It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis

Strategy of Enhancing Built-in Field to Promote the Application of C-TiO2 /SnO2 Bilayer Electron Transport Layer in High-Efficiency Perovskite Solar Cells (24.3%)

Combining two kinds of electron transport layer (ETL) which have complementary advantages into a bilayer structure to form a bilayer ETL is an effective way to transcend inherent limitations of single-layer ETL, which is very helpful in the development of perovskite solar cells (PSCs). In this work, a strategy is proposed to break constraints on the application of the staggered bilayer ETL in high-efficiency PSC, namely utilizing a built-in field to overcome the dilemma in E_CBM making it possible to improve V_OC and FF simultaneously by tuning the Fermi level of ETLs properly. According to the strategy, a bilayer ETL structure comprised of C-TiO₂ and SnO₂ layer and corresponding Li-doping process are developed, and the characterization results confirm the effectiveness of the strategy, making the potentials of the C-TiO₂ (Li)/SnO₂ bilayer ETL fully released for its application in high-efficiency PSCs: a V_OC of 1.201 V for an ordinary triple-cation-perovskite-based PSC and a photoelectric conversion efficiency of 24.3% for a low-bandgap-perovskite-based PSC with high haze FTO superstrate are successfully achieved, indicating that the C-TiO₂ (Li)/SnO₂ bilayer ETL is a successful application paradigm of the proposed strategy and very promising in the application of high-efficiency PSCs.

NiN-Passivated NiO Hole-Transport Layer Improves Halide Perovskite-Based Solar Cell

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiO_x, an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiO_x can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiO_x and HaPs is still not well-controlled, which leads at times to a lack of stability and V_oc losses. Here, we use RF sputtering to fabricate NiO_x and then cover it with a Ni_yN layer without breaking vacuum. The Ni_yN layer protects NiO_x doubly during PSC production. Firstly, the Ni_yN layer protects NiO_x from Ni³⁺ species being reduced to Ni²⁺ by Ar plasma, thus maintaining NiO_x conductivity. Secondly, it passivates the interface between NiO_x and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.

A post-deposition annealing approach for organic residues control in TiO2 and its impact on Sb2Se3/TiO2 device performance

We report a systematic investigation on the influence of the two-step post-deposition treatments (PDTs) on TiO2 buffer layers deposited by ultrasonic spray pyrolysis (USP) for emerging Sb2Se3 photovoltaics. Air annealing...

Compositional and Interfacial Engineering Yield High-Performance and Stable p-i-n Perovskite Solar Cells and Mini-Modules

Through the optimization of the perovskite precursor composition and interfaces to selective contacts, we achieved a p-i-n-type perovskite solar cell (PSC) with a 22.3% power conversion efficiency (PCE). This is a new performance record for a PSC with an absorber bandgap of 1.63 eV. We demonstrate that the high device performance originates from a synergy between (1) an improved perovskite absorber quality when introducing formamidinium chloride (FACl) as an additive in the "triple cation" Cs_0.05FA_0.79MA_0.16PbBr_0.51I_2.49 (Cs-MAFA) perovskite precursor ink, (2) an increased open-circuit voltage, V_OC, due to reduced recombination losses when using a lithium fluoride (LiF) interfacial buffer layer, and (3) high-quality hole-selective contacts with a self-assembled monolayer (SAM) of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) on ITO electrodes. While all devices exhibit a high performance after fabrication, as determined from current-density voltage, J-V, measurements, substantial differences in device performance become apparent when considering longer-term stability data. A reduced long-term stability of devices with the introduction of a LiF interlayer is compensated for by using FACl as an additive in the metal-halide perovskite thin-film deposition. Optimized devices maintained about 80% of the initial average PCE during maximum power point (MPP) tracking for >700 h. We scaled the optimized device architecture to larger areas and achieved fully laser patterned series-interconnected mini-modules with a PCE of 19.4% for a 2.2 cm² active area. A robust device architecture and reproducible deposition methods are fundamental for high performance and stable large-area single junction and tandem modules based on PSCs.

