Limiting Efficiency of Perovskite Solar Cells

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– Limiting Efficiency of Perovskite Solar Cells –

ABSTRACT

The power conversion efficiency of perovskite solar cells has risen from as low as 3.8% to as high as 19.3% in just five years with yet a projected value of over 20% in the next few years by experimentalists.
Such a tremendous breakthrough is one of its kind in photo-voltaic research with thin-film solar cells as the only major competitor.

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The light-harvesting layer in these new devices has a crystalline structure called the perovskite structure which is capable of absorbing photons in both the visible and near infra-red regions of the solar radiation spectrum.
In this study, we carried out theoretical studies based on the detailed balance theory originally proposed by Shockley and Queisser, and on a semi-empirical approach based on the measured optical absorption spectrum of the three most widely used perovskite absorbers: CH3NH3SnI3, CH3NH3PbI3, and CH3NH3PbI3-xClx.
We arrived at an upper conversion efficiency limit for a single planar heterojunction(PHJ) perovskite solar cell with anti-reflection capabilities considering radiative losses as the only carrier loss mechanism within the cell.
The limiting efficiency was found to be 29.2% for CH3NH3PbI3, 27.5% for CH3NH3PbI3-xClx, and 24.8% for CH3NH3SnI3 under AM1.5 solar spectrum. Issues such as the effect of exciton diffusion length and absorber thickness on the efficiency are also discussed.

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Background of Study
An exciton is a bound state of an electron and an electron-hole held together by the Coulombic interaction.It is a quantum mechanical particle found in both organic (e.g;     in the dye mentioned above), and inorganic semiconductors (e.g; silicon).
The binding energy of an exciton which is the minimum energy required to split the exciton into an individual electron-hole pair can give useful insights into whether a semiconductor will be In an organic-inorganic hybrid semiconductor such as methylammonium lead tri-iodide (CH3NH3PbI3),
Only Wannier type excitons exist with diffusion lengths in the order of ≈ 1µm which is favourable for absorber thicknesses in the range of ≤ 100nm.
Besides, the exciton lifetime in CH3NH3PbI3 powder is high, up to 10ns [27]. The combination of these two effects means that the excitons in the CH3NH3PbI3 film can travel a longer distance beforere wardful as a photon absorber for photo-voltaic applications in addition to other required optical properties.
For  instance,  in inorganic semiconductors, the exciton binding energy  is 10meV, and the electron-hole separation is 10nm Fig.(1.1a); with exciton diffusion lengths of the order of 50nm 100nm.
These are called Wannier excitons [19]. However, organic semiconductors such as polymer blends are made of Frenkel excitons with binding energies of the order of  1eV, electron-hole separation of the order of  1nm Fig.(1.1b);   and diffusion lengths of the order of  10nm [27].

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Heo et al.Efficient inorganic–organic hybrid hetero-junction solar cells containing perovskite compound and polymeric hole conductors, nature photonics, doi: 10.1038/npho- ton.2013.80,5 MAY 2013.

Shockley and H.J. QueisserDetailed Balance Limit of Efficiency of pn Junction Solar Cells, Journal of Applied Physics 32, 510 (1961); doi: 10.1063/1.1736034.
Tiedje et al.Limiting efficiency of silicon solar cells, IEEE Transactions on electron devices, Vol.ED-35,No. 5, may 1984.
R Abrams et al.Theoretical efficiency of 3rd generation solar cells: Comparison between carrier mul- tiplication and down-conversion, Solar Energy Materials and Solar Cells (2012), doi:10.1016/j.solmat.2011.12.019.
WurfelThe Physics of solar cells, from principles to new concepts (WILEY-VCH, Berlin;Germany, 2005).

Tvingstedt et al.Radiative efficiency of lead iodide based  perovskite solar cells,      Scientific re- ports,4:6071.DOI:10.1038/srep06071, August, 2014.

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