In this perspective, we explore the insights into the device physics of perovskite solar cells gained from modeling and simulation of these devices. We discuss a range of factors that influence the modeling of perovskite solar cells, including the role of ions, dielectric constant, density of states, and spatial distribution of recombination losses. By focusing on the effect of non-ideal energetic alignment in perovskite photovoltaic devices, we demonstrate a unique feature in low recombination perovskite materials - the formation of an interfacial, primarily electronic, self-induced dipole that results in a significant increase in the built-in potential and device open-circuit voltage. Finally, we discuss the future directions of device modeling in the field of perovskite photovoltaics, describing some of the outstanding open questions in which device simulations can serve as a particularly powerful tool for future advancements in the field.
Ni et al. report minimum bulk trap densities of 10^11 cm-3 and 1-4 orders change in interfacial trap densities derived from drive-level capacitance profiling of lead halide perovskites. From basic electrostatic arguments, we show that such bulk trap densities cannot be resolved for a p-i-n perovskite solar cell for the reported layer thicknesses, while the apparent interfacial charge densities are a consequence of the geometrical capacitance and of charge injection into the perovskite layer.
Perovskite solar cells have shown power conversion efficiencies (PCE) comparable to cystalline silicon solar cell despite involving low-temperature, solution based synthesis processes outside clean room environment. As the theoretical PCE of a perovskite solar cell with band gap 1.55 eV is capped to 33 % due to Shockley-Queisser limit, tandem configurations are being investigated to go beyond this limit. Here, we propose a two-terminal (2T) tandem solar cell structure consisting of perovskite and multilayer transition metal dichalcogenide as the absorber layers of the top and the bottom subcel…
We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx solar cells due to oxygen exposure occurs over shorter timescales than FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these oxygen-induced losses in the power conver…
Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface, due to its rapid oxidation. In addition, at t…
We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS) monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick), light-weight, and flexible polyimide foil with a steady-state power conversion efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard test condition.
One of the most significant features of lead-halide perovskites are their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to…
Perovskite solar cells employing self assembled monolayers such as Me-4PACz as hole transport layer has been reported to demonstrate high device efficiency. However, the poor perovskite wetting on the Me-4PACz caused by poor perovskite ink interaction with the underlying Me-4PACz presents significant challenges for fabricating efficient perovskite devices. A triple co-solvent system comprising of dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite ink-substrate interaction and obtain a uniform perovskite layer. In comparison…
Capacitance measurements as a function of voltage, frequency and temperature are useful tools to identify fundamental parameters that affect solar cell operation. Techniques such as capacitance-voltage (CV), Mott-Schottky analysis and thermal admittance spectroscopy (TAS) measurements are therefore frequently employed to obtain relevant parameters of the perovskite absorber layer in perovskite solar cells. However, state-of-the-art perovskite solar cells employ thin electron and hole transport layers that improve contact selectivity. These selective contacts are often quite resistive in nature…
We show that the addition of 1 % (v/v) nitrobenzene within the perovskite formulation can be used as a method to improve the power conversion efficiency and reliability performance of methylammonium-free (CsFA) inverted perovskite solar cells. Addition of nitrobenzene increased PCE due to defect passivation and provides smoother films resulting in PVSCs with narrower PCE distribution. Moreover, the nitrobenzene additive methylammonium-free hybrid PVSCs exhibit prolonged lifetime compare to additive free PVSCs due to enhanced air and moisture degradation resistance.
