Friday, November 07, 2014

Regenerative Agriculture: The Transition.

In the face of peak oil and in order to curb carbon emissions, methods of farming that depend less on oil and natural gas, respectively to run machinery and to make synthetic fertilizers, must be sought. Such options are to be found within the framework of regenerative agriculture, but the transition from current industrialised agriculture to these alternative strategies will prove testing.

It is an illusion to think we can continue to use as much energy as we do now. No one can entirely rule-out that some extravagant technology will be forthcoming, e.g. solar power or nuclear fusion on the full-scale of more than 500 EJ/year as we get through now, but the particular issue of matching liquid fuels derived currently almost entirely from petroleum appears insurmountable. The "solution" is probably the collective of individual solutions, and this means adopting a completely different paradigm of human philosophy and intention. The most pressing demand is how to feed the population of the world, and how to adapt industrialised conurbations, with cities provided for entirely from external regions for their food and electricity. If oil is the most vulnerable element in the energy-mix as the life-blood of transportation, then we must aim to live with less transportation, and this includes the means and distribution implicit to modern food production.

In methods of regenerative agriculture and permaculture, much of the energy involved is provided quite naturally by native soil flora and fauna fed ultimately by photosynthesis, since the fuel for good soil derives from plants as the factories that supply carbon-rich nutrients, and in a wonderful symbiosis, the living soil microbes, especially fungi can draw other nutrients and water from the soil to nourish the plants. The individual elements of life feed one another in a mutually dependent and beneficial manner.

While the two approaches can be defined and envisaged rather clearly, the intermediate means for transition from industrial agriculture, with its acknowledged unsustainable impact on the soil, to regenerative agriculture (permaculture, agro-ecology) is rather more nebulous, since it has not been done before, or at least not in the degree that necessity now demands. So how might we perform this revolution in the least painful way?

For a start, a decolinising and restructuring of present industrialised agriculture is necessary along with an appreciation and magnification of native and traditional food systems Overall, a change in thinking and concept is required from conflict and limit to cooperation and abundance.

The scale of the transition may be compared with other milestone transitions throughout human history, such as the hunter-gatherers becoming farmers, and then modern industrial societies. It is the latter that are under threat and unsustainable, and a compromise devolution to a more localised collective of small communities (pods) is required, supplied by local farms and infrastructure with (probably) rail links between them for essential movement of goods and people. The maintenance of the Internet and electronic communications would seem desirable since ideas and knowledge can be transmitted from pod to pod and between countries and continents.

In the 1970s, there were studies done that evaluated the massive inefficiency in energy requirements for food production. It was concluded that 10 Calories of energy are expended to bring 1 calorie of food onto the dinner plate It has been stressed that essential agricultural production is to yield food and fibre - i.e. the essential elements of biomass. One might also add-in fuel as a product, if the consideration also includes fermentation of sugars form starch into ethanol, or hydrothermal production of liquid and gaseous fuels from biomass by heating it under pressure in the presence of water.

The impending stress of "climate change" is well acknowledged, e.g. sea-level rise and the spreading desertification of formerly green lands, but its impact on agriculture is rarely mentioned by climate-modellers. However, as a for-instance, it is speculated that the Colorado River basin could dry up. It's mighty dams would then look something like the pyramids of Egypt, maybe leaving future generations to speculate as to what their purpose was, and upon the nature of the civilization that created them. As climate zones shift, it is the variability of the weather that will have greater impact than ramping "mean temperatures" on the enormous investment made by humans in agriculture. The capital outlays required for new dams, irrigation supplies and the retraining of farmers will need to be contrasted with that for flood-defences in vulnerable locations (e.g. New Orleans and the east coast of England). Most likely both cannot be supported and it may prove expedient to simply let some regions "go to the sea".

Biodiversity is a natural means for evening-out the losses and gains of living systems It is cooperative in the sense that pests are not encouraged as they are by growing single strains of crop, and that suitably matched plants help each other to grow - the holistic whole being more robust than the simple measure of its components. The term "global village" tends to signify an interconnected unity of trade or electronic communication, while aspects of cultural diversity and biodiversity seldom enter the line of thinking. However, it is a necessity to preserve and expand the traditional food and fibre production systems that are tried and tested and whose regenerative capabilities have been demonstrated over millennia. We may adapt to or readopt cultures that have been lost, as industrial civilization has supplanted them, and it is the latter that we must seek to break away from in order to arrive at a sustainable future, that is if we are to survive as a human species.

