Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells

Aluminium doped ZnO films have been developed by RF-magnetron sputtering at 350 °C substrate temperature on glass substrate and commercially available SnO₂-coated glass substrate. The developed ZnO and SnO₂/ZnO films can be used as the substrates of microcrystalline silicon based solar cell. The electrical, optical properties and surface morphologies of ZnO film and SnO₂/ZnO bi-layer films have been investigated and they are compared with the commercially available SnO_2-coated glass substrate. The resistivities of ZnO and SnO₂ films are comparable (10⁻⁴ Ω-cm). Surface morphologies of different transparent conducting oxide coated substrates before and after H-plasma exposure were studied by scanning electron microscopy. The optical transmission of ZnO, SnO₂/ZnO and SnO₂ films are comparable and varies from 85 to 90% in the visible region. The optical transmission reduces drastically to less than 20% in SnO₂ films and for ZnO film it remains almost unchanged after H-plasma exposure. For SnO₂/ZnO film transmission decreases slightly but remains considerably high (80%). The performance of microcrystalline silicon solar cells fabricated on different transparent conducting oxides as substrates (ZnO/glass, SnO₂/glass and ZnO/SnO₂/glass double layer) is investigated in detail.     

Surface textured MF-sputtered ZnO films for microcrystalline silicon-based thin-film solar cells

Highly conductive and transparent aluminum-doped zinc oxide (ZnO:Al) films were prepared by reactive mid-frequency (MF) magnetron sputtering at high growth rates. By varying the deposition pressure, pronounced differences with respect to film structure and wet chemical etching behavior were obtained. Optimized films develop good light-scattering properties upon etching leading to high efficiencies when applied to amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon-based thin-film solar cells and modules. Initial efficiencies of 7.5% for a μc-Si:H single junction and 9.7% for an a-Si:H/μc-Si:H tandem module were achieved on an aperture area of 64 cm².     

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%.     

TCO and light trapping in silicon thin film solar cells

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) or microcrystalline (μc-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. This paper discusses ways to realize efficient light trapping schemes by using textured transparent conductive oxides (TCOs) as light scattering, highly conductive and transparent front contact in silicon p–i–n (superstrate) solar cells. Focus is on the concept of applying aluminum-doped zinc oxide (ZnO:Al) films, which are prepared by magnetron sputtering and subsequently textured by a wet-chemical etching step. The influence of electrical, optical and light scattering properties of the ZnO:Al front contact and the role of the back reflector are studied in experimentally prepared a-Si:H and μc-Si:H solar cells. Furthermore, a model is presented which allows to analyze optical losses in the individual layers of a solar cell structure. The model is applied to develop a roadmap for achieving a stable cell efficiency up to 15% in an amorphous/microcrystalline tandem cell. To realize this, necessary prerequisites are the incorporation of an efficient intermediate reflector between a-Si:H top and μc-Si:H bottom cell, the use of a front TCO with very low absorbance and ideal light scattering properties and a low-loss highly reflective back contact. Finally, the mid-frequency reactive sputtering technique is presented as a promising and potentially cost-effective way to up-scale the ZnO front contact preparation to industrial size substrate areas.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Deposition pressure effects on material structure and performance of micromorph tandem solar cells

Tandem solar cells represent an elegant way of overcoming the efficiency limits of single-junction solar cells and reducing the light-induced degradation of amorphous silicon films. Stacked structures consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell allow a good utilization of the solar spectrum due to the band gap values of the two materials. These devices, firstly introduced by the IMT research group, were designated as “micromorph” tandem solar cells. To better exploit this concept, it is important to tune parameters like the band gaps and the short-circuit currents.In this work, we have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layer of both the amorphous top cell and the microcrystalline bottom cell is grown by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 °C). Finally, a ZnO reflector and a metal contact complete the structure. No intermediate optical mirror between the two cells is used at this stage. Undiluted a-Si:H, with reduced band gap when compared to H₂-diluted amorphous silicon, is used as absorber layer in the top cell. As for the bottom cell, the high-pressure–high-power regime (up to 267 Pa–80 W) has been explored aiming at growing high-quality microcrystalline silicon at large deposition rates. The effect of the structural composition of the microcrystalline absorber layer on the current–voltage characteristic and spectral response of tandem devices has been investigated. An efficiency of 11.3% has been obtained with short-circuit current densities around 13 mA/cm², open-circuit voltages 1.34 V and fill factors 66%.     

