(wasting energy). The absorbing active layer alters cavity behavior drastically. Such
fundamental differences in principles and materials require significant different cavity design
and operation. Separately, it is unclear how well the metallic disk/wire layer in an nMAM
cavity could replace ITO film as a transparent electrode in solar cells.
In searching for efficient photoelectron cathodes inspired by 3D cavity antenna design for
ultrasensitive sensors [41], we developed a new plasmonic cavity that has a ultrathin
sandwich structure of m
etal-mesh electrode with subwavelength holes/absorbing-
semiconductor/metal-sheet (MESH/A/M), and found that such cavity is extremely efficient in
coupling incident light into the absorbing semiconductor rather than the metals [42]. Here we
report an implementation of the MESH/A/M cavity design into a new organic solar cell with
MESH replacing ITO film (termed “plasmonic cavity with subwavelength hole-array
(PlaCSH) solar cell”), the first experimental study and demonstration, and the observation of
several unique properties of PlaCSH-SC.
2. PlaCSH solar cell structure and operation
A PlaCSH solar cell is a ultrathin plasmonic cavity, consisting of primarily, a thin MESH as a
transparent front electrode, a thin metal back electrode, and in-between a thin light absorbing
photovoltaic active material layer (namely, an MESH/A/M plasmonic cavity). All features of
the MESH (i.e. the pitch, hole size and hole separation) are subwavelength (i.e. less than light
wavelength). For the optical property of PlaCSH, the thickness of the back electrode is
insignificant, as long as it is a good metallic light reflector.
The PlaCSH is designed in such way that, for a given light wavelength range (i.e.
bandwidth), a significant portion of incoming light is transmitted through the MESH into the
cavity, and then the most of the transmitted light is trapped and absorbed by the cavity. As
shown later, the experimentally demonstrated total light absorption by the PlaCSH (the
lumped effect of transmission, trapping and absorption) is as high as 96% (average 90%) and
broad bandwidth (e.g. nearly constant from 400 nm to 900 nm). Moreover, we observed that
the PlaCSH also can reduce the light reflection and absorption by MESH by 2 to 6 fold
compared to that when the MESH is alone; and that the sheet resistance of a 30 nm thick
MESH is 2.2 ohm/sq or less–4.5 fold or more lower than a 100 nm thick ITO film. All of
these have led to a significant enhancement of solar cell performances (external quantum
efficiency, open-circuit voltage, short-circuit current density, fill factor and a power
conversion efficiency) as discussed below. In this demonstration of PlaCSH-SCs, the
photovoltaic active layer is a sub-absorption length thick layer of a bulk hetero-junction of
poly (3-hexylthiophene) / [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM). Our
current study has focused on the effects of PlaCSH-SC structure parameters on solar cell
performance, rather than the effects of polymer materials, compositions or processing
conditions. We used all polymers as received from commercial venders and processed them
using conventional methods, except filtering with 450 nm filter size.
A PlaCSH solar cells with an optimized structure has a 30 nm thick gold MESH with a
hole array of 175 nm diameter and 200 nm period (as transparent electrode) on a fused silica
substrate, 10 nm thick poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic-acid
(PEDOT:PSS) layer (electron blocking layer), 85 nm P3HT/PCBM film (active layer), 5 nm
thick TiOx (hole blocking layer) and 100 nm thick Al film (back electrode) (Fig. 1(a) and
1(b)). The incident light enters the solar cell from transparent silica substrate side through the
transparent MESH. Excluding the thickness of the Al electrode and the substrate, the total
thickness of the device (the active layer, the transparent front contact, and charge blocking
layers) is 130 nm.
Received 12 Sep 2012; revised 14 Nov 2012; accepted 14 Nov 2012; published 28 Nov 2012
14 January 2013 / Vol. 21, No. S1 / OPTICS EXPRESS A63