Axel Straub

Large open-circuit voltage improvement by rapid thermal annealing of evaporated solid-phase-crystallized thin-film silicon solar cells on glass

In this letter, we investigate the impact of rapid thermal annealing (RTA) on thin-film polycrystalline silicon (pc-Si) solar cells on glass made by evaporation of amorphous silicon (a-Si) and subsequent solid-phase crystallization (SPC). These devices have the potential to deliver low-cost photovoltaic electricity and are named EVA cells (SPC of EVAporated a-Si). The RTA is used to perform a high-temperature (>700°C) process for point defect annealing and dopant activation. RTA processes have predominantly been developed for wafer-based devices yet also have great potential for low-temperature devices such as thin-film pc-Si on glass solar cells. Parameter variations are performed on EVA solar cells to determine optimum values for point defect removal and dopant activation while minimizing dopant diffusion causing junction smearing. The 1-Sun open-circuit voltage, Voc, of the as-crystallized pc-Si devices is rather modest (135mV). However, after RTA and subsequent hydrogen passivation in a rf PECVD plasm...

Impedance analysis : A powerful method for the determination of the doping concentration and built-in potential of nonideal semiconductor p-n diodes

An impedance analysis method is introduced that enables the reliable determination of the doping concentration and the built-in potential of nonideal semiconductor p‐n diodes featuring poor values for the shunt resistance, the series resistance, and∕or the diode saturation current. The sample doping concentration on the lightly doped side of the p‐n junction and the built-in potential are determined using the classic 1∕C2 vs V representation. The small-signal capacitance C for each reverse bias voltage V is directly extracted from the measured frequency dependence of the sample’s impedance Z. A crucial feature of the method is the determination of the diode’s series resistance and shunt resistance for each reverse bias voltage used. The method is verified using high-quality p‐n junction diodes fabricated in silicon wafer substrates and its capabilities are demonstrated on nonideal p‐n junction diodes fabricated in polycrystalline silicon thin films on glass substrates.

Recent advances in polycrystalline silicon thin-film solar cells on glass at UNSW

Polycrystalline Si (pc-Si) thin-film solar cells on glass are a very promising approach for lowering the cost of photovoltaic solar electricity. This paper reports on the status of three distinctly different pc-Si thin-film solar cells on glass under development at the University of New South Wales (UNSW). The cells are termed EVA, ALICE and ALICIA, are less than 3 microns thick, and are made by vacuum evaporation, a fast and inexpensive Si deposition method. EVA cells are made on non-seeded glass, whereas ALICE and ALICIA are both made on a thin large-grained pc-Si seed layer formed on glass by metal-induced crystallisation. All three solar cells seem to be capable of voltages of over 500 mV and, owing to their potentially inexpensive and scalable fabrication process, have significant industrial appeal.

Light trapping properties of evaporated poly-silicon films on AIT-textured glass substrates

This paper explores the optical properties of e-beam evaporated polycrystalline silicon films deposited on a variety of glass surface morphologies produced by the aluminium-induced texturing (AIT) process. We find that very small features laterally one micron or smaller most effectively promote light trapping in films 1-3 /spl mu/m thick. Light trapping, assessed from reflectance and transmittance measurements, is almost random when the HF concentration required in an etch solution of the AIT process is minimised.

The influence of defects and postdeposition treatments on the free carrier density in lightly phosphorus-doped large-grained polycrystalline silicon films

The influence of postdeposition treatments (rapid thermal annealing and hydrogenation) on the doping of large-grained polycrystalline silicon p+nn+ thin-film diodes on glass substrates is investigated using resistivity and impedance analysis measurements. Whereas in the lightly phosphorus-doped base region both treatments are found to cause an increase in the active doping concentration, hydrogenation decreases the active doping concentration of both heavily doped layers (Al and P). The different behavior is attributed to acceptorlike defects which are present in the nonhydrogenated base region in a similar concentration as the atomic phosphorus concentration and which are well passivated by hydrogenation. From posthydrogenation annealing experiments and temperature-dependent impedance analysis measurements, different temperature dependences and activation energies (depending on the posthydrogenation annealing temperature) are found for the lightly doped base region. The temperature dependences are quanti...

