Ohmic contacts fabricated on moderately doped p-type GaAs by sputtering deposition and a laser-firing process

A novel approach is used to achieve ohmic contacts on moderately doped p-type GaAs substrates. A laser-firing process is used instead of the conventional annealing step. The morphology of the crater created by the laser-firing process and the electrical response of the metal–semiconductor contact are characterized.Peer ReviewedPostprint (published version

Capacitance study of a polystyrene nanoparticle capacitor using impedance spectroscopy

In this study, a metal-insulator-metal capacitor structure is fabricated using polystyrene nanoparticles. Impedance spectroscopy is used to evaluate the performance of this capacitor in which we found a significant magnitude increment in capacitance and loss tangent compared with an equivalent ideal capacitor with continuous polystyrene layer and same geometry. Capacitance values up to 11.7 and loss tangent values up to 387 (at 0.1 Hz) larger than the expected for a continuous polystyrene MIM capacitor are achieved. The capacitor shows a good stable capacitive behaviour in the frequency range from 0.1 Hz to 100 kHz at room temperature, 30 °C, 40 °C and 50 °C without an effective relaxation process. Nyquist, capacitance, loss tangent and normalized powers curves are analysed by modified Randles model. Also, a slight decrease in the capacitance value at 50 °C is observed, which that may be attributed to space charge localized at the nanoparticles interface and that are affected by the temperature changes.Peer ReviewedPostprint (author's final draft

TCO-free low-temperature p+ emitters for back-junction c-Si solar cells

In this work, we report on the fabrication and characterization of n-type c-Si solar cells whose p+ emitters are based on laser processed aluminum oxide/silicon carbide (Al2O3/SiCx) films. The p+ emitter is defined at the rear side of the cell and it consists of point-like laser-diffused p+ regions with a surface charge induced emitter in between based on the high negative charge located at the Al2O3/c-Si interface. These emitters are fabricated at low temperature (1000 nm) that reach the rear surface of the cell resulting in an excellent back reflector. We fabricated solar cells with distance between p+ regions or pitch ranging from 200 to 350 µm with a front surface based on silicon heterojunction technology. Best efficiency (18.1%) is obtained for a pitch of 250 µm as a consequence of the trade-off between Voc and FF values.Peer ReviewedPostprint (published version

Impedance modeling of silica nanoparticle metal insulator metal capacitors

In this study, we have fabricated metal-insulator-metal (MIM) capacitors where the insulator layer is made of 255¿nm diameter silica nanospheres. The MIM devices have been characterized and modeled by electrochemical impedance spectroscopy (EIS) and charge-discharge transients. Fitting the results with modified Randles models agreed well with three constant phase elements, three leakage resistors, and a Warburg element. According to the results of the fitting of the charge-discharge measurements and of the modified Randles model, values of real capacitances up to thousand times larger than the theoretical capacitance of a similar capacitor with a continuous layer dielectric are found. These unexpected high capacitances seemed to be related to the ability of the nanospheres to trap electric charges due to surface hydroxyl groups that are originated by the adsorption of water molecules, thereby indicating that the environmental humidity plays a role. This has been ascertained by measurements at several temperatures above the ambient and the resulting capacitance decreases as temperatures increases. Furthermore, active and reactive parts of the complex power have been measured showing capacitive or resistive behavior depending on the frequency. These results suggest that this novel MIM device based on nanospheres may be a new baseline technology for supercapacitor technology.Peer ReviewedPostprint (author's final draft

Numerical simulations of rear point-contacted solar cells pn 2.2 Wcm p-type c-Si substrates

Rear surface of high-efficiency crystalline silicon solar cells is based on a combination of dielectric passivation and point-like contacts. In this work, we develop a 3D model for these devices based on 2.2 Ωcm p-type crystalline silicon substrates. We validate the model by comparison with experimental results allowing us to determine an optimum design for the rear pattern. Additionally, the 3D model results are compared with the ones deduced from a simpler and widely used 1D model. Although the maximum efficiency predicted by both models is approximately the same, large deviations are observed in open-circuit voltage and fill factor. 1D simulations overestimate open-circuit voltage because Dember and electrochemical potential drops are not taken into account. On the contrary, fill factor is underestimated because of higher ohmic losses along the base when 1D analytical model is used. These deviations are larger for relatively low-doped substrates, as the ones used in the experimental samples reported hereby, and poor passivated contacts. As a result, 1D models could mislead to too short optimum rear contact spacing.Peer ReviewedPostprint (published version

