Simone Mastroianni

Physical and Electrochemical Analysis of an Indoor–Outdoor Ageing Test of Large‐Area Dye Solar Cell Devices

A long-term life test (3200 h) on large-area dye-sensitized cells is performed both under outdoor conditions, in the sunny Mediterranean climate in Rome (Italy), and under continuous light soaking (1 Sun, 85 °C). Different degradation rates are investigated for the outdoor samples with horizontally and vertically oriented cells (azimuth South, tilt angle 25°). Thirty identical photocells (active area=3.6 cm(2), conversion efficiencies=(4.8±0.2)%) are aged using a robust master-plate configuration. After the first 1000 h of testing in open-circuit conditions, some of the test samples are set near the maximum power point (MPP) and the life test continued further until 3200 h. A detailed analysis of the physical parameters obtained by electrochemical impedance is given together with electrolyte transmittance variation with time as a function of the ageing conditions. Faster degradation in devices working at the MPP is observed, due mainly to a progressive decrease of the triiodide concentration in the electrolyte and a likely alteration at the titania/electrolyte interface. Outdoor devices working with vertically oriented cells show clearly that the orientation of long-striped cells can affect the lifetime. The aged cells suffer an increase of recombination rate, change in the chemical capacitance, and positive shift of the titania conduction band level. A strong correlation between the increase of the electrolyte diffusion resistance and degradation phenomena is found.

Effect of electrolyte bleaching on the stability and performance of dye solar cells

Degradation of dye solar cells (DSCs) under severe ageing conditions may lead to loss of the tri-iodide in the electrolyte - a phenomenon known as electrolyte bleaching. Monitoring changes in the tri-iodide concentration as a result of degradation mechanisms and understanding their causes and effects are fundamental for improving the long-term stability of DSCs. In this contribution a strongly accelerated ageing test (1 Sun visible light, 1.5 Suns UV light, T = 110 °C for 12 h) was performed on DSCs in a double-sealed masterplate configuration to purposely induce severe electrolyte bleaching, and its effects on the performance and stability of DSCs with different initial tri-iodide concentrations [I3(-)]0 were investigated. The cells with low [I3(-)]0 suffered a severe loss in short circuit current density JSC (up to 85%). Also a significant loss of open circuit voltage VOC was observed and this loss was proportional to [I3(-)]0 with the highest VOC drop observed with the highest [I3(-)]0. Non-destructive analysis techniques based on the limited current density, JSCvs. light intensity, and photographic image analysis, were used to quantify the [I3(-)] loss, which was found to be ca. 50 mM and independent of [I3(-)]0. Quantitative model based VOC analysis in terms of changing [I3(-)] revealed that the degradation responsible for the VOC drop was dominated by an unknown mechanism that is unrelated to [I3(-)]0. The methods and results reported here help separating and identifying different degradation mechanisms related to electrolyte bleaching in DSCs.

Electrochemistry in Reverse Biased Dye Solar Cells and Dye/Electrolyte Degradation Mechanisms

Mismatched or shadowed individual cells in a module can operate in the reverse bias (RB) regime. We investigate and identify key mechanisms for RB operation and degradation in dye solar cells (DSCs). Current-voltage characteristics in RB are sensitive to the type of dye utilised and to TiCl(4) substrate treatment. Subjecting the cell to a RB of 0.4 V over 740 h has little effect on conversion efficiency whereas a significant lowering is observed for the harsher stress tests at 0.6 V and by forcing a constant current equal to its I(SC). For more prolonged reverse biases at I(SC) (>740 h), we show that depletion of [I(3)(-)] inside the DSC can lead the reverse bias potentials across the cells to considerably increase in time. Electrochemical impedance measurements show that the overpotentials at the counter electrodes (CEs) can eventually reach values high enough to cause hydrogen evolution. Clear evidence of gas bubbles forming inside a complete dye solar cell under reverse bias stress, leading to severe device degradation, is presented. We also show that reactions of iodine with water present in the electrolyte can play an important role in [I(3)(-)] depletion and in the formation of hydrogen at the Pt CE.

