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I-V characteristics at AM1.5G, 1000 W/m2 of a five series-connected dye solar cell module: unshaded module (squares) formed by the 5 unshaded cells (down triangles), and the module (circles) after one of the cells is fully shaded at initial time t0. The I-V characteristics of the fully shaded cell are shown at initial time t0 (up triangles) and after the cell is subjected to a prolonged RB stress at time t* (continuous line). Negative voltages represent the reverse bias region. Vth represents the threshold voltage value where a large current starts flowing in reverse bias in the shaded cell calculated as the intersection of the fitted line taken in the linear region of the I-V curve, at reverse biases, with the voltage axis. Ilim represents the diffusion-limited current. The reverse bias voltage VRB is the operating point of the shaded cell at ISC. At the initial time t0 (fresh cell) VRB ranges from 0 V to VRB0 as function of the connected load on the module. When the module works at its ISC, the shaded cell at t0 is reverse biased at VRB0 = −0.6 V, while the 4 unshaded cells are working at a forward bias of 0.15 V. The curve at t* shows the effects of a reverse bias stress after an extended time applied to a DSC on its I-V characteristics in RB: the slope, Vth, and Ilim undergo changes which cause |VRB| to increase significantly in time (VRB* at ISC).
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(a) Reverse bias voltage VRB vs time for 3 different pairs of dye solar cells fabricated with different batches of materials and stressed in the dark forcing a current equal to their ISC = −50 mA, so as to have the cell working under reverse bias. Cells from the same batch are represented with similar colors and curve style and they show comparable behaviours; (b) efficiency vs time of the same cells under AM1.5G 1000 W/m2 illumination measured at specific time intervals; in this case the lines connecting the symbols are simply a guide to the eye. (c) Relation between VRB and efficiency η and linear fits. A slow initial degradation is followed by a dramatically steeper one when VRB reaches (−1.65 ± 0.15)V which leads to cell breakdown at tbr (−2.5 V for VRB is the equipment-imposed limit). The cells' average electrical parameters before stress were: VOC = 680 mV, |ISC| = 50 mA, FF = 56%, η = 5.3%.
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(a) Absorbance spectra of a dye solar cell subjected to reverse bias stress at −90 mA measured on the active area (closed symbols) and on the electrolyte area (open symbols) (corresponding to the white rectangles indicated in (b) and (c)). t0 represents the time before the stress test, t* the time where the current applied approaches the Ilim and tbr the time after complete breakdown. (b) Photographs of the same cell at time t0 (b) and after breakdown tbr (c) where degradation of the dye layer and electrolyte leakage are clearly visible. (d) Sheet resistance of the transparent conducting oxide (TCO) RS_TCO, platinum/electrolyte charge transfer resistance (Rpt), and electrolyte diffusion resistance (Rd) vs. time. t1, t2, and t3 represent intermediate times. The resistances were extracted from impedance measurements performed in the dark at a forward bias of 0.8 V (see supplementary material 11 ).
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Gas chromatographic analysis performed in a 2-Pt-electrode cell using the electrolyte solution methoxypropionitrile (MPN) + 2M 1-methyl-3-propyl-imidazolium iodide (PMII) (dashed line) and HSE electrolyte (continuous line) and applying a fixed potential of 2 V for 10 min (see supplementary material 11 ).
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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.
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