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Perovskite solar cells (PSCs) employing organic–inorganic hybrid lead perovskite have attracted much attention as promising next generation solar cells because of their low fabrication cost and extremely high power conversion efficiency (PCE). Exploring new perovskite materials and additives is one of the effective strategies to improve the performance of PSCs. Here, we synthesized formamide iodide (FoAI) and applied it as both a cation material and additive. Although it was revealed that FoAI is not incorporated in the A-site of the perovskite structure, we found that the FoAI additive suppresses δ-FAPbI3 formation and improved the performance of FAPbI3 based PSCs. The PCE was improved from 12.29% to 14.49% by adding 5 mol% of FoAI in the precursor solution. Meanwhile, we found that FoAI additive can also improve the performance of triple-cation PSCs. We believe that FoAI is one of the promising additives to boost the PCE of PSCs without any influence on the composition of the perovskite materials.

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI2) to the precursor solution to increase the device performance. However, recent reports have shown that an excess of PbI2 can accelerate performance loss. In addition, PbI2 is photoactive (band gap ∼2.3 eV), which may lead to parasitic absorption losses in a solar cell. Here we show that devices using small quantities of excess PbI2 exhibit better device performance as compared with stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing the charge extraction. However, the photolysis of PbI2 negates the beneficial effect on charge extraction by leaving voids in the perovskite film and introduces trap states that are detrimental for device performance. We propose that although excess PbI2 provides a good template for enhanced performance, the community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI2, but without the photolysis that negates these beneficial effects under long-term device operation.

Halide perovskites of the form ABX3 have shown outstanding properties for solar cells. The highest reported compositions consist of mixtures of A-site cations methylammonium (MA), formamidinium (FA) and cesium, and X-site iodide and bromide ions, and are produced by solution processing. However, it is unclear whether solution processing will yield sufficient spatial performance uniformity for large-scale photovoltaic modules or compatibility with deposition of multilayered tandem solar cell stacks. In addition, the volatile MA cation presents long-term stability issues. Here, we report the multisource vacuum deposition of FA0.7Cs0.3Pb(I0.9Br0.1)3 perovskite thin films with high-quality morphological, structural, and optoelectronic properties. We find that the controlled addition of excess PbI2 during the deposition is critical for achieving high performance and stability of the absorber material, and we fabricate p-i-n solar cells with stabilized power output of 18.2%. We also reveal the sensitivity of the deposition process to a range of parameters, including substrate, annealing temperature, evaporation rates, and source purity, providing a guide for further evaporation efforts. Our results demonstrate the enormous promise for MA-free perovskite solar cells employing industry-scalable multisource evaporation processes.

Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7 and with local strain8, both of which make devices less stable9. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process10,11, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices. Photoemission electron microscopy images of trap states in halide peroskites, spatially correlated with their structural and compositional factors, may help in managing power losses in optoelectronic applications.

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF4 - , introduced via methylammonium tetrafluoroborate (MABF4 ) in a surface treatment for MAPbI3 perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 106 to 2.5 × 105 s-1 in BF4 - -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). 19 F, 11 B, and 14 N solid-state NMR is used to elucidate the atomic-level mechanism of the BF4 - additive-induced improvements, revealing that the BF4 - acts as a scavenger of excess MAI by forming MAI-MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF4 - .

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 need for current matching is relaxed when the high-bandgap sub-cell is a luminescent perovskite compared to calculations that do not consider luminescence coupling. We show luminescence coupling becomes important in all-perovskite tandems when charge carrier trapping rates are < 10$^{6}$ s$^{-1}$ (corresponding to carrier lifetimes longer than 1 $��$s at low excitation densities) in the high-bandgap sub-cell, which is lowered to 10$^{5}$ s$^{-1}$ in the better-bandgap-matched perovskite-silicon cells. We demonstrate luminescence coupling endows greater flexibility in both sub-cell thicknesses, increased tolerance to different spectral conditions and a reduction in the total thickness of light absorbing layers. To maximally exploit luminescence coupling we reveal a key design rule for luminescent perovskite-based tandems: the high-bandgap sub-cell should always have the higher short-circuit current. Importantly, this can be achieved by reducing the bandgap or increasing the thickness in the high-bandgap sub-cell with minimal reduction in efficiency, thus allowing for wider, unstable bandgap compositions (>1.7 eV) to be avoided. Finally, we experimentally visualise luminescence coupling in an all-perovskite tandem device stack through cross-section luminescence images., 20 pages, 5 figures

