Dongdong Cai

Asymmetric-Indenothiophene-Based Copolymers for Bulk Heterojunction Solar Cells with 9.14% Efficiency

The first two asymmetric-indenothiophene-based donor-acceptor copolymers (PITBT and PITFBT) are prepared through Stille coupling reactions between distannyl indenothiophene and brominated benzothiadiazole derivatives. The best performing solar cell fabricated from PITFBT exhibits a power conversion efficiency of 9.14% which demonstrates a great potential of the asymmetric indenothiophene for high-performance copolymers.

Interface control of semiconducting metal oxide layers for efficient and stable inverted polymer solar cells with open-circuit voltages over 1.0 volt.

Inverted polymer solar cells (PSCs) with high open-circuit voltages of 1.00-1.06 V are fabricated by using an indenofluorene-containing copolymer (PIFTBT8) as an electron donor material and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor material. To improve the photovoltaic performance, interface control of various low-temperature processed ZnO films as cathode buffer layers is systematically investigated for effective electron transportation, while transition metal oxides including MoO3, WO3, NiO, and Cu2O are employed as anode buffer layers for hole-extraction. Incorporation of optimized semiconducting metal oxide interlayers can minimize interfacial power losses, which thus affords large open-circuit voltages (Voc), increased short-circuit current densities (Jsc), and fill factors (FF), eventually contributing to higher power conversion efficiencies (PCEs) as well as better device stability. Due to the improved interfacial contacts and fine-matching energy levels, inverted PSCs with a device configuration of ITO/ZnO/PIFTBT8:PC71BM/MoO3/Ag exhibit a high PCE of 5.05% with a large Voc of 1.04 V, a Jsc of 9.74 mA cm(-2), and an FF of 50.1%. For the single junction inverted PSCs with efficiencies over 5.0%, 1.04 V is the largest Voc ever achieved. By controlling the processing conditions of the active layer, the Voc can further be improved to 1.05 and 1.06 V, with PCEs of 4.70% and 4.18%, respectively. More importantly, the inverted PSCs are ascertained to maintain a PCE of 4.55% (>90% of its initial efficiency) and a Voc of 1.05 V over 180 days, demonstrating good long-term stability, which is much better than that of the conventional devices. The results suggest that the interface engineering of metal oxide interlayers is an important strategy to develop PSCs with good performance.

Solution-derived poly(ethylene glycol)-TiOx nanocomposite film as a universal cathode buffer layer for enhancing efficiency and stability of polymer solar cells

Highly efficient and stable polymer solar cells (PSCs) have been fabricated by adopting solution-derived hybrid poly(ethylene glycol)-titanium oxide (PEG-TiOx) nanocomposite films as a novel and universal cathode buffer layer (CBL), which can greatly improve device performance by reducing interface energy barriers and enhancing charge extraction/collection. The performance of inverted PSCs with varied bulk-heterojunctions (BHJs) based on this hybrid nanocomposite CBL was found to be much better than those of control devices with a pure TiOx CBL or without a CBL. An excellent power conversion efficiency up to 9.05% under AM 1.5G irradiation (100 mW·cm−2) was demonstrated, which represents a record high value for inverted PSCs with TiOx-based interface materials.

Diindenocarbazole-based large bandgap copolymers for high-performance organic solar cells with large open circuit voltages

Three donor–acceptor alternating copolymers abbreviated as PC1, PC2, and PC3, respectively, were designed and synthesized using diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) units. Through backbone manipulation, copolymers with large bandgaps (∼2.0 eV) and deep-lying HOMO energy levels (below −5.41 eV) were obtained. The side chains were also investigated to tune the intermolecular interactions and morphology of the copolymers blended with PC71BM. Polymer solar cells (PSCs) based on PC2 : PC71BM exhibit an outstanding power conversion efficiency (PCE) of 7.26%, which represents one of the highest PCEs ever reported for PSCs while combining an open-circuit voltage (Voc) of 0.93 V and a large optical bandgap of 2.01 eV. Under similar device fabrication conditions, regular PSCs based on PC1 and PC3 achieve PCEs of 2.45% and 6.68%, respectively. Moreover, inverted PSCs derived from PC2 also exhibit an attractive PCE of 6.17% with a high Voc of 0.92 V. In view of its similar optical profiles to P3HT, but a deeper-lying HOMO energy level, PC2 should be a promising candidate as a short wavelength absorbing material for tandem solar cells.

