Practical Efficiency Limit of Methylammonium Lead Iodide Perovskite (CH3NH3PbI3) Solar Cells

Enhanced efficiency of bifacial perovskite solar cells using computational study

The most rapidly expanding type of solar cells are the Perovskite Solar Cells (PSCs), because of its high device performance, ease of synthesis, high open-circuit voltage, and affordability. Despite these advantages, the development of perovskite-based solar cells continues to be impeded by the issues with perovskite stability and the utilization of the hazardous heavy element lead (Pb). The study emphasizes on the bifacial structure that maintains the conventional absorber layer and electron transport layer (ETL) in the optimized PSC structure. This study employs SCAPS software for device simulation to comprehensively analyze how various parameters affect the performance of solar cells. Additionally, doping concentration variation in both ETL and HTL are explored. The simulation reveals that changing device structure from monofacial to bifacial significantly influences PSC performance, demonstrating that optimizing individual layers effectively enhances overall solar cell performance. The optimized structure achieves impressive PSC performance metrics through parametric analysis, such as voltage (VOC) of 1.18 V, fill factor (FF) of 82.24%, current density (JSC) of 27.12 mA/cm2, power conversion efficiency (PCE) of 27.90% for an incident solar spectrum from the ETL side, and power conversion efficiency (PCE) of 19.86% for an incident solar spectrum from the HTL side, the calculated bifaciality factor (BF) for this structure is 71.18%.

Experimental and Theoretical Investigations of MAPbX3 -Based Perovskites (X=Cl, Br, I) for Photovoltaic Applications

This work mainly focuses on synthesizing and evaluating the efficiency of methylammonium lead halide-based perovskite (MAPbX3 ; X=Cl, Br, I) solar cells. We used the colloidal Hot-injection method (HIM) to synthesize MAPbX3 (X=Cl, Br, I) perovskites using the specific precursors and organic solvents under ambient conditions. We studied the structural, morphological and optical properties of MAPbX3 perovskites using XRD, FESEM, TEM, UV-Vis, PL and TRPL (time-resolved photoluminescence) characterization techniques. The particle size and morphology of these perovskites vary with respect to the halide variation. The MAPbI3 perovskite possesses a low band gap and low carrier lifetime but delivers the highest PCE among other halide perovskite samples, making it a promising candidate for solar cell technology. To further enrich the investigations, the conversion efficiency of the MAPbX3 perovskites has been evaluated through extensive device simulations. Here, the optical constants, band gap energy and carrier lifetime of MAPbX3 were used for simulating three different perovskite solar cells, namely I, Cl or Br halide-based perovskite solar cells. MAPbI3 , MAPbBr3 and MAPbCl3 absorber layer-based devices showed ~13.7 %, 6.9 % and 5.0 % conversion efficiency. The correlation between the experimental and SCAPS simulation data for HIM-synthesized MAPBX3 -based perovskites has been reported for the first time.

Low-Cost Antimony Selenosulfide with Tunable Bandgap for Highly Efficient Solar Cells

About 10% efficient antimony selenosulfide (Sb₂ (S,Se)₃ ) solar cell is realized by using selenourea as a hydrothermal raw material to prepare absorber layers. However, tailoring the bandgap of hydrothermal-based Sb₂ (S,Se)₃ film to the ideal bandgap (1.3-1.4 eV) using the selenourea for optimal efficiency is still a challenge. Moreover, the expensive selenourea dramatically increases the fabricating cost. Here, a straightforward one-step hydrothermal method is developed to prepare high-quality Sb₂ (S,Se)₃ films using a novel precursor sodium selenosulfate as the selenium source. By tuning the Se/(Se+S) ratio in the hydrothermal precursor solution, a series of high-quality Sb₂ (S,Se)₃ films with reduced density of deep defect states and tunable bandgap from 1.31 to 1.71 eV is successfully prepared. Consequently, the best efficiency of 10.05% with a high current density of 26.01 mA cm^-2 is achieved in 1.35 eV Sb₂ (S,Se)₃ solar cells. Compared with the traditional method using selenourea, the production cost for the Sb₂ (S,Se)₃  devices is reduced by over 80%. In addition, the device exhibits outstanding stability, maintaining more than 93% of the initial power conversion efficiency after 30 days of exposure in the atmosphere without encapsulation. The present work definitely paves a facile and effective way to develop low-cost and high-efficiency chalcogenide-based photovoltaic devices.

