Rear Surface Recombination Velocity Research Articles

Over 100 mV VOC Improvement for Rear Passivated ACIGS Ultra‐Thin Solar Cells

AbstractA decentralized energy system requires photovoltaic solutions to meet new aesthetic paradigms, such as lightness, flexibility, and new form factors. Notwithstanding, the materials shortage in the Green Transition is a concern gaining momentum due to their foreseen continuous demand. A fruitful strategy to shrink the absorber thickness, meeting aesthetic and shortage materials consumption targets, arises from interface passivation. However, a deep understanding of passivated systems is required to close the efficiency gap between ultra‐thin and thin film devices, and to mono‐Si. Herein, a (Ag,Cu)(In,Ga)Se2 ultra‐thin solar cell, with 92% passivated rear interface area, is compared with a conventional nonpassivated counterpart. A thin MoSe2 layer, for a quasi‐ohmic contact, is present in the two architectures at the contacts, despite the passivated device narrow line scheme. The devices present striking differences in charge carrier dynamics. Electrical and optoelectronic analysis combined with SCAPS modelling suggest a lower recombination rate for the passivated device, through a reduction on the rear surface recombination velocity and overall defects, comparing with the reference solar cell. The new architecture allows for a 2% efficiency improvement on a 640 nm ultra‐thin device, from 11% to 13%, stemming from an open circuit voltage increase of 108 mV.

Impact of different parameters on the performance of GaAs solar cell using PC1D simulation

In this work, GaAs solar cell has been analyzed by using PC1D simulation software. Optimization of thickness and doping concentration of emitter and absorber layer of solar cell is done. Effect of front and rear surface recombination velocity is analyzed in this work. When surface recombination velocity increases, the value of efficiency decreases due to increase in recombination process. The optimum value of surface recombination velocity are Sn = Sp = 102 cm/s. Front surface reflection is minimized by the use of TiO2 single layer ARC and double layer LiF/HfO2 ARC. Increase in efficiency around 2.71% is achieved as compared to cell without ARC after applying anti-reflection coating of optimum thickness. Simulation results shows GaAs solar cell with efficiency of 26.68%.

Uniqueness of parameter estimates obtained from fitting free carrier absorption data of silicon wafers

In this paper, a data processing method is employed to improve the uniqueness of the electronic transport parameters (the carrier lifetime, carrier diffusion coefficient, and front and rear surface recombination velocities) obtained from fitting free carrier absorption data of silicon wafers. By employing the mean square variance graph or map, the influence of initial values on multi-parameter estimation greatly decreases. Theoretical simulations are performed to investigate the dependence of the uniqueness of the estimated parameters on the number of free parameters by choosing different initial values during multi-parameter fitting. Simulation and experimental results show that the proposed method can significantly improve the uniqueness of the fitted electronic transport parameters.

Nickel Oxide Hole‐Selective Heterocontact for Silicon Solar Cells: Role of SiOx Interlayer on Device Performance

Carrier‐selective contact‐based silicon heterojunction solar cells are fabricated using nickel oxide (NiOx) as a hole‐selective layer by thermal evaporation. The highest power conversion efficiency of ≈15.20% with a chemically grown SiOx interlayer is achieved from a Ag/ITO/NiOx/n‐Si/LiFx/Al cell structure in comparison with ≈12.43% without SiOx. The cells without and with the SiOx layer are analyzed by considering crucial parameters for conversion efficiency, such as minority carriers' diffusion lengths, lifetimes, recombination resistance, and density of interface defect states at the NiOx/n‐Si junction, by studying the dark/light current density–voltage, quantum efficiency, impedance, and parallel conductance characteristics. Device analysis provides evidence for the cell's open‐circuit voltage and short‐circuit current enhancement with the SiOx interlayer. This is due to an improvement in minority carrier lifetimes from ≈8.6 to ≈48.27 μs (photo‐conductance decay analysis), which is also estimated from ≈7.45 to ≈49.20 μs by impedance spectra analysis, increased minority carrier diffusion length from ≈171 to ≈952 μm, and decreased rear surface recombination velocity from ≈1106 to ≈170 cm s−1 (quantum efficiency analysis). These investigations reveal that engineering the n‐Si/LiFx interface by the SiOx interlayer is more important than the NiOx/n‐Si interface because of a thin unintentionally grown SiOx layer during NiOx evaporation simultaneously mediating silicon surface passivation.

