HIT Solar Cells Research Articles

Simulation of High-Efficiency Crystalline Silicon Solar Cells With Homo–Hetero Junctions

A novel solar cell structure consisting of both homojunction and heterojunction (homo-hetero junctions), which possesses a potential to realize high photoelectric conversion efficiency, is investigated by the numerical simulation tool AFORS-HET. We demonstrate that the homo-hetero junctions solar cell has a higher fill factor than the solar cell with heterojunction with intrinsic thin layer (HIT), due to the reduced series resistance, which results in a better conversion efficiency, whereas their interfacial density of states (DOS) values are identical. Through a detailed study of the effect of inserting a homojunction, we find that the field-effect passivation can adequately explain the interesting behaviors that the open-circuit voltage increases and the emitter saturation current density declines when increasing the doping concentration in the P-type crystalline silicon layer. In addition, as compared with the HIT solar cell, the homo-hetero junctions solar cell is less sensitive to the DOS due to the field-effect passivation, leading to a comparable open-circuit voltage even if its total interfacial DOS is 10 times higher.

The Research and Development of the Third Generation of Photovoltaic Modules

ABSTRACTIn order to make high efficiency and low cost solar cell modules, the concept of third generation of photovoltaic modules have been provided. The first generation solar cell: Crystal Si solar cell including single crystal and poly-crystal Si solar cell；The second generation solar cell：Thin film solar cell including Si base thin film, CIGS, CdTe and III-V thin films; The third generation solar cell is the future high efficiency and low cost solar cell modules, such as low cost quantum dots solar cell, Si base thin film tandem and triple cell modules, III-V solar cell on Si, HIT solar cell and nanotechnology with no vacuum technique such as printable technologies and etc. This paper reviewed the advantages and disadvantages of each generation of the solar cell modules and technologies and discussed the research and development of the third generation of photovoltaic modules including the detail technology developments.

梯度掺杂对n型异质结太阳能电池性能的影响

梯度掺杂对n型异质结太阳能电池性能的影响

High-Efficiency Heterojunction with Intrinsic Thin-Layer Solar Cells: A Review

Heterojunction with intrinsic thin layer (HIT) solar cell has attracted attention of photovoltaic research community due to its low process temperature, as compared to crystalline silicon (c-Si) solar cell and relatively high efficiency (η). In this solar cell structure, thin intrinsic amorphous silicon layers are used as surface passivator, which also acts as buffer layer at the top as well as bottom surfaces of n-type crystalline silicon (c-Si) wafer. A thin p-type a-Si top layer acts as an emitter while highly doped n-type bottom layer is used as a back surface field. There have been suggestions that the device performance comes with significant tunnelling transport of the charge carriers across junction barriers, whereas other investigation suggests that the diffusion of the carriers may be adequate. Various experimental as well as theoretical investigations were carried out to characterize and improve performance of HIT solar cell. For example, it was reported that a thicker p-type emitter (up to 40nm) shows higher open circuit voltage, although short circuit current density (Jsc) and cell efficiency decreases. Recent investigation showed that when the p-type and i-type layer thickness increased (within 4 nm) the open circuit voltage (Voc) as well as the efficiency improves. Using thinner c-Si absorbed is another interesting aspect in which a light trapping scheme can be combined to achieve a better performance characteristics. There has been several theoretical as well as experimental investigations on the use of various types of emitter layer and intrinsic layer of the cell, and their role on the Jsc, Voc, η and fill factor (FF). Investigations of HIT solar cell on p-type c-Si wafers have also been carried out. In this article we will present a brief review on some of the important aspects of the HIT solar cell.

Panasonic’s Thin Film Silicon Technologies for Advanced Photovoltaics

ABSTRACTWe have fabricated high-efficiency a-Si/µc-Si tandem solar cells and modules with a very high µc-Si deposition rate using Localized Plasma Confinement CVD to give very high-rate deposition (>2.0 nm/s) of device-grade µc-Si layers. For further progress in productive plasma-CVD techniques, we have studied plasma phenomena by combining newly developed plasma simulation and plasma diagnosis techniques that reveal the importance of non-emissive atomic hydrogen. We also have proposed a model of defective µc-Si formation on highly textured substrates in which the atomic H in plasma is assumed to play an important role. We are also developing a non-vacuum deposition technique that we term “Liquid Si Printing.” A new record conversion efficiency for HIT solar cells of 24.7% has been achieved using a very thin c-Si wafer (Thickness: 98 µm, Area: 102 cm2).

