Contact Silicon Solar Cells Research Articles

Application of polycrystalline silicon carbide thin films as the passivating contacts for silicon solar cells

In this paper, silicon rich polycrystalline silicon carbide (poly-SiCx) thin films were prepared for passivating contact of silicon solar cells. The effect of different carbon doping ratios R [methane flow (sccm)/silane flow (sccm)] and the annealing temperatures on the passivation quality was investigated. The lifetime test showed that, given a lower annealing temperature, the passivation quality of poly-SiCx thin films was very poor at R over 0.2, and it can be improved by elevating the annealing temperature. The effective lifetime of over 1.8 ms can be obtained with implied open-circuit voltages (iVoc) of 715 mV and saturated dark current (J0) of 18 fA/cm2. The microstructure and optical property of poly-SiCx films was studied by Raman spectra, X-ray photoelectron spectroscopy and UV-VIS spectrophotometer. The results showed that the incorporation of carbon lowered the crystalline fraction of poly-SiCx and a higher annealing temperature was needed to achieve high crystallinity. The optical band gap of poly-SiCx was widened with increase in R, and got up to 2.3eV when R was 0.4. A proof-of-concept top/rear TOPCon solar cells, featuring a N+-poly-SiCx (R = 0.2) front contact, was fabricated to demonstrate the potential of this SiCx passivation contact. The decrease in the parasitic absorption of light at the front side resulted in higher photogenerated current. And the conversion efficiency of 20.17% was achieved.

Thermal stability improvement of metal oxide-based contacts for silicon heterojunction solar cells

Metal oxides are interesting materials for use as carrier-selective contacts for the fabrication of doping-free silicon solar cells. In particular, MoOx and TiOx have been successfully used as hole and electron selective contacts in silicon solar cells, respectively. However, it is of paramount importance that good thermal stability is achieved in such contacts. In our work, we combined i-a-Si:H/MoOx based hole contacts with electron contacts featuring i-a-Si:H/TiOx/low work function metal (ATOM) to fabricate doping-free cells, termed MolyATOM cells. We found that the thermal stability of the ATOM contact was improved when the i-a-Si:H was annealed (300 °C for 20 min in N2) before depositing TiOx (i.e. pre-TiOx annealing), which reduces the hydrogen content in i-a-Si:H by about 27 %rel, and thereby the H-related degradation of the ATOM contact characteristics. Moreover, it was found that reducing the thickness of the low-work function metal on top of the TiOx enhanced the thermal stability of the ATOM contact. With these adaptations, the MolyATOM cell efficiency was improved by 3.5 %abs, with the highest efficiency of 17.6%. Moreover, the cells show improved thermal stability after the above-mentioned pre-TiOx annealing, which is confirmed by annealing tests at cell level as well as damp-heat tests at module level. The insights of this study could be used to tailor other metal-oxide based electron or hole contacts.

Contributions to the Contact Resistivity in Fired Tunnel-Oxide Passivating Contacts for Crystalline Silicon Solar Cells

In this article, we investigate the contact resistivity of p -type passivating contacts for silicon solar cells. Our contact structures are compatible with firing, a rapid annealing process similar to the one used for sintering of the screen-printed metallization in solar cell manufacturing. We find that the short firing process crystallizes the doped layers and incorporates active boron dopants up to the solubility concentration at the chosen firing temperature. The dependence of our contact resistivities on carrier density and temperature suggest that the hole transport is a combination of tunneling through the oxide at the wafer surface and of thermionic field emission over the Schottky barrier to the metallization. For ideal firing conditions, we find implied open-circuit voltages up to 720 mV and contact resistivities as low as 15 m·Ω·cm2.

Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Studies of Hole-Selective Molybdenum Oxide Contacts in Silicon Solar Cells.

