Optoelectronic Properties of a Series of P3HT/Dye Pairs

A series of polymer/dye composite pairs, where the same polymer Poly (3-hexylthiophene2,5-diyl) or P3HT paired with a series of three molecular dyes [Meso-Tetra(4-carboxyphenyl) porphine (TCPP), Ferriprotoporphyrin IX chloride (Hemin), and Protoporphyrin IX (Proto)] of different frontier orbitals and absorption coefficients are systematically investigated for potential optoelectronic conversion applications. A facile method to distinguish the charge and energy transfer contributions between emissive P3HT and dyes under certain conditions via concentration dependent photoluminescence (PL) is described. The experimental results from this study also revealed that the optoelectronic energy conversion efficiencies of the P3HT/dye based OE devices appear to be affected more critically by the P3HT/dye pair PL quenching than the optical absorption coefficients of the dyes, and that there appears to exist an optimal LUMO offset of P3HT/dye pair for most severe PL quenching or the best optoelectronic conversion efficiency. These study could be very useful for the design and development of next generation high efficiency soft materials based optoelectronic devices such as solar cells and photodetectors.


Introduction
Photo induced charge transfer (CT) is a process in which an electron is transferred from an electron donating species (D) to an electron accepting species (A) upon photo excitation. On the contrary, photo induced energy transfer (ET) usually takes place through the non-radiative multiple interactions or couplings between an exciton (a tightly bond electron-hole pair) donating (D) and accepting (A) species (also referred to as the exciton transfer). Both transfers are distinct from each other but sometimes occur simultaneously, and this may lead to certain misinterpretation or confusion of physical processes. Basic difference between the two transfers is that the CT process involves an electron (or charge) transfer, whereas the ET process involves an exciton transfer [1][2][3][4]. The main problem or challenge is to distinctly separate the two processes or contributions so better solar photoelectric energy conversion materials and devices can be developed or optimized.
Abramavicius and Mukamel [5] proposed a tight-binding two-band model and used it to simulate the energy-transfer and charge-separation dynamics without separating the two processes. Sontakke et al.
[4] used a dynamic rate equation approach to distinguish the electron and energy transfer processes between lanthanide ions in dielectric hosts. Chen et al. [6] studied charge and energy transfer using Time-Resolved pulse Laser spectroscopic experiments at nanosecond/picosecond levels without separating the two processes. Kandada et al. [7] analyzed charge and energy transfer between P3HT and PCBM in different timescale using the multi-pass transient absorption technique (TrAMP) again without separating the two processes.
In this study, a series of polymer/dye composite pairs, where the same polymer Poly(3-hexylthiophene-2,5-diyl) or P3HT paired with a series of molecular dyes (Figure 1) of different frontier orbitals are being investigated for potential optoelectronic conversion applications. A facile or straight forward method to analyze and separate the charge and energy transfer processes between P3HT and dyes via concentration dependent PL spectroscopy under certain conditions is described. To our best knowledge this is the first of such report. Solar cell devices fabrication and testing: 10.00 mg P3HT, 10.00 mg PCBM and dye (1.15 mg TCPP, 0.95 mg Hemin and 0.82 mg Protoporphyrin, respectively) were dissolved in 1 mL mixture solvents of 1,2-Dichlorobenzene (DCB) and Dimethylformamide (DMF) (3:1, v/v). The mixture solution was stirred for 24 h at room temperature. P3HT (5.80 × 10 -2 M), PCBM (1.10 × 10 -2 M) and dye (1.45 × 10 -3 M). The ITO glass (Rs=8-12 Ω) was cleaned by sequential 5 minus sonication in solutions of detergent, acetone, isopropyl alcohol. Then ITO glass was rinsed with deionized water and dried with N 2 . PEDOT:PSS aqueous solution (Clevious PVPAI 4083, from H.C. Starck) was spun onto clean ITO glasses in 5000 rpm for 30 s forming 30 nm thin film, followed by drying in vacuum oven at 90°C for 1 hour. After cooling down to room temperature, the mixture solution was spun on the top of "ITO/ PEDOT:PSS" film in 1000 rpm for 60 s. Samples were kept in a high vacuum oven at 80°C for 30 mins. All the sample solutions were filtered by 0.2 μm filter. After cooling down to room temperature, evaporation of metal Al was performed in a glove box with the pressure of 1.0 × 10 -7 mbar. The film thickness was 100 nm. I-V measurement was performed by a solar simulator (1.5 AM simulated sunlight radiation), a current-voltage source measure unit (Keithley SMU-237).

