Rafael Gόmez and José L. Segura’s ‘Plastic Solar Cells: A Multidisciplinary Field to Construct Chemical Concepts from Current Research’ provides an overview of organic solar cells; their structure, principles and how to improve their performance . I have tried to highlight particular strengths displayed within the text and regions where improvement could be made. The review article is comprised of the following sub sections, titled: • Characteristics of Organic Solar Cells, • Structure and Principles of Operation of an Organic Solar Cell • Design of a Successful Material for Organic Photovoltaics • Conclusion. A brief introduction is provided in the review, whereby, the writers enthusiastically discuss the importance of organic photovoltaics to various scientific fields of study and reinforce the importance of educating students on this topic . Gόmez, R. Segura, J, L. (2007, pp 253) claim that the aim of their review is to ‘propose examples of plastic solar-cell technology to illustrate core concepts in chemistry’ going on to state that this information will be of interest to teachers . The aim of the review is consistent with the purpose of the journal in which it is published; Journal of Chemical Education, which as stated on its website, is a ‘resource to those in the field of chemical education’ . Within the Characteristics of Organic Solar Cells segment, the authors provide a synopsis of the processes and general parameters which affect the operation of a photovoltaic device and the basis of organic solar cells . The writers seem to describe organic solar cells as possessing a p-n junction; two contrastively doped inorganic semiconductor crystals connected at a continuous interface, sandwiched between a substrate and oxidised layer . Figure 1 below illustrates charge migration within a typical p-n junction consisting of extrinsic, inorganic semiconductors . Gόmez, R. et al (2007, pp 253) explains that the electron concentration is ‘much larger on the n side than on the p side’ . Peer reviewed literature on organic solar cells describe the device as possessing donor type and acceptor type organic substances sandwiched between a cathode and anode . Also, electrons are also present in the same concentration on the p side; however, these electrons are immobile . Hence, it would have been increasingly correct to state that the majority carriers on the p-type semiconductor are holes, whereas, those on the n-type region are electrons .
Figure 1 displays a typical p-n junction within an inorganic solar cell . Within the Structure and Principles of Operation of an Organic Solar Cell segment of the review, the writers provide a simple overview of the typical conformations of organic photovoltaic cells (OPVs), and outline the main processes which occur within the cell . Gόmez, R. Segura, J, L. (2007, pp 254) presents an accurate, clear, sequential summary of the processes which occur within an OPV: ‘(i) absorption (ii) generation of charge carriers (iii) transport of charge carriers to the electrodes (iv) collection of charges’. The summary above is illustrated pictographically within Figure 2 below . Like, Gόmez, R. Segura, J, L. (2007), Clarke, T, M. et al. (2010) chronologically summarises the processes involved in an OPV device following photo irradiation from a light source . Gόmez, R. Segura, J, L. (2007, pp 255) goes on to state that there is a ‘small chance’ for recombination of charge carriers ‘as they do not have to diffuse through the same material’ . Clarke, T, M. et al.(2010, pp 6737) opposes this view affirming that within polymer: fullerene OPVs ‘despite being located on different materials, the electron hole pairs are still expected to exhibit significant Coulomb attraction’ which could result in the unideal recombination of charges . Günes, S. et al. (2007, pp 1327-1328) emphasises the possibility of recombination in the ‘operating principles’ segment of their review, stating that the ‘transport of charges is affected by recombination during the journey to the electrodes’ . There is a clear likelihood of charge recombination as the charges travel towards the electrodes, as outlined by Clarke, T, M et al. and Günes, S et al . However, the views of Gόmez, R. Segura, J, L. (2007) may cause the reader to misinterpret this information and believe that recombination is not an issue within OPVs where in actuality, it is .
