Multi Junction Solar Cells Stacked with Transparent and Conductive Adhesive

Abstract

We propose a polyimide transparent adhesive layer dispersed with In 2 O 3 -SnO 2 (ITO) conductive particles (polyimide-ITO) to be used in the mechanical stacking of solar cells. A 20-m-thick polyimide-ITO layer had a high transmissivity from 78 to 80% for wavelengths ranging from 500 to 1000 nm and a low connecting resistivity of 2.3 cm 2 at minimum. The fabrication of stacked cell consisting of a top hydrogenated amorphous silicon (a-Si:H) p-i-n cell and a bottom hetero-junction with an intrinsic thin-layer (HIT)-type silicon cell was demonstrated using an intermediate polyimide-ITO layer. A high open circuit voltage of 1.34 V was experimentally obtained. Simultaneous electric power generation from the top and bottom solar cells was achieved. #

Adhesive bonding for mechanically stacked solar cells

“…Mechanical stacking of solar cells through adhesive wafer bonding has already been demonstrated as a possible fabrication route for high‐performance multi‐junction solar cells . An efficiency of 31% under concentration (×347 suns) was reported by Gee and Virshup for an adhesive‐bonded GaAs/Si MSSC in 1985, which was a world record at the time.…”

“…An efficiency of 31% under concentration (×347 suns) was reported by Gee and Virshup [23] for an adhesive-bonded GaAs/Si MSSC in 1985, which was a world record at the time. Sameshima et al [22] outlined the optical performance of an adhesive layer used in a dual-junction stack, although the thickness of the adhesive was primarily limited by the size of conducting In 2 O 3 -SnO 2 ITO particles used to electrically interconnect the subcells. The effect of adhesive thickness on the optical transmission of an interface layer in an AlGaAs/Si MSSC was also simulated by Siegel, but the optimum thickness was not experimentally demonstrated [24].…”

“…Polyimides are generally highly absorbing at wavelengths below 500 nm but would be suitable for use in a GaInP/GaAs-InGaAs stack, as absorption in the relevant wavelength range of 850-1700 nm is low. 90% void-free bond areas have been achieved for 4″ Si wafer pairs at a curing temperature of 300°C [26], and polyimides have already been used to a form a seriesconnected Si to a-Si tandem solar cell [22]. PDMS is a low-cost polymeric compound that can be cured at temperatures as low as 25°C, thus minimising the risk of residual stress in the layers post-bonding as happens with high temperature bonding of layers with different thermal expansion co-efficients.…”

Reduction in optical reflection loss at intermediate adhesive layer for mechanical stacked multi-junction solar cells

“…Multi-junction solar cell with different band gaps is attractive to effectively collect sun light which has wide wavelength from ultraviolet to infrared [4][5][6][7]. We have developed transparent and conductive adhesive layer for fabricating mechanically stacked multi-junction solar cells [8][9][10][11][12][13][14]. A low connecting resistivity below 1 Ωcm 2 was achieved using 20-μm-sized Indium Tin Oxide (ITO) conductive particles dispersed in commercial epoxy type Cemedine adhesive [13].…”

“…This connecting resistivity was enough low value to fabricate solar cells with high conversion efficiency about 30% [12][13]. Although the adhesive was transparent in visible and infrared regions [10], there is a problem of optical reflection loss at the interfaces between semiconductor and adhesive layer caused by difference of their refractive indexes. In this paper, we report reduction in optical reflection loss at intermediate layer in the case of stacking Si and Ge substrate, which have different band gaps.…”