Spectroscopic Characterization on Excitonic Properties in Organic Photovoltaics


Student thesis: Doctoral Thesis

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Award date6 Sep 2018


Since 1980s, organic semiconductors and devices have been widely investigated due to its potential application on high-efficiency and low-cost device. Particularly, organic photovoltaics (OPVs) have achieved significant advancement in both science and technology. It has great potential for the next generation photovoltaic applications due to their potentially low-cost, solution processable, tunable energetic properties, possible mechanical flexibility and comparably easy in fabrication. With tremendous efforts and work devoted, OPVs have already developed progressively in power conversion efficiency over the last two decades. Recently, some efficient non-fullerene acceptors further boost the efficiency of OPVs over 13%. However, wide application based on this efficiency is not enough for the commercial market. Further breakthrough in efficiency is still needed and that relies on the understanding on this excitonic type solar cell.

Owing to low dielectric constant (ε ~ 2 – 4) of organic materials, OPV is regarded as an excitonic solar cell that excitons are generated upon photo-excitation. Such intrinsic small dielectric constant in organic materials results in large exciton binding energy (Eb). Overcoming the strong excitonic effect in organic photovoltaics (OPVs) is one of the key strategies for the designs of materials and device structures in OPVs. Despite the matter of importance, there are limited reports on measuring the Eb and ε in organic photovoltaic materials and the explanation of how the dielectric related to the exciton binding energy is still insufficient. Herein, we demonstrate a facile access to the transport gap and exciton binding energies by studying electrical and optical quantum efficiencies in a series of conjugated photovoltaic polymer, fullerene and non-fullerene materials. As a result, Eb varies from 0.3 eV to 1.2 eV in those prototypical materials and it apparently follows a second power law with the inverse of the dielectric constant of the materials, i.e. Eb ∝ 1 / ε2. Instead of the widely assumed first-order dependence, this second order dependent relationship is firstly reported. In short, our result can clarify this relationship on how the exciton binding energy in organics depends on the material dielectric constant. It further provides the guidance in the future development of reducing the excitonic effect, including integration of high-dielectric organic semiconductor in OPVs for high efficiency organic photovoltaic cells.

Apart from the intrinsic low dielectric materials in the OPVs, another major factor that severely limits the performance of organic solar cells is the high photovoltage loss. With the current understanding, the photovoltage loss in OPVs is larger than inorganics. To investigate the voltage loss mechanism in OPVs, finding the energy bandgap (Eg) in these materials is first and foremost step. However, probing such energy bandgap in donor-acceptor bulk heterojunctions (BHJ) is challenging. Traditional methods in determining Eg may ignore the interfacial dipole and/or energy level bending in the bulk heterojunction blends. Such ignorance would severely introduce error while interpreting the physics of corresponding photovoltage losses. Herein, temperature dependent open-circuit voltage (VOC) measurement is employed in studying its energy bandgap among several prototypical polymeric bulk heterojunction blends systems. Our results show the extrapolated VOC at 0 K of these systems is equal to their effective energy gap (Eeff), which is commonly known as the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO−D) and lowest unoccupied molecular orbital of the fullerene acceptor (ELUMO−A). Similarly, the photoemission results confirm that the simple extrapolation of VOC at 0 K is providing an easy access to the energy level alignment at organic donor:acceptor interfaces. On the success of this demonstration, we can immediately determine the VOC loss on these systems. More importantly, this approach can also simply be used to study energetics in the device structure.

Moreover, the VOC of OPVs is related to the energy of charge-transfer state (CTS). As demonstrated in the classic Shockley-Queisser limit, VOC is determined by the energy gap of the materials. Here we study CTS energy in different donor:acceptor heterojunction devices. Due to the excitonic properties owned by the OPVs, Charge-Transfer (CT) state is presented as the intermediate state in the charge dissociation process. To better understand the photo-physical process in these devices, direct identification of CT states is needed from the perspective of energetics. As the probability for direct charge generation in the sub-gap regime is far more less than the above gap excitation. CT states are difficult to be observed by the conventional optical absorption method. Advanced spectroscopic techniques are required to probe this low lying CT states. Although there is increasing number of characterization approaches presented on this topic recent years, there are still controversies between each other to interpret the formation of CT states energetics. In here, we combine simple and feasible experimental set-up, photothermal deflection spectroscopy (PDS) and external quantum efficiency (EQE) to probe these charge-transfer excitons in the energetic levels interested. Both techniques provide a comprehensive picture of the nature of CT state formation in the energetics as they demonstrate the evolution from delocalization to localization of charges. The sharpness of both spectroscopic curves directly reveals the nature of state in responsible to charge transport properties. A broad feature indicates more deep states presence in the sub gap region of materials that is corresponding to the excitonic properties in the OPVs. In addition, the experimental results show that the energy of charge-transfer state (ECT) in the organic solar cells lies on 1.0 – 1.6 eV. It elucidates the deviation of finding this parameter from previous reported literature values. In summary, our study in here provides better understanding on the properties of these excitonic solar cells both on the perspectives of materials and devices physics.