Flexible Organic Photovoltaics Employ laser produced metal nanoparticles into the absorption layer 1. An Introduction Among the renewable energy sources that are called to satisfy the continuously increased worldwide demand of energy are Photovoltaics (PVs). The latter devices convert solar energy into electricity. The same operational principle is used by the human vision system. The latter converts the captured illuminated images into electrical signal and the human eye system transfers the information data to the human brain. There the information is decoded and we realize that we have seen a 'friend' for an example. Thus it can be said that a PV device imitates the human vision system. One of the main features that characterize a Photovoltaic (PV) device is its external quantum efficiency that addresses the number of photons that are converted into electrons. The PVs that are made by organic semiconductors, organic photovoltaics (OPVs), employ thin films made by organic compounds and are characterized by low fabrication cost, straight forward fabrication processes, flexibility and continuously increased efficiencies through the years. Nanoparticles are defined as small objects that are sized between 1 100 nm and mainly their dimensions determine their mechanical, electrical and optical properties. The size dependent properties of nanoparticles have been realized since the 9 th century. For an example spherical gold nanoparticles with diameters from 25 to 100 nm look red, green and orange respectively. Nanoparticles act as amplifiers or scattering centers for the sunlight that incidents into photovoltaics, through the plasmonic effect and this has reported extensively. One technique that has followed in order to increase the efficiency of organic photovoltaics, today registered maximum value around 12%, is to employ into the photoactive layer of a OPV stable uncapped metal nanoparticles (NPs) or bimetal compounds. The NPs enhance the light harvesting in the visible region of the solar spectrum due to plasmon excitations. The localized surface plasmon resonance (LSPR) of metallic nanoparticles leads to 1
electromagnetic field enhancement nearby. The very high absorption coefficient of the metal nanoparticle around the plasmon resonance could be advantageously used to increase the absorbance at wavelengths where the absorption coefficient of the organic material is dropping off. Embedding metal nanoparticles inside the organic photovoltaic devices and exploiting the near-field enhanced absorption, enhancement of the exciton yield and the overall efficiency can be achieved. The employed NPs, depending on their sizes, operate as scattering or absorption centers for the visible light and as a consequence they trap stronger part of the solar spectrum within the OPV. In this presentation we present the development of such device sensor. 2. The Sensor The photovoltaic effect that first introduced in 1839 by Becquerel describes the conversion of light into electricity [1]. Since then the photovoltaic effect has studied in various elements such as crystalline silicon, GaAs, peroskites, organic semiconductors. PVs are emerging as a clean and sustainable energy sources that are expected to play a major role in meeting the worldwide energy needs. The efficiencies of PVs are expected in 2015 to reach efficiencies as high as 45%. The crystalline silicon based devices have dominated todays solar cell market. However intensive investigations in other materials occur in order to reduce the cost of the produced electricity by increasing the device power efficiency, reducing the amount of absorbing material needed and lowering the assembly cost of modules. Among other materials the last two decades the technology of organic semiconductors has advanced rapidly offering a number of solid state devices such as Organic Light Emitting Diodes (OLEDs) [2], field effect transistors [3], photodiodes [4] and photovoltaic devices [5]. The use of organic semiconductors as parts of PVs has allowed the production of flexible devices that the use of inorganic materials prohibited. This allowed the fabrication of lightweight, low cost, transparent and flexible organic PVs. These properties combined with the freedom to tune the properties of the 2
selected organic semiconductor during its fabrication constitute the main initiatives to research and market interest in organic photovoltaics. The organic photovoltaics that are called third generation photovoltaics have presented current external efficiency record close to 12%. Figure 1: Best research efficiencies of 3 rd generation PVs (http://juanbisquert.wordpress.com/2013/05/) In order to understand better the operation of an organic solar cell, figure 2 depicts the cross section of such a device. Figure 2: Cross section of 3 rd generation PVs (http://www.eight19.com/technology/printed-plastic-solar-cells) One of the major goals of the organic solar cell technology is the increase of the external efficiency. Different approaches have been followed towards the 3
