Eurig W. Jones

Impedance spectroscopy of thin-film CdTe/CdS solar cells under varied illumination

The electrical properties of CdTe/CdS solar cells grown by metal organic chemical vapor deposition were investigated by a technique of impedance measurements under varied intensity of AM1.5 illumination. A generalized impedance model was developed and applied to a series of CdTe/CdS cells with variations in structure and doping. The light measurements were compared to the conventional ac measurements in dark under varied dc bias, using the same methodology for equivalent circuit analysis in both cases. Detailed information on the properties of the device structure was obtained, including the properties of the main p-n junction under light, minority carrier lifetime, back contact, as well as the effect of the blocking ZnO layer incorporated between the transparent conductor and CdS layers. In particular, the comparison between samples with different chemical concentrations of As has shown that the total device impedance and the series resistance are strongly increased at lower As densities, resulting in th...

Development of selective, ultra-fast multiple co-sensitization to control dye loading in dye-sensitized solar cells

Enhancing the spectral response of dye-sensitized solar cells (DSC) is essential to increasing device efficiency and a key approach to achieve this is co-sensitization (i.e. the use of multiple dyes to absorb light from different parts of the solar spectrum). However, precise control of dye loading within DSC mesoporous metal oxide photo-anodes is non-trivial especially for very rapid processing (minutes). This is further complicated by dyes having very different partition (Kd) and molar extinction (e) coefficients which strongly influence dye uptake and spectral response, respectively. Here, we present a highly versatile, ultra-fast (ca. 5 min) desorption and re-dyeing method for dye-sensitized solar cells which can be used to precisely control dye loading in photo-electrode films. This method has been successfully applied to re-dye, partially desorb and re-dye and selectively desorb and re-dye photo-electrodes using examples of a Ru-bipy dye (N719) and also organic dyes (SQ1 and D149) giving η up to 8.1% for a device containing the organic dye D149 and re-dyed with the Ru dye N719. The paper also illustrates how this method can be used to rapidly screen large numbers of dyes (and/or dye combinations) and also illustrates how it can also be used to selectively study dye loading.

Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells

In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.

Multiple linker half-squarylium dyes for dye-sensitized solar cells; are two linkers better than one?

The synthesis and full characterization of new half-squaraine dyes (Hf-SQ) containing two or three carboxylate-based linker units is reported and these dyes tested in dye-sensitized solar cell (DSC) devices. The data show improved device efficiency for a Hf-SQ dye with two linkers (η = 5.5%) compared to the highest efficiency Hf-SQ previously reported which had only a single linker (η = 5.0%); this is mainly due to improved Voc. To understand the effects of using multiple dye linker groups, device I–V data have been correlated with single crystal X-ray structural analysis and dye electrical properties (both in solution and adsorbed to TiO2) using UV-visible and ATR-IR spectroscopy along with cyclic voltammetry, and also theoretical studies using density functional theory (DFT) calculations. These data show that positioning the linkers near the dye LUMO and so that this enables complete linker chemisorption are key factors for device performance.

SIMS analysis of intentional in situ arsenic doping in CdS/CdTe solar cells

A series of CdTe/CdS devices with different tris(dimethylamino)arsine (TDMAAs) partial pressures were grown by metal organic chemical vapour deposition (MOCVD) to investigate the incorporation of arsenic into the bulk. Characterization of the growth layers using secondary ion mass spectrometry (SIMS) showed arsenic concentrations ranging from 1 × 1016 to 1 × 1019 atoms cm−3. A square law dependence of arsenic concentration on the TDMAAs vapour concentration was observed. A reaction mechanism for the decomposition of TDMAAs precursor via dimerization is presented and discussed in terms of reaction kinetics.

Comparative study of trap densities of states in CdTe∕CdS solar cells

Density of deep and shallow states has been investigated in three different kinds of CdTe∕CdS samples, two of which were grown by metal-organic chemical vapor deposition (MOCVD) and one by close-space sublimation (CSS) methods. The MOCVD samples were p doped by As and grown either with or without a ZnO buffer layer between the transparent conductor and CdS layers. Capacitance-voltage, admittance spectroscopy, and quantum efficiency measurements show pronounced effects of As doping and ZnO incorporation. It is found that A centers and vacancies of Cd, usually observed in CSS devices, are absent in the defect spectra of MOCVD samples.

