The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
a-Si Related Photovoltaics I
Chair: Nazir Kherani, University of Toronto
Mo-C1.1 (invited) 14:00–14:30
Ultra-thin Silicon Films and Device Architectures for Transparent Photovoltaics
1. Center for Advanced Photovoltaic Devices and Systems (CAPDS), Waterloo, Ontario, Canada
2. Electrical and Computer Engineering Department, University of Waterloo, Waterloo, Ontario, Canada
Transparent photovoltaics (PV) is an important component in building integrated PV (BIPV), which is a rapidly growing PV sector. Thin film based transparent solar panels have strongest potential for window and skylight applications, and the main challenge is to incorporate good transparency with acceptable conversion efficiency. Doped layers with high conductivity, highly efficient yet thin absorber layers, and high conductive transparent conductive oxides are key issues in developing large area, transparent devices. Ultra-thin nanocrystalline silicon (nc-Si), due to its higher electron mobility and acceptable levels of transparency and optical absorption, is a potential candidate for transparent PV devices. Additionally, incorporating nc-Si material as the absorber layer in single or tandem thin film PV cells greatly enhances the spectral efficiency and device stability.
New transparent nc-Si solar cell with p-i-n configuration is developed on glass substrates incorporating highly conductive aluminium doped zinc oxide (AZO) films as top and bottom electrodes. The main experimental processes involved, (i) development of a high deposition rate process in a modified PECVD system, (ii) optimization of doped and undoped micro and nanocrystalline Si layers at temperature 300°C, (iii) process development for AZO films by sputter deposition at low pressure and moderate power with enhanced conductivity and transparency, and (iv) process integration for single junction p-i-n device process on glass substrate.
For the nc-Si deposition, an inductively coupled plasma (ICP) PECVD chamber has been modified to include a grounded, perforated, separator plate below the high density plasma source. A low-temperature (150°C) ICP-PECVD process been developed using the modified plasma chamber that resulted in nc-Si deposition with high growth rates (> 6 angstroms/second) and without having to dilute the SiH4 precursor with H2. The films are characterized by Raman spectroscopy, XRD, and photoconductivity measurements. Additionally, highly conductive (17 S/cm) n-type μc-Si:H (SiH4, PH3, 98% H2 dilution) and p type μc-Si:H (SiH4, B2H6, 98.5% H2 dilution) films have been developed at 300°C.
RF sputtering process has been optimized for highly conductive and transparent aluminum doped ZnO films. The effect of deposition pressure, power, and in-situ heating were systematically investigated to develop a film with a record low sheet resistivity of 5.4 ohm/square at 690 nm film thickness. For the device integration, the individually optimized film processes were used. Different absorber layers were tested in fabrication of single junction devices. The optimum photovoltaic performance is achieved with an absorber layer right at the transition from amorphous to nanocrystalline silicon. Conversion efficiencies exceeding 5%–6% have been obtained.
In addition to the experimental work, new cell architectures have been designed incorporating nanoplasmonic structures with the purpose of further trapping the light in devices especially with thin absorber layers. Numerical simulations carried out (using FDTD Solution software) underline the strong potential of nanoplasmonic metallic structures to further enhance the performance of ultra-thin solar cells for transparent and BIPV applications. The simulation results show the overall 25–30% percent increase in absorption efficiency for ultra-thin film silicon (100 nm thickness) by incorporating appropriate nanoplasmonic metallic array architecture.
Keywords: nanocrystalline silicon, thin film, photovoltaics, PECVD, nanoplasmonic
High Deposition Rate Amorphous Silicon Solar Cells
Ovshinsky Innovation LLC, 1050 East Square Lake Rd., Bloomfield Hills, MI 48304 USA
We describe a new method of depositing solar grade amorphous silicon thin films at deposition rates ranging from 200 Å/s to above 300 Å/s. We use remote microwave plasma deposition method at 2.45 GHz frequency and a commercial ASTEX 7610 applicator with a sapphire tube. Fluorine was added to some films in a range of between 0.2–0.6 at.%. To further increase deposition rates and improve film quality, varying amounts of Si2H6 were mixed with SiH4 in some recipes. The films were analyzed for density of states in the gap using photo-thermal deflection spectroscopy (PDS), as well as constant photo-current method (CPM). Temperature dependence of the μτ product was measured in the range from LN to 150°C. Film composition and morphology was studied by SIMS profile analysis and cross-section SEM. Amorphicity was confirmed with Raman spectroscopy. Staebler-Wronski type degradation was measured and compared in films both with and without fluorine. Doped films (both p- and n-type) were deposited successfully at high deposition rates by adding BF3 and PH3 to the SiH4/Si2H6 plasma. The best recipes were used for making single junction dot cells which gave efficiencies in the range between 5%–6.5%. The total time to deposit a single junction cell amounted to 22 seconds.