If "global village" means "global supermarket", the term lends acceptance to the concomitant rule of multinational corporations. If we restructure societies to become self-sustaining, rather than dependent on inputs and indeed outputs, as they are now, we also must abandon "limited liability" and the legal designation of "corporations" as "persons" with the same rights as individual citizens. Traditional food systems are storehouses both of biodiversity and cultural diversity. It is a pity that the seedbanks around the world contain no information about the culture, economy, details of cultivation methods, flavour or other human aspects of the crops and the food they produce. Including my own musings on the topic, most commentators on the post peak oil world refer to the need to localise food systems, such that small populations are provided for locally by means of community farms. However, establishing regenerative systems to grow food and fibre must include cities too, the design of which must be analysed in terms of the natural mechanisms that interweave them.

It is mostly not realised that the rural development or redevelopment urged by the industrialised nations for the developing world are precisely those they need to adopt themselves. E.F.Schumacher's "Buddhist Economics" which he describes in the bestselling "Small is Beautiful - A Study of Economics as if People mattered", applies equally to the industrialised world as it must of needs de-industrialise, and take lessons from simpler societies which consume far less per head of population. The example of Cuba may be taken as a benchmark for progress, as it has survived and indeed thrived through implementing a system of community gardens, in the abrupt absence of cheap and plentiful oil and fertilizers gifted from the Soviet Union when that regime collapsed in 1989. "Small is Beautiful" has been updated for the 21st Century context by Fritz Schumacher's Daughter-in-Law, Diana:

James Lovelock's Gaia hypothesis has acted as an iconic beacon to the environmental movement, drawing-in a range of people who are dissatisfied with the industrial and materialistic way of life, and seek alternative, more natural and or spiritually rewarding lifestyles, and with less detriment to the planet and life upon it. "Gaia" is holistic in nature and is based on ecology. Rather than an indstrialised "global village" it implies a "globe of villages". Food and fibre production one of the most important features of the transition to a post-fossil fuel era, to which the establishment of regenerative food systems is essential. Underpinning all of this is the need to grow and protect perhaps the most fundamental element of the life-nexus, the soil itself, which is truly "our children's earth"

Thursday, October 02, 2014

Is Peak Oil Now a Phantom?

While this edited article of mine (on page 4) was published in The Professional Engineer a while ago, I have only just discovered its existence!I remember being asked to write a piece for them, by someone in the audience at a talk that I gave to the Guildford, Cafe' Scientifique a while back on "What happens When the Oil Runs Out?", but I had heard nothing further. However, it provides a reasonable summary of the oil situation, which is probably worth emphasising here.

All engineers should recognise the formula m.g.h as representing potential energy. Oil is one of the most used sources of such energy, and once it has been released from the form in which it is found, it is gone. Professor Rhodes’ article relates to his concern that the rate of finding new sources of energy may not keep up with the rate of diminution of existing sources, and this concern ought to be of the greatest interest to Professional Engineers.
It has been claimed that the United States has enough natural gas to last for 100 years, and that by 2017 the nation will be producing more oil than Saudi Arabia. Much of this bounty, it is asserted, will come from horizontal drilling, combined with hydraulic fracturing (“fracking”). Therefore, so runs the rhetoric, peak oil can now be relegated to a myth. Indeed, to quote from an article in The Daily Mail (8-12-12):

“…the Earth can now provide us with about 250 years’ worth of gas supplies.
The so-called ‘peak oil’ theory, which suggests that within the foreseeable future the world will run out of fossil fuels — coal, oil and gas — has never looked more absurd.”

Peak oil does not mean we will abruptly “run out of oil”, but that the rate of production will reach a maximum and thereafter relentlessly fail demand for it. For a global civilization, entirely dependent on crude oil for its food, materials, transportation, and economy, the unplanned consequences could be dire. Many of the more cornucopian conclusions are arrived at by confounding resources with reserves, and ignoring the fact that it is not only the quantity that might be available, which determines “peaking”, but the rate at which it can be recovered, over time. A useful analogy is that it is the size of the tap not the size of the tank that matters. In gauging a resource, all known, proved, probable and theoretical quantities are tallied together, not only ignoring technical and economic factors, but the uncertainty of whether the material is there to be had in the first place. Thus resources are considerably “larger” than reserves.

