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Large-grained, n + n-type polycrystalline silicon (poly-Si) films were obtained on alumina substrates by combining the aluminium induced crystallization (AIC) process of amorphous silicon and chemical vapour deposition (LPCVD) at high temperature (1000 °C) for the epitaxial thickening. The n + seed layer was obtained by phosphorus doping of the AIC layer. The electron backscattering diffraction (EBSD) technique was used for the crystallographic analysis of the poly-Si thin films. Seed layers with an average grain size of 7.6 μm were obtained on alumina substrates by exchange annealing at 475 °C for 6 h. Heterojunction emitter (HJE) solar cells were fabricated on such layers and their characteristics were monitored. IQE measurements show that n-type material based solar cells led to a much higher current collection over a large part of the spectrum compared to p-type cells. Accordingly a high effective diffusion length of about 2 μm for n-type heterojunction solar cells was obtained while it is about 0.9 μm for the p-type cell. As a result, the first n-type solar cells showed efficiencies above 5%, which is a very promising result considering that no optimization nor texturing have been applied so far.

In this work, we investigated the formation of n-type polysilicon films on alumina substrates by overdoping a p-type silicon layer obtained by aluminium induced crystallization of amorphous silicon (AIC), and subsequent epitaxy. The phosphorus doping of the AIC was carried out by thermal diffusion from a solid source. The structural quality of the n-type Si film was monitored by optical microscope and scanning electron microscope (SEM). The doping efficiency was determined by resistivity measurements and secondary ion mass spectroscopy (SIMS). The sheet resitivity changed from 2700Ω/sq to 19.6Ω/sq after thermal diffusion at 950°C for 1h, indicating the overdoping effect. The SIMS profile carried out after the high temperature epitaxy exhibits a two steps phosphorus distribution, indicating the formation of an n+n structure.

There are three ways in which the cell efficiency of silicon solar cells may be improved by better exploitation of the solar spectrum: down-conversion (cutting one high energy photon into two low energy photons), photoluminescence (shifting photons into wavelength regions better accepted by the solar cell) and up-conversion (combining low energy photons to one high energy photon). In this paper, we present the state of the art of these three methods and discuss the suitability of materials available today for application to silicon solar cells.

Two approaches can be followed to reduce thermalisation and transmission losses in solar cells and thereby better exploit the solar spectrum. Firstly, modification may be done to lower energies, which can be via down-conversion, where one high energy photon is split into two or more low energy photons, or photoluminescence, where photons are shifted into different wavelength regions. Secondly, modification may be done to higher energies using up-conversion, where two or more low energy photons are combined to form one high energy photon. In this paper, the state of the art of these methods and the suitability of materials available today for application to silicon solar cells are presented.

Hydrogen is released from hydrogenated silicon nitride (SiNx:H) during thermal treatments. The formation of molecular hydrogen (H2) in SiNx:H layers with low mass density is confirmed by Raman spectroscopy. However, no H2 is observed in layers with a high mass density despite clear evidence that hydrogen diffuses through those layers. Therefore hydrogen migrates in those layers in a different form. This is consistent with the observed improvement of the hydrogen passivation of silicon substrates using thermally treated high density SiNx:H antireflection coatings.

We give an overview and analysis of research on thin-film polycrystalline Si solar cells on foreign substrates, with layers formed at intermediate temperatures (700–1300 °C), covering substrates, deposition techniques and solar cell processing. The main deposition techniques that have been investigated are solution growth (SG) and chemical vapour deposition (CVD). Insufficient nucleation on foreign substrates is an important problem with SG, which could be solved with appropriate surface preparation techniques and growth conditions. With CVD, continuous layers are achieved routinely, but the electronic quality of the material is usually very low. Solar cell performance appears to be limited by a very large recombination activity of grain boundaries. Improvement can be achieved reducing the grain boundary density and recombination activity, and experimental examples are given. Devices have been demonstrated with efficiencies up to 5.5%.