Efficient Wide-Bandgap Mixed-Cation and Mixed-Halide Perovskite Solar Cells by Vacuum Deposition

Abstract

of development, 15 with a record power conversion efficiency (PCE) now exceeding 25%. 16 An important property of perovskites is the possibility of readily tune their bandgap, 17-20 making them a suitable candidate for applications in single-junction as well as multijunction solar cells, 21-24 e.g. in combination with narrow-bandgap absorbers such as Cu(In,Ga)Se2, 25-28 silicon, 29-35 or by using complementary perovskites. 36-42 Perovskite alloys of the type ASn1-xPbxI3 (where A is an organic or inorganic cation, or a mixture of them) have bandgaps in the 1.20-1.25 eV range for lead content 0.25 ≤ x ≤ 0.5. 43,44 This requires perovskite compositions with wide bandgaps in the 1.75-1.85 eV range in order to aim at perovskite-perovskite tandem devices that can exceed the theoretical efficiency limit of single junction solar cells. 21-24 Perovskite films with wide bandgaps suitable for perovskiteperovskite tandems can be readily obtained by using mixed iodide/bromide formulations, 17 and mixed A-site cations are also employed to improve the photo-and thermal stability of the compounds. 45-47 The study of wide-bandgap perovskite materials and solar cells is a booming field of research, well summarized in recent reviews 23,48 and in research articles containing some of the best performing devices to date. 49-51 In comparison with narrower-bandgap materials, 52 wide-bandgap perovskite solar cells suffer from a larger open-circuit voltage (Voc) deficit, i.e. the Voc does not scale linearly with the bandgap as predicted by the Shockley-Queisser (SQ) limit. This deviation is due to non-radiative recombination in the perovskite bulk and at the interface with the transport layers. 51,53-55 For this reason, a large number of studies aimed at developing bulk and surface passivation strategies, as well as to identify suitable transport layers and contacts. 56-60 The vast majority of studies on wide-bandgap perovskite solar cells relied on solution-processed perovskite thin-films. Vacuum deposition is an alternative method with superior control over the film thickness and composition; it is compatible with large areas and eliminates the processing concerns related with the use of solvents. 61-63 This is especially relevant for the fabrication of complex multilayer architectures, necessary for tandem solar cells. 37,64 Moreover, vacuum deposition allows the deposition of pinhole-free, uniform and smooth films. 65-68 Early reports on vacuum-deposited widebandgap perovskites used the simplest formulation, methylammonium lead iodide-bromide, MAPb(I1-xBrx)3. We showed that these type of compounds with bandgap (Eg) up to 1.7 eV (x ≈ 0.2), are stable even at high irradiance levels, and the corresponding perovskite solar cells exhibited PCE up to 15.9%. 69 When the amount of bromide is increased (x ≥ 0.3), the perovskite demixes into iodide-and bromide-rich phases in a process known as "halide segregation", 45,70,71 which can be readily monitored from the red-shifted perovskite photoluminescence (PL) spectrum. 69,72 The iodide-rich, nar...

Effects of the PbBr2:PbI2 Molar Ratio on the Formation of Lead Halide Thin Films, and the Ratio’s Application for High Performance and Wide Bandgap Solar Cells

“…In this study, we employ a mixed solution consisting of PbI 2 and PbBr 2 to improve the morphology and microstructures of lead halide films. As reported in the preparation of the photoactive layer of perovskite-based solar cells by employing mixed-halide elements [26][27][28][29], adding PbBr 2 to a PbI 2 solution can have significant impacts on the microstructure and growth of PbI 2 , depending on the molar ratio (x) of PbBr 2 to PbI 2 . For example, Br ions dissolved in PbI 2 structures modify the interlayer interaction of the sandwiches, affecting the growth of PbI 2 in the c-direction [30,31].…”

Halide-driven formation of lead halide perovskites: insight from ab initio molecular dynamics simulations

“…38,39,[42][43][44] Vapor-phase deposition has been recently used to fabricate wide-bandgap mixed-cation and mixed-halide perovskite films with suppressed halide segregation by simultaneous sublimation of four precursors. 45 Perovskite crystallization by vapor-phase deposition occurs by a deconstruction-reconstruction mechanism due to high thermal energies, while solution-processed perovskites crystallize by a dissolution-reconstruction mechanism. 46 Brenner et al proposed that the crystallization process within vapor-phase deposition is initiated by the attack to PbI 2 sheets by highly energetic MAI vapor that forms randomly oriented crystal seeds; additional energy (e.g.…”

Simple approach for an electron extraction layer in an all-vacuum processed n-i-p perovskite solar cell

“…Vacuum processing of thin films has several advantages, such as good process stability enabling large area, uniform films, and good control over the film depositions. Therefore, vacuum processed perovskites and all-vacuum processed PSCs have recently attracted considerable attention [12][13][14][15][16][17][18][19][20][21][22][23][24] . In all-vacuum processed PSCs, the p-i-n structure is more commonly used than the n-i-p structure [17,18,[22][23][24][25][26][27] .…”

“…Therefore, vacuum processed perovskites and all-vacuum processed PSCs have recently attracted considerable attention [12][13][14][15][16][17][18][19][20][21][22][23][24] . In all-vacuum processed PSCs, the p-i-n structure is more commonly used than the n-i-p structure [17,18,[22][23][24][25][26][27] . This is related to the limited choice of materials for the interfacial layer, or contact layer between the electron extraction layer (EEL) and the electrode.…”