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The efficiency of perovskite solar cells is affected by open-circuit voltage losses due to radiative and non-radiative charge recombination. When estimated using sensitive photocurrent measurements that cover the above- and sub-bandgap regions, the radiative open-circuit voltage is often unphysically low. Here we report sensitive photocurrent and electroluminescence spectroscopy to probe radiative recombination at sub-bandgap defects in wide-bandgap mixed-halide lead perovskite solar cells. The radiative ideality factor associated with the optical transitions increases from 1, above and near the bandgap edge, to ~2 at mid-bandgap. Such photon energy-dependent ideality factor corresponds to a many-diode model. The radiative open-circuit voltage limit derived from this many-diode model enables differentiating between radiative and non-radiative voltage losses. The latter are deconvoluted into contributions from the bulk and interfaces via determining the quasi-Fermi level splitting. The experiments show that while sub-bandgap defects do not contribute to radiative voltage loss, they do affect non-radiative voltage losses.

Crystallization of low-dimensional perovskites is a complex process that leads to multidimensional films comprising two-dimensional (2D), quasi-2D, and three-dimensional (3D) phases. Most quasi-2D perovskite films possess a regular gradient with 2D phases located at the bottom of the film and 3D phases at the top. Recently, multiple studies have reported reverse-graded perovskite films, where the location of the 2D and 3D structures is inverted. The underlying reasons for such a peculiar phase distribution are unclear. While crystallization of regular-graded quasi-2D perovskites has been described as starting with 3D phases from the liquid-air interface, the film formation of reverse-graded films has not been investigated yet. Here, we examine the impact of the alkyl chain length on the formation of regular- and reverse-graded perovskites using n-alkylammonium ions. We find that long alkyl chains reverse the phase distribution gradient. By combining photoluminescence spectroscopy with in situ optical absorption measurements, we demonstrate that crystallization starts at the liquid-N2 interface, though as 3D phases for short-chain n-alkylammonium ions and as quasi-2D phases for long chains. We link this behavior to enhanced van der Waals interactions between long-chain n-alkylammonium ions in polar solvents and their tendency to accumulate at the liquid-N2 interface, creating a concentration gradient along the film thickness.

Wide-bandgap (>1.6 eV) mixed-halide perovskites tend to experience notable open-circuit voltage losses in solar cells due to non-radiative recombination. Here, the effects of defects and their passivation on the non-radiative recombination of charge carriers in mixed-halide perovskite solar cells are studied. By determining the quasi-Fermi level splitting via absolute photoluminescence measurements of perovskite layers with and without charge transport layers, bulk and interface contributions are disentangled and compared to the radiative open-circuit voltage. For wide-bandgap perovskites, non-radiative recombination present in the pristine perovskite layers increases with increasing bandgap. The most prominent loss, located at the perovskite – electron transport layer interface (ETL), can be reduced by interface passivation for the different bandgaps studied (1.58 to 1.82 eV) to a level close to that of the intrinsic losses. By combining light-intensity-dependent absolute photoluminescence spectroscopy with sensitive spectral photocurrent measurements it is found that different passivation agents result in a similar decrease of the non-radiative recombination for different bandgaps. This suggests that the gained open-circuit voltage is not due to an improved energy level alignment at the perovskite – ETL interface. Instead, passivation involves eliminating the direct contact between the perovskite semiconductor and the ETL.

Single junction crystalline silicon (c-Si) solar cells are reaching their practical efficiency limit whereas perovskite/c-Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c-Si solar cells. Next to low-thermal budget silicon heterojunction architecture, high-thermal budget carrier-selective passivating contacts (CSPCs) based on polycrystalline-SiOx (poly-SiOx) also constitute a promising architecture for high efficiency perovskite/c-Si tandem solar cells. In this work, we present the development of c-Si bottom cells based on high temperature poly-SiOx CSPCs and demonstrate novel high efficiency four-terminal (4T) and two-terminal (2T) perovskite/c-Si tandem solar cells. First, we tuned the ultra-thin, thermally grown SiOx. Then we optimized the passivation properties of p-type and n-type doped poly-SiOx CSPCs. Here, we have optimized the p-type doped poly-SiOx CSPC on textured interfaces via a two-step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.

