Doping internal layers of perovskite solar cells with CO₂ reduces processing time

A simple procedure can dramatically reduce the time required to process a promising type of solar cell. The procedure may reduce manufacturing time and costs and uses just one inexpensive reagent: CO₂.

The ability of photovoltaic (PV) cells based on light-absorbing perovskite compounds to generate electricity from sunlight has soared in the past decade. Some of these low-cost devices now boast power conversion efficiencies—a measure of sunlight in and electricity out—of 25%, putting them on par with costly cells made of crystalline silicon, the standard PV material.

One factor impeding broad commercialization of perovskite devices is a slow doping procedure that improves the cells’ ability to transport electrical charges, one of the keys to generating a usable current. The bottleneck affects a thin charge-extraction layer, often a blend of an organic semiconductor known as spiro-OMeTAD and lithium bis(trifluoromethane)sulfonimide (LiTFSI), a common lithium salt.

Researchers typically cast films of this blend on the perovskite light absorber then let it sit for hours, sometimes overnight, exposed to air and light. The long exposure enables oxygen to slowly diffuse into the film, which boosts the film’s knack for shuttling positive charges, also referred to as holes, to an electrode that’s applied to that side of the solar cell. (Electrons travel to the cell’s other electrode.)

Now, a team of researchers has shown that the doping process can be cut from hours to just 1 minute by bubbling CO₂ into a solution of spiro-OMeTAD and LiTFSI while irradiating the mixture with ultraviolet light and then casting the film(Nature 2021, DOI: 10.1038/s41586-021-03518-y).

The team, which includes Jaemin Kong and André D. Taylor of New York University and Yongwoo Shin of Samsung Semiconductor, explains that UV light excites spiro-OMeTAD molecules, causing them to transfer electrons to dissolved CO₂. The resulting negatively charged CO₂ reacts with lithium ions from LiTFSI. That reaction produces carbonates and other species and releases TFSI, which stabilizes the oxidized spiro-OMeTAD and increases the longevity of the device.

Tests show that the procedure quickly improves the properties of the film. The electrical conductivity of CO₂-treated films is 10 times as high as that of films exposed to oxygen and 100 times as high as untreated films. And compared with a series of control cells in which the maximum output power fell below 75% of the initial value within 6 h, cells with CO₂-treated films retained 80% of the initial power after 500 h.

The team also found that the CO₂ treatment can be used to quickly dope π-conjugated polymers, increasing their conductivity and extending the method’s application beyond solar cells to organic light-emitting diodes and other types of organic electronics.

Given that spiro-OMeTAD is “the workhorse material” that many researchers use to extract positive charges in perovskite solar cells, “this is a big deal” that will stimulate further study, says Juan-Pablo Correa-Baena of the Georgia Institute of Technology, who was not involved in the study.

Yet as he and other solar cell specialists including Tsutomu Miyasaka of Toin University of Yokohama point out, the CO₂ treatment is not the final advance needed to bring perovskite cells to market. To improve the longevity of these devices, scientists are searching for materials that readily extract and transport positive charge without the need for any dopants, which eventually could diffuse through the cells and hamper performance.