Bandgap adjustment assisted preparation of >18% CsyFA1−yPbIxBr3−x-based perovskite solar cells using a hybrid spraying process

The preparation of Cs_yFA_1−yPbI_xBr_3−x-based perovskite by ultrasonic spraying has valuable application in the preparation of tandem solar cells on textured substrates due to its excellent stability and the advantages of large-area uniform preparation from the spraying technology. However, the bandgap of perovskite prepared by spraying method is difficult to adjust, and perovskites with a wide bandgap have the issue of phase instability. Here, we improved the crystallinity of the perovskite by simply controlling the post-annealing temperature. The results show that perovskite film prepared by hybrid spray method has the best crystallinity and device performance at a post-annealing temperature of 170 °C. On this basis, the bandgap of perovskite was changed from 1.53 eV to 1.76 eV by controlling the ratio of the organic halogen precursor solution. When the bandgap is 1.57 eV, a perovskite solar cell with an efficiency of 18.31% is obtained.

High-efficiency perovskite solar cells with good grain morphology and adjustable band gap were prepared by ultrasonic spray.     

Bandgap tuning strategy by cations and halide ions of lead halide perovskites learned from machine learning

Bandgap engineering of lead halide perovskite materials is critical to achieve highly efficient and stable perovskite solar cells and color tunable stable perovskite light-emitting diodes. Herein, we propose the use of machine learning as a tool to predict the bandgap of the perovskite materials from their compositions. By learning from the experimental results, machine learning algorithms present reliable performance in predicting the bandgap of the lead halide perovskites. The linear regression model can be used to manually predict the bandgap of the perovskite with the formula of Cs_aFA_bMA_(1−a−b)Pb(Cl_xBr_yI_(1−x−y))₃ (FA = formamidinium, MA = methylammonium). The neural network (NN) algorithm, which takes the interplay of cations and halide ions into account in predicting the bandgap, presents higher accuracy (with a RMSE of 0.05 eV and a Pearson coefficient larger than 0.99). Furthermore, the compositions of the mixed halide perovskites with desirable bandgaps and high iodide ratio for suppressing halide segregation are predicted by NN algorithm. These results highlight the power of machine learning in predicting the bandgap of the perovskites from their compositions and provide bandgap tuning directions for experiments.

Bandgap engineering of lead halide perovskite materials is critical to achieve highly efficient and stable perovskite solar cells and color tunable stable perovskite light-emitting diodes.     

The lattice reconstruction of Cs-introduced FAPbI1.80Br1.20 enables improved stability for perovskite solar cells

Inorganic–organic hybrid perovskite solar cells (PSCs) have stirred up a new research spree in the field of photovoltaics due to its high photoelectric conversion efficiency and simple preparation process. In recent years, the research of inorganic–organic hybrid PSCs has been widely reported, among which FA⁺/Cs⁺ PSCs are especially outstanding. However, there are few reports explaining the lattice structural change mechanism of Cs_xFA_1−xPbI_1.80Br_1.20 PSCs from the view of chemical bonds. In this work, a facile method of 15% Cs⁺ cations partially substituting FA⁺ cations has been presented to enhance the structural stability and photovoltaic performances of FAPbI_1.80Br_1.20 PSCs. The partial incorporation of Cs⁺ in FAPbI_1.80Br_1.20 resulted in a more beneficial tolerance factor and inhibited the deep defect state of elemental Pb. More importantly, it inhibited the phase transition from the cubic black α-phase to the hexagonal yellow δ-phase of FAPbI_1.80Br_1.20. Moreover, the power conversion efficiency (PCE) of Cs_0.15FA_0.85PbI_1.80Br_1.20 PSCs achieved a substantial improvement. The stability also achieved a remarkable promotion, which was demonstrated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Nuclear Magnetic Resonance (NMR). These analyses indicate that 15% Cs⁺ can induce the lattice shrinkage, reduce the specific traps and inhibit the phase transition, thus improving the structural stabilities of Cs_0.15FA_0.85PbI_1.80Br_1.20 PSCs under atmosphere and calefaction. These results provide an effective way for fabricating stable and efficient inorganic–organic perovskite solar cells with promising properties.

