The increase in energy demand due to increase in population and reduction of fossil fuels

has led to the search of alternative energy sources. Solar energy, which is an alternative source of

energy, has been in the fore front of this research. Various methods of converting solar energy into

electricity has been attained by silicon solar cells, thin film solar cells, dye sensitized solar cells and

perovskite solar cells. Unlike dye sensitized solar cells, perovskite solar cells based on lead halide

perovskite or organic-inorganic halide perovskite have had a significant impact on photovoltaic devices.

A power conversion efficiency of approximately 20.1% has been attained by perovskite solar cells

compared to organic solar cells. Also, several fabrication techniques, hole and electron transport

materials have been developed for high performance. Although, some issues need to be addressed

before commercialization is possible. Issues like stability of the cells under moisture and temperature.

In this review, fundamental aspects of the perovskite device and recent breakthroughs are illustrate

Akinyemi, M. L.

Boyo, A. O.

Kesinro, O. R.

Covenant University Repository

Journal of Informatics and Mathematical SciencesVol. 9, No. 2, pp. 485–491, 2017ISSN 0975-5748 (online); 0974-875X (print)Published by RGN Publications http://www.rgnpublications.comProceedings ofInternational Conference on Science and Sustainable Development (ICSSD)“The Role of Science in Novel Research and Advances in Technology”Center for Research, Innovation and Discovery, Covenant University, NigeriaJune 20-22, 2017Survey ArticleAdvances in Perovskite-Based Solar CellsO.R. Kesinro1, M.L. Akinyemi1 and A.O. Boyo21Department of Physics, Covenant University, Ota, Ogun State, Nigeria2Department of Physics, Lagos State University, Lagos, Nigeria*Corresponding author: olakesinro02@gmail.comAbstract. The increase in energy demand due to increase in population and reduction of fossil fuelshas led to the search of alternative energy sources. Solar energy, which is an alternative source ofenergy, has been in the fore front of this research. Various methods of converting solar energy intoelectricity has been attained by silicon solar cells, thin film solar cells, dye sensitized solar cells andperovskite solar cells. Unlike dye sensitized solar cells, perovskite solar cells based on lead halideperovskite or organic-inorganic halide perovskite have had a significant impact on photovoltaic devices.A power conversion efficiency of approximately 20.1% has been attained by perovskite solar cellscompared to organic solar cells. Also, several fabrication techniques, hole and electron transportmaterials have been developed for high performance. Although, some issues need to be addressedbefore commercialization is possible. Issues like stability of the cells under moisture and temperature.In this review, fundamental aspects of the perovskite device and recent breakthroughs are illustrated.Keywords. Hole transport material; Electron transport material; Perovskite; StabilityMSC. 68Q80Received: May 20, 2017 Revised: July 1, 2017 Accepted: July 26, 2017Copyright © 2017 O.R. Kesinro, M.L. Akinyemi and A.O. Boyo. This is an open access article distributed underthe Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in anymedium, provided the original work is properly cited.486 Advances in Perovskite-Based Solar Cells: O.R. Kesinro, M.L. Akinyemi and A.O. Boyo1. IntroductionSolar energy is a form of renewable energy, which is an alternative source to traditional resourcessuch as coal. In this regard, developing solar cells is one of the most important approaches togain electrical energy from the conversion of solar energy with sunlight being the source ofenergy. The source of energy would be sunlight. Silicon-based solar cells have been used due totheir efficient solar to power generation (∼30%) particularly crystalline silicon [1]. The cost ofsilicon based solar cells is relatively high and difficult to utilize in large scale industrial areas.Generally solar cells can be classified as (i) first generation solar cells (mono crystalline and polycrystalline silicon), (ii) second