Junction Amorphous Silicon Solar Cells Research Articles

Towards upconversion for amorphous silicon solar cells

Upconversion of subbandgap light of thin film single junction amorphous silicon solar cells may enhance their performance in the near infrared (NIR). In this paper we report on the application of the NIR–vis upconverter β-NaYF 4:Yb 3+(18%) Er 3+(2%) at the back of an amorphous silicon solar cell in combination with a white back reflector and its response to infrared irradiation. Current–voltage measurements and spectral response measurements were done on experimental solar cells. An enhancement of 10 μA/cm 2 was measured under illumination with a 980 nm diode laser (10 mW). A part of this was due to defect absorption in localized states of the amorphous silicon.

The AM1.5 absorption factor of thin-film solar cells

Both for photovoltaic and photovoltaic/thermal applications insight is required in the mechanisms that determine the effective absorption factor A eff . A eff is the part of the incident irradiation that is converted into heat, taking into account that part of the energy is withdrawn as electricity. A eff was studied for five different solar cell technologies using an optical simulation model and ranges from 74% for single junction amorphous silicon solar cells to 82% for CIGS solar cells. The simulations also show that the longer wavelength part of the spectrum is hardly absorbed by the active semiconductors, but mostly by free carrier absorption in the transparent conductive oxide film present in these devices.

Polar coordinated fuzzy controller based real-time maximum-power point control of photovoltaic system

It is crucial to improve the photovoltaic (PV) system efficiency and to develop the reliability of PV generation control systems. There are two ways to increase the efficiency of PV power generation system. The first is to develop materials offering high conversion efficiency at low cost. The second is to operate PV systems optimally. However, the PV system can be optimally operated only at a specific output voltage and its output power fluctuates under intermittent weather conditions. Moreover, it is very difficult to test the performance of a maximum-power point tracking (MPPT) controller under the same weather condition during the development process and also the field testing is costly and time consuming. This paper presents a novel real-time simulation technique of PV generation system by using dSPACE real-time interface system. The proposed system includes Artificial Neural Network (ANN) and fuzzy logic controller scheme using polar information. This type of fuzzy logic rules is implemented for the first time to operate the PV module at optimum operating point. ANN is utilized to determine the optimum operating voltage for monocrystalline silicon, thin-film cadmium telluride and triple junction amorphous silicon solar cells. The verification of availability and stability of the proposed system through the real-time simulator shows that the proposed system can respond accurately for different scenarios and different solar cell technologies.

RF Sputter Deposition of Indium Oxide / Indium Iron Oxide Thin Films for Photoelectrochemical Hydrogen Production

AbstractThis project focuses on using indium oxide and indium iron oxide as an alloy to make a protective thin film (transparent, conductive, and corrosion resistant or TCCR) for amorphous silicon based solar cells, which are used in immersion-type photoelectrochemical cells for hydrogen production. From the work completed, the results indicate that samples made at 250 °C with 30 Watt of indium and 100 Watt of indium iron oxide, and a sputter deposition time of ninety minutes produced optimal results when deposited directly on single junction amorphous silicon solar cells. At 0.65 Volts, the best sample displays a maximum current density of 21.4 mA/cm2.

Simulation studies on the effect of a buffer layer on the external parameters of hydrogenated amorphous silicon p-i-n solar cells

Device modeling of p-i-n junction amorphous silicon solar cells has been carried out using the amorphous semiconductor analysis (ASA) simulation programme. The aim of the study was to explain the role of a buffer layer in between the p-and i-layers of the p-i-n solar cell on the external parameters such as dark current density and open circuit voltage. Investigations based on the simulation of dark I–V characteristics revealed that as the buffer layer thickness increases the dark current for a given voltage decreases.

