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In this work, structure designs and the corresponding energy loss analysis are conducted to achieve the high-efficiency n-type rear-junction solar cells with polysilicon passivated contact. We focus on the front-side structure design of solar cells, considering that the primary efficiency loss of the conventional n-type polysilicon passivated contact cells with boron-diffusion emitter is from the front side. A well-designed rear-junction solar cell with front localized n-type and rear full-area p-type polysilicon passivated contacts is expected to overcome these problems. However, the efficiency of rear-junction solar cells is sensitive to the front-side electrode contact resistivity. To accurately assess the practically achievable efficiency that the current technology can reach, we develop a simple modified TLM with wet-chemical etching to measure the contact resistivity of the n-type polysilicon contact. This method does not require relatively expensive photolithography and reactive ion etching tools and is easy to be used. When the contact resistivity of the front-side localized n-type polysilicon contact reaches 0.002 Ω·cm2 with a saturation current density of ~10 fA/cm2 in the front-side un-diffused area, the efficiency of the rear-junction n-type solar cell is expected to be ~26%, showing its potential for application in mass-production of high-efficiency crystalline silicon solar cells.

Hydrogenated amorphous silicon (a-Si:H) deposited using plasma-enhanced chemical vapor deposition (PECVD) is used as the passivation layer for p-type multi-crystalline black silicon (p-mc-b-Si) wafers. The effects of deposition conditions on passivation quality are investigated. The optimized a-Si:H passivation layer enables the best surface passivation with an implied open-circuit voltage (iVoc) of 710 mV, which is better than other conventional passivation materials. The excellent passivation quality possibly resulted from that the activated hydrogen from a-Si:H at ~200 °C was able to passivate both surface defect states and bulk trap states of p-mc-b-Si effectively. Moreover, an additional SiNx capping layer helps to reduce reflectance significantly and to keep passivation quality. This work suggests that b-Si passivated by a-Si:H/SiNx stack film shows potential for various optoelectronic devices that requires both excellent optical anti-reflectance and surface passivation.

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.

The junction properties of tunnel silicon oxide (SiOx) passivated contact (TOPCon) with n-type poly-Si on p-type c-Si wafer are characterized using current-voltage (J-V) and capacitance-voltage (C-V) measurements. The dark J-V curves show a standard diode characteristic with a turn-on voltage of ∼0.63 V, indicating a p-n junction is formed. While the C-V curve displays an irregular shape with features of 1) a slow C increase with the decrease of the magnitude of reverse bias voltage, being used to estimate the built-in potential (Vbi), 2) a significant increase at a given positive bias voltage, corresponding to the geometric capacitance crossing the ultrathin SiOx, and 3) a sharp decrease to negative values, resulting from the charge tunneling through the SiOx layer. The C of depleting layer deviates from the normal linear curve in the 1/C2-V plot, which is caused by the diffusion of P dopants from the n-type poly-Si into the p-type c-Si wafer as confirmed by the electrochemical capacitance-voltage measurements. However, the 1/C2+γ-V plots with γ > 0 leads to linear curves with a proper γ and the Vbi can still be estimated. We find that the Vbi is the range of 0.75–0.85 V, increases with the increase of the doping ratio during the poly-Si fabrication process, and correlates with the passivation quality as measured by the reverse saturated current and implied open circuit voltage extracted from transient photoconductivity decay.

A precise characterization of thin film silicon in devices is a great challenge for device optimization because the material properties depend on the substrate and change with film thickness. A specific situation is thin poly-Si layer in tunnel oxide passivated contact (TOPCon) structure in high-efficiency solar cells. In this paper, we present a systematic study using a wide range UV-Raman spectroscopy to measure the crystalline structure and chemical bonding configurations in the TOPCon structure. Because of the high absorption coefficient of 325-nm laser in silicon films, the Raman probes only the top surface region and minimizes the contribution from c-Si substrate. However, the comparison with the Raman spectra collected with a green laser of 532 nm, the UV-Raman spectrum of a-Si:H shows a very different spectral shape with the disappearance of the LA mode of Si-Si vibration and an appearance of additional signal at 325 cm−1. In addition, the UV-Raman spectrum of the pre-crystallized P doped a-Si:H layer shows most of the vibration modes of Si–Si, Si–H, Si–O–Si and Si–O–H bonds, where the Si–O–H bonds are in the top surface oxide layer. After the crystallization, the Si-Si TO mode shows a high degree of crystallization, the Si-H vibration modes disappear, and an additional Si-O-Si mode is observed which is believed from the ultrathin SiOx passivation layer. In addition, we provide a set of UV-Raman measurements on the samples made with the same layer structure but at different annealing temperatures to show the correlation between the UV-Raman spectra and the passivation quality of the TOPCon structure on the n-type c-Si wafers. These observations prove that UV-Raman spectroscopy is a useful characterization tool for high efficiency c-Si solar cells with TOPCon structure. In addition, the impacts of the material properties on the passivation quality are discussed.

