Polymeric Solar Cells (PSCs)

Novel materials are developed for organic optoelectronic devices, such as polymeric solar cells (PSCs), which is a popular research topic in recent decades, because they are low cost and green materials for sustainable resources to reduce consumptions of fossil energy and nuclear power.² In particularly, bulk heterojunction (BHJ) solar cells consisting of electron-donating conjugated polymers blended with electron-accepting fullerenes are fabricated in solid thin

films.³ Up to now, regio-regular poly[2-methoxy-5-(3’,7’- dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV)⁴ and poly(3-hexylthiophene) (P3HT)⁵ as electron donors blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor approached high power conversion efficiency (PCE) values of 5.0% in PSCs. More recently, the PCE values of BHJ solar cells using new low-band gap conjugated polymers have reached 6 to 8%.^6,7 The PCE values of BHJ solar cells were affected by, for example, the energy band gaps of polymers, which is related to the chemical structure of the conjugated polymers.

1.2 Hybrid Polymer-Inorganic Solar Cells

These hybrid polymer-inorganic solar cells utilize the high electron mobility of the inorganic phase to overcome charge-transport limitations associatedwith organic materials. The efficient BHJ solar cells made of ZnO nanoparticles and a conjugated polymer have been reported previously.⁸ The ZnO nanoparticles were blended with poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMOPPV) to possess a highest power conversion efficiency (PCE) approaching 1.6% in PSCs.⁹ Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and thiophene units (PFT) wereinvestigated. The CdSe/PFT devices showed

very low photocurrent and fill factor values, which were attributed to the poor charge transport in the trioctylphosphine oxide (TOPO)-capped CdSe nanoparticle phase.

Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were investigated. It was observed that for the optimized composition of 20 wt.% PFT+40 wt.% CdSe+40 wt.%

PCBM the devices presented higher photocurrents and efficiencies.(Figure 1.2.1) The use of inorganic nanoparticles, such as TiO₂, ZnO, CuInS₂, PbSe, CdSe, and CdTe, have some advantages, related to the versatility of these materials, which often can be easily synthesized in a great variety of sizes and shapes, according to the desired properties.¹⁰

Figure 1.2.1 The energy levels of in the ternary system solar cell showing the HOMO and LUMO levels of the materials and work function of the electrodes. The arrows indicate the expected charge transfer and charge transport processes.

1.3 Supramolecular H-bond Polymers for Organic Solar Cells

Nanostructured materials with tailor-made properties and functions can be developed by exploiting the supramolecular approach through molecular recognition.In fact, the hierarchical self-assembly of multivalent molecular modules through the concerted action of multiple noncovalent interactions represents a very powerful approach as it makes possible the simultaneous organization of various molecular systems into intrinsically defect-free 2D architecture featuring a long-range order.¹¹

Hydrogen bonds (H-bonds) are ideal noncovalent interactions to form self-assembled architectures due to their selectivity and directionality. A numerous advantages of H-bonded polymers, such as stronger light absorptions, lower HOMO levels, higher V_oc values, higher hole mobilities, and higher crystallinities, were utilized for organic solar cells.¹² Therefore, great efforts have been taken toward the preparation and characterization of photo- and electroactive noncovalent assemblies based on hydrogen bonds (H-bonds). Wurthner,^12(a,b) El-ghayoury et al., and Jonkheijm et al.^12(c,d) reported H-bonded assemblies of perylene bisimide and melamine derivatives.

In addition, El-ghayoury et al. reported a PCE value of 0.39% for PSCs by utilizing a H-bonded polymer containing oligo(p-phenylene vinyene) and ureido-pyrimidinone units.^12(c) Because of several advantages in polymers, including low cost, easy processing, and tunable chemical properties, the conjugated polymers consisting of different heteroaromatic rings, such as thiophene and carbazole, exhibit an

electrochromic behavior as well as photovoltaic properties.

Moreover, connecting the electrooptical properties in organic devices have been established through the supramolecular interactions, e.g. H-bonds, in organic, dendritic, and polymeric H-bonded complex systems. This was illustrated by a recent report on a triple hydrogen-bonded triad consisting of a central perylene that was connected to two C₆₀ chromophores (Figure 1.3.1).¹³

Figure 1.3.1 Superstructure of self-assembly of [60]fullerene derivative 1 with perylene bisimide 5 by H-bonding.

