Crystalline Silicon Heterojunction Research Articles

27.09%-efficiency silicon heterojunction back contact solar cell and going beyond

Crystalline-silicon heterojunction back contact solar cells represent the forefront of photovoltaic technology, but encounter significant challenges in managing charge carrier recombination and transport to achieve high efficiency. In this study, we produced highly efficient heterojunction back contact solar cells with a certified efficiency of 27.09% using a laser patterning technique. Our findings indicate that recombination losses primarily arise from the hole-selective contact region and polarity boundaries. We propose solutions to these issues and establish a clear relationship between contact resistivity, series resistance, and the design of the rear-side pattern. Furthermore, we demonstrate that the wafer edge becomes the main channel for current density loss caused by carrier recombination once electrical shading around the electron-selective contact region is mitigated. With the advanced nanocrystalline passivating contact, wafer edge passivation technologies and meticulous optimization of front anti-reflection coating and rear reflector, achieving efficiencies as high as 27.7% is feasible.

Low-cost deposition of tunable band gap Zn(O,S) as electron transport layer for crystalline silicon heterojunction solar cells

In recent years, the research on silicon heterojunction (HJT) solar cells based on dopant-free contacts has experienced rapid development. Zn(O,S) is a low work function n-type semiconductor compound with tunable band gap that can be used as the electron transport layer (ETL) in HJT solar cells. In our work, we choose Zn(O,S) as ETL and deposit it via the low-cost chemical bath deposition (CBD) method. Uniform and fast growth of Zn(O,S) films can be obtained by regulating the concentration of the complexing agent and the ratio of reactants to reduce the generation of impurities. To further achieve band matching with c-Si, the Zn(O,S) films are processed with air-annealing to modify the elemental ratios. The air-annealing lowers the interfacial electron transport barrier, which promotes charge separation and transport, thus reducing carrier recombination at the interface. Finally, the PCE of HJT solar cell based on CBD-Zn(O,S) ETL is close to 15 %, providing a new potential route for the development of low-cost HJT solar cells.

Heavy Boron-Doped Silicon Tunneling Inter-layer Enables Efficient Silicon Heterojunction Solar Cells.

P-type hydrogenated nanocrystalline silicon (nc-Si:H) has been used as a hole-selective layer for efficient n-type crystalline silicon heterojunction (SHJ) solar cells. However, the presence of an additional valence band offset at the interface between intrinsic amorphous hydrogenated silicon and p-type nc-Si:H films will limit the hole carrier transportation. In this work, it has been found that when a heavily boron-doped silicon oxide layer deposited with high hydrogen dilution to silane (pB) was inserted into their interface, the fill factor of SHJ solar cells increases 3% absolutely because of the reduced valence band offset and the increased opportunity to provide a hopping tunnel assisted by the doping energy level and valence band tail states. Furthermore, the additional boron incorporation in intrinsic amorphous silicon adjacent to pB helps to enhance the built-in electric field, thus increasing the hole selectivity. By these means, the power conversion efficiency was improved from 23.9% to approximately 25%.

Bifacial silicon heterojunction solar cells using transparent‐conductive‐oxide‐ and dopant‐free electron‐selective contacts

AbstractThe development of transparent electron‐selective contacts for dopant‐free carrier‐selective crystalline silicon (c‐Si) heterojunction (SHJ) solar cells plays an important role in achieving high short‐circuit current density (JSC) and consequently high photoelectric conversion efficiencies (PCEs). This becomes even more important when focusing on the development of bifacial solar cells. In this study, bifacial SHJ solar cells using a transparent‐conductive‐oxide‐free and dopant‐free electron‐selective passivating contacts are developed, showing a JSC bifaciality of up to 97%. Intrinsic ZnOX layer deposited by atomic layer deposition was used in this structure, which simultaneously provides negligible passivation loss after annealing and enables a low contact resistivity on the electron‐selective contact. With both side finger metal electrodes contact, this bifacial solar cell shows an efficiency of 21.2% under front‐side irradiation and 20.4% under rear‐side irradiation, resulting in an estimated output power density of 24.1 mW/cm2 when considering rear‐side irradiance of 0.15 sun.

Highly Efficient and Highly Flexible Thin Crystalline Silicon Heterojunction Solar Cells Based on Dopant-Free Carrier-Selective Contacts Fabricated with Simple Processes

Highly Efficient and Highly Flexible Thin Crystalline Silicon Heterojunction Solar Cells Based on Dopant-Free Carrier-Selective Contacts Fabricated with Simple Processes

A novel silver-doped nickel oxide hole-selective contact for crystalline silicon heterojunction solar cells

A novel silver-doped nickel oxide hole-selective contact for crystalline silicon heterojunction solar cells

Exceptional Hole-Selective Properties of Ta2O5 Films via Sn4+ Doping for High Performance Silicon Heterojunction Solar Cells.

Carrier-selective passivating contacts using transition metal oxides (TMOs) have attracted great attention for crystalline silicon (c-Si) heterojunction solar cells recently. Among them, tantalum oxide (Ta2O5) exhibits outstanding advantages, such as a wide bandgap, good surface passivation, and a small conduction band offset with c-Si, which is typically used as an electron-selective contact layer. Interestingly, it is first demonstrated that solution-processed Ta2O5 films exhibit a high hole selectivity, which blocks electrons and promotes hole transport simultaneously. Through the ozone pre-treatment of Ta2O5/p-Si interface and optimization of the film thickness (≈9nm), the interfacial recombination is suppressed and the contact resistivity is reduced from 178.0 to 29.3mΩcm2. Moreover, the Sn4+ doping increases both the work function and oxygen vacancies of the film, contributing to the improved hole-selective contact performance. As a result, the photoelectric conversion efficiencies of Ta2O5/p-Si heterojunction solar cells are significantly improved from 14.84% to 18.47%, with a high thermal stability up to 300°C. The work has provided a feasible strategy to explore new features of TMOs for carrier-selective contact applications, that is, bipolar carrier transport properties.

Optical, Electrical and Structural Properties of ITO/IZO and IZO/ITO Multilayer Transparent Conductive Oxide Films Deposited via Radiofrequency Magnetron Sputtering

In this study, we investigated the potential of multilayer TCO structures, specifically those made up of Indium Tin Oxide (ITO) and Indium Zinc Oxide (IZO), for crystalline silicon heterojunction solar cells (SHJ). We used the radiofrequency (RF) magnetron sputtering method to deposit various thin-film structures under various deposition temperatures and evaluated their electrical, optical, and morphological properties. The objective was to obtain films with lower sheet resistances and higher transmittances than those of single-layer thin films. Our results show that the ITO/IZO/ITO/IZO/ITO multilayer film structure deposited at 200 °C achieves the best sheet resistance of 18.5 Ohm/sq and a high optical transmittance of over 90% at a 550 nm wavelength. This indicates that multilayer TCO structures have the potential to be more optically and electrically efficient, and that they can improve the performance of optoelectronic devices. Finally, a power conversion efficiency of 17.46% was obtained for a silicon heterojunction (SHJ) solar cell fabricated using an ITO/IZO/ITO/IZO/ITO multilayer film structure deposited at 200 °C as a front TCO. Our study provides valuable insights into the field of TCOs and offers a promising avenue for future research.

Beyond 25% efficient crystalline silicon heterojunction solar cells with hydrogenated amorphous silicon oxide stacked passivation layers for rear emitter

Hydrogenated amorphous silicon oxide (a-SiOx:H) alloy is considered as a promising alternative to hydrogenated amorphous silicon (a-Si:H) in crystalline silicon heterojunction (SHJ) solar cells due to its reduced parasitic absorption induced by the incorporation of oxygen. However, the large valence band offset (ΔEV) between a-SiOx:H and crystalline silicon (c-Si) usually leads to low fill factor (FF) for the solar cells. Here, we designed a trilyer a-SiOx:H(i) stacked passivation scheme underneath the rear emitter (p) for the SHJ solar cell on n-type c-Si wafer via depositing the a-SiOx:H(i) layers with different oxygen contents by plasma-enhanced chemical vapor deposition (PECVD). Highly efficient SHJ solar cells with the average efficiency of 25.37% were obtained by optimizing the trilayer a-SiOx:H(i) passivation structure with an oxygen-rich layer inserted between two oxygen-less layers. Comparing to the solar cells with complete a-Si:H passivation layers, higher short-circuit current density (JSC) and open-circuit voltage (VOC) of the solar cells were achieved, which indicated excellent passivation and low parasitic absorption contributed from the a-SiOx:H(i) stacked layers. At the same time, FF of the solar cells was also increased slightly. The corresponding improvement mechanisms were analyzed carefully. As a demonstration, the highest efficiency of the SHJ solar cells passivated by the trilayer a-SiOx:H(i) scheme for rear emitter was up to 25.44%.

