The performance of non-fullerene acceptor (NFA)- based organic solar cells (OSC) has shown continuous increase in recent years, reaching power-conversion efficiencies up to 17% through the design and synthesis of efficient acceptor materials. Recent research is directed towards achieving higher efficiency of OSC, which is limited by the open-circuit voltage (Voc) which is lower than the Voc values achieved in inorganic or perovskites solar cells with comparable bandgaps. In this work, voltage losses in NFA based OSC were calculated by investigating charge-transfer state energy (ECT) using electroluminescence spectroscopy and sensitive external quantum efficiency in three polymer:non-fullerene bulk heterojunction solar cells. PCE10:ITIC device acquired the highest ECT with a Voc of 0.82V, and a a power conversion efficiency (PCE) of 7.91%. While PCE10:O-IDTBR obtained the highest Voc of 1.03V, a PCE of 8.02% compared to PCE10:O-IDTBCN solar cell that has a lower Voc of 0.73V with a PCE of 7.98%. Both radiative and non-radiative voltage losses were calculated. In this thesis, the high open circuit voltage of PCE10:O-IDTBR is explained by the low non-radiative voltage losses compared to PCE10:O-IDTBCN and PCE10:ITIC devices.

In organic photovoltaics (OPVs) the key processes relevant to device operation such as exciton dissociation and free carriers recombination occur at the donor-acceptor (D-A) interface. OPV devices require the bulk heterojunction (BHJ) architecture to function efficiently. In these BHJs, D-A interfaces are arranged in three dimensions, which makes molecular arrangements at these interfaces ill defined and hard to characterize. In addition, molecular materials used in OPVs are inherently disordered and may exhibit variable degrees of structural order in the same BHJ. Yet, D-A molecular arrangements and structure are crucial because they shape the energy landscape and photovoltaic (PV) performance in OPVs. Studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. We have used in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored bilayers and BHJs based on small molecule donors. We show preferential miscibility at the D-A interface depending on molecular conformation in zinc phthalocyanine (ZnPc)/ C60 bilayers and we derive implications for exciton dissociation. Using sexithiophene (6T), a crystalline donor, we show that the energy landscape at the D-A interface varies markedly depending on the molecular composition of the BHJ. Both the ionization energies of sexithiophene and C60 shift by over ~0.4 eV while the energy of the charge transfer state shifts by ~0.5 eV depending on composition. Such shifts create a downward energy landscape that helps interfacial excitons to overcome their binding energies. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form in ordered phases and significantly limit the Voc in devices. Overall our work underlines the importance of the aggregation and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPV devices. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in depth characterization of planar heterojunction bilayer devices recreating model interfaces.

Organic photovoltaics (OPVs) have received tremendous attention in recent years due to their numerous attractive attributes such as, the potential for high power conversion efficiency (PCE), mechanical flexibility, and the potential for large-scale manufacturing via low-cost techniques. To date, the record PCE values for bulk-heterojunction (BHJ) OPVs exceed 18% for single-junction cells thanks to the rapid development of donors and acceptors materials for active layer. However, the progress of hole-transporting layer (HTL) systems, which is a key device component to reduce the additional performance losses of OPVs, has been limited with only a handful of materials available like PEDOT:PSS and MoOX. In this thesis, I introduce serval materials to unitize as hole-selective contact in high-performance OPVs. Firstly, the application of liquid-exfoliated two-dimensional transition metal disulfides (TMDs) is demonstrated as the HTLs in OPVs. The solution processing of few-layer WS2 suspensions was directly spun onto transparent indium-tin-oxide (ITO) electrodes yield solar cells with superior power conversion efficiency (PCE), improved fill-factor (FF), enhanced short-circuit current (JSC), and lower series resistance than devices based on PEDOT:PSS. Based on PM6:Y6:PC71BM BHJ layer, the cells with WS2 HTL exhibit the highest PCE of 17% thanks to the favorable photonic structure and reduced bimolecular recombination losses in WS2-based cells. Next, the self-assembled monolayer (SAM) namely 2PACz is utilized as hole-selective contact directly onto the ITO anode. The 2PACz modifies the work function of ITO while simultaneously affecting the BHJ layer’s morphology deposited atop. This ITO-2PACz anode is utilized in OPV with PM6:BTP-eC9:PC71BM, showing a remarkable PCE of 18.0%. The enhanced performance is attributed to reduced contact-resistance, lower bimolecular recombination losses, and improved charge transport within the BHJ layer. Lastly, the previously 2PACz SAM was functionalized with bromide functional groups, namely Br-2PACz, which is investigated as hole-extracting interlayers in OPVs. The highest occupied molecular orbital (HOMO) energy of Br-2PACz was measured at -6.01 eV, and significant changes the work function of ITO electrodes upon chemical functionalization. OPV cells based on PM6:BTP-eC9:PC71BM using ITO/Br-2PACz anodes exhibit a maximum PCE of 18.4%, outperforming devices with ITO/PEDOT:PSS (17.5%), resulting from lower interface resistance, improved hole transport, and longer carrier lifetimes.

