The Research On Planar Heterojunction Perovskite Solar Cells

In recent years, methylamino lead iodine compounds(CH3NH3PbI3) represented organicinorganic hybrid perovskite materials have became as a new "star" in the PV field. Energy conversion efficiency of solar cells based on these materials soar from 3.8% in 2009 to 20.1% currently, the fast progress of efficiency is impressive. More importantly, abundant material with low price, low temperature solution processing and the ompatibility with large-area flexible devices make it a promising material for low cost solar cells. To achieve better reproducibility of perovskite thin films and solar cells device, we explore the processing for high-quality, high coverage, highly reproducible preparation of perovskite thin films, thus achieving high efficiency, better reproducibility in planar heterojunction perovskite solar cell.The main contents in this work include:1. By comparing the structure morphology and optical characteristics of the perovskite film and performance of the corresponding photovoltaic devices treated with two different annealing method(direct annealing step and multi-step slow annealing), we found that multi-step annealing is a effective and efficient method for universal reproducible planar heterojunction solar cell preparation. Device prepared using this method gave the highest efficiency of 13.58%, compared with the 8.65% efficiency prepare with direct annealing a 57% enhancement in efficiency is demonstrated. Our optimized annealing method can guarantee reproducible preparation of high quality perovskite films. As a universal method for preparing perovskite thin films, multi-step slow annealing method is not only applicable to prepare efficient planar heterojunction solar cell, but also applicable to other perovskite material based semiconductor microelectronic or optoelectronic devices, such as light emitting diodes, lasers, photodetectors and thin film transistor and so on.2. By further finely control the annealing temperature and duration, we have prepared almost 100% coverage CH3NH3PbI3-xClx perovskite films with a high degree of crystal orientation. Thus an efficiency of 12.0% on average and a highest efficiency of 15.17% in planar heterojunction perovskite solar cell is demonstrated. High coverage ensures sufficient light absorption and avoids the possibility of short circuit. Highly oriented crystals is very conducive to efficient electron and hole transport towards the corresponding electrodes. Now that the film prepared in this manner shows high degree of crystal orientation, a model of anisotropy of electron transport properties was first proposed to explain the observed excellent photovoltaic performance. On the basis of this preparation method, by optimizing the other functional layers we can expect better device performance.3. By conducting comparative experiments, we found part of the reasons for the relatively poor stability of the conventional planar heterojunction device. Both solvent and solute that incorporated into the spiro-MeOTAD solution have more or less destructive effect on perovskite, especially extremely hygroscopic lithium salt(Li-TFSI) can severely decomposed perovskite by absorbing moisture in the air thus worsen the overall performance of the device. Acetonitrile can also dissolve and decompose perovskite. Alternatively, the use of strong oxidant F4-TCNQ that can co-dissolved in chlorobenzene with spiro-MeOTAD as a p-type dopant in hole transport layer resulted in a maximum efficiency of 10.59% in planar heterojunction perovskite solar cell, generally the device has better stability than the similar device with Li-TFSI doped spiroMeOTAD as hole transport layer.