Advanced optical characterization of disordered semiconductors by Fourier transform photocurrent spectroscopy

The paper summarizes progress in the Fourier transform photocurrent spectroscopy (FTPS) since 2001 when it was introduced for the first time for evaluation of the spectral dependence of the optical absorption coefficient in microcrystalline silicon thin films. We concentrate on the appropriate measuring conditions and evaluation procedures for a correct data interpretation of various thin films and solar cell structures. We show how the measured and finally evaluated absorption spectra differ in case of single junction amorphous and microcrystalline silicon solar cells as well as for a micromorph tandem. The key issue of the tandem structure diagnostics is a separation of FTPS signals from amorphous and microcrystalline parts of a stacked structure and their correct interpretation. For an appropriate simulation of the light propagation within the structure and confirmation of our measuring approach the computer model cell has been used.

ZnO:Al films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells

Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are usually achieved by RF sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, on the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented.

The effect of spectral variations on the performance parameters of single and double junction amorphous silicon solar cells

We present the results of an experimental investigation into the effects of spectral variations in a maritime climate on the performance parameters of single and double junction amorphous silicon solar cells. Such considerations are important for accurate modelling of system performance. It is shown that one can distinguish between two effects: a primary effect that results from variations in the total irradiance in the spectrally useful range of the device, and a secondary (mismatch) effect observed in double junction devices that is related to details of device structure. Results showing the impact on the short-circuit current, the current at the maximum-power-point, the fill factor and the overall efficiency are discussed.

Development of High Quality P-Type Hydrogenated Amorphous Silicon Oxide Film and Its Use in Improving the Performance of Single Junction Amorphous Silicon Solar Cells

By using radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) method (13.56 MHz) we have developed high quality wide band gap p-type hydrogenated amorphous silicon oxide (p-a-SiO:H) films having characteristics suitable for use as the window layer for single and multijunction amorphous silicon (a-Si) solar cells. The films have been characterized in detail. The p-a-SiO:H films having thickness ≥100 Å has photoconductivity (σph) one order of magnitude higher than that for p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) films having similar band gap. However, at thickness ∼100 Å, required for window layer, the σph of p-a-SiO:H film is ∼102 times higher than that of p-a-SiC:H film. This difference in thickness dependence of σph has been attributed to the structural difference of the two types of window layers. We have fabricated single junction p-i-n structure a-Si solar cells on transparent conducting oxide (TCO) coated glass substrates with both p-a-SiO:H and p-a-SiC:H film as the window layer. In the former case the Fill Factor of the solar cell is higher by ∼10%. The improvement in Fill Factor is due to the higher σph of p-a-SiO:H resulting in lower series resistance.

Absence of a Steady State in Hydrogen Diluted Silane Plasmas Due to Mass Dependent Gas Pumping Speeds and Its Consequences

AbstractWe have investigated the cause for continued changes in the silane related species observed using mass spectroscopy during i-layer deposition process lasting several tens of minutes. Silane decomposition is found to influence the mass spectrometer filament and therefore, pressure changes were compared to ascertain differences in pumping speed using a rotary, mechanical pump. The cathode voltage was used as a measure of effective hydrogen to silane ratio in real time and deliberate tailoring of this ratio is seen to result in a discernible difference in the degradation of single junction amorphous silicon solar cells. Some obvious consequences of this subtle, pump-type dependent transient behavior are discussed.

High-Efficiency Amorphous Silicon Solar Cells with ZnO as Front Contact

B-doped ZnO films deposited by the photo-induced metalorganic chemical vapor deposition (photo-MOCVD) method were applied to p-i-n single junction amorphous silicon (a-Si) solar cells as front contacts. The thickness and the doping level of the p-layer were optimized in order to increase the open-circuit voltage (Voc) of the fabricated a-Si solar cells. As a result, a stabilized conversion efficiency of 8.7% (Voc: 0.926 V, Jsc: 14.6 mA/cm2, FF: 0.646) was achieved under AM 1.5 (100 mW/cm2) illumination. Furthermore, the electrical properties of ZnO films were improved by employing an atomic layer deposition (ALD) technique instead of the conventional MOCVD method, and a lower resistivity of 5×10-4 Ωcm was achieved. It was also found that the stability of the electrical properties of ZnO films was improved by the ALD technique. The performance of the a-Si solar cells was further improved by applying the obtained high-quality ZnO films.

