Surface Recombination Velocity Research Articles

The impact of interface recombination on the external quantum efficiency of silicon solar cells

In various types of organic/inorganic solar cells, optical response enhancement is consistently observed within the external quantum efficiency spectra owing to the improvement in interface passivation and the suppression of carrier recombination. In this study, we focused on crystalline silicon solar cells and systematically investigated the impact of interface recombination on the optical response upon dual-side illumination using numerical simulations. The results shed light on the interesting phenomenon that the surface recombination velocity has a significant impact on the external quantum efficiency, and it changes as the illumination direction changes. Moreover, from a practical perspective, the spectra of external quantum efficiency under dual-side illumination conditions can act as a powerful tool for the quick diagnosis of the passivation quality at the top and bottom interfaces.

Effect of films thickness and hydrogen annealing on passivation performance of plasma ALD based Hafnium oxide films

We investigate the silicon surface passivation property of Plasma Atomic Layer Deposited (PALD) hafnium oxide thin films and study its dependence on silicon (Si) doping type, film thickness, and post-deposition annealing conditions. Our results demonstrate that as-deposited HfOx films exhibit poor passivation quality that can be improved by performing post-deposition annealing at 450 °C in hydrogen ambient. We demonstrate that the films can effectively passivate p-Si surfaces as compared to n-Si, where the surface passivation quality of the films improves with increasing film thickness for both silicon doping types. The best performance with a minority carrier lifetime of 1.7 ms, corresponding surface recombination velocity (SRV) ∼10 cm s−1, is achieved for HfOx films thickness ∼23 nm deposited on the p-Si substrate. The Capacitance-Voltage (C–V) measurements give an insight into the passivation mechanism of the studied films. Field effect passivation is found to be an important passivation mechanism in PALD-deposited HfOx films, as revealed by C–V measurements. The films are also characterized using Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS), which reveals the chemical passivation provided by hydrogen ambient annealing. Overall, the impact of hafnium oxide film thickness and hydrogen ambient annealing conditions on silicon surface passivation is investigated. Our findings will help in utilizing plasma ALD process based HfOx films for silicon solar cell device application.

Comparison of three titanium-precursors for atomic-layer-deposited TiO2 for passivating contacts on silicon

Atomic layer-deposited (ALD) TiO2 thin films on silicon were deposited using titanium tetrachloride (TiCl4), titanium tetraisopropoxide (TTIP), and tetrakis(dimethylamino)titanium (TDMAT) together with water vapor as the oxidant at temperatures ranging between 75 and 250 °C. The Si surface passivation quality of as-deposited and isothermally annealed samples was compared using photoconductance lifetime measurements in order to calculate their effective surface recombination velocities Seff. A low Seff of 3.9 cm/s (J0s=24fA/cm2) is achieved for as-deposited TiCl4-TiO2 at 75 °C when a chemically grown (i.e., from RCA cleaning) SiOx interface layer is present. Depositing TTIP-TiO2 at 200 °C on a chemically grown SiOx interface layer yields equivalent Seff values; however, in this case, TTIP-TiO2 requires a 5–15 min postdeposition forming gas anneal at 250 °C. In contrast, TDMAT-TiO2 was not found to provide a similar level of passivation with/without a chemically grown SiOx interface layer and postdeposition anneal. Modeling of the effective lifetime curves was used to determine the magnitude of the effective charge densities Qf in the TiO2 films. In all cases, Qf was found to be of the order of ∼1011 q cm−2, meaning field-effect passivation arising from ALD TiO2 is relatively weak. By comparing the material properties of the various TiO2 films using ellipsometry, photothermal deflection spectroscopy, Raman spectroscopy, elastic recoil detection analysis, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we find experimental support for the role of Cl (in conjunction with hydrogen) playing a beneficial role in passivating dangling bond defects at the Si surface. It is concluded that low deposition temperature TiCl4 processes are advantageous, by providing the lowest Seff without any postanneal and a comparatively high growth per cycle (GPC).

