LBIC Measurements Research Articles

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Analysis of Surface Passivation and Laser Firing on Thin-Film Silicon Solar Cells Via Light-Beam Induced Current

Liquid phase crystallized silicon solar cells on glass have recently demonstrated 15.1% efficiency using a heterojunction interdigitated back contact cell architecture and an absorber thickness of 14 $\mu$ m. One of the key enabling developments was a new method to first passivate electron contact fingers with a-Si:H(i) and then locally laser fire them to maintain a low contact resistance. In this work, high resolution, light-beam induced current measurements (LBIC) were used to analyze this approach. Charge collection was observed to have increased from 0.13 ${\text{mAcm}}^{-2}$ to 0.9 ${\text{mAcm}}^{-2}$ under the electron contact which is a sevenfold increase. Using 520, 642, 932, and 1067 nm wavelengths of incident light, external quantum efficiency was mapped in regions including grain boundaries, dislocation defects, shunts, defect-free regions, and laser fired spots. Reduction of charge collection in the laser fired spots was limited to diameters of 50–20 $\mu$ m, depending on whether electrical recombination or optical losses dominated. Effective minority carrier diffusion length under the majority carrier contacts was obtained by fitting of LBIC measurements. It was observed to have improved from 20.5 $\mu$ m to 22.7–40.4 $\mu$ m and up to 89.0 $\mu$ m in the best case. Based on this, wider contact fingers and improved surface passivation at the electron contact is encouraged in the near future to achieve efficiencies $\geq$ 16%.

Front-floating Emitter Voltage Mapping of IBC Mercury Cells

Standard characterization techniques such as LBIC are difficult to apply to IBC solar cells with a front floating emitter (FFE). This is because the cells need to be under bias illumination, and this is often impossible in commercial LBIC measurement tools. To the best of our knowledge, LBIC measurements on FFE IBC cells have not been published so far. In this work we present a experimental method to spatially characterize the FFE of IBC cells and gain insights on electrical shading losses, without using LBIC. This method makes use of the commercially available CoreScan device, mapping the FFE voltage relative to the shorted back-contacts over the cell area. Using this technique we were able to resolve busbar, pads and finger features of both polarities with a scan of the front side. The scan shows that BSF features have higher voltage than emitter features, thus giving a direct evidence of the FFE pumping effect. If coupled to circuit simulation, FFE voltage maps can be the base for estimating electrical shading losses in IBC cells. The FFE voltage maps also give information on the homogeneity of the pumping effect across the wafer, e.g. affected by diffusion non-uniformity, and at least qualitatively its effect on the cell performance.

The impact of thermal treatment on gettering efficiency in silicon solar cell

This paper describes a study to understand the impact of heat treatment on the crystal Silicon (c-Si) with developed surface by Aluminum /Porous silicon (Al/PS) and evaluating their influence on the solar cell performance. In fact, the heat treatment of silicon surfaces with PS/Al is an effective means of structural and electrical adjustment and performance improvement of c-Si solar cells. The new process could be accomplished by a two-step annealing in order to benefit from both the high and low temperature processes. The advantages of such a new getter in comparison with traditional getter were demonstrated in various devices. We can estimate that the resistivity using the standard Van Der Pauw, and LBIC measurements have been performed to determine the diffusion length. The resistivity of crystalline silicon decreased when the porous layer was removed at about a depth around 70μm. As a result, we found amelioration in the I–V characteristics and an enhancement of minority carrier lifetime. It has been shown that this simple method leads to improve the charge carrier collection and the electrical properties of c-Si based solar cells.

Effect of thermal treatments on the morphological and optoelectronic properties of the silicon nanowires

This work presents additional arguments about the enhancement of optical and electrical properties of SiNWs obtained by chemical etching and the beneficial effect of the formation of ultrathin film of SiO2 by a heat treatment at 900 and 1000 °C reported recently by Karyaoui et al. in [Eur. Phys. J. Appl. Phys. 58 , 20103 (2012)]. In the actual study, we report the effect of this thermal oxidation treatment on the morphological and optoelectronic properties of silicon nanowires (SiNWs). The SEM and AFM observations exhibit indeed a remarkable change of the surface shape after this oxidation. On the other hand, the LBIC measurements of the SiNWs reveal that the thermal treatment improves the optoelectronic response at 900 °C and the effective diffusion length increases from 150 μ m for untreated SiNWS to 235 μ m after thermal treatment.