The Cs_0.15FA_0.85PbI_1.80Br_1.20 perovskite shows excellent structural stability, while 15% Cs⁺ can reduce specific traps such as Pb⁰ and I⁰.     

Localized incorporation of cesium ions to improve formamidinium lead iodide layers in perovskite solar cells

For the perovskite solar cells with formamidinium lead iodide (FAPbI₃) as a light harvester, cesium ions (Cs⁺) can be used to stabilize the perovskite crystal structure of FAPbI₃. However, the incorporation of Cs⁺ ions usually reduces the grain size and degrades the crystallization of FAPbI₃ layers, and this is harmful to the photovoltaic performance of solar cells. In this work, we incorporate Cs⁺ ions into FAPbI₃ layers using the interfacial doping method, which is different from the mixed solution doping method in previous reports. Elemental analysis indicates that Cs⁺ dopants cannot be detected at the outer surfaces of perovskite layers, and the majority of Cs⁺ dopants should be localized in the vicinity of TiO₂/perovskite interfaces, which is remarkably different from the distribution of Cs⁺ dopants in the perovskite layers prepared using the mixed solution doping method. It is found that interfacial doping method can avoid the blue shift of the light absorption edge and can improve the crystallization of FAPbI₃ layers. For the optimized conditions, Cs_xFA_1−xPbI₃ solar cells prepared using the interfacial doping method achieve a power conversion efficiency (PCE) of 17.1%, which is better than the PCE of Cs_xFA_1−xPbI₃ devices prepared using the mixed solution doping method.

An interfacial doping method leads to a localized profile of dopants at interfaces, which results in improved photovoltaic performance.     

Enhancing the thermal stability of the carbon-based perovskite solar cells by using a CsxFA1−xPbBrxI3−x light absorber

Despite the impressive photovoltaic performance with a power conversion efficiency beyond 23%, perovskite solar cells (PSCs) suffer from poor long-term stability, failing by far the market requirements. Although many efforts have been made towards improving the stability of PSCs, the thermal stability of PSCs with CH₃NH₃PbI₃ as a perovskite and organic hole-transport material (HTM) remains a challenge. In this study, we employed the thermally stable (NH₂)₂CHPbI₃ (FAPbI₃) as the light absorber for the carbon-based and HTM-free PSCs, which can be fabricated by screen printing. By introducing a certain amount of CsBr (10%) into PbI₂, we obtained a phase-stable Cs_xFA_1−xPbBr_xI_3−x perovskite by a “two-step” method and improved the device power conversion efficiency from 10.81% to 14.14%. Moreover, the as-prepared PSCs with mixed-cation perovskite showed an excellent long-term stability under constant heat (85 °C) and thermal cycling (−30 °C to 85 °C) conditions. These thermally stable and fully-printable PSCs would be of great significance for the development of low-cost photovoltaics.

Mixed-cation Cs_xFA_1–xPbBr_xI_3–x perovskite was used as light absorber for the carbon-based perovskite solar cells, and the as-prepared solar devices showed excellent long-term stability under constant heat (85 °C) and thermal cycling (−30 °C to 85 °C) condition.     

Investigations aimed at producing 33% efficient perovskite–silicon tandem solar cells through device simulations

The conversion efficiencies for silicon-based photovoltaic devices have become stagnant, with the record conversion efficiency of 26.7% achieved in 2017. This record efficiency is also close to the theoretical Auger limit of 29.4% for single-junction silicon solar cells. Therefore, it is anticipated that further enhancement in conversion efficiency could only be achieved by adopting multijunction or tandem concepts for silicon PV devices. In this context, perovskites are widely preferred for tandem application with silicon solar cells to mitigate thermalization and non-absorbed photon losses to achieve higher conversion efficiencies. The perovskite–silicon (PVK–Si) tandem design can deliver 45.1% efficiency, and currently, this design holds a record conversion efficiency of 29.5%. Therefore, critical research and development activities are required to unlock the potential of such devices. Thus, we have designed and investigated enhanced hole extraction PVK–Si monolithic tandem solar cells with 33% power conversion efficiency (PCE) to make a humble contribution in this field. The device is facilitated with Me-4PACz and ITO-based ideal tunnel recombination junctions for current matching, with parasitic absorption losses. Detailed standalone and tandem analysis has been carried out in terms of absorber layer thickness variation, illuminated current density–voltage (J–V) curves, external quantum efficiency (EQE), energy band diagrams (EBDs), filtered spectra, filtered integrated power, current matching, and tandem PV parameters to finalize the conversion efficiency. The device constructed using a 1.68 eV perovskite top cell and 1.12 eV c-Si-based heterojunction with an intrinsic thin layer (HIT) based bottom cell showed an open-circuit voltage, V_OC, of as high as 2.02 V. The comprehensive analysis of PVK–Si tandem devices reported in this work may pave the way for developing high-efficiency tandem solar cells in the future.

