Dana C. Olson

Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer

Inverted organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerene have been developed by inserting a solution-processed ZnO interlayer between the indium tin oxide (ITO) electrode and the active layer using Ag as a hole-collecting back contact. Efficient electron extraction through the ZnO and hole extraction through the Ag, with minimal loss in open-circuit potential, is observed with a certified power conversion efficiency of 2.58%. The inverted architecture removes the need for the use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as an ITO modifier and for the use of a low-work-function metal as the back contact in the device.

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.

Improvement of interfacial contacts for new small-molecule bulk-heterojunction organic photovoltaics.

The influence of protonation reactions between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a thiadiazolo[3,4-c]pyridine small-molecule donor are reported; these result in poor solar-cell performance due to a barrier for charge extraction. The use of a NiO(x) contact eliminates such deleterious chemical interactions and results in substantial improvements in open-circuit voltage, fill factor, and an increased power conversion efficiency from 2.3% to 5.1%.

Surface Modification of ZnO Using Triethoxysilane-Based Molecules

Zinc oxide (ZnO) is an important material for hybrid inorganic-organic devices in which the characteristics of the interface can dominate both the structural and electronic properties of the system. These characteristics can be modified through chemical functionalization of the ZnO surface. One of the possible strategies involves covalent bonding of the modifier using silane chemistry. Whereas a significant body of work has been published regarding silane attachments to glass and SiO2, there is less information about the efficacy of this method for controlling the surface of metal oxides. Here we report our investigation of molecular layers attached to polycrystalline ZnO through silane bonding, controlled by an amine catalyst. The catalyst enables us to use triethoxysilane precursors and thereby avoid undesirable multilayer formation. The polycrystalline surface is a practical material, grown by sol-gel processing, that is under active exploration for device applications. Our study included terminations with alkyl and phenyl groups. We used water contact angles, infrared spectroscopy, and X-ray photoemission spectroscopy to evaluate the modified surfaces. Alkyltriethoxysilane functionalization of ZnO produced molecular layers with submonolayer coverage and evidence of disorder. Nevertheless, a very stable hydrophobic surface with contact angles approaching 106 degrees resulted. Phenyltriethoxysilane was found to deposit in a similar manner. The resulting surface, however, exhibited significantly different wetting as a result of the nature of the end group. Molecular layers of this type, with a variety of surface terminations that use the same molecular attachment scheme, should enable interface engineering that optimizes the chemical selectivity of ZnO biosensors or the charge-transfer properties of ZnO-polymer interfaces found in oxide-organic electronics.

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.

Direct synthesis of CdSe nanoparticles in poly(3-hexylthiophene).

A new approach was developed for the synthesis of nearly monodisperse CdSe nanoparticles directly in a polymer-containing solution in the absence of any other surface capping molecules. The comparatively high synthesis temperature reaction produces good quality crystalline CdSe nanoparticles. Time-resolved microwave conductivity measurements show that photoinduced charge separation occurs at the interface between the CdSe quantum dots and the polymer. This method can be extended to the synthesis of other II-VI semiconductor nanomaterials directly in a polymer-containing solution.

Photoinduced Carrier Generation and Decay Dynamics in Intercalated and Non-intercalated Polymer:Fullerene Bulk Heterojunctions

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

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.

Stability of inverted organic solar cells with ZnO contact layers deposited from precursor solutions

We report on investigations of the stability of inverted organic solar cells with ZnO electron collecting interlayer that are solution-processed from zinc acetate (ZnAc) or diethylzinc (deZn) precursors. Characterization of the respective solar cells suggests that the two materials initially function similarly in devices, however, we find that devices with ZnO from the deZn precursor are more stable under long-term illumination and load than devices with ZnO from the ZnAc precursor. A dipolar phosphonic acid that reduces the ZnO work function also improved device performance and stability when compared with unmodified ZnAc-based ZnO, but was problematic for deZn-based ZnO. The long-term device degradation analyses shows that the improved devices had increased and significantly more stable open-circuit voltage and fill factor characteristics. Chemical analyses suggests that defects in the ZnO films, most likely interstitial zinc, may be responsible for the observed disparities in stability within organic solar cells.

Efficient Modification of Metal Oxide Surfaces with Phosphonic Acids by Spray Coating

We report a rapid method of depositing phosphonic acid molecular groups onto conductive metal oxide surfaces. Solutions of pentafluorobenzyl phosphonic acid (PFBPA) were deposited on indium tin oxide, indium zinc oxide, nickel oxide, and zinc oxide by spray coating substrates heated to temperatures between 25 and 150 °C using a 60 s exposure time. Comparisons of coverage and changes in work function were made to the more conventional dip-coating method utilizing a 1 h exposure time. The data show that the work function shifts and surface coverage by the phosphonic acid were similar to or greater than those obtained by the dip-coating method. When the deposition temperature was increased, the magnitude of the surface coverage and work function shift was also found to increase. The rapid exposure of the spray coating was found to result in less etching of zinc-containing oxides than the dip-coating method. Bulk heterojunction solar cells made of polyhexylthiophene (P3HT) and bis-indene-C60 (ICBA) were tested with PFBPA dip and spray-modified ITO substrates as well as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)-modified ITO. The spray-modified ITO solar cells showed a similar open circuit voltage (VOC) and fill factor (FF) and a less than 5% lower short circuit current density (JSC) and power conversion efficiency (PCE) than the dip- and PEDOT:PSS-modified ITO. These results demonstrate a potential path to a scalable method to deposit phosphonic acid surface modifiers on metal oxides while overcoming the limitations of other techniques that require long exposure and post-processing times.