Graphene is the thinnest two-dimensional (2D) carbon material and has many advantages including high carrier mobilities and conductivity, high optical transparency, excellent mechanical flexibility ...and chemical stability, which make graphene an ideal material for various optoelectronic devices. The major applications of graphene in photovoltaic devices are for transparent electrodes and charge transport layers. Several other 2D materials have also shown advantages in charge transport and light absorption over traditional semiconductor materials used in photovoltaic devices. Great achievements in the applications of 2D materials in photovoltaic devices have been reported, yet numerous challenges still remain. For practical applications, the device performance should be further improved by optimizing the 2D material synthesis, film transfer, surface functionalization and chemical/physical doping processes. In this review, we will focus on the recent advances in the applications of graphene and other 2D materials in various photovoltaic devices, including organic solar cells, Schottky junction solar cells, dye-sensitized solar cells, quantum dot-sensitized solar cells, other inorganic solar cells, and perovskite solar cells, in terms of the functionalization techniques of the materials, the device design and the device performance. Finally, conclusions and an outlook for the future development of this field will be addressed.
2D materials have been successfully used in various types of solar cells as transparent electrodes, interfacial and active materials.
All‐inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their ...future commercial application. The mixed‐halide inorganic CsPbI2Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI2Br perovskites limit the open‐circuit voltage (Voc < 1.2 V), short‐circuit current density (Jsc < 15 mA cm−2), and fill factor (FF < 75%) of CsPbI2Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI2Br perovskite is doped by Eu(Ac)3 to obtain a high‐quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable Voc of 1.25 V, a reasonable Jsc of 15.44 mA cm−2, and a high FF of 79.00% are realized for CsPbI2Br solar cells. Moreover, the CsPbI2Br solar cells with Eu(Ac)3 doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35–40%) for 30 days.
CsPbI2Br perovskite is doped by Eu(Ac)3 to obtain high‐quality perovskite films with low defect density and long carrier lifetime. A high efficiency of 15.25%, an open‐circuit voltage of 1.25 V, a short‐circuit current density of 15.44 mA cm−2, and a fill factor of 79.00% are realized for CsPbI2Br solar cells. The devices with Eu(Ac)3 doping demonstrate excellent air stability.
Poor stability of organic-inorganic halide perovskite materials in humid condition has hindered the success of perovskite solar cells in real applications since controlled atmosphere is required for ...device fabrication and operation, and there is a lack of effective solutions to this problem until now. Here we report the use of lead (II) thiocyanate (Pb(SCN)2) precursor in preparing perovskite solar cells in ambient air. High-quality CH3NH3PbI3-x(SCN)x perovskite films can be readily prepared even when the relative humidity exceeds 70%. Under optimized processing conditions, we obtain devices with an average power conversion efficiency of 13.49% and the maximum efficiency over 15%. In comparison with typical CH3NH3PbI3-based devices, these solar cells without encapsulation show greatly improved stability in humid air, which is attributed to the incorporation of thiocyanate ions in the crystal lattice. The findings pave a way for realizing efficient and stable perovskite solar cells in ambient atmosphere.
With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before ...perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb2+ and passivate under‐coordinated Pb2+ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.
Aided by theoretical calculations, a multifunctional 2,2‐difluoropropanediamide (DFPDA) molecule that bears carbonyl, amino, and fluorine groups is first introduced into the perovskite precursor, serving as a crystal growth mitigator, grain boundaries passivator, and surface protection material. With the help of the combined effects of multifunctional groups in DFPDA, the perovskite cells deliver an efficiency of 22.21% and improved stability.
Despite being a promising candidate for next‐generation photovoltaics, perovskite solar cells (PSCs) exhibit limited stability that hinders their practical application. In order to improve the ...humidity stability of PSCs, herein, a series of ionic liquids (ILs) “1‐alkyl‐4‐amino‐1,2,4‐triazolium” (termed as RATZ; R represents alkyl chain, and ATZ represents 4‐amino‐1,2,4‐triazolium) as cations are designed and used as additives in methylammonium lead iodide (MAPbI3) perovskite precursor solution, obtaining triazolium ILs‐modified PSCs for the first time (termed as MA/RATZ PSCs). As opposed to from traditional methods that seek to improve the stability of PSCs by functionalizing perovskite film with hydrophobic molecules, humidity‐stable perovskite films are prepared by exploiting the self‐assembled monolayer (SAM) formation of water‐soluble triazolium ILs on a hydrophilic perovskite surface. The mechanism is validated by experimental and theoretical calculation. This strategy means that the MA/RATZ devices exhibit good humidity stability, maintaining around 80% initial efficiency for 3500 h under 40 ± 5% relative humidity. Meanwhile, the MA/RATZ PSCs exhibit enhanced thermal stability and photostability. Tuning the molecule structure of the ILs additives achieves a maximum power conversion efficiency (PCE) of 20.03%. This work demonstrates the potential of using triazolium ILs as additives and SAM and molecular design to achieve high performance PSCs.
Humidity‐stable perovskite films are prepared by exploiting the self‐assembled monolayer formation of water‐soluble triazolium ionic liquids (ILs) on a hydrophilic perovksite surface. This strategy means that the devices exhibit good humidity stability, maintaining around 80% initial efficiency for 3500 h under 40 ± 5% humidity. Tuning the molecule structure of the ILs achieves a maximum power conversion efficiency (PCE) of 20.03%.
