The effect of interposing nanocrystalline Si(B) P plus layer on the photovoltaic properties of a-Si: H tandem solar cells

Tandem amorphous silicon solar cells have attracted extensive interest because of better performance than single junction counterpart. As n/p junctions play an important role in the current transportation of tandem solar cells, it is important to design and fabricate good n/p junctions.The properties of the n/p junction of amorphous silicon (a-Si) were studied. We investigate the effect of interposing a nanocrystalline p(+) layer between n (top cell) and p (bottom cell) layers of a tandem solar cell. The crystalline volume fraction, the band gap, the conductivity and the grain size of the nanocrystalline silicon (nc-Si) p(+) layer could be modulated by changing the deposition parameters.Current transport in a-Si based n/p ("tunnel") junctions was investigated by current-voltage measurements. The voltage dependence on the resistance (V/J) of the tandem cells was examined to see if n/p junction was ohmic contact. To study the affection of different doping concentration to the properties of the nc-Si p(+) layers which varied the properties of the tunnel junctions, three nc-Si p(+) film samples were grown, measured and analyzed.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Improved diphasic nc-si/a-si : H I-layer materials using PECVD

Two series of films has been prepared by using a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystalline state. The photoelectronic properties of the films have been investigated as a function of crystalline fraction. In comparison with typical a-Si:H, these diphasic films with a crystalline fraction less than 0.3 show a similar optical absorption coefficient, higher mobility life-time product ( LT) and higher stability upon light soaking. By using the diphasic nc-Si/a-Si films as the intrinsic layer, a p-i-n junction solar cell has been prepared with an initial efficiency of 9. 10 % and a stabilized efficiency of 8.56 % (AM 1.5, 100 mW/cm(2)).

EFFECTS OF 1-MEV-ELECTRON IRRADIATION ON A-SI SOLAR-CELLS

In this report we present the effects of 1 MeV-electron irradiation on i a-Si:H films and solar cells. It is observed that in the dose range of 1.4-8.4 x 10(15) cm(-2) the defect creation has not reached its saturation level and the metastable defects caused by the irradiation cannot be completely removed by a two hour annealing at 200 degrees C for i a-Si:H films or at 130 degrees C for a-Si:H solar cells. The results may be understood in terms of a model based on two kinds of metastable defects created by 1 MeV-electron irradiation.

A NEW METHOD FOR DETECTING SUBTLE CHANGES IN SI-H BONDS OF A-SI-H

A new method developed for detecting possible subtle changes in Si-H bonds of a-Si:H after light soaking is described in detail. The method promises a sensitivity orders of magnitude higher than that reached by the usual IR spectrometer. Some preliminary results on phosphorus doped a-Si:H are given.

HYDROGEN DIFFUSION IN A-SI-H/A-SI STRUCTURE UNDER ELECTRICAL BIAS

By using the technique of elastic recoil detection (ERD), we have measured the hydrogen profiles in a-Si:H/a-Si structure samples annealed at various temperatures with and without electrical bias, and investigated the influence of electrical bias on hydrogen diffusion. The results show that hydrogen diffusion in a-Si is significantly enhanced by the action of electrical bias. The existence of the excess carriers, which are introduced by electrical injection, is considered to be responsible for the enhancement of hydrogen diffusion, and the microprocess of hydrogen transport has been exploited.

PHOTOEMISSION-STUDY ON THE SNO2/P A-SICX-H INTERFACE

Ultraviolet and X-ray photoemission spectroscopies (UPS and XPS) have been employed to SnO2 and its interface with P-type a-SiCx:H. The HeI valence band spectra of SnO2 show that the valence band maximum (VBM) shifts from 4.7 eV to 3.6 eV below the Fermi level (E(F)), and the valence band tail (VBT) extends up to the E(F), as a consequence of H-plasma treatments. The work function difference between SnO2 and P a-SiCx:H is found to decrease from 0.98 eV to 0.15 eV, owing to the increase of the work function of the treated SnO2. The reduction of SnO2 to metallic Sn is also observed by XPS profiling, and it is found that this leads to a wider interfacial region between the treated SnO2 and the successive growth of P a-SiCx:H.

A NEW RESULT IN TRANSIENT CURRENT EXPERIMENT ON A-SI-H P/I/N AND N/I/N DEVICES

The transient current response of a-Si:H in both p/i/n and n/i/n structures has been measured as a function of pulse intermittence and pulse amplitude. The results are consistent with the picture that in p/i/n samples the peculiar current response is caused by the competing contributions of electrons and holes which show themselves in different time scales.

