Bulk Heterojunction Photovoltaic Cells Research Articles

Solid-state showdown: Comparing the photovoltaic performance of amorphous and crystalline small-molecule diketopyrrolopyrrole acceptors

A novel glass-forming diketopyrrolpyrrole (DPP) derivative (DPP-glass) and a crystalline isoelectronic analogue were synthesized and their photophysical, electrochemical and photovoltaic properties were investigated. The compounds were processed into thin films that were further studied by Atomic force microscopy (AFM) and X-ray diffraction (XRD). Solution-processed inverted and regular bulk heterojunction photovoltaic cells were fabricated in different blend ratios with P3HT as donor. Both DPP derivatives show appreciable photovoltaic performance with power conversion efficiencies (PCE) ranging from 0.30 to 2.50%. The regular device with amorphous DPP-glass showed the highest efficiency (2.50%) in the blend ratio 1:2 whereas the device with crystalline DPP derivative shows an efficiency of 1.56% under simulated 1.5 solar irradiation of 100 mWcm−2. The better performance of DPP-glass in comparison to its crystalline analogue is attributed to a more uniform film morphology and fewer defects, leading to improved electron mobility.

Negligible effect of processing additive in polymer bulk heterojunction photovoltaic cells with unmodified fullerene

Although processing additives such as 1,8-diiodooctane are known to effectively modify the microscopic structure of polymer bulk heterojunction composite using PCBM-type fullerene derivatives and yield efficient photovoltaic cells, it seems not yet to be clarified which part of the PCBM-type fullerene derivatives, i.e. bucky ball or side group, is essential to the phenomena. Here, the effect of processing additive 1,2-dimethylnaphthalene (DMN), which is reported to be effective for polymer bulk heterojunction composite film with [6,6]-phenyl-C71-butyric acid methyl ester (C70-PCBM) coated from 1,2,4-trimethylbenzene, on the composite film with unmodified C70 is examined. It has been observed that DMN shows negligible effects on the photovoltaic performance including the low-light performance as well as the surface morphology. Therefore, not the bucky ball but the side group in the fullerene derivatives seems to be critical to the positive effect of processing additives such as DMN.

Interface engineering on phenanthrocarbazole/thienopyrroledione-based conjugated polymer for efficient organic photovoltaic devices with ideal nano-morphology and improved charge carrier dynamics

We synthesized PP-TPD, based on a phenanthro[1,10,9,8-cdefg]carbazole unit as a donor and a thieno [3,4-c]pyrrole-4,6-dione (TPD) unit as an acceptor for efficient bulk heterojunction photovoltaic cells. The copolymer achieves a relatively low bandgap (1.6 eV) by its internal charge-transfer, which concurs with the results of density functional theory (DFT) calculation for the distribution of HOMO and LUMO levels. In addition, we fabricated the bulk-heterojunction (BHJ) photovoltaic cells as a function of the blend ratio of the donor (PP-TPD) and acceptor (PC70BM), and applied a solvent additive and an interlayer to the device, with the optimized ratio used in this study. To comprehend the relationships between the device performance and the synergetic effects of 1,8-diiodooctane (DIO) and titanium oxide (TiOx) within the PP-TPD: PC70BM blends, we investigated the hole mobility related to the charge carrier transport, the charge generation, and the charge transport resistance using space-charge limited current (SCLC) measurement, photocurrent analysis, and impedance spectroscopy, respectively. Especially, we implemented fluorescence imaging by Raman spectroscopy, which related to the charge generation and recombination dynamics within the surface of photoactive areas.

Effect of vacuum treatment in diketopyrrolopyrrole (DPP) based copolymer with ratio controlled toluene- and benzene- functional groups for efficient organic photovoltaic cells: Morphological and electrical contribution

Solution-processed organic bulk-heterojunction (BHJ) photovoltaic cells using random copolymeric donor materials have been extensively reported due to their suitable film-forming characteristics and phase-separated nano-morphology. Here, ratio-controlled toluene-versus benzene-chemical group based diketopyrrolopyrrole (DPP) donor polymers mixed with a fullerene acceptor were investigated to fabricate an efficient photovoltaic active layer with improved electrical properties through a vacuum treatment. The vacuum process leads to an increase in the phase-separation with a low surface roughness and nanoscale-distributed crystallinity due to securing the dry time of the residual solvent and solvent additive within the active layer. Moreover, the optimized DPP-based donor with toluene (T) versus benzene (B) linkers and electron transporting layer leads to an improvement in the power conversion efficiencies of up to 6.31% under AM 1.5G illumination due to the contributions of an efficient charge transfer and reduced series resistance. Therefore, the organic semiconductor obtained with the ratio-controlled molecular structure and proper solvent drying process plays an important role in increasing the electrical and morphological properties to produce efficient organic solar cells.