A Novel Multi-Sulfur Source Collaborative Chemical Bath Deposition Technology Enables 8%-Efficiency Sb2 S3 Planar Solar Cells

Sb₂ S₃ as a light-harvesting material has attracted great attention for applications in both single-junction and tandem solar cells. Such solar cell has been faced with current challenge of low power conversion efficiency (PCE), which has stagnated for 8 years. It has been recognized that the synthesis of high-quality absorber film plays a critical role in efficiency improvement. Here, using fresh precursor materials for antimony (antimony potassium tartrate) and combined sulfur (sodium thiosulfate and thioacetamide), a unique chemical bath deposition procedure is created. Due to the complexation of sodium thiosulfate and the advantageous hydrolysis cooperation between these two sulfur sources, the heterogeneous nucleation and the S^2- releasing processes are boosted. As a result, there are noticeable improvements in the deposition rate, film morphology, crystallinity, and preferred orientations. Additionally, the improved film quality efficiently lowers charge trapping capacity, suppresses carrier recombination, and prolongs carrier lifetimes, leading to significantly improved photoelectric properties. Ultimately, the PCE exceeds 8% for the first time since 2014, representing the highest efficiency in all kinds of Sb₂ S₃ solar cells to date. This study is expected to shed new light on the fabrication of high-quality Sb₂ S₃ film and further efficiency improvement in Sb₂ S₃ solar cells.

Phase Stability and Electronic Properties of Hybrid Organic-Inorganic Perovskite Solid Solution (CH(NH2)2) x (CH3NH3)1-x Pb(Br y I1-y )3 as a Function of Composition

Compositional mixing provides the means to maintain the structural stability of a hybrid organic-inorganic perovskite for efficient and robust photovoltaic applications. Here we present a theoretical, first-principles study of the electronic and energetic properties of the solid solution (CH(NH₂)₂) _x (CH₃NH₃)_1-x PbBr _y I_1-y , the mixing of two organic molecules with various orientations, formamidinium and methylammonium, and two halides, bromide and iodide. Our results show the variation in the band gap as a function of composition (x and y) provides several candidates that exceed the 27.5% Schockley-Queisser efficiency. The variation in the composition of hybrid perovskite shows specific regions where either the hexagonal or cubic phase dominates. We discuss the balance between the band gap and phase stability and indicate regions where the phase transition temperature between cubic and hexagonal phases is far from room temperature, indicating that these compositions are more robust at room temperature against phase transitions.

Physics of defects in metal halide perovskites

Metal halide perovskites are widely used in optoelectronic devices, including solar cells, photodetectors, and light-emitting diodes. Defects in this class of low-temperature solution-processed semiconductors play significant roles in the optoelectronic properties and performance of devices based on these semiconductors. Investigating the defect properties provides not only insight into the origin of the outstanding performance of perovskite optoelectronic devices but also guidance for further improvement of performance. Defects in perovskites have been intensely studied. Here, we review the progress in defect-related physics and techniques for perovskites. We survey the theoretical and computational results of the origin and properties of defects in perovskites. The underlying mechanisms, functions, advantages, and limitations of trap state characterization techniques are discussed. We introduce the effect of defects on the performance of perovskite optoelectronic devices, followed by a discussion of the mechanism of defect treatment. Finally, we summarize and present key challenges and opportunities of defects and their role in the further development of perovskite optoelectronic devices.

Heterojunction Annealing Enabling Record Open-Circuit Voltage in Antimony Triselenide Solar Cells

Despite the fact that antimony triselenide (Sb₂ Se₃ ) thin-film solar cells have undergone rapid development in recent years, the large open-circuit voltage (V_OC ) deficit still remains as the biggest bottleneck, as even the world-record device suffers from a large V_OC deficit of 0.59 V. Here, an effective interface engineering approach is reported where the Sb₂ Se₃ /CdS heterojunction (HTJ) is subjected to a post-annealing treatment using a rapid thermal process. It is found that nonradiative recombination near the Sb₂ Se₃ /CdS HTJ, including interface recombination and space charge region recombination, is greatly suppressed after the HTJ annealing treatment. Ultimately, a substrate Sb₂ Se₃ /CdS thin-film solar cell with a competitive power conversion efficiency of 8.64% and a record V_OC of 0.52 V is successfully fabricated. The device exhibits a much mitigated V_OC deficit of 0.49 V, which is lower than that of any other reported efficient antimony chalcogenide solar cell.