While oil or gas is not going to “run out” any time soon, continuing to produce 30 billion barrels of conventional crude oil every year is unlikely to be possible for very much longer. We have already run out of cheap oil, and at some near point, production will reach a maximum, and then fall relentlessly. It must – this is the nature of a finite reserve. So long as the enlarging “hole” in the supply of conventional crude oil can be filled from unconventional sources, all is well, but once it exceeds them, the overall sum will pass into the negative; i.e., global oil production will have peaked.

New technologies – horizontal drilling combined with fracking – have made it both practically and economically viable to exhume gas and oil from previously inaccessible reservoirs. In principle, shale gas can be recovered all over the world, although until an actual well is drilled, both the quantity and quality of it are unknown – e.g. from nine such wells drilled in Poland, came a gas so heavily contaminated with nitrogen that it wouldn’t burn. Both shale gas and shale oil wells tend to play-out more rapidly than their conventional counterparts, and after two years, production has typically decreased by 80%, meaning more wells must be drilled continually to maintain the overall output of a field. If shale gas production is to be enhanced, they must be drilled even faster, and at a typical unit cost in the region of $5-10 million. Ultimately, the strategy must run up against material limits in financial investment, infrastructure, equipment and trained personnel that can be brought to bear in the effort.

As to how much shale gas the United States has, detailed inspection of the available figures reveals the “100 years worth” claim to relate to a resource – i.e. the most optimistic set of accounts – while the reserve (proved plus probable) is enough for only 20 years. To surpass Saudi Arabia, by 2017, a total production of 11 million barrels a day (mbd), ramped up from just under 6 mbd currently, would be necessary. The projected production of shale oil (for which the correct term is “tight oil”) falls far short of this. The term “liquids”, is now often used, by which biofuels, natural gas plant liquids (NGPLs) and refinery gains are reckoned together with crude oil. This obfuscates the truth, since the other liquids have different properties from crude oil - in particular, a lower energy density. While world production of liquids has increased by around 3 mbd since 2004, actual production of crude oil has remained almost flat at 72 mbd, and so the global production limit may have been reached.

It is claimed there are two trillion tonnes “oil” under the U.S., in the form of oil shale, but really, this refers to a resource, not a reserve. Moreover, oil shale is not the same thing as shale oil. Shale oil (tight oil) is actual crude oil that if recovered, e.g. through horizontal drilling and fracking, can be refined in the normal way. Oil shale does not contain oil as such, but a solid organic material called kerogen, which is heated to around 500 oC, in order to crack it into liquid form. The process also uses large amounts of freshwater, and churns-out an equal volume of contaminated wastewater which needs to be dealt with.

There is, as yet, no commercial scale production of oil from “oil shale”, and there may never be, since it takes almost as much energy to get oil from it as will be delivered by the oil itself, i.e. pointless. The returns are better on “oil sands”, maybe 3 to 1, in energy terms - once the material has been “upgraded” to provide a liquid fuel - but here too, vast quantities of water are needed, and sufficient energy is required to extract the bitumen in the first place, that installing nuclear reactors in such locations is being considered seriously as a source of heat, currently generated  by burning natural gas.

Since the total “oil” contains five times the amount of carbon reckoned to raise the mean global temperature by 2 oC - modelled as the limit, to avoid dangerous climate change - even if it could all be accessed and burned, the effect on the climate would most likely be catastrophic.

Professor Rhodes has outlined a problem on which Professional Engineers are in an eminently well qualified position to hold a view.


Tuesday, September 16, 2014

Perovskites – and their Potential Use in Solar Energy Applications: A Current Commentary.

The following has just been published in the most recent (September 2014) issue of the journal Science Progress, of which I am an editor.The article may be downloaded for free from this link:

Science Progress welcomes proposals and manuscripts from potential authors on most aspects of science and technology, guided by the following description:

"The journal's objective is to provide reviews of a range of current topics, which are both in-depth in their content, and of general appeal, presenting the reader with an overview of contemporary science and technology, and its impacts on humanity." The style of the following article is intended to illustrate this.

