CORDIS - Forschungsergebnisse der EU



In the framework of the INPHOFLEX project, the first examples of integration of optimized optical designs within flexible optoelectronic devices have been realized. This is a relevant milestone as the efforts of the optoelectronics manufacturers towards a more versatile, light weight and adaptable technology are increasing every day. Ordered and disordered photonic materials, like the ones targeted in the project, have been shown to increase significantly the performance of both semitransparent and opaque flexible dye solar cells, in the former case leading to the highest photocurrent ever reported for a flexible solar cell of this kind. The project has also served to strengthen the links to some of the world leader groups in the field working in Korea. During this period, the grantee, Dr. Yuelon Li, has also visited different world class laboratories specialized in photovoltaics, which have provided him with a broader network of professional contacts and a wider overview of the field. As a result of the work developed under the Marie Curie program, he has been appointed a position as Assistant Professorship in one of the most competitive and prestigious universities of China, Nankai University.

From a technical perspective, the project presented several phases, which are commented next. In order to prepare highly efficient dye-sensitized solar cells (DSC) on plastic substrates amenable for the integration of the different photonic designs foreseen at the beginning of the project, several key materials and processes were synthesized or developed by the grantee during his first months in the host institute. First of all, the organic binder-free paste has been formulated by mixing home-synthesized nanoglue nanoparticle and main titania particles following a sol-gel process developed by the grantee, which has enough viscosity for coating a thick film onto flexible substrate as photoanode of flexible DSCs. Finally, some plastic DSCs with high efficiency have been prepared by the grantee with combination of chemical and physical sintering, which was implemented based on physical compression but with heating simultaneously. By taking dual benefits of chemical sintering due to nanoglue and physical sintering of physical compression, the efficiency of plastic DSCs has reached 6.4% without further optimization of optical design by inclusion of photonic crystal or diffusion scattering centers as shown in Figure 1. A paper has been published recently in RSC Advances, 2015, 5, 76795, as seen in the attachment 1. Task 1 devoted to the fabrication of plastic DSCs through physical compression process, has therefore been successfully completed.
An in-depth analysis of the compatibility of PVDF nanofibers with potential photonic designs led us to take the decision of not pursuing Task 2 anymore. However, we found an alternative way to prepare similar highly bendable electrodes based on plastic substrates for DSCs, as described in the activities related to Task 3.
From the beginning, the grantee started to learn how to prepare inorganic optical lattices of high dielectric contrast on slide glass with standard procedures and materials developed in the host group. He managed to master the techniques in a relatively short time, being capable of attaining multilayers displaying a well-defined, intense optical reflection peak at the desired spectral ranges. In order to integrate photonic crystal stacks into photoanode of flexible DSCs, the grantee developed a method to directly deposit photonic crystals onto titania photoanode as back reflector by forming photonic crystal multilayers with home-synthesized acidic titania nanoparticles, which solved the incompatibility of basic titania nanoparticles originally used in the host group with main titania particles grown in acidic condition. A representative schematic is depicted in Figure 2. The spectral response of photonic crystals was finely tuned in different ranges to cover blue, green and red regions of visible light. The grantee fabricated flexible DSCs with those photonic crystal integrated titania photoanodes and found that the photonic crystal based flexible DSCs presented better short circuit current density and power conversion efficiency than a reference cell of similar size but without integrating a photonic crystal. This is due to the light harvesting enhancement caused by the back reflection provided by the photonic crystal. Moreover, the mechanical stability of photonic crystal integrated flexible DSCs was investigated and found that >96% of initial cell efficiency was maintained after bending flexible DSCs, indicating the bendability of photonic crystal integrated flexible DSCs. Therefore, Task 3 has been successfully completed and integration of flexible back mirrors into flexible solar cells has been achieved. This result is the first example, to the best of our knowledge, of the integration of a flexible photonic structure in an optoelectronic device, which has been published in Advanced Optical Materials, 2016, 4, 464-471, as seen in the attachment 2. We have demonstrated that it is possible to obtain flexible, low weight solar cells with tunable color and enhanced efficiency, which we believe may be of great interest for applications in building integrated photovoltaics or rechargeable portable devices.
As scheduled for the second-half of INPHOFLEX project which are the consecution of one of the main objectives, the grantee focused in the realization of the activities comprised in Task 4: the rational integration of random distributions of light scatterers within plastic DSCs. The grantee have maximized light harvesting and optimized charge collection properties of plastic DSCs by integrating random distributions of light scatterers within plastic DSCs, which are followed the guidelines of a rigorous and complete optical and electrical model of the cell, as shown in Figure 3. The optimized design includes a dispersion of scatterers to reinforce light absorption over the whole spectral range at which the dye absorbs (leading to saturation absorption), and structuring of the electrode incorporating an adhesion layer to enhance charge collection. A synthetic route to prepare these electrodes at low temperature based on a combination of chemical and physical sintering is similar to the one developed during the realization of Task 1. The grantee have demonstrated that by using this approach it is possible to build DSCs on plastic electrodes with record PCE as high as 8.55%, and fully flexible DSCs with efficiencies as high as 7.79%. In this latter case, the photocurrent reaches a value of 15.94 mA/cm2, above the highest value previously reported for a flexible DSC device (15.2 mA/cm2). Moreover, bending tests prove that the optimized design of flexible DSCs is mechanically stable, photovoltaic performance being preserved after 150 bending cycles. These results have been published in a very decent journal Energy & Environmental Science, 2016, 9, 2061-2071, as seen in the attachment 3. We foresee our work may have an impact in the fields of building integrating photovoltaics and in that of rechargeable portable devices, for which light weight and adaptability of energy suppliers are key aspects.
Task 5, which is devoted to activities focused in the characterization of the photovoltaic performance (IV curves, EQE) and optical properties of the cells, has a transversal character, it is related to all objectives during the realization of Task 1, 2, 3 and 4. The grantee has been performing all necessary experimental characterization during the realization of this project, including not only the photovoltaic and optical characterization such as IV curves, EQE, UV-Visible, but also XRD, SEM, TEM, AFM, particle size distribution. So, Task 5 has been accomplished successfully.
Up to now, most of relevant objectives proposed at the beginning of INPHOFLEX project have been successfully addressed by the grantee within the past two years. During his stay in host institute the grantee frequently discussed with colleagues of the host institute about many aspects related to the project, such as preparation and optical characterization of photonic crystals and other optical designs based on random distribution of scatterers. The interaction between the grantee and the colleagues in the host institute has demonstrated to be extremely beneficial for both sides. Until now, the grantee is capable of fabricating high quality photonic crystal independently, which is completely new field for him. At the same time, the grantee learned to optically characterize solar materials. As proposed in the agreement, the grantee has transferred the knowledge of synthesis of key materials, and the fabrication technique of organic binder-free paste and plastic DSCs to the host institute. Moreover, the grantee shared his knowledge on rigid DSCs, which has further promoted the related research in the host institute. Reciprocally, he has received training in the preparation and characterization of optical materials of interest for the field of photovoltaics. At the same time, the conclusions obtained during the realization of INPHOFLEX project may have an impact in the fields of building integrating photovoltaics and in that of rechargeable portable devices, for which light weight and adaptability of energy suppliers are key aspects.