Review: photovoltaics Research

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Understanding perovskite-based solar cells The Essential Physics of Organometallic Halide Perovskite solar cells Dr Ghous B Narejo Abstract During the last couple of years there has been a large interest in perovskite based photovoltaics. A lot of experimental and theoretical research has been conducted but the critical factor that link the higher efficiency of these solar cells with the underlying physics has not been understood. This is attributed to the complex physics of these materials. An attempt is made to interpret the critical physics contributing to the higher diffusion coefficient in these materials. Introduction The world is facing energy shortage. With the passage of time the fossil fuels are shrinking. The shirking of the primary and traditional resources of energy would lead to a possible energy crisis of international proportions. We have a sign at the end of the tunnel as there is a massive investment into the search for the alternative energy resources impressed by the nano materials. The great thrust in the modern research on the photovoltaic has been witnessed leading to the less known and very complex materials called perovskites. Challenges:

description

There has been considerable interest in the organometallic perovskite halide solar cells due to variety of properties. The physics embedded within these materials is controlling their long diffusion coefficients. This review attempts to explore the unexplored physics. The investment in the future solar cells based upon these materials would be less risk free if we know the critical parameters controlling the cost and efficiency of solar cells based upon these materials.

Transcript of Review: photovoltaics Research

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Understanding perovskite-based solar cellsThe Essential Physics of Organometallic Halide Perovskite solar cells

Dr Ghous B Narejo

Abstract

During the last couple of years there has been a large interest in perovskite based photovoltaics. A lot of experimental and theoretical research has been conducted but the critical factor that link the higher efficiency of these solar cells with the underlying physics has not been understood. This is attributed to the complex physics of these materials. An attempt is made to interpret the critical physics contributing to the higher diffusion coefficient in these materials.

Introduction

The world is facing energy shortage. With the passage of time the fossil fuels are shrinking. The shirking of the primary and traditional resources of energy would lead to a possible energy crisis of international proportions.

We have a sign at the end of the tunnel as there is a massive investment into the search for the alternative energy resources impressed by the nano materials. The great thrust in the modern research on the photovoltaic has been witnessed leading to the less known and very complex materials called perovskites.

Challenges:

The role of the mesoporus film is huge as there is large surface to volume ratio resulting in the lifting of the degenracyt. The electrons transporting through the perovskite across the mesocopic TiO2 has the energy favourting the transport to the contact and not the recombination. –[Ref: “However, which electron or hole injection process occurs first and, in the latter case, whether electron injection into and transport within the oxide mesoporous film (equation (2b)) is playing any role, remain unclear.” ][2]There has been a major role of the mesoporous TiO2 thin film in the enhancement of the efficiency of the solar cell.[Ref: The precise role of titanium dioxide in efficient perovskite solar cells based on mesoporous films of this particular metal oxide has indeed not been established clearly.] [2]

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[Ref Evidence has been found for electron injection from the mixed halide CH3NH3PbI2Cl into TiO2 [2]”]The crystalline structure, electronic properties and the charge transport in these materials has been widely investigated by the theoretical approaches. But there has been severe challenges given by these materials with respect to their electronic structure instability leading to unstable chemical composition resulting in numerous crystalline arrangements.

Organo-metallic halide perovskites:

The scientists have always sought the support of the modern computational science when understanding the complex scientific phenomenon is impossible through the experimentation. Same may be true for organo-metallic halide perovskite based solar cells. These interesting material composites have resulted in an increase of 15 % in the efficiency as well as the lowest cost associated in the fabrication of solar cells. This is a great breakthrough in the recent history of the solid state solar cells.

Crystalline structure of perovskites:

The electronic structure of perovskites is ABX3 type. In ABX3 halide perovskites the electronic properties such as the band gap and the electronic transport are greatly controlled by the X elements. Further, the role of the B and X is modified depending upon the type of the perovskite. If we look at the electronic band gap of these materials we find that there is a medium to low band gap which is expected for these types of perovbskites. Further, the position of the top of the electronic valence band and the bottom of the electronic conduction band is occupied by the transition metal and the halide respectively.