Black silicon (b-Si) with nanotextures is a promising light-trapping scheme for potentially achieving high conversion efficiency at reduced manufacturing cost in crystalline-silicon solar cells. However, the inherently high aspect-ratio and tiny feature size of the nanostructures are subject to severe surface (large surface areas) and Auger recombination (worse doping profile). These will abate the cost values of b-Si since one has to adopt a comprise strategy of applying shallow nanotextures with antireflection and passivation layers. Here, we show that silicon microwire solar cells featuring well-defined radial junctions can extensively suppress both surface and Auger recombination by providing excellent all-around electrical field. The radially doped silicon micropillar devices even show an internal quantum efficiency as good as that of planar substrate and their measured minority carrier lifetimes become nearly independent of total surface area. A great reduction in short-circuit current density loss was further identified as the junction abruptly changed from a fully diffused to a core-shell configuration, manifesting the powerful effectiveness of radial p-n+ junction on the suppression of Auger recombination. Furthermore, silicon microwire solar cell with a radial junction demonstrates 37% increase in efficiency compared with the reference cell, suggesting a feasible strategy towards high-efficiency solar devices.

In this work, the application of the p-type and n-type polysilicon passivated contact on industrial-level p-type silicon solar cell is studied using numerical simulation. The effects of (i) the structure design, (ii) the bulk lifetime and resistivity of the p-type wafer, and (iii) the carrier selectivity of polysilicon passivated contact on cell performances are investigated. Furthermore, the corresponding energy-loss pathways are classified by using free energy loss analysis (FELA). In essence, the rear-junction solar cell with the n-type polysilicon passivated-contact generates more internal power because of the better surface passivation and less front metallization shading, but the efficiency potential is limited by the low lifetime of the state-of-the-art p-type Czochralski (Cz) wafer. Thus, the p-type polysilicon passivated contact serving as the back-surface field would be more favorable if the lifetime of the p-type Cz silicon were less than 350 μs. Over the long term, the lifetime of the p-type wafer possibly becomes the bottleneck of the high-efficiency polysilicon passivated-contact solar cells. Finally, we present the roadmap toward the 23% industrial p-type silicon solar cell with the p-type or n-type polysilicon passivated contact.

The carrier transport through the silicon-oxide (SiOx) layer in tunnel oxide passivated contact (TOPCon) c-Si solar cells has been studied experimentally and by simulation. The current intensity versus voltage (J-V) characteristics of GaIn/n-c-Si/SiOx/n+-poly-Si/Al structures shows a linear Ohmic characteristic, while a non-Ohmic behavior is observed in the samples without the n+-poly-Si contact layer. Conductive Atomic Force Microscopy (c-AFM) images reveal some current spikes on the surface of the samples, which could be related to the transport through pinholes. The simulation results show that 1) a rectification characteristic is obtained when only the tunneling mechanism is included, 2) both the reverse saturation current and the forward current increase when a small amount of transport through pinholes is introduced, and 3) finally a linear Ohmic behavior is observed when the pinhole transport component reaches a certain level. Furthermore, the simulation for whole TOPCon solar cells provides some useful results. For very thin SiOx ( 1.2 nm) without the transport through pinholes, the TOPCon solar cell shows a poor fill factor (FF) with a high series resistance (Rs) because the tunneling does not provide a sufficient high transport channel for carrier transport, and the introduction of a small number of transports through pinholes improves the FF and reduces the Rs, hence improves the PCE. However, a high possibility for carrier going through pinholes reduces all of the performance parameters and degrades PCE for all the cases simulated. Therefore, an optimized pinhole density and size distribution is critical engineering for solar cell performance optimization. However, the establishment of an optimized method to precisely control the pinhole formation and characterization is still on the way.

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.

