Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.

Photovoltaics literature survey (no. 149)

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Indonesia is an archipelagic nation that has many small islands where the average load is low and currently supplied by diesel power generators. The drawbacks from these generators are cost constraints from its operation. Solar cells are the solution of this problem with the support of daily average radiation in Indonesia of 4.8 kWh/m2/day. There has been a lot of technology for the construction of solar cells such as silicon based, copper indium gallium selenide (CIGS), which was already successfully commercialized. However, these technologies have been obsolete and started to reach its maximum potential. Perovskite solar cells have a very high future potential, due to the increase on the efficiency of this technology in a relatively short amount of time. The current challenge for the fabrication of perovskite solar cell is the material cost and fabrication cost. This paper discussed the low-cost fabrication of perovskite solar cell using only spin coating deposition method and relatively also low-cost materials for the structure of the perovskite solar cell itself. As a result, we achieve perovskite solar cell with VOC of 0.6 V, ISC of 13 mA, FF of 0.28, and 1.2% efficiency.

Lead-free tin perovskite solar cells

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

There are many materials for making solar cells such as silicon, copper indium gallium selenide (CIGS), which have been successfully commercialized. However, this technology has been obsolete and has begun to reach its maximum efficiency potential. Perovskite solar cells (PSCs) have a very high future potential, because in a relatively short time this technology produces the efficiency of solar cells that are almost equal to silicon-based solar cells. Organic-metal halide perovskite material has been studied in several years because of its ability as an active layer of perovskite solar cells. The current challenge for the fabrication of perovskite solar cell based on organic-metal halide material is the device stability, and lead toxicity of perovskite. By optimizing the spin coating rotational speed of perovskite and hole transport layers we describe in this paper performance of fabricated perovskite solar cell which uses organic-metal halide as an active layer. This paper discusses also the inexpensive fabrication of perovskite solar cells using spin coating deposition methods and is also relatively low-cost materials for the perovskite solar cells structure themselves.

Recently, hybrid processing using evaporation and spin-coating has been applied for the fabrication of perovskite silicon tandem solar cells with record power conversion efficiencies (PCEs) of 25.2%. To achieve large area, high-throughput production of solar cells, spray-coating, a well-known scalable method used in the industry, has advantages over spin-coating. Therefore, we apply a hybrid method using the evaporation of PbI2 and spraying of methylammonium iodide (MAI) for the fabrication of perovskite solar cells. This article presents a systematic study of sprayed MAI amount and processing parameters on the performance of solar cells. We find clear correlations between the processing parameters of the sprayed MAI, the crystallinity of CH3NH3PbI3 and the PCE of the resulting PSCs. High crystallinity and morphology of the CH3NH3PbI3 film is achieved using a slow nozzle speed of 30 mm·s−1. An average PCE of 11.6% ± 3.1% (maximum PCE of 14.3%, stabilized PCE of 10%) was achieved with a normalized MAI amount of 3.4, which represent the flow rate of 0.17 ml·min−1, nozzle speed of 30 mm·s−1, MAI concentration of 9 mg·mL−1, and one single spray pass. Moreover, we successfully show a proof-of-concept of this method for the deposition of homogenous perovskite layers on pyramidal textured silicon.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Recently, there has been a remarkable improvement in the efficiency of organic-inorganic perovskite solar cells (PSCs). However, the complexity of the fabrication process for these PSCs has increased due to the post-processing of the perovskite layer and the use of antisolvents, among others. In this investigation, a straightforward method for fabricating high-performance and CH3NH3+-free [CH(NH2)2PbI3]0.95[CsPbBr3]0.05 solar cells using fluorinated phosphoric acid passivating additives in combination with mixed solvents was presented. This study required neither post-treatment nor anti-solvent for solar cell fabrication. Fluorinated phosphoric acid was introduced into the perovskite layer, which then self-organized on the perovskite surface and exhibited a passivating effect. Planar PSCs fabricated at reduced temperatures without fluorinated phosphoric acid exhibited power conversion efficiencies (PCEs) of 19.9 % and 17.3 % during reverse and forward scanning, respectively. Conversely, PSCs fabricated with fluorinated phosphoric acid as an additive exhibited a remarkable PCE of 20.5 % without hysteresis and a stabilized power output of 20.4 %. This study will facilitate the fabrication of high-efficient PSCs in future research efforts, thereby promoting the advancement of these devices.

