The study of a new n/p tunnel recombination junction and its application in a-Si:H/μc-Si:H tandem solar cells

This paper reports that a double N layer （a-Si：H/μc-Si：H） is used to substitute the single microcrystalline silicon n layer （n-μc-Si：H） in n/p tunnel recombination junction between subcells in a-Si：H/μc-Si：H tandem solar cells. The electrical transport and optical properties of these tunnel recombination junctions are investigated by current voltage measurement and transmission measurement. The new n/p tunnel recombination junction shows a better ohmic contact. In addition, the n/p interface is exposed to the air to examine the effect of oxidation on the tunnel recombination junction performance. The open circuit voltage and FF of a-Si：H/μc-Si：H tandem solar cell are all improved and the current leakage of the subcells can be effectively prevented efficiently when the new n/p junction is implemented as tunnel recombination junction.

An analytical model to explore open-circuit voltage of a-Si:H/c-Si heterojunction solar cells

The effect of the parameters on the open-circuit voltage, V_(OC) of a-Si:H/c-Si heterojunction solar cells was explored by an analytical model. The analytical results show that V_(OC) increases linearly with the logarithm of illumination intensity under usual illumination. There are two critical values of the interface state density(D_(it)) for the open-circuit voltage(V_(OC)), D_(it)^(crit,1) and D_(it)crit,2(a few 1010 cm^(-2)·e V^(-1)). V_(OC) decreases remarkably when D_(it) is higher than D_(it)^(crit,1). To achieve high V_(OC), the interface states should reduce down to a few 1010 cm^(-2)·e V^(-1). Due to the difference between the effective density of states in the conduction and valence band edges of c-Si, the open-circuit voltage of a-Si:H/c-Si heterojunction cells fabricated on n-type c-Si wafers is about 22 mV higher than that fabricated on p-type c-Si wafers at the same case. V_(OC) decreases with decreasing the a-Si:H doping concentration at low doping level since the electric field over the c-Si depletion region is reduced at low doping level. Therefore, the a-Si:H layer should be doped higher than a critical value of 5×10^(18) cm^(-3) to achieve high V_(OC).

Preparation and Properties of a-Si:H Thin Films Deposited on Different Substrates

The effects of different substrates on the structure and hydrogen evolution from a-Si： H thin films deposited by plasma enhanced chemical vapour deposition were studied, as well as the similar films exposed to an hydrogen plasma. Spectroscopic ellipsometry and hydrogen evolution measurements were used to analyse the effects of the substrate and hydrogen plasma on the films microstructure, thickness, hydrogen content, hydrogen bonding and hydrogen evolution. The hydrogen evolution spectra show a strong substrate dependence. In particular on crystalline silicon substrate, the formation of bubbles was observed. For different substrates, hydrogen plasma treatments lightly affected the hydrogen evolution spectra. These results indicate that the action of hydrogen in a-Si：H was modified by the nature of the substrate.

Numerical simulation of a triple-junction thin-film solar cell based on μc-Si_(1-x)Ge_x :H

In this paper, a-Si：H/a-SiGe：H/μc-SiGe：H triple-junction solar cell structure is proposed. By the analyses of mi- croelectronic and photonic structures （AMPS-1D） and our TRJ-F/TRJ-M/TRJ-B tunneling-recombination junction （TRJ） model, the most preferably combined bandgap for this structure is found to be 1.85 eV/1.50 eV/1.0 eV. Using more realistic material properties, optimized thickness combination is investigated. Along this direction, a-Si：H/a-SiGe：H/μc-SiGe：H triple cell with an initial efficiency of 12.09% （Voc = 2.03 V, FF = 0.69, Jsc = 8.63 mA/cm^2, area = 1 cm^2） is achieved in our laboratory.

Study on stability of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon （a-Si：H） films with high and same order of magnitude photosensitivity （-10^5） but different stability were prepared by using microwave electron cyclotron resonance chemical vapour deposition system under the different deposition conditions. It was proposed that there was no direct correlation between the photosensitivity and the hydrogen content （CH） as well as H-Si bonding configurations, but for the stability, they were the critical factors. The experimental results indicated that higher substrate temperature, hydrogen dilution ratio and lower deposition rate played an important role in improving the microstructure of a-Si：H films. We used hydrogen elimination model to explain our experimental results.

Solid-phase Crystallization of Hydrogenated Amorphous Silicon on Glass Substrates

Amorphous silicon films prepared by PECVD on glass substrate have been crystallized by conventional furnace annealing and rapid thermal annealing(RTA), respectively. From the Raman spectra, X-ray diffraction and scanning electron microscope, it is found that the grain size is crystallized at 850℃ in both techniques. The thin film made by RTA is smooth and of perfect structure, the thin film annealed by FA has a highly structural disorder. An average grain size of about 30nm is obtained by both techniques.