2H-Benzotriazol-2-ylethylammonium bromide and iodide and its difluorinated derivatives are synthesized and employed as interlayers for passivation of formamidinium lead triiodide (FAPbI3) solar cells. In combination with PbI2 and PbBr2, these benzotriazole derivatives form two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) as evidenced by their crystal structures and thin film characteristics. When used to passivate n-i-p FAPbI3 solar cells, the power conversion efficiency improves from 20% to close to 22% by enhancing the open-circuit voltage. Quasi-Fermi level splitting experiments and scanning electron microscopy cathodoluminescence hyperspectral imaging reveal that passivation provides a reduced nonradiative recombination at the interface between the perovskite and hole transport layer. Photoluminescence spectroscopy, angle-resolved grazing-incidence wide-angle X-ray scattering, and depth profiling X-ray photoelectron spectroscopy studies of the 2D/three-dimensional (3D) interface between the benzotriazole RPP and FAPbI3 show that a nonuniform layer of 2D perovskites is enough to passivate defects, enhance charge extraction, and decrease nonradiative recombination.

Monolithic perovskite/c-Si tandem solar cells have attracted enormous research attention and have achieved efficiencies above 30%. This work describes the development of monolithic tandem solar cells based on silicon heterojunction (SHJ) bottom- and perovskite top-cells and highlights light management techniques assisted by optical simulation. We first engineered (i)a-Si:H passivating layers for (100)-oriented flat c-Si surfaces and combined them with various (n)a-Si:H, (n)nc-Si:H, and (n)nc-SiOx:H interfacial layers for SHJ bottom-cells. In a symmetrical configuration, a long minority carrier lifetime of 16.9 ms was achieved when combining (i)a-Si:H bilayers with (n)nc-Si:H (extracted at the minority carrier density of 1015 cm–3). The perovskite sub-cell uses a photostable mixed-halide composition and surface passivation strategies to minimize energetic losses at charge-transport interfaces. This allows tandem efficiencies above 23% (a maximum of 24.6%) to be achieved using all three types of (n)-layers. Observations from experimentally prepared devices and optical simulations indicate that both (n)nc-SiOx:H and (n)nc-Si:H are promising for use in high-efficiency tandem solar cells. This is possible due to minimized reflection at the interfaces between the perovskite and SHJ sub-cells by optimized interference effects, demonstrating the applicability of such light management techniques to various tandem structures.

Solar-assisted water electrolysis is a promising technology for storing the energy of incident solar irradiation into hydrogen as a fuel. Here, an integrated continuous flow electrochemical reactor coupled to a monolithic perovskite-silicon tandem solar cell is demonstrated that provides light-driven electrochemical solar-to-hydrogen conversion with an energy conversion efficiency exceeding 21% at 1-Sun equivalent light intensity and stable operation during three simulated day-night cycles.

Photoinduced halide segregation hinders widespread application of three-dimensional (3D) mixed-halide perovskites. Much less is known about this phenomenon in lower-dimensional systems. Here, we study photoinduced halide segregation in lower-dimensional mixed iodide-bromide perovskites (PEA2MAn-1Pbn(BrxI1-x)3n+1, with PEA+: phenethylammonium and MA+: methylammonium) through time-dependent photoluminescence (PL) spectroscopy. We show that layered two-dimensional (2D) structures render additional stability against the demixing of halide phases under illumination. We ascribe this behavior to reduced halide mobility due to the intrinsic heterogeneity of 2D mixed-halide perovskites, which we demonstrate via 207Pb solid-state NMR. However, the dimensionality of the 2D phase is critical in regulating photostability. By tracking the PL of multidimensional perovskite films under illumination, we find that while halide segregation is largely inhibited in 2D perovskites (n = 1), it is not suppressed in quasi-2D phases (n = 2), which display a behavior intermediate between 2D and 3D and a peculiar absence of halide redistribution in the dark that is only induced at higher temperature for the quasi-2D phase.