Focussed Review of Utilization of Graphene-Based Materials in Electron Transport Layer in Halide Perovskite Solar Cells: Materials-Based Issues

The present work applies a focal point of materials-related issues to review the major case studies of electron transport layers (ETLs) of metal halide perovskite solar cells (PSCs) that contain graphene-based materials (GBMs), including graphene (GR), graphene oxide (GO), reduced graphene oxide (RGO), and graphene quantum dots (GQDs). The coverage includes the principal components of ETLs, which are compact and mesoporous TiO2, SnO2, ZnO and the fullerene derivative PCBM. Basic considerations of solar cell design are provided and the effects of the different ETL materials on the power conversion efficiency (PCE) have been surveyed. The strategy of adding GBMs is based on a range of phenomenological outcomes, including enhanced electron transport, enhanced current density-voltage (J-V) characteristics and parameters, potential for band gap (Eg) tuning, and enhanced device stability (chemical and environmental). These characteristics are made complicated by the variable effects of GBM size, amount, morphology, and distribution on the nanostructure, the resultant performance, and the associated effects on the potential for charge recombination. A further complication is the uncertain nature of the interfaces between the ETL and perovskite as well as between phases within the ETL.

Gradient Energy Alignment Engineering for Planar Perovskite Solar Cells with Efficiency Over 23

An electron-transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open-circuit voltage (V_OC ). Herein, a simple low-temperature-processed In₂ O₃ /SnO₂ bilayer ETL is developed and used for fabricating a new PSC device. The presence of In₂ O₃ results in uniform, compact, and low-trap-density perovskite films. Moreover, the conduction band of In₂ O₃ is shallower than that of Sn-doped In₂ O₃ (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing V_OC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high V_OC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO₂ layers achieve 21.42% efficiency with a V_OC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N₂ without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices.

Charge Transfer in c-Si(n++)/TiO2(ALD) at the Amorphous/Anatase Transition: A Transient Surface Photovoltage Spectroscopy Study

Electronic properties and charge transfer processes were studied in an n-Si(n⁺⁺)/TiO₂(ALD) system at an amorphous TiO₂/anatase transition by transient surface photovoltage spectroscopy at constant photon flux. The TiO₂ layers were deposited by atomic layer deposition (ALD) onto highly doped silicon (c-Si(n⁺⁺)), and the phase composition of the TiO₂ layers changed with increasing thickness from amorphous to the anatase polymorph as anatase crystallites started to grow at the surface. Depending on phase composition, the band gap of TiO₂ correlated with the characteristic energy of exponential tails. In most cases, photogenerated electrons were separated toward the back contact. For photogeneration in c-Si(n⁺⁺), electron back transfer was limited by Auger recombination with holes in the surface space charge region of c-Si(n⁺⁺), and by electron transfer across the interface, either via exponentially distributed states near the conduction band edge of amorphous TiO₂ or via distance-dependent recombination with holes trapped in anatase. For photogeneration in TiO₂, electron back transfer was limited by trapping in TiO₂. Under strong light absorption in amorphous TiO₂ with anatase crystallites on top, electrons were preferentially separated toward the TiO₂ surface.

Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V_OC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V_OC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome-paving the way to the thermodynamic efficiency limit.

Interfacial Engineering for High-Efficiency Nanorod Array-Structured Perovskite Solar Cells

TiO₂ nanorod (NR) array for perovskite solar cells (PSCs) has attained great importance due to its superb power conversion efficiency (PCE) compared to that of the traditional mesoporous TiO₂ film. A TiO₂ compact layer for the growth of TiO₂ NR array via spin-coating cannot meet the requirements for efficient NR-based PSCs. Herein, we have developed and demonstrated the insertion of a bifunctional extrathin TiO₂ interlayer (5 nm) by atomic layer deposition (ALD) at the interface of the fluorine-doped tin oxide (FTO)/TiO₂ compact layer to achieve alleviated electron exchange and a reduced energetic barrier. Thus, an accelerated extraction of electrons from TiO₂ NR arrays via the compact layer and their transfer to the FTO substrate can improve the PSC efficiency. The thickness of the spin-coated TiO₂ compact layer on the ALD-deposited TiO₂ layer is spontaneously optimized. Finally, an outstanding efficiency of 20.28% has been achieved from a champion PSC with negligible hysteresis and high reliability. To the best of our knowledge, this is the first study demonstrating the superiority of TiO₂-NR-based PSCs withstanding the dry heat and thermal cycling tests. The results are of great importance for the preparation of efficient and durable PSCs for real-world applications.

