Session H11: Organic Electronics - Organic Photovoltaics

Charge transfer states for organic opto-electronics

Intermolecular charge transfer (CT) states at the interface between electron-donating and electron-accepting materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive, or generate free carriers at high yield. The former can result in rather efficient organic light emitting diodes, via thermally activated delayed fluorescence, while the latter property is exploited in organic photovoltaic devices and photodetectors. In this contribution, we will discuss the fundamental properties of CT states and link them to organic opto-electronic device performance. Furthermore, we introduce a new device concept, using an optical cavity resonance effect to boost CT absorption at photon energies below the optical gap of both donor and acceptor, enabling narrow-band, near infrared (NIR) photo-detection. This new type of photodetector can compete with standard organic photodetectors but extends their detection range to longer wavelengths.   

Origin of Non-Radiative Voltage Losses in Fullerene-Based Organic Solar Cells

The open-circuit voltage of organic solar cells (OSCs) is low as compared to the optical gap of the absorber molecules, indicating high energy losses per absorbed photon. These voltage losses arise only partly due to necessity of an electron transfer event to dissociate the excitons. A large part of these voltage losses is due to recombination of photo-generated charge carriers, including inevitable radiative recombination. In this work, we study the non-radiative recombination losses and we find that they increase when the energy difference between charge transfer (CT) state and ground state decreases. This behavior is in agreement with the ”energy gap law for non-radiative transitions”, which implies that internal conversion from CT state to ground state is facilitated by skeletal molecular vibrations. This intrinsic loss mechanism, which until now has not been thoroughly considered for OSCs, is different in its nature as compared to the commonly considered inorganic photovoltaic loss mechanisms of defect, surface, and Auger recombination. As a consequence, the theoretical upper limit for the power conversion efficiency of a single junction OSC reduces by ~25\% as compared to the Shockley-Queisser limit for an optimal optical gap of the main absorber between (1.45-1.65)\,eV.   

Role of a pure acceptor phase on charge generation and recombination in BHJ solar cells

Electric properties of organic photovoltaics arise not only from molecular donor/acceptor pairs but the nanostructure brought about through device processing. Characterization of this structure has been a challenging endeavor due to low levels of both crystallinity and material contrast that hinder traditional methods such as electron microscopy and hard x-ray scattering. Increased popularity in resonant x-ray techniques have allowed for a stronger understanding of the complicated morphology found in these systems. Recent studies have qualitatively measured relative domain purity in addition to domain spacing to investigate device performance. Here we utilize Resonant Soft X-ray Scattering to track the shifting morphological balance of a known 3-phase OPV. We find an increasing volume fraction of a pure acceptor phase promotes both increased charge generation as well as reduced bimolecular recombination. The importance of a pure acceptor phase will be discussed.   

Incorporating fluorinated moieties in fully conjugated donor-acceptor block copolymers

Fully conjugated donor-acceptor block copolymers are promising candidates for photovoltaics due to their ability to microphase separate at length scales commensurate with exciton diffusion lengths. These materials can also serve as model systems to study the relationship between molecular structure, microstructure, and optoelectronic properties of conjugated polymers. The development of new donor-acceptor block copolymers relies on the manipulation of the chemical structure to fine tune properties and improve overall performance when employed in photovoltaic devices. To this end, we have demonstrated the incorporation of fluorinated moieties in conjugated block copolymers. The introduction of fluorine, a strong electron withdrawing element, is known to influence phase separation and the bandgap, and as a result, optoelectronic properties. Fluorine was introduced to the acceptor block of poly(3-hexylthiophene-2,5-diyl)-\textit{block}-poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-\textit{alt}-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5$\prime $,5?-diyl) (P3HT-$b$-PFTBT). PFTBTs were prepared with di-fluorinated and mono-fluorinated TBT. We find that fluorination impacts the bandgap, morphology and performance in devices.   

