Recombination Current Density Research Articles

Impact of Selective BSF on Performance of Bifacial nPERT Cells with Ni/Ag Co-Plated Contacts

In this work, we evaluate the impact of implementing a selective back surface field (BSF) on the performance of bifacial n-type Passivated Emitter and Rear Totally diffused (PERT) cells with Ni/Ag co-plated contacts. For this, we compare bifacial nPERT cells featuring a homogeneous BSF etched-back to ~100 Ω/sq and patterned by laser ablation to cells featuring an identical front side design and a selective BSF obtained by means of laser doping. In the first part of this paper, we demonstrate using test structures that selective BSF cells benefit from drastically lower area weighted recombination current density (J0) contributions at the BSF side resulting in estimated Voc improvement from ~657 mV to ~ 681 mV. In the second part of this paper, we confirm at cell level that implementing a selective BSF results in gains in Voc ~ 30 mV, jsc~ 0.2 mA/cm2, and Fill Factors FF ~ 0.9%abs. resulting in average cell efficiencies improving from 20.7±0.4 % to 22.0±0.2 %. This clearly confirms that a selective BSF is required in nPERT cells with Ni/Ag co-plated contacts to maximize cell efficiencies. Finally, we fabricated a 9-cells bifacial glass-glass laminate for proof-of-concept and discuss several possible improvements to bring 60-cells module front maximum power values under standard test conditions (STC) above 340 Wp.

Passivation of Phosphorus Diffused Black Multi‐Crystalline Silicon by Hafnium Oxide (Phys. Status Solidi RRL 12/2017)

Black silicon texturing formed by reactive ion etching has great promise in providing strong optical gains for multicrystalline silicon solar cells, especially when coupled with diamond wire sawing. However, the resulting nano-scale surface structures are challenging to passivate with silicon nitride by plasma enhanced chemical vapor deposition, the conventional method used in industry. Researchers from the Australian National University and Trina Solar (see article no. 1700296 by Jie Cui et al.) use hafnium oxide, which contains positive charge, appropriate to passivate the phosphorus diffused surface of solar cells, together with atomic layer deposition, to obtain a conformal coating of the sharp surface features. By optimizing the deposition and annealing conditions, the authors have achieved not only a low surface recombination, but also an effective passivation of bulk defects in multi-crystalline silicon wafers, as shown by photoluminescence imaging. They report a recombination current density of J0n+ = 98 fA cm-2 for the phosphorus diffused black silicon surface, which equates to an attainable open circuit voltage of 691 mV, thus proving that hafnium oxide offers a viable alternative for passivating the surface and the bulk of black-textured multi-crystalline silicon solar cells.

Performance variation of dark current density-voltage characteristics for PID-affected monocrystalline silicon solar modules from the field

Potential-induced degradation (PID) has caught considerable attention in recent years because of its detrimental influence on output power of solar module and energy yield of photovoltaic power plant. In this paper, performance variation of dark current density-voltage (dark J-V) characteristics for PID-affected monocrystalline silicon solar modules dismounted from photovoltaic power plant was investigated. By measuring dark J-V characteristics of the PID-affected modules, the deterioration trend of dark J-V characteristics for PID-affected modules was found for the first time. Based on diffusion theory of metal ions, a new field-assisted diffusion model of metal ions was built, and the deterioration of open-circuit voltage and increase of recombination current density were explained for solar modules affected by PID. The thermal characteristics of solar modules with different level of PID were revealed under forward and reverse bias. There is an obvious temperature difference between the substrings neighboring the frame and the middle substring within the PID-affected modules. The results obtained in this paper give out the reasonable explanation about mechanism of PID and the deterioration trend of dark J-V for PID-affected modules.

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.

Fabrication and Modeling of High-Efficiency Front Junction N-Type Silicon Solar Cells With Tunnel Oxide Passivating Back Contact

