Organic Polymer Solar Cells Research Articles

A Theoretical Research on the Impact of Pyridine Based and Fused Cyclic Based Polymer on the Properties of Donor Polymer for Organic Solar Cells

A Theoretical Research on the Impact of Pyridine Based and Fused Cyclic Based Polymer on the Properties of Donor Polymer for Organic Solar Cells

The effect of side-chain substitution and hot processing on diketopyrrolopyrrole-based polymers for organic solar cells.

The effects of cold and hot processing on the performance of polymer-fullerene solar cells are investigated for diketopyrrolopyrrole (DPP) based polymers that were specifically designed and synthesized to exhibit a strong temperature-dependent aggregation in solution. The polymers, consisting of alternating DPP and oligothiophene units, are substituted with linear and second position branched alkyl side chains. For the polymer-fullerene blends that can be processed at room temperature, hot processing does not enhance the power conversion efficiencies compared to cold processing because the increased solubility at elevated temperatures results in the formation of wider polymer fibres that reduce charge generation. Instead, hot processing seems to be advantageous when cold processing is not possible due to a limited solubility at room temperature. The resulting morphologies are consistent with a nucleation-growth mechanism for polymer fibres during drying of the films.

Novel benzodithiophene-based polymer acceptors for efficient organic solar cells.

All polymer organic solar cells afford unique potentials due to the tunable chemical and electronic properties of both polymer donors and polymer acceptors. Compared with the rapid development of polymer donors, the development of polymer acceptors lags far behind. To seek high-performance polymer acceptors used in organic solar cells, based on the experimentally reported D-A polymer acceptor (NDI2OD-T2)n (P1), a series of novel acceptors, designated as (BDTNDI2OD-T2)n(P2), (BDTNDTI)n(P3), (BDTNDI2OD-Tz2)n(P4), and (BDTNDTzI)n(P5), were designed by introduction of a benzodithiophene (BDT) unit and the nitrogen atom in the bridged thiophene ring. The density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods were applied to study the effect of the BDT unit and the nitrogen atom on the geometrical, optical, electronic, and charge transport properties. The obtained results show that incorporation of the electron-donating BDT unit into P1 and the replacement of a carbon atom by the nitrogen atom in the bridged thiophene ring are effective strategies to lower the lowest unoccupied molecular orbital (LUMO) energy and exciton binding energy, and enhance light-absorbing capacity and electron mobility. Moreover, among the investigated molecules, P2 and P5 exhibit stronger and broader light absorption, higher light absorption efficiency and exciton separation ability as well as electron mobility; therefore they are recommended as promising polymer acceptors for future high-efficiency organic solar cells.

Furan-containing conjugated polymers for organic solar cells

Development of organic semiconductors is one of the most intriguing and productive topics in material science and engineering. Many efforts have been made on the synthesis of aromatic building blocks such as benzene, thiophene and pyrrole due to the facile preparation accompanied by the intrinsic environmental stability and relatively efficient properties of the resulting polymers. In the past, furan has been less explored in this field because of its high oxidation potential. Recently, furan has attracted obsession due to its weaker aromaticity, the greater solubilities of furan-containing π-conjugated polymers relative to other benzenoid systems and the accessibility of furan-based starting materials from renewable resources. This review elaborates the advancements of organic photovoltaic polymers containing furan building blocks. The uniqueness and advantages of furan-containing building blocks in semiconducting materials are also discussed.

Effect of bisalkylthio side chains on benzo[1,2-b:4,5-b′]dithiophene-based polymers for organic solar cells

The length and conformation of solubilizing side chains in π-conjugated polymers impact polymers' self-assembling properties in thin-film devices. To reveal the effect of different bisalkylthio side chains, four copolymers containing bisalkylthio side chain modifying two-dimensional benzo[1,2-b:4,5-b′]dithiophene (BDT) donor with (5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BDff) acceptor have been developed and studied in this report. The different absorption maxima (λmax) of PBDTT-DS1-BTff, PBDTT-DS2-BTff, PBDTT-DS3-BTff and PBDTT-DS4-BTff are 578, 574, 591 and 594 nm in CHCl3 solution, respectively, indicating different aggregation behaviors. The devices based on PBDTT-DS1-BTff didn't reveal high PCEs due to its poor miscibility with PC71BM compared to those with other polymers. The devices blending PBDTT-DS2-BTff with PC71BM exhibited a highest PCE of 5.82%, with an enhanced JSC of 11.06 mA cm−2, a VOC of 0.92 V and a FF of 57.2%. This work demonstrates that suitable side chain lengths in π-conjugated polymers are of great significance in optimizing the performance of polymers based solar cells.