The conversion efficiencies for silicon-based photovoltaic devices have become stagnant, with the record conversion efficiency of 26.7% achieved in 2017.     

Formation of cubic perovskite alloy containing the ammonium cation of 2D perovskite for high performance solar cells with improved stability

The perovskite solar cells have demonstrated to be strong competitors for conventional silicon solar cells due to their remarkable power conversion efficiency. However, their structural instability is the biggest obstacle to commercialization. To address these issues, we prepared (CH₃NH₃)_1−x(HC(NH₂)₂)_xPbI₃ (CH₃NH₃ = MA, HC(NH₂)₂ = FA) perovskite alloys that contain ethylammonium (EA, CH₃CH₂NH₃⁺) and benzylammonium (BA, C₆H₅CH₂NH₃⁺) cations with no new additional two-dimensional (2D) perovskite phases. The crystal structures of alloy perovskites exhibit the cubic phase, which decreased the cation disorder and the intrinsic instability compared to 3D MAPbI₃ perovskite. The band gaps of the alloy perovskites are almost the same as the corresponding 3D perovskites, which exhibit a high refractive index, a large absorption coefficient, and paramagnetic properties for the production of high performance photovoltaic devices. After we constructed the solar cell with the configuration of regular (n–i–p) solar cells using the alloy perovskites, the power conversion efficiencies (PCE) of the MA_0.83EA_0.17PbI₃ perovskite solar cell showed the highest efficiency, which was 10.22%, under 1 sun illumination.

We prepared (MA)_1−x(FA)_xPbI₃ (CH₃NH₃ = MA, HC(NH₂)₂ = FA) perovskite alloys that contain ethylammonium (CH₃CH₂NH₃⁺) and benzylammonium (C₆H₅CH₂NH₃⁺) cations with no new additional two-dimensional perovskite phases.     

Synergistic effect of the anti-solvent bath method and improved annealing conditions for high-quality triple cation perovskite thin films

One step solution processing together with anti-solvent engineering is a tested route in producing high-quality perovskite films due to its simplicity and low fabrication costs. Commercialization of perovskites will require replacing the anti-solvent drip process and lowering annealing temperatures to decrease the energy payback time. In this work, we successfully replace the anti-solvent drip with the anti-solvent bath (ASB) method through balancing the methylammonium (MA) and formamidinium (FA) cations to produce high-quality cesium (Cs)/FA/MA triple cation perovskite films. Furthermore, the annealing parameters of Cs_0.05FA_0.16MA_0.79PbI_2.7Br_0.3 are enhanced to allow for a low-temperature fabrication process when paired with the ASB method. This resulted in the formation of remarkable films with micrometer grains and few defects. Self-powered photodetectors were constructed using the improved conditions, resulting in devices that exhibited a low dark current, an on/off ratio of >10³, and a rapid rise time of 12.4 μs. The conclusion of this work shows that ASB can be applied to triple cation perovskites and in using this method, the previously established optimal annealing temperature is lowered.

High quality triple cation perovskite thin films realized through the combination of the anti-solvent bath method and low temperature annealing.     

