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In recent years, the power conversion efficiencies of halide perovskite solar cells (PSCs) have reached 25.7%. Further commercialization puts forward higher requirements for the stability of PSCs under different stresses. Current research on the stability of PSCs predominantly focuses on investigating the effects of temperature, humidity, oxygen, ultraviolet light, and electrical bias stress, while stability under reverse bias has been poorly studied among the many factors. When large‐area application of PSCs panels occurs, the shading effect and local hot spots of PSCs modules in real scenarios all indicate that the research on reverse bias stability of PSCs is urgently needed. Within this review, a bird's‐eye view of the recent advances on the reverse bias stability issue and reverse bias behavior of PSCs is obtained, and the related mechanisms behind that have been proposed so far are reviewed and discussed. Furthermore, future directions are provided for further improving the reverse bias stability of PSCs via increasing the reverse breakdown voltage (VRB). This review aims to promote the establishment of well‐established reverse bias degradation and reverse breakdown mechanisms of PSCs as well as to establish standardized test procedures for reverse bias stability issues on the road to commercialization of PSCs. [ABSTRACT FROM AUTHOR]

Efficient electron transport layer–free perovskite solar cells (ETL‐free PSCs) with cost‐effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small‐molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier‐free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state‐of‐the‐art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL‐free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification. [ABSTRACT FROM AUTHOR]

To realize high-performance perovskite solar cells (PSCs) ensuring low-cost flexible application with low energy consumption, efficient electron transport layers (ETL) that can be processed at low temperatures are desired. In this contribution, low temperature (about 70 °C) UV/ozone irradiation was applied to synthesis crystallized TiO 2 films as ETL. The UV exposure induces photo-catalytic removal of the organic ligands and spontaneous coalescence of the TiO 2 framework, resulting in excellent TiO 2 films with considerable crystallinity. UV irradiation induces both complete removal of organic media and more efficient charge collection. Accordingly, planar heterojunction (PH) PSCs with such ETL exhibited outstanding photovoltaic performance up to 17.03%, which is even higher than the value (12.98%) of counterpart device with high temperature thermal sintered TiO 2 ETL (without TiCl 4 treatment). These low temperature solution-processed TiO 2 films by facile UV irradiation are suitable for the low-cost, large yield solution process on a flexible substrate for photoelectric devices. [ABSTRACT FROM AUTHOR]

Rough dense sol-gel-derived titanium dioxide (TiO 2 ) electron-transport layers (ETLs) and smooth organolead halide perovskite (PVK) films for pseudo-planar heterojunction perovskite solar cells (P-PH PVKSCs) were fabricated by a facile one-step dip-coating method. The highly compact TiO 2 ETLs and uniform PVK films endow the device a high power conversion efficiency (PCE) of over 11%, which was nearly identical to that of a reference device (12%) fabricated by conventional spin-coating. Furthermore, the device showed no pronounced hysteresis when tested by scanning the voltage in a forward and backward direction, showing the potential of facile and waste-free dip-coating in replacing of spin-coating for large area perovskite solar cells preparation. Lastly, the hysteresis was compared and discussed and models regarding the abnormal hysteresis, roll-over and current peak phenomena were proposed as well. [ABSTRACT FROM AUTHOR]

Device modeling of CH 3 NH 3 PbI 3−x Cl x based electron transport layer-free planar perovskite solar cells was performed. The simulation was conducted by the program SCAPS (Solar Cell Capacitance Simulator). With appropriate physical parameters, a high open-circuit voltage of 1.04 V close to results reported experimentally was successfully reproduced in the simulation. Simulation results revealed a great dependence of PCE on the thickness and defect density of the perovskite layer. An optimum perovskite thickness of about 500 nm was confirmed and it well consistent with the thickness range of real devices was derived. Meanwhile, parameters including the FTO/perovskite interface defect density as well as the doping concentration of the front contact (FTO) were identified to significantly influence the performance of the device. In particular, the interface quality at the FTO/perovskite interface has greater impact on the device parameters than that at the perovskite/HTL interface, which suggests that more attention should be paid to the front FTO/perovskite interface to further enhance the performance of electron transport layer-free device. Appropriate interface defect passivation to reduce the interface defect density to the order of ~10 15 cm −3 is necessary and urgently needed. Lastly, the effect of the electron and hole mobility and carrier diffusion length of CH 3 NH 3 PbI 3−x Cl x were also analyzed and the results revealed that the mobility and diffusion length experimentally reported (~1.0 µm) are large and long enough for high efficiency. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PVKSCs) are an attractive technology that finds their potential in the field of renewable energy sources. Transparent conductive oxides including fluorine-doped tin dioxide (FTO) and indium tin oxide (ITO) with high optical transmittance and low electrical resistivity are key components in PVKSCs. While commercial FTO or ITO either requires high temperature and high vacuum process or contains rare indium element, which will increase the production cost of PVKSCs. Here we report efficient electron-transport layer (ETL) free planar PVKSCs using the recycled FTO/glass substrates from degraded devices. By simple and low-temperature processes including organic solvent washing, ultrasonic cleaning and UV ozone treatment, the discarded substrates can be readily reused for fabricating ETL-free planar PVKSCs. The UV–vis optical transmission, crystal structure, sheet resistance, surface morphology, elemental composition and static contact angles measurement of the original and recycled FTO/glass substrates (one time and two times) were measured and compared. Planar ETL-free devices with power conversion efficiencies of about 10% have been achieved by adopting the recycled FTO/glass substrates, which are comparable to that of the devices based on the original FTO substrates, suggesting the feasibility of recycling the FTO/glass substrates from degraded devices for fabricating ETL-free PVKSCs. [ABSTRACT FROM AUTHOR]

