Study Of Photovoltaic Device Performance Based On Porphyrin Small Molecules And Asymmetric Non-fullerene Acceptors

Currently China focus on adjusting energy structure and developing clean energy and low-carbon technology in order to construct a clean,efficient,safe and sustainable modern energy system.Organic solar cells?OSCs?are a promising candidate for converting solar energy,stemming from tunable energy levels and charge transport properties of organic photovoltaic materials in addition to the good compatibility with inkjet printing,roll-to-roll fabrication processes.By designing molecules with narrow bandgaps,the absorption region of organ active layers are broadened,which benefit to maximize the efficiency for utilization of solar energy.So far,two main types of highly efficient narrow-bandgap organic photovoltaic materials are developed,one is narrow-bandgap donor materials and the other is non-fullerene small molecular acceptors?SMAs?.Porphyrin and their derives are promising narrow-bandgap small molecular donor materials with excellent device performance through molecular engineering.While in addition to rational molecular design,photoelectric properties of active layer together with interface engineering have a great impact on the resulted device performance.Since ITIC was reported in 2015,non-fullerene acceptor with an A-D-A configuration have received tremendous progress due to constant improvement of material properties.Among them,in addition to maintaining the advantages of symmetric non-fullerene SMAs,non-fullerene SMAs with asymmetric structures may additionally exhibit more excellent properties than symmetric non-fullerene SMA counterparts,rendering them a promising class of non-fullerene for OSCs.Therefore,the studies in this thesis can be divided into three parts.In the first part we try to optimize porphyrin-based active layers and study the effects on photoelectric properties and morphology of active layers as well as device performances under various processing conditions.In the second part,we broaden application of porphyrins.We apply porphyrin-based small molecular cathode interlayer materials into polymer solar cells and study the relationship between chemical structure and property.The last part focus on non-fullerene acceptor materials with novel side chain structure and its application.By modulating side chains on central cores,changes of optical properties and morphology of non-fullerene based active layers as well as device performance are well studied.The details are shown as below:In the second section,strong electron-withdrawing materials of TFSA and LITFS are incorporated into the active layers of a porphyrin small molecule TDPPEZn P and PC₆₁BM for bulk heterojunction solar cells.While the solar cells based on the undoped devices show a power conversion efficiency?PCE?of 6.11%,doping the active layers with very low loadings of LITFSI and TFSA leads to improved PCEs of 6.85%and 7.01%,accounting for 14%and16%PCE enhancement,respectively.The improved performance is ascribed to the enhanced charge carrier transport,which is supported by charge mobility,impedance spectroscopy and transient spectroscopy analyses.In the third section,we demonstrate that methanol soaking of the active layers is efficient to improve the performance of porphyrin small molecule-based bulk heterojunction photovoltaic devices.Based on this,we fabricate porphyrin-based solar cells without a cathode interlayer?CIL-free?.An OSC usually contains a cathode interlayer?CIL?to enhance the device performance.Compared with the CIL-free devices without treatment,the champion of the CIL-free OSCs upon 15 s Me OH soaking treatment is improved by 18.7%from 7.55%to 8.96%.Methanol soaking of the active layers improves the exciton dissociation and charge carrier transport and collection in the devices.This work provides a simple approach for preparing highly efficient CIL-free organic solar cells,which can be beneficial for large-scale and commercial applications.In the fourth section,we mainly focus on three porphyrin cathode interlayer materials Br^-FNEZn P-OE,Br^-FNEZn P-TC₁₆ and SO₃^-FNEZn P-TC₁₆ with the same conjugated backbones but different ionic side chains at the fluorene units and different substituents at the porphyrin cores.Because Br^-FNEZn P-TC₁₆ and SO₃^-FNEZn P-TC₁₆ are substituted with hydrophobic alkyl-thienyls while Br-FNEZn P-OE contains more hydrophilic di-ethanediol ester phenyls at the porphyrin cores,Br^-FNEZn P-TC₁₆ and SO₃^-FNEZn P-TC₁₆ are soluble in methanol but insoluble in water while Br^-FNEZn P-OE soluble in not only methanol but also water.The bulk heterojunction OSCs based on PTB7:PC₇₁BM with Br^-FNEZn P-TC₁₆ and SO₃^-FNEZn P-TC₁₆ cathode interlayers?CILs?exhibit high power conversion efficiencies of9.07%and 8.86%,which are enhanced by 66.4%and 62.6%,respectively,with simultaneously improved open circuit voltages,short circuit currents and fill factors compared with those of the devices without any CIL?PCE:5.45%?,and even higher than those of the devices with widely used PFN CILs?PCE:7.74%?.In contrast,the OSCs with Br^-FNEZn P-OE CILs only show a slightly enhanced PCE of 6.55%with a reduced Jsc compared with that of the control devices with no CILs.We also investigate why the performance of the devices with Br^-FNEZn P-OE CILs is inferior to those with Br^-FNEZn P-TC₁₆and SO₃^-FNEZn P-TC₁₆ CILs.In the fifth section,firstly,three new small molecules based on the benzo[1,2-b:4,5-b']dithiophene?BDT?fused central core with different side-chains,namely DPBDT-4Cl,POBDT-4Cl and COBDT-4Cl,are selected to investigate the side-chain effect on the properties of nonfullerene acceptors.DPBDT-4Cl has symmetrical phenylalkyl side-chains on the central BDT unit.In order to narrow the bandgap and reduce the steric hindrance,the phenyl-alkyl chains are systematically replaced with the flexible electron-donating alkoxy side-chain?POBDT-4Cl?and alkyl side-chain?COBDT-4Cl?.As a result,POBDT-4Cl and COBDT-4Cl are characterized with asymmetry-featured side-chains.From DPBDT-4Cl to POBDT-4Cl to COBDT-4Cl,their light absorption abilities,molecular packing behaviors and crystallinity are gradually enhanced.The devices based on these three acceptors all show power conversion efficiencies?PCEs?over 11%with energy loss below0.55e V.Compared to DPBDT-4Cl,POBDT-4Cl and COBDT-4Cl obviously exhibit enhanced device performance with improved short-circuit current densities?Jsc?and fill factors?FFs?,which is mainly ascribed to their reduced charge recombination and enhanced charge transport.In addition,the COBDT-4Cl-based devices achieved a high efficiency of 13.5%.This result is among the best performance obtained from asymmetry-featured small molecules.In the sixth section,a new small molecule non-fullerene acceptor based on the benzo[1,2-b:4,5-b']dithiophene?BDT?fused central core with asymmetrical alkoxy and thienyl side-chains,namely TOBDT,is selected to construct solar cells.Alkoxy unit helps narrow the bandgap and thienyl side-chain helps enhance the intermolecular interaction.As a result,TOBDT is suitable to match the deep-lying highest occupied molecular orbital?HOMO?of polymer donor PM6.Then a strong crystalline acceptor IDIC is introduced as the third component to fabricate as-cast non-fullerene ternary devices to achieve absorption and morphology control.Addition of IDIC not only mixes well with TOBDT but modulates the morphology of the blend film,which helps to balance the charge transport properties and reduce the photo-voltage loss of ternary device.All these contribute to synergetic improvement of J_sc,V_oc and FF parameters,leading a power conversion efficiency?PCE?of14.0%for the as-cast fullerene-free ternary device.