In order to regulate the in-room environment in buildings and homes, materials researchers have been interested in the possibility of utilizing room-windows. Smart windows—windows that employ specially designed glass that can regulate the amount of sunlight passing through—have attracted significant research attention over the past few years. Electrochromic windows (ECWs) undergo changes to their light transmission properties in response to an applied voltage. A cheaper, sustainable and “greener” route to realizing and operating ECWs would be to supply the required voltage from sunlight. Solar-powered ECWs have been explored recently; however, a majority of the demonstrations have relied on utilizing near-infrared (near-IR) and visible solar photons for powering the windows, leaving a lesser portion of the incoming sunlight for in-room environment regulation. In addition, the low photovoltages produced by solar cells that target near-IR and visible photons has limited the optical transmittance contrast of ECWs paired with these cells.

Now, the research group of Yueh-Lin Loo at Princeton University has demonstrated a different approach to powering ECWs, using photovoltaic (PV) devices that absorb only near-ultraviolet (near-UV) solar photons. These near-UV solar cells leave the remaining 93% of sunlight for lighting and heating, meeting the requirements of an ideal smart window.

“Our technology allows utilizing and tuning the visible and near-IR portions of the solar spectrum independently for in-room heating, cooling, and lighting, by exploiting near-UV photons to power the ECWs,” says Nick Davy, a graduate student in the Loo group and the first-author on the article published recently in Nature Energy. Utilizing derivatives of organic molecules reported earlier by Davy, larger bandgaps and near-UV absorption were realized. These modified donor molecules when blended with complementary non-fullerene acceptor molecules resulted in solar cells with record photovoltages in excess of 1.6 V, which were then utilized to power organic ECWs. Organic layers for the near-UV PV cells were fabricated using industrially viable vacuum thermal evaporation, mimicking the organic thin-film deposition procedure employed for example by Heliatek, a company that currently leads global industrial fabrication of organic PVs.

Davy and co-researchers anticipate significant commercial utility for transparent near-UV solar cells given their compatibility with a range of existing and emerging ECWs. When scaled-up to device areas of market interest, the near-UV PV cells were found to sustain their performance; the overall generated power increased linearly with area. The researchers explain that these devices avoid significant resistive losses by virtue of their high voltages. Also, the thin films are amorphous and pinhole-free, helping to avoid device shorting during scale-up.

Although the current demonstration employs a side-by-side operation of the near-UV PV cell and the organic ECW, the next step is to pair these novel architectures vertically in a flexible assembly, says Davy, to mimic the requirement of a final market product. This challenge will require the team to determine a suitable transparent top electrode for the near UV-cells, which currently employ aluminum. Demonstrating the long-term stability of their near-UV cells is another challenge that the team aims to overcome as research efforts proceed.

Read the abstract in Nature Energy