The Photovoltaic Properties of Carbon Nanotube Network p-n Diodes

Abstract

Single Walled Carbon nanotubes (SWCNTs) are quasi one dimensional rolled up sheets of graphene with amazing optical and electronic properties. Depending on their diameter and roll up angle, SWCNTs come in varying chiralities with multiple bandgaps giving them exceptional properties that make them attractive for photovoltaic applications. One of such properties is the absorption of light across the broad solar spectrum, a highly desirable property in semiconducting solar cell absorbers. In this dissertation, we will be exploring our attempt to fabricate a fundamental device that enables us harness the full sunlight potential of semiconducting SWCNT (s-SWCNT) networks and have a better understanding of its photovoltaic properties. To fabricate this fundamental device, we look to nature for inspiration on solar energy conversion. We use the process of photosynthesis as a model for building our solar energy conversion device. Nature, through centuries of evolution, has perfected the harvesting of light for energy conversion through the process of photosynthesis by employing two main mechanisms carried out by distinct proteins: excitation energy transfer, where light harvesting complexes capture light from multiple regions of the solar spectrum and funnel photoexcitations to a reaction center, and charge separation, where the photoexcitations become free charges in the reaction center. As we will see in this dissertation, SWCNTs have similar properties to that of photosynthetic systems, one of which is the varying chiralities of SWCNTs with different diameters, analogous to the distinct proteins in photosynthetic systems absorbing light at different wavelengths. We fabricate p-n diodes on various networks of s-SWCNTs, we study the intrinsic electronic and optical properties of nearly monochiral and polychiral s-SWCNT networks and form a fundamental understanding of the best s-SWCNT films required to make more ideal diodes. We examine the current-voltage characteristics of these diodes in the dark and find correlations between the key figure of merits, including the diode leakage current and the ideality factor, to different s-SWCNT networks. We also examine their optical properties by measuring wavelength-dependent photocurrent spectroscopy to gain insights into the dynamics of excitons in a network of s-SWCNTs. We achieve ideal diodes, for the first time in a homogenous network of s-SWCNTs. We discuss the limitations of using ideal diodes in the measurement of the electronic bandgap of s-SWCNT networks and then use non – ideal diodes to measure the electronic bandgaps of the s-SWCNT networks for the first time. After a more in-depth understanding of the dark diode characteristics of the s-SWCNT networks, we progress to fabricating a fundamental solar energy conversion device, modelled after photosynthesis. We fabricate photovoltaic diodes mimicking photosynthetic systems. Using different s-SWCNT chiralities, we create an energy funnel in our diodes by layering different s-SWCNT networks according to their bandgaps. The photo excitations in the larger bandgap s-SWCNTs are funneled down to the smallest bandgap s-SWCNT, allowing us to increase the spectral response of our diodes. We show that the photocurrent generation in our energy funnel is more efficient than in diodes formed using single chirality s-SWCNT networks. Finally, we show that our device architecture increases the photocurrent without increasing the highly undesirable dark leakage current. Using the analogy to photosynthetic systems, we use the smallest bandgap s-SWCNT network to create the diode (Reaction Center). The larger bandgap s-SWCNT networks act as light harvesters. We demonstrate an increase in short circuit current and the open circuit voltage as we add these nanotubes sequentially. We use this device to implement the mechanisms of exciton energy transfer in our p-n diodes and study its properties as it applies to s-SWCNT networks. We see some new and exciting physics which we will cover in this dissertation.