Woojun Yoon

Enhanced open-circuit voltage of PbS nanocrystal quantum dot 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 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.

Impact of nanocrystal spray deposition on inorganic solar cells.

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.

Sintered CdTe Nanocrystal Thin Films: Determination of Optical Constants and Application in Novel Inverted Heterojunction Solar Cells

In this paper, we report a novel heterojunction solar cell based on sintered CdTe and CdSe nanocrystal thin films using solution-based deposition. For the absorber layer, CdTe thin films were made using a layer-by-layer deposition process consisting of spin coating colloidal CdTe nanocrystals followed by a sintering step. The optical constants for these sintered CdTe films were accurately determined through a combination of optical modeling and measurements. For the all-solution processed p-n heterojunction, we focus on CdSe nanocrystal thin films due to their excellent compatibility with solution processing. For the optimized inverted structure (glass/ITO/CdSe/CdTe/Cr/Au), a high open-circuit voltage (Voc) of 593 ± 32 mV with an efficiency of 1.9 ± 0.2% was obtained under simulated one sun illumination. These preliminary results demonstrate that this novel inverted heterojunction structure has the potential to produce a high-quality CdSe/CdTe junction for utilization in heterojunction solar cells.

Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells

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.

Inorganic Photovoltaic Devices Fabricated Using Nanocrystal Spray Deposition

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.

Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth

Inorganic CdSe/CdTe nanocrystals for solid-state photovoltaic devices are typically sintered into a bulk-like material after annealing in the presence of solid cadmium chloride. As in commercial CdTe devices, this salt exposure is a key component to improve device performance by promoting grain growth. However, in contrast to vapor depositions, we demonstrate that the role of the salt treatment also involves crucial ligand removal reactions, which are a unique challenge facing nanocrystal ink depositions. After testing other salts such as CdF2, CdCl2, CdBr2, CdI2 and Cd(NO3)2 for oleate ligand removal as determined by FTIR, SEM imaging of CdTe grain growth revealed the largest grains were observed from reactions with CdCl2 (142 ± 26 nm) and, to a lesser extent, CdBr2 (131 ± 19 nm). These results were used to identify cadmium-free alternatives. Trimethylsilyl chloride (28.0 ± 5.1 nm), NH4Br (75.5 ± 31 nm) and NH4Cl (136 ± 39 nm) were also tested, demonstrating comparable ligand removal and grain growth to the cadmium halides. In order to validate these observations, heterojunction photovoltaic devices were fabricated from CdSe/CdTe nanocrystals treated with non-toxic NH4Cl in place of the conventional CdCl2. Under AM 1.5G illumination, open circuit voltages (Voc), short circuit currents (Jsc) and efficiencies (η) of solar cells processed with evaporated Au and commercial ITO were found to be Voc = 0.46 ± 0.02 V, Jsc = 9.27 ± 0.6 mA cm−2, and η = 1.73 ± 0.24 demonstrating minimal differences in film morphology and device performance compared to those fabricated using cadmium chloride. Specific properties of the salts (solubility, reactivity, melting point and the identity of both the cation and the anion) were found to have a profound impact on grain growth and consequently device performance, suggesting the need for further investigation of additional non-toxic metal halide salts for this reaction.

Solution processing of CdTe nanocrystals for thin-film solar cells

We describe solution-processed sintered nanocrystal solar cells. As the absorber layer, thin-films of CdTe nanocrystals were deposited using a layer-by-layer spin coating process. For Schottky barrier solar cells (ITO/CdTe/Ca/Al), an efficiency of 3.0±0.3% with V_oc= 0.53±0.03 V, J_sc= 13.2±0.2 mA/cm², and FF=43.1±4.2% was measured under AM1.5G conditions. In order to overcome the limitations associated with the Schottky barrier structure, heterojunction solar cells incorporating solution-processed CdSe nanocrystals as an n-type layer were fabricated and characterized. Under darkness, heterojunction devices (ITO/CdTe/CdSe/Al) showed an improved current rectification ratio of 4.53×10² at ±1V in comparison to Schottky barrier solar cells, indicating that a well-defined junction is formed between CdSe and CdTe. Under illumination (AM1.5G), the devices exhibited an average efficiency of ~2% with V_oc=0.50±0.02 V.

Transparent conducting oxide-based, passivated contacts for high efficiency crystalline Si solar cells

In this work, we investigate a transparent conducting oxide (TCO)-based, passivated contact for the potential use as a passivated tunnel contact to p-type Si. As a surface passivation layer, the Al₂O₃ films with varying the thickness are deposited using plasma-enhanced atomic layer deposition (PEALD) at 200 °C, followed by post-deposition annealing. For a ~15 nm thick Al₂O₃ layer, a high level of surface passivation is achieved, characterized by the effective surface recombination velocity (S_eff,max) of <;30 cm/s. The samples with ultrathin Al₂O₃ layer <;3 nm, however, shows degradation in passivation quality, reaching the S_eff,max<;500 cm/s. When Al-doped zinc oxide (ZnO:Al) as TCO contact is directly deposited onto a ~10.6 nm thick Al₂O₃ coated p-Si via RF magnetron sputtering, the final passivation quality (p-Si/Al₂O₃/ZnO:Al) is characterized by the saturation current density at contact (J_0,contact) of 92.1 fA/cm² with the implied open-circuit voltage (iVoc) of 653 mV, showing the passivation quality is not severely degraded after sputtering without thermal treatment. Further process optimization of PEALD is in progress to produce an improved quality of surface passivation with the S_eff,max<;10 cm/s for ultrathin passivation layers less than 2 nm, enabling a passivated tunneling contact.

Enhanced surface passivation of epitaxially grown emitters for high-efficiency ultrathin crystalline Si solar cells

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped emitters by replacing thermal SiO₂ as a passivation layer of p⁺-emitter employed in a 16.8% efficient 18-μm Si solar cell on stainless steel with plasma assisted atomic layer deposition (ALD) Al₂O₃/PECVD SiN_x stack. For the Al₂O₃/SiN_x stacks on epitaxial p⁺-emitter after post-deposition anneal, the emitter saturation current density (J_0e) values were decreased to 19.5 fA/cm² with the corresponding iV_oc of 688 mV By using advanced surface passivation scheme, further improvement in the V_oc of a present 16.8% efficient ultrathin Si solar cell on steel can be expected.

Rapid thermal annealing of InAlAsSb lattice-matched to InP for top cell applications

The effect of rapid thermal annealing on the optical properties of In_xAl_1-xAs _ySb _1-y was analyzed and compared to that for In_0.52 Al_0.48As. Initial ellipsometry and photoluminescence experiments performed before the annealing indicate the presence of a low energy Urbach tail in the absorption spectrum. Rapid thermal annealing produces a blue-shift in the PL emission when annealed at 650°C for 60s and a decrease in the full-width-half-maximum, which originates from a reduction of the emission from the longer wavelength states. For the In_0.52 Al_0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in In_0.52 Al_0.48As is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.