Transition metal oxides (TMOs) like MoOx are increasingly explored as hole transport layers for perovskite-based solar cells. Due to their large work function, the hole collection mechanism of such solar cells are fundamentally different from other materials like PEDOT: PSS, and the associated device optimizations are not well elucidated. In addition, the prospects of such architectures against the challenges posed by ion migration are yet to be explored - which we critically examine in this contribution through detailed numerical simulations. Curiously, we find that, for similar ion densities…
The enhancement of the fill factor in the current generation of perovskite solar cells is the key for further efficiency improvement. Thus, methods to quantify the fill factor losses are urgently needed. A classical method to quantify Ohmic and non-Ohmic resistive losses in solar cells is based on the comparison between the voltage in the dark and under illumination analysed at equal recombination current density. Applied to perovskite solar cells, we observe a combination of an Ohmic series resistance with a voltage-dependent resistance that is most prominent at short circuit and low forward …
Inorganic lead halide perovskite materials have attracted great attention recently due to their potential for greater thermal stability compared to hybrid organic perovskites. However, the high processing temperature to convert from the non-perovskite phase to cubic perovskite phase in many of these systems has limited their application in flexible optoelectronic devices. Here, we report a room temperature processed inorganic PSC based on CsPbI2Br as the light harvesting layer. By combing this composition with key precursor solvents, we show that the inorganic perovskite film can be prepared b…
The scalability of highly efficient organic-inorganic perovskite solar cells (PSCs) is one of the remaining challenges of solar module manufacturing. Various scalable methods have been explored to strive for uniform perovskite films of high crystal quality on large-area substrates. However, each of these methods have individual drawbacks, limiting the successful commercialization of perovskite photovoltaics. Here, we report a fully scalable hybrid process, which combines vapor- and solution-based techniques to deposit high quality uniform perovskite films on large-area substrates. This two-ste…
Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in t…
Here we use time-resolved and steady-state optical spectroscopy on state-of-the-art low- and high-bandgap perovskite films for tandems to quantify intrinsic recombination rates and absorption coefficients. We apply these data to calculate the limiting efficiency of perovskite-silicon and all-perovskite two-terminal tandems employing currently available bandgap materials as 42.0 % and 40.8 % respectively. By including luminescence coupling between sub-cells, i.e. the re-emission of photons from the high-bandgap sub-cell and their absorption in the low-bandgap sub-cell, we reveal the stringent n…
Perovskite solar cells (PSCs) are attracting great attention as the most promising candidate for the next generation solar cells. This is due to their low cost and high power conversion efficiency in spite of their relatively short period of development. Key components of PSCs are a variety of halide perovskites with ABX3 stoichiometry used as a photoabsorber, which brought the factual breakthrough in the field of photovoltaic (PV) technology with their outstanding optoelectronic properties. To commercialize PSCs in the near future, however, these materials need to be further improved for a be…
Stabilizing Solution-Substrate Interaction of Perovskite Ink on PEDOT:PSS for Scalable Blade Coated Narrow Bandgap Perovskite Solar Modules by Gas Quenching
Successful commercialization of Perovskite/Si tandem solar cells (P/Si TSCs) need a-priori estimation of technological benchmarks to outperform c-Si based technologies under field conditions. To this end, through detailed numerical simulations and analytical modeling, here we identify the limits of ion migration and lifetime degradation till which P/Si TSCs remain competitive. Our results unravel a unique scaling law for the evolution of the efficiency and the temperature coefficient of P/Si TSCs which allows us to anticipate the limiting annual degradation rates. Interestingly, we find that 4…
Ravishankar et al. claimed the drive-level capacitance profiling (DLCP) method cannot resolve trap density along depth direction in perovskites with given thickness, and explained the measured charges to be a consequence of geometrical capacitance and diffusion capacitance. We point out that the trap densities in DLCP method are derived from the differential capacitance at different frequencies, and thus the background charges caused by diffusion and geometry capacitance has been subtracted. Even for the non-differential doping analysis by DLCP, the contribution from diffusion capacitance is s…
Despite the youthfulness of hybrid halide perovskite solar cells, their efficiencies are currently comparable to commercial silicon and have surpassed quantum-dots solar cells. Yet, the scalability of these devices is a challenge due to their low reproducibility and stability under environmental conditions. However, the methods reported to date to tackle such issues recurrently involve the use of solvent methods that would further complicate their transfer to industry. Herein we present a reliable alternative relaying in the implementation of an ultrathin plasma polymer as passivation interfac…
