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In part I, we explored the charge transfer properties and energy loss mechanisms vs. the morphological and molecular structures of organic donor-acceptor HJs. The studies extended from fullerene to NFA-based HJs to assist in our understanding of the photophysical origins of the recent increases of OPV efficiency. Indeed, the state- of-the-art power conversion efficiency exceeding 15% presents a very bright future for OPVs. However, to also realize long term stability and scalable low-cost fabrication of the devices, many compromises in the choice of materials, HJ morphologies, pro- cessing, and encapsulation techniques have to be made. To date, no OPV has reached a market-viable balance in every aspect of device performance. Furthermore, even after OPVs reach market-entry-level properties, challenges remain in gaining enough market share to compete with mature silicon and other thin film PV techniques.

One of the lessons we have learned from OLED commercialization is that the ini- tial curiosities of customers may come from a “premium”-like product feature that creates new customer expectations such as a decorative appearance, and new func- tionality, even though such a product may have a higher price and lower reliability than conventional products. Future development of OPVs should therefore focus on the unique characteristics of organic materials, such as the intense and narrow-band

Figure 9.1: The narrow absorption bands of a-d-a-type NFAs at different wavelength. Figure courtesy of Yongxi Li

absorption, lightweight form and flexibility, and improve the performance of semi- transparent OPVs for building-integrated, car-integrated, and clothing-integrated power generation applications. Current acceptor-donor-acceptor(a-d-a)-type NFAs, as shown in Fig. 9.1, are candidates for these purposes. The big energy gap between the first (S1) and second (S2) excited state manifolds gives rise to transparency in

the visible. The energy of narrow absorption bands can be varied from visible to near infrared by either changing the conjugation length or the electron donating and withdrawing groups of the molecules.

Organic HJs employing NFAs, therefore, exhibit potential for future OPV appli- cations. To further understand this new class of organic HJs from a photophysical perspective, we highlight a few open questions worthy of further study:

Open-circuit voltage of ternary bulk HJs vs. blend ratio

Due to the tunability of the NFA absorption wavelength, ternary bulk HJ com- prised of either one donor and two acceptors, or two donors and one acceptor has been applied in OPVs. The extra donor or acceptor component increases the solar

spectral covergae compared to bianry mixtures. As a result, power conversion effi- ciencies of ∼13% from ternary OPVs have been reported.[82,280] _{Ternary OPVs have}

been shown to have a nonlinear change of open-circuit voltage (VOC) as a function

of the blend ratio between three constituents.[281–284] _{That is, the short-circuit cur-}

rent (JSC) increases with the ratio of the third component while the VOC remains

relatively unchanged. The mechanisms behind has not been conclusively established. One intriguing model claims that the formation of a “molecular alloy” by mixing the two acceptors in the active region gives rise to a nonlinear shift of the CT state energy.[285] _{In contrast to alloys comprising atomic constituents in inorganic semi-}

condcutors, the chemical bonds between neighbors in organic materials are replaced by far weaker van der Waals bonds. Indeed, the formation of a molecular alloy has been proposed in several instances, e.g. charge transfer complexes TTF-7,7,8,8- tetracyanoquinodimethane (TCNQ).[286–288] Solid evidence has been shown that the electronic structure of the charge transfer complex is distinguished from that of a sim- ple blend. However, no such results have been seen in ternary HJs. The mechanisms to explain the change of VOC in ternary OPVs have to be more fully studied.

The role of singlet and triplet energy splitting in NFAs

The small electron and hole wavefunction overlap of CT excitons at the HJ signif- icantly reduces the exchange energy splitting 2J (c.f. Eq. 1.12) between the singlet and triplet CT states, making them nearly degenerate. Fast intersystem crossing between them facilitates a spin-conservative back transfer from the CT state to T1

of acceptor or donor molecules, where non-radiative recombination loss from T1 fol-

lows, as shown in Fig. 9.2(a). This transfer is more significant in NFA-based HJs, where ∆ECT = ES1 − ECT is small, and hence the energy difference between the CT

and T1 states is potentially large. Therefore, design of a thermally activated delayed

Figure 9.2: (a) Schematic of the transfer process from the singlet (S1) state of accep-

tor molecules to the charge separated state (P) via the charge transfer state (CT). The energy level of the triplet state (T1) determines the back

transfer rate from CT to T1, i.e. one of the non-radiative recombination

loss pathways at the HJ (b) Low temperature (17K) PL of the singlet and triplet states of IT-IC diluted in PMMA matrix at a concentration of 0.07%. The triplet spectrum is fit by a Gaussian function to extract the peak position

is necessary for decreasing the transfer probability from CT to T1.

A PL measurement of singlet and triplet excitons of an a-d-a-type NFA, IT-IC, is shown in Fig. 9.2(b). The spectra of the sample were taken at 17 K with an optical chopper coupled to the spectrometer. The timing of the chopper opening was coupled to a the pulsed diode laser at λ = 405nm using a delay generator. The singlet emission was measured at zero delay, while the triplet emission was measured at 20 ± 10 ms after the excitation. The singlet and triplet energy difference of IT-IC is only 0.20 ± 0.02 eV, which is much smaller than for common organic molecules. Further studies of the transfer rate between the singlet and triplet states, and the value of 2J as a function of molecular structure are yet to be done.

Stability of HJs using NFAs

While HJs employing NFAs have presented high power conversion efficiencies, their stability are as yet not thoroughly investigated. Since a high operational lifetime has been reported in all-carbon-based DBP/C70 HJs,[289] questions remain as to whether

the photochemical stability of NFAs, comprising relatively weak bonded heteroatoms and highly electron deficient moieties, is acceptable for long-term device stability. Ad- ditionally, the thermal stability of solution-processed blended HJs comprising NFAs has to be studied as another important figure of merit. Morphological changes as a function of temperature and aging time should be systematically measured using TEM and X-ray diffraction techniques, and interface sensitive spectroscopic tools.

9.2

Future Work on 2D Organic/Transition Metal Dichalco-