Vamsi K. Komarala

Reduced ultraviolet light induced degradation and enhanced light harvesting using YVO4:Eu3+ down-shifting nano-phosphor layer in organometal halide perovskite solar cells

We report a simple method to mitigate ultra-violet (UV) degradation in TiO2 based perovskite solar cells (PSC) using a transparent luminescent down-shifting (DS) YVO4:Eu3+ nano-phosphor layer. The PSC coated with DS phosphor showed an improvement in stability under prolonged illumination retaining more than 50% of its initial efficiency, whereas PSC without the phosphor layer degraded to ∼35% of its initial value. The phosphor layer also provided ∼8.5% enhancement in photocurrent due to DS of incident UV photons into additional red photons. YVO4:Eu3+ layer thus served a bi-functional role in PSC by reducing photo-degradation as well as enhancing energy conversion efficiency.

Influence of surface plasmon resonances of silver nanoparticles on optical and electrical properties of textured silicon solar cell

Here, we report average reflectance reduction of ∼8% in wavelength range of 300–1100 nm after coupling surface plasmon resonances (SPRs) of silver nanoparticles (NPs) to textured silicon (T-Si) surface. The enhancement of photocurrent from T-Si solar cell in off-resonant SPR region observed due to better radiative efficiency of NPs leading to outflow of scattered far-field into silicon maximized power generating electrons. Improvement in series resistance, fill factor, and open-circuit voltage (insensitive NPs size and morphology) are also observed with NPs along with photocurrent enhancement (sensitive to NPs sizes), which resulted cell efficiency enhancement from 4.49% to 6.42% for large area of 12.24 cm2.

Improved stability and enhanced efficiency of dye sensitized solar cells by using europium doped yttrium vanadate down-shifting nanophosphor

Through detailed experiments, it has been deduced that ultraviolet (UV) light is a major factor in the degradation of dye-sensitized solar cell (DSSC) performance due to generation of surface defects in nanoporous TiO2 and dye structural modification with free radical formation. We describe a simple spray deposition method to coat europium doped yttrium vanadate (YVO4:Eu3+) down-shifting (DS) phosphor nanoparticles (NPs) on the front side of DSSCs in order to enhance photocurrent and mitigate UV induced degradation. The nanophosphor provides an enhancement in short-wavelength spectral response of solar cells, and long-term stability is also improved under illumination, due to down-shifting of high energy UV photons to the visible region. Our observations demonstrate that the DS nanophosphor layer can be used as an optical filter with visible light transmission and UV light absorption, through placement on the front surface of DSSCs to provide stability, as well as for improving the performance.

Improving the Short-Wavelength Spectral Response of Silicon Solar Cells by Spray Deposition of YVO 4 :Eu 3+ Downshifting Phosphor Nanoparticles

Europium-doped yttrium vanadate downshifting phosphor nanoparticles (NPs) have been coated on top of monocrystalline silicon solar cells, having efficiency more than 15%, by a spray deposition technique. The effects of phosphor NPs on solar cells with antireflection coating (ARC) have been studied. The optimized quantity of phosphor NPs provides a photocurrent enhancement of ~2.1% for monosilicon solar cells. External quantum efficiency data of high-efficiency ARC layer-coated silicon solar cells conclusively show that the enhancement in short-wavelength spectral response is mainly due to downshifting effects of the phosphor NPs. A small increment in a long-wavelength spectral response is also observed due to the scattering effects of phosphor NPs, which also results in a small enhancement of effective diffusion length of minority carriers in the base region.

Effect of graphene and Au@SiO2 core–shell nano-composite on photoelectrochemical performance of dye-sensitized solar cells based on N-doped titania nanotubes

We have investigated the role of graphene and Au@SiO2 core–shell nano-composite (NC), on the performance of dye-sensitized solar cells (DSSC) based on nitrogen doped TiO2 nanotubes (N-TNTs) as photoanodes. The N-TNTs were synthesized by an environmentally-friendly solvothermal method. The photoelectrochemical performance of DSSCs with N-TNTs improved compared to undoped TNTs; due to extended absorption in the visible part of the solar spectrum. An improved open circuit voltage was also observed with N-TNTs due to a change in the TiO2 Fermi energy level with increased electron density. After that, we investigated DSSC performance using graphene in N-TNTs with varying concentration from 0.2 to 1.0 wt%. With an optimal concentration of graphene (0.6 wt%), we have achieved 6.33% energy conversion efficiency, which is ∼47.5% enhancement in performance compared to pure N-TNTs. The enhanced device performance with graphene is mainly due to better dye loading, improved electron transport and charge collection process. To further boost the conversion efficiency of the DSSC based on graphene/N-TNTs NC, we introduced Au@SiO2 core–shell nanoparticles (NPs) of different concentration into the device structure. Finally, we are able to fabricate a DSSC having an energy conversion efficiency of 7.01% with 1.8% (w/w) of Au@SiO2 NPs, due to an improved excitation of dye molecules by generated strong near-fields around the Au NPs along with incident light far-fields.

