Davit Yeghikyan

Back Amorphous-Crystalline Silicon Heterojunction (bach) photovoltaic device

Back Amorphous-Crystalline Silicon Heterojunction (BACH) photovoltaic device integrates a range of high efficiency features while using low temperature (∼200 °C) fabrication processes. High efficiency features include back contact metallization, optimum interfacial passivation, and front-side and rear-side reflective coatings and scatterers for maximum light trapping. Two types of BACH cell prototypes were fabricated in this work with the best performing cell having an AM1.5G conversion efficiency of 8.1%, V OC = 0.536 V, J SC = 20 mA/cm2 and FF = 75.5%. Theoretically, the BACH solar cell concept has the potential of attaining energy conversion efficiencies in the 20 – 25% range. In this first set of cells, the BACH cell performance is limited by poor surface passivation and un-optimized cell design. Recent experiments with various passivation procedures has led to high minority carrier lifetimes in the ms range while carrying out the processes at low temperature, thus opening the way to higher efficiency BACH cells.

Excellent low temperature passivation scheme with reduced optical absorption for back amorphous-crystalline silicon heterojunction (BACH) photovoltaic device

Low temperature processing of silicon photovoltaic (PV) solar cells with excellent passivation quality enables the effective use of ultra-thin wafers for solar cell manufacturing, thus paving the way for high-efficiency low-cost silicon photovoltaics. This article presents Back Amorphous-Crystalline Silicon Heterojunction (BACH) cell performance using low temperature (<;= 400°C) facile native oxide-PECVD silicon nitride (SiNx) dual layer passivation scheme. The cell performance is also compared with the BACH cells fabricated using intrinsic hydrogenated amorphous silicon (i-aSi:H) and PECVD SiNx layer passivation. Reduced optical absorption in the native oxide-SiNx passivation layer resulted in a higher short-circuit current, JSC, compared to the i-aSi:H-SiNx passivated cells. The fill-factor also improved for the native oxide-SiNx passivated cells owing to the improved transport properties. The i-aSi:H-SiNx passivated cells exhibited optimum cell performance of 10.9% efficiency with VOC of 598.7 mV, JSC of 34.3 mA/cm2 and fill-factor of 0.531. In contrast, a maximum cell efficiency of 16% is obtained for native oxide-SiNx passivated cells with VOC of 651 mV, JSC of 35.4 mA/cm2 and fill-factor of 0.694 for a 1 cm2 untextured cell (all measurements having been performed under AM 1.5 global spectrum illumination). The above untextured cell performance is a record efficiency for a back amorphous-crystalline silicon heterojunction PV device synthesized using all low temperature processes, exceeding the previously reported highest cell efficiency of ~15%.

Influence of Hydrogen Dilution on Properties of Silicon Films Prepared by D.C. Saddle-Field Glow-Discharge: Observation of Microcrystallinity

ABSTRACT In the D.C. saddle field glow discharge deposition the transition from amorphous to microcrystalline silicon thin films occurs when the silane concentration in the gas phase drops below 10%. We report here the results of Raman spectroscopy, SEM, TEM, and HRTEM studies of the film morphology. We estimate the average crystallite size to be in the range of 5 to 7 nm and the crystalline volume fraction of 25 to 35%. INTRODUCTION Amorphous Si is widely used for large area photovoltaic and microelectronic applications [1]. The use of microcrystalline Si is expected to improve stability against light-induced degradation and provide more efficient doping over that offered by amorphous silicon. Recently, we reported on the growth of mixed phase amorphous-microcrystalline silicon using the D.C. saddle field glow discharge deposition method [2]. The films were grown using hydrogen dilution of silane during the deposition. We were able to identify the growth conditions and the types of substrates that promote microcrystallinity. In this work we present the structural properties of saddle field glow discharge deposited microcrystalline Si films as a function of hydrogen dilution. The films were studied using Raman spectroscopy, SEM, TEM and high-resolution TEM.

1/f noise in p-type amorphous silicon

We have measured conductance fluctuations in four samples of p-type hydrogenated amorphous silicon, two doped at 10−4 and the other two at 5×10−2, at temperatures between 22 and 200 °C. The noise power density varies for the most part as 1/fα in the frequency range 2 Hz to 1 kHz, although deviations from a strict power law are observed. In all samples, the magnitude of the noise trends higher with temperature typically increasing by a factor of 5 over the temperature range. α also increases with temperature from near unity to over 1.4. The magnitude of the noise decreases as the Fermi level moves toward the valence band with increased doping. The dependence on doping and temperature is inconsistent with generation-recombination noise. Above 180 °C for the 10−4 doped samples, the noise fails to scale as the square of the bias current at low frequencies.

