Organic Solar Cells: Fundamentals, Working Principle and Device Structures

Abstract

New photovoltaic energy technologies are helping to provide ecologically acceptable renewable energy sources while also lowering carbon dioxide emissions from fossil fuels and biomass. Organic photovoltaic (OPV) technology is a novel type of solar technology based on conjugated polymers and small molecules. These solar cells have enticed triable attention in recent years due to their potential of providing mechanical flexible, light weight, low cost and environmental friendly solar cells with highly tunable electrical and chemical properties. In particular, bulk-heterojunction organic solar cells (OSCs) made up of a blend of a p-type conjugated polymer as a donor and an n-type semiconductor as an acceptor is thought to be a viable method. The fundamental physics of OSCs, their operating mechanism, novel materials used and device architectures are discussed in this chapter. The technological development for large-area fabrication and the studies on stability issues of the flexible OSCs will be the main focus of the researchers in the next step. The chapter also reviews the present state of OSC production and the problems that it faces, as well as issues of stability and deterioration.

Charge-Transfer State Formation and Recombination Dynamics in Deep Absorbing Porphyrin–PCBM Blends

“…The peak at 797 nm is significantly red-shifted compared to the analogous peak in solution at 741 nm by 56 nm and is attributed to the strong intermolecular π−π stacking in the closely packed neat film and indicates the formation of optically active J-aggregates. 34,35 The absorption spectrum of the blend film of ZnP/ [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) is shown in Figure 1g. The blend film was prepared from a mixed solution in chlorobenzene (1:1.2, w/ w), and it shows absorption peaks at 466, 573, and 795 nm, with the shoulder peak appearing at 731 nm.…”

“…On excitation, excitons are bound strongly with binding energies ranging from 100 to 1000 meV and are classified as Frenkel excitons. 6 These bound excitons require an intricate architecture of the donor− acceptor interface to produce free carriers. 5 The most studied architecture in OPVs that is easy to implement is the wellknown bulk heterojunction (BHJ) solar cells.…”

“…Organic materials possess a low dielectric constant, thus inducing poor screening of charge carriers in the lattice. On excitation, excitons are bound strongly with binding energies ranging from 100 to 1000 meV and are classified as Frenkel excitons . These bound excitons require an intricate architecture of the donor–acceptor interface to produce free carriers .…”

“…The donor and acceptor layers are phase-separated in the active layers in nanometersized domains; thus, a huge interface remains for charge separation. 6 These domains are interpenetrated and form a bicontinuous pathway to the electrodes where charges are collected giving overall currents. These electron-donating and -accepting materials in the photoactive layer are phaseseparated at a typical length scale of a few tens of nanometers, imposed by the limited exciton diffusion length in organic semiconductors.…”

“…BHJ solar cells incorporate a solution-processed light-absorbing active layer which is an intimately mixed blend composed of electron donor and acceptor organic molecules. , The electron donor is a panchromatic absorber, and the acceptor serves as the interface for charge dissociation. The donor and acceptor layers are phase-separated in the active layers in nanometer-sized domains; thus, a huge interface remains for charge separation . These domains are interpenetrated and form a bicontinuous pathway to the electrodes where charges are collected giving overall currents.…”