Hybrid organic/inorganic perovskite for high efficient solar cells

In the broad context of new generation photovoltaic, perovskite technology gained the interest of the scientific community thanks to the surprising power conversion efficiency (PCE) approaching that of already commercialized thin film photovoltaic. Hybrid organic/inorganic Perovskites are semiconductors with general formula ABX3, where A is the organic molecule such as CH3NH3 or CH(NH2)2, B the metal (Pb, Sn), and X is the halogen (I, Cl, Br). Such Organometal halide perovskites have attracted attention due to very interesting electrical/optical properties such as good electron/hole diffusion lengths of up to 1 μm [1,2,3] and strong light harvesting properties across the entire visible solar spectrum [4,5].
Moreover, hybrid perovskite have a simple synthesis [6] and can be deposited on a substrate by using solution processes, vacuum processes as well as vapor processes. Hybrid perovskite constitute the absorbing layer of the Pervoskite Solar Cell (PSC) and an Electron Transporting Layer (ETL), such as TiO2, SnO2, PCBM and a Hole Transporting Layer (HTL) such as Spiro-OMeTAD, P3HT, PTAA are needed to finalize the cell. Two families of PSC have been so far proposed, namely the Mesoscopic PSC based on mesoporous scaffold (TiO2, Al2O3) or the planar structure, which resembles the Organic PV structure. In a standard mesoscopic configuration, after the realization of a compact TiO2 (cTiO2) layer onto the conductive (FTO) glass substrate, a mesoscopic TiO2 (mTiO2) layer is deposited as a scaffold to realize CH3NH3PbI3 active layer. The counter electrode is subsequently realized by depositing a hole conductive polymer, typically spiro-OMeTAD and finally by evaporating gold metal contact.

The perovskite revolution started with the seminal work of Kojima and co-workers [4] who employed, for the first time, hybrid perovskites as sensitizers in a conventional liquid-electrolyte Dye Solar cell. Subsequently, a similar structure was optimized [5] showing an efficiency of 6.5%. However, only in 2012 the solid-state Perovkiste Solar Cells was introduced in three reports in which perovskites where employed to substitute active components in a typical solid-state dye sensitized solar cells. [6,7]. Starting from these works, perovskites have been employed in a plethora of different devices and a certified efficiency up to 22.7% has been recently demonstred. Similarly, perovskite solar modules demonstrated for the first time in 2014 have now reached and efficiency of 13%.[8]. Hybrid Perovskite, however, have a profound impact on the silicon PV technology. In fact, they are the ideal candidate in a tandem Pervoskite/Silicon solar cells and an efficiency of 23.6% have been already reported for such a structure [9]. It is expected that tandem cells could reach efficiency exceeding 30%. [10]

Being a new PV technology, several issues need to be verified and further developed before PSCs could be ready for commercialization. Stability of PSC is a concern and several degradation mechanisms (temperature, moisture, iodine migration, photobleaching etc.) [11] have been identified. Few solution strategies have been proposed, such as proper sealing, replace of Organic HTL with a carbon based one [12], optimized structure of the perovskite crystal [13] etc. In that context, interfaces engineering (IE) approach have been demonstrated to be a winning strategy to finely control the active layer realization and the final device’s performance and stability. Owing to the bi-dimensional nature of Graphene and Related Materials (GRM), a new paradigm to tailor interface properties based on GRM was recently proposed and applied to PSC and modules with the aim to increase both power conversion efficiency and stability of PSCs. [14]. In fact 2D materials have already exhibited excellent charge transport properties when employed in organic photovoltaics devices while GRMs have been successfully introduced as dopant or interlayer in the PSCs in order to improve the charge injection and/or collection at the electrodes. The Graphene Interface Engineering, has several advantages:

  1. the 2D nature of the materials matches the dimensionality of interface,
  2. ii) there is a large library of 2D materials and
  3. iii) 2D materials properties can be easily tuned by proper functionalization.

Several strategies have been used to master interface properties with GRM both at the anode and cathode parts of the cell. By dispersing Graphene flakes, produced by liquid

 phase exfoliation of pristine graphite,into the mesoporous TiO2 layer and by inserting graphene oxide (GO) as interlayer between perovskite and Spiro-OMeTAD layers, we demonstrate a PCE of 18.2% with the two-step deposition procedure, carried out in air. The proposed interface engineering strategy based on GRM has been exploited for the fabrication of state-of-the-art large area perovskite modules. We indeed demonstrated a PCE of 12.6% on a monolithic module with an active area exceeding 50 cm2. The use of GRM permitted to increase the PCE by more than 10% with respect to “conventional” modules, i.e. without GRM interfaces.

There is at the present a great interest in the scientific community concerning the HTL, typically the spiro-electronrich derivative spiro-OMeTAD, adapted from solid-state DSSC. Unfortunately spiro-OMeTAD has a number of serious drawbacks which limit its industrial scale-up such as many-step tedious synthesis, difficulty on the purification and, accordingly, high fabrication costs. The research community is engaged in finding new alternatives to spiro-OMeTAD which combine high power conversion efficiencies and easier preparation with lower manufacturing costs. In recent times this target has been achieved in a few cases although much research in this field is still needed.[15] Recently PSC with efficiencies higher than 20% have been prepared using a new HTM different than spiro-OMeTAD.[16]
From the point of view of electrical characteristics, PSCs are affected by hysteresis and usually forward and reverse I-V measurements produce different efficiency. The physical/chemical nature of the hysteresis is still debated and several hypothesis has been formulated such as ion migration, charge unbalancing, ferroelectricity, giant dielectric constant. Among them, ion migration seems to be one of the fundamental reason for such effect. [17]


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