(Invited) High Performance Nano-Carbon/Silicon Solar Cells Via Strategic Doping Processes

Hybrid approaches that combine silicon (Si) and nano-carbon (nano-C) materials-based thin films (i.e., single walled carbon nanotube (SWNT) films or graphene) have recently led to novel and promising photovoltaic devices. The main advantage is that the intrinsically high photovoltaic efficiency of Si can be realized in a cost-effective manner owing to the low-temperature solution processibility inherent to the fabrication of nano-C/Si junctions. Indeed, efficiencies up to 15% have recently been reported, which surpasses the best efficiency of any polymer/Si hybrid solar cells developed with a similar motivation towards reduced fabrication cost. This superiority is mainly due to the excellent electrical properties of the nano-C materials in comparison to conventional conjugated polymers. In this talk, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope SWNT thin films, using a simple spin coating process [1]. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future [1]. In a separate approach we will show how HF treatment of SWNTs followed by current stimulation is a very effective method for oxygen removal with broad applications for carbon electronics [2]. Finally, we will describe how direct solution casting of silver nanowires (AgNWs) onto the SWNT/Si junctions leads to a significant improvement of photovoltaic properties owing to enhanced carrier transport in the bilayer AgNW/SWNT composites [3]. This improvement becomes particularly more pronounced with increasing junction area, which results in a significantly high efficiency of ~11 % in SWNT/Si solar cells with the largest junction area (49 mm²) developed so far [3].

[1] X. Li, L.M. Guard, J. Jiang, K. Sakimoto, J.-S. Huang, J. Wu, J. Li, L. Yu, R. Pokhrel, G.W. Brudvig, S. Ismail-Beigi, N. Hazari and A.D. Taylor (2014) Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells, Nano Letters, 14 (6), 3388–3394. DOI: 10.1021/nl500894h

[2] X. Li, J-S. Huang, S. Nejeti, L. McMillon, S. Huang, C.O. Osuji, N. Hazari, A.D. Taylor (2014), The Role of HF in Oxygen Removal from Carbon Nanotubes: Implications for High Performance Carbon Electronics, Nano Letters, 14 (11), 6179-6184. DOI: 10.1021/nl502401c

[3] X. Li, Y. Jung, J-S. Huang, T. Goh , and A.D. Taylor (2014), Device Area Scale-Up and Improvement of SWNT/Si Solar Cells Using Silver Nanowires, Advanced Energy Materials, 4 (12), 1400186. DOI: 10.1002/aenm.201400186