Organic bulk heterojunction (BHJ) solar cells offer the potential advantages as low-cost, lightweight, flexible, and large area devices that can be fabricated in the roll-to-roll printing method. Despite the recent progress in the increase of power conversion efficiency (PCE) of organic BHJ solar cells, fundamental breakthrough has to be made in order to develop high efficiency solar cells especially those working for wide light incident angles. This EAGER project will explore the concept of integrating novel plasmonic wide angle light concentrators as transparent electrodes in BHJ solar cells to tune and enhance transmitted light to match the energy band gaps of the donor and acceptor in the active layer in a wide range of incident angles. Electromagnetic finite-difference time-domain (FDTD) simulations will be applied to rationally design the plasmonic nanostructures and the transfer matrix (TM) optical modeling will be used to design the entire device architecture to ensure the maximum light absorption in the active layer. The designed plasmonic nanostructures will be made on glass substrates via the nanoimprinting method which can be extended to the low-cost roll-to-roll printing method. The effects of far-field light transmission and near-field electric field enhancement induced by plasmonic nanostructures on the performance of BHJ solar cells and the fundamental physical processes in solar energy conversion will be elucidated by conducting the experimental measurements on photocurrent density-voltage curve, reflectance and UV-Vis absorption spectroscopy, steady state and dynamic photoluminescence (PL), and external quantum efficiency (EQE). The objective of this work is to fundamentally understand the physical principles and processes governing the tuning of wide angle light absorption and the enhancement of charge transport and collection in BHJ solar cells by plasmonic nanostructures via a combined electromagnetic simulation and experimental approach.
By integrating plasmonic wide angle light concentrators as transparent electrodes in BHJ solar cells, it will allow one to (1) replace the expensive ITO; (2) tune and concentrate far-field transmitted light to match the band gaps of donor and acceptor in the active layer; (3) enable wide angle absorption without mechanical moving parts; and (4) understand near-filed electric field enhancement on exciton generation and charge separation, transport and collection. A wide angle light concentrator enabled by plasmonic nanostructures will be developed and integrated into organic BHJ solar cells to enhance the solar energy conversion efficiency even at large oblique incident angles. The fundamental investigation and experimental findings from this work can be generalized for guiding the development of other types of solar cells and novel optoelectronic and plasmonic devices. Graduate and undergraduate students from underrepresented groups such as female will receive training and participate in this highly interdisciplinary research project.
Solar cells have great promise for sustainable energy but remain expensive relative to other technologies. Organic bulk heterojunction (BHJ) solar cells offer the potential advantages as lightweight, flexible, and large area devices that can be fabricated in the low cost roll-to-roll printing method. However, the power conversion efficiency (PCE) has to be increased to over 15% for practical uses. Integrating plasmonic nanostructures as transparent electrodes and wide angle light concentrators in BHJ solar cells provide a possible solution to increase PCE as well as enable omnidirectional light absorption. Through this one-year project, gold thin films with nanoholes in a squared arrangement were designed using electromagetic simulations. The following criteria have been used to assess the designed plasmonic nanostructures: (1) increased light absroption in the wavelength that the materilas in the active layer can be absorbed and can generate charges; (2) electric field intensity through the charge collection layer to the active layeris is increased at the wavelength matching the band gaps of the materilas in the active layer; and (3) absorption spectra remain unchanged in a large light incident angle. By deploying this film on top of glass or flexible plastic substrate, it functions as an electrode. This film can increase light absoprtion in the active layer to generte more charges and thereby, convert more soalr energy into electric energy. The solar cells with such plasmonic nanostructured electrode can also absorb ligth in a wide angle while reamining high efficiency. The procedures for making gold thin film with nanoholes on glass and PET substrates were developed, which can be adopted to roll-to-roll fabrications. The organic BHJ solar cells with conventional and inverted configurations were made and the device performence was investigated by measuring the current-voltage curves. Three graduate students and one undergraduate students participated in this interdiscipline research project and received trainings on both simulations and experiments with the knowledge ranging from fundamental physics and chemistry to electric, materials and chemical engineerings and to photovoltaic technologies. The findings and results have been disseminated through the presentations at the international conferences and departmental graduate symposium. Manuscripts are in preparation and will be submitted for publication. Students participated in the Environment Innovation and Business Plan Competitions. The findings and results will lead to new technology that will be transferred to products by seeking the commercialization gap fund from the UW Center for Commerciallization (C4C).