Efficient alternating polymer based on benzodithiophene and di-fluorinated quinoxaline derivatives for bulk heterojunction photovoltaic cells

A new donor-acceptor polymer (PBDTT-DFTQ) based on two-dimensional conjugated alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene and two fluorine substituted quinoxaline derivatives was synthesized and applied as donor materials for bulk heterojunction solar cells. Thermal stability, absorption properties, energy levels and photovoltaic properties of the polymer were intensively investigated. This polymer exhibited an optical bandgap of 1.76 eV and a deep highest occupied molecular orbital (HOMO) energy level (−5.45 eV). Conventional devices were fabricated using PBDTT-DFTQ as donor blending with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as acceptor, a high open-circuit voltage (Voc) 0.95 V and a power conversion efficiency (PCE) of 5.52% were obtained for the optimal donor and acceptor weight ratio (1:3). After tetrahydrofuran vapor annealing, the device delivers balanced charge transport property and enhanced absorption in the visible region, which are benefit to obtain improved short-circuit current density (Jsc) and fill factor (FF). Consequently, an enhanced power conversion efficiency (PCE) of 7.76% with a high Voc of 0.92 V, Jsc of 11.43 mA cm−2 and FF of 72.1% was achieved, which among one of the best values of benzo[1,2-b:4,5-b′]dithiophene and quinoxaline derivatives based alternating polymers.

Efficient, large area organic photovoltaic modules with active layers processed with non-halogenated solvents in air

Organic photovoltaic (OPV) modules with excellent power conversion efficiencies of 4.20% and 5.87% were obtained with blends of novel low bandgap polymers, POD2T-DTBT and PDTBT-alt-TT, respectively, with PCBM as the active layers. Large area polymer:PCBM blend films were deposited by blade coating with high film uniformity, fast processing speed, and low material consumption, without the need for an inert atmosphere. With the requirements of future manufacturing in mind, the effects of air exposure on the active layers and the substitution of chlorinated solvents for polymer processing were investigated. While the decline of Voc due to air exposure was recovered by appropriate treatment of the completed devices, FF and Jsc were significantly affected due to interfacial effects, indicating that air exposure should be limited in duration. Active layers of POD2T-DTBT:PCBM processed with less toxic, non-halogenated mixed solvents, comprising non-polar o-xylene with small amounts of polar additive, showed a fine, interpenetrating nanofibrous network that facilitates charge transport in bulk heterojunction photovoltaic cells. The performance of POD2T-DTBT:PCBM photovoltaic cells and modules processed with the new ink formulations were on par with that obtained with chlorinated solvents, with the modules exhibiting impressive power conversion efficiencies above 4%. The results indicated that POD2T-DTBT was an outstanding candidate for future OPV production due to its high performance, processibility in different ink formulations and compatibility with the large area coating techniques.

Soluble single-walled carbon nanotubes for photovoltaics

Single wall carbon nanotubes (SWCNTs) have attracted great attentions in the fields of nanoscience and nanotechnology due to their unprecedented physical/chemical/mechanical stability and high dimensional aspect ratio resulting superior electrical properties. However, their poor solubility in both organic and aqueous solvents refrain their solution processability and hence practical applications. In the present work, we report one-pot functionalization of SWNCT based on the nitrene chemistry of two structurally different aromatic azides having ester functional groups (very low concentrations 10% w/w), by thermal activation which renders them soluble in organic solvents and water. The resulting functionalized SWCNTs (f-SWCNT I &f-SWCNT II) covalent linkage and the strong electronic interaction between the P3HT and f-SWCNTs are confirmed by various spectroscopic measurements and electrochemical analysis. The bulk heterojunction (BHJ) photovoltaic cell based on the solution cast P3HT:PCBM with doping concentrations of f-SWCNTs, demonstrate up to 30% increase in the power conversion efficiency with respect to the reference cell under AM 1.5 illumination (100mW/cm2).

Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing.

Here we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of the rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made with DTBDT-Rho:PC71BM blends showed an extremely high power conversion efficiency (PCE; 7.10%); notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.

Synergetic Solvent Engineering of Film Nanomorphology to Enhance Planar Perylene Diimide-Based Organic Photovoltaics.

Solvent additive has proven as a useful protocol for improving the film nanomorphology of polymer donor (D): fullerene acceptor (A) blends in bulk heterojunction (BHJ) photovoltaic cells. By contrast, the effect of such solvent additive on nonfullerene BHJ cells based on perylene diimide acceptor, for instance, is less effective because of their highly planar structure and strong π-aggregation in solid state. Here we choose N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) and thieno[3,4-b]thiophene-alt-benzodithiophene (PTB7) as a model D:A blend system to investigate how solvent engineering strategy synergistically impacts the blend film nanomorphology. Based on the differences of solvent volatility and solubility, various host solvents-chloroform (CF) and chlorobenzene (CB) and solvent additives-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) are selected for comparative studies. It is found that the π-aggregation of PDIs can be largely suppressed by using low-boiling point (Tb) CF solvent, yet enlarged by using high-Tb CB. Moreover, CN additive provides good solubility of PDI molecules and hence reduces large PDI aggregates in CB system, while DIO exhibiting poor solubility works oppositely. By contrast, DIO that presents larger Tb difference with CF prolongs the film-forming, which assists in optimizing the PDI aggregation and increases the intermixed PTB7:PDI phases more significantly than CN in CF system, yielding the finest phase-separation morphology and balanced charge mobility. Consequently, the inverted BHJ cells based on CF-processed PTB7:PDI blend film with 0.4 vol % DIO exhibit the highest PCE of 3.55% with a fill factor of 56%, both of which are among the best performance for such a paradigm PTB7:PDI blend-based BHJ cells.

Morphological engineering via processing additive in thin film bulk-heterojunction photovoltaic cells: A systematic understanding of crystal size and charge transport

In this research, organic photovoltaic devices based on bulk-heterojunction (BHJ) from Poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were successfully fabricated to determine the optimization of solvent additive of 1,8-diiodooctane (DIO). The BHJ devices with DIO show improved short circuit current (JSC) correlated to the improved device performance, since the DIO in P3HT:PC71BM BHJ affects the morphology in terms of the surface roughness and crystal size. Furthermore, the DIO contributes to the enhanced charge carrier transport of the BHJ device, because the decreased fluorescence and increased hole mobility as found through Raman spectra analysis and SCLC measurement, respectively. The device with optimized concentration (1 vol %) of DIO accomplishes 5% improved photovoltaic performance due to the favorable structural property and enhanced carrier mobility.

“Click” polymerization: A convenient strategy to prepare designer fullerene materials

“Click” polymeric assemblies of fullerenes, thiophenes and aryl compounds were prepared for the first time to study their spectroscopic/chemical/electronic characteristics and possible applications in photovoltaic cells. The GPC and NMR spectroscopic data confirmed a moderate to high degree of polymerization while microscopic pictures showed well-defined arrangements of fullerenes. The FTIR studies indicated that the product is free from starting azide materials, whereas thermogravimetric analysis data suggested a good stability. Band gap of the material was determined using UV–Vis spectroscopy, cyclic voltammetric techniques and compared with computational investigation. The fullerene copolymer was fabricated as acceptor material in a bulk heterojunction photovoltaic cell.