To obtain significant increases in the Power Conversion Efficiency (PCE) of solar cells, future cell research and development should be based on the concomitant improvement of multiple material properties, rather than on the state-of-the-art one or two-dimensional improvements. In this context, researchers should know, which combined material properties and cell design parameters lead to the highest efficiency increase. For the same objective, it should also be known which relationships in-between these variables have to be adjusted. Such knowledge becomes available by simulation and numerical…
Solution-processed quantum dots (QDs) have a high potential for fabricating low cost, flexible and large-scale solar energy harvesting devices. It has recently been demonstrated that hybrid devices employing a single monovalent cation perovskite solution for PbS QD surface passivation exhibit enhanced photovoltaic performance when compared to standard ligand passivation. Herein we demonstrate that the use of a triple cation Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3 perovskite composition for surface passivation of the quantum dots results in highly efficient solar cells, which maintain 96 % of th…
Perovskite solar cells have achieved a substantial breakthrough via advanced interface engineerings. Reports have emphasized that combining the hybrid perovskites with Lewis base and graphene improve the performance; the underlying mechanisms are not yet fully understood. Here, using density functional theory, we show that upon the formation of CH3NH3PbI3 interfaces with three different Lewis base molecules and graphene, the binding strength with S-donors thiocarbamide and thioacetamide is higher than with O-donor dimethyl sulfoxide, while the interface dipole and work function reduction tend …
Tandem perovskite solar cells beckon as lower cost alternatives to conventional single junction solar cells, with all-perovskite tandem photovoltaic architectures showing power conversion efficiencies up to 26.4%. Solution-processing approaches for the perovskite layers have enabled rapid 2optimization of perovskite solar technologies, but new deposition routes are necessary to enable modularity and scalability, facilitating further efficiency improvements and technology adoption. Here, we utilise a 4-source vacuum deposition method to deposit FA$_{0.7}$ Cs$_{0.3}$Pb(I$_x$Br$_{1-x}$)$_3$ perov…
Perovskite (CH3NH3PbI3) solar cells have made significant advances recently. In this paper, we propose a bilayer heterojunction solar cell comprised of a perovskite layer combining with a IV-VI group semiconductor layer, which can give a conversion efficiency even higher than the conventional perovskite solar cell. Such a scheme uses a property that the semiconductor layer with a direct band gap can be better in absorption of long wavelength light and is complementary to the perovskite layer. We studied the semiconducting layers such as GeSe, SnSe, GeS, and SnS, respectively, and found that Ge…
Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings sug…
Using cross-correlation current noise spectroscopy, we have investigated carrier dynamics in methylammonium lead triiodide solar cells. This method provides a space selectivity for devices with planar multi-layered structure, effectively amplifying current noise contributions coming from the most resistive element of the stack. In the studied solar cells, we observe near full-scale shot noise, indicating the dominance of noise generation by a single source, likely the interface between the perovskite and the spiro-OMeTAD hole-transport layer. We argue that the strong 1/f noise term has contrib…
Organometal trihalide perovskite solar cells have been rapidly developed and attracted much attention in recent years due to their high photoelectric conversion efficiency and low cost. Pulsed laser deposition (PLD) is a widely adopted technology which is used in the preparation of thin films, especially oxide thin films. With this technology, the thickness and composition of films can be conveniently and accurately controlled. In the structure of perovskite solar cells, TiO$_2$ layer working as the n-type semiconductor is used to block holes and transport electrons into electrode, which is cr…
The current utmost challenge for commercialization of perovskite solar cells is to ensure long-term operation stability. Here, we developed the pulsatile therapy which can prolong device lifetime by addressing accumulation of both charges and ions in the middle of maximum power point tracking (MPPT). In the technique, reverse biases are repeatedly applied for a very short time without any pause of operation, leading to stabilization of the working device. The observed efficacies of our pulsatile therapy are delaying irreversible degradation as well as restoring degraded photocurrent during MPP…
Inverted perovskite solar cells (PSCs) using a Cu:NiOx hole transporting layer (HTL) often exhibit stability issues and in some cases J/V hysteresis. In this work, we developed a \b{eta}-alanine surface treatment process on Cu:NiOx HTL that provides J/V hysteresis-free, highly efficient, and thermally stable inverted PSCs. The improved device performance due to \b{eta}-alanine-treated Cu:NiOx HTL is attributed to the formation of an intimate Cu:NiOx/perovskite interface and reduced charge trap density in the bulk perovskite active layer. The \b{eta}-alanine surface treatment process on Cu:NiOx…