Microcrystalline silicon thin film solar modules on glass

We developed microcrystalline silicon (μc-Si:H) thin film solar modules on textured ZnO-coated glass. The single junction (p–i–n) cell structure was prepared by plasma-enhanced chemical vapour deposition (PECVD) at substrate temperatures below 250 °C. Front ZnO and back contacts were prepared by sputtering. A process for the monolithic series connection of μc-Si:H cells by laser scribing was developed. These microcrystalline p–i–n modules showed aperture area efficiencies up to 8.3% and 7.3% on aperture areas of 64 and 676 cm², respectively. The temperature coefficient of the efficiency was −0.4%/K.     

Development of high efficiency p-i-n amorphous silicon solar cells with the p-μc-Si:H/p-a-SiC:H double window layer

A structure is developed to help improve the TCO/p contact and efficiency of the solar cell. A p-i-n amorphous silicon (a-Si:H) solar cell with high-conversion efficiency is presented via use of a double p-type window layer composed of microcrystalline silicon and amorphous silicon carbide. The best efficiency is obtained for a glass/textured TCO/p-μc-Si:H/p-a-SiC:H/buffer/i-a-Si:H/n-μc-Si:H/GZO/Ag structure. Using a SnO₂/GZO bi-layer and a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) layer between the TCO/p-a-SiC:H interface improves the photovoltaic performance due to reduction of the surface potential barrier. Layer thickness, B₂H₆/SiH₄ ratio and hydrogen dilution ratio of the p-μc-Si:H layer are studied experimentally. It is clearly shown that the double window layer can improve solar cell efficiency. An initial conversion efficiency of 10.63% is achieved for the a-Si:H solar cell.     

Hydrogenated nanocrystalline silicon p-layer in amorphous silicon n–i–p solar cells

This paper describes an investigation into the impacts of hydrogenated nanocrystalline silicon (nc-Si:H) p-layer on the photovoltaic parameters, especially on the open-circuit voltage (V_oc) of n–i–p type hydrogenated amorphous silicon (a-Si:H) solar cells. Raman spectroscopy and transmission electron microscopy (TEM) analyses indicate that this p-layer is a diphasic material that contains nanocrystalline grains with size around 3–5 nm embedded in an amorphous silicon matrix. Optical transmission measurements show that the nc-Si:H p-layer has a wide band gap of 1.9 eV. Using this nanocrystalline p-layer in n–i–p a-Si:H solar cells, the cell performances were improved with a V_oc of 1.042 V, whereas the solar cells deposited under similar conditions but incorporating a hydrogenated microcrystalline silicon (μc-Si:H) p-layer exhibit a V_oc of 0.526 V.     

A comparison of fill factor and recombination losses in amorphous silicon solar cells on ZnO and SnO2

Effects of ZnO and SnO₂ TCO (Transparent Conductive Oxide) substrate materials on hydrogenated amorphous silicon (a-Si:H) p-i-n solar cell performances and recombination kinetics have been investigated. DC and Frequency-resolved photocurrent measurements in a-Si:H p-i-n solar cells of 6 have been carried out experimentally. In particular, the I–V characteristics in the dark and light, the quantum efficiency spectra, the intensity-, bias voltage- and frequency-dependence of photocurrent were obtained. Fill factor (FF) values were determined from I–V characteristics for both types of substrate cells under various illumination levels. The exponent v in the power–law relationship, I_ph α G^v, between generating flux density and photocurrent were determined at different bias voltages (DC) and modulation frequencies. High values of V_oc (open-circuit voltage), FF, and DC exponent v for the a-Si:H p-i-n solar cell with SnO₂ were obtained, but the integrated QE (quantum efficiency), the modulated exponent v were found to be low compared to cells prepared on ZnO substrates. Our results show that these parameters are sensitive to the ZnO and SnO₂ substrate materials which act as a window layer allowing most of the incident light to pass into the i-layer of p-i-n cells.     

Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.     

