Low pressure chemical vapour deposition of ZnO layers for thin-film solar cells: temperature-induced morphological changes

Zinc oxide (ZnO) is now often used as a transparent conductive oxide for contacts in thin-film silicon solar cells. This paper presents a study of ZnO material deposited by the low-pressure chemical vapour deposition technique, in a pressure range below the pressures usually applied for the deposition of this kind of material. A temperature series has been deposited, showing a morphological transition around 150 °C. ZnO samples deposited with temperatures just higher than this transition are constituted of large grains highly oriented along a single crystallographic orientation. These “monocrystals” lead to low resistivity values, showing a clear correlation between the size of the surface grains and the electrical performance of corresponding films. Additionally, these large grains also yield ZnO layers with high transparency and high light-scattering power, specially suitable for solar cell technology based on thin-film silicon.     

Large area ZnO films optimized for graded band-gap Cu(InGa)Se2-based thin-film mini-modules

In this study, two deposition methods (i.e. MOCVD and sputtering methods) to prepare n-type ZnO window layers for CIGS-based thin-film solar cells are discussed. In order to make ZnO : Al transparent conductive oxide (TCO) films prepared by DC magnetron sputtering comparable to ZnO : B TCO prepared by MOCVD, a new ZnO sputtering process is proposed by introducing a multilayer structure. Using these films, CIGS thin-film solar cells with efficiencies of greater than 14% have been fabricated with an active area of 3.2 cm². This structure was adapted to fabricate CIGS thin-film mini-modules with efficiencies around 11% having aperture area of 50 cm².     

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.     

Microcrystalline silicon and the impact on micromorph tandem solar cells

Intrinsic microcrystalline silicon opens up new ways for silicon thin-film multi-junction solar cells, the most promising being the “micromorph” tandem concept. The microstructure of entirely microcrystalline p–i–n solar cells is investigated by transmission electron microscopy. By applying low pressure chemical vapor deposition ZnO as front TCO in p–i–n configurated micromorph tandems, a remarkable reduction of the microcrystalline bottom cell thickness is achieved. Micromorph tandem cells with high open circuit voltages of 1.413 V could be accomplished. A stabilized efficiency of around 11% is estimated for micromorph tandems consisting of 2 μm thick bottom cells. Applying the monolithic series connection, a micromorph module (23.3 cm²) of 9.1% stabilized efficiency could be obtained.     

Experimental studies and limitations of the light trapping and optical losses in microcrystalline silicon solar cells

This study addresses the potential of different approaches to improve the generated current in silicon thin-film solar cells and modules. Decreasing the carrier concentration in the front contact has proven to increase the quantum efficiency and the cell-current density significantly. Additionally, an optically improved ZnO/Ag back reflector and the optimized light incoupling by anti-reflection layers were studied. In this contribution, we show the potential of the different optical components and discuss combinations thereof in order to obtain a maximized cell-current density in silicon thin-film solar cells. Limitations of the cell-current density are discussed with respect to theoretical calculations.     

The application of grating couplers in thin-film silicon solar cells

As an alternative to randomly textured transparent conductive oxides as front contact for thin-film silicon solar cells, the application of periodic light grating couplers was studied. The periods and groove depths of transparent gratings made of zinc oxide were tuned independently from each other and varied between 1 and 4 μm and 100 and 600 nm, respectively. The one-dimensional grating couplers were realized using photolithography. We have analysed the optical properties of the gratings and the properties of amorphous and microcrystalline silicon solar cells incorporating these grating couplers. The achieved results are discussed with respect to the performance of cells deposited on flat and randomly textured substrates.     

Commercial white paint as back surface reflector for thin-film solar cells

In this work, commercially available white paint is applied as a pigmented diffuse reflector (PDR) on the rear surface of thin-film crystalline silicon (c-Si) solar cells with a silicon thickness in the 1–2 μm range. We show that white paint increases the short-circuit current density of the solar cells enormously, with a boost of 41% observed for very thin planar solar cells illuminated with the global AM1.5 solar spectrum. We also show that white paint is a better back surface reflector (BSR) than aluminium, air, a transparent conductive oxide (TCO)/aluminium stack, and even a detached aluminium mirror. While previous studies have investigated the influence of PDRs on silicon solar cells with thicknesses of over 27 μm, this work closes the gap that has existed for much thinner cells.     

Optical analysis of textured plastic substrates to be used in thin silicon solar cells

Light confinement strategies in thin-film silicon solar cells play a crucial role in the performance of the devices. In this work, the possible use of Ag-coated stamped polymers as reflectors to be used in n–i–p solar cells is studied. Different random roughnesses (nanometer and micrometer size) have been transferred on poly(methylmethacrylate) (PMMA) by hot embossing. Morphological and optical analyses of masters, stamped polymers and reflectors have been carried out evidencing a positive surface transference on the polymer and the viability of a further application in solar cells.     

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.     

Quality control and characterization of Cu(In,Ga)Se2-based thin-film solar cells by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/ Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect “hard failures” following CIGS deposition, and both “hard” and “soft” failures following CdS deposition and following ZnO deposition. In addition, a semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS/CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO/CIGS structures is found.     