Broadband emission from zero-dimensional Cs4PbI6 perovskite nanocrystals

To overcome the drawbacks in three-dimensional (3D) perovskites, such as instability, surface hydration, and ion migration, recently researchers have focused on comparatively stable lower-dimensional perovskite derivatives. All-inorganic zero-dimensional (0D) perovskites (e.g., Cs₄PbX₆; X = Cl⁻, Br⁻, I⁻) can be evolved as a high performing material due to their larger exciton binding energy and better structural stability. The clear understanding of carrier recombination process in 0D perovskites is very important for better exploitation in light-emitting devices. In this work, we comprehensively studied the light emission process in 0D Cs₄PbI₆ nanocrystals (NCs) and interestingly we observe intense white light emission at low temperatures. According to our experimental observations, we conclude that the white light emission contains an intrinsic exciton emission at 2.95 eV along with a broadband emission covering from 1.77 eV to 2.6 eV. We also confirm that the broadband emission is related to the carrier recombination of both self-trapped excitons (STE) and defect state trapped excitons. Our investigations reveal the carrier recombination processes in Cs₄PbI₆ NCs and provide experimental guidelines for the potential application of white light generation.

The broadband white light emission is realized in zero dimensional (OD) Cs₄PbI₆ nanocrystals at low temperatures. The white light emission originates from recombination of both self-trapped excitons and defect state trapped excitons.     

Unexpected bowing band evolution in an all-inorganic CsSn1−xPbxBr3 perovskite

We theoretically investigated the structural and electronic properties of the all-inorganic perovskite CsSn_1−xPb_xBr₃, compared with the mixed perovskite compound MA_yCs_1−ySn_1−xPb_xBr₃, based on first-principle calculations. It has been demonstrated that Pb and Sn atoms are inclined to occupy the lattice sites uniformly in the all-inorganic perovskite, and this is distinguished from the most stable configurations observed in the mixed Cs-MA system. It is interesting that small Sn atoms prefer to stay close to the large MA⁺ cations, leading to smaller local structural distortion. Through spin-orbital coupling calculations, we found non-linear bowing band evolution in the all-inorganic mixed Sn–Pb system with a small bowing parameter (b = 0.35), while the band gap of MA_yCs_1−ySn_1−xPb_xBr₃ was clearly reduced as the ratio of MA was around 0.5 (y ≥ 0.25). We determined the bowing band evolution in the mixed cation perovskites and the intrinsic electronic deficiency of the all-inorganic perovskite to obtain the optimal band gap.

We theoretically investigated the structural and electronic properties of the all-inorganic perovskite CsSn_1−xPb_xBr₃, compared with the mixed perovskite compound MA_yCs_1−ySn_1−xPb_xBr₃, based on first-principle calculations.     

Hot carrier relaxation in Cs2TiIyBr6−y (y = 0, 2 and 6) by a time-domain ab initio study

Cs₂TiI_yBr_6−y is a potential light absorption material for all-inorganic lead free perovskite solar cells due to its suitable and tunable bandgap, high optical absorption coefficient and high environmental stability. However, solar cells fabricated based on Cs₂TiI_yBr_6−y do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs₂TiI_yBr_6−y (y = 0, 2 and 6) were investigated by a time-domain density functional theory combined with the non-adiabatic molecular dynamics method. It was found that the relaxation time of the hot carriers in Cs₂TiI_yBr_6−y ranges from 2–3 ps, which indicates that the hot carriers within 10 nm from the Cs₂TiI_yBr_6−y/TiO₂ interface can be effectively extracted before their energy is lost completely. The carrier-phonon non-adiabatic coupling (NAC) analyses demonstrate that the longer hot electron relaxation time in Cs₂TiI₂Br₄ compared with that in Cs₂TiBr₆ and Cs₂TiI₆ originates from its weaker NAC strength. Furthermore, the electron–phonon interaction analyses indicate that the relaxation of hot electrons mainly comes from the coupling between the electrons distributed on the Ti–X bonds and the Ti–X vibrations, and that of hot holes can be attributed to the coupling between the electrons distributed on the X atoms and the distortions of [TiI_yBr_6−y]²⁻. The simulation results indicate that Cs₂TiI₂Br₄ should be better than Cs₂TiBr₆ and Cs₂TiI₆ to act as a light absorption layer based on the hot carrier energy loss, and the hot electron relaxation time in Cs₂TiI_yBr_6−y can be adjusted by tuning the proportion of the I element.

The hot carriers within 10 nm from the Cs₂TiI_yBr_6−y/TiO₂ interface can be extracted effectively due to their 2–3 ps relaxation time.     

