Organic photovoltaics

Energy inflation, the constant encouragement to economize on energy consumption and the huge investments in developing alternative energy resources might seem to suggest that there is a global shortage of energy. Far from it, the energy the Sun beams on the Earth each hour is equivalent to a year’s supply, even at our increasingly ravenous rate of global energy consumption [1]. But it’s not what you have got it’s what you do with it. Hence the intense focus on photovoltaic research to find more efficient ways to harness energy from the Sun. Recently much of this research has centred on organic solar cells since they offer simple, low-cost, light-weight and large-area flexible photovoltaic structures. This issue with guest editors Frederik C Krebs and Hongzheng Chen focuses on some of the developments at the frontier of organic photovoltaic technology. Improving the power conversion efficiency of organic photovoltaic systems, while maintaining the inherent material, economic and fabrication benefits, has absorbed a great deal of research attention in recent years. Here significant progress has been made with reports now of organic photovoltaic devices with efficiencies of around 10%. Yet operating effectively across the electromagnetic spectrum remains a challenge. ‘The trend is towards engineering low bandgap polymers with a wide optical absorption range and efficient hole/electron transport materials, so that light harvesting in the red and infrared region is enhanced and as much light of the solar spectrum as possible can be converted into an electrical current’, explains Mukundan Thelakkat and colleagues in Germany, the US and UK. In this special issue they report on how charge carrier mobility and morphology of the active blend layer in thin film organic solar cells correlate with device parameters [2]. The work contributes to a better understanding of the solar-cell characteristics of polymer:fullerene blends, which form the material basis for some of the most successful solution processable organic photovoltaic devices at present. Andrey E Rudenko, Sangtaik Noh, and Barry C Thompson at the University of Southern California, Los Angeles, combine two approaches to broaden the absorption of conjugated polymers [3]. In atomistic bandgap control, a heavier chalcogen heteroatom is introduced into the aromatic repeat unit to decrease the HOMO–LUMO gap. In the semi-random donor–acceptor polymer architecture, small amounts of electron deficient monomers are incorporated at random. ‘We have successfully established the concept of extending photon absorption through the combination of atomistic bandgap control and the donor–acceptor-based semi-random platform using a family of three new semi-random selenophene-based polymers’, explain Thompson and colleagues. They add that the polymers exhibit extended and enhanced photon absorption compared with their polythiophene analogues while maintaining semicrystallinity. The benefits of various fabrication treatments are also reported, such as methanol rinsing for modifying the active layer interface [4] and annealing to achieve bicontinuous nanoscale phase separation for efficient exciton dissociation and charge collection [5]. The issue highlights how successfully structure and morphology can be manipulated to optimize solar-cell efficiencies while retaining advantageous material properties, with reports of innovative studies of bulk heterojunction [6–9] and inverse [10–13] structures, as well as innovative replacements for the traditional ITO transparent conducting electrode [14, 15]. Thomas Edison is famously quoted as saying ‘I’d put my money on the Sun and solar energy, what a source of power! I hope we don’t have to wait until oil and coal run out, before we tackle that’. Born in the wake of the industrial revolution when coal was king, the words seem strangely anachronistic and ahead of his time. As an undisputed genius of inventions it should not surprise us that he had such remarkable foresight, nor that the present generation of innovators are ‘tackling’ the opportunity with such promise and success, as the work in this special issue clearly demonstrates. References [1] http://environment.nationalgeographic. co.uk/environment/global-warming/solar-power-profile [2] Muth M-A, Mitchel W, Tierney S, Lada T A, Xue X, Richter H, Carrasco-Orozco M and Thelakkat M 2013 Influence of charge carrier mobility and morphology on solar cell parameters in devices of mono- and bis-fullerene adducts Nanotechnology 24 484001 [3] Rudenko A E, Noh S and Thompson B C 2013 Influence of selenophene on the properties of semi-random polymers and their blends with PC61BM Nanotechnology 24 484002 [4] Zhang K, Hu Z, Duan C, Ying L, Huang F and Cao Y 2013 The effect of methanol treatment on the performance of polymer solar cells Nanotechnology 24 484003 [5] Meng B, Fang G, Fu Y, Xie Z and Wang L 2013 Fine tuning of the PCDTBT-OR:PC71BM blend nanoscale phase separation via selective solvent annealing toward high-performance polymer photovoltaics Nanotechnology 24 484004 [6] Arar M et al 2013 Influence of morphology and polymer:nanoparticle ratio on device performance of hybrid solar cells—an approach in experiment and simulation Nanotechnology 24 484005 [7] Yu B, Wang H and Yan D 2013 Efficient organic photovoltaic cells with vertical ordered bulk heterojunction Nanotechnology 24 484006 [8] Chen G, Sasabe H, Sano T, Wang X-F, Hong Z, Kido J and Yang Y 2013 Chloroboron (III) subnaphthalocyanine as an electron donor in bulk heterojunction photovoltaic cells Nanotechnology 24 484007 [9] Cheng P, Li Y and Zhan X 2013 DMF-assisted solution process boosts the efficiency in P3HT:PCBM solar cells up to 5.31% Nanotechnology 24 484008 [10] Chen H-Y, Lin S-H, Sun J-Y, Hsu C-H, Lan S and Lin C-F 2013 Morphologic improvement of the PBDTTT-C and PC71BM blend film with mixed solvent for high-performance inverted polymer solar cells Nanotechnology 24 484009 [11] Peng J, Sun Q, Zhai Z, Yuan J, Huang X, Jin Z, Li K, Wang S, Wang H and Ma W 2013 Low-temperature, solution-processed alumina for organic solar cells Nanotechnology 24 484010 [12] Chen F, Chen Q, Mao L, Wang Y, Huang X, Lu W, Wang B and Chen L 2013 Tuning ITO work function with solution-processed alkali carbonate interfacial layers for high-efficiency inverted organic photovoltaic cells Nanotechnology 24 484011 [13] Wu Z, Song T, Xia Z, Wei H and Sun B 2013 Enhanced performance of polymer solar cell with ZnO nanoparticle electron transporting layer passivated by in situ cross-linked three-dimensional polymer network Nanotechnology 24 484012 [14] Urbina A, Park J-S, Lee J-M, Kim S-O and Kim J-S 2013 Work function engineering of ZnO electrodes by using p-type and n-type doped carbon nanotubes Nanotechnology 24 484013 [15] van de Wiel H J et al 2013 R2R embedded conductive structures integrated into OPV devices Nanotechnology 24 484014