Implementation of full-area-deposited electron-selective TiOx layers into silicon solar cells

We examine two different silicon solar cell designs featuring full-area electron-selective contacts based on ultrathin (2–3 nm) titanium oxide (TiOx) films deposited by atomic layer deposition. The first cell design applies a layer stack to the cell front, which is composed of an ultrathin intrinsic amorphous silicon (i-a-Si:H) layer for interface passivation, the TiOx film and an indium tin oxide (ITO) layer to provide a good lateral conductance for electrons to the metal fingers. Whereas carrier lifetime measurements on test structures promise high implied open-circuit voltages Voc up to 726 mV, the realized solar cells achieve disappointingly low Voc values <400 mV. The J-V parameters of this cell type are negatively affected by a reverse diode occurring due to the contacting of the TiOx by the high-work function ITO layer. In the second cell type, we implement a layer stack to the cell rear, which is composed of an ultrathin silicon oxide (SiOy) layer, the TiOx film and a full-area-deposited aluminum (Al) layer. Initial Voc values of these cells are relatively low (<600 mV), but improve significantly after annealing at 350°C. The best cell featuring a SiOy/TiOx/Al rear contact achieves an open-circuit voltage of 661 mV and an efficiency of 20.3%. No reverse diode is observed, which is attributed to the lower work function of the Al compared to ITO in the first cell design. From internal quantum efficiency measurements, we extract a rear surface recombination velocity Srear of (52±20) cm/s for our best cell, which is well compatible with efficiencies exceeding 23%.

Superiority of random inverted nanopyramid as efficient light trapping structure in ultrathin flexible c-Si solar cell

Abstract In this work, random inverted nanopyramids (INPs) are fabricated as light trapping structures on ultrathin c-Si through a simple and cost-effective wet chemical method, followed by a systematic investigation of the photo-capturing properties of INPs combining experiments and simulations. In comprehensive consideration of thickness loss and light trapping performance, random INPs are applied onto 45 μm ultrathin c-Si solar cell and a high short-current density (Jsc) (36.6 mA/cm2) and energy-conversion efficiency (17.0%) are achieved, which are 0.3 mA/cm2 and 0.13% respectively higher than that in micro pyramid textured one, and our electrical simulation also demonstrates that the advantages of INPs are more obvious on thinner c-Si compared with conventional micro pyramids. Finally, through electrical simulation, INPs textured 45 μm c-Si solar cell is expected to have a large improvement room for efficiency by controlling the front and rear surface recombination velocity. All the findings not only offer additional insight into the light-trapping mechanism in the random INPs but also provide controllable and efficient broadband light harvesters for next-generation cost effective flexible photovoltaics.

Large‐Area PEDOT:PSS/c‐Si Heterojunction Solar Cells With Screen‐Printed Metal Contacts

A large‐area BackPEDOT solar cell with a phosphorus‐diffused emitter and a high‐temperature‐fired screen‐printed Ag grid on the front surface and PEDOT:PSS as hole‐collecting and passivating layer at the cell rear is developed. As base material, 15.6 × 15.6 cm2 pseudo‐square industrial‐type boron‐doped p‐type Czochralski‐grown silicon wafers are used. The set‐peak firing temperature (Tset) is varied from 850 to 870 °C with a total number of 32 processed solar cells. The optimum Tset of 870 °C results in a median solar cell efficiency of 19.0%. The best large‐area BackPEDOT solar cell achieves an efficiency of 20.2%. Based on external quantum efficiency measurements, a rear surface recombination velocity Srear < 70 cm s−1 is determined, a value which is on a par with today's industrial high‐efficiency solar cells. Furthermore, a low‐temperature metal paste is introduced, which is shown to be capable of metalizing the PEDOT:PSS‐covered rear surface of the solar cells without damaging the rear surface passivation. The principle feasibility of such a rear metallization scheme is demonstrated. The parasitic absorption of infrared light within the PEDOT:PSS layer is identified as the major loss mechanism in the current cells, which might be overcome in the future by adding infrared‐transparent additives to the PEDOT:PSS dispersion.

