Front Junction Research Articles

Electroluminescence analysis for spatial characterization of parasitic optical losses in silicon heterojunction solar cells

Electroluminescence (EL) coupled with reflection measurements are used to spatially quantify optical losses in silicon heterojunction solar cells due to plasmonic absorption in the metal back contacts. The effect of indium tin oxide back reflector in decreasing this plasmonic absorption is found to increase the reflection from the back nickel (Ni)-aluminum (Al) and Al metals by ∼12% and ∼41%, respectively, in both bifacial and front junction silicon solar cells. Losses due to back reflection are calculated by comparison between the EL emission signals in high and low back reflection samples and are shown to be in agreement with standard reflection measurements. We conclude that the optical properties of the back contact can significantly influence the EL intensity which complicates the interpretation of EL as being primarily due to recombination especially when comparing two different devices with spatially varying back surface structures.

Accuracy of Simplifications for Spectral Responsivity Measurements of Solar Cells

The determination of the spectral responsivity is an essential part of solar cell characterization. Since solar simulators only approximate the reference spectrum, a spectral mismatch correction has to be performed. This correction procedure requires spectral responsivity data. Apart from the complete differential spectral responsivity procedure, the IEC 60904-8 standard defines four simplifications. In this paper, we provide information on the variations in the spectral responsivity curves for these simplifications. We show that for nonlinear front junction cells, deviations predominantly occur at wavelengths above 700 nm and become largest around 1000 nm. While we found a maximum deviation of 30% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations below 10%. For a nonlinear cell measured relative to a world photovoltaic scale reference cell using a class A solar simulator, this transfers to a deviation below 0.01% in the spectral mismatch factor. If one depends on the use of a simplification, we recommend using the multicolor approach. Even though the singlecolor approach might yield lower deviations, this approach requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

Principles of dopant-free electron-selective contacts based on tunnel oxide/low work-function metal stacks and their applications in heterojunction solar cells

Dopant-free carrier-selective contacts (CSCs) are considerably attractive on highly efficient crystalline silicon (c-Si) based heterojunction solar cells (HSCs) owing to their combined advantages of low thermal budget, ease of processing and simplicity of the device architecture. However, due to inferior passivation property and relatively high resistivity, the silicon oxide (SiOx) thin film is always ruled out as a viable interfacial layer in a CSC. Here, we demonstrate that the functionalization of nanoscale SiOx films with low work-function metals (WFMs) can simultaneously achieve moderate-level passivation and low contact resistivity on lightly doped n-type c-Si. Simulation results on the CSC comprising SiOx/low-WFM reveal that the tunnel SiOx layer alleviates Fermi-level pinning, and the low WFM helps to reduce the effective barrier height and contact resistivity. This kind of bi-functional electron-selective design possesses high tolerance to the surface state density on the c-Si surface as well as the SiOx layer thickness, as experimentally confirmed by the SiOx/Mg stack achieving an implied open-circuit voltage of 661 mV and a contact resistivity of 35.2 mΩ·cm2 for a SiOx film thickness up to 1.7 nm. A comprehensive assessment suggests that this kind of rear electron transport layer (ETL) enables a theoretical power conversion efficiency (PCE) up to 27.4% if assuming an ideal front junction. Ultimately, a fully dopant-free c-Si HSC featuring a SiOx/low-WFM electron-selective contact and a MoOx hole-selective contact was proposed, yielding a PCE of 21.8%. The relatively high performance and facile integration with c-Si substrates enabled by the SiOx/low-WFM contact may open up new possibilities in designing and fabricating low-cost dopant-free SCs toward high efficiencies competitive with conventionally diffused pn junction SCs.

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.

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.

Metal-organic semiconductor interfacial barrier height determination from internal photoemission signal in spectral response measurements

Accurate and convenient evaluation methods of the interfacial barrier ϕb for charge carriers in metal semiconductor (MS) junctions are important for designing and building better opto-electronic devices. This becomes more critical for organic semiconductor devices where a plethora of molecules are in use and standardised models applicable to myriads of material combinations for the different devices may have limited applicability. In this paper, internal photoemission (IPE) from spectral response (SR) in the ultra-violet to near infra-red range of different MS junctions of metal-organic semiconductor-metal (MSM) test structures is used to determine more realistic MS ϕb values. The representative organic semiconductor considered is [6, 6]-phenyl C61 butyric acid methyl ester, and the metals considered are Al and Au. The IPE signals in the SR measurement of the MSM device are identified and separated before it is analysed to estimate ϕb for the MS junction. The analysis of IPE signals under different bias conditions allows the evaluation of ϕb for both the front and back junctions, as well as for symmetric MSM devices.

