The possibility of
lead (Pb) contamination and the volatility of
the organic cations in the prevailing Pb-based organic-inorganic perovskite
(HP) light absorbers are the two key issues of concern in the emerging
perovskite solar cells (PSCs). The majority of the Pb-free HP candidates
that are being explored for PSCs either suffer from instability issues
and have unfavorable defect properties or have unsuitable bandgaps
for PSC applications. We report the prediction of a promising new
family of all-inorganic HPs based on the nontoxic, earth-abundant,
ultrastable Ti(IV) for use in PSCs. We show that the Ti-based HPs
possess a combination of several desirable attributes, including suitable
bandgaps, excellent optical absorption, benign defect properties,
and high stability. In particular, we show experimentally that representative
members of the Ti-based HP family, Cs2TiI
x
Br6–x
, have bandgaps that
can be tuned between the ideal values of 1.38 and 1.78 eV for single-junction
and tandem photovoltaic applications, respectively.
The power-conversion efficiency (PCE) of lead halide perovskite photovoltaics has reached 22.1% with significantly improved structural stability, thanks to a mixed cation and anion strategy. However, the mixing element strategy has not been widely seen in the design of lead-free perovskites for photovoltaic application. Herein, we report a comprehensive study of a series of lead-free and mixed tin and germanium halide perovskite materials. Most importantly, we predict that RbSnGeI possesses not only a direct bandgap within the optimal range of 0.9-1.6 eV but also a desirable optical absorption spectrum that is comparable to those of the state-of-the-art methylammonium lead iodide perovskites, favorable effective masses for high carrier mobility, as well as a greater resistance to water penetration than the prototypical inorganic-organic lead-containing halide perovskite. If confirmed in the laboratory, this new lead-free inorganic perovskite may offer great promise as an alternative, highly efficient solar absorber material for photovoltaic application.
The discovery and development of new Pb-free perovskite light-absorber materials that are eco-friendly and stable has become an active research area in the field of photovoltaics (PVs). These perovskites are being considered for possibly replacing the Pb-based organic-inorganic halide perovskites in state-of-the-art perovskite solar cells. While the recent effort in this area has led to certain progress, some scientific and technological issues still remain unresolved. Here we provide perspectives on the comprehensive understanding of perovskite toxicity/instability, followed by design strategies for new nontoxic, stable perovskites. We also envision unprecedented challenges in the processing of the promising candidate perovskites that bridges materials design and actual devices. Future research in these directions will open up new possibilities in realizing eco-friendly and stable perovskite PVs for real-world applications.
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