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non-exposed feature. Herein, the mystery of the buried interface in full device stacks is deciphered by combining advanced in situ spectroscopy techniques with a facile lift-off strategy. By establishing the microstructure-property relations, the basic losses at the contact interfaces are systematically presented, and it is found that the buried interface losses induced by both the sub-microscale extended imperfections and lead-halide inhomogeneities are major roadblocks toward improvement of device performance. The losses can be considerably mitigated by the use of a passivation-molecule-assisted microstructural reconstruction, which unlocks the full potential for improving device performance. The findings open a new avenue to understanding performance losses and thus the design of new passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

In this work, we demonstrate the optimum utilization of porphyrin-based hole-transporting materials (HTMs), namely, WT3 and YR3, for fabricating triple-cation perovskite solar cells. These newly designed HTMs based on dimeric porphyrin structure exhibit a good HOMO level, high hole mobility, and great charge extraction ability for perovskite solar cells. Moreover, through proper molecular engineering, dimeric porphyrins WT3 and YR3 are capable of forming films free of pinholes, with more uniform and dense surfaces leading to enhanced device performance. Perovskite solar cells using a WT3 HTM achieve a power conversion efficiency (PCE) of 19.44%, which is higher than that using YR3 (17.84%) and even spiro-OMeTAD (18.62%) under 1 Sun AM 1.5G illumination. In addition, WT3-based devices show better stability than spiro-based counterparts under moisture, light-soaking, and thermal testing conditions.

A robust and recyclable monolithic substrate applying all-inorganic metal-oxide selective contact with a nanoporous (np) Au:NiOx counter electrode is successfully demonstrated for efficient perovskite solar cells, of which the perovskite active layer is deposited in the final step for device fabrication. Through annealing of the Ni/Au bilayer, the nanoporous NiO/Au electrode is formed in virtue of interconnected Au network embedded in oxidized Ni. By optimizing the annealing parameters and tuning the mesoscopic layer thickness (mp-TiO2 and mp-Al2O3), a decent power conversion efficiency (PCE) of 10.25% is delivered. With mp-TiO2/mp-Al2O3/np-Au:NiOx as a template, the original perovskite solar cell with 8.52% PCE can be rejuvenated by rinsing off the perovskite material with dimethylformamide and refilling with newly deposited perovskite. A renewed device using the recycled substrate once and twice, respectively, achieved a PCE of 8.17 and 7.72% that are comparable to original performance. This demonstrate...

A clean and flexible synthesis of TiO 2 nanocrystallites for dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) was proposed. Anatase nanocrystallites with different morphologies and sizes ranging from 5 nm to 30 nm were synthesized in one pot via the microwave solvothermal method using different types of alcohol solvents, namely, n-propanol (NPA), isopropanol (IPA), and octanol (OCT). No ionic additive was added during the synthesis. The influences of crystallite size and morphology on the photovoltaic performances of the DSSCs and PSCs were investigated. Results showed that using OCT (5–7 nm) as a photoanode offered the highest power conversion efficiency (PCE), which gave rise to the apparent high dye loading capability and long electron recombination lifetime of OCT. The best PCE of the OCT-based DSSC device reached 9.58%, with the film thickness being only 10.6 μm where no scattering layer was needed. By contrast, the IPA (20–30 nm)-based photoanode showed good adaptability to PSC applications because of its homogeneity and large mean pore size that facilitated the infiltration of the perovskite sensitizer. The IPA-based PSC device attained the highest PCE of 15.3%, with J sc being 21.55 mA/cm 2 and V OC being 1.07 V.

Although perovskite solar cells have achieved extremely high performance in just a few years, their device stability and fabrication cost are still of great concern. For inverted p–i–n perovskite solar cells, the commonly used electron-transporting layers are C60 and PCBM, which have stability issues and are very expensive. Here, we report a novel and highly stable perovskite solar cell using an inorganic electron-transporting layer made of microwave-assisted solution-processed indium-doped zinc oxide (IZO) nanoparticles. With NiO as the hole-transporting layer, the perovskite solar cells with all-inorganic selective contacts demonstrate a decent power conversion efficiency of over 16%. More importantly, the IZO-based perovskite solar cells demonstrate impressive long-term stability under air or light-soaking conditions. With encapsulation, our device retained 85% of the initial power conversion efficiency after 460 hours of light soaking. This result reveals that good device performance, low fabrication cost and impressive light-soaking stability can be realized simultaneously by employing facile microwave-synthesized oxides (IZO and NiO in this work) as inorganic selective contacts.

