| Abstract [eng] |
In 2015 in Paris 196 countries approved the agreement before 2050 to make greenhouse gas emissions to no more than the Earth can absorb. Thus, during the years 2011-2050 emission of carbon dioxide should not be higher than 1100 gigatons. This means that, by the year 2050 it should be unused two-thirds of current fossil fuel reserves, i.e. one-third of the oil, half of the gas and 80% of coal reserves. Moreover, the growth of these energy sources usage will stop and continue to decline. After Fukushima nuclear power plant accident in 2011 the attitude to nuclear energy has changed. Thus, the growing energy needs will be satisfied by increasingly important renewable energy sources, of which the sun has the greatest energy potential. The photovoltaic effect was discovered in 1839 by A. E. Becquerel. In 1954 in Bell laboratory D. M. Chapin and his colleagues have developed the first practical silicon p-n junction solar cell. One year later, Hoffman Electronics started to sell a 2% efficiency cells for 1785 USD/W. Currently, commercial monocrystalline silicon solar cell cost is reduced to 0.5 USD / W and efficiency reached 20%. Although silicon solar cells are the most popular, they have several disadvantages: the high-purity silicon wafer production is expensive, elements are rigid, brittle and opaque, which limits their application in niche areas. Organic Bulk Heterojunction (BHJ) solar cells do not posses these shortcomings. BHJ can be a variety of colors, partially transparent, cheaply printed on a flexible substrate. Due to these properties, they can be integrated into portable electronic devices, used as a colorful, electricity-generating windows and etc. Charge carriers photogeneration, separation, transport, collection at the corresponding electrodes and the efficiency of recombination processes depend on the composition and morphology of bulk heterostructure layer. In order to obtain BHJ solar cells with maximum efficiency, among other things it is necessary to ensure effective charge carrier transport. One of the most important parameters are charge carrier mobility and recombination mechanisms. There are several different techniques to study these processes, such as Time-of-flight (ToF), Charge Extraction by Linearly Increasing Voltage (CELIV), photogenerated (injected) Charge Extraction by Linearly Increasing Voltage (photo-CELIV (i-CELIV)). However, TOF is suitable only for low conductivity thick samples; moreover, the rate of the recombination is strongly influenced by the load resistance, CELIV can only be used to measure the mobility of the majority charge carriers and using photo-CELIV errors may occur due to the influence of the initial distribution and recombination of the photogenerated charge carriers, in i-CELIV technique the estimation of the transit time and herewith mobility is influenced by the ratio of semiconductor’s and insulator’s capacitances and by using i-CELIV as well as CELIV or photo-CELIV determination of the mobility dependence on electrical field is extremely approximate. |