A material may be described as having a perovskite structure1 if it has same type of crystal structure as perovskite - calcium titanium oxide (CaTiO3) – itself does (Figure 1). Perovskite was first discovered in 1839 by Gustav Rose, in the Ural mountains in Russia, and is named after the Russian mineralogist L. A. Perovski (1792–1856), who first characterised the material. The general formula for perovskites is ABX3, with 'A' and 'B' being two cations of significantly different sizes ('A' > 'B'), while X is an anion that binds with both cations. The perovskite structure is adopted by many oxides which have an elemental composition: ABO3. The ideal cubic-structure has the ‘B’ cation in a 6-fold coordination, surrounded by an octahedron of anions, with the ‘A’ cation in a 12-fold cuboctahedral coordination. Cations 'A' occupy the cube corner positions (0, 0, 0), while cations 'B' occupy the body centred positions (1/2, 1/2, 1/2) with oxygen anions ‘O’ being located at face centred positions (1/2, 1/2, 0). Figure 1 shows edges for an equivalent unit cell with ‘A’ in body centre, ‘B’ at the corners, and ‘O’ in mid-edge. The requirements of relative ionic radii are quite exacting to maintain a stable cubic structure, meaning that even relatively minor degrees of buckling and distortion can result in a number of alternative versions with lower symmetry, in which the coordination numbers of either the ‘A’ cations, ‘B’ cations, or both, are reduced. Tilting of the BO6 octahedron reduces the coordination of a too-small ‘A’ cation from 12 down to as low as 8. Conversely, when a small ‘B’ cation is brought off-centre, within its octahedral coordination, a stable bonding arrangement can be obtained. Such distortions can create an electric dipole, and it is for this reason that perovskites such as BaTiO3, which distort in this manner, exhibit the property of ferroelectricity. The most usual non-cubic forms of perovskites are the orthorhombic and tetragonal variants. There are also some more complex perovskite structures which contain two different ‘B’-site cations, with the result that ordered and disordered variants are possible.
Figure 1 here.
Under the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSiO3, is converted to a more dense perovskite-type polymorph, and indeed it is speculated that this particular phase of the material might be the most common mineral in the Earth.2 It has a perovskite structure, with an orthorhombic distortion, and is stable at pressures from ~24 GPa to ~110 GPa. [For comparison, we may note that the pressure at the centre of the Earth is ca 300 GPa]. However, it is stable only at depths of several hundred kilometres and could not be transported back to the Earth’s surface without reforming into less dense materials. At yet greater pressures, MgSiO3 perovskite undergoes a transformation to form post-perovskite. Although the most common perovskite compounds contain oxygen, perovskites containing fluoride anions are known, e.g. NaMgF3. Metallic perovskite compounds also exist1, with the general formula RT3M, where R represents a rare-earth or other relatively large cation, T is a transition metal ion and M represents light metalloids (anions) which occupy the octahedrally coordinated ‘B’ sites, e.g. RPd3B, RRh3B and CeRu3C. MgCNi3 is a metallic perovskite compound, and is of particular interest on account of its superconducting properties. As a further category, are mixed oxide-aurides of Cs and Rb, such as Cs3AuO, which contain large alkali metal cations in the traditional "anion" sites, bonded to O2− and Au anions.


Properties of perovskites.

As noted, the perovskite structure is imparted with an appreciable element of structural pliancy, and the ideal cubic structure (Figure 1) can be distorted in many different ways. Thus, the octahedra may become tilted, the cations be displaced from the centres of their coordination polyhedra, and the octahedra might be distorted at the behest of electronic factors (e.g. Jahn-Teller distortions).2 Accordingly, perovskite materials exhibit many unusual properties that are of theoretical interest, but which may also furnish practical applications. Such phenomena as colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin-dependent transport, high thermopower and the interleaving of structural, magnetic and transport properties are those typically observed from this family of materials. Thus, applications are found for perovskites in sensors and in catalyst electrodes for certain types of fuel cells, and they might play a future role in memory devices and spintronics devices.2 Many superconducting ceramic materials (high temperature superconductors) have perovskite-like structures, generally incorporating three or more metals, copper often being one of them, and with some prevailing oxygen vacancies. In the latter regard, yttrium barium copper oxide can be made either insulating or superconducting according to its oxygen content. It is also of note that a cobalt-based perovskite material is being developed, intended to replace platinum in the catalytic converters of diesel vehicles.2 In view of the limited availability of platinum, this would be a major advance. As we shall see, perovskites also offer the potential to be incorporated in efficient, and low-cost photovoltaic cells.


