Dithienocarbazole- and benzothiadiazole-based donor-acceptor conjugated polymers for bulk heterojunction polymer solar cells

Donor-acceptor(D-A)-conjugated polymers P(BT-C1)and P(BT-C2),with dithieno[2,3-b;7,6-b]carbazole(C1)or dithieno[3,2-b;6,7-b]carbazole(C2)as D-unit and benzothiadiazole(BT)as A-unit,were synthesized.The optical bandgaps of the polymers are similar(1.84 and 1.88 e V,respectively).The structures of donor units noticeably influence the energy levels and backbone curvature of the polymers.P(BT-C1)shows a large backbone curvature;its highest occupied molecular orbital(HOMO)energy level is 5.18 e V,whereas P(BT-C2)displays a pseudo-straight backbone and has a HOMO energy level of 5.37 e V.The hole mobilities of the polymers without thermal annealing are 1.9×10 3 and 2.7×10 3 cm2 V 1 s 1 for P(BT-C1)and P(BT-C2),respectively,as measured by organic thin-film transistors(OTFTs).Polymer solar cells using P(BT-C1)and P(BT-C2)as the donor and phenyl-C71-butyric acid methyl ester(PC71BM)as the acceptor were fabricated.Power conversion efficiencies(PCEs)of 4.9%and 5.0%were achieved for P(BT-C1)and P(BT-C2),respectively.The devices based on P(BT-C2)exhibited a higher Voc due to the deeper HOMO level of the polymer,which led to a slightly higher PCE.     

Photovoltaic properties of bulk heterojunction solar cells incorporating 2-hydroxylethyl- and fullerene-functionalized conjugated polymers

We have synthesized a series of maleimide–thiophene copolymers presenting pendent 2-hydroxylethyl and fullerene units for use as photo-energy conversion materials in polymer solar cells (PSCs), which we fabricated from blends of these maleimide–thiophene copolymers and the fullerene derivative [6, 6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). A too-homogenous distribution of the 2-hydroxylethyl-functionalized copolymer and PCBM inhibited charge separation and transport in the photoactive layer. Introducing fullerenes as pendent units of the copolymer promoted the formation of phase-separated interpenetrating networks with sizable PCBM domains in the photoactive layer, favorable for transporting charges to the electrodes. The photovoltaic performance and operational stability of PSCs based on the fullerene-functionalized copolymer/PCBM blends were superior to those based on the hydroxyethyl-functionalized copolymer/PCBM blends.     

Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.     

Novel conjugated copolymers based on dithiafulvalene moiety for bulk heterojunction solar cells

New conjugated copolymers, P1‐P3 , based on dithiafulvalene‐fused entity and different conjugated segments have been synthesized. Incorporation of electron‐deficient conjugated segments into the conjugated copolymers results in red shifting the absorption band and lowering the hole mobility. Bulk heterojunction solar cells using on these polymers as the donor and [6,6]‐phenyl‐C61 ‐butyric acid methyl ester (PC₆₁BM) as the acceptor were fabricated by solution process. The cells based on the blend of P1‐P3 /PC₆₁BM (1:1, w/w) have power conversion efficiencies (PCEs) ranging from 0.53 to 0.93%. Among these, the cell of P1 /PC₆₁BM exhibited the highest open‐circuit voltage at 0.85 V, and the cell of P3/PC₆₁BM exhibited the best PCE at 0.93% with the short‐circuit current (J_SC) of 4.88 mA/cm². © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

二噻吩并吡咯-苯并噻二唑D-A型共轭齐聚物的合成、表征及其在体相异质结太阳能电池中的应用

以二噻吩[3,2-b:2',3'-d]并吡咯为电子给体单元、2,1,3-苯并噻二唑为电子受体单元.通过Stille偶联反应合成了4个含不同烷基取代基的给体-受体(D-A)型共轭齐聚物,即O-D3,O-D2P1,O-D1P2和O-P3,它们分别含有3～0个正十二烷基(D=dodecyl)和0～3个支化烷基链戊基己基(P=...     

Parallel-like bulk heterojunction polymer solar cells

Here we demonstrate a conceptually new approach, the parallel-like bulk heterojunction (PBHJ), which maintains the simple device configuration and low-cost processing of single-junction BHJ cells while inheriting the major benefit of incorporating multiple polymers in tandem cells. In this PBHJ, free charge carriers travel through their corresponding donor-polymer-linked channels and fullerene-enriched domain to the electrodes, equivalent to a parallel-like connection. The short-circuit current (J(sc)) of the PBHJ solar cell is nearly identical to the sum of those of the individual "subcells", while the open-circuit voltage (V(oc)) is between those of the "subcells". Preliminary optimization of the PBHJ devices gives improvements of up to 40% in J(sc) and 30% in overall efficiency (η) in comparison with single-junction BHJ devices.     

