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.
Defects in metal halide perovskites contribute to nonradiative recombination of photo‐carriers. On device level, such recombination undesirably inflates the open‐circuit voltage deficit and acts as a significant roadblock toward the theoretical efficiency limit of 30% perovskite solar cells. Such voltage‐limiting mechanisms are assessed by focusing on their origin and possible mitigation strategies.
The performance of state‐of‐the‐art perovskite solar cells is currently limited by defect‐induced recombination at interfaces between the perovskite and the electron and hole transport layers. These ...defects, most likely undercoordinated Pb and halide ions, must either be removed or passivated if cell efficiencies are to approach their theoretical limit. In this work, a universal double‐side polymer passivation approach is introduced using ultrathin poly(methyl methacrylate) (PMMA) films. Very high‐efficiency (≈20.8%) perovskite cells with some of the highest open circuit voltages (1.22 V) reported for the same 1.6 eV bandgap are demonstrated. Photoluminescence imaging and transient spectroscopic measurements confirm a significant reduction in nonradiative recombination in the passivated cells, consistent with the voltage increase. Analysis of the molecular interactions between perovskite and PMMA reveals that the carbonyl (CO) groups on the PMMA are responsible for the excellent passivation via Lewis‐base electronic passivation of Pb2+ ions. This work provides new insights and a compelling explanation of how PMMA passivation works, and suggests future directions for developing improved passivation layers.
Interface recombination is a dominant loss mechanism in perovskite solar cells. Double‐side passivation of perovskite solar cells using ultrathin films of poly(methyl methacrylate) (PMMA) can boost open circuit voltages up to 1.22 V. This results from the carbonyl (CO) group of the Lewis‐base polymer PMMA, which effectively passivates under‐coordinated Pb atoms (Pb2+) at the perovskite/transport layer interfaces.
With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics ...from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures - the interdigitated back-contact silicon cell and the silicon heterojunction cell - both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic-inorganic perovskite materials.
This article reviews key factors for the success of crystalline silicon photovoltaics and gives an update on promising emerging concepts for further efficiency improvement and cost reduction.
Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; however, reported PCE values for tandems have ...so far lain below their potential due to inefficient photon harvesting. Here we increase the optical path length in perovskite films by preserving smooth morphology while increasing thickness using a method we term boosted solvent extraction. Carrier collection in these films - as made - is limited by an insufficient electron diffusion length; however, we further find that adding a Lewis base reduces the trap density and enhances the electron-diffusion length to 2.3 µm, enabling a 19% PCE for 1.63 eV semi-transparent perovskite cells having an average near-infrared transmittance of 85%. The perovskite top cell combined with solution-processed colloidal quantum dot:organic hybrid bottom cell leads to a PCE of 24%; while coupling the perovskite cell with a silicon bottom cell yields a PCE of 28.2%.
Perovskite solar cells increasingly feature mixed‐halide mixed‐cation compounds (FA1−x−yMAxCsyPbI3−zBrz) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the ...underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti‐solvent processing window for the fabrication of high‐quality films and efficient solar cells. This window widens from seconds, in the case of single cation/halide systems (e.g., MAPbI3, FAPbI3, and FAPbBr3), to several minutes for mixed systems. In situ X‐ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol–gel and to the number of crystalline byproducts; the processing window therefore depends directly on the precise cation/halide composition. Moreover, anti‐solvent dripping is shown to promote the desired perovskite phase with careful formulation. The processing window of perovskite solar cells, as defined by the latest time the anti‐solvent drip yields efficient solar cells, broadened with the increasing complexity of cation/halide content. This behavior is ascribed to kinetic stabilization of sol–gel state through cation/halide engineering. This provides guidelines for designing new formulations, aimed at formation of the perovskite phase, ultimately resulting in high‐efficiency perovskite solar cells produced with ease and with high reproducibility.
The role of cation and halide mixing is revealed using in situ X‐ray scattering measurements during spin‐coating. Modulating the cation/halide composition directly impacts the lifetime of the sol–gel precursor film and its easy and reproducible conversion to the perovskite phase to yield solar cells with 20% power conversion efficiency.
Minimizing carrier recombination at contact regions by using carrier‐selective contact materials, instead of heavily doping the silicon, has attracted considerable attention for high‐efficiency, ...low‐cost crystalline silicon (c‐Si) solar cells. A novel electron‐selective, passivating contact for c‐Si solar cells is presented. Tantalum nitride (TaN
x
) thin films deposited by atomic layer deposition are demonstrated to provide excellent electron‐transporting and hole‐blocking properties to the silicon surface, due to their small conduction band offset and large valence band offset. Thin TaNx interlayers provide moderate passivation of the silicon surfaces while simultaneously allowing a low contact resistivity to n‐type silicon. A power conversion efficiency (PCE) of over 20% is demonstrated with c‐Si solar cells featuring a simple full‐area electron‐selective TaNx contact, which significantly improves the fill factor and the open circuit voltage (Voc) and hence provides the higher PCE. The work opens up the possibility of using metal nitrides, instead of metal oxides, as carrier‐selective contacts or electron transport layers for photovoltaic devices.
Tantalum nitride (TaNx) thin films are demonstrated to be an excellent hole‐blocking, electron‐selective passivating contact for crystalline silicon solar cells. An efficiency of over 20% is achieved on crystalline silicon solar cell featuring a full‐area TaNx heterocontact. This work opens up the possibility of using transition metal nitrides, instead of metal oxides, as electron transport layers for photovoltaic devices.
The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations. Using ultrafast pump-probe spectroscopy, we demonstrate that in state-of-the-art mixed-cation ...MHPs ultrafast photo-induced bandgap narrowing occurs with a linear to super-linear dependence on the excited carrier density ranging from 10
cm
to above 10
cm
. Time-domain terahertz spectroscopy reveals carrier localization increases with carrier density. Both observations, the anomalous dependence of the bandgap narrowing and the increased carrier localization can be rationalized by photo-induced lattice fluctuations. The magnitude of the photo-induced lattice fluctuations depends on the intrinsic instability of the MHP lattice. Our findings provide insight into ultrafast processes in MHPs following photoexcitation and thus help to develop a concise picture of the ultrafast photophysics of this important class of emerging semiconductors.
Hole‐transporting layers (HTLs) are an essential component in inverted, p–i–n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, ...perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low‐cost HTLs is of vital importance for propelling p–i–n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self‐assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole‐transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.
Inverted perovskite solar cells (PSCs) attract great attention due to their low‐temperature processing, negligible hysteresis, and superior stability. For these devices, hole‐transport layers play a decisive role in carrier extraction, transport, and perovskite crystallization. This review provides a comprehensive overview of the structural engineering of organic hole‐transport layers utilized in inverted PSCs including conductive polymers, small molecules, and emerging self‐assembled monolayers.
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