Monday, June 15, 2015

Perovskite Thin-film Solar Cells

 

This innovation will make for excellent solar cells.  It may well be the breakthrough we have waited decades for.  Best it surfaces silicon and is far superior in terms of managing weathering.  


Its rapid climb in efficiency is actually rightly described as astounding.


Perovskite is a mineral familiar to geology and it is an refractory and otherwise difficult to work with, unlike silicon.  After all we have been manipulating silicon since the Bronze Age.  .

Oxford PV has pioneered the development of perovskite thin-film solar cells


Oxford PV is developing and commercialising thin-film perovskite solar cells, which can be printed directly onto silicon solar cells, CIGS solar cells or glass. This will drive a paradigm shift in the aesthetics, performance and cost of both current solar panels and Building Integrated Photovoltaic (BIPV) systems.

Pioneering work developing perovskite thin-film solar cells has delivered a route to boosting the efficiency of current commercial cells; using a high efficiency coating in a multi-junction or “tandem” cell architecture. In addition, printing perovskites directly onto glass has led to a semi-transparent coating ideal for BIPV applications and, once integrated into the glazing units of a building, the technology is capable of providing a significant percentage of the building’s electrical energy requirements directly from sun light, as can be seen from our case studies.

By employing well known and well understood printing processes, focused on inexpensive and abundant raw materials, Oxford PV has developed a highly cost effective technology.

Perovskite materials have astounded the solar cell community with a steep rise in efficiency from ~4% in 2010 to a certified efficiency of 20.1% (NREL) in 2014- already surpassing many other solar technologies.

To reach this remarkable efficiency in such a short time, many iterations of the cell architecture were trialled through to the current planar thin-film devices – more akin to other thin-film technologies.



Perovskite (pronunciation: /pəˈrɒvskt/) is a calcium titanium oxide mineral composed of calcium titanate, with the chemical formula CaTiO3. The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792–1856).[1]
It lends its name to the class of compounds which have the same type of crystal structure as CaTiO3 (XIIA2+VIB4+X2−3) known as the perovskite structure.[9] The perovskite crystal structure was first described by Victor Goldschmidt in 1926, in his work on tolerance factors.[10] The crystal structure was later published in 1945 from X-ray diffraction data on barium titanate by Helen Dick Megaw.[11]

Occurrence

Found in the Earth’s mantle, perovskite’s occurrence at Khibina Massif is restricted to the under-saturated ultramafic rocks and foidolites, due to the instability in a paragenesis with feldspar. Perovskite occurs as small anhedral to subhedral crystals filling interstices between the rock-forming silicates.[7]
Perovskite is found in contact carbonate skarns at Magnet Cove, Arkansas, in altered blocks of limestone ejected from Mount Vesuvius, in chlorite and talc schist in the Urals and Switzerland.[12] and as an accessory mineral in alkaline and mafic igneous rocks, nepheline syenite, melilitite, kimberlites and rare carbonatites. Perovskite is a common mineral in the Ca-Al-rich inclusions found in some chondritic meteorites.[2]
A rare earth-bearing variety, knopite, (Ca,Ce,Na)(Ti,Fe)O3) is found in alkali intrusive rocks in the Kola Peninsula and near Alnö, Sweden. A niobium-bearing variety, dysanalyte, occurs in carbonatite near Schelingen, Kaiserstuhl, Germany.[12][13]

Special characteristics

The stability of perovskite in igneous rocks is limited by its reaction relation with sphene. In volcanic rocks perovskite and sphene are not found together, the only exception being in an atindite[dubious ] from Cameroon.[4]

Physical properties

Perovskites have a cubic structure with general formula of ABO
3
. In this structure, an A-site ion, on the corners of the lattice, is usually an alkaline earth or rare earth element. B site ions, on the center of the lattice, could be 3d, 4d, and 5d transition metal elements. A large number of metallic elements are stable in the perovskite structure, if the tolerance factor t is in the range of 0.75 – 1.0.[14]
 t = \frac{R_A + R_O}{\sqrt2(R_B+R_O)}
where RA, RB and RO are the ionic radii of A and B site elements and oxygen, respectively.
Perovskites have sub-metallic to metallic luster, colorless streak, cube like structure along with imperfect cleavage and brittle tenacity. Colors include black, brown, gray, orange to yellow. Crystals of perovskite appear as cubes, but are pseudocubic and crystallize in the orthorhombic system. Perovskite crystals have been mistaken for galena; however, galena has a better metallic luster, greater density, perfect cleavage and true cubic symmetry.[5]

Layered perovskites

Perovskites may be structured in layers, with the above ABO
3
structure separated by thin sheets of intrusive material. Different forms of intrusions, based on the chemical makeup of the intrusion, are defined as:[15]
  • Aurivillius phase: the intruding layer is composed of a [Bi
    2
    O
    2
    ]2+ ion, occurring every n ABO
    3
    layers, leading to an overall chemical formula of [Bi
    2
    O
    2
    ]-A
    (n−1)
    B
    2
    O
    7
    . Their oxide ion-conducting properties were first discovered in the 1970s by Takahashi et al., and they have been used for this purpose ever since.[16]
  • Dion−Jacobson phase: the intruding layer is composed of an alkali metal (M) every n ABO
    3
    layers, giving the overall formula as M10+A
    (n−1)
    B
    n
    O
    (3n+1)
  • Ruddlesden–Popper phase: the simplest of the phases, the intruding layer occurs between every one (n = 1) or two (n = 2) layers of the ABO
    3
    lattice. Ruddlesden−Popper phases have a similar relationship to perovskites in terms of atomic radii of elements with A typically being large (such as La[17] or Sr[18]) with the B ion being much smaller typically a transition metal (such as Mn,[17] Co[19] or Ni[20]).

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