Solution processed perovskite solar cells

Today’s carbon-based economy will not be sustainable in the future. Not only will the known reserves of fossil fuels, like oil, natural gas or coal, be significantly reduced within the next 100 years, but the continued burning of fossil fuels also emits greenhouse gases, which have led to a global increase in temperature, called global warming. To preserve the environment for future generations and to prepare for the time when we will inevitably run out of fossil fuel, we have to change the way we produce our primary energy and focus research and investments on renewable energy sources. While energy from wind and water is already harvested with very high efficiencies, the utilization of solar energy still offers big room for improvements. Although conventional crystalline silicon cells achieve efficiencies around 25 %, their production is very energy intensive and relies on advanced production technologies, which makes them still rather expensive. To make photovoltaics a major part of our energy landscape, an easily prepared type of solar cell consisting of cheap and abundant materials is required. Novel organometal halide perovskite-type materials fulfill these requirements and have proven to be serious competitors for conventional photovoltaics. After only four years of research they already achieve power conversion efficiencies above 20 %. This thesis introduces a fast and easy way to prepare planar heterojunction solar cells based on methylammonium lead iodide (MAPbI3). The photoactive layer is deposited in a 2-step deposition approach, where a thin film of the lead precursor is converted into the final perovskite simply by immersing it into a solution of the other component. The resulting films consist of individual crystals sizes a few 100 nm and covering the whole substrate without significant gaps or holes. Solar cells prepared by this method achieve power conversion efficiencies of 15 %. Furthermore, by adjusting the temperature of the immersion bath, the orientation of the perovskite crystals can be controlled. The orientation, together with the resulting change in efficiency and resistance, gives interesting insights into the anisotropic charge transport properties of this class of materials. Additionally, the conventionally used hole blocking layer, titanium dioxide, was replaced by one made of fullerene molecules. The efficiencies achieved by solar cells employing this kind of electron selective contact reached almost 10 %, although the reproducibility was initially very low. This was attributed to a partial dissolution of the fullerene film during the subsequent preparation steps. To increase the stability of the layer, it was photo-polymerized using UV radiation. This not only reduces the solubility and therefore increases the fraction of solar cells achieving high efficiencies; it also changed the energy levels close to the bandgap. The bandgap energy of organic lead halide perovskite materials is strongly dependent on the composition. By exchanging some or all of the iodide in MAPbI3 with bromide, the difference between valence and conduction band can be changed from 1.5 eV (pure iodide) to 2.25 eV (pure bromide). This substitution can be performed gradually, so that phase pure materials with properties in between the two extremes are obtained. The pure bromide MAPbBr3 perovskite, however, does not perform efficiently in a planar heterojunction solar cell. Its close relative based on formamidinium FAPbBr3 has also been investigated for its suitability as active solar cell material. Although it is structurally very similar to MAPbBr3, with equivalent light absorption and emission properties, a 10 fold higher efficiency was observed for the FA-based compound. This striking difference is mainly attributed to an increased photoluminescence lifetime, resulting in an increased diffusion length of the free charge carriers. Apart from their application as light absorbing materials in solar cells, perovskites have also been investigated for their application as light emitters. Depending on the perovskite used, it was possible to demonstrate red light emission (MAPbI3) or green emission (MAPbBr3)

Solution processed perovskite solar cells

Today’s carbon-based economy will not be sustainable in the future. Not only will the known reserves of fossil fuels, like oil, natural gas or coal, be significantly reduced within the next 100 years, but the continued burning of fossil fuels also emits greenhouse gases, which have led to a global increase in temperature, called global warming. To preserve the environment for future generations and to prepare for the time when we will inevitably run out of fossil fuel, we have to change the way we produce our primary energy and focus research and investments on renewable energy sources. While energy from wind and water is already harvested with very high efficiencies, the utilization of solar energy still offers big room for improvements. Although conventional crystalline silicon cells achieve efficiencies around 25 %, their production is very energy intensive and relies on advanced production technologies, which makes them still rather expensive. To make photovoltaics a major part of our energy landscape, an easily prepared type of solar cell consisting of cheap and abundant materials is required. Novel organometal halide perovskite-type materials fulfill these requirements and have proven to be serious competitors for conventional photovoltaics. After only four years of research they already achieve power conversion efficiencies above 20 %. This thesis introduces a fast and easy way to prepare planar heterojunction solar cells based on methylammonium lead iodide (MAPbI3). The photoactive layer is deposited in a 2-step deposition approach, where a thin film of the lead precursor is converted into the final perovskite simply by immersing it into a solution of the other component. The resulting films consist of individual crystals sizes a few 100 nm and covering the whole substrate without significant gaps or holes. Solar cells prepared by this method achieve power conversion efficiencies of 15 %. Furthermore, by adjusting the temperature of the immersion bath, the orientation of the perovskite crystals can be controlled. The orientation, together with the resulting change in efficiency and resistance, gives interesting insights into the anisotropic charge transport properties of this class of materials. Additionally, the conventionally used hole blocking layer, titanium dioxide, was replaced by one made of fullerene molecules. The efficiencies achieved by solar cells employing this kind of electron selective contact reached almost 10 %, although the reproducibility was initially very low. This was attributed to a partial dissolution of the fullerene film during the subsequent preparation steps. To increase the stability of the layer, it was photo-polymerized using UV radiation. This not only reduces the solubility and therefore increases the fraction of solar cells achieving high efficiencies; it also changed the energy levels close to the bandgap. The bandgap energy of organic lead halide perovskite materials is strongly dependent on the composition. By exchanging some or all of the iodide in MAPbI3 with bromide, the difference between valence and conduction band can be changed from 1.5 eV (pure iodide) to 2.25 eV (pure bromide). This substitution can be performed gradually, so that phase pure materials with properties in between the two extremes are obtained. The pure bromide MAPbBr3 perovskite, however, does not perform efficiently in a planar heterojunction solar cell. Its close relative based on formamidinium FAPbBr3 has also been investigated for its suitability as active solar cell material. Although it is structurally very similar to MAPbBr3, with equivalent light absorption and emission properties, a 10 fold higher efficiency was observed for the FA-based compound. This striking difference is mainly attributed to an increased photoluminescence lifetime, resulting in an increased diffusion length of the free charge carriers. Apart from their application as light absorbing materials in solar cells, perovskites have also been investigated for their application as light emitters. Depending on the perovskite used, it was possible to demonstrate red light emission (MAPbI3) or green emission (MAPbBr3)

Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI(3) perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (<150 degrees C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 degrees C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance

Highly Efficient Reproducible Perovskite Solar Cells Prepared by Low-Temperature Processing

In this work, we describe the role of the different layers in perovskite solar cells to achieve reproducible, similar to 16% efficient perovskite solar cells. We used a planar device architecture with PEDOT:PSS on the bottom, followed by the perovskite layer and an evaporated C-60 layer before deposition of the top electrode. No high temperature annealing step is needed, which also allows processing on flexible plastic substrates. Only the optimization of all of these layers leads to highly efficient and reproducible results. In this work, we describe the effects of different processing conditions, especially the influence of the C-60 top layer on the device performance

Preparation of Single-Phase Films of CH3NH3Pb(I1-xBrx)3 with Sharp Optical Band Edges.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH3NH3PbI1.2Br1.8. We demonstrate a bromide-iodide lead perovskite film (CH3NH3PbI1.2Br1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.We acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme (Cambridge) for the Physics of Sustainability. THT acknowledges funding from Cambridge Australia Scholarships and the Cambridge Commonwealth Trust. D.C. acknowledges support from St. John's College Cambridge and the Winton Programme (Cambridge) for the Physics of Sustainability.This is the final published version. It's also available at: http://pubs.acs.org/doi/abs/10.1021/jz501332v

Bright light-emitting diodes based on organometal halide perovskite.

Solid-state light-emitting devices based on direct-bandgap semiconductors have, over the past two decades, been utilized as energy-efficient sources of lighting. However, fabrication of these devices typically relies on expensive high-temperature and high-vacuum processes, rendering them uneconomical for use in large-area displays. Here, we report high-brightness light-emitting diodes based on solution-processed organometal halide perovskites. We demonstrate electroluminescence in the near-infrared, green and red by tuning the halide compositions in the perovskite. In our infrared device, a thin 15 nm layer of CH3NH3PbI(3-x)Cl(x) perovskite emitter is sandwiched between larger-bandgap titanium dioxide (TiO2) and poly(9,9'-dioctylfluorene) (F8) layers, effectively confining electrons and holes in the perovskite layer for radiative recombination. We report an infrared radiance of 13.2 W sr(-1) m(-2) at a current density of 363 mA cm(-2), with highest external and internal quantum efficiencies of 0.76% and 3.4%, respectively. In our green light-emitting device with an ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag structure, we achieved a luminance of 364 cd m(-2) at a current density of 123 mA cm(-2), giving external and internal quantum efficiencies of 0.1% and 0.4%, respectively. We show, using photoluminescence studies, that radiative bimolecular recombination is dominant at higher excitation densities. Hence, the quantum efficiencies of the perovskite light-emitting diodes increase at higher current densities. This demonstration of effective perovskite electroluminescence offers scope for developing this unique class of materials into efficient and colour-tunable light emitters for low-cost display, lighting and optical communication applications.This is the author accepted manuscript and will be under embargo until 3/2/15. The final version is published in Nature Nanotechnology: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.149.html

Towards optimum solution-processed planar heterojunction perovskite solar cells

No abstract available

Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI3 perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (&lt;150 °C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 °C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance

Highly Efficient Reproducible Perovskite Solar Cells Prepared by Low-Temperature Processing

In this work, we describe the role of the different layers in perovskite solar cells to achieve reproducible, similar to 16% efficient perovskite solar cells. We used a planar device architecture with PEDOT:PSS on the bottom, followed by the perovskite layer and an evaporated C-60 layer before deposition of the top electrode. No high temperature annealing step is needed, which also allows processing on flexible plastic substrates. Only the optimization of all of these layers leads to highly efficient and reproducible results. In this work, we describe the effects of different processing conditions, especially the influence of the C-60 top layer on the device performance

Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide

Formamidinium lead iodide (FAPbI₃) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbI₃), while maintaining a higher stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (<i>P</i>6₃<i>mc</i>) instead of the desired trigonal (<i>P</i>3<i>m</i>1) black phase formed at a higher temperature (130 °C). Our results show that employing a mixture of MAI and FAI in films deposited via a two-step approach, where the MAI content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 °C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices