There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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

Low-cost photovoltaic energy conversion using conjugated molecular materials has become increasingly feasible through the development of organic ‘bulk heterojunction (BHJ)’ structures1,2,3,4,5,6,7, where efficient light-induced charge separation is enabled by a large-area donor–acceptor interface2,3. The highest efficiencies have been achieved using blends of poly(3-hexylthiophene) (P3HT) and a fullerene derivative8,9,10,11,12, but performance depends critically on the material properties and processing conditions. This variability is believed to be influenced by the self-organizing properties of P3HT, which means that both optical13,14 and electronic15,16 properties are sensitive to the molecular packing. However, the relationship between molecular nanostructure, optoelectronic properties of the blend material and device performance has not yet been demonstrated. Here we focus on the influence of polymer regioregularity (RR) on the molecular nanostructure, and hence on the resulting material properties and device performance. We find a strong influence of RR on solar-cell performance, which can be attributed to enhanced optical absorption and transport resulting from the organization of P3HT chains and domains. Further optimization of devices using the highest RR material resulted in a power conversion efficiency of 4.4%, even without optimization of electrodes7.

A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells

Here we report enhanced efficiency bulk heterojunction organic solar cells using blend films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) that are subjected to a thermal annealing process. Blend films (P3HT:PCBM=1:1 by weight) were prepared using chlorobenzene and 1,2-dichlorobenzene in order to investigate the role of the solvent. Irrespective of the chosen solvent, the optimal device annealing temperature was found to be 140 °C. The highest power conversion efficiency, 3% under air mass 1.5 simulated solar illumination (100mW∕cm2), was achieved by device annealing at 140 °C for 15 min using blend films prepared from chlorobenzene (2.3% for 1,2-dichlorobenzene).

Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene

The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. The authors report the creation of an efficient electron selective bottom contact based on a solution-processed titanium oxide interfacial layer on the top of indium tin oxide. The use of o-xylene as a solvent creates an efficient carrier collection network with little vertical phase segregation, providing sufficient performance for both regular and inverted solar cells. The authors demonstrate inverted layer sequence OPVs with AM 1.5 calibrated power conversion efficiencies of over 3%.

Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact

High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends

The technological attraction in organic solar cells is their compatibility to printing processes. However, up to today, nearly no literature on "printed" organic solar cells have been published and the major body of the research work was done by spin coating or blading techniques. Transferring the spin-coating or doctor blading process currently used for the fabrication of bulk heterojunction solar cell to a printing process holds morphological challenges that have not been observed or reported up to today. We highlight these challenges and we show that inkjet printing of organic bulk heterojunction solar cells requires completely novel approaches and skill sets compared to the current state of the art. By adjusting the chemical properties of the poly(3-hexylthiophene) polymer donor and by using our recently developed inkjet solvent mixture, we have gained control over the nanomorphology of poly(3-hexylthiophene):fullerene blends during the printing process and report a new record power conversion efficiency of 3.5% for inkjet printed poly(3-hexylthiophene):fullerene based solar cells.

Stelios A. Choulis

Papers

A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells

Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene

Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact

High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends

Printing highly efficient organic solar cells.