Planar perovskite solar cells with a p–i–n structure use both hole-transport layers and electron-transport layers to promote collection of photogenerated holes and electrons for achieving high performance, wherein a high temperature processed compact and mesoporous titanium dioxide are usually required. We report here efficient perovskite solar cells grown directly on ultraviolet–ozone treated fluorine-doped tin oxide (FTO) substrates without using any electron-transport layers. The morphology, structure, optical–electrical properties of perovskite films deposited on FTO substrates with and without ultraviolet–ozone treatment and their corresponding devices׳ performance have been studied and compared. Ultraviolet–ozone treatment of FTO substrates improves the smoothness and coverage of CH 3 NH 3 PbI 3− x Cl x films, which avoids direct contact between FTO and hole-transport layer. A planar electron-transport layer free device with a power conversion efficiency of over 10% has been achieved, suggesting that the widely adopted electron-transport layer is not a requirement for an efficient device. [ABSTRACT FROM AUTHOR]

We report planar heterojunction perovskite solar cells with tetrafluoro-tetracyanoquinodimet-hane (F4-TCNQ) doped spiro-MeOTAD by solvent treatment as hole transport layer. The non-hygroscopic F4-TCNQ doped spiro-MeOTAD plays the same role as Li-bis (trifluoromethanesulfonyl) imide doped spiro-MeOTAD in perovskite solar cells, endowing the device both high efficiency and good stability. The F4-TCNQ dopant was optimized to enhance the conductivity and adjust the highest occupied molecular orbital of spiro-OMeTAD to match the valence band maximum of perovskite for efficient hole extraction, which results in a high fill factor, open circuit voltage and short current density, simultaneously. Device including the F4-TCNQ doped spiro-MeOTAD exhibits a power conversion efficiency of 10.59%, which is a 160% improvement relative to that of the control device with the pristine spiro-MeOTAD as hole transport layer. Time-dependent solar cell performance measurements revealed significantly improved air stability for perovskite solar cells with the F4-TCNQ doped spiro-MeOTAD hole transport layer. [ABSTRACT FROM AUTHOR]

The morphology, structure, optical and electrical properties of perovskite films treated by two different annealing methods with different annealing temperature ramp and their corresponding device performance have been studied and compared. Annealing temperature ramp significantly influences the surface morphology and optical properties of perovskite films which determines the performance of solar cells which determines the performance of solar cells . The perovskite films treated by one-step direct annealing method tend to exhibit irregular and weak ultraviolet–visible absorption spectrum, which can easily result in great variation in the final performance of solar cells. While multi-step slow annealing is beneficial for preparing highly uniform and well-crystallized perovskite films, and thus these devices present tightly-distributed performance parameters. The best device treated by multi-step slow annealing method showed a short circuit current density of 21.49 mA/cm 2 , an open circuit voltage of 0.988 V, a fill factor of 64.86%, and a power conversion efficiency (PCE) of 13.58%, which is a 57% enhancement of the overall PCE relative to 8.65% of the device treated by one-step annealing method. These findings suggest that optimized slow temperature ramp is necessary to prepare high-efficient and well-reproducible perovskite solar cells. [ABSTRACT FROM AUTHOR]

