Date Published: January 31, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Sungjun In, Namkyoo Park.
The application of nanophotonic structures for organic solar cells (OSCs) is quite popular and successful, and has led to increased optical absorption, better spectral overlap with solar irradiances, and improved charge collection. Significant improvements in the power conversion efficiency (PCE) have also been reported, exceeding 11%. Nonetheless, with the given material properties of OSCs with low optical absorption, narrow spectrum, short transport length of carriers, and nonuniform photocarrier generations resulting from the nanophotonic structure, the PCE of single‐junction OSCs has been stagnant over the past few years, at a barrier of 12%. Here, an ultrathin inverted OSC structure with the highest efficiency of ≈13.0%, while being made from widely used organic materials, is demonstrated. By introducing a smooth spatial corrugation to the vertical plasmonic cavity enclosing the active layer, in‐plane propagation modes and hybridized Fabry–Perot cavity modes inside the corrugated cavity are derived to achieve an ultralow Q, uniform coverage of optical absorption, in addition to uniform photocarrier generation and transport. As the first demonstration of ultra‐broadband absorption with the introduction of spatial corrugation to the ultrathin metal film electrode–cathode Fabry–Perot cavity, future applications of the same concept in other light‐harvesting devices utilizing different materials and structures are expected.
Significant improvements have been made in the performance of organic solar cells (OSCs) with the introduction of nanophotonic effects,1, 2, 3, 4, 5, 6, 7, 8 material‐interface synthesis,9, 10 and the development of high‐performing organic materials.11, 12 The state‐of‐the‐art single‐junction OSCs with plasmonic cavity structures,1, 2, 3 textured light trapping structures,4, 5 multiplasmonic effects,6, 7, 8 morphological implementation,9, 10 bandgap tuning,11, 12 and solution fabrication method13 now provide over 8–11% of power conversion efficiency (PCE) based on the above approaches or a combination of these approaches. However, over the past few years, a serious difficulty has also been observed in the increase of the PCE of single‐junction OSCs to over 12%. The narrow absorption band of organic materials, incomplete spectral engineering matching to the absorption band of the active layer, recombination losses from low charge carrier mobilities, and nonuniform photocarrier generation/transport caused by subwavelength nanostructures together comprise the current bottleneck and therefore an opportunity in PCE improvement.
In conclusion, the highest performance of an organic solar cell of ≈13.0% PCE was numerically demonstrated by identifying the limitation of conventional UTMF OSC through optical–electrical multiphysics analysis and then introducing a corrugated cavity structure which perfects the spectral engineering over the optical absorption of the ultrathin PBDTT‐F‐TT:PC71BM active layer. Ultralow Q (330–775 nm), highly uniform (>85%) coverage of optical absorption is derived from the hybridized Fabry–Perot cavity mode and multipeak in‐plane propagation modes, without any penalty in the electrical carrier transport dynamics. Compared to the flat UTMF OSC, large improvements of ≈21.5% in optical absorption, ≈21.0% in the short‐circuit current, and ≈19.7% in the PCE enhancements are confirmed from optical–electrical multidisciplinary numerical analysis, thereby realizing 13% of PCE for the first time for the OSC, notably with widely used organic materials. Clearly revealing the significance of spectral engineering, driven by the corrugated cavity, we anticipate the application of the same concept to ultrathin OSCs based on other materials, such as PBDB‐T:IT‐M for the realization of even higher PCE. Moreover, it is emphasized that the use of optical–electrical multiphysics analysis should be considered in the proper identification and overcoming strategy of the performance limiting factor (e.g., electrical transport vs optical engineering), for any type of solar cells.
Structure of the Reference UTMF OSC and Corrugated Cavity UTMF OSCs: The reference OSC and suggested CC‐UTMF OSC assumed the same material compositions, except for the corrugation. The reference flat UTMF OSC was constructed with a structure of TeO2/UTMF‐Ag/FPI‐PEIE/PBDTT‐F‐TT:PC71BM/MoO3/Ag (Figure 1a). For the CC‐UTMF OSC, a smooth sine‐shape spatial modulation was introduced to derive a corrugated cavity structure (Figure 2c), where P is the major axis period (in‐plane) and A is the amplitude of the minor axis (vertical). For the efficient electrical charge transport and optical transparency, TeO2 was selected as a capping layer.1, 6 The thicknesses of FPI‐PEIE, MoO3, UTMF‐Ag, and TeO2 were set as 10, 6, 12, and 50 nm, respectively.
The authors declare no conflict of interest.