Simulation approach for optimization of device structure and thickness of HIT solar cells to achieve ∼27% efficiency

Optimization of thicknesses of n-type a-Si:H emitter layer, front a-Si:H i-layer and p-type c-Si base wafer as well as optimum heterojunction (HJ) and HJ with intrinsic layer (HIT) solar cells are performed using AFORS-HET simulation software. By optimization, we realized record efficiency of 27.02% in bifacial HIT solar cell at emitter layer, front i-layer and c-Si base wafer thicknesses of 6nm, 3nm and 200μm, respectively. Interestingly when the thickness of c-Si wafer was reduced to 58μm, while keeping the thicknesses of emitter and front i-layers as same as 6nm and 3nm, respectively, efficiency in bifacial cell got reduced to 26.45%. All cell structures generated highest efficiency at emitter layer and front i-layer thicknesses of 6nm and 3nm, respectively. However, optimum c-Si base wafer thickness was varied according to the following cell structures: simple HJ and HIT cells showed highest efficiency at 300μm, HJ with BSF layer cell at 98μm, HIT with BSF layer at 58μm. It is worth mention that, efficiency in bifacial cell at 58, 98 and 200μm was varied nominally. These optimizations may help in producing low cost high efficiency HJ and HIT solar cells technology.

Crystalline silicon interface passivation improvement with a-Si1−xCx:H and its application in hetero-junction solar cells with intrinsic layer

Excellent passivation of an n-type Czochralski crystalline silicon surface is made possible by the deposition of hydrogenated silicon carbide (Si1−xCx:H) layers in the electron cyclotron resonance chemical vapor deposition. We investigate the structural effect with various CH4/SiH4 dilution ratios, and the lowest effective surface recombination velocity (21.03 cm/s) that can be obtained. We also demonstrate that the Voc can be improved more than 200 mV by inserting Si1−xCx:H layers to form hetero-junction with intrinsic thin layer (HIT) solar cells. The conversion efficiency of the planar HIT solar cell with μc-Si emitter can reach 13%.

Effects of LiF/Al back electrode on the amorphous/crystalline silicon heterojunction solar cells

Abstract To improve the quantum efficiency (QE) and hence the efficiency of the amorphous/crystalline silicon heterojunction solar cell, we have employed a LiF dielectric layer on the rear side. The high dipole moment of the LiF reduces the aluminum electrode's work–function and then lowers the energy barrier at back contact. This lower energy barrier height helps to enhance both the operating voltage and the QE at longer wavelength region, in turn improves the open-circuit voltage (Voc), short-circuit current density (Jsc), and then overall cell efficiency. With optimized LiF layer thickness of 20 nm, 1 cm2 heterojunction with intrinsic thin layer (HIT) solar cells were produced with industry-compatible process, yielding Voc of 690 mV, Jsc of 33.62 mA/cm2, and cell efficiencies of 17.13%. Therefore LiF/Al electrode on rear side is proposed as an alternate back electrode for high efficiency HIT solar cells.

Study on the ITO work function and hole injection barrier at the interface of ITO/a-Si:H(p) in amorphous/crystalline silicon heterojunction solar cells

For this study we focused on the front contact barrier height of HIT (ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)) solar cell. The ITO films with low resistivity of 1.425×10−4Ωcm were deposited by pulsed DC magnetron sputtering as a function of substrate temperature (Ts). There were improvement in ΦITO from 4.15 to 4.30eV and delta hole injection barrier from 0 to 0.129eV for the HIT solar cell. The results show that the high values of ΦITO and the delta hole injection barrier at the front interface of ITO/p-layer lead to an increase of open circuit voltage (Voc), fill factor (FF) and efficiency (η). The performance of HIT device was improved with the increase of Ts and the best photo voltage parameters of the device were found to be Voc=635mV, FF=0.737 and η=14.33% for Ts=200°C.

Future directions for higher-efficiency HIT solar cells using a Thin Silicon Wafer

To reduce the power generation cost of heterojunction with intrinsic thin layer (HIT) solar cells, it is necessary to use a thinner crystalline silicon wafer, as well as to improve the conversion efficiency. We have experimentally confirmed that VOC of the HIT solar cell increases with decreasing the wafer thickness, and can reach a very high VOC of 747mV with a 58-μm-thick wafer owing to a sufficiently low surface recombination velocity. We also indicate the future directions for improving the efficiency. The uniformization of the texture size of the silicon surface and reduction of the carrier density in TCO film while maintaining an equal or lower conductivity are effective for improving the optical confinement.

