Alex C. Mayer

Polymer-based solar cells

A significant fraction of the cost of solar panels comes from the photoactive materials and sophisticated, energy-intensive processing technologies. Recently, it has been shown that the inorganic components can be replaced by semiconducting polymers capable of achieving reasonably high power conversion efficiencies. These polymers are inexpensive to synthesize and can be solution-processed in a roll-to-roll fashion with high throughput. Inherently poor polymer properties, such as low exciton diffusion lengths and low mobilities, can be overcome by nanoscale morphology. We discuss polymer-based solar cells, paying particular attention to device design and potential improvements.

Structure of pentacene thin films

Grazing incidence x-ray diffraction, x-ray reflectivity and atomic force microscopy have been performed to study the structure of pentacene thin films on oxidized Si substrates from submonolayer to multilayer coverages. The volume of the unit cell in the thin film phase is almost identical to that of the bulk phase, thus the molecular packing efficiency is effectively the same in both phases. The structure forming from the first monolayer remains the same for films at least 190A thick. The in-plane structure of the submonolayer islands also remains unchanged within a substrate temperature range of 0<Tsub<45°C while the island size changes by more than a factor of 4.

Organic bulk heterojunction solar cells using poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2,-b]thiophene)

By transitioning to semicrystalline polymers, the performance of polymer-based solar cells has recently increased to over 5% [W. Ma et al., Adv. Fund. Mater. 15, 1665 (2005); G. Li et al., Nat. Mater. 4, 864 (2005); M. Reyes-Reyes et al., Org. Lett. 7, 5749 (2005); J. Y. Kim et al., Adv. Mater. (Weinheim, Ger.) 18, 572 (2005); J. Peet et al., Nat. Mater. 6, 497 (2007)]. Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) has caused recent excitement in the organic electronics community because of its high reported hole mobility (0.6cm2V−1s−1) that was measured in field effect transistors and its ability to form large crystals. In this letter, we investigate the potential of pBTTT as light absorber and hole transporter in a bulk heterojunction solar cell. We find that the highest efficiency of 2.3% is achieved by using a 1:4 blend of pBTTT and[6,6]-phenyl C61-butyric acid methyl ester. The hole mobility as measured by space charge limited current modeling was found to be 3.8×10−4cm2V−1s...

Postfabrication annealing of pentacene-based photovoltaic cells

We studied the effects of postfabrication annealing on heterojunction photovoltaic cells made from vacuum deposited pentacene and C60. The maximum power conversion efficiency under 115mW∕cm2 illumination increases from 0.45% to 1.07% after annealing the cells at 200°C. The increased performance is a result of better molecular ordering, which leads to an increased shunt resistance and built-in potential.

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.

Transistor and solar cell performance of donor–acceptor low bandgap copolymers bearing an acenaphtho[1,2-b]thieno[3,4-e]pyrazine (ACTP) motif

We report the performance of low-bandgap polymers with a new ACTP acceptor in organic transistors (max. field-effect mobility 0.2 cm2V−1s−1), and solar cells (max. efficiency 1.4%).

Structure of a pentacene monolayer deposited on SiO2: Role of trapped interfacial water

In situ synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO2 substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70°C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum.

Using atomic steps to induce texture in polycrystalline pentacene films

The authors have studied the effect of substrate atomic steps on the azimuthal alignment of crystals in polycrystalline pentacene films. The substrates used were Si (111) with a low miscut angle and these were annealed at high temperature in ultrahigh vacuum before the pentacene deposition to produce surfaces with atomically flat terraces and arrays of parallel atomic steps. By depositing pentacene on these heated samples, at a low deposition rate, the authors have succeeded in obtaining significant azimuthal alignment of the pentacene crystals relative to the atomic steps.

Development of a Compact System for In‐situ X‐ray Scattering Studies of Organic Thin Film Deposition

We have developed a compact vacuum deposition chamber for in‐situ x‐ray scattering studies of organic thin film growth. The system is based on a small cylindrical chamber that can be mounted on a standard four‐circle diffractometer. Incident and scattered x‐rays enter and exit the chamber through a curved Be foil window that covers 200 degrees, and is sealed to the body of the chamber. The sample is mounted on a support tube with heating and cooling from liquid nitrogen temperature to >100°C. Integral to the sample stage is a multi‐wire feedthrough to facilitate in‐situ electrical transport characterization of organic semiconductor thin films. This is one of the novel capabilities of the system. In addition, the sample stage is mounted on a rotary vacuum feedthrough, which is mechanically coupled to the “phi” stage of the diffractometer. An effusion cell, shutter, and quartz oscillator thickness monitor are also incorporated into the system, which is pumped by a small turbomolecular pump. The system thus ...

Using Atomic Steps to Control Pentacene Crystal Orientation Texture

We have studied the effect of substrate atomic steps on the azimuthal alignment of vapor-deposited pentacene crystals. Si(111) substrates with a low miscut angle were annealed at high temperature in ultra-high vacuum before the pentacene deposition; this produced surfaces with atomically flat terraces and arrays of parallel atomic steps. AFM analysis shows that pentacene deposited on these heated samples, at a low deposition rate, results in significant alignment of the pentacene crystals along the atomic steps.