Low temperature sintering of binder-containing TiO2/metal peroxide pastes for dye-sensitized solar cells

Nano-structured metal oxide films are key components of dye-sensitized (DSC) solar cells. Scaling such devices requires lower temperature processing to enable cheaper substrates to be used. In this context, we report a new and scalable method to sinter binder-containing metal oxide pastes to make DSC photo-electrodes at lower temperatures. Metal peroxide powders (CaO2, MgO2, or ZnO2) were added to terpineol-based P25 pastes containing ethyl cellulose binder or to commercial TiO2 paste (DSL18NR-T). Thermal analysis shows that binder decomposition occurs at 300 °C instead of the standard 450 °C for a TiO2-only paste and suggests that the metal peroxides act as combustion promoters releasing heat and oxygen within the film while heating. The data show that this heat and oxygen release coincide best with binder combustion for ZnO2 and DSC device tests show that adding ZnO2 to TiO2 pastes produces the best performances affording η = 7.5% for small devices (0.26 cm2) and η = 5.7% at 300 °C or 450 °C for DSL18NR-T/ZnO2 for larger (1 cm2) devices. To the best of our knowledge, the performance of the (0.26 cm2) cells is comparable to the highest efficiency devices reported for DSCs fabricated using low temperature methods. The device efficiency is most strongly linked with Jsc; BET and dye sorption measurements suggest that Jsc is linked with the metal oxide surface area and dye loading. The latter is linked to the availability of surface sorption sites for dye molecules which is strongly negatively affected by any residual organic binder which resulted from incomplete combustion.

Solvent issues during processing and device lifetime for perovskite solar cells

This paper considers the manufacturing issues associated with dimethyl formamide, γ-butyrolactone, dimethyl sulfoxide and chlorobenzene solvents, in particular the health and safety issues of using these solvents in scaled perovskite photovoltaic processing. Issues of device lifetime are also considered, for example the effects of atmospheric conditions (e.g. humidity).

Factors leading to low cost dye sensitised solar cells with ‘go faster’ stripes

Abstract Dye sensitised solar cells are a potential low cost photovoltaic technology because they can be manufactured by roll to roll processing. However, to achieve this, each manufacturing step must be extremely rapid (taking minutes at most). This paper addresses ultrafast dye sensitisation that can now be achieved in <5 min using one or multiple dyes. The use of multiple dyes can effectively give rise to any almost any desired colour in minutes, giving rise to ‘go faster’ stripes. This paper describes key parameters affecting dye uptake using metal containing (N719) and organic dyes (triphenylamine or squaraines) as examples.

Digital imaging to simultaneously study device lifetimes of multiple dye-sensitized solar cells

In situ degradation of multiple dyes (D35, N719, SQ1 and SQ2) has been investigated simultaneously using digital imaging and colour analysis. The approach has been used to study the air stability of N719 and squaraine dyes adsorbed onto TiO2 films with the data suggesting this method could be used as a rapid screening technique for DSC dyes and other solar cell components. Full DSC devices have then been tested using either D35 or N719 dyes and these data have been correlated with UV-vis, IR and XPS spectroscopy, mass spectrometry, TLC and DSC device performance. Using this method, up to 21 samples have been tested simultaneously ensuring consistent sample exposure. Liquid electrolyte DSC devices have been tested under light soaking including the first report of D35 testing with I−/I3− electrolyte whilst operating at open circuit, short circuit, or under load, with the slowest degradation shown at open circuit. D35 lifetime data suggest that this dye degrades after ca. 370 h light soaking regardless of UV filtering. Control, N719 devices have also been light soaked for 2500 h to verify the imaging method and the N719 device data confirm that UV filtration is essential to protect the dye and I3−/I− electrolyte redox couple to maintain device lifetime. The data show a direct link between the colour intensity and/or hue of device sub-components and device degradation, enabling “real time” diagnosis of device failure mechanisms.