generation solar cells [amorphous silicon (a-Si), microcrystallinesilicon, copper indium gallium selenide (CIGS), and cadmium telluride (CdTe)] and (iii) thirdgeneration solar cells (polymer solar cells, oligomers, dye synthesized solar cells (DSSCs), andorganic/inorganic hybrid perovskite solar cells). The third generation photovoltaic devices, whichhave been developed based on various dye sensitizers, organic-inorganic materials have reachedefficiency up to ∼ 15-20% ([2], [3]).Perovskites are materials used in the fabrication of solar cells and have reached an efficiencyof 20% within 5 years, due to substantial improvement of power conversion efficiency (PCE) andlow processing cost [4]. Perovskite named after the Russian mineralogist L. A. Perovski has aspecific crystal structure with MAX3(X = oxygen or halogen). The larger M cation considered asan organic, which is generally methylammonium (CH3NH3+) Ethylammonium (CH3CH2NH3+)and Formamiinium (NH2CH=NH2) occupies a cubo-octahedral site shared with twelve X anions([5], [6]). The cation B, Lead (Pb) is universally considered for stabilization efficiency in solarcells. Although Tin (Sn) has similar band gap (1.55 eV) and in the same group as Pb, due to theease of oxidation and lack of stability, it performs poorly compared to Pb in efficiency.The most studied perovskite are oxides due to their electrical properties of ferroelectricityor superconductivity. Halide perovskites received little attention until layered organometalhalide perovskite were reported to exhibit a semiconductor-to-metal transition with increasingdimensionality [7]. The working principles for perovskite solar cells are different to that ofDSSC, they are more like a thin film semiconductor solar cell rather than a device which hasa separate sensitizer. The perovskite solar cell is made up of different layers namely ElectronTransport Layer (ETL), Hole Transport Layer (HTL) and Electrode.The first reported perovskite solar cell employing CH3NH3PbI3 achieved an efficiencyof 3.81% [8], changes to the device geometry and architecture allowed for a rise in deviceperformance for which the highest reported value is 20.1% ([4], [10]).Unlike silicon solar cells, perovskite solar cells are solution processable with diversity,abundance and large scale manufacturing. Also, they have advantages including mechanicalflexibility, light weight, low material consumption, color turning and less toxicity. However, poorstability of perovskite solar cells to temperature and moisture has been its major limitations.Journal of Informatics and Mathematical Sciences, Vol. 9, No. 2, pp. 485–491, 2017Advances in Perovskite-Based Solar Cells: O.R. Kesinro, M.L. Akinyemi and A.O. Boyo 487Figure 1. (a) Generic perovskite crystal unit cell structure. (b) Typical structure of PSCs2. Progress In Organic – Inorganic Halide Perovskite Solar CellsReference [11] formed a planar perovskite solar cell by replacing TiO2 with SnO2 as the electronselective layer. It was formed by spin coating SnO2 nanoparticles and sintering at 200oC for1 hour which is a lower temperature than that required for TiO2. The perovskite layer wasdeposited using two step deposition process. A solvent vapor annealing step was included tocrystallize the PbI2 crystallites into the SnO2/CH3NH3PbI3 interface. A cell efficiency of 13%was achieved. Also, SnO2 devices that were not encapsulated and stored in ambient conditions,the measured PCE were quite stable for 700 hours. This is significantly more stable than TiO2based devices.Reference [12] made use of chemical engineering to alter the stability of the commonlyused CH3NH3PbI3. They included an additive butylphosphonic acid 4-ammonium chloride (4-ABPACl) into a one step spin coating method to modify the CN3NH3PbI3 surface. It was observedthat these additive molecules bond to the surface of CH3NH3PbI3, rather than becomingintegrated into the perovskite crystal structure. The inclusion of this cross linking additiveleads to the formation of improved infiltration into the underlying TiO2. The improvementgives rise to an increased efficiency higher than 16%, almost double that of the control device.Measuring the device stability at 