Piezoelectric-polarization-enhanced photovoltaic performance in depleted-heterojunction quantum-dot solar cells.

Colloidal quantum-dot (QD) photovoltaics (PVs) have shown a promising future with merits of low-cost processing, tunable spectral absorption, long-lifetime hot carriers, and multipleexciton generation by a single photon. [ 1–5 ] For instance, tailoring the semiconductor bandgap of QDs by the quantum-size effect enables multijunction PVs in single-junction QD solar cells (QDSCs). [ 4 ] The ability of collecting hot carriers could theoretically raise the solar-energy-conversion effi ciency of QDSCs to as high as 66%. [ 4 , 6 ] However, the performance of QDSCs is seriously hampered by the charge-extraction/transport problem due to the material and electrical discontinuity among organic molecule-capped QDs. Endeavors have been carried out to mitigate electron-hole recombination and improve the charge-extraction effi ciency from various aspects. To alleviate charge recombination at the QD-anode interface, an ultradeep-work-function layeŕ, such as MoO 3 , was utilized to provide a back surface fi eld to repel electrons. [ 7 ] Depletion-heterojunction (DH) QDSCs have been developed with a highest solar-energy-conversion effi ciency of 7% using p-type QDs as absorbers and n-type wide-bandgap semiconductors as electron collectors. [ 8 ] The built-in electric fi eld at the p-n junction effectively facilitates the dissociation of excitons and extraction of electrons from the QD thin fi lm. [ 9 ] One critical obstacle for achieving highly effi cient DH QDSCs is the divergence between their effective regions of charge extraction (effective depletion width ≈ 100–50 nm) and light absorption ( ≈ 1–2 μ m thick fi lm to fully absorb abovebandgap solar illumination). [ 10 ] Addressing this issue requires either a wider and steeper depletion region in the QD layer or a well-designed photomanagement structure of the chargecollecting electrode. Depletion bulk-heterojunction (DBH) QDSCs were exploited to reduce the compromise between light absorption and charge extraction using a porous charge collector. [ 11 ] However the very large QD/electrode interfacial area signifi cantly increased the bimolecular recombination rate and thus reduced the charge-extraction effi ciency. A great number of studies have shown that the interfacial electronic band structure can be engineered by the permanent polarization induced by ionic displacement (e.g., the ferroelectric or piezoelectric effect). [ 12–17 ] This is typically implemented