The incomplete physics of Perovskites:

The band gap for the material cannot express the physics as can be clearly seen. The band gap either provides the atomic positions which are based upon the underestimation or the overestimating of the position of the transition metals and the halides. The DFT or ab-initio methods [1] as well as the experimentation done for these materials suffer from numerous challenges. Moreover, the atomic physics calculations cannot alone be able to provide the macroscopic picture to connect of with the causes of the longer diffusion coefficient for these materials.

The comparative strength and weakness of theory and experiment:

We may however, utilize the available theoretical and the experimental results and connect these with the macroscopic physics models as have always been done by the scientist and engineers in understanding the relation between the physics of the material and the device made out of the former.

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Type of perovskite and its crystalline property:

For the lead based perovskites there would be the cages formed between the lead at the centre and the halide at the closed neighborhood. As these cages would be unstable due to the physics associated with the lead based perovskites, these materials would exhibit a combination of electronic band gaps and not a single one to complete the picture. Moreover, the vibration spectra of these materials would provide the true picture.

Causes of Physical and chemical instability:

For the Ti based cages there would be the complete absence of these instabilities and the chemical st abilities of the Ti based Halide cages in these photovoltaics may be better in terms of the physical and chemical instabilities and understanding these Ti based cages and their impact on the longer diffusion coefficients would be far easier.

Fig 1: Scanning electron microscopy of a Perovskite-solar cell: on a glass substrate (glass and FTO) highly porous titanium dioxide is deposited, which is impregnated with perovskite. This film is covered by an organic hole transporting material (HTM) and gold contact. (Image: EPFL)”

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Fig2: The band gaps [Ref to be typed]

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Fig 4: The Raman Spectra [Ref to be typed]

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Fig 5: The positions of atoms in the cage

Fig 6: Perovskite and its possible crystalline phases

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The dependence of diffusion coefficient on the critical factors:

The position and the distiches between the transition metal in the cage which supplies the electron and the cage and the halide whose electrons lie in the lower edge of the conduction band controls the physics. The electrons lying with the transition metals are essentially d orbital electrons having a highly localized electron wave function. These do electrons can chemically bond with the halide surrounding the former. The d orbital electrons have different positions where they are facing electrons at x, y and z coordinates. The position of d orbital vis a vis the electrons of halides form the sp-d orbital interactions.

The d orbital’s electrons, highly localized are controlled by the position of the central atom in the cage. The crystalline arrangement of the transition metal at the centre of the cage and the position and the type of the halide atom further controls the geometry. The resulting geometry decides the exchange interactions which are indirect interactions.

The indirect interactions between the d and the s-p electrons of the halides have long diffusion lengths.

The DFT calculations done on these materials have till now been successful in getting the band gap and the spin-orbit coupling in these materials showing indirectly the critical factors leading to the long diffusion conefficient. There are however the weaknesses of the DFTR and ab-inityio and for these complex materials its quite challenging if not diffuiclt to determine thphysics of these heterjuctions

[ Ref: -Thanks to density functional theory calculations, we show that the band gap of these compounds is dominated by a giant spin–orbit coupling (SOC) in the conduction-band (CB). [3,4]]

This is very complex and highly critical factor in the physics of these composites and may not be left at mere clueless argument. We are sure that the recombination rates are very weak as compared to the photogenreated electrons. The major reason behind these photogenreated electrons are obviously the extrernally generated electrons that are injkected into and are tarnsporterd across the perovskite through TioO2. Thesae electrons are favoured by the exchange interactions which are exdentually taking place between the Transition metal and Halide across the cage favour these electrons to remain hopping via the s-p-d orbital interactions.[Ref: “ Undesired reactions such as exciton annihilation, leading to photoluminescence (equation (3)) or to non-radiative recombination (equation (4)), as well as recombination of the charge carriers at the three interfaces (equations (5) to (7)) compete with the extraction of the

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photogenerated charges. The kinetics of these processes is thus expected to control, to a large extent, the overall photovoltaic conversion efficiency of the system. “ ]

In these material systems, the electrons generated through photons do not recombine as often as in other materials thanks to the spin-orbital coupling supporeted by the d orbital localization. [Ref: - On alumina and in the absence of HTM, the only possible pathway for energy conservation after light absorption is electron–hole recombination, either through luminescence or non-radiative processes (equations (3) and (4)).“A high power conversion efficiency of the photovoltaic device must obviously imply that the charge recombination processes (equations (3) to (7)) occur on a much slower timescale than the charge separation and extraction processes (equations (1) and (2)). It is therefore of crucial interest to determine the kinetics of these reactions so as to ultimately improve the cell’s performance.”]