Philosophiae Doctor - PhD / The commercial need to improve the performance of low-cost organic solar cells has led to the idea for this research. The study discusses the synthesis of one dimensional TiO2 and ZnO nanowire arrays synthesised using a hydrothermal autoclave method and their application in bulk heterojunction inverted organic solar cells. Previous literature has shown that the precise manipulation, positioning and assembly of 1D nanostructures remain one of the greatest challenges in the field of nanotechnology, with much of the difficulty arising primarily from the lack of size and scale of the materials as well as the inability to visualise the nanostructures. In particular, one dimensional metal-oxides such as TiO2, ZnO and Fe2O3 have emerged as attractive alternatives to traditional semiconductor structures such as Si and GaAs as they are simple and inexpensive to manufacture, with research showing that application of ZnO nano-cones yield efficiencies of 8.4%, which is very attractive given the scope that exists in optimising the metal-oxide architecture. Much is still to be learned from the precise structural features of these materials and their influence on device performance. In this regard, this work largely focuses on this aspect of metal-oxide nanowires prior their application in organic solar cells.

Research in recent years on novel materials in organic solar cells (OSCs) have contributed to a rapid advancement in OSC efficiency. Here, OSCs with the well-studied organic semiconductors poly(3-hexylthiophene):[6,6]-phenyl C60-butyric acid methylester (P3HT:PCBM) in a bulk heterojunction structure were prepared to establish a baseline procedure for fabrication and spin-sensitive spectroscopy. Spin-sensitive spectroscopy can be used to probe the spin-dependent processes and loss mechanism in OSCs. Understanding the microscopic processes enables research targeting loss mechanisms directly, which opens up for higher efficiency OSCs. Measurements with continuous wave electrically detected magnetic resonance (cwEDMR), continuous wave electron paramagnetic resonance (cwEPR) and transient (tr)EDMR were set up and followed by an initial study on the spin-dependent processes and their influence on the photocurrent. Signals from spin-dependent processes in the samples were seen for the three experiment series carried out, and well-functioning OSCs with consistent results were prepared. This thesis and the presented baseline fabrication and experimental setup procedures provide with valuable learnings for future research in the group AG Behrends enabling fabrication and spin-sensitive studies on novel materials in OSCs. It was found that post-processing annealing at 120 C for 5 min reduces S-shape behaviour in current-voltage curves, and over all improves poor current-voltage characteristics of the OSC samples fabricated. Further, influence from PEDOT:PSS was seen in cwEPR spectroscopy, indicating the presence of traps or free radicals in the PEDOT:PSS. In cwEDMR spectroscopy of the OSCs it was shown that the spin processes of bias-induced and photo-induced charge carriers influence the photocurrent in the same way. Finally, the authors suggest that results from spin-sensitive spectroscopy of degraded OSCs cannot be applied to non-degraded OSCs.

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Organic solar cells have shown great promise of becoming a cheaper alternative to inorganic solar cells. Additionally, they can also be made semitransparent. To avoid using expensive indium tin oxide electrodes in organic solar cells the electrodes can be made from conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). However, these so-called PEDOT-PEDOT solar cells are prone to short-circuiting. The work behind this thesis thus aimed to find the cause of these short circuits. The initial working hypothesis assumed the hygroscopic PSS in the bottom electrode could attract water across the active layer when the top electrode layer was applied. This would then swell the bottom electrode and cause the active layer to crack leading to short circuits. Accordingly, swelling was investigated as it was suspected to be the main cause of the shorts. This was achieved by coating reflective substrates with different layers from the solar cell, dropping water on top of the stack and then filming the thin film interference effects. SEM, AFM and IR were also used for further analysis. Although the bottom electrode swells, it was found that water does not cause permanent cracking. Instead, the research unveiled that water causes a formation of blisters, which are suspected to be made of PSS. The exact mechanism for the formation of the shorts remains unclear however.

<p>Nanomaterials have been heavily studied in the past two decades. Previous findings have demonstrated that the characteristics of nanocomposites and the performance of nanomaterial-based devices are both determined by the interfacial characteristics of the nanomaterials. However, there are still some remaining challenges from interfacial characteristics to device applications, which are specified as follows: the difficulty in identifying the interfacial contacts of nanostructured surfaces, the instability of nanocomposite surfaces, and the under-researched mechanism of the correlation between interfacial characteristics and the performance of devices.</p> <p>Therefore, the main theme of this thesis is to investigate the interfacial contacts of nanostructured solid-liquid interfaces by direct observation, and to develop a stable nanocomposite based on which the direct observation of the interfacial contact can be better conducted, and to eventually investigate the effect of interfacial contacts on the performance of organic solar cells.</p> <p>As the previous identification of the solid-liquid interface is limited to a microscale range, a direct method of tracing the different wetting states of water was developed, on nanostructured surfaces. This method provided an answer to a long standing question of, whether there is a transition from Wenzel to Cassie states in the sliding angle drop on nanocomposite thin films. In order to complete the observation of the wetting states of water, a stable superhydrophobic nanocomposite thin film with hierarchical structure was developed.</p> <p>Furthermore, with the knowledge of identifying the wetting states and the preparing procedures of the nanocomposites, a surfactant-free small-molecule nanoparticle organic solar cell with a much improved fill factor was developed by spin coating. The inverse correlation of series resistance and parallel resistance was discovered, due to the morphology change and the variation of the charge carrier concentration near the donor-acceptor interface in small-molecule organic solar cells.</p> / Doctor of Philosophy (PhD)