Perovskite semiconductors have demonstrated outstanding external luminescence quantum yields, enabling high power conversion efficiencies (PCE). However, the precise conditions to advance to an efficiency regime above monocrystalline silicon cells are not well understood. Here, we establish a simulation model that well describes efficient p-i-n type perovskite solar cells and a range of different experiments. We then study important device and material parameters and we find that an efficiency regime of 30% can be unlocked by optimizing the built-in potential across the perovskite layer by usi…
Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide ($MAPbI_3$) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potentia…
Perovskite solar cell (PSC) is one of the most promising next generation photovoltaic technologies, and there are considerable interests in the role of possible polarization of organic-inorganic halide perovskites (OIHPs) in photovoltaic conversion. The polarity of OIHPs, however, is still hotly debated. In this review, we examine recent literature on the polarity of OIHPs from both theoretical and experimental points of view, and argue that they can be both polar and nonpolar, depending on compositions, processing, and environments. Implications of OIHP polarity to photovoltaic conversion is …
One of the key challenges for future development of efficient and stable metal halide perovskite solar cells is related to the migration of ions in these materials. Mobile ions have been linked to the observation of hysteresis in the current--voltage characteristics, shown to reduce device stability against degradation and act as recombination centers within the band gap of the active layer. In the literature one finds a broad spread of reported ionic defect parameters (e.g. activation energies) for seemingly similar perovskite materials, rendering the identification of the nature of these spe…
All-inorganic halide perovskites have attracted a great interest as a promising light harvester of perovskite solar cells due to their enhanced chemical stability. In this work we investigate the material properties of solid solutions CsPb(I$_{1-x}$Br$_x$)$_3$ in cubic phase by applying the virtual crystal approximation approach within a density functional theory framework. First we check the validity of constructed pseudopotentials of the virtual atoms (X = I$_{1-x}$Br$_x$) by verifying that the lattice constants follow the linear function of mixing ratio. We then suggest an idea of using the…
Fundamental electronic processes such as charge-carrier transport and recombination play a critical role in determining the efficiency of hybrid perovskite solar cells. The presence of mobile ions complicates the development of a clear understanding of these processes as the ions may introduce exceptional phenomena such as hysteresis or giant dielectric constants. As a result, the electronic landscape, including its interaction with mobile ions, is difficult to access both experimentally and analytically. To address this challenge, we applied a series of small perturbation techniques including…
Perovskite light emitting devices have drawn considerable attention for their favorable optoelectronic properties. High carrier mobilities make perovskites excellent candidates as host materials in electroluminescent devices. To achieve high performance in a simple single layer device, we employed a CsPbBr3 perovskite host and a novel ionic iridium complex guest along with a polyelectrolyte to demonstrate efficient light emitting electrochemical cells (PeLECs). For an optimal guest/host blend, 10600 cd/m2 luminance at 11.6 cd/A and 9.04 Lm/W are achieved at 4.1 V. These devices showed voltage-…
Lead halide perovskite-based photoactive layers are nowadays employed for a large number of optoelectronic applications, from solar cells to photodetectors and light-emitting diodes, because of their excellent absorption, emission and charge-transport properties. Unfortunately, their commercialization is still hindered by an intrinsic instability towards classical environmental conditions. Water in particular promotes fast decomposition, leading to a drastic decrease in device performance. An innovative functional approach to overcome this major issue could derive from integrating water-splitt…
In this work, perovskite solar cells (PSCs) with different transport layers were fabricated to understand the hysteresis phenomenon under a series of scan rates. The experimental results show that the hysteresis phenomenon would be affected by the dielectric constant of transport layers and scan rate significantly. To explain this, a modified Poisson and drift-diffusion solver coupled with a fully time-dependent ion migration model is developed to analyze how the ion migration affects the performance and hysteresis of PSCs. The simulation model was optimized for carrier transportation of organ…
A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light-absorption dependent short-circuit current, contact and transport-layer modified carrier transport, as well as recombination and photon-recycling influenced open-circuit voltage. Our theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, non-radiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors fo…
