The role of hole transport between dyes in solid-state dye-sensitized solar cells

In dye-sensitized solar cells (DSSCs) photogenerated positive charges are normally considered to be carried away from the dyes by a separate phase of hole-transporting material (HTM). We show that there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid-state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy, we demonstrate that the rate of interdye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO₂ surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Interdye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2 μm thick films. Finally, we use hole transport in the dye monolayer to characterize the spatial distribution of the HTM phase in the pores of the dyed mesoporous TiO₂

Dye Regeneration by Spiro-MeOTAD in Solid State Dye-Sensitized Solar Cells Studied by Photoinduced Absorption Spectroscopy and Spectroelectrochemistry

Photoinduced absorption spectroscopy is presented as a tool for the systematic study of dye regeneration and pore filling in solid state dye-sensitized solar cells. Oxidn. potentials and extinction coeffs. for oxidized species of the perylene dye, ID28, on TiO2 and of the hole conductor, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD), were detd. by spectroelectrochem. The onset of oxidn. of a solid film of spiro-MeOTAD was found to be 0.15 V vs. ferrocene/ferrocenium and extinction coeffs. of spiro-MeOTAD+ were found to be 33,000 M-1/cm-1 at 507 nm and 8500 M-1/cm-1 at 690 nm. Electrons in TiO2 films were shown to alter the ground-state absorption spectra of ID28 attached to TiO2. Photoinduced absorption measurements indicated a good contact between ID28 and spiro-MeOTAD for different spiro-MeOTAD concns. for both 2- and 6-μm thick TiO2 films. We discuss the possibility of estg. the quality of pore filling from the positions of absorption peaks. Results suggested that with a spiro-MeOTAD concn. of 300 mg/mL in chlorobenzene, a uniform distribution of spiro-MeOTAD in the pores of the 6-μm thick TiO2 film could be achieved

Oxygen-Induced Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar Cells and Its Impact on Device Performance

Solid state dye-sensitized solar cells (sDSCs) employing the hole conductor 2,2′7,7′-tetrakis-(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD) require the presence of oxygen during fabrication and storage. In this paper, we determine the concentrations of oxidized spiro-MeOTAD within devices under different operating and storage conditions by UV–vis spectroscopy. Relative concentrations of spiro-MeOTAD⁺ were found to be greater than 10% after illumination for standard sDSCs, where no chemical dopant had been used in the solar cell fabrication but oxygen and lithium ions were present. We suggest that oxidized spiro-MeOTAD is created as a byproduct of oxygen reduction at the TiO₂ surface during cell illumination. Furthermore, we studied the effect of light soaking under different conditions and associated changes in spiro-MeOTAD⁺ concentration on the solar cell measurements. Our findings give insights to photochemical reactions occurring within sDSCs and provide guidelines for which doping levels should be used in device fabrication in absence of oxygen

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 prodn. and its flexibility in terms of color and design. Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs. A unexplained feature obsd. 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, this feature can be attributed to a change of the local elec. field affecting the absorption spectrum of the dye, an effect related to the Stark effect. The authors present a method for measuring the effect of an externally applied elec. field on the absorption of dye monolayers adsorbed on flat TiO2 substrates. The measured signal has the shape of the 1st deriv. 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 TiO2 surface. A similar signal is obsd. in photoinduced absorption spectra of dye-sensitized TiO2 electrodes under solar cell conditions, demonstrating that the elec. field across the dye mols. changes upon illumination. This result has important implications for the anal. of transient absorption spectra of DSCs and other mol. optoelectronic devices and challenges the interpretation of many previously published results

Simple Method for Efficient Slot-Die Coating of MAPbI(3) Perovskite Thin Films in Ambient Air Conditions

Scalable methods for deposition of lead halide perovskite thin films are required to enable commercialization of the highly promising perovskite photovoltaics. Here, we have developed a slot-die coating process under ambient conditions for methylammonium lead iodide (MAPbI(3)) perovskite on heated substrates (about 90 degrees C on the substrate surface). Dense, highly crystalline perovskite films with large grains (100-200 mu m) were obtained by careful adjustment of the deposition parameters, using solutions that are similar but more dilute than those used in typical spin-coating procedures. Without any further after treatments, such as antisolvent treatment or vapor annealing, we achieved power conversion efficiencies up of 14.5% for devices with the following structure: conducting tin oxide glass (FTO)/TiO2/MAPbI(3)/spiro-MeOTAD/Au. The performance was limited by the significant roughness of the deposited films, resulting from the hot-casting method, and the relatively high deposition temperature, which led to a defect-rich surface due to loss of MAI

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

X-ray photoelectron spectroscopy for understanding molecular and hybrid solar cells

X-ray photoelectron spectroscopy is a powerful tool for the characterization of molecular and hybrid solar cells. This technique allows for atomic-level characterization of their components as well as for the determination of the electronic structure that governs the key conversion processes. In this chapter, we introduce the basic concepts of electronic structure in molecules and semiconducting materials followed by a description of the concepts of photoelectron spectroscopy and how they relate to electronic structure. Finally, we give examples of the application of photoelectron spectroscopy to different types of molecular and hybrid solar cell materials demonstrating the type of information that can be obtained, to gain fundamental understanding and to further develop such devices

Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials

The valence band of lead halide hybrid perovskites with a mixed I/Br composition is investigated using electronic structure calculations and complementarily probed with hard X-ray photoelectron spectroscopy. In the latter, we used high photon energies giving element sensitivity to the heavy lead and halide ions and we observe distinct trends in the valence band as a function of the I : Br ratio. Through electronic structure calculations, we show that the spectral trends with overall composition can be understood in terms of variations in the local environment of neighboring halide ions. From the computational model supported by the experimental evidence, a picture of the microheterogeneity in the valence band maximum emerges. The microheterogeneity in the valence band suggests that additional charge transport mechanisms might be active in lead mixed halide hybrid perovskites, which could be described in terms of percolation pathways