Once these electrons going through the perovskite reach TiO2, they are continuying the exzchange interactions. Therse indirect exchange interactions facvour the electron hopping across the energy split off on the d orbital degeneracy supported by the spin-orbital –lattice structure couplings.[Ref: In systems prepared on TiO2 films, a small contribution to the transient absorption signal of conduction-band electrons injected in TiO2 cannot be excluded. Similarly, for all systems containing spiro-OMeTAD HTM, contributions to the transient absorption signal of oxidized HTM species cannot be omitted and will be discussed later. We will assume in the following discussion that charge recombination within the perovskite is similar on TiO2 and Al2O3 samples.]

The critical factors controlling the exciton diffusion length, carrier mobility, nature and density of trap states, and the energetics of the band are the type of the metal in the cnetre of the cage and those surrounding the cages formed by the Halides around the Pb. [Ref:The intrinsic physical properties of the perovskite material, such as exciton diffusion length, carrier mobility, nature and density of trap states, and the energetics of the bands, are obviously important in explaining why this exceptional material appears to be working in a variety of configurations. Rather than dealing with the intrinsic properties of the perovskite semiconductor, t present work focuses on interfacial charge transfer processes occurring at the junctions between the light absorber and the electron- and/or hole-conducting materials. We can then take for granted that the exciton diffusion length and carrier mobility within the perovskite are easily sufficient and that all energy levels are conveniently aligned18,19. A sequence of interfacial electron transfer steps is derived from the kinetics determined experimentally in configurations that have already been proven to function efficiently in solar cells.]

The typ[e of the crysatalline quality of the perovskite and the quality of the interfaces between the perovskite absorber and the oxide electrodes formed by the perovskite oxide TiO2 and Al2O3 are the controlling factors which decide the type of the physical interactions [Ref: In samples containing the perovskite absorber deposited on a titanium dioxide electron acceptor with HTM infiltrating into the pores of the mesoscopic film, non-ideal morphologies obtained during the preparation will lead to various local situations within the same sample, where

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perovskite domains could be insulated, in contact with only TiO2 or the HTM, or form two junctions with both the TiO2 and the HTM. All these different scenarios are encountered in current solar energy conversion devices of various architectures. Interest in discussions about the charge transfer mechanism taking place in TiO2 perovskite|spiro-OMeTAD solidstate solar cells therefore extends beyond this particular example to other types of perovskite-based photovoltaic systemsFour different cases related to the morphology of the sample could coexist in perovskite-based devices:(1) All the perovskite is conformally deposited on top of the metal oxide (either the insulating Al2O3 or the TiO2 electron acceptor) as a thin light-harvesting film, with a thickness of at most a few nanometres.(2) Part of the perovskite is not in direct contact with the oxide substrate but is present in the form of isolated crystalline domains in the pores and/or a capping layer on top of the mesoporous network. All perovskite-based devices deposited on a mesoporous structure, regardless of preparation type, should fall into one of the two categories (1) or (2). When adding an HTM, these cases can be coupled with two additional cases in relation to the junction:(3) All the HTM is conformally deposited into the pores, forming a continuous junction with the perovskite material.(4) Not enough HTM is present in the pores, or the interfacial contact between the perovskite and the HTM is only partial as a result of infiltration problems.]

