Rear Surface Recombination Research Articles

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.

Passivation of Phosphorus Diffused Black Multi‐Crystalline Silicon by Hafnium Oxide

A key challenge for the success of the recent trend to adopt diamond wire sawing for multi‐crystalline silicon wafers is the texturing and passivation of their surfaces. The various so‐called “black silicon” texturing technologies show great promise in providing strong optical gains, but the nano‐scale surface structures resulted from dry etching are challenging to passivate with the conventional plasma enhanced chemical vapor deposition of silicon nitride. In this work, a single layer of atomic layer deposited hafnium oxide, in combination with a short anneal, has been used to demonstrate not only low surface recombination on reactive ion etched and phosphorus diffused (90 Ω/□) surfaces but also effective passivation of bulk defects in p‐type 1.5 Ω cm multi‐crystalline silicon wafers. A combination of photoluminescence imaging and quasi‐steady state photoconductance measurements shows that the recombination via bulk defects such as grain boundaries is mitigated due to hydrogenation during rapid thermal annealing at mid to high temperatures in nitrogen. A further activation anneal at low temperature following hydrogenation improves the effective carrier lifetime by 40%. A recombination current density of J0n+ = 98 fA cm−2 per side at an injection level of Δn = 1015 cm−3 has been obtained, which equates to an attainable 1‐sun implied open circuit voltage of 691 mV when bulk and rear surface recombination are excluded. These results prove that hafnium oxide offers a viable alternative for passivating the surface and the bulk of multi‐crystalline silicon solar cells.

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 the effects of rear surface recombination on the Cu(In,Ga)Se2 solar cell performances

This study investigated the band profile design of Cu(In,Ga)Se2 (CIGS) solar cells by considering the rear surface recombination. We compared the structures assuming the back surface field (BSF), passivation and graded band profile by using device simulator. As a result, it was found that the band structure of a combination of a flat-band and a single-graded profile is the suitable structure for CIGS solar cells with the absorber thickness of around 1.0 μm. In addition, the back passivation technique is unnecessary in the case of CIGS solar cells with a band profiling technique. We proposed that the band structure of a combination of a flat-band and a single-graded profile is the most practical and effective way for CIGS solar cells.

Improving Performance of Organic-Silicon Heterojunction Solar Cells Based on Textured Surface via Acid Processing.

PSS) is widely applied in organic-photoelectronic devices due to its excellent transparency and conductivity. However, when it is used in the organic-silicon heterojunction solar cells with traditional pyramid texturing surface, the device performance is limited by the contact between the PSS and silicon wafer at the bottom of the pyramids. We optimized the structure of the bottom of the pyramids via acid isotropic etching (AIE) method with mixed acid solution to ensure that the silicon wafer is fully covered by the PSS. In addition, hydrogenated amorphous silicon thin films were deposited with PEVCD method as the passivation and back surface field (BSF) layer to decrease the rear surface recombination rate, thus increasing the long wavelength response. Finally, a power conversion efficiency of 13.78% was achieved after depositing MoO3 on the front of the device as the antireflection layer.

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.

Rapid Testing of External Quantum Efficiency using LED Solar Simulators

A new generation of solar simulators is based on light emitting diode illumination sources. These measurement systems offer the opportunity to adjust the light spectrum as close as possible to the AM1.5G reference spectrum. Additionally, they provide the technical basis to combine power measurements with a spectral resolved analysis. Such an application is the determination of the quantum efficiency, which results in valuable additional cell information such as front and rear surface recombination, diffusion length, or emitter dead layer thickness. In this work, a fast method to determine the external quantum efficiency (EQE) of a solar cell using a LED solar simulator is presented. The measurement time of our LED-EQE approach could be reduced to less than half a second as no mechanical parts such as monochromators are involved. Due to the finite spectral band-width of the LEDs an adapted data analysis approach has been developed, which leads to results that show excellent agreement with standard EQE measurements.

The Passivated Emitter and Rear Cell (PERC): From conception to mass production

Improved solar cell efficiency is the key to ongoing photovoltaic cost reduction, particularly as economies of scale propel module-manufacturing costs towards largely immutable basic material costs and as installation costs become an increasingly large contributor to total system costs. To enable manufacturers to move past the 20% cell energy conversion efficiency figure in production, high-efficiency PERC (Passivated Emitter and Rear Cell) sequences are being increasingly brought online. Most new photovoltaic manufacturing capacity added in the second half of 2014 was PERC-based, making PERC now the cell technology with second-highest production capacity, with the latest industry roadmap anticipating PERC will become the dominant commercial cell technology by 2020. The first paper describing the PERC cell appeared in 1989, although the structure was conceived several years earlier. The attractive technical features were the reduction of rear surface recombination by a combination of dielectric surface passivation and reduced metal/semiconductor contact area while simultaneously increasing rear surface reflection by use of a dielectrically displaced rear metal reflector. The key issues in the development of this technology and its commercial implementation are described, including a review of recent adoption rates and the way these are likely to evolve in the future.

