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.

PROCEED NOW TO DOWNLOAD PAGE

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.

PROCEED NOW TO DOWNLOAD PAGE

INTRODUCTION

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 before 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].

PROCEED NOW TO DOWNLOAD PAGE

REFERENCES

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.