Conduction Band Offset Effect on the Cu2ZnSnS4 Solar Cells Performance

Among the causes of the degradation of the performance of kesterite-based solar cells is the wrong choice of the n-type buffer layer which has direct repercussions on the unfavorable band alignment, the conduction band offset (CBO) at the interface of the absorber/buffer junction which is one of the major causes of lower VOC. In this work, the effect of CBO at the interface of the junction (CZTS/Cd(1-x)ZnxS) as a function of the x composition of Zn with respect to (Zn+Cd) is studied using the SCAPS-1D simulator package. The obtained results show that the performance of the solar cells reaches a maximum values (Jsc = 13.9 mA/cm2, Voc = 0.757 V, FF = 65.6%, ɳ = 6.9%) for an optimal value of CBO = -0.2 eV and Zn proportion of the buffer x = 0.4 (Cd0.6Zn0.4S). The CZTS solar cells parameters are affected by the thickness and the concentration of acceptor carriers. The best performances are obtained for CZTS absorber layer, thichness (d = 2.5 µm) and (ND = 1016 cm-3). The obtained results of optimizing the electron work function of the back metal contact exhibited an optimum value at 5.7 eV with power conversion efficiency of 13.1%, Voc of 0.961 mV, FF of 67.3% and Jsc of 20.2 mA/cm2.

Space charge capacitance study of GaP/Si multilayer structure grown by plasma deposition

Microcrystalline GaP/Si multilayer structures grown on GaP substrates using combination of PE-ALD for GaP and PECVD for Si layers deposition are studied by three main space charge capacitance techniques: C-V profiling, admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS), which have been used on Schottky barriers formed on the GaP/Si multilayer structures. C-V profiling qualitatively demonstrates an electron accumulation in the Si/GaP wells. However, quantitative determination of the concentration and spatial position of its maximum is limited by the strong frequency dependence of the capacitance caused by electron capture/emission processes in/from the Si/GaP wells. These processes lead to signatures in AS and DLTS with activation energies equal to 0.39±0.05 eV and 0.28±0.05 eV, respectively, that are linked to the energy barrier at the GaP/Si interface. It is shown that the value obtained by AS (0.39±0.05 eV) is related to the response from Si/GaP wells located in the quasi-neutral region of the Schottky barrier, and it corresponds to the conduction band offset at the GaP/Si interface, while DLTS rather probes wells located in the space charge region closer to the Schottky interface where the internal electric field yields to a lowering of the effective barrier in the Si/GaP wells. Two additional signatures were detected by DLTS, which are identified as defect levels in GaP. The first one is associated to the SiGa+VP complex, while the second was already detected in single microcrystalline GaP layers grown by PE-ALD.

Terahertz Pulse Emission from Semiconductor Heterostructures Caused by Ballistic Photocurrents

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.

Electrothermal modeling for nano AlGaN/GaN HEMTs using dual-phase-lag theory

The combined dependence of the electronic and thermal characteristics in the AlGaN/GaN HEMTs supported in nano-elctronic devices was studied theoretically and numerically. The Schrödinger-Poisson equations coupled with Dual phase lag (DPL) thermal transfer equation was undertaken. Simultaneous impacts of the conduction band offset and polarization charge between the AlGaN/GaN heterointerface induce the production of the two-dimensional electron gas density (2DEG). The simulation results showed that the 2DEG density at the heterointerface increased with increase of Aluminum fraction. In addition, the simulation results of the thermalization process were found to be in good agreement with the literature. As a result, the maximum heat generation as well the maximum temperature at the heterointerface increased. The obtained result could to be useful in assessing thermal transfer in the AlGaN/GaN HEMTs nano-devices to improve their performance.

Analytical study of effect of energy band parameters and lattice temperature on conduction band offset in AlN/Ga2O3 HEMT

Apart from other factors, band alignment led conduction band offset (CBO) largely affects the two dimensional electron gas (2DEG) density ns in wide bandgap semiconductor based high electron mobility transistors (HEMTs). In the context of assessing various performance metrics of HEMTs, rational estimation of CBO and maximum achievable 2DEG density is critical. Here, we present an analytical study on the effect of different energy band parameters-energy bandgap and electron affinity of heterostructure constituents, and lattice temperature on CBO and estimated 2DEG density in quantum triangular-well. It is found that at thermal equilibrium, ns increases linearly with ?EC at a fixed Schottky barrier potential, but decreases linearly with increasing gate-metal work function even at fixed ?EC, due to increased Schottky barrier heights. Furthermore, it is also observed that poor thermal conductivity led to higher lattice temperature which results in lower energy bandgap, and hence affects ?EC and ns at higher output currents.

Energy band alignment at the heterointerface between CdS and Ag-alloyed CZTS

Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its alloys plays a crucial role in determining the efficiency of the solar cells. Whereas Ag alloying of CZTS has been shown to reduce anti-site defects in the bulk and thus rise the efficiency, the electronic properties at the interface with the CdS buffer layer have not been extensively investigated. In this work, we present a detailed study on the band alignment between n-CdS and p-CZTS upon Ag alloying by depth-profiling ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Our findings indicate that core-level peaks and the valence band edge of CdS exhibit a significant shift to a lower energy (larger than 0.4 eV) upon the etching of the CdS layer, which can be assigned due to band bending and chemical shift induced by a change in the chemical composition across the interface. Using a simplified model based on charge depletion layer conservation, a significantly larger total charge region depletion width was determined in Ag-alloyed CZTS as compared to its undoped counterpart. Our findings reveal a cliff-like band alignment at both CdS/CZTS and CdS/Ag-CZTS heterointerfaces. However, the conduction-band offset decreases by more than 0.1 eV upon Ag alloying of CZTS. The approach demonstrated here enables nanometer-scale depth profiling of the electronic structure of the p–n junction and can be universally applied to study entirely new platforms of oxide/chalcogenide heterostructures for next-generation optoelectronic devices.