Analysis of the interdigitated back contact solar cells:The n-type substrate lifetime and wafer thickness

The n-type silicon integrated-back contact(IBC) solar cell has attracted much attention due to its high efficiency,whereas its performance is very sensitive to the wafer of low quality or the contamination during high temperature fabrication processing, which leads to low bulk lifetime τbulk. In order to clarify the influence of bulk lifetime on cell characteristics, two-dimensional(2D) TCAD simulation, combined with our experimental data, is used to simulate the cell performances, with the wafer thickness scaled down under various τbulk conditions. The modeling results show that for the IBC solar cell with high τbulk,(such as 1 ms–2 ms), its open-circuit voltage V oc almost remains unchanged, and the short-circuit current density J sc monotonically decreases as the wafer thickness scales down. In comparison, for the solar cell with low τbulk(for instance, 500 μs) wafer or the wafer contaminated during device processing, the V oc increases monotonically but the J sc first increases to a maximum value and then drops off as the wafer's thickness decreases. A model combing the light absorption and the minority carrier diffusion is used to explain this phenomenon. The research results show that for the wafer with thinner thickness and high bulk lifetime, the good light trapping technology must be developed to offset the decrease in J sc.     

Activated carbon as back contact for HTM-free mixed cation perovskite solar cell

Carbon materials have potential applications in perovskite solar cell because of their excellent electronic properties and low cost. In this paper, we report, for the first time, activated carbon as a back contact for hole transport layer-free mixed halide perovskite solar cells. The ability of activated carbon to form conducting chain-like structure when dispersed in a polymeric solution makes it a possible candidate for back contact. A composite of activated carbon and PEMA was optimized with varying concentration. Mixed cation was used as a perovskite absorber and was analysed for its structural and optical properties. The fabricated devices were studied for their electrical performance. They were also subjected to stability study and showed promising results.     

Front and back surface cw CO2-laser annealing of arsenic ion-implanted silicon

The annealing behavior of arsenic-implanted silicon under scanned cw CO₂-laser irradiation from front and back surfaces is investigated. Ellipsometry, Hall effect, Rutherford backscattering measurements and neutron activation analysis indicate an enhancement of annealing efficiency by laser irradiation from the back surface, which provides complete recovery of crystal damage, high substitutionality and electrical activation of implanted arsenic atoms without redistribution of concentration profile. The enhancement of annealing efficiency under back-surface irradiation is explained by the difference in laser reflection from the front and back surface of silicon wafers. No differences in the results are found for scanned and static annealing.     

Performance improvement for epitaxially grown SiGe on Si solar cell by optimizing the back surface field

This letter reports on the performance improvement of an epitaxially grown SiGe on Si solar cell by optimizing the back surface field (BSF). First, a Si_0.18Ge_0.82 on silicon (Si) solar cell was fabricated with a 0.25 μm BSF layer. A 25 mV open‐circuit voltage (V_OC) improvement was observed on this BSF solar cell compared with the reference solar cell without BSF layer. Then, a Si_0.18Ge_0.82 on Si solar cell with double BSF layers was designed and fabricated. The measured efficiency of this solar cell is 3.4% when filtered by a GaAs_0.79P_0.21 top cell. To the best of the authors' knowledge, the 3.4% efficiency reported here is the highest efficiency for SiGe on Si solar cells when filtered by a GaAs_0.79P_0.21 top cell. The previous best reported efficiency for high Ge composition SiGe on Si solar cell was only 1.7% when filtered by a GaAs_0.79P_0.21 top cell.     