The conduction band energy, conductivity, mobility, and electronic trap states of electron transport layer (ETL) are very important to the efficiency and stability of a planar perovskite solar cell ...(PSC). However, as the most widely used ETL, TiO2 often needs to be prepared under high temperature and has unfavorable electrical properties such as low conductivity and high electronic trap states. Modifications such as elemental doping are effective methods for improving the electrical properties of TiO2 and the performance of PSCs. In this study, Nb-doped TiO2 films are prepared by a facile one-port chemical bath process at low temperature (70 °C) and applied as a high quality ETL for planar PSCs. Compared with pure TiO2, the Nb-doped TiO2 is more efficient for photogenerated electron injection and extraction, showing higher conductivity, higher mobility, and lower trap-state density. A PSC with 1% Nb-doped TiO2 yielded a power conversion efficiency of more than 19%, with about 90% of its initial efficiency remaining after storing for 1200 h in air or annealing at 80 °C for 20 h in a glovebox.
Inorganic CsPbI2Br perovskite solar cells (PSCs) have gained enormous research interest due to their excellent thermal and light stabilities. However, their unsatisfactory power‐conversion efficiency ...and poor intrinsic phase stability remain roadblocks to their further development. Herein, Cs2PbI2Cl2 nanosheets (NSs) with the Ruddlesden–Popper (RP) structure are synthesized, and an NSs/CsPbI2Br/NSs heterostructure is employed to enhance both the stability and efficiency of CsPbI2Br solar cells. The novel Cs2PbI2Cl2 NSs can not only passivate the top and bottom surfaces of the perovskite film and top surface of the TiO2 film but also enhance the stability of the perovskite film. Based on the heterostructured NSs/CsPbI2Br/NSs inorganic perovskite film, the efficiency of the CsPbI2Br PSCs is improved from 15.02% to 16.65%. Moreover, the unencapsulated CsPbI2Br devices with the NSs/CsPbI2Br/NSs heterostructure sustain over 90% of their original efficiencies after being exposed to ambient conditions (≈25 °C and ≈35% RH) for 648 h. Both the UV‐light‐soaking stability (100 mW cm−1 365 nm UV light) and thermal stability (T = 85 °C) of the optimized devices are dramatically improved in comparison with their counterparts with only a 3D active layer. Therefore, this work promotes the application of RP inorganic perovskite nanocrystals in a range of perovskite optoelectronic devices.
Herein, novel Ruddlesden–Popper Cs2PbI2Cl2 nanosheets are synthesized and creatively employed as a multifunctional interface optimization material to improve the performance of CsPbI2Br solar cells. Based on the heterostructured NSs/CsPbI2Br/NSs inorganic film, an efficiency of 16.65% is obtained, which is one of the best reported for CsPbI2Br solar cells, along with much‐enhanced UV, air, and thermal stabilities.
The inferior power conversion efficiency (PCE) compared to their regular counterparts (n-i-p) and undesirable stability issues of inverted (p-i-n) perovskite solar cells (PSCs) are the foremost ...issues hindering their commercialization. Here, for the first time, we demonstrate a polymeric room-temperature molten salt (poly-RTMS), namely poly(1-vinyl-3-ethyl-acetate) imidazole tetrafluoroborate (PEa), as a novel type of additive to modulate the perovskite crystallization and its electronic properties. The PEa poly-RTMS containing multiple chemical anchoring sites along with strong bonding stability can firmly bond to Pb ion defects at grain boundaries and the interface of the perovskite film
via
coordination bond, which effectively passivates the electronic defects and enhances the photo-, thermal-, and moisture-stability of perovskite films. As a result, the PEa-modified inverted PSCs show striking performance improvements over the control with the PCE exceeding 21.4% and excellent long-term operational stability, maintaining over 92% of the initial efficiency for 1200 hours under continuous full sun illumination at 70-75 °C. This strategy opens a new avenue to modulate the properties of perovskites for optoelectronic applications.
We for the first time report the application of a polymeric room-temperature molten salt in high efficiency and excellent stability inverted perovskite solar cells.
Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin‐film versions. Herein, we report the use of a simple ...sol–gel‐processed NiO nanocrystal (NC) layer as the hole‐transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well‐crystallized CH3NH3PbI3 in a two‐step solution process. The hole‐extraction and ‐transport capabilities of this film interfaced with the CH3NH3PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30–40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole‐extracting layer.
The advantages are crystal clear: A transparent layer of NiO nanocrystals fabricated by a simple sol–gel processing method acted as an efficient hole‐transport layer in an inverted perovskite solar cell. With the NiO film (see picture), a cell efficiency of 9.11 % was observed: by far the highest for a planar perovskite solar cell based on an inorganic hole‐extraction layer.
High temperature stable inorganic CsPbX3 (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top ...cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX3 perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI3, and HPbBr3) is developed to replace the traditional precursors (CsI, PbI2, and PbBr2) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH•Cs+) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI2Br film deposition. The CsPbI2Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI2Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.
Herein, a novel precursor (HCOOCs and HPbX3) for deposition of high‐quality CsPbI2Br films, irrespective of humidity is presented. CsPbI2Br cells prepared in an atmosphere with 30% and 91% relative humidity exhibit efficiencies of 16.1% and 15.1%, respectively, which are the highest among all inorganic CsPbX3 (X: I, Br, or mixed halides) PSCs prepared in a medium or high humid atmosphere.
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