Simulation and Fabrication of HfO2 Thin Films Passivating Si from a Numerical Computer and Remote Plasma ALD

Recombination of charge carriers at silicon surfaces is one of the biggest loss mechanisms in crystalline silicon (c-Si) solar cells. Hafnium oxide (HfO2) has attracted much attention as a passivation layer for n-type c-Si because of its positive fixed charges and thermal stability. In this study, HfO2 films are deposited on n-type c-Si using remote plasma atomic layer deposition (RP-ALD). Post-annealing is performed using a rapid thermal processing system at different temperatures in nitrogen ambient for 10 min. The effects of post-annealing temperature on the passivation properties of the HfO2 films on c-Si are investigated. Personal computer one dimension numerical simulation for the passivated emitter and rear contact (PERC) solar cells with the HfO2 passivation layer is also presented. By means of modeling and numerical computer simulation, the influence of different front surface recombination velocity (SRV) and rear SRV on n-type silicon solar cell performance was investigated. Simulation results show that the n-type PERC solar cell with HfO2 single layer can have a conversion efficiency of 22.1%. The PERC using silicon nitride/HfO2 stacked passivation layer can further increase efficiency to 23.02% with an open-circuit voltage of 689 mV.

Optoelectronic Evaluation and Loss Analysis of PEDOT:PSS/Si Hybrid Heterojunction Solar Cells

The organic/silicon (Si) hybrid heterojunction solar cells (HHSCs) have attracted considerable attention due to their potential advantages in high efficiency and low cost. However, as a newly arisen photovoltaic device, its current efficiency is still much worse than commercially available Si solar cells. Therefore, a comprehensive and systematical optoelectronic evaluation and loss analysis on this HHSC is therefore highly necessary to fully explore its efficiency potential. Here, a thoroughly optoelectronic simulation is provided on a typical planar polymer poly (3,4-ethylenedioxy thiophene):polystyrenesulfonate (PEDOT:PSS)/Si HHSC. The calculated spectra of reflection and external quantum efficiency (EQE) match well with the experimental results in a full-wavelength range. The losses in current density, which are contributed by both optical losses (i.e., reflection, electrode shield, and parasitic absorption) and electrical recombination (i.e., the bulk and surface recombination), are predicted via carefully addressing the electromagnetic and carrier-transport processes. In addition, the effects of Si doping concentrations and rear surface recombination velocities on the device performance are fully investigated. The results drawn in this study are beneficial to the guidance of designing high-performance PEDOT:PSS/Si HHSCs.

Effective Diffusion Length and Bulk Saturation Current Density Imaging in Solar Cells by Spectrally Filtered Luminescence Imaging

Most methods for interpreting electroluminescence (EL) or photoluminescence (PL) images of solar cells evaluate the local diode voltages but not the local luminescence intensity itself. One exception is the Fuyuki approximation, which assumes that the local value of the luminescence signal is proportional to the local effective diffusion length. This dependence has been derived for infinitely thick solar cells and neglects self-absorption of the luminescence photons. However, for real solar cells and imaging conditions, with increasing diffusion length, the luminescence signal approaches a limiting value; hence, the Fuyuki approximation no longer holds. In this paper, we compare EL and PL images of multicrystalline solar cells using different kinds of light filtering and find that gentle shortpass filtering is useful for avoiding optical artifacts. Based on earlier calculations, a physically founded formula for the dependence of the gently shortpass-filtered luminescence signal on the bulk diffusion length, for a given rear surface recombination velocity, is presented. Since this formula only barely allows us to calculate the diffusion length from the luminescence signal, a simplified approximate formula is proposed, and its accuracy is checked. This method is tested on EL and ${V_{{{\rm oc}}}}$ PL images of solar cells. We find that for a typical industrial multicrystalline Al-backside solar cell, the obtained effective diffusion length images correlate well with such images obtained by spectral LBIC image evaluation. In addition, the saturation current density images correlate well with such images obtained by dark lock-in thermography, which show a much lower spatial resolution. The main limitation of the proposed method is that it is basically approximate and needs some fitting parameters.