In this work, the tunnel oxide passivated contact (TOPCon) with a low-work-function electron-selective-collection (ESC) layer is studied using a numerical simulation method. An exhaustive comparison between the low-work-function ESC TOPCon and the heavily-doped-Si TOPCon solar cell is carried out to find out the differences between these two kinds of devices. The work function modulated ESC TOPCon with a work function of typically

The use of Ga-doped Czochralski (CZ) silicon wafers in Passivated Emitter and Rear Cells (PERC) has been confirmed to have a prominent advantage in suppressing light-induced degradation (LID), which will attract considerable attention for the application of Ga-doped wafers in more efficient photovoltaic devices. In this work, we investigate the passivation quality and address the issue of LID in Ga-doped CZ Si wafers equipped with p-type polysilicon passivating contact that consists of an ultrathin SiOx and a heavily doped polysilicon. We also present the modeling results for solar cells using this type of contact. The experiments show that samples with Ga-doped CZ Si wafers have superior anti-LID properties when compared to the samples with B-doped wafers. An excellent passivation performance with a high implied open-circuit voltage (iVoc) of 705 mV and a low single-sided saturation current density (J0,s) of 9 fA/cm2 was achieved. Moreover, with the help of the numerical simulations, we predict that the p-type Ga-doped CZ Si solar cells with p-type polysilicon passivating contacts have the potential to achieve a high efficiency of 23.8%, an ~0.5% absolute efficiency improvement over that of PERC solar cells. The results demonstrated in this study suggest that Ga-doped CZ Si wafers combined with p-type polysilicon passivating contacts could resolve the LID issue while maintaining good passivation properties, providing a promising alternative for p-type solar cells in the photovoltaic industry.

A comparison study of ultrathin atomic layer deposited AlOx, wet-chemically oxidized SiOx, and their combination as the tunnel layer in TOPCon structure with both B-doped and P-doped poly-Si contact layers on n-type c-Si wafers was carried out. The passivation quality with the three types of tunnel layers was examined as a function of thickness and annealing temperature. Ideally, the high density of negative fixed charge in the AlOx is expected to provide a positive benefit on the passivation quality with p-type poly-Si as the contact layer, however, it is surprisingly observed that the AlOx and AlOx/SiOx do not yield a better passivation than the SiOx. Searching for the mechanisms behind, an interesting phenomenon is observed that the AlOx, especially the AlOx/SiOx bi-layer, significantly enhances B diffusion and suppresses P diffusion. It is also found a remarkable accumulation of B in the AlOx and AlOx/SiOx region, forming a reservoir for B diffusion. Furthermore, the free carrier assistant extrinsic diffusion is an additional factor for the enhanced B and retarded P diffusions by the AlOx/SiOx. The enhanced B diffusion causes extra Auger recombination as well as recombination through B–O pair defects and degrades the p-TOPCon passivation quality; additionally, a quantity of Al diffuse into the c-Si wafer could be another potential factor degrading passivation quality, because Al forms deep-level defects and induces a significant SRH recombination.

A rapid thermal anneal (RTA) is used to crystallize the plasma-enhanced chemical vapor deposition (PECVD) deposited hydrogenated amorphous silicon (a-Si:H) thin film to form the phosphorus-doped polysilicon passivated contact in tunnel oxide passivated contact (TOPCon) solar cells. The effects of annealing temperature, annealing time, cooling time, and the polysilicon thickness on the surface passivation are investigated. The primary advantage of the RTA is reducing the whole crystallization period to ~15 min, shorter than the conventional tube-furnace annealing period of >60 min. We find that the RTA is a robust method to prepare high-quality polysilicon passivated contact without introducing blistering when the thickness of the a-Si:H is less than 40 nm. The optimized RTA process leads to an implied open-circuit voltage (iVoc) of 712 mV and a single-sided dark saturation current density (J0,s) of 12.5 fA/cm2 in the as-annealed state, which is inferior to the surface passivation of the controlled one prepared by a tube furnace annealing. Fortunately, a subsequent Al2O3 capping hydrogenation improves the iVoc and J0,s to 727 mV and 4.7 fA/cm2, respectively. The champion conversion efficiency of 23.04% (Voc = 679.0 mV, Jsc = 41.97 mA/cm2 and FF = 80.86%) is achieved, which demonstrates the effectiveness of RTA for preparing a high-efficiency polysilicon passivated-contact solar cell.