Universal Nanoparticle Wetting Agent for Upscaling Perovskite Solar Cells

Solution-processed perovskite solar cells reach efficiencies over 23% on lab-scale. However, a reproducible transfer of these established processes to upscaling techniques or different substrate surfaces requires a highly controllable perovskite film formation. Especially, hydrophobic surfaces cause severe dewetting issues. Such surfaces are particularly crucial for the so-called standard n-i-p cell architecture when fullerene-based electron transport layers are employed underneath perovskite absorber films. In this work, a unique and universally applicable method was developed based on the deposition of size-controlled Al₂O₃ or SiO₂ nanoparticles. By enhancing the surface energy, they act as a universal wetting agent. This allows perovskite precursor solutions to be spread perfectly over various substrates including problematic hydrophobic Si-wafers or fullerene self-assembled monolayers (C₆₀-SAMs). Moreover, the results show that the perovskite morphology, solar cell performance, and reproducibility benefit from the presence of the nanoparticles at the interface. When applied to 144 cm² C₆₀-SAM-coated substrates, homogenous coverage can be realized via spin coating resulting in average efficiencies of 16% (maximum 18%) on individualized cells with 0.1 cm² active area. Modules in the same setup reached maximum efficiencies of 11 and 7% on 2.8 and 23.65 cm² aperture areas, respectively.

Halide Perovskites: Is It All about the Interfaces?

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especially the often-questionable validity of vacuum level alignment, the importance of interface dipoles, and band bending as the result of interface formation (section 3). We follow this by elaborating on the impact of the interface formation on device performance, considering effects such as chemical reactions and surface passivation on interface energetics and stability. On the basis of these concepts, we propose a roadmap for the next steps in interfacial design for HaP semiconductors (section 4), emphasizing the importance of achieving control over the interface energetics and chemistry (i.e., reactivity) to allow predictive power for tailored interface optimization.

Efficiency Enhancement and Hysteresis Mitigation by Manipulation of Grain Growth Conditions in Hybrid Evaporated-Spin-coated Perovskite Solar Cells

Perovskite solar cells have become a game changer in the field of photovoltaics by reaching power conversion efficiencies beyond 23%. To achieve even higher efficiencies, it is necessary to increase the understanding of crystallization, grain formation, and layer ripening. In this study, by a systematic variation of methylammonium iodide (MAI) concentrations, we changed the stoichiometry and thereupon the crystal growth conditions in MAPbI₃ perovskite solar cells, prepared by a two-step hybrid evaporation-spin-coating deposition method. Utilizing X-ray diffraction, scanning electron microscopy, atomic force microscopy, photoluminescence, and current-voltage ( J- V) characterization, we found that a relatively lower concentration of MAI, or in other words higher content of remnant and unconverted PbI₂, correlates with smaller and stronger interconnected grains, as well as with an improved optoelectronic performance of the solar cells and mitigation of hysteresis. The possible explanations are grain and interface passivation by the excess PbI₂ and a positive contribution of the grain boundaries to current extraction.

Modification of dry/wet hybrid fabrication method for preparing a perovskite absorption layer on a PCBM electron transport layer

The fabrication method of a perovskite absorption layer on a [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) electron transport layer was investigated for application to perovskite/Si tandem photovoltaic devices. A dry/wet hybrid method that involves thermal evaporation of a PbI₂ precursor followed by spin coating of an organic cation solution was found to be a useful means to form a perovskite layer without destruction of the underlying PCBM layer. To form the perovskite layer densely packed with crystals of large grain size and having smooth surface morphology, the rotational speed of the spin coating and the mixing ratio of organic cations were carefully modified. By using this modified absorption layer, a photovoltaic device fabricated under atmospheric exposure conditions showed a power conversion efficiency of ∼10.3% without any large hysteresis. These results have great importance to develop a production technology for high performance tandem solar cells.

A dry/wet hybrid method was modified and improved for fabricating a high quality perovskite absorption layer on a PCBM electron transport layer.