Self‐Assembled Monolayer Ti3C2TxMXene as Electron Selective Heterocontact for Crystalline Silicon Solar Cells

Carrier‐selective contact is an essential strategy to improve the power conversion efficiency (PCE) of crystalline silicon (c‐Si) solar cells by enhancing carrier transport and reducing recombination. Many two‐dimensional transition metal carbides/nitrides (MXene) possess excellent electrical conductivity and tunable work function, enabling them as potential carrier‐selective contact materials for c‐Si solar cells. Herein, low work function (3.71 eV) monolayer Ti3C2Txthin films are fabricated with an ordered arrangement of nanosheets by interfacial self‐assembly method, and the Ti3C2Tx/n‐type c‐Si heterostructure achieves extremely low contact resistivity (22.64 mΩ cm2). Dopant‐free heterojunction crystalline silicon solar cell with self‐assembled monolayer Ti3C2Txelectron‐selective contact is reported for the first time, showing the highest PCE of 16.46% among MXene monolayer‐based c‐Si solar cells, which is mainly attributed to the greatly increased fill factor and open circuit voltage by the accession of self‐assembled monolayer Ti3C2Txas electron transport layer. This work opens up the possibility of applying MXene as electron transport material for photovoltaic devices.

Open circuit voltage reduction due to recombination at the heterojunction solar cell edge

Today to achieve high efficiency solar cells, the crystalline silicon (c-Si) heterojunction (HJ) represents one of the best available options. The key factor of its success is the high open circuit voltage (Voc) achievable, because of the excellent surface passivation due to the intrinsic amorphous silicon (a-Si:H) layers. During cells manufacturing, the a-Si:H film deposition is simultaneously performed on both c-Si wafer sides, and consequently also on the edges of the wafer. However, the wafer edges could result non-passivated in many cases such as the so-called shingling manufacturing route.Moreover, at laboratory level it is quite common to manufacture small area cells from larger wafers. When a silicon edge is left uncovered by cutting, a recombining region is created due to the silicon non-passivated surface. This, in principle, leads to a reduction in the Voc of the cell.Nevertheless, this is not experienced for large area cells cut in half but it is commonly observed that cutting silicon HJ edges results in a lowering in Voc. In this work we have analyzed the correlation between the Voc, the cell area and the recombining surface introduced by cutting the cell into a smaller one. We have monitored the Voc as a function of the cell area during time, and we also have investigated the possibility of a re-passivation of the cell edges by depositing a thick a-Si:H layer after masking the sun exposed cell surface, exploring different deposition conditions to avoid re-annealing of the existing a-Si:H layers.

Tunable Hole‐Selective Transport by Solution‐Processed MoO3−x Via Doping for p‐Type Crystalline Silicon Solar Cells

Molybdenum oxide (MoO3−x, x < 3) has been successfully used as an efficient hole‐selective contact material for crystalline silicon heterojunction solar cells. The carrier transport capability strongly depends on its work function, that is, oxygen vacancies; however, there are lack of effective methods to modulate the multiple oxidation states. Herein, the oxidation states of solution‐processed MoO3−x by doping Nb5+ to improve its hole‐selective contact performance with silicon are tuned. With the optimum doping concentration of 5%, both the reduced Mo5+ and oxygen vacancies increase, resulting in a decrease in the contact resistivity between the MoO3−x film and p‐type silicon from 161.1 to 62.9 mΩ·cm2 and an increase of the effective carrier lifetime from 165.4 to 391.0 μs simultaneously. Similarly, the doping of Ta5+ or V5+ in MoO3−x improves the passivated contact performance with silicon, while the former increases the concentration of oxygen vacancies and the latter reduces it. The solar cell with the structure of Ag/MoO3−x:Nb/p‐Si exhibits a conversion efficiency of 18.37%, which is the highest so far reported for the solution‐processed MoO3−x/silicon heterojunction. This work demonstrates a feasible strategy of tuning hole selectivity in MoO3−x by doping for high‐efficiency solar cells and other optoelectronic device applications.

