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Wednesday, March 1, 2023

Market share

The popularity of online shopping continues to erode sales of conventional retailers. For example, Best Buy, the largest retailer of electronics in the U.S. in August 2014 reported its tenth consecutive quarterly dip in sales, citing an increasing shift by consumers to online shopping.^[50] Amazon.com has the largest market share in the United States. As of May 2018, a survey found two-thirds of Americans had bought something from Amazon (92% of those who had bought anything online), with 40% of online shoppers buying something from Amazon at least once a month. The survey found shopping began at amazon.com 44% of the time, compared to a general search engine at 33%. It estimated 75 million Americans subscribe to Amazon Prime and 35 million more use someone else's account.^[51]

There were 242 million people shopping online in China in 2012.^[52] For developing countries and low-income households in developed countries, adoption of e-commerce in place of or in addition to conventional methods is limited by a lack of affordable Internet access.

Advantages

Convenience

Online stores are usually available 24 hours a day, and many consumers in Western countries have Internet access both at work and at home. Other establishments such as Internet cafes, community centers and schools provide internet access as well. In contrast, visiting a conventional retail store requires travel or commuting and costs such as gas, parking, or bus tickets, and must usually take place during business hours. Delivery was always a problem which affected the convenience of online shopping. Additionally, the online shopping industry has not only involved the concept of providing convenience for customers but also improved perceptions of social inclusion.^[53] However to overcome this many retailers including online retailers in Taiwan brought in a store pick up service. This now meant that customers could purchase goods online and pick them up at a nearby convenience store, making online shopping more advantageous to customers.^[54] In the event of a problem with the item (e.g., the product was not what the consumer ordered or the product was not satisfactory), consumers are concerned with the ease of returning an item in exchange for the correct product or a refund. Consumers may need to contact the retailer, visit the post office and pay return shipping, and then wait for a replacement or refund. Some online companies have more generous return policies to compensate for the traditional advantage of physical stores. For example, the online shoe retailer Zappos.com includes labels for free return shipping, and does not charge a restocking fee, even for returns which are not the result of merchant error. (Note: In the United Kingdom, online shops are prohibited from charging a restocking fee if the consumer cancels their order in accordance with the Consumer Protection (Distance Selling) Act 2000).^[55] A 2018 survey in the United States found 26% of online shoppers said they never return items, and another 65% said they rarely do so.^[56] Merchants may benefit from online shopping due to low sales inventory pressure, low operating costs, and the scale of operation is not limited by the site.

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User interface

An automated online assistant, with potential to enhance user interface on shopping sites.

The most important factors determining whether customers return to a website are ease of use and the presence of user-friendly features.^[49] Usability testing is important for finding problems and improvements in a web site. Methods for evaluating usability include heuristic evaluation, cognitive walkthrough, and user testing. Each technique has its own characteristics and emphasizes different aspects of the user experience.^[49]

Market share

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Una celda fotovoltaica es un diodo semiconductor especializado que convierte la luz

Física [ editar ]

Fig. 3: Ejemplos de materiales fotovoltaicos orgánicos

Una celda fotovoltaica es un diodo semiconductor especializado que convierte la luz en electricidad de corriente continua (CC). Dependiendo de la brecha de banda del material que absorbe la luz, las células fotovoltaicas también pueden convertir fotones infrarrojos (IR) de baja energía o ultravioleta (UV) de alta energía en electricidad de CC. Una característica común tanto de las moléculas pequeñas como de los polímeros (Fig. 3) utilizados como material absorbente de luz en la energía fotovoltaica es que todos tienen grandes sistemas conjugados . Se forma un sistema conjugado donde los átomos de carbono se unen covalentementeenlace con enlaces simples y dobles alternantes. Los orbitales pz de los electrones de estos hidrocarburos se deslocalizan y forman un orbital π enlazante deslocalizado con un orbital antienlazante π* . El orbital π deslocalizado es el orbital molecular ocupado más alto ( HOMO ), y el orbital π* es el orbital molecular desocupado más bajo ( LUMO ). En la física de semiconductores orgánicos, el HOMO asume el papel de la banda de valencia mientras que el LUMO sirve como la banda de conducción . La separación de energía entre los niveles de energía HOMO y LUMO se considera la brecha de banda de los materiales electrónicos orgánicos y normalmente está en el rango de 1 a 4 eV . ^[11]

Toda la luz con energía mayor que la banda prohibida del material puede ser absorbida, aunque existe una compensación para reducir la banda prohibida ya que los fotones absorbidos con energías mayores que la banda prohibida emitirán térmicamente su exceso de energía, lo que resultará en voltajes más bajos. y eficiencias de conversión de energía. Cuando estos materiales absorben un fotón , se crea un estado excitado y se limita a una molécula o una región de una cadena polimérica. El estado excitado puede considerarse como un excitón , o un par electrón-hueco unido por acción electrostática .interacciones. En las células fotovoltaicas, los campos efectivos descomponen los excitones en pares de huecos de electrones libres. Los campos efectivos se establecen creando una heterounión entre dos materiales diferentes. En la energía fotovoltaica orgánica, los campos efectivos rompen los excitones al hacer que el electrón caiga de la banda de conducción del absorbedor a la banda de conducción de la molécula aceptora. Es necesario que el material aceptor tenga un borde de la banda de conducción más bajo que el del material absorbente. ^[12]^[13]^[14]^[15]

Fig. 4: Cadena polimérica con polarón en difusión rodeado de moléculas de fullereno

Las celdas solares poliméricas generalmente consisten en una capa de bloqueo de electrones o huecos encima de un vidrio conductor de óxido de indio y estaño (ITO), seguido de un donante de electrones y un aceptor de electrones (en el caso de las celdas solares de heterounión masiva), un hueco o un bloqueador de electrones. capa y electrodo de metalen la parte superior. La naturaleza y el orden de las capas de bloqueo, así como la naturaleza del electrodo de metal, depende de si la celda sigue una arquitectura de dispositivo regular o invertida. En una celda invertida, las cargas eléctricas salen del dispositivo en la dirección opuesta a la de un dispositivo normal porque los electrodos positivo y negativo están invertidos. Las celdas invertidas pueden utilizar cátodos de un material más adecuado; Las OPV invertidas disfrutan de una vida útil más larga que las OPV estructuradas regularmente y, por lo general, muestran eficiencias más altas en comparación con las contrapartes convencionales. ^[dieciséis]

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Célula solar orgánica

Una celda solar orgánica ( OSC ^[1] ) o celda solar de plástico es un tipo de fotovoltaica que utiliza electrónica orgánica , una rama de la electrónica que trata con polímeros orgánicos conductores o pequeñas moléculas orgánicas, ^[2] para la absorción de luz y el transporte de carga para producir electricidad de la luz solar por el efecto fotovoltaico . La mayoría de las células fotovoltaicas orgánicas son células solares poliméricas .

Fig. 2. Fotovoltaica Orgánica fabricada por la empresa Solarmer.

Las moléculas utilizadas en las células solares orgánicas son procesables en solución con un alto rendimiento y son económicas, lo que genera bajos costos de producción para fabricar un gran volumen. ^[3] En combinación con la flexibilidad de las moléculas orgánicas, las células solares orgánicas son potencialmente rentables para aplicaciones fotovoltaicas. ^[4] La ingeniería molecular ( p. ej., cambiar la longitud y el grupo funcional de los polímeros ) puede cambiar la brecha de banda , lo que permite la sintonización electrónica. El coeficiente de absorción ópticade moléculas orgánicas es alto, por lo que se puede absorber una gran cantidad de luz con una pequeña cantidad de materiales, generalmente del orden de cientos de nanómetros. Las principales desventajas asociadas con las células fotovoltaicas orgánicas son la baja eficiencia , la baja estabilidad y la baja resistencia en comparación con las células fotovoltaicas inorgánicas, como las células solares de silicio .

