Highly-Transparent and True-Colored Semitransparent Indoor Photovoltaic Cells

Organic photovoltaic (OPV) cells are promising indoor light energy harvesters because organic materials absorb strongly in the visible range. An indoor photovoltaic (IPV) device is an effective tool for the remote off-grid wireless charging. However, as the indoor light fluxes are much weaker than the 1-Sun condition, high-performance indoor cells should have large areas in order to generate appreciable energies. They would then appear as flat, but expansive and dark objects if deployed indoors. Their presence would then alter the indoor lighting environment and affect visual perceptions. This work addresses the lighting and perception issues of IPV cells in three ways. i) A concept is proposed such that a high-efficiency, semitransparent indoor OPV cell should possess an absorbance which is mismatched with the emission spectra of the light sources. ii) It is demonstrated that bulk heterojunction (BHJ) OPV solar cells with porphyrin donors can serve as high-transparency and high-efficiency indoor light harvesters. iii) Quantitative assessment criteria are presented for the transparency and chromaticity of an indoor semitransparent OPV cell, demonstrating that porphyrin-based P2:PC₇₁BM semitransparent BHJ cells can achieve a power conversion efficiency (PCE) exceeding 10%, and an illuminance transparency ~65%, while preserving the color perception of the light sources.