Experimental evidence of boron induced charged defects in amorphous silicon materials

We present an experimental proof of the existence of charged defects related to boron diffusion at the p–i interface of single junction amorphous silicon solar cells. Two different test devices were manufactured substituting the intrinsic absorber layer of a standard p–i–n structure with a boron lightly doped layer or a microcompensated region. Presence of trap states lying around 0.3–0.4 eV above the valence band was demonstrated through spectral photocapacitance response under medium infrared radiation and through capacitance versus frequency measurements that show the typical response of trapping kinetics process. Differences in the experimental results between our test devices and standard p–i–n solar cells show that boron atoms are the main cause of charged defect formation.

Calculation of collection efficiency for amorphous silicon solar cells

A simple method for calculating the collection efficiency of p–i–n type single junction amorphous silicon (a-Si:H) solar cells is presented. Using a modified admittance analysis method, the absorption profile of the intrinsic layer (i-layer) of a cell is calculated to determine the recombination function, which is then used for calculating the collection efficiency. It is expected that the present results of the collection efficiency are more accurate in comparison with the previous ones calculated either using a constant generation rate or an empirical exponential function for the generation of charge carriers throughout the i-layer. The collection efficiency thus calculated is used to obtain the short circuit current as a function of the thickness of the i-layer. Hence it can be used to determine the optimal thickness of the i-layer of a cell.

High-Efficiency Photoelectrochemical Hydrogen Production Using Multijunction Amorphous Silicon Photoelectrodes

Photoelectrochemical solar-to-hydrogen conversion efficiencies as high as 7.8% (based on the lower heating value of hydrogen) have been demonstrated in outdoor testing using a photocathode fabricated from triple junction amorphous silicon-solar cells and a separate catalytic anode. The tests were conducted in a specially designed Teflon-sealed reactor in 1 N KOH with a photoactive area of 0.27 cm2 and anode and cathode areas of 1 cm2. The hydrogen production rates, inferred from direct measurement of the anodic/cathodic currents and confirmed by independent volumetric and gas chromatographic measurements of the evolved hydrogen, were in excellent agreement with the rates expected from the measured solid-state JV behavior of the solar cell and the overpotentials of the thin-film catalysts. The thin-film catalysts, CoMo hydrogen catalysts deposited by sputtering from a compound target and NiFeyOx oxygen catalysts deposited from nickel−iron Permalloy target by reactive sputtering, have, in separate tests, shown no degradation after over 7200 h of operation in 1 N KOH electrolyte. During outdoor testing, the solar-to-hydrogen conversion efficiency decreased in the late afternoon as the blue portion of the spectrum decreased, a result of the spectral sensitivity of the solar cell used to construct the photoelectrode. Detailed modeling of the multijunction amorphous silicon cells is being conducted to identify structures that are better load-matched to the catalyst performance and that could yield higher hydrogen production efficiencies. Future work to advance this technology includes development of improved thin-film catalysts and development of transparent protective coatings that will allow complete immersion of the active electrode into the electrolyte.

A single p-i-n junction amorphous-silicon solar cell with conversion efficiency of 12.65%

A conversion efficiency of 12.65% was obtained at AM-1.5, 100 mW/cm/sup 2/ for a single p-i-n junction amorphous-Si solar cell of 1.05 cm/sup 2/, with a glass/SnO/sub 2//p-i-n/back-electrode structure. The solar cell had a short-circuit current of 19.13 mA/cm/sup 2/, an open-circuit voltage of 0.885 V, and a fill factor of 74.7%. The reason why the efficiency of the produced solar cell was enhanced by hot-annealing treatment is considered to be due to the reduction of the minute leaks caused by the pinhole treatment and the reduction of the number of pinholes by reverse-bias annealing treatment. It is believed that because of the minute irregularities on the surface of SnO/sub 2/, the diffuse light in the solar cell increased the effective optical length in the i layer. Therefore the collection efficiency in the long-wavelength region was enhanced.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>