Publication date: 21 November 2018
Source: Joule, Volume 2, Issue 11
Author(s): Yanfa Yan
In a recent paper published in Energy & Environmental Science, Ávila et al. report a fully vacuum-processed dual-junction CH3NH3PbI3/CH3NH3PbI3 tandem solar cell featuring an unprecedentedly high open-circuit voltage of 2.30 V. This work demonstrates the promise of vacuum-based process for fabricating light-weight and flexible all-perovskite tandem solar cells with ultra-high power-conversion efficiencies.
2D organolead halide perovskite field effect transistors, which are fabricated based on phase‐pure homologous (n = 1, 2, and 3) Ruddelsden–Popper perovskite (BA)2(MA) n− 1Pb n I3 n +1 single crystals are demonstrated. A strong dependence of carrier transport behavior of the 2D organolead halide hybrid perovskites on the n value is revealed.
This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase‐pure homologous (n = 1, 2, and 3) Ruddelsden–Popper perovskite (RPP) (BA)2(MA) n −1Pb n I3n+1 single crystals. The 2D perovskite field effect transistors with high‐quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single‐crystal flakes with various n values under different temperatures. Unipolar n‐type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10−3, 8.3 × 10−2, and 1.25 cm2 V−1 s−1 at 77 K, respectively. Furthermore, by combining temperature‐dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic–inorganic hybrid perovskites based on phase‐pure homologous single crystals.
Solution‐processed metal oxide nanocrystals present unique properties as efficient carrier transport layers in photovoltaic devices. In this review, solution‐processed metal oxide nanocrystal‐based carrier transport layers in organic solar cells and perovskite solar cells, and their low‐temperature solution‐processed synthesis approaches are summarized.
There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.
An ultrathin ferroelectric oxide PbTiO3 layer is incorporated between the electron transport material and the halide perovskite in the hole transport material (HTM) free carbon‐based perovskite solar cell (C‐PSCs). The achieved power conversion efficiency is as high as 16.37%, which is the highest record for HTM‐free C‐PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage.
The hole transport material (HTM) free carbon based perovskite solar cells (C‐PSCs) are promising for its manufactural simplicity, but they currently suffer from low power conversion efficiencies (PCE) largely because of the voltage loss. Here, a new strategy to increase the PCE by incorporating an ultrathin ferroelectric oxide PbTiO3 layer between the electron transport material and the halide perovskite is reported. The resulting C‐PSCs have achieved PCEs up to 16.37%, which is the highest record for HTM‐free C‐PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage. Detail measurements and analysis show an enhanced built‐in potential in the C‐PSCs as well as suppression of the non‐radiative recombination due to the ferroelectric PbTiO3 layer incorporation, accounting for the boosted V OC and photovoltaic performance.
PbTiO3 (PTO) with suitable band alignment is a promising electron‐selective layer in hybrid perovskite solar cells. Reversal of the local polarization of PTO upon alternating external poling can tune the transfer direction of the photogenerated carriers in the active layer, thereby improving the photovoltaic performance of the solar cells.
PbTiO3 (PTO) is explored as a versatile and tunable electron‐selective layer (ESL) for perovskite solar cells. To demonstrate effectiveness of PTO for electron–hole separation and charge transfer, perovskite solar cells are designed and fabricated in the laboratory with the PTO as the ESL. The cells achieve a power conversion efficiency (PCE) of ≈12.28% upon preliminary optimization. It is found that the PTO ferroelectric layer can not only increase the PCE, but also tune the photocurrent via tuning PTO's ferroelectric polarization. Moreover, to understand the physical mechanism underlying the carrier transport by the ferroelectric polarization, the electronic structure of PTO/CH3NH3PbI3 heterostructure is computed using the first‐principles methods, for which the triplet state is used to simulate charge transfer in the heterostructure. It is shown that the synergistic effect of type II band alignment and the specific ferroelectric polarization direction provide the effective extraction of electrons from the light absorber, while minimize recombination of photogenerated electron–hole pairs. Overall, the ferroelectric PTO is a promising and tunable ESL for optimizing electron transport in the perovskite solar cells. The design offers a different strategy for altering direction of carrier transport in solar cells.