The cumulative global photovoltaic generating capacity had reached around 100 GWp (gigawatts) by the end of 2012, 85% of which is derived from crystalline Si-cells, the remainder being from polycrystalline thin film cells, mostly containing cadmium telluride/cadmium sulfide3. While thin-film cells tend to be cheaper to make and offer a shorter energy payback time4, most of them rely upon rare elements such as tellurium (which is as rare as gold), indium, and gallium, all of which have issues over their future supply5, certainly if the global photovoltaic generating capacity is to be extended into the Terawatt (TW) realm3. On grounds of their relative cheapness and that a conversion efficiency of 15% has been achieved from them (i.e. as is competitive with thin-film photovoltaic technology4), synthetic perovskites are being explored as foundation materials for the manufacture of high-efficiency commercial photovoltaic devices (e.g. Figure 2). As a further convenience, they can be produced using the same thin-film methodology as is used to make thin film silicon solar cells.6 Organic-inorganic perovskite-structured semiconductors have shown promise as high-performance light-harvesting materials in solar cells, most commonly methylammonium lead triiodide (CH3NH3PbI3), initially used as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures6. Such materials are found to possess both a high charge carrier mobility and a high charge carrier lifetime, meaning that light-generated electrons and holes can move over sufficiently long distances that an electric current may be extracted from them, as opposed to the excitation energy being merely dissipated as heat within the cell. The effective diffusion lengths are close to 100 nm for both electrons and holes in CH3NH3PbI3.7,8 Since low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole diffusion lengths (typically about 10 nanometers), this result is significant and deserves explanation. By applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective-electron or selective-hole extraction materials, balanced long-range electron-hole diffusion lengths of at least 100 nanometers have been confirmed to exist in solution-processed CH3NH3PbI3.  It is concluded that the high photoconversion efficiencies of these systems are a result of the fact that the optical absorption length and charge-carrier diffusion lengths are comparable, so obviating the traditional constraints of solution-processed semiconductors8. Low-temperature solution methods (spin-coating) are employed for the deposition of the latter kind of perovskites. This approach is likely to lead to cheaply produced devices on account of the low temperature solution methods per se, and that there is no requirement for rare elements. Solution-processed films produced by other low-temperature (< 100 oC) methods have the disadvantage that the resulting diffusion lengths are considerably shorter.
Figure 2 here.
Stranks et al. have reported nanostructured photovoltaic cells made with CH3NH3PbI3-xClx (essentially the triiodide, but containing a small quantity of chloride) which in one case gave a conversion efficiency of 11.4%, but this was increased to 15.4 % when vacuum evaporation was employed. A diffusion length of > 1 µm was determined7 for CH3NH3PbI3-xClx, which is an order of magnitude greater than that in the pure iodide. The carrier lifetimes are also increased in the mixed halide perovskite from those in the pure iodide.7 The open-circuit voltage (VOC) typically approaches 1 V in CH3NH3PbI3, while for CH3NH3PbI3-xClx , a VOC > 1.1 V has been observed. The band gaps (Eg) of both materials are 1.55 eV, and so the VOC-to-Eg ratios are higher than those usually measured for similar third-generation cells. A VOC of 1.3 V has been demonstrated for perovskites with higher band-gap energies.3 However, under working conditions, the cell presently lacks sufficient durability to be used as an actual commercial device.3 New strategies are being explored to obtain an even greater VOC, using CH3NH3PbBr3 which, when employed as a film containing Cl ions, can be as high as 1.5 V9. Vapour-deposition has been employed to make planar heterojunction perovskite solar cells containing simplified device architectures (i.e. with complex nanostructures absent), which show a 15% solar-to-electrical power conversion, as determined under simulated full sunlight.6
The importance of the field is indicated by the recent ACS Selects collection (, and that both the highly reputable journals Nature and Science, have highlighted9 perovskite photovoltaics as one of the major scientific advances of the year 2013. On the basis of the rapid developments that have been witnessed in CH3NH3PbX3 (X = Cl, Br, or I) perovskite photo-sensitizers as used in solid-state mesoscopic solar cells, it is anticipated that a power-production efficiency as high as 20% might be obtained, using optimized perovskite-based solid-state solar cells.10 Indeed, by means of a low-temperature (70 °C) solution processing to make TiO2/CH3NH3PbI3 based solar cells, a power conversion efficiency (PCE) of 13.7% has been obtained, along with a high open circuit potential (VOC) of 1110 mV, which is claimed to be the highest VOC value measured for solution-processed TiO2/CH3NH3PbI3 solar cells. A nanocrystalline TiO2 (rutile) hole-blocking layer was deposited on a fluorine-doped tin oxide (FTO) conducting glass substrate via hydrolysis of TiCl4 at 70 °C, to create an electron selective contact with the photoactive CH3NH3PbI3 film. It was reported that this nanocrystalline rutile is superior in its performance to a planar TiO2 (anatase) film which was prepared by high-temperature spin coating of TiCl4, and gave a much reduced power conversion efficiency of 3.7%. This result is explained in terms of an intimate junction being formed with a large area, so providing an effective interface between the nanocrystalline rutile TiO2 and the CH3NH3PbI3 layer, with an enhanced ability to extract and mobilise electrons.11 A series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX3, has been prepared whose optical properties may be tuned according to the nature and ratio of the halides employed (X =  Cl, Br, and I), and with different cell archetectures: thin film, and mesoporous scaffold (TiO2 and Al2O3). Using impedance spectroscopy, it is found that the the charge recombination rates are decreased in the light absorber film, when Cl and Br are included in the perovskite lattice. The charge recombination rates are lower, as prepared on a mesoporous Al2O3 electrode, than those devices prepared on mesoporous TiO2. In all the devices measured, an efficiency was preserved to at least 80% of the initial value one month after their preparation.12

