Test samples of a monolithic perovskite-silicon multijunction solar cell produced by the MIT-Stanford University team. Credit: Felice Frankel, MIT
Halide perovskites, with a higher band gap than silicon, have become a favorite semiconductor of scientists seeking to boost photovoltaic efficiency past the theoretical limit of silicon alone. Researchers at two American universities recently created a new method of combining perovskite and silicon to form a two-terminal, multijunction solar cell with an open-circuit voltage of up to 1.65 V (Appl. Phys, Lett., doi:10.1063/1.4914179).
The team from the Massachusetts Institute of Technology (MIT) and Stanford University layered the two types of semiconductor monolithically, with the perovskite—methylammonium-lead(II)-iodide, or CH3NH3PbI3—on top to catch the higher-energy visible photons in sunlight. In between the perovskite and the silicon sub-cell, which absorbs lower-energy infrared photons that pass through the perovskite, the scientists deposited layers of doped amorphous silicon to form an n++/p++ tunnel junction, which facilitates carrier recombination.
In addition, a layer of titanium dioxide immediately under the perovskite acts as an electrical contact for that material, with less energy loss than transparent conducting oxide used in other cells. A semi-transparent mesh of nanowires on top of the perovskite provided the other electrode.
Earlier types of “tandem” solar cells using these two semiconductors had separate circuitry for the two materials, but the architecture of the MIT-Stanford prototype allows both layers to be operated together. The stack of perovskite, TiO2 and silicon layers can be fabricated using standard deposition techniques.
Graphs of current versus voltage for the 1-cm2 prototype tandem cells revealed that the perovskite layer limits the overall efficiency of the cells to 13.7 percent, still below the record efficiency levels of either perovskite or silicon alone. The researchers suggest several ways to improve the tandem cells' efficiency: better hole-transport material, higher-quality perovskite and improvements to the silicon fabrication part of the manufacturing process. Such changes could boost overall efficiency to 29 percent, according to the team.