A case of polymer photovoltaics in India

Abstract

Polymer solar cells are light weight, flexible and inexpensive. There has been great progress in three important aspects of polymer solar cells in the last decade, i.e. efficiency, stability and processing. The efficiencies of single-junction solar cells have been improved by stacking two or more complementary single cells in the form of tandem cells. There has been tremendous progress in roll-to-roll processing of single and tandem cells resulting in the setting up 1 GW solar power parks in Denmark and south of Spain. India has an ambitious programme of setting up solar energy power houses up to 300 GW by 2030. However, subcritical research and development is taking place in the field of polymer solar cells in India. It is high time we decide to pursue intensive polymer photovoltaics in the country. Following the commentary by Bose, I discuss here polymer photovoltaics in India. The progress in polymer photovoltaics has no parallel in any technological growth in the world. Polymer solar cells being light weight, flexible and inexpensive can be produced by cheap low-temperature solution processing. Three important aspects of polymer solar cells, i.e. efficiency, stability and processing have shown tremendous progress. Tang reported the first polymer solar cell in 1986 using bilayer structure demonstrating a power conversion efficiency of 1%. The last decade witnessed the impressive development in bulk heterojunction concept in polymer solar cell technology leading to the increase in single-junction cell efficiency from 4% in 2005 (refs 8 and 9) to state-of-the-art present-day efficiencies of 8–9% (refs 10–15; Figure 1). By optimization of materials with proper band gap (necessarily low band gap), energy levels and carrier mobility, efficiencies up to 10–12% are achievable in single-junction solar cells. The organic tandem solar cells comprising two series-connected single cells and covering complementary solar spectra can reach theoretical Power Conversion Efficiency (PCE) of 15% (refs 17 and 18). You et al. have demonstrated an efficiency of 10.6% in solution-processed tandem cells (Figure 2). Heliatek Co, Germany has recently demonstrated certified 12% efficiency in a vacuumprocessed small-molecule organic triplejunction device with an active area of > 1 cm. There has been remarkable progress in the stability of bulk heterojunction solar cells in the last ten years. There are various degradation phenomena taking place in polymer solar cells. When the device is illuminated, complex chemical reactions of organic semiconductors with oxygen and moisture have been observed. Similarly, interface is another source of degradation. The diffusion of the electrode materials into an active layer, acidity of the poly(3,4-ethylene-dioxythiophene)– polystyrene-para-sulfonic acid (PEDOT : PSS) layer and change of morphology of active layer are some reasons for the degradation of polymer solar cells. These instabilities can be significantly reduced by appropriate encapsulation. Flexible poly(3-hexyl thiophene) : (6,6)-phenylC61-butyric methyl ester) (P3HT : PC61BM), organic photovoltaics (OPV) devices encapsulated with food-quality packaging barrier film have shown a lifetime over 1200 h. Where a higher quality barrier film was used, lifetimes of 4000 h (at 65C/85% relative humidity) have been achieved. Peters et al. recently reported a lifetime of 7 years for poly[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl2,5-thiophenediyl]–(6,6)-phenyl-C61 butyric methyl-ester (PCDTBT–PCBM) solar cells. An inter-laboratory outdoor stability study of flexible roll-to-roll coated organic photovoltaic modules (P3HT : PCBM inverted architecture) in different geographical locations from the southern and northern hemispheres has been undertaken. The most stable modules have demonstrated lifetime of more than 10,000 h and sub-cell analyses revealed stability of up to 17 months. Development has also taken place in the processing of polymer solar cells resulting in the setting up 1 GW solar power parks in southern Spain and Denmark. Spin coating is the most popular method in OPV device fabrication in research laboratories. Various printing and coating techniques have been found compatible to fast roll-to-roll processing. In the past it was confined to single donor– acceptor polymer solar cells. Frederic Krebs and his research team at the Technical University of Denmark have demonstrated for the first time the successful roll-to-roll manufacture of tandem OPV modules, each comprised of a stack of 14 discrete layers which are rapidly printed, coated or deposited on top of another by a machine reminiscent of a printing press (Figure 3). The processing was carried out under simple conditions; it is very fast with single solar cell module being printed onto blank foil each second. The whole processing is cheap