Photostability of perovskite solar cells

Organometal halide perovskite solar cells (PSCs) have emerged as a new disruptive technology in the photovoltaic landscape in recent years. Their power conversion efficiencies (PCE) soared up to over 25% as of now from as little as 3.8% in 2009. Despite the promising device performance, the limited operational stability hampers their commercialization. The instability problem is manifold, mainly due to their vulnerability to thermal decomposition, photo-degradation, and oxidation. The thermal stability is already lifted to the recommended industrial levels by employing compositional engineering. The oxygen and humidity ingress can be controlled by proper encapsulation – as already demonstrated for established PV technologies – leaving behind the problem of photo-instability to be taken care of.

Therefore, in this dissertation, the mechanisms governing the photo-degradation in perovskite solar cells are investigated. Among various spectral regimes in air mass 1.5 global (AM 1.5 G) spectrum, ultraviolet (UV) radiation is identified to contribute most to the photo-degradation, lowering the PCEs by 64% (relative).  The most impacted PV parameter is the short-circuit current density (JSC), decreasing by up to ~80% in worst case scenario, with the open-circuit voltage (VOC) being the least affected parameter (20 – 30% decrease). Furthermore, a varying trend of degradation is observed upon exposing the solar cells to spectra belonging to UV-A (315 – 400 nm) and UV-B (280 – 315 nm) radiation, with UV-B being more destructive. In addition to UV triggered instability, investigation of photostability beyond UV radiation revealed that a steady decrease in the degradation exists upon the decreasing energy of the incident photons. This can be observed for wavelengths between 300 nm and 500 nm. The reduction in PCE is noted to be lowest (7%) when exposed to the wavelengths > 500 nm, and highest (43%) for the wavelengths < 400 nm over a period of 250 hours. The spectral selectivity is realized via multiple long-pass and short-pass optical filters. In addition to wavelength-dependent degradation, the intensity dependence of photo-instability is also investigated using neutral density filters and custom-made light intensity moderators. Furthermore, the mechanisms and causes of the photo-degradation are probed by measuring the ideality factors, thermally stimulated currents (TSC), photo spectroscopy, and x-ray crystallography. With TSC measurements it is established that the trap states of different activation energies (265±5 meV and 425±10 meV) are generated upon photo-degradation, stimulating the recombination via trap states thus influencing the device performance.

To counter the severe instability in PSCs towards UV radiation, two protective measures are proposed. These measures include the use of UV-filters to block the troublesome wavelengths. However, this strategy comes at the cost of JSC (up to 1.3 mA/cm2) since the energy content of the blocked photons is not harnessed. The other method of improving the UV triggered instability involves doping of luminescent downshifting material in the encapsulant or the front covering glass sheet. With this approach, a recovery of JSC in amount of 0.6 mA/cm2 can be realized in a realistic scenario.

In addition to the role of photon-energy in degradation, bias-dependent photo-instability is also investigated. The worst performance deterioration is found in solar cells maintained at open-circuit condition (77%), while least is observed in the ones held at short-circuit condition (34%). On the other hand, the solar cells maintained at constant voltage near the maximum power point showed a medium degradation of 57% in PCE. Altogether, these findings are important to lay the basis for the improvement of the photo-stability of multi-cation perovskite solar cells.


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