Air-stable Organic Solar Cells Research Articles

Unfused-ring small molecule acceptors based on A1-D-A2-D-A1 architecture with low non-radiative energy loss and excellent air stability

Organic solar cells (OSCs) have made enormous progress in recent years. However, OSCs still suffer from the low price–quality ratio since the complex synthesis of photovoltaic materials and the poor stability of the devices. Comparing with traditional large conjugated ladder-type non-fullerene acceptors, the unfused-ring acceptors with non-covalent intramolecular interactions could notably simplify the synthetic routes while maintaining the device performance. Here, we designed and synthesized two A1-D-A2-D-A1 type non-fullerene acceptors named BDTC-F and BDTC-Cl based on 1,3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5c']dithiophene-4,8-dione (BDD) as A2 unit. With cyclopenta[2,1-b:3,4-b']dithiophene functions as the D unit, the weak O⋯S interaction between A2 and D units is formed, which reduces the torsion angle between them and increases the planarity of the molecular backbone. Furthermore, the participation of BDD could deepen the energy levels of these two acceptors, makes it possible to decrease the energy loss of the corresponding devices. When blending with polymer donor PM6, devices based on both acceptors exhibit PCEs over 10% with low energy losses of 0.59 and 0.57 eV for BDTC-F and BDTC-Cl, respectively. Moreover, the devices based on PM6:BDTC-F and PM6:BDTC-Cl demonstrate excellent air stability. After stored in the ambient atmosphere without encapsulation for 1,200 h, devices based on these two acceptors retained over 96% of their initial PCEs. These results demonstrate that designing the A1-D-A2-D-A1 type acceptor with non-covalent intramolecular interactions is an effective strategy to construct low-cost and air-stable OSCs.

Non-fullerene acceptors based on multiple non-covalent interactions for low cost and air stable organic solar cells

Significant progresses have been made on organic solar cells (OSCs) in last few years, but the industrialization of OSCs is still hindered by high cost and poor device stability. Recently, the most successful OSCs are usually based on fused-ring small molecule non-fullerene acceptors (SM-NFAs) which often involve multiple complex synthetic steps and result in low overall yields and high cost. Herein, two easily accessible non-fused NFAs based on BT , CPDT and IC with different halogen atoms named BT-F and BT-Cl were designed and synthesized. Non-covalent interactions or conformation locks between BT and adjacent CPDT units provided planar conformations, which is beneficial for molecular packing and charge transport. When blended with low cost polymer donor PTQ-10 , optimal devices based on BT-F provided a PCE of 8.26%. A higher PCE of 8.65% was obtained from BT-Cl -based devices. Low voltage loss of 0.57 and 0.59 V were observed for BT-F - and BT-Cl -based devices. It is noteworthy that devices based on PTQ-10 : BT-F and PTQ-10 : BT-Cl exhibited excellent stabilities when stored in ambient atmosphere for 10 days without encapsulation. This work demonstrates an example of pursuing low cost OSCs with excellent air stability. • Two non-fused non-fullerene acceptors named BT-F and BT-Cl were reported. • When blend with PTQ-10, BT-F and BT-Cl show PCEs of 8.26% and 8.65%, respectively. • The devices exhibit excellent air stabilities without encapsulation.

Highly efficient organic solar cells enabled by a porous ZnO/PEIE electron transport layer with enhanced light trapping

In this study, a porous inorganic/organic (ZnO/PEIE, where PEIE is polyethylenimine ethoxylated) (P-ZnO) hybrid material has been developed and adopted in the inverted organic solar cells (OSCs). The P-ZnO serving as the electron transport layer (ETL) not only presents an ameliorative work function, but also forms the cratered surface with increased ohmic contact area, revealing suppressed charge recombination and enhanced charge extraction in devices. Particularly, P-ZnO-based OSCs show improved light trapping in the active layer compared with ZnO-based ones. The universality of P-ZnO serving as ETL for efficient OSCs is verified on three photovoltaic systems of PBDB-T/DTPPSe-2F, PM6/Y6, and PTB7-Th/PC71BM. The enhancements of 8% in power conversion efficiency (PCE) can be achieved in the state-of-the-art OSCs based on PBDB-T/DTPPSe-2F, PM6/Y6, and PTB7-Th/PC71BM, delivering PCEs of 14.78%, 16.57%, and 9.85%, respectively. Furthermore, a promising PCE of 14.13% under air-processed condition can be achieved for PZnO/PBDB-T/DTPPSe-2F-based OSC, which is among the highest efficiencies reported for air-processed OSCs in the literature. And the P-ZnO/PBDB-T/DTPPSe-2F-based device also presents superior long-term storage stability whether in nitrogen or ambient air-condition without encapsulation, which can maintain over 85% of its initial efficiency. Our results demonstrate the great potential of the porous hybrid PZnO as ETL for constructing high-performance and air-stable OSCs.

