Approaches for enhancing the efficiency of organic solar cells

Abstract

Solution-processed organic solar cells (OSCs) have attracted much attention in recent two decades in virtue of their advantages, including light weight, low cost, high mechanical flexibility and easy fabrication, over conventional solar cells. However, the power conversion efficiencies (PCEs) of OSCs are still lower than those of inorganic counterparts due to the insufficient light absorption, low carrier mobilities and short exciton diffusion lengths in organic semiconductors, etc. Nowadays, lots of techniques have been developed to enhance the PCEs of OSCs based on different principles, such as the introduction of buffer layers, additives, nanoparticles with a surface plasmon resonance effect, ternary device structure, and tandem architecture, etc. with a view to enhancing the light absorption, facilitating carrier transport, and tuning the energy band structure of the devices, etc. Since the introduction of additives into not only the buffer layers but also the active layers of OSCs is a promising strategy to improve device performance, we have tried to incorporate different types of functional materials, including Au@Ag core-shell nanocuboids (NCs), ultrathin black phosphorus (BP), BP quantum dots (BPQDs), and high-mobility conjugated polymers, into OSCs to enhance the device efficiencies. Polythieno-thiophene/benzodithiophene (PTB7) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b0]dithiophene-2, 6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PBDTTT-EFT) were selected as the representative donor materials and [6,6]-phenylC71-butyric acid methylester (PC₇₁BM) was chosen as the acceptor in our OSCs. The detailed research works include the following four parts. Part 1 is for the study of Au@Ag NCs incorporated OSCs. Au@Ag NCs exhibit broadband and tunable plasmonic enhancement due to the coexistence of longitudinal and transverse dipole modes and octupolar modes. We designed and tuned the thickness of the Ag shells of Au@Ag NCs to match the light absorption region of the solar cells. It is notable that plasmonic effect including both light scattering and near field enhancement induced by the NCs can substantially improve the device performance when the NCs are incorporated in the active layers. Under optimum conditions, the PCEs of the OSCs can be relatively improved by up to 22.8% by the NCs. The maximum average PCE of the OPVs we obtained is 10.42%, which is much higher than those of the previously reported plasmonic OSCs. In Part 2, we focused on high-efficient OSCs modified with BP flakes. BP is attractive for the applications in optoelectronics devices due to its high carrier mobilities. We demonstrated for the first time that solution exfoliated BP flakes can be served as an effective electron transport layer in OSCs and lead to pronounced enhancement of the device performance. The PCEs of the BP-incorporated devices with inverted structure can be improved from 7.37% to 8.18% in average. The incorporation of BP flakes with the optimum thickness of ~10 nm can form cascaded band structure in OSCs, which is favorable for electron transport and can prohibit carrier recombination near the cathodes. Moreover, BP incorporated in OSCs shows good stability in air due to the encapsulation of the devices. In Part 3, we synthesized 0-dimensional (0D) BPQDs using a convenient probe sonication method and incorporated them into the active layers of OSCs. BPQDs show a tunable band structure and broadband light absorption, and can be easily introduced in the active layers by a solution process. It is interesting to find that the addition of BPQDs can increase the electron mobility and facilitate charge transport in the devices, giving rise to the increases in Jsc and FF of the OSCs and thus the enhancement of the device efficiency. In part 4, we introduced a high-mobility conjugated polymer in the active layer as an additive. Under optimum conditions, the efficiencies of one type of OSCs can be improved from 8.75% to 10.08%, which is mainly attributed to the improved hole mobilities and carrier lifetimes in the devices. We also find that, besides the high hole mobility, the band structure of the conjugated polymer is critical to the efficiency enhancement, which should form a cascade band structure in OSCs and have a similar HOMO level to that of the donor material. Because the energy levels of the molecular orbitals can influence carrier recombination and lifetimes in the devices and further determine the device performance. In summary, we demonstrated that various functional materials, including novel plasmonic NCs with broadband absorption, BP flakes, BPQDs with tunable size and high-mobility polymers with matchable energy levels, can be introduced into OSCs to improve the efficiencies of devices. These techniques are convenient and cost effective, which pave the ways to realizing high-efficiency solar cells including not only OSCs but also many other new generation thin film solar cells.