We present a systematic study of highly boron (B)-doped poly-silicon (p+-poly-Si) and ultrathin silicon oxide (SiOx) bi-layer structure, also named as p-TOPCon, as the hole-selective passivated contact on n-type c-Si wafer, where the SiOx layer is made with three methods of hot nitric acid oxidation SiOx (NAOS-SiOx), plasma-assisted nitrous-oxide (N2O) gas oxidation (PANO-SiOx), and thermal oxidation (Thermal-SiOx). We demonstrate that the SiOx has a strong influence on the passivation quality. The best result is achieved using the Thermal-SiOx, while the NAOS-SiOx is slightly inferior, but better than the PANO-SiOx. The p+-poly-Si/SiOx structures with the three SiOx layers achieve the optimized passivation quality at different annealing temperatures of 820 °C for the NAOS-SiOx, 880 °C for the PANO-SiOx, and 930 °C for the Thermal-SiOx. The other potential factors affecting the passivation quality are also studied. The most important observation is that the optimized p-TOPCon structures with the three SiOx layers have a similar B diffusion profile, which penetrates into the c-Si wafer about 50 nm with B concentration decreasing to ~1 × 1018 cm−3. However, the overall p+-poly-Si/SiOx is still much poorer than n+-poly-Si/SiOx in terms the passivation quality. The comparison of the τeff versus carrier injection intensity spectra suggests that the B–O complex is the passivation killer possibly, and the approaches to improve the p-TOPCon are searching the other elements to reduce the B–O defects. In addition, contact resistivity (ρc) measurements show that the Thermal-SiOx leads a higher ρc than the others, but its value is still low enough for high-efficiency solar cells.

We develop a plasma-assisted nitrous-oxide (N2O) gas oxidation (PANO) method to prepare the ultrathin silicon oxide (SiOx) for polysilicon (poly-Si) passivated contact. The effects of preparation conditions, including the substrate temperature, processing time, and plasma power, are studied. Afterwards, we integrate the PANO SiOx into the polysilicon passivated contact and optimize the passivation and contact performances. Excellent surface passivation with the n-type poly-Si and PANO SiOx on the n-type c-Si wafer is achieved by 880 °C annealing, which shows competitive passivation quality to the one with NASO SiOx. Champion implied open-circuit voltage (iVoc) and single-sided recombination saturated current (J0) reach 730 mV and 4.3 fA/cm2 after crystallization; and they are further improved to 747 mV and 2.0 fA/cm2 (3 × 1015cm−3) after subsequent AlOx/SiNx hydrogenation. Using transmission electron microscopy (TEM), we find that the thickness of PANO SiOx ranges 1.1–2.4 nm and the controlled nitric acid oxidized SiOx (NAOS) ranges 1.3–1.8 nm. The contact resistivity (ρc) is typically 820 °C. Also, the crystallinity, phosphorous in-diffusion profile, and current-leaking density of the passivated contacts are investigated. In general, the PANO SiOx and in-situ doping amorphous silicon precursor can be fabricated in one PECVD system without additional equipment or transfer procedures, which is favorable for the high-efficiency, low-cost industrial manufacture.

We report an industrial compatible tunnel oxide passivated contact (TOPCon) structure on solar-grade p-type c-Si wafer as the rear emitter for high-efficiency solar cells, where the ultrathin silicon oxide (SiOx) is made by plasma-assisted oxidation and the P-doped n-type poly-Si (n+-poly-Si) contact layer by plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) with in-situ doping with PH3 and following the high-temperature annealing. The fabrication processes of SiOx and n+-poly-Si layers are systematically optimized. It is found that the optimized annealing temperature for the crystallization and dopant activation is in the range of 850 °C and 880 °C, which is higher than the similar structure with chemically oxidized SiOx (820 °C). In addition, the samples made with higher PH3/SiH4 doping ratio during the a-Si:H deposition need a lower annealing temperature to reach the similar dopant distribution within the samples made with lower doping ratio but annealed at higher temperature, it means that the optimized annealing temperature decreases with the increase of the PH3/SiH4 doping ratio during the deposition of a-Si:H precursor. The optimized process with post-hydrogenation yields excellent surface passivation on the p-type Si substrate with the implied open-circuit voltage (iVoc) of ∼742 mV, the single-side saturated recombination current density (J0) of ∼3.0 fA/cm2, the contact resistivity (ρc) of ∼2–4 mΩ cm2, and the effective minority carrier lifetime (τeff) of ∼1050 μs (Δn = 1 × 1015 cm-3). With these passivation parameters, a simulation study demonstrates the advantage of rear emitter over the conventional front emitter TOPCon solar cell with ∼22.8% achievable efficiency on solar-grade p-type wafers with current industrial constraints, and ∼24.9% on high quality p-type c-Si wafers with the optimized conditions in R&D laboratories. Our study suggests that the p-type c-Si solar cell with a rear n+-poly-Si TOPCon emitter is a viable structure for high-efficiency solar cell production.