We present a solvothermal synthetic route to produce monodispersed CuO nanoparticles (NPs) in the range of 5-10 nm that can be used as hole selective interfacial layer between indium tin oxide (ITO) and perovskite active layer for p-i-n perovskite solar cells by a spin casting the dispersions at room temperature. The bottom electrode interface modification provided by spherical CuO-NPs at room temperature promotes the formation of high quality perovskite photoactive layers with large crystal size and strong optical absorption. Furthermore, it is shown that the nanoparticulate nature of the CuO…
Light Management (LM) is essential for metal-halide perovskite solar cells in their race for record performance. In this review, criteria on materials, processes and photonic engineering are established such as to enhance mainly the short circuit current density, towards high energy yields. These criteria are used to analyse a large panel of solutions envisaged in the literature for single junction cells. Moreover, a perspective based on rigorous electromagnetic simulations performed on various comparable structures is proposed in order to clarify the conclusions, and to pave the way to furthe…
Whether dispersal of plasmonic nanoparticles (NPs) within a perovskite active layer can increase the efficiency of solar cells is a long-standing question. It is well known that inclusion of metallic NPs in an active layer can boost the surrounding near-field intensity around them owing to the dipolar localized surface plasmon resonance (LSPR, also called antenna effect), which can increase light absorption by solar cells. However, the use of plasmonic NPs in perovskite solar cells has been barely reported, and it is not known whether inserting plasmonic NPs into a perovskite active layer prod…
Process induced efficiency variation is a major concern for all thin film solar cells, including the emerging perovskite based solar cells. In this manuscript, we address the effect of pinholes or process induced surface coverage aspects on the efficiency of such solar cells through detailed numerical simulations. Interestingly, we find the pinhole size distribution affects the short circuit current and open circuit voltage in contrasting manners. Specifically, while the Jsc is heavily dependent on the pinhole size distribution, surprisingly, the Voc seems to be only nominally affected by it. …
Solvent engineering offers fine control over the photovoltaic efficiency, film morphology, and crystallization quality of perovskite films and also enables to optimize light transmittance and absorbance in solar cell applications. In the present work, the band gap and reflectance were reduced through solvent engineering. We found that perovskite thin films produced using DMF (Dimethyl formamide) solvent had a band gap that was 0.24 eV less than those produced using IPA (Isopropyl Alcohol) solvent. Perovskite thin films produced using DMF solvent also exhibited considerably lower solar spectrum…
Fundamental working mechanisms of perovskite solar cells remain an elusive topic of research. Impedance Spectroscopy (IS) application to perovskite-based devices generates uncommon features and misleading outputs, mainly due to the lack of a stablished model for the interpretation of the results. In this work we control the perovskite precursor concentration to fabricate a series of perovskite-based solar cells with different amounts of perovskite absorber. Low concentration devices present the well-known dye sensitized solar cell (DSSCs) impedance pattern. As the amount of perovskite is incre…
Electric fields arising from the distribution of charge in metal halide perovskite solar cells are critical for understanding the many weird and wonderful optoelectronic properties displayed by these devices. Mobile ionic defects are thought to accumulate at interfaces to screen electric fields within the bulk of the perovskite semiconductor on application of external bias, but tools are needed to directly probe the dynamics of the electric field in this process. Here we show that electroabsorption measurements allow the electric field within the active layer to be tracked as a function of fre…
Cui et al. describe the fabrication and characterization of planar pn-junction solar cells based on lead-halide perovskites. The doping densities measured using Hall effect measurements vary from $N_D = 10^{12} cm^{-3}$ to $8\times 10^{12} cm^{-3}$ for the solution-processed n-type layer and $N_A = 8\times 10^9 cm^{-3}$ for the evaporated p-type layer. While these devices outperform their counterparts, that are supposedly un-doped, the results raise three important questions: (i) Are the reported doping densities high enough to change the electrostatic potential distribution in the device from…
The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via transient and absolute photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer and specifically at the perovskite/C60 interface. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the…
Hybrid organic-inorganic halide perovskites are low-cost solution-processable solar cell materials with photovoltaic properties that rival those of crystalline silicon. The perovskite films are typically sandwiched between thin layers of hole and electron transport materials, which efficiently extract photogenerated charges. This affords high-energy conversion efficiencies but results in significant performance and fabrication challenges. Herein we present a simple charge transport layer-free perovskite solar cell (PSC), comprising only a perovskite layer with two interdigitated gold back-cont…
