Coming of age of ferroic photovoltaics

18 11 2014

A few days ago an important paper has been published


Bandgap tuning of multiferroic oxide solar cells

Nechache, C. Harnagea, S. Li, L. Cardenas, W. Huang, J. Chakrabartty &F. Rosei

Nature Photonics (2014) doi:10.1038/nphoton.2014.255


The paper demonstrates 8.1% power conversion efficiency (PCE) at 1 sun light intensity, using a double perovskite thin film Bi2FeCrO6. This means that the paper reports the first serious efficiency of “ferroic photovoltaics”, a possible new class of photovoltaic (PV) solar energy converters that was incipiently developed so far and now shows itself with great force.

Ferroic photovoltaics came to general notice by the appearance of ferroic effects in hybrid halide perovskites (that reached officially 20% PCE also last week).1-3 In the organic-lead-iodide perovskite CH3NH3PbI3, however, ferroic behavior has been associated more to a nuisance than to a real advantage, although some, including this writer, think that there are new phenomena in there that may show great potential for development, and indeed this has been shown by Rosei and coworkers, albeit not with hybrid but with inorganic oxide perovskites.

Ferroelectric materials are materials with a low symmetry phase below a certain phase transition temperature, that are able to maintain spontaneous electric polarization. The polarization, may be not unique, as often there are equivalent types of distortions of the lattice, that allows the formation regions with different orientation of polarization. These are called domains, and are separated by a very thin domain wall. When an external electric field is applied, domains are oriented preferentially along with the applied field producing a macroscopic polarization. It can be switched if the field is applied in the opposite direction. In general, materials are called ferroic if they have the ability to experience substantial polarization even if it is not stable over a long time. It is also very important to take into account the properties of contacts, since they hold free electronic charge that cancels out the ionic bound charge. Tiny spatial distribution of the charge at contacts may create a large depolarizing field that counteracts the permanent polarization and finally removes it.

The PV effect in ferroelectric materials depends on the polarization-induced internal electric field, but the principles that cause the observed response are unclear yet.  Regular inorganic perovskites, mainly oxides ABO3, often show internal enhanced electrical fields by ferroelectric effect that can be exploited for PV applications. It is known since 50 years that ferroelectric perovskites may produce a large photovoltage exceeding by far the bandgap on single crystal materials.4,5  However, most metal oxide perovskites with ferroelectric properties ABO3 are unsuitable to harvest of the solar spectral photons, as they have a large bandgap of 3-4 eV. This property is due to the fact that the main optical transition occurs between the oxygen 2p states and the B transition metal d states, that involves a large difference of electronegativity. Recently there has been success in the synthesis of materials closer to the required 1.5 eV by using elements that form a less ionic bond with O, showing promising PV properties.6,7 In general these materials are single crystal oxides with very low PV performance due to very small current and low fill factor, as the mechanism of photovoltage generation is ohmic and only works well in high resistance cells.

The larger than bandgap voltage cannot be explained by the regular mechanism of separation of Fermi levels in a homogeneous absorber layer. One assumption to explain these phenomena relies on a local disposition of electric fields formed by spontaneous polarization over short distances of a few nm. This structure causes local steps of built-in potential as suggested by recent observations of domain wall structure at the nanoscale.8 However other experimental results indicate that the large voltage observed in epitaxial BiFeO3 films may be attributed to the “Bulk photovoltaic effect”, that is created by an asymmetric generation of current, or photogalvanic effect.9

At present it is still under discussion, if a ferroic PV material can form a directional current by local microscopic reasons so that the current might not be driven by the gradient of electrochemical potential. This would be a convincing reason for distinction with respect to all the rest of solar cells that operate with thermalized carriers.

The singular result of the paper by Rosei et al. is to develop a ferroic oxide Bi2FeCrO6 that for the first time provides a non-negligible photocurrent and PCE. The result is based on two combined breakthroughs. The first is a control of long range ordering of the cationic components of the perovskite, Fe and Cr. In this way they reduce the absortpion edge from aprox. 2.5 eV of the parent compositions with Fe or Cr, to an impressive 1.5 eV, thanks to charge transfer excitations between Fe and Cr. Nevertheless, the absorption of each layer is too low to provide sufficient photocurrent.