Perovskite solar cells have become one of the most studied systems in the quest for new, cheap and efficient solar cell materials. Within a decade device efficiencies have risen to >25% in single-junction and >29% in tandem devices on top of silicon. This rapid improvement was in many ways fortunate, as e. g. the energy levels of commonly used halide perovskites are compatible with already existing materials from other photovoltaic technologies such as dye-sensitized or organic solar cells. Despite this rapid success, fundamental working principles must be understood to allow concerted further improvements. This thesis focuses on a comprehensive understanding of recombination processes in functioning devices. First the impact the energy level alignment between the perovskite and the electron transport layer based on fullerenes is investigated. This controversial topic is comprehensively addressed and recombination is mitigated through reducing the energy difference between the perovskite conduction band minimum and the LUMO of the fullerene. Additionally, an insulating blocking layer is introduced, which is even more effective in reducing this recombination, without compromising carrier collection and thus efficiency. With the rapid efficiency development (certified efficiencies have broken through the 20% ceiling) and thousands of researchers working on perovskite-based optoelectronic devices, reliable protocols on how to reach these efficiencies are lacking. Having established robust methods for >20% devices, while keeping track of possible pitfalls, a detailed description of the fabrication of perovskite solar cells at the highest efficiency level (>20%) is provided. The fabrication of low-temperature p-i-n structured devices is described, commenting on important factors such as practical experience, processing atmosphere & temperature, material purity and solution age. Analogous to reliable fabrication methods, a method to identify recombination losses is needed to further improve efficiencies. Thus, absolute photoluminescence is identified as a direct way to quantify the Quasi-Fermi level splitting of the perovskite absorber (1.21eV) and interfacial recombination losses the transport layers impose, reducing the latter to ~1.1eV. Implementing very thin interlayers at both the p- and n-interface (PFN-P2 and LiF, respectively), these losses are suppressed, enabling a VOC of up to 1.17eV. Optimizing the device dimensions and the bandgap, 20% devices with 1cm2 active area are demonstrated. Another important consideration is the solar cells’ stability if subjected to field-relevant stressors during operation. In particular these are heat, light, bias or a combination thereof. Perovskite layers – especially those incorporating organic cations – have been shown to degrade if subjected to these stressors. Keeping in mind that several interlayers have been successfully used to mitigate recombination losses, a family of perfluorinated self-assembled monolayers (X-PFCn, where X denotes I/Br and n = 7-12) are introduced as interlayers at the n-interface. Indeed, they reduce interfacial recombination losses enabling device efficiencies up to 21.3%. Even more importantly they improve the stability of the devices. The solar cells with IPFC10 are stable over 3000h stored in the ambient and withstand a harsh 250h of MPP at 85◦C without appreciable efficiency losses. To advance further and improve device efficiencies, a sound understanding of the photophysics of a device is imperative. Many experimental observations in recent years have however drawn an inconclusive picture, often suffering from technical of physical impediments, disguising e. g. capacitive discharge as recombination dynamics. To circumvent these obstacles, fully operational, highly efficient perovskites solar cells are investigated by a combination of multiple optical and optoelectronic probes, allowing to draw a conclusive picture of the recombination dynamics in operation. Supported by drift-diffusion simulations, the device recombination dynamics can be fully described by a combination of first-, second- and third-order recombination and JV curves as well as luminescence efficiencies over multiple illumination intensities are well described within the model. On this basis steady state carrier densities, effective recombination constants, densities-of-states and effective masses are calculated, putting the devices at the brink of the radiative regime. Moreover, a comprehensive review of recombination in state-of-the-art devices is given, highlighting the importance of interfaces in nonradiative recombination. Different strategies to assess these are discussed, before emphasizing successful strategies to reduce interfacial recombination and pointing towards the necessary steps to further improve device efficiency and stability. Overall, the main findings represent an advancement in understanding loss mechanisms in highly efficient solar cells. Different reliable optoelectronic techniques are used and interfacial losses are found to be of grave importance for both efficiency and stability. Addressing the interfaces, several interlayers are introduced, which mitigate recombination losses and degradation.

Cuprous thiocyanate (CuSCN) emerges as a prime candidate among inorganic hole-transport materials, particularly suitable for the fabrication of perovskite solar cells. Nonetheless, there is an Ohmic contact degradation between the perovskite and CuSCN layers. This is induced by polar solvents and undesired purities, which reduce device efficiency and operational stability. In this work, we introduce amidinothiourea (ASU) as an intermediate layer between perovskites and CuSCN to overcome the above obstacles. The characterization results confirm that ASU-modified perovskites have eliminated trap-induced defects by strong chemical bonding between -NH- and C═S from ASU and under-coordinated ions in perovskites. The interfacial engineering based on the ASU also reduces the potential barrier between the perovskite and CuSCN layers. The ASU-treated perovskite solar cells (PSC) with a gold electrode obtains an improved power conversion efficiency (PCE) from 16.36 to 18.03%. Furthermore, after being stored for 1800 h in ambient air (relative humidity (RH) = 45%), the related device without encapsulation maintains over 90% of its initial efficiency. The further combination of ASU and carbon-tape electrodes demonstrates its potential to fabricate low-cost but stable carbon-based PSCs. This work finds a universal approach for the fabrication of efficient and stable PSCs with different device structures.

