Eta Solar Cell Research Articles

Microstructure Control of Absorber Sb2S3 and p-Type Semiconductor CuSCN for Semiconductor-Sensitized Solar Cells (TiO2/Sb2S3/CuSCN)

Semiconductor-sensitized solar cells (SSSCs) were fabricated with a planar n-type TiO2 film, an antimony sulfide (Sb2S3) thin film as the absorber, and a transparent p-type CuSCN film. The deposition amount of Sb2S3 and the pn junction interface condition significantly affected the generation and recombination of photo carriers. Analysis of the microstructure and chemical state of the Sb2S3, and the microstructure of CuSCN films showed that the homogeneity of the Sb2S3 and CuSCN films, the oxidation of Sb2S3 and the CUSCN thickness determined the optimal amount of Sb2S3 and the highest photoelectric conversion efficiency. For the ETA solar cell with the highest photoelectric conversion efficiency (1.67%), Sb2S3 was deposited by sputtering (SP) and CuSCN by wiping (WI).

All-Solid Semiconductor-Sensitized Solar Cells Made by Electrodeposition

It would be very attractive to fabricate (Extremely Thin Absorber) ETA-solar cells using one single technique of deposition such as the electrochemical one, as this would facilitate the assembly of the solar cells and reduce cost of production. In the ZnO-NWs/CdSe/CuSCN solar cells, the ZnO nanowires (NWs) and the absorber, CdSe, were already made by electrochemical deposition. In this paper, the electrodeposition of the 2D ZnO buffer layer and p-type CuSCN was investigated as well as the possibility to use them in the solar cell assembly. Formation of vertically free standing CuSCN nanowires opened the route to built ETA-solar cells with an inverted configuration.

Facile synthesis of phosphine free ultra-small PbSe nanocrystals and their light harvesting studies in ETA solar cells.

Ultra-small PbSe nanocrystals (NCs) were synthesized via a 'one-pot' approach in olive oil as the reaction medium and capping agent. The optical spectra showed discernible blue shifts in the absorption band edges (570-780 nm) due to strong quantum confinement effects and photoluminescence (PL) spectra showed significant strong emission peaks in the range of 780-850 nm. The broad peaks in the powder X-ray diffraction (p-XRD) pattern indicate the ultra-small size of the as-prepared NCs. These NCs were used to construct an extremely thin absorber (ETA) solar device after surface modification. The preliminary results indicate their potential as light harvesting entities in nanostructure based solar cells.

Fabrication of ZnO/Absorber Heterostructures for Nanostructured Solar Cells

The development of low-band-gap semiconducting materials for the extremely thin absorber (eta) solar cell using cheap and scalable methods is the main objective of this work. The eta-solar cell is composed of all inorganic materials consisting of an extremely thin layer of absorbing material (1.1 < Eg < 1.8 eV) sandwiched between nanostructured transparent electron and hole conductors (Eg ≥ 3.3 eV). The photosensitization of the electrodeposited ZnO nanowires with conformal layers of CdS and CdSe prepared by successive ionic layer adsorption and reaction (SILAR) is proposed. The improvement of the absorber structural and optical properties by annealing and chemical treatment is achieved. SILAR method insures a conformal layer formation on the entire nanowire surface. Thus prepared heterostructures exhibit optimal optical features. The effects of the core/shell heterostructure interface (ZnO/absorber) were studied in a photoelectrochemical cell using the 3I-/I3- redox electrolyte as in a standard DSSC. The effects of the chemical treatment and annealing step of the photosensitizing shell are studied.

Effect of Recombination on Series Resistance in eta Solar Cell Modified with In(OH)xSy Buffer Layer

Transport mechanism studies in TiO<SUB>2</SUB>/In(OH)<SUB>x</SUB>S<SUB>y</SUB>/Pb(OH)<SUB>x</SUB>S<SUB>y</SUB>/PEDOT:PSS eta solar cell have been carried out. The characterizations have been performed both in the dark and under varying illumination intensity for temperature range 200 K – 320 K. Calculations from ideality factor have shown that the recombination process of the eta solar cell in the dark to be tunneling enhanced, while under illumination it is thermally activated and takes place through exponentially distributed energy recombination levels. The temperature has been found to influence series resistance of the solar cell. Series resistance has been found to be high at low temperature and low at higher temperature, thus we can conclude that the recombination is thermally activated.