Theoretical and spectroscopic studies of perovskites.
The low-frequency resonant Raman spectrum of methylammonium lead-triiodide, adsorbed on mesoporous Al2O3 has been obtained. On the basis of DFT calculations of appropriate related systems, the bands at 62 cm-1 and 94 cm–1 are assigned respectively to the bending and stretching vibrations of the Pb–I bonds, while the bands at 119 cm-1 and 154 cm-1 are ascribed to librations of the organic cations. There is also a broad, unstructured band spanning the range 200–400 cm–1 which is assigned to torsional vibrations of the methylammonium cations, and serves as a marker of the orientational disorder of the material.13 Electronic structure calculations have been employed to interpret the fundamental properties of bulk perovskites. Hybrid perovskites are predicted to show spontaneous electric polarization, a phenomenon which might be fine-tuned according to the selection of the organic cation. It is concluded that the presence of ferroelectric domains will form internal junctions that might assist the separation and segregation of photoexcited electron and hole pairs, with an according lowering of the recombination rate. The Wannier-Mott exciton separation and effective ionization of donor and acceptor defects are both promoted as a result of high dielectric constant and low effective mass, and it is proposed that the photoferroic effect could be used to generate a higher open circuit voltage in nanostructured films too and may be responsible  for the current–voltage hysteresis that is measured in perovskite solar cells.14 A determination was made of photovoltaic conversion in high-performing perovskite-based mesostructured solar cells, with a particular focus on the part played by the mesoporous oxide/perovskite interface. Using a number of different spectroscopic methods, in particular Stark spectroscopy, the existence of oriented permanent dipoles, consistent with the hypothesis of an ordered perovskite layer, close to the oxide surface, was demonstrated. It is concluded that one of the decisive reasons for the highly efficient transport of electrons and holes in perovskite films, could be the presence of such interfacial ordering, as promoted by specific local interactions.15 The electronic structure and chemical composition of efficient photoelectron spectroscopy with hard X-rays, was used to study CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2, so being able to measure the occupied energy levels of the perovskite in addition to the underlying TiO2, so to determine the energy level matching at the interface. A good agreement was found between the simulated density of states and the measured valence levels, and it was concluded that similar electronic structures were formed, despite two different deposition methods being used.16 DFT calculations have been employed to investigate the intrinsic defects in CH3NH3PbI3 and their relation to its photovoltaic properties. Schottky defects, as are vacancies on PbI2 and CH3NH3I, do not form stable trapping sites, and so they can reduce the lifetime of carriers. However, vacancies on Pb, I, and CH3NH3 which originate from Frenkel defects, can act as dopants, such that methylammonium lead halides (MALHs) can become unintentionally doped. That there are no intrinsic defects in MALHs is accounted for by the ionic bonding that results from organic–inorganic hybridization.17