This paper reports on in-depth understanding, modeling, and fabrication of 23.8% efficient 4 cm2 n-type Float Zone (FZ) silicon cells with a selective boron emitter and photolithography contact on front and tunnel oxide passivating contact on the back. Tunnel oxide passivating contact composed of a very thin chemically grown silicon oxide (∼15 A) capped with plasma-enhanced chemical vapor deposition (PECVD) grown 20 nm n+ poly Si gave excellent surface passivation and carrier selectivity with very low saturation current density (∼5 fA/cm2). A high-quality boron selective emitter was formed using ion implantation and solid source diffusion to minimize metal recombination and emitter saturation current density. Process optimization resulted in a cell $V_{{\rm{oc}}}$ of 712 mV, $J_{{\rm{sc}}}$ of 41.2 mA/cm2, and FF of 0.811. A simple methodology is used to model these cells which replaces tunnel oxide passivating contact region by electron and hole recombination velocities extracted from measured saturation current density of tunnel oxide passivating contact region and analysis. Using this approach and two-dimensional device modeling gave an excellent match between the measured and simulated cell parameters and efficiency, supporting excellent passivation and carrier selectivity of these contacts. Extended simulations showed that 26% cell efficiency can be achieved with this cell structure by further optimization of wafer quality, emitter profile, and contact design.

Contact resistivity reduction on lowly-doped n-type Si using a low work function metal and a thin TiOX interfacial layer for doping-free Si solar cells

Eliminating a doping process could be an effective way to reduce the production cost of c-Si cells. However, in absence of highly doped Si, the formation of a high quality contact is not straightforward. The lack of field-effect passivation from a lowly doped region can lead to a high recombination current density at the contacts (J0,metal) and moreover, contact resistivity (ρC) typically increases when doping level is decreasing. In this work we focus on reducing the contact resistivity of an electron-selective contact for doping-free cells. Although the effect of low work function metals (LWMs) in combination with an i-a-Si:H layer has already been reported, the synergy effect of a LWM and a MIS (Metal-Insulator-Semiconductor) contact structure on top of the i-a-Si:H has not been reported yet. Here, we demonstrate a new ATOM (i-a-Si:H / TiOX / low workfunction metal) contact structure as an electron-selective contact using an i-a-Si:H layer, a TiOX interfacial layer and Ca (Φ = 2.9eV) without requiring an additional n+ doping process. The addition of TiOX in between the i-a-Si:H layer and the Ca decreases the ρC by about 2 orders of magnitude. Despite of increased J0,metal due to e-beam processing of TiOX, the Ca based ATOM contact increases the potential max efficiency up to 25 %. To the best of our knowledge, this is the first demonstration of an electron-selective contact comprising a low work function metal, an interfacial TiOX and an i-a-Si:H passivation layer. This type of contact could be a promising route for the optimization of doping-free cells

Loss analysis of 22% efficient industrial PERC solar cells

The efficiency record of industrial type PERC solar cells exceeded 22% at the turn of the year 2015 to 2016. Our best screen-printed PERC solar cell reached 22.04% efficiency while the best cell batch showed a very narrow efficiency distribution. A detailed electrical and optical loss analysis of those industrial type high efficiency PERC solar cells is carried out which enables further optimization and strategic improvements. A variety of characterization data allows for a recombination current density, resistance and optical loss analysis based on numerical device simulation, analytical calculations and raytracing, respectively. The main recombination losses at maximum power point (MPP) occur in the homogenous and selective diffused regions of the emitter. A series resistance loss analysis is analytically performed. The emitter contribution to the lumped series resistance dominates the series resistance losses. The optical loss analysis performed with raytracing shows main reflection and absorption losses in the rear metal layer which is partly due to the light trapping capability of the PERC cells.

Quantification of pn-Junction Recombination in Interdigitated Back-Contact Crystalline Silicon Solar Cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn -junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn -junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn -junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn -junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J 02 of ∼61 nA·junction–1·cm–1 was measured, whereas for surfaces which were passivated by either SiN $_{x}$ or Al 2O3/SiN $_{x}$ , J 02 was reduced to ∼0.4 nA·junction–1·cm–1. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p - and n -type dopant diffusion process recipes, J 02 recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ∼1% absolute.

A Fast Strategy to determine the Physical and Electrical Parameters of Photovoltaic Silicon Cell

This paper proposes a fast strategy to extract and exploit the electrical parameters of photovoltaic cell using the double-diode model. The polycrystalline silicon (poly-Si) junction is chosen in this work due to the importance of its proprieties in industrial and economic fields. The proposed method to extract the solar cell electrical parameters contains two steps. The first is based on the graphical adjustments to choose the initial values of these parameters, and the second is numerical, using Modified Newton-Raphson’s algorithm. The obtained parameters extractions values are compared to the others methods and give a considerable agreement. Furthermore, we have developed a complementary analytical method to deduce both the minority’s carrier’s lifetime and the diffusion lengths through the diffusion and recombination current densities. The found values of these parameters are precisely comparable with theoretical models, and give very useful informations on the intrinsic quality of the studied cell.

