Photoconductive decay lifetime and Suns-Voc diagnostics of efficient heterojunction solar cells
Minority carrier lifetime and Suns-Voc measurements are well-accepted methods for characterization of solar cell devices. We use these methods, with an instrument from Sinton Consulting, as we fabricate and optimize state-of-the-art all hot-wire chemical vapor deposition (HWCVD) silicon heterojunction (SHJ) devices. For double-sided SHJ devices, lifetime measurements were performed immediately after hydrogenated amorphous silicon (a-Si:H) deposition of the front emitter and back base contacts on a Silicon wafer, and also after indium tin oxide (ITO) deposition of transparent conducting oxide contacts. We report results of minority carrier lifetime measurements for double-sided p-type Si heterojunction devices and compare Suns-Voc results to Light I–V measurements on 1-cm2 solar cell devices measured on an AM1.5 calibrated XT-10 solar simulator.
Synthetisierung und Charakterisierung von Si/SiO2-Nanostrukturen als Hetero-Emitter für siliziumbasierte Solarzellen
In the present study the fabrication and characterization of nanostructured multilayer systems are investigated in order to construct a Si/SiO2-nanodot-hetero-emitter (Si/SiO2-ND-HE) which can be integrated as a functional layer in silicon-based solar cells. The Si/SiO2-ND-HE consists of crystalline Si (c-Si) nanodots surrounded by a SiO2-matrix. Compared with conventional emitters consisting of amorphous Si (a-Si) or c-Si, the Si/SiO2-ND-HE offers higher band offsets and lower parasitic light absorption. In addition the Si/SiO2-ND-HE holds the potential to improve conversion efficiency by exploiting quantum effects. A prerequisite for the integration of a Si/SiO2-ND-HE in a Si solar cell device is to ensure the passivation of the adjacent c-Si absorber and the induction of a band bending by doped Si nanodots. In addition, the charge carrier transport through the emitter has to be provided. In order to fulfill these conditions a precise control over the growth process of the Si-nanodots and the SiO2-matrix is required. This study examines two different methods to produce a Si/SiO2-ND-HE: (i) the dewetting of thin a-Si layers in combination with a plasma-assisted oxidation (plasma oxidation) and (ii) the decomposition of non-stoichiometric SiOx layers. This work contains systematic studies of the physical properties of Si nanodots and the surrounding SiO2 matrix and provide founded analysis on their structural properties as well as the passivation mechanisms and the charge carrier transport. In the present study the dewetting is performed for the first time onto oxidized c-Si wafers with an ultra-thin oxide (≤ 2 nm). In combination with the plasma oxidation process a new concept for producing a Si/SiO2-ND-HE is developed which allows a precise control of the Si nanodot growth and the formation of the SiO2 matrix. By this new method a high density of c-Si nanodots with short inter-dot distances (≤ 2 nm) at a process temperature of 600 ◦C can be obtained. This leads to an increment of the charge carrier transport through the Si/SiO2-ND-HE and allows the fabrication of temperatureand process-stable passivation layers. In comparison to the more established decomposition method, the dewetting technique is superior in terms of controlling the density and size of Si nanodots, the quality of the substrate passivation and the current transport through the emitter itself. For the decomposition technique, a band bending of more than 700meV induced by nand p-doped SiOx-films with O/Si ≤ 0.4 onto c-Si wafers is presented. This high band bending remains the same even after the decomposition of the films. The n-doped SiOx films are integrated for the first time as Si/SiO2-ND-HE in a polycrystalline Si thin film solar cell. For an unoptimized test structure an open-circuit voltage above 400mV is presented. In addition, the SiOx-films protect an underlying ZnO:Al layer from thermal degradation. The results obtained in this work make significant contributions to the modern research field of Si nano-photovoltaics and show new ways for the fabrication of a nanostructured Si/SiO2-hetero-emitter.
Application of infrared camera to bituminous concrete pavements: measuring vehicle
Infrared thermography (IR) has been used for decades in certain fields. However, the technological level of advancement of measuring devices has not been sufficient for some applications. Over the recent years, good quality thermal cameras with high resolution and very high thermal sensitivity have started to appear on the market. The development in the field of measuring technologies allowed the use of infrared thermography in new fields and for larger number of users. This article describes the research in progress in Transport Research Centre with a focus on the use of infrared thermography for diagnostics of bituminous road pavements. A measuring vehicle, equipped with a thermal camera, digital camera and GPS sensor, was designed for the diagnostics of pavements. New, highly sensitive, thermal cameras allow to measure very small temperature differences from the moving vehicle. This study shows the potential of a high-speed inspection without lane closures while using IR thermography.