Introduction

Plasmonic nanoparticles emerged recently as a possible solution to enhance solar cells efficiency1,2,3,4. High losses of photons due to both incomplete absorption of solar spectrum and thermalization are the main drawbacks that have been addressed to overcome limitations in photovoltaics efficiency. Over the past decades several improvements in solar cells were introduced but the highest conversion efficiency achieved so far for a single junction cell is 25% for crystalline silicon and 10.5% for amorphous silicon5.

Surface Plasmon Resonance (SPR) arises in metal NPs as a collective oscillation of the surface electrons when subjected to the electric field associated to the incident light. Also, NPs should be smaller than the wavelength of the incident light in order to achieve SPR. As a result, the nanoparticles polarize. The scattering and absorption of light depends on the polarizability of the NPs, which is a function of the size, refractive index, shape, density and surrounding medium, as demonstrated in several theoretical studies6,7. These parameters should be tuned in order to maximize the NPs light scattering, relatively to absorption. Scattering efficiencies lower than 50% may lead to heat dissipation and absorption suppression. This promotes undesired reduction of photocurrent for solar cells having high response in the blue region8. However a change in shape from spherical to ellipsoidal or a change in the NPs medium affects the polarizability with possible red shift in the resonance frequency. In this case, NPs used as front electrode lead to an enhancement of the intrinsic layer absorption above 59% in the infrared region9.

The influence of shape: spheroid10, cylindrical11, nanowires12, size6 and also the distribution13 have been studied mainly for Au and Ag nanoparticles. Other nanoparticles such as Sn9 has also shown an enhancement of light absorption up to 59% when Al-NPs were deposited on the top of the TCO layer of a-Si:H solar cells. Nevertheless, our Au-NPs are embedded in a-Si:H film and the prove is the coincidence of TT and TSp spectra obtained by irradiating the samples from the glass side or from a-Si:H film side (reflectance was not shown also because no difference was observed). As reported by Pillai and Green2, if a semiconductor is close to electric field of an emitting dipole, the absorption is enhanced. Other non-linear processes such as up/down-conversion can be originated by the enhancement of electric field around the surface plasmons. In our results the photoluminescence shows that down-conversion (in terms of energy) is obtained in a-Si:H films deposited on Au-NPs (5 nm Au layer). In the literature, the luminescence of Au-NPs, deposited on glass substrate, has shown a red shift of the peak in the visible wavelength range, which was attributed to the reduction in quantum confinement of Au nanoparticles28. Due to the excitation energy in these measurements (2.71 eV, 457 nm), the excitation should involve just one photon. Besides that, a stronger luminescence signal is observed in the near-infrared region and it increases as the Au-NPs decrease. According to the literature, the above two emission bands in the visible and infrared can be attributed to interband and intraband radiative transitions involving electrons in the sp-conduction band and holes in the d-band of NPs21,23,29. However, NPs embedded in a-Si:H has a different behaviour since the emitted photons can be absorbed by gap states of a-Si:H, followed by recombination or it can induce radiative transitions in the a-Si:H. That would explain the difference in the PL intensity of a-Si:H with Au-NPs 2 nm and Au-NPs 5 and 7 nm. In this analysis we exclude any effect of a-Si:H induced recrystallization during the film deposition, since micro Raman and spectroscopic ellipsometry measurements were performed (standard techniques to evaluated the crystallinity of Si:H film) and no difference was observed in samples with and without NPs. Although the a-Si:H deposited on 7 nm Au layers has high photoluminescence in the near infrared region the low transmission in the visible range unsuited its application to front layers of solar cells.

Overall, we showed a straightforward method to deposit Au-NPs with controllable particle size which consists in the deposition of very thin Au layer by thermal evaporation in heated substrates. The a-Si:H films deposited on Au-NPs layers has enhanced absorption in the infrared region (700–900 nm) opening doors to enhance amorphous silicon based solar cells efficiency through improvement of absorption close to the band gap of silicon and up-conversion of photons. The deposition of NPs can be made in any part of the solar cell, intra or inter layers, according to the size of NPs, by a compatible process.

Methods

Preparation of the Au-NPs and a-Si:H films

Au-NPs were deposited onto glass substrates by e-beam evaporation at substrate temperature of 250°C and deposition rate of 0.1-0.2 nm/s. The layer thickness was controlled, after opening the shutter of the system, by the deposition time and by a calibrated quartz crystal detector. This procedure enable the control of the layer thickness and three different layers were reproduced (2, 5 and 7 nm).

A 200 nm film of hydrogenated amorphous silicon (a-Si:H) was deposited by radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) on the samples (2, 5 and 7 nm layers). During the a-Si:H deposition (20 min) the substrate was heated to 150°C. Details about the a-Si:H films can be found in previous works30,31.

Structural and morphological characterization

To estimate the crystallite size and discard eventual cross-contaminations during the depositions, X-ray diffraction (XRD) measurements of the samples were performed in Panalytical X'Pert PRO with Cu Kα line radiation (λ = 1.540598 Å) using the Bragg-Brentano configuration with a 2Theta range of 15 to 80° and a step size of 0.02°.

The SEM images of samples were performed in a Zeiss Auriga Crossbeam microscope with a Schottky Field Emitter column. The Everhart-Thornley type SE and InLens detectors were used with an aperture size of 30.00 μm, working distance between 4.8 and 5.6 nm and an acceleration voltage from 1 to 20 kV. The transmission electron microscopy (TEM) was performed in a Hitachi H-8100 microscope which runs at 200 kV in samples deposited directly in the Cu TEM grid with 400 nm mesh.

Optical characterization

The transmittance and reflectance spectra were obtained in double beam UV-VIS-NIR Shimadzu spectrophotometer with an integrating sphere for reflectance and total transmittance measurements, in the range of 300-800 nm. The specular transmittance was measured in the range of and 300–2500 nm.

The PL measurements were performed both in the visible and in the infrared regions. In the first case, a Spex 1704 monochromator fitted with a cooled Hamamatsu R928 photomultiplier was used. The excitation source was the 457 nm line of a Melles–Griot laser with a power of ~30 mW. In the second region, a Bruker IFS 66v Fourier transform infrared (FTIR) spectrometer, equipped with a Ge diode detector, was used. The excitation source was a 514.5 nm line of an Ar+ ion laser, with a power of 151 mW. The samples were always inserted in a helium gas flow cryostat to allow the change of the temperature in the range 69–300 K. The excitation power was measured at the front of the cryostat window.