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Two classes of thin film solar cells, hybrid perovskite and polymer solar cell are introduced. Research advancement and prospects of respective solar cells will be discussed. From the conceptual understanding point of view, transient spectroscopy studies, charge-transport studies, optical and electrical modelling of thin-film solar cells devices (device physics), and structural, morphological studies using Neutron and X-ray probe scattering and diffraction methods; need to be looked into. Hybrid perovskite solar cells emerged at the forefront by surpassing existing cost-effective low temperature processed solar cell technologies, with certified efficiencies reached from 3.0% to 22.1 % within eight years of their first introduction (2009-2016) [1, 2]. Largest optical absorption coefficient (105 cm-1) with direct-band gap (1.4-1.6 eV) of methylammonium lead iodide (CH3NH3PbI3) hybrid semiconductor material pave the way for successful application of it used as a best solar absorber material in the photovoltaic applications. Furthermore low excition binding energy (<10 meV) and micrometer order carrier (e/h) diffusion lengths of CH3NH3PbI3 bring efficient charge (e/h) generation and transport of free charges (e/h) for extraction to the respective electrodes respectively [3, 4]. Electrodes with matching work-function to the CH3NH3PbI3 with interface modifying layers complete the thin film solar cell device structure (e.g. Glass/ITO/PCDTBT/CH3NH3PbI3/PCBM/Ag). Using Transfer Matrix Optical model with the inputs of optical constants (n & k); optimised thickness values of individual layers can be predicted prior to the (hybrid perovskite) solar cell device making to achieve efficient photoconversion efficiencies [5]. Inspire of their (hybrid perovskite solar cells) highest efficiencies reported so far, there are few serious issues: toxicity of lead (Pb) usage, instability (chemical, atmospheric, photo, and thermal); challenging the researchers to study and understand fundamental properties of hybrid perovskite semiconductors; instead of focussing on climbing the photoconversion efficiency ladder. Polymer solar cells are flexible as they are made with polymers (contain only less weight elements C, H, O, N etc.), cost-effective in making with solution processing methods, large area deposition feasibility, and low-temperature process (less than 200 degree Celsius) making. However photo conversion efficiencies are less than single digit; and only recently [6] it is reported 11.7% certified efficiency (single junction) using the novel PffBT4T-C9C13 donor polymer semiconductor blended with PC71BM acceptor semiconductor in the bulk heterojunction (BHJ) geometry. Over the years, synthesis of novel polymer semiconductors (e.g. P3HT, PCDTBT, PTB7, DTPyT etc.) drove the journey of polymer solar cells towards increased photo conversion efficiencies, yet, major hurdle of polymer solar cells is the high-exciton binding energy (200-400 meV is more than room-temperature thermal energy 25 meV). Inventing new high-dielectric constant (exciton binding energy inversely proportional to square of dielectric constant) polymers with improved mobility will lead to the advancement and commercialisation of polymer solar cells.
References:
1. Kojima, A., K. Teshima, Y. Shirai and T. Miyasaka, Journal of the American Chemical Society, 2009, 131, 6050-6051.
2. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
3. Q. Lin, A. Armin, Ravi Chandra Raju. N, P.L. Burn, and P. Meredith, Nature Photonics, 2015, 9, 106-112.
4. S. D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. P. Alcocer, T.Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science, 2013, 341-344.
5. http://web.stanford.edu/group/mcgehee/transfermatrix/index.html
6. J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Nature Energy, 2016, 1, 1-7. |