Inorganic n-type NiO and p-type ZnO has been used as hole transport layers (HTL) and electron transport layers (ETL), respectively, for the fabricated perovskite solar cells. The sols of NiO and ZnO have been synthesized using sol–gel method. The structural, morphological, and optical analysis of the synthesized ZnO and NiO nanoparticles prepared from sol has been done. The spin coating technique was used for the fabrication of HTL, ETL, and perovskite layers. The thickness of the fabricated solar cell is about 2 μm. The current density versus voltage (JV) characteristics of the fabricated solar cell was investigated. The JV characteristics revealed the value of various parameters such as short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), power conversion efficiency, and maximum power (Pmax) is 15.90 mA/cm2, 0.93 V, 73%, 10.70%, and 10.70 W, respectively. The fabricated solar cell has been used to design a special electronic circuit with maximum power point tracker (MPPT) for monitoring of patient suffering from the heart diseases. The performance parameters of MPPT circuit have been tested under “direct sunlight” and “artificial shadow.” The energy consumption and 1-day performance load of the designed circuit is tested with the solar cell. The e-monitoring of patient suffering from heart diseases is done with the help of bluetooth low energy (BLE) to measure blood pressure (BP), heartbeat, photoplethysmography (PPG), electrocardiogram (ECG) signals.

Perovskite solar cells (PSCs) have recently demonstrated the power conversion efficiencies (PCEs) larger than most organic solar cells and comparable to those of commercial silicon solar cells. In this work, we have thoroughly studied the performance of mixed cation PSCs fabricated inside the N2-filled glove box using anti-solvent spin-coating method. In these experiments, the perovskite solution consists of PbI2, PbBr2, MABr, FAI, and CsI with a final formula of Cs0.07MA0.1581FA0.7719Pb1I2.49Br0.51. We fabricated PSCs with efficiencies ~17% using the glove box. We investigated the effect of DI water in the cleaning process on the performance of device. After sonication of samples with DI water, ethanol, and IPA, rinsing with DI water produces some black spots on the surface of FTO glasses which are visible under the microscope. These spots cannot be cleaned by O2 plasma etching for even 15 min. Thus, the c-TiO2 layers deposited by spray pyrolysis have many defects remained on the surface of FTO substrates, which decrease the fill factor, short circuit current and the overall performance of device. We have also investigated the effect of adding hydrochloric acid 37% (HCl) acid to the c-TiO2 solution in spray pyrolysis method. We found that the devices with HCl additive in their c-TiO2 solution show lower open circuit voltage compared to the control devices. The normal Voc is 1.05-1.10 V for the control cells, however it decreases to 0.7-0.85 V for the samples contain HCl. We measured the resistance of c-TiO2 layer by using 4-probe measurement system. The high resistance of c-TiO2 layer means a low number of defects and pinholes in it. When we do a comparison between samples with different resistances of c-TiO2 layers, we found that the fill factor is higher in samples with the high-resistance c-TiO2 layer. The resistance of the ETL layer was easily controlled by the spray time on considering that all other parameters such as solution volume, density, annealing time, distance, and temperature were fixed. Generally, samples with longer spraying time showed higher resistivity compared to the samples with shorter spray time. Dryness of glove box and spin-coater environment plays an important role in getting a high quality perovskite layer. All researchers are using the two-step spin-coating method with 1000 rpm for 10 s and 4000 rpm for 20 s for deposition of perovskite layer, however it doesn’t work in our fabrication setup. Our spin-coater doesn’t have a cap, so the samples will get dry quickly during the first spin step. Thus, the 10 s spinning at low speed doesn’t provide a shiny, black, and smooth perovskite layer. In this case, we use 1000 rpm for 2-3 s and 4000 rpm for 20 s. Following this strategy, we obtained shiny, black, and thick enough perovskite layers with very good perovskite peak intensity according to the XRD results. We used the thermal evaporation for deposition of the gold layer. The surface of gold layer under microscope has lots of cracks in the size of 300-500 nm. These cracks may deteriorate the performance of the final cells in many ways. First, the gold layer may not collect the holes effectively from the HTM layer due to its poor adhesion to the HTM layer. Secondly, the cracks inside the gold layer reduce the active area of the device and thus underestimate the real performance. In addition, cracks inside the gold layer can also provide a pathway for the moisture to find a way to contact the HTM layer and lower the stability performance of the device. Using SEM cross-sectional imaging of our samples, we found that there are always some vapor particles and solvents inside the glove box that distribute on the surface of our samples before we finish the spin-coating of c-TiO2, mp-TiO2, and perovskite layer. Two tests are performed to deposit c-TiO2 layer on the clean FTO substrates. Series A were deposited right after cleaning the glove box, and series B were deposited after a month of using the glove box. We couldn’t find any defects between FTO and c-TiO2 in the SEM cross-sectional imaging of the A samples, but there are many defects and black spots in SEM images at the interface of FTO and c-TiO2 for the series B. These defects prevent the strong adhesion between FTO and c-TiO2 layer, so the overall performance of the device deteriorated.

