Efficiency of photovoltaic cells
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This year's Nobel Prize has been awarded to an American physicist and chemist whose work paved the way to built efficient and low-cost polymer photovoltaic cells.
Professor A. J. Heeger of University of California at Santa Barbara, US received the prestigious Nobel Prize for his research on polymer photovoltaic solar cells over the past two decades.
In 1995 Heeger published a paper (science 270 1789), in which he proposed a new approach to fabricate photovoltaic devices, which led to the development of efficient solar cells. This enabled to fabricate renewable, sustainable, and recyclable, low cost photovoltaic devices which are used to convert light energy into electric current.
This approach is enhanced and widely used in commercial applications to produce flexible organic solar cells. The increasing demand for energy has created a need for low cost and eco-friendly energy source. Solar power, which is a renewable energy source holds good for producing energy at low cost.
Efficiency of Photovoltaic cells depends on the energy conversion and charge collection of the device, which are high in inorganic based photovoltaic device, however organic photovoltaic devices have major advantages over inorganic photovoltaic devices i.e., low-cost fabrication, mechanical flexibility and disposability. This led many researches to focus on polymer photovoltaic cells, hence several approaches have been proposed for fabricating photovoltaic cells like mono and bilayered organic solar cells by using photo induced electron transfer in composites of conducting polymers as donors(D) and Buckminsterfullerene and its derivatives as acceptors(A). However the conversion efficiency is limited by the carrier collection efficiency at the D-A interface. A major breakthrough in the field of organic photovoltaic cells is achieved by overcoming the limitation of the efficiency in the bilayer heterojunction, which is proposed by Heeger. A high interfacial area is achieved within a bulk material by carefully controlling the morphology of the phase separation into an interpenetrating bicontinuous D-A network, which yields efficient photo induced charge separation, this obtained interfacial area is known as "Bulk D-A heterojunction". Even though the bulk heterojunction is previously proposed by Hiramoto et al, [J. Appl. Phys. 72, 3781, 1992] but the fabrication of solar cells is far more difficult than that of Heeger approach.
Heeger used composite film of poly (2-methoxy-5-(2-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) and soluble derivatives of buckminsterfullerene namely [6,6]PCBM and [5,6]PCBM to form a polymer blend. To overcome the limited solubility of C60, a series of soluble C60 derivatives are used, this concept of soluble C60 derivatives enabled to realize new device concept. The structure of bulk heterojunction consists of metal electrode contacts (Ca or Al) of different work function to optimise the efficiency of carrier collection of holes from donor phase and electrons from acceptor phase.
Indium tin oxide (ITO) is used as anode and Ca or Al is used as anode which automatically extracts electrons and holes from the polymer blend. The performance of bulk heterojunction photovoltaic cells depends on the phase separation in polymer blend devices; so much of the research is concentrated on the precise control of the phase separation. The film formation has to be very fast, in order to obtain fine structures i.e., Phase separation has to be arrested earlier, which results in smaller domains than exciton diffusion length. This can be achieved [Adv. Mater. 12, 498, 2000] by spin coating on a heated substrate, so that the solvent evaporates faster.
The quantum efficiency (percentage of photos hitting the photo reactive surface that will produce electron-hole pair) of up to 2.9% is achieved by Heeger, which is further enhanced by using different low molecular weight materials [Adv. Mater. 12, 1270, 2000]. Quantum efficiency can be improved up to 10% by using different materials.
A wide range of research is carried out based on bulk heterojunction approach, which resulted in efficient photovoltaic cells; however organic solar cells degrade when exposed to ultraviolet light, which effects the life time of the cells. Energy conversion efficiency is also low when compared to its inorganic counterparts. Fig 1 shows the efficiency achieved by different research groups in the last decade. 7.9% efficiency is achieved by Solarmer Company, which is certified by National renewable energy laboratory (NREL). Whereas Heeger achieved a quantum efficiency of 2.9%, this indicates a rapid development in this field over a decade. Solarmer produce photovoltaic commercial products using bulk heterojunction approach.
Another company named Konarka which is founded by Heeger also manufactures plastic electronics and solar cells with bulk heterojunction. Konarka offer conventional products like sensors, portable battery charging for PDA, mobiles and other small devices, microelectronics, portable power, remote power, building integrated photovoltaic.
Plextronics is another company developing and selling pre-formulated inks as well as the know-how to print them, which are extensively tested for outdoor lifetime. Device using these products have high lifetimes of the order of years.
However the efficiency of polymer photovoltaic cells is low when compared to the silicon based photovoltaic devices. In order to compete with other available technologies, the efficiency of polymer photovoltaic cells should be increased to 15% with a lifetime of 15-20 years [Solar energy, 2009, 1224]. Heeger made a significant contribution to polymer solar cells field by proposing the bulk Heterojunction approach, which has many potential applications in renewable energy.
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