Principal Investigator Vladimir Bulovic
Co-investigator Moungi Bawendi
Colloidal quantum dot (QD) systems offer distinct optical and electronic properties that are not easily attained by other nanostructured semiconductors, such as highly saturated emission in QD light-emitting-diodes, access to infrared radiation in QD photodetectors, and the prospect of optically optimized solar cell structures. The prevailing deposition method for colloidal QD systems is spin casting, which introduces limitations such as solvent incompatibility with underlying films and the inability to pattern side-by-side pixels for multispectral photodetector arrays. In the present work we employ a non-destructive microcontact printing method, which allows for deposition of a thin quantum dot films onto a wide-band-gap organic hole transport layer, N,N'-Bis (3-methylphenyl)-N,N'-bis-(phenyl)-9,9-spiro-bifluorene (spiro-TPD), thus producing an inorganic/organic heterojunction that serves to enhance charge separation in the device. The top and bottom contacts are provided by ITO electrodes, allowing for near-transparency.
Restrictions imposed by transport losses in the QD film are found to limit charge generation. Measurements of the external quantum efficiency (EQE) and internal quantum efficiency (IQE) as a function of QD film thickness, reveal a marked dependence on thickness. The IQE is determined by dividing the EQE by the absorption of the QD film, all of which are measured at the first absorption peak of the QD film (? = 590 nm). Following excitation and exciton diffusion to an interface, dissociation of the exciton produces free carriers that must diffuse to opposite electrodes in order to produce a photocurrent. A model that accounts for both exciton and charge diffusion reproduces the general thickness trend, assuming an exciton diffusion length LEx = 43 nm, an electron diffusion length LEl = 61 nm, and near-zero contribution from the first two QD monolayers. Further development will require reducing exciton and charge transport losses in order to permit efficient charge-generation from thicker QD films with improved absorption.