Catalyst: Chemistry’s Role in Providing Clean and Affordable Energy for All

Image Leif Hammarstrom is a professor of chemical physics at Uppsala University, Sweden. He is one of the leaders of the Swedish Consortium for Artificial Photosynthesis, founded in the mid-1990s. He is chair of the Swedish Solar Energy Platform and represents Uppsala University as a core member of the Solar Fuels Institute.

Hexa-peri-hexabenzocoronene and diketopyrrolopyrrole based D-A conjugated copolymers for organic field effect transistor and polymer solar cells

Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped conjugated molecule with strong π-π stacking property, high intrinsic charge mobility and good self-assembly property. But for a long time, the organic photovoltaic (OPV) solar cells based on HBC small organic molecules demonstrated low power conversion efficiencies (PCEs). In this study, a series of polymers named as PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT were designed and synthesized through copolymerization of HBC with bulky mesityl substituents and strong electron-withdrawing diketopyrrolopyrrole (DPP) with different alkyl side chains and various π-bridges. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowered the band gaps. Bulky mesityl substituents attached to periphery of HBC prevented polymers from self-aggregating into too large domain size in the blend films of photovoltaic devices. The different π-bridges have significant effect on the structure conformation of the polymers. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated the broadest absorption for its extensive π-conjugation and more coplanar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave field-effect hole mobilities of 1.35 × 10−3, 2.31 × 10−4, 2.79 × 10−4 and 8.60 × 10−3 cm2 V−1 s−1, respectively. The solar cells based on these polymers displayed PCEs of 2.12%, 2.85%, 1.89% and 2.74%. To our knowledge, 2.85% is the highest PCE for the HBC-based photovoltaic materials till now.

Quaternary Organic Solar Cells Enhanced by Cocrystalline Squaraines with Power Conversion Efficiencies >10%

The incorporation of multiple donors into the bulk‐heterojunction layer of organic polymer solar cells (PSCs) has been demonstrated as a practical and elegant strategy to improve photovoltaics performance. However, it is challenging to successfully design and blend multiple donors, while minimizing unfavorable interactions (e.g., morphological traps, recombination centers, etc.). Here, a new Förster resonance energy transfer‐based design is shown utilizing the synergistic nature of three light active donors (two small molecules and a high‐performance donor–acceptor polymer) with a fullerene acceptor to create highly efficient quaternary PSCs with power conversion efficiencies (PCEs) of up to 10.7%. Within this quaternary architecture, it is revealed that the addition of small molecules in low concentrations broadens the absorption bandwidth, induces cocrystalline molecular conformations, and promotes rapid (picosecond) energy transfer processes. These results provide guidance for the design of multiple‐donor systems using simple processing techniques to realize single‐junction PSC designs with unprecedented PCEs.

Analysis of diverse direct arylation polymerization (DArP) conditions toward the efficient synthesis of polymers converging with stille polymers in organic solar cells

ABSTRACTDespite the emergence of direct arylation polymerization (DArP) as an alternative method to traditional cross‐coupling routes like Stille polymerization, the exploration of DArP polymers in practical applications like polymer solar cells (PSCs) is limited. DArP polymers tend to have a reputation for being marginally inferior to Stille counterparts due to the increased presence of defects that result from unwanted side reactions in direct arylation, such as unselective C‐H bond activation and homocoupling. We report ten DArP protocols across the three major classes of DArP to generate poly[(2,5‐bis(2‐hexyldecyloxy)phenylene)‐alt‐(4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole)] (PPDTBT). Through evaluation of the method and resulting photophysical and electronic properties, we show not all DArP methods are suitable for generating device‐quality alternating copolymers. When DArP PPDTBT was synthesized in superheated THF with Cs2CO3, neodecanoic acid, and P(o‐anisyl)3, it generated polymers of exceptional quality that performed comparably to Stille counterparts in both roll coated ITO‐free and spin‐coated ITO devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2907–2918

