[Image above] Much next-generation photovoltaic research focuses on perovskite materials. Two researchers in Korea investigate the potential of antiperovskite oxides instead. Credit: Pixabay
When it comes to the future of solar energy, one technology that gets a lot of attention is perovskite solar cells.
Perovskite materials are compounds with the same type of crystal structure as calcium titanium oxide, the mineral traditionally referred to as perovskite. The general chemical formula for perovskite compounds is ABX3, where “A” and “B” are two cations and “X” is an anion that bonds to both cations.
On their own, perovskite materials face several challenges to application in solar cells, such as the fact that they degrade when exposed to ultraviolet light. However, when combined with silicon, perovskite materials can greatly increase the power conversion efficiency of solar cells, which is why many perovskite studies focus on tandem solar cells.
To date, metal halide perovskites, or perovskites that have halogen elements in the “X” position of the chemical formula, are the category of perovskites that has achieved the most success in solar cell applications due to their tunable band gaps and high structure tolerance, among other properties. Ferroelectricity is one property, though, that metal halide perovskites do not always possess.
Ferroelectricity refers to the spontaneous electric polarization exhibited by certain materials. In solar cells, ferroelectricity is useful because electric polarization helps facilitate the separation of photoexcited electron–hole pairs.
Perovskite oxides, or perovskites that have oxygen in the “X” position of the chemical formula, are known as excellent ferroelectric materials. Unfortunately, most perovskite oxides exhibit unfavorably large band gaps (>2.5 eV), which limits their use in solar cell applications.
Researchers have explored tuning the band gap of perovskite oxides via structural and chemical modiﬁcations. Yet solar cells incorporating these modified materials still lag in performance compared to conventional p−n junction solar cells based on silicon and gallium arsenide.
In a recent study, two researchers in Korea focused on modifying the structure to achieve a smaller band gap. However, their modification is more than a simple adjustment—they suggest that inversing the perovskite structure completely may lead to desired results.
Antiperovskites: Another promising material for photovoltaics
Antiperovskites, or materials with an inverse perovskite structure, are the lesser-known counterparts to perovskites. They share a similar structure to perovskites, but positions of the cation and anion constituents are reversed in the unit cell structure.
Less research exists on antiperovskites compared to perovskites. However, more research is beginning to emerge, and some of the findings suggest that antiperovskites may be promising materials in solar applications.
“[Fully] inorganic antiperovskite oxides with a composition of Ba4Pn2O (Pn = As or Sb) … were reported to have proper band gaps for PV [photovoltaic] applications in a previous high-throughput computational study,” assistant professor Youngho Kang (Incheon National University) and professor Seungwu Han (Seoul National University) write in the recent paper.
So they decided to conduct their own simulation study on the potential of Ba4Pn2O antiperovskites as materials in solar applications.
Using density functional theory calculations and many-body quantum-mechanical calculations, they showed that Ba4As2O and Ba4Sb2O exhibit moderate macroscopic polarization, which is enough for charge carrier separation, and have direct band gaps of 1.1 eV and 1.3 eV, respectively, which are close to the optimal Shockley−Queisser value of 1.3 eV.
In addition, by investigating optical absorption coeﬃcients and resulting short-circuit currents, they demonstrated that a very thin layer of Ba4As2O or Ba4Sb2O can yield large photocurrents. They also found the eﬀective masses of charge carriers in antiperovskite oxides to be fairly small, thus suggesting easy extraction of photocarriers when the materials are applied to actual devices.
“In light of the favorable simulation results and conﬁrmed synthesizability, we believe that Ba4Pn2O can pave a new avenue for the development of next-generation PVs,” they write in the conclusion.
The paper, published in ACS Applied Materials & Interfaces, is “Antiperovskite oxides as promising candidates for high-performance ferroelectric photovoltaics: First-principles investigation on Ba4As2O and Ba4Sb2O” (DOI: 10.1021/acsami.0c13034).