In this study, substochiometric hole-selective molybdenum oxide (MoOx) contacts in crystalline silicon (c-Si) solar cells were investigated by a combination of transmission electron microscopy (TEM) and spatially resolved electron energy-loss spectroscopy (SR-EELS). It was observed that a ≈ 4 nm SiOx interlayer grows at the MoOx/c-Si interface during the evaporation of MoOx over a c-Si substrate. SR-EELS analyses revealed the presence of a 1.5 nm diffused MoOx/indium tin oxide (ITO) interface in both as-deposited and annealed samples. Moreover, the presence of a 1 nm thin layer with a lower oxidation state of Mo was detected at the SiOx/MoOx interface in an as-deposited state, which disappears upon annealing. Overall, it was evident that no hole-blocking interlayer is formed at the MoOx/ITO interface during annealing and homogenization of the MoOx layer takes place during the annealing process. Furthermore, device simulations revealed that efficient hole collection is dependent on MoOx work function and that reduction in the work function of MoOx results in loss of band bending and negatively impacts hole selectivity.

High‐Efficiency Interdigitated Back Contact Silicon Solar Cells with Front Floating Emitter

Silicon interdigitated back contact (IBC) solar cells with front floating emitter (FFE‐IBC) put forward a new carrier transport concept of “pumping effect” for minority carriers compared with traditional IBC solar cells with front surface field (FSF‐IBC). Herein, high‐performance FFE‐IBC solar cells are achieved theoretically combining superior crystalline silicon quality, front surface passivation, and shallow groove structure using 2D device model. The improvement of minority carrier transport capacity is realized in the conductive FFE layer through optimizing the doping concentration and junction depth. It is shown that the shallow groove on the rear side of FFE‐IBC solar cells can effectively enhance the carrier collection ability by means of minimizing the negative impact of undiffused gap or surface p–n junction. The high efficiency exceeding 25% can be realized on silicon FFE‐IBC solar cells with the novel cell structure and optimized cell parameters, where the back surface field and emitter region width can be made for the same with only a slight sacrifice of photocurrent density and conversion efficiency. It is demonstrated theoretically that the realization of high‐efficiency and low‐cost silicon IBC solar cells is feasible due to the increase of the module fabrication tolerance.

Impact of phosphorus diffusion on n-type poly-Si based passivated contact silicon solar cells

This paper describes the impact of phosphorus dopant concentrations in n-type passivated contact structures. Free carrier absorption (FCA) of the phosphorus doped poly-Si layers was studied by using different phosphorus dopant concentrations, with a range of from 9 × 1019 to 2.3 × 1020 cm−3, and different poly-Si layer thicknesses, with a range of from 30 to 200 nm. In the worst case, the effect of FCA reduce the short circuit current density by 0.89 mA/cm2. The impacts of tunnel SiO2 thicknesses, poly-Si thicknesses and diffusion temperatures on the performances of the poly-Si passivated contacts were studied. Phosphorus concentrations induced silicon surface field passivation play an important role in passivated contact structure. On the planar n-type mono crystalline-silicon substrates, excellent J0 value of 6.5 fA/cm2 was achieved by optimizing phosphorus doping in poly-Si films and silicon substrate. Different poly-Si thickness and diffusion temperature were studied simultaneously to optimize the phosphorous dopants in the poly-Si passivated contacts for the specified poly-Si thickness. N-type silicon solar cell have been fabricated by using the optimized poly-Si passivated contacts, as addressed above. Average efficiency of 22.52%, and best cell efficiency of 23.04% were demonstrated.

TEM Study of MoOx/Ni and MoOx/Al Contacts for Silicon Solar Cells

TEM Study of MoO_x/Ni and MoO_x/Al Contacts for Silicon Solar Cells

Direct Determination of Total Hemispherical Emittance of Perovskite and Silicon Solar Cells