Enhancement of minority carrier lifetimes in n- and p-type silicon wafers using silver nanoparticle layers

The quasi-steady state photo conductance technique is employed to probe effective minority carrier lifetime (τ eff) modifications after integrating silver nanoparticles (Ag NPs) on n-type and p-type silicon wafers with a native oxide surface. Our observations reveal that τ eff modification is very sensitive to Ag NPs size, surface coverage and also wafer type. With an optimized Ag NPs, τ eff is enhanced from 4.4 μs to 10 μs for a p-type silicon wafer, and from 8.1 μs to 14 μs for an n-type silicon wafer. We attributed the enhancement in τ eff to the partial field effect passivation of the silicon surface by the surface plasmon resonance near-fields of Ag NPs after excitation. Our investigations demonstrate that an optimized Ag NPs on any silicon wafer with a native oxide layer can work as both a light trapping and a surface-passivating layer.

Graphene/ZnO nanocomposite as an electron transport layer for perovskite solar cells; the effect of graphene concentration on photovoltaic performance

Perovskite solar cells (PSCs) have been fabricated by a graphene/ZnO nanocomposite (G/ZnO NC) as an electron transporting layer. We use a novel spray deposition method compatible with large area processing methods for deposition of pristine ZnO and G/ZnO NC films. We show the effect of varying the graphene concentration in the G/ZnO NC films from 0 to 1 wt% on the photovoltaic performance of PSCs. We find that a 0.75 wt% graphene concentration in the G/ZnO NC films gives an optimum PSC performance with short circuit current density and power conversion efficiencies going up from 15.54 to 19.97 mA cm−2, and 7.01 to 10.34% respectively as compared to pristine ZnO. The enhancement in photovoltaic performance is attributed to the superior growth of the perovskite thin-film and enhanced electron transport/extraction on using the graphene network in the NC.

Role of Textured Silicon Surface in Plasmonic Light Trapping for Solar Cells: The Effect of Pyramids Width and Height

Silicon solar cells with different front texturization are used for understanding pyramidal size influence on plasmonic light trapping. Cells with different pyramidal heights and widths have shown strong light back scattering in the surface plasmon resonance (SPR) region and minimal light forward scattering in the off-resonance region of silver nanoparticles (NPs). On the other hand, cell surface with similar pyramidal heights and widths has shown reduced back scattering in the SPR region, as well as enhanced light forward scattering in the off-resonance region of NPs with good optical impedance matching. The reason for these types of light interaction with NPs (nanoscale) and textured silicon (micrometer-scale) is explained, and plasmonic textured silicon solar cell performance with different pyramidal sizes using quantum efficiency measurements is verified.

Efficiency enhancement of silicon solar cells with vertically aligned ZnO nanorod arrays as an antireflective layer

Vertically aligned ZnO nanorods grown by the hydrothermal method have been explored as an antireflection layer on polished, textured, and antireflection coating (ARC) coated textured silicon (Si) wafers. Average reflectance (from 380 to 1100 nm) of polished and textured Si wafers reduced from 32 to 9% and 14 to 2%, respectively. With nanorods, multiple light interactions and good optical impedance matching with graded refractive index (effective medium) from air to Si favored for light confinement in Si. Optimized nanorods on ARC coated textured Si cell led to an enhancement of photocurrent from 34.30 to 36.38 mA/cm2 and efficiency from 15.11 to 16.43%.

Engineered optical properties of silver-aluminum alloy nanoparticles embedded in SiON matrix for maximizing light confinement in plasmonic silicon solar cells

Self-assembled silver-aluminum (Ag-Al) alloy nanoparticles (NPs) embedded in SiO2, Si3N4, and SiON dielectric thin film matrices explored as a hybrid plasmonic structure for silicon solar cells to maximize light confinement. The Ag2Al NPs prepared by ex-vacuo solid-state dewetting, and alloy formation confirmed by X-ray diffraction and photoelectron spectroscopy analysis. Nanoindentation by atomic force microscopy revealed better surface adhesion of alloy NPs on silicon surface than Ag NPs due to the Al presence. The SiON spacer layer/Ag2Al NPs reduced silicon average reflectance from 22.7% to 9.2% due to surface plasmonic and antireflection effects. The SiON capping layer on NPs reduced silicon reflectance from 9.2% to 3.6% in wavelength region 300–1150 nm with preferential forward light scattering due to uniform Coulombic restoring force on NPs’ surface. Minimum reflectance and parasitic absorptance from 35 nm SiON/Ag2Al NPs/25 nm SiON structure reflected in plasmonic cell’s photocurrent enhancement from 26.27 mA/cm2 (of bare cell) to 34.61 mA/cm2 due to the better photon management. Quantum efficiency analysis also showed photocurrent enhancement of cell in surface plasmon resonance and off-resonance regions of NPs. We also quantified dielectric thin film antireflection and alloy NPs plasmonic effects separately in cell photocurrent enhancement apart from hybrid plasmonic structure role.