Matthew T. Lloyd

Stability Assessment on a 3% Bilayer PbS/ZnO Quantum Dot Heterojunction Solar Cell

We provide the first NREL-certified efficiency measurement on an all-inorganic, solution-processed, nanocrystal solar cell. The 3% efficient device is composed of ZnO nanocrystals and 1.3 eV PbS quantum dots with gold as the top contact. This configuration yields a stable device, retaining 95% of the starting efficiency after a 1000-hour light soak in air without encapsulation.

Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoOx) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air yield continuous thin films of MoOx. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoOx and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoOx thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo5+ gap states in the MoOx HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoOx HCL.

Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment

Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. In contrast to established methods that are used in solution or vacuum environments, here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply this new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enables us to answer the outstanding question: why previous attempts to make OPV devices using 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor were not successful.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Improved performance of poly(3-hexylthiophene)/zinc oxide hybrid photovoltaics modified with interfacial nanocrystalline cadmium sulfide

To improve zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) hybrid solar cell performance, we introduce a nanocrystalline cadmium sulfide (CdS) film at the ZnO/P3HT heterojunction, creating a cascading energy band structure. Current-voltage characteristics under AM1.5 illumination show that, compared to unmodified ZnO/P3HT devices, CdS modification leads to an approximate doubling of the open-circuit voltage and a mild increase in fill factor, without sacrificing any short-circuit current. These characteristics double the power conversion efficiency for devices with an interfacial CdS layer. External quantum efficiency spectra reveal definite photocurrent contributions from the CdS layer, confirming the cascading band structure. The mechanisms behind open-circuit voltage increase are discussed.

Impact of interfacial polymer morphology on photoexcitation dynamics and device performance in P3HT/ZnO heterojunctions

To understand the critical factor(s) that influence short-circuit current in poly(3-hexylthiophene) (P3HT)/ZnO solar cells, we investigate the morphology of the interfacial polymer layer and the photoexcitation dynamics in the picosecond regime. Thin (∼6 nm) films of P3HT deposited on bare ZnO and ZnO modified with an alkanethiol monolayer are used as model systems for the heterojunction interface. Results are compared with thin P3HT films on glass for the behavior of the polymer alone. Synchrotron grazing incidence X-ray diffraction spectra of P3HT thin films deposited on glass and on an alkanethiol-modified ZnO surface identify a crystalline P3HT interfacial layer, while an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay dynamics of initial photoexcited states, the samples are interrogated by pump–probe spectroscopy with sub-picosecond time resolution. Compared to P3HT/ZnO composite films, the decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly slower with alkanethiol modification, indicating a reduction in early-stage charge recombination. These experiments demonstrate how the interfacial polymer morphology has a critical role in determining device performance.

Open-Circuit Voltage Improvement in Hybrid ZnO–Polymer Photovoltaic Devices With Oxide Engineering

We present strategies to improve low open-circuit voltage (V_oc) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting V_oc variability is the ZnO bandgap (E_g), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO E_g by 0.14 eV and V_oc of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in V_oc by depositing conformal amorphous TiO_x films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiO_x coatings monotonically increased V_oc from 0.4 to 0.8 V for devices with increasing TiO_x thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiO_x coating significantly decreases the reverse-bias current density, leading to an improvement in V_oc, in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiO_x. In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.

Enhanced lifetime in unencapsulated organic photovoltaics with air stable electrodes

Organic photovoltaics (OPVs) are realizing power conversion efficiencies that are of interest for commercial production. Consequently, understanding device lifetime and mitigating degradation pathways have become vital to the success of a new industry. Historically, the active organic components are considered vulnerable to photo-oxidation and represent the primary degradation channel. We present several (shelf life and light soaking) studies pointing to the relative stability of the active layers and instabilities in commonly used electrode materials. We show that engineering of the metal electrode and hole/electron injection layer can lead to environmentally stable devices without encapsulation.

Overcoming degradation in organic photovoltaics: Illuminating the role of fullerene functionalization

Photobleaching rates are investigated for thin films of poly(3-hexylthiophene) (P3HT) blends employing either an indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor. Relative to the bisindene, PCBM significantly enhances resistance to photobleaching of the P3HT donor polymer. We tentatively attribute a decrease in the charge transfer rate as the mechanism responsible for the more rapid photobleaching in the sample containing the bisindene adduct. In order to elucidate the influence of the photobleaching rate on the initial performance of unencapsulated devices, we also monitored the time-dependent behavior for P3HT:fullerene inverted devices. Under conditions of constant illumination, we observe essentially identical behavior in device performance parameters regardless of the energy levels of the electron acceptor. We conclude that over the time frame measured for these devices, the primary degradation mechanism of the active layer is independent of the electron acceptor, despite the enhanced tolerance to photobleaching it may impart to the donor material.

Combined characterization techniques to understand the stability of a variety of organic photovoltaic devices: the ISOS-3 inter-laboratory collaboration

This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPVs) devices prepared by leading research laboratories. All devices have been shipped to and degraded at the Danish Technical University (DTU, formerly RISO-DTU) up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work we present a summary of the degradation response observed for the NREL sample, an inverted OPV of the type ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag/Al, under full sun stability test. The results reported from the combination of the different characterization techniques results in a proposed degradation mechanism. The final conclusion is that the failure of the photovoltaic response of the device is mainly due to the degradation of the electrodes and not to the active materials of the solar cell.