Potential induced degradation of n‐type crystalline silicon solar cells with p+ front junction

AbstractN‐type silicon‐based solar cells are currently being used for achieving high efficiency. However, most of the photovoltaic modules already constructed are based on p‐type silicon solar cells, and there are few studies on potential induced degradation (PID) in n‐type solar cells. In this study, we investigated PID in n‐type silicon solar cells with a front p+ emitter. Further, the PID characteristics of n‐type solar cells are compared with those of p‐type solar cells. The electrical properties of PID in solar cells are observed with the light I‐V, quantum efficiency (QE), and electroluminescence (EL). The possible causes for the change in the external quantum efficiency (EQE) after PID are interpreted using PC1D and are discussed by comparing the experimental results with the simulation results.

High-efficiency selective boron emitter formed by wet chemical etch-back for n-type screen-printed Si solar cells

Front metal contact induced recombination and resistance are major efficiency limiting factors of large-area screen-printed n-type front junction Si solar cells with homogeneous emitter and tunnel oxide passivated back contact (TOPCON). This paper shows the development of a selective boron emitter (p+/p++) formed by a screen-printed resist masking and wet chemical etch-back process, which first grows a porous Si layer and subsequently removes it. Various wet-chemical solutions for forming porous Si layer are investigated. An industrial compatible process with sodium nitrite (NaNO2) catalyst is developed to uniformly etch-back the ∼47 Ω/◻ atmospheric pressure chemical vapor deposited heavily doped boron emitter to ∼135 Ω/◻ by growing a 320 nm porous Si layer within 3 min and subsequently removing it. After etching back, the boron emitter was subjected to a thermal oxidation to lower the surface concentration and the emitter saturation current density J0e. Various etched-back emitters were evaluated by measuring J0e on symmetric test structures with atomic layer deposited aluminum oxide (Al2O3) passivation. Very low J0e of 21, 14, and 9 fA/cm2 were obtained for the 120, 150, and 180 Ω/◻ etched-back emitters, respectively. A solar cell with a selective emitter (65/180 Ω/◻) formed by this etch-back technology and with an Al/Ag contact on the front and TOPCON on the back gave an open-circuit voltage (Voc) of 682.8 mV and efficiency of 21.04% on n-type Czochralski Si wafer. This demonstrates the potential of this technology for next generation high-efficiency industrial n-type Si solar cells.

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.

A magnesium/amorphous silicon passivating contact for n-type crystalline silicon solar cells

Among the metals, magnesium has one of the lowest work functions, with a value of 3.7 eV. This makes it very suitable to form an electron-conductive cathode contact for silicon solar cells. We present here the experimental demonstration of an amorphous silicon/magnesium/aluminium (a-Si:H/Mg/Al) passivating contact for silicon solar cells. The conduction properties of a thermally evaporated Mg/Al contact structure on n-type crystalline silicon (c-Si) are investigated, achieving a low resistivity Ohmic contact to moderately doped n-type c-Si (∼5 × 1015 cm−3) of ∼0.31 Ω cm2 and ∼0.22 Ω cm2 for samples with and without an amorphous silicon passivating interlayer, respectively. Application of the passivating cathode to the whole rear surface of n-type front junction c-Si solar cells leads to a power conversion efficiency of 19% in a proof-of-concept device. The low thermal budget of the cathode formation, its dopant-less nature, and the simplicity of the device structure enabled by the Mg/Al contact open up possibilities in designing and fabricating low-cost silicon solar cells.

