Dark I–V–T measurements and characteristics of (n) a-Si/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si:H on p-type 1–2 Ω cm Cz single-crystalline silicon substrates. An efficiency of 14.2% has been obtained for 1 cm² solar cells by using a simple (Al/(p) c-Si/(n) a-Si:H/ITO/metal grid) structure. With an additional surface texturing, we have reached an efficiency of 15.3% for 1 cm² solar cells. We have investigated the dark IV-curves in order to contribute to a better understanding of the basis of solar cells.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH₄) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)⁻¹ and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.     

Design optimization of bifacial HIT solar cells on p-type silicon substrates by simulation

Heterojunction with intrinsic thin layer (HIT) solar cells fabricated on p-type silicon substrates usually demonstrate inferior performance than those formed on n-type substrates. The influence of various structure parameters on the performance of the c-Si(p)-based bifacial HIT solar cell, i.e., the TCO/a-Si:H(n)/a-Si:H(i)/c-Si(p)/a-Si:H(i)/a-Si:H(p⁺)/TCO solar cell, was investigated in detail by computer simulation using the AFORS-HET software. The work function of the transparent conductive oxide was found to be a key factor to affect the solar cell performance. Detailed influence mechanisms were analysed. Accordingly, the design optimization of the bifacial HIT solar cells on c-Si(p) substrates was provided.     

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type a-Si:H/n-type c-Si (P⁺ a-Si:H/N⁺ c-Si) heterojunction was simulated for developing solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency of P⁺ a-Si:H/N⁺ c-Si solar cells. With the optimized parameters set, the P⁺ a-Si:H/N⁺ c-Si solar cell reaches a high efficiency (η) up to 21.849% (FF: 0.866, V_OC: 0.861 V, J_SC: 29.32 mA/cm²).     

Highly and quickly stabilized p–i–n/p–i–n-type protocrystalline silicon multilayer tandem solar cells

We have developed p–i–n/p–i–n-type protocrystalline silicon (pc-Si:H) multilayer tandem solar cells. The purpose of this work is to make a thin film silicon solar cell with low degradation by combining the virtues of a pc-Si:H multilayer and tandem structure. The usefulness of the pc-Si:H multilayer as a low degradation top and bottom cell was confirmed when we achieved a low degradation ratio of 10.0%. Notably, this tandem cell stabilized rapidly, within 1 h. Nanocrystalline silicon (nc-Si) grains embedded in a pc-Si:H multilayer were detected with the aid of a planer transmission electron microscope. The isolated nc-Si grains may suppress the photocreation of dangling bonds due to non-radiative recombination in an a-Si:H matrix. Because of these embedded nc-Si grains, the pc-Si:H multilayer has a fast and high light-induced metastability.     

Application of real-time spectroscopic ellipsometry for characterizing the structure and optical properties of microcrystalline component layers of amorphous semiconductor solar cells

Over the past few years, we have applied real-time spectroscopic ellipsometry (RTSE) to probe hydrogenated amorphous silicon (a-Si:H)-based solar cell fabrication on the research scale. From RTSE measurements, the microstructural development of the component layers of the cell can be characterized with sub-monolayer sensitivity, including the time evolution of (i) the bulk layer thickness which provide the deposition rates, and (ii) the surface roughness layer thickness which provide insights into precursor surface diffusion. In the same analysis, RTSE also yields the optical properties of the growing films, including the dielectric functions and optical gaps. Results reported earlier have been confined to p-i-n and n-i-p cells consisting solely of amorphous layers, because such layers are found to grow homogeneously, making data analysis relatively straightforward. In this study, we report the first results of an analysis of RTSE data collected during the deposition of an n-type microcrystalline silicon (μc-Si:H) component layer in an a-Si:H p-i-n solar cell. Such an analysis is more difficult owing to (i) the modification of the underlying i-layer by the H₂-rich plasma used in doped μc-Si:H growth and (ii) the more complex morphological development of μc-Si:H, including surface roughening during growth.     

Optimum design and its experimental approach of a-Si/ /poly-Si tandem solar cell

A series of technical data on four-terminal a-Si/ /poly-Si stacked solar cells has been reported. The developed device has some unique significances such as high achievable efficiency, and low cost with almost no light induced degradation. It has been shown on a poly-Si bottom cell that an efficiency of 17.2% has been obtained by employing high conductivity with wide optical band-gap p-type μc-SiC as a window material and n-type μc-SiC as a back ohmic contact with BSF effects. On the optically transparent a-Si top cell, an optimum design has been experimentally made with the device structure of p μc-SiC/p a-SiC/i a-Si/n μc-Si/ITO, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer, while the best efficiency is 12.3% for p-i-n single-junction solar cell with 500 nm i-layer thickness deviced by Ag back-electrode. With the 100 nm thick ultrathin top cell, a total conversion efficiency as high as 21.0% has been achieved on a-Si/ /poly-Si four-terminal tandem solar cells.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Boron-doped zinc oxide layers grown by metal-organic CVD for silicon heterojunction solar cells applications

In this work, we focus on ZnO:B layers as an alternative TCO for application on a-Si:H/c-Si heterojunction solar cells. First, the optimization of the material has been done in terms of optical and electrical properties. We have also studied the behaviour of ZnO:B against ageing. Finally, complete heterojunction solar cells have been fabricated with different back-side TCO configurations to understand the ageing mechanisms and to deeply study the influence of degradation on the solar cells parameters. Stable efficiencies up to 16.6% on polished n-type c-Si were obtained on 25 cm² heterojunction solar cells fabricated at low temperatures.     

