspective for the solid phase charge separation.
 
 
 
 

Dye-sensitised Nano-crystalline Solar Cells: Major Factors Determining Solar Efficiency and Long-term Ageing

Andreas Hinsch, Rainer Kern and Joachim Luther

Fraunhofer Institute for Solar Energy Systems, Oltmannsstr. 22, D-79100 Freiburg, Germany

Jan M. Kroon

The Netherlands Energy Research Foundation ECN, Westerduinweg 3, NL-1755 Petten, The Netherlands

Today, nano-crystalline dye sensitised solar cells (DSC) are a vastly growing research field. The reason that makes DSC so interesting for fundamental and industrial related research and development is the wide range of new materials, which have not been related to photovoltaic use before. Solar efficiencies of up-to 11 % have been reported on small areas (0.25 cm2). Recently AM1.5 solar efficiencies of 7.8 - 8.2 % have been reached reproducibly on 2.25 cm2. Also, great progress has been made in terms of stability, in large part due to work on electrolyte composition and sealing. In this paper, we discuss several relevant factors which determine solar efficiency as well as long-term stability. It turns out, that electron recombination losses at the interface between the nano-crystalline TiO2 and the electrolyte are of major importance for the future optimisation of DSCs. Various suggestions both on materials and device level are made for the improvement of the long-term stability and efficiency.