(Invited) Diffuse Light Harvesting to Structured Information with Hybrid Photovoltaics
Authors: Freitag Marina 
Published: September 2023

By 2025 about 75 billion IoT devices will be installed, of which the majority will reside indoors. It is therefore crucial to find an energy source that yields high efficiencies in this environment.1,2 At 34% efficiency under ambient light, while being more environmentally friendly, sustainable to produce and to recycle. Dye-sensitized solar cells (DSCs) are known for efficient conversion of ambient light. Fast charge separation in a variety of organic dyes and tuneable energy levels in CuII/I redox systems combined with negligible recombination processes allow DSCs to maintain a high photovoltage under ambient light.1

We tailored dye-sensitized photovoltaic cells based on a copper (II/I) electrolyte for power generation under ambient lighting with an unprecedented conversion efficiency (PCE 34 %, 103 mW cm-2 at 1000 lux; 32.7 percent, 50 mW cm-2 at 500 lux; and 31.4 percent, mW cm-2 at 200 lux from a fluorescent lamp) using a novel co-sensitization strategy.2 Though there are additional factors to consider when converting ambient light. First, rather of simply extending the absorption range, the DSC's spectral conversion response should be tailored to the source of ambient lighting. Second, at low light intensities, recombination suppression is critical for DSC performance.3 Though there are additional factors to consider when converting ambient light. First, rather of simply extending the absorption range, the DSC's spectral conversion response should be tailored to the source of ambient lighting. Second, at low light intensities, recombination suppression is critical for DSC performance. Herein, we developed DSCs with CuII/I(tmby)2 (tmby = 4,4′,6,6′-tetramethyl-2,2′-bipyridine) hole transport material with a combination of organic sensitizers XY1 and L1. Under simulated sunlight (AM 1.5G, 100 mW cm−2), the best device reached a photovoltage of 1080 mV, a photocurrent density of 15.9 mA cm−2, a fill factor of 0.67 and a PCE of 11.5%.4,5

Under 1000 lux lighting, 64 cm2 photovoltaic area gives 152 J or 4.41 1020 photons sufficient for training and testing of an artificial neural network in less than 24 hours. Ambient light harvesters enable a new generation of self-powered and "smart" IoT gadgets to be fueled by a hitherto untapped energy source.2

Muñoz-García, A. B. et al. Dye-sensitized solar cells strike back. Chemical Society Reviews
50, 12450–12550 (2021).

Michaels, H. et al. Dye-sensitized solar cells under ambient light powering machine learning: towards autonomous smart sensors for the internet of things. Chemical Science
11, 2895–2906 (2020).

Freitag, M. et al. Dye-sensitized solar cells for efficient power generation under ambient lighting. Nature Photonics
11, 372–378 (2017).

Michaels, H., Benesperi, I. & Freitag, M. Challenges and prospects of ambient hybrid solar cell applications. Chemical Science (2021) doi:10.1039/D0SC06477G.

Benesperi, I., Michaels, H. & Freitag, M. The researcher’s guide to solid-state dye-sensitized solar cells. Journal of Materials Chemistry C (2018) doi:10.1039/C8TC03542C.