Improved Performance of Silicon-Germanium Solar Cell Based on Optimization of Layer Thickness

Electrical energy has become an essential part of our life. Therefore, its supply must be sustainable, economical, and environment-friendly. The conversion of sunlight into electricity is made possible through the solar cell, a semiconductor device, however, the conversion efficiency of these cells is low which can be further improved. This research work presents the design and performance analysis of silicon-germanium (Si-Ge) solar cells. Amorphous silicon / crystalline silicon Heterojunction (a-Si/c-Si HIT) solar, Ge, Si-Ge alloy with 25% Si concentration solar cells are designed in Afors-Het software. An improved conversion efficiency (?) of 25.23%, 5.125%, and 11.53%, respectively is achieved.

Monolithic perovskite/silicon tandem solar cells with optimized front zinc doped indium oxides and industrial textured silicon heterojunction solar cells

The power conversion efficiency (PCE) of monolithic perovskite/silicon tandem solar cells has surpassed that of silicon single-junction solar cells recently. Industrial textured crystalline silicon heterojunction solar cells are considered as competitive candidate bottom cells because of their high efficiency and simple process flow. Meanwhile, the front transparent conductive window layers with industrial compatibility and low processing temperature are worth investigating in detail. In this work, zinc-doped indium oxide (IZO) thin films achieved by room-temperature sputtering are studied as a function of sputtering powers. The IZO thin film with the resistivity and electron mobility of 7.2 × 10−4 Ω cm and 40.03 cm2/Vs are obtained under 85 W, which is implemented into the monolithic perovskite/silicon tandem solar cells. A PCE of 19.38% is realized based on the industrial textured silicon heterojunction bottom solar cell and optimized IZO films.

Optimization of the Electrode Formation Mechanism for Crystalline Silicon Heterojunction Solar Cells

The screen-printing process for making good contact of electrodes with the top layer of solar cells is crucial for enhancing the electrical properties of a solar cell. This paper reports the experimental approach adopted for the process of electrode formation and the resulting shape of electrodes in silicon-based heterojunction (SHJ) solar cells. It was observed that good contact between electrodes and the top transparent conductive oxide (TCO) layer strongly depends on the squeegee pressure, curing temperature, and curing time. By optimizing the squeegee pressure at 0.350 MPa, snap-off distance of 1.4 mm, squeegee speed of 80 mm sec−1, curing temperature of 180 °C, and curing time of 30 min, respectively for which the height to width ratio (aspect ratio) of the fabricated electrodes was achieved around 0.66. The results have been verified through 3D laser profiler, field emission scanning electron microscopy (FE-SEM), transfer length method (TLM), and Light current-voltage (LIV) measurements. The SHJ solar cells were fabricated using an optimized condition and successfully achieved splendid properties of short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and efficiency (η) up to 40.57 mA cm−2, 723 mV, 81.03%, and 23.79%, respectively.

HAXPESによる結晶シリコンヘテロ接合型太陽電池の課題解明

HAXPESによる結晶シリコンヘテロ接合型太陽電池の課題解明

Enabling Transparent‐Conductive‐Oxide Free Efficient Heterojunction Solar Cells by Flexibly Using Dopant‐Free Contact

AbstractMinimizing the optical parasitic absorption loss in hydrogenated amorphous silicon (a‐Si:H) and transparent conductive oxide (TCO) layers is considered an effective approach to further improve the power conversion efficiencies (PCEs) of crystalline silicon heterojunction (SHJ) solar cells. In this work, carrier‐selective passivating contacts for both polarities, e.g., alkali fluoride/aluminum electron‐selective contact and transition metal oxide/silver hole‐selective contact, are used to completely replace the doped a‐Si:H layers in SHJ solar cell, forming a totally TCO‐ and dopant‐free asymmetric structure. By minimizing the charge carrier transporting and recombination losses on the front‐side electron‐selective contact through interface engineering, an improved photovoltaic performance with a short‐circuit current density of 40.5 mA cm−2, an open‐circuit voltage of 0.709 V, and a fill factor of 79.6% is realized, resulting in a final PCE exceeding 22.9%. The successful demonstration of this work provides an effective way to further boost the efficiency, as well as reduce the cost of SHJ solar cells with a TCO‐free and doped a‐Si:H‐free configuration, using a moderate and simplified fabrication process.

