The movement to using renewable energy for power generation is leading to the growing use of solar power. While progress has been made, research is ongoing to find new materials that can increase power conversion efficiency while remaining stable during use. 

Geoffroy Hautier, Hodgson Family associate professor of engineering at Dartmouth College in Hanover, N.H., says, “The main solar material in use is silicon, which only achieves a certain efficiency level but has limitations including energy needs for production. Research has been conducted to identify different materials that could be used alone or even combined with silicon to improve performance.” 

Some of these alternative materials are cadmium telluride, CIGS (prepared from copper, indium, gallium and selenium) and perovskites. Hautier says, “One concern with CIGS is the sourcing of the various materials used in its preparation. Lead halide perovskites have emerged as another option but are not stable. They degrade when exposed to oxygen and water. There is no clear approach for overcoming these issues.” 

A previous TLT article¹ addressed the issue of perovskite solar cell stability by discussing work conducted by researchers to change the architecture from a n-i-p to an inverted p-i-n structure. The difference between the two orientations is the positioning of the electron transportation and hole transportation layers with respect to each other and to the perovskite. The surface modification was accomplished through treating a p-i-n perovskite thin film with an amine-based pyridine derivative. Performance testing at 55°C in ambient air led to an 87% maximum power conversion efficiency after 2,428 hours of sunlight exposure. A conventional perovskite solar cell only attained a 76% efficiency after less than half the sunlight exposure. Better resistance from moisture also was obtained with the cell maintaining 94% of its efficiency after being exposed to 85% relative humidity at 85°C for 850 hours. 

Hautier says, “Alternative solar cell materials are needed that meet the criteria of high performance, good stability under ambient conditions, abundant and easy to source, low cost and can be considered to be sustainable.” 

Hautier and his colleague Jifeng Liu, professor of engineering at Dartmouth College, believe that following a conventional research strategy to make small changes to existing solar cell materials will probably not lead quickly to the identification of a new solar cell material. Hautier says, “Currently, computer capabilities have greatly expanded allowing for a more efficient approach for screening many materials in a much shorter period of time.” 

Liu adds, “The computational tool provides big value from an empirical standpoint. Synthesizing a new potential candidate in the laboratory and evaluating its potential for photovoltaic applications can take months if not years for most solar materials. Finding a way to better identify suitable solar materials can significantly reduce the time need to produce and evaluate them.” 

Hautier led a team of researchers that used high-throughput computational screening to find a new material that has potential as a stable solar absorber. 

Zintl phosphide 
The researchers utilized the U.S. Department of Energy’s (DOE’s) Materials Project Database, which contains approximately 40,000 inorganic materials to identify a potential new solar material. Hautier says, “We knew from previous studies that parameters such as band gap and carrier transport needed to be factored into our evaluation. Finding a material with the right size band gap for absorbing photons and displaying good carrier transport is essential and easy to compute. But we also decided to better understand the type and size of defects that can form in these materials.” 

Solar materials can contain bad defects, which will in effect disable their ability to harvest electricity from sunlight. Liu says, “Defects also can be detrimental in fabricating solar cells on a large commercial scale especially those known as ‘deep defects’ with energy levels right in the middle of the bandgap of solar materials. If defects are ‘shallower’ and do not impact performance in certain new solar material candidates, then this will lead to a lower fabrication cost, and a potential winner in the future.” 

In completing the screening study, the researchers identified a Zintl phosphide with the following chemical structure as a potential high-efficiency solar absorber candidate: BaCd₂P₂. Hautier says, "Zintl compounds are hybrid materials that exhibit both ionic and metallic characteristics. They are very useful as thermoelectric materials that can convert heat to electricity.” 

The Zintl phosphide was synthesized by reacting the three elements (barium-Ba, cadmium-Cd and phosphorus-P) in a solid- state process at the correct stoichiometry in a sealed silica ampoule. Crystal structure analysis shows alternating layers of tetrahedrally coordinated cadmium and octahedrally coordinated barium. 

Liu says, “Our team produced the Zintl phosphide at a purity of 99.9% initially, which is lower than what is customarily found for solar absorbers (typically purity is in the 99.99999% range). But we were very pleased with the initial evaluation for band gap, carrier transport and the defects. Defects can be a potential eliminator of a solar material, but we found no significant, deep defects that would act as strong nonradiative recombination centers.” 

The one parameter that the researchers could not predict from modeling was stability. Hautier says, “Gaining a theoretical understanding of a compound’s stability is complicated. Stability can be measured in different ways and materials can exhibit stability both from a thermodynamic and kinetic standpoint.” 

The researchers found the Zintl phosphide to be very stable in ambient air (for more than six months at room temperature) and after immersion in water for 12 hours. Treatment with a potassium hydroxide solution resulted in little change after 72 hours due to passivation of the surface with hydroxide. The Zintl phosphide was only vulnerable to strong acids such as hydrochloric. In all cases, stability was determined by looking for significant changes in the material’s crystal structure (see Figure 1). 


Figure 1. The stability of Zintl phosphide was analyzed through evaluation of the material’s crystal structure, which consists of alternating layers of tetrahedrally coordinated cadmium and octahedrally coordinated barium. Figure courtesy of Dartmouth College.

Comparison of the performance of the Zintl phosphide was made with a high-performance solar cell material, gallium arsenide. Liu says, “We found that our candidate even at a less pure level compared favorably to gallium arsenide, which is a wellknown, high-performance, solar absorber.” 

Hautier and Liu indicated that a more environmentally friendly version of the Zintl phosphide is desirable due to the toxic nature of cadmium. Hautier says, “We do not believe that the presence of cadmium in the Zintl phosphide will eliminate future work. Other solar materials such as the perovskites exhibit toxicity due to the presence of lead.” 

The researchers believe the Zintl phosphide may be the first of a new potential series of solar cell materials. Hautier says, “Our intention is to continue to better understand the performance characteristics of high purity versions of the Zintl phosphide and to substitute other metals such as zinc for cadmium to make new versions that may be less toxic.” 

Additional information on this research can be found in a recent paper² or by contacting Hautier at Geoffroy.hautier@dartmouth.edu and Liu at jifeng.liu@dartmouth.edu.

REFERENCES
1. Canter, N. (2023), “Surface modification of perovskite solar cells,” TLT, 79 (1), pp. 16-17. Available here.
2. Yuan, Z., Dahliah, D., Hasan, M., Kassa, G., Pike, A., Quadir, S., Claes, R., Chandler, C., Xiong, Y., Kyveryga, V., Yox, P., Rignanese, G., Dabo, I., Zakutayev, A., Fenning, D., Reid, O., Bauers, S., Liu, J., Kovnir, K. and Hautier, G. (2024), “Discovery of the Zintl-phosphide BaCd2P2 as a long carrier lifetime and stable solar absorber,” Joule, 8 (5), pp. 1412-1429.