Ute B. Cappel

The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells.

The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost production and its flexibility in terms of color and design. Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs. A surprising, unexplained feature observed in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, we demonstrate that this feature can be attributed to a change of the local electric field affecting the absorption spectrum of the dye, an effect related to the Stark effect first reported in 1913. We present a method for measuring the effect of an externally applied electric field on the absorption of dye monolayers adsorbed on flat TiO(2) substrates. The measured signal has the shape of the first derivative of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO(2) surface. A very similar signal is observed in photoinduced absorption spectra of dye-sensitized TiO(2) electrodes under solar cell conditions, demonstrating that the electric field across the dye molecules changes upon illumination. This result has important implications for the analysis of transient absorption spectra of DSCs and other molecular optoelectronic devices and challenges the interpretation of many previously published results.

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance

In this work, molecular tuning of metal xanthate precursors is shown to have a marked effect on the heterojunction morphology of hybrid poly(3-hexylthiophene-2,5-diyl) (P3HT)/CdS blends and, as a result, the photochemical processes and overall performance of in situ fabricated hybrid solar cells. A series of cadmium xanthate complexes is synthesized for use as in situ precursors to cadmium sulfide nanoparticles in hybrid P3HT/CdS solar cells. The formation of CdS domains is studied by simultaneous GIWAXS (grazing incidence wide-angle X-ray scattering) and GISAXS (grazing incidence small-angle X-ray scattering), revealing knowledge about crystal growth and the formation of different morphologies observed using TEM (transmission electron microscopy). These measurements show that there is a strong relationship between precursor structure and heterojunction nanomorphology. A combination of TAS (transient absorption spectroscopy) and photovoltaic device performance measurements is used to show the intricate balance required between charge photogeneration and percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. This study presents a strong case for xanthate complexes as a useful route to designing optimal heterojunction morphologies for use in the emerging field of hybrid organic/inorganic solar cells, due to the fact that the nanomorphology can be tuned via careful design of these precursor materials.

Light-driven transformation processes of anisotropic silver nanoparticles

The photoinduced formation of silver nanoprisms from smaller silver seed particles in the presence of citrate anions is a classic example of a photomorphic reaction. In this case, light is used as a convenient tool to dynamically manipulate the shape of metal nanoparticles. To date, very little is known about the prevailing reaction mechanism of this type of photoreaction. Here we provide a detailed study of the shape transformation dynamics as a function of a range of different process parameters, such as photon energy and photon flux. For the first time, we provide direct evidence that the photochemical synthesis of silver nanoprisms from spherical seed nanoparticles proceeds via a light-activated two-dimensional coalescence mechanism. On the other hand, we could show that Ostwald ripening becomes the dominant reaction mechanism when larger silver nanoprisms are grown from photochemically synthesized smaller nanoprisms. This two-step reaction proceeds significantly faster and yields more uniform, sharper nanoprisms than the classical one-step photodevelopment process from seeds. The ability to dynamically control nanoparticle shapes and properties with light opens up novel synthesis avenues but also, more importantly, allows one to conceive new applications that exploit the nonstatic character of these nanoparticles and the ability to control and adjust their properties at will in a highly dynamic fashion.

Removing Cosmic Ray Features from Raman Map Data by a Refined Nearest Neighbor Comparison Method as a Precursor for Chemometric Analysis

An algorithm to remove cosmic ray (CR) features from Raman spectra collected in mapping experiments using a charge-coupled device (CCD) is presented. Each spectrum is compared to spectra collected from adjacent points in space using correlation values. The most similar neighbor (MSN) spectrum is selected, offset, and used for identification of CRs. The offset values are defined in terms of the noise level for data with a low signal-to-noise ratio and in terms of the peak height for data with a high signal-to-noise ratio. Scaled intensity values of the MSN spectra are used for replacement of contaminated pixels, allowing for full recovery of underlying spectral features. The algorithm is applicable for any Raman map where the particle sizes within the analyzed mixture are larger than the sampling size or to any other data where the sampling is more frequent than the variation, e.g., time series or temperature profiles. Its application to several maps of pharmaceutical samples is discussed here. With an appropriate offset value for the MSN spectra, no misdetections occur, and all CRs more intense than the offset are removed, which includes the CRs that would have hampered subsequent chemometric analysis by methods such as principal component analysis (PCA).