Fabrication and Characterization of Hybrid Organic-Inorganic Electron Extraction Layers for Polymer Solar Cells toward Improved Processing Robustness and Air Stability.

Organic-inorganic hybrid materials composed of bismuth and diaminopyridine are studied as novel materials for electron extraction layers in polymer solar cells using regular device structures. The hybrid materials are solution processed on top of two different low band gap polymers (PTB7 or PTB7-Th) as donor materials mixed with fullerene PC70BM as the acceptor. The intercalation of the hybrid layer between the photoactive layer and the aluminum cathode leads to solar cells with a power conversion efficiency of 7.8% because of significant improvements in all photovoltaic parameters, that is, short-circuit current density, fill factor, and open-circuit voltage, similar to the reference devices using ZnO as the interfacial layer. However when using thick layers of such hybrid materials for electron extraction, only small losses in photocurrent density are observed in contrast to the reference material ZnO of pronounced losses because of optical spacer effects. Importantly, these hybrid electron extraction layers also strongly improve the device stability in air compared with solar cells processed with ZnO interlayers. Both results underline the high potential of this new class of hybrid materials as electron extraction materials toward robust processing of air stable organic solar cells.

High-efficiency and air stable fullerene-free ternary organic solar cells

Fullerene-free acceptors and ternary strategy have aroused extensive attention due to their great potential for improving efficiency and stability of organic solar cells (OSCs). Here, we demonstrate high-efficiency and air stable OSCs by combining the benefits of fullerene-free acceptors and ternary strategy. A polymer acceptor N2200 as the third component is incorporated with PBDB-T:ITIC to fabricate fullerene-free ternary OSCs, which achieve an outstanding power conversion efficiency (PCE) of 11.41% and excellent stability in air conditions. Compared with the binary cells, the performance improvement of ternary OSCs is mainly attributed to the enhanced photon harvesting and optimized morphology of active layers. Replacing ITIC by a similar acceptor ITIC-Th or IT-M in the ternary OSCs, the optimized PCE of 11.40% or 12.10% can be achieved for PBDB-T:ITIC-Th:N2200-based or PBDB-T:IT-M:N2200-based cells, respectively.

Size‐Controlled Soft‐Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials

Size-controlled soft-template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue-shifted photoluminescence (PL) or so-called an inverse PL shift. Using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND-based optoelectronic devices, including multicolored light-emitting diodes and air-stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.

MoO3–Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) blend active layer have been fabricated with a MoO3–Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO3–Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO3 and PEDOT:PSS as AIL. The surface morphology of the MoO3–Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08nm to 1.81nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO3–Au composite, MoO3 and PEDOT:PSS) were studied and it was found that the MoO3–Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO3 layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO3 component.

Modified electrode architecture for efficient and air-stable polymer solar cells based on P3HT:PCBM

Data are presented on high performance and air-stable organic solar cells with modified electrode architecture (MAOSCs), which have a patterned transparent anode determining the real active area of devices and a large area of reflective cathode covering the whole area of an active layer based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Based on the finite-difference time-domain (FDTD) scheme as a numerical method, improved light trapping within the photoactive layer, resulting from the efficient reflection of incident light at the large area of cathode, can be attained by modifying the conventional organic solar cell structures. Here, an improved power conversion efficiency of 4.3% was obtained in the case of MAOSCs under 1 Sun with air mass (AM) 1.5 global (G) condition. In addition, the stability of MAOSCs in air was remarkably improved due to the limited exposure of their active layers to air.