Synthesis and characterization of a new phenanthrenequinoxaline‐based polymer for organic solar cells

ABSTRACTTwo donor–acceptor (D‐A) conjugated polymers, PQx and PphQx, composed of alkylthienyl‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTT) as the electron donor and the new electron acceptors quinoxaline (Qx) or phenanthrenequinoxaline (phQx), were synthesized with Stille cross‐coupling reactions. The number‐averaged molecular weights (Mn) of PQx and PphQx were found to be 25.1 and 23.2 kDa, respectively, with a dispersity of 2.6. The band‐gap energies of PQx and PphQx are 1.82 and 1.75 eV, respectively. These results indicate that, because phQx units have highly planar structures, their inclusion in D‐A polymers will be a very effective method for increasing the polymers' effective conjugation lengths. The hole mobilities of PQx and PphQx were determined to be 5.0 × 10−5 and 2.2 × 10−4 cm2 V−1 s−1, respectively. A polymer solar cell device prepared with PphQx as the active layer was found to exhibit a power conversion efficiency (PCE) of 5.03%; thus, the introduction of phQx units enhanced both the short circuit current density and PCE of the device. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2804–2810

DFT and TD-DFT study on the structural and optoelectronic characteristics of chemically modified donor-acceptor conjugated oligomers for organic polymer solar cells

The structural and optoelectronic properties of several substituted poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) conjugated oligomers have been investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Various electron-withdrawing and electron-donating groups were attached to the 5′ position of the benzothiadiazole unit in the PCDTBT oligomers. HOMO energies (EHOMO), LUMO energies (ELUMO), and the fundamental energy gaps (EGap) were calculated using DFT while the first singlet excited states (EOpt) were calculated using TD-DFT. Results show that the structural properties of the oligomers greatly affect the optoelectronic properties. Results also show that both electron-withdrawing and electron-donating groups are capable of lowering the EGap of PCDTBT. Several properties of substituted PCDTBT, which may affect the solar cell properties, were then calculated and compared with the calculated values for PCDTBT. From the calculated properties, PCDTBT-F showed improvement in the EGap, open-circuit voltage (VOC), and ionization potential (IP), which may lead to solar cells with improved power conversion efficiency (PCE) compared to PCDTBT.

Design of highly soluble PCDTBTBT-type conjugated polymers for organic solar cells

Two novel polymers comprising TBTBT units bearing alkyl side chains in different arrangements were synthesized and investigated as electron donor materials in organic solar cells. The presence of alkyl chains significantly improves the material solubility with simultaneous increase in the material band gap and causes minor improvement in the photovoltaic performance.

Synthesis of pyrrolidinofullerenes and their applications as an n-type component in organic transistors and polymer solar cells

We synthesized pyrrolidinofullerenes functionalized with an electron donor or acceptor group via Prato reaction. Their structures were studied using 1H-NMR, FT-IR and MALDI-TOF mass spectrometry. The electronic energy levels were calculated using quantum chemical calculation based on the density functional theory, showing that the lowest unoccupied molecular orbitals were in the range of −3.7 and −3.9 eV. The electron mobility of one of the fullerene derivatives was as high as 0.05 cm2/(V s), suggesting that pyrrolidinofullerenes can be promising n-type semiconductors. The fullerene derivatives were also blended with P3HT to fabricate bulk heterojunction polymer solar cells, and the performance of solar cells as a function of structural variations of the fullerene derivatives was investigated.

Study on the Doped 1-Bromo-4-Nitrobenzene at Active Layer in Improving the Performance of Organic Polymer Solar Cells

The paper studied the effect on the performance of organic polymer solar cells at the active layer based on P3HT/PCBM from the addition of a solid derivative of nitrobenzene, 1-Bromo-4-Nitrobenzene, carried out experiment on 1-Bromo-4-Nitrobenzene doped with different doses in P3HT/PCBM active layer, and comparatively analyzed the performance parameters, absorption intensity, external quantum efficiency, fluorescence intensity, morphology characterization, and transient absorption of devices under different doses. Results showed that the organic polymer solar cells reached the maximum energy conversion efficiency, the energy conversion efficiency was improved from 2.2% to 3.4%, with an increase of 54 %, when the additive volume of 1-Bromo-4-Nitrobenzene was 25wt%. Addition of 1-Bromo-4-Nitrobenzene helped to improve the photoelectric conversion efficiency of organic polymer solar cells.

