Title: Three-Dimensional Carbon Nanotube Based Photovoltaics

SUMMARY: Photovoltaic (PV) cells with a three dimensional (3D) morphology are an exciting new research thrust with promise to create cheaper, more efficient solar cells. This work introduces a new type of 3D PV device based on vertically aligned carbon nanotube (CNT) arrays. These arrays are paired with the thin film heterojunction, CdTe/CdS, to form a complete 3D PV device (3DCNTPV). 

The main benefit of the 3DCNTPV cell is the ability to utilize multiple photon interactions with the solar cell surface. This three dimensionality allows photons to interact multiple times with the photoactive material, which increases the absorption and the overall power output over what is possible with a two dimensiona (2D) morphology. To quantify the increased power output arising from these multiple photon interactions, a new absorption efficiency term, h3D, is introduced. The theoretical basis behind this new term and how it relates to the absorption efficiency of a planar cell, h2D, is derived.  This theory is validated by monte carlo simulations.

A series of 3DCNTPV prototype cells was developed. Marriage of a complicated 3D structure with production methods traditionally used for planar CdTe solar cell is challenging. This work examines the problems associated with manufacturing these types of cells and systematically alters production methods and architecture of the semiconductor layers and electrodes to increase the short circuit current (Isc), eliminate parasitic shunts, and increase the open circuit voltage (Voc).  Original prototype cells suffered from very low power output.  Isc and Voc in later cells was increased by an order of magnitude and 300%, respectively. Output power of the 3DCNTPV cells was measured at varying incident angles of light and these cells show an increase in the normalized power output compared to similar planar cells when the solar flux is at off-normal angles. Experimental power output vs. zenith angle of the 3DCNTPV cells shows very good agreement with the theory proposed in this work.