Room-Temperature Atomic-Layer-Deposited Al2 O3 Improves the Efficiency of Perovskite Solar Cells over Time

Electrical characterisation of perovskite solar cells consisting of room-temperature atomic-layer-deposited aluminium oxide (RT-ALD-Al₂ O₃ ) film on top of a methyl ammonium lead triiodide (CH₃ NH₃ PbI₃ ) absorber showed excellent stability of the power conversion efficiency (PCE) over a long time. Under the same environmental conditions (for 355 d), the average PCE of solar cells without the ALD layer decreased from 13.6 to 9.6 %, whereas that of solar cells containing 9 ALD cycles of depositing RT-ALD-Al₂ O₃ on top of CH₃ NH₃ PbI₃ increased from 9.4 to 10.8 %. Spectromicroscopic investigations of the ALD/perovskite interface revealed that the maximum PCE with the ALD layer is obtained when the so-called perovskite cleaning process induced by ALD precursors is complete. The PCE enhancement over time is probably related to a self-healing process induced by the RT-ALD-Al₂ O₃ film. This work may provide a new direction for further improving the long-term stability and performance of perovskite solar cells.

Nitrogen substitution improves the mobility and stability of electron transport materials for inverted perovskite solar cells

A suitable electron transport material (ETM) plays key roles in efficient perovskite solar cells (PSCs), because it is beneficial for exciton dissociation and charge transport at the interface thus increasing the short circuit current density. Based on the experimentally reported efficient electron transport molecule 10,14-bis(5-(2-ethylhexyl)thiophen-2-yl)-dipyrido[3,2-a:2',3'-c][1,2,5]thiadiazolo[3,4-i]phenazine (TDTP), we theoretically design a set of new ETMs (TDTP-1, TDTP-2a, TDTP-2b, TDTP-3a, and TDTP-3b) by introducing a nitrogen atom into the thiophene ring or replacing a hydrogen atom on the methyl with an amino group. Quantum-chemical calculations reveal that the designed molecules behave much better than TDTP in terms of electron mobility, air stability, and solubility, where the electron mobility of TDTP-3b is two orders of magnitude higher than that of TDTP owing to the extra SN interactions in TDTP-3b that lead to the quasi two-dimensional π packing motif which facilitates electron transport evidently. Moreover, we find that the substitution effect of the nitrogen atom strongly depends on the position, where the nitrogen atom at the β-position of the thiophene ring (TDTP-2b and TDTP-3b) is more conducive to electron transport. Importantly, our calculations show that the ETM/perovskite interface interaction is enhanced after the introduction of the nitrogen atom and amino group thanks to the added NPb interaction, which favors electron transport with the newly designed ETMs. Our results not only report a set of novel promising ETMs, but also provide a useful design strategy for efficient ETMs.

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.

Balancing the carrier selectivity and extraction by the selective contacts is of vital importance to the performance of the nanowire solar cells. Here Oener et al. employ a permanent local gate to overcome this tradeoff and substantially increase the open-circuit voltage by 335 mV.

Detailed Investigation of Evaporated Perovskite Absorbers with High Crystal Quality on Different Substrates

Dual-source vapor-phase deposition enables low-temperature fabrication of high-performance planar structure perovskite (CH₃NH₃PbI₃) solar cells (PSCs), applicable in tandem devices or for industrial production with high homogeneity. Herein, we report low-temperature fabrication of high-efficiency PSCs by dual-source vapor-phase deposition and significance of TiO₂ surface modification with [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) on cell performance. Co-evaporation of PbI₂ and CH₃NH₃I, as confirmed by X-ray diffraction and high-resolution transmission electron microscopy analyses, results in CH₃NH₃PbI₃ layers with a well-crystallized tetragonal phase formed on both TiO₂ and TiO₂/PCBM electron-transport layers (ETLs). The devices with PCBM interlayer between TiO₂ and CH₃NH₃PbI₃ showed remarkably higher performance than those with TiO₂ only, which was attributed to enhance charge extraction and reduced recombination at the TiO₂/PCBM/CH₃NH₃PbI₃ interface. The devices composed of evaporated CH₃NH₃PbI₃ on top of the TiO₂/PCBM and [2,2',7,7'-tetrakis( N, N-di- p-methoxyphenyl-amine)-9,9'-spirobifluorene] (Spiro-OMeTAD) as hole-transport material demonstrated power conversion efficiencies of 17.1% (reverse scan) and 13.4% (forward scan) with stabilized efficiency of over 16%, which is, to the best of our knowledge, the highest efficiency reported for evaporated perovskite solar cells using low-temperature fabrication method involving compact TiO₂ layer as ETL. Furthermore, we show that this process can be used to deposit a CH₃NH₃PbI₃ layer on top of a textured silicon substrate, which is the first step for preparing perovskite-silicon tandem devices with enhanced antireflection and light-trapping properties.