Light Soaking Induced Increase in Conversion Efficiency in Solar Cells Based on In(OH)&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;S&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;/Pb(OH)&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;S&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;

Light soaking characterization on complete SnO2:F/TiO2/In(OH)xSy/Pb(OH)xSy/PEDOT:PSS/Au, eta solar cell structure as well as on devices which do not include one or both TiO2 and/or PEDOT:PSS layers has been conducted. Additionally, studies of SnO2:F/In(OH)xSy/Pb(OH)xSy/PEDOT:PSS/Au solar cell have been performed. The power conversion efficiency and the short circuit current density have been found to increase with light soaking duration by a factor of about 1.6 - 2.7 and 2.1 - 3, respectively. The increase in these two parameters has been attributed to the filling up of trap states and/or charge-discharge of deep levels found in In(OH)xSy. These effects take place at almost fill factor and open circuit voltage being unaffected by the light soaking effects.

Extremely thin absorber solar cells based on nanostructured semiconductors

Extremely thin absorber (eta) solar cells aim to combine the advantages of using very thin, easily and cheaply produced absorber layers on nanostructured substrates with the stability of all-solid-state solar cells using inorganic absorber layers. The concept of using nanostructured substrates originated from the dye-sensitised solar cell, where having a very high surface area allows the use of very thin layers of dye while still absorbing sufficient sunlight. However, both the dye and liquid electrolyte used in these devices demonstrated poor stability, and efforts were made to replace them with very thin inorganic absorber layers and solid state hole collectors respectively. The combination of these concepts – a nanostructured substrate coated with a very thin inorganic absorber and completed with a solid state hole collector – is known as an eta solar cell. This review summarises the development of both the inorganic absorbers and solid state hole collectors in porous TiO2 and ZnO nanorod based cells, focusing on the material properties and growth/deposition methods. Future possibilities for eta solar cells are discussed, including utilisation of a wider range of materials, synthesis methods and novel materials such as quantum dots to produce tuned band gap and multijunction solar cells.

(Research Award of the Energy and Technology Division) (Photo)Electrochemistry in the Service of Solar Energy Conversion

The paper is a short summary of the lecture delivered for the 2011 Research Award of the Energy Technology Division. The main expertise field of my group is materials science and (photo)electrochemistry for energy conversion and storage. The motivations that oriented my research are described, spanning from photoelectrochemistry of single crystal semiconductors, p-n photoelectroelectrochemical cells for solar hydrogen generation, to the photoetching of semiconductors, electrochemical processing of silicon surface including porous silicon, electrodeposition of polycrystalline semiconductor thin films and monocrystalline nanowires and development of Eta (extremely thin absorber) solar cells.

ZnO Buffer Layers and Nanowires Electrodeposition for Extremely Thin Absorber Solar Cells

In this manuscript we are reporting on the electrodeposition of ZnO nanowires on potentiostatically and galvanostatically prepared ZnO 2D layers (called buffer layers) on glass substrates covered with transparent conductive oxide (TCO). It is shown that the buffer layer morphology and thickness could be used as parameters to control the ZnO nanowires dimensions. From the other side the supporting electrolyte concentration and the passed charge density during the electrodeposition are also playing role on tailoring their dimensions. The obtained nanowires exhibit interesting optical and structural properties and therefore they could be further used as nanostructured transparent n-type semiconductor for the extremely thin absorber (eta) solar cells, dye sensitised solar cells or hybrid polymer solar cells.

Modeling and characterization of extremely thin absorber (eta) solar cells based on ZnO nanowires

Extremely thin absorber (eta)-solar cells based on ZnO nanowires sensitized with a thin layer of CdSe have been prepared, using CuSCN as hole transporting material. Samples with significantly different photovoltaic performance have been analyzed and a general model of their behavior was obtained. We have used impedance spectroscopy to model the device discriminating the series resistance, the role of the hole conducting material CuSCN, and the interface process. Correlating the impedance analysis with the microstructural properties of the solar cell interfaces, a good description of the solar cell performance is obtained. The use of thick CdSe layers leads to high recombination resistances, increasing the open circuit voltage of the devices. However, there is an increase of the internal recombination in thick light absorbing layers that also inhibit a good penetration of CuSCN, reducing the photocurrent. The model will play an important role on the optimization of these devices. This analysis could have important implications for the modeling and optimization of all-solid devices using a sensitizing configuration.