Indirect to direct band gap transition through order to disorder transformation of Cs2AgBiBr6via creating antisite defects for optoelectronic and photovoltaic applications

Non-toxic lead free inorganic metal halide cubic double perovskites have drawn a lot of attention for their commercial use in optoelectronic and photovoltaic devices. Here we have explored the structural, electronic, optical and mechanical properties of lead-free non-toxic inorganic metallic halide cubic double perovskite Cs₂AgBiBr₆ in its ordered and disordered forms using first-principles density functional theory (DFT) to verify the suitability of its photovoltaic and optoelectronic applications. The indirect bandgap of Cs₂AgBiBr₆ is tuned to a direct bandgap by changing it from an ordered to disordered system following the disordering of Ag⁺/Bi³⁺ cations by creating antisite defects in its sublattice. In the disordered Cs₂AgBiBr₆, the Bi 6p orbital modifies the conduction band significantly and leads to a shift the conduction band minimum (CBM) from L to Γ-point. Consequently, the system changes from indirect to direct band gap material. At the same time the band gap reduces significantly. The band gap of Cs₂AgBiBr₆ decreases from 2.04 eV to 1.59 eV. The absorption edge towards the lower energy region and strong optical absorption in the visible to the UV region indicate that the disordered direct band gap material Cs₂AgBiBr₆ is appropriate for use in solar cells and optoelectronic and energy harvesting devices. Dielectric function, reflectivity and refractive index of disordered direct band gap material Cs₂AgBiBr₆ is favorable for its optoelectronic and photovoltaic applications. However, its stability and ductility favor its thin film fabrication. The creation of antisite defects in the sublattice of double perovskites opens a new avenue for the design of photovoltaic and optoelectronic materials.

The bandgap of Cs₂AgBiBr₆ is tuned to a direct bandgap by the disordering of Ag⁺/Bi³⁺ cations, creating antisite defects. The creation of antisite defects in the sublattice of double perovskites opens a new avenue for the design of photovoltaic and optoelectronic materials.     

Introduction of cadmium chloride additive to improve the performance and stability of perovskite solar cells

With the increase in the importance of using green energy sources to meet the world''s energy demands, attempts have been made to push perovskite solar cell technology toward industrialization all around the world. Improving the properties of perovskite materials as the heart of PSCs is one of the methods to fabricate favorable photovoltaic (PV) solar cells based on perovskites. Here, cadmium chloride (CdCl₂) was used as an additive source for the perovskite precursor to improve its PV properties. Results indicated CdCl₂ improves the perovskite growth and tailors its crystalline properties, suggesting boosted charge transport processes in the bulk and interfaces of the perovskite layer with electron–hole transport layers. Overall, by incorporation of 1.0% into the MAPbI₃ layer, a maximum power conversion efficiency of 15.28% was recorded for perovskite-based solar cells, higher than the 12.17% for the control devices. The developed method not only improved the PV performance of devices but also boosted the stability behavior of solar cells due to the passivated domain boundaries and enhanced hydrophobicity in the CdCl₂-based devices.

With the increase in the importance of using green energy sources to meet the world''s energy demands, attempts have been made to push perovskite solar cell technology toward industrialization all around the world.     

Theoretical and experimental investigations on the bulk photovoltaic effect in lead-free perovskites MASnI3 and FASnI3

Perovskite solar cells based on the lead free hybrid organic–inorganic CH₃NH₃SnI₃ (MASnI₃) and CH₄N₂SnI₃ (FASnI₃) perovskites were fabricated, and the photoelectric conversion efficiency (PCE) was assessed. FASnI₃''s PCE was higher than MASnI₃''s efficiency. To study the different photovoltaic properties, we calculated their structural, electronic, and optical properties using density functional theory via the Perdew–Burke–Ernzerhof and spin–orbit coupling (PBE-SOC) methods. The results show that FASnI₃ exhibits an appropriate band gap, substantial stability, marked optical properties, and significant hole and electron conductive behavior compared with MASnI₃. The interaction of organic cations (FA⁺) with the inorganic framework of FASnI₃ was stronger than that with MASnI₃, so they affected the band length and band angle distribution, causing the structure of the FASnI₃ and MASnI₃ to change. The calculations also demonstrated that energy splitting was evident in FASnI₃ due to the spin–orbit coupling effect, however, it was moderate in MASnI₃, which was caused by the H bond effect. This research not only furthers the understanding of these functional materials, but also can assist the development of highly efficient and stable non-lead perovskite solar cells.