To solve the energy crisis, humans are actively searching for renewable energy to replace fossil fuels. Solar energy has attracted much attention due to its clean and inexhaustible characteristics. The photovoltaic effect can be used to convert solar energy into electrical energy for human use; thus, it is important to develop high-performance, stable solar cells. Perovskite solar cells have been the subject of attention since the first perovskite solar cell was reported in 2009. The power conversion efficiency of perovskite solar cells has increased from 3.8% to 22.1% at present. Perovskite is considered a candidate material for a new generation of solar cells because of its high-power conversion efficiency, its simple fabrication process, its flexible characteristics and its controllable band gap. Several methods exist for the fabrication of perovskite solar cells, such as the one-step method and the two-step method as well as vapor deposition. Herein, we used a novel method, the flash method, to deposit perovskite film. After the perovskite precursor was deposited onto an electron transport layer by spin coating, the substrate was transferred into a vacuum chamber. The solvent was removed, and an interphase was formed after pumping. In this work, we fabricated perovskite devices by the flash method through a homemade vacuum flash system. The structure of the devices was ITO/SnO2/perovskite/spiro-OMeTAD/Ag, where spiro-OMeTAD is 2,2′,7,7′-tetrakis( N , N -di- p -methoxyphenylamine)- 9,9′-spirobifluorene. The fabrication parameters for depositing a perovskite active layer were studied. Through the analysis of the results, we show that the pumping pressure should be controlled under 20 Pa and that the time for laying up precursor solution needs to be less than 1 h. The open-circuit voltage decreased, and the short-circuit current density increased, with pumping. The filling factor reached a maximum at approximately 20 Pa. The films were observed by a scanning electron microscope to recognize the growth process. It was found that the perovskite begins to crystallize following pumping and that the surface of the perovskite film is relatively rough at this time, and defect pinholes can be observed. The film became uniform and showed no defect pinholes when decreasing pressure in the vacuum chamber. The interphase was formed when the pressure reached 20 Pa. After optimization, the highest power conversion efficiency reached was 18.09%. In addition, it was possible to control the perovskite bandgap width, to reduce the leakage current and to increase the carrier diffusion length by doping other halogen ions. Therefore, this work demonstrates improved performance of perovskite solar cells by doping of CH3NH3Cl into a perovskite precursor. A series of perovskite devices was fabricated by adjusting the doping ratio of CH3NH3Cl. The doping of the Cl element significantly improved the short circuit current density and fill factor of the devices. To further explore the influence of Cl doping, we observed the morphology of perovskite films with varying doping ratios with a scanning electron microscope. It was found that perovskite grains were enlarged after Cl doping, which increased the length of carrier diffusion. The highest power conversion efficiency of 18.69% was reached when the Cl doping ratio was 10%. For flexible devices, the highest power conversion efficiency obtained was 15.10%. This work has value for researchers in terms of the use of the vacuum flash method to fabricate perovskite devices.

We introduced a new approach to deposit perovskite layer with no need for dissolving perovskite precursors. Deposition of Solution-free perovskite (SFP) layer is a key method for deposition of perovskite layer on the hole or electron transport layers that are strongly sensitive to perovskite precursors. Using deposition of SFP layer in the perovskite solar cells would extend possibility of using many electron and hole transport materials in both normal and invert architectures of perovskite solar cells. In the present work, we synthesized crystalline perovskite powder followed by successful deposition on TiO2 and cuprous iodide as the non-sensitve and sensitive charge transport layers to PbI2 and CH3NH3I solution in DMF. The post compressing step enhanced the efficiency of the devices by increasing the interface area between perovskite and charge transport layers. The 9.07% and 7.71% cell efficiencies of the device prepared by SFP layer was achieved in respective normal (using TiO2 as a deposition substrate) and inverted structure (using CuI as deposition substrate) of perovskite solar cell. This method can be efficient in large-scale and low cost fabrication of new generation perovskite solar cells.