Low defect interface study of intrinsic layer for c-Si surface passivation in a-Si:H/c-Si heterojunction solar cells

High quality hydrogenated intrinsic amorphous silicon [a-Si:H(i)] layer with adequate hydrogen content and lesser void fraction is the key to obtaining good surface passivated crystalline silicon (c-Si), with high open-circuit voltage (Voc), which will ultimately make the heterojunction with intrinsic thin layer (HIT) solar cell highly efficient. In this study, we performed good surface passivation of a HIT solar cell by depositing a-Si:H(i) layers at different working pressures from 26.7 to 107Pa by using very high frequency of 60MHz plasma-enhanced chemical vapor deposition. Based on spectroscopic ellipsometry and gas depletion analysis, we discuss the influence of the working pressure on the deposition mechanism, interface passivation and ultimately cell efficiency. Highest minority lifetime of about 4ms was achieved at the highest working pressure of 107Pa. The decrease in working pressure results in less denser and/or incorporation of epitaxy layer inside the a-Si:H(i) films, and leads to decrease in c-Si surface passivation. The performance of heterojunction solar cell device was improved with the increase of working pressure and the best photo voltage parameters of the device were found to be Voc of 647mV, short-circuit current density of 32.28mA/cm2 and efficiency of 15.57% at working pressure of 107Pa.

Modeling solar cells: A method for improving their efficiency

Abstract After a brief discussion on the theoretical basis for simulating solar cells and the available programs for doing this we proceed to discuss two examples that show the importance of doing numerical simulation of solar cells. We shall concentrate in silicon Heterojunction Intrinsic Thin film aSi/cSi (HIT) and CdS/CuInGaSe 2 (CIGS) solar cells. In the first case, we will show that numerical simulation indicates that there is an optimum transparent conducting oxide (TCO) to be used in contact with the p-type aSi:H emitter layer although many experimental researchers might think that the results can be similar without regard of the TCO film used. In this case, it is shown that high work function TCO materials such as ZnO:Al are much better than smaller work function films such as ITO. HIT solar cells made with small work function TCO layers (

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.

Ultrathin Oxide Passivation Layer by Rapid Thermal Oxidation for the Silicon Heterojunction Solar Cell Applications

It is difficult to deposit extremely thin a-Si:H layer in heterojunction with intrinsic thin layer (HIT) solar cell due to thermal damage and tough process control. This study aims to understand oxide passivation mechanism of silicon surface using rapid thermal oxidation (RTO) process by examining surface effective lifetime and surface recombination velocity. The presence of thin insulating a-Si:H layer is the key to get highVocby lowering the leakage current (I0) which improves the efficiency of HIT solar cell. The ultrathin thermal passivation silicon oxide (SiO2) layer was deposited by RTO system in the temperature range 500–950°C for 2 to 6 minutes. The thickness of the silicon oxide layer was affected by RTO annealing temperature and treatment time. The best value of surface recombination velocity was recorded for the sample treated at a temperature of 850°C for 6 minutes at O2flow rate of 3 Lpm. A surface recombination velocity below 25 cm/s was obtained for the silicon oxide layer of 4 nm thickness. This ultrathin SiO2layer was employed for the fabrication of HIT solar cell structure instead of a-Si:H, (i) layer and the passivation and tunneling effects of the silicon oxide layer were exploited. The photocurrent was decreased with the increase of illumination intensity and SiO2thickness.

The characterization of hydrogenated amorphous silicon and epitaxial silicon thin films grown on crystalline silicon substrates by using spectroscopic ellipsometry

Two series of silicon films on c-Si substrates with different thicknesses are deposited by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD). The structures of samples are investigated by spectroscopic ellipsometry (SE) with fixed angle. The results show that the films are all amorphous for the first series of samples. In such a case an abrupt a-Si:H/c-Si heterojunction is formed which is beneficial for the passivation of the interface of HIT solar cell. For amorphous silicon films, the fitting result is acceptable by using Tauc-Lorentz Genosc model. For the second series, the films are of epitaxial Si at the initial deposition stages, the amorphous fraction increases with the increase of thickness. When the film thickness reaches a critical value of 46 nm, a transition to pure amorphous phase occurs. The epitaxial film shows excellent fitting by using the effective medium approximation (EMA) model under the assumption of the mixture of c-Si phases and a-Si:H. However, we obtain better fitting result by using a three-layer model, whose bulk layer is divided into EMA layer and a-Si layer after the transition to pure amorphous phase. This study indicates that the SE analysis, operated in different models, is effective to characterize different structures of silicon films on c-Si substrate.

