Improved short-circuit current density of a-Si:H thin film solar cells with n-type silicon carbide layer

In this work, the performance of p–i–n hydrogenated amorphous silicon thin film solar cells by adopting n-type silicon carbide (n-SiC_x:H) layer was investigated. By varying CH₄/SiH₄ gas flow ratio, refractive index and electrical conductivity of n-SiC_x:H thin films were changed in the range of 3.4 to 3.8 and 1.48E?5 to 1.24 S/cm, respectively. Compared with solar cells with n-Si:H/Ag configuration, short-circuit current density (J _sc ) of solar cells with n-SiC_x:H/Ag configuration was improved up to 8.4%, which was comparable with that of solar cells with n-Si:H/ZnO/Ag configuration. Improved J _sc was related with enhanced spectral response at long wavelength of 500–800 nm. It was supposed that the decreased refractive index of n-SiC_x:H layer resulted in the increased back reflectance, which contributed to the improved J _sc. Our experiments demonstrated that n-SiC_x:H thin films were attractive choice because they functioned both as n-layer and interlayer in back reflector, and their deposition method was compatible with preparation process of solar cells.     

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm² were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br-Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm² (22 Ω cm²). In order to make more detailed study, the solar cell with an area of 1 cm² was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance.     

Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ _c  = 2.15 × 10⁻⁶ Ω-cm² indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ _c  = 8.34 × 10⁻⁵ Ω-cm². These results indicate that the ρ _c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ _c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.     

聚光背结背接触太阳电池发射区半宽度的优化研究

利用TCAD半导体器件仿真软件对中低倍聚光光伏系统中应用的N型叉指背接触(IBC)单晶硅太阳电池的电学性能进行了仿真研究。全面系统地分析了在不同聚光比情况下,单元电池发射区半宽度对聚光IBC太阳电池短路电流密度、开路电压、填充因子及转换效率的影响。仿真结果表明:聚光IBC太阳电池的电学性能受到单元电池发射区半宽度和聚光比的显著影响。在不同的聚光比情况下,存在最优的发射区半宽度,使得聚光IBC太阳电池转换效率最高。随着聚光比的增大,最优的发射区半宽度减小。虽然增大聚光比可提高聚光IBC太阳电池的转换效率,但同时减小了最优的发射区半宽度及参考范围,增加了聚光IBC太阳电池的制备难度。     

Heterojunction solar cells based on single-crystal silicon with an inkjet-printed contact grid

Technical Physics Letters - Results on the creation of a current-collecting grid for heterojunction silicon solar cells by ink-jet printing are presented. Characteristics of the obtained solar...     

SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si₃N₄) deposited over the n⁺-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si₃N₄ layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n⁺-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n⁺-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρ _c (in Ω-cm²) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρ _c  = 2.53 × 10⁻⁵ Ω-cm² is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρ _c is two orders of magnitude lower than the typical value of ρ _c  = 3 × 10⁻³ Ω-cm² reported previously for the Ag contacts of the solar cell. Such low value of ρ _c for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρ _c versus impurity doping level (N _s ) shows that measured value of the ρ _c follows a linear relationship with the N _s as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n⁺-Si surface of the silicon solar cell.

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Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρ_c) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρ_c of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10⁻⁵ Ω−cm ² is measured for the Ag metal contacts formed on the n⁺ silicon surface. This value is much lower than the ρ_c data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρ_c of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

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Simulation of forward dark current voltage characteristics of tandem solar cells

