Development of an ultralight, flexible a-Si solar cell submodule

An ultralight, flexible a-Si solar cell fabricated on a polyimide film substrate has been developed. It was found that prebaking the polyimide film substrate improved the output performance of the cell by reducing the amount of H₂0 and polymers released from the substrate. Also, using an i-layer fabricated at high temperature (250°C) and a p-layer fabricated at low temperature (80°C) improved the cell's output performance to a level of over 10%. A maximum output of 782.4 mW and a power-to-weight ratio of 340 mW/g were obtained for an integrated-type flexible a-Si solar cell submodule with a size of 113 mm × 120 mm.     

Development of high-efficiency a-Si solar cell submodule with a size of 30 cm × 40 cm

We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Si/a-SiGe solar cell for a size of 1 cm², which are the world's highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of E_opt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of ± 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm × 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm × 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm × 40 cm double-junction a-Si/a-SiGe submodule.     

Current-induced degradation method for stabilization of a-Si solar cell

Light-induced degradation and light-induced degradation after current-induced degradation of several types of a-Si solar cells were compared in order to investigate the difference between current-induced degradation and light-induced degradation. Very little difference in the efficiency after long-term light irradiation is observed between samples which were degraded by light- after current-induced degradation and those which were degraded by light only. Little difference in the collection efficiency spectra and the depth profiles of the DICE (dynamic inner collection efficiency) is also observed between the cells degraded by these two processes. The possibility of applying the current-induced degradation method to the stabilization of a-Si solar modules is indicated.     

Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method

The solid phase crystallization (SPC) method has been studied for fabricating polycrystalline (poly) Si thin films for solar cells. The approach was to optimize the “partial doping structure” (nondoped a-Si/phosphorus(P)-doped a-Si) which we proposed as a starting structure before SPC. A conversion efficiency of 6.3% was obtained by using nondoped a-Si with a large structural disorder. This cell showed a collection efficiency of 51% at a wavelength of 900 nm. In order to significantly reduce the incubation time which is the important factor for the enlargement of the grain size, P doping of more than 10²⁰ cm⁻³ was required for the P-doped layer.     

A new characterization method for solar cell dynamic impedance

The paper describes a new method to characterize solar cell dynamic impedances in the dark with forward biasing using square wave inputs. The solar cell dynamic impedance is calculated from the output responses by MATLAB using the FFT technique to analyze harmonic contents. The impedances are shown as impedance loci plotted in the complex plane. Comparison is made with results obtained from available reports using sinusoidal inputs. The advantages of this method, compared with the sinusoidal input method, are less expensive equipment setup, simpler measurement, and reduction in measuring steps, yielding comparable results.     

Fabricating high performance a-Si solar cells by alternately repeating deposition and hydrogen plasma treatment method

The performance of a p-i buffer layer in pin amorphous silicon solar cell was improved by the “alternately repeating deposition and hydrogen plasma treatment method (ADHT)”. The optical bandgap of the a-Si film was increased by hydrogen plasma treatement. The wide optical bandgap and the high photoconductive a-Si:H films without carbon could be fabricated by the ADHT method. The conversion efficiency of the solar cell with a-Si:H buffer layer was almost the same as that using an a-SiC:H buffer layer. Second, the a-Si (ADHT) films were applied to the n-i buffer layer. The insertion of a-Si (ADHT) films between the i-layer and the n-layer was effective to improve the cell performance, especially the fill factor. With the use of high performance a-Si p-i and n-i buffer layer deposited by ADHT method, a cell conversion efficiency of 12.9% was obtained.     

Preparation of (n) a-Si: H/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si: H on p-type 1–2Ω cm CZ single crystalline silicon substrates. Although our cell structure is very simple - neither a BSF nor a surface texturing is used - a conversion efficiency of 13.1% has been achieved on an area of 1 cm². In this paper the technology is described and the dependence of the solar cell parameters on the properties of the n-type a-Si: H layer is discussed. It is shown that this cell type exhibits no degradation under light exposure.     

The evaluation of accelerated test for degradation a stacked a-Si solar cell and EVA films

We studied the photothermal stability of ethylene-vinyl acetate (EVA) laminates using accelerated testing in order to identify the reasons for its performance degradation on stacked amorphous silicon (a-Si) solar cells and EVA films. Stacked a-Si solar cells and EVA films were aged in accelerated tests under conditions of 1-SUN and 4-SUN. The results evaluated the deterioration of each separate level for solar cells made of three-layer stacked structure and EVA films. Spectral sensitivity and conversion efficiency were used as evaluation parameters. Spectral sensitivity could be used as an evaluation parameter after the accelerated test caused degradation on the EVA films. EVA films under 4-SUN accelerated testing revealed a phenomenon in which light transmission increased on the short-wavelength side of the spectrum.     

Experimental and theoretical investigations of a new potential barrier due to sharp a-Si/c-Si heterointerfaces buried in the solar cell emitter

Several new concepts to increase the conversion efficiency of solar cells have been presented over the last few years. One possibility is the multi-interface novel device solar cell with a highly doped amorphised substructure inserted in the emitter. This active nanostructure is created by P ion implantation followed by an adequate thermal treatment necessary to form two sharp a-Si/c-Si heterointerfaces. After an incomplete initial thermal treatment at 500°C, dark current–voltage (I–V) characteristics were measured after each of several complementary thermal treatments. In this paper, we show that the classical two-diode model has to include a voltage reduction resulting from the two low–high-type interfaces in order to correctly fit the experimental curves.     