The pursuit of high power conversion efficiency (PCE) and cost‐effective perovskite solar cells (PSCs) has spawned many innovative device structure designs. Compared with the traditional NIP type PSCs, electron transport layer (ETL) free PSCs have attracted growing attention due to their enormous potential in large area, low‐cost flexible application. However, there is still a lack of in‐depth understanding of the energy level arrangement indium tin oxide (ITO)/perovskite interface, resulting in poor device performance. Here, a metal/insulator/n‐type perovskite/p‐type spiro‐MeOTAD (MINP) structure is proposed to elaborate on the influence of the apparent work function and contact barrier change of the metal–semiconductor (MS) and metal–insulator–semiconductor (MIS) junction on the carrier transfer and collection. Common and easily available 5‐amino‐valeric acid is inserted into the ITO/perovskite interface to form an insulating dipole layer and to ensure quasi‐ohmic contact at the front interface. Consequently, the device changes from Schottky/PN cascade heterojunction type to a single PN heterojunction device with its ideal factor decreasing from 3.35 to 2.05. Accordingly, the champion device achieves 19.37% PCE with significantly increased Voc and FF compared to the pristine device. This work provides a facile and effective method to improve the application potential of novel ETL‐free PSCs. [ABSTRACT FROM AUTHOR]

Silver nanowire (Ag NW)-based transparent back electrodes prepared via all solution-process are promising for bifacial and semitransparent perovskite solar cells (PSCs). However, the inadequate electrical contact between the silver nanowires themselves and the contact with substrate leads to reduced overall solar cell performance; meanwhile, serious corrosion of Ag NW in ambient environment restricting the further applications. In this work, polyvinylpyrrolidone (PVP) as a binder in Ag NW electrode significantly enhances the conductivity and transparency at the same time improving the interface electric contact. The PSC with PVP modified Ag NW electrode demonstrated frontside and backside efficiency approaching 18 % and 14.1 % (AM1.5, 1 Sun), respectively, which achieves 82 % efficiency with normal Ag device and bifacial factor of 79 %. The long-term storage stability of PSC is also improved by PVP protection. The final device structure is ITO/Ni O x /MeO/(FA0.92MA0.08)0.95Cs0.05Pb(I0.9Br0.1)3/PEAI/PC61BM/BCP/ Ag NW. This work highlights the method to fabricate robust transparent Ag NW electrode for bifacial perovskite solar cells. • Polyvinylpyrrolidone (PVP) in Ag nanowire (Ag NW) electrode significantly enhances the conductivity and transparency. • PVP modified Ag NW electrode improves the interface electric contact and charge transport of the solar cell. • The Perovskite solar cell (PSC) with PVP modified Ag NW electrode demonstrated high efficiency and bifacial factor. • The long-term storage stability of PSC is also improved by PVP protection. [ABSTRACT FROM AUTHOR]

A facile and low‐temperature solution processed UV/ozone process has been proposed for fabricating TiO2 layer, making it possible to fabricate planar perovskite solar cells at low temperature. UV/ozone photo‐annealing treatment could easily eliminate the organic components, resulting in high quality amorphous TiO2 films with high transparency, electrical conductivity, and hydrophilicity. In addition, perovskite solar cells with the UV/ozone‐treated TiO2 electron transport layer exhibiting the best power conversion efficiency of 11.58% for rigid substrate and 7.33% for flexible substrate, respectively. The results clearly indicate that UV/ozone photo‐annealing treatment is a simple and effective process of fabricating high performance flexible perovskite solar cells. Fully low‐temperature solution‐processed perovskite solar cells (PSCs) are fabricated using UV/ozone (UV/O3) treated TiO2 film as an electron transport layer. UV/O3 treatment increases the electrical conductivity and wettability of the TiO2 film, and therefore improves the performance of PSCs. By optimizing the processing time of UV/O3 treatment, the power conversion efficiency of the best rigid and flexible device is 11.58% and 7.33%, respectively. [ABSTRACT FROM AUTHOR]

• The oxidative I 2 /I 3 − in Sn-Pb perovskite is effectively eliminated to stabilize Sn2+. • The movable I− is successfully neutralized, protecting the Ag counter electrode from corrosion. • The solar cell device shows promoted photovoltage and efficiency with enhanced stability. In recent hotly researched tin–lead (Sn-Pb) mix perovskite solar cells (PSCs), the oxidation of Sn2+ is not an isolated process but accompanied by the oxidation of I− to I 2 , which are both responsible for the severe p-doping and complex defect situation. Diffusion of interstitial I further severely corrodes the metal electrode to fail the PSC. Thereby, managing the I− oxidation and meanwhile neutralizing the movable I− together with suppressing Sn2+ oxidation is quite a challenge for efficient and stable Sn-Pb PSCs. In this work, a natural reductive amino acid (DOPA) is introduced into perovskite layer to successfully inhibit the oxidation of both Sn2+ and I−, at the same time, neutralize the diffusive I− to protect the Ag electrode from corrosion. Benefiting from the improved chemistry situation, the DOPA doped PSC shows significantly promoted photovoltage and efficiency at AM1.5, with enhanced light and shelf stability. This work highlights the importance of I defects management to achieve efficient and stable Sn-Pb PSCs. [ABSTRACT FROM AUTHOR]

