Cu-In Halide Perovskite solar absorbers (1611.10036v3)

Abstract: The long-term chemical instability and the presence of toxic Pb in otherwise stellar solar absorber APbX${3}$ have hindered their large-scale commercialization. Previously explored ways to achieve Pb-free halide perovskites involved replacing Pb$^{2+}$ with other similar M$^{2+}$ cations in ns$^2$ electron configuration, e.g., Sn$^{2+}$ or by Bi$^{3+}$ (plus Ag$^+$), but unfortunately this showed either poor stability (M = Sn) or weakly absorbing oversized indirect gaps (M = Bi), prompting concerns that perhaps stability and good optoelectronic properties might be contraindicated. Herein, we exploit the electronic structure underpinning of classic Cu[In,Ga]Se${2}$ (CIGS) chalcopyrite solar absorbers to design Pb-free halide perovskites by transmuting 2Pb to the pair [B$^{IB}$ + C$^{III}$]. The resulting group of double perovskites with formula A$_2$BCX$_6$ (A = K, Rb, Cs; B = Cu, Ag; C = Ga, In; X = Cl, Br, I) benefits from the ionic, yet narrow-gap character of halide perovskites, and at the same time borrows the advantage of the strong and rapidly rising Cu(d)/Se(p) $\rightarrow$ Ga/In(s/p) valence-to-conduction-band absorption spectra known from CIGS. This constitutes a new group of CuIn-based Halide Perovskite (CIHP). Our first-principles calculations guided by such design principles indicate that the CIHPs class has members with clear thermodynamic stability, showing rather strong direct-gap optical transitions, and manifesting a wide-range of tunable gap values (from zero to about 2.5 eV) and combination of light electron and heavy-light hole effective masses. Materials screening of candidate CHIPs then identifies the best-of-class Rb$_2$[CuIn]Cl$_6$, Rb$_2$[AgIn]Br$_6$ and Cs$_2$[AgIn]Br$_6$, having direct band gaps of 1.36, 1.46 and 1.50 eV, and a theoretical spectroscopic limited maximal efficiency comparable to chalcopyrites and CH$_3$NH$_3$PbI$_3$.

• The paper introduces a transmutation strategy that replaces toxic Pb2+ with Cu+/Ag+ and Ga3+/In3+ to develop lead-free perovskite absorbers.
• It employs first-principles calculations to identify stable A2BCX6 compounds with optimal direct band gaps ranging from 1.36 to 1.50 eV for solar absorption.
• Results indicate practical potential with spectroscopic limited maximum efficiencies near 30% at a 2 µm film thickness, advancing sustainable solar technology.

An Analysis of Cu-In Halide Perovskite Solar Absorbers

The paper presented by Zhao et al. proposes a promising new class of lead-free halide perovskites as potential substitutes for toxic lead-based solar absorbers, which have hindered widespread commercial application due to their chemical instability and inherent toxicity. This research is rooted in a strategic shift from conventional methods that replace lead with other similar cations, to an innovative approach leveraging electronic characteristics of the chalcopyrite Cu(In,Ga)Se2 (CIGS) solar absorbers.

In addressing the limitations of traditional lead-based perovskites, such as MAPbI3, the researchers introduce a transmutation technique where two Pb2+ cations are substituted with a pair of IB (Cu+, Ag+) and III (Ga3+, In3+) group cations. This approach is designed to address previous challenges with cation replacements seen in alternatives like Sn or Bi by offering a pathway to stable materials with favorable optoelectronic properties.

Key Findings and Methodology

Using first-principles calculations, the paper identifies halide double perovskites of the formula A2BCX6, where A = K, Rb, Cs; B = Cu, Ag; C = Ga, In; and X = Cl, Br, I. The research is rigorous in its evaluation, detailing the thermodynamic and dynamic stabilities of these compounds while also offering insights into their band structures and optoelectronic properties.

Key compounds identified for their advantageous properties include Rb2CuInCl6, Rb2AgInBr6, and Cs2AgInBr6, each exhibiting direct band gaps of 1.36, 1.46, and 1.50 eV, respectively. These values are close to the optimal range for solar absorption, comparable to the well-regarded CIGS and hybrid perovskites. Furthermore, the calculated spectroscopic limited maximum efficiencies (SLME) reveal that these materials potentially achieve efficiencies nearing 30% at a 2 µm film thickness.

Theoretical and Practical Implications

The implications of this research are twofold: theoretical understanding and practical application. Theoretically, the work extends the design paradigm for photovoltaic materials by using a comprehensive electronic structure perspective to guide compound formation, which may influence future materials science research beyond the scope of solar absorbers.

Practically, the development of non-toxic, stable, and efficient perovskite solar materials suggests a viable pathway to greener and more sustainable photovoltaic technologies. This could have significant implications in solar energy markets, pending successful synthesis and validation of these compounds.

Conclusion

By leveraging insights from both the electronic structure of established CIGS absorbers and modern computational methods, this paper offers a significant advancement in the search for reliable, lead-free photovoltaic materials. Future work will likely focus on experimental synthesis, further validating these predictions and optimizing the materials for potential commercial use. The work by Zhao et al. epitomizes a forward-thinking approach in material science, aiming to reconcile efficiency, stability, and environmental safety in solar technology.