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

The power conversion efficiency of an organic solar cell has now exceeded the 10% mark, which is a significant improvement in the last decade. This has been made possible due to the development of low-band-gap polymers with tunable electron affinity, ionization potential, solubility, and miscibility with the fullerene acceptor, and the improved understanding of the factors affecting the critical device parameters such as the V_OC and the J_SC. This review examines the latest strategies, results, and trends that have evolved in the design of solar cells with better efficiency and durability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Self-doped n-type water/alcohol-soluble conjugated polymers ETL for high-performance polymer and perovskite solar cells

正Electron transporting layer(ETL)materials have attracted much attention in recent years in the research fields of organic/polymer solar cells(OSCs/PSCs)and planar perovskite solar cells(pero-SCs),because ETL materials play very important role in improving the photovoltaic performance of devices.Among the ETL materials reported in literatures,conjugated polymer poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN)show good film-forming property and excellent device performance for the small area PSCs.But the ETL of PFN     

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.     

New narrow-band-gap thiazoloquinoxaline-containing polymers and their use in solar cells with bulk heterojunction

Two new regioregular polymers P1 and P2 with structure of type D–A1–D–A2 have been prepared. The polymers exhibit strong light absorption in the range 300–1100 nm and have band gaps of 1.09 and 1.11 eV, respectively. The HOMO and LUMO energies for P1 and P2 are–5.08/–3.81 and–5.16/–3.85 eV, respectively. Polymer solar cells (PSC) based on P1: PC₇₁BM (1: 2, v/v) and P2: PC₇₁BM (1: 1, v/v) have open-circuit voltage V _oc, short circuit current J _sc, and efficiency of 0.79 and 0.84 V, 8.32 and 9.54 mA/cm², 3.5 and 4.7%, respectively. The PSC based on P2 exhibits higher characteristics due to the presence of fluorine atoms in the structure: their strong electron-withdrawing properties decrease the HOMO level of polymer P2 as compared with that of P1, which increases the V _oc value. Moreover, the formation of additional S???F contacts leads to the growth of ordering and crystallinity of polymer P2 as compared with P1, which favors an increase in the values of J _sc and filling factor.     

Naphthodithiophene-2,1,3-benzothiadiazole copolymers for bulk heterojunction solar cells

Two newly synthesized naphthodithiophene-based copolymers, PNB, exhibit a low optical bandgap of ～1.64 eV with which the solar cells fabricated from the blend of PNB and PC(71)BM afforded a power conversion efficiency of 5.3% with external quantum efficiency over 60% in a broad spectral range.     

Synthesis and characterization of new low bandgap polyfluorene copolymers for bulk heterojunction solar cells

Two new low bandgap alternating polyfluorene copolymers based on dioctylfluorene and donor‐acceptor‐donor monomers have been synthesized via a Suzuki polymerization reaction. The resulting copolymers have low optical bandgaps at 1.99–1.98 eV. The bulk heterojunction polymer solar cells were fabricated with the conjugated polymers as the electron donor and 6.6‐phenyl C₆₁‐butyric acid methyl ester as the electron acceptor. The power conversion efficiencies of the solar cells based on copolymers 1 and 2 are 0.37 and 0.42%, respectively, under the illumination of AM 1.5, 100 mW/cm². The results indicate that the two copolymers are promising conjugated polymers for polymer solar cells. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5336–5343, 2009     

Absorption tuning of monosubstituted triazatruxenes for bulk heterojunction solar cells

A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption coefficients and fluorescence quantum yields as well as interesting electrochemical properties. The blue dyes of this series exhibit interesting photovoltaic effects (V(OC) = 0.83 V, J(SC) = 3.6 mA/cm(2), efficiency 0.9%) in bulk heterojunction solar cells, due to the good hole mobility imported by the TAT entity.     

The effect of photocrosslinkable groups on thermal stability of bulk heterojunction solar cells based on donor–acceptor‐conjugated polymers

Three different types of photocrosslinkable groups into a low band‐gap donor–acceptor‐conjugated polymer, namely poly{benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐ thieno[3,4‐b]thiophene} (PBT), were developed to comparatively investigate the effect of the photocrosslinkable groups on the thermal stability of bulk heterojunction solar cells. Compared with vinyl groups, bromine‐ and azide‐ photocrosslinkable groups are more prompt for photocrosslinking to yield a denser crosslinking network, probably due to the different crosslinking mechanisms and reaction rates. In contrast to the reference device decreasing to less than 10% of its initial efficiency value after 80 h of annealing at 150 °C, a great improvement in the thermal stability of performance of all these crosslinked functional copolymers devices demonstrates that photocrosslinking can effectively improve the thermal stability of the active layer by suppressing [6,6]‐phenyl‐C₆₁‐butyric acid methyl diffusion and phase separation. Furthermore, the solar cells with crosslinked bromine‐ and azide‐functionalized PBT polymers showed very thermally stable photovoltaic device performance by retaining 78 and 66% of their initial device efficiency, respectively, whereas vinyl‐functionalized PBT devices retained only 51% of its initial value after long‐time thermal annealing. This suggests that an appropriate crosslinking network with homogenous active morphology could dramatically enhance the device stability without sacrificing the performance. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4156–4166     