1.4 Dye-Sensitized Solar Cells (DSSCs)¹⁴

Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy. In this context, dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years. Efforts in the synthesis of sensitizers for DSSCs can be grouped into two broad areas: 1) Functional ruthenium(II)–polypyridyl complexes such as N3,¹⁵ N719, Z907, and black dye; and 2) metal-free organic donor–acceptor (D–A) dyes. The

former class of compounds contains expensive ruthenium metal and requires careful synthesis and tricky purification steps. On the other hand, the second class can be prepared rather inexpensively by following established design strategies.

Conventional DSSCs typically contain five components: 1) a photoanode, 2) a mesoporous semiconductor metal oxide film, 3) a sensitizer (dye), 4) an electrolyte/hole transporter, and 5) a counter electrode. In DSSCs, the incoming light is absorbed by the sensitizer, which is anchored to the surface of semiconducting TiO₂ nanocrystals. Charge separation takes place at the interface through photoinduced electron injection from the excited dye into the conduction band of the TiO₂. Holes are created at the dye ground state, which is further regenerated through reduction by the hole-transport material (HTM), which itself is regenerated at the counterelectrode by electrons through an external circuit. In principle, for efficient DSSCs the regeneration of the sensitizer by a hole transporter should be much faster than the recombination of the conduction band electrons with the oxidized sensitizer.

Additionally, the highest occupied molecular orbital (HOMO) of the dye should lie below the energy level of the hole transporter, so that the oxidized dyes formed after electron injection into the conduction band of TiO₂ can be effectively regenerated by accepting electrons from the HTM. The general operating principle of a dye-sensitized solar cell is depicted in Figure 1.4.1. The research area dealing with

DSSCs is expanding very rapidly and attracting scientist from different disciplines: 1) Chemists to design and synthesize suitable donor–acceptor dyes and study structure–property relationships; 2) physicists to build solar cell devices with the novel materials, to characterize and optimize their performances, and to understand the fundamental photophysical processes; and 3) engineers to develop new device architectures. The synergy between all the disciplines will play a major role for future advancements in this area.

Figure 1.4.1 a) Fundamental processes in a dye-sensitized solar cell. b) Energy-level diagram of a DSSC. TCO=transparent conducting oxide.¹⁴

1.4.1 Metal-Free Organic Dyes in

The development of novel materials for use in organic optoelectronic devices, such as dye-sensitized solar cells (DSSCs),¹⁶ has become a popular research topic in the quest for low-cost, green materials for sustainable use and a decrease in demand for fossil fuels and nuclear power. DSSCs based on Ru-photosensitizers,^17-18 such as cis-bis(isothiocyanato)bis(2,2´-bipyridyl-4,4´-dicarboxylato)-ruthenium(II) (N3)¹⁵and related derivatives, have been applied very successfully with high power conversion efficiencies (PCEs) of 9–12%.^15,19-23 Recently, it has been demonstrated that DSSCs can also be constructed from metal-free organic dyes.¹⁴ Because of the high cost of rare Ru metal and the relatively low molar extinction coefficients and tedious purification of Ru-photosensitizes,¹⁹ metal-free organic sensitizers have become increasingly attractive and widely developed.^24,25 Nevertheless, the ability to reach higher efficiencies when using metal-free organic dyes remains a challenge, although great progress has been made in this field.^26-29 The key characteristics for a dye to be used in a DSSC are high absorption over a wide range of the solar spectrum with high molar extinction coefficients, efficient charge separation, redox stability, andsuitable functional groups to interact with the electron sink (TiO₂). Metal-free organic dyes featuring a donor/acceptor structural design were synthesized have particularly wide absorption ranges for DSSC applications.^24-32

Some general principles to construct an efficient dye and efficient DSSCs are as follows: 1) The absorption range of the dye should cover the whole visible and some of the near-infrared region, and its molar extinction coefficient must be as high as possible to enable efficient light harvesting with thinner TiO₂ layers (panchromatic absorption). 2) For efficient electron injection into the anode, the lowest unoccupied molecular orbital (LUMO) of the dye should be localized near the anchoring group (usually a carboxylic or phosphonic acid) and above the conduction band edge of the semiconductor electrode (typically TiO₂). 3) The HOMO of the dye should lie below the energy level of the redox mediator to allow efficient regeneration of the oxidized dye. 4) To minimize charge recombination between the injected electrons and the resulting oxidized dye, the positive charge resulting after electron injection should be localized on the donor part, which is further away from the TiO₂ surface. 5) The periphery of the dye should be hydrophobic to minimize direct contact between the electrolyte and the anode to prevent water-induced desorption of the dye from the TiO₂ surface and consequently enhance the longterm stability. 6) The dye should not aggregate on the surface to avoid nonradiative decay of the excited state to the ground state, which often occurs with thicker films.