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS₂ absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS₂ absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS₂ absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS₂ absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS₂ surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS₂ absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS₂ absorbers can be used to assess the quality of CuInS₂ absorbers prior to solar cell fabrication.     

a-Si:H(p)/c-Si(n)异质结太阳电池的模拟优化

通过AFORS-HET软件模拟了TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/Ag结构的硅异质结电池中硅衬底电阻率、本征非晶硅薄膜厚度、发射极材料特性以及TCO功函数对电池性能的影响。结果表明:在其它参数不变的条件下,硅衬底电阻率越低,转换效率越高;发射极非晶硅薄膜厚度对短路电流有较大影响,发射极掺杂浓度低于7.0×10¹⁹cm^-3时,电池各项性能参数都极差;TCO薄膜功函数应大于5.2 eV,以保证载流子的输运收集。     

Study of the effect of introducing a bottom ITO layer in an a-Si:H p-i-n type solar cell

An optical admittance method is applied to design a thin film a-Si:H solar cell with the structure glass/TCO/p-i-n/TCO/metal. We have optimised the thicknesses of top and bottom TCO for Al and Ag as a rear metal contacts. It is shown that with such optical optimisation of structure, one can increase the integrated absorbance in the active layer and, thus, improve the photovoltaic performance of a solar cell. The implication of using a glass cover with higher refractive index is also discussed.     

Evidence of electron trapping in undoped hydrogenated amorphous silicon (a-Si:H) Schottky barriers by the surface photovoltage (SPV) technique — A study of stability in a-Si:H solar cells

Surface photovoltage (SPV) measurements on a-Si:H/metal Schottky devices under forward bias are presented. The space-charge density decreases with applied forward bias while both the space-charge width and the apparent diffusion length are essentially constant. These are all attributed to electron trapping in Schottky barrier region. Furthermore, we explore electron trapping effect in a-Si:H on light-induced degradation of a-Si:H solar cells.     

Dye-sensitized solar cell made of mesoporous titania by surfactant-assisted templating method

Nanocrystalline mesoporous titania was obtained by surfactant-assisted templating method using tetraisopropyl orthotitanate modified with acethylacetone and laurylamine hydrochloride as template. This material was applied for the electrode of dye-sensitized solar cell. The mesoporous TiO₂ (MP-TiO₂) cells exhibited higher short-circuit photocurrent density and solar energy conversion efficiency compared to P25 (a typical commercial titania powder) cells. The incident photon to current conversion efficiency spectrum of MP-TiO₂ can be improved by using the cell made with 5% P25 additive. Double-layer titania cells were fabricated to further improve cell performance by increasing the film thickness and light scattering. The solar conversion efficiency up to 8.06% was obtained by using the double-layer titania cell sintered at 450 °C for 2 h.     

Deep defect determination by the constant photocurrent method (CPM) in annealed or light soaked amorphous hydrogenated silicon (a-Si:H)

We present systematic measurements of CPM on two independent series of slightly phosphorous and boron doped films. For “n-type” samples of both series, the CPM deep defect absorption is proportional to the square root of the gas dopant ratio. For these samples we discuss the influence of Fermi level on the CPM spectra. For slightly “p-type” samples, CPM deep defect absorption as evaluated by CPM becomes higher than the corresponding PDS-values. This fundamental problem can be traced back to the violation of two basic conditions necessary for a correct evaluation of the absorption from CPM measurements: (1) the power law exponent γ (Rose factor) of the photoconductivity must be spectrally independent, and (2) the generation rate G, which corresponds to the CPM photocurrent, also has to be spectrally independent. Further, we compare the annealed and the “saturated” light soaked states of selected slightly doped samples and an undoped sample: the variations in the CPM deep defect absorption and in photoconductivity due to light-soaking are discussed.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.     

Performances of fluorescent solar concentrators doped with a new dye (benzoxazinone derivative)

A benzoxazinone derivative, DFSBO (7-dimethylamino-3-(p-formyl-stryryl)-1,4-benzoxazine-2-one) is compared to DCM and Rhodamine 640 for applications in fluorescent solar concentrators (FSC). Plates made of polymethylmethacrylate doped with these dyes are investigated regarding spectral characteristics, gain in current intensity of the photovoltaic cell, concentration factor in terms of power and photochemical stability. Among the three dyes investigated, DFSBO offers the best performances.     

Design and simulation of a new energy-conscious system (CFD and solar simulation)

This paper presents the use of a validated CFD programme (FLUENT) and a solar simulator, for designing a solar water-heater. The water-heater is part of a new passive cooling and heating system introduced for buildings in North Africa. CFD transient simulations were carried out using a small time-step of 10 s and a set of fine body-fitted computational grids (1770–4740 nodes). FLUENT results were then verified against indoor testing employing a solar simulator. Good agreement was achieved.     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.