• Energy Research

In this paper we present new results on doped μc-Si:H thin films deposited by hot-wire chemical vapour deposition (HWCVD) in the very low temperature range (125-275°C). The doped layers were obtained by the addition of diborane or phosphine in the gas phase during deposition. The incorporation of boron and phosphorus in the films and their influence on the crystalline fraction are studied by secondary ion mass spectrometry and Raman spectroscopy, respectively. Good electrical transport properties were obtained in this deposition regime, with best dark conductivities of 2.6 and 9.8 S cm -1 for the p- and n-doped films, respectively. The effect of the hydrogen dilution and the layer thickness on the electrical properties are also studied. Some technological conclusions referred to cross contamination could be deduced from the nominally undoped samples obtained in the same chamber after p- and n-type heavily doped layers.

The metastable defects of a-Si:H samples annealed at temperatures in the 300-550°C range have been studied by photothermal deflection spectroscopy (PDS). The light-soaked samples show an increase in optical absorption in the 0.8 to 1.5 eV range. The metastable defect density decreases when the annealing temperature increases, while the defect density increases. This decrease in the metastable defect density shows an almost linear correlation with the decrease in the hydrogen content of the samples, determined by IR transmission spectroscopy and thermal desorption spectroscopy.

In this work, we study bifacial heterojunction silicon solar cells fabricated by hot-wire CVD on both p- and n-type crystalline silicon substrates. In these devices, hydrogenated microcrystalline silicon doped layers are combined with thin intrinsic amorphous silicon buffers to form the heterojunction emitter and the low-temperature back surface field contact. The maximum temperature achieved in the whole fabrication process is 200 °C. A comprehensive electrical characterization has been done, including current density-voltage characteristics and external quantum efficiency curves with illumination from both sides. These results evidence the feasibility of efficient bifacial heterojunction silicon solar cells fully processed at low temperature. Peer Reviewed

Abstract Using different methods, intrinsic a-Si:H/a-Si:H interfaces were artificially generated in hydrogenated amorphous silicon films. Three types of interfaces were studied: (i) interfaces in which only the regrowth mechanisms were present; (ii) interfaces where both the regrowth mechanisms and the initial transient state of the discharge were present; and (iii) interfaces in which the initial transient state of the discharge was the dominant mechanism. The resulting material was characterized by measuring the hydrogen content, the electronic density of states, the transport properties and the film stability as a function of the interface type and density. It was found that samples containing interfaces of the first type are not very different from samples without interfaces and that transient discharge processes produce accumulation of hydrogen in the interface, which could relieve strains in the network. In order to investigate the influence of this strain-relieving mechanism on the Staebler-Wronski effect, p-i-n solar cells containing these H-rich interfaces were prepared and measured, together with “normal” cells. It was found that the former cells are more stable than the “normal” cells. The results of the present paper provide a new and promising way to circumvent the problem of the long term stability of hydrogenated amorphous solar cells.

Abstract The modification of the electrical and optical properties of a-Si : H by the incorporation of aluminium has been studied. The a-Si : H,Al films were obtained by means of dc glow discharge decomposition of gas silane and simultaneous evaporation of alumunium using the same discharge conditions and varying only the evaporation rate of Al. The study of the films properties includes V-NIR transmission spectroscopy, temperature dependence of dark conductivity and steady state photoconductivity. From these measurements an important variation of both optical gap and dark conductivity has been observed as the aluminium content in the films increases. Electrical conduction probably takes place, in the analyzed temperature range (300–450 K), through extended states of the valence band. It has also been observed that the normalized photoconductivity of the slightly Al doped films is three or four orders of magnitude lower than that of undoped a-Si : H film.

Abstract Although the beneficial effect of the buffer layer between the p- and i-layer of amorphous silicon solar cells has been known for many years, the role of this layer is controversial. This paper examines the effect of the buffer layer using a new equivalent circuit for these devices (Merten et al. IEEE Trans. Electron Dev. 45 (1988) 423–429 [1] ). The parameters of this model can be easily assessed by variable illumination measurements (VIM) of the devices' I ( V )-curve. With the model, collection of carriers in the bulk of the cell is easy and clearly separated from the diode behaviour of the device. The VIM-method allows for a complete analysis of the thin film cells, covering both technological and physical topics. It is shown that the dominant effect increasing the efficiency of the cells with buffer layer is the reduction of the hole injection from the p-layer which leads to a reduced diode term. The buffer layer only slightly reduces the recombination in the i-layer. This reduction mainly occurs in a region close to the p/i-interface and cannot be observed with red light (homogeneous carrier generation).

Undoped hydrogenated microcrystalline silicon was obtained by hot-wire chemical vapour deposition at different silane-to-hydrogen ratios and low temperature (<300 °C). As well as technological aspects of the deposition process, we report structural, optical and electrical characterizations of the samples that were used as the active layer for preliminary p-i-n solar cells. Raman spectroscopy indicates that changing the hydrogen dilution can vary the crystalline fraction. From electrical measurements an unwanted n-type character is deduced for this undoped material. This effect could be due to a contaminant, probably oxygen, which is also observed in capacitance-voltage measurements on Schottky structures. The negative effect of contaminants on the device was dramatic and a compensated p-i-n structure was also deposited to enhance the cell performance.