Hole-transport material-free perovskite-based solar cells
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Introduction Over the past few years, a breakthrough has occurred in the photovoltaic (PV) field by using inorganic–organic hybrid layers called perovskites as the light harvester in solar cells. The inorganic layers consist of sheets of corner-sharing metal halide octahedra. The cation (M) is generally a divalent metal that satisfies charge balancing and adopts octahedral anion coordination. The inorganic layers are usually called perovskite sheets, because they are derived from the three-dimensional AMX3 perovskite structure (where A = organic molecule, X = halide, and M = metal). The organic component consists of a bilayer or a monolayer of organic cations. The inorganic–organic arrangement is self-assembled as alternate layers by a simple, low-cost procedure. These inorganic–organic hybrids promise several benefits that are not provided by the separate constituents,1–3 including mechanical rigidity, chemical stability and conductivity of the inorganic sheets, adjustable inter-sheet coupling by the organic part, and large absorption coefficient of the entire hybrid structure. These properties show good potential for the use of these materials as light harvesters in heterojunction solar cells. Due to intensive work on the perovskite structure, the solar-cell structure, and the deposition techniques (one-step/two-step deposition and coevaporation of perovskite films) of the different active layers,4–18 the efficiency currently achieved is approximately 20.1%.19
Exceptional properties of the perovskite are the high mobility of charges (mobility refers to how the charge moves through a semiconductor under an electric field) and long diffusion lengths of electrons and holes (diffusion length is the distance a carrier travels between generation and recombination), which are some of the reasons for the high solar-cell efficiency achieved.20,21 However, prior to the discovery of these significant properties, pioneering work13 was published that reported on the use of HTM-free perovskite heterojunction solar cells. The authors found that a lead halide perovskite can transport holes, in addition to its functionality as a light harvester. Avoiding the use of a HTM in the solar cell has several advantages, including reducing the cost, avoiding oxidation, simplifying fabrication of the solar cell, and providing consistent results. However, it must be noted that when eliminating an important layer in the solar-cell structure (such as the HTM), the PV performance decreased. Nevertheless, recently, perovskite solar cells without a hole conductor achieved a power-conversion efficiency (PCE) of 10.85%.22–26 In addition, a fully printable mesoscopic perovskite solar cell using a porous carbon film reportedly achieved 12.8% efficiency.27 This article presents an overview on perovskite solarcell structures without a HTM. The following topics are discussed: the mechanism in HTM-free solar cells; the possibility of gaining high voltage even without the HTM in
Lioz Etgar, The Institute of Chemistry, The Hebrew University of Jerusalem, Israel; li
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