Solution-processed Planar Perovskite Solar Cells Research Articles

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

NDI-based small molecules as electron transporting layers in solution-processed planar perovskite solar cells

High-temperature preparation of metal oxide-based electron transporting materials is considered to be a potential obstacle toward the commercialization of perovskite solar cells. Inverted perovskite solar cells can overcome this problem by employing metal-oxide free, low-temperature-fabricated, and solution-processed electron transporting materials. However, the conventionally-used electron transporting materials (e.g. phenyl-C61-butyric acid methyl ester (PCBM)) has several drawbacks including poor morphology control and high cost, which make its application impractical. Thus, scientists are actively searching novel organic small molecules to replace PCBM because these small compounds have tunable frontier molecular orbitals as well as good film morphology control. More importantly, these molecules can be prepared through inexpensive synthesis routes. Herein, we report the synthesis of two novel naphthalenediimide (NDI)-based electron transporting materials (4,4′-(piperazine-1,4-diyl)bis(2,7-dioctylbenzo[lmn]-[3,8]phenanthroline-1,3,6,8(2 H,7 H)-tetraone) (PDPT) and 9,9′-(piperazine-1,4-diyl) bis(4-(4-methylpiperidin-1-yl)-2,7 dioctylbenzo [lmn]-[3,8]phenanthroline-1,3,6,8(2 H,7 H)-tetraone) (PMDPT)), and found that the inverted perovskite solar cells with PMDPT as an electron transporting layer can reach a power conversion efficiency up to 9.2% while the efficiency of PSCs based on PDPT can only approach 7.6%. We believe that this improvement in the efficiency of PMDPT-based PSCs ascribes to the increased number of nitrogen atoms in the framework of PMDPT, which passivates the electron trap centers on the surface of the perovskite layer. This passivation results in less charge recombination, therefore delivering a higher Voc and PCE.

Influence of Polymer Additives on the Efficiency and Stability of Ambient‐Air Solution‐Processed Planar Perovskite Solar Cells

AbstractIt is challenging to fabricate efficient and stable perovskite solar cells (PSCs) in ambient air conditions without special controlled humidity. Here, we report a facile polymer additive route to overcome the problem. We find that the polyvinyl butyral (PVB) and polyvinyl alcohol (PVA) additives can both obviously enhance the quality of the perovskite films compared with the pristine one. Especially, the perovskite film with PVB additive shows clearer improved quality because of the formation of larger crystal grains which can go from bottom to top and smoother surface with the lower root‐mean‐square roughness of 23.4 nm than that of the one with PVA additive (45.6 nm). The improved quality of the perovskite films with PVB additive contributes to better photovoltaic performance and long‐term stability of the devices. The PSCs with PVB additive can achieve a champion power conversion efficiency (PCE) of 19.10 % and a small hysteresis index of 0.039, which are both better than those of the pristine ones (15.14 % and 0.148, respectively). Furthermore, the PSC with PVB additive can maintain a relatively high PCE of 12.43 % after being stored in a humid ambient air condition without any encapsulation, whereas the pristine one drops to a very low PCE of 5.18 %. This work demonstrates the valuable PVB additive route for developing efficient and stable PSCs in ambient air conditions.

Electrode Design to Overcome Substrate Transparency Limitations for Highly Efficient 1 cm2 Mesoscopic Perovskite Solar Cells

Summary Fluorine-doped tin oxide glass substrate is typically used for state-of-the-art perovskite solar cells (PSCs). However, indium-doped tin oxide (ITO) is better due to higher transparency for a given conductivity, although it has lower tolerance to high-temperature processes required for the compact and mesoporous TiO2 layers. Here we overcome this challenge by developing and utilizing a new electrode design. We successfully demonstrate high-efficiency mesoscopic PSCs on annealed ITO substrates showing improved photocurrent without sacrificing fill factor. After further optimizations of cell geometry and substrate conductivity guided by simulation, a certified 19.63% efficiency is achieved on 1 cm2 for ITO-based mesoscopic PSC, which is the highest among PSCs prepared by gas quenching. This work is useful for providing design principles and methods for optimizing cell geometry, metal electrode design, and substrate conductivity requirements for large-area PSCs.

Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Summary Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity.

One-step facile synthesis of a simple carbazole-cored hole transport material for high-performance perovskite solar cells

A carbazole-based hole-transporting material named 4,4′,4′′,4′′′-(9-octylcarbazole-1,3,6,8-tetrayl) tetrakis(N,N-bis(4-methoxyphenyl)aniline) (CZ-TA) has been developed through a one-step facile synthesis approach. The solution-processed planar perovskite solar cells (PVSCs) with 50nm thick CZ-TA hole selective layer (HSL) contributes a power conversion efficiency of 18.32%, comparable to the most commonly used 200nm spiro-OMeTAD (18.28%) HSLs. The improved hole extraction, transport and reduced recombination are found to endow CZ-TA-based devices with impressive fill factors over 81.0%. Importantly, the unit cost of HSL in PVSCs using CZ-TA can be as low as only 1/80 of that of spiro-OMeTAD, indicating that CZ-TA could be a promising candidate as HSLs for commercialization of the low-cost PVSC technology.

Efficient and stable solution-processed planar perovskite solar cells via contact passivation.

Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).

Degradation Mechanisms of Solution-Processed Planar Perovskite Solar Cells: Thermally Stimulated Current Measurement for Analysis of Carrier Traps.

Degradation mechanisms of CH3 NH3 PbI3 -based planar perovskite solar cells (PSCs) are investigated using a thermally stimulated current technique. Hole traps lying above the valence-band edge of the CH3 NH3 PbI3 are detected in PSCs degraded by continuous simulated solar illumination. One source of the hole traps is the photodegradation of CH3 NH3 PbI3 in the presence of water.

Efficient and non-hysteresis CH3NH3PbI3/PCBM planar heterojunction solar cells

Highly efficient and non-hysteresis organic/perovskite planar heterojunction solar cells was fabricated by low-temperature, solution-processed method with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al. The high-quality perovskite thin film was obtained using a solvent-induced-fast-crystallization deposition involving spin-coating the CH3NH3PbI3 solution followed by top-dropping chlorobenzene with an accurate control to induce the crystallization, which results in highly crystalline, pinhole-free, and smooth perovskite thin film. Furthermore, it was found that the molar ratio of CH3NH3I to PbI2 greatly influence the properties of CH3NH3PbI3 film and the device performance. The equimolar or excess PbI2 was facile to form a flat CH3NH3PbI3 film and produced relatively uniform perovskite crystals. Perovskite solar cells (PSCs) with high-quality CH3NH3PbI3 thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 13.49% was achieved, which is one of the highest PCEs obtained for low-temperature, solution-processed planar perovskite solar cells based on the structure ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al. More importantly, PSCs fabricated using this method didn’t show obvious hysteresis under different scan direction and speed.