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

Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

https://yylab.seas.ucla.edu/papers/yy-pv-7-slow growth-nm-05.pdf

High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends

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

By applying the specific fabrication conditions summarized in the Experimental section and post-production annealing at 150 °C, polymer solar cells with power-conversion efficiency approaching 5 % are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron-collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells.

Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology

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

The monochromatic external quantum efficiency of a bulk heterojunction photodetector based on a blend of poly-3(hexylthiophene) with a methanofullerene is reported to be as high as 76% at the peak maximum at 25 °C. Analysis of the temperature dependence, the illumination intensity dependence together with absorption measurements in reflection geometry, allow calculation of the internal quantum efficiency of the device close to 100% at the peak maximum. Recombination of photoinduced carriers is negligible or even absent in these photodetectors when operated in the photovoltaic mode. Optical losses in these bulk heterojunction devices are analyzed.

Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors

We investigate thin poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl C61 butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing-incidence X-ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low-photon-energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.

Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells†

The essential cost-driving factor for the production of classical photovoltaic devices is the expensive investment in costly semiconductor processing technologies. This unfavorable cost structure has so far prohibited the technology from having a significant impact on global energy production. Nevertheless, the continued high interest in photovoltaics originates from the fact that they represent the only truly portable renewable-energy conversion technology available today. Therefore, the potential of fabricating organic photovoltaic elements on low-cost, thin plastic substrates by standard printing and coating techniques and packaged by lamination is not only intriguing, but highly attractive from a cost standpoint. In this article, we discuss the economic and technical production aspects for organic photovoltaics.

Production Aspects of Organic Photovoltaics and Their Impact on the Commercialization of Devices

An extended replacement circuit describing the current–voltage characteristics of bulk heterojunction polymer solar cells at different light bias levels is introduced and discussed. A one diode-model is expanded by an extraction model for photogenerated carriers taking into account the effective reduction of the mean distance which the charge carriers cover when sweeping the electrical bias through the fourth quadrant of the solar cell. The model properly describes the current–voltage behavior of bulk heterojunction solar cells over more than three orders in light intensity with one set of parameters.

Christoph Waldauf

Papers

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

Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors

Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells†

Production Aspects of Organic Photovoltaics and Their Impact on the Commercialization of Devices

Simulation of light intensity dependent current characteristics of polymer solar cells