Charge generation in polymer:fullerene and oligomer:fullerene blends for organic photovoltaics

Polymer:fullerene blends have attracted attention as efficient organic photovoltaic (OPV) materials promising over 10{\%} power conversion efficiency. It has recently been shown that the polymers can be replaced by small molecules or oligomers, which have better chemical reproducibility. In this talk, I present results obtained with a variety of ultrafast spectroscopic techniques (transient absorption, terahertz and electro-modulated differential absorption spectroscopy) that have allowed to correlate the mechanism of charge generation in donor:acceptor blends to the phase morphology, i.e. to the arrangement of the donor and acceptor into phase-pure (neat) and intermixed domains. In particular, I describe how varying the miscibility between the components (by changing the fullerene acceptor, or using ternary blends containing two different fullerenes), and replacing the polymer with the corresponding dimer, affect the phase morphology and charge generation.   

Energetic disorder induced ultrahigh open circuit voltage loss at low temperatures in organic bulk heterojunction solar cells

In organic bulk heterojunction solar cells, the open circuit voltage ($V_{oc}$) suffers from an ultra-high loss at low temperatures. In this work we investigate the origin of the loss through calculating the $V_{oc}-T$ plots with the device model method systematically and comparing it with experimentally observed ones. When the energetic disorder is incorporated into the model by considering the disorder-suppressed temperature dependent charge carrier mobilities, it is found that for nonselective contacts the $V_{oc}$ reduces drastically under the low temperature regime, while for selective contacts the $V_{oc}$ keeps increasing with the decreasing temperature. The main reason is revealed that as the temperature decreases, the reduced mobilities give rise to low charge extraction efficiency and small bimolecular recombination rate for the photogenerated charge carriers, so that in the former case they can be extracted from the wrong electrode to form a leakage current which counteracts the photocurrent and increase quickly with voltage, leading to the anomalous reduction of $V_{oc}$.   

Proficient ternary blend organic photovoltaic device with PC$_{\mathrm{\mathbf{61}}}$BM as an interface modulator

Ternary organic solar cells (OSCs) with inverted structure of ITO/ZnO/PTB7-Th:PC$_{\mathrm{71}}$BM:PC$_{\mathrm{61}}$BM/MoO$_{\mathrm{x}}$/Ag were fabricated in ambient using PC$_{\mathrm{61}}$BM in place of ICBA to seize electron-cascade effect. Since spherically-shaped PC$_{\mathrm{61}}$BM molecules are more symmetric than elliptical PC$_{\mathrm{71}}$BM molecules, results in better precipitation kinetics. The ternary devices with additional 20{\%} PC$_{\mathrm{61}}$BM content exhibit average efficiency of 7.8{\%}, higher than (6.3{\%}) that of binary blend. The observed lowering of built-in potential and defect states at the ZnO/BHJ interface can be attributed to the interface modulation due to the percolation of PC$_{\mathrm{61}}$BM towards ZnO interface, leads to better transport. Also AFM image shows better morphology with suitable phase separation for 20{\%} PC$_{\mathrm{61}}$BM loaded active layer. This homogeneous blend may be evolved due to the differential chemical kinetics of two fullerenes in the blend. Thus the ternary blend OPV witnessed with efficiency upto 8.2{\%} with mere addition of PC$_{\mathrm{61}}$BM as interface modulator.   

Hybrid ZnO-organic semiconductor interfaces in photodetectors: a comparison of two near-infrared donor-acceptor copolymers

Hybrid organic-inorganic photodiode interfaces have gained significant interest due to their unique physical properties such as mechanical flexibility and high photosensitivity. In this work, two diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers with different backbone conformations were characterized in an inverted photodiode architecture using ZnO nano-patterned films as the electron transport layer. The DPP copolymer with a thienothiophene unit (PTTDPP-TBDT) is more planar and rigid compared to the DPP system with a thiophene unit connecting the donor and acceptor moieties within the monomer (PTDPP-TBDT). The hybrid interfaces were optimized by using poly(3-hexylthiophene) (P3HT) as the p-type layer for monitoring the critical thickness and morphology of the ZnO layer. The maximum photo-responsivity from a P3HT:ZnO photodiode was found to be 42 mA/W. The photo-responsivity of PTTDPP-TBDT:ZnO photodiodes were found to be at least three orders of magnitude higher than PTDPP-TBDT:ZnO photodiodes, which is related to an enhanced transport of carriers due to the planar backbone conformation of the TTDPP-TBDT copolymer. Capacitance-voltage measurements from hybrid Schottky barrier interfaces further shed light into the nature of photo-carriers and device parameters.