Combined EL and LBIC Study of the Electrical Activity of Defects in Solar Cells Based on Innovative Wafers Grown by Casting Methods

In this paper we combine LBIC and EL measurements of commercially multi-crystalline silicon solar cells, in order to obtain detailed information about the electrical activity around defect areas. This integrated analysis is suitable for the study of different crystal defects at both micrometric and full wafer scale. In particular, the electrical activity of some defect areas is studied in detail by means of highly spatially-resolved LBIC maps, showing important differences in their behaviours. A discussion about the origin of these differences is presented.

Improved efficiency of multicrystalline silicon solar cells by TiO2 antireflection coatings derived by APCVD process

This paper reports about the adaptation of the chemical vapor deposition (CVD) thin films technology to the fabrication process of multicrystalline silicon solar cells as a simple, low cost and very effective technology for efficiency device improvement by reducing reflection and improving the light-generated current. In this contribution, the higher reflection of a mc-Si solar cell surface is strongly reduced by the deposition of TiO2 antireflection coating (ARC) on the front using the atmospheric pressure chemical vapor deposition method (APCVD). The surface morphology and elemental composition of the TiO2 antireflective layers were revealed using scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy . The reflectivity was then reduced from 35% to 8.6% leading to the increase of the short circuit current Jsc which was 33.86mA/cm² with a benefit of 5.23mA/cm² (surface area=25cm²) compared to the reference cell (without ARC). This simple and low cost technology induces a 14.26% conversion efficiency which is a gain of +3% absolute in comparison to the reference cell. The LBIC measurements of a typical multicrystalline cell confirmed the uniformity of the photocurrent distribution throughout the device. These results are encouraging and prove the effectiveness of the APCVD method for efficiency enhancement in silicon solar cells.

Simulation and Characterization of Epitaxial n- and p-Type Emitters on Silicon Wafer Solar Cells

Epitaxial emitters deposited by atmospheric pressure CVD have been studied using different characterization methods such as spreading resistance profiling (SRP) or electrochemical capacitance voltage profiling (ECV). Comparing simulations with measured data, very low minority carrier lifetimes of τeff = 20μs have been determined for FZ-material after a standard epitaxy process. LBIC measurements showed very low and inhomogeneous distributed diffusion lengths in the range of 100-250μm before process optimization. Therefore an emitter epitaxy process working at lower temperatures was developed [1] leading to diffusion lengths of 750μm. The effective minority carrier lifetimes and emitter saturation currents were determined using QSSPC measurements. For samples with an emitter, deposited at optimized temperature, a best value for the effective minority carrier lifetime of τeff = 212μs was measured and an emitter saturation current could be determined to J0e= 46 fA/cm2. In order to investigate the stress in the epitaxial layers μRaman and μPL measurements were performed showing no increased values at the interface. The doping density in boron doped emitters was determined with μPL and in good agreement with measured ECV-profiles. The Shockley-Read-Hall lifetime in the emitter bulk showed no increased values at the interface between substrate and epitaxial layer.

Limited laser beam induced current measurements: a tool for analysing integrated photovoltaic modules

A new limiting laser beam induced current (li-LBIC) measurement technique is described which allows the recovery of the true signal in an integrated photovoltaic (PV) module. It is a measurement based on optically exciting the device by scanning with a laser, where the probed cell is brought under limiting condition by means of manipulating the background illumination. The signal obtained is directly proportional to the laser strength, which was previously unobtainable by typical LBIC measurements in a module. The advantages are shown to be that it gives an opportunity to measure the real properties of investigated cells and is usable for fully encapsulated, series connected photovoltaic modules. It is also unaffected by non-uniform background illumination. The system provides spatially resolved measurements and has shown to be an effective characterization tool to reveal spatial non-uniformity in the PV devices with sub-millimetre resolution. It can be employed as a characterization tool for commercial modules functioning not only for quality control of newly produced PV devices but also for loss analysis during various stages of their operating lives.

