The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Conjugated polymer-based organic solar cells

There has been an intensive search for cost-effective photovoltaics since the development of the first solar cells in the 1950s. [1–3] Among all alternative technologies to silicon-based pn-junction solar cells, organic solar cells could lead the most significant cost reduction. [4] The field of organic photovoltaics (OPVs) comprises organic/inorganic nanostructures like dyesensitized solar cells, multilayers of small organic molecules, and phase-separated mixtures of organic materials (the bulkheterojunction solar cell). A review of several OPV technologies has been presented recently. [5] Light absorption in organic solar cells leads to the generation of excited, bound electron– hole pairs (often called excitons). To achieve substantial energy-conversion efficiencies, these excited electron–hole pairs need to be dissociated into free charge carriers with a high yield. Excitons can be dissociated at interfaces of materials with different electron affinities or by electric fields, or the dissociation can be trap or impurity assisted. Blending conjugated polymers with high-electron-affinity molecules like C60 (as in the bulk-heterojunction solar cell) has proven to be an efficient way for rapid exciton dissociation. Conjugated polymer–C60 interpenetrating networks exhibit ultrafast charge transfer (∼40 fs). [6,7] As there is no competing decay process of the optically excited electron–hole pair located on the polymer in this time regime, an optimized mixture with C60 converts absorbed photons to electrons with an efficiency close to 100%. [8] The associated bicontinuous interpenetrating network enables efficient collection of the separated charges at the electrodes. The bulk-heterojunction solar cell has attracted a lot of attention because of its potential to be a true low-cost photovoltaic technology. A simple coating or printing process would enable roll-to-roll manufacturing of flexible, low-weight PV modules, which should permit cost-efficient production and the development of products for new markets, e.g., in the field of portable electronics. One major obstacle for the commercialization of bulk-heterojunction solar cells is the relatively small device efficiencies that have been demonstrated up to now. [5] The best energy-conversion efficiencies published for small-area devices approach 5%. [9–11] A detailed analysis of state-of-the-art bulk-heterojunction solar cells [8] reveals that the efficiency is limited by the low opencircuit voltage (Voc) delivered by these devices under illumination. Typically, organic semiconductors with a bandgap of about 2 eV are applied as photoactive materials, but the observed open-circuit voltages are only in the range of 0.5–1 V. There has long been a controversy about the origin of the Voc in conjugated polymer–fullerene solar cells. Following the classical thin-film solar-cell concept, the metal–insulator–metal (MIM) model was applied to bulk-heterojunction devices. In the MIM picture, Voc is simply equal to the work-function difference of the two metal electrodes. The model had to be modified after the observation of the strong influence of the reduction potential of the fullerene on the open-circuit volt

Design Rules for Donors in Bulk‐Heterojunction Solar Cells—Towards 10 % Energy‐Conversion Efficiency

A polymer solar-cell based on a bulk hetereojunction design with an internal quantum efficiency of over 90% across the visible spectrum (425 nm to 575 nm) is reported. The device exhibits a power-conversion efficiency of 6% under standard air-mass 1.5 global illumination tests.

Bulk heterojunction solar cells with internal quantum efficiency approaching 100

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer-based organic photovoltaic systems hold the promise for a cost-effective, lightweight solar energy conversion platform, which could benefit from simple solution processing of the active layer. The function of such excitonic solar cells is based on photoinduced electron transfer from a donor to an acceptor. Fullerenes have become the ubiquitous acceptors because of their high electron affinity and ability to transport charge effectively. The most effective solar cells have been made from bicontinuous polymer–fullerene composites, or so-called bulk heterojunctions. The best solar cells currently achieve an efficiency of about 5 %, thus significant advances in the fundamental understanding of the complex interplay between the active layer morphology and electronic properties are required if this technology is to find viable application.

Polymer–Fullerene Composite Solar Cells

This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Polymer solar cells

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200690091

High Photovoltaic Performance of a Low‐Bandgap Polymer

High Photovoltaic Performance of a Low-Bandgap Polymer. Adv Mater 18:2884

We designed and synthesized a series of conjugated polymers containing alternating electron-donating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5‘-[2,2‘]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]), and their absorption characteristics can be tuned by adjusting the ratio of the two electron-donating units:  (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene) and 5,5‘-[2,2‘]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as ...

Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications

The design of novel conjugated polymers suitable for use in plastic solar cells is one of today's challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3
				(poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5
				(poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61
				(PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm−2 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current–voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.

David Mühlbacher

Papers

Design Rules for Donors in Bulk‐Heterojunction Solar Cells—Towards 10 % Energy‐Conversion Efficiency

High Photovoltaic Performance of a Low‐Bandgap Polymer

High Photovoltaic Performance of a Low-Bandgap Polymer. Adv Mater 18:2884

Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications

Photovoltaic action of conjugated polymer/fullerene bulk heterojunction solar cells using novel PPE-PPV copolymers