Having shown that one may tune the bandgap by controlling the cationic ordering, the second important development is to fabricate multilayer structures with a graded bandgap. This realization enhances the current to 20 mA cm-2, giving the PCE of 8.1%. However, the device needs to be under very strong polarization, and the energy cost of maintaining such polarization is unclear from the report.

The results of this paper show the enormous potential for this new class of solar cells. Of course many new questions are open about how it works, and what it may achieve. It seems very likely that a community will be formed soon to investigate these interesting ferroic photovoltaics.


(1)        Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells, Nano Letters 2014, 14, 2584-2590.

(2)        Sanchez, R. S.; Gonzalez-Pedro, V.; Lee, J.-W.; Park, N.-G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Slow dynamic processes in lead halide perovskite solar cells. Characteristic times and hysteresis, The Journal of Physical Chemistry Letters 2014, 5, 2357−2363.

(3)        Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films, The Journal of Physical Chemistry Letters 2014, 5, 3335-3339.

(4)        Chynoweth, A. G. Surface Space-Charge Layers in Barium Titanate, Physical Review 1956, 102, 705-714.

(5)        Neumark, G. F. Theory of the Anomalous Photovoltaic Effect of ZnS, Physical Review 1962, 125, 838-845.

(6)        Grinberg, I.; West, D. V.; Torres, M.; Gou, G.; Stein, D. M.; Wu, L.; Chen, G.; Gallo, E. M.; Akbashev, A. R.; Davies, P. K.; Spanier, J. E.; Rappe, A. M. Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials, Nature 2013, 503, 509-512.

(7)        K. Rezaie, F.; Panjwani, D.; Nath, J.; Fredricksen, C. J.; Oladeji, I. O.; Peale, R. E. “Junctionless thin-film ferroelectric oxides for photovoltaic energy production”; Energy Harvesting and Storage: Materials, Devices, and Applications V, Baltimore, Maryland, USA | May 05, 2014, 2014.

(8)        Yang, S. Y.; Seidel, J.; Byrnes, S. J.; Shafer, P.; Yang, C. H.; Rossell, M. D.; Yu, P.; Chu, Y. H.; Scott, J. F.; Ager, J. W.; Martin, L. W.; Ramesh, R. Above-bandgap voltages from ferroelectric photovoltaic devices, Nat Nano 2010, 5, 143-147.

(9)        Bhatnagar, A.; Roy Chaudhuri, A.; Heon Kim, Y.; Hesse, D.; Alexe, M. Role of domain walls in the abnormal photovoltaic effect in BiFeO3, Nat Commun 2013, 4.


Energy diagram calculations in photoelectrochemical cells

17 11 2014

Peter Cendula and coworkers have published a thorough paper about the energy diagram of semiconductor/ electrolyte interface for photoelectrochemical cells

Fe2O3 Energy-Band Diagram of Photoelectrochemical (PEC) Water Splitting

P. Cendula, D. Tilley, S. Gimenez, J. Bisquert, M. Schmid, M. Gratzel, and J. O. Schumacher, J. Phys. Chem. C (2014)

A simulation program is available to everyone to design the energy diagram with arbitrary parameters.

Peter Cendula

Peter Cendula


Perovskite solar efficiency over 20%

15 11 2014

KRICT has produced the first certified perovskite solar cell with energy conversion efficiency 20.1%.

It is published in NREL chart.

Serious independence games

12 11 2014

José Fernández Albertos nos planteó una pregunta muy interesante en una entrada pasada: ¿es la independencia un asunto de países pobres? Los datos parecerían indicar que así es. Como muestra “ningún país más rico que la República Checa en 1993 ha logrado nunca independizarse de su Estado matriz”. Y la República Checa, junto con Eslovenia, es casi la única excepción. De hecho los países que se independizan desde 1975 tienen una renta per cápita media de 6.300 dólares, lo que equivale aproximadamente a la España de 1962.

