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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Sol-Pro (Solution Processed Next Generation Photovoltaics)

Teaser

Summary

Light is one of the most important goods on earth: The sun is the single most clean and sustainable energy source which can solve all the energy needs of our world. The energy in sunlight striking the earth for 40 min is equivalent to global energy consumption for a year. During the last years, increasing concern about global warming has led to an intense search for cost-effective alternative energy sources such as printed photovoltaics. The ERC project aims to re-design solution processed PV components relevant to printed PVs product development targets. Consequently, several high-level objectives concerning materials/electrodes/interfaces/processing, relevant to product development targets (efficiency, lifetime, cost) of printed inverted PVs, are aimed for within the proposed ERC program.

Work performed

High power conversion efficiencies (PCEs) and prolonged lifetimes are essential for organic photovoltaic (OPV) and Perovskite PVs commercialization. While exciting PCEs of over 10% and 20 % respectively have been demonstrated, long device stabilities are the next barrier that needs to be overcome. Understanding the degradation mechanisms that influence the stability of inverted device configurations under various environmental stress factors is therefore a crucial task of the Sol-Pro Project. During the second period of the Sol-Pro project the influence of hole transporting layers on the stability of inverted OPVs was investigated. High PCE inverted OPVs usually use thermally evaporated MoO3 as a hole transporting layer (HTL). As shown in figure 1, we monitored the accelerated lifetime performance of non-encapsulated inverted OPVs using thiophene-based active layer materials and evaporated MoO3 under the ISOS D-2 protocol (heat conditions 65 Â°C). By using a series of measurements and device/reverse engineering methods, the results presented indicate that although the interface between MoO3 and Ag contributes to degradation, the interface between the organic active layer and MoO3 is the main origin of failure of inverted OPVs under intense heat conditions. We concluded, that PEDOT:PSS HTL resulted in more stable inverted OPV performance compared to MoO3 HSC under ISOS-D-2 heat protocol. The MoO3 hole selective contact of the record PCE inverted structure OPVs has been identified as one of the most vulnerable parts of the device under various environmental stress factors. (F. Hermerschmidt et al, ACS Appl. Mat. Inter, 2017). As indicated within the Sol-Pro program perovskite solar cells gave breath of fresh air in the printed photovoltaic (PV) technology landscape. They have amazed with an incredibly fast PCE improvement, going from below 10% in 2009 to over 20 % in 2018. However, these cells are facing issues towards commercialization, such as the need to achieve long-term stability and the development of a low cost environmentally friendly electrodes for the reproducible fabrication of high-performance devices. The Sol-Pro Project focus on inverted perovskite solar cell (PVSC) device architecture due to its completability with low cost printing manufacturing. The typical inverted PVSC are based on ITO/PEDOT:PSS/ Perovskite/PCBM)Al. According to the description of the Sol-Pro ERC action low cost and environmentally friendly metal-oxide based charge selective contacts were developed for this product development related device structure. The realization of solution processed conductive and transparent p-type metal oxide thin films to be used as HTLs is a challenge since we have shown that one of the most critical factors affecting the crystallinity of the perovskite sensitizer is the layer on which the perovskite photoactive layer is grown. The metal-oxide under-layers has a great influence on the perovskite crystal orientation, grain size and grain boundaries (please see figure 2). All these elements play a significant role to the overall PCE of the device. The ERC (Sol-Pro) project has shown the effect of hole transporting layers (HTLs) in charge accumulation properties of inverted perovskite solar cells by using impedance spectroscopy (F. Galatopoulos et al, APL Materials 2017). According the description of the Sol-Pro ERC action Solution Combustion Synthesized (SCS) metal-oxide based charge selective contacts were synthesized for next generation PVs aiming the development of reproducible and reliable functional optoelectronic devices. The Sol-Pro ERC work developed an SCS based nickel cobaltite (NiCo2O4) which exhibits better conductivity and is more environmentally friendly than commonly used NiOx. As shown in figure 3 the utilization of tartaric acid, for the first time, as fuel and nitrate as oxidizer agent enabled the formation of NiCo2O4 nanoparticles (NPs) with an average size of ~5 nm and narrow particle-size dist

Final results

To summarize a systematic understanding of the relationship between metal oxide-based interlayers, processing and device performance relevant to Hybrid Perovskite and OPV photovoltaics product development targets were performed during the first 30 months of the Sol-Pro ERC program. The potentials of applying the low-temperature SCS for fabrication of highly reproducible and reliable metal oxides provided high PCE and reliable PVSCs. Furthermore, as shown in figure 4 we have achieved thermally long-lived inverted PVSCs incorporating fullerene diffusion blocking Layer (DBL) . We have shown that the interaction of the top metal electrode (TME) with the perovskite active layer (AL) through diffusion mechanisms is the main thermal degradation mechanism for inverted PVSCs. Using fullerene DBL we were able to isolate the AL from the TME and report accelerated heat lifetime for inverted PVSCs of over 1000 hours (F. Galatopoulos et al., Adv. Mater. Inter., 2018). Relevant to the designing solution processed ITO-free and evaporation free electrodes ERC action plans we have shown that the metal grid-based ITO-free and evaporation free electrodes work effectively with the combination of PEDOT:PSS but using doped metal oxides as replacement of PEDOT:PSS remain a challenge. Relevant to OPVs, during the last year a shift to non-fullerene acceptors was performed. Non-fullerene acceptor based OPVS have potential to increase the PCE in the range of 15 % but degradation issues already identified by the Sol-Pro ERC program under various environmental stress factors need to be resolved. In parallel to high performance hybrid perovskites, solution processed inorganic perovskites including metal oxide-based perovskites will be investigated aiming to identify novel solution processed inorganic active layers for next generation PVs.