The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Conjugated polymer-based organic solar cells

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

-phenylenevinylene)s L4. Fluorene-Based Conjugated Polymers L4.1. Fluorene-Based Copolymers ContainingElectron-Rich MoietiesM4.2. Fluorene-Based Copolymers ContainingElectron-Deﬁcient MoietiesN4.3. Fluorene-Based Copolymers ContainingPhosphorescent ComplexesQ5. Carbazole-Based Conjugated Polymers R5.1. Poly(2,7-carbazole)-Based Polymers R5.2. Indolo[3,2-

https://ir.nctu.edu.tw:443/bitstream/11536/6508/1/000271856900020.pdf

Synthesis of Conjugated Polymers for Organic Solar Cell Applications

Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* tra...

Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy

In this article we study the interface between poly (9,9-dioctylfluorene) (PFO) and different alkali metals (Cs, K, Na, and Li) by photoelectron spectroscopy. The low work-function alkali metals led to low or no electron injection barrier at the PFO interface. From the ultraviolet photoelectron spectroscopy, alteration of electronic structures upon Cs, K, Na, or Li doping into PFO represented a charge transfer process among them. Two new gap states known as bipolaron states were found above the highest-occupied molecular orbital of PFO. Variations in the intensity and feature of these gap states with increasing coverage of the alkali metals were correlated with changes of C 1s shakeup peaks acquired from x-ray photoelectron spectroscopy. From the deduced energy level diagram, it is suggested that the new gap states may reduce the radiative recombination of holes and electrons in the polymer light-emitting devices. Films exposed either to residual gases at a pressure of 2.0×10−9 mbar for 3 h or to small am...

Interface between poly (9,9-dioctylfluorene) and alkali metals: cesium, potassium, sodium, and lithium

Photoelectron spectroscopic studies of poly(9,9-dioctylfluorene)-potassium interface, and its influence by oxygen

The surface of poly (9,9-dioctylfluorene) (PFO) film was modified by 1.0 keV Ar+ irradiation with a dose of 6.5×1014 ions/cm2 prior to Ca deposition. Ultraviolet and x-ray photoelectron spectroscopic studies indicated that the modified surface could effectively block Ca diffusion into the PFO film and prevent the formation of doping-induced bipolaron states in the former forbidden energy gap. As a result, a sharper metal contact on the surface of the PFO film could be formed, compared to that on the surface without Ar+ irradiation. The results suggest that the judicial surface modification of polymer surfaces may be useful for the improvement of metal/polymer contacts and thus device performance.

Improvement of interface formation between metal electrode and polymer film by polymer surface modification using ion sputtering

The effects of ambient storage, thermal annealing, and ion irradiation on the electronic structure of poly (9,9-dioctylfluorene) (PFO) films were investigated using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. It was shown that PFO is a rather stable material used for light-emitting devices.

M. Inbasekaran

Papers

Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy

Interface between poly (9,9-dioctylfluorene) and alkali metals: cesium, potassium, sodium, and lithium

Photoelectron spectroscopic studies of poly(9,9-dioctylfluorene)-potassium interface, and its influence by oxygen

Improvement of interface formation between metal electrode and polymer film by polymer surface modification using ion sputtering

51.2: Invited Paper: Stability Study of Poly (9,9‐Dioctylfluorene) Film Using Photoelectron Spectroscopy