Correlating the Phase Behavior with the Device Performance in Binary P3HT: NFA Blend Using Optical Probes of Microstructure
ELHAM REZASOLTANI a, Anne Guilbert a, Jun Yan a, xabier Rodriguz b, Mohammed Azzuzi a, Sachetan Tuladhar a, Andrew Wadsworth c, Iain Mcculloch c d, Mariano Campoy b, Jenny Nelson a
a Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK.
b Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain
c Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, United Kingdom
d King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
Oral, ELHAM REZASOLTANI, presentation 038
DOI: https://doi.org/10.29363/nanoge.iperop.2020.038
Publication date: 14th October 2019

The performance of polymer: acceptor blends for use as a light-harvesting layer in organic photovoltaic (OPV) cells depends strongly on the structural features of the active layer, including the extent of intermixing, vertical phase segregation, and generally, its phase morphology. Recent studies show that in most popular bulk heterojunction (BHJ) systems the phases are not pure and a significant volume is occupied by mixed domains that may be rich in either donor or acceptor. [1] Therefore, one crucial aspect of understanding optimized OPV performance is to find characterization techniques to gain a detailed picture of the mixed phases in BHJ systems. This is especially important with the introduction of multicomponent systems such as ternaries where multiple acceptors (A), or donors (D), with different mixing behavior with the other (donor or acceptor) component can be used. [2]

In this study, we present the observations of the phase behavior of non-fullerene acceptors (NFAs), namely non-planar O-IDFBR, planar O-IDTBR, and its branched side-chain analog EH-IDTBR, in semi-crystalline P3HT using thermal and optical characterization techniques and evaluate the optical methods we have used as probes of microstructure. We use a combination of non-destructive optical probes to analyze the microstructure of blend films of P3HT: NFAs as a function of composition. Spectroscopic ellipsometry (SE) helps analyze the optical properties of multicomponent systems in terms of the composition, and Raman spectroscopy allows us to understand the state of order of the conjugated backbone, and chemical structure.  Moreover, e demonstrate how UV-vis and PL can be used to capture the degree of mixing of the thin films. We interpret the optical properties of binary blends of different composition in terms of the phase behavior of the blends and compare our findings with a picture of the phase behavior obtained using differential scanning calorimetry (DSC). The detailed picture of the microstructure allows us to correlate the impact of NFA composition and crystallinity on the microstructure with photocurrent generation by photovoltaic devices.

These optical studies demonstrate that the less planar NFA (O-IDFBR) mixes finely into semicrystalline P3HT at weight contents up to 40 wt%, beyond which it disrupts the order of the P3HT. However, the more planar NFAs (O-IDTBR and EH-IDTBR), could be accommodated up to 70 wt% in the blend without disrupting the polymer. We observe the maximum of the power conversion efficiency (PCE) of P3HT: O-IDTBR peaks at the eutectic composition where crystals of both D/A form and based on the optical probes the binary is phase-separated. Surprisingly, we observe the maximum of the PCE of P3HT: O-IDFBR to lie around 30-50 wt% O-IDFBR, which is far below the apparent eutectic composition, as shown in the figure attached. This is assigned to the loss of P3HT transport before reaching the eutectic, based on Raman analysis.

The results show that device performance is dictated by short circuit current density (Jsc). In order to focus on understanding the interplay between the blend film morphology and the Jsc of the corresponding devices we estimate composition-dependent internal quantum efficiency (IQE) and compare with our results for phase behavior. We find that the O-IDTBR contribution to IQE is notably higher than that of O-IDFBR, which can be assigned to the higher crystallinity of O-IDTBR compared to O-IDFBR as well as due to P3HT crystallinity is rather disrupted when mixed with O-IDFBR than O-IDTBR. It appears that the optimized performance is strongly dependent on the degree of polymer and NFA crystallization.

 

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