Tailored PEDOT:PSS hole transport layer for higher performance in perovskite solar cells: Enhancement of electrical and optical properties with improved morphology

Precise control over the charge carrier dynamics throughout the device can result in outstanding performance of perovskite solar cells(PSCs).Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)is the most actively studied hole transport material in p-i-n structured PSCs.However,charge transport in the PEDOT:PSS is limited and inefficient because of its low conductivity with the presence of the weak ionic conductor PSS.In addition,morphology of the underlying PEDOT:PSS layer in PSCs plays a crucial role in determining the optoelectronic quality of the active perovskite absorber layer.This work is focused on realization of a non-wetting conductive surface of hole transport layer suitable for the growth of larger perovskite crystalline domains.This is accomplished by employing a facile solventengineered(ethylene glycol and methanol)approach resulting in removal of the predominant PSS in PEDOT:PSS.The consequence of acquiring larger perovskite crystalline domains was observed in the charge carrier dynamics studies,with the achievement of higher charge carrier lifetime,lower charge transport time and lower transfer impedance in the solvent-engineered PEDOT:PSS-based PSCs.Use of this solventengineered treatment for the fabrication of MAPbI3 PSCs greatly increased the device stability witnessing a power conversion efficiency of 18.18%,which corresponds to^37%improvement compared to the untreated PEDOT:PSS based devices.

Nanoscale control of grain boundary potential barrier, dopant density and filled trap state density for higher efficiency perovskite solar cells

In this work,grain boundary(GB)potential barrier(ΔφGB),dopant density(Pnet),and filled trap state density(PGB,trap)were manipulated at the nanoscale by exposing the fabricated perovskite films to various relative humidity(RH)environments.Spatial mapping of surface potential in the perovskite film revealed higher positive potential at GBs than inside the grains.The averageΔφGB,Pnet,and PGB,trap in the perovskite films decreased from 0%RH to 25%RH exposure,but increased when the RH increased to 35%RH and 45%RH.This clearly indicated that perovskite solar cells fabricated at 25%RH led to the lowest average GB potential,smallest dopant density,and least filled trap states density.This is consistent with the highest photovoltaic efficiency of 18.16%at 25%RH among the different relative humidities from 0%to 45%RH.

Synergistic engineering of hole transport materials in perovskite solar cells

In this work,methylammonium lead triiodide(CH3NH3PbI3)perovskite solar cells with efficiencies higher than 18%were achieved using a new nanocomposite hole transport layer(HTL)by doping poly(ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)with a mixed dopant of polyaniline(PANI)and graphene oxide(GO).A synergistic engineering between GO,PANI,and PEDOT:PSS was accomplished to introduce additional energy levels between perovskite and PEDOT:PSS and increase the conductivity of PEDOT:PSS.Kelvin probe force microscope results confirmed that adding GO to PEDOT:PSS/PANI composite significantly reduced the average surface potential.This increased the open circuit voltage(Voc)to 1.05 V for the GO/PEDOT:PSS/PANI nanocomposite perovskite solar cells from the pristine PEDOT:PSS(Voc=0.95 V)and PEDOT:PSS/PANI(Voc=0.99 V).In addition,adding PANI to the HTLs substantially enhanced short circuit current density(Jsc).This was supported by the current sensing-atomic force microscopy(CS-AFM)and conductivity measurements.The PANI doped films showed superior electrical conductivity compared with those without PANI as indicated by CS-AFM results.PANI can fill the gaps between the microflakes of GO and give rise to more compact hole transport material(HTM)layer.This led to a higher Jsc after doping with PANI,which was consistent with the incident photon-to-current efficiency and electrochemical impedance spectroscopy results.The results of X-ray diffraction(XRD)and AFM indicated the GO/PANI doped HTMs significantly improved the crystallinity,topography,and crystal size of the perovskite film grown on their surface.A higher efficiency of 18.12%for p-i-n perovskite solar cells has been obtained by adding the mixed dopant of GO,PANI,and PEDOT:PSS,demonstrating better stability than the pristine PEDOT:PSS cell.