A Fast Strategy to determine the Physical and Electrical Parameters of Photovoltaic Silicon Cell

This paper proposes a fast strategy to extract and exploit the electrical parameters of photovoltaic cell using the double-diode model. The polycrystalline silicon (poly-Si) junction is chosen in this work due to the importance of its proprieties in industrial and economic fields. The proposed method to extract the solar cell electrical parameters contains two steps. The first is based on the graphical adjustments to choose the initial values of these parameters, and the second is numerical, using Modified Newton-Raphson’s algorithm. The obtained parameters extractions values are compared to the others methods and give a considerable agreement. Furthermore, we have developed a complementary analytical method to deduce both the minority’s carrier’s lifetime and the diffusion lengths through the diffusion and recombination current densities. The found values of these parameters are precisely comparable with theoretical models, and give very useful informations on the intrinsic quality of the studied cell.

UV radiation hardness of photovoltaic modules featuring crystalline Si solar cells with AlOx/p+‐type Si and SiNy/n+‐type Si interfaces

We report on the UV radiation hardness of photovoltaic modules with bifacial n‐type Passivated Emitter and Rear Totally diffused crystalline Si cells that are embedded in an encapsulation polymer with enhanced UV transparency. Modules with front junction cells featuring an AlOx/p+‐type Si passivation interface at the illuminated side are stable for a UV irradiation dose of 598 kWh m−2. In contrast, irradiating modules with back junction cells featuring an a‐SiNy/n+‐type Si passivation interface at the illuminated side reduces the output power by 15%. The quantum efficiency of the a‐SiNy‐passivated module degrades in the spectral range between 300 and 1000 nm, which we ascribe to a degradation of the surface passivation. Modeling the experimental data shows that photons with an energy above 3.4 eV contribute to the degradation effect and enhance the front surface recombination current density by a factor of 15.

Surface passivation investigation on ultra-thin atomic layer deposited aluminum oxide layers for their potential application to form tunnel layer passivated contacts

The surface passivation performance of atomic layer deposited ultra-thin aluminium oxide layers with different thickness in the tunnel layer regime, i.e., ranging from one atomic cycle (∼0.13 nm) to 11 atomic cycles (∼1.5 nm) on n-type silicon wafers is studied. The effect of thickness and thermal activation on passivation performance is investigated with corona-voltage metrology to measure the interface defect density Dit(E) and the total interface charge Qtot. Furthermore, the bonding configuration variation of the AlOx films under various post-deposition thermal activation conditions is analyzed by Fourier transform infrared spectroscopy. Additionally, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) is used as capping layer on ultra-thin AlOx tunneling layers to further reduce the surface recombination current density to values as low as 42 fA/cm2. This work is a useful reference for using ultra-thin ALD AlOx layers as tunnel layers in order to form hole selective passivated contacts for silicon solar cells.

Effect of the Ti/Si ratio of spin coating solutions on surface passivation of crystalline silicon by TiOx–SiOx composite films

Passivation films or antireflection coatings are generally prepared using costly vacuum or high-temperature processes. Thus, we report the preparation of TiOx–SiOx composite films by novel spin coatable solutions for the synthesis of low-cost passivation coating materials. The desired films were formed by varying the mixing ratios of TiOx and SiOx, and the resulting films exhibited excellent surface passivation properties. For the p-type wafer, an optimal effective surface recombination velocity (Seff) of 93 cm/s was achieved at , while a surface recombination current density (J0s) of 195 fA/cm2 was obtained. In contrast, for the n-type wafer, an Seff of 27 cm/s and a J0s of 38 fA/cm2 were achieved at . This excellent surface passivation effect could be attributed to the low interface state density and high positive fixed charge density. Furthermore, the thickness of the interfacial SiOx layer was determined to be important for obtaining the desired surface passivation effect.

