Hybrid heterojunction solar cells using single-walled carbon nanotubes and amorphous silicon thin films

Abstract

Single-walled carbon nanotubes possess extraordinary optical, electrical, chemical, and mechanical properties. Thin films of randomly oriented SWCNTs have a great potential in many opto-electro-mechanical applications. Moreover, recent developments in photovoltaics have been largely contributed by SWCNTs as a p-type transparent conductor that fulfill the requirements for continuous, fast, and cheap film manufacturing process compatible with the roll-to-roll technology.

The scope of this thesis is the development of a conductive p-type SWCNT transparent conductor and its application in hybrid heterostructure solar cell based on amorphous silicon. For successful implementation of SWCNTs film in solar cells, it is very critical for the SWCNTs to have good physical contact with the material on which it is deposited. At first, quantitative measurements of the adhesion of SWCNT films with substrate materials in air and inert Ar atmosphere using atomic force microscopy was performed. It was found that adhesion of SWCNT films depends on the atmospheric conditions under which it is stored and deposited on a substrate material. The SWCNT film was measured to have higher adhesion in an inert atmosphere. With this understanding, a simple fabrication method of hybrid heterostructure solar cells was proposed in which the SWCNT-PEDOT:PSS composite p-type film forms a coupled continuous hybrid heterojunction with a-Si:H absorber. The optical and electrical properties of this composite was extensively characterized and further optimized by introducing multifunctional components like ultrathin MoO3 and SWCNT fibers. A rationally designed p-type transparent conductor with a combination of SWCNTs-MoO3-PEDOT:PSS-SWCNT fibers composite resulted in a state-of-the-art sheet resistance of 17 Ω/sq at 90% transmittance. Moreover, SWCNT fibers by itself can be used as replacement for traditional metal contacts as demonstrated here. This opens a new avenue in widespread energy technologies, where high hole conductivity and transparency of the material are prerequisites for their successful implementation.

Integrating the developed p-type transparent conductor as a window layer and top electrode on a-Si:H in a nip configuration resulted in a dramatic increase in its power conversion efficiency reaching up to 8.8%. The energy level alignment of these solar cells is carefully engineered at a-Si:H and SWCNTs interface by introducing a ultrathin MoO3 layer that shows the carrier transport by means of band-to-band or trap-assisted tunneling.