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Home > Nanotechnology Columns > Emerging Techniques for Organic Photovoltaics > Nanoimprint Lithography: A Promising Technique for Efficient Polymer Solar Cells

Yi Yang
Sr. Engineer/Ph.D. Researcher
GlobalFoundries Inc./UT-Dallas

Abstract:
Nanoimprint lithography has emerged as a new technique to simultaneously control the active layer morphology and polymer chain orientation for efficient charge separation and transport in polymer solar cells.

July 29th, 2014

Nanoimprint Lithography: A Promising Technique for Efficient Polymer Solar Cells

In recent years polymer solar cells have drawn considerable amounts of research interest due to their many attractive features including the potential to be flexible, semitransparent and manufactured in a cost-effective continuous printing process. However, one challenge limiting their commercialization is the relatively low efficiency (7-8%) when compared to their inorganic counterparts (>10%). One of the causes for their low performance is the difficulty to simultaneously realize donor-acceptor phase separation within the short exciton diffusion length (~10 nm) and high charge mobility, especially hole mobility, which are critical for charge separation and transport, respectively.

The bulk heterojunction has shown the possibility to solve these two issues through its large donor-acceptor interface area as well as thermal or solvent vapor annealing assisted polymer crystallization. However, the active layer morphology in this structure cannot be precisely controlled. Significant charge recombination is caused by discrete and randomly distributed phases and thus far from the ideal design.

Nanoimprint lithography is an emerging technique to fabricate high resolution nanostructures at high throughput and low cost. During nanoimprint, the topological patterns on a mold are transferred to the imprint resist (thermoplastic polymer) on a substrate. This technique is hoped to replace the most widely used photolithography in the integrated circuit (IC) fabrication industry. However, in Dr. Yi Yang's research, it is applied to the area of polymer solar cells and he finds that it has a great potential to solve those two challenges for polymer solar cells as described above.

Fig. 1 Process flow of the thermal nanoimprint lithography: schematic of (a) a mold is pressed onto a thin layer of polymer on a substrate heated to a temperature above the polymer's glass transition temperature, and (b) polymer nanostructures of negative replication to the mold are formed after de-molding. SEM images of (c) Si nanolined mold and (d) imprinted P3HT nanogratings. (Reprinted with permission by the American Chemical Society)


"A bicontinuous and interdigitized heterojunction, which has been regarded as the optimal architecture for polymer solar cells, can be realized by infiltrating fullerene into nanoimprint lithography defined solar cell polymer nanostructures. Moreover, we find there is a nanoimprint induced chain alignment in imprinted polymer nanostructures, which can enhance the hole mobility," said Dr. Yi Yang, a senior engineering at GlobalFoundries Inc. and former Ph.D. researcher at UT Dallas in the USA. His research is supported by Dr. Walter Hu, an associate professor of electrical engineering at UT Dallas and Dr. Anvar Zakhidov, the American Physics Society (APS) Fellow as well as a professor of physics at the same university. "It is the first technique that has shown the capability to precisely define the nanostructured heterojunction and enhance the hole mobility simultaneously."

In their paper "Nanoimprinted polymer solar cell" published in ACS Nano, which is the 1st review to this type of solar cells in the field, these researchers demonstrate how nanoimprint lithography can be used to pattern the active layer of polymer solar cells to form the well-ordered heterojunction. They also preview this approach's current challenges and future tasks. According to Dr. Yang, the goal of this article is to help develop a better understanding of nanoimprinted solar cells so as to unleash the full potential of this emerging technique toward significant improvements of device performance. Many solar cell scientists have been inspired by this paper and it has been cited by 40+ journal articles thus far.

Among the future tasks introduced in their review paper, a fundamental understanding of the nanostructure geometry effects on the polymer chain alignment and solar cell performance remains critical. "In literature, inconsistent geometries of the imprinted polymer nanostructures are used by different research groups, which result in different sizes and shapes of donor/acceptor heterojunctions as well as differently oriented polymer chain alignments which in turn affect the device performance. This gap makes the conclusions from different groups sometimes contradictory to each other." Dr. Yang explains.

In Dr. Yang's new 1st author paper "Effects of nanostructure geometry on nanoimprinted polymer photovoltaics" published in Nanoscale, he and his teammates have demonstrated the effects of nanostructure geometry on the nanoimprint-induced chain alignment in the standard solar cell polymer Poly(3-hexylthiophene-2,5-diyl) (P3HT), as well as the performance of nanoimprinted photovoltaic devices. They observe the dependence of the P3HT crystallite orientation on nanostructure geometry such that a larger width of nanogratings leads to more edge-on chain alignment, which is non-preferred for hole transport, while the increase in height gives more vertical alignment, which is preferred for hole transport. "The highest density and aspect ratio P3HT nanostructures in solar cell enable the most efficient charge separation, transport and light absorption, resulting in the highest power conversion efficiency among others," Yang concludes the findings. "To obtain a better efficiency, one can further increase the aspect ratio of P3HT nanostructures, i.e. decrease the width and increase the height, as predicted by the trend found in this study," he says. This work has also for the first time clarified the origins of different solar cell performances as observed by different groups in literature. More importantly, it shows people how to achieve highly efficient polymer solar cells by nanostructure geometry control.

Fig. 2. Schematic of edge-on, face-on and vertical chain orientations of P3HT molecules in a nanoimprinted polymer solar cell with vertically interdigitized and bi-continuous P3HT and PCBM heterojunction. Face-on and vertical orientations are preferred for hole transport due to their short hopping distances b and c, respectively, along the vertical direction of electric field E, compared to the non-preferred edge-on with a large hopping distance a. (Reprinted with permission by the Royal Society of Chemistry)


Fig. 3. Schematic of imprinted P3HT nanostructures with edge-on orientation close to the mold trench bottoms and vertical orientation close to mold sidewalls. (Reprinted with permission by the Royal Society of Chemistry)


Dr. Yang's next goal is to fabricate nanostructured solar cells with a feature size close to the exciton diffusion length (~10-20 nm) to further push the exciton dissociation, charge collection and solar cell efficiency. Therefore the future work will focus on the mold fabrication and de-molding process improvement.

Dr. Yi Yang can be reached via . He started working on low cost flexible polymer solar cells in 2007. His focus has been on nanoimprint lithography defined polymer electronics since 2009. His current role in GlobalFoundries Inc. is to develop 20 nm Cu/ULK interconnect process flows, which will be utilized in next generation of computer and smartphone chips. His work has been published in scientific journals such as ACS Nano, Nanoscale and Nanotechnology and well cited in literature.

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