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Quantum Dot Photovoltaics
Nanocrystals are an ideal light harvester in photovoltaic devices. The band gap can be exquisitely tuned by controlling the size of the nanocrystal, thus the proper choice of size and material allows one to create a device with absorption that matches the spectral distribution of sunlight. Nanocrystals absorb sunlight more strongly than dye molecules and bulk semiconductor material, therefore high optical densities can be achieved while maintaining the requirement of thin films. A nanocrystal is also an artificial reaction center, separating the electron hole pair on a femtosecond (10-15 sec) timescale. Nanocrystals have an intrinsic dipole moment originating from the top and bottom terminating faces of the crystal having different chemical compositions: one cationic and one anionic. Carriers are rapidly localized to the surface of the crystal where they remain for up to 290ns before recombining. All of these properties combined – size-tunable band gap, large absorption coefficient, intrinsic electron hole pair separation, long exciton lifetime, and chemical robustness – make nanocrystals the ideal material for solar cells. Additionally, the photovoltaic devices we make in our laboratory are solution-processed, making them amenable to large coverage areas at considerably lower temperatures and much lower costs than silicon based devices.
In the Rosenthal research group we are interested in optimization of device architectures and developing new techniques for device characterization that enable rational design of quantum dot photovoltaics. The nanocrystal-based hybrid bulk heterojunction device has a photoactive layer that is a blend of semiconductor nanocrystals or nanorods and a charge-accepting polymer. Commonly poly(3-hexylthiophene) (P3HT) is used because it is a semiconductor polymer with a type-II band alignment with CdSe; that is, the valence and conduction bands of the materials are staggered such that excited electrons remain in CdSe while the holes are separated into P3HT. The charge separation prevents carrier loss due to recombination and the high hole mobility of P3HT makes it an ideal transport layer for hole extraction. Recently, we have pioneered a new technique combining scanning electron microscopy (SEM) and electron beam induced current (EBIC) on cross sections of a hybrid device that allows us to map out electronically deficient and proficient regions within the photoactive layer with <100nm resolution. Our new SEM-EBIC technique gives us an unprecedented detailed understanding of the device physics within the photoactive layer and across the entire device.
We are also investigating the effects of different nanocrystal materials in quantum dot-sensitized solar cells (QD-SSCs). The Rosenthal research group is the first to directly interrogate the influence of plasmon-on-semiconductor architectures with respect to excitonic absorption in photovoltaic systems. We have demonstrated enhanced performance in QD-SSCs that utilize CuxInyS2 nanocrystals with localized surface plasmon resonance (LSPR) modes in the near infrared frequencies. The increased incident photon conversion efficiencies are attributed to augmented charge excitation as a result of near-field “antenna” effects in the plasmonic QD-SSCs.
Niezgoda, J. S.; Yap, E.; Keene, J. D.; McBride, J. R.; Rosenthal, S. J., Plasmonic CuxInyS2 Quantum Dots Make Better Photovoltaics Than Their non-Plasmonic Counterparts. Nano. Lett. 2014, 14 (6), 3262-3269.
Ng, A.; Poplawsky, J. D.; Li, C.; Pennycook, S. J.; Rosenthal, S. J., Direct Electronic Property Imaging of a Nanocrystal-Based Photovoltaic Device by Electron Beam-Induced Current via Scanning Electron Microscopy. J. Phys. Chem. Lett. 2014, 5 (5), 856-860.
Smith, N. J.; Emmett, K. J.; Rosenthal, S. J., Photovoltaic cells fabricated by electrophoretic deposition of CdSe nanocrystals. Appl. Phys. Lett. 2008, 93 (4), 043504/1-043504/3.
Erwin, M. M.; Kadavanich, A. V.; McBride, J.; Kippeny, T.; Pennycook, S.; Rosenthal, S. J., Material characterization of a nanocrystal based photovoltaic device. Eur. Phys. J. D 2001, 16 (1-3), 275-277.