Silicon Heterojunction Structure Research Articles

Electrical Performance, Loss Analysis, and Efficiency Potential of Industrial‐Type PERC, TOPCon, and SHJ Solar Cells: A Comparative Study

ABSTRACTCurrently, the efficiency of p‐type passivated emitter and rear contact (PERC) cells has been growing at an absolute efficiency of 0.5% per year and has reached 23%–23.5% in mass production while getting closer to its theoretical efficiency limit. n‐Type tunnel oxide passivated contact (TOPCon) and silicon heterojunction (SHJ) cells with their superior “passivating selective contacts” technology were the most interesting photovoltaics (PV) technology in the industry. The effect of different passivated contact layers with respect to their influence on the J0, J0,metal, ρc, and the carrier selectivity (S10) and the loss analysis and efficiency potential of industrial‐type PERC, TOPCon, and SHJ solar cells were studied and compared. The results showed that TOPCon structure with a high passivation performance and good optical performance is more suitable for bifacial solar cell and the highest theoretical limiting efficiency with metal shading on the n‐type Si wafer (ηb,e,h,m,max) can be achieved to 27.62%. Although SHJ structure with the highest passivation performance but the worst optical performance owing to the parasitic absorption of a‐Si:H layer and high contact resistivity, the value of ηb,e,h,m,max is 0.7% lower than that of TOPCon solar cells. PERC structure has superior optical performance than SHJ structure, but due to poor passivation performance, the ηb,e,h,m,max is only 26.42%. The next‐generation products may be heterojunction back‐contact (HBC) and TOPCon back‐contact (TBC) cells with high ηb,e,h,m,max of 28.12% and 27.99%, respectively. Exploiting a perfect passivation of the noncontact area, the wide process window and low cost are required and transferring these new concepts to industrial solar cell production will be the next major challenge.

Silicon–Hydrogen Bonding Configuration Modified by Layer Stacking Sequence in Silicon Heterojunction Solar Cells

Recent improvements in highly efficient crystalline silicon (c-Si) solar cells have relied on surface passivation. Hydrogen plays a crucial role in the surface passivation of silicon heterojunction (SHJ) solar cells because Si–H bonds passivate dangling bonds in amorphous silicon and on the c-Si surface. In this work, we demonstrate that the Si–H bonding configuration is modified by layer stacking sequence for SHJs. The quality of surface passivation strongly correlates with low-temperature hydrogen effusion from the SHJ structure. Our results show that the deposition of doped layers on intrinsic amorphous silicon supplies additional hydrogen to the amorphous/crystalline heterostructure. Moreover, the deposition of a p-layer modifies the microstructure of the intrinsic layer underneath, whereas depositing an n-layer does not induce structural changes. We suggest that the low-temperature hydrogen effusion characteristics can be used as a sensitive indicator for examining the passivation quality of SHJ solar cells.

Silicon Heterojunction Solar Cells and p‐type Crystalline Silicon Wafers: A Historical Perspective

The first reports of both boron–oxygen (BO)‐related light‐induced degradation (BO‐LID) and amorphous/crystalline silicon heterojunction (SHJ) solar cell fabrication date back to the early 1970s. However, the complete development of the “modern” SHJ structure took place well before BO defect stabilization processes were developed. Due to the susceptibility of p‐type Czochralski (Cz)‐grown silicon to BO‐LID, such wafers were deemed unsuitable for SHJ solar cells. In addition to stability issues, lower charge carrier lifetimes due to contamination and challenges with surface passivation posed barriers to the adoption of p‐type wafers in SHJ applications. Herein, these three key challenges are discussed in detail. Kinetic modeling and experimental results reveal the severe impact of BO‐LID in p‐type SHJ solar cells and provide possible explanations as to why earlier attempts using p‐type wafers might have failed. The role of gettering and advanced hydrogenation in stabilizing BO defects in SHJ solar cells is demonstrated experimentally. Finally, a summary of the effective surface recombination velocities reported in the literature for hydrogenated intrinsic amorphous silicon passivation of p‐ and n‐type crystalline silicon wafers is presented. Based on these findings, the potential of p‐type wafers to enable a next‐generation of high‐efficiency solar cells featuring carrier‐selective contacts is discussed.

