Alessandro Lanuti

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.

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.