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Although impressive performance has been obtained, PSCs are still far from commercial or real-life availability due to serious issues such as toxicity [15,16] and poor stability to heat, [17] oxygen, [18] moisture, [19,20] electric field, [21] and light. [18,22] The toxic nature of hybrid organic-inorganic lead halide perovskites has been traced to the presence of Pb in its chemical composition. [15,16,[23][24][25] Pb 2+ readily dissolves in water (e.g., rain water) to form a toxic solution capable of causing serious environmental pollution, harmful to human beings and the ecosystem. Besides their sensitivity to moisture, oxygen, light, electric field, or thermal stress, an existing self-degradation pathway [26,27] is also a big issue in hybrid organic-inorganic lead halide perovskites. Mixed-halide and mixed-cation perovskites have been investigated to address these issues. [28][29][30][31][32] The group IV elements, tin (Sn) [23,[33][34][35] and germanium (Ge), [34,36] have been employed as the replacements for Pb. However, the device performance through this approach has fallen short of the Pb-based ones. For example, the PCEs reported for Sn-based perovskite solar cells are usually less than 10%. [23,[33][34][35][37][38][39][40] In addition, the easy oxidation of Sn and Ge from the +2 state to the +4 state due to their high energy 5s and 4s orbitals makes them less promising for application in stable and long-term PSCs. [41] High throughput calculations also demonstrate that these substitutions are likely to compromise the ideal optoelectronic properties of MAPbI 3 . [42,43] Furthermore, low dimensional (e.g., 2D, 1D, and 0D) perovskites have also been used to address the stability issues in PSCs. [44][45][46][47][48] Recently, a stabilized PCE of 21.7% resulting from a 2D/3D bilayer PSC was reported. [49] However, the highest certified PCE in a 2D-only planar PSC is 15.3%, [50] which is far below that of their 3D perovskite-based counterparts. It thus stimulates the interest to develop new classes of materials which can solve the issues of toxicity and stability while still maintaining the fascinating properties of lead-based perovskite materials.Recent theoretical calculations demonstrate that a halide double perovskite structure, A 2 B′B″X 6 , which could be formed through a replacement of two toxic Pb 2+ in the crystal lattice with a pair of nontoxic heterovalent (i.e., monovalent and trivalent) metal cations, is a promising alternative to realize high-performance, lead-free, and stable PSCs. [51,52] Although, spectroscopic limited maximum efficiency (SLME) calculations revealed an efficiency limit less than 8% for the most prominent member www.advancedsciencenews.com double perovskites with a vacancy ordered structure. [88] In particular, the unit cell axis of Cs 2 AgBiBr 6 is given to be ≈11.25 Å, [25] which is two times larger than that of MAPbBr 3 (≈5.92 Å). [89] Both Ag + and Bi 3+ occupy the B-site of the crystal lattice with slightly varied metal-halide bond lengths. The dissimilar bond lengths st...

Tandem organic light-emitting diodes (OLEDs), with multiple electroluminescent (EL) units connected electrically in series, have been fabricated. Using an optically transparent doped organic “p-n” junction as the connecting unit between adjacent EL units, excellent light out-coupling and carrier-injection properties have been realized. The luminous efficiency is found to scale almost linearly with the number of EL units in the stack, giving values as high as 32 or 136 cd/A for a three-unit tandem OLED using a fluorescent or a phosphorescent emitter, respectively.

Addressing the toxicity issue in lead-based perovskite compounds by seeking other nontoxic candidate elements represents a promising direction to fabricate lead-free perovskite solar cells. Recently, Cs 2 AgBiBr 6 double perovskite achieved by replacing two Pb 2+ with Ag + and Bi 3+ in the crystal lattice has drawn much attention owing to the convenient substitution of its chemical compositions. Herein, the dependence of the optoelectronic properties and corresponding photovoltaic performance of Cs 2 AgBiBr 6 thin films on the deposition methods of vacuum sublimation and solution processing is investigated. Compared to the vacuum sublimation based one, the solution-processed Cs 2 AgBiBr 6 shows inherently higher crystallinity, narrower electronic bandgap, longer photoexcitation lifetime, and higher mobility. The excellent optoelectronic properties are attributed to the accurate composition stoichiometry of Cs 2 AgBiBr 6 films based on solution processing. These merits enable the corresponding perovskite solar cells to deliver a champion power conversion efficiency (PCE) of 2.51%, which is the highest PCE in the Cs 2 AgBiBr 6based double perovskite solar cells to date. The finding in this work provides a clear clue that a precise composition stoichiometry could guarantee the formation of high quality multicomponent perovskite films.