Effect of Thermal Annealing of Atomic-Layer-Deposited AlOx/ Chemical Tunnel Oxide Stack Layer at the PEDOT : PSS/n-type Si Interface to Improve its Junction Quality

概要

High-efficiency c-Si solar cells are attracting interest in research, in which a metal oxide or organic polymer thin film having electron/hole transporting ability is bonded to crystalline Si (c-Si) as an anode and a cathode. In our previous study, a transparent and high hole-transporting conductive polymer poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) was used to plane n-type crystal Si (We have achieved an efficiency of 13.3% for pristine PEDOT:PSS device), and have achieved an efficiency of 15.5% by using an antireflection film together. Furthermore, we have prototyped 10 series modules for 2 cm2 and 4-inch size elements and have demonstrated the potential as an independent power supply for surveillance cameras. However, in order to further improve the device performance, it is necessary to promote the passivation of the c-Si/PEDOT:PSS anode-cathode interface and further enhance the internal electric field at the anode-cathode interface. In this doctoral dissertation, aluminum oxide (AlOx), which is expected to have high Si passivation ability and high fixed charge density, among high-k materials with high dielectric constant, and PEDOT:PSS/n-Si interface were selected. The effect of increasing the electric field strength at the interface was investigated by inserting the film into the PEDOT:PSS/n-Si interface. Firstly, by arranging the amorphous (a-) AlOx films prepared by atomic layer deposition (ALD) in islands, the efficiency of hole collection at the anode, and the improvement of AlOx to increase the electric field strength at the anode interface. Secondly, Si passivation by short-time heat treatment (RTA) at 425℃ in a reduced pressure environment and an atmospheric pressure heat treatment (FGA) in an N₂/H₂ environment of a coated structure of a 1-2 nm thick AlOx layer and a chemically oxidized SiOx layer. And the effects on PEDOT:PSS/n-Si interface local chemical bond state, electronic structure, and device performance were investigated. This doctoral dissertation consists of 6 chapters. The outline of each chapter is described below:
Chapter 1, "Introduction", describes the background of research and development of crystalline Si solar cells and the purpose of this paper.
In Chapter 2, the effect of a TiOx layer as a hole blocking layer at the Si/Al interface on the back cathode of a c-Si solar cell is discussed from steady-state photocurrent and current transient response characteristics. Specifically, focusing on TiO₂, which has a small work function, a titanium oxide (TiO₂) synthesized by hydrolysis of TiCl₂ was used to insert a TiOx layer with a thickness of 1-2 nm at the n-Si/Al interface by spin coating. It has been demonstrated that the device structure is effective in improving the conversion efficiency and the efficiency of collecting photo-generated carriers in the 600-1200 nm region as compared with the device performance without insertion. Furthermore, in order to quantitatively evaluate the hole blocking ability, the hole current pushed back into the c-Si by the hole blocking layer by instantaneously applying the reverse bias from the steady current when the forward bias was applied in the dark. We established a Transient Reverse Recovery measurement to determine the recombination velocity S from the waveform. As a result of a comparative study of this technique with existing techniques for evaluating the recombination rate, such as the QSSPC and μ-PCD methods, it has been clarified that it can be sufficiently used as a technique for evaluating the performance of the hole blocking layer.
Chapter 3, “Experimental procedure”, describes the fabrication method and the evaluation method of the ultra-thin aluminum oxide (AlOx) film, which is expected as a high dielectric constant material (high-k), by the ALD method for the purpose of enhancing the electric field strength at the PEDOT:PSS/n-Si anode interface. It also describes the photolithography process for forming a 15 × 15 μm² island array, chemical oxide layer formation, RTA, and FGA heat-treatment process. About the in-plane distribution of minority carrier lifetime (τeff) by μ-photoconductive decay method (μ-PCD method), X-ray photoelectron spectroscopy (XPS) method, and infrared absorption spectroscopy (FTIR) evaluation method for the above ultrathin films.
