Progress in Energy and Combustion Science 70 (2019) 1–21 Contents lists available at ScienceDirect Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs Multifunctional graphene and carbon nanotube films for planar heterojunction solar cells Kehang Cui a,∗ , Shigeo Maruyama b,c,∗ a Department of Mechanical Engineering and Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan c Energy NanoEngineering Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8564, Japan a r t i c l e i n f o Article history: Received 7 April 2018 Accepted 5 September 2018 Available online 26 September 2018 Keywords: Graphene Carbon nanotubes Solar cell Energy conversion Thin film a b s t r a c t Graphene and carbon nanotubes, featured with outstanding electronic, photonic and mechanical prop- erties as well as Earth abundancy, are perfect for use as carrier-selective transport and collecting layers in photovoltaics. In recent years, graphene and carbon nanotube films have underpinned significant ad- vancement in the planar heterojunction (PHJ) solar cells, with reduced fabrication cost, improved power conversion efficiencies approaching 20%, and the great potential for scalable deployment. Here we dis- cuss the state-of-art progress in graphene-based and carbon nanotube-based PHJ solar cells leveraging advanced nanocarbon technologies as well as industrial-compatible solar cell design and processing. Fab- rication and functionalization strategies of graphene and carbon nanotube films for electronic and pho- tonic optimization of PHJ solar cells are systematically reviewed. We also envision technological pathways and future prospects to exploit multifunctionality of graphene and carbon nanotubes to realize ubiquitous application of high-performance, flexible PHJ solar cells. © 2018 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Fermi level tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Molecular adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Electrochemical gating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Solid-state functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Electrical property tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Morphological design and hybrid structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2. Junction interface engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4. Optical property tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Antireflection coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. Plasmonic-enhanced absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5. Towards flexible PHJ solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6. Challenges and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 ∗ Correspondence authors. E-mail addresses: cuik@mit.edu, cuikehang@gmail.com (K. Cui), maruyama@photon.t.u-tokyo.ac.jp (S. Maruyama). 1. Introduction Solar cells can directly convert solar energy to electric power without carbon footprint, and thus are promising to meet world energy consumption demands and fulfil low carbon scenario in fu- https://doi.org/10.1016/j.pecs.2018.09.001 0360-1285/© 2018 Elsevier Ltd. All rights reserved.