            By means of highly sensitive photothermal deflection and photocurrent spectroscopies, the absorption spectra of CH3NH3PbI3 perovskite thin films were measured at room temperature, yielding a high absorption coefficient with an unusually sharp onset. The presence of a well-ordered microstructure is suggested by the fact that below the bandgap, the absorption is exponential over more than four decades and the Urbach energy is down to 15 meV. No evidence for deep states was found at least at the detection limit of 1 cm–1. These results accord with the well established electronic properties of perovskite thin films, and the relatively high open-circuit voltages measured for perovskite solar cells. Evidence for a change in the composition of the material, caused by the deliberate introduction of moisture, is given by the strong reduction in the absorption at photon energies below 2.4 eV.18 It has been shown that trap states at the perovskite surface give rise to charge accumulation and consequent recombination losses in working solar cells. It is found that undercoordinated iodine ions within the perovskite structure are responsible for this effect, and supramolecular halogen bond complexation can be utilised as a means to passivate these sites successfully. Thus, solar cells are fabricated with a maximum power conversion efficiency of 15.7% and a stable power output of > 15%, under a constant 0.81 V forward bias, in simulated full sunlight. It is concluded that such means of surface passivation may pave the way to constructing more efficient perovskite solar cells.19 On the basis of DFT calculations, it is shown that that the band gap in three-dimensional (3D) hybrid perovskites CH3NH3PbX3 (X = Br, I) is dominated by a massive spin–orbit coupling (SOC) in the conduction-band (CB). Both direct and isotropic optical transitions, at ambient temperature, are associated with a spin–orbit split-off band that is related to the triply degenerate CB of the cubic lattice in the absence of SOC. As a result of the dominance of the SOC, the electronic states involved in the optical absorption are but weakly perturbed by local lattice distortions.20 Again using DFT calculations, the observed absorption blue shift along the I → Br → Cl series was accounted for in CH3NH3PbX3 and mixed halide CH3NH3PbI2X perovskites (X = Cl, Br, I). It was found that CH3NH3PbI3 and the mixed CH3NH3PbI2Cl or CH3NH3PbI3–xClx perovskites exhibited a similar absorption onset at 800 nm, whereas CH3NH3PbI2Br absorbs light below 700 nm. A good accord was met between the calculated band structures and the experimental trend of optical absorption frequencies. The existence of two different structural types with different electronic properties was indicated for the mixed perovskites (CH3NH3PbI2X), with a relative stability that depended on the nature of the group, X. For these systems, the calculated energies of formation decrease in the order I > Br > Cl, which would accord with the observed miscibility of CH3NH3PbI3 and CH3NH3PbBr3, while suggesting that the degree of chlorine incorporation into CH3NH3Pb(I1–xClx)3 should be smaller. The calculations further indicate that that in the PbI4X2 octahedra, the Cl atoms preferentially occupy the apical positions while Br atoms may occupy both apical and equatorial positions, in agreement with reported lattice parameters.21 Transient laser spectroscopy and microwave photoconductivity measurements were made22 on TiO2 and Al2O3 mesoporous films impregnated with CH3NH3PbI3 perovskite and the organic hole-transporting material spiro-OMeTAD. The results show that primary charge separation occurs at both junctions, involving simultaneously TiO2 and spiro-OMeTAD, with ultrafast electron and hole injection occurring from the photoexcited perovskite over similar timescales. It is observed that charge recombination is appreciably slower on TiO2 films than on Al2O3.