1.5 Metalloplymer³³

The rapid growth of supramolecular chemistry since the 1970s has led to many new opportunities to take advantage of reversible interactions. A major contribution to the diversity of the field of metal-containing polymers has involved the development of metallosupramolecular polymers, in which the metal ions are bound by non-covalent coordination interactions that allow for reversible, ‘dynamic’, binding analogous to hydrogen bonding (figure panel b).^34-35 The recent developments are illustrated by the formation of metallosupramolecular polymers that involve labile multidentate ligation and metallophilic interactions. As another key contributor to structural diversity, metal containing polymers can contain a variety of metal centres, from transition-metal ions and main-group metals through to lanthanides and actinides. In addition, the metal centres can be located either in the polymer main chain or in the side-chain structure (figure panels c and d) (Figure 1.5.1). As examples of further subdivisions, metallopolymers can be linear, star-shaped, highly branched or dendritic^36-42 (figure panels e, f and g) (Figure 1.5.1). Significantly, with all of these materials the typical classical polymer processing possibilities, such as spin coating, inkjet printing, extrusion, compounding and film blowing, are maintained.

Metal-ligand coordination seems to be particularly attractive in past few decades because of searching for new smart materials.^40-42 In recent years, the researches on

supramolecular metallo-polymers applied to electro-optical materials have been commonly conducted, because the advantages of these materials, such as easy processability, cheap fabrication, rapid coordination, and tunability of the optical band gap, can promote long-range electrons or energy transfers.⁴³ Supramolecular metallo-architecture is formed with coordination ability of transition metal ions and chelating ligands because of their self-recognition and self-assembly.^44-46 Moreover, metal-ligand complexes realized ideal conditions from self-assembly to form the kinetically labile but nevertheless thermodynamically stable bonds.⁴⁷ In the meanwhile, metallo-polymers are also good candidates to study for their electrochromic properties during the redox processes.^48-49 2,2’:6’,2’’-Terpyridine(terpy) and bipyrdine (bpy) derivatives have been utilized recently for multinuclear supramolecular interactions.^50-52 The transport of energy and electrons within nanoscale ordered materials is significant to optoelectronics. It needs to control over both of their physical and chemical properties in the self-assembled organization.

Figure 1.5.1 Structural diversity of metal-containing polymers.

1.5.1 Nanocomposite Systems Base on Metallopolymer and

Nanoparticles

Nanocomposite systems are that combine the favorable features of, for example, fullerenes and porphyrins as electron acceptors and donors, respectively.⁴⁹ They have received interest in the areas of light-induced electron-transfer chemistry and solar energy conversion.⁵³ Common electron donor-acceptor systems are based on covalent linkages. However, much less is, known about noncovalent electron donor-acceptor nanocomposites and the function of the intervening spacers.⁵⁴ Compared with other intermolecular forces, such as van der Waals, π-π stacking, or Coulombic interactions, hydrogen bonds are particularly attractive as they are directional and do not possess electronic energy levels that interfere with those in materials for organic electro-optical applications.^55-56 Therefore, that great efforts have been expended toward the preparation and characterization of photo- and electro-active noncovalent assemblies based on hydrogen bonds (H-bonds). (Figure 1.5.2)

Figure 1.5.2 Characterization of photo- and electro-active noncovalent assemblies based on hydrogen bond.

Chapter 2

Applications of novel dithienothiophene- and 2,7-carbazole-based conjugated

polymers with surface-modified ZnO nanoparticles for organic photovoltaic cells

Two kinds of novel conjugated polymers containing 2,7-carbazole, thiophene, and

fused-dithienothiophene rings as backbones bearing acid-protected and benzoic acid

pendants (PCA and PCB, respectively) were utilized for organic solar cell

applications. The absorption spectra of these polymers (in both solutions and solid

films) showed an absorption range at 300–580 nm. Furthermore, ZnO nanoparticles

were synthesized and surface-modified with pyridyl surfactants (ZnOpy) to be ca.