Based on the high humidity stable [(NH4)2.4(FA)8Pb9I28.4]0.85(MAPbBr3)0.15 mixed‐dimensional perovskite, the authors investigated its aging properties under different humidity levels. Through analyzing the performance changes during aging, the authors speculated, and verified the possible mechanism of its high moisture resistance, which is a result from the formation of NH4PbX3*(H2O)2 and two‐dimensional protective layers, and the conversion of δ‐phase FAPbI3 into α‐phase under continuous illumination.
Recently, perovskite materials are widely applied in the photovoltaic field, whereas its practical application is hindered by the humidity instability. To solve this problem, the authors prepared a [(NH4)2.4(FA)8Pb9I28.4]0.85(MAPbBr3)0.15 mixed‐dimensional (MD) perovskite with superior humidity stability. Herein, we investigated the aging properties of three‐dimensional (3D) and MD perovskite under different humidity levels. Through analyzing the performance changes before and after aging tests, the possible mechanism of high moisture resistance for MD perovskite is speculated and verified. After undergoing cation exchange, the surface NH4 + combines with H2O to form NH4PbX3*(H2O)2 (X = I or Br), and then the two‐dimensional (2D) perovskite protective layers are formed on the surface of perovskite, which prevent H2O from destroying the 3D perovskite structure. Meanwhile, under continuous illumination, the δ‐phase FAPbI3 produced from inside FA+ may change into α‐phase FAPbI3. Therefore, the MD perovskite maintains great 3D perovskite structure and displays outstanding humidity stability under high humidity. The devices retain their starting photoelectric conversion efficiency (PCE) for 4000 h under 40% relative humidity (RH) and 80% of PCE over 2000 h under 70% RH. This finding provides a promising prospect for solving the humidity instability of perovskite materials and will promote the development of PSCs.
Non‐peripheral octamethyl‐substituted copper (II) phthalocyanine nanowires are incorporated in poly(3‐hexylthiophene) to form nanocomposite, which exhibited higher hole mobilities and well‐matched energy levels. A power conversion efficiency of 16.61% is achieved for a perovskite solar cell based on composite hole‐transport material which retains 90% of their initial efficiencies after 800 h of storage at 25 °C with a relative humidity of 75% without any encapsulations.
New efficient hole‐transport material (HTM) composites based on low‐cost easy‐preparation non‐peripheral octamethyl‐substituted copper (II) phthalocyanine (N‐CuMe2Pc) nanowire and poly(3‐hexylthiophene) (P3HT) are developed for CH3NH3PbI3 (MAPbI3)‐based perovskite solar cells (PSCs). Compared with pristine P3HT, the prepared nanocomposite HTMs provided thin films with better qualities and reduced trap densities, and exhibited higher hole mobilities and well‐matched energy levels with the perovskite layer. Depending on the ratio of the two components, the power conversion efficiency (PCE) reached up to 16.61%, which is higher than the efficiency of the standard device based on doped spiro‐OMeTAD (16.13%). Moreover, the long‐term stability of the PSCs is also improving greatly. The best performing devices based on P1C1 HTM retained 90% of their initial efficiencies after 800 h of storage with a relative humidity of 75%. These results indicate N‐CuMe2Pc nanowire/P3HT nanocomposites can be an effective HTM to realize superior performance in PSCs.
By combining optical modeling and experimental results, the authors provide a full optical analysis for semitransparent organic solar cells (STOSCs). Defined as the sum of external quantum efficiency and transmittance, the term “quantum utilization efficiency (QUE)” is proposed as a parameter to describe light energy use in the semitransparent devices, which provides a new angle for analyzing STOSCs.