The augmented saddle field discharge characteristics and its applications for plasma enhanced chemical vapour deposition

A high ion flux parallel electrode plasma is proposed and studied in its DC configuration. By cascading a diode source region which supplies electrons and a saddle field region where these seed electrons are energized and amplified, the energy of ion bombardment on the substrate can be decoupled from the plasma density. The sufficiently large density of electrons and holes in the vicinity of the substrate raises the possibility to perform plasma enhanced chemical vapour deposition on insulating materials, at low sheath voltages (around 40 V in the configuration studied), at low temperatures in which the surface mobility of film growth species may be provided by the bombardment of moderate energy ions. As a benchmarking exercise, experiments are carried out on silane discharge characteristics and deposition of hydrogenated amorphous silicon (a-Si:H) on both silicon wafer and glass. The films grown at low anode voltages have excellent microstructures with predominantly monohydride bonds, sharp band tails, b...

Amorphous-Microcrystalline Silicon Films Obtained Using Hydrogen Dilution in a DC Saddle-Field Glow-Discharge

Amorphous-microcrystalline Si has been grown with hydrogen dilution using the DC saddle-field glow-discharge deposition technique. The five-electrode saddle-field system allows for independent control of the discharge parameters and of the substrate bias. The film structure was studied using Raman spectroscopy and SEM. We find that the structure of the films depends mainly on hydrogen dilution, substrate bias, electrical conductivity of the substrate, and chamber pressure. The deposition conditions, which promote growth of microcrystals, have been identified. It was found that the local electric field at the substrate surface plays a key role in obtaining microcrystallinity.

Amorphous-crystalline silicon heterojunction solar cells formed by the DC saddle field PECVD system: A deposition parameter optimization

The DC Saddle Field PECVD system was used to deposit hydrogenated amorphous silicon (a-Si:H) layers for high efficiency amorphous-crystalline silicon heterojunction (ACSHJ) solar cells. The plasma controlling parameters; including the chamber pressure, gas phase dopant concentration for the p-type a-Si:H (a-Si:H(p+)) emitter, and substrate temperature were varied. The substrate temperature was found to be a critical parameter for the deposition of intrinsic a-Si:H as epitaxial formation can occur with just a temperature increase of 10°C. The processing capabilities have been developed to construct ACSHJ solar cells with 15.5% conversion efficiency for a 4.2 cm2 area.

Defect Density in Doped Amorphous Layer and Interface of Silicon Heterojunction Devices Obtained with the Constant Photocurrent Method

We present here the application of CPM for the examination of defect density in the doped amorphous silicon layer and the amorphous-crystalline silicon interface of silicon heterojunction photovoltaic devices. CPM derived absorption and internal quantum efficiency (QE) spectra of the devices are measured. A simple model is proposed wherein the amorphous layer and the interface constitute one absorbing layer while the crystalline substrate forms the other absorbing layer. On the basis of this model, we obtain the combined defect density in the amorphous film and interface. Also, an estimate of the defect density at the interface is inferred using an independent measure of the defect density in a similar amorphous film

Amorphous-crystalline silicon interface prepared using DC saddle-field pecvd

The DC saddle field glow discharge method was used to deposit a-Si∶H in order to passivate c-Si surfaces. The process temperature and the thickness of the a-Si∶H films were varied. In addition subsequent annealing of the smaples were studied. Passivation quality of the a-Si∶H overlayers were studied by measuring the effective minority carrier lifetime in the heterostructures as a function of the minority carrier density in the c-Si wafer. These results are then used to model the surface recombination mechanism in our samples. The defect density and the charge density at the interface are inferred which helps us to distinguish between the effects of electric field and chemical passivation at the interface. It is shown that for our intrinsic a-Si∶H samples improvements in surface passivation are directly correlated with the reduction of interface defect density and field effect passivation is minimal. We have achieved surface passivation with effective carrier lifetime ≫ 5 ms for a 40 nm intrinsic a-Si∶H sample deposited at a process temperature of 200 °C. It is also demonstrated that subsequent annealing, at 240 °C, of the samples which were prepared at process temperatures ≪ 240 °C drastically increases the effective lifetime.