The experimental results have supported the fact that the crystalline geometry and the type of the materials support the photogenrearion as compared with the recombination. The underlying physics although have been attempted butr could not be fully understood. There has been an attempt at understanding the physics through ab-initio but there remain some opened ended questions that are not clear that. [Ref: “The efficiency of charge extraction in a perovskite solar cell depends on the ratio between the rate constants for charge recombination and charge separation. It is thus important to determine the timescale for charge recombination processes in the cell. Results obtained by flash photolysis show that the recombination reaction for electrons and oxidized spiro-OMeTAD (equations (6) and (7)) is slow, taking place in the microsecond range. This indicates a factor of at least 106 between charge separation and recombination rate constants at the HTM interface, ensuring a quantitative yield for sustained charge separation. Additionally, reaction (7), being slower than reaction (6), proves that the use of TiO2 as electron acceptor and transporter in conjunction with an organic HTM in contact with the perovskite is indeed quite beneficial.Experimental data showed that charge recombination with oxidized HTM species, which occurs over a microsecond timescale, is delayed on TiO2 films with respect to Al2O3, indicating that the mechanism involves recombination of charges separated by a longer distance.Conformal coating of the TiO2 surface with CH3NH3PbI3 facilitates charge separation by ensuring direct electron injection into the oxide. These findings highlight the advantage of employing two heterojunctions with titanium dioxide and the HTM while using perovskite as a solid-state light absorber.”][Ref:

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]

The transport of the photogenerated carriers has been efficient through the semiconductor heterojunctions. The reason s behind these are the type of the heteropstructures and band alignments across them[Ref: Obviously, the design of novel and/or efficient PV devices requires a realistic modeling of underlying material’s properties including chemical composition, mechanical, electrical and optical features. This can be gained with state-of-the-art ab-initio approaches. In addition, such knowledge is desirable to reach PV cells composed of earth abundant elements based materialInterestingly, whereas conduction-band and valence-band alignments between absorbers and TiO2 are of crucial importance in understanding charge transfer and charge transport,7,8,16,18 their modeling is still scanty.]

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Fig8: Overview of the crystal structures of MAPbI3(left) and MAPbBr3(right) at low temperature. The structures are both orthorhombic (space group Pnma), with a cell doubling when compared to the room temperature cubic phase. [Ref to be supplied]

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Fig9 Electronic band structures of MAPbI3 (top) and MAPbBr3 (bottom), without (a) and

with (b) the spin-orbit coupling interaction. The origin of the energy scale is taken at the top ofthe valence band (VBM).[ref to be supplied]

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Fig10: Energy level diagram derived from the position of Pb-5d orbitals, computed VBM(figure 4) and experimental band gaps of MAPbI3 (1.5eV), MAPbBr3 (2.3eV)5,14 and an alkyl ammonium (AA) 2D hybrid perovskite (2,5 eV).24 Commonly accepted values for TiO2 electron affinity of -4.1eV and absolute valence band energy of -7.3eV are used [ref to be supplied][Ref. It clearly demonstrates that the conduction-band offsets are favorable for the electron injection from absorber to electrode.]

References:

1, First-Principles Modeling of Mixed Halide Organometal Perovskites for Photovoltaic Applications

Edoardo Mosconi *†, Anna Amat †, Md. K. Nazeeruddin ‡, Michael Grätzel ‡, and Filippo De Angelis *†

2, Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells, Arianna Marchioro1,2, Joe¨l Teuscher2, Dennis Friedrich3, Marinus Kunst3, Roel van de Krol3, Thomas Moehl2, Michael Gra¨tzel2 and Jacques-E. Moser1*

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3, Importance of Spin–Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications Jacky Even *†, Laurent Pedesseau †, Jean-Marc Jancu †, and Claudine Katan *‡

4, PIN-STRAIN COUPLING IN 3-D TRANSITION METAL OXIDES, GHOUS BAKHSH NAREJO, Paul Bergstrom, Warren F Perger, MTU, 2010

5, Importance of spin-orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications, Jacky Even,†Laurent.Pedesseau,†Jean-marcJancu,† and ClaudineKatan

6, A Novel Coupling between the Electron Structure and Properties of Perovskite Transition Metal Oxides, Ghous Narejo, Warren F. Perger Electrical Engineering Department, Michigan Tech University, Houghton, USA Email: [email protected]