Interstellar photovoltaics

Abstract

The term 'Solar Cell' is commonly used for Photovoltaics that convert light into electrical energy. However, light can be harvested from various sources not limited to the Sun. This work considers the possibility of harvesting photons from different star types, including our closest neighbor star Proxima Centauri. The theoretical efficiency limits of single junction photovoltaic devices are calculated for different star types at a normalized light intensity corresponding to the AM0 spectrum intensity with AM0 = 1361 W/m². An optimal bandgap of > 12 eV for the hottest O5V star type leads to 47% Shockley-Queisser photoconversion efficiency (SQ PCE), whereas a narrower optimal bandgap of 0.7 eV leads to 23% SQ PCE for the coldest red dwarf M0, M5.5Ve, and M8V type stars. Organic Photovoltaics (OPVs) are the most lightweight solar technology and have the potential to be employed in weight-restricted space applications, including foreseeable interstellar missions. With that in mind, the Sun's G2V spectrum and Proxima Centauri's M5.5Ve spectrum are considered in further detail in combination with two extreme bandgap OPV systems: one narrow bandgap system (PM2:COTIC-4F, E_g = 1.14 eV) and one wide bandgap system (PM6:o-IDTBR, E_g = 1.62 eV). Semi-empirically modeled JV-curves reveal that the absorption characteristics of the PM2:COTIC-4F blend match well with both the G2V and the M5.5Ve spectrum, yielding theoretical PCEs of 22.6% and 12.6%, respectively. In contrast, the PM6:o-IDTBR device shows a theoretical PCE of 18.2% under G2V illumination that drops sharply to 0.9% under M5.5Ve illumination.

Figure 1

Stellar types and their spectra. a Visually representative examples of O, B, A, F, G, K and M stellar types (hot to cold) within our galaxy (source: NASA54). b Spectral distributions of stars ranging from O5V to M8V.

Figure 2

Theoretical photovoltaic performance limits for the different stellar types. a PCE as a function of the band gap for spectral star types O5V, B0V, and A0V. b PCE as a function of the band gap for spectral star types F0, K1V, and G2V. c) PCE as a function of the band gap for spectral star types M0, M5.5Ve, and M8V.

Figure 3

OPV model systems. a Chemical structures of the organic semiconducting donor polymers PM6 and PM2 as well as of the NFAs O-IDTBR and COTIC-4F. b Energy diagram for the materials. c Schematic device structure for inverted solar cells based on PM6:o-IDTBR and PM2:COTIC-4F active layers.

Figure 4

Comparison of the spectral match of the G2V and M5.5Ve spectra with two BHJ OPV blends. a G2V spectrum (black) and the fraction of light absorbed by PM6:o-IDTBr (blue dashed) and the amount of light absorbed (blue area). b M5.5Ve spectrum (black) and the fraction of light absorbed by PM6:o-IDTBr (blue dashed) and the amount of light absorbed (blue area). c) G2V spectrum (black) and the fraction of light absorbed by PM2:COTIC-4F (red dashed) and the amount of light absorbed (red area). d) M5.5Ve spectrum (black) and the fraction of light absorbed by PM2:COTIC-4F (red dashed) and the amount of light absorbed (red area).

Figure 5

JV-characteristics of OPVs with PM6:o-IDTBR (blue) and PM2:COTIC-4F (red) active layers under M5.5Ve (dashed line) and G2V (solid lines) illumination.