Los datos que presenta son especialmente relevantes porque de alguna manera contradicen los argumentos de Alesina y Spolaore según los cuales la globalización económica impulsa la aparición de países pequeños. La idea es que en un entorno globalizado el punto fuerte de los países grandes, la fuerza militar y las economías de escala, pierde peso relativo con respecto a su talón de Aquiles, la heterogeneidad. Por ejemplo, en la Europa contemporánea la OTAN se encarga de la seguridad y la UE de las economías de escala. Así que, según la teoría de Alesina y Spolaore, deberíamos ver una ebullición de secesionismo en el seno de la UE. Sin embargo hasta ahora ninguna región ha conseguido independizarse de un país occidental democrático, ni dentro de la UE ni fuera de ella.

La razón podría está en la falta de apoyo popular. Si nos fijamos en los movimientos independentistas exitosos veremos que el apoyo a la independencia siempre ha sido abrumador, como Alberto Penadés tan bien nos mostró. Claro, en Eslovenia a muy poca gente le apetecía quedarse en la Yugoslavia de Milosevic, pero seguir siendo canadiense o británico no parece tan mal dada la incertidumbre asociada al proceso de independencia.

Por eso, si algo sorprende (y mucho) en el caso de Cataluña es el vigor del apoyo a la independencia, en torno al 50%, sobre todo si se tiene en cuenta que sólo la mitad del otro 50% parece estar inequívocamente en contra. Además, una clarísima mayoría entre los menores de 30 años es independentista. De seguir así podríamos tener un 70% de independentistas de aquí a quince años.

¿Cómo es posible que un país desarrollado y democrático haya tanta gente que se quiera ir? Alguien debería aportar una respuesta seria. Nosotros tenemos algo que decir sobre la otra gran pregunta: ¿veremos una Cataluña independiente? En un artículo de próxima aparición en Nations & Nationalism argumentamos que bien puede ser que sí. En tanto que una parte del apoyo a la independencia en Cataluña viene determinado por la crisis económica que atraviesa España, mantener el statu quo ya no es tan atractivo para una parte importante de la población, el coste de la incertidumbre se reduce. Este razonamiento se ha presentado en varias ocasiones: en la medida en que la crisis empeora la situación de los ciudadanos, los partidos ofrecen ‘soluciones’, y la independencia puede aparecer como una de estas.

Creemos que si se celebrara un referéndum (llámese consulta, elecciones plebiscitarias o lo que sea) y el apoyo a la independencia fuera mayoritario, Cataluña se separaría de España y devendría miembro de facto, como mínimo, de la UE. El principal elemento disuasivo que emplea el Gobierno central es que Cataluña no sería reconocida internacionalmente y quedaría excluida de la UE. Esto último nos parece una amenaza no creíble.

La novedad del trabajo -que escribimos entre finales de 2013 e inicios de 2014- reside en, por un lado, detallar mediante teoría de juegos la secuencia que seguiría el camino a la independencia y, por otro, demostrar que la amenaza del Gobierno español no es creíble. De hecho, mucho nos tememos que el Gobierno central se encuentra ante la espada y la pared. En los siguientes párrafos desarrollamos tanto la secuencia como demostramos las limitaciones del Gobierno central.

Nuestro modelo se sustenta en dos supuestos bastante realistas: a) un apoyo mayoritario (creciente) a la opción independentista; b) una situación económica difícil. Respecto a lo primero, las encuestas muestran una escalada importante, de la que ya se habló aquí. Sobre la situación económica española, lamentablemente, no hace falta extenderse. Es la conjunción de estos dos factores lo que hace viable la independencia de Cataluña (o lo que muestra las limitaciones del Gobierno central).

Simplificando, la secuencia es la siguiente: el Gobierno de Cataluña convoca un referéndum (o unas elecciones anticipadas con sabor plebiscitario). Ante esto, el Gobierno de España puede o bien no hacer nada, o bien poner una oferta sobre la mesa para que Cataluña decida no seguir con el referéndum. Una buena oferta podría socavar el apoyo a la independencia tanto que Cataluña decidiese aceptarla. Si no hay oferta, o no es lo suficientemente buena, entonces se celebra el referéndum. Si la mayoría vota sí a la independencia, o a los partidos independentistas, el nuevo Parlamento de Cataluña puede decidir buscar un acuerdo con España (que podría parecerse a la oferta realizada y algo más) o proclamar la independencia.