Investigation of Firing Conditions for Optimizing Aluminum-Doped p+-layer of Crystalline Silicon Solar Cells

Screen printing technique followed by firing has commonly been used as metallization for both laboratory and industrial based solar cells. In the solar cell industry, the firing process is usually conducted in a belt furnace and needs to be optimized for fabricating high efficiency solar cells. The printed-Al layer on the silicon is rapidly heated at over 800°C which forms a layer of back surface field (BSF) between Si-Al interfaces. The BSF layer forms p-p + structure on the rear side of cells and lower rear surface recombination velocity (SRV). To have low SRV, deep p + layer and uniform junction formation are required. In this experiment, firing process was carried out by using conventional tube furnace with N₂ gas atmosphere to optimize V oc of laboratory cells. To measure the thickness of BSF layer, selective etching was conducted by using a solution composed of hydrogen fluoride, nitric acid and acetic acid. The V oc and pseudo efficiency were measured by Suns-V oc to compare cell properties with varied firing condition.

Photocarrier radiometry for noncontact evaluation of space monocrystalline silicon solar cell under low-energy electron irradiation

A space monocrystalline silicon (c-Si) solar cell under low-energy (< 1 MeV) electron irradiation was investigated using noncontact photocarrier radiometry (PCR). Monte Carlo simulation (MCS) was employed to characterize the effect of different energy electron irradiation on the c-Si solar cell. The carrier transport parameters (carrier lifetime, diffusion coefficient, and surface recombination velocities) were obtained by best fitting the experimental results with a theoretical one-dimensional two-layer PCR model. The results showed that the increase of the irradiation electron energy caused a large reduction of the carrier lifetime and diffusion length. Furthermore, the rear surface recombination velocity of the Si:p base of the solar cell at the irradiation electron energy of 1 MeV was dramatically enhanced due to 1 MeV electron passing through the whole cell. Short-circuit current (Isc) degradation evaluated by PCR was in good agreement with that obtained by electrical measurement.

Aluminum doped zinc oxide wide band-gap n-type optical window for μc-Si superstrate solar cell

ZnO:Al thin films were deposited onto glass substrates by RF-magnetron sputtering system. The crystallographic orientation of the films, determined using an X-ray diffractometer (XRD), had a high c-axis orientated crystalline structure along (002) plane. The grains are densely packed as shown in the surface micrograph. The electrical parameters were carried out using Hall Effect measurements. The optical band-gap of the films was estimated based on the thickness and the optical transmittance data and is about 3.78 eV for 50 W RF-power. All parameters obtained were used to simulate a new solar cell structure based on p-type microcrystalline silicon as an absorber and n-ZnO:Al as an optical window. The excellent optical properties of this layer result in a high light trapping yielding to efficiencies about 19%. In order to improve efficiency, we have used a p+-μc-Si thin layer highly doped as a back surface field which minimizes significantly the impact of rear surface recombination velocity on voltage and current leading to a high efficiency of 22%. Optoelectronic parameters were determined using the current density-voltage (J-V) curve by means of an AMPS-1D device simulator.

Study of the Electron Lifetime in Crystalline and Multicrystalline Si Solar Cells

Crystalline (including multicrystalline) silicon based devices dominate the present solar cell industry due to their durability, relatively low cost, and the vast silicon knowledge base developed by the microelectronics industry. Multicrystalline silicon is the becoming dominating substrate material of the photovoltaic cells. This is obviously due to the significant cost savings compared with the single crystalline silicon wafers. Consequently, the conversion efficiency of the mc-Si solar cell is suffering because of the low cost requirement of the starting materials. One contributor to the lower performance of the mc-Si cells is the grain boundaries which reduce the charge carrier collection properties. Thus, reducing solar cell fabricate is an attractive way to reduce material costs. However, the model calculations in the presented paper show that the rear surface recombination velocity (S) in multicrystalline silicon cell is greater than (mc-Si) cell and the efficiency of mc-Si cell is lower than the efficiency of a crystalline cell.