Status of Dye Solar Cell Technology as a Guideline for Further Research

Recently, the first commercial dye solar cell (DSC) products based on the mesoscopic principle were successfully launched. Introduction to the market has been accompanied by a strong increase in patent applications in the field during the last four years, which is a good indication of further commercialization activity. Materials and cell concepts have been developed to such extent that easy uptake by industrial manufacturers is possible. The critical phase for broad market acceptance has therefore been reached, which implies focusing on standardization-related research topics. In parallel the number of scientific publications on DSC is growing further (>3500 since 2012), and the range of new or renewed fundamental topics is broadening. A recent example is the introduction of the perovskite mesoscopic cell, for which an efficiency of 14.1% has been certified. Thus, a growing divergence between market introduction and research could be the consequence. Herein, an attempt is made to show that such an unwanted divergence can be prevented, for example, by developing suitable reference-type cell and module concepts as well as manufacturing routes. An in situ cell manufacturing concept that can be applied to mesoscopic-based solar cells in a broader sense is proposed. As a guideline for future module concepts, recent results for large-area, glass-frit-sealed DSC modules from efficiency studies (6.6% active-area efficiency) and outdoor analysis are discussed. Electroluminescence measurements are introduced as a quality tool. Another important point that is addressed is sustainability, which affects both market introduction and the direction of fundamental research.

Multiscale Modeling of Dye Solar Cells and Comparison With Experimental Data

In this paper, we investigate the electrical properties of dye solar cells (DSCs) under illumination and in dark conditions when an external bias is applied. The measurements performed on the cells will be compared with theoretical calculations. The modeling is made using two approaches: a finite-element code based on Tiber computer-aided design (CAD) software to describe in detail the electrical properties of the cell and a circuital model implemented on PSpice. The latter has been developed in the perspective of simulating larger systems, such as modules and panels. It should be a phenomenological model to fit I-V characteristics of real cells. The CAD instead allows to calculate steady-state properties and I-V characteristics of the cell, solving a set of differential equations on meshes in one, two, and three dimensions. The two models are compared to experimental values, and the microscopic model is used to shine light over the fitting parameters of the circuital model.

Reverse bias degradation in dye solar cells

A prolonged reverse bias (RB) stress forcing a short-circuit current through a dye solar cell, corresponding to the harshest test a shadowed cell may experience in real conditions, can cause the RB operating voltage VRB to drift with time, initially slowly but accelerating for VRB < (−1.65 ± 0.15)V when gas bubbles, identified as H2 (gas chromatography), are produced inside the cell, leading to breakdown. A close connection between VRB, cell performance, and stability was established. Contributions to RB degradation include triiodide depletion and impurities, in particular water. Acting upon these components and setting up protection strategies is important for delivering long-lasting modules.

Micro-Raman analysis of reverse bias stressed dye-sensitized solar cells

The degradation mechanisms of Reverse Bias (RB) stressed Dye Solar Cells (DSCs), sensitized with cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719, Red Dye) and with cis-dicyano-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) (Ru505, Orange Dye) have been studied by means of resonance micro-Raman and UV-Vis spectroscopy. For N719 sensitized devices, the visible degradation induced by the stress tests involves both electrolytic solution and the sensitizer: the electrolyte suffers gas bubble formation and loss of solvent, while the dye cannot be regenerated and undergoes irreversible chemical changes. Confocal Raman imaging and UV-Vis absorption spectra confirmed that in regions where the electrolyte was absent, the detachment of the thiocyanate ligand (SCN−) from the dye is favored. On the other hand, measurements carried out on DSCs realized with the bis-cyano dye (Ru505) do not show dye modifications during the RB stress. We also clarify that the apparent N719 dye bleaching in particular zones of the cell active area, is not related to dye desorption from the TiO2 layer, but to loss of solvent and to dye chemical changes, which are responsible for a characteristic blue shift in the absorption spectrum.

Fabrication and reliability of dye solar cells: A resonance Raman scattering study

Abstract In this work, the effect of reverse bias stress tests on Dye Solar Cells (DSCs) based on N719 dye was investigated in detail using resonant micro-Raman spectroscopy. First the Raman lines were assigned to vibrations from the different constituents in a fresh solar cell. Then the mechanism of thiocyanato (SCN − ) loss under stress conditions was reported.

PSPICE models for Dye solar cells and modules

A Circuital model for Dye Sensitized Solar Cell is proposed. Experimental comparisons and module design are carried out with the present model.