Effect of the acceptor or PL quencher concentration
The charge transport properties in conjugated materials critically depend on the packing order of the chains in the solid state as well as on the density of impurities and structural defects [10]. In this study, both P3HT (donor) and dye (acceptor) in THF solution have a strong tendency of aggregation with increasing concentration, and this would affect significantly the PL quenching as a result of charge or energy transfers. In order to eliminate the potential PL quenching effect due to P3HT aggregation, pristine P3HT emission measurement versus concentration was carried out first to determine an optimal concentration range where PL quenching due to aggregates are minimum.

Charge and energy transfer analysis
The UV-Vis absorption peak of P3HT in THF is at 450 nm, so 440 nm is used as excitation wavelength to excite P3HT. Figures 5 and 6 exhibited PL emission spectra (PL intensity as arbitrary units or au) of P3HT quenched by different concentrations of Proto and TCPP, respectively. The emission peaks of Proto at 620 nm ( Figure 5, where P/P-8 means P3HT at 1.04 x 10 -4 M/Proto at 8 th concentration) and TCPP at 720 nm ( Figure 6, where P/T-8 means P3HT at 1.04 x 10 -4 M//TCPP at 8 th concentration) were also observed, and this can be due to 440 nm excitation also exciting the acceptor dyes, and it can also be due to possible energy transfer from P3HT to the emissive dyes. In Figure 7, no emission peak of Hemin was observed possibly due to Hemin is not emissive and/or no energy transfer occurs, only charge transfer occurs as PL quenching was observed. The contributions of accepter PL highest at concentration of 1.00 × 10 -4 M, then decreases toward a concentration of 1.00 × 10 -3 M. The decrease could be attributed mainly to P3HT aggregations [11]. P3HT PL peak exhibited a steady increase in the concentrations below 1.00 × 10 -4 M, where it is believed that P3HT chains were without aggregation. As a result, P3HT/THF (Tetrahydrofuran) in a concentration range of 5.00 × 10 -6 M-4.00 × 10 -5 M was selected in this study.  Stern-Volmer plot of P3HT quenched by TCPP in broad concentration range where aggregations at higher concentrations could be severe.    Finally, solar cells using the three P3HT/dye pairs combined with a common acceptor, phenyl-C 61 -butyric acid methyl ester (PCBM) were also fabricated and tested as shown in Figures 12 and 13, where Figure 12 exhibits solar cell device fabrication scheme, and Figure 13 exhibits the photo JV curves of the solar cells, and solar cell device data are also tabulated in Table 3. The frontier orbital (HOMO/LUMO) levels of P3HT and the three dyes were measured using Cyclic Voltammetry (CV) method combined with UV-visible spectroscopy and the data are shown in Figure 12b and Table 4. The molar absorption coefficients (Abs-Coeff, or ɛ) of the three dyes were determined via UV-Vis. The molar absorption coefficients (Abs-Coeff), photoluminescence quenching constants (K -sv ), lowest unoccupied molecular orbital offsets (d-LUMO), and fabricated solar cell power conversion efficiencies (η, or PCE) of the three P3HT/dyes (Proto, Hemin, and TCPP) are listed in Table  4, and normalized values are compared in Figure 14.
As shown in Figure 14    dissociation efficiency versus ΔLUMO [13][14][15][16][17]. As a matter of fact, a time resolved microwave conductivity (TRMC) studies on a series of polymer/fullerene thin film OE devices also observed an optimum driving force corresponding to highest photo induced microwave conductivity [18].

Conclusion
Photo induced charge transfer (CT) and energy transfer (ET) could occur simultaneously in a molecular donor/acceptor weakly electronically coupled pair. If both donor and acceptor are PL emissive, under certain conditions and approximations, CT and ET contributions may be separated and quantified via concentration dependent PL spectra. The experimental results from this study also revealed that the optoelectronic conversion