Efficiency Of Silicon Heterojunction Solar Cells Research Articles

Dipoles and defects caused by CO2 plasma improve carrier transport of silicon solar cells

AbstractCarrier‐selective contact is a fundamental issue for solar cells. For silicon heterojunction (SHJ) solar cells, it is important to improve hole transport because of the low doping efficiency of boron in amorphous silicon and the barrier stemming from valence band offset. Here, we develop a carbon dioxide (CO2) plasma treatment (PT) process to form dipoles and defect states. We find a dipole moment caused by longitudinal distribution of H and O atoms. It improves hole transport and blocks electron transport and thus suppresses carrier recombination. In the meantime, the CO2 PT process also results in defect states, which reduce passivation performance but improve hole hopping in the intrinsic amorphous layer. As a balance, an appropriate CO2 PT process at the i/p interface increases fill factor and power conversion efficiency of SHJ solar cells. We emphasize, based on sufficient evidences, this work finds a distinct role of the CO2 plasma in SHJ solar cells opposed to reported mechanisms.

Effects of Al2O3 Thickness in Silicon Heterojunction Solar Cells

In this paper, we investigate the effects of aluminum oxide (Al2O3) antireflection coating (ARC) on silicon heterojunction (SHJ) solar cells. Comprehensive ARCs simulation with Al2O3/ITO/c-Si structure is carried out and the feasibility to improve the short circuit current density (JSC) is demonstrated. Based on the simulation results, we apply Al2O3 ARC on SHJ solar cells, and the increasement in JSC to 1.5 mA/cm2 is observed with an Al2O3 layer thickness of 20 nm. It is because the total reflectance of SHJ solar cells is decreased by the shifting of the wavelength range on constructive and destructive light interference. As a result, we believe that the proposed Al2O3 ARC can support an effective engineering technic to increase JSC and efficiency of SHJ solar cells.

Reassessment of silicon heterojunction cell performance under operating conditions

Temperature coefficient (TC) is a critical operational parameter for solar modules. Amorphous/crystalline silicon heterojunction (SHJ) solar cell is usually considered as a good choice for installation in hot climates because of its lowest TC amongst silicon solar cells. Recently, we have found that light soaking can activate efficient boron doping in hydrogenated amorphous silicon (a-Si:H), which improves the efficiency of SHJ solar cells by enhanced field effect passivation and declined series resistance. In this study, we further report that high temperature can boost this light-induced doping effect. This implies that SHJ solar cells can generate more electricity than estimated by indoor International Electrotechnical Commission (IEC) standards in light abundant locations with hot climates. As a consequence, a 100 MW solar plant comprising SHJ solar modules will gain an extra income of more than 2 million U.S. dollars in a 25-year operation period. Additionally, for the first time, we demonstrate that the dark conductance of phosphorus-doped a-Si:H (n-a-Si:H) increases more greatly than that of boron-doped a-Si:H (p-a-Si:H) after one-sun illumination. This finding has an important implication for bifacial solar modules.

Intensive light soaking improves electricity generation of silicon heterojunction solar cells by the anomalous Staebler–Wronski effect

Silicon heterojunction (SHJ) solar cells are usually considered to be a good choice for power plants owing to their high power-conversion efficiency. A recent work reports light soaking can activate boron doping in hydrogenated amorphous silicon (a-Si:H), improving the efficiency of SHJ solar cells. Here we further show high light intensity can boost the dark conductivity of the phosphorus-doped a-Si:H and the boron-doped a-Si:H, this has significant implications, i.e., SHJ panels can generate more power than expected, especially for bifacial SHJ solar panels illuminated by intensive light soaking. In this regard, SHJ solar cells exhibit advantages in concentrator photovoltaic applications.

