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The ever-growing energy demand of the world necessitates the research and development of alternate sustainable energy sources that are cheaper and greener. Solar energy is the unmatched forerunner of all the available clean energy sources with an extensive research being done in the past few decades. The commercially available first generation solar cells are made of Silicon; Although they have high power conversion efficiencies (PCEs), they require high energy manufacturing process, suffer from material inavailability and the need for sophisticated device fabrication processes; Also, the first generation solar cell technology is so mature that there is little room for improvement. Later, the second generation photovoltaics evolved into amorphous Si, II-IV and III-V thin film photovoltaics, which also suffer from high cost, material scarcity, complex manufacturing process and material toxicity, in spite of having reasonably high PCEs. In the third generation photovoltaics, the hybrid and multi-junction third generation solar cells, which include dye-sensitized (DSSC), organic solar cells (OSC) and quantum-dot/nanomaterial based solar cells have the advantages of simple and low cost manufacturing. Organic-inorganic hybrid (OIH) materials have the combined properties of inorganic semiconductors and organic (polymer or small molecules) materials. OIH solar cells could adopt the merits of inorganic materials, such as stability, enhanced light absorption, high carrier mobility and compatible fabrication process, and utilize the advantages of organics, such as light weight, flexiblility, adjustable molecular structures for energy band alignment and solution processability.
The hot topic of current generation photovoltaics consist of organo metal halide perovskites that has attained an unprecedented growth from 3.8% to 22.1% of power conversion efficiency (PCE) in a short span of 8 years. Such high PCEs in perovskite solar cells (PSCs) are benefited from the unique properties of the perovskites such as high absorption coefficient, large carrier diffusion lengths, long carrier lifetimes, small exciton binding energies and a direct bandgap. The path to achieve such high PCEs comprised of understanding crystallization kinetics, factors affecting crystallization, controlling the device processing parameters to obtain defect-free crystals and interface engineering between different layers. This resulted in unexpected PCEs approaching its theoretical limits, rivalling Si-based solar cells. PSCs are more favorable for commercialization due to their simple processability, compatibility to large-scale production, material abundance and remarkable optoelectronic properties. In spite of favorable properties and high efficiencies, there are a few factors which hinder their commercialization. They are toxicity of lead, instability to air and moisture, use of fullerenes which are costlier and not very eco-friendly and use of PEDOT:PSS that is slightly acidic. To address a few of these problems, we propose alternative materials for fullerene-based electron transport layers (ETLs) and alternate non-toxic metal, Antimony (Sb) to replace toxic Lead (Pb)-based perovskites.
PCBM, a fullerene derivative, which is the commonly used electron transport layer (ETL) has the following disadvantages- high production costs, photochemical instability, tendency to aggregate at high temperatures leading to morphological instability, postfabrication crystallisation and synthetic inflexibility. N-type conjugated Perylene diimide based molecules are chemically robust, resistant to photodegradation, relatively easy to manipulate synthetically, possess tunable energy levels, cheaper and they are in industrial use as pigments. We propose the use of n-type conjugated perylene diimide based molecules as alternative materials for PCBM ETL in inverted planar heterojunction perovskite solar cells. A decent PCE of 11% was achieved with PDI small molecule in place of PCBM.
Later, we studied the use of benzo[ghi]perylenetriimide (BPTI) derivatives as novel ETL materials in a series of PSCs. The BPTI is expanded on the π-conjugated plane by a five-membered imide ring along the short axis on the original PDI backbone. Compared to PDI, BPTI shows straightforward access to chemical functionality through substituted groups attached on the five-membered imide position. This inspired us to explore possible strategies on making new non-planar π-conjugated electron acceptors. We achieved an efficiency of about 11.7% with the twisted-BPTI as the alternate ETL.
Another major concern regarding the commercialization of lead-based perovskite solar cells is the presence of environmentally toxic metal lead (Pb) and its chemical instability in air and moisture. Initially, Tin (Sn) was proposed to replace Lead as it can homogeneously substitute Pb due to similar valence state, but failed to achieve efficiencies as high as that of Pb-based ones till date. Also, the fact that Sn gets oxidized easily imposes hindrance to consider it as an alternative for Pb in terms of stability. To replace lead-based perovskites which are environmentally toxic, we propose the use of antimony-based perovskite materials, which forms A3B2X9 type perovskites by heterogeneous substitution of Pb. Our work is one of the pioneer reports on Sb-based perovskite solar cells. Initially, we achieved 1% efficiency with a simple solution process to form MA3Sb2I9 perovskite layer. Later, we introduced an additive to enhance the film quality and anti-solvent treatment with Chlorobenzene to enhance the crystallinity resulting in 2.1% PCE. Finally, we introduced hydrophobic interlayer between the hole transport layer and the perovskite layer to facilitate the formation of large-grain size crystals and reached 2.8% PCE, which is one of the highest PCEs reported for this material until now.
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