Poly(pentacyclic lactam-alt-diketopyrrolopyrrole) for field-effect transistors and polymer solar cells processed from non-chlorinated solvents

A highly crystalline conjugated polymer incorporating two electron-deficient units was applied in high performance organic field-effect transistors and polymer solar cells.

First-principle study of the optical absorption spectra of chalcogen on D-A and D--A copolymers

D-A type copolymer as an organic polymer solar cell electronic material in recent years has attracted wide attention. In order to improve the efficiency of energy conversion, many active layer materials, especially the donor materials, have been designed and synthesized. By inducing the different donor and acceptor units, the absorption spectrum can better match with the solar spectrum and the carrier mobility can increase. In this paper, by using the density functional theory method, we investigate the electronic structures and optical absorption spectra of D-A and D--A copolymers. Benzodithiophene (BDT) as the electron donor unit, and dibenzothiophene (BT) as the electron acceptor unit are used to simulate D-A (PBDT-BX, X = O, S, Se, Te) copolymer systems; and D--A (PBDT-DTBX, X = O, S, Se, Te) structures are constructed with thiophene ring as a bridge between D and A. Firstly, our calculation results indicate that when X is replaced separately by elements O, S, Se and Te in D-A copolymers, the LUMO levels move close to the Fermi level, while the changes of the HOMO energy levels are relatively small, resulting in the band gap decreasing gradually. Then, the analysis of the density of states (DOS) shows that the contribution of LUMO comes from the BT unit and HOMO from the BDT unit. Also the difference in charge density shows that the thiophene ring enhances the charge transfer between BT and BDT. Besides, the studies of the optical absorption spectrum reveal that there appear two strong absorption peaks with the increase of atomic number of X, of which one is at about 4.0 eV and has no obvious change, and the other increases intensively and is red-shifted. Moreover, compared with the D-A structure, the D--A structure has the band gap that will decrease obviously and has a lowest value when X is Te. The optical absorption peak also increases significantly as the atomic number of oxygen group elements increases and peak position is red-shifted. The range of optical absorption peak is mainly from 703.9 to 519.4 nm. According to the absorption spectrum and DOS the optical absorption peak at about 4.0 eV is mainly contributed by the BDT unit. Overall, our findings provide a good understanding of mechanism about the red-shift of optical absorption spectra of copolymers and can serve as guidance for organic polymer design in photovoltaic cell experimentally.

Polytriacetylenes bearing directly attached functional groups with tunable physical and electronic properties

We report the synthesis and characterization of a unique class of conjugated polymers, polytriacetyelenes (PTAs), from a set of trans-enediyne (EDY) monomers bearing alkyl and aromatic substituents. Two different methods, i.e., Glaser-Hay coupling and Pd-catalyzed oxidative coupling, were employed depending on the nature of the side-chains. Our methodology is highly versatile in that it allows direct attachment of aromatic substituents to the PTA main-chains, allowing enhanced tunability in polymer electronic properties, which has not been achievable through previously reported methods. The newly synthesized PTAs displayed strong ability of quenching the fluorescence of poly(3-hexylthiophene) (P3HT), a prototypical conjugated polymer in organic solar cells, indicating possible applications of these PTAs in optoelectronic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1391-1395