Plasma enhanced chemical vapor deposition of excellent a-Si：H passivation layers for a-Si：H/c-Si heterojunction solar cells at high pressure and high power

The intrinsic a-Si：H passivation layer inserted between the doped a-Si：H layer and the c-Si substrate is very crucial for improving the performance of the a-Si：H/c- Si heterojunction （SHJ） solar cell. The passivation performance of the a-Si：H layer is strongly dependent on its microstructure. Usually, the compact a-Si：H deposited near the transition from the amorphous phase to the nanocrystalline phase by plasma enhanced chemical vapor deposition （PECVD） can provide excellent passivation. However, at the low deposition pressure and low deposition power, such an a-Si：H layer can be only prepared in a narrow region. The deposition condition must be controlled very carefully. In this paper, intrinsic a- Si：H layers were prepared on n-type Cz c-Si substrates by 27.12 MHz PECVD at a high deposition pressure and high deposition power. The corresponding passivation perfor- mance on c-Si was investigated by minority carrier lifetime measurement. It was found that an excellent a-Si：H passivation layer could be obtained in a very wide deposition pressure and power region. Such wide process window would be very beneficial for improving the uniformity and the yield for the solar cell fabrication. The a-Si：H layer microstructure was further investigated by Raman and Fourier transform infrared （FTIR） spectro-scopy characterization. The correlation between the microstructure and the passivation performance was revealed. According to the above findings, the a-Si：H passivation performance was optimized more elaborately. Finally, a large-area SHJ solar cell with an efficiency of 22.25% was fabricated on the commercial 156 mm pseudo-square n-type Cz c-Si substrate with the opencircuit voltage （Voc） of up to 0.732 V.

Computer modelling and analysis of the photodegradation effect in a-Si:H p–i–n solar cell

Using a previous model, which was developed to describe the light-induced creation of the defect density in the a-Si：H gap states, we present in this work a numerical modelling of the photodegradation effect in the a-Si：H p–i–n solar cell under continuous illumination. We first considered the simple case of a monochromatic light beam with a wavelength λ between 530–540 nm non uniformly absorbed, then the global standard solar spectrum（AM1.5） illumination is taken into account. The photodegradation is analysed on the basis of the resulting changes in the free carrier＇s densities, recombination rate, band structure, electrical potential and field, space charge, and current densities. Changes in the cell＇s external parameters： the open circuit voltage Voc, the short circuit current density Jsc, the fill factor FF and the maximum power density Pmaxare also presented.

Numerical simulation of the effect of the free carrier motilities on light-soaked a-Si:H p–i–n solar cell

Using a previous model, which was developed to describe the light-induced creation ofthe defect density in the a-Si：H gap states, we present in this work a computer simulation of the a-Si：H p-i-n solar cell behavior under continuous illumination. We have considered the simple case of a monochromatic light beam nonuniformly absorbed. As a consequence of this light-absorption profile, the increase of the dangling bond density is assumed to be inhomogeneous over the intrinsic layer （i-layer）. We investigate the internal variable profiles during illumination to understand in more detail the changes resulting from the light-induced degradation effect. Changes in the cell external parameters including the open circuit voltage, Voc, the short circuit current density, Jsc, the fill factor, FF, and the maximum power density, Pmax, are also presented. This shows, in addition, the free carrier mobility influence. The obtained results show that Voc seems to be the less affected parameter by the light-induced increase of the dangling bond density. Moreover, its degradation is very weak-sensitive to the free carrier mobility. Finally, the free hole mobility effect is found to be more important than that of electrons in the improvement of the solar cell performance.

Hydrogen bonding in hydrogenated amorphous silicon thin films prepared at different precursor gas temperatures with undiluted silane

Hydrogen bonding configurations and hydrogen content in hydrogenated amorphous silicon (a-Si:H) thin films prepared at different precursor gas temperatures with undiluted silane have been investigated by means of Fourier transform infrared (FTIR) spectroscopy.The results show that the gas temperature before precursor gases entering the glow-discharge zone re-markably influences the hydrogen bonding configurations and the hydrogen content in a-Si:H thin films.The hydrogen content decreases from 18% down to 11% when increasing the gas temperature from room temperature (RT) to 433 K.Meanwhile,the clustered hydrogen at the physical film surface or at the internal surfaces of the microvoids decreases,indicating that a-Si:H thin films are densified at higher precursor gas temperatures.For a-Si:H thin films deposited at gas temperature of 433 K,the isolated silicon-hydrogen bonding configuration is predominant in the testing films.

Structural evolution and electronic properties of phosphorus-doped hydrogenated amorphous silicon thin films deposited by PECVD

The relationship between structure and electronic properties of phosphorus-doped hydrogenated amorphous silicon(a-Si:H) thin films was investigated.Samples with different features were prepared by plasma enhanced chemical vapor deposition(PECVD) at various substrate temperatures.Raman spectroscopy and Fourier transform infrared(FTIR) spectroscopy were used to evaluate the structural evolution,meanwhile,electronic-spin resonance(ESR) and optical measurement were applied to explore the electronic properties of P-doped a-Si:H thin films.The results revealed that the changes in materials structure affect directly the electronic properties and the doping efficiency of dopant.

Influence of zinc phthalocyanines on photoelectrical properties of hydrogenated amorphous silicon

Composites consisting of hydrogenated amorphous silicon （a-Si： H, inorganic） and zinc phthalocyanine （ZnPc, organic） were prepared by vacuum evaporation of ZnPc and sequential deposition amorphous silicon via plasma enhanced chemical vapor deposition （PECVD）. The optical and electrical properties of the composite film have been investigated. The results demonstrate that ZnPc can endure the temperature and bombardment of the PECVD plasma and photoconductivity of the composite film was improved by 89.9% compared to pure a-Si： H film. Electron mobility-lifetime products/lr of the composite film were increased by nearly one order of magnitude from 6.96 × 10^-7 to 5.08 × 10^-6 cm2/V. Combined with photoconductivity spectra of the composites and pure a-Si： H, we tentatively elucidate the improvement in photoconductivity of the composite film.