Intrinsic layer modification in silicon heterojunctions: Balancing transport and surface passivation

In this work, the intrinsic hydrogenated amorphous silicon (a-Si:H) layer of silicon heterojunction (SHJ) solar cells was modified to improve carrier transport while maintaining excellent passivation of the c-Si absorber surface. The microstructure of different multilayer intrinsic a-Si:H films was measured and its influence on contact resistance and passivation quality was investigated thoroughly. We show that a pronounced trade-off between passivation and transport exists and that this trade-off is governed by the a-Si:H properties close to the c-Si surface. The fact that the same trend was observed for hole and electron contact suggests that the transport barrier formed by the interfacial a-Si:H layer is governed by a higher resistivity of the void-rich interfacial layer or a less pronounced induced junction and not by asymmetric hole or electron barriers (band offsets, tunnel efficiencies, …). Modified intrinsic layers have been tested on cell level, resulting in a series resistance ( R s ) reduction by about 0.3 Ωcm 2 and an increase in fill factor ( FF ) by roughly 1.0 % abs . The power conversion efficiency ( η ) was improved by about 0.3 % abs with respect to our baseline. Further, the beneficial effect of a hydrogen plasma treatment (HPT) on passivation and transport of the hole contact was shown on device level. • Void-rich a-Si(i) is essential to prevent epitaxial growth for good surface passivation, but is also the main contributor to a-Si:H(i)-induced transport losses for both electron and hole contact. • Both thickness and microstructure of the void-rich a-Si:H(i) are decisive for the cell’s series resistance, small differences can have a huge impact on transport and surface passivation. • The microstructure of void-rich a-Si:H(i) is carefully tailored towards denser films via a-Si:H deposition parameters and a post-deposition hydrogen plasma treatment to significantly improve transport and the conversion efficiency of SHJ solar cells.

IZO/ITO Double-Layered Transparent Conductive Oxide for Silicon Heterojunction Solar Cells

Zinc-doped indium oxide (IZO)+tin-doped indium oxide (ITO) is proposed in this study as a double-layered transparent conductive oxide (TCO) for the application of silicon heterojunction (SHJ) solar cells. IZO is consecutively deposited via radio frequency sputtering at room temperature. The electrical and optical characteristics of IZO are examined depending on the thickness and the O₂ flow rate, which ranged from 0 to 5 sccm. The improved photocurrent characteristic for the proposed SHJ solar cell with IZO+ITO is achieved from 40.64 to 41.3 mA/cm² by an OPAL2 solar cell simulation. The sheet resistance of IZO slightly increases from 120 to 130 Ω/□ , which is caused by a decrease in the O₂ vacancy due to an increase in the O₂ flow rate from 1 to 5 sccm. The SHJ solar cells with 4.84 nm IZO deposited at the O₂ flow rate of 4 sccm show the lowest solar weighted reflectance of 4.36%. Therefore, these results indicate that IZO+ITO is expected to increase the efficiency of SHJ solar cells.

Influence of TCO and a-Si:H Doping on SHJ Contact Resistivity

Resistive losses in silicon heterojunction (SHJ) solar cells are partly linked to transport barriers at the amorphous silicon/crystalline silicon (a-Si:H/c-Si) and transparent conductive oxide (TCO)/a-Si:H interfaces. A key parameter is the position of the Fermi-level on either side of the junction which we modify by a systematic doping variation of the amorphous silicon and the transparent conductive oxide. We identify the charge carrier concentration to be the main driver for low contact resistance. For a-Si:H, this is achieved by using a sufficient but not too high doping gas concentration during deposition. For indium tin oxide (ITO) and aluminum zinc oxide (AZO), no or only a very low oxygen (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas concentration during deposition is needed. We show that a stack of low-oxygen ITO interlayer and an oxygen-rich ITO “bulk” layer is not only an effective means to combine efficient transport and low TCO absorption but also to improve the thermal stability of the a-Si:H/TCO/metal contact resistivity (ρ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sup> ). Such a layer stack helps to relax the constraints regarding the optoelectrical performance and improves the efficiency of SHJ solar cells.