3–4 nm. The pyridyl surfactants of ZnOpy nanoparticles (as electron acceptors to

partially replace expensive electron acceptor PCBM) not only induce supramolecular

interactions with benzoic acid pendants of polymer PCB via H-bonds, but also

enhance the homogeneous dispersions of ZnOpy nanoparticles in polymer PCB. Thus,

the ternary systems of PCA,PCB/ZnOpy/PCBM in weight ratios of 1:0.05:1 and

1:0.1:1 were investigated in bulk heterojunction polymer solar cells (PSCs). Under

the standard illumination of AM 1.5, 100 mW/cm², the best power conversion

efficiency (PCE) of the PSC cell containing a polymer blend of

PCB/ZnOpy/PCBM=1:0.05:1 reached PCE=0.55%, with Jsc=2.11 mA/cm²,

Voc=0.88 V, and FF=29.4%.

2.1 Introduction

The developments of new materials to be used in organic optoelectronic devices such as polymeric solar cells (PSCs) have become dramatically attractive because they represent a green and renewable energy alternative to fossil energy and nuclear power.

In particular, the so-called bulk heterojunction (BHJ) concept² has been established in thin films of organic solar cell devices utilizing electron-donating conjugated polymers blended with electron-accepting species, such as fullerenes,³ dicyano-based polymers,^4,57 or n-type nanoparticles.⁵⁸ These hybrid polymer-inorganic solar cells utilize the high electron mobility of the inorganic phase to overcome charge-transport limitations associated with organic materials. The efficient BHJ solar cells made of ZnO nanoparticles and a conjugated polymer have been reported previously.⁸ The

ZnO nanoparticles were blended with poly[2-

methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMOPPV) to possess a highest power conversion efficiency (PCE) approaching 1.6% in PSCs.⁹ Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and thiophene units (PFT) wereinvestigated. The CdSe/PFT devices showed very low photocurrent and fill factor values, which were attributed to the poor charge transport

in the trioctylphosphine oxide (TOPO)-capped CdSe nanoparticle phase. Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were investigated. It was observed that for the optimized composition of 20 wt.% PFT+40 wt.% CdSe+40 wt.%

PCBM the devices presented higher photocurrents and efficiencies. The use of inorganic nanoparticles, such as TiO₂, ZnO, CuInS₂, PbSe, CdSe, and CdTe, have some advantages, related to the versatility of these materials, which often can be easily synthesized in a great variety of sizes and shapes, according to the desired properties.¹⁰ In parallel, oligo- and poly(2,7-carbazole)⁶¹ derivatives have been successfully used in polymer light emitting diodes (PLEDs)⁶² and organic field-effect transistors (OFETs)^62-63, demonstrating good p-type transport properties.⁶⁰ Recently, Müllen and co-workers⁶¹ have reported solar cells consisting of poly(N-alkyl-2,7-carbazole)with a PCE value of 0.6%. Moreover, in contrast to the fluorene unit the carbazole moiety is fully aromatic, providing a better chemical and environmental stability. Taking all of these results into account, the development of new copolymers based on carbazoles should therefore lead to interesting features for photovoltaic applications. A class of polymers that have to date received little attention as p-type materials for use in solar cells is polycarbazoles. Carbazole is a well-known electron-donating unit, and thus poly(2,7-carbazole)s are attractive

