Yi-Feng Lai

Multi-Source/Component Spray Coating for Polymer Solar Cells

A multi-source/component spray coating process to fabricate the photoactive layers in polymer solar cells is demonstrated. Well-defined domains consisting of polymer:fullerene heterojunctions are constructed in ambient conditions using an alternating spray deposition method. This approach preserves the integrity of the layer morphology while forming an interpenetrating donor (D)/acceptor (A) network to facilitate charge transport. The formation of multi-component films without the prerequisite of a common solvent overcomes the limitations in conventional solution processes for polymer solar cells and enables us to process a wide spectrum of materials. Polymer solar cells based on poly(3-hexylthiophene):[6,6]-phenyl C(61) butyric acid methyl ester spray-coated using this alternating deposition method deliver a power conversion efficiency of 2.8%, which is comparable to their blend solution counterparts. More importantly, this approach offers the versatility to independently select the optimal solvents for the donor and acceptor materials that will deliver well-ordered nanodomains. This method also allows the direct stacking of multiple photoactive polymers with controllable absorption in a tandem structure even without an interconnecting junction layer. The introduction of multiple photoactive materials through multisource/component spray coating offers structural flexibility and tenability of the photoresponse for future polymer solar cell applications.

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells

Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.

Efficiency Enhancement of InGaN-Based Multiple Quantum Well Solar Cells Employing Antireflective ZnO Nanorod Arrays

Antireflective ZnO nanorod arrays (NRAs) by a scalable chemical method have been applied for InGaN-based multiple quantum well solar cells. The length of the NRAs plays an important role in photovoltaic characteristics. It was found that the 1.1-μm-long NRA results in enhanced conversion efficiency due to the suppressed surface reflection. However, the 2.5- μm-long NRAs, although exhibiting the lowest reflection, lead to slightly deteriorated performances, possibly due to the increased absorption of the NRAs. The results indicate that the absorption of lengthened NRAs should be considered when optimizing their antireflection performances. We demonstrated a viable efficiency-boosting way for photovoltaics.

Origin of Hot Carriers in InGaN-Based Quantum-Well Solar Cells

InxGa1-xN/GaN multiple quantum-well (QW) (MQW) solar cells with x = 0.30 and 0.15 were characterized. The MQWs with x = 0.30 show deteriorated performances due to the inferior crystal qualities. At the temperatures above 200 K, the conversion efficiency (η) for x = 0.30 exhibits an abrupt increase led by the thermally activated carriers. Two potential origins are proposed for the hot carriers: 1) the native shallow donors in the MQWs and 2) the shallow QWs due to the compositional fluctuations. According to the distinct behavior of the device with x = 0.15, it is believed that the shallow QWs lead to the abrupt increase in η.

Efficiency enhancement of InGaN multi-quantum-well solar cells via light-harvesting SiO2 nano-honeycombs

Periodic sub-wavelength SiO2 nano-honeycombs are fabricated on GaN-based multiple quantum well solar cells by self-assembly polystyrene nanosphere lithography and reactive ion etching. The nano-honeycombs are found to be effective in suppressing the undesired surface reflections over a wide range of wavelengths. Under the illumination of air mass 1.5G solar simulator, conversion efficiency of the solar cell is enhanced by 24.4%. Simulations based on finite-difference time-domain method indicate that the improved performances result from the enhanced optical absorption in the active region due to the reflection suppression and enhanced forward scattering.

Defect-induced negative differential resistance of GaN nanowires measured by conductive atomic force microscopy

We report on negative differential resistance (NDR) from individual GaN nanowires prepared without catalysts by thermal chemical vapor deposition. Conductive atomic force microscopy was used to characterize the electron transport behavior and transmission electron microscopy was employed to characterize the microstructure of the GaN nanowires. The current-voltage curve exhibits two clear NDR regions in the forward bias. The defect assisted inelastic tunneling process resulting in the NDR behavior and the related mechanism for energy band diagram is proposed and discussed.

Correlation of In–Ga intermixing with band-tail states in InAs∕GaAs quantum dots

We have investigated the shape and composition profiles of buried and surface InAs∕GaAs Stranski–Krastanov quantum dots (QDs) by using the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In–Ga intermixing in the vicinity of the buried QDs.

Structural and optical properties of cubic-InN quantum dots prepared by ion implantation in Si (100) substrate

The authors have synthesized InN quantum dots by ion implantation into a Si (100) substrate followed by a postannealing process. X-ray photoemission spectroscopy data verified the formation of In–N bonding in both as-implanted and postannealed samples. Diffraction patterns from transmission electron microscopy (TEM) confirm that the dots are of cubic crystal (zinc-blende phase) with no presence of wurtzite InN. The silicon matrix provides a constraint for the formation of the InN cubic metastable phase. However, dislocations were revealed by high resolution TEM at the interfaces between the dots and the silicon. In addition, the authors found that as the annealing temperature or time increases, dot size increases and dot density decreases. Furthermore, they demonstrate that the main emission energy of zinc-blende InN dots is about 0.736eV.

Improving the Brightness and Reliability of InGaN/GaN Near Ultraviolet Light-Emitting Diodes by Controlling the Morphology of the GaN Buffer Layer

A facile method for fabricating near ultraviolet light-emitting diodes (NUV LEDs) with better crystal quality and enhanced light emission is demonstrated by tuning total working pressure to modify the morphology of the buffer layer. With the conditions favoring the transition from 2-D to 3-D island growth in the GaN buffer layer, the crystal quality of the epitaxial GaN film is improved, leading to a decrease in the full-width at half-maximum of the (1 0 2) peak in X-ray rocking curve measurement from 293 (arcsec) to 230 (arcsec). Scanning electron microscopy images imply the morphology of GaN grain boundaries at the buffer stage is different. The transmission electron microscopy analysis shows the edge dislocation can be bended due to the controlled morphology of buffer layer, leading to a better crystal quality of the GaN epitaxial film. Therefore, the reverse current is reduced with the reverse bias of -40 V. Moreover, the output power in lamp-formed NUV LEDs at a driving current of 20 mA can be enhanced by about 12%.

Comparison of the Microstructure and Optical Properties of InAs Quantum Dots Grown with/without an AlAs Insertion Layer

Be microstructure and optical properties of InAs quantum dots (QDs) grown on a GaAs buffer with a 30 nm thick AlAs insertion layer are investigated and compared with those grown on a plain GaAs buffer by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The former InAs QDs exhibit larger dot sizes of 20 nm and higher aspect ratios of 0.4, compared to 15 nm and 0.2, respectively, for the latter. Temperature-dependent PL spectra of the larger dots show that the main emission is dominated by band-tail state transitions at low temperatures and ground state transitions at high temperatures. The ground state transition energy in such quantum dots is significantly red-shifted compared to the smaller InAs QDs. Lower thermal activation energy is also observed for the larger QDs with an AlAs layer. All of the phenomena are caused by different In-Ga intermixing behavior occurring during capping, which is discussed in detail.