Ante la proclamación de la independencia de Cataluña, España (y la Unión Europea) tienen dos opciones: rechazar dicha proclamación de independencia o aceptarla. Las declaraciones tanto del Gobierno central (como de la UE) indican que la opción es rechazarla. Sin embargo, ¿se pueden permitir esta opción? El Gobierno podría legal y constitucionalmente suspender la autonomía de Cataluña, meter mucha o poca gente en la cárcel, enviar la brigada paracaidista, etc. A nadie se le debería escapar que un movimiento así generaría mucha incertidumbre (manifestaciones, huelgas, desobediencia civil) y por lo tanto una presión tremenda en los mercados de deuda.  La UE le vería las orejas al lobo y forzaría una solución negociada.

Un escenario alternativo de rechazo pasa por que España y la UE permitan la independencia pero no la reconozcan. En ese caso Cataluña no se vería obligada a asumir su parte proporcional de la deuda asumida por el Gobierno de España que aumentaría de golpe algo así como un 20%, reduciéndose además su base impositiva entre el 5% y el 8,5%. España, un país demasiado grande para ser rescatado, se  iría a la quiebra poniendo en claro peligro al euro y a la UE. Sin duda, este escenario sería también devastador para Cataluña, pero queremos que queden claras las consecuencias para España y la UE una vez Cataluña ya haya declarado la independencia.

La otra opción es que España acepte la independencia y se pacten las condiciones de la salida: porcentaje de deuda que se lleva el nuevo país, etc. En definitiva, nuestro modelo ofrece dos resultados (técnicamente equilibrios de Nash perfectos en subjuegos): uno en el que España hace una oferta suficientemente interesante para desactivar el referéndum o, si dicha oferta no es factible (ejemplo: no se puede extender el sistema de cupos vasco y navarro sin quebrar la caja); dos, un referéndum que lleva a una independencia consensuada con España (y la UE).

Al fin y al cabo lo que decimos es que, dada la debilidad económica de España, si una mayoría clara de catalanes de verdad quiere la independencia, la obtendrá en condiciones favorables. El primer supuesto, debilidad económica, parece bastante realista. Lo segundo, mayoría clara, no está tan claro ahora, pero podría estarlo en unos cuantos años. Si Rajoy sale airoso por el momento, tendrá que apretar el cinturón de los españoles (¡aún más!) para que en unos años los hijos políticos de Junqueras no vuelvan a las andadas. El precio de mantener a Cataluña en el redil podría ser demasiado alto.


Tandem perovskite solar cell

9 11 2014

Coupling two solar cells in series (in “tandem”) allows one to split the solar spectrum into separate blue and red absorbers, which enables to match the bandgap at which solar photons are absorbed, to the energy of the photons. This recipe is able in principle to notably enhance the efficiency of solar cells with respect to single absorber devices. However, the construction of a monolithic tandem device on a substrate, with adequate intermediate layer to add up the voltages of the two layers, is challenging. There has been a lot of talk about the prospect of highly efficient tandem solar cells using the organometal halide perovskite as front absorber. It is believed that the metal halide perovskite, due to the properties of tunable bandgap and low temperature processing, is a good complement to enhance the efficiency of inorganic high efficiency solar cells as crystal silicon or CIGS. Another important target is to develop tandem solar cells entirely by solution process routes, that have the potential of lower production cost, albeit at a cost of lower conversion efficiency. A high voltage monolithic solar cell is also a good candidate to drive fuel production reaction as water splitting, if combined with suitable catalysis layers.

Progress in this direction is reported now in Applied Physics Letters by our good friend Teo Todorov and his coworkers at IBM T. J. Watson Research Center. In the past Teo contributed with David Mitzi the highest efficiency inorganic Cu2ZnSn(S,Se)4 (CZTSSe) kesterite solar cells. In this work the IBM researchers present a monolithic cell CZTSSe/NH3CH3PbI3 with PEDOT:PSS intermediate recombination layer and aluminium front contact. A nice result is that the obtained photovoltage 1353 mV is practically the addition of the separate voltages of the components. On the other hand the current is relatively low and the tandem device does not improve the separate efficiencies. This is explained in terms of the  drastically reduced illumination in all layers and high sheet resistance of the Al contact. This is nevertheless and important result that shows further the versatility of perovskite solar cells and solution processed photovoltaics.

