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Multilayer Carbon Nanotube Films

Stage: Development
Chromic materials can change their colors in response to heat (thermochromic) or voltage (electrochromic). Incorporation of these materials in smart windows enables the intelligent regulation of window transparency, allowing for control of transmitted light and heat into building interiors. This control allows improved energy efficiency and occupant comfort. With the application of heat or voltage, state-of-the-art chromic windows can switch from transparent to opaque and reject unwanted solar energy as heat. However, the National Renewable Energy Laboratory’s (NREL’s) chromic photovoltaic (PV) window technologies can transform a portion of the rejected or transmitted energy into electricity, using perovskite materials commonly found in PV panels of traditional architectures. To switch the transparency state of NREL’s chromic PV, heat or voltage trigger molecules to absorb into or desorb from the perovskite structure, causing a reversible change in bandgap properties.

To realize chromic photovoltaic and conventional photovoltaic architectures, carbon-nanotube films have been identified as a promising replacement for Spiro-OMeTAD as hole-selective transport layers in perovskite photovoltaic devices. While Spiro works well as a hole-selective transport layer, there is serious concern that it is also a significant source of perovskite-cell-performance degradation over time due to migration of Li atoms into the perovskite bulk.

Researchers at the National Renewable Energy Laboratory (NREL) have created a novel  bilayer of carbon nanotubes, which is used as a hole transport layer. Specifically, a first carbon-nanotube layer (wrapped in a P3HT polymer) provides an energetically favorable interface with the perovskite absorber layer, while the second carbon-nanotube layer doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is optimized for hole transport out of the perovskite device. This bilayer improves the stability of the absorber layer and enables perovskite device architectures desiring a transparent hole-selective contact (e.g. for  building-integrated photovoltaic window applications).

For further information about this technology or advances in NREL's broader perovskite portfolio, please visit NREL's Chromic Technologies for Photovoltaic Applications Portfolio Summary or Perovskite Patent Portfolio Website or contact Bill Hadley at bill.hadley@nrel.gov.

ROI 17-54.

Applications and Industries

  • Perovskites
  • Photovoltaics
  • Thermochromic or electrochromic photovoltaic windows
  • Building integrated photovoltaics (BIPV)


  • Improved absorber stability
  • Can be used in transparent perovskite device architectures