Photodilatation effect of undoped a-Si:H films

A photodilatation effect of undoped a-Si:H films has been discovered by a differential dilatometric method. The film thickness has been found to increase instantaneously when the sample is exposed to light. The dilatation weakens with illumination time, following a stretched exponential law, and finally reaches a saturation value. The dilatation disappears when light is off. The results unambiguously show that the whole structure of the film becomes less compact and less stable under light exposure. The metastable change (Staebler-Wronski effect) could be a redistribution of different configurations after this photodilatation in the a-Si:H films.

Influence of the rapid thermal annealing on the properties of thin a-Si films

The variation of the structure, morphology and the electrical properties of thin amorphous silicon films caused by Rapid Thermal Annealing is studied. The films annealed at 1200degreesC for 2 minutes change their structure to polycrystalline and as a result their resistivity decreases by 4 orders of magnitude. Due to the small thickness of the as deposited amorphous silicon the obtained poly-Si is strongly irregular and has many discontinuities in its texture.

Characterization of diphasic nc-Si/a-Si : H thin films and solar cells

Hydrogenated silicon films with diphasic structure have been prepared by using a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystal. line state. The photoelectronic and microstructural properties of the films have been characterized by the constant photocurrent method (CPM), Raman scattering and nuclear magnetic resonance (NMR). In comparison with typical hydrogenated amorphous silicon (a-Si:H), these diphasic films with a crystalline fraction less than 0.3 show a similar optical absorption coefficient, lower deep-defect densities and higher stability upon light soaking. By using the diphasic nc-Si/a-Si films a p-i-n junction solar cell has been prepared With an initial efficiency of 8.51 % and a stabilized efficiency of 8.02 % on an area of 0.126 cm(2) (AM1.5, 100 mW/cm(2)).

Light-excited structural instability of a-Si : H.

With the accumulation of experimental data, it has been recognized by many that the light-induced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.

Preparation and photoluminescence of nc-Si/SiO2 MQW

The deposition rate and refractive index for a-Si(amorphous silicon) and SiO2 grown by PECVD were studied under different pressure, power and proportion of reactant source gases. a-Si/SiO2 MQW(multi-quantum well) with high quality was deposited under suitable conditions, in which the thickness of the a-Si layers is several nanometers. The sample of a-Si/SiO2 MQW was crystallized by laser annealing. Because of the confinement of the SiO2 layers, crystalline grains were formed during the a-Si layers were being crystallized. The size of the crystalline grains were not more than the thickness of the a-Si layers. The a-Si layers were crystallized to be nanometer crystalline silicon(nc-Si), therefore, nc-Si/SiO2 MQW was formed. For the a-Si/SiO2 MQW with 4.0nm a-Si wells separated by 5nm SiO2 barries, most of the a-Si were crystallized to silicon grains after laser annealing,and the size of the grains is 3.8nm. Strong photoluminescence with three peaks from the nc-Si/SiO2 MQW was detected at 10K. The wavelength of the peaks were 810nm, 825nm and 845nm, respectively.

The diphasic nc-Si/a-Si : H thin film with improved medium-range order

A series of silicon film samples were prepared by plasma enhanced chemical vapor deposition (PECVD) near the threshold from amorphous to nanocrystalline state by adjusting the plasma parameters and properly increasing the reactions between the hydrogen plasma and the growing surface. The microstucture of the films was studied by micro-Raman and Fourier transform infrared (FTIR) spectroscopy. The influences of the hydrogen dilution ratio of silane (R-H = [H-2]/[SiH4]) and the substrate temperature (T-s) on the microstructural and photoelectronic properties of silicon films were investigated in detail. With the increase of RH from 10 to 100, a notable improvement in the medium-range order (MRO) of the films was observed, and then the phase transition from amorphous to nanocrystalline phase occurred, which lead to the formation of diatomic hydrogen complex, H-2* and their congeries. With the increase of T-s from 150 to 275 degreesC, both the short-range order and the medium range order of the silicon films are obviously improved. The photoconductivity spectra and the light induced changes of the films show that the diphasic nc-Si/a-Si:H films with fine medium-range order present a broader light spectral response range in the longer wavelength and a lower degradation upon illumination than conventional a-Si:H films. (C) 2004 Elsevier B.V. All rights reserved.