Study Of Interfacial Engineering For High Performance Advanced Thin-film Solar Cells

Bulk heterojunction（BHJ）organic solar cells（OSCs）have drawn much research interest in the past decades due to their potential advantages in achieving efficient,flexible,lightweight,large-scale,and low-cost device manufacture.The rapid development of OSCs has recently led to remarkable power conversion efficiencies（PCE）of 16%by exploiting interfacial engineering,device structure optimization,and rational material synthesis.It has been demonstrated that the interfacial engineering plays a significant role in improving charge transportation and collection efficiency for the inverted OSCs.Meanwhile,perovskite solar cells（PSCs）also have attracted great scientific and technological interest due to their unparalleled properties such as solution processability,broad optical absorption,large carrier diffusion lengths,efficient ambipolar carrier transport,and very large PCEs.Improvements in both the perovskite active layer processing and composition,as well as device architecture have afforded steady PSC PCE increases from 3.8%to 24%.Because of the large carrier diffusion lengths of perovskites,the electron transporting layer（ETL）and hole transporting layer（HTL）play crucial roles in enhancing charge separation and transport as well as reducing interfacial recombination favoring charge collection at the electrodes.In this Ph.D.dissertation,4 parts are included as following:1.A novel hybrid ETL consisting of in-situ thermal reduced graphene oxide（ITR-GO）and zinc oxide（ZnO）in OSC was studied,and high performance OSC was realized.A dual-nozzle spray coating system and facile one-step in situ thermal reduction/annealing method are introduced to precisely control the components of the ETL,assemble ZnO with graphene oxide,and reduce graphene oxide into in-situ thermal reduced graphene oxide（IT-RGO）,simultaneously.The ZnO:IT-RGO hybrid ETL shows high electric conductivity,interconnecting nanostructure,and matched energy level,which leads to a significant enhancement in the power conversion efficiency from 6.16%to 8.04%for PTB7:PC₇₁BM and from 8.02%to 9.49%for PTB7-Th:PC71BM-based OSCs,respectively.This newly developed spray-coated ZnO:IT-RGO hybrid ETL based on one-step ITR/ITA treatment has the high potential to provide a facile pathway to fabricate the large-scale,fast fabrication,and high performance OSCs.Then,An innovative ETL with a nanostructure consisting of ITR-GO and pol y[（9,9-bis（30-（N,N-dimethylamion）propyl）-2,7-fluorene）-alt-2,7-（9,9-dioctyl）fluorene]（PFN）has been fabricated for inverted OSCs.A novel approach to prepare an ETL of high electronic quality by using ITR-GO as a template to modulate the morphology of the interface between the active layer and electrode and to further reduce the work function of the electrode has also been realized.This bilayer ITR-GO/PFN ETL is processed by a spray-coating method with facile in situ thermal reduction.Meanwhile,the ETL shows an excellent charge transport efficiency and less charge recombination,which leads to a significant enhancement of the power conversion efficiency from 6.47%to 8.34%for PTB7:PC₇₁BM based OSCs.These results indicate that the bilayer ITR-GO/PFN ETL is a promising way to realize high-efficiency and stable OSCs by using water-soluble conjugated polymer electrolytes such as PFN.2.Carbon nano-onions（CNOs）as a functional dopant to modify the hole transporting layers were investigated,and the stability and performance of planar perovskite solar cells were improved.Poly（3,4-ethylenedioxythiophene）polystyrene sulphonate（PEDOT:PSS）is the most widely used HTL in planar perovskite solar cells.Nevertheless,the acidic and hygroscopic property of PEDOT:PSS restricts its film conductivity and leads to the degradation of device stability.Herein,for the first time,we introduce the unprecedentedly zero-dimensional dopant of CNOs and the functionalized oxidized carbon nano-onions（ox-CNOs）to modify the PEDOT:PSS HTL.Besides the merits of high conductivity and suitable energy level,the CNOs and ox-CNOs modified PEDOT:PSS HTLs could provide a superior perovskite crystalline film with large-scale grains and orderly grain boundaries exhibiting a high surface tension with the hydrophobic property,resulting in a significant enhancement of PCE from 11.07%to 15.26%.Moreover,by suppressing the corrosion effect of PEDOT:PSS on ITO electrode,a dramatic improvement in the device stability has also been obtained.3.Combustion synthesized ZnO ETL for efficient and stable perovskite solar cells were processed,and its effect on the performace of PSC was studied.Due to the long diffusion lengths of charge carriers in the photoactive layer,a PSC device architecture comprising of an ETL is essential to optimize charge flow and collection for maximum performance.Here we report a novel approach to low temperature,solution-processed ZnO ETLs for PSCs using combustion synthesis.Due to the intrinsic passivation effects,high crystallinity,matched energy levels,ideal surface topography,and good chemical compatibility with the perovskite layer,this combustion-derived ZnO enables PCEs approaching 17-20%for three types of perovskite materials systems with no need for ETL doping or surface functionalization.4.The modification of non-conjugated small-molecule electrolytes（NSEs）for ETL were studied,and highly efficient planar perovskite solar cells with simultaneous bottom-up interfacial and bulk defect passivation were achieved.PSC advances have pursued strategies for reducing interfacial energetic mismatches to mitigate energy losses,as well as to minimize interfacial and bulk defects and ion vacancies to maximize charge transfer.Here we introduce non-conjugated multi zwitterionic NSEs that act not only as charge extracting layers for barrier-free charge collection at planar triple cation perovskite solar cell cathodes,but also passivate charged defects at the perovskite bulk/interface via a spontaneous bottom-up passivation effect.Implementing these synergistic properties affords NSE-based planar PSCs which deliver a remarkable PCE of 21.18%with a maximum V_OC=1.19 V,in combination with suppressed hysteresis and enhanced environmental,thermal and light soaking stability.Thus,this work demonstrates that the bottom-up,simultaneous interfacial and bulk trap passivation using NSE modifiers is a promising strategy to overcome outstanding issues impeding further PSC advances.In summary,this work explored new processes and new methods for preparing carrier transport layers,and studied the effects on performance of thin-film solar cell by interface engineering,thereby improving the device performance.This dissertation will provide a facile pathway to the large-scale and fast fabrication of the OSCs and PSCs as well as high potential or low-cost manufacturing and application versatility in the future.