Keywords: amorphous silicon, solar cells, Staebler Wronski effect, defect density
Role of a Disperse Carbon Inter-monolayer on the Performances of Tandem a-Si Solar Cells
1. CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP-UNINOVA, 2829-516 Caparica, Portugal
2. INNOVNANO, Materiais Avançados, SA, 3040-570 Antanhol, Portugal
3. CQFM and IN, Departamento de Eng. Química, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal
4. Solar Plus, Produção de Painéis Solares SA, 3770-305 Oliveira do Bairro, Portugal
The aim of this paper is to present the role of a carbon disperse interlayer laying between the n-a-Si:H and a AZO back contact layers on amorphous silicon solar cells, compared these results with the ones achieved in conventional solar cells. This work also reports the influence of non-sintered and sintered nanostructured powders of aluminum zinc oxide (AZO) targets prepared to fabricate transparent conductive oxide (TCOs) by RF magnetron sputtering at room temperature as the back contact layer. The data achieved show that solar cells fabricated on glass substrates using as back contact TCO/metal, improves the overall device performance when a disperse highly insulator inter-nanolayer of carbon is formed.
The incorporation of this inter-layer leads to solar cells exhibiting an initial efficiency of 11%, Voc = 1.6 V, Jsc = 11 mA/cm2, and FF = 63%. Compared to the standard solar cell this improvement leads to a 10% increase in Jsc and 20% in efficiency of the solar cell and also in the external quantum efficiency.
This data are quite encouraging for the development of high efficiency solar cells by exploiting the back contact structure and composition role on overall device performances.
Keywords: photovoltaics, amorphous silicon solar cells, disperse dielectric interlayers, oxide interfaces, composition
Progress in Processing of Hydrogenated Amorphous Silicon Thin Film Solar Cells Using the Expanding Thermal Plasmas
1. National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
2. Delft University of Technology, Building 36, Mekelweg 4, 2628 CD Delft, The Netherlands
In this contribution, we report on progress in processing of hydrogenated amorphous silicon (a-Si:H) solar cells using expanding thermal plasma chemical vapor deposition (ETP-CVD) method. The ETP-CVD results in a-Si:H with different nanostructure compared to conventional RF-PECVD method. The density of smallest volume deficiencies in the a-Si:H matrix is significantly smaller, while we have indications that the passivation degree of the volume deficiencies appears to be worse. We focus on the study of the relation between the metastability of p-i-n a-Si:H solar cells and the nanostructure of the a-Si:H layer deposited using ETP-CVD.
A breakthrough in the processing of a-Si:H based solar cells using ETP-CVD has been established. So far, additional DC pulse and RF bias on the substrate holder under ETP-CVD conditions were required to achieve conversion efficiencies in the order of ~6%. One of the short-comings recognized under these typical ETP-CVD conditions is the possible negative effect of the higher order polysilane molecules, radical and ions, generated by the reaction of silane with the Ar ions and excited Ar* in the reactor chamber. By decreasing both the Ar dilution and Arc current—or in other words increasing the H2 dilution in Arc head—device grade a-Si:H solar cells with an initial conversion efficiency of 9.7% was obtained using purely remote processing conditions without any additional DC pulse or RF voltage biasing of the substrate.
The stability of p-i-n solar cells were studied in great detail. First the degradation of the external parameters was studied under 1 sun AM1.5 conditions, while the devices were kept at a temperature of 25°C. The short (fast defect states) and long term (slow defect states) degradation is the same for all a-Si:H solar cells and appears to be independent of the above mentioned ETP-CVD growth conditions. Secondly, the evolution of the defect states in the band gap of the intrinsic a-Si:H in the solar cells is measured versus light soaking using Fourier Transform Photoconductivity Spectroscopy (FTPS) in combination with external quantum efficiency (EQE) measurements on the solar cell. At least 3 different defect states can be resolved in the FTPS-EQE spectra in the range of 0.6 down to 1.4 eV from the conduction band. The density of the defect states non-linearly increases versus light soaking time. Especially, the defect state at 0.6 eV from the conduction band, as resolved from the FTPS-EQE spectrum, is significantly enhanced during light soaking compared to the other two defect states. The most significant increase in density of defect states is observed within 100 hours of light exposure and correlates with the degradation of the external parameters of the solar cells. We consider that light degradation in a-Si:H solar cells prepared by ETP strongly relates with these sub-gap absorptions. The differences and agreements with the stability of a-Si:H p-i-n cells deposited using the conventional RF-PECVD techniques will be discussed.
Keywords: expanding thermal plasma, hydrogenated amorphous silicon, solar cells, FTPS