N-type emitters passivation through antireflective phosphorus doped a-SiCxNy:H(n) stacks

This paper studies the passivation of industrially textured deep silicon emitters using amorphous silicon carbonitride layers in stack configuration, deposited by plasma enhanced chemical vapor deposition. With this technique, emitter saturation current density can be decreased to values around 250 fA middot cm-2. As a consequence, open circuit voltages can be increased 25 mV achieving values around 640 mV.Postprint (published version

Optimization of laser processes in n+Emitter formation for c-Si solar cells

Punctual phosphorus diffused emitters were achieved by laser patterning phosphorus doped a-SiCx:H films deposited by PECVD as a doping source. Two different lasers at wavelengths of 1064 nm and 532 nm were used. Phosphorus diffusion was confirmed by Secondary Ion Mass Spectroscopy. We explored the effect of pulse energy and number of pulses per diffused point. The results show that a fine tune of the energy pulse is critical while the number of pulses has minor effects. Scanning Electron Microscopy (SEM) pictures and optical profilometry showed a laser affected area where the c-Si is melted, ejected and solidified quickly again. Typically, the diameter of the affected area for 1064 nm laser is between two and four times greater than for 532 nm laser. Optimum parameters for both lasers were determined to obtain best J-V curves nearly to ideal diode behavior. Comparing best J-V results, lower emitter saturation current density (Jo) and contact resistance are obtained with 532 nm laser. The improvement in Jo can be related mainly to the smaller affected areas observed by SEM while lower contact resistance can be attributed to that 532 nm laser has a more superficial action resulting in higher phosphorus concentration at the surface. The expected open voltage circuit for finished solar cells using these emitters is in the range of 640 mV for 532 nm laser and 620 mV for 1064 nm one.Postprint (published version

Laboratori d'electrònica i física

Manual de les pràctiques de laboratori de les Assignatures Fonaments de Física i de Fonaments d'Electrònica2011/201

Low temperature back-surface-field contacts deposited by hot-wire CVD for heterojunction solar cells

The growing interest in using thinner wafers (< 200 µm) requires the development of low temperature passivation strategies for the back contact of heterojunction solar cells. In this work, we investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by Hot-Wire CVD. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. The microstructure of the deposited thin films has been comprehensively studied by Spectroscopic Ellipsometry in the UV–visible range. The effective recombination velocity at the back surface has been measured by the Quasi-Steady-State Photoconductance technique. Complete double-side heterojunction solar cells (1 cm2) have been fabricated and characterized by External Quantum Efficiency and current–voltage measurements. Total-area conversion efficiencies up to 14.5% were achieved in a fully low temperature process (< 200 °C).Peer ReviewedPostprint (author's final draft

Surface passivation and optical characterization of Al2O3/a-SiCx stacks on c-Si substrates

The aim of this work is to study the surface passivation of aluminum oxide/amorphous silicon carbide (Al2O3/a-SiCx) stacks on both p-type and n-type crystalline silicon (c-Si) substrates as well as the optical characterization of these stacks. Al2O3 films of different thicknesses were deposited by thermal atomic layer deposition (ALD) at 200 °C and were complemented with a layer of a-SiCx deposited by plasma-enhanced chemical vapor deposition (PECVD) to form anti-reflection coating (ARC) stacks with a total thickness of 75 nm. A comparative study has been carried out on polished and randomly textured wafers. We have experimentally determined the optimum thickness of the stack for photovoltaic applications by minimizing the reflection losses over a wide wavelength range (300–1200 nm) without compromising the outstanding passivation properties of the Al2O3 films. The upper limit of the surface recombination velocity (Seff,max) was evaluated at a carrier injection level corresponding to 1-sun illumination, which led to values below 10 cm/s. Reflectance values below 2% were measured on textured samples over the wavelength range of 450–1000 nm.Postprint (published version