Investigation of light-induced degradation in N-Type silicon heterojunction solar cells during illuminated annealing at elevated temperatures

In this paper, we study a light-induced degradation (LID) mechanism observed in commercial n-type silicon heterojunction (SHJ) solar cells at elevated temperatures using dark- and illuminated annealing for a broad range of illumination intensities (1–40 kWm−2) at temperatures from 25 to 180 °C. Three key results are identified. Firstly, an increase in solar conversion efficiency (η) of up to 0.3% absolute is observed after 3 min of dark annealing at 160 °C, attributed to improved surface passivation and a reduction in series resistance. Secondly, a temperature-dependent light-induced degradation behaviour is observed at temperatures as low as 85 °C under 1-sun equivalent illumination, with increasing degradation extent and rate for increasing temperatures. At 160 °C, an average η loss of 0.5 ± 0.3% absolute is observed after only 5 min and exceeding 0.8% in some cells. Thirdly, a subsequent light intensity-dependent recovery occurs with continued illumination exposure. Under 1-sun illumination at 160 °C, a reduction in net η loss up to 0.05 ± 0.1% absolute is observed after 2 h. Increasing the illumination intensity to 40 kWm−2 accelerates the recovery and can result in a net η improvement of 0.2% absolute at 150 °C within 100 s. The results suggest that attempts to improve the efficiency of SHJ solar cells using illuminated annealing could be detrimental to cell performance if not carefully optimised. Further investigation is required to identify the exact nature of the underlying defect mechanism(s) and develop appropriate mitigation strategies on commercially suitable timescales.

Effect of residual water vapor on the performance of indium tin oxide film and silicon heterojunction solar cell

In this study, we investigate the effect of residual H2O pressure (PH2O) on the structural, electrical, and optical properties of indium tin oxide (ITO) film and the current-voltage (I-V) performance of silicon heterojunction (SHJ) solar cells. The ITO films and SHJ solar cell were prepared under different PH2O (3.75 × 10−7 Pa, 6 × 10−7 Pa, 10.25 × 10−7 Pa) by varying the pumping time before starting the fabrication process. The experimental results revealed the crystallinity of ITO films was suppressed with the increase in PH2O. Further, the sheet resistance of ITO films was reduced from 103 Ω/sq to 60 Ω/sq due to the enhanced carrier mobility, and the transmittance in near- infrared (NIR) region exhibited a slight increase, but the conversion efficiency of SHJ solar cells dropped dramatically from 23.3% to 22.79% due to the degradation of fill factor (FF). The analysis on n-a-Si:H/ITO and ITO/Ag contact properties indicated that such FF degradation could be attributed to the increased specific contact resistance for the two contacts, when PH2O was high. Finally, SHJ solar cell with conversion efficiency of 23.53% was achieved by restraining the residual H2O vapor during ITO deposition along with the optimization of transmittance in NIR region by adjusting the amount of O2 flow.

Role of hydrogen in modifying a-Si:H/c-Si interface passivation and band alignment for rear-emitter silicon heterojunction solar cells

Boosting the contact property of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) film is pivotal to achieving high-efficiency silicon heterojunction (SHJ) solar cells. Here, the microstructure of a-Si:H(i) film is modified with hydrogen dilution ratio using hot wire chemical vapor deposition (HWCVD) for the application into rear-emitter SHJ solar cells. A higher hydrogen content associated with high valence band offset was found to decrease the fill factor FF for low dilution, while high interface defect densities related to epitaxial growth are responsible for the deterioration of both FF and open-circuit voltage VOC for high dilution. In particular, the most compact film prepared at a moderate dilution exhibits the most compact structure with most hydrogen located as isolated hydrogen rather than clustered hydrogen. Finally, high efficiency of SHJ solar cells up to 22.5% was yielded using the optimized a-Si:H(i) layer thanks to a significant enhancement of FF, which is attributed to improved passivation quality and rational band alignment at the a-Si:H(i)/c-Si interface. This work clearly interpreted the correlation between SHJ device parameters and a-Si:H(i)/c-Si interface properties, which might guide the design of a-Si:H passivation layers in pursuit of high-efficiency SHJ solar cells.