Analysis of dry-spot behavior in the pressure field of a liquid composite molding process

Due to industrial automation of liquid composite molding processes and increasing geometrical complexity of composite components, dry-spots from flow front junctions have become increasingly difficult to avoid. The impact and behavior of voids (microscopic or small macroscopic gas entrapments) during preform impregnation is well known, but no attention is given to dry-spots (large macroscopic gas entrapments). Experiments show that formation of a dry-spot in an early stage of an injection process does not necessarily lead to scrap parts. Therefore, simulation-based predictions of dry-spots are no sufficient condition for identification of unsuitable injection strategies. In this paper, the resolution mechanisms of dry-spots under controlled process conditions are investigated and the resulting findings of fundamental formation- and dispersion-mechanisms are presented.

PID- and UVID-free n-type Solar Cells and Modules

In this paper we report on the high stability of our n-type front junction solar cells (n-PERT) exposed to potential-induced degradation (PID) and UV-induced degradation (UVID) conditions. These intrinsically stable n-Pasha cells enable PID- and UVID-resistant modules even with industrially low-cost standard EVA encapsulant, independent of system grounding and system voltage. Based on intentional modifications of the Boron emitter and/or the dielectric layer in the PID-free and UVID-free n-Pasha solar cells, we are able to replicate reported degradation effects and study the mechanisms behind it. A combination of altering the boron profile and the dielectric properties together with increasing the interface defect density Dit is detrimental for the stability. Applying our standard optimal B-diffusion and passivation scheme assure that the UV radiation and system voltage have virtually no effect on our n-Pasha cell and module performance.

Parasitic Absorption in Polycrystalline Si-layers for Carrier-selective Front Junctions

We investigate the optical properties of n- and p-type polycrystalline silicon (poly-Si) layers. We determine the optical constants n and k of the complex refractive index of polycrystalline silicon by using variable-angle spectroscopic ellipsometry. Moreover, we investigate the effect of different doping levels in the poly-Si on free carrier absorption (FCA). Thereby, we demonstrate that the FCA in poly-Si can be described by a model developed for crystalline silicon (c-Si) at a first approximation. The optical properties of hydrogenated amorphous silicon layers (a-Si:H) are also investigated as a reference. With ray tracing simulations the absorption losses of poly-Si and of the a-Si:H layers are quantified with respect to the film thickness. Based on this approach we find that the short-circuit current density losses due to parasitic absorption of poly-Si layers are significantly lower when compared to a-Si:H layers of the same thickness. For example the short-circuit current density loss due to a 20 nm thick p-type poly-Si layer is around 1.1 mA/cm2, whereas a 20 nm thick p-type a-Si:H layer leads to a loss of around 3.5 mA/cm2.

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.

High potential for the optimum designs for a front contact and junction: A route to heterojunction solar cell

The present work describes a method for the development of an efficient silver nanowire (AgNWs) embedded indium tin oxide (ITO) based silicon heterojunction solar cell. The working mechanism of the heterojunction solar cell is studied by using the current-voltage (J-V) and Impedance spectroscopy (IS) techniques. The charge transfer mechanism and recombination process of the solar cell were explained by resistance, capacitance and ideality factor derived from both these techniques. A relatively high efficiency has been achieved for AgNWs embedded ITO-Si heterojunction solar cell in which AgNWs network acts as a transparent buried contact. This study also provides a new architecture for various heterojunction solar cells with a simple route of fabrication.

Development of a large area n-type PERT cell with high efficiency of 22% using industrially feasible technology

A high-efficiency front junction n-type passivated emitter and a rear total diffusion (n-type PERT) solar cell with the front boron diffusion passivated by a Al2O3/SiNx stack layer deposited by plasma enhanced chemical vapor method and the rear with phosphorus total diffusion and Al evaporated local contact are presented in this paper. The main purpose of this study was to develop industrially feasible front junction n-type PERT solar cells with high-efficiency; these were realized on a large area of n-type industrial 5- and 6-in. wafers. An average of 21.85% cell efficiency was achieved on 5-in. wafers, and the highest cell convert efficiency of 21.98% was achieved with Voc of 683.8mV, Jsc of 40.13mA/cm2, and FF of 80.11%. For 6-in. cells, we get Fraunhofer independently confirmed efficiency of 21.49% with a Voc of 674.1mV, a Jsc of 39.77mA/cm2, a FF of 80.18%. Details of cell fabrication and results are presented, followed by a best cell power loss analysis on Voc, Isc and FF.

