A Facile Method to Enhance Photovoltaic Performance of Benzodithiophene-Isoindigo Polymers by Inserting Bithiophene Spacer

This study describes the synthesis and characterization of four polymers based on benzo[1,2-b:4,5-b']dithiophene (BDT) and isoindigo with zero, one, two, and three thiophene spacer groups. Results have demonstrated that the use of bithiophene as a spacer unit improves the geometry of the polymer chain, making it planar, and hence, potentially enhanced π- π stacking occurs. Due to favorable interaction of the polymer chains, enhanced absorption coefficient, and optimal morphology, PBDT-BTI, which possesses bithiophene as a spacer, revealed high current and fill factor leading to a power conversion efficiency of 7.3% in devices, making this polymer the best performing isoindigo-based material in polymer solar cells (PSCs). Also, PBDT-BTI could still maintain efficiency of over 6% with the active layer thickness of 270 nm, making it a potential candidate for a material in printed PSCs. These results demonstrate that the use of thiophene spacers in D-A polymers could be an important design strategy to produce high-performance PSCs

A transparent electrode based on solution-processed ZnO for organic optoelectronic devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm-1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices

Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells

The work presented in this thesis focuses on the two critical issues of bulk-heterojunction polymer solar cells: morphology of active layers and energy loss during charge transfer process at electron donor/acceptor interfaces. Both issues determine the performance of polymer solar cells through governing exciton dissociation, charge carrier recombination and free charge carrier transport. The morphology of active layers (spatial percolation of the donor and acceptor) is crucial for the performance of polymer solar cells due to the limited diffusion length of excitons in organic semiconductors (5-20 nm). Meanwhile, the trade-off between charge generation and transport also needs to be considered. On the one hand, a finely mixed morphology with a large donor/acceptor interface area is preferred for charge generation because efficient exciton dissociation only occurs at the interface, but on the other hand, proper phase separation is needed to reduce charge carrier recombination and facilitate free charge carrier transport to the electrodes. In this thesis, morphologies of the active layers based on different polymeric donors and fullerene acceptors are correlated to the performance of solar cells with various microscopic and spectroscopic techniques including atomic force microscope, transmission electron microscope, grazing incidence x-ray diffraction, photoluminescence, electroluminescence and Fourier transform photocurrent spectroscopy. Furthermore, methods to manipulate the morphologies of solution processed active layers to achieve high performance solar cells are also presented. Processing solvents, chemical structures of the donor and the acceptor materials, and substrate surface properties are found critically important in determining the nanoscale phase separation and performance of polymer solar cells. Optimizing morphology of active layers alone does not guarantee high performance devices. In addition to spatial percolation, energy arrangements of donors and acceptors are also essential due to contrary requests of the photocurrent and the photovoltage: Efficient exciton dissociation or charge transfer at donor/acceptor interfaces requires large enough energetic driving force, which is also known as energy loss for charge transfer. However, the energy loss due to charge transfer will unavoidably reduce the photovoltage. In this thesis the balance between the photocurrent and the photovoltage in polymer solar cells due to charge transfer at donor/acceptor interfaces is investigated for different active material systems. The driving force tuned by synthesizing series of polymers is determined by directly measuring the optical band gap via UV-Vis spectroscopy and probing the charge transfer recombination via electroluminescence measurements. Influences of driving force on the photocurrent and the photovoltage are characterized via field dependent photoluminescence and internal quantum efficiency measurements. The results correlated well with the performance of the solar cells

An alternating copolymer of fluorene donor and quinoxaline acceptor versus a terpolymer consisting of fluorene, quinoxaline and benzothiadiazole building units: Synthesis and characterization

An alternating polyfluorene copolymer based on fluorene donor and quinoxaline acceptor (P1) and an alternating terpolymer (P2) with fluorene (50 %) donor and quinoxaline (25 %) and benzothiadiazole (25 %) acceptor units were designed and synthesized for use as photoactive materials in solar cells. The presence of benzothiadiazole unit in P2 increased the optical absorption coverage in the range of 350–600 nm, which is an interesting property and a big potential for achieving improved photovoltaic performances with judicious optimization of the devices. Solar cells were fabricated from 1:4 blends of polymers-PCBM[70] using o-dichlorobenzene (o-DCB) as processing solvent, and P1 showed a power conversion efficiency (PCE) of 3.18 %, with a short-circuit current density (J SC) of 7.78 mA/cm2, an open-circuit voltage (V OC) of 0.82 V, and a fill factor (FF) of 50 % while P2 showed an overall PCE of 2.14 % with corresponding J SC of 5.97 mA/cm2, V OC of 0.84 V and FF of 42 %. In general, P2 gave lower J SC and FF presumably due to the fine domain sizes of the polymer–PCBM[70] blend as seen from the atomic force microscopy (AFM) image which might have affected the charge carrier transport

An underestimated photoactive area in organic solar cells based on a ZnO interlayer