稀土掺杂CdTe太阳电池背接触层ZnTe的第一性原理研究

结合CdS/CdTe太阳电池背接触层的制备要求考虑, 利用基于密度泛函理论平面波超软赝势方法和广义梯度近似, 计算了未掺杂ZnTe、稀土Y、Gd掺杂ZnTe的能带和电子态密度, 得到了不同体系下系统总能和晶格常数. 研究表明, 稀土Y和Gd掺杂后ZnTe结构的稳定性均提高, 掺杂Y使ZnTe与CdTe的晶格匹配更好. 计算表明, 掺杂可使载流子发生简并, 掺Y比掺Gd电子有效质量小, 掺Y与掺Gd的载流子浓度数量级相同. 根据计算结果分析了稀土掺杂对ZnTe背接触层的影响. 关键词： ZnTe 稀土掺杂 第一性原理 太阳电池背接触层     

CdTe太阳电池的不同背电极和背接触层的特性研究

用Ni替代Au来作为CdTe太阳电池的背电极，比较了Ni,Ni/Au,Au/Ni及Au背电极对电池性能的影响.发现Ni作为背电极和ZnTe/ZnTe:Cu复合层接触，电池的开路电压V_oc略有降低，填充因子FF有增有减，变化幅度不大，但因短路电流I_sc有较大的提高，转换效率η平均增长4％.测试了不同背电极的CdTe太阳电池的暗I-V和C-V特性，对背电极剥离后的样品进行了XPS测试分析.结果表明，Ni扩散到ZnTe/ZnTe:Cu复合层的深度比Au多，且大多呈离子态，与ZnTe/ZnTe:Cu复合层中的富Te离子形成Ni_xTe，提高了掺杂浓度，使电池性能获得改善. 关键词： 金属背电极 复合背接触层 转换效率 CdTe太阳电池     

牺牲Ni退火对硅衬底GaN基发光二极管p型接触影响的研究

本文通过在硅衬底发光二极管(LED)薄膜p-GaN表面蒸发不同厚度的Ni覆盖层,将其在N₂ ∶O₂=4 ∶1的气氛中、400℃—750℃的温度范围内进行退火,在去掉薄膜表面Ni覆盖层之后制备Pt/p-GaN欧姆接触层.实验结果表明:退火温度和Ni覆盖层厚度均对硅衬底GaN基LED薄膜p型欧姆接触有重要影响,Ni覆盖退火能够显著降低p型层中Mg受主的激活温度.经牺牲Ni退火后,p型比接触电阻率随退火温度的升高呈先变小后变大的规律,随Ni覆盖层厚度的增加呈先变小后变 关键词： 氮化镓 发光二极管 牺牲Ni退火 p型接触     

Effects of annealing on gap states in amorphous Si films

Annealing behaviors of the activation energy for electrical conduction, the optical gap, and the spin density in amorphous Si are investigated. It is found that the Fermi level shifts downwards with the decrease of the spin density, increasing the decay length of the wave function of localized state at the Fermi level. The downward shift of the Fermi level suggests that the localized states in the upper energy region in the gap are annealed out more easily than those in the lower one.     

Alkali-treated Si nanowire array for improving solar cell performance

The Si nanowire array (SiNWA) was achieved by metal-assisted etching. The effect of alkali-treating of SiNWA on the photovoltaic performance of solar cells was investigated. We found that the post-chemical treatment of SiNWA on solar cells could reduce (i) the surface reflection and (ii) the thickness of “dead layer” so that the short-wavelength spectral response is enhanced. As a result, a solar-spectrum-averaged reflectance of less than 3 % has been reached, a 3-fold enhancement on the spectral response of post-treated SiNWA cell at the wavelength of 400 nm has been observed, together resulting in a remarkable improvement of 45 % in the conversion efficiency.     

High-temperature laser annealing for thin film polycrystalline silicon solar cell on glass substrate

Thin film polycrystalline silicon films grown on glass substrate were irradiated with an infrared continuous wave laser for defects annealing and/or dopants activation. The samples were uniformly scanned using an attachment with the laser system. Substrate temperature, scan speed and laser power were varied to find suitable laser annealing conditions. The Raman spectroscopy and Suns-V _oc analysis were carried out to qualify the films quality after laser annealing. A maximum enhancement of the open circuit voltage V _oc of about 100?mV is obtained after laser annealing of as-grown polysilicon structures. A strong correlation was found between the full width half maximum of the Si crystalline peak and V _oc. It is interpreted as due to defects annealing as well as to dopants activation in the absorbing silicon layer. The maximum V _oc reached is 485?mV after laser treatment and plasma hydrogenation, thanks to defects passivation.     