Ion-induced interface defects in a-Si:H/c-Si heterojunction: possible roles and kinetics of hot mobile hydrogens

Interface defects in state-of-the-art semiconductors have a strong impact on device performance. These defects are often generated during device fabrication, in which a variety of plasma processing is used for deposition, etching and implantation. Here, we present the ion-induced defects in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) heterojunction. The experiments of argon ion (Ar+) irradiation over an a-Si:H/c-Si stack are systematically performed. The results suggest that the defects are generated not only by the impact of Ar+ (i.e. well-known effects), but also by another unique effect associated with “hot” mobile hydrogens (H). The mobile H atoms generated near the a-Si:H surface by the impact of Ar+ diffuse deeper, and they generate the a-Si:H/c-Si interface defects such as dangling bonds. The diffusion length of mobile H is determined to be 2.7 ± 0.3 nm, which indicates efficient reactions of mobile H with weak bonds in an a-Si:H network structure.

Multilayer emitter of molybdenum oxide/silver/molybdenum oxide thin films for silicon heterojunction solar cells: Device fabrication and electrical characterization

In a simplified heterojunction crystalline silicon (c-Si) solar cell, a molybdenum oxide/silver/molybdenum oxide (MoOx/Ag/MoOx) stack has been investigated as an alternative to the well-known p-type amorphous silicon hydrogenated (a-Si: H) layer or a single layer of MoOx. The optical characteristics of the MoOx/Ag/MoOx multilayer were studied and found to have acceptable features for embedding on the solar cell's front side. Using a straightforward fabrication technique, thermal evaporation, a device structure composed of Al/Ag/MoOx/Ag/MoOx/n-Si/MoOx/Mg/Al has been fabricated. Impedance spectroscopy measurements were performed and Nyquist plots revealed single semicircles in the frequency range (102 Hz - 106 Hz) and at different temperatures, indicating the formation of a junction between MoOx/Ag/MoOx and n-Si substrate and the establishment of good Ohmic contacts for the front/rear electrodes. Moreover, capacitance-voltage characteristics were used to determine the temperature dependency of the built-in potential, doping concentration, and depletion width. The temperature-dependent current-voltage plots for the cell suggested that the predominant carrier transport mechanisms in the forward bias regime are trap-assisted tunneling at V> 0.35 V and e-h pair recombination at 0.35–0.75 V. For the reverse bias regime, carriers transport is dominated by the current generation in the space-charge region. Additionally, the photovoltaic performance of the solar cell was measured under illumination.

Greatly enhanced hole collection of MoO x with top sub-10 nm thick silver films for gridless and flexible crystalline silicon heterojunction solar cells.

Greatly enhanced hole collection of MoOx is demonstrated experimentally with a top sub-10 nm thick Ag film, allowing for an efficient dopant-free contacted crystalline silicon (c-Si) heterojunction solar cell without a front grid electrode. With the removal of shadows induced by the front grid electrode, the gridless solar cell with the MoOx/Ag hole-selective contact (HSC) shows an increment of ∼8% in its power conversion efficiency (PCE) due to the greatly improved short-circuit current density (Jsc) as well as the almost undiminished fill factor (FF) and open-circuit voltage (Voc), while the gridless solar cells with the conventional MoOx/ITO and pure MoOx HSCs exhibit ∼20% and ∼43% degradations in PCE due to the overwhelming decrease in their FF and Jsc, respectively. Through systematic characterizations and analyses, it is found that the ultrathin Ag film (more conductive than ITO) provides an additional channel for photogenerated holes to transport on MoOx, contributing to the great enhancement in the hole collection and the great suppression of the shunt loss in the gridless solar cells. A 50 μm thick gridless c-Si heterojunction solar cell with the MoOx/Ag HSC is 75% thinner but is 86% efficient compared to its 200 μm thick counterpart (while the 50 μm thick gridless solar cell with the MoOx/ITO HSC is much less efficient). It is over 82% efficient after being bent to a curvature radius as small as 4 mm, also showing superior mechanical flexibility to its counterpart with the MoOx/ITO HSC. Our MoOx/Ag double-layer HSC can be easily fabricated through thermal evaporation without breaking the vacuum, saving both the time and cost of the fabrication of the whole device. Therefore, this work provides a guide for the design of efficient HSCs for high-efficiency, low-cost, and flexible solar cells.