Taming light with plasmons – theory and experiments Aliaksandr Rahachou, ITN, LiU Kristofer Tvingstedt, IFM, LiU , Hjo

OUTLINE Introduction to plasmonics Optical excitation of plasmons Plasmons in organic solar cells Experimental results for APFO3:PCBM on Al gratings Theoretical results for APFO3:PCBM on Al gratings

p-polarization: E-field is parallel to the plane of incidence s-polarization: E-field is perpendicular to the plane of incidence (German senkrecht = perpedicular) x z y 11 HxHx 22 z=0 11 22 EyEy H HzHz x z y 11 ExEx EzEz 22 11 22 HyHy E INTRODUCTION TO PLASMONICS

x z y z=0 11 22 E 1x E 1z H 1y E1E1 p-polarized incident radiation will create polarization charges at the interface. We will show that these charges give rise to a surface plasmon modes E 2x E 2z H 2y E2E2 creation of the polarization charges if one of the materials is metal, the electrons will respond to this polarization. This will give rise to surface plasmon modes Boundary condition: (a) transverse component of E is conserved, (b) normal component of D is conserved

Polarization charges are created at the interface between two material. The electrons in metal will respond to this polarization giving rise to surface plasmon modes

x z y z=0 11 22 H 1x H 1z E 1y H1H1 s-polarized incident radiation does not create polarization charges at the interface. It thus can not excite surface plasmon modes H 2x H 2z E 2y H2H2 no polarization charges are created  no surface plasmon modes are excited! In what follows we shall consider the case of p-polarization only Boundary condition (note that E-field has a transverse component only): transverse component of E is conserved, compare with p-polarization:

More detailed theory Let us check whether p-polarized incident radiation can excite a surface mode x z y z=0 dielectric  1 metall  2 E 1x E 1z H 1y E1E1 wave propagating in x-direction intensity z we are looking for a localized surface mode, decaying into both materials components of E-, H-fields: E = (E x, 0, E z ); H = (0, H y, 0) Thus, the solution can be written as

x z y z=0 dielectric  1 metall  2 E 1x E 1z H 1y E1E1 solution for a surface plasmon mode: Let us see whether this solution satisfies Maxwell equation and the boundary conditions: + condition imposed on k-vector

What is the wavelength of the surface plasmon ? let us find k: substitute k  kxkx light cone  = c k The surface plasmon mode always lies beyond the light line, that is it has greater momentum than a free photon of the same frequency 

Ideal case:  r1 and  r2 are real (no imaginary components = no losses) Dielectric:  r1 >0 Metal:  r2 >  r1 k resonant width = 0  lifetime =  k x is real

Realistic case:  r1 is real, and  r2 is complex, imaginary part describes losses in metal k resonant width (gives rise to losses) Dielectric functions of Ag, Al

surface plasmon length scales: dielectric  1 metall  2 z decay into metal decay into dielectric propagation length

dielectric  1 metall  2 is it possible to excite a plasmon mode by shining light directly on a dielectric/metal interface? k  kxkx light cone  = c k The surface plasmon mode always lie beyond the light line, that is it has greater momentum than a free photon of the same frequency . This makes a direct excitation of a surface plasmon mode impossible! OPTICAL EXCITATION OF PLASMONS

metal coupling gap prism  Otto geometry metal prism  Kretschmann-Raether geometry Grating METHODS OF PLASMON EXCITATION

Observation of plasmon enhanced absorbtion in Apfo3/PCBM

Introduction Prescence of periodic metal gratings in a dielectric environment triggers surface plasmons and creates an intense optical near field An absorbing layer on top of the grating should therefore be exposed to a strong field Plasmons are traveling along the interface (not perpendicular as the impinging light) Introducing Surface plasmons in solar cells may hence increase the absorption

Grating manufacturing Optical diffraction gratings are replicated via PDMS replica molding The PDMS replica is subsequently imprinted in a photocureable resin. Very high replication throughput 1 2 3

Grating Manufacturing Grating is metallized by thermal evaporation of ~90 nm Al

Grating Characterization Period: 277 nm Depth: ~48 nm Rougness ~5 nm

Samples *Metal gratings coated with ~150 nm Apfo3/PCBM 1:4 mixture *Planar mirror reference samples manufactured *Reflectance measured in integrating sphere (all angels collected)

Grating mirror reflectance Different orientation/polarization shows very different reflectance in the UV region. *Polarized reflection *Air metal SP

Sample reflectance New absorption peaks! SP? Waveguide?

Initial results: Photocurrent from inverted cells

CLEAN GRATING MIRROR Al-air plasmonic peak

ESTIMATING THE POSITION OF A PLASMON PEAK APF03:PCBM 1:4-Al dispersion relation normal incidence where d is a period of grating (sinusoidal, tiranglar or step-like) Dielectric function of APFO3:PCBM 1:4 in direction normal to the surface

NUMERICAL RESULTS (Green’s function method) Al APFO3:PCBM 1:4 Air ~120nm TE (P)-polarized light HzHz EyEy ExEx Flat surface…

Flat surface and experiment once again...

THEORETICAL RESULTS (Ideal sinosoidal surface) Al APFO3:PCBM 1:4 Air ~120nm TE (P)-polarized light HzHz EyEy ExEx 46nm 277nm

THEORETICAL RESULTS (Sinusoidal surface)

Realistic surface Al APFO3:PCBM 1:4 Air ~120nm TE (P)-polarized light HzHz EyEy ExEx 46nm 277nm Roughness ~ 6x4nm Smooth surface variation

Realistic surface 25nm

Absoptance peaks ~250 nm thick polymer ?

CONCLUSIONS We demonstrated both experimentally and theretically enchanced absorptance of light in APFO3:PCBM 1:4 solar-cells with Al gratings Easy manufacturing with soft lithography. The theoretical and experimental data agree very well! THANK YOU!

Acknowledgements Nils-Christer Persson for optical characterization of the materials Chalmers for materials