The effect of electrolyte filling method on the performance of dye-sensitized solar cells

The effect of electrolyte filling method on the performance of the dye-sensitized solar cells is investigated with the segmented cell method, a recent technique which is very simple but effective as it can be used to examine all the photovoltaic characteristics. The electrolyte filling techniques compared were single injection, which is typically used in small laboratory cells, and pumping the electrolyte through the cell several times, which is often used for larger cells and modules. Significant photovoltage and photocurrent variations occur with the repeated pumping of the electrolyte in the cell preparation. Transient and charge extraction measurements confirmed that the differences in open circuit voltage were due to the shifts of the TiO2 conduction band and time correlated single photon counting confirmed that the reduction of short circuit current was largely due to reduced electron injection correlated with the increasing conduction band edge in the studied cases. This was interpreted as an effect of molecular filtering by the TiO2 causing an accumulation of electrolyte additives (4-tert-butylpyridine and benzimidazole) near the electrolyte filling hole, the concentration of which increased with repeated pumping of the electrolyte. Interestingly, spatial variations were seen not only in the relative TiO2 conduction band energy but also in the density of trap states. In this contribution it is demonstrated how the changes in the conduction band can be separated from the changes in the density of trap states which is an essential for the correct interpretation of the data.Peer reviewe

Effect of molecular filtering and electrolyte composition on the spatial variation in performance of dye solar cells

It is demonstrated that the molecular filtering effect of TiO2 has a significant influence on dye solar cell (DSC) performance. As electrolyte is injected to a DSC, some of the electrolyte components adsorb to the surface TiO2 (here 4-tert-butylpyridine and 1-methyl-benzimidazole) and accumulate near the electrolyte filling hole resulting in varying electrolyte composition and performance across the cell. The spatial performance distribution was investigated with a new method, the segment cell method. Not only is the segmented cell method simple and cheap when compared to the only other method for examining spatial variation (photocurrent mapping), it also has the major advantage of allowing the spatial variation in all other operating parameters to be assessed. Here the molecular filtering effect was to influence the cell performance in case of all the five studied electrolytes causing up to 35% losses in efficiency. Raman spectra indicated that the loss in photocurrent in the electrolyte filling was in correlation with the loss of thiocyanate ligands suggesting that dye regeneration may also be a significant factor in addition to electron injection in some of the cells. There were also shifts in the absorption spectra the photoelectrodes which further supported changes in the thiocyanate ligands. Besides absorption changes, there were additional shifts in the IPCE spectra which may relate to deprotonation of the dye. The efficiency losses were reduced to ∼10% with contemporary electrolyte compositions.Peer reviewe

Dye solar cells as optically random photovoltaic media

20 páginas, 5 figurasIn order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize conversion efficiency for electrodes of different thickness containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thickness can be understood as the result of the randomization of light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparible to the thickness of the film. This simple relation could also be used as a design rule to attain the optimum optical design in other photovoltaic materials.The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° 307081 (POLIGHT), the Spanish Ministry of Economy and Competitiveness under grants MAT2011-23593 and CONSOLIDER HOPE CSD2007-00007, and the Junta de Andalucía under grants FQM3579 and FQM5247. PB is grateful to funding from the EPSRC APEX project (EP/H040218/1) and fellowship (EP/J002305/1. The authors also thank the Unit of Information Resources for Research (URICI-CSIC) for the co-financing of this publication in Open Access.Peer reviewe

Dye sensitized solar cells as optically random photovoltaic media

In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize the conversion efficiency of electrodes of different thicknesses containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thicknesses can be understood as the result of the randomization of the light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparable to the thickness of the film. This simple relationship could also be used as a design rule to attain the optimum optical design in other photovoltaic materials. This journal is © The Royal Society of Chemistry.Peer Reviewe