Effect of hydrogen plasma treatment on the passivation performance of TiOx on crystalline silicon prepared by atomic layer deposition

The authors report on the effect of hydrogen plasma treatment (HPT) on the passivation performance of titanium oxide (TiOx) on crystalline silicon (c-Si) fabricated by atomic layer deposition. Recently, TiOx has gathered attention as an electron-selective contact material for silicon heterojunction (SHJ) solar cells due to its preferable work function and band lineup. Moreover, TiOx has excellent light transmission properties due to its large bandgap energy. In order to improve the power conversion efficiency of SHJ solar cells with TiOx, it is necessary to enhance the passivation performance. The effective carrier lifetime representing the passivation performance is enhanced by HPT and amounts to 407.2 μs after HPT at 200 °C for 1 min. This value is twice as high as after forming gas annealing, which is the standard method to enhance the passivation performance of TiOx/c-Si heterostructures. Nuclear reaction analysis clarifies that the hydrogen concentration (CH) at the TiOx/c-Si interface of the HPT-processed sample is higher than that of an as-deposited sample and that the peak position of the CH distribution is shifted closer to the TiOx/c-Si heterointerface after HPT. Moreover, thermal desorption spectroscopy shows that Si–H and Si–H2 hydrogen bonds increase with HPT. These results indicate that the atomic hydrogen produced by the hydrogen plasma diffuses toward the TiOx/c-Si interface and terminate the local dangling bonds, which is responsible for the improved passivation performance.

Study of Double Layer Indium Tin Oxide in Silicon Hetero-Junction Solar Cells.

In this literature, we discussed the effect of anti-reflection coating of silicon heterojunction (SHJ) solar cells with different characteristics of double layered indium tin oxide (ITO/ITO) structure. Firstly, the OPAL 2 simulation was performed to optimize the values of the photo generation-current density of ITO/ITO/Si device structures. Afterwards, experimental work was conducted by depositing ITO on the SHJ solar cell to analyze the anti-reflection coating effect. ITO was deposited on the SHJ solar cell for 90 to 180 seconds by varying the oxygen flow rate. The highest short-circuit current density of 39.25 mA/cm² was obtained when ITO was deposited for 150 seconds, which was higher than the short-circuit current density of non-deposited cell of ITO (38 mA/cm²). The efficiency of the SHJ solar cell increased by about 2% after additional ITO deposition to 20.75%, which was due to the improvement of short-circuit current density by ITO deposition. The double layer ITO helped to improve the efficiency of SHJ solar cell by increasing light absorption in a silicon wafer.

Effects of oxygen related thermal donors on the performance of silicon heterojunction solar cells

Absract The influence of oxygen-related thermal donors (TDs) on the performance of silicon heterojunction (SHJ) solar cells was explored experimentally in this paper. It is found that a certain number of thermal donors could do much harm to the performance of SHJ solar cells. The efficiency of SHJ solar cells is usually reduced by a value of ~ 1% absolute in the case of thermal donors with a concentration of nearly 1015 cm−3. The microwave photoconductance decay and Hall Effect studies have proved that the TDs can significantly reduce the carrier lifetime of n-type silicon substrate, but have no influence on the carrier mobility. Deep level transient spectroscopy (DLTS) measurements have further demonstrated that the TDs cause an energy level at Ec-0.13 eV with carrier capture cross-section of 10−15 cm2, which is responsible for the reduction of carrier lifetime and solar cell efficiency.

Influence of interface states, conduction band offset, and front contact on the performance of a-SiC:H(n)/c-Si(p) heterojunction solar cells**Project supported by the National High Technology Research and Development Program of China (Grant No. 2012AA050301) and Scientific Research of Hebei Education Department, China (Grant No. QN2017135).

P-type silicon heterojunction (SHJ) solar cells with a-SiC:H(n) emitters were studied by numerical computer simulation in this paper. The influence of interface states, conduction band offset, and front contact on the performance of a-SiC:H(n)/c-Si(p) SHJ solar cells was investigated systematically. It is shown that the open circuit voltage () and fill factor (FF) are very sensitive to these parameters. In addition, by analyzing equilibrium energy band diagram and electric field distribution, the influence mechanisms that interface states, conduction band offset, and front contact impact on the carrier transport, interface recombination and cell performance were studied in detail. Finally, the optimum parameters for the a-SiC:H(n)/c-Si(p) SHJ solar cells were provided. By employing these optimum parameters, the efficiency of SHJ solar cell based on p-type c-Si was significantly improved.