candidates as p-type materials for solar cells.⁶⁴ Dithieno[3,2-b:2′,3′-d]thiophene (DTT) is a sulfur rich (three-S atoms) and electron rich segment, and serves as an important building block of a wide variety of materials for electronic and optical applications, such as electroluminescence, two photon absorptions, nonlinear optics, photochromism, OFETs, and OPVs.⁶⁵ Besides, the fused aromatic rings can make the polymer backbones more rigid and coplanar, therefore enhancing effective π-conjugation lengths, lowering band gaps, and extending absorption lengths. Powder X-ray diffraction (XRD) analyses suggested that these copolymers formed self-assembled π–π stacking and pseudo-bilayered structures.⁶⁶ Molecules containing fused ring systems intend to maximize the π-orbital overlaps by restricting intramolecular rotation in these systems and possibly to induce face-to-face π–π stackings, facilitating intermolecular hoppings and charge transports.⁶⁵ In order to increase the solubility in poly(DTT) without causing any additional twisting of the repeating units in the resulting polymers, alkylsubstituted thiophene units were incorporated into the polymer backbones as copolymers to fabricate OPVs¹² and OFETs.⁶⁷ Based on this concept, two different moieties, i.e., fused dithienothiophene and carbazole, were utilized as donor monomers to synthesize fused dithienothiophene-based polymers PCA and PCB (see Figure 2.1). In order to integrate electron donor polymers (PCA and PCB) with electron acceptors,

pyridyl-surface-modified ZnO nanoparticles (ZnOpy) were synthesized according to Scheme 2.1. Compared with those reported fused dithienothiophene-based polymers, polymers PCA and PCB showed much improved open circuit voltage (V_oc) values with a highest open-circuit voltage of up to 0.88 V (in PCB) as well as suitable electronic energy levels and good processabilities for PSC applications. So far, the preliminary PSC performance of these structurally related copolymers showed the best PCE value of up to 0.55% while blended with ZnOpy and PCBM in a weight ratio of 1:0.05:1, with a short circuit current density (I_sc) of 2.11 mA/cm², an open circuit voltage (V_oc) of 0.88 V, and a fill factor (FF) of 0.29 under the solar simulator adjusted to give 100 mW/cm² of AM 1.5 G irradiation. Although the results for the PCE values of these nonoptimized PSCs are not sufficiently high enough, this research affords a new concept to incorporate electron donor polymers and electron acceptor surface-modified ZnO nanoparticles to the nanocomposite design.

Figure 2.1 Chemical structures of polymers PCA and PCB.

2.2 Experimental

2.2.1 Materials

All chemicals and solvents were used as received.

2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole,⁶⁰ 2,7-dibromo-carbazole,⁶⁰ and 3,5-didecanyldithieno[3,2-b:2′3′-d]thiophene)^68-69 were synthesized according to the literature procedures. The detailed synthetic routes of polymers PCA and PCB will be published somewhere later. The synthetic routes of surface-modified ZnO nanoparticles (ZnOpy) are shown in Scheme 1. ZnO nanoparticles were synthesized by following the literature procedures.⁷⁰ Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Toluene, tetrahydrofuran, and diethyl ether were distilled to keep anhydrous before use.

2.2.2 Measurements and characterization

1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl₃ solvents. Elemental analyseswere performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperature were determined by differential scanning calorimetry (DSC, Perkin-Elmer Pyris 7) with a heating and cooling rate of 10 °C/min. Thermogravimetric analyses (TGA) were conducted with a TA instrument Q500 at a heating rate of 10 °C/min under nitrogen. Gel permeation chromatography (GPC) analyses were conducted on a Waters 1515 separation module using

polystyrene as a standard and THF as an eluent. UV–visible absorption and photoluminescence (PL) spectrawere recorded in dilute chlorobenzene solutions (10⁻⁶ M) on a HP G1103A and Hitachi F-4500 spectrophotometer, respectively. Solid films of UV–vis and PL measurements were spin-coated on quartz substrates from chlorobenzene solutions with a concentration of 10 mg/mL. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard threeelectrode electrochemical cell in a 0.1M tetrabutylammonium hexafluorophosphate (TBAPF₆) solution (in chorobenzene) at room temperature with a scanning rate of 50mV/s. In each case, a carbon working electrode coated with

polystyrene as a standard and THF as an eluent. UV–visible absorption and photoluminescence (PL) spectrawere recorded in dilute chlorobenzene solutions (10⁻⁶ M) on a HP G1103A and Hitachi F-4500 spectrophotometer, respectively. Solid films of UV–vis and PL measurements were spin-coated on quartz substrates from chlorobenzene solutions with a concentration of 10 mg/mL. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard threeelectrode electrochemical cell in a 0.1M tetrabutylammonium hexafluorophosphate (TBAPF₆) solution (in chorobenzene) at room temperature with a scanning rate of 50mV/s. In each case, a carbon working electrode coated with