The transport mechanisms tailoring the shape of dark current-voltage characteristics of amorphous and microcrystalline silicon based tandem solar cell structures are explored with numerical simulations. Our input parameters were calibrated by fitting experimental current voltage curves of single and double junction structures measured under dark and illuminated conditions. At low and intermediate forward voltages the dark current-voltage characteristics show one or two regions with a current-voltage exponential dependence. The diode factor is unique in tandem cells with the same material in both intrinsic layers and two dissimilar diode factors are observed in tandem cells with different materials on the top and bottom intrinsic layers. In the exponential regions the current is controlled by recombination through gap states and by free carrier diffusion. At high forward voltages the current grows more slowly with the applied voltage. The current is influenced by the onset of electron space charge limited current (SCLC) in tandem cells where both intrinsic layers are of amorphous silicon and by series resistance of the bottom cell in tandem cells where both intrinsic layers are of microcrystalline silicon. In the micromorph cell the onset of SCLC becomes visible on the amorphous top sub-cell. The dark current also depends on the thermal generation of electron-hole (e-h) pairs present at the tunneling recombination junction. The highest dependence is observed in the tandem structure where both intrinsic layers are of microcrystalline silicon. The prediction of meaningless dark currents at low forward and reverse voltages by our code is discussed and one solution is given.     

Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.     

PEDOT:PSS as back contact for CdTe solar cells and the effect of PEDOT:PSS conductivity on device performance

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was studied as the back contact of Cadmium telluride (CdTe) solar cells and was compared with conventional Cu-based back contact. A series of PEDOT:PSS aqueous solutions with different conductivities were spin coated onto the glass/SnO2:F/SnO2/CdS/CdTe structures as back contact, and the PEDOT:PSS conductivity dependence of device performance was studied. It was found that PEDOT:PSS back contact with higher conductivity produces devices with lower series resistance and higher shunt resistance, leading to higher fill factor and higher device efficiencies. As the conductivity of PEDOT:PSS increased from 0.03 to 0.24 S/cm, the efficiency of the solar cell increased from 2.7 to 5.1 %. Methanol cleaning also played an important role in increasing the device performance. The efficiency of our best device with PEDOT:PSS back contact has reached 9.1 %, approaching those with conventional Cu/Au back contact (12.5 %).     

Improvement of the short-circuit current density and efficiency in micromorph tandem solar cells by an anti-reflection layer

An anti-reflection layer has been fabricated and applied in micromorph tandem (a-Si:H/μc-Si:H) solar cells. In this work, the porous anti-reflection layers are produced on glass substrates by plasma enhanced chemical vapor deposition using a CF₄ and O₂ gas mixture. The process is simple and easily controlled. The tandem solar cells with the anti-reflection layer show the increased short-circuit current density of the solar cells due to increased light transmittance from air/glass interface. With the anti-reflection layer, the short-circuit current density of the tandem cell increases by 0.29 mA/cm². Meanwhile, the solar cell efficiency increases from 11.15% to 11.55% (3.5% in relative) which allows us to develop more efficient a-Si based solar cells.     

Controlling number of indene solubilizing groups in multiadduct fullerenes for tuning optoelectronic properties and open-circuit voltage in organic solar cells

The ability to tune the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels of fullerene derivatives used as electron acceptors is crucial in controlling the optical/electrochemical properties of these materials and the open circuit voltage (V(oc)) of solar cells. Here, we report a series of indene fullerene multiadducts (ICMA, ICBA, and ICTA) in which different numbers of indene solubilizing groups are attached to the fullerene molecule. The addition of indene units to fullerene raised its LUMO and HOMO levels, resulting in higher V(oc) values in the photovoltaic device. Bulk-heterojunction (BHJ) solar cells fabricated from poly(3-hexylthiophene) (P3HT) and a series of fullerene multiadducts-ICMA, ICBA, and ICTA showed V(oc) values of 0.65, 0.83, and 0.92 V, respectively. Despite demonstrating the highest V(oc) value, the P3HT:ICTA device exhibited lower efficiency (1.56%) than the P3HT:ICBA device (5.26%) because of its lower fill factor and current. This result could be explained by the lower light absorption and electron mobility of the P3HT:ICTA device, suggesting that there is an optimal number of the solubilizing group that can be added to the fullerene molecule. The effects of the addition of solubilizing groups on the optoelectrical properties of fullerene derivatives were carefully investigated to elucidate the molecular structure-device function relationship.     

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

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 (V_oc), short-circuit current density (J_sc), 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 V_oc of 690 mV, J_sc 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.     

