EFFECT OF SURFACE PASSIVATION OF NANOCRYSTALS ON ELECTRICAL PROPERTIES OF POLYMER-NANOCRSYTAL SOLAR CELL

Abstract

Colloidal semiconductor nanocrystals (NCs) have been widely investigated due to their tunable optical and electrical properties through shape- and size control. Moreover, low cost solution based device fabrication processes such as inkjet printing or spin-casting are applicable to build NCs based optoelectronic devices. Tunable band gap and possibility to harvest excess energy as additional charge carriers via multiple exciton generation attracted much attention toward colloidal NCs based solar cells. The surface states originating from the non-bonding orbital of surface atoms has been regarded as the critical issues to deteriorate electrical and optical properties of NCs. several modification methods have been suggested to passivate surface traps and to reduce an insulating organic shell thickness. However, in spite of the improvement in device performance, the chemical status of surface modified NCs still has not been clear

remnant or exchanged surface ligands and thickness of surface ligand shell are sensitively altered by temperature, concentration of additional ligands, and reaction time. Because of this complexity most reports have only described about the device performance before and after surface modification of NCs. In this thesis, we investigated the relationships between surface chemistry changed by surface treatment (i.e., amount of surface ligands exchanged and average ligand chain length) and correlated device physics (i.e., charge separation, extraction, and recombination) in polymer-nanocyrstal hybrid solar cells. Through the 1H-NMR analysis, we observed different nature of ligands exchange according to functional group (carboxylic acid and amine group) and process sequence, and correlated their effect on Poly(3-hexylthiophene) (P3HT):CdSe quantum dot (QD) BHJ solar cell performances (current density-voltage (J-V) characteristics, incident photon-to-current conversion efficiency (IPCE)). Photogenerated charge carrier collection properties in active layers were studied using incident light intensity dependent J-V characteristics. Device performances depending on the thickness of active layer and donor: accepter blend ratio and the effect of thermal annealing were also investigated. By reducing insulating ligand shell thickness with sequential ligand exchange and passivating surface trap states with 1-hexylamine ligand, carrier transport and collection property, and solar cell performance were significantly enhanced with 2% of power conversion efficiency (PCE) and 72% of carrier collection efficiency ( ). However surface ligand modification in solution phase could alter the colloidal stability of NCs, and therefore, inevitable change of P3HT:NCs BHJ morphology is arisen. Since surface chemistry of NCs as well as nanoscopic morphology of the active layer are correlated with device performances, systematical study needs to clarify the effect of surface ligand on the device performances with fixed nanoscopic morphology. To prevent morphology change followed by surface ligand modification and resolve reliability problem, we propose a modular fabrication procedure to assemble CdSe TP nanocrystal/P3HT hybrid heterojunction. Such a modular approach enables the independent control of nanoscopic morphology and surface chemistry of the nanocrystals in reproducible manner, which are generally known to exhibit complex correlations. This modular fabrication separates the breakwaters-like nanocrystal network formation (step 1), the nanocrystal surface modification via chemical treatments (step 2), and the intrusion of polymers into the nanocrystal network (step 3). Accordingly, CdSe TP/P3HT hybrid heterojuctions could be assembled with their nanoscopic morphology and surface chemistry is simply under control without any morphology change. Thanks to the modular fabrication, nanocrystal could be surface modified simply with pyridine and 1-hexylamine and solar cell performance could be systematically examined through temperature-dependent J-V) measurements at varying illumination light intensities. We found that a 2-fold increase in short circuit current with 1-hexylamine ligands, compared with the value based on pyridine ligands, originates from the reduced depth of trap states, minimizing the trap-assisted bimolecular recombination process. This thesis demonstrates the practical approaches to enhance efficiency of polymer-NC hybrid solar cells, covering the tailored surface modification of NCs, suitable device structure, and novel processing method. This approach is believed that could be applied not only polymer-NC hybrid solar cells but also other kinds of optoelectronic devices.