The effect of front and rear surface recombination velocities on the photocurrent of a buried emitter silicon solar cell

Analytical work on the buried emitter solar cell (BESC) has been carried out based on the model presented previously by various researchers. The minority carrier distribution and the photocurrent contribution from the front region and also the rear region of the cell have been studied, taking into consideration the effects of front surface and rear surface recombination velocities. It is observed from these theoretical studies that reducing these recombination velocities, there is significant improvement in the photocurrent contributions form these regions. This emphasizes the role of a back surface field (BSF) in such devices. It may be mentioned here that in some recent experimental studies on new types of BESC solar cells, heavily doped BSF regions have been included by investigators.

Impact of Carrier Profile and Rear-Side Reflection on Photoluminescence Spectra in Planar Crystalline Silicon Wafers at Different Temperatures

The increasing use of spectral photoluminescence as an advanced and accurate diagnostic tool motivates a comprehensive assessment of the effects of some important optical and electrical properties on the photoluminescence spectra from crystalline silicon wafers. In this paper, we present both modeling results and measurements to elucidate the effects of the internal reflectance at the planar wafer surfaces, as well as the carrier profile varying across the sample thickness due to an increased rear-surface recombination velocity, as a function of temperature. These results suggest that the accuracy of existing spectral PL techniques may be improved by using higher temperatures due to the increased effect of the carrier profile at higher temperatures. They also show that changes in the photoluminescence spectrum shape caused by the addition of a rear-side specular reflector offset those caused by changes in the carrier profile due to increased rear surface recombination, and therefore, considerable care needs to be taken when changing the rear-side optics. Finally, the possible impact of variations in the rear-side reflectance on the band-band absorption coefficient and radiative recombination coefficient, which have previously been determined using the spectral photoluminescence technique, is assessed and demonstrated to be insignificant in practice.

Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se2 solar cells.

Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells.

Optimization of the back surface structure of a crystalline silicon solar cell by using a simulation method

The back surface passivation, including the formation of the local contact through the dielectric passivation layer, is regarded as one of the core technologies for enhancing the efficiencies of crystalline silicon solar cells. In this study, the effects of the rear local contact structure on the performances of the cell were simulated by varying the size of the contact area and the distance between the contacts. Because of the symmetry of the current flow in the cell, a simulation domain was defined, and the geometric parameters, such as the rear contact width and the rear contact pitch, were optimized for a high-efficiency solar cell. The behaviors of the cell’s output parameters also depended on the local doping concentration at the rear local contact. The influence of the rear surface recombination velocity on the efficiency of the cell was confirmed by comparing it with the simplified analytic equation for the short circuit-current density in the solar cell.

Inductively coupled plasma chemical vapour deposited AlOx/SiNy layer stacks for applications in high-efficiency industrial-type silicon solar cells

Passivated emitter and rear cells (PERC) are considered to be the next generation of industrial-type screen-printed silicon solar cells. Deposition methods for rear passivation layers have to meet both the high-throughput and low-cost requirements of the PV industry in combination with high-quality surface passivation properties. In this paper, we evaluate and optimise a novel deposition technique for AlOx passivation layers by applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. The ICP AlOx deposition process enables high deposition rates up to 5nm/s as well as excellent surface recombination velocities below 10cm/s after firing. A fixed negative charge of −4×1012cm−2 is measured for ICP AlOx single layers with an interface state density of 11.0×1011eV−1cm−2 at midgap position. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. We achieve independently confirmed conversion efficiencies of up to 20.1% for large-area (15.6×15.6cm2) PERC solar cells with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency reveals an effective rear surface recombination velocity Srear of (90±30)cm/s and an internal rear reflectance Rb of (91±1)% which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack.

Surface Recombination Velocity Measurements of Metallized Surfaces by Photoluminescence Imaging

QSSPC-calibrated photoluminescence imaging (PLI) was used to determine the rear surface recombination velocity Srear,eff of p-type Si FZ wafers after processing of large area screen printed Al contacts with varying paste thickness and firing temperatures. The paste thickness was varied by changing the snap off in the screen printer, and the resulting back surface field (BSF) thickness was investigated by scanning electron microscopy. BSF thicknesses of between 1.4 and 6.2μm were found. The BSF thickness saturated at high paste amounts combined with high firing temperatures while an increase in the eutectic layer thickness was observed. Several luminescence images were taken of each sample with different reference samples to assess the stability of the technique, and uncertainties are discussed.