55% relative humidity within the first week, the control devicedegraded rapidly indicative of the moisture induced decomposition of the perovskite layer. The4-ABPACl-modified devices remained relatively constant during the testing period.Reference [13] described the role of different layers in perovskite solar cells to achieve aPCE of 16%. Vacuum Assisted One- Step solution (VAOS) method was reported for producinghigh quality perovskite layers. A compact and pin-hole free perovskite layer by accelerating thecrystallization process by the VAOS method. The mixed halide perovskite solution was spincoated on the Indium doped tin oxide (ITO) / poly-(2,3-dihydrothieno-1, 4-dioxin)-poly(styrene-sulfonate) (PEDOT: PSS) substrate. C60 layer is used to act as a selective electron extractionJournal of Informatics and Mathematical Sciences, Vol. 9, No. 2, pp. 485–491, 2017488 Advances in Perovskite-Based Solar Cells: O.R. Kesinro, M.L. Akinyemi and A.O. Boyolayer on the solar cell. C60 layers with different thickness have been thermally evaporatedon top of the perovskite film. For further optimization, lithium fluoride (LiF) was used as aninterfacial layer between ETL and metal contact. A PCE of 13.3% was achieved with LiF and12.6% without LiF.Reference [14] also developed a simple and robust process to prepare efficient perovskitesolar cells. Pinhole free thin films of CH3NH3PbI3−xClx were coated on high conductivityPEDOT:PSS. The process utilized PCBM as ETL with a PCE greater than 12%. Differentdeposition procedures were tested and characterized using scanning electron microscope. Also,the PEDOT:PSS was deposited together with DMSO and zonyl additives. A device architectureof ITO/PEDOT:PSS/perovskite/PCBM/Al was fabricated. Different PCEs were determined bycurrent-voltage characterization by sweeping from negative to positive values and vice versa.The devices showed low leakage currents at reverse bias and good stability when operatedunder continuous solar illumination. A PCE in the range of 12-12.5% was attained.Reference [15] used thin film PT instead of PEDOT:PSS film. It was deposited byelectrochemical polymerization. The thickness of the PT layer was 5nm, significantly thickerthan other HTLs. A PCE of 15.4% was attained for the device which is higher than referencePEDOT:PSS. The stability of the PT/CH3NH3PbI3 device remained stable over a 816 hour testperiod compared to PEDOT:PSS reference. Zhang et al, 2015 investigated a new structure ofCH3NH3PbI2Br and P3HT. Spiro-OMeTAD was replaced with P3HT. The devices using thiscombination attained a device performance of 6.64%. The stability of such devices was testedfor non-encapsulated devices over a test period of 250h, the efficiency remained stable.Reference [13] showed that the crystallization of the initial PbI2 layer can be retarded by theuse of N,N-dimethylsulfoxide (DMSO) as the solvent instead of N,N-dimethylformamide (DMF),which is typically, used resulting in more controlled particle sizes, more complete conversion andincreased reproducibility of film quality. Burschka et al. [7] infiltrated a mesoporous TiO2 filmwith solution containing PbI2 and subsequently converted the film to CH3NH3PbI3 perovskiteby exposing it to a solution of MAPI. Devices fabricated using the mesostructured TiO2 designsachieved a PCE of 15.3%.Reference [18] demonstrated the use of solution processed Cu-doped NiOx HTL in theconstruction of high performance planar heterojunction perovvskite (CH3NH3PbI3) solar cells.It was noted that the electrical conductivity and favorable crystallization of the perovskite wasdue to the Cu-doping. A PCE of up to 15.40% was obtained with decent environmental stability.Also, different HTLs were fabricated and compared with Cu:NiOx, they included NiOx andPEDOT:PSS. The different HTLs were spin coated on substrates. Then, perovskite layer wasspin coated afterward PCBM and C60-bis surfactant were spin coated sequentially as ETL andcathode interfacial layer respectively. A PCE of up to 15.40%, 11.16% and 8.94% was obtainedfor Cu:NiOx, PEDOT:PSS and NiOx respectively.Journal of Informatics and Mathematical Sciences, Vol. 9, No. 2, pp. 485–491, 2017Advances in Perovskite-Based Solar