Perovskite solar cells have attracted much attention as next-generation solar cells because of their high efficiency and low fabrication costs. Moreover, perovskite solar cells are a promising candidate for indoor energy harvesting. We investigated the effect of bandgap tuning on the characteristics of triple cation-based perovskite solar cells under fluorescent lamp illumination. According to the current density–voltage curves, perovskite solar cells with a wider bandgap than the conventional one exhibited improved open-circuit voltage without sacrificing short-circuit current density under fluorescent lamp illumination. Moreover, the wider bandgap perovskite films including a large amount of bromine in the composition did not show phase segregation, which can degrade the photovoltaic performance of perovskite solar cells, after fluorescent lamp illumination. Our results demonstrate the facile strategy to improve the performance of perovskite solar cells under ambient lighting and great potential of perovskite solar cells for indoor applications such as power sources for the internet of things.

New zinc porphyrins Y2 and Y2A2 have been utilized in perovskite solar cells specifically as hole-transporting materials (HTMs) rather than photosensitizers. The combination of MAPbI3 as photosensitizer and porphyrins as HTMs is a potential alternative to well-known MAPbI3/Spiro-OMeTAD hybrids owing to high performance and versatility toward molecular engineering of porphyrin families. A high efficiency of 16.60% is achieved by n-butyl tethered Y2 HTM (VOC = 0.99 V; JSC = 22.82 mA cm–2) which is comparable to that of Spiro-OMeTAD of 18.03% (VOC = 1.06 V; JSC = 22.79 mA cm–2). Both materials possess similar highest occupied molecular orbital level and the same order of magnitude of hole mobility at 10–4 cm2 V–1 s–1. The slightly poorer performance of 10.55% (VOC = 1.01 V; JSC = 17.80 mA cm–2) is obtained for n-dodecyl tethered Y2A2 HTM. This is believed to stem from more surface pinholes when deposited on perovskite leading to an order of magnitude slower mobility.

In this report, we fabricated thiocyanate-based perovskite solar cells with low-pressure vapor-assisted solution process (LP-VASP) method. Photovoltaic performances are evaluated with detailed materials characterizations. Scanning electron microscopy images show that SCN-based perovskite films fabricated using LP-VASP have long-range uniform morphology and large grain sizes up to 1 μm. The XRD and Raman spectra were employed to observe the characteristic peaks for both SCN-based and pure CH3NH3PbI3 perovskite. We observed that the Pb(SCN)2 film transformed to PbI2 before the formation of perovskite film. X-ray photoemission spectra (XPS) show that only a small amount of S remained in the film. Using LP-VASP method, we fabricated SCN-based perovskite solar cells and achieved a power conversion efficiency of 12.72 %. It is worth noting that the price of Pb(SCN)2 is only 4 % of PbI2. These results demonstrate that pseudo-halide perovskites are promising materials for fabricating low-cost perovskite solar cells.

Novel organic–inorganic hybrid perovskite compounds composed of mixed A-site cation (Formamidinium and Cesium) and pseudohalides (SCN and I) ions are successfully synthesized. These new classes of hybrid perovskites photovoltaics exhibited remarkable power conversion efficiency of more than 16% with excellent stability against moisture in ambient environment and under low-light storage condition. The existence of SCN– ion inclusion is confirmed by secondary ion mass spectrometry and Fourier transform infrared spectroscopy. The SCN–-doped pseudohalide is advantageous for the formation of large perovskite grains, as well as the performance and stability of the device.

Transient absorption in the Visible and in the deep-UV is performed on MAPbBr3 thin films with 3.1 eV pump excitation. The UV probe can access higher order transitions in the material exploring different high-symmetry points of the Brillouin zone. Uncorrelated electron-hole pairs are generated within the instrument response function of 150 fs. The photobleaching at 3.3 eV shows that electron cooling happens in ~ 1 ps.

Vapor-based deposition technique is considered as a promising approach for preparing a high-quality and uniform perovskite thin film. With evolution from coevaporation deposition to a low-pressure vapor-assisted solution process, both energy budget and reaction yield for perovskite film fabrications are improved. In this work, a low-pressure hybrid chemical vapor deposition (LPHCVD) method is applied to fabricate CH 3 NH 3 PbI 3 perovskite films. The crucial dependence of working pressure on the perovskite formation is revealed. Moreover, the reaction time plays an important role in controlling the quality of the synthesized perovskite film. Efficient mesoscopic perovskite solar cells of 14.99% and perovskite modules (active area of 8.4 cm2) of 6.22% are achieved by this LPHCVD method

In this article, we apply indium zinc oxide (IZO) transparent conductive oxide film for the fabrication of bifacial-illuminated perovskite solar cells (PSCs) and two substrate type PSCs based on non-fluorine-doped tin oxide (FTO) and ultrathin substrates. The Si/perovskite tandem solar cell system employing bifacial-illuminated PSCs delivers a decent power conversion efficiency (PCE) of 19.51% with 1 cm2 active area. The investigation of PSCs without FTO substrate is further demonstrated using sputtered Ti metal as a substrate. The devices employing a Ti cathode coating on a slide and ultrathin glass could deliver 13.5% and 13.6% of PCE, respectively, revealing a promising application for cost-effective and light-weight non-FTO devices.