Solution-processed ZnO is commonly used as a charge-selective interlayer between an absorber and electrode in organic solar cells. In this work, the impact of the resistance of the sol–gel grown ZnO interlayer on solar cell performance is investigated. We find that the UV-induced doping effect leads to a significantly reduced ZnO resistance, which gives rise to an underestimated photoactive area and thus overestimated short-circuit current density (Jsc) for the solar cell measured without an aperture. Moreover, we show that this so far mostly overlooked issue can be unintentionally triggered during common fabrication and characterization processes, because the UV photons flux from a solar simulator, or from a light source for encapsulating the solar cell, are already sufficient in leading to the too much increased lateral conductivity of the ZnO. Finally, we demonstrate that interlayers with rather high sheet resistance can lead to an overestimation of Jsc (e.g. by 10% for a 10 MΩ per square interlayer in a 2 mm2 device). Therefore, the validity of the argument that high-resistance interlayers do not lead to overestimated Jsc should always be carefully evaluated

Optimizing sequence structures by stepwise-feeding terpolymerization for high-performance organic solar cells

Terpolymerization is a feasible approach to optimize the device performance of organic semiconductors. Yet, since most reported terpolymers utilize a one-pot polymerization method, the regularity of the polymer backbone is severely disrupted, and the sequence structure is roughly unclear. Herein, a novel stepwise-feeding terpolymerization approach is developed to finely adjust the terpolymerization process by utilizing the merits of stereoregular donor-acceptor alternative polymerization as well as irregular random copolymerization. In the novel stepwise-feeding terpolymerization, a pre-polymer of the host polymer D18 was synthesized initially, followed by adding a third unit BDD into the polymer backbone. The obtained H9 terpolymer has a more regular main chain with a more definite sequence distribution than its analogs (H6) prepared by one-pot polymerization. As a result, the stepwise-feeding-based terpolymer H9 offered significantly reduced energy loss in the combination with the Y6 acceptor in blend, yielding a record power conversion efficiency (PCE) of up to 18.50% in the terpolymer-based organic solar cells (OSCs). The results demonstrate that stepwise terpolymerization is an effective approach to developing regular terpolymeric semiconductors for high-performance OSCs

Structural similarity induced improvement in the performance of organic solar cells based on novel terpolymer donors dagger

Terpolymers have been proven to be promising polymer donors for organic solar cells (OSCs). However, the aperiodic sequence distribution caused by random copolymerization dramatically interrupts the orderly stacking of the conjugated main chain. In this work, we reported three terpolymers by introducing Si and Cl functionalized benzodithiophene (BDT-SiCl) into a polymer PM6 matrix with F functionalized benzodithiophene (BDT-2F) by random copolymerization. BDT-SiCl has the same skeleton as BDT-2F, but only differs in functional atoms on the side chain. The structural similarity of the two building blocks not only can minimize the disturbance of the molecular orderly packing caused by random copolymerization, but also can enhance the face-on orientation of the active layer to facilitate charge transport. Moreover, the alkylsilyl and chlorine atom enable the terpolymers to have a lower HOMO level and broadened light absorption compared to PM6. As a result, the terpolymer-based OSCs achieve the best efficiency of 16.22% with a small energy loss of 0.50 eV, yielding overall improved device parameters compared to PM6-based devices. In addition, thanks to the chemical bond linkage, the terpolymer-based device has a higher thermal stability than its corresponding ternary OSC. These results demonstrate that the design concept of structural similarity is a promising strategy to construct terpolymers for high-performance OSCs

Alkylsilyl Fused Ring-Based Polymer Donor for Non-Fullerene Solar Cells with Record Open Circuit Voltage and Energy Loss

The energy loss (E-loss), especially the nonradiative recombination loss and energetic disorder, needs to be minimized to improve the device performance with a small voltage (V-OC) loss. Urbach energy (E-U) of organic photovoltaic materials is related to energetic disorder, which can predict the E-loss of the corresponding device. Herein, a polymer donor (PBDS-TCl) with Si and Cl functional atoms for organic solar cells (OSCs) is synthesized. It can be found that the V-OC and E-loss can be well manipulated by regulation of the energy level of the polymer donor and E-U, which is dominated by the morphology. A low energetic disorder with an E-U of 23.7 meV, a low driving force of 0.08 eV, and a low E-loss of 0.41 eV are achieved for the PBDS-TCl:Y6-based OSCs. Consequently, an impressive open circuit voltage (V-OC) of 0.92 V is obtained. To the best of knowledge, the V-OC value and E-loss are both the record values for the Y6-based device. These results demonstrate that fine-tuning the polymer donor by functional atom modification on the side chain is a promising way to reduce E-U and energy loss, as well as obtain small driving force and high V-OC for highly efficient OSCs

Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells

Here, we demonstrate the correlation between the surface roughness of the ZnO interlayer used as an electron transporting interlayer (ETL) in organic inverted solar cells (ISCs) and the photovoltaic performance of the ISCs. Three different surfaces of the ZnO ETL are studied in ISCs with the polymer poly­[2,3-bis-(3-octyloxyphenyl)­quinoxaline-5,8-diyl-<i>alt</i>-thiophene-2,5-diyl] (TQ1) mixed with [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as the active layer. The results obtained from these ISCs show that power conversion efficiency increases from 2.7% to 3.9% when the root-mean-square roughness of the ZnO layer decreases from 48 to 1.9 nm. Moreover, it is found that the short-circuit current density is higher in the ISC based on the smoother ZnO interlayer, with a larger donor/acceptor (D/A) interfacial area in the active layer that facilitates exciton dissociation. The reduced effective interfacial area between the photoactive layer and the ZnO interlayer with decreased ZnO surface roughness leads to an observed improvement in both fill factor and open-circuit voltage, which is ascribed to a reduced concentration of traps at the interface between the ZnO interlayer and the active layer