Modeling the potential of screen printed front junction CZ silicon solar cell with tunnel oxide passivated back contact

AbstractCarrier selective passivated contacts composed of thin oxide, n + polycrystalline Si and metal on top of a n‐Si absorber can significantly lower the recombination current density (Jorear ≤8 fA/cm2) under the contact while providing excellent specific contact resistance (5–10 mΩ‐cm2); 25.1% efficient small area cells with photolithography front contacts on boron doped selective emitter and Fz wafers have been achieved by Fraunhofer ISE using their tunnel oxide passivated contact (TOPCon) approach. This paper shows a methodology to model such passivated contact cells using Sentaurus device model, which involves replacing the TOPCon region by carrier selective electron and hole recombination velocities to match the measured Jorear of the TOPCon region as well as all the light IV values of the cell. We first validated the methodology by modeling a 24.9% reference cell. The model was then extended to assess the efficiency potential of large area TOPCon cells on commercial grade n‐type Cz material with screen‐printed front contacts. To use realistic input parameters, a 21% n‐type PERT cell was fabricated on Cz wafer (5 Ω‐cm, 1.5‐ms lifetime). Modeling showed that the cell efficiency will improve to only 21.6% if the back of this cell is replaced by the above TOPCon, and the performance is limited by the homogenous emitter. Efficiency was then modeled to improve to 22.6% with the implementation of selective emitter (150/20 Ω/sq). Finally, it is shown that screen printing of 40‐µm‐wide lines and improved bulk material (10 Ω‐cm, 3‐ms lifetime) can raise the single side TOPCon Cz cell efficiency to 23.2%. Copyright © 2016 John Wiley & Sons, Ltd.

IBC c-Si(n) Solar Cells Based on Laser Doping Processing for Selective Emitter and Base Contact Formation

In the last years, there is a clear trend in c-Si solar cell fabrication to place both emitter and base contacts on the back side leading to the so-called Interdigitated Back Contacted (IBC) solar cell structure. This solar cell architecture requires excellent front and rear surface passivation as well as a very low recombination emitter. Moreover, laser doping may be an attractive technique to create both selective emitter and base contacts using appropriate dielectric layers as dopant sources, i.e. Al2O3 and a-SiCx(n) stacks for the p+ and n+ regions respectively. In this study, we report on a simplified fabrication process for IBC n-type c-Si solar cells combining laser doping and a conventional boron emitter passivated by Al2O3 films. Results show very low emitter recombination currents in the ∼10-50 fA/cm2 range before laser processing. In addition, selective emitter contacts can be created by laser doping with recombination current densities at each contact point around 4.4 pA/cm2 in relatively low and shallow doped boron doped profiles (sheet resistance ∼400Ω/sq). Finally, IBC solar cells, 3 x 3 cm2 device area, were fabricated combining selective laser-doped emitter and base contacts reaching efficiencies up to 20.8%.

PC1Dmod 6.2 – Improved Simulation of c-Si Devices with Updates on Device Physics and User Interface

In this paper we present a new update to PC1Dmod, which extends the original PC1D program by implementing Fermi-Dirac statistics and a range of state-of-the-art models in order to improve the accuracy of c-Si device simulation. In PC1Dmod 6.2 the list of models is further expanded to include a parameterization of incomplete ionization of dopants (Altermatt et al., J. Appl. Phys.100, 113715, 2006), with parameters for phosphorus, boron, arsenic, gallium and aluminum. The results have been verified against a previous implementation of the model. We show that the inclusion of incomplete ionization is of particular importance for moderately doped surface regions with doping densities in the range ∼5 × 1017 to ∼5×1019 cm-3, resulting in a deviation of up to 15% in the simulated recombination current density and sheet resistance. The effect of incomplete ionization is also shown to be more pronounced in devices made from compensated Si material. Furthermore, the default solar spectrum has also been updated in PC1Dmod 6.2, taking advantage of the increased maximum file size to include a more realistic representation. The generation profiles calculated using PC1Dmod 6.2 together with input from OPAL 2 agree well with results from Sentaurus TCAD for both planar and pyramidally textured surfaces, thus facilitating a more direct comparison between PC1Dmod 6.2 and other simulation programs utilizing standard spectra. The simulation comparison is also extended to the electrical performance of the modeled devices, showing good agreement in the calculated short-circuit current density for a range of planar and textured solar cells with varying emitter doping. Finally, several new output options, including emitter saturation current and injection-dependent lifetime curves have been added, enabling a simpler and more direct way for users to access and plot important device properties.