Cells: O.R. Kesinro, M.L. Akinyemi and A.O. Boyo 4893. Methods for Fabricating Perovskite Solar Cells3.1 Solution Processed Two-Step MethodTwo step sequential depositions was first proposed by [18], where PbI2 was deposited onsubstrate prior to CH3NH3I treatment by either vacuum evaporation or spin coating. The PbI2coated substrate will be dipped in CH3NH3I solution. Saturated methanol solution of PbI2 willbe used as precursor solution for spin coating process. A similar procedure will be performed,where PbI2 layer is formed on the mesoporous TiO2 film by spin coating a PbI2 solution in N,N-dimethylformamide (DMF) at 70oC. The dried PbI2 film is dipped in a solution of CH3NH3Iin 2-propanol for 20s. A certified PCE of 14.1% has been achieved using two step method.3.2 Solution-Processed Single Precursor and anti Solvent MethodOne step method was discovered to be better in terms of minimizing the processing step.However, solar cell performance made by simple a one step method is inferior to two step resultsbecause of significant difference in morphology of the CH3NH3PbI3 [10].The precursor of CH3NH3I and PbI2 will be dissolved in the mixed solution of N,N-dimethylsulfoxide (DMSO) and GBL, which will be spin coated on the substrate. Anti-solventsuch as toluene will be dripped while spin coating the precursor solution.3.3 Vacuum Deposition MethodOrgano lead halide film was prepared by vacuum deposition process. The vapor process isargued to be better than the solution process in planar heterojunction layout because the formerproduces a flat and even surface. A sequential layer by layer vapor deposition is producedsimilar to the two step solution process, where PbCl2 or PbI2 will be first deposited by thermalevaporation, which will be followed by vapor deposition of CH3NH3I. The sequential depositionwas developed because of difficulty in monitoring CH3NH3I deposition rate in co-depositionprocess.4. ConclusionPerovskite materials are in position to disrupt the present photovoltaic landscape. They will besignificant for research and commercialization of solar cells in the next few years. However, fewproblems need to be minimized with respect to commercialization: (1) Cost effectiveness. (2)stability in moisture and (3) toxicity of Pb atoms. Alternatively, perovskite absorber materialscan be coupled with dye sensitized solar cells or silicon solar cells to form higher efficiencies.Also different combination deposition methods can be worked on to determine higher powerconversion efficiencies.AcknowledegmentThe authors appreciate Covenant University for partial sponsorship.Journal of Informatics and Mathematical Sciences, Vol. 9, No. 2, pp. 485–491, 2017490 Advances in Perovskite-Based Solar Cells: O.R. Kesinro, M.L. Akinyemi and A.O. BoyoCompeting InterestsThe authors declare that they have no competing interests.Authors’ ContributionsAll the authors contributed significantly in writing this article. The authors read and approvedthe final manuscript.References[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta and E.D. Dunlop, Solar cell efficiency tables (version39), Prog. Photovoltaics: Res. Appl. 20 (2012), 12 – 20.[2] G. Hodes and D. Cahen, Photovoltaics: perovskite cells roll forward, Nature Photon. 8 (2014), 87 –88.[3] R.F. Service, Perovskite solar cells keep on surging, Science 344 (2014), 458.[4] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo and S.I. Seok, High-performancephotovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015),1234 – 1237.[5] J.-H. Im„ J. Chung, S.-H. Kim and N.-G. Park, Synthesis, structure, and photovoltaic property ofa nanocrystalline 2H perovskitetype novel sensitizer (CH3CH2NH3)Pbl3, Nanoscale Res. Lett. 7(2012), 353.[6] P. Umari, E. Mosconi and F. De Angelis, Relativistic GW calculations on CH3NH3PbI3 andCH3NH3SnI3 perovskites for solar cell applications, Sci. Rep. 4 (2014), 4467.[7] A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Organometal halide perovskites as visible-lightsensitizers for photovoltaic cells, Journal American Chemical Society 131 (2009), 6050 – 6051.[8] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. 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