04/01596 CELLO: an advanced LBIC measurement technique for solar cell local characterization: Carstensen, J. et al. Solar Energy Materials and Solar Cells, 2003, 76, (4), 599–611

04/01596 CELLO: an advanced LBIC measurement technique for solar cell local characterization: Carstensen, J. et al. Solar Energy Materials and Solar Cells, 2003, 76, (4), 599–611

CELLO: Advanced LBIC measurement technique for solar cell local characterization

CELLO: Advanced LBIC measurement technique for solar cell local characterization

Silicon tubes by a closed molten zone: a characterisation study

We report on the characterisation of silicon tubes recrystallised by closed molten zone , a technique developed as a step to a possible process for thin silicon sheet production. The tube faces are quite flat and have a smooth surface. For the electrical characterisation, samples were cut from the tube faces and simple photovoltaic solar cells were formed. The average diffusion length of minority carriers was found, from spectral response, to be around 100 μm. Low-resolution LBIC measurements showed lower diffusion lengths (around 40 μm) in the regions close to the tube edges. This behaviour was correlated to measurements of residual thermal stresses using infrared photoelasticity. Measurements of the changes of spectral response with increasing bias light intensity reveal an increase in the effective diffusion length, a known effect that is interpreted in terms of a density of trapping states.

Characteristics of gradually doped LWIR diodes by hydrogenation

The hydrogenation effects on HgCdTe diode performance are presented and the mechanism of hydrogenation is revealed. By the hydrogenation, R0A is increased by 30 times and photo-response is also improved. It is supposed that these are explained by the increased minority carrier lifetime by the hydrogenation. However, it is found from LBIC measurements that the minority carrier lifetime doesn’t increase by the hydrogenation. An important clue that explains the hydrogenation effects is found from Hall measurements. It is found that, after the hydrogenation, the doping concentration of Hg-vacancy doped substrate decreases and the mobility increases. For the heavily hydrogenated bulk substrate, it is also found that the hydrogen passivates the whole Hg-vacancy and reveals the residual impurity and p-type doping concentration is exponentially graded. From these measurements, the diffusion current model of gradually doped diode is proposed. This model shows that the diffusion current of the graded junction diode is 2 orders of magnitude smaller than that of the abrupt junction diode, which clearly explains the R0A increase by the hydrogenation. Medicisimulation to investigate the change of LBIC signal by the doping grading also coincides with the measurements. From these measurements and model, the hydrogenation effects are attributed to the grading of Hg-vacancy doped p-type substrate by the diffused hydrogen.

Study of the Radiative and Non-Radiative Recombination Processes at Dislocations in Silicon by Photoluminescence and LBIC Measurements

AbstractIt is well known that the sharp, room temperature luminescence emission at 1.54 μm from dislocated silicon has set off a great interest for this material in view of its applications in the third window of optical telecommunications. For this reason the dislocation related luminescence in silicon addressed recently a number of investigation aimed at understanding the mechanism of light emission. The problem is still unsolved as most of the experiments done gave contradictory answers to the main questions open, which concern the intrinsic or extrinsic nature of dislocation luminescence and the effect on it of reconstruction, interaction or passivation processes, possibly assisted by metallic or non-metallic impurities.In order to go more insight on the problem, we started a systematic work on CZ silicon, aimed at understanding the properties of dislocation luminescence. The identification of the energy levels involved in the different dislocation PL bands has been obtained.

Calculation of beam induced current in a solar cell structure

The excess carrier concentrations and currents in EBIC and LBIC measurement situations are calculated in a solar cell structure. The surface recombination velocities and following boundary conditions are taken into account by using extra deltafunction like generation or recombination points located outside the layer.

LBIC measurements of the recombining activity of dislocations in float zone silicon

The electrical activity of dislocations has been evidenced by LBIC scanning measurements through two different distributions of dislocations for which theoretical models leading to the recombination velocity could be applied. A close experimental correlation was found between photocurrent and dislocation density. The models led to the same recombination velocity for dislocations Sd ≈ 10 3 cm s −1 . The role of oxygen and of temperature on the activity of dislocations is discussed.