The performance and lifetime of a solar cell in the field is sensitive to its operating temperature. A knowledge of the total radiative heat transfer is required to predict the operating temperature. This requires an accurate knowledge of the total hemispherical thermal emittance as a function of temperature. Calorimetric methods performed under vacuum are considered to be the “gold standard” method for determining total hemispherical emittance as they are direct and absolute. Here, we report a novel, cost-effective and rapid transient calorimetric method to measure directly the total hemispherical thermal emittance of a solar cell. Examples of standard and high efficiency passivated-emitter-rear-localised contact (PERL) silicon solar cell and perovskite solar cells with and without encapsulation were measured over their operating temperature range for both front and rear surfaces. Results show that the encapsulation material of a cell and the type of rear electrode strongly influence the total hemispherical thermal emittance of the cell. Specifically, using soda-lime glass as a cover layer in encapsulated cells increases the emittance due to the high infrared (IR) absorptance of glass. Metal rear electrodes in a semi-transparent cell decrease the emittance considerably by creating a broad band of high reflectance. These findings demonstrate the importance of front and rear materials selection in the optimization of radiative cooling for efficiency and lifetime. Additionally, total hemispherical emittance of solar cells measured in this work is considerably lower than spectral emittance values previously measured which were used to calculate solar cell operating temperature in the context of radiative cooling. Therefore, radiative heat transfer calculation of photovoltaics will need to be re-examined in the light of this work.

26%-efficient and 2 cm narrow interdigitated back contact silicon solar cells with passivated slits on two edges

Perimeter recombination is a relevant loss mechanism, in particular for cells with a large perimeter-to-area ratio and with poorly passivated edges, e.g., cut or cleaved solar cells for shingled modules. We experimentally demonstrate that cut edges can be well passivated during front-end processing. The resulting cells have an efficiency of 26%.The designated cell area of our lab-type highly efficient cells is smaller than the total area of the wafer. This causes recombination losses in the masked perimeter region. We separate the active cell area from the wafer on two sides of the cell by slits to reduce the transport of carriers into the perimeter region. We apply a diffusion model to describe impact of the slits on the perimeter recombination. The slits have an effective surface recombination velocity of down to 9 cm/s, depending on the resistivity of the base. For a base resistivity of 80 Ωcm, the average cell efficiency increases by 0.7 %abs as compared to embedded cells and by 2.3 %abs as compared to laser-cut cells due to the passivated slits.

Polysilicon Passivating Contacts for Silicon Solar Cells: Interface Passivation and Carrier Transport Mechanism

Polysilicon passivating contacts, consisting of a stack of tunnel-oxide and doped polysilicon layers, can simultaneously provide excellent surface passivation and low contact resistivity for silico...

Life Cycle Assessment of tandem LSC-Si devices

Given the increasing interest in tandem silicon-based solar cells and the recent advances in luminescent solar concentrators, the luminescent solar concentrators/silicon tandem structure has been proposed as an option for a four-terminal tandem structure. As part of the evaluation of a new type of solar cell, it is important to conduct a Life Cycle Assessment to effectively guide research efforts towards cell designs with minimum environmental impacts. Here, we carry out a process-based Life Cycle Assessment to assess global warming, human toxicity (carcinogenic and non-carcinogenic), freshwater eutrophication and ecotoxicity and abiotic depletion potential impacts associated with three luminescent solar concentrators/silicon tandem cell structures, considering a bottom layer as being a passivated emitter rear contact silicon solar cell. The results are based on experimental parameters and show that the increase in the performance of the cells and modules using the studied tandem structure can produce lower environmental effects than the passivated emitter rear contact technology (single-junction) for the impact categories studied. These results encourage the studies on cell and module performance improvements using such tandem luminescent solar concentrators/silicon structures.

Activating and optimizing evaporation-processed magnesium oxide passivating contact for silicon solar cells