Distribution of soluble impurities in cold glacial ice

Understanding the microstructure of ice underpins the interpretation of ice-core measurements and many ice-sheet properties. A detailed study of polar snow and ice using scanning electron microscope (SEM) and X-ray analysis revealed the microstructural distribution of soluble impurities. Sublimation under vacuum (etching) concentrated impurity from both the bulk and grain boundaries on to the specimen surfaces in detectable quantities. Sublimation in the cold room before examination (preetching) collected previously unobservable quantities of impurity at triple junctions. A heterogeneous distribution of impurities was observed. Chloride was frequently found to originate from the lattice, but not usually at triple junctions. Other impurities, particularly sodium chloride, were detected at grain boundaries and bubble surfaces. Sulphate was often found at triple junctions in specimens containing a high bulk concentration of the acid, frequently in conjunction with cations. This suggests the possibility that veins were only filled with significant amounts of impurity when the surrounding grain boundaries were saturated. The model of impurity arrangement inferred from the results reconciles differences between previous SEM studies; additionally it is consistent with and explains recent electrical conduction observations. The disconnected arrangement of impurity-filled grain boundaries and veins limits opportunities for significant post-depositional solute movement

Etching channels and grain-boundary grooves on ice surfaces in the scanning electron microscope

Short communication - no abstrac

Experimental and theoretical optical properties of methylammonium lead halide perovskites

Leguy, Aurélien M. A. et al.The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).We thank Jarvist M. Frost and Paul N. Stavrinou for useful discussions, Xiao Li for assistance with sample preparation and Stefan A. Maier for access to equipment. PB and AL are grateful to the EPSRC (EP/J002305/1, EP/M014797/1 and EP/M023532/1) for financial support. JN thanks the EPSRC for funding (EP/J017361 and EP/K030671/1). MCQ and MIA acknowledge financial support from MINECO through project MAT2012- 37776. The theory portion of this work was supported by EPSRC (EP/M009602/1, EP/K016288/1 and EP/M009580/1).Peer reviewe

Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites

Leguy, Aurélien M. A. et al.We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations.1 Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamics effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity and supressed electron-phonon scattering events.The authors thank Juraj Sibik and Axel Zeitler for their contribution in the measurement and interpretation of the terahertz absorption spectra, and Philip Calado and Davide Moia for their helpful discussions regarding the TOC graphic. PB and AL are grateful to the EPSRC (EP/J002305/1, EP/M014797/1 and EP/M023532/1) for financial support. The work at Bath was supported by the EPSRC (EP/K016288/1, EP/K004956/1, EP/L000202, and EP/M009580/1) and the ERC (Grant 277757). ARG, MIA and MCQ thank the Spanish Ministry of Economy and Competitiveness (MINECO) for its support through Grant No. CSD2010-00044 (Consolider NANOTHERM) and MAT2015-70850-P (HIBRI2). The work at ICMAB was carried out under the auspices of the Spanish Severo Ochoa Centre of Excellence program (grant SEV-2015-0496). F. B. is funded through the EU DESTINY Network (Grant No. 316494). JN acknowledges the EPSRC for founding (EP/J017361). AP would like to thank the Royal Irish Academy for the Charlemont grant for funding the research visit that made the terahertz work possible.Peer reviewe

Signal variability in replicate ice cores

Replicate ice cores have been drilled about 10 m apart for the top 790 m of the ice sheet at Dome C, Antarctica. This provides an opportunity to examine inter-core variation of the signal for identical events, based on dielectric profile (DEP) comparisons. Comparison of the signal from the same core (a section 48 m long), measured 1 year apart, showed good reproducibility, with peak heights varying by around 10% between the two measurements. For the two replicate cores, identical peaks were matched and showed variability between cores of typically a factor 1.5. This can be explained based on the likelihood of significant time periods of missing accumulation in any single core at sites with such low snow accumulation rate. To synchronize core depths by matching peaks, it is essential to use the pattern of peaks, rather than just widely spaced individual strong peaks. To derive a quantitative volcanic index from these low-accumulation rate sites, it will be necessary to combine or average the results from several closely spaced parallel cores