Two dibenzo[Def, Mno]chrysene‐based polymeric semiconductors: Surprisingly opposite device performances in field‐effect transistors and solar cells

ABSTRACTA new series of conjugated polymers containing dibenzo[def, mno]chrysene units were successfully designed and synthesized to investigate their physical properties and device performances in field‐effect transistors and photovoltaic cells. Two polymers, namely poly(4,10‐bithiophene‐6,12‐bis(2‐decyltetradecyloxy)‐dibezo[def, mno]chrysene) (PTTC) and poly(2,2′‐thiophenevinylenthiophene‐4,10‐[6,12‐bis(2‐decyltetradecyloxy)‐dibenzo[def, mno]chrysene]) (PTVTC), exhibited similar light absorption, electrochemical characteristics, and theoretical electronic structures. However, they behaved very differently when used in thin‐film transistors and solar cells. The PTTC polymer with two thiophene groups had better charge transport behavior, whereas the PTVTC polymer with two thiophene units connected by a vinyl group exhibited higher efficiency in bulk heterojunction photovoltaic cells. These results were discussed in terms of their nanostructural bulk morphologies established from transmission electron microscopy and two‐dimensional grazing incidence wide angle X‐ray scattering analyses. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2559–2570

Enhanced photovoltaic performance of ternary solar cells by doping a new squaraine derivative

Ternary blend solar cells have been proved to be an efficient way to harvest more photons over the near-IR region. In this work, the effect of adding a NIR absorbing squaraine derivative (TPE-SQ) into a conventional P3HT:PC71BM based bulk heterojunction photovoltaic cell is investigated. The new NIR dye exhibits a quite broad absorption band with full-width at half maximum of 169 nm in the film state, with the absorption edge extending to more than 850 nm. The near infrared absorption of the ternary system is enhanced by the blended squaraine component, and the energy level of the third component matched well with the donor and the acceptor material, leading to increased short-circuit current as well as elevated power conversion efficiency compared to the P3HT:PC71BM binary solar cell. The highest efficiency of 3.93% is achieved for a P3HT:PC71BM blend with 2.4 wt % of TPE-SQ.

Synthesis, characterization and photovoltaic performance of novel glass-forming perylenediimide derivatives

Novel glass-forming perylene-3,4,9,10-tetracarboxylic diimide (PDI) derivatives were synthesized from 1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimides by nucleophilic substitution with a glass-forming mexylaminotriazine unit, characterized, and their performance as electron acceptors in bulk heterojunction photovoltaic cells with P3HT as donor were studied. Imide groups (octyl and 2,6-diisopropylphenyl) and bay substituents (bromo or pyrrolidinyl) were used to study the impact of sterics and electronics on device performance. The HOMO and LUMO levels of the materials were determined using cyclic voltammetry. The morphology and packing behavior of molecules in films of blends with P3HT were studied using Atomic Force Microscopy and X-ray diffraction, and in all four cases, the PDI derivatives remain in the amorphous phase, while the P3HT portion of the blends crystallizes. The devices gave photovoltaic performances ranging from 0.2 to 0.6%, and while the bay substituents showed negligible impact on device performance, switching from bulky 2,6-diisopropylphenyl imide groups to linear octyl chains improved the efficiency of the devices by 36%, current density by 66% and fill factor by 16%. The performances observed for devices incorporating these PDI glasses are comparable to that of devices with crystalline PDI acceptors.

Magnetophotocurrent in Organic Bulk Heterojunction Photovoltaic Cells at Low Temperatures and High Magnetic Fields

Magnetophotocurrent in Organic Bulk Heterojunction Photovoltaic Cells at Low Temperatures and High Magnetic Fields

Solution-processed tBu4-ZnPc:C61 bulk heterojunction organic photovoltaic cells

A bulk heterojunction (BHJ) organic photovoltaic cell employing tetra-tert-butyl zinc phthalocyanine as electron donor and [6,6]-phenyl C61-buturic acid methyl ester as electron acceptor has been fabricated. The effect of TiOx cathode interlayer, the weight ratio of donor:acceptor in the photoactive layer, and the thermal annealing of photoactive layer on the performance of the cells were investigated. The results show that the insertion of TiOx layer leads to an increase in the photocurrent density of the cells by 11 times compared to those without cathode interlayer. Atomic force microscopy images reveal the formation of well-connected percolated pathways for each phase separated component (donor and acceptor) upon annealing of the film at 150 °C. An important aspect of the present BHJ photovoltaic cell is that it has been obtained by simple wet processes, and most of the fabrication steps have been carried out in ambient air without the use of a glove box.