Irrespective of the success on reduction of contact resistivity, lack of chemical passivation of evaporated metal oxides heavily hinders their applications as passivating contacts, such contacts can be an alternative route for high efficiency and cost effective silicon solar cells. Here, we demonstrate that electron beam evaporated magnesium oxide (MgOx) thin film can work as a promising electron-selective passivating contact for n-Si solar cells after a post-annealing treatment and an alumina-initiated atomic hydrogenation. 10 nm MgOx on n-Si provided a surface recombination velocity down to 14.9 cm/s while 1 nm MgOx showed a low contact resistivity of 14 mΩ cm2. Comprehensive characterizations revealed the formation of Si–O–Mg bonds and the activation of atomic hydrogens were the main reasons for such high-level passivation. A PERC-like dopant-free rear contact was formed by using the 1 nm-MgOx as electron-collector and the 10 nm-MgOx as passivating layer, the resultant solar cells achieved 27% increment in efficiency and 51 mV increase in open-circuit voltage in comparison with reference devices. The ways of improving passivation quality of MgOx and novel design of contact structure open up the possibility of using evaporation-processed metal oxides as effective and low-cost carrier-selective passivating contacts for n-Si photovoltaic devices.

Enhancing the efficiency of silicon solar cells using ZnO nanostructures prepared by microwave-assisted hydrothermal method

In this study, zinc oxide (ZnO) nanostructures were grown on interdigitated back contact silicon solar cells (IBC-SSCs) by using the microwave-assisted hydrothermal method. The effect of these ZnO nanostructures grown by different precursor concentrations on the conversion efficiency of solar cells was investigated. The as-prepared ZnO products were analyzed by XRD, SEM, EDS, UV–vis, and PL, then were grown onto IBC-SSCs. The IBC-SSCs conversion efficiency without any ZnO nanostructure was 8.88%, and with a ZnO nanostructure, it reached 12.15%. which effectively enhances the conversion efficiency of IBC-SSCs.

Computational Modeling of Polycrystalline Silicon on Oxide Passivating Contact for Silicon Solar Cells

Polycrystalline silicon on oxide (POLO) junction passivating contacts have recently been demonstrated as carrier selective contacts for high-efficiency silicon solar cells. The carrier transport through these contacts has been attributed to two competing mechanisms: 1) carrier tunneling through ultrathin oxide and 2) transport through weak spots (pinholes) – the nanoscale regions where oxide thickness has been completely or partially compromised during the processing. In this paper, we use two dimensional device simulations to compare the relative effects of these mechanisms on solar cell characteristics with n-type POLO contact. We show that variation in pinhole areal density ( $ {D}_{\text {ph}}$ ) or the tunnel oxide thickness ( $ {t}_{\text {ox}}$ ) both result in qualitatively similar trends in the cell characteristics under dark and light. For a given $ {t}_{\text {ox}}$ , an exponential variation in $ {D}_{\text {ph}}$ results in trends that are similar to those for a linear variation in $ {t}_{\text {ox}}$ . The effect of pinholes on contact resistance ( $\rho _{\text {c}}$ ) and saturation current density ( ${J} _{\text {o}}$ ) is most significant for relatively thicker oxides (≥ 2 nm). For $ {t}_{\text {ox}}~\leq1$ nm, $\rho _{\text {c}}$ and ${J} _{\text {o}}$ become essentially insensitive to pinholes for $ {D}_{\text {ph}}\,\, . The modeling results are compared with a set of published experimental data to predict the possible roles of various mechanisms.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Utilization of 2D gahnite nanosheets as highly conductive, transparent and light trapping front contact for silicon solar cells

The current scenario illustrates distinct interest in developing renewable energy sources for power generation. In this regard, several researches are performed in enhancing the power conversion efficiency of solar cells. The present work focuses on utilizing ZnAl2O4 (gahnite) spinel as antireflection coating material to improve the power conversion efficiency of silicon solar cells. Gahnite was synthesized using two precursors namely zinc nitrate hexahydrate and aluminum nitrate nonahydrate through sol–gel technique. The thickness of the prepared gahnite sheets measured through atomic force microscopy was around 50 nm. Single to quintuple layers of gahnite was deposited on silicon solar substrate using spin coating technique. The influence of gahnite coating on the structural, optical, electrical properties and cell temperature of silicon solar cells are analyzed. The synthesized gahnite bears spinel crystal structure in the form of two dimensional nanosheet. Increment in layer thickness proves the deposition of single to quintuple layer on silicon substrate. A maximum of 93% transmittance and 20.72% power conversion efficiency at a low cell temperature (39.4 °C) has been achieved for triple layer deposition proving diffusion of more photons on the substrate. The obtained results prove gahnite as suitable anti-reflection coating material for enhancing the power conversion efficiency of silicon solar cells.