Targeted strategies to overcome defects in organic semiconductors require insight into their identity and origin. Here the formation, nature, and location of defects is studied in PM6:Y6 organic solar cells by sensitive EQE measurements. Exposure of the active layer to ambient atmosphere and H2O-saturated compressed air indicates that a trace constituent in ambient air causes the formation of defects. By exposing the active layer to O3-enriched air, O3 is identified as the species creating defects in PM6:Y6 blends. Aging of complete inverted (n–i–p) configuration solar cells in H2O-saturated compressed air also increases the defect response. This is attributed to a reduced band bending at the PM6:Y6 | MoO3 hole-collecting contact, caused by a change in work function of MoO3 interacting with the H2O, which allows more defect states to be filled and available for photoexcitation. By measuring energy resolved-electrochemical impedance spectroscopy and by fabricating semitransparent cells, regular architecture cells, and semitransparent cells with an optical spacer−mirror stack it is found that defects originate predominantly from PM6 and are located near the top electrode, independent of device polarity. Because O3 is omnipresent in ambient atmosphere, albeit in small amounts, it likely causes defects in many organic semiconductors exposed to ambient air.

Interface layers used for electron transport (ETL) and hole transport (HTL) often significantly enhance the performance of organic solar cells (OSCs). Surprisingly, interface engineering for hole extraction has received little attention thus far. By finetuning the chemical structure of carbazole-based self-assembled monolayers with phosphonic acid anchoring groups, varying the length of the alkane linker (2PACz, 3PACz, and 4PACz), these HTLs were found to perform favorably in OSCs. Compared to archetypal PEDOT:PSS, the PACz monolayers exhibit higher optical transmittance and lower resistance and deliver a higher short-circuit current density and fill factor. Power conversion efficiencies of 17.4% have been obtained with PM6:BTP-eC9 as the active layer, which was distinctively higher than the 16.2% obtained with PEDOT:PSS. Of the three PACz derivatives, the new 3PACz consistently outperforms the other two monolayer HTLs in OSCs with different state-of-the-art nonfullerene acceptors. Considering its facile synthesis, convenient processing, and improved performance, we consider that 3PACz is a promising interface layer for widespread use in OSCs.

Fabricating efficient perovskite solar cells on steel substrates could enable easy building integration of this photovoltaic technology. Herein, an n–i–p perovskite solar cell is developed on steel substrates for top illumination. The optimized stack uses a Ti bottom electrode, covered with an indium tin oxide (ITO) interlayer and a SnO2 electron transport layer passivated by [6,6]-phenyl-C61-butyric acid. The active layer is a triple-cation perovskite. A thermally evaporated tris(4-carbazoyl-9-ylphenyl)amine)/MoO3 bilayer acts as hole transport layer. The transparent top contact consists of ITO with a MgF2 antireflective coating. Optical analysis shows small parasitic absorption and reflectance losses for this stack, which provides 15.9% power conversion efficiency when fabricated on glass. On steel, covered with a polyamide imide planarization coating to moderate the surface roughness (R p), the highest efficiency is 15.2% for high-gloss steel (R p≈ 200 nm), 14.9% for battery steel (R p≈ 500 nm), 14.2% for packaging steel (R p≈ 1500 nm), and 13.8% for construction steel (R p≈ 2500 nm). While the short-circuit current density and open-circuit voltage are invariant, the fill factor decreases with increasing R p due to increasing series resistance and decreasing shunt resistance. The yield of working devices remain high, also for the roughest substrates.

Photoluminescence spectroscopy is a simple and powerful characterization technique to determine material properties and dynamic effects in metal–halide perovskite optoelectronic systems. However, self-absorption and thin film cavity effects, amplified due to their high refractive indices, can have a significant impact on spectral lineshapes, affecting the inferences drawn from such characterization. In this work, key variables extrinsic to the perovskite material influencing the photoluminescence spectrum of perovskite thin films are identified and an optical model to account for these factors is proposed. The model is used to extract the intrinsic spectrum of a film using complex refractive indices and film thickness as input variables. Lastly, the use of the model is extended to multilayer systems to demonstrate its applicability to complex device-relevant stacks.