Technical innovations for transformative perovskite solar cells.
A low-temperature vapour-assisted solution process has been introduced23 to make polycrystalline perovskite thin films with full surface coverage, small surface roughness, and up to microscale grain sizes. Thus, it should be possible to fabricate perovskite films and devices in a simple and highly reproducible fashion. A power conversion efficiency of 12.1% has been achieved, which is the best so far obtained from CH3NH3PbI3 using a planar heterojunction configuration, and the critical kinetic and thermodynamic parameters attendant to the film growth were also investigated.23 A method has been reported for the preparation of 6 nm-sized nanoparticles of CH3NH3PbBr3 perovskites, which employs an ammonium bromide containing a chain of sufficient length (steric size) that the nanoparticles remain dispersed in a wide range of organic solvents. Since the nanoparticles are stable both in the solid state and in concentrated solutions, with no requirement for a mesoporous support, homogeneous nanoparticle thin films can be made by spin-coating onto a quartz substrate. Since both the colloidal solution and the thin film emit light over a narrow wavelength region of the visible spectrum and with a high quantum yield (ca. 20%), it is thought that the nanoparticles might find particular applications in the fabrication of optoelectronic devices.24 In (CH3NH3)PbI3-sensitized solar cells containing iodide-based electrolytes, (CH3NH3)PbI3 is relatively stable in a nonpolar solvent, such as ethyl acetate, so long as the iodide concentration is kept low (e.g., 80 mM). According to frequency-resolved modulated photocurrent/photovoltage spectroscopy when the TiO2 film thickness is increased from 1.8 to 8.3 μm, the transport is barely altered, but the electron-hole recombination is increased by above a factor of 10, which reduces the electron diffusion distance from 16.9 to 5.5 μm. An explanation for this is the greater degree of iodide depletion within the TiO2 pores as the film thickness increases. Thus, for the development of (CH3NH3)PbI3 or similar perovskites in potential photoelectrochemical applications, it will be necessary to find alternative, compatible redox electrolytes.25 In another study, the effect of TiO2 film thickness on charge transport and recombination in solid-state mesostructured perovskite CH3NH3PbI3 (via one-step coating) solar cells, using spiro-MeOTAD as the hole conductor, has been investigated using intensity-modulated photocurrent/photovoltage spectroscopies. It is demonstrated that charge transport in perovskite cells is not dominated by electron conduction from the perovskite layer, but within the mesoporous TiO2 network. The film-thickness was found to have little influence on the electron-hole transport and recombination processes, and yet the efficiency of perovskite cells increases as the TiO2 film increases in thickness from 240 nm to ca 650–850 nm. This effect is a consequence of enhanced light harvesting by the thicker films, although a drop-off in the cell efficiency is found as the film thickness is further increased, which is thought to be connected with a reduced fill factor or photocurrent density26. A novel metal-halide perovskite has been produced, based on the formamidinium cation (HC(NH2)2+), which displays a favourable band gap (1.47 eV) and has a broader absorption than light absorbing materials that contain the methylammonium cation (CH3NH3+), as previously documented. The high open-circuit voltage (Voc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%. The formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal nonperovskite (P63mc) polymorph is also reported, and it is concluded that to develop the cell further would necessitate the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagonal nonperovskite (P63mc) polymorph.27
Perovskite (CH3NH3)PbI3-sensitized solid-state solar cells have been reported which contain spiro-OMeTAD, poly(3-hexylthiophene-2,5-diyl) (P3HT) and 4-(diethylamino)benzaldehyde diphenylhydrazone (DEH) as hole transport materials (HTMs), which yield a respective light-to-electricity conversion efficiency of 8.5%, 4.5%, and 1.6%, under AM 1.5G illumination with an intensity of 1000 W/m2. Measurements made using photoinduced absorption spectroscopy (PIA) show that the hole transfer occurs from the (CH3NH3)PbI3 to the particular HTM, following an initial excitation of (CH3NH3)PbI3. The electron lifetimes (τe) in these devices decrease in the order spiro-OMeTAD > P3HT > DEH, so explaining the lower efficiency of the cells containing P3HT and DEH; however, the charge transport time (ttr) is relatively insensitive to the nature of the HTM.28 Copper iodide (CuI) has emerged as a potential new inorganic hole conducting material for perovskite-based thin film photovoltaics, since it has been used in a cell to yield a power conversion efficiency of 6.0%, and with excellent photocurrent stability. However ,the open-circuit voltage is much lower than is obtained from the best spiro-OMeTAD devices, as is attributed to a higher degree of recombination in CuI devices, according to results obtained from impedance spectroscopy. However, the latter technique also disclosed that the electrical conductivity in CuI is two orders of magnitude higher than in spiro-OMeTAD, meaning that significantly greater fill factors are possible29.  The optical and electronic structures of three N,N-di-p-methoxyphenylamine-substituted pyrene derivatives were investigated by UV/vis spectroscopy and cyclic voltammetry, to be used as hole-transporting materials (HTMs) in mesoporous TiO2/CH3NH3PbI3/HTMs/Au solar cells. A short-circuit current density of 20.2 mA/cm2, an open-circuit voltage (Voc) of 0.886 V, and a fill factor of 69.4% were measured under an illumination of 1 sun (100 mW/cm2), which gave an overall light to electricity conversion efficiency of 12.4%. Accepting that the Voc is slightly lower, the performance of the pyrene analogue is comparable with that of the well-studied spiro-OMeTAD, and may find future applications as an HTM in perovskite-based solar cells.30

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Captions to Figures.
Figure 1. Structure of a perovskite with a chemical formula ABX3. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green spheres are the A-atoms (a larger metal cation, such as Ca2+). Pictured is the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites.
Figure 2. Diagram showing (left) the device configuration and (right) energy levels of each layer in the device. Al: Aluminum. BCP: Bathocuproine. C60: Fullerene. PEDOT:PSS: Conducting polymer.