McNeill Research Group

Group Members

Leader

Ph.D. Students

Research Areas

Device Physics

The operation of organic semiconductor devices is markedly different to that of inorganic devices. Organic semiconductors (in particular conjugated polymers) are characterised by a relatively high degree of structural disorder, strong electron-phonon coupling and low dielectric constants. Organic solar cells in particular have rich device physics with tightly bound excitons the initial product of photoexcitation. These excitons must diffuse to an interface with another semiconductor with different electron affinity in order to dissociate into electron-hole pairs (see figure 1a). Due to the low exciton diffusion length (~ 10 nm) compared to the optical absorption length (~ 100 – 200 nm) a blended or “bulk heterojunction” architecture is used (see figure 1b). These bulk heterojunction films are produced by solution deposition of solutions with donor and acceptor materials co-dissolved. Key challenges to the field are the characterisation of the microstructure of bulk heterojunction films and the relationship between film structure and device operation. Interfacial processes are also important as the separation electron-hole pairs from the donor/acceptor interface is another key process that can limit device efficiency.

Figure 1: (a) Schematic diagram of the photocurrent generation mechanism in organic photovoltaic devices. Step 1: Photogeneration of an exciton. Step 2: Exciton diffusion to and dissociation at a donor/acceptor interface. Step 3: Separation of the interfacial electron–hole pair. Step 4: Transport of charges to the electrodes. (b) Schematic representation of the device structure and nanoscale bicontinuous blend morphology employed in polymer solar cells. Adapted from reference [1].

Figure 2: (a) Influence of interfacial roughness on the performance of bilayer solar cells (see reference [2]). (b) Change in the photocurrent dynamics of polymer blend solar cells with increasing intensity. The device is subject to a square light pulse (on at 0 s, off at 100 microseconds) of varying intensity (see reference [3]).

Structural Characterisation

The structure of organic semiconductor thin films is hard to characterise. Distinguishing two different organic materials in a blend is challenging due to the similar elemental make-up and mass and electron densities of the constituent materials. Organic materials also have low scattering cross-sections and a high degree of disorder limiting the application of conventional techniques used to study inorganic materials. In collaboration with Prof. Harald Ade of North Carolina State University and Dr. Ben Watts of the Swiss Light Source, we have pioneered the use of soft x-ray techniques to study the structure of organic semiconductors. The different carbon K-edge x-ray absorption spectra of donor and acceptor materials used in organic solar cells provides a means for chemical contrast in the absence of differences in elemental composition or mass/electron density. Both microscopy and scattering methods have been utilised to probe film nanomorphology with high resolution and unprecedented chemical contrast (see figure 3).

Figure 3. (a) Relative scattering intensity (SI) for a PFB:F8BT thin film with a thickness of a fraction of an absorption length (see reference [4]). (b) Scattering profiles of PFB:F8BT blends acquired at 284.7 eV (see reference [4]). (c) X-ray microscopy image of a PFB:F8BT blend (see reference [5]).

Figure 4: (a) Map of local resonance direction (colour wheel) and backbone orientation (lines) of a polycrystalline F8BT film. (b) Corresponding map of local polymer order (see reference [6])

References

[1] C. R. McNeill and N. C. Greenham "Conjugated polymer blends for optoelectronics" Adv. Mater., 21, 3840-3850 (2009).
[2] H. Yan, S .Swaraj, C. Wang, I. Hwang, N. C. Greenham, C. Groves, H. Ade and C. R. McNeill, "Influence of annealing and interfacial roughness on the performance of bilayer donor/acceptor polymer photovoltaic devices" Adv. Funct. Mater., in press (2010).
[3] C. R. McNeill, I. Hwang and N. C. Greenham, "Photocurrent transients in all-polymer solar cells: Trapping and detrapping effects" J. Appl. Phys., 106, 024507 (2009).
[4] S .Swaraj, C. Wang, H. Yan, B. Watts, J. Lüning, C. R. McNeill and H. Ade, "Nanomorphology of bulk heterojunction photovoltaic thin films probed with resonant soft x-ray scattering" Nano Letters, 10, 2863-2869 (2010).
[5] C. R. McNeill, B. Watts, S. Swaraj, H. Ade, L. Thomsen, W. Belcher and P. C. Dastoor, "Evolution of the nanomorphology of photovoltaic polyfluorene blends: Sub-100 nm resolution with X-ray spectromicroscopy" Nanotechnology 19, 424015 (2008).
[6] B. Watts, T. Schuettfort and C. R. McNeill, submitted (2010).