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 performance 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) spectroscopy 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 (V_oc) of up to 0.732 V.     

Intrinsic microcrystalline silicon: A new material for photovoltaics

Microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) has been investigated as material for absorber layers in solar cells. The deposition process has been adjusted to achieve high deposition rates and optimized solar cell performance. In particular, already moderate variations of the crystalline vs. amorphous volume fractions were found to effect the electronic material – and solar cell properties. Such variation is readily achieved by changing the process gas mixture of silane to hydrogen. Best cell performance was found for material near the transition to the amorphous growth regime. With this optimized material efficiencies of 7.5% for a 2 μm thick μc-Si:H single solar cell and 12% for an a-Si:H/μc-Si:H stacked solar cell have been achieved.     

Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

Enhanced absorption in Au nanoparticles/a-Si:H/c-Si heterojunction solar cells exploiting Au surface plasmon resonance

Au nanoparticles (NPs)/(n-type)a-Si:H/(p-type)c-Si heterojunctions have been deposited combining plasma-enhanced chemical-vapour deposition (PECVD) with Au sputtering. We demonstrate that a density of 1.3×10¹¹ cm⁻² of Au nanoparticles with an approximately 20 nm diameter deposited onto (n-type)a-Si:H/(p-type)c-Si heterojunctions enhance performance exploiting the improved absorption of light by the surface plasmon resonance of Au NPs. In particular, Au NPs/(n-type)a-Si:H/(p-type)c-Si show an enhancement of 20% in the short-circuit current, J_SC, 25% in the power output, P_max and 3% in the fill factor, FF, compared to heterojunctions without Au NPs. Structures have been characterized by spectroscopic ellipsometry, atomic force microscopy and current–voltage (I–V) measurements to correlate the plasmon resonance-induced enhanced absorption of light with photovoltaic performance.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

Light trapping effect of patterned back surface reflectors in substrate-type single and tandem junction thin-film silicon solar cells

Light trapping is a key issue to boost the efficiency of thin-film Si solar cells including μc-Si:H. In this paper, effect of textured back reflectors on light trapping in μc-Si:H cells has been investigated with self-orderly patterned Al substrates obtained by anodic oxidation. With increase in the period of the patterned substrates from 0 to 1.1 μm, the short circuit current densities of 1-μm-thick μc-Si:H cells on the patterned substrates significantly increase from 18 to over 24 mA/cm², which is attributed to the improved light trapping in the infrared region. It has been clarified that this enhanced light-trapping effect in longer wavelengths is mainly attributed to the improved light scattering at the rear side by comparing μc-Si:H solar cells with polished and as-deposited front surfaces. The effectiveness of the patterned Al substrates has also been demonstrated in an a-Si:H/μc-Si:H tandem cell with a bottom cell thickness of 1 μm, showing a higher conversion efficiency than the reference cell.     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

Deposition of thin film silicon using the pulsed PECVD and HWCVD techniques

We report on the use of pulsed plasma-enhanced chemical vapor deposition (P-PECVD) technique and show that “state-of-the-art” amorphous silicon (a-Si:H) materials and solar cells can be produced at a deposition rate of up to 15 Å/s using a modulation frequency in the range 1–100 kHz. The approach has also been developed to deposit materials and devices onto large area, 30 cm×40 cm, substrates with thickness uniformity (<5%), and gas utilization rate (>25%). We have developed a new “hot wire” chemical vapor deposition (HWCVD) method and report that our new filament material, graphite, has so far shown no appreciable degradation even after deposition of 500 μm of amorphous silicon. We report that this technique can produce “state-of-the-art” a-Si:H and that a solar cell of p/i/n configuration exhibited an initial efficiency approaching 9%. The use of microcrystalline silicon (μc-Si) materials to produce low-cost stable solar cells is gaining considerable attention. We show that both of these techniques can produce thin film μc-Si, dependent on process conditions, with 1 1 1 and/or 2 2 0 orientations and with a grain size of approx. 500 A. Inclusion of these types of materials into a solar cell configuration will be discussed.     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

Low-temperature deposition of amorphous silicon solar cells

We develop amorphous silicon (a-Si:H)-based solar cells by plasma-enhanced chemical vapor deposition (PECVD) at deposition temperatures of T_s=75°C and 100°C, compatible with low-cost plastic substrates. The structural and electronic properties of low-temperature standard PECVD a-Si:H, both doped and undoped, prevent the photovoltaic application of this material. In this paper, we demonstrate how to achieve device-quality a-Si:H even at low deposition temperatures. In the first part, we show the dependence of structural and carrier transport properties on the deposition temperature. The sub-band gap absorption coefficient and the Urbach energy increase when the deposition temperature declines from T_s=150°C to 50°C, the conductivity of doped layers and mobility-lifetime product of intrinsic a-Si:H drop drastically. Therefore, in the second part we investigate the impact of increasing hydrogen dilution of the feedstock gases on the properties of low-temperature a-Si:H. We restore n-type a-Si : H device-quality conductivity while the p-type a-Si:H conductivity is still inferior. For undoped layers, we depict the hole diffusion length, the mobility-lifetime product for electrons, the Urbach energy, and sub-band gap absorption coefficient as a function of the hydrogen dilution ratio. We incorporate these optimized materials in solar cell structures of single and multilayer design and record initial efficiencies of η=6.0% at a deposition temperature of T_s=100°C, and η=3.8% at T_s=75°C. For prospective opaque polymer substrates we develop, in addition to our conventional pin cells, devices in nip design with similar performance.