N-Type polysilicon passivating contact for industrial bifacial n-type solar cells

We present a high-performance bifacial n-type solar cell with LPCVD n+ polysilicon (polySi) back side passivating contacts and fire-through screen-printed metallization, processed on full area 6″ Cz wafers. The cells were manufactured with low-cost industrial process steps yielding a best efficiency of 20.7%, and an average Voc of 674mV. We analysed effects of variation of doping level, thickness, and oxide properties of the n-type polySi/SiOx layers, as well as hydrogenation from a PECVD SiNx:H coating, which led to recombination current densities down to ~2fA/cm2 and ~4fA/cm2 on planar and textured surface, respectively. Analysis shows that the wafer bulk lifetime in the cell is high and that the Voc of the cell is limited by the Jo of the uniform diffused boron emitter and its contacts. Ways to improve the efficiency of the cell to >22% are indicated.

Magnesium Fluoride Electron-Selective Contacts for Crystalline Silicon Solar Cells.

In this study, we present a novel application of thin magnesium fluoride films to form electron-selective contacts to n-type crystalline silicon (c-Si). This allows the demonstration of a 20.1%-efficient c-Si solar cell. The electron-selective contact is composed of deposited layers of amorphous silicon (∼6.5 nm), magnesium fluoride (∼1 nm), and aluminum (∼300 nm). X-ray photoelectron spectroscopy reveals a work function of 3.5 eV at the MgF2/Al interface, significantly lower than that of aluminum itself (∼4.2 eV), enabling an Ohmic contact between the aluminum electrode and n-type c-Si. The optimized contact structure exhibits a contact resistivity of ∼76 mΩ·cm(2), sufficiently low for a full-area contact to solar cells, together with a very low contact recombination current density of ∼10 fA/cm(2). We demonstrate that electrodes functionalized with thin magnesium fluoride films significantly improve the performance of silicon solar cells. The novel contacts can potentially be implemented also in organic optoelectronic devices, including photovoltaics, thin film transistors, or light emitting diodes.

Titanium oxide: A re-emerging optical and passivating material for silicon solar cells

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by a thin layer of thermal atomic layer deposited (ALD) titanium oxide (TiO2). Surface recombination velocities of 0.8cm/s, 2.5cm/s and 9.8cm/s have been obtained on n-type 10cm, 1Ωcm and p-type 1Ωcm undiffused wafers, respectively. Recombination current densities of 19 fA/cm2 and 780 fA/cm2 have been measured on 120Ω/□ boron diffused p+ and 100Ω/□ phosphorus diffused n+ regions. In addition to providing a superior passivation on p+ over n+ (c-Si) surfaces, the ALD TiO2 layers produces a strong injection-dependent effective lifetime on the n-type substrates, , both of which are consistent with the possible presence of negative charge in the passivating layer. The recombination at the interface between TiO2 and planar <100>, planar <111> and alkaline textured (c-Si) surfaces with upright pyramids is compared. We find that (i) a planar <111> surface exhibits a 1.39 times higher recombination than a planar <100> surface, and (ii) after area correction, the ratio of recombination on the textured and planar <111> surfaces is 1.37. A thin film of TiO2 deposited by ALD has been applied to the front surface of a rear locally diffused p+nn+ front junction solar cell, performing the dual role of surface passivation and single-layer antireflection coating on the textured p+ diffusion. The best solar cell achieved Voc=655mV, FF=79.9% and efficiency=20.45%. The results presented in this work demonstrate that TiO2 re-emerging as a suitable optical and passivating material to produce high performance solar cells.

Criteria for validating polaron pair dissociation in polymer-fullerene bulk heterojunction solar cells

The dissociation of polaron pairs into free charge carriers in organic bulk heterojunction solar cells is a fundamental step in generating photocurrent and is still in debate. In this study, we propose two simple criteria that can be used to test the validity of any polaron pair dissociation model for polymer-fullerene bulk heterojunction solar cells. The first criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density should increase as a function of applied voltage. The second criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density at short circuit should not be larger than 1. We apply these criteria to test the validity of the widely used Onsager-Braun model by using the experimental current-voltage data of poly[2-methoxy-5-(3′-7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells. We find that our numerical analysis is not suitable to employ these criteria. Our analytical analysis, on the other hand, clearly demonstrates that the Onsager-Braun model simply cannot fulfill the first criteria. The reason is because the polaron pair dissociation given by the Onsager-Braun model is too strongly influenced by the electric field (i.e., decreases too rapidly as the electric field decreases). The analysis provides a further evidence against the widely used Onsager-Braun model. The proposed criteria can help us to determine the correct model for polaron pair dissociation by serving as a guideline on how strongly the electric field is allowed to influence the polaron pair dissociation.