| Polymer solar cells (PSCs) have been intensively investigated in recent years due to their advantages of low cost, light weight, and flexibility. Currently, the bulk-heterojunction (BHJ) PSCs, in which an intermingled network of a polymer as the donor semiconductor and a fullerene derivative as the acceptor semiconductor, have demonstrated power conversion efficiencies (PCEs) exceed 10%. The progress of the polymer donor materials plays a key role in boosting the performances of the PSCs devices. From the optical bandgap perspective, there are wide-bandgap (>1.8 eV), medium-bandgap (1.6-1.8 eV) and narrow-bandgap (<1.6 eV) polymers, and the synergistic effect of the defferent bandgap polymers should contribute to the developments of the PSCs.In this thesis, we designed and synthesized a series of new polymers with different optical bandgap as the donor materials of the active layer.In chapter one, we introduced the working mechanism and the associated parameters of the PSCs. Then, we reviewed the latest developments of the polymer donors according to the bandgap of the polymers, which are divided into wide, medium and narrow bandgap polymers. The design strategies and main contents of the thesis are outlined.In chapter two, we designed and synthesized a new weak electron-donor unit, 4-(2-octyldodecyl)-dithieno[3,2-b:2',3'd]pyridin-5(4H)-one (DTPO). The homopolymer of PDTPO and conjugated polymers of PDTPO-BDTO and PDTPO-BDTT based on DTPO were synthesized. The optical bandgaps of PDTPO, PDTPO-BDTO and PDTPO-BDTT are 1.86,2.02 and 1.95 eV, respcetively, and the HOMO levels of the three polymers are-5.53,-5.38 and-5.44 eV, respectivley. The homopolymer of PDTPO revealed a high hole mobility of 0.19 cm2 V-1 s-1 in field-effect transistor. The photovoltaic properties of PDTPO-BDTO and PDTPO-BDTT were characterized in PSCs devices. In conventional devices, PCE of 6.19% was achieved for PDTPO-BDTO based devices, and 6.84% for PDTPO-BDTT based devices. In inverted devices, PCE of 6.84% was achieved for PDTPO-BDTO based devices, and 6.61% for PDTPO-BDTT based devices. The PCE of 6.84% of both PDTPO-BDTO and PDTPO-BDTT are among the highest ever reported for wide bandgap PSCs. We further investigated the stability of the devices under ambient condition, and the stability of the inverted devices was much better than that of the conventional devices.In chapter three, we designed and synthesized a new D-A conjugated polymer P4TFBT by using mono-fluorinated benzothiadiazole (FBT) as the acceptor unit and quaterthiophene (4T) as the donor unit. Two samples of P4TFBT with the same polydispersity index but different molecular weights (low molecular weight: L-P4TFBT; high molecular weight:H-P4TFBT) were obtained. Our results reveled that the molecular weights of the polymer have a little effect on the optical and electrochemical properties. However, the molecular weights significantly affect the Jsc and FF of the polymer-based devices. The higher molecular weights lead to the higher PCE. The device based on L-P4TFBT achived a PCE of 5.80%. The device based on H-P4TFBT achived a PCE of 7.45%, which is one of the highest PCE among the mono-fluorinated benzothiadiazole (FBT) copolymers. Additionally, H-P4TFBT revealed a high p-channel organic field-effect transistor mobility of 0.39 cm2 V-1, which is the highest hole mobility among the reported mono-fluorinated benzothiadiazole based devices.In chapter four, we designed and synthesized two copolymers P4T14BO with wide optical bandgap and P4T20BO with narrow optical bandgap based on benzooxadiazole as the electron-acceptor unit and quaterthiophene as the electron-donor unit. The absorption of the polymer is red-shifted with the decreased steric hindrance through the modulation of the position and number of the alkyl of the polymer. The maximum absorption of the P4T20BO with little steric hindrance showed a 136 nm of redshift compared to that of P4T14BO with large steric hindrance, and the bandgap was 1.60 eV for P4T20BO, and 2.06 eV for P4T14BO. The HOMO level was increased from -5.44 eV for P4T14BO to -5.25 eV for P4T20BO. The hole mobility of P4T20BO was measured by the space-charge-limited current (SCLC) method, and a mobility of 1.49×10-4 cm2 V-1 s-1 was demonstrated. The photovoltaic properties of P4T20BO was investigated in PSCs devices, and a medium PCE of 4.44% was achieved.In chapter five, we designed and synthesized four medium bandgap copolymers PNBT?PNFB?PN2FBT and PB2FBT with benzene or naphthalene as the electron-donor unit and fluorinated benzothiadiazole as the electron-acceptor unit. The optical bandgaps of the four polymers are 1.77?1.77?1.81 and 1.75 eV, which belong to medium bandgap polymers. The HOMO levels of the four polymers are-5.46,-5.49,-5.43 and -5.22 eV. The naphthalene-based copolymers with ?-extended conjugation length compared to the benzene-based copolymer exibited deeper HOMO energy levels. The extended coplanarity and rigidity of the donor unit significantly impact the transporting properties, and the SCLC hole mobilities of the naphthalene-based copolymers are higher than that of the benzene-based copolymer at least one order of magnitude. The PCEs of the optimal devices based PNBT?PNFBT? PN2FBT and PB2FBT are 0.46,2.57,2.59 and 0.44%, respectively. The morphology of the active layer plays an important role in improving the PCE of the devices. The active layer's morphology of the PN2FBT based device is in favor of the transport and dissociation of the exctions, leading to the highest PCE.In chapter six, a new weak electron-acceptor building block, dithieno-[3',2':3,4;2",3":5,6]benzo[1,2-c][1,2,5]thiadiazole (fDTBT), was designed by applying a fusion strategy on 4,7-dithienyl-2,1,3-benzothiadazole (DTBT). A new copolymer P3TfDTBT based on fDTBT and terthiophene was synthesized. P3TfDTBT has a wide bandgap of 1.98 eV due to the weak electron-accepting capacity of the fDTBT unit. P3TfDTBT own a deep HOMO level of -5.48 eV due to the high degree of planarity of the fDTBT unit, which offers enhanced electron delocalization. In conventional devices, the PSCs based on P3TfDTBT as the electro-donor and PC71BM as the electron-acceptor demonstrated a PCE of 0.71% with the P3TfDTBT/PC7iBM weight ratio of 1:2. In inverted devices, when the P3TfDTBT/PC7iBM weight ratio of the active layer is 1:1, the PSCs achieved the best PCE of 1.16%. The additive DIO was used to optimized the morphology, and the PCE based on P3TfDTBT was increased to 1.77%.In chapter seven, two narrow bandgap copolymers of PBDTO-PO and PBDTT-PO based on the strong electron-acceptor unit l,2,5-oxadiazolo[3,4-c]pyridine (PO) and BDT were designed and synthesized. PBDTO-PO and PBDTT-PO have not only narrow bandgap of 1.46 and 1.47 eV, respectively, but also deep HOMO levels of-5.40 and-5.39 eV, respectively, due to the strong electron-accepting capacity. The hole mobility of PBDTO-PO and PBDTT-PO were measured by the SCLC method, and mobility of 2.65×10-4 cm2 V-1 s-1 was achieved for PBDTO-PO and 2.24×10-6 cm2 V-1 s-1 for PBDTT-PO. The photovoltaics of PBDTO-PO and PBDTT-PO were characterized in PSCs, and a PCE of 1.49% was achieved for the PBDTO-PO based devies and 1.79% for the PBDTT-PO based devices with the polymer/PCBM weight ratio of 1:1. |
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