| The incident monochromatic photon-to-electron conversion efficiency (IPCE) is an essential characterization method for the photoelectrical performance of solar cells. An IPCE measurement system involving alternating current (AC) and direct current (DC) methods was set up independently by our research group, which is suitable for most types of solar cells. The features of this IPCE measurement system are high power, high precision and high stability. A 1000 W xenon lamp coupled to a monochromator creates an incident monochromatic light beam. For AC method, the lock-in technique is applied to measure the short-circuit photocurrent of solar cells. While for DC method, a high-precision digital source meter is used for recording the short-circuit photocurrent of solar cells. The incident monochromatic light power is measured by a monochromatic optical power meter. The measurement principle, procedure and feature of the IPCE measurement system were described in detail. Through the IPCE measurement instances of solar cells, the measurement accuracy and repeatability of this IPCE measurement system were investigated. In addition, several examples were given to illustrate that the IPCE measurement system can be applied to the development of new type solar cells.Dye-sensitized solar cell (DSSC) is a kind of photovoltaic device of majority carrier transport which is driven by the gradient of photogenerated carrier concentration (i.e. the gradient in the chemical potential of electrons). The working principle of DSSCs is essentially different from the traditional inorganic p-n junction solar cells. Thus, DSSCs will certainly show some special characteristics during the IPCE measurement. The IPCE spectrum of DSSC was measured by the self-built IPCE measurement system. A frequency effect on IPCE measurements for DSSCs was founded, which involves two aspects:(1) IPCE spectrum values drop significantly with the increase of chopper frequency; (2) There are different degrees of decline for the different bands of IPCE spectrum with the increase of chopper frequency, and the peak in the visible band of IPCE spectrum decreases more than the peak in the ultraviolet band. The frequency effect on IPCE measurement for DSSCs in the dark was discussed in detail through measuring the short-circuit current waveform, the extinction spectrum of dye-coated TiO2 photoelectrode film and the electrochemical impedance spectroscopy (EIS). The mechanisms of frequency effect were investigated from electron transport and extinction spectrum. For DSSCs, AC method is remarkably influenced by trap states of electrons and the optical penetration depth; while DC method is a steady state measurement avoiding the impact of these two factors. The short-circuit current density (JSC) was calculated by integrating the product of the incident monochromatic photon flux density of AM 1.5 global solar spectrum and DC IPCE spectrum of DSSC, which matched with JSC derived from I-V testing. Therefore, a conclusion can be drawn that DC method is more suitable than AC method for IPCE measurement of DSSCs.A number of DSSCs were fabricated, which were sensitized by bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2’-bipyridine-4-carboxylic acid, 4’-carboxylate)ruthenium(Ⅱ) (N719). A 1080-hour thermal stability testing of DSSCs was conducted. The focus is a systematic study on exploring and understanding degradation mechanisms for long-term thermal stability of DSSCs. Two different temperature conditions were selected to deeply explore the degradation mechanisms of DSSCs performance. One temperature condition is at 25℃ (i.e. room temperature); the other is under a temperature cycle from -20℃ to 25℃. The cells maintained ca. 80% of its initial overall power conversion efficiency (η) after 1080-hour aging. It is found that the decrease of η was ascribed to the decrease of JSC while the open-circuit photovoltage and the fill factor increased with time; and the decrease rate of JSC for the cell under the temperature cycle from -20℃ to 25℃ was slower, compared with the cell at 25℃. From the physical point of view, the reason for the drop in JSC was analyzed. Several possible degradation mechanisms, which are stability of the dye, aging of the photoelectrode film, change of the electrolyte components, and degradation of the counter electrode, were discussed systematically. Particularly, the long-term stability of N719 adsorbed on the TiO2 electrode permeated in the electrolyte was investigated by using UV-visible absorption spectrum and resonance Raman scattering. Density functional theory calculations were applied to illustrate the deterioration of N719 which caused the decrease of the light harvesting efficiency of photoelectrode and subsequently decreased JSC. Moreover, high temperature will accelerate the deterioration. Additionally, the change of the electron transport/transfer at the interfaces in DSSCs, which was caused by aging of the photoelectrode film, change of the electrolyte components, and degradation of the counter electrode, was analyzed with EIS. The analysis result of EIS shows that the change of the electron collection efficiency of photoelectrode with time was not large. As a conclusion, the decline of long-term stability for N719 is found to be the main reason for the degradation of DSSCs. |
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