Perovskite solar cells (PSCs) have shown impressive power conversion efficiencies (PCE). To become commercially successful, also challenges regarding large-scale manufacturing and stability must be addressed. In this talk we present two studies in which advanced electro-optical characterization techniques are combined to assess the influence of cell design parameters on the performance and degradation of PSCs.

In the first study, we investigate mixed self-assembled monolayers (SAMs) composed of Me-4PACz and F-4PACz and evaluate their impact on device performance and stability. The combination of the two SAMs offers the potential to combine the benefits of Me-4PACz (high PCE) with F-4PACz (good wettability), but it also poses the question of how the SAM properties are altered in the mixed state. It is found that optoelectronic characteristics of mixed SAM films and devices are primarily dominated by a variable combination of the pure SAMs properties. Interestingly, some measurements indicate a tendency toward increased variability for mixed SAMs compared to their pristine counterparts which suggests that SAM mixture do not form a homogeneous hybrid layer. This hypothesis is further analysed by a combination of device simulations (Setfos),[1] ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy measurements.

In a second part, we evaluate performance and stability of a series of mesoscopic carbon-based perovskite solar cells with systematically varied thickness of the titania and the zirconia mesoporous layers. The results are consistent with previously reported decrease in collection efficiency when decreasing the mesoporous titania (m-TiO2) thickness. [2] By comparing current-voltage (JV) and impedance results with device simulations using Setfos, we infer the relative importance of bulk recombination and of recombination at the perovskite/carbon interface as a function of bias. [3,4] Next, an operational stability test (maximum power point tracking, MPPT) on such series of mesoscopic solar cells is performed under 1 sun illumination and controlled temperature using Litos Lite. While the cells show good stability at 25 °C for > 500 hours, further tests at higher temperatures reveal irreversible instability of the devices already after 100 hours. Interestingly, the extent of such a degradation is dependent on the solar cell structure, with a drop in performance in the order of 30% for cells with thicker m-TiO2 layer and an almost unchanged efficiency recorded for cells with thin m-TiO2. Different degradation scenarios are discussed and complemented with scan-rate dependent JV curves, impedance measurements as well as with photo- and electroluminescence imaging (Vitios). [2,5] The results indicate that both photo-inactive areas are formed and recombination and ionic properties of the PSC are altered during degradation.