Sheng Dong

n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells

With the demonstration of small-area, single-junction polymer solar cells (PSCs) with power conversion efficiencies (PCEs) over the 10% performance milestone, the manufacturing of high-performance large-area PSC modules is becoming the most critical issue for commercial applications. However, materials and processes that are optimized for fabricating small-area devices may not be applicable for the production of high-performance large-area PSC modules. One of the challenges is to develop new conductive interfacial materials that can be easily processed with a wide range of thicknesses without significantly affecting the performance of the PSCs. Toward this goal, we report two novel naphthalene diimide-based, self-doped, n-type water/alcohol-soluble conjugated polymers (WSCPs) that can be processed with a broad thickness range of 5 to 100 nm as efficient electron transporting layers (ETLs) for high-performance PSCs. Space charge limited current and electron spin resonance spectroscopy studies confirm that the presence of amine or ammonium bromide groups on the side chains of the WSCP can n-dope PC71BM at the bulk heterojunction (BHJ)/ETL interface, which improves the electron extraction properties at the cathode. In addition, both amino functional groups can induce self-doping to the WSCPs, although by different doping mechanisms, which leads to highly conductive ETLs with reduced ohmic loss for electron transport and extraction. Ultimately, PSCs based on the self-doped WSCP ETLs exhibit significantly improved device performance, yielding PCEs as high as 9.7% and 10.11% for PTB7-Th/PC71BM and PffBT4T-2OD/PC71BM systems, respectively. More importantly, with PffBT4T-2OD/PC71BM BHJ as an active layer, a prominent PCE of over 8% was achieved even when a thick ETL of 100 nm was used. To the best of our knowledge, this is the highest efficiency demonstrated for PSCs with a thick interlayer and light-harvesting layer, which are important criteria for eventually making organic photovoltaic modules based on roll-to-roll coating processes.

Donor–Acceptor Copolymers Based on Thermally Cleavable Indigo, Isoindigo, and DPP Units: Synthesis, Field Effect Transistors, and Polymer Solar Cells

A series of donor-acceptor type of π-conjugated copolymers based on tert-butoxycarbonyl (t-Boc) substituted indigo, isoindigo or diketopyrrolopyrrole as the acceptor unit and a benzodithiophene derivative as the donor unit was designed and synthesized. These copolymers can be readily dissolved in organic solvents and can produce uniform films by solution deposition. Thermal treatment of copolymer films at 200 °C for 10 min resulted in elimination of t-Boc side groups in nearly quantitative yield as suggested by thermogravimetric analysis and Fourier transform infrared spectroscopy. The elimination of the bulky t-Boc side groups resulted in the emergence of N-H···O═C hydrogen bonding interactions by virtue of the lactam structures of the indigo, isoindigo and diketopyrrolopyrrole units. Of particular interests is the distinctly increased field-effect mobility of these copolymers after thermal treatment, which may arise from the enhanced coplanarity and intermolecular ordering of the indigo, isoindigo or diketopyrrolopyrrole units after elimination of the bulky t-Boc side groups. These results demonstrate that the incorporation of latent side groups provides a viable strategy to construct conjugated polymers that can attain more ordered intermolecular stacking by simple thermal treatments. On the other hand, despite the thermal cleavage of t-Boc groups can also lead to increased ordering of polymer chains when blending with [6,6]-phenyl C71 butyric acid methyl ester, the photovoltaic performances of the resulting bulk heterojunction solar cells did not obviously increase due to the serious phase separation and coarsening of the film morphology.

