The optimization of open circuit voltage upon interface energetics in organic photovoltaic devices

Mats Fahlman^a, Slawomir Braun^a, Qinye Bao^a, Xianjie Liu^a

^aDepartment of Physics, Chemistry and Biology, Linkoping University, 58183 Linkoping,

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Xianjie Liu, presentation 258
Publication date: 5th February 2015

To optimize and improve the efficiency of organic photovoltaic (OPV) devices such as polymer:fullerene bulk heterojunction (BHJ) solar cells to enable their successful commercialization, much attention has been paid to increase two photovoltaic parameters: short circuit current density (Jsc) and open circuit voltage (Voc). The upper limit of Voc in the device is the energy difference between the hole-transporting level of the donor and the electron-transporting level of the acceptor, which can be tuned by the synthesis of new polymers and/or new acceptor/fullerene derivatives to optimize donor-acceptor (D-A) energy level offsets. On the other hand, the photogenerated D-A charge transfer complexes have a significant influence on Voc.[1] However, strategies for Voc (and overall efficiency) improvement based on this effect are less explored. Here, we explore the effect of energy level alignment at the BHJ on device efficiency using a variety of experimental techniques and interface modeling, in which we focused on a series of regioregular-poly(3-hexylthiophene):fullerene BHJ OPV devices with pinned electrodes. It demonstrated that integer charge transfer states [2] present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on Voc [3]. The effective energy gap including possible potential steps at the BHJ that is the relevant parameter to the Voc upper limit. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor–acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in Voc for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Furthermore, EPR measurements confirm the creation of integer charge transfer states at the BHJ in the dark and together with photo-induced absorption measurements which show that the D-A coupling strength and recombination-induced loss can be estimated using the so-called pinning energies. Based on the results, novel design rules for optimizing Voc of BHJ solar cells are proposed, and the design rules are tested against a series of high performing donor polymer: fullerene solar cell devices available in literature, and excellent correlation is obtained.

[1] K. Vandewal, et al. Nat. Mater.2014, 13, 64. [2] S. Braun, et al. Adv. Mater. 2009, 21, 1450. [3] Q. Y. Bao, et al. Adv. Funct. Mater. 2014, 24, 6309.

nanoGe is a prestigious brand of successful science conferences that are developed along the year in different areas of the world since 2009. Our worldwide conferences cover cutting-edge materials topics like perovskite solar cells, photovoltaics, optoelectronics, solar fuel conversion, surface science, catalysis and two-dimensional materials, among many others.

MATSUS

Previously nanoGe Spring Meeting (NSM) and nanoGe Fall Meeting (NFM), MATSUS is a multiple symposia conference focused on a broad set of topics of advanced materials preparation, their fundamental properties, and their applications, in fields such as renewable energy, photovoltaics, lighting, semiconductor quantum dots, 2-D materials synthesis, charge carriers dynamics, microscopy and spectroscopy semiconductors fundamentals, etc.

International Conference on Hybrid and Organic Photovoltaics

International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.

Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics

The main topics of the Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP) are discussed every year in Asia-Pacific for gathering the recent advances in the fields of material preparation, modeling and fabrication of perovskite and hybrid and organic materials. Photovoltaic devices are analyzed from fundamental physics and materials properties to a broad set of applications. The conference also covers the developments of perovskite optoelectronics, including light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.

International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics

The International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics (NIPHO) is the best place to hear the latest developments in perovskite solar cells as well as on recent advances in the fields of perovskite light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.