The materials scientists have increased the stability and efficiency of wide-bandgap perovskite solar cells that can be used as the upper part of the tandem solar module. Additive salts of substituted piperidine in the perovskite layer prevents oxidation of anions of iodine to molecular iodine, resulting in solar cells retain 95 percent of its efficiency after 1000 hours of operation. The results of a study published in the journal Science.
Tandem solar modules consist of two semiconductor solar cells, placed one below the other. First ray of sunlight passes through the upper semi-transparent semiconductor with a larger width of a forbidden zone, where the absorbed high-energy photons (with shorter wavelength). Photons with less energy pass through and are absorbed in the lower semiconductor with lower gap. Thus, the tandem architecture maximizes the efficient use of the entire spectrum of solar radiation.
As the lower part of the tandem, the most commonly used silicon, and to the upper one of the best materials are considered wide-gap perovskite semiconductors based on mixed halides of lead with the addition of caesium and bromine. In the beginning of 2020 tandem silicon-perovskite has achieved efficiency at 29.1 percent is nearly 3 percent more than the record efficiency silicon solar cells. For commercial use of silicon-perovskite tandem until that lacks stability. The “weak link” of such tandem — of course, perovskite: if amorphous silicon solar cells retain their efficiency for decades, the best perovskites stand until only a few thousand hours of operation.
Solar cells based on wide bandgap perovskites are considered to be more stable than the standard perovskites — they do not contain in its composition of methylamine, which is very sensitive to thermal stress, however, such devices have a serious drawback. The fact that the manufacture of monolithic tandems all the layers of the perovskite solar cell should be consistently applied to the finished silicon solar cell — the lower part of a future tandem. To avoid damage to the silicon element, the synthesis is carried out under mild conditions, at temperatures above 200 degrees Celsius. This temperature is sufficient for crystallization of the perovskite active layer, but not always sufficient for the application transport layers, which are located above and below the active layer and provide a charge separation in a solar cell. In tandem solar cells cannot be used oxide transport layers for crystallization which requires a high temperature (400 degrees Celsius), so they have to be replaced by organic and polymeric materials, which may be less stable.
New work by materials scientists from five countries, led by Henry Snaith (Henry J. Snaith) from the University of Oxford devoted to the stabilization of wide-gap perovskite solar cells prototype the upper part of the tandem. They made Cs0.17FA0.83Pb(I1-xBrx)3 with different content of bromine and different values of the forbidden zone, from 1,56 to 1,76 electron volts. As the electron-transport layer, the researchers used a fullerene derivative PCBM, and the hole-transport layer poly(4-butylphenyl-diphenylamine) (polyTPD). All of these materials can be applied at low temperature (not higher than 130 degrees Celsius), so a solar cell, it is possible to synthesize as a second layer on top of a silicon solar cell.
To increase stability, the researchers added in the active layer tetrafluoroborate 1-butyl-1-methyl piperidine (BMP) — the amount varied from 0 to 0.3 mole percent, the optimum effect provided a Supplement of 0.25 per cent. Cells with BMP do additives was much more stable: they retain 95 percent of its efficiency after 1000 hours of irradiation with sunlight at a temperature of 60 degrees Celsius, while the same elements without additives lost more than half its efficiency during the first 200 hours of the experiment. In addition additive BMP improved the open-circuit voltage and efficiency of solar cells — an average of one and a half percent.