Figure 2 displays an organic solar cell: displaying the  absorption  generation of charges  recombination  charge carrier migration/collection of charges . In the final segment of the review the authors discuss methods which can be employed in order to effectively increase: solar absorbance, charge carrier mobility and dissociation of excitons within an OPV . Significantly, each of the factors mentioned is essential in formulating an efficient solar cell . The authors delegate a key issue in organic solar cell devices which are low intrinsic mobilities . To resolve this issue, Gόmez, R. et al (2007 pp 255) suggests the addition of inorganic compounds, in a bid to increase ‘electron mobility and transport’ within an OPV . Hybridised solar cells (solar devices containing both inorganic and organic photoactive compounds) have been discussed by Günes, S. et al (2007, pp 1334-1335) as an efficient charge carrier producer . Nevertheless, Günes, S. et al (2007) highlights a key solubility limitation of hybrid solar cells; inorganic substances cannot be dissolved in organic solvents, unless ligand exchange is induced to increase the inorganic substances solubility . However, the ligand will then obstruct charge transportation; furthermore, it will need to be removed . Moreover, Gόmez, R. Segura, J, L. (2007), should have provided the limitations of hybridisation in order for the reader to evaluate whether or not this technique is suitable for its particular application. In the conclusion of the review, Gόmez, R. Segura, J, L. (2007) summarises the immense promise of developing OPV devices through collaborative work . Gόmez, R. Segura, J, L. (2007, pp 257-258) refers to their initial aim, stating that the design of novel materials which will enhance the performance of OPVs should be ‘incorporated into the chemistry curriculum’ . However, the writers haven’t clarified how the readers should implement this. Furthermore, a clear guide should have been provided within the text, of important, curriculum based chemical principles that should be taught. This was the aim of the text and could have been referred to in its main body to emphasise its intent. I have reasoned that while the review provides a clear, summative, comprehensible insight into organic photovoltaic devices, there are a few incidences where the writer provides inadequate information on a particular matter. Such as, underestimating the effects of charge recombination within OPVs, not providing the limitations of the hybridisation of inorganic compounds and not meeting the criterion of the aim . Further development on the direct significance of organic photovoltaics in the chemical discipline is required for educators and students alike.
By Naeema Ebrahim
References  Gόmez, R. Segura, J, L. Plastic Solar Cells: A Multidisciplinary Field to Construct Chemical Concepts from Current Research, Journal of Chemical Education, 84 (2), 253-285, American Chemical Society, Journal of Chemical Education, [online], Available at: http://pubs.acs.org/page/jceda8/about.html [Accessed date: 15/10/2014]  Zambuto, M. Semiconductor Devices, 1989, international ed, McGraw-Hill inc.  University of Wisconsin-Madison [online] Available at: http://education.mrsec.wisc.edu/SlideShow/slides/pn_junction/pn_junction_solar.html [Accessed date: 29/10/2014] Clarke, T, M. Durrant, J, R. Charge Photogeneration in Organic Solar Cells, 2010, Chemical Reviews, 110, 6736-6767.  Elite Network of Bavaria, [online] Available at: https://www.elitenetzwerk.bayern.de/uploads/tx_templavoila/Text_Foto_560_350.jpg [Accessed date: 29/10/2014] Günes, S. Neugebauer, H. Et al. Conjugated Polymer-Based Organic Solar Cells, Chemical Review, 2007, 107, 1324-1338.  Brabec, C, J. Sariciftci, N, S. Et al. Plastic Solar Cells, Advanced Functional Materials, 2001, 11, 15-26.  Brabec, C, J. Gowrisanker, S. et al. Polymer-Fullerene Bulk-Heterojunction Solar Cells, Advanced Materials, 2010, 22, 3839-3856. Yang, X. Uddin, A. Effect of thermal annealing on P3HT: PCBM Bulk-Heterojunction Organic Solar Cells: A Critical Review, Renewable and Sustainable Energy Reviews, 2014, 30, 324-336.  Gan, Q. Bartoli, F, J. et al. Research Highlights on Organic Photovoltaics and Plasmonics, Institute of Electrical and Electronics Engineers, 2012, 4, 620-624.  Zhu, H. Wei, J. et al. Application of Carbon Materials in Photovoltaic Solar Cells, Solar Energy Materials & Solar Cells, 2009, 1461-1470.  Streetman, B, G. Solid State Electronic Devices, 1995, 4th ed, Prentice-Hall International Inc.  Kenyon, A, J. Recent Developments in Rare earth Doped Materials for Optoelectronics, Progress in Quantum Electronics, 2002, 26, 225-284, Elsevier.  Schmidt-Mende, L. et al. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics, Science, 2001, 293, 1119-1122.  Carrasco-Orozco, M. Tsoi, W, C. et al. New Photovoltaic Concept: Liquid-Crystal Solar Cells Using a Nematic Gel Template, Advanced Materials, 2006, 18, 1754-1758.  Benati, T, L. Venkataraman, D. Organic Solar Cells: An Overview Focusing on Active Layer Morphology, Photosynthesis Research, 2006, 87, 73-81.