Novel ladder-type heteroheptacene-based copolymers for bulk heterojunction solar cells

Four semiconducting copolymers were designed and synthesized based on a novel ladder-type heteroheptacene building block, which was prepared in four steps from 3,7-dibromodibenzo[b,d]thiophene. The copolymers were characterized by 1H NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Field effect transistors (FETs) fabricated from these four polymers exhibit semiconducting behavior in air. The highest mobility of 2.21 × 10−4 cm2 V−1 s−1 was found in the polymer copolymerized from the ladder-type heteroheptacene and terthiophene. Low band-gap donor–acceptor copolymers with a deep-lying HOMO energy level were synthesized by a copolymerization between the ladder-type heteroheptacene donor and the 2,1,3-benzothiadiazole acceptor. Polymer solar cells made from one of these polymers gave a power conversion efficiency of 4.17%, a current density of 9.2 mA cm−2, an open-circuit voltage of 0.79 V, and a fill factor of ∼57.4% under 100 mW cm−2 AM 1.5 G sunlight illumination.

Improving the photovoltaic performance of ladder-type dithienonaphthalene-containing copolymers through structural isomerization

A ladder-type angular-shaped dithienonaphthalene (aDTN), an isomer of ladder-type linear-shaped dithienonaphthalene (DTN), was designed and synthesized as an electron-rich unit to construct donor–acceptor copolymers with deep-lying highest occupied molecular orbital (HOMO) energy levels. Benzo[c][1,2,5]thiadiazole (BT) with various substituents were used as electron deficient units for synthesizing the target copolymers (PaDTNBTO, PaDTNBTH, and PaDTNBTF) via the Stille coupling reaction. Incorporating different substituents onto the BT moiety has significant effects on the photophysical and electrochemical properties of the copolymers, as well as on the roughness of the polymer/PC71BM blends. With four solubilizing alkyl chains on the aDTN unit, all its three copolymers have good solubility in common solvents. The synthesized copolymers exhibit deep-lying HOMO energy levels, leading to high open circuit voltages (Voc ≥ 0.90 V) of the resulting polymer solar cells. The bulk heterojunction solar cell based on the aDTN-containing copolymers (PaDTNBTO) shows an improved efficiency of 6.44% and an increased Voc of 0.92 V compared to that based on the linear-shaped DTN containing counterpart (efficiency = 4.78%, Voc = 0.86 V). Whereas, under the same device fabrication conditions, PaDTNBTH- and PaDTNBTF-based devices exhibit efficiencies of 5.22% and 1.73%, respectively. Our results demonstrate that aDTN is a better building block in constructing p-type copolymers for high open circuit voltage devices compared to the linear-shaped DTN.

A ternary conjugated D–A copolymer yields over 9.0% efficiency in organic solar cells

Ternary conjugated D–A copolymers are designed and synthesized by using indenothiophene and benzodithiophene as the weak and the strong donors, respectively, and fluorinated benzodithiazole as the strong acceptor. The best performance copolymer delivers a high PCE of 9.08%, which is a record efficiency for ternary conjugated D–A copolymers reported to date.

