Characterization of nanocrystal-based photovoltaics: electron microscopy & electron beam-induced current via scanning electron microscopy
The work presented here is the first of its kind where nanocrystal-based photovoltaics are characterized by directly imaging the electronic properties and correlating them to the structure of the sample. Through electron beam-induced current (EBIC), a finite beam of high energy electrons mimics photons to generate a photoresponse in the device, thus enabling the measurement of the current being generated in a precise region of the specimen. Two different nanocrystal-based architectures were studied in this manner: (1) a hybrid bulk heterojunction composed of CdSe nanorods dispersed in a conductive polymer matrix and (2) a PbS quantum dot depleted-heterojunction device. Both yielded significant results, such as high hole mobility in the case of (1), and changes in the EBIC signature as a result of defects in (2); the electronic shortcomings of the devices believed prior to this work were also confirmed. Another PbS quantum dot-based device was characterized using high resolution elemental mapping; this architecture comprised of titania nanotubes rather than nanoparticles as used in the photovoltaic described in (2). Preliminary work on CdTe thin film solar cells was also conducted as a test bed for higher resolution EBIC. With EBIC, maps of nanocrystal-based, solid state photovoltaics were collected and analyzed to reveal important information regarding the electronic properties of the devices as well as areas of improvement. As a result of the latter, more efficient solar cell technology can be developed to help meet the energy demands of the global population.