ZnO–SnO 2 composite anodes in extremely thin absorber layer (ETA) solar cells

ZnO–SnO 2 composite electrodes have been deposited on fluorine-doped tin oxide (FTO) substrates by aerosol assisted chemical vapour deposition (AACVD) from a single source precursor solution. The electrodes were characterised using X-ray diffraction (XRD), atomic force microscopy (AFM), field emission gun scanning electron microscopy (FEGSEM) and energy dispersive X-ray analysis (EDX). The composite electrodes were used to construct ETA solar cells with the following structure; FTO/ZnO–SnO 2/In 2S 3/PbS/PEDOT:PSS/C graphite/FTO. Performance of the cells were characterised by measuring the current–voltage ( I– V) and incident photon to electron conversion efficiencies (IPCE). The effect of Zn:Sn ratio in the precursor and effect of post deposition annealing temperature on the morphology of the composite layers, in relation to the performance of the fabricated cells were investigated. The highest performing cells were fabricated using the composite anode deposited from 50:50 mol% Zn:Sn in the precursor with post deposition annealing at 400 °C. I– V characterisation under AM 1.5 solar simulated light reveals that the cell had an open circuit voltage ( V oc) ∼ 0.32 V, short circuit current density ( J sc) ∼ 8.2 mA cm −2, a fill factor (FF) ∼ 0.26, an overall efficiency ( η) ∼ 0.68% and a maximum IPCE ∼ 30%. The experimental IPCE agrees well with theoretically estimated IPCE when the PbS surface coverage is about 0.1–0.2.

Intensity and temperature dependent characterization of eta solar cell

AbstractTemperature‐dependent electrical characterization of a highly structured TiO2/In(OH)x Sy /Pb(OH)x Sy /PEDOT:PSS eta solar cell has been carried out. The transport mechanism in this type of solar cell has been investigated. A schematic energy band diagram which explains the photoelectrical properties of the device has been proposed. The solar cell has been characterized in the temperature range 200–320 K at illumination intensities between 0.05 mW/cm2 and 100 mW/cm2. The diode ideality factor A under illumination has been found to vary between 1.2 and 1.6, whereas in the dark 6.9 ≤ A ≤ 10.1. The device has been found to undergo a thermally activated recombination under illumination, while tunnelling enhanced recombination has been established to dominate the current in the dark. The solar cell efficiency shows a logarithmic dependence on illumination in the whole temperature range investigated, achieving its maximum at an illumination of ∼45 mW/cm2. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Implications of the Negative Capacitance Observed at Forward Bias in Nanocomposite and Polycrystalline Solar Cells

Four different types of solar cells prepared in different laboratories have been characterized by impedance spectroscopy (IS): thin-film CdS/CdTe devices, an extremely thin absorber (eta) solar cell made with microporous TiO2/In(OH)xSy/PbS/PEDOT, an eta-solar cell of nanowire ZnO/CdSe/CuSCN, and a solid-state dye-sensitized solar cell (DSSC) with Spiro-OMeTAD as the transparent hole conductor. A negative capacitance behavior has been observed in all of them at high forward bias, independent of material type (organic and inorganic), configuration, and geometry of the cells studied. The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices. An equivalent circuit model is suggested to explain the impedance and capacitance spectra, with an inductive recombination pathway that is activated at forward bias. The deleterious effect of negative capacitance on the device performance is discussed, by comparison of the results obtained for a conventional monocrystalline Si solar cell showing the positive chemical capacitance expected in the ideal IS model of a solar cell.

Fabrication and characterization of ZnO nanowires/CdSe/CuSCN eta-solar cell

Des cellules solaires extremely thin absorber (eta) ZnO/CdSe/CuSCN a base de nanofils de ZnO ont ete realisees avec succes en utilisant des techniques faciles d'acces de depot en solution et par voie electrochimique. Une couche constituee de nanofils de ZnO monocristallin de plusieurs microns de hauteur et de 100-200 nm de diametre a ete deposee sur un substrat de verre conducteur (SnO 2 :F) recouvert d'une couche de ZnO formee par pyrolyse de spray agissant comme couche electronique bloquante. Une couche mince de 30-40 nm de CdSe jouant le role d'un absorbeur a ete electrodeposee de maniere conforme le long des nanofils de ZnO. Les espaces entre les nanofils composite de ZnO/CdSe ont ete remplis par du CuSCN semiconducteur de type p ; l'ensemble formant une jonction p-i-n. La couche de nanofils composite de ZnO/CdSe presente un important effet de confinement optique de la lumiere, avec une absorption effective de ∼ 89% et une reflectivite effective de ∼ 8% dans la region 400-800 nm du spectre solaire (AMI.5). L'influence du recuit sur la taille des grains de CdSe et sur le rendement de conversion de l'energie de la cellule eta-solar cell a ete analysee. Le rendement de conversion observe des cellules apres recuit de la couche de CdSe est, pour la meilleure d'entre elles, de 2,3%, demontrant ainsi que la nanoheterostructure ZnO/CdSe/CuSCN constitue une option interessante pour developper un nouveau type de cellules photovoltaiques.