Long lifetime stable and efficient semitransparent organic solar cells using a ZnMgO-modified cathode combined with a thin MoO3/Ag anode

Semitransparent organic solar cells (STOSCs) have shown great promise as advanced window integrated photovoltaics for architectural and automotive utilization. In the applications, STOSCs should possess both long-term stability and high power conversion efficiency (PCE). Here, long lifetime stable and efficient STOSCs with good transparency color perception are demonstrated by combining a transparent ZnMgO-modified cathode with thin MoO3/Ag anodes. The resulting devices exhibit high PCEs (6.83–8.15%) with tunable average visible transmittances (21.6–3.8%) and good color perception close to white light. The combination of the ZnMgO-modified cathode with the MoO3/Ag anode in the STOSCs leads to long-term device shelf lifetime due to their barrier effects for oxygen and water. The STOSCs can maintain over 90% of their original PCEs over two-month storage under ambient conditions. More importantly, record high PCEs of 7.08% and 7.02% were retained for the STOSCs after storage for 1 year and 2 years, respectively, demonstrating the long lifetime stability for high-efficiency STOSCs. These findings offer a promising path to develop STOSCs with high efficiencies, long lifetime and good color perception towards practical applications.

Side-chain engineering of diindenocarbazole-based large bandgap copolymers toward high performance polymer solar cells

Five wide bandgap conjugated polymers based on diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) alternating units have been designed and prepared to investigate the effects of side chains on the photovoltaic performance of the polymers. All the polymers are soluble in common organic solvents and they show similar optical bandgaps of around 2.0 eV as well as deep-lying highest occupied molecular orbital (HOMO) energy levels (below −5.48 eV). Bulk heterojunction (BHJ) polymer solar cells (PSCs) using phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor material were fabricated. The side chains on the polymer backbone have a strong impact on the film morphology of the polymer:PC71BM blends. A phase separation with a relatively larger domain size was found for the polymers with longer side chains, while those with relatively shorter alkyl chains could form uniform films featuring a phase separation with a smaller domain size. Finally, PSCs based on PC1BT6:PC71BM exhibited an outstanding power conversion efficiency of 7.34% with a high open-circuit voltage (Voc) of 0.95 V. Our results demonstrate that the judicious design of side-chains is effective in improving the photovoltaic performance of DIC-based polymers. With a larger bandgap than P3HT but an improved power conversion efficiency (PCE) with a large Voc, this type of DIC-based polymer should be a promising bottom layer material for tandem solar cells.

Improved synthesis and photovoltaic performance of donor–acceptor copolymers based on dibenzothiophene-cored ladder-type heptacyclic units

We have developed a facile synthetic route to a ladder-type donor unit (SDCT) wherein two outer thiophene subunits are covalently fastened to the central dibenzothiophene core through two sp3-hybridized bridging carbons. An innovative transformation from an aryl ketone group to an aryl ester group was applied to construct the ladder-type molecular skeleton, and the overall synthetic yield toward the donor unit has been significantly improved by choosing aryl side chains instead of aliphatic ones to avoid competing dehydration reactions. To reveal the effects of π-spacers and heteroatom substituents, three donor–acceptor (D–A) copolymers containing SDCT and acceptor units of 2,1,3-benzothiadiazole (BT), 2,1,3-benzoxadiazole (BO), or 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DTBT) were synthesized, characterized and used for polymer solar cells (PSCs). All polymers exhibit blue-shifted absorption spectra and deeper-lying HOMO energy levels compared to the previous carbazole-based skeleton analogues. In comparison with its analogous polymer with the same π-conjugated backbone, the polymer with alkoxy-substituted BT as the acceptor unit (PSBT) shows an order of magnitude higher OFET mobility (1.8 × 10−4versus 1.25 × 10−5 cm2 V−1 s−1). An optimal device based on PSBT : PC71BM (1/3 in wt%) delivers a respectable PCE of 5.18% and a high Voc of 0.94 V, all of which are superior to those of the carbazole-based analogue (PCE = 3.7%, Voc = 0.80 V) and greatly surpass the values of a previously reported dibenzothiophene-based polymer (PCE = 0.76%, Voc = 0.64 V). These results demonstrate that SDCT is a promising building block for constructing photovoltaic polymers and the synthetic strategy developed herein can be used to prepare other dibenzothiophene-cored ladder-type heptacyclic units.