Perovskite Twin Solar Device with Estimated 50% Bifacial PCE Potential   and New Solar Material Options
  There are recent investigations regarding tandem solar cells with a top perovskite cell and a bottom silicon one with a potential of > 25% power conversion efficiency. Because of still high production costs of silicon cells it is believed that this tandem cell does not satisfy future requirements. Here the construction of a low-cost FAPbI3 twin solar cell is proposed with assumed PCE of 30%. With an ingenious rear illumination even 50% bifacial power conversion efficiency should be feasible. Importantly, a single twin cell can deliver the minimum potential difference of 1.7 V to conduct water splitting in practice. Avoiding additional electrodes, the twin cell device may be expanded by a large band gap sensitizer film at the mirror plan, derived from known electrically isolating phosphor storage materials that can capture otherwise wasted high-energetic radiation. Further, the substitution ot TiO2 (rutile) by ferroelectric and photo-catalytically active Bi2SiO5 with its comparable energy gap is sugested. CuO1-x could serve as new back electrode material due to its high electric conductivity. In addition, an environmentally benign (Cs,FA)2(Na,Cu,Ag)Bi(I,Br)6 elpasolite solar absorber material is discussed. Alternatively, pyroelectric hexagonal bismuth sulfide iodide with its complex superstructure may deliver a promising multiple band gap feature with suggested singlet fission capability as intrinsic property to overcome its lower efficiency. 
Hans Hermann Otto
cond-mat.mtrl-sci