Adhesion strength of plated Ni/Cu metallization in Si solar cells

Replacing silver paste contacts in silicon solar cells by electroplated nickel and copper (Ni/Cu) connections offer potential advantages of more exceptional grid lines, lower series resistance, and reduced costs in industrial silicon solar cell manufacturing. To achieve acceptance in the market for an electroplated Ni/Cu contacts sufficient contact adhesion has become the significant issue. In this work, a thin nickel layer was deposited on textured (001) silicon by an electroless plating method and form silicides by annealing treatment—this substrate designed as Ax–Si. Afterward, the Cu(15µ m)/Ni(240–1440 nm)/Ax–Si were prepared by electroplating techniques. The structures of Cu/Ni/Ax–Si characterized by scanning electron microscopy, transmission electron microscope and powder X-ray diffraction. Peel force tests were conducted using a universal testing machine. The peel force tests performed at an angle of 90° with a constant speed of 30 mm/min. Excellent adhesion force with 5.2 N/mm achieved.

Silicon Contact Defects Get Fired

The large majority of installed photovoltaics today are based on silicon. Improvements in their performance and reductions in their manufacturing cost can have a direct global impact on the photovoltaic market, provided this can be realized with minimal disruption to existing manufacturing lines. Recently in Nature Energy, Ballif and colleagues demonstrate an easily integrated method based on a single firing thermal anneal to passivate rear contacts in silicon solar cells.

A strong-oxidizing mixed acid derived high-quality silicon oxide tunneling layer for polysilicon passivated contact silicon solar cell

We developed a new wet-chemical method to grow the high-quality tunnel silicon oxide (SiOx) layer by using a strong-oxidizing mixed acid, which consists of three volumes of HNO3 (68 wt%) and one volume of H2SO4 (98 wt%), named as the CNS (concentrated nitric and sulfuric) acid for short. In comparison with the HNO3 acid, the CNS acid grows high-quality SiOx layer with the higher oxidized state at 60 °C, where the relatively low temperature avoids the significant volatilization of acid and remains the quality of acid during the extending process. The results prove that the SiOx grown in the CNS acid benefits for the surface passivation of the n-type polysilicon passivated contact structure. An average gain of implied open circuit voltage (iVoc) by 2–10 mV and a reduction of single-side surface saturated dark current (J0) by 1–7 fA/cm2 are obtained by using the 60 °C CNS-acid grown SiOx to replace the 60 °C HNO3-acid grown one. Also, in comparison with the 90 °C HNO3 acid, the 60 °C CNS acid exhibits improved stability and repeatability for preparing the SiOx during the extending process. The CNS-acid grown SiOx helps the polysilicon passivated contact solar cell to raise the efficiency by ~ 0.15% on average. In summary, the CNS acid has shown the potential for industrial application, which improves not only the manufacturing process but also the device performances.

A passivating contact for silicon solar cells formed during a single firing thermal annealing

Passivating contacts are indispensable for achieving high conversion efficiency in crystalline-silicon solar cells. Their realization and integration into a convenient process flow have become crucial research objectives. Here, we report an alternative passivating contact that is formed in a single post-deposition annealing step called ‘firing’, an essential step for current solar cell manufacturing. As firing is a fast ( 750 °C) anneal, the required microstructural and electrical properties of the passivating contact are stringent. We demonstrate that tuning the carbon content of boron-doped silicon-based thin films inhibits firing-induced layer delamination without preventing a partial crystallization. The latter promotes charge-carrier selectivity, even in the absence of a diffused doped region beyond the oxide, by inducing hole accumulation near the wafer surface. We fabricated proof-of-concept solar cells employing the developed technology, demonstrating an open circuit voltage of 698 mV and an efficiency of 21.9%, and show how it could be a drop-in replacement for today’s rear contacts based on locally opened dielectric passivation stacks.