Probing the charge generation and recombination in thin-film, optoelectronic devices

Sustainably and environment-friendly manufactured semiconductors are at-tractive candidates for next generation electronic and optoelectronic appli-cations ranging from memory storage and computation, to power manage-ment and energy generation. In this regard, organic semiconductors, i.e., semiconductors based on conjugated carbon-based molecules and polymers derived from earth abundant elements, are the subject of intense basic re-search and technological development efforts. Understanding the funda-mental processes governing these low-mobility and disordered semiconduct-ing materials is therefore key to establish next generation applications based upon flexible and solution-processible organic semiconductors as global com-mercial technologies.The work presented in this thesis focuses on the investigation of charge generation and recombination processes on thin film optoelectronic devices based upon organic semiconductors. A suite of experimental techniques, im-proved measurement setups, and expanded approaches are presented, and form the basis of comprehensive studies on state-of-the-art, high-efficiency organic photovoltaic systems. Specifically, an external quantum efficiency measurement technique with unprecedented dynamic range will be detailed. Using this enhanced apparatus, an approach allowing one to accurately de-termine charge generation quantum yields is introduced. After this, an extended technique to probe photogenerated charge carrier densities is out-lined and applied to thin-film solar cells. Having emphasized the importance of studying charge generation, a combined theoretical and experimental ex-ploration of the light intensity dependence of photocurrent and charge col-lection efficiency under the influence of various loss mechanisms is described. These insights provide the basis of a comprehensive study on organic so-lar cells, where recombination caused by localized trap states is found to be universally present under operational conditions limiting photocurrent and power-conversion efficiency. Overall, the work presented in this thesis expands on existing techniques and approaches, and yields important new understanding as to the device physics of thin-film, optoelectronic applica-tions

Understanding the Role of Order in Y‐Series Non‐Fullerene Solar Cells to Realize High Open‐Circuit Voltages

Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing temperature-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage VOC on temperature and illumination intensity. Disorder is found to influence the value of the VOC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500–600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20–30% at wavelengths of 900–1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones

Understanding Performance Limiting Interfacial Recombination in pin Perovskite Solar Cells

Perovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto‐electronic properties and their successful integration into multijunction cells. However, the performance of single‐ and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first‐principle numerical simulations. It is found that the most significant contribution to the total C60‐induced recombination loss occurs within the first monolayer of C60, rather than in the bulk of C60 or at the perovskite surface. The experiments show that the C60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells

Determining Ultralow Absorption Coefficients of Organic Semiconductors from the Sub‐Bandgap Photovoltaic External Quantum Efficiency

Energy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross-section, the absorption coefficient of these states is challenging to measure using conventional transmission reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. In this work, it is shown that the subbandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device, making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor-donor systems (PCE12ITIC and PBTTTPC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding sub-gap states and their utilization in organic optoelectronic devices

Light intensity dependence of the photocurrent in organic photovoltaic devices

The competition between recombination and extraction of carriers defines the charge collection efficiency and, therefore, the overall performance of organic photovoltaic devices, including solar cells and photodetectors. In this work, we describe different components of the steady-state light intensity-dependent photocurrent (IPC) and charge collection efficiency under operational conditions. Further, we demonstrate how different loss mechanisms can be identified based on their unique signatures in the IPC. In particular, we show how IPC can be used to distinguish first-order, trap-assisted recombination from other first-order photocurrent loss mechanisms, which dominate at the low-intensity characteristic of indoor light-harvesting applications. The theoretical framework is presented and verified by a one-dimensional drift-diffusion device model. Finally, the extended IPC methodology is validated on organic thin-film photovoltaic devices. We conclude that the relatively straightforward measurement of IPC over a large dynamic range can be a powerful tool for understanding solar and indoor device fundamentals

Stark Effect of Hybrid Charge Transfer States at Planar ZnO/Organic Interfaces

We investigate the bias-dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photo-doping allow us to confirm that this bias-induced spectral shifts relate to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image

Organic solar cells with near-unity charge generation yield

The subtle link between photogenerated charge generation yield (CGY) and bimolecular recombination in organic semiconductor-based photovoltaics is relatively well established as a concept but has proven extremely challenging to demonstrate and probe especially under operational conditions. Received wisdom also teaches that charge generation in excitonic systems will always be lower than non-excitonic semiconductors such as GaAs – but this view is being challenged with the advent of organic semiconductor blends based upon non-fullerene acceptors (NFAs) with power conversion efficiencies exceeding 18%. Using a newly developed approach based upon temperature dependent ultra-sensitive external quantum efficiency measurements, we observe near unity CGY in several model NFA-based systems measured with unprecedented accuracy. We find that a relatively small increase in yield from 0.984 to 0.993 leads to a reduction in bimolecular recombination from 400 times to 1000 times relative to the Langevin limit. In turn, this dramatic reduction delivers the best thick junction performance to date in any binary organic solar cell – notably 16.2% at 300 nm. The combination of high efficiency and thick junction is the key for industrial fabrication of these devices via high-throughput deposition processing such as roll-to-roll, and thus central to a viable solar cell technology. These results also clearly reveal and elucidate the relationship between photo-generation and recombination in excitonic semiconductor photovoltaics thus providing an important bridge between basic device physics and practical cell engineering

On the Impact of Cadmium Sulfide Layer Thickness on Kesterite Photodetector Performance

Kesterites are currently viewed as one of the most promising candidates for earth abundant and benign elements to substitute critical raw materials in photovoltaic technologies and may also be suitable for low-noise, room-temperature, self-powered photodetectors. However, while the impact of buffer layers on kesterite solar cell efficiency has been an active area of investigation, links between photodetector performance and intermediate layers are yet to be addressed. Herein, the impact of cadmium sulfide buffer layers on the performance of kesterite (Cu2ZnSnS4) photodetectors is probed. Specifically, the effect of buffer layer thickness on various photodetector performance metrices is clarified, including noise current, spectral responsivity, noise equivalent power, frequency response, and specific detectivity. Devices with a 100 nm cadmium sulfide layer perform the best, achieving a linear dynamic range of 180 dB and frequency responses in the range of tens of kHz. The key loss mechanisms are identified, and it is found that the photodetector performance to be primarily limited by shunt resistance-induced thermal noise and defect-induced nonradiative losses. Furthermore, we estimate the upper radiative limit of specific detectivity to be approximately 10(19) Jones. Our results highlight the potential of kesterites to be used as an interesting earth abundant candidate for photodetection applications

Direct observation of trap-assisted recombination in organic photovoltaic devices

Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records. The trap-assisted recombination is found to be induced by states lying 0.35-0.6 eV below the transport edge, acting as deep trap states at light intensities equivalent to 1 sun. Apart from limiting the photocurrent, we show that the associated trap-assisted recombination via these comparatively deep traps is also responsible for ideality factors between 1 and 2, shedding further light on another open and important question as to the fundamental working principles of organic solar cells. Our results also provide insights for avoiding trap-induced losses in related indoor photovoltaic and photodetector applications