High-efficiency Carrier Collection In Bulk-heterojunction PbS Colloidal Quantum Dot Solar Cells

Lead sulfide（PbS）colloidal quantum dot solar cells（CQDSCs）are promising photovoltaic technology due to their long-term air stability,multiple exciton generation and room-temperature solution processability.In the ordered bulk heterojunction（OBH）PbS CQDSCs employing Zn O nanowires（Zn O NWs）as electron transport layer（ETL）,the interpenetration of Zn O NWs and PbS light-harvesting layer capped with tetrabutylammonium iodide ligand（PbS-I）realized the direction orthogonalization of light harvesting and electron extraction,and this could dramatically enhance minority（electron）extraction of PbS CQDSCs.Therefore the thickness of PbS CQD light-harvesting layer could be raised to over 1μm for the high-efficiency application of solar light.However,this OBH structure enlarged the distance of majority（hole）extraction,restricting hole collection.Thus the imbalance between electron collection and hole collection becomes more apparent in the OBH PbS CQDSCs compared with that in the solar cells with planar structure.For the urgent need of carrier collection,especially hole collection in OBH PbS CQDSCs,pointed researches are as follows:1.Annealing treatment was employed to promote fast carrier transport and enhance carrier collection of OBH PbS CQDSCs.On account of restricted hole transport and collection in OBH PbS CQDSCs,we proposed to improve performance by thermal annealing treatment.Mild annealing（60℃）accelerated surface oxidation doping of PbS,increased the work function of PbS from 4.8 e V to 5.0 e V,thus enhancing electric field strength and promoting carrier drift in depletion region.60℃annealing treatment boosted the short-circuit current density(J_sc)from 12.74 to 15.72m A/cm²,and OBH PbS CQDSCs exhibited a great increase of 33.4%in power conversion efficiency（PCE）.But open-circuit voltage(V_oc)degraded because of oxidation-induced traps growing charge recombination loss.These results proved mild annealing could improve carrier transport and collection.2.A new hole transport layer（HTL）CuS_x was demonstrated to improve PCE of OBH PbS CQDSCs.Much attention has been taken on electron transport layer of OBH PbS CQDSCs,while hole collection and hole transport layer（HTL）were neglected.Seeking for new HTL suitable for the PbS CQDSCs was still an open question.Herein,we reported a new p-type metal-organic material CuS_x could act as HTL of OBH PbS CQDSCs.Compared with PbS-I,CuS_x displayed a shallower valence band energy at-5.48 e V,conduction band energy at-3.19 e V and a proper Fermi level at-5.02 e V,inducing a proper band alignment at the PbS-I/CuS_x interface.This not only enhanced hole extraction proved by the increase of J_sc from 19.12 to22.33 m A/cm²,but also blocked back electron demonstrated by the extended carrier lifetime.Consequently,cells with CuS_x HTL generated a PCE of 5.3%,leading to a PCE increase of 15.2%compared with that of reference HTL-free cell（4.6%）.CuS_xwas a promoting HTL and showed great potential application in quantum-dot-based devices.3.Atomic layer deposition（ALD）an aluminum oxide（Al₂O₃）was used to passivate PbS-EDT for enhancement of OBH PbS CQDSCs.Though PbS-EDT was widely used as HTL in PbS CQDSCs and the device with PbS-EDT was reported to possess the highest PCE till now,PbS-EDT presented high surface-trap density inducing charge recombination risk and blocking hole extraction at the PbS-EDT/Au interface.Herein,we demonstrated a method to passivate the surface traps of PbS-EDT film by post-depositing ALD-Al₂O₃.This ALD-Al₂O₃ treatment efficiently decreased surface traps of PbS-EDT by～50%and successfully kept the proper band alignment at PbS-I/PbS-EDT interface for the fast hole extraction of CQDSCs.Consequently,this method allowed the efficient carrier extraction at the PbS-EDT/Au interface through suppressing trap-induced reverse Schottky barrier.A power conversion efficiency of 7.5%was finally obtained in the PbS CQDSCs with ALD-Al₂O₃.These findings suggested ALD-Al2O3 was an new passivation strategy for the post-treatment of quantum dot optic and electronic device.