Large area tunnel oxide passivated rear contact n‐type Si solar cells with 21.2% efficiency

AbstractThis paper reports on the implementation of carrier‐selective tunnel oxide passivated rear contact for high‐efficiency screen‐printed large area n‐type front junction crystalline Si solar cells. It is shown that the tunnel oxide grown in nitric acid at room temperature (25°C) and capped with n+ polysilicon layer provides excellent rear contact passivation with implied open‐circuit voltage iVoc of 714 mV and saturation current density J0b′ of 10.3 fA/cm2 for the back surface field region. The durability of this passivation scheme is also investigated for a back‐end high temperature process. In combination with an ion‐implanted Al2O3‐passivated boron emitter and screen‐printed front metal grids, this passivated rear contact enabled 21.2% efficient front junction Si solar cells on 239 cm2 commercial grade n‐type Czochralski wafers. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Tunnel oxide passivated rear contact for large area &lt;em&gt;n&lt;/em&gt;-type front junction silicon solar cells providing excellent carrier selectivity

Carrier-selective contact with low minority carrier recombination and efficient majority carrier transport is mandatory to eliminate metal-induced recombination for higher energy conversion efficiency for silicon (Si) solar cells. In the present study, the carrier-selective contact consists of an ultra-thin tunnel oxide and a phosphorus-doped polycrystalline Si (<em>poly</em>-Si) thin film formed by plasma enhanced chemical vapor deposition (PECVD) and subsequent thermal crystallization. It is shown that the <em>poly</em>-Si film properties (doping level, crystallization and dopant activation anneal temperature) are crucial for achieving excellent contact passivation quality. It is also demonstrated quantitatively that the tunnel oxide plays a critical role in this tunnel oxide passivated contact (TOPCON) scheme to realize desired carrier selectivity. Presence of tunnel oxide increases the implied <em>V_oc</em> (<em>iV_oc</em>) by ~ 125 mV. The <em>iV_oc</em> value as high as 728 mV is achieved on symmetric structure with TOPCON on both sides. Large area (239 cm²) <em>n</em>-type Czochralski (Cz) Si solar cells are fabricated with homogeneous implanted boron emitter and screen-printed contact on the front and TOPCON on the back, achieving 21.2% cell efficiency. Detailed analysis shows that the performance of these cells is mainly limited by boron emitter recombination on the front side.

Characterization of n-type Mono-crystalline Silicon Ingots Produced by Continuous Czochralski (Cz) Technology

Continuous Czochralski (Cz) technology has been developed to address the high cost drivers of the traditional Cz technology for producing n-type wafers which are used to make the silicon based solar cells with the highest energy conversion efficiency. Continuous Cz technology overcomes the shortcomings of the traditional Cz in low ingot output from each crucible and large axial variation in resistivity and interstitial oxygen across the ingot, two of the drivers for the high wafer cost. In this work, five 2-meter long ingots pulled by continuous Cz from a single crucible were characterized for the minority carrier lifetime, resistivity, interstitial oxygen and substitutional carbon concentrations. In addition, the wafers cut from these ingots were made into cells at both Georgia Institute (nPERT with front junction) and ECN (n-Pasha). Equivalent cell performance from these ingots has been achieved.

Potential-induced degradation in photovoltaic modules based on n-type single crystalline Si solar cells

Potential-induced degradation (PID) in photovoltaic (PV) modules based on n-type single crystalline Si solar cell (front junction cell) was experimentally generated by applying negative voltage from an Al plate, which was attached on the front cover glass of the module, to the Si cell. The solar energy-to-electricity conversion efficiency of the standard n-type Si PV module decreased from 17.8% to 15.1% by applying −1000V at 85°C for 2h. The external quantum efficiency in the range from 400 to 600nm significantly decreased after the PID test, although no change was observed from 800 to 1100nm. PID in n-type Si PV modules can be basically explained by enhanced front surface recombination between electron and hole on the Si cell, whereas the polarity of voltage leading to PID depends on structure of Si cell. An ionomer encapsulant instead of EVA has significantly suppressed PID in n-type Si PV modules.