In Chapter 4, 4.1 describes the fundamental physical properties of amorphous (a-)AlOx insulator thin films produced by an alternate supply of TMA[Al(CH₃)₃] and water by the ALD method. Film formation was performed by using TMA, water supply time, time sequence, substrate temperature as variables, the film thickness, surface morphology, and local chemical bonding state were evaluated. In addition, the physical properties of n-Si interface bonding were evaluated by short-time heat treatment (RTA) at 425°C for 15 minutes in a reduced pressure environment after film formation. As a result, it was clarified that the surface roughness and the Al(OH) bond remaining in the film were reduced most at the substrate temperature of 200°C, and a dense amorphous structure was formed.
In 4.2, in order to achieve both passivation of the c-Si surface and hole trapping ability, a 15 × 15 μm2 size of 20 nm thick ALD a-AlOx with different lattice spacing was formed on the c-Si surface by photolithography. We investigated the PEDOT:PSS/a-AlOx/n-Si junction characteristics and device performance of spin-coated 80 nm thick PEDOT:PSS layer on the island array. The Si passivation ability was improved, and the diffusion potential was increased to 1.4V with the increase of the a-AlOx/PEDOT:PSS area ratio, but the fill factor in the solar cell element was significantly decreased by the increase of the parallel resistance, which deteriorated the conversion efficiency.
Therefore, in Section 4.3, we investigated the effect of inserting a 2-3 nm thick AlOx ultrathin layer formed by the ALD method as a tunnel layer and Si passivation. In the RTA of 20-nm-thick a-AlOx, τeff was reduced from 150 μs to 15-30 μs. On the other hand, τeff can be improved to 600-700 μs by FGA treatment of a-AlOx/ch-SiOx/n-Si coated structure with ch-SiOx of 1-2 nm thickness inserted at the a-AlOx/n-Si interface. It is revealed that, from the evaluation of the sheet resistance, as a result of FGA processing of the coated structure, it decreases from PEDOT:PSS/c-Si junction 162Ω/□ to PEDOT:PSS/a-AlOx/ch-SiOx/n-Si coated structure 105Ω/□. In addition that, from the evaluation of the capacitance-voltage (C-V) characteristics, the fixed charge density was changed from 3.2 × 10¹² cm-² to 5.7 × 10¹² cm-² by applying FGA processing from a-AlOx/n-Si to a-AlOx/ch-SiOx/n-Si coated structure, and the interface state density decreased from 4.5 × 10¹¹ cm-²eV-¹ to 2 × 10¹¹ cm-²eV-¹. Moreover, the conversion efficiency of the device on the planarized n-Si with PEDOT:PSS/a-AlOx/ch-SiOx/n-Si coated structure as anode is 13.08% without insertion and 14.91% (open voltage: 0.645V, Fill factor: 0.77, short-circuit current density increased to 30 mA/cm²).
In Chapter 5, the electronic structure of the interface in the a-AlOx/ch-SiOx/n-Si layered structure by ch-SiOx insertion and RTA and FGA treatment was evaluated by XPS, UV spectroscopy, and Kelvin probe method. FGA treatment revealed that ch-SiOx contained a large proportion of Si* complexes near 103 eV, which did not belong to Si+, Si²+, Si³+, and Si⁴+, compared to RTA. In RTA, the oxidation of the a-AlOx layer on the surface became dominant, whereas in FGA, the oxidation of the ch-SiOx layer was promoted as the reduction of the AlOx layer progresses, and as a result, the passivation ability of the c-Si surface was improved. Besides, the band level diagram of the PEDOT:PSS/a-AlOx/ch-SiOx/n-Si interface was determined, and the a-AlOx/ch-SiOx/n-Si coated structure was inserted, and the subsequent FGA treatment was performed to form the anode interface. It was clarified for the first time that the electric field strength was enhanced.
In Chapter 6, we summarized the doctoral dissertation, gave conclusions, and mentioned future prospects.

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