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The investigation of degradation of seven distinct sets (with a number of individual cells of n $ 12) of state of the art organic photovoltaic devices prepared by leading research laboratories with a combination of imaging methods is reported. All devices have been shipped to and degraded at Risø DTU up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. Imaging of device function at different stages of degradation was performed by laser-beam induced current (LBIC) scanning; luminescence imaging, specifically photoluminescence (PLI) and electroluminescence (ELI); as well as by lock-in thermography (LIT). Each of the imaging techniques exhibits its specific advantages with respect to sensing certain degradation features, which will be compared and discussed here in detail. As a consequence, a combination of several imaging techniques yields very conclusive information about the degradation processes controlling device function. The large variety of device architectures in turn enables valuable progress in the proper interpretation of imaging results—hence revealing the benefits of this large scale cooperation in making a step forward in the understanding of organic solar cell aging and its interpretation by state-of-the-art imaging methods.

Abstract Nb-doped TiO2 films have been fabricated by RF magnetron sputtering as protective material for transparent-conducting oxide (TCO) films used in Si thin film solar cells. It is found that TiO2 has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO2 and ZnO. Further, the minimum thickness of TiO2 film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO2 in the as-deposited film is found to be ∼10−6 S/cm due to the Nb doping. Higher conductivity of ∼10−2 S/cm is achieved in thermally annealed films. Nitrogen treatments of Nb-doped TiO2 film have been also performed for improvements of optical and electric properties of the film. The electrical conductivity becomes 4.5×10−2 S/cm by N2 annealing of TiO2 films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO2 films can be controlled by nitrogen doping (from n=2.2 to 2.5 at λ = 550 nm) using N2 as a reactive gas. The controllability of n implies a better optical matching at the TCO/p-layer interface in Si thin film solar cells.

The use of Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) has gained popularity in the last decade. The success of this technique relies on the possibility to deposit thin films in a fast, vacuum-free, low-cost, low-damage, and high throughput way. In this work, we present ZnO and Aluminium doped ZnO (AZO) films deposited by AP-SALD at low temperature (<220 °C) with high uniformity and conformity. The ZnO films present a high transparency of 80%–90% in the visible range, with a tuneable band-gap, between 3.30 eV and 3.55 eV, controlled by the deposition temperature. The carrier density reaches values greater than 3 × 1019 cm−3, while the electron mobility of the films is as high as 5.5 cm2 V−1 s−1, resulting in an optimum resistivity of 5 × 10−2 Ω cm. By doping ZnO with aluminium, the resistivity decreases down to 5.57 × 10−3 Ω cm, as a result of a significant increase in the carrier density up to 4.25 × 1020 cm−3. The combination of ZnO thin films with p-type cuprous oxide (Cu2O), deposited by aerosol assisted metal organic chemical vapor deposition, allowed the formation of oxide-based pn junctions. The dark I-V characteristic curve confirms a rectifying behaviour, opening the window for the production of all-oxide solar cells completely by chemical vapour deposition methods. We also show the potential of AP-SALD to deposit AZO as a transparent conductive oxide layer for silicon heterojunction solar cells.

AbstractWe recently demonstrated at imec a relatively simple process sequence for the formation of copper based front contacts consisting of: i) defining the front contact pattern by laser ablation ii) plating of the contacts using Ni/Cu/Ag in a single plating sequence and finally iii) contact sintering. In this paper, we conduct a power loss analysis of the best 20.5% (confirmed by ISE CalLab) PERC solar cell produced on 125mm magnetically pulled CZ (m-CZ) Si with Ni/Cu/Ag contacts. Based on this power loss analysis, we estimate potential improvements in cell design enabling >21% energy conversion efficiencies.

The complexity involved in obtaining pure Kesterite Cu2ZnSnSe4 (CZTSe) thin film primarily arises due to its narrow region of stability, leading to the presence of unavoidable binary selenides of the constituent metals. This study offers an insight on the formation of Cu selenides when the amount of Cu is varied in the precursor from Cu poor to Cu rich. The amount of Cu selenides was found to decrease when the composition of CZTSe absorber approached Cu rich conditions but functional devices were not obtained. Detailed characterizations also showed that the Cu and Sn binary phases were present at the backside interface of CZTSe solar cells. However with an increase in the selenization temperature it was found that the amount of Cu selenides and other secondary phases could be drastically minimized or even eliminated leading to high efficiency devices.

Abstract Recently, the impurity photovoltaic effect (IPV) was proposed to improve the solar cell performance. Free electron–hole pairs can be generated in a two-step process involving an impurity level in the energy gap and two lower-energy photons: first electrons are optically excited from the valence band to the defect level and then from the defect level to the conduction band. The IPV effect will thus enhance the long-wavelength response of the cell. A significant amount of theoretical work has been carried out on IPV effect in the literature, particularly on silicon solar cells with indium impurities as defect. However, the lack of an easily available solar cell simulator including the IPV effect is a handicap. In this work, the numerical solar cell simulator SCAPS of the ELIS group was extended to include IPV in collaboration between the ELIS and the LPDS groups. Also, some special features are implemented, such as the calculation of electron and hole photoemission cross-sections of the impurity using the model of Lucovsky. The functionality of new SCAPS version was checked against existing results in the literature. Also, new results are presented such as the evolution of solar cell parameters with the indium density. We find that increasing indium concentration can improve silicon solar cell parameters, especially the short-circuit current and the efficiency, without drastically decreasing the open-circuit voltage. This is possible if a suitable structure for the cell is chosen. The optimum indium density should be equal around the base region density to obtain a positive benefit from the IPV effect. Light trapping, which is related to the internal reflectance at the front and the back of the cell, is very important in the IPV study. Reflectivity at the front and the back should exceed 99.9% to obtain a real efficiency increase. We calculate an improvement of about 6 mA/cm 2 in the photocurrent, and about 2% for the efficiency, which is due to the enhancement of long-wavelength absorption by the IPV effect.

Tandem perovskite cells The ready processability of organic-inorganic perovskite materials for solar cells should enable the fabrication of tandem solar cells, in which the top layer is tuned to absorb shorter wavelengths and the lower layer to absorb the remaining longer-wavelength light. The difficulty in making an all-perovskite cell is finding a material that absorbs the red end of the spectrum. Eperon et al. developed an infrared-absorbing mixed tin-lead material that can deliver 14.8% efficiency on its own and 20.3% efficiency in a four-terminal tandem cell. Science , this issue p. 861