Progress in the Next Generation Non‐silicon Thin Film Solar Cells

Since the breakthrough of the solar energy conversion efficiency by dye-sensitization of mesoscopic TiO2 films in early 1990s, there were huge growth and progress side-by-side in the field of sensitization-based solar cells and purely organic based thin layer hetero-junction type solar cells. These cells were continuously evolving toward higher efficiency by introducing novel strategies to develop efficient organic/inorganic absorbers and electron/hole conductors as well as the development of well-ordered nanostructure of photoelectrodes and highly catalytic and economical counter electrodes. This research encompassed the use of many inorganic quantum dots as sensitizers and it led to the initiation of a significant efficiency jump in solid-state solar cells. The recent reports of highly efficient perovskite solar cells have opened a new chapter in the next generation solar cell area, while lots of major contributions were produced in Korea. This special issue was designed to encompass the recent progress and state of the art research by leading scientists in Korea in the area of sensitization-based solar cells and organic solar cells, which were presented at the Global Photovoltaic Conference 2014 (GPVC 2014) held in Busan, Republic of Korea, in November 2014. Recent progress in the development of counter electrode for T-mediated DSSCs is discussed (Prof. Sining Yun, Xi'an University of Architecture and Technology, China) along with the development of highly transparent counter electrode (Dr. Min Jae Ko, Korea Institute of Science and Technology, Korea) and a cost-effective strategies with reduced graphene oxide based materials (Prof. Yongseok Jun, Konkuk University, Korea). Tandem strategy for high open-circuit voltage with novel cobalt-based redox couple (Prof. Hwan Kyu Kim, Korea University, Korea) and the novel strategy of core/shell type photoelectrode engineering to boost up the efficiency of DSSCs (Prof. Jin Hhyeok Kim, Chonnam National University, Korea) are presented. For organic polymer solar cells, the development of fullerene type acceptors (Prof. Do-Hoon Hwang, Pusan National University, Korea) and a strategy to enhance the thermal stability (Prof. Yong-Sang Kim, Sungkyunkwan University, Korea) are discussed. In addition to two reviews regarding the recent progress in the perovskite hybrid solar cells (Prof. Sang Hyuk Im, Kyung Hee University, Korea) and the electrochemical impedance spectroscopic analysis of sensitization based solar cells (Prof. Jae-Joon Lee, Konkuk University, Korea), a comprehensive review of R&D strategies in Korean photovoltaics (Prof. Chang-Sik Son, Silla University, Korea) is featured in this special issue. Finally, we would like to thank Dr. Ehud Keinan as Editor-in-Chief and Dr. Brian Johnson as Publisher of Israel Journal of Chemistry at Wiley-VCH for helping us edit this special issue. 1 1 1 Prof. Dr. Jae-Joon Lee Prof. Dr. Do-Hoon Hwang Prof. Dr. Hwan Kyu Kim Konkuk University Pusan National University Korea University Guest Editor Guest Editor Guest Editor

Coevaporated Bisquaraine Inverted Solar Cells: Enhancement Due to Energy Transfer and Open Circuit Voltage Control

There is currently enormous interest in the development of small molecule organic solar cells (SMSC), as in principle, these systems offer advantages over both conventional Si photovoltaics and organic polymer solar cells. Here, we report Förster Resonance Energy Transfer (FRET) enhanced inverted SMSC fabricated by coevaporating two different squaraine donors, a symmetrical squaraine (SQ, 2,4-bis-4-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine). ASSQ absorbs blue light (λmax 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (λmax 690 nm). Therefore, by utilizing a thin film containing the two squaraine donors as the active layer in a SMSC, we can both broaden the photovoltaic absorption spectrum, and reduce recombination loss as a result of FRET. This strategy has resulted in SMSC with power conversion efficiencies (PCE) which are up to 46% greater than those obtained by using a single squaraine donor. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy provide clear evidence of FRET between the small molecules, with a rapid energy transfer time of ∼1 ps. At optimal blending, which correlates to the highest PCE measured, the efficiency of energy transfer is as high as 85%. Furthermore, in the devices containing two different squaraine molecules, the open circuit voltage (VOC) is proportional to the fraction of the two donors in the blend, allowing us to predict the VOC as the ratio of the two donors is changed. SMSC with inverted structures also demonstrate long-term stability in ambient conditions compared to devices employing a conventional architecture.

Comparison of selenophene and thienothiophene incorporation into pentacyclic lactam-based conjugated polymers for organic solar cells

In this work, we compare the effect of incorporating selenophene versus thienothiophene spacers into pentacyclic lactam-based conjugated polymers for organic solar cells.