Development of n-μc-SiOx:H as cost effective back reflector and its application to thin film amorphous silicon solar cells

Development of doped silicon oxide based microcrystalline material as a potential candidate for cost-effective and reliable back reflector layer (BRL) for single junction solar cells is discussed in this article. Phosphorus doped μc-SiO_x:H layers with a refractive index ∼2 and with suitable electrical properties were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, using the conventional capacitively coupled reactors. Optoelectronic properties of these layers were controlled by varying the oxygen content within the film. The performance of these layers as BRL have been investigated by incorporating them in a single junction amorphous silicon solar cell and compared with the conventional ZnO:Al based reflector layer. Single junction thin film a-Si solar cells with efficiency ∼9.12% have been successfully demonstrated by using doped SiO:H based material as a back reflector. It is found that the oxide based back reflector shows analogous performance to that of conventional ZnO:Al BRL layer. The main advantage with this technology is that, it can avoid the ex-situ deposition of ZnO:Al, by using doped μc-SiO:H based material grown in the same reactor and with the same process gases as used for thin-film silicon solar cells.     

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design:

(1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f_exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment.

(2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon.

Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.     

High efficiency, magnetron sputtered CdS/CdTe solar cells

Polycrystalline II–VI semiconductor materials show great promise for efficient, low-cost photovoltaics. Large-area deposition of the II–VI semiconductors such as CdTe is possible by a variety of methods but the use of a plasma-based method such as magnetron sputtering can have significant advantages. Here we present recent results in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that accrue from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses issues related to the fabrication of tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell having 14.0% efficiency at one sun with an air mass 1.5 global spectrum. We have also studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or as a recombination junction with ZnO:Al for II–VI based alloy top cells in a tandem solar-cell configuration. Finally, some advances have been made in substrate-configuration CdTe cells on Mo using sputter deposition that hold promise for flexible CdTe-based PV.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Low-temperature growth of thick intrinsic and ultrathin phosphorous or boron-doped microcrystalline silicon films: Optimum crystalline fractions for solar cell applications

The growth kinetics and optoelectronic properties of intrinsic and doped microcrystalline silicon (μc-Si:H) films deposited at low temperature have been studied combining in situ and ex situ techniques. High deposition rates and preferential crystallographic orientation for undoped films are obtained at high pressure. X-ray and Raman measurements indicate that for fixed plasma conditions the size of the crystallites decreases with the deposition temperature. Kinetic ellipsometry measurements performed during the growth of p-(μc-Si:H) on transparent conducting oxide substrates display a remarkable stability of zinc oxide, while tin oxide is reduced at 200°C but stable at 150°C. In situ ellipsometry, conductivity and Kelvin probe measurements show that there is an optimum crystalline fraction for both phosphorous- and boron-doped layers. Moreover, the incorporation of p-(μc-Si:H) layers produced at 150°C in μc-Si:H solar cells shows that the higher the crystalline fraction of the p-layer the better the performance of the solar cell. On the contrary, the optimum crystalline fraction of the p-layer is around 30% when hydrogenated amorphous silicon (a-Si:H) is used as the intrinsic layer of p–i–n solar cells. This is supported by in situ Kelvin probe measurements which show a saturation in the contact potential of the doped layers just above the percolation threshold. In situ Kelvin probe measurements also reveal that the screening length in μc-Si:H is much higher than in a-Si:H, in good agreement with the good collection of microcrystalline solar cells     

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.     

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO₂ (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO₂-coated float glass as used for module production.     

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.     

PEN as substrate for new solar cell technologies

The possible use of polyethylene naphthalate as substrate for low-temperature deposited solar cells has been studied in this paper. The transparency of this polymer makes it a candidate to be used in both substrate and superstrate configurations. ZnO:Al has been deposited at room temperature on top of PEN. The resulting structure PEN/ZnO:Al presented good optical and electrical properties. PEN has been successfully textured (nanometer and micrometer random roughness) using hot-embossing lithography. Reflector structures have been built depositing Ag and ZnO:Al on top of the stamped polymer. The deposition of these layers did not affect the final roughness of the whole. The reflector structure has been morphologically and optically analysed to verify its suitability to be used in solar cells.     

Microcrystalline/micromorph silicon thin-film solar cells prepared by VHF-GD technique

Hydrogenated microcrystalline silicon prepared at low temperatures by the glow discharge technique is examined here with respect to its role as a new thin-film photovoltaic absorber material. XRD and TEM characterisations reveal that microcrystalline silicon is a semiconductor with a very complex morphology. Microcrystalline p–i–n cells with open-circuit voltages of up to 560–580 mV could be prepared. “Micromorph” tandem solar cells show under outdoor conditions higher short-circuit currents due to the enhanced blue spectra of real sun light and therefore higher efficiencies than under AM1.5 solar simulator conditions. Furthermore, a weak air mass dependence of the short-circuit current density could be observed for such micromorph tandem solar cells. By applying the monolithic series connection based on laser patterning a first micromorph mini-module (total area of 23.6 cm²) with 9% cell conversion efficiency could be fabricated.