In Situ Grown Nanocrystalline Si Recombination Junction Layers for Efficient Perovskite-Si Monolithic Tandem Solar Cells: Toward a Simpler Multijunction Architecture.

The perovskite-Si tandem is an attractive avenue to attain greater power conversion efficiency (PCE) than their respective single-junction solar cells. However, such devices generally employ complex stacks with numerous deposition steps, which are rather unattractive from an industrial perspective. Here, we develop a simplified tandem architecture consisting of a perovskite n-i-p stack on a silicon heterojunction structure without applying the typically used indium-tin-oxide (ITO) recombination junction (RJ) layer between the top and bottom cells. It is demonstrated that an n-type hydrogenated nanocrystalline silicon (nc-Si:H) grown in situ on an amorphous silicon hole contact layer of the bottom cell acts as an efficient RJ layer, leading to a high open-circuit voltage (VOC) of >1.8 V and a PCE of 21.4% without optimizing the optical design. Compared to the tandem cell with an ITO RJ layer, the nc-Si:H RJ layer not only improves light management but also achieves a higher VOC due to superior contact properties with an overlying SnO2 electron transport layer of the perovskite top cell. Omitting the costly material and its deposition step offers the opportunity toward realizing industrially feasible high-efficiency tandem solar cells.

Amorphous silicon PEC-PV hybrid structure for photo-electrochemical water splitting

Abstract The paper presents fabrication and characterization of amorphous silicon carbide (a-SiC:H) based structures for photo-electrochemical (PEC) water splitting. The increase of the photocurrent of PEC upon the decreased of CH4 flow during the deposition is associated with the decrease of the band gap and increased absorption of light in a-SiC:H. Photocurrent of 50 µA/cm2 is achieved for PEC structure prepared with the lowest CH4 flow during the deposition. An ITO/a-SiC:H/Si silicon heterojunction structure forming a simple photovoltaic cell (PV) with efficiency of 9.66% was prepared to support additional voltage hereby forming a hybrid PEC-PV system. ASA simulation revealed that a photocurrent of 0.62 mA/cm2 and solar to hydrogen efficiency of 0.76% can be achieved for hybrid a PEC-PV structure with 5 PVs connected in series behind the PEC cell. Further opportunities for increasing the performance are discussed and summarized.

Light-Induced Effects on the a-Si:H/c-Si Heterointerface

Light-induced effects on the minority carrier lifetime of silicon heterojunction structures are studied through multiple-exposure photoconductance decay (MEPCD). MEPCD monitors the effect of the measurement flash from a photoconductance decay setup on a sample over thousands of measurements. Varying the microstructure of the intrinsic hydrogenated amorphous silicon (a-Si:H) used for passivation of n-type crystalline silicon (c-Si) showed that passivating films rich in voids produce light-induced improvement, while denser films result in samples that are susceptible to light-induced degradation. Light-induced degradation is linked to an increase in dangling bond density at the a-Si:H/c-Si interface, while light-induced improvements are linked to charging at the a-Si:H/c-Si interface. Furthermore, doped a-Si:H is added to make samples with an emitter and back surface field (BSF). These doped layers have a significant effect on the light-induced kinetics on minority carrier lifetime. Emitter samples exhibit consistent light-induced improvement, while BSF samples exhibit light-induced degradation. This is explained through negative charging at the BSF and positive charging at the emitter. Full precursors with a BSF and emitter exhibit different kinetics based on which side is being illuminated. This suggests that the light-induced charging at the a-Si:H/c-Si interface can only occur when a-Si:H has sufficient generation.

Temperature dependency of the silicon heterojunction lifetime model based on the amphoteric nature of dangling bonds

This work adapts a model to simulate the carrier injection dependent minority carrier lifetime of crystalline silicon passivated with hydrogenated amorphous silicon at elevated temperatures. Two existing models that respectively calculate the bulk lifetime and surface recombination velocity are used and the full temperature dependency of these models are explored. After a thorough description of these temperature dependencies, experimental results using this model show that the minority carrier lifetime changes upon annealing of silicon heterojunction structures are not universal. Furthermore, comparisons of the temperature dependent model to using the room temperature model at elevated temperatures is given and significant differences are observed when using temperatures above 100 °C. This shows the necessity of taking temperature effects into account during in-situ annealing experiments.