Modeling the impact of grain size on device characteristics of Sb2Se3 solar cells

In this paper, we developed an analytical model to study the impact of grain size and the associated electrical parameters on the device characteristics and spectral response of Sb2Se3 antimony solar cells. Current-voltage characteristics as well as internal and external quantum efficiencies of the cell were modelled and the impact of grain size, effective intra-grain diffusion length, surface recombination velocity at grain boundaries, and carrier lifetime have been investigated. The profile of Electric field at the grain boundaries, the impact of grain size on the performance parameters of the cell have been investigated and the intra-grain diffusion length as an effective parameter on device performance parameters has been studied. The results indicate that a larger grain size is essential for surpassing recombination loss and improving the open-circuit voltage while a better carrier collection and spectral response can improve the short-circuit current density. A practical outcome of our study confirms the necessity of post-deposition surface passivation of Sb2Se3 absorber layers to surpass the recombination loss at grain boundaries and develop larger grains from an optimized deposition process to enhance the carrier collection for a more efficient and stable Sb2Se3 solar cell.

Optimization of a Solution-Processed TiOx/(n)c-Si Electron-Selective Interface by Pre- and Postdeposition Treatments.

Developing a vacuum-free and low-temperature deposition technique for dopant-free carrier-selective materials without sacrificing their performance can reduce the fabrication cost and CO2 footprint of silicon heterojunction (SHJ) solar cells. In this contribution, to activate the full capacity of the solution-processed TiOx as an electron-selective passivation contact, the effects of various pre- and postdeposition treatments on the passivation quality and contact resistivity are investigated simultaneously. It is demonstrated that the electrical properties of a thin TiOx layer spin-coated on an n-type silicon substrate can be remarkably improved through tailor-made pre- and postdeposition treatments. A notable low surface recombination velocity (SRV) of 6.54 cm/s and a high implied open-circuit voltage (iVoc) of 706 mV are achieved. In addition, by inserting a 1 nm LiFx buffer layer between TiOx and Al metal contact, a low contact resistivity (ρc) of 15.4 mΩ·cm2 is extracted at the n-Si/SiOx/TiOx heterojunction. Our results bring the solution-processed TiOx electrical properties to a level on par with those of state-of-the-art pure TiOx layers deposited by other techniques. Chemical and electrical characterizations elucidate that the improved electrical properties of the investigated Si/SiOx/TiOx heterojunction are mediated by the concomitant involvement of chemical and field-effect passivation.

Quantifying recombination and charge carrier extraction in halide perovskites via hyperspectral time-resolved photoluminescence imaging

Identifying sources of nonradiative recombination and quantifying charge carrier extraction in halide perovskite solar cells are important in further developing this thin-film technology. Steady-state and time-resolved photoluminescence (TRPL), in combination with analytical modeling, have emerged as non-destructive tools to achieve the desired results. However, the exact location of the recombination and charge carrier extraction losses in devices is often obscured by various competing processes when photoluminescence measurements are analyzed. Here, we show via absolute-photon-calibrated hyperspectral photoluminescence and TRPL imaging how surface passivation and inhomogeneities at interfaces impact the photoluminescence quantum yields and minority carrier lifetimes. Laser illumination from the perovskite and glass/TiO2 sides allows us to disentangle changes in surface recombination velocity from the charge carrier extraction at the electron transport layer. We find that charge extraction is spatially modulated due to an inhomogeneous mesoporous (mp)-TiO2 film thickness. Our results show that the mp-TiO2 layer is not fully optimized since the electronic properties are spatially modified, leading to lateral changes in quasi-Fermi-level splitting, minority carrier lifetime and, consequently, a reduction in open-circuit voltage.

Study of silicon surface passivation by ZnOx/AlOx stack prepared using super-cycle approach in thermal ALD process

High quality surface passivating films are essential for high efficiency solar cells. In this work, we present our study about the silicon surface passivation performance of thermal atomic layer deposited (T-ALD) ZnOx/AlOx stack deposited using super-cycle approach. Super-cycle approach is composed of three cycles of DEZ-DI water system followed by one cycle of TMA-DI water system. The number of super-cycles is varied to change the film thickness. Excellent passivation with surface recombination velocity ∼ 7 cm/s was achieved under this study for hydrogen annealed films. The passivation mechanism is related to the saturation of defects by hydrogen after annealing and further improvement in fixed oxide charges by one order of magnitude (1010 to 1011 cm−2). Hydrogenation at the optimum temperature (450⁰C) involves the transport of hydrogen atoms towards the interface and their interaction with dangling bonds at the Si surface leading to the effective passivation. The optical transmittance of the film in the visible region of the spectrum is found to be >95 % for all the deposited films. XPS data reveals that deposited films are Al rich which is possible due to ligand exchange reactions during the growth process. Annealing of ZnOx/AlOx stacks in hydrogen ambient and their Al-rich behavior helps in the realization of good passivation performance. XPS study also shows that Al-O bond improves with film thickness. According to valence band spectra, a relative shift in Fermi level position with regard to the valence band improves band bending, which improves field effect passivation. Such high-quality ZnOx/AlOx passivating films may improve the device efficiency.

