Spray-Coated MoO3 Hole Transport Layer for Inverted Organic Photovoltaics.

This study focuses on the hole transport layer of molybdenum trioxide (MoO3) for inverted bulk heterojunction (BHJ) organic photovoltaics (OPVs), which were fabricated using a combination of a spray coating and low-temperature annealing process as an alternative to the thermal evaporation process. To achieve a good coating quality of the sprayed film, the solvent used for solution-processed MoO3 (S-MoO3) should be well prepared. Isopropanol (IPA) is added to the as-prepared S-MoO3 solution to control its concentration. MoO3 solutions at concentrations of 5 mg/mL and 1 mg/mL were used for the spray coating process. The power conversion efficiency (PCE) depends on the concentration of the MoO3 solution and the spray coating process parameters of the MoO3 film, such as flow flux, spray cycles, and film thickness. The results of devices fabricated from solution-processed MoO3 with various spray fluxes show a lower PCE than that based on thermally evaporated MoO3 (T-MoO3) due to a limiting FF, which gradually increases with decreasing spray cycles. The highest PCE of 2.8% can be achieved with a 1 mg/mL concentration of MoO3 solution at the sprayed flux of 0.2 mL/min sprayed for one cycle. Additionally, S-MoO3 demonstrates excellent stability. Even without any encapsulation, OPVs can retain 90% of their initial PCE after 1300 h in a nitrogen-filled glove box and under ambient air conditions. The stability of OPVs without any encapsulation still has 90% of its initial PCE after 1300 h in a nitrogen-filled glove box and under air conditions. The results represent an evaluation of the feasibility of solution-processed HTL, which could be employed for a large-area mass production method.

Highly Efficient Flexible Roll-to-Roll Organic Photovoltaics Based on Non-Fullerene Acceptors.

The ability of organic photovoltaics (OPVs) to be deposited on flexible substrates by roll-to-roll (R2R) processes is highly attractive for rapid mass production. Many research teams have demonstrated the great potential of flexible OPVs. However, the fabrication of R2R-coated OPVs is quite limited. There is still a performance gap between the R2R flexible OPVs and the rigid OPVs. In this study, we demonstrate the promising photovoltaic characteristics of flexible OPVs fabricated from blends of low bandgap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T) and non-fullerene 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC). We successfully R2R slot-die coated the flexible OPVs with high power conversion efficiency (PCE) of over 8.9% under irradiation of simulated sunlight. Our results indicate that the processing parameters significantly affect the PCE of R2R flexible OPVs. By adjusting the amount of solvent additive and processing temperature, as well as optimizing thermal annealing conditions, the high PCE of R2R slot-die coated OPVs can be obtained. These results provide significant insights into the fundamentals of highly efficient OPVs for the R2R slot-die coating process.

Dramatic improvement in the stability and mechanism of high-performance inverted polymer solar cells featuring a solution-processed buffer layer.

In this study, we demonstrate inverted PTB7:PC71BM polymer solar cells (PSCs) featuring a solution-processed s-MoO3 hole transport layer (HTL) that can, after thermal aging at 85 °C, retain their initial power conversion efficiency (PCE) for at least 2200 h. The T80 lifetimes of the PSCs incorporating the novel s-MoO3 HTL were up to ten times greater than those currently reported for PTB7- or low-band-gap polymer:PCBM PSCs, the result of the inhibition of burn-in losses and long-term degradation under various heat-equivalent testing conditions. We used X-ray photoelectron spectroscopy (XPS) to study devices containing thermally deposited t-MoO3 and s-MoO3 HTLs and obtain a mechanistic understanding of how the robust HTL is formed and how it prevented the PSCs from undergoing thermal degradation. Heat tests revealed that the mechanisms of thermal inter-diffusion and interaction of various elements within active layer/HTL/Ag electrodes controlled by the s-MoO3 HTL were dramatically different from those controlled by the t-MoO3 HTL. The new prevention mechanism revealed here can provide the conceptual strategy for designing the buffer layer in the future. The PCEs of PSCs featuring s-MoO3 HTLs, measured in damp-heat (65 °C/65% RH; 85 °C per air) and light soaking tests, confirmed their excellent stability. Such solution-processed MoO3 HTLs appear to have great potential as replacements for commonly used t-MoO3 HTLs.

Performance Improvement and Characterization of Spray-Coated Organic Photodetectors.

Low dark current organic photodetectors (OPDs) with a conventional structure consisting of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as active layer have been fabricated by spray-coating. Tuning the thickness of active layer and thermal annealing process for the spray-coated OPDs results in a remarkable performance with a low dark current density ( Jd) of 2.90 × 10-8 A/cm2 at reverse bias of 1 V. The impact of thermal annealing on the performance of sprayed OPDs is also investigated by the impedance analysis for mechanistic understanding. Our results demonstrate that the optimization of PCBM cluster and interfacial contact between the active layer and the metal electrode tailored by thermal annealing, respectively, could effectively reduce the Jd and increase the sensitivity of sprayed OPDs. The control of PCBM cluster is more important than the interfacial contact between the layers for improving Jd. In addition, structural characterization of the active layer studied by synchrotron small-angle X-ray scattering technique reveals why the spray-coated process can achieve the lowest dark current due to the favorable structure.

Correlation between Hierarchical Structure and Processing Control of Large-area Spray-coated Polymer Solar Cells toward High Performance.

The formation mechanism of a spray-coated film is different from that of a spin-coated film. This study employs grazing incidence small- and wide-angle X-ray Scattering (GISAXS and GIWAXS, respectively) quantitatively and systematically to investigate the hierarchical structure and phase-separated behavior of a spray-deposited blend film. The formation of PCBM clusters involves mutual interactions with both the P3HT crystal domains and droplet boundary. The processing control and the formed hierarchical structure of the active layer in the spray-coated polymer/fullerene blend film are compared to those in the spin-coated film. How the different post-treatments, such as thermal and solvent vapor annealing, tailor the hierarchical structure of the spray-coated films is quantitatively studied. Finally, the relationship between the processing control and tailored BHJ structures and the performance of polymer solar cell devices is established here, taking into account the evolution of the device area from 1 × 0.3 and 1 × 1 cm(2). The formation and control of the special networks formed by the PCBM cluster and P3HT crystallites, respectively, are related to the droplet boundary. These structures are favorable for the transverse transport of electrons and holes.