A consequence of the environmental problems associated with fossil fuel use is that there is a great need for clean alternative energy sources. The sun is the world’s major energy source and each hour approximately 4.3 x 1020 J of energy from the sun strikes the Earth. As a result, solar energy could meet the world’s energy requirements and provide an environmentally benign and sustainable alternative to fossil fuels. Currently, most commercial solar cells are fabricated from silicon (Si) and convert 15-20% of the sunlight into electricity. Despite these impressive efficiencies, Si solar cells are often difficult to fabricate and do not always fit into the integrated photovoltaics landscape, where solar cells can be directly incorporated into the windows or façade of buildings or into portable electronic devices. Therefore, in collaboration with Professor André Taylor’s group in the Department of Chemical and Environmental Engineering at Yale, our group is actively working on two projects to develop alternatives to purely Si based photovoltaics. Information on both projects is summarized below.
I. Rational development of more efficient hybrid carbon nanotube/Si solar cells.
Single-walled carbon nanotubes (SWNTs) possess fascinating electrical properties and offer new entries into a wide range of electronics applications including solar cells. In particular, hybrid photovoltaics which combine p-type SWNTs and n-type Si in the active layer, have given high power conversion efficiencies. The main advantage of this unusual solar cell design is that the intrinsically high photovoltaic efficiency of Si can be realized in a cost-effective manner owing to the low-temperature solution processing which can be used in the fabrication of SWNT/Si bulk heterojunctions. At this stage only a small number of molecules can be used to dope SWNTs, and it is difficult to predict doping levels. Recently, we prepared SWNT thin films doped with either electron deficient early transition metal metallocenes or electron rich late transition metal metallocenes. This resulted in controllable p-type doping in the case of early transition metal metallocenes and n-type doping in the case of the late transition metal metallocenes. In fact, these studies have led to the highest level of n-type doping observed in any SWNT film, with results which are significantly better than those obtained with hydrazine, which was the best previous small molecule dopant. Furthermore, using the n-type doped SWNTs, n-type SWNT/p-type Si photovoltaics were developed which are over 450 times more efficient than those previously reported in the literature (Figure 1).
During the doping study it was observed that dioxygen absorbed onto SWNTs affects the properties of the solar cells and its removal is important for high efficiency devices. The conventional method to remove dioxygen from SWNTs involves heating at high temperature under vacuum for approximately 72 hours. A new low temperature process for oxygen removal, which involves treatment of the SWNTs with HF and a cathodic current was developed. The new process is significantly faster than the traditional method and has the potential to become the method of choice for removing oxygen from SWNTs. The next stage in this research is to generalize the results of both the doping studies and oxygen removal process to graphene, and to develop new molecules for the controlled n-type doping of carbon materials.
II. Understanding energy transfer in organic solar cells.
Another alternative to Si based photovoltaics are organic polymer solar cells (PSCs), which have the potential to provide low-cost, lightweight, high surface area, and mechanically flexible energy conversion devices. However, currently the efficiency of PSCs is too low for commercialization. In 2013, we reported ternary PSCs, containing a donating polymer (P3HT), an acceptor (PCBM) and a squaraine dye. In these devices it was established that due to overlap between the polymer emission and the squaraine absorption, Förster Energy Resonance Transfer (FRET) occurs from the polymer to the dye and limits recombination (Figure 2). As a result of both FRET and increased absorption due to the dye, a 38% increase in the power conversion efficiency was demonstrated, when the squaraine dye was present. This was the first time FRET was unequivocally shown in PSCs, and suggests that it can be used as a general design principle to improve PSC efficiency. In addition, it was established that the squaraine dye assists in developing an ordered nano-morphology for enhanced charge transport. This is particularly noteworthy given that in other ternary blend PSCs the third component has a harmful effect on morphology. In future work, fundamental studies to further understand FRET in PSCs using transient spectroscopy will be performed. Furthermore ‘cascade’ solar cells, which feature a series of organic dyes and/or polymers, with overlapping absorption and emission spectra, will be developed, to create PSCs with multiple FRET pairs.