fabrication-fully-solution-processed-inorganic-nanocrystal

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Naval Research Laboratory

Solution-synthesized inorganic cadmium telluride nanocrystals (∼4 nm; 1.45 eV band gap) are attractive elements for the fabrication of thin-film-based low-cost photovoltaic (PV) devices. Their encapsulating organic ligand shell enables them to be easily dissolved in organic solvents, and the resulting solutions can be spray-cast onto indium-tin oxide (ITO)-coated glass under ambient conditions to produce photoactive thin films of CdTe. Following annealing at 380 °C in the presence of CdCl2(s) and evaporation of metal electrode contacts (glass/ITO/CdTe/Ca/Al), Schottky-junction PV devices were tested under simulated 1 sun conditions. An improved PV performance was found to be directly tied to control over the film morphology obtained by the adjustment of spray parameters such as the solution concentration, delivery pressure, substrate distance, and surface temperature. Higher spray pressures produced thinner layers (<60 nm) with lower surface roughness (<200 nm), leading to devices with improved open-circuit voltages (Voc) due to decreased surface roughness and higher short-circuit current (Jsc) as a result of enhanced annealing conditions. After process optimization, spray-cast Schottky devices rivaled those prepared by conventional spin-coating, showing Jsc = 14.6 ± 2.7 mA cm(-2), Voc = 428 ± 11 mV, FF = 42.8 ± 1.4%, and Eff. = 2.7 ± 0.5% under 1 sun illumination. This optimized condition of CdTe spray deposition was then applied to heterojunction devices (ITO/CdTe/ZnO/Al) to reach 3.0% efficiency after light soaking under forward bias. The film thickness, surface morphology, and light absorption were examined with scanning electron microscopy, optical profilometry, and UV/vis spectroscopy.

Impact of nanocrystal spray deposition on inorganic solar cells.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating Voc(mV)=553Eg/q-59 as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells.

Soluble inorganic nanocrystals offer a potential route to the fabrication of all-inorganic devices using solution deposition techniques. Spray processing offers several advantages over the more common spin- and dip-coating procedures, including reduced material loss during fabrication, higher sample throughput, and deposition over a larger area. The primary difference observed, however, is an overall increase in the film roughness. In an attempt to quantify the impact of this morphology change on the devices, we compare the overall performance of spray-deposited versus spin-coated CdTe-based Schottky junction solar cells and model their dark current-voltage characteristics. Spray deposition of the active layer results in a power conversion efficiency of 2.3 ± 0.3% with a fill factor of 45.7 ± 3.4%, Voc of 0.39 ± 0.06 V, and Jsc of 13.3 ± 3.0 mA/cm(2) under one sun illumination.

Inorganic photovoltaic devices fabricated using nanocrystal spray deposition.

In this work solar cell anti-reflection coatings tuned to give a specific hue under solar illumination are investigated. We demonstrate that it is possible to form patterned coatings with large color contrast and high transmittance. We use colorimetric and thin film optics models to explore the relationship between the color and performance of bilayer anti-reflection coatings on Si, and predict the photocurrent generation from an example Si solar cell. The colorimetric predictions were verified by measuring a series of coatings deposited on Si substrates. Finally, a patterned Si sample was produced using a simple, low-cost photolithography procedure to selectively etch only the top layer of a bilayer coating to demonstrate a high-performance anti-reflection coating with strong color contrast.

Article	Year
Impact of nanocrystal spray deposition on inorganic solar cells. ACS applied materials & interfaces\| PubMed ID: 24755091	2014
Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells. Scientific reports\| PubMed ID: 23868514	2013
Inorganic photovoltaic devices fabricated using nanocrystal spray deposition. ACS applied materials & interfaces\| PubMed ID: 24007356	2013
Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells. Optics express\| PubMed ID: 24104486	2013

Woojun Yoon

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