Structural quality of smooth AIC poly-Si films on glass substrates

A method for removing Si precipitations from the surface of AIC (aluminium-induced crystallisation) poly-Si films is introduced. The basic idea is to remove the aluminium oxide layer that is present between the poly-Si film and the Si precipitates and thus enable a lift-off process. Furthermore, a detailed structural investigation of the resulting smooth AIC poly-Si film prepared on SiN-coated planar glass is performed. Based on Raman, UV reflectance and plan-view TEM, the overall crystal quality is found to be excellent for a poly-Si film on glass. Especially the fact that 10-micron grains without dendritic growth pattern are obtained is encouraging. However, there is still room for improvement as both Raman and plan-view TEM reveal areas (/spl sim/10% of total surface area) of sub-micron grains and/or twinned grains. Work is in progress to understand the mechanism behind these results.

Towards 400 mV ALICIA thin-film silicon solar cells on glass

Polycrystalline silicon (pc-Si) is a promising candidate for thin-film photovoltaics. In this paper, the advantages, evolution of the technology, and the latest results of ALICIA pc-Si solar cells on glass are presented. ALICIA stands for aluminium-induced crystallisation ion-assisted deposition. In the ALICIA technology, a polycrystalline seed layer is formed on borosilicate glass by AIC (aluminium-induced crystallisation) and epitaxially thickened by non-ultra-high vacuum ion-assisted deposition (IAD). The key factors which led to the improvement of the open-circuit voltage from 130 mV in June 2003 to 386 mV in June 2004 are discussed. Furthermore, an ALICIA solar cell with a 1-Sun energy conversion efficiency of 2.2 % is presented. The short-circuit current density is 11.4 mA/cm/sup 2/ and the V/sub oc/ is 380 mV. These results were achieved by an optimisation of the temperature profile of the epitaxial growth process, by rapid thermal annealing (RTA), and by an increased hydrogenation temperature (/spl sim/480 /spl deg/C). The presented results confirm that ALICIA is a very promising thin-film PV technology.

V/sub oc/ improvement of evaporated SPC thin-film Si solar cells on glass by rapid thermal annealing

Rapid thermal processing has predominantly been developed for wafer-based devices yet also has great potential for low-temperature devices such as thin-film polycrystalline silicon (pc-Si) on glass solar cells. The present paper investigates the impact of rapid thermal annealing (RTA) on thin-film pc-Si solar cells on glass made by evaporation of a-Si and subsequent solid-phase crystallization (SPC). These devices are referred to by us as EVA cells (SPC of evaporated a-Si). RTA parameter variations are performed to determine optimum values for point defect removal and dopant activation, and to maximize the open-circuit voltage (V/sub oc/). Upon hydrogenation in a RF PECVD hydrogen plasma, a 1-Sun V/sub oc/ of 443 mV is realized. Through optimization, a V/sub oc/ of 500 mV is clearly within the reach of the EVA technology.

EVA polycrystalline silicon thin-film solar cells on textured glass

EVA solar cells are a novel type of poly-Si thin-film solar cell on glass that has recently been developed at UNSW (EVA = solid-phase crystallisation of evaporated a-Si). In this paper, EVA solar cells are for the first time made on textured glass. The glass substrates are textured with the UNSW-developed AIT process (AIT = aluminium-induced texture). The AIT texture is shown to have a beneficial effect on the absorption in EVA films (antireflection and light trapping effect). A SiN barrier layer on the AIT-textured glass substrates is shown to significantly improve both the crystalline quality of EVA poly-Si films and the energy conversion efficiency of the resulting solar cells. The results reported in this paper represent a good basis for a thorough optimization study of this promising poly-Si thin-film PV technology on glass.