Semitransparent organic solar cells (STOSCs) show great potential for application as power generating windows for buildings. The power conversion efficiency (PCE) and the average visible transmittance (AVT) are both important parameters with which to evaluate the overall performance of STOSCs. However, it is very challenging to simultaneously improve these two performance parameters because they are intrinsically contradictory to each other. In this work, the optical and photovoltaic properties of STOSCs are investigated based on two model samples including PTB7‐Th:PC61BM and PTB7‐Th:PC71BM by systematically tuning their device structures. By combining optical modeling and experimental results, a full optical analysis is provided for the STOSCs with details on photon harvesting, optical losses, transmission properties, energy distribution spectrum, electric field intensity distribution, and photon absorption rate distribution within the devices. Defined as the sum of the external quantum efficiency and the transmittance, the term “quantum utilization efficiency” is used as a subjective parameter to describe the light energy use in the semitransparent devices, which provides an alternative angle for analyzing STOSCs.
Conventional PbI2 is replaced by PbI2/I2 mixed precursor during the first step of sequential deposition, causing the formation of a PbI2 porous nanostructure. By changing the content of I2 in the precursor, the morphology of the PbI2 film as well as the resulting perovskite film can be successfully modulated. With an optimal content of I2, a high‐quality perovskite film with a pure phase and smooth surface is achieved, enabling the high performance of perovskite solar cells.
The quality of the perovskite film has a vital influence on the performance of perovskite solar cells and it is quite desirable to simultaneously manipulate the crystallization and morphology of the perovskite film. In this study, conventional PbI2 is replaced with a PbI2/I2 mixed precursor during the first step of sequential deposition, causing the formation of a PbI2 porous nanostructure. By changing the content of I2 in the precursor, the morphology of the PbI2 film as well as the resulting perovskite film can be successfully modulated. With an optimal content of I2, a high‐quality perovskite film with a pure phase and smooth surface can be achieved. As a result, the conversion efficiency of perovskite solar cells using a PbI2/I2 mixed precursor can be as high as 18.63%, compared to 16.89% for the reference device through traditional sequential deposition with a pure PbI2 precursor.
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A crystallization kinetics management “L‐I” process based on co‐regulation of Lewis adduct and the ion exchange process is developed to obtain high‐quality low‐dimensional Ruddlesden–Popper (LDRP) lead‐free perovskite films with large grain size and low trap density. The corresponding devices exhibit promising efficiency of 4.03% and improved stability of 94 d in nitrogen.
Low‐dimensional Ruddlesden–Popper (LDRP) lead‐free perovskite has great potential due to its improved stability and oriented crystal growth, which is mainly attributed to the effective control of crystallization kinetics. However, the crystallization kinetics of LDRP lead‐free perovskite films are highly limited by Lewis theory. Here, the management of the crystallization kinetics of LDRP tin (Sn) perovskite films jointly controlled by Lewis adducts and the ion exchange process using a mixture of polar aprotic solvent dimethyl sulfoxide (DMSO) and ion liquid solvent methylammonium acetate (MAAc) (the process named as “L‐I”) is demonstrated. Homogeneous nucleated LDRP Sn perovskite films with average grain size close to 9 µm are achieved. Both low electron and hole defect density with a magnitude of 1016, high carrier mobility, and excellent electrical performance are obtained. As a result, the LDRP Sn perovskite solar cell (PSC) with power conversion efficiency (PCE) of 4.03% is achieved using a simple one‐step method without antisolvents, which is one of the best LDRP Sn PSCs. Most importantly, the PSC exhibits excellent stability with no degradation in PCE after 94 d in a nitrogen atmosphere owing to the high‐quality film and the inhibition of the oxidation of Sn2+.