Crystal Growth Promotion and Defects Healing Enable Minimum Open-Circuit Voltage Deficit in Antimony Selenide Solar Cells

Antimony selenide (Sb₂ Se₃ ) is an ideal photovoltaic candidate profiting from its advantageous material characteristics and superior optoelectronic properties, and has gained considerable development in recent years. However, the further device efficiency breakthrough is largely plagued by severe open-circuit voltage (V_OC ) deficit under the existence of multiple defect states and detrimental recombination loss. In this work, an effective absorber layer growth engineering involved with vapor transport deposition and post-selenization is developed to grow Sb₂ Se₃ thin films. High-quality Sb₂ Se₃ with large compact crystal grains, benign [hk1] growth orientation, stoichiometric chemical composition, and suitable direct bandgap are successfully fulfilled under an optimized post-selenization scenario. Planar Sb₂ Se₃ thin-film solar cells with substrate configuration of Mo/Sb₂ Se₃ /CdS/ITO/Ag are constructed. By contrast, such engineering effort can remarkably mitigate the device V_OC deficit, owing to the healed detrimental defects, the suppressed interface and space-charge region recombination, the prolonged carrier lifetime, and the enhanced charge transport. Accordingly, a minimum V_OC deficit of 0.647 V contributes to a record V_OC of 0.513 V, a champion device with highly interesting efficiency of 7.40% is also comparable to those state-of-the-art Sb₂ Se₃ solar cells, paving a bright avenue to broaden its scope of photovoltaic applications.

Reducing Defects in Organic-Lead Halide Perovskite Film by Delayed Thermal Annealing Combined with KI/I2 for Efficient Perovskite Solar Cells

This study improved quality of CH₃NH₃PbI₃ (MAPbI₃) perovskite films by delaying thermal annealing in the spin coating process and introducing KI and I₂ to prepare MAPbI₃ films that were low in defects for high-efficiency perovskite solar cells. The influences of delayed thermal annealing time after coating the MAPbI₃ perovskite layer on the crystallized perovskite, the morphology control of MAPbI₃ films, and the photoelectric conversion efficiency of solar cells were investigated. The optimal delayed thermal annealing time was found to be 60 min at room temperature. The effect of KI/I₂ additives on the growth of MAPbI₃ films and the corresponding optimal delayed thermal annealing time were further investigated. The addition of KI/I₂ can improve perovskite crystallinity, and the conductivity and carrier mobility of MAPbI₃ films. Under optimized conditions, the photoelectric conversion efficiency of MAPbI₃ perovskite solar cells can reach 19.36% under standard AM1.5G solar illumination of 100 mW/cm².

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72-11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm²V^-1s^-1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

Defect and Contact Passivation for Perovskite Solar Cells

Metal-halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that may enable high-performance solar cells at affordable cost. Thanks to the appealing optoelectronic properties of these materials, tremendous progress has been reported in the last few years in terms of power conversion efficiencies (PCE) of perovskite solar cells (PSCs), now with record values in excess of 24%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can also be present, which all contribute to nonradiative recombination of photo-carriers. On device level, such recombination undesirably inflates the open-circuit voltage deficit, acting thus as a significant roadblock toward the theoretical efficiency limit of 30%. Herein, the focus is on the origin of the various voltage-limiting mechanisms in PSCs, and possible mitigation strategies are discussed. Contact passivation schemes and the effect of such methods on the reduction of hysteresis are described. Furthermore, several strategies that demonstrate how passivating contacts can increase the stability of PSCs are elucidated. Finally, the remaining key challenges in contact design are prioritized and an outlook on how passivating contacts will contribute to further the progress toward market readiness of high-efficiency PSCs is presented.

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, the perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related two-dimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of molybdenum disulfide (MoS₂) and reduced graphene oxide (RGO) hybrids both as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH₃NH₃PbI₃) perovskite (MAPbI₃)-based PSCs. We show that zero-dimensional MoS₂ quantum dots (MoS₂ QDs), derived by liquid phase exfoliated MoS₂ flakes, provide both hole-extraction and electron-blocking properties. In fact, on one hand, intrinsic n-type doping-induced intraband gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS₂ (from 1.4 eV for the flakes to >3.2 eV for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of the conduction band of MAPbI₃ (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS₂ QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional nature of RGO effectively plugs the pinholes of the MoS₂ QD films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS₂ QD/graphene hybrids enables MAPbI₃-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs.

High Electrical Conductivity 2D MXene Serves as Additive of Perovskite for Efficient Solar Cells

MXenes, a newly intriguing family of 2D materials, have recently attracted considerable attention owing to their excellent properties such as high electrical conductivity and mobility, tunable structure, and termination groups. Here, the Ti₃ C₂ T_x MXene is incorporated into the perovskite absorber layer for the first time, which aims for efficiency enhancement. Results show that the termination groups of Ti₃ C₂ T_x can retard the crystallization rate, thereby increasing the crystal size of CH₃ NH₃ PbI₃ . It is found that the high electrical conductivity and mobility of MXene can accelerate the charge transfer. After optimizing the key parameters, 12% enhancement in device performance is achieved by 0.03 wt% amount of MXene additive. This work unlocks opportunities for the use of MXene as potential materials in perovskite solar cell applications.