Processing additives for improved efficiency from bulk heterojunction solar cells

Two criteria for processing additives introduced to control the morphology of bulk heterojunction (BHJ) materials for use in solar cells have been identified: (i) selective (differential) solubility of the fullerene component and (ii) higher boiling point than the host solvent. Using these criteria, we have investigated the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphology by selective solubility of the fullerene component is demonstrated using these high boiling point processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the reference device) to 5.1%.     

Synthesis of the (X-DADAD)n-type conjugated polymers with 2,1,3-benzoxadiazole acceptor blocks and their application in organic solar cells

Two polymers with benzoxadiazole acceptor units were synthesized and investigated as electron donor materials in organic solar cells. Variation of the alkyl substituents was shown to significantly affect the optoelectronic properties of the polymers. In particular, the polymer HOMO energy level was lowered by 0.1 eV, while maintaining the same band gap, by replacement of the 2-ethylhexyl side-chains with the 2-hexyldecyl group. This modification also resulted in a higher open circuit voltage of the solar cells.     

Time-resolved structural evolution of additive-processed bulk heterojunction solar cells

Solution deposition using high-boiling-point additives such as octanedithiol (ODT) provides a simple and widely used fabrication option for improving the power conversion efficiencies of solar cells composed of narrow-band-gap conjugated polymer donor/fullerene acceptor blends. Previous examination of the resulting device active layers has shown that the use of additives influences the degree of phase segregation within the bulk heterojunction (BHJ) blend and also improves ordering within the polymeric domains. In this work, in situ grazing-incidence wide-angle X-ray scattering as a function of time was used to explore the dynamics of the BHJ evolution. These studies showed that a small percentage of ODT in chlorobenzene (CB) induced the nucleation of polymeric crystallites within 2 min of deposition, increased the orientational order of specific polymorphs, and promoted further crystallite nucleation over a period longer than 40 min after casting. Similar structural changes did not occur when the same BHJ blend was cast from pure CB.     

Significance of miscibility in multidonor bulk heterojunction solar cells

Ternary organic blends have potential in realizing efficient bulk heterojunction (BHJ) organic solar cells by harvesting a larger portion of the solar spectrum than binary blends. Several challenging requirements, based on the electronic structure of the components of the ternary blend and their nanoscale morphology, need to be met in order to achieve high power conversion efficiency in ternary BHJs. The properties of a model ternary system comprising two donor polymers, poly(3-hexylthiophene) (P3HT) and a furan-containing, diketopyrrolopyrrole-thiophene low-bandgap polymer (PDPP2FT), with a fullerene acceptor, PC₆₁BM, were examined. The relative miscibility of PC₆₁BM with P3HT and PDPP2FT was examined using diffusion with dynamic secondary ion mass spectrometry (dynamic SIMS) measurements. Grazing incidence small and wide angle X-ray scattering analysis (GISAXS and GIWAXS) were used to study the morphology of the ternary blends. These measurements, along with optoelectronic characterization of ternary blend solar cells, indicate that the miscibility of the fullerene acceptor and donor polymers is a critical factor in the performance in a ternary cell. A guideline that the miscibility of the fullerene in the two polymers should be matched is proposed and further substantiated by examination of known well-performing ternary blends. The ternary blending of semiconducting components can improve the power conversion efficiency of bulk heterojunction organic photovoltaics. The blending of P3HT and PDPP2FT with PC₆₁BM leads to good absorptive coverage of the incident solar spectrum and cascading transport energy levels. The performance of this ternary blend reveals the impact of the miscibility of PC₆₁BM in each polymer as a function of composition, highlighting an important factor for optimization of ternary BHJs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 237–246     

Design and characterization of Bodipy derivatives for bulk heterojunction solar cells

Two electron rich Bodipy dyes with strong absorptivities in the visible region were designed and synthesized as potential electron donors in bulk heterojunction photovoltaic constructs. Overall efficiency is above 1%, with impressive responsiveness at both UV and near-IR ends of the visible spectrum. Computational studies reveal an unexpected effect of meso-substituents on the electron transfer efficiency.