In article number 1801963, Bin Ding, Hakyong Kim, and co‐workers prepare Al2O3‐La2O3 nanofibrous membranes via facile electrospinning. The self‐assembly “plum‐pudding” like CsPbBr3/Cs4PbBr6 perovskite crystals anchor onto the Al2O3‐La2O3 nanofibers via a supersaturated recrystallization process. The (CsPbBr3/Cs4PbBr6)@Al2O3‐La2O3 (CCAL) membranes display outstanding mechanical flexibility and luminescence properties. CCAL membranes have potential applications in green light devices through remote packaging of the green‐emissive CCAL membranes with a GaN light‐emitting device chip.
A general nondestructive passivation approach is developed for perovskite solar cells by using 4‐fluoroaniline (FAL). FAL is not only an antisolvent surface modifier, but also a large dipole molecule with a directional field to separate charge at the interface, thus delivering a 20.48% power conversion efficiency with improved stability. Micro/time‐resolved photoluminescence reveals the impact picture of boundary on the local carriers after passivation.
Hybrid perovskite thin films are prone to producing surface vacancies during the film formation, which degrade the stability and photovoltaic performance. Passivation via post‐treatment can heal these defects, but present methods are slightly destructive to the bulk of 3D perovskite due to the solvent effect, which hinders fabrication reproducibility. Herein, nondestructive surface/interface passivation using 4‐fluoroaniline (FAL) is established. FAL is not only an effective antisolvent candidate for surface modification, but also a large dipole molecule (2.84 Debye) with directional field for charge separation. Density functional theory calculation reveals that the nondestructive properties are attributed to both the conjugated amine in aromatic ring and the para‐fluoro‐substituent. A hot vapor assisted colloidal process is employed for the post‐treatment. The molecular passivation yields an ultrathin protection layer with a hydrophobic fluoro‐substituent tail and thus enhances the stability and optoelectronic properties. FAL post‐treated perovskite solar cell (PSC) delivers a 20.48% power conversion efficiency under ambient conditions. Micro‐photoluminescence reveals that passivation activates the dark defective state at the surface and interface, delivering the impact picture of boundary on the local carriers. This work demonstrates a generic nondestructive chemical approach for improving the performance and stability of PSCs.
Perovskite solar cells (PSCs) have undergone rapid development, but the performance degradation accompanied by device upscaling urgently needs a solution. This review covers the research progress on each functional material of large‐area PSCs. A conclusion on the main challenges and an outlook on the research direction of large‐area PSCs are provided.
Perovskite solar cells (PSCs) are promising candidates for the next generation of photovoltaic technologies due to their constantly improved efficiencies, which gain much attention from both the scientific and industrial communities. Although the performance of PSCs is dramatically enhanced, most certified or reported high‐efficiency PSCs are still limited to a relatively small active area. The degradation of efficiency and stability accompanied by upscaling must be solved, being a bottleneck toward industrialization. This review focuses on the research progress, challenges, and strategies on large‐area PSCs, especially each functional material in various device architectures, including perovskites, hole transport materials, electron transport materials, and electrodes. Finally, the main issues related to each functional layer of PSCs from laboratory to industry are presented and an outlook on the research direction of large‐area PSCs is given.
The sensitive detection of X‐rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. In article number 1804550, Julian A. Steele, Maarten B. J. Roeffaers, and co‐workers detail the photophysical properties contributing to the impressive performance of highly sensitive X‐ray detectors based on double perovskite Cs2AgBiBr6. Upon cooling the device, enormous performance enhancements are realized and are tracked via optical‐based studies, closing the gap between the interpretation of high‐ and low‐energy photogenerated charges.
The utilization of poly(vinylpyrrolidone) (PVP) as a cathode interlayer is demonstrated in inverted and conventional devices via both the self‐organization method and the step‐by‐step preparation method. The driving forces for PVP migration are the high surface energy of the PVP and the strong intermolecular interaction between the PVP and the bottom cathode. In addition, the PVP‐modified devices have excellent stability in air and show insensitivity to PVP molecular weight.