Electroluminescent study of the efficiency of silicon heterostructural solar cells

A strong (by more than an order of magnitude) change in the electroluminescence intensity is observed for the first time in high-quality heterojunction solar cells that are based on a single-crystal silicon and have an efficiency of 18 to 20.5%. This effect occurs due to the sharp change in the concentration of the recombination centers on the surface of single-crystal silicon wafers in the course of their pyramidal texturing and also due to the rise in the series resistance. The effect can be used for a quantitative highly sensitive characterization of the texturing, which is a fundamentally important stage in fabricating highly efficient silicon solar cells.     

Effect of dispersibility of silver powders in conductive paste on microstructure of screen-printed front contacts and electrical performance of crystalline silicon solar cells

Effect of dispersibility of silver powders in conductive paste on microstructure of screen-printed front contacts and electrical performance of crystalline silicon solar cells was investigated. Two different dispersed degree silver powders were experimented. It is found that the dispersibility of the silver powders strongly affects the microstructure of thick film and electrical performance of solar cells and highly dispersed silver powders can form even compact thick film and exhibit higher open-circuit voltage (V _oc ), short-circuit current (I _sc ), shunt resistance (R _sh ), maximum power (P _max ), optimum operating voltage (V _mp ), optimum operating current (I _mp ), fill factor (FF) and photoelectric conversion efficiency (Eff) than seriously agglomerated silver powders under otherwise identical conditions.     

Influence of the sensitizer adsorption mode on the open-circuit potential of dye-sensitized solar cells

We report a combined experimental and theoretical study on the origin of the different open circuit potentials observed in dye-sensitized solar cells using Ru(II)-polypyridyl homoleptic and heteroleptic sensitizers. We have measured the photovoltaic data of different sensitizers and used DFT calculations to analyze the electronic structure of dye-sensitized TiO(2) nanoparticles. Heteroleptic sensitizers adsorb onto TiO(2) via a single bipyridine, leading to a TiO(2) conduction band downshift and overall reduction of the cell open circuit potential.     

The influence of the electronic current on the measurement of ion polarization in evaporated silicon oxide

In this paper the influence of the electronic leakage current on the measurement of the ion polarization mechanism in thin film capacitors is investigated. A simple theory is given and is applied to small signal and large signal cases. The theoretical results are compared with the experimental data. In order to fit the theoretical analysis to the experimental results, a new hypothesis concerning ion enhancement is introduced.     

Influences of oxygen contamination on evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The influences of the oxygen contaminations on the crystal quality and performances of the evaporated polycrystalline silicon (poly-Si) thin-film solar cells prepared by solid-phase epitaxy were investigated by applying different deposition rates and base pressures. The experimental results show that although the evaporated poly-Si thin-film solar cell obtained at high base pressures (9.33 × 10^− 5 Pa) and high deposition rate (300 nm/min) has small amount of SiO₂ precipitations, it still shows the similar good material quality and performances as the cell prepared at low base pressure (1.33 × 10^− 6 Pa) and high deposition rate (300 nm/min) with oxygen interstitials. On the other hand, the poly-Si thin-film solar cell deposited at low base pressure (1.33 × 10^− 6 Pa) and low deposition rate (50 nm/min) has large amount of SiO₂ precipitations and resulting worse material quality and hence cell performances. Therefore, the high deposition rate is desirable to maximize the solar cell performance, as well as the throughput. It is a more influential factor than the base pressure.     

A laser based process for the formation of a local back surface field for n-type silicon solar cells

We report on the development of a laser doping process for the formation of a local back surface field (LBSF) for n-type silicon solar cells. Local point contacts are formed by applying a laser process to a doped, passivating layer of amorphous silicon carbide (PassDop layer). The exposure to laser radiation results in local doping and opening of the passivation layer at the same time. By variation of the laser parameters and the dopant content in the layer, strength and depth of the doping can be controlled. Both parameters are varied and the formation of a LBSF structure on lifetime samples is investigated. High dopant content in the passivation layer and comparably low laser fluencies yield the best electrical results, evidencing the formation of an effective LBSF in combination with a restricted laser induced damage in the silicon crystal.