Photonic coatings for Light Trapping in organic photovoltaic solar cells – PhotoLighT

The method used for this project is based on the partner skills for the elaboration of structured films and modeling of parameters to be controlled. Indeed, it is essential to predict the necessary dimensions of pores needed to get the desired photonic effect. Then, these films are prepared optimizing the nature of the material used and preparation conditions. One of the challenge is to structure the active layer to increase the absorption by light trapping while avoiding an additional interface layer in the device.

Different structured films have been already elaborated and deposited on different substrates, in particular representative active layer of an organic solar cell. The first results are promising and showed an improvement of the absorption. Simultaneously, modeling tools were constructed to determine the key parameters for photonics structures. A particular effort was put on the preparation process of these interlayers to integrate them in an organic device.

Find materials and interfacial layers to play with light to trap it or another way to extract it for application other than photovoltaics.

Two papers were published in peer reviews in line with this project, on predictive aspects of light trapping for organic solar cells.

Photovoltaic efficiencies in organic photovoltaic OPV solar cells are reduced by the conflict of scales between exciton diffusion lengths (~10 nm), that are an order of magnitude smaller than typical absorption lengths in solids (~100 nm). This intrinsic contrariness can be improved the creation of a bicontinuous donor/acceptor bulk heterojunction phase (BHJ). Optimized BHJ devices have active layer thicknesses of ~100 nm due to the device-limiting trade-off between optical absorption and electrical performance. These thicknesses are adequate to absorb most photons of the solar spectrum due to the strong extinction coefficient of BHJ materials. However, the sun’s maximum photon ?ux is located around ?=700 nm, which is near the band-edge of many BHJ materials. This weakness can be slightly counterbalanced via the custom synthesis of low-bandgap (LBG) polymers which exhibit broader absorption tails toward near-infrared. The active layer thickness constraints imposed on BHJ solar cells make it imperative to develop innovative ways to enhance absorption or light trapping inside the solar cell photoactive layer.
The origin of functional materials in Nature is attributed to the presence of highly structured arrangements at the submicron-scale level to create photonic crystals (PCs) for generating colors as in butterfly wings. Similarly, Honeycombs (HC) films can generate opalescence when sub-micrometer pores are easily organized into well-ordered two-dimensional (2D) arrays. HC films are one of the most fascinating structures which are obtained by a fast and simple solution evaporation of building blocks due to the elaboration low cost of the bottom-up process and the subsequent applications.
Conventional light trapping exploits the effect of total internal re?ection by roughening the entrance interface and randomizing the light propagation direction inside the active materials which results in a much longer interaction distance inside the material and hence a more substantial absorption enhancement. Among many attempts to bypass limitations, light trapping schemes of highly ordered and periodic structures can be conceived and designed to manipulate light ?ow to enhance light absorption. From a ray optic approach, light trapping schemes have been investigated by top-down processes, to improve light trapping efficiency. Indeed, diffracted light can result in localization and subsequent light trapping with the periodicity and the refractive index contrast. The fascinating ability of PCs to control the ?ow of light has been exploited to improve performance of OPV active layer via top-down processes.
In the present project, we want to focus on the elaboration of HC coatings deposited on a BHJ active layer in an OPV solar cell using the self-assembly of various building blocks by a Breath Figure (BF) process. This low-cost bottom-up process permits to target photonic coatings for light trapping into the sub-micrometer pores of the HC.
For achieving our ultimate goal, 5 main tasks are identified:
1) Optical modeling of the HC photonic structure inserted into the multilayer solar cell (LOMA/IMS),
2) Elaboration of well-defined nano-particles with various refractive indices (ICMCB),
3) Design of functional building blocks, i.e. core@shell colloids or block copolymers (IPREM),
3) Tailoring the 2D HC coatings on BHJ active layer by self-assembly of building blocks (IPREM),
5) Preparation/testing of the photonic coating effects on the light trapping in OPV solar cells devices (IMS/LOMA/IPREM).
It is important to mention that among the few groups actively working in the world on HC films no applications for OPV solar cells have up till now been developed.
The researchers associated in this project have published several papers justifying the present strategy. This 4 years project requires the skills of the ICMCB, IMS, LOMA and IPREM partners which will be carried out in the framework of a collaboration requiring 4 post-doc (54 months).