Analysis of low temperature output parameters for investigation of silicon heterojunction solar cells

This paper presents an ASA simulation analysis of temperature dependent output parameters of the silicon heterojunction (SHJ) solar cell. The analysis has shown that low temperature behaviors of the open circuit voltage and fill factor are strongly connected with the presence of barriers for collection of photogenerated carriers in the SHJ structure. Previous experimentally observed saturation of the open circuit voltage at low temperature was attributed to the presence of a parasitic Schottky barrier at the transparent conductive oxide/amorphous emitter contact. A comprehensive simulation study is provided to describe the mechanism of such saturation and to define the conditions under which the saturation of the open circuit voltage can be used as relevant identification of the parasitic Schottky barrier in SHJ structures with both n-type and p-type amorphous emitters. In addition, the presented study provides the first guideline on the possible utilization of SHJ in low temperature applications such as space applications.

Triple Layer Antireflection Design Concept for the Front Side of c-Si Heterojunction Solar Cell Based on the Antireflective Effect of nc-3C-SiC:H Emitter Layer

We investigated the antireflective (AR) effect of hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) emitter and its application in the triple layer AR design for the front side of silicon heterojunction (SHJ) solar cell. We found that the nc-3C-SiC:H emitter can serve both as an emitter and antireflective coating for SHJ solar cell, which enables us to realize the triple AR design by adding one additional dielectric layer to normally used SHJ structure with a transparent conductive oxide (TCO) and an emitter layer. The optimized SHJ structure with the triple layer AR coating (LiF/ITO/nc-3C-SiC:H) exhibit a short circuit current density (Jsc) of 38.65 mA/cm2and lower reflectivity of about 3.42% at wavelength range of 300 nm–1000 nm.

Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell

The crystalline silicon heterojunction structure adopted in photovoltaic modules commercialized as Panasonic's HIT has significantly reduced recombination loss, resulting in greater conversion efficiency. The structure of an interdigitated back contact was adopted with our crystalline silicon heterojunction solar cells to reduce optical loss from a front grid electrode, a transparent conducting oxide (TCO) layer, and a-Si:H layers as an approach for exceeding the conversion efficiency of 25%. As a result of the improved short-circuit current (J sc ), we achieved the world's highest efficiency of 25.6% for crystalline silicon-based solar cells under 1-sun illumination (designated area: 143.7 cm 2 ).

Radial p-n junction macroporous silicon solar cell on p-type upgraded metallurgical-grade silicon substrate

Macroporous silicon was fabricated on p-type upgraded metallurgical-grade (UMG) c-Si substrate with the resistivity of about 0.1–3 Ω cm, by metal-catalyzed electrochemical etching (MCECE). Ag nanoparticle catalyst was first fabricated on the c-Si substrate. Then, the electrochemical etching was performed. By optimizing the MCECE processes carefully, the macroporous silicon was successfully obtained with the pore width up to about 400 nm and the pore depth of 2–5 μm. After that, the radial amorphous/crystalline silicon heterojunction (SHJ) structure was prepared by conformally depositing a-Si:H i-layer and n-layer on the macroporous silicon via plasma-enhanced chemical vapor deposition (PECVD) at 200 °C, followed by depositing indium tin oxide front contact and Ag back contact via magnetron sputtering and thermal evaporation, respectively. As a result, the demonstrated radial p-n junction macroporous silicon solar cell with the unoptimized SHJ structure gave out a conversion efficiency of 3.67% under the standard AM1.5 illumination, which indicated a potential to make high performance radial p-n junction solar cell on UMG c-Si substrate.

Directional Breakdown of Metal/a-Si:H/c-Si Heterostructures and its Application to PROMs

AbstractThis paper gives the first report of a directional breakdown phenomenon present in specially constructed amorphous silicon / crystalline silicon heterojunction structures. After breakdown, the forward current can be greatly increased, but the reverse leakage current remains unchanged or even lowered. We use the Analysis of Microelectronic and Photonic Structures (AMPS) [1] device simulation tool to study this phenomenon and show that the structures can provide a new approach to producing Programmable Read Only Memory (PROM).