Examining And Controlling The Morphology Of The Photoactive Layer Of Organic Photovoltaic Devices

Electronic devices such as solar cells, transistors and light-emitting diodes (LEDs) fabricated using organic semiconductors offer a potential feasible alternative to their inorganic counterparts due to several advantages such as ease of processing (ink-jet printing, roll-to-roll processing), flexibility and excellent control over the electronic properties through chemical modifications. Compared to the inorganic semiconductors, however, the performance of organic semiconductor-based electronic devices are much lower. For example, in the case of photovoltaic devices (solar cells), the power-conversion efficiencies are still lower (7%-10%) compared to that of inorganic solar cells (> 25%). The efficiency of a solar cell is determined, among other factors, to a significant extent by the morphology of the active layer, the thin film where photons are absorbed and charges generated. Even though significant improvement in the efficiencies have been achieved, mainly through band-gap engineering and processing optimization, a fundamental understanding of the structural and morphological effects of the active layer on the performance of organic photovoltaic devices remains obscured. In this work, the focus is on examining the structure-function relationships in solution-processed bulk-heterojunction organic photovoltaic devices and development of processing techniques for device optimization. A bulk-heterojunction device is formed by mixing of donor-acceptor semiconductors, and the subsequent structure formed in the active layer is dictated by the miscibility and crystallization of the components, which are functions of processing conditions. Excitons (electron-hole pairs bound by coulombic forces) formed in the donor semiconductor upon absorption of light have a diffusion length of around 5-10 nm before recombination occurs. Thus the structural length scales formed in the active layer determine the number of excitons that can dissociate into charges. We have examined the microstructure of poly(3-hexyl thiophene) (P3HT) donor and phenyl-C61-butyric acid methyl ester (PC61BM) acceptor mixture using grazing incidence small angle X-ray scattering (GISAXS) and energy-filtered transmission electron microscopy (EFTEM) to characterize the in-plane structural length scales for various processing conditions such as annealing temperatures and spin-casting solvents. Our results show that the structural length scales are driven by self-limiting P3HT crystallization upon thermal annealing, which correlate to the internal quantum efficiencies of the devices. In contrast, it has been reported in the case of poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT)/ fullerene mixtures that thermal annealing results in crystallization of PBTTT with unconstrained lateral dimensions causing coarsening of the in-plane characteristic length scales. Thus the morphological evolution in polymer/fullerene solar cells, and consequently device performance, depends on the crystallization motif of the polymer. The microstructure resulting from mixing of donor-acceptor semiconductors can yield distinctive donor-acceptor interfaces that affect charge separation and recombination. Our studies utilizing a low band-gap poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT) donor and PC71BM acceptor examine the effects of mixing on the charge generation in a device. Composition of mixed phases ascertained qualitatively and quantitatively using EFTEM and resonance soft X-ray scattering (RSOXS) show that the concentration of polymer in the mixed phase decreases as fullerene concentration in the mixture is increased. This resulted in a concomitant increase in the device performance. Similarly, photo-induced absorption studies carried out using ultrafast spectroscopy show increase in polaron concentration with increase in purity of the mixed phase. Grazing-incidence wide-angle X-ray scattering (GIWAXS) data show a change in fullerene aggregation with increase in fullerene concentration in the mixture. This indicates that adding polymer to the mixed phase results in dispersal of fullerene, and consequently, changing the local environment of the polymer affects formation of charge-transfer states and subsequent dissociation into individual charges. Thus, high interfacial area that is formed upon intimate mixing of polymer/fullerene, considered ideal for efficient exciton dissociation, counteracts through high charge recombination. Our results show that the composition of mixed phases affects charge separation at the interface consequently affecting device performance of organic photovoltaics. Another important aspect that has been shown to affect device performance of organic photovoltaics is the orientation of polymer crystals with respect to the substrate. For example, P3HT predominantly orients in an edge-on configuration, i.e., with the [pi]-[pi] bond stacking direction parallel to the substrate. It is hypothesized that out of plane [pi]-[pi] stacking, called face-on orientation, is important for effective charge transport. One way to achieve enhancement of face-on orientation is by directional crystallization which has been shown to be very effective for P3HT -- in this case, directional crystallization from solution. In this context, 'zone-annealing' is relevant as it has been employed to directionally crystallize polymers. In this work, we designed and developed the zone-annealing equipment, which can yield thermal gradients greater than 60°C/mm. Preliminary results from GIWAXS experiments on P3HT/PC61BM thin films show anisotropy in the structure and a moderate enhancement of face-on orientated P3HT crystallites. This technique was extended to organic field-effect transistors (OFET) to enhance charge mobilities through directional crystallization of organic semiconductors. In case of P3HT, the increment in charge mobilities was by a factor of 2 upon zone-annealing. However, in the case of organic small molecule semiconductor, 2,7-dioctyl[1]benzo- thieno[3,2-b][1] benzothiophene (C8-BTBT) , highly aligned crystalline domains were obtained -- a very promising result for fabricating high mobility OFETs. Thus, the zone-annealing technique provides a handle for controlling the morphology of organic thin film electronic devices.