(637c) Metal Oxide Infilling of Quantum Dot Thin Films: Increased Stability and Carrier Mobility for Device Applications

Colloidal semiconductor quantum dot (QD) thin films have attracted great interest for their applications in photovoltaic devices and as light emitters. This is due to the controlled variation in their band-gap and ease of fabrication. However, these thin films suffer from instability due to their inherent inclination towards oxidative and photothermal degradation. Thus, for fabricating QD-based electronic devices encapsulation or pore infilling processes are desirable to increase the stability of these materials. The encapsulation process should provide protection against oxidation without hindering the electron transport properties or causing sintering of the QDs. The pore infilling process has been shown to also enhance the conductivity and carrier mobility.

Atomic layer deposition (ALD) is an ideal candidate for deposition of conformal inorganic thin films on nonplanar substrates with subnanometer thickness control. ALD of metal oxides allows for a low-temperature process of infilling and capping of QD thin films. In this work we examine two different metal oxide ALD processes for infilling the InP QD thin films and their effects on the stability and carrier mobility of the coated QD films. InP QDs show promise as light emitters (i.e. phosphors) across the visible spectrum due to a favorable bandgap and high photoluminescence quantum yield. Furthermore, InP QDs exhibit multiple exciton generation where more than one electron-hole pair is created per absorbed photon theoretically allowing for higher solar cell efficiency than traditional materials. However, carrier mobility in QD films was too low for practical applications until now.

We compare the results of amorphous TiO₂ with crystalline ZnO films deposited at various temperatures. The effects of film thickness and pore infilling versus capping of QD film on stability and carrier mobility are studied with spectrophotometry and time-resolved microwave conductivity (TRMC) measurements. We show that the inorganic matrix reduces the size of tunnel barriers within the QD thin film hence increasing the carrier mobility through the film without causing sintering of the QDs. Furthermore, our results confirm that the stability of QD thin films is improved when the inorganic ALD coating is applied. We also show prototype QD solar cells with this work opening up the possibility of fabricating robust InP QD thin films for many optoelectronic devices.