Demonstration of Excellent Crystalline Silicon Surface Passivation (S < 1.27 cm s−1) by High‐Rate DC‐Sputtered Hydrogenated Amorphous Silicon

The surface passivation quality of intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) layers deposited by constant DC power‐facing target sputtering (FTS) technology is investigated. An n‐type crystalline silicon wafer passivated with a 42 nm‐thick i‐a‐Si:H shows an effective carrier lifetime of 11 ms, which corresponds to a low upper limit effective surface recombination velocity Seff of 1.27 cm s−1. An ultrathin 5 nm‐thick i‐a‐Si:H film achieves an implied open‐circuit voltage (iVoc) of 730 mV at a high deposition rate of 31 nm min−1. It is found that constant DC‐FTS does not cause sputtering damage under high deposition rate conditions. This work demonstrates that i‐a‐Si:H deposited by constant DC‐FTS can achieve the same passivation quality as i‐a‐Si:H deposited by plasma‐enhanced chemical vapor deposition or catalytic chemical vapor deposition, paving the way for the fabrication of silicon heterojunction solar cells without the use of explosive and toxic gases.

A highly efficient n‐CdS/p‐Ag2S/p+‐SnS thin film solar cell: Design and simulation

AbstractSilver sulfide (Ag2S) chalcogenide compound can be a viable absorber in the applications of thin film solar cells owing to its optimum bandgap of 1.1 eV and high absorption coefficient. Herein, we propose a novel Ag2S‐based n‐CdS/p‐Ag2S/p+‐SnS double‐heterojunction solar cell. The numerical analysis of the device has been performed with SCAPS‐1D (Solar Cell Capacitance Simulator). In the case of single heterojunction, n‐CdS/p‐Ag2S manifests an efficiency of 19.75%, where VOC = 0.66 V, JSC = 36.99 mA/cm2 and FF = 81.50%. However, Ag2S‐based double‐heterojunction device with optimized structure provides the efficiency of 29.51% with VOC = 0.81 V, JSC = 42.81 mA/cm2 and FF =85.24%. The noteworthy augmentation of VOC and JSC in double‐heterojunction results from the reduction in surface recombination velocity and rise in built‐in voltage in the p‐Ag2S/p+‐SnS hetero‐interfaces that promote the higher efficiency of the device. These theoretical insights indicate a path for fabrication of an efficient Ag2S based thin film solar cell.

Approach to determining the limiting recombination mechanism in CdTe-based solar cells

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data.

Enhancing minority carrier lifetime in Ge: Insights from HF and HCl cleaning procedures

Efficiently passivating germanium (Ge) surfaces is crucial to reduce the unwanted recombination current in high-performance devices. Chemical surface cleaning is critical to remove surface contaminants and Ge oxides, ensuring effective surface passivation after dielectric deposition. However, Ge oxides can rapidly regrow upon air exposure. To understand the surface evolution after wet cleaning, we present a comprehensive study comparing HF and HCl deoxidation steps on p-type Ge surfaces and monitor the surface as a function of air exposure time. Distinct oxide regrowth dynamics are observed: HF-treated samples exhibit swift regrowth of all Ge oxide states, whereas HCl-treated Ge surfaces exhibit a lower concentration of low degrees of oxidation and slower or no regrowth of high oxide states even after 110 min of air exposure. In addition, the presence of Ge–Cl bonds induces different oxidation dynamics compared to the Ge–OH bonds resulting from HF cleaning. This leads to varying surface electronic band structures, with HF-treated Ge exhibiting a strong positive band bending (+0.20 eV). Conversely, HCl-treated samples display a lower band curvature (+0.07 eV), mostly due to the presence of Ge–Cl bonds on the Ge surface. During air exposure, the increased GeOx coverage significantly reduces the band bending after HF, while a constant band bending is observed after HCl. Finally, these factors induce a reduction in the surface recombination velocity after wet etching. Combining both chemical and field-induced passivation, HF-treated Ge without rinsing exceeds 800 μs.

Modeling the performance of perovskite solar cells with inserting porous insulating alumina nanoplates

Peng et al. [Science 379 683 (2023)] reported an effective method to improve the performance of perovskite solar cells by using thicker porous insulator contact (PIC)-alumina nanoplates. This method overcomes the trade-off between the open-circuit voltage and the fill factor through two mechanisms: reduced surface recombination velocity and increased bulk recombination lifetime due to better perovskite crystallinity. From arguments of drift-diffusion simulations, we find that an increase in mobility and carrier recombination lifetime in bulk are the key factors for minimizing the resistance-effect from thicker PICs and achieving a maximum power conversion efficiency (PCE) at approximately 25% reduced contact area. Furthermore, the partially replacement of perovskite films with thicker PICs would result in a reduction in short-current density, but the relative low refractive index of the PICs imbedded into the high refractive index perovskite creates light trapping structures that compensate for this loss.

Au Nanocluster Assisted Microstructural Reconstruction for Buried Interface Healing for Enhanced Perovskite Solar Cell Performance.

The heterogeneity of perovskite film crystallization along the vertical direction leads to voids and traps at the buried interfaces, hampering both efficiency and stability of perovskite solar cells. Here, bovine serum albumin-functionalized Au nanoclusters (ABSA), combined with heavy gravity, high surface charge density, and strong interactions with the electron transport layer, are designed to reconstruct the buried interfaces for not only high-quality crystallization, but also improved carrier transfer. The ABSA macromolecules with amine function groups and larger surface charge density interact with the perovskite to improve the crystallinity, and gradually migrate towards the buried interface, healing the defective voids, hence suppressing surface recombination velocity from 3075 to 452 cm s-1 . The healed buried interface and the higher surface potential of ABSA-modified TiO2 lead to improved carrier extraction at the interface. The resulting solar cell attains a power conversion efficiency of 25.0% with negligible hysteresis and remarkable stability, maintaining 92.9% of their initial efficiency after 3200 h of exposure to the ambient atmosphere, they also exhibit better continuous irradiation stability compared to control devices. These findings provide a new metal-protein complex to eliminate the deleterious voids and defects at the buried interface for improved photovoltaic performance and stability.

Computational investigation towards highly efficient Sb2Se3 based solar cell with a thin WSe2 BSF layer

A novel structure of chalcogenide-based antimony selenide (Sb2Se3) which is the most promising absorber materials in the field of thin film solar cells with a tungsten diselenide (WSe2) back surface field (BSF) has been proposed in this paper. Antimony selenide (Sb2Se3) with a BSF has been observed by one-dimensional solar cell capacitance simulator (SCAPS-1D). Its recognizable properties make it one of the most usable for non-toxic solar cell absorbers. This paper demonstrated the planning and modeling of Al/FTO/n-TiO2/p-Sb2Se3/Ni (without BSF) and Al/FTO/n-TiO2/p-Sb2Se3/p+-WSe2/Ni (with BSF) structures. The capability of Al/FTO/n-TiO2/p-Sb2Se3/p+-WSe2/Ni heterojunction solar cell structure shows the promising performances. We have numerically simulated and observed the performance parameters such as power conversion efficiency (PCE), open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) by tuning the different parameters such as thickness, doping concentration, defect density, the interface defect density, surface recombination velocity, series and shunt resistances along with temperature. The PCE of 20.61% with an absorber’s thickness of 800 nm for without BSF has been achieved. By inserting the WSe2 BSF with thickness of 100 nm, highly improving efficiency of 32.35% has been demonstrated. Our simulation and numerical analysis also provide valuable and important information that is very effective for further implementation and achieving high efficiency of thin film solar cell.

Surface recombination velocities for 4H–SiC: Dependence of excited carrier concentration and surface passivation

Carrier lifetime plays a crucial role in optimizing the performance and reliability of SiC devices. Surface recombination is one of the factors affecting carrier lifetime. However, the reported values for surface recombination velocity (S) have been inconsistent. This study aimed to elucidate the discrepancy in S observed in various reports by conducting microwave photoconductivity decay measurements on n-type 4H–SiC freestanding epitaxial layers at different excitation carrier concentrations. Additionally, we investigated the impact of passivation by examining the surfaces subjected to oxidation and post-oxidation annealing. We observed an increase in S with increasing excited carrier concentration for the surfaces, irrespective of the crystal faces (S- and C-faces) and passivation. This suggests the presence of a potential barrier or shallow recombination centers on the surfaces. Furthermore, we observed that S remained nearly constant with increasing temperature, contrary to the behavior observed under low-injection conditions. This difference also suggests the existence of a potential barrier or shallow recombination centers on both Si- and C-faces. The comprehensive insights gained from our study have significant implications for the design of SiC devices with enhanced performance and reliability.