Quaternisation-polymerized N-type polyelectrolytes: synthesis, characterisation and application in high-performance polymer solar cells

Perylene diimide (PDI) based semiconductors with high mobility are promising electron-transporting materials (ETMs), which are used to fabricate polymer solar cells (PSCs) using roll-to-roll (R2R) processing. However, PDI-based molecular semiconductors have a strong tendency to aggregate, which hinders their use as ETMs in the fabrication of high-performance thin-film devices. Additionally, multi-layer organic opto-electronic devices require that the materials for different layers should possess orthogonal solubility. Here, we develop an in situ polymerisation method to successfully prepare ion-containing PDI-based polyelectrolytes with good water/alcohol solubility, which can enable high-performance PSCs. The doping behaviour, self-assembling and charge-transporting properties of these polyelectrolytes can be fine-tuned by their anions, which allows the fabrication of high-quality and high-mobility electron-transporting thin films for PSCs. PSCs with these polyelectrolytes can maintain high power conversion efficiencies of over 8% when the thickness of the polyelectrolyte is up to 50 nm, which offers a remarkable processing window for the mass-fabrication of PSCs using R2R techniques. Our findings on the structure–property–performance relationships of these polyelectrolytes provide insights and guidelines for the design of high-performance n-type polyelectrolytes for organic opto-electronic devices.

Supramolecular Sky-Blue Phosphorescent Polymer Iridium Complexes for Single-Emissive-Layer Organic Light-Emitting Diodes

Novel supramolecular phosphorescent polymers (SPPs) are synthesized as a new class of solution-processable electroluminescent emitters. The formation of these SPPs takes advantage of the efficient non-bonding assembly between bis(dibenzo-24-crown-8)-functionalized iridium complex monomer and bis(dibenzylammonium)-tethered co-monomer, which is monitored by (1) H NMR spectroscopy and viscosity measurements. These SPPs show good film morphology and an intrinsic glass transition with a Tg of 94-116 °C. Noticeably, they are highly photoluminescent in solid state with quantum efficiency up to ca. 78%. The photophysical and electroluminescent properties are strongly dependent on the molecular structures of the iridium complex monomers.

Cross-Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High-Performance Inverted Polymer Solar Cells

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC71 BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Donor-acceptor-type copolymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) scaffold for high-efficiency polymer solar cells.

Four donor-acceptor-type low-bandgap conjugated polymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) acceptor and different donors bridged by a bithiophene spacer have been synthesized through Suzuki or Stille polymerization reactions. Fluorene (F), carbazole (Cz), alkylidene fluorene (AF), and benzodithiophene (BDT) were selected as the donor units to produce a series of new conjugated polymers. Owing to the different electron-donating ability of the donor units, the energy levels, absorption spectra, bandgaps, and carrier mobilities of the resulting polymers were systematically tuned. Bulk-heterojunction-type polymer solar cells based on the new polymers and [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM) or [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM) were investigated and all of the devices exhibited good photovoltaic performance, with power-conversion efficiencies (PCEs) over 3 %. The best device performance was achieved by PF-C12NT, with an open-circuit voltage (Voc ) of 0.87 V, a short-circuit current density (Jsc ) of 12.19 mA cm(-2) , a fill factor (FF) of 61.36 %, and a PCE of 6.51 % under simulated sunlight (100 mW cm(-2) , AM 1.5G).

The influence of amino group on PCDTBT-based and P3HT-based polymer solar cells: Hole trapping processes

Polymer solar cells (PSCs) based on aliphatic-amino-functionalized materials presented low performance with negligibly small efficiency, the prime mechanism of which is found to be hole trapping induced by the amine end groups. We propose that such hole trapping behavior depends on the relative energetic position of the hole transport states and the trapping states. Herein, we comparatively study the photovoltaic properties of PSCs based on amino-functionalized fullerene derivative blended with poly [N-9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole)] (PCDTBT) or poly (3-hexylthiophene) (P3HT). The former polymer has a lower-positioning highest occupied molecular orbital (HOMO) level, whereas the latter has a comparable HOMO level relative to the ionization state of tertiary aliphatic amine in energy. Our investigation confirms our proposition, revealing an ultrafast trapping process in PCDTBT:amino-group-functionalized fullerene derivative film, which seriously crippled hole transport, consequently results in very poor device performance. In contrast, trapping process is almost negligible in P3HT systems.

11.2% All-Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All-polymer solar cells (all-PSCs) that contain both p-type and n-type polymeric materials blended together as light-absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular-based organic solar cells. In this work, the interfacial stability is studied by using highly stable all-polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high-glass-transition-temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all-PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.