Teodor Todorov, Talia Gershon, Oki Gunawan, Charles Sturdevant and Supratik Guha
Appl. Phys. Lett. 105, 173902 (2014).
todorov et al APL
Teodor Todorov

Teodor Todorov

Views on independence of catalans

8 11 2014

Vicent Partal


Homage to Catalans

In modern Europe there is no democratic movement of the scope, force and radicality of the Catalan movement for independence. Four years ago, the Spanish courts modified the Statute of autonomy approved in a referendum of the people of Catalonia, which was also signed by the Catalan and Spanish parliaments. This was a unique and extraordinary case, a veritable judicial coup d’état that crushed the pact between the Catalan and Spanish democratic forces at the end of Francoism. Since Jose Maria Aznar’s arrival in power, the Partido Popular, the ideological and biological heir of Francoism, has degraded democracy to simply unbearable extremes; not only in Catalonia, but especially in Catalonia.

The clearest sign of this is precisely its prohibition of the referendum on independence called for by the Catalan political forces. The Parliament of Catalonia approved a law on popular consultations with 106 votes in favour and only 28 against which was to enable the independence vote of 9-November to be held in a situation of normality. Before this, the Catalan parliament had officially asked the Spanish parliament either to organise a referendum in Catalonia or, as happened in Scotland, to give Catalonia the chance to organise one. The request to the Spanish parliament was answered with a firm No by the PP and the PSOE parties and the consultation law was repealed by the Constitutional Court under the orders of the government.  When the Catalan government finally chose to turn the consultation into a participative process as a way to overcome the obstacles, the State prohibited it once more.

Spain simply says No to everything and thinks that this will make us give in. It says that the Spanish constitution does not allow independence and in an exercise of cynicism difficult to surpass, challenges the Catalans to change it. But the present autonomous community of Catalonia accounts for 16% of the population of the Spanish state, and even though 100% of Catalans wanted to change it, it would be impossible.

Maybe in some other place the people would have given in, but not the Catalans. The Catalans have risen and for the last four years we have led a true political epic. The whole world has seen us holding hands and demonstrating on the streets of Barcelona. The Catalan flag today is instantly recognised by any reader of the world’s newspapers. We have put our country on the map of information and also on the political map.  Governments from all around the world are paying special attention to what is happening here.

But what is difficult to see from outside is the daily work that this democratic revolution involves. In all of the towns, districts and cities, thousands and thousands of volunteers spend hours and hours driving the movement on. Men and women of all social orders, of all origins and of all ideologies for four years have worked with a smile on their faces, vowing to create a new country.

And it is precisely this extraordinary work that the social movement for sovereignty is doing that is drawing out a different future for this country. We have learnt to talk and overcome our differences. We have learnt to mercilessly criticise ourselves but always for the better. We have analysed the finest details of each of our faults and we have strived to improve. We have created a new culture where the people order the politicians and where the politicians work to find legal outlets for the people’s demands.

Europe is battling through a terrible decade and it would do well to ask itself how it is that in Catalonia the people in the street are smiling. We smile because we know we are building a better country with our democratic and radically free fight.  We always defend plurality from a position of inclusion and discussion, therefore the whole of the campaign of those supporting the Yes vote always includes a cry to vote in freedom, to consider whether it might be best to vote No. Therefore on 9-N the vote will also be for the immigrants, with whom we joyfully share our country and therefore from Christian democracy to left wing anti-capitalism, the Catalan parties are capable of agreeing and moving forward together, giving a lesson of democratic dignity and loyalty to our country.

When George Orwell wrote his ‘Homage to Catalonia’, he did so under the impact of a Catalan Revolution which was different from any other event of his time. Europe has to know today that the Catalans are back; we are back to create a much more democratic country than the one we have today. We are back to place the citizens at the centre of political life and to make ourselves the owners of our own future.  We are back to remind the world of something obvious which we are not willing to ignore: that rights and freedoms cannot be threatened and much less denied forever.

Today’s combat, this effort that will soon take us to independence, is the homage that we Catalans are making to a tradition of centuries of fighting for personal and collective decency.  In some way, therefore, you might think that in itself it is a homage to ourselves; to our eternal passion for freedom.