Study On Carbon Nanomaterials And Their Photovoltaic Application

With the development of technology,the energy shortage has become one of the biggest obstacles to the intelligentization of human society.The photovoltaic devices are thought to be the most promising way to solve the energy demand and environmental problem.Recently,the third generation of photovoltaic technology has raised increasing interest due to their high efficient and low cost.Among them,polymer solar cells and perovskite solar cells have become two important branches because of their advantages of low cost,low temperature solution preparation,and flexible compatibility.Nowadays,the certificated efficiency of polymer solar cells and perovskite solar cells have reached 18% and 25.2%,respectively,which make them the most potential photovoltaic system in the future.However,many challenges such as efficiency,stability,large-scale production,and cost must be taken into account for a commercialized application.In order to solve these problems,carbon nanomaterials have drawn increasing attention due to their virtues of low cost,diverse structures,high conductivity and stability.Meanwhile,carbon nanomaterials with different dimensions can also play unique roles on the electrode,transport layer,and active layer of the device.The defects of photovoltaic devices can be overcome by designing carbon nanomaterials and optimizing the device.The carbon nanomaterial offers an abundant candidate for manufacturing novel electrodes for photovoltaic devices.In Chapter 2,we synthesize the onion-like ordered carbon nanospheres?OLCNS?and fabricate OLCNS/Ag counter electrodes for polymer solar cells.For this carbon counter electrode,OLCNS graphite is contained as the main components,and an ultrathin Ag is used as bridge to fill up the mesoporous and improve the adhesion of OLCNS.Compared with traditional metal electrode,our OLCNS/Ag electrodes reduce the dosage of precious Ag.Meanwhile,OLCNS/Ag counter electrodes improve charge extraction capacity of electrode owing to the high conductivity of carbon materials.Additionally,OLCNS/Ag nanocomposite layer regulates the optical electric field distribution of the device,which increases the light harvesting of devices.As a result,the improved efficiency of 9.81% for PTB7:PC₇₁BM and 6.95% for PCDTBT:PC₇₁BM and good device stability are achieved.In Chapter 3,we tend to reduce their dimensions and further explore the application of carbon nanorods due to the poor solubility of 3-dimensional carbon spheres.Herein,the antimony?Sb?doped carbon nanorods?Sb-CNRs?are prepared by ball-milling of CNTs and incorporated into PCBM as 1-dimensional electron channel to accelerate charge collectron in perovskite solar cells.The N-type doping effect of Sb-CNRs extends the built-in electric field between perovskite and PCBM,which facilitates electron/hole-pair separation.Meanwhile,Sb-CNRs generate many discontinuous bands in PCBM,which promote electrons transfer through 1-dimensional Sb-CNRs network.Therefore,the high efficiency of 19.26% with little hysteresis are achieved.Next,the oxidized carbon nanorods?OCNRs?with high work function are synthesized using a chemical-oxidation procedure of Sb-CNRs and doped into PEDOT:PSS as dopants.The incorporation of OCNRs improves the work function of PEDOT:PSS,which avoids the energy-level mismatch between PEDOT:PSS andperovskite,leading an increasedV_ocof 1.01 V?0.92 V for control device?.Moreover,the employment of OCNRs increases the perovskite grain size and uniformity,resulting in the charge transport improvement.As a result,the fill factor is increased from 75.4% to 81.7%,and the best efficiency of 19.02% is achieved.In Chapter 4,we further reduce their dimensions and explore the application of carbon nanodots?CDs?.Herein,a new strategy was proposed to improve the electron transfer of polymer solar cells by developing a kind of polymer-functionalized CDs electron transfer layer with surface charge engineering.CDs with sufficient hydroxyl/carboxyl were synthesize as the core by microwave process,and polyethyleneimine?PEI?self-assembled on the surface of CDs to give CDs@PEI.CDs absorb the ultraviolet light and produce low-energy blue light for reutilization.Meanwhile,the local states of CDs@PEI inhibit the leakage current under dark and accelerate electron transfer after illumination via light-induced charge filling.Therefore,the improved efficiency of 9.53% for PTB7:PC₇₁BM device is achieved.In Chapter 5,regulatory perovskite crystallization and defects passivation were achieved by doping K⁺modified carbon nanodots?CDs@K?into precursor solution.CDs bounds K⁺at the boundary and prevents them from entering into the interstitial sites,which decreases perovskite microstrain.Besides,CDs@K can tailor crystal size and suppresse grain boundary defects,which facilitate charge transfer,lengthen lifetime,and decrease trap density,leading to a boosted efficiency of 21.01% with a high fill factor of 84%.In the following,Na⁺functionalized carbon nano-dots?CNDs@Na?were designed and employed as interfacial layer of perovskite solar cells to suppresse the interfacial recombination.CNDs@Na can modify surface wettability of PTAA hole transfer layer,which improves perovskite crystal sizes and ordering.As a result,the restrained ion diffusion and improved interfacial contact reduces interfacial charge recombination,resulting in the enhanced photovoltaic performance and long-term stability of PSCs.CDs show enormous potential in chemical decoration,crystal modification,and surface passivation of perovskite materials.However,the hydrophilic/hygroscopic nature of CDs clashes with moisture sensitive perovskite.In Chapter 6,hydrophobic CDs-SAM layers are obtained by self-assembling C₃H₄Cl₃F₃Si on the top of CDs passivation layers.CDs can tailor perovskite grains size and passivate grain boundary defects,which improves the carrier lifetime and transport.Additionally,C₃H₄Cl₃F₃Si insulating layer works as tunneling junction between the perovskite and electron transport layer.This C₃H₄Cl₃F₃Si tunneling junction can conduct electrons and block holes,which accelerates photo-generated carrier separation and suppresses their recombination.Consequently,the optimized devices deliver the increased efficiency of 21.12% with an improved fill factor of 82.86%.Besides,hydrophobic CDs-SAM layers protect the perovskite solar cells from the moisture,which enhanced the stability by maintaining 90% device efficiency for over 30 days' exposured in air.