Band matched transport layers and intrinsically stable perovskite solar cells for application to perovskite Si tandem cells

Abstract

Hybrid perovskite/silicon tandem solar cells offer low-cost alternatives to the commercially established silicon solar cells. In this thesis, we present the material and device optimizations of the subcells that can be used to fabricate tandem cells: Methylammonium Lead Halide (MAPbI3) based perovskite solar cells and Silicon/metal oxide type-II heterojunction based solar cells. Specifically, we focus on (a) improving the Voc of (MAPbI3) solar cells using band-matched polymer hole transporting layers (HTL), (b) improving the intrinsic stability of MAPbI3 by introducing Acetamidinium (AA) cation in the matrix, (c) studying the effect of Magnesium and Bromine substitution in MAPbI3, and (d) developing a Silicon/Cu2O type-II heterojunction solar cell that can act as a bottom cell in the proposed perovskite/silicon tandem solar cell. a) The fermi-splitting (in the absorber) and consequently the Voc of a thin film heterojunction cell depends on the fermi-level of the adjacent transport layers. The most widely used HTL: Spiro-OMeTAD has a HOMO of -5.0 eV, a 0.5 eV valance band maxima (VBM) offset with MAPbI3. In this part, we examine whether a p-type semiconducting polymer: Poly-4-(5-(9,9-dihexyl-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-5,6-difluoro-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (PF-DTDFBT) having a HOMO level of -5.6 eV: exactly matched to the VBM of MAPbI3 leads to the enhancement of the Voc of the cell. The increased fermi-splitting directly contributed to the improvement of Voc from 1.04 V in standard Spiro-OMeTAD HTL devices to 1.11 V in PF-DTDFBT interlayer devices. In addition, the polymer being hydrophobic leads to an increase in device stability by reducing the seepage of moisture into the active perovskite layer slowing down its degradation. b) One of the major obstacles in the commercialization of perovskites is its instability towards moisture, optical and thermal stimulus...