Inorganic nickel oxide (NiO x)-based inverted perovskite solar cells (PSCs) are reported to be more stable compared to those using organic hole transport layer materials. However, the Fermi level of NiO x is relatively away from the valence band of perovskite (PVK) layer, causing a large energy level shift at the NiO x /PVK interface and a weak built-in electric field strength. These increase the possibility of recombination, reduce the effective charge collection, seriously affect the efficiency of the device. In this work, an interface dipole with -CF 3 end group is anchored to the NiO x /PVK interface, leading to a more matched energy level structure and greatly reducing the energy barrier at the NiO x /PVK interface. Moreover, the addition of Si-CF 3 significantly improves the conductivity of NiO x and blunts the defects to large extent. The dipole interlayer significantly improves the built-in electric field and thus increases the photovoltage. Ultimately, the modified device achieves a score of 19.58% efficiency, much better than 17.98% of the control device. [Display omitted] [ABSTRACT FROM AUTHOR]

• Formation of (TMA) x FA 1-x Pb(PF 6) x I 3-x /FAPbI 3 heterojunction through interface ion exchange. • Strong hydrogen bonding and ion size effect stabilize the phase structure. • A champion efficiency of 21.43% was obtained with greatly enhanced device stability. Though formamidinium lead triiodide FAPbI 3 is hotly researched, it still suffers from chemical and phase instabilities, arising from inherent weak chemical bonding and easily transformed crystal configures respectively. This induces depraved opto-electronic properties and device instability, which is imperative to be solved. Here, chemical interaction enhancement in heterostructure of (TMA) x FA 1-x Pb(PF 6) x I 3-x /FAPbI 3 simultaneously improves the chemical and phase stability with preserving its inherent bandgap. Strong hydrogen bonding between PF 6 - and FA+ drastically reduces the α phase formation energy and diminishes the interface trap states. The resulting perovskite solar cells achieve improved efficiency of 21.43% compared with 19.32% of the control device. Meanwhile, ∼90% and 80% of the initial efficiency are maintained for unpacked devices after 1200 h storage (25 °C, 30% relative humidity) and 300 h maximum power point tracking (1 sun, 25 °C, N 2), outperforming the control one. This work highlights a robust interface construction method to overcome the inherent problem of FAPbI 3. [ABSTRACT FROM AUTHOR]

• The fluorine atom has induced electrostatic potential and dipole moment change. • Cl- doping can enhance crystallization and prevent phase change. • The champion device achieves a PCE of 16.29% and the humidity stability is improved. [Display omitted] All-inorganic CsPbI 2 Br material has attracted considerable attention due to its good balance between phase stability and absorption capacity. However, the power conversion efficiency (PCE) and device stability of CsPbI 2 Br-based perovskite solar cells (PSCs) still lag far behind those of their counterparts, mainly deriving from the notoriously poor moisture stability under humid environment and large serious charge recombination. Herein, a typical phenethylammonium chlorine (PEACl) and its fluorinated derivative 4-fluoro-phenethylammonium chlorine (F-PEACl) are individually incorporated to modify the surface of CsPbI 2 Br film to ameliorate the above issues. Benefiting from the comprehensive passivation effect of ammonium halogen, the trap states of treated CsPbI 2 Br are vastly suppressed. In particular, fluorination significantly increased the dipole moment and hydrophobic property of F-PEACl. Thus, the present CsPbI 2 Br PSC achieves a champion PCE of 16.29% with high open-circuit voltage of 1.272 V, while the reference device and PEACl-treated device yield PCEs of 14.30% and 15.64%, respectively. More importantly, the PSC with F-PEACl capping layer retains 87.2% of its initial PCE after 1000 h storage under 20% RH ambient without encapsulation. Our results reveal the underlying mechanism of performance improvement by surface passivation strategy and provide an effective approach to further boost the CsPbI 2 Br PSC efficiency and stability. [ABSTRACT FROM AUTHOR]