The fabrication of multidimensional organometallic halide perovskite via a low-pressure vapor-assisted solution process is demonstrated for the first time. Phenyl ethyl-ammonium iodide (PEAI)-doped lead iodide (PbI2 ) is first spin-coated onto the substrate and subsequently reacts with methyl-ammonium iodide (MAI) vapor in a low-pressure heating oven. The doping ratio of PEAI in MAI-vapor-treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, UV-vis absorption and photoluminescence spectra, and the resultant device performance. Multiple photoluminescence spectra are observed in the perovskite film starting with high PEAI/PbI2 ratio, which suggests the coexistence of low-dimensional perovskite (PEA2 MAn-1 Pbn I3n+1 ) with various values of n after vapor reaction. The dimensionality of the as-fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI2 ratio varies from 2 to 0. Scanning electron microscopy images and Kelvin probe force microscopy mapping show that the PEAI-containing perovskite grain is presumably formed around the MAPbI3 perovskite grain to benefit MAPbI3 grain growth. The device employing perovskite with PEAI/PbI2 = 0.05 achieves a champion power conversion efficiency of 19.10% with an open-circuit voltage of 1.08 V, a current density of 21.91 mA cm-2 , and a remarkable fill factor of 80.36%.

With the rapid progress in deposition techniques for hybrid organic-inorganic perovskite (HOIP) thin films, this new class of photovoltaic (PV) technology has achieved material quality and power conversion efficiency comparable to those established technologies. Among the various techniques for HOIP thin films preparation, vapor based deposition technique is considered as a promising alternative process to substitute solution spin-coating method for large-area or scale-up preparation. This technique provides some unique benefits for high-quality perovskite crystallization, which are discussed in this research update.

Vapor-based deposition technique is considered as a promising approach for preparing a high-quality and uniform perovskite thin film. With evolution from coevaporation deposition to a low-pressure vapor-assisted solution process, both energy budget and reaction yield for perovskite film fabrications are improved. In this paper, a low-pressure hybrid chemical vapor deposition (LPHCVD) method is applied to fabricate CH3NH3PbI3 perovskite films. The crucial dependence of working pressure on the perovskite formation is revealed. Moreover, the reaction time plays an important role in controlling the quality of the synthesized perovskite film. Efficient perovskite solar cells of 14.99% (mesoscopic), 15.37% (planar), and perovskite modules (active area of 8.4 cm2) of 6.22% are achieved by this LPHCVD method.

Small-sized single crystalline silicon solar cells (ca. 25 mm2) were fabricated by a non-vacuum process as an energy supply for small devices (ubiquitous devices: a wristwatch, desktop calculator etc.) and processed for a tandem solar-cell research. A side-edge etching procedure was performed in order to eliminate detrimental cracks to improve photovoltaic properties. Moreover, the new structure of the small-sized solar cell with side contact provides reduced shadow loss of contacts. After the structural and procedural optimization, a conversion efficiency of 16.4% was achieved by a non-vacuum process with 3 mm × 8 mm surface dimension solar cell. Finally, the photovoltaic characteristics of small silicon cells, as a function of light intensity for the ubiquitous purposes were compared with amorphous silicon solar cells. In addition, the facile processed silicon solar cell was integrated into mechanically-stacked tandem solar cells with perovskite solar cells by direct contact of TCO layers of each sub-cell. The counter electrode MoOX/IZO over HTM (spiro-OMeTAD) in top perovskite cells is physically placed on top of the bottom Si solar cell to form a series tandem configuration.

Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (=180 ��C to be used to coat the perovskite. This is >=70 ��C higher than achievable by current methods and results in more conductive TiOx films, boosting solar cell efficiencies by >2%. Likewise, when AP-CVD SnOx (x ~ 2) is grown on perovskites, there is also minimal damage to the perovskite beneath. The SnOx layer is pinhole-free and conformal, which reduces shunting in devices, and increases steady-state efficiencies from 16.5% (no SnOx) to 19.4% (60 nm SnOx), with fill factors reaching 84%. This work shows AP-CVD to be a versatile technique for growing oxides on thermally-sensitive materials., R.D.R and R.A.J contributed equally. 23 pages. 6 figures