• Energy Research

The modulated surface photovoltage (SPV) experiment in metal-insulator- semiconductor configuration (MIS) uses application of a chopped light beam upon the surface of a sample. The induced variation of the surface potential is recorded by a transparent conductive probe placed on top of the sample. We investigate the impact of the light modulation frequency on the response of untreated p-type silicon, used as a model system. Due to the absence of buried heterojunctions in the sample, we assume that the SPV response originates at the Si surface due to the presence of a defect-induced space charge region (SCR). Experimentally, we observe an increase of the phase shift of SPV with respect to the incoming beam, and a reduction in the SPV amplitude, when we increase the modulation frequency from 29 to 494 Hz. This trend is qualitatively explained by the timescale of charge carrier trapping and detrapping at the surface of the sample. We have employed drift-diffusion simulations to quantitatively evaluate the impact of key surface defects’ properties, and to show the power of frequency modulation in the MIS- SPV technique. Following the Shockley–Read-Hall non-radiative recombination mechanism [1,2], parameters such as defect density and carrier capture cross sections have been evaluated. From our preliminary results, a large asymmetry in the role of the electron and hole capture cross sections arise. This can be ascribed to the doping nature of the semiconductor. Under illumination, photogenerated electrons are driven towards the surface, where they subsequently trap, with a speed which depends on the electron capture cross section. During the dark semi-period, these electrons are released by recombining with holes in the valence band, a process which mainly depends on the hole capture cross section. On the one hand, dynamics under light are impacted by the large photogenerated carrier density, which increases the electron trapping rate on the surface. On the other hand, during the dark semi-period, the hole density at the surface quickly decreases. This translates into a much higher impact on the SPV phase shift of the hole capture cross section, with respect to the electron one.

AbstractThis paper presents a simple and nondestructive method to determine doping densities and built‐in potential of subcells by adapting the well‐known capacitance‐voltage (C‐V) technique to two‐terminal (2 T) tandem solar cells. Because of the electrical coupling between the two subcells in a monolithic 2 T tandem solar cell, the standard method using a Mott‐Schottky plot (1/C2 vs V) cannot be applied. Using numerical modeling, it is demonstrated that, by under chosen illumination conditions where only one subcell can absorb the light, it is possible to explore the bias dependence of the capacitance and to extract the parameters of the other subcell if the appropriate frequency conditions are present. This method is experimentally applied to an AlGaAs/Si tandem cell, and parameters of both AlGaAs and Si cells are extracted. Finally, the validity of that method is assessed by the very good agreement obtained when comparing the values extracted from our measurements on the tandem cell to those extracted from measurements on isotype cells and to the values targeted during the fabrication process of the AlGaAs/Si tandem solar cell.

It is now well established that halide perovskite materials, such as Methyl Ammonium Lead Iodide (MAPI), contain low-mobility ions that affect the device operation and performance. Ionic motion is believed to be primarily responsible for the long transients observed in dark J-V measurements (hysteresis) of perovskite devices.In this work, we use a drift-diffusion numerical simulation to evaluate the main characteristics of the current transients produced by a mobile ion after biasing simple Metal-Semiconductor-Metal (MSM) structures. We compare the theoretical results with experimental measurements performed in monocrystals and thin-films devices of halide perovskites. We observe that most of the transient characteristics can be explained with our model and we present a discussion of the possible causes for some observed discrepancies.We relate the semiconductor parameters, such as the concentration of dopants and ionic species, with the current transient shape and time position. We deduce two analytical formulas for extracting the ionic mobility and the ratio between ionic and dopant concentrations from the current transient. Finally, we perform measurements at different temperatures for extracting the activation energy of the ionic mobility.

We present some advanced characterization techniques developed to investigate on the opto-electronic properties of thin film semiconductors and apply them to perovskite layers. These techniques are the steady state photocarrier grating (SSPG) and the Fourier transform photocurrent spectroscopy (FTPS). The SSPG was developed to study the ambipolar diffusion length of carriers and the FTPS was imagined to measure the variations of the below gap absorption coefficient with the light energy, giving information on the defect densities of the gap responsible for this absorption. The potentialities of these techniques are first detailed and then exemplified by their application to thin film perovskites. To study their stability, these films were exposed to different environments, air or vacuum, and in their as-deposited state or after light-soaking with heavy light. We find that the diffusion length and density of states are quite stable, even after light-soaking, and suggest that the degradation of devices exposed to 1 sun mainly comes from the evolution of the contacts instead of the perovkite itself.

Amorphous silicon–germanium (a-Si1−xGex:H) alloy thin films were studied over a wide range of Ge content (x=0–1.00) grown by radio frequency plasma CVD (rf PECVD) technique with variation of different deposition parameters such as H2 dilution, rf power and square wave pulse modulation (SWPM) of rf amplitude. Structural properties like microstructure factor (R⁎) and AFM surface roughness (RRMS) were correlated with the transport properties such as mobility-lifetime product (μτ) and ambipolar diffusion length (Ld) of these films. Near the middle composition range (x=0.32–0.70), the R⁎ in these films varies between 0.20 and 0.42 and Ld ranges between 50 and 60 nm. Films deposited near the pure silicon and pure germanium ends have improved structural and transport properties. By SWPM method we have been able to significantly lower the R⁎ value of the a-Si1−xGex:H films to 0.15 with x=0.40–0.45 resulting in Ld=100 nm and μτ=1.0×10−6 cm2 V−1.

We propose a very simple system to be adapted to a Fourier Transform Infra-Red (FTIR) spectrometer with which three different types of characterizations can be done: the Fourier transform photocurrent spectroscopy, the recording of reflection-transmission spectra of thin film semiconductors, and the acquisition of spectral responses of solar cells. In addition to gather three techniques into a single apparatus, this FTIR-based system also significantly reduces the recording time and largely improves the resolution of the measured spectra compared to standard equipments.

Very high conversion efficiency is reached with triple junction solar devices integrated in concentrator photovoltaic (CPV) modules. However, reduction of the active area for micro-CPV applications increases the perimeter/area ratio, enhancing losses linked to the edges. It is therefore important to characterize the perimeter influence on the final conversion efficiency. For this purpose, I(V) characterization under dark and/or light could be used as a test of the sidewalls influence. We have designed an experiment to perform I(V) curves using the light of three lasers with adjustable powers at 405, 785, and 980 nm, preferentially absorbed by the top, middle or bottom junction of the device, respectively. This experiment was applied to commercial devices made from a stack of GaInP/GaAs/Ge. In parallel we have developed a numerical calculation modeling the device to reproduce the behaviors observed during I(V) experiments. Junction parameters and influence of leakage resistances are deduced from the fit of experimental results with the numerical calculation. The I(V) experiment as well as the numerical calculation are presented in details. It is also underlined that, combining both experiment and calculation, the I(V) characteristic of each junction as if it was isolated can be determined.