(Un)avoidable energy loss during carrier extraction in polymer:fullerene solar cells

Martijn Kemerink^a, Olle Inganäs^a, Armantas Melianas^a, Frédéric Laquai^b, Fabian Etzold^b, Tom Savenije^c

^aLinköping University, Sweden, SE-581 83, Linköping, Sweden

^bResearch Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Mainz, Ackermannweg, 10, Mainz, Germany

^cDelft University of Technology, The Netherlands, Julianalaan, 136, Delft, Netherlands

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, Martijn Kemerink, presentation 030
Publication date: 5th February 2015

Compared to other photovoltaic systems, organic bulk-heterojunction solar cells (OSC) suffer from large voltage losses. Although various rules of thumb exist to estimate the magnitude of this loss, its origin is still debated. Here we investigate the energy loss due to relaxation of photogenerated charge carriers in the disordered density of states (DOS). Combining experimental and numerical techniques we find that this loss channel can amount to 0.1-0.3 eV for both the electrons and the holes, i.e. 0.2-0.6 eV in total, with large variation between materials. More specifically, by a combination of time-resolved optical/electro-optical experiments and kinetic Monte Carlo simulations we show that as photo-generated charge carriers are transported to the extracting electrode they gradually lose energy to continuous thermalization in the DOS. In fact, the charge carriers are extracted before even reaching equilibrium. The free charge carrier thermalization mechanism is fundamentally different from that in inorganic photovoltaic devices. We identify it as a two-step process: 1) Following charge transfer a fast (1-100 ns) thermalization loss of the order of 1-2σ occurs (σ≈0.05-0.1 eV the DOS width), during which an average charge carrier makes multiple hops but does effectively not move any closer to the extracting electrode. The excess energy is wasted to undirected (diffusive) motion, which dominates over directed (drift) motion at early time scales. This fraction of the loss is unavoidable. 2) Gradually charge carrier drift overcomes diffusion. The charge carrier is transported to the extracting electrode, its remaining excess energy is continuously lost by further thermalization in the DOS; an additional loss of the order of 0.5-1σ occurs. This loss is reduced in thinner devices. Since thermalization occurs downhill in energy it boosts charge carrier motion, leading to a strongly time-dependent mobility, which we independently confirm by direct experiments. We identify the time and distance scales which are relevant for charge extraction in OPV devices and show that experimental techniques which probe the mobility of (almost) relaxed charge-carriers are not meaningful to OPV device operation. Therefore to correctly describe the device physics of operating OPV devices non-equilibrium effects related to the gradual thermalization of the charge carrier populations must be taken into account.
Schematic description of charge carrier thermalization in OPV. First, most of the excess energy is lost by diffusion, as indicated by the red arrow going mostly downward. At later time scales the drift component of motion gradually overcomes diffusion and extraction begins. During transport to the electrode (yellow, mostly horizontal arrow) the remainder of the excess energy is continuously lost by further thermalization. Charges are extracted from the photovoltaic device before reaching equilibrium.

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.