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.

Effect of the Back Surface Reflector and Passivated Rear Contacts With PERT Solar Cells

We use Silvaco software (atlas tcad) simulation to investigate the effect of dielectric layer deposed on rear surface of solar cells passivated emitter and rear totally diffused (PERT). For an improved performance for this solar cell, several physical factors must be considered, such as the light trapping behavior, and the resulting passivation performance and rear surface recombination currents were investigated. Particular consideration will be given to the back surface reflector (BSR) impact on reflection surface, interface passivation, and on the I–V characteristics. Numerical simulations show that using a layer of two dielectrics (SiNx/SiO2) with optical indices and thickness optimized in combination with contacts located (optimized metallization fraction f) at the rear surface allow for energy conversion efficiencies of 21.26% compared to a single layer of dielectric SiN 21.01%.

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.

Optimized conditions for the improvement of thin film CdS/CdTe solar cells

Efficient thin film CdS/CdTe solar cell performance requires optimum parameters of each layer of this cell and of the barrier structure. Moreover, the effect of optical losses, recombination losses at front and back surface of CdTe and recombination losses in the space-charge region (SCR) must be considered in order to really analyze the role of these parameters on the performance of these cells. This work is focused on studying theoretically the effect of the thickness of the front contact (ITO), thickness of the window layer (CdS), thickness of the absorber layer (CdTe), width of the space-charge region and electron lifetime on the efficiency of CdS/CdTe solar cells. The reflection losses from interfaces and absorption losses in ITO and CdS, front and rear surface recombination losses of CdTe as well as recombination losses in SCR have been studied. It has been observed that the short-circuit current strongly depends on the thickness of ITO, thickness of CdS, thickness CdTe and electron lifetime. The concentration of uncompensated impurities (Na−Nd) in CdTe, which determines the width of SCR, plays a key role in the generation of photocurrent. The recombination losses in the SCR decrease rapidly with increasing the carrier lifetime in this region and can be ignored at lifetime of 10−7s. The reflectivity from the back contact introduces a small influence in increasing the short-current density particularly at thick absorber layer (5–8μm). Under the conditions of Na−Nd~1016cm−3, τn=10−6s, dCdTe=8μm, dITO=100nm, and dCdS=100nm, the recombination and optical losses record their minimum ratio of 27%. Most of these losses (24%) are due to the optical losses. The efficiency of CdS/CdTe under these parameters is about 18.2% which is exactly matching with the recent experimental studies. Moreover, an ultrathin CdTe (=1μm) is sufficient to introduce high efficiency of 16.4%.

Ultra-Thin Monocrystalline Silicon Solar Cell with 12.2% Efficiency Using Silicon-On-Insulator Substrate.

Single side heterojunction silicon solar cells were designed and fabricated using Silicon-On-Insulator (SOI) substrate. The TCAD software was used to simulate the effect of silicon layer thickness, doping concentration and the series resistance. A 10.5 µm thick monocrystalline silicon layer was epitaxially grown on the SOI with boron doping concentration of 2 x 10(16) cm(-3) by thermal CVD. Very high Voc of 678 mV was achieved by applying amorphous silicon heterojunction emitter on the front surface. The single cell efficiency of 12.2% was achieved without any light trapping structures. The rear surface recombination and the series resistance are the main limiting factors for the cell efficiency in addition to the c-Si thickness. By integrating an efficient light trapping scheme and further optimizing fabrication process, higher efficiency of 14.0% is expected for this type of cells. It can be applied to integrated circuits on a monolithic chip to meet the requirements of energy autonomous systems.

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.

Modeling the efficiency of multijunction solar cells

The efficiency of multijunction solar cells (MSCs) η is calculated taking into account radiative recombination, Shockley-Read recombination, front and rear surface recombination, recombination in the space-charge regions, and recombination at heterojunctions. Calculation is performed by self-consistent solution of the photocurrent, photovoltage, and heat-balance equations. MSC cooling by increasing the numbers of cells n and improvement in the conditions of heat removal is taken into account. An effect leading to a decrease in the photocurrent with increasing n, associated with narrowing of the energy ranges of photons incident on the MSC cell, is considered. It is found that a significant increase in the MSC efficiency can be achieved by improving the heat-removal conditions, in particular, through the use of radiators and increasing the MSC grayness factor to unity. The results obtained are compared to those of other authors. It is shown that the calculated dependences η(n) are in agreement with experimental values.

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.