Herein, poly(vinylpyrrolidone) (PVP) is used as the cathode interlayer (CIL) through the self‐organization method in inverted organic solar cells (OSCs). By coating a solution of PVP and active layer materials onto a glass/indium tin oxide (ITO) substrate, the PVP can segregate to the near ITO side due to its high surface energy and strong intermolecular interaction with the ITO electrode. The power conversion efficiency (PCE) of the obtained OSC device reaches 13.3%, much higher than that of the control device with a PCE of only 10.1%. The improvement results from the increased exciton dissociation efficiency and the depressed trap‐assisted recombination, which can be attributed to the reduced work function of the cathode by the self‐organized PVP. Additionally, the molecular weight of the PVP has almost no influence on the device performance, and the PVP‐modified device presents superior stability. This method can also be applied in other highly efficient fullerene‐free OSCs, and with a fine selection of the active layer, a high PCE of 14.0% is obtained. Overall, this work demonstrates the great potential of the PVP‐based CIL in inverted OSCs fabricated via the self‐organization method.
An important design principle for perovskite light‐emitting diodes is discovered regarding optimal perovskite thickness. Adopting a thinner perovskite layer is beneficial for both device efficiency and stability, with external quantum efficiency (EQE) as high as 17.6% being achieved. The improved EQE is primarily due to better light outcoupling, and the improved stability is correlated with reduced Joule heating.
Hybrid organic–inorganic perovskite semiconductors have shown potential to develop into a new generation of light‐emitting diode (LED) technology. Herein, an important design principle for perovskite LEDs is elucidated regarding optimal perovskite thickness. Adopting a thin perovskite layer in the range of 35–40 nm is shown to be critical for both device efficiency and stability improvements. Maximum external quantum efficiencies (EQEs) of 17.6% for Cs0.2FA0.8PbI2.8Br0.2, 14.3% for CH3NH3PbI3 (MAPbI3), 10.1% for formamidinium lead iodide (FAPbI3), and 11.3% for formamidinium lead bromide (FAPbBr3)‐based LEDs are demonstrated with optimized perovskite layer thickness. Optical simulations show that the improved EQEs source from improved light outcoupling. Furthermore, elevated device temperature caused by Joule heating is shown as an important factor contributing to device degradation, and that thin perovskite emitting layers maintain lower junction temperature during operation and thus demonstrate increased stability.
Charge and triplet exciton dynamics in neat fullerene and hybrid solar cells with fullerene as the only light absorber are investigated by transient spectroscopy. The efficiency and mechanism of charge generation and triplet formation depend on the photoactive layer composition. Formation of a hybrid bulk heterojunction leads to ultrafast exciton dissociation, fast charge extraction, and power conversion efficiencies in excess of 5%.
Organic solar cells that use only fullerenes as the photoactive material exhibit poor exciton‐to‐charge conversion efficiencies, resulting in low internal quantum efficiencies (IQE). However, the IQE can be greatly improved, when copper(I) thiocyanate (CuSCN) is used as a carrier‐selective interlayer between the phenyl‐C70‐butyric acid methyl ester (PC70BM) layer and the anode. Efficiencies of ≈5.4% have recently been reported for optimized CuSCN:PC70BM (1:3)‐mesostructured heterojunctions, yet the reasons causing the efficiency boost remain unclear. Here, transient absorption (TA) spectroscopy is used to demonstrate that CuSCN does not only act as a carrier‐selective electrode layer, but also facilitates fullerene exciton dissociation and hole transfer at the interface with PC70BM. While intrinsic charge generation in neat PC70BM films proceeds with low yield, hybrid films exhibit much improved exciton dissociation due to the presence of abundant interfaces. Triplet generation with a rate proportional to the product of singlet and charge concentrations is observed in neat PC70BM films, implying a charge–singlet spin exchange mechanism, while in hybrid films, this mechanism is absent and triplet formation is a consequence of nongeminate recombination of free charges. At low carrier concentrations, the fraction of charges outweighs the population of triplets, leading to respectable device efficiencies under one sun illumination.
