RAINBOW Organic Solar Cells: Implementing Spectral Splitting in Lateral Multi‐Junction Architectures

Abstract

While multi‐junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub‐cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub‐cell. Using the resulting design rules, three systems are selected, with narrow, medium, and wide effective bandgaps, and their potential as sub‐cells in this geometry is experimentally investigated. With the aid of a custom‐built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub‐cell. Finally, a working proof‐of‐concept monolithic device consisting of two sub‐cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept.

Spectrum on Demand Light Source (SOLS) for Advanced Photovoltaic Characterization

“…There is one notable exception for this requirement: the mentioned multijunction solar cells based on the spectrally split (Rainbow) concept. 21 Here, a spectrally split beam is required, so each sub-cell can be illuminated with the appropriate fraction of the sun spectrum. Notably, what defines the appropriate fraction is the bandgap of the materials used in each sub-cell, and thus a highly tunable setup is required to investigate and optimize this geometry.…”

“…(5) In the path and at the focal plane where light is being concentrated (where element ( 5) is placed in Figure 1), the equipment delivers a focused but spectrally split beam. This could be used for characterization of a RAINBOW solar cell multi-junction device 21 by placing the solar cell at the correct distance before or beyond that focal plane. Alternatively, a homogenizing device consisting in a diffusing element in front of a homogenizing light pipe can be placed at the focal point (5).…”

“…19,20 Recently, multi-junction solar cells based on the spectral splitting concept have attracted renewed attention. 21,22 In this case, an optical element separates the sunlight into a number of beams exhibiting different spectra in order to match the spectral range with the bandgap of each sub-cell. In order to optimize this geometry, each sub-cell should be investigated as a function of a fraction of the sun spectrum.…”

Bypassing the Single Junction Limit with Advanced Photovoltaic Architectures

“…Spectral sharing strategies are also studied, by applying optical elements such as beam splitters [3] or phase masks, either planar [4] or focusing, [5] which lead to a lateral splitting of the solar spectrum. Thus, "rainbow solar cells" [6] can be constructed by a lateral multijunction architecture. These rainbow cells can be built more easily than vertical architectures, however, the Kirchhoff rules must still be respected.…”

High‐Performance Filterless Blue Narrowband Organic Photodetectors

“…[ 11,12 ] Other promising approaches employing dispersive elements, like prisms and gratings, also encounter challenges like spatial resolution and high cost, despite their high performance. [ 12,13 ] Given the advantages of molecular building blocks and device engineering, OPDs have showcased superior filterless narrowband detection by means of charge collection narrowing (CCN), [ 14,15 ] charge injection narrowing (CIN), [ 16,17 ] novel narrowband absorbing materials, [ 18,19 ] self‐filtering, [ 20,21 ] and microcavity integration. [ 22,23 ] Those methods propel the realization of spectroscopic sensing with phenomenal full‐width half‐maximum down to 7 nm.…”