4 research outputs found
Charge Dynamics in Solution-Processed Nanocrystalline CuInS<sub>2</sub> Solar Cells
We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS<sub>2</sub> (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or <i>in situ</i> solâgel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the solâgel synthesized CIS/CdS cell
Device Performance of Small-Molecule Azomethine-Based Bulk Heterojunction Solar Cells
Recently, we demonstrated that small-molecule
azomethines are promising
candidates as electron-donating materials for organic photovoltaic
(OPV) devices. Azomethines can be prepared via well-known condensation
chemistry, with water being the sole byproduct. Here we present a
record power conversion efficiency for azomethine-based small-molecule
OPV devices of 2.2%. To understand the underlying physics limiting
device performance, devices of the small-molecule azomethine TPAâTBTâTPA
were characterized using a range of spectroscopic and electro-optical
techniques. Light-intensity-dependent current-density measurement
showed the presence of nongeminate charge recombination, which is
most likely the result of poor charge mobility. In addition, transient
absorption measurements showed a relatively short lifetime for the
exciton (120 ps). However, due to the very fast charge dissociation
(<300 fs), charge separation is relatively efficient. This knowledge
presents a guideline for preparing subsequent generations of compounds
with improved device performance
Ultrafast Charge- and Energy-Transfer Dynamics in Conjugated Polymer: Cadmium Selenide Nanocrystal Blends
Hybrid nanocrystalâpolymer systems are promising candidates for photovoltaic applications, but the processes controlling charge generation are poorly understood. Here, we disentangle the energy- and charge-transfer processes occurring in a model system based on blends of cadmium selenide nanocrystals (CdSe-NC) with poly[2-methoxy-5-(3â˛,7â˛-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) using a combination of time-resolved absorption and luminescence measurements. The use of different capping ligands (<i>n</i>-butylamine, oleic acid) as well as thermal annealing allows tuning of the polymerânanocrystal interaction. We demonstrate that energy transfer from MDMO-PPV to CdSe-NCs is the dominant exciton quenching mechanism in nonannealed blends and occurs on ultrafast time scales (<1 ps). Upon thermal annealing electron transfer becomes competitive with energy transfer, with a transfer rate of 800 fs independent of the choice of the ligand. Interestingly, we find hole transfer to be much less efficient than electron transfer and to extend over several nanoseconds. Our results emphasize the importance of tuning the organicânanocrystal interaction to achieve efficient charge separation and highlight the unfavorable hole-transfer dynamics in these blends
Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide
Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway