Sergey Varlamov

Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons

Significant photocurrent enhancement has been achieved for evaporated solid-phase-crystallized polycrystalline silicon thin-film solar cells on glass, due to light trapping provided by Ag nanoparticles located on the rear silicon surface of the cells. This configuration takes advantage of the high scattering cross-section and coupling efficiency of rear-located particles formed directly on the optically dense silicon layer. We report short-circuit current enhancement of 29% due to Ag nanoparticles, increasing to 38% when combined with a detached back surface reflector. Compared to conventional light trapping schemes for these cells, this method achieves 1/3 higher short-circuit current.

Progress in Laser-Crystallized Thin-Film Polycrystalline Silicon Solar Cells: Intermediate Layers, Light Trapping, and Metallization

Diode laser crystallization of thin silicon films on the glass has been used to form polycrystalline silicon layers for solar cells. Properties of an intermediate layer stack of sputtered SiOx/SiNx/SiOx between the glass and the silicon have been improved by reactively sputtering the SiNx layer, which result in enhanced optical and electrical performance. Light trapping is further enhanced by texturing the rear surface of the silicon prior to metallization. An initial efficiency of 11.7% with VOC of 585 mV has been achieved using this technique, which are the highest values reported for poly-Si solar cells on glass substrates. Cells suffer a short term, recoverable degradation of VOC, and fill factor. The magnitude of the degradation is reduced via the repeated thermal treatment. A selective p+ metallization scheme has been developed which eliminates the degradation altogether.

Combined plasmonic and dielectric rear reflectors for enhanced photocurrent in solar cells

To reduce the use of fossil fuels and to fulfill the increasing energy demand for the fast growing population of our planet, renewable energies need to be more efficient and cheaper. A way of reducing the silicon solar cell cost for today’s technology is by using thinner layers and therefore less of the silicon material that accounts for about 50% of the photovoltaic module cost. However, due to weak absorption of infrared light in crystalline Si (c-Si), a significant fraction of solar energy is not converted into electricity in thin-film devices. Proper light management can potentially lead to ultra-efficient thin solar cells. 1 One method of achieving absorption enhancement in thin-film solar cells is through the excitation of localized surface plasmons in metal nanoparticles. These can be used as subwavelength scattering elements to couple a large fraction of incident light into trapped modes within a nearby semiconductor layer. 2‐10

Intermediate Layer Development for Laser-Crystallized Thin-Film Silicon Solar Cells on Glass

The intermediate layer (IL) between the glass and silicon plays an important role in laser-crystallized thin-film silicon solar cells. SiO_X, SiN_X, and SiC_X deposited by RF sputtering or plasma-enhanced chemical vapor deposition, either as single layers or in stacks, have been tested as ILs with regard to silicon wettability and silicon crystal quality and the effect of hydrogen passivation. SiC_X is the best wetting layer, allowing a larger laser crystallization process window than SiO_X or SiN_X. SiN_X layers are limited by pinholing, which increases in severity with laser fluence. SiO_X ILs result in lower silicon grain-boundary density compared with SiC_x-based layers and to SiN_X-based layers. Hydrogen passivation of laser-crystallized silicon on single layer SiO_X has no impact on V_OC, while an improvement of around 60 mV is found for samples on SiO_x/SiN_x/SiO_x stacks. Diffusion of dopants from the IL are found to create a uniformly doped absorber with no evidence of a front-surface field.

Enhanced light trapping in solar cells using snow globe coating

A novel method, snow globe coating, is found to show significant enhancement of the short circuit current JSC (35%) when applied as a scattering back reflector for polycrystalline silicon thin-film solar cells. The coating is formed from high refractive index titania particles without containing binder and gives close to 100% reflectance for wavelengths above 400 nm. Snow globe coating is a physicochemical coating method executable in pH neutral media. The mild conditions of this process make this method applicable to many different types of solar cells. Copyright

Influence of the absorber doping for p-type polycrystalline silicon thin-film solar cells on glass prepared by electron beam evaporation and solid-phase crystallization

A systematic investigation of the influence of the absorber doping on the performance of planar, p-type, evaporated, solid-phase crystallized polycrystalline silicon thin-film solar cells on glass is presented. It is found that the optimum Suns-Voc parameters (open-circuit voltage and pseudo fill factor) are achieved at intermediate absorber doping of Nabs∼1–2 × 1017 cm−3, while high short-circuit currents are achieved at the lowest absorber doping of Nabs ≤ 6 × 1015 cm−3. Since the short-circuit current is the dominating factor to achieve high conversion efficiencies for evaporated polycrystalline silicon cells, the maximum pseudo efficiencies are achieved at very low absorber doping. The Suns-Voc characteristics of lightly doped cells can be adequately described by a modified two-diode model with n1=1 and n2≈1.5, which is in contrast to the value of 2 for n2 commonly quoted in the literature. PC1D modeling demonstrates that such a low ideality factor for space charge region recombination can be modeled ...

Effect of Nanoparticle Size Distribution on the Performance of Plasmonic Thin-Film Solar Cells: Monodisperse Versus Multidisperse Arrays

The effect of the silver nanoparticle size distribution on the performance of plasmonic polycrystalline Si thin-film solar cells is studied. Monodisperse particle arrays are fabricated using nanoimprint lithography. Multidispersed particle arrays are fabricated using thermal evaporation followed by annealing. The short-circuit current enhancement for the cells without a back reflector is 24% and 18% with the multidisperse array and the monodispersed array, respectively. For the cells with a back reflector, the current enhancement increases to 34% and 30%, respectively, compared with 13% enhancement due to the reflector alone. Better performance of multidisperse Ag nanoparticle arrays is attributed to a broader scattering cross section of the array owing to a broad particle size distribution and a higher nanoparticle coverage.

Large-Area Diode Laser Defect Annealing of Polycrystalline Silicon Solar Cells

A new process was presented to anneal crystallographic defects in solid-phase crystallized silicon that produces higher Suns-Voc voltages than the conventional belt-furnace annealing (BFA) process. A few-millisecond continuous-wave diode laser treatment anneals defects present in the polycrystalline silicon and activates dopants. It is shown that the silicon/glass-interface system reaches an effective steady state at every point during the laser treatment, and the relative temperature profile is determined by the laser beam profile rather than its power or scanning speed. The peak temperature that is reached during a few-millisecond laser exposure is shown to increase and then level off with laser dose, indicating partial recrystallization of the film. A reduction in boron-doped p-type sheet resistance and phosphorus-doped emitter resistance is reported for a wide range of laser dose and scanning speed. The highest substrate temperature required during the process sequence is reduced from 960°C to 620°C. A peak 1-sun voltage of 492 mV is achieved using a 4-ms exposure at 500-J·cm-2 laser dose, which is an improvement of 32 mV over the voltage after optimized BFA.

Optimum surface condition for plasmonic Ag nanoparticles in polycrystalline silicon thin film solar cells

Excitation of surface plasmons in silver nanoparticles is a promising method for increasing the light absorption in solar cells and hence, the cell photocurrent. The optical environment is an important key factor to consider when designing plasmonic solar cells because it affects the surface plasmon characteristics. In this paper, we applied the silver nanoparticles on the rear side of polycrystalline silicon thin film solar cells and systematically investigated the optimum surface condition for maximising plasmonic enhanced light absorption in the cells. Three different environments, thermal silicon dioxide (SiO2), native SiO2, and oxide-free silicon surface were investigated. We found that the existence of the SiO2 layer between Si and nanoparticles has a major effect on Qscat and therefore, the absorption in the cells. We also found that nanoparticles on the thermal SiO2 layer showed that the peak of Qscat is located at the visible light wavelengths <700 nm, nanoparticles on the native SiO2 layer and d...

Light Absorption Enhancement in Laser-Crystallized Silicon Thin Films on Textured Glass

Liquid-phase crystallization of Si films on textured glass by line-focus diode laser produces a material with large highquality grains of several hundred micrometers in width and up to centimeters in length, similar to the films crystallized on planar glass. A combination of glass texturing, rear Si texturing, white paint rear reflector, and a front moth-eye antireflection foil on a 7-μm absorber results in significant broadband absorption enhancement in the Si film with potential short-circuit current densities in solar cells of 27.8 mA/cm2 or 36.3% enhancement compared with a planar reference film without light-trapping features.