Neodimium laser pulse annealing and decomposition of ion-implanted GaAs

Summary The amorphous-to-single crystal transition induced by high-power Nd laser pulse has been studied in ion-implanted GaAs by taking advantage of the high-absorption coefficient of amorphous material. A threshold energy density of about 0.8 J/cm² has been measured for both 50 keV Te⁺ and 100 Ar⁺ implants. Channelling effect and X-ray photoelectron spectroscopy techniques indicate that no appreciable As loss occurs in a narrow energy density window just above threshold. High-energy densities cause instead As loss from the near surface region. Visiting scientist on the basis of I.N.F.N. Academia Sinica cultural exchange plan.     

Dependence of solar cell performance on Si:H nanostructure

Nanocrystalline Si:H (nc-Si:H) films were prepared to fabricate solar cells, of which the output properties were investigated. Nanocrystals are indicated in the films by electron microscopy. Raman experimental data of phosphorus doped nc-Si:H films were well simulated by Fano effect (FE) profiles due to Fano interference between discrete optical phonons and a continuum of electronic excitations in the nanocrystals. Raman measurement signals of the phosphorus doped and intrinsic nc-Si:H layers can be fitted well by a strain-calibrated phonon confinement effect (S-PCE) model owing to incorporated effects of phonon confinement and tensile strain. For the Raman signals from intrinsic films, the fit according to the S-PCE model is better than that based on FE. The output properties of P–I–N type nc-Si:H solar cells, depending on the material structures, including the mean size of the nanocrystals, crystalline volume fraction and disorder, were analyzed.     

Influence of laser power on POCl3 diffused back surface field on n-type PERT silicon solar cells with local back contact

We report n-type passivated emitter rear totally diffused (PERT) silicon solar cells with local back contacts (LBC) formed by laser process. With passivated back surface field (BSF), the PERT cell design shows an improved open circuit voltage (V_oc) with reduced recombination at the rear due to improved optical confinement. The rear side was diffused by POCl₃ diffusion with low sheet resistance (R_s) BSF and passivated using SiN_x. Laser ablation was used to open the SiN_x on the rear for LBC. The Nd:YAG laser power (mW) parameters and POCl₃ doping temperature were varied to obtain the BSF with lower sheet resistance. Laser power of 44 mW with 10 kHz resulted in 30 Ω/sq BSF layer with effective lifetime (τ_eff) of 90 μs and a higher V_oc of 646 mV. With the optimized laser parameters the best electrical results yielded a short circuit current density (J_sc) of 36 mA/cm² and efficiency of 18.54%.     

Dependence of the solar cell performance on nanocarbon/Si heterojunctions

Solar cells that combine single-crystalline silicon(Si) with graphene(G) have been widely researched in order to develop next-generation photovoltaic devices. However, the power conversion efficiency(PCE) of G/Si solar cell without chemical doping is commonly low due to the relatively high resistance of graphene. In this work, through combining graphene with carbon nanotube(CNT) networks, we fabricated three kinds of hybrid nanocarbon film/Si heterojunction solar cells in order to increase the PCE of the graphene based Si solar cell. We investigated the characteristics of different nanocarbon film/Si solar cells and found that their performance depends on the heterojunctions. Specifically, a doping-free G-CNT/Si solar cell demonstrated a high PCE of 7.9%, which is nearly equal to the combined value of two individuals(G/Si and CNT/Si). This high efficiency is attributed to the synergistic effect of graphene and CNTs, and can be further increased to 9.1% after applying a PMMA antireflection coating. This study provides a potential way to further improve the Si based heterojunction solar cells.     

