A study on organic and hybrid emerging photovoltaics: modeling and reliability

Photovoltaic solar cells can be classified into three generations depending on the technology used: single junction (multi) crystalline (1G), single junction thin film (2G) and hetero-junctions (3G). Organic and hybrid solar cells belong to the 3G generation. Low temperature and typographic printing processing, tunable colors, flexibility and short payback time are but a few of the advantages these technologies possess, making organic and hybrid photovoltaics appealing in terms of both cost and architectonic integration. Despite the several advantages, organic and hybrid photovoltaics have lower efficiency, lifetime and reliability compared to inorganic technologies. Several issues must be addressed in order to improve organic/hybrid solar cells. We investigated the response of roll coated organic solar cells at different bias voltages and illumination levels to implement a detailed impedance model. We analyzed both fresh and intentionally degraded cells. The impedance spectra show different peaks evolutions, depending on the degradation of the cells. To describe the cell impedance behaviors we suggest an electrical model based on distributed elements. The main differences between fresh and degraded samples are underlined. In addition, we subjected P3HT:PCBM solar cells to electrical constant current stress and thermal storage. We identified and separated different contributions affecting the open circuit voltage and short circuit current during both stresses. A model applied to photocurrent experimental data allows the estimation of parameters such as generation, recombination, dissociation rate and nearly zero field voltage within the active layer as a function of the stress time. Extrapolated parameters show that the stress mainly affects the recombination rate of the polaron charge transfer states. On hybrid solar cells, the anomalous electrical behavior of lead halide perovskite opto-electronic devices still prevents their full reliability. We studied the hysteresis and electric field effects on planar CH3NH3PbI3 perovskite devices, synthetized from laser-ablated precursors, by means of electrical characterizations at different scan rates and optical measurements. The aim of our investigation is to characterize the phenomena behind perovskite degradation under prolonged applied electric field. The results point to the presence of ions migrating in the perovskite when the device is biased. In order to explain the details of the mechanisms concurring to the observed behaviors, we presented a qualitative model. The same degradation dynamics occur on vertical devices, typical on perovskite solar cells. Furthermore, we subjected to both storage and thermal stress solid state solar cells based on organ-metal perovskites and using Spiro-OMeTAD as hole transport material. We applied two different sealing techniques to encapsulate the devices. We correlated the results obtained during the experiment to different degradation dynamics within the cell structure. The correlation allows us to distinguish at least two possible sources of degradation that can help understanding loss mechanisms of perovskite solar cells. Finally, accurate determination of the lifetime of novel hybrid and organic solar cells is often rather challenging due to very dynamic behavior of such cells over time and ageing curves with shapes of varying nature. Therefore, in order to accurately and reproducibility determine the lifetime of photovoltaic (PV) devices with such a behavior a novel elaboration algorithm was developed, which enables automatic smoothing, filtering and extrapolation of the real lifetime data and reproducibility determining the lifetime parameters defined in the ISOS guiding standards. The algorithm is also capable of predicting the lifetime of devices, not tested until the end of sample life, given that there is sufficient amount of measured data points for performing reliable extrapolation of ageing curves (to a limited time frame).