The Study of Improving the Conversion Efficiency and Reducing the Thickness of the HIT Solar Cell

ABSTRACTIn order to reduce the power-generating cost of silicon solar cells, it is necessary to achieve a high conversion efficiency using a thinner crystalline silicon (c-Si) substrate. The HIT solar cell is an amorphous silicon (a-Si) /crystalline silicon (c-Si) heterojunction solar cell that makes it possible to realize excellent surface passivation and hence high open circuit voltage (Voc). In addition, its symmetrical structure and a low-temperature fabrication process that is under 200°C provide advantages in reducing thermal and mechanical stresses within the device so that it can easily be applied to thinner solar cells. We fabricated HIT solar cells using thin wafers from 58-98 μm, and achieved a 22.8% conversion efficiency with a HIT solar cell using a 98-μm-thick wafer, and an excellent Voc value of 0.747 V with a HIT solar cell using a 58-μm-thick wafer.

Influence of ITO deposition and post annealing on HIT solar cell structures

Heterojunction silicon with intrinsic thin layer (HIT) solar cells that combine advanced thin-film hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) technologies are promising because of the high performance at low cost. Due to the low conductivity of a-Si:H, indium tin oxide (ITO) needs to be used as a front contact layer on top of a-Si:H in order to collect photogenerated currents. The thin a-Si:H layer requires the ITO deposition to be soft so that the passivation is maintained after deposition. Otherwise, the passivation degradation resulting from ITO deposition should be recovered by some post processing. In this contribution, we investigate how the power density and the temperature during ITO deposition as well as post annealing influence the passivation quality of HIT solar cells as characterised by the open-circuit voltage (Voc) and minority carrier lifetime. Firstly, ITO sputtering with lower power density can reduce the degradation of the passivation quality after ITO deposition. Secondly, we have investigated the simultaneous annealing during ITO deposition at elevated temperature. On one hand, simultaneous annealing can recover some of the degradation resulting from sputtering. On the other hand, there is a temperature threshold above which degradation of the passivation is observed, probably by hydrogen effusion. Thirdly, we observe that post annealing can fully recover the degradation resulting from ITO sputtering at room temperature (RT).

SIMULATION OF HETERO-JUNCTION SILICON SOLAR CELLS

Solar modules based on single crystal and polycrystalline silicon have almost a 90% share of the total world market, primarily because of their stable, robust and reliable characteristics. For these reasons, a great deal of attention is devoted to improving their characteristics and especially their efficiency. One of the many approaches includes HIT solar cells (hetero-junction solar cell with an intrinsic thin amorphous layer) fabricated at temperatures below 300C. In this paper, the electrical characteristics of HIT solar cells are simulated by using the TCAD Silvaco software package. The structure of HIT solar cell is optimized in order to obtain their maximum efficiency. In addition, the modifications of the simulation procedure, which allow the simulation of HIT solar cell, are described.

I– V characteristics of a-Si–c-Si hetero-junction diodes made by hot wire CVD

p-Type hydrogenated amorphous silicon (a-Si:H) was deposited on n-type crystalline silicon (c-Si) substrates to obtain hetero-junction diodes. Additionally, a thin intrinsic a-Si:H layer was inserted between both the p-type film and the n-type substrate to study its passivation effect on the c-Si surface. The amorphous films were obtained by the hot wire chemical vapor deposition (HWCVD) technique, using a tungsten filament and silane (SiH 4), hydrogen (H 2) and diborane (B 2H 6) gases, where the deposition parameters such as gas flow, substrate temperature and filament temperature were varied. Optical band gap, deposition rate and conductivity were measured for all the films. We studied the influence of the quality of the amorphous films upon the performance of the hetero-junction diodes. In particular, the diode ideality factor ( n) and the saturation current density ( J 0) were determined by measuring the current–voltage characteristics in dark conditions. It is shown that the presence of the intrinsic layer is fundamental for making good diodes, since devices made without this film cause the diodes to have high saturation current density and ideality factor ( J 0>10×10 −6 A/cm 2, n>4) as compared to diodes with a good intrinsic layer ( J 0=5×10 −9 A/cm 2, n=1.39). The results obtained are encouraging, but the quality of the intrinsic films still should be improved for applying them to HIT solar cells.

Effect of hydrogen plasma passivation on performance of HIT solar cells

We studied the performance improvement of HIT solar cells by optimizing H 2 plasma exposure and deposition of thin a-S:H layer on c-Si. With increasing H 2 treatment time, the V OC increases until 80 s and then decreases, indicating the optimum time is 80 s. It is found that the cell performance is almost the same with and without a thin a-Si:H layer when 80 s plasma is treated on the c-Si before i-layer deposition. The conversion efficiency of 14.04% was achieved at the substrate temperature of 160 °C.