Perovskite-based multijunction solar cells are a potentially cost-effective technology that can help surpass the efficiency limits of single-junction devices. However, both mixed-halide wide-bandgap perovskites and lead-tin narrow-bandgap perovskites suffer from non-radiative recombination due to the formation of bulk traps and interfacial recombination centers which limit the open-circuit voltage of sub-cells and consequently of the integrated tandem. Additionally, the complex optical stack in a multijunction solar cell can lead to losses stemming from parasitic absorption and reflection of incident light which aggravates the current mismatch between sub-cells, thereby limiting the short-circuit current density of the tandem. Here, an integrated all-perovskite tandem solar cell is presented that uses surface passivation strategies to reduce non-radiative recombination at the perovskite-fullerene interfaces, yielding a high open-circuit voltage. By using optically benign transparent electrode and charge-transport layers, absorption in the narrow-bandgap sub-cell is improved, leading to an improvement in current-matching between sub-cells. Collectively, these strategies allow the development of a monolithic tandem solar cell exhibiting a power-conversion efficiency of over 23%.

An efficient substrate-configuration p-i-n metal-halide perovskite solar cell (PSC) is fabricated on a polymer-coated steel substrate. The optimized cell employs a Ti bottom electrode coated with a thin indium tin oxide (ITO) interlayer covered with a self-assembled [2-(9H-carbazol-9-yl)ethyl]phosphonic acid monolayer as a hole-selective contact. A triple-cation perovskite is used as the absorber layer. Thermally evaporated C60and atomic layer deposited SnO2layers serve to create an electron-selective contact. The cells use an ITO top electrode with an antireflective MgF2coating. The optimized cell fabricated on a polymer-coated steel substrate reaches a power conversion efficiency of 16.5%, which approaches the 18.4% efficiency of a p-i-n reference superstrate-configuration cell that uses a similar stack design. Optical simulations suggest that the remaining optical losses are due to the absorption of light by the ITO top electrode, the C60layer, the Ti bottom electrode, and reflection from the MgF2coating in almost equal amounts. The major loss is, however, in the fill factor as a result of an increased sheet resistance of the top ITO electrode.

Thin-film organic and perovskite solar cells have seen tremendous advances in recent years. A common technique to characterize the low-energy states in these solar cells is electroluminescence spectroscopy. Stemming from their thin film nature, however, the outcoupled electroluminescence spectra of these devices are affected by cavity effects and self-absorption. Herein, a modeling approach is developed taking into account self-absorption, cavity effects, and a nonhomogeneous emission profile, to correct the outcoupled spectrum, thereby yielding the intrinsically emitted spectrum. The modeling approach is then employed to structurally investigate the impact of these effects for a variety of active layer thicknesses, complex refractive indices, device structures, and emission profiles. The data presented uncover trends and provide guidelines that enable gauging the impact of self-absorption and cavity effects on the electroluminescence spectra of a large set of thin-film devices.

Low-dimensional perovskites attract increasing interest due to tunable optoelectronic properties and high stability. Here, it is shown that perovskite thin films with a vertical gradient in dimensionality result in graded electronic bandgap structures that are ideal for photodiode applications. Positioning low-dimensional, vertically-oriented perovskite phases at the interface with the electron blocking layer increases the activation energy for thermal charge generation and thereby effectively lowers the dark current density to a record-low value of 5 × 10−9 mA cm−2 without compromising responsivity, resulting in a noise-current-based specific detectivity exceeding 7 × 1012 Jones at 600 nm. These multidimensional perovskite photodiodes show promising air stability and a dynamic range over ten orders of magnitude, and thus represent a new generation of high-performance low-cost photodiodes.

Solar-assisted water electrolysis is a promising technology for storing the energy of incident solar irradiation into hydrogen as a fuel. Here, an integrated continuous flow electrochemical reactor coupled to a monolithic perovskite-silicon tandem solar cell is demonstrated that provides light-driven electrochemical solar-to-hydrogen conversion with an energy conversion efficiency exceeding 21% at 1-Sun equivalent light intensity and stable operation during three simulated day-night cycles., Photovoltaic Materials and Devices