Time-resolved optical transmission during pulsed-laser annealing of ion-implanted amorphous silicon-on-sapphire

Time-resolved optical transmission of annealing dye laser pulse (wavelength : 590 nm and pulse width : 250 nsec) itself has been measured in thin layers of Si⁺-implanted amorphized silicon-on-sapphire (SOS) with several energy densities in the range of 0.02 to 2.64 J/cm². At the energy densities where the amorphized SOS is recrystallized, the transmission exhibits a sudden drop and subsequently remains at a detectable level. This laser-induced absorption is inconsistent with the presence of ordinary molten silicon but suggests an electronic structural-change from amorphous to crystalline state.     

Nanosecond optical transmission studies of laser annealing in ion-implanted silicon-on-sapphire

Time-resolved optical transmission has been studied using 633 and 514 nm CW probes on ion-implantation-amorphized silicon-on-sapphire during annealing by a 10 nsec, ⊥ J/cm² pulse at either 532 nm or 485 nm. As recrystallization sets in the transmitted signal at 514 nm rises by ⊥ 10³ in ⊥ 60 nsec and provides a measure of regrowth velocity. Beyond 200 nsec the much slower transmission rise is used to provide an estimate of the Si cooling rate. The difference in transmission observed between initially crystalline and initially amorphous Si provide an estimate of the latent heat of recrystallization of the amorphous phase.     

The influence of the annealing conditions on the photoluminescence of ion-implanted SiO2:Si nanosystem at additional phosphorus implantation

The influence of P ion doping on the photoluminescence (PL) of the system of nanocrystals in SiO₂ matrix (SiO₂:Si) both without annealing and after annealing at various temperatures (provided before and after additional P implantation) is investigated. The Si and P implantation was carried out with ion energies of 150 keV and doses Φ_Si=10¹⁷ cm⁻² and Φ_P=(0.1–300)×10¹⁴ cm⁻² (current density j3 μAcm⁻²). The system after Si implantation was formed at 1000°C and 1100°C (2 h). For the case of SiO₂:Si system as-implanted by P, the intensity of PL was drastically quenched, but partially retained. As for the step-by-step annealing (at progressively increased temperatures) carried out after P implantation, the sign and degree of doping effect change with annealing temperature. The possible mechanisms of these features are discussed.     

Dual N/Pb ion-implanted Si: Temperature dependence of the novel shift of the Pb peak under electron beam annealing

(1 0 0) Si was dual ion-implanted with 24 keV N and 7 keV Pb to peak concentrations ∼10 at.%. Implanted samples were then electron beam annealed (EBA) at a peak temperature T for 30 s with T ranging from 100 °C to 900 °C and for 15 s at 1000 °C. Pb profiles were measured using RBS and surfaces characterised by AFM. For T up to 500 °C there was no shift in the profile from the implanted depth ∼10 nm. For higher values of T a striking feature was the large movement of the Pb profile away from the surface without a significant change in width or Pb content. The profile depths were: ∼40 nm for 600 °C, ∼68 nm for 700-900 °C and ∼80 nm for 1000 °C. The response to EBA was found to be strongly dependent on both ion implantation order and Si starting structure. For (1 0 0) Si nanowhiskers formed on the treated surfaces for T = 900 °C and 1000 °C. A model is developed based on the restructuring of the amorphous implanted layer under EBA. It is proposed that a compaction starting at the surface sweeps the Pb before it via a stress interaction as it advances into the Si.     

Photoconductivity in ion-implanted thin films of Si:In and Si:Tl

The extrinsic photoconductivity of Si:In and Si:Tl is studied on ion-implanted thin films. Additional boron implants are used to compensate the phosphorus substrate doping for obtaining high photocurrents. The spectral photocurrent response and ionisation energies determined are identical in the implanted film and in bulk wafers. The ionisation energiesE _In=157±2 meV andE _Tl=252±2 meV are obtained by a novel evaluation which excludes photothermal excitation via exited bound states.