satisfaction of this goal from the selection of different organic materials, organic inorganic composites and better trapping of the incident solar radiation photons within the active layer of the cell. Figure 2 depicts a cross section of an OPV. The heart of such device is the active medium where the sun light photons incident. Next to the active layer there are two individual layers that sit next to each other (see figure 4): the electron transport layer and the hole transport layer. The absorbed photon by the medium in the active layer generates an exciton. Figure 3: Formation of excitons in OPVs The latter is a bound couple of electron and hole. Under the influence of strong internal electric fields found at polymer/metal interfaces the electron and the holes separate and follow their ways towards the cathode and the anode respectively. A great deal is paid for the electrons and the holes to reach the electrodes before recombine. The distance that an exciton diffuses into the active region before recombine is called exciton diffusion length. Typical values are around 10 nm. The thinner the active medium (typical values of the order of 100 nm) is the higher the chances are for an electron to reach the cathode, through the electron transfer layer. But thinner active 4
medium means lower number of potential light generated free charge carriers. Figure 4: Device Architectures in 3 rd generation OPVs Another way to increase the number of free electrons that reach the cathode, is to increase the number of photons that are absorbed by the active medium: a transparent electrode is used in order the photons to reach the active medium. The selection of the electrodes is strongly dependent to the energy levels of the electron donor and electron acceptor layers and the work functions of the cathode and the anode electrodes respectively. Figure 5 shows how the LUMO and the HOMO of the electron and hole transport layers should be aligned compared to the work functions of cathode and anode for efficient extraction of electrons and holes through cathode and anode respectively. 5
Figure 5: LUMO & HOMO levels alignment compared to the metal electrodes in 3 rd generation OPVs Until recently the major material for the anode in organic PVs that use PEDOT:PSS as a hole transport layer is the ITO. The main disadvantages of ITO are the following: (a) is based in rare earth element which is mined in limited geographical areas and (b) its operation is degraded a lot if it bends which means is not compatible with flexible electronics technology. Nowadays GO electrodes are called to replace ITO as a transparent anode electrode in flexible electronics [6]. To maximize absorption of the active layer and at the same time to keep its thickness as low as possible metal nanoparticles are added into the active layer. The expected increase in conversion efficiency can be attributed to Localized Surface Plasmon Resonance at the small diameter metal nanoparticles (diameter smaller than 50 nm) and to efficient scattering by the large diameter NPs (diameter bigger than 50 nm). Metal NPs like gold (Au) and aluminium (Al) exhibit strong absorption in UV visible spectrum that lies within the absorption spectrum of conjugated polymers that we use in OPVs. The metal nanoparticles can be generated by ps and fs laser ablation in liquids that contain the NPs. This technique provides the possibility of generating a large variety of NPs that are free of both surface-active substances and counter-ions. Solutions that contain Al and Au NPs placed into ethanol are irradiated with short laser pulses for few seconds and a mixture of various diameters Au and Al NPs are expected to generate. The 6
Au absorbs strongly at 535 nm whereas the Al absorbs strongly at UV. Compounds of these two NPs thus absorb from UV to Visible Spectrum. Figure 6 shows the setup using silver NPs into the active region. Figure 6: Incorporation of NPs into the active region of OPVs [http://photonicsforenergy.spiedigitallibrary.org/article.aspx?articleid=1166231] The metal NPs can be placed above the active layer in order to couple the escaped photons back into the active region as shown in figure 7. Figure 7: Incorporation of NPs on top of the active region of OPVs [http://en.wikipedia.org/wiki/plasmonic_solar_cell] In summery we described how an organic solar solar that incorporates laser produced metal nanoparticles operate. This device uses the same operational principles as this one that our eye uses: conversion of light to electrical signal. 7
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