Charge Generation Dynamics in CdS:P3HT Blends for Hybrid Solar Cells

Development of design rules for hybrid inorganic-organic solar cells through understanding charge generation and recombination dynamics is an important pathway for the improvement of solar cell conversion efficiencies. In this Letter, we study the dynamics of charge generation in CdS:polymer blends by transient absorption spectroscopy. We show that charge generation following excitation of the inorganic component is highly efficient and can occur up to a few nanoseconds after excitation, allowing for diffusion of charges within the inorganic component to an interface. In contrast, charge generation following excitation of the organic component occurs on subpicosecond time scales but suffers from two loss processes, incomplete exciton dissociation and geminate recombination.

Photoinduced electron and hole transfer in CdS:P3HT nanocomposite films: effect of nanomorphology on charge separation yield and solar cell performance

The influence of morphology on the photophysical properties of blend films containing in situ grown CdS and poly(3-hexylthiophene-2,5-diyl) (P3HT), fabricated utilising a metal xanthate single source precursor, is reported. A combination of transient absorption spectroscopy (TAS), transmission electron microscopy (TEM) and photovoltaic device measurements are employed to study the relationship between the efficiency of charge separation, photocurrent generation and thin film morphology. We identify that a significant proportion of the extractable charge originates from the direct excitation of CdS followed by hole-transfer to the P3HT polymer. The yield of this hole-transfer step from the inorganic CdS to the organic polymer is largely unaffected by the films nanomorphology, while the dissociation of P3HT excitons into free charges at the CdS:P3HT interface is found to be strongly dependent on this parameter with high yields of charge transfer only being achieved at high CdS loadings. The present study elucidates design rules for the optimization of hybrid inorganic–organic solar energy conversion devices.

Low‐Temperature Solution Processing of Mesoporous Metal–Sulfide Semiconductors as Light‐Harvesting Photoanodes

Low-Temperature Solution Processing of Mesoporous Metal-Sulfide Semiconductors as Light-Harvesting Photoanodes

Characterization techniques for dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) have been widely studied in the last two decades and start to be commercialized in the photovoltaic market. Comprehensive characterization is needed to fully understand and optimize the device performance and stability. In this review, we summarize different characterization methods for dye-sensitized solar cells with liquid redox electrolytes or solid state hole transporting materials, most of which can also be used for similar devices such as perovskite based thin film solar cells. Limitations and advantages of relevant methods for studying the energy levels and time scales involved in charge transfer processes as well as charge transport related characteristic lengths are discussed. A summary of recent developments in DSCs and the importance of measured parameters for the device optimization procedure are mentioned at the end.

Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application

The most common material for dye-sensitized photocathodes is mesoporous NiO. We transformed the usual brownish NiO to be more transparent by reducing high valence Ni impurities. Two pretreatment methods have been used: chemical reduction by NaBH4 and thermal reduction by heating. The power conversion efficiency of the cell was increased by 33% through chemical treatment, and an increase in open-circuit voltage from 105 to 225 mV was obtained upon heat treatment. By optical spectroelectrochemistry, we could identify two species with characteristically different spectra assigned to Ni3+ and Ni4+. We suggest that the reduction of surface Ni3+ and Ni4+ to Ni2+ decreases the recombination reaction between holes on the NiO surface with the electrolyte. It also keeps the dye firmly on the surface, building a barrier for electrolyte recombination. This causes an increase in open-circuit photovoltage for the treated film.

Evidence for photo-induced charge separation between dye molecules adsorbed to aluminium oxide surfaces

Excited state dynamics and photo-induced charge transfer of dye molecules have been widely studied due to their relevance for organic and dye-sensitised solar cells. Herein, we present a femtosecond transient absorption spectroscopy study of the indolene dye D131 when adsorbed to inert Al2O3 substrates for different surface concentration of the dye. Surprisingly, we find that at high surface concentrations, the first singlet excited state of the dye is converted into a new state with an efficiency of about 80%. We assign the absorption features of this state to the oxidised dye and discuss the possibility of photo-induced charge separation between neighboring dye molecules. Our study is the first to show that this process can be highly efficient without the use of donor and acceptor molecules of different chemical structures.