Perovskite solar cells based on the lead free hybrid organic–inorganic CH₃NH₃SnI₃ (MASnI₃) and CH₄N₂SnI₃ (FASnI₃) perovskites were fabricated, and the photoelectric conversion efficiency (PCE) was assessed.     

Mechanochemical synthesis of pure phase mixed-cation/anion (FAPbI3)x(MAPbBr3)1−x hybrid perovskite materials: compositional engineering and photovoltaic performance

Organic–inorganic hybrid perovskites have emerged as promising light harvesting materials for many optoelectronic devices. Here, we present a facile mechanochemical synthesis (MCS) route for the preparation of a series of pure phase mixed-cation/anion (FAPbI₃)_x(MAPbBr₃)_1−x (0 ≤ x ≤ 1) hybrid perovskite materials for high-efficiency thin-film perovskite solar cells (PSCs). The use of (α-FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite prepared by MCS for the thin-film PSCs achieves a maximum PCE of 15.9% from a current–voltage (J–V) scan, which stabilises at 15.4% after 120 s of the maximum power point output. Furthermore, PSCs based on (KPbI₃)_0.05(FAPbI₃)_0.9(MAPbBr₃)_0.05 perovskite prepared by MCS exhibit higher photovoltaic performance and lower hysteresis compared with (α-FAPbI₃)_0.95(MAPbBr₃)_0.05, with a maximum PCE of 16.7%. These results indicate that the use of mechanochemically synthesised perovskites provides a promising strategy for high performance PSCs and superior control in optoelectronic properties, leading to improved control in fabrication approaches and facilitating the development of efficient and stable PSCs in the future.

Pure phase mixed-cation/anion (α-FAPbI₃)x(MAPbBr₃)1−x (0 ≤ x ≤ 1) hybrid perovskites are efficiently prepared via MCS, and the band gaps can be tuned easily. PSCs based on 5% K-doped perovskite exhibit low I–V hysteresis, with a maximum PCE of 16.7%.     

Electronic and optical properties of perovskite compounds MA1−αFAαPbI3−βXβ (X = Cl,Br) explored for photovoltaic applications

As outstanding light harvesters, solution-processable organic–inorganic hybrid perovskites (OIHPs) have been drawing considerable attention thanks to their higher power conversion efficiency (PCE) and cost-effective synthesis relative to other photovoltaic materials. Nevertheless, their further development is severely hindered by the drawbacks of poor stability and rapid degradation in particular. First-principles calculations based on density functional theory (DFT) are hence performed towards the perovskite compounds MA_1−αFA_αPbI_3−βX_β (X = Cl, Br), with the aim of exploring more efficient and stable OIHPs. In addition to that, a hybrid density functional is adopted for exact electronic properties, and their band structures indicate that the doped series are all direct band-gap semiconductors. Moreover, the defect formation energies indicate that the stability of perovskite compounds can be significantly enhanced via ion doping. Meanwhile, it is unveiled that the optical performance of the doped perovskite series is also effectively improved through ion doping. Therefore, the investigated perovskite compounds MA_1−αFA_αPbI_3−βX_β (X = Cl, Br) are promising candidates for enhancing solar-energy conversion efficiency. Our results pave a way in deeper understanding of the inherent characteristics of OIHPs, which is useful for designing new-type perovskite-based photovoltaic devices.

The absorption performance of perovskite CH₃NH₃PbI₃ can be significantly improved via mono-, or co-doping of organic cations and halide ions.     

Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer

We herein demonstrate n-i-p-type planar heterojunction perovskite solar cells employing spin-coated ZnO nanoparticles modified with various alkali metal carbonates including Li₂CO₃, Na₂CO₃, K₂CO₃ and Cs₂CO₃, which can tune the energy band structure of ZnO ETLs. Since these metal carbonates doped on ZnO ETLs lead to deeper conduction bands in the ZnO ETLs, electrons are easily transported from the perovskite active layer to the cathode electrode. The power conversion efficiency of about 27% is improved due to the incorporation of alkali carbonates in ETLs. As alternatives to TiO₂ and n-type metal oxides, electron transport materials consisting of doped ZnO nanoparticles are viable ETLs for efficient n-i-p planar heterojunction solar cells, and they can be used on flexible substrates via roll-to-roll processing.

Planar formamidinium perovskite solar cells have been fabricated with an alkali carbonate-doped zinc oxide layer.     

Atomic layer deposition of metal oxides for efficient perovskite single-junction and perovskite/silicon tandem solar cells

Aluminum-doped and undoped zinc oxide films were investigated as potential front and rear contacts of perovskite single and perovskite/silicon tandem solar cells. The films were prepared by atomic layer deposition (ALD) at low (<200 °C) substrate temperatures. The deposited films were crystalline with a single-phase wurtzite structure and exhibit excellent uniformity and low surface roughness which was confirmed by XRD and SEM measurements. Necessary material characterizations allow for realizing high-quality films with low resistivity and high optical transparency at the standard growth rate. Spectroscopic ellipsometry measurements were carried out to extract the complex refractive index of the deposited films, which were used to study the optics of perovskite single junction and perovskite/silicon tandem solar cells. The optics was investigated by three-dimensional finite-difference time-domain simulations. Guidelines are provided on how to realize perovskite solar cells exhibiting high short-circuit current densities. Furthermore, detailed guidelines are given for realizing perovskite/silicon tandem solar cells with short-circuit current densities exceeding 20 mA cm⁻² and potential energy conversion efficiencies beyond 31%.

The necessity of thin and highly doped metal oxide films is discussed for realizing efficient perovskite single and perovskite/silicon tandem solar cells.     

First-principles study on optoelectronic properties of Cs2PbX4–PtSe2 van der Waals heterostructures

In order to achieve low-cost, high efficiency and stable photoelectric devices, two-dimensional (2D) inorganic halide perovskite photosensitive layers need to cooperate with other functional layers. Here, we investigate the structure, stability and optical properties of perovskite and transition metal dichalcogenide (TMD) heterostructures using first-principles calculations. Firstly, Cs₂PbX₄–PtSe₂ (X = Cl, Br, I) heterostructures are stable because of negative interface binding energy. With the halogen varying from Cl to I, the interface binding energies of Cs₂PbX₄–PtSe₂ heterostructures decrease rapidly. 2D Cs₂PbCl₄–PtSe₂, Cs₂PbBr₄–PtSe₂ and Cs₂PbI₄–PtSe₂ heterostructures have an indirect bandgap with the value of 1.28, 1.02, and 1.29 eV, respectively, which approach the optimal bandgap (1.34 eV) for solar cells. In the contact state, the electrons transfer from the PtSe₂ monolayer to Cs₂PbX₄ monolayer and only the Cs₂PbBr₄–PtSe₂ heterostructure maintains the type-II band alignment. The Cs₂PbBr₄–PtSe₂ heterostructure has the strongest charge transfer among the three Cs₂PbX₄–PtSe₂ heterostructures because it has the lowest tunnel barrier height (ΔT) and the highest potential difference value (ΔEP). Furthermore, the light absorption coefficient of Cs₂PbX₄–MSe₂ heterostructures is at least two times higher than that of monolayer 2D inorganic halide perovskites. With the halogen varying from Cl to I, the light absorption coefficients of the Cs₂PbX₄–PtSe₂ heterostructures increase rapidly in the visible region. Above all, the Cs₂PbX₄–MSe₂ heterostructures have broad application prospects in photodetectors, solar cells and other fields.

Energy level graphs of the monolayer PtSe₂ and Cs₂PbX₄ in the (a) precontact and (b) contact. The Cs₂PbBr₄–PtSe₂ heterostructure has a type-II level alignment which is conducive to spontaneously driving the holes and electrons to move forward in opposite directions.     

Potassium iodide reduces the stability of triple-cation perovskite solar cells

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current–voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO₂ or TiO₂ electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite–ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and – at concentrations >5 mol% in the precursor solution – modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices.