Francesco Di Giacomo

Progress, challenges and perspectives in flexible perovskite solar cells

Perovskite solar cells have attracted enormous interest since their discovery only a few years ago because they are able to combine the benefits of high efficiency and remarkable ease of processing over large areas. Whereas most of research has been carried out on glass, perovskite deposition and synthesis is carried out at low temperatures (<150 °C) to convert precursors into its final semiconducting form. Thus, developing the technology on flexible substrates can be considered a suitable and exciting arena both from the manufacturing view point (e.g. web processing, low embodied energy manufacturing) and that of the applications (e.g. flexible, lightweight, portable, easy to integrate over both small, large and curved surfaces). Research has been accelerating on flexible PSCs and has achieved notable milestones including PCEs of 15.6% on laboratory cells, the first modules being manufactured, ultralight cells with record power per gram ratios, and even cells made on fibres. Reviewing the literature, it becomes apparent that more work can be carried out in closing the efficiency gap with glass based counterparts especially at the large-area module level and, in particular, investigating and improving the lifetime of these devices which are built on inherently permeable plastic films. Here we review and provide a perspective on the issues pertaining progress in materials, processes, devices, industrialization and costs of flexible perovskite solar cells.

Vertical TiO2 Nanorods as a Medium for Stable and High-Efficiency Perovskite Solar Modules

Perovskite solar cells employing CH3NH3PbI3-xClx active layers show power conversion efficiency (PCE) as high as 20% in single cells and 13% in large area modules. However, their operational stability has often been limited due to degradation of the CH3NH3PbI3-xClx active layer. Here, we report a perovskite solar module (PSM, best and av. PCE 10.5 and 8.1%), employing solution-grown TiO2 nanorods (NRs) as the electron transport layer, which showed an increase in performance (∼5%) even after shelf-life investigation for 2500 h. A crucial issue on the module fabrication was the patterning of the TiO2 NRs, which was solved by interfacial engineering during the growth process and using an optimized laser pulse for patterning. A shelf-life comparison with PSMs built on TiO2 nanoparticles (NPs, best and av. PCE 7.9 and 5.5%) of similar thickness and on a compact TiO2 layer (CL, best and av. PCE 5.8 and 4.9%) shows, in contrast to that observed for NR PSMs, that PCE in NPs and CL PSMs dropped by ∼50 and ∼90%, respectively. This is due to the fact that the CH3NH3PbI3-xClx active layer shows superior phase stability when incorporated in devices with TiO2 NR scaffolds.

Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination

This is the first report of an investigation on flexible perovskite solar cells for artificial light harvesting by using a white light-emitting diode (LED) lamp as a light source at 200 and 400 lx, values typically found in indoor environments. Flexible cells were developed using either low-temperature sol–gel or atomic-layer-deposited compact layers over conducting polyethylene terephthalate (PET) substrates, together with ultraviolet (UV)-irradiated nanoparticle TiO2 scaffolds, a CH3NH3PbI3–xClx perovskite semiconductor, and a spiro-MeOTAD hole transport layer. By guaranteeing high-quality carrier blocking (via the 10–40 nm-thick compact layer) and injection (via the nanocrystalline scaffold and perovskite layers) behavior, maximum power conversion efficiencies (PCE) and power densities of 10.8% and 7.2 μW·cm–2, respectively, at 200 lx, and 12.1% and 16.0 μW·cm–2, respectively, at 400 lx were achieved. These values are the state-of-the-art, comparable to and even exceeding those of flexible dye-sensitized solar cells under LED lighting, and significantly greater than those for flexible amorphous silicon, which are currently the main flexible photovoltaic technologies commercially considered for indoor applications. Furthermore, there are significant margins of improvement for reaching the best levels of efficiency for rigid glass-based counterparts, which we found was a high of PCE ~24% at 400 lx. With respect to rigid devices, flexibility brings the advantages of being low cost, lightweight, very thin, and conformal, which is especially important for seamless integration in indoor environments.

Reconsidering figures of merit for performance and stability of perovskite photovoltaics

The development of hybrid organic-inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, reversible improvement and degradation of PSC efficiency have been reported under illumination-darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T80 as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell.

Device architectures with nanocrystalline mesoporous scaffolds and thin compact layers for flexible perovskite solar cells and modules

Hybrid organometallic halide perovskite photovoltaics has seen remarkable growth in world wide research and power conversion efficiencies (PCEs) over the last two years. Key advantages of perovskites devices, together with high PCEs typical of inorganic semiconductors, are represented by the ease of deposition of the precursors of the perovskite (via ink solutions) and their low temperature processing (<; 140°C), more typical of organic semiconductors. These values enable coating the active layers on plastic substrates, which can make the technology compatible with continuous roll to roll manufacturing. Flexible photovoltaics is drawing strong interest as it can also bring advantages to applications where flexibility, conformability, and being lightweight and easy-to-integrate are sought. Development of the technology on flexible substrates is far from trivial. Especially important is identifying materials and techniques that are low-temperature and compatible with plastic films even for the other components of the cell like the nanometer-thin electron extraction/blocking layers and the mesoporous nanocrystalline scaffolds. We will present effective strategies and formulations that enable the realization of efficient flexible perovskite cells and the first ever CH3NH3PbI3-xClx perovskite module on plastic film.