Solar cells have been around for many years but so far, they haven't contributed a significant portion of overall electricity production. Solar farms have popped up in recent years, but polluting fossil fuels still dominate. Even so, research in improving efficiency and reducing the cost of solar cells continues.

In July 2018, Imec and EnergyVille announced the development of solar cells with a record efficiency of 27.1%. It's possible that by press time, that record will be broken. Indeed, 30% efficiency is now within reach, according to Imec's press release (Ref. 1). Imec researchers used perovskite in tandem with silicon solar cells to achieve the result.

What is perovskite?
Perovskite is a class of materials that take on the same crystal structure as calcium titanite (Ref. 2). The perovskite material is placed on top of a silicon solar cell to increase efficiency by minimizing thermalization losses in the silicon while also generating its own power from sunlight. "Perovskites are expected to become an add-on to the existing silicon solar cells, which can significantly boost their efficiency at little added cost," said Imec/Energyville researcher Manoj Jaysankar.

"Perovskite is promising because of its remarkable electronic properties (as good as Si or GaAs), cheap material cost, low fabrication cost of solar cell and, being a thin-film technology, it can also be made on flexible substrates," added Manoj. "Although the electrical performance of perovskite solar cells is comparable to established solar cell technologies, we don't expect it to replace the existing technologies. We do expect it to make an impact in new markets such as flexibles, building integration, etc."

The researchers used a spin-coating technique — one of several possible techniques — to deposit the liquid perovskite material on a substrate. Methods such as blade coating, dot-slide coating, and ink-jet printing have also been used (Ref. 3). To achieve the 27.1% efficiency, researchers used a 0.13 cm² spin-coated Perovskite cell atop a 4 cm² silicon solar cell. Because efficiency drops as cell size increases, the efficiency measurements dropped to 25.3% when the perovskite module size was increased 4 cm2 and placed over the 4 cm2 silicon cell. Even at that size, the combined solar cell's efficiency surpassed that of the stand-alone silicon cell.

The researchers at Imec constructed a four-terminal solar cell shown in Figure 1. The top cell is the perovskite solar cell. It's optically coupled to the silicon lower cell and power from both cells is separately extracted. Combining perovskite and silicon increases overall efficiency. According to Manoj, the stand-alone silicon cell's efficiency stands at 23%. There is also a two-terminal version of the tandem solar cell, where the top and bottom cells are electrically integrated. While this does reduce the amount of electronics needed to extract power, it results in a lower energy yield depending due to variable spectrum of sunlight at different times of the day. With a four-terminal configuration, the two cells are only connected optically. From these and previous results, it appears that the efficiency gains will outweigh the additional manufacturing costs of solar cells and their electronics.

textFigure 1: Imec researchers used a two-part solar cell consisting of perovskite (top) and silicon (bottom). The two cells are optically linked, leading to higher power conversion efficiency than silicon alone. Source: Imec.

Imec's test setup
Imec researchers tested the cell using the configuration in Figure 2. The measurement setup consists of a calibrated “Sun simulator” and a Keithley SourceMeter. The simulator illuminates the solar cell with an optical power of 1000 W/m2 and the SourceMeter measures the cell's electrical output (voltage and current). Commercial software communicates with the SourceMeter, displaying the output and calculating efficiency. In this setup, the SourceMeter was used as a measuring instrument only.

textFigure 2: Imec's test setup uses a calibrated sun simulator to illuminate the cell under test. Source: Imec.

Manoj explained the test procedure.

  1. Do a voltage sweep of the devices up to 2V.
  2. Measure the current at each voltage to get the I-V characteristics of the solar cell.
  3. Extract short-circuit current density, open-circuit voltage, and fill factor from the I-V curve (Ref. 3).

The software calculates an I-V curve on the cell to find the point of maximum power, which usually occurs at the curve's knee (Figure 3).

current voltage power curveFigure 3: The maximum power point on a solar cell's I-V curve usually occurs at the knee.

The output current density is typically from few tens of milliamps per square centimeter while the voltage output can reach 2.2 V (0.7 V for silicon, 1.5 V for perovskite.

Efficiency = Short-circuit current density × open-circuit voltage × Fill factor

Perovskite issues
As with any new technology, perovskite-enhanced solar cells have some issues. The most concerning is the presence of lead. While the lead in solar cells is not as accessible as lead in paint, it's still a concern that chemists and physicists will need to work out.

Short lifetime was another issue that initially arose. Early perovskite materials lost their crystalline structure in just minutes. Today, stability of the structures can hold for thousands of hours as researchers have found better material formulas that have increased stability while maintaining the absorption needed to improve power efficiency (Ref. 4). Lifetimes and efficiencies will likely continue to improve and once the lead issue is resolved, perovskite materials could provide the boost that solar cells need to increase our use of renewable energy.

References

  1. Imec Beats Silicon PV with 27.1 Percent Perovskite-Silicon Tandem, press release, July 24, 2018.
  2. Snaith, Henry J., "Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells," The Journal of Physical Chemistry, 2013.
  3. News Release: Perovskite Technology Scalable, But Questions Remain about the Best Methods, National Renewable Energy Laboratory, U.S. Dept. of Energy, April 6, 2018.
  4. Video: The IV Curve, Technical University of Denmark.
  5. Werner, Jérémie, Boern Niesen, and Christophe Ballif, "Perovskite/Silicon Tandem Solar Cells: Marriage of Convenience or True Love Story? – An Overview," Advanced Materials Interfaces, 2017.

Martin Rowe covers test and measurement for EDN and EE Times. Contact him at martin.rowe@AspenCore.com

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