Perovskite: A Replacement for Silicon Solar Cells
For decades Silicon based solar cells have ruled the PV industry, but a different crystalline material, perovskite, have recently attracted considerable attention due to the advantages of high performance, low cost, easy solution-processablity, light weight, and flexibility . The intrinsic properties of perovskites — such as their broad absorption spectrum, fast charge separation, long transport distance of electrons and holes, long carrier separation lifetime — make them very promising materials for solid-state solar cells. But, there are issues such as reproducibility and long term stability that need to be addressed before they can be used on large scale for solar cells fabrication.
Perovskites, named after the mineral CaTiO3, are the materials that have a generic chemical formula ABX3 and a cubic structure. In general, A and B sites can accommodate inorganic cations of various valency and ionic radius. Alternatively, suitable organic species can replace cation A and create organic–inorganic hybrid materials. X sites are occupied by different halides such as iodine, chlorine etc. Perovskite has shown a number of exciting physical properties, like colossal magnetoresistance, ferroelectricity and superconductivity, but its use for solar cell fabrication was very limited till 2009. In 2009 Miyasaka et al reported use of halide perovskite as visible light sensitizers in liquid DSCs and an efficiency of only 3.8% obtained with the perovskite CH3NH3PbI3. This work attracted attention of co-workers in the same field and within a span of six years perovskite solar cells efficiency crossed over 20%.
The highest confirmed power conversion efficiency of solid-state perovskite solar cells has rapidly raised to over 20% under 1 Sun (100 mW cm–2) illumination [3, 5]. First DSC (Dye Sensitized Solar Cell) was reported in 1991, which used dye to sensitize a wide band gap semiconducting material, and since then researchers have been in search of dyes with a broader absorption spectrum and a higher molar extinction coefficient. A conversion efficiency of 6.5% was reported in 2011, also relying on CH3NH3PbI3, this time as quantum dots. A major breakthrough was obtained in 2012 with the use of CH3NH3PbI3 for sensitization in an all-solid-state thinfilm (submicrometre) mesoscopic solar cell, which showed an efficiency of 9.7%. Almost at the same time, efficient hybrid solar cells based on mesoporous super-structured organometallic halide perovskite were also demonstrated that showed power conversion efficiencies up to 10.9% in a single-junction device. During 2014 there have been a number of publications with reported efficiencies more than16%, along with predictions of efficiency crossing over 20%. Recently, yang et al reported efficiencies crossing over 20% for perovskite solar cells based on FAPbI3. A graphical representation of growth of perovskite based solar PV in past few years is shown in fig. 1.
The precursors required to make a perovskite solar cell are abundant and inexpensive, and their conversion into thin films can be achieved by using simple solution and vacuum-based techniques. Therefore, in principle, perovskite photovoltaics can generate electricity with a few easy steps and at a very low cost . A cross sectional scheme of solar cell fabricated using solution processing has been shown in Fig. 2, an electron transport material (ETM) is coated on a conductive substrate (FTO coated glass for example), and a photoactive layer of perovskite is spin-coated over the ETM. Another layer of hole transport material (HTM) is then spin-coated on top of the perovskite, and a metal thin-film, which serves as the contact, is subsequently deposited .
Under illumination of light, the perovskite material absorbs light and creates charge separation: electrons move to the conductive substrate through the ETM and holes move to the metal contact through the HTM. Those light-generated electrons and holes can then do the work and generate electricity through an external circuit (Fig. 2). Recently Noh et al. have reported a colourful perovskite solar cell based Stable Inorganic? Organic Hybrid Nanostructured. These kind of colourful solar cells can be an excellent choice for making of attractive commercial solar cells.
There is no doubt that perovskite materials have shown excellent overall performance, both in device and commercial viability, and in future can be a good replacement for Si solar cells. The present efficiency record already shows promise for practical commercial applications, though there is still room for further improvement (theoretical values >30%) by changing the precursor chemistry and using different technologies . But there are major problem and pitfalls that need to be taken care of in order to make it commercially viable option for solar cells. Problems listed below needs immediate attention.
1. Being hygroscopic in nature perovskite gets degraded in presence of humidity
2. Weak metal-halogen bonds compromises stability and durability of solar cells for outdoor applications. 3. Lead toxicity is other major disadvantage with methyl ammonium lead halide PSCs.
·Sun L., “PEROVSKITE SOLAR CELLS Crystal crosslinking” Nat Chem. 7 2015 684-685.
·Jeon N. J. et al., “Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells” Nat Mater.13 2014 897-903.
·Sessolo M., and Bolink J H., “PEROVSKITE PHOTOVOLTAICS: Hovering solar cells”, Nat Mater. 14 2015 964-966.
·Huang C. et al., “Insight into Evolution, Processing and Performance of Multilength- scale Structures in Planar Heterojunction Perovskite Solar Cells”, Sci Rep. 5 2015 1-11.
·Yang et al., “High-performance photovoltaic perovskite layers fabricated through intramolecular exchange”, Science 348 2015 pp. 1234-1237.