ZnO/CdTe/CuSCN, a promising heterostructure to act as inorganic eta-solar cell

The ZnO/CdTe/CuSCN heterostructure was analyzed as a candidate to act as an inorganic eta-solar cell. A ZnO film consisting of single crystal nanocolumns was electrodeposited on a transparent conducting substrate which acts as n-type material. As absorber material we used CdTe, which was deposited on the ZnO columnar film by Metal Organic Chemical Vapor Deposition. In order to complete the eta-solar cell we deposited a CuSCN layer by chemical solution deposition. A conformal and uniform CdTe coverage of the ZnO columns was achieved, producing a very efficient light trapping effect. The effective absorption (∼87%) and effective reflectance (∼10%) of the complete heterostructure in the 400–800 nm (AM1.5) solar spectrum range are very favorable for the use of this heterostructure in photovoltaic devices. Preliminary current–voltage characterization made on this ZnO/CdTe/CuSCN heterostructure shows rectifying behavior and a photovoltaic effect.

Theoretical study on the effect of an intermediate layer in CIS-based ETA-solar cells

A theoretical treatment is given of the mechanisms that might be responsible for reported improvements in solar cell characteristics of, in particular, TiO 2/CuInS 2 based nanostructured ETA-solar cells. Especially, the possible benefits of the insertion of an intermediate tunnel barrier (Al 2O 3, MgO) or buffer layer (CdS) are investigated. Three mechanisms to improve V oc without compromising J sc are discussed: (i) introduction of an energy barrier which is higher for dark current than for light current; (ii) reduction of the interface state density at either side of the intermediate layer (IL) compared to the structure without IL, for materials reasons; and (iii) reduction of the effect of interface states due to a more favourable position of the Fermi levels at the interface, with respect to the band edges. The dark saturation current and hence V oc turn out to be determined by interface recombination, and chemical and electrostatic interactions at the interfaces.

Comparison of different thin film absorbers used in eta-solar cells

Alloying with mercury (Hg) has been used in electrodeposition as a means to reduce the bandgap of CdTe absorber films from 1.5 eV for CdTe to 1.07 eV for Cd x Hg 1− x Te (CMT). Alloy films have been incorporated as extremely thin absorbers in solar cells with the structure glass∣SnO 2∣microporous–TiO 2∣CMT∣Au. The decrease in the CMT bandgap is expected to reduce the conduction band offset at the interface to TiO 2, which is 0.6 eV for the TiO 2∣CdTe interface. This high band offset was believed to be responsible for the low fill-factor observed in the J– V characteristics of the former interface. Lowering the band offset is expected to lead to higher fill-factor values. Our results show that using a CMT as absorber gives place to both higher quantum efficiencies and a spectral response spectrum extended towards longer wavelengths, but not to a fill-factor enhancement.

Solar cell with extremely thin absorber on highly structured substrate

We report the optical, structural and photovoltaic properties of an extremely-thin-absorber (eta) solar cell on highly porous TiO2 substrates. Nano-crystalline absorber layers with a local thickness of only 150 nm have been prepared. This small absorber thickness allows good carrier collection even for absorber material with poor transport properties. The morphology of the highly structured TiO2 substrate produces strong internal light scattering, resulting in an enhancement of the optical path length by a factor 5. With 150 nm thick CdTe absorber layers, the eta solar cells produce an open-circuit voltage of 0.67 V and a short-circuit current of 8.9 mA cm−2 for 100 mW cm−2 illumination. Alloying the CdTe with Hg improves the short-circuit current to 15 mA cm−2.

The eta-solar cell with CuInS 2: A photovoltaic cell concept using an extremely thin absorber (eta)

Diffusion length of charge carriers within the absorbing material is one of the important restricting properties for the efficiency of solar cell devices. A new cell design using an extremely thin absorber (eta-solar cell) is prepared to obtain an effective separation of charge carriers within the depletion layer. It could be figured out that the properties of CuInS 2 (CIS) strongly depend on the porosity of the base layer. Multiple scattering within the porous structure is evident. Moreover, it can be demonstrated that there is a maximum in short-circuit current density for a medium thickness of the absorbing layer.

Solar cell using a highly structured pin-junction and CuInS2 as an extremely thin absorber

A new cell design using CuInS2 (CIS) as an extremely thin absorber (ETA-solar cell) is explored. The new cell allows the collection of photogenerated charge carriers over short transport distances. The charge separation is field-assisted and hence the diffusion length is of less importance for ETA-cells. It is shown in this paper that the properties of the (CIS)layer strongly depend on the porosity of the substrate. From transmission and reflection measurements we find that scattering of light within the porous structure results in high light absorption. Photoluminescence spectra indicate, that states in the band gap of CIS depend on the porosity of the structure. The short circuit current density of the complete ETA solar cell shows a maximum with increasing thickness of the absorbing layer.