Perovskites-Based Solar Cells: A Review of Recent Progress, Materials and Processing Methods

With the rapid increase of efficiency up to 22.1% during the past few years, hybrid organic-inorganic metal halide perovskite solar cells (PSCs) have become a research “hot spot” for many solar cell researchers. The perovskite materials show various advantages such as long carrier diffusion lengths, widely-tunable band gap with great light absorption potential. The low-cost fabrication techniques together with the high efficiency makes PSCs comparable with Si-based solar cells. But the drawbacks such as device instability, J-V hysteresis and lead toxicity reduce the further improvement and the future commercialization of PSCs. This review begins with the discussion of crystal and electronic structures of perovskite based on recent research findings. An evolution of PSCs is also analyzed with a greater detail of each component, device structures, major device fabrication methods and the performance of PSCs acquired by each method. The following part of this review is the discussion of major barriers on the pathway for the commercialization of PSCs. The effects of crystal structure, fabrication temperature, moisture, oxygen and UV towards the stability of PSCs are discussed. The stability of other components in the PSCs are also discussed. The lead toxicity and updated research progress on lead replacement are reviewed to understand the sustainability issues of PSCs. The origin of J-V hysteresis is also briefly discussed. Finally, this review provides a roadmap on the current needs and future research directions to address the main issues of PSCs.

Influence of the Grain Size on the Properties of CH3NH3PbI3 Thin Films

Hybrid perovskites have already shown a huge success as an absorber in solar cells, resulting in the skyrocketing rise in the power conversion efficiency to more than η = 22%. Recently, it has been established that the crystal quality is one of the most important parameters to obtain devices with high efficiencies. However, the influence of the crystal quality on the material properties is not fully understood. Here, the influence of the morphology on electronic properties of CH₃NH₃PbI₃ thin films is investigated. Postannealing was used to vary the average grain size continuously from ≈150 to ≈1000 nm. Secondary grain growth is thermally activated with an activation energy of E_a = 0.16 eV. The increase in the grain size leads to an enhancement of the photoluminescence, indicating an improvement in the material quality. According to surface photovoltage measurements, the charge-carrier transport length exhibits a linear increase with increasing grain size. The charge-carrier diffusion length is limited by grain boundaries. Moreover, an improved morphology leads to a drastic increase in power conversion efficiency of the devices.

Novel Low-Temperature Process for Perovskite Solar Cells with a Mesoporous TiO2 Scaffold

The most efficient organic-inorganic perovskite solar cells (PSCs) contain the conventional n-i-p mesoscopic device architecture using a semiconducting TiO₂ scaffold combined with a compact TiO₂ blocking layer for selective electron transport. These devices achieve high power conversion efficiencies (15-22%) but mainly require high-temperature sintering (>450 °C), which is not possible for temperature-sensitive substrates. Thus far, comparably little effort has been spent on alternative low-temperature (<150 °C) routes to realize high-efficiency TiO₂-based PSCs; instead, other device architectures have been promoted for low-temperature processing. In this paper the compatibility of the conventional mesoscopic TiO₂ device architecture with low-temperature processing is presented for the first time with the combination of electron beam evaporation for the compact TiO₂ and UV treatment for the mesoporous TiO₂ layer. Vacuum evaporation is introduced as an excellent deposition technique of uniform compact TiO₂ layers, adapting smoothly to the rough fluorine-doped tin oxide substrate surface. Effective removal of organic binders by UV light is shown for the mesoporous scaffold. Entirely low-temperature-processed PSCs with TiO₂ scaffold reach encouraging stabilized efficiencies of up to 18.2%. This process fulfills all requirements for monolithic tandem devices with high-efficiency silicon heterojunction solar cells as the bottom cell.