Advanced Functional Materials and Devices (AFMD) Group

Abstract:

The role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S-shaped current-voltage ( J-V ) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric fi eld and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the ? s to ms regime. In combination with drift-diffusion simulation data, we demonstrate a pile-up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S-kinks in the J-V curve and can be distinguished from each other and from other reasons for S-kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S-shaped J-V curve close to open circuit to evaluate whether their electrodes are optimized. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Ternary composite inverted organic solar cells based on poly(3-hexylthiophen-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blended with two different near-infrared absorbing benzannulated aza-BODIPY dyes, difluoro-bora-bis-(1-phenyl-indoyl)-azamethine (1) or difluoro-bora-bis-(1-(5-methylthiophen)-indoyl)-azamethine (2), were constructed and characterized. The amount of these two aza-BODIPY dyes, within the P3HT and PCBM matrix, was systematically varied, and the characteristics of the respective devices were recorded. Although the addition of both aza-BODIPY dyes enhanced the absorption of the blends, only the addition of 1 improved the overall power conversion efficiency (PCE) in the near-infrared (IR) region. The present work paves the way for the integration of near-infrared absorbing aza-BODIPY derivatives as sensitizers in ternary composite solar cells.

Abstract:

In the photoactive film of organic solar cells, the orientation of the absorber molecules is one of the key parameters to achieve high absorption and high photocurrents as well as efficient exciton and charge transport. However, most organic absorber small molecules, such as zinc-phthalocyanine (ZnPc) or diindenoperylene (DIP) grow more or less upright standing in crystalline thin films. Considering absorption, this molecular alignment is unfavorable. In this work we control the orientation of ZnPc and DIP in crystalline absorber films by varying the substrate or organic underlayer appropriately. For this purpose, a precise evaluation of the molecular orientation and packing is important. We find that a combination of the methods variable angle spectroscopic ellipsometry (VASE) and grazing incidence X-ray diffraction (GIXRD) can fulfill this requirement. The combination of these complementary methods shows that the growth of DIP and ZnPc is nearly upright standing on weakly interacting substrates, like glass or amorphous charge transport films. In contrast, on strongly interacting metal sublayers and PTCDA templating layers, both molecules arrange in a strongly tilted orientation (mean tilt angle 54 -71 with respect to the substrate normal), inducing a significant enhancement of absorption (maximum extinction coefficient from 0.72 to 1.3 for ZnPc and 0.14 to 0.4 for DIP). However, even when deposited on metal or PTCDA sublayers, not all ZnPc and DIP molecules in the film are oriented in the desired flat-lying fashion. This highlights that classifying organic films into either solely flat lying structures or solely upright standing structures, as often made in literature, is a too simplified picture. © 2013 American Chemical Society.