Realization of high efficiency micromorph tandem silicon solar cells on glass and plastic substrates: Issues and potential

High conversion efficiency for (amorphous/microcrystalline) "micromorph" tandem solar cells requires both a dedicated light management, to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. Efficient light trapping is achieved here by use of textured front and back contacts as well as by implementing an intermediate reflecting layer (IRL) between the individual cells of the tandem. This paper discusses the latest developments of IRLs at IMT Neuchâtel: SiO_x based for micromorphs on glass and ZnO based IRLs for micromorphs on flexible substrates were successfully incorporated in micromorph tandem cells leading to high, matched, current above 13.8 mA/cm² for p-i-n tandems. In n-i-p configuration, asymmetric intermediate reflectors were employed to achieve currents of up to 12.5 mA/cm². On glass substrates, initial and stabilized efficiencies exceeding 13% and 11%, respectively, were thus obtained on 1 cm² cells, while on plastic foils with imprinted gratings, 11.2% initial and 9.8% stable efficiency could be reached. Recent progress on the development of effective front and back contacts will be described as well.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Novel etching method on high rate ZnO:Al thin films reactively sputtered from dual tube metallic targets for silicon-based solar cells

Highly transparent and conductive aluminum-doped zinc oxide thin films (ZnO:Al) were reactively sputtered from metallic targets at high rate of up to 90 nm m/min. For the application as transparent light scattering front contact in silicon thin film solar cells, a texture etching process is applied. Typically, it is difficult to achieve appropriate etch features in hydrochloric acid as the deposition process must be tuned and the interrelation is not well understood. We thus introduce a novel two-step etching method based on hydrofluoric acid. By tuning the etch parameters we varied the surface morphology and achieved a regular distribution of large craters with the feature size of 1-2 μm in diameter and about 250 nm in depth. Microcrystalline silicon single junction solar cells (μc-Si:H) and amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with high efficiency of up to 8.2% and 11.4%, respectively, were achieved with optimized ZnO:Al films as light scattering transparent front contact.     

Characterization and simulation of a-Si:H/μc-Si:H tandem solar cells

We simulated device characteristics of a-Si:H single junction, μc-Si:H single junction and a-Si:H/μc-Si:H tandem solar cells with the numerical device simulator Advanced Semiconductor Analysis (ASA). For this purpose we measured and adjusted electrical and optical input parameters by comparing measured and simulated external quantum efficiency, current−voltage characteristic and reflectivity spectra. Consistent reproducibility of experimental data by numerical simulation was achieved for all types of cells investigated in this work. We also show good correspondence between the experimental and simulated characteristics for a-Si:H/μc-Si:H tandem solar cells with various absorber thicknesses on both Asahi U-type SnO₂:F and sputtered/etched (Jülich) ZnO:Al substrates. Based on this good correlation between experiment and theory, we provide insight into device properties that are not directly measurable like the spatially resolved absorptance and the voltage-dependent carrier collection. These data reveal that the difference between tandem solar cells grown on Asahi U-type and Jülich ZnO substrates primarily arises from their optical properties. In addition, we find out that the doped layers do not contribute to the photocurrent except for the front p-layer. We also calculated the initial efficiencies of a-Si:H/μc-Si:H tandem solar cells with different combinations of a-Si:H and μc-Si:H absorber layer thicknesses. The maximum efficiency is found at 260 nm/1500 nm for tandem solar cells on Asahi U-type substrates and at 360 nm/850 nm for tandem solar cells on Jülich ZnO substrates.     

Optimization of ZnO for front and rear contacts in a-Si solar cells

The enhancement of the reflection from the rear contact of p-i-n a-Si solar cells using ZnO combined with metals (Ag/Al) as a back reflector was demonstrated theoretically and experimentally. Futhermore, the incorporation of unreacted H₂O as source gas in the ZnO films was clearly observed through the thermal evolution measurement, suggesting the need for employing the pre-annealing technique for ZnO films before using them as a front contact in p-i-n a-Si solar cells. By using these approaches, the a-Si solar cells with glass/annealed-ZnO/delta-doped p/buffer/i/n/ZnO/metals(Ag/Al) structure were successfully fabricated and a conversion efficiency of 12.1% (AM-1.5, area 3×3 mm²) was obtained. Moreover, the solar cells with a structure of AR coated glass/SnO₂/delta-doped/p/buffer/i/n/ZnO/metals(Ag/Al) were also fabricated and by optimizing the use of the ZnO layer at the rear contact, a conversion efficiency of 12.6% was obtained. To make the ZnO films more appropriate for solar cells application, the growth rate of the ZnO films was increased by increasing the flow rate of diethylzinc used as a source gas.     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.     

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.     

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.