On the packing and the orientation of P(NDI2OD-T2) at low molecular weight

Impact of the Electron Acceptor Nature on the Durability and Nanomorphological Stability of Bulk Heterojunction Active Layers for Organic Solar Cells

A systematic study is conducted to compare the performances and stability of active layers employing a high performance electron donor (PBDB-T) combined with state-of-the-art fullerene (PC₇₁ BM), nonfullerene (ITIC), and polymer (N2200) electron acceptors. The impact of the chemical nature of the acceptor on the durability of organic solar cells (OSCs) is elucidated by monitoring their photovoltaic performances under light exposure or dark conditions in the presence of oxygen. PC₇₁ BM molecules exhibit a higher resistance toward oxidation compared to nonfullerene acceptors. Unencapsulated PBDB-T:PC₇₁ BM OSCs display relatively stable performances at room temperature when stored in air for 3 months. However, when exposed to temperatures above 80 °C, their active materials demix causing notable reductions in the short-circuit densities. Such detrimental demixing can also be seen for PBDB-T:ITIC active layers above 120 °C. Although N2200 chains irreversibly degrade when exposed to air, thermally induced demixing does not occur in PBDB-T:N2200 active layers annealed up to 200 °C. In summary, fullerene OSCs may be the best currently available choice for unencapsulated room temperature applications but if oxidation of the polymer acceptors can be avoided, all polymer active layers should enable the fabrication of highly durable OSCs with lifetimes matching the requirements for OSC commercialization.

Decreased domain size of p-DTS(FBTTh2)2/P(NDI2OD-T2) blend films due to their different solution aggregation behavior at different temperatures

Nanoscale interpenetrating networks play a key role in determining the optoelectrical properties of functional blends. However, phase separated large domain sizes could probably be observed in pristine films composed of two crystalline components. For example, p-DTS(FBTTh₂)₂/P(NDI2OD-T2) 3/2 blend films with interpenetrating networks are obtained, however, large domain sizes are found when they are prepared from a 20 °C solution due to the simultaneous process of crystallization and phase separation during solvent evaporation. In this paper, we proposed to reduce the domain size of p-DTS(FBTTh₂)₂/P(NDI2OD-T2) blend films using their different solution aggregation behaviors at different temperatures. The aggregation of p-DTS(FBTTh₂)₂ molecules in chlorobenzene (CB) was insensitive to the solution temperature. However, the in situ absorption spectra of the neat P(NDI2OD-T2) solution from 80 °C to room temperature indicated that P(NDI2OD-T2) aggregation increased with decreasing temperature due to intrachain interactions. Therefore, in order to reduce the domain size, we employed a hot solution to prepare the blend films. During the solidification process, the majority of p-DTS(FBTTh₂)₂ molecules were confined in the P(NDI2OD-T2) networks prior to occurrence of severe p-DTS(FBTTh₂)₂ aggregation. Thus, the domain size of the p-DTS(FBTTh₂)₂ phase became smaller than that of the pristine films, leading to a decrease in the corresponding photoluminescence intensity of the blend films. In addition, the crystallinity of the blend films improved after thermal annealing, which resulted from the ordered alignment of p-DTS(FBTTh₂)₂ molecules facilitated by their enhanced diffusion ability. Based on the various morphologies, a possible phase diagram of the p-DTS(FBTTh₂)₂/P(NDI2OD-T2) blend system was depicted, which could be a guide to directly control the morphology of blend films.

Transfer-printing of active layers to achieve high quality interfaces in sequentially deposited multilayer inverted polymer solar cells fabricated in air

Polymer solar cells (PSCs) are greatly influenced by both the vertical concentration gradient in the active layer and the quality of the various interfaces. To achieve vertical concentration gradients in inverted PSCs, a sequential deposition approach is necessary. However, a direct approach to sequential deposition by spin-coating results in partial dissolution of the underlying layers which decreases the control over the process and results in not well-defined interfaces. Here, we demonstrate that by using a transfer-printing process based on polydimethylsiloxane (PDMS) stamps we can obtain increased control over the thickness of the various layers while at the same time increasing the quality of the interfaces and the overall concentration gradient within the active layer of PSCs prepared in air. To optimize the process and understand the influence of various interlayers, our approach is based on surface free energy, spreading parameters and work of adhesion calculations. The key parameter presented here is the insertion of high quality hole transporting and electron transporting layers, respectively above and underneath the active layer of the inverted structure PSC which not only facilitates the transfer process but also induces the adequate vertical concentration gradient in the device to facilitate charge extraction. The resulting non-encapsulated devices (active layer prepared in air) demonstrate over 40% increase in power conversion efficiency with respect to the reference spin-coated inverted PSCs.