Study On Carrier Trapping Effect And Relevant Device Physics In Organic Solar Cells

Along with a growing world-wide population,mankind's demand for energy supply is increasing every day.To solve the conflicts between energy shortage and the increasing energy demand,scientific and industrial field are actively exploring new ways of energy utilization,which includes tremendous research on solar cells.Until now,solar cells have been developed to the third generation based on solution processing.As a typical representative,bulkheterojunction?BHJ?organic thin film solar cells play critical roles in advancing 3^rdgeneration photovoltaics revolution due to their unique advantages of low-cost,solution processability,high throughput roll-to-roll production.Thanks to the deep exploration of relevant molecular engineering,interfacial engineering and device processing in organic solar cells,large number of novel,highly-efficient donor/acceptor materials and interface modifiers have been proposed.They increase the charge generation in active layer and charge extraction capability at interface,so that the opto-electronic properties are substantially improved.Meanwhile,the rich of functional materials also promotes researchers to focus their attention on studies of inside working mechanism.Understanding and recognizing multiple complex physical processes and their limiting factors become the key to guide material design,device optimization and enhance performance.The physical processes in organic solar cells contains excitons generation/diffusion,charge transfer,polaron pair separation,separated polarons transport,polarons extraction and collection.The core thing is energy transfer that is in the medium of carrier transport between molecules.In this process,carrier is easy to be trapped by molecular localized states or interfacial defect states,transport ends.Such carrier trapping effect and concomitant charge recombination losses exist throughout,resulting in reduced carriers that are effectively transit to the external circuit thus limiting efficiency enhancement.We focus on this issue,demonstrate the influence of active layer molecular packing,transport layer/interface structural defects on carrier trapping effect in organic solar cells.We use steady-state and transient opto-electronic tests in combination with theoretical simulation to study trap states distribution,the generation,transport and recombination properties of photocarriers.Specifically,we optimize the aggregation structure of active layers that contain benzodithiophene?BDT?based donors and study related carrier dynamics.In addition,we propose novel doping and surface passivation strategies for ZnO that is thought to be very potential as electron transport layer.Ultimately,high efficiency and storage stability are achieved in several blend systems.This thesis is divided into the following three parts,which corresponds to the contents in Chapter Two,Three and Four.In first prat,we systematically optimize the aggregation structure of active layer containing conjugated polymer PBDT-DTffBT and PC₇₁BM by controlling polymer aggregate state in solution,solvent evaporation rate and film post-treatment.Results show that spin-coating hot solution to prevent large amorphous phase from forming is the prerequisite for device fabrication,which is related to the temperature-dependent aggregation behaviour of PBDT-DTffBT.The application of DIO additive results in nanofibers and interpenetrating network formation.Further thermal annealing treatment promotes more orderly?-?and lamella stacking,and preferential face-on orientation of polymer domains.By multiple steady-state and transient opto-electronic test,in combination with numerical simulation,we found that these structural advantages push density of tail states distribution towards low energy region.This increases carrier mobility,relaxes transport dispersion and suppresses carrier recombination losses.Power conversion efficiency of 9.03%is obtained in conventional structure.Further matching vertical phase separation,ZnO/PBDT-DTffBT:PC₇₁BM/Mo O₃/Ag based device affords high fill factor of75%and efficiency of 9.72%.We also compare the properties of a series of BDT based small molecules,it turns out to be that the more conjugated the molecule the better crystallinity of active layer.Phase separation size and continuity are improved with increased hole mobility by one order of magnitude.Small molecules-based devices afford open-circuit voltage of 1.04 V and fill factor of 65%.In second part,aiming at the issue that surface structural defects of ZnO nanocrystals degrade the device performance,we proposed plasmonically sensitized ZnO nanocrystals as electron transport layer.To this end,we synthesize anisotropic Au nanorods?AuNRs?for bulk doping of ZnO film.The photoluminescence?PL?study shows that irradiating ZnO/AuNRs composite structure only by visible light can effectively suppress the ZnO defects emission,which is attributed to the plasmonically excited AuNRs generate“hot electrons”,electrons transfer to ZnO matrix and fill their trap states.Carrier mobility,transient photocurrent,charge extraction by linearly increasing voltage?CELIV?measurements shows that the trap-filled devices exhibit better charge transport and longer carrier lifetime.Besides,AuNRs preserve light scattering effect thus enhance light trapping of active layer.The synergistic electrical and optical enhancement make the power conversion efficiency of a low-bandgap PTB7 based solar cell increase from 7.91%to9.36%.The storage stability of device is obviously improved.In third part,controllable deposition of ultra-thin polyethylenimine?PEI?modifier is achieved by using N,N-Dimethylformamide?DMF?post-treatment strategy.The application of DMF effectively passivates structural defects on low-temperature processed ZnO surface,demonstrating champion efficiency of 10.35%based on several blend systems.Results indicates that re-dissolving PEI and strong hydrogen bound between PEI and DMF induce PEI re-construction on ZnO surface,form a smooth ultra-thin PEI sheet with good substrate coverage.This increases PEI-ZnO interaction thus provides more thorough passivation of the exposed dangling bonds,oxygen vacancies and absorptive O₂.DMF treatment also increases surface dipole effect of PEI,which not only contributes to electron injection from active layer into ZnO,but also depletes hole density at the cathode side.The carrier recombination thus reduces.Furthermore,due to removing the requirement of exact thickness control of the PEI layer,the ZnO/PEI/DMF cathode would be suitable for mass production.