Valerio Zardetto

Progress in flexible dye solar cell materials, processes and devices

Flexible Dye Solar Cells (FDSCs), in their most widespread architecture, are assembled with two opposing planar films or foil substrates in metal–plastic or plastic–plastic combinations. The use of one metal electrode enables the convenient utilization of materials and high temperature processes but is accompanied by issues including partial opacity of the electrolyte and catalyst layer. Constraints on the stability of plastic substrates have led to the development of a variety of alternative material formulations and processes to guarantee performance even at low temperatures compatible with plastic films. Recently, efforts in doing without transparent conducting oxides have led to the development of new unconventional architectures. Review of the operation of DSCs shows that initial target markets are represented by indoor applications where power output densities have been shown to outperform competing flexible photovoltaic technologies. Whereas performance, stability in particular, needs to be significantly improved for the adoption in long term outdoor installations, commercial products integrating FDCSs for indoor or portable use have already been launched. Issues pertaining progress in materials, processes, devices and industrialization of FDSCs will be analyzed and discussed in this review.

Atomic layer deposition for perovskite solar cells: research status, opportunities and challenges

Atomic layer deposition is widely acknowledged as a powerful technique for the deposition of high quality layers for several applications including photovoltaics (PV). The capability of ALD to generate dense, conformal, virtually pinhole-free layers becomes attractive also for the emerging organo-metal halide perovskite solar cells (PSCs), which have garnered the interest of the PV community through their remarkable efficiency gains, now over 20%, in just a few years of research. Until now, the application of ALD layers in PSCs has almost exclusively been restricted to the stages of device fabrication prior to perovskite deposition. Researchers have mainly focused on fabricating efficient electron and hole transport layers (TiO2, SnO2, ZnO, NiO) and ultra-thin Al2O3 or TiO2 passivation layers for several device configurations. The first section of this contribution reviews the current state-of-the-art ALD for perovskite solar cells. Then, we explore other potential opportunities, such as the fabrication of doped metal oxide selective contacts and transparent electrodes, also for use in tandem solar cell architectures, as well as barrier layers for encapsulation. Finally, we present our own experimental investigation of the challenges involved in depositing directly on perovskite absorbers in view of replacing organic electron and hole transport layers with ALD metal oxides (MOs). Therefore, the effects of temperature, oxidizing agents and metal precursors on perovskite are studied. A number of insights are gained which can lead to the development of ad hoc ALD processes that are compatible with the underlying perovskite, in this case, methylammonium lead iodide, MAPbI3. The phase purity and surface chemistry of the perovskite were used as metrics to quantify the feasibility of depositing selected MOs which can be adopted as selective contacts and passivation layers.

Formulations and processing of nanocrystalline TiO2 films for the different requirements of plastic, metal and glass dye solar cell applications

We carried out a systematic study on the effect of nanocrystalline TiO2 paste formulations and temperature treatment on the performance of dye solar cells (DSCs) over a large temperature range, to provide useful information for the fabrication of both plastic and metal flexible devices. We compared conventional screen-printable and binder-free TiO2 pastes with a new formulation which includes hydroxylethyl cellulose (HEC), enabling the study of the effect of organic materials in the TiO2 layer in the whole 25-600 °C temperature range. Differently from the binder-free formulations where the device efficiency rose monotonically with temperature, the use of cellulose binders led to remarkably different trends depending on their pyrolysis and decomposition thresholds and solubility, especially at those temperatures compatible with plastic foils. Above 325 °C, where metal foil can be used as substrates, the efficiencies become similar to those of the binder-free paste due to effective binder decomposition and inter-nanoparticle bonding. Finally, we demonstrated, for the first time, that the simultaneous application of both temperature (110-150 °C) and pressure (100 MPa) can lead to a large improvement (33%) compared to the same mechanical compression method carried out at room temperature only.

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.

Anti-stiction coating for mechanically tunable photonic crystal devices

A method to avoid the stiction failure in nano-electro-opto-mechanical systems has been demonstrated by coating the system with an anti-stiction layer of Al2O3 grown by atomic layer deposition techniques. The device based on a double-membrane photonic crystal cavity can be reversibly operated from the pull-in back to its release status. This enables to electrically switch the wavelength of a mode over ~50 nm with a potential modulation frequency above 2 MHz. These results pave the way to reliable nano-mechanical sensors and optical switches.

Low-temperature plasma-assisted atomic-layer-deposited SnO2 as an electron transport layer in planar Perovskite solar cells

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO2 layers, with the aim of identifying key material properties of SnO2 to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO2 films are fabricated at 50 °C, while a SnO2 film with a low electrical resistivity of 1.8 × 10–3 Ω cm, a carrier density of 9.6 × 1019 cm–3, and a high mobility of 36.0 cm2/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I2.7Br0.3) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO2 and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO2 deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO2 retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO2 shows a decrease in PCE of approximately 50%.

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.