Incorporation of diketopyrrolopyrrole dye to improve photovoltaic performance of P3HT:PC71BM based bulk heterojunction polymer solar cells

Ternary blend solar cells have been intensively studied in recent years to harvest more photons over the near-IR region. In this work, the effects of adding a diketopyrrolopyrrole dye (py-DPP) into a conventional P3HT:PC71BM based bulk heterojunction photovoltaic cell are investigated. The near infrared absorption of the blend is enhanced by the doped py-DPP dye, leading to more than 20% increased power conversion efficiency compared to the P3HT:PC71BM binary system. The highest efficiency of 4.05% is achieved for a P3HT:PC71BM blend with 2.4 wt % of py-DPP.

Comprehensive study of pyrido[3,4‐b]pyrazine‐based D–π–a copolymer for efficient polymer solar cells

ABSTRACTTwo D–π–A copolymers, based on the benzo[1,2‐b:4,5‐b′]‐dithiophene (BDT) as a donor unit and benzo‐quinoxaline (BQ) or pyrido‐quinoxaline (PQ) analog as an acceptor (PBDT‐TBQ and PBDT‐TPQ), were designed and synthesized as a p‐type material for bulk heterojunction (BHJ) photovoltaic cells. When compared with the PBDT‐TBQ polymer, PBDT‐TPQ exhibits stronger intramolecular charge transfer, showing a broad absorption coverage at the red region and narrower optical bandgap of 1.69 eV with a relatively low‐lying HOMO energy level at −5.24 eV. The experimental data show that the exciton dissociation efficiency of PBDT‐TPQ:PC71BM blend is better than that in the PBDT‐TBQ:PC71BM blend, which can explain that the IPCE spectra of the PBDT‐TPQ‐based solar cell were higher than that of the PBDT‐TBQ‐based solar cell. The maximum efficiency of PBDT‐TPQ‐based device reaches 4.40% which is much higher than 2.45% of PBDT‐TBQ, indicating that PQ unit is a promising electron‐acceptor moiety for BHJ solar cells. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1822–1833

Dithieno[2,3‐d:2′,3′‐d′]naphtho[2,1‐b:3,4‐b′]dithiophene based medium bandgap conjugated polymers for photovoltaic applications

ABSTRACTThree novel medium band gap (MBG) conjugated polymers (CPs) (named as P1, P2, and P3, respectively) were developed by copolymerizing 2,7‐dibromo‐10,11‐di(2‐hexyldecyloxy)dithieno[2,3‐d:2′,3′‐d′]naphtho[2,1‐b:3,4‐b′]dithiophene (NDT‐Br) with three different units: 2,5‐bis(tributylstannyl)thiophene, 2,5‐bis(trimethylstannyl)thieno[3,2‐b]thiophene and trans−1,2‐bis(tributylstannyl)ethene, respectively. The thermal, optical, and electrochemical properties of the polymers were investigated. All of the polymers have good thermal stability and medium band gap (∼ 1.9 eV). Prototype bulk heterojunction photovoltaic cells based on the blend P1/P2/P3 and [6, 6] phenyl‐C61 butyric acid methyl ester (PC61BM) were assembled and the photovoltaic properties were assessed. Power conversion efficiencies (PCEs) of 1.61% ∼ 2.43% have been obtained under 100 mW cm−2 illumination (AM1.5). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43288.

Reappraising the validity of poly(3-hexylthiophene) nanostructures in interdigitated bilayer organic solar cells

Interdigitated nanostructures can increase power conversion efficiency (PCE) by providing large interfaces for the dissociation of excitons and arrays for carrier transportation without recombination. Here, we reappraise the validity of donor–acceptor nanostructures composed of well-aligned porphyrin–fullerene nanopillars based on their contribution to the PCE compared with bulk heterojunction (BHJ) and bilayer (BL) organic photovoltaic solar cells (OPVs) with high efficiencies. Contrary to expectations, the results showed that poly(3-hexylthiophene) (P3HT):[6, 6]-phenyl C61 butyric acid methyl ester (PCBM) interdigitated nanostructures hardly enhanced the PCE. We suggested three reasons focused on the morphological characteristics: the misbalanced carrier transit time caused by differences in the travel distances and carrier mobilities, detrimental effects on carrier transportation due to the overlap of the pathways to the electrodes, and the dependence of the PCE on the shapes and sizes of the nanostructures.