Highly reflective and passivated ohmic contacts in p-Ge by laser processing of aSiCx:H(i)/Al2O3/aSiC films for thermophotovoltaic applications

Crystalline germanium (c-Ge) has historically been regarded as a cost-effective alternative to III-V semiconductors for thermophotovoltaic (TPV) device fabrication. However, Ge-based devices have not yet reported high efficiencies, partially due to the lack of an efficient back-surface reflector that turns back to the heat source out-band (sub-bandgap) thermal radiation. The difficulty of implementing back surface reflectors in Ge TPV cells is related to the simultaneous requirement of good back surface passivation, low electrical resistivity, and high out-band optical reflectivity. In this study, we demonstrate a highly reflective ohmic contact to p-type c-Ge (doping concentration of 2 × 1015 cm−3) made of an aSiCx(1 nm)/Al2O3 (50 nm)/aSiC (45 nm) stack that is laser processed using Nd:YVO4 laser emitting at 355 nm to create punctual p+ contacts (locally diffused Al regions). This stack is finally caped with a thick (1000 nm) Al layer that behaves as a metallic mirror and back electrode. As the laser processed area increases from 0.1 to 3 %, which is the typical range in the final devices, the surface recombination velocity increase from 10.5 to 60.0 cm/s, while the effective contact resistance reduces from 0.462 to 0.036 Ω cm2. Moreover, a sub-bandgap reflectance of 90–98 % is achieved. Simulations assuming ideal device configuration indicate that implementing these back contacts could potentially enable TPV cell conversion efficiencies comparable to the reported high-efficiency c-Ge TPV cells operating at similar illumination temperature.

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

Numerical investigation on the role of ZnTe back surface layer in an efficient CuInS2 thin film solar cell

This paper presents the modeling and numerical inspection of an efficient CuInS2-based n-CdS/p-CuInS2/p+-ZnTe thin film solar cell applying the SCAPS-1D simulator. The various parameters used in the simulation have been obtained from existing literature. The optimization of the device has considered the width, doping concentration, and defect density of individual layer. The optimized standalone CuInS2 device shows an efficiency of 16.83%. Addition of ZnTe in the device gives an impressive efficiency of approximately 28.67%, having a current density, JSC of 25.58 mA cm−2, a VOC of 1.25 V, and an FF of 89.12%. The superior VOC is a result of the increased built-in potential formed at the hetero-interfaces of the device and the decrease in surface recombination velocity by back surface field effect in the ZnTe layer. The highest JSC is ascribed to the enhancement of the absorption of vis-infrared photons by back surface field (BSF) effect in the ZnTe layer. These findings demonstrate the potential for manufacturing high efficiency CuInS2-based thin film solar cells in the future.

Enhancing the Efficiency of Inverted Organic Solar Cells with Treatment Techniques: Numerical and Experimental Study

The combination of UV-ozone and Al-doping enhancements in the ZnO ETL layer was studied for fullerene-based (P3HT:PCBM) and nonfullerene-based (J71:m-ITIC) solar cells. The study focused on morphological and interfacial defects and their relation to the electrical and optical properties of different Al-doped ZnO layers. The study also investigated the surface recombination velocity at the interface of the ZnO/active layer, as well as the carrier lifetime and diffusion length of the active layer theoretically. The most effective Al-doping dosage was 3% for P3HT:PCBM and 2% for J71:m-ITIC, attributed to the varying sensitivity of the active layer materials to morphological enhancement.

An Investigation of The J-V Curve for N-AlGaAs/p-GaAs Heterojunction Solar Cell Due to Influence of Surface Recombination Velocity

Theoretical analysis can substantially reduce the time and costs required for developing a specific device such as solar cell, by allowing the designer to take a suitable geometry and doping profile prior to the fabrication stage. In this paper, theoretical analysis of electric field and electric potential within depletion layer of N-AlGaAS/p-GaAs has been made by solving Poisson's equation numerically using finite difference method (FDM) with Fortran program. Continuity equations of charge carriers in quasi- nature regions in N and p-layers were solved numerically to obtain the current density curve (J-V curve). Boundary conditions related to surface recombination velocity were implemented in this analysis. The effect of front and back surface recombination velocity on the excess minority carriers distribution and photocurrent have been studied. The aim of this work is to find the influence of surface recombination velocity in the output of N-AlGaAs/p-GaAs heterojunction solar cell. The analysis showed that, the low surface recombination velocity produce high efficient solar cell.