En comparación con los dispositivos basados en silicio , las células solares poliméricas son livianas (lo cual es importante para los sensores autónomos pequeños), potencialmente desechables y económicas de fabricar (a veces usando electrónica impresa ), flexibles, personalizables a nivel molecular y potencialmente tienen un impacto ambiental menos adverso. Las celdas solares de polímero también tienen el potencial de exhibir transparencia, lo que sugiere aplicaciones en ventanas, paredes, electrónica flexible, etc. En la Fig. 1 se muestra un dispositivo de ejemplo. Las desventajas de las celdas solares de polímero también son serias: ofrecen alrededor de 1/3 de la eficiencia de los materiales duros y experimentan una degradación fotoquímica sustancial. ^[5]

Los problemas de ineficiencia y estabilidad de las células solares poliméricas, ^[6] combinados con su promesa de bajos costos ^[7] y mayor eficiencia ^[8] las convirtieron en un campo popular en la investigación de células solares. A partir de 2015, las células solares de polímero pudieron lograr una eficiencia de más del 10% a través de una estructura en tándem. ^[9] En 2018, se alcanzó una eficiencia récord para la energía fotovoltaica orgánica del 17,3 % a través de la estructura en tándem. ^[10]

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Vitamin A plays an essential role

Vitamin A plays an essential role in developing the immune system in children, therefore, it is considered an essential micronutrient that can greatly affect health.^[2] However, because of the expense of testing for deficiencies, many developing nations have not been able to fully detect and address vitamin A deficiency, leaving vitamin A deficiency considered a silent hunger.^[2] According to estimates, subclinical vitamin A deficiency, characterized by low retinol levels, affects 190 million pre-school children and 19 million mothers worldwide.^[112]

The WHO estimates that 5.2 million of these children under five are affected by night blindness, which is considered clinical vitamin A deficiency.^[113] Severe vitamin A deficiency (VAD) for developing children can result in visual impairments, anemia and weakened immunity, and increase their risk of morbidity and mortality from infectious disease.^[114] This also presents a problem for women, with WHO estimating that 9.8 million women are affected by night blindness.^[115] Clinical vitamin A deficiency is particularly common among pregnant women, with prevalence rates as high as 9.8% in South-East Asia.^[112]

Estimates say that 28.5% of the global population is iodine deficient, representing 1.88 billion individuals.^[116] Although salt iodization programs have reduced the prevalence of iodine deficiency, this is still a public health concern in 32 nations. Moderate deficiencies are common in Europe and Africa, and over consumption is common in the Americas.^[90] Iodine-deficient diets can interfere with adequate thyroid hormone production, which is responsible for normal growth in the brain and nervous system. This ultimately leads to poor school performance and impaired intellectual capabilities.^[2]

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Vitamin and mineral malnutrition

Vitamins and minerals are essential to the proper functioning and maintenance of the human body.^[105] There are 20 trace elements and minerals that are essential in small quantities to body function and overall human health.^[105]

Iron deficiency is the most common inadequate nutrient worldwide, affecting approximately 2 billion people.^[106] Globally, anemia affects 1.6 billion people, and represents a public health emergency in mothers and children under five.^[107] The World Health Organization estimates that there exists 469 million women of reproductive age and approximately 600 million preschool and school-age children worldwide who are anemic.^[108] Anemia, especially iron-deficient anemia, is a critical problem for cognitive developments in children, and its presence leads to maternal deaths and poor brain and motor development in children.^[2] The development of anemia affects mothers and children more because infants and children have higher iron requirements for growth.^[109] Health consequences for iron deficiency in young children include increased perinatal mortality, delayed mental and physical development, negative behavioral consequences, reduced auditory and visual function, and impaired physical performance.^[110] The harm caused by iron deficiency during child development cannot be reversed and result in reduced academic performance, poor physical work capacity, and decreased productivity in adulthood.^[3] Mothers are also very susceptible to iron-deficient anemia because women lose iron during menstruation, and rarely supplement it in their diet.^[3] Maternal iron deficiency anemia increases the chances of maternal mortality, contributing to at least 18% of maternal deaths in low and middle income countries.^[111]

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Thursday, March 2, 2023

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Wednesday, March 1, 2023

Market share

Market share