Stability Improvement of Perovskite Solar Cells by the Moisture-Resistant PMMA:Spiro-OMeTAD Hole Transport Layer.

Perovskite solar cells (PSCs) based on the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) hole transport layer have exhibited leading device performance. However, the instability caused by this organic function layer is a very important limiting factor to the further development of PSCs. In this work, the spiro-OMeTAD is doped with polymethyl methacrylate (PMMA), which is further used as the hole transport layer to improve the device stability. It is shown that the PMMA can effectively improve the moisture and oxygen resistance of spiro-OMeTAD, which leads to improved device stability by separating the perovskite layer from moisture and oxygen. The device efficiency can maintain 77% of the original value for PSCs with the PMMA-doped spiro-OMeTAD hole transport layer, under a natural air environment (RH = 40%) for more than 80 days. The results show that the moisture- and oxygen-resistant PMMA:spiro-OMeTAD hole transport layer is effective at improving the device performance.

Synchronous Interface Modification and Bulk Passivation via a One-Step Cesium Bromide Diffusion Process for Highly Efficient Perovskite Solar Cells.

Perovskite film modification is one of the most effective methods to improve the performance of perovskite solar cells. The modification should follow its characters of an asymmetric structure and the corresponding charge transportation and extraction. In this work, it is shown that synchronous interface modification and bulk passivation for highly efficient PSCs can be achieved by a one-step cesium bromide (CsBr) diffusion process because it is more suitable for an asymmetric structure. The synchronous interface modification and bulk asymmetric passivation can be better applied to the asymmetric PSC structure and can boost the power conversion efficiency apparently from 19.5 to 22.1%. It is shown that the perovskite crystallization is improved and the charge extraction is also enhanced obviously due to the better band alignment matching. The diffusion of CsBr into the perovskite bulk could form a gradient distribution, which is more applicable to the asymmetric charge transport and extraction. Thus, the CsBr at the interface between the electronic transport layer (ETL) and perovskite, as well as in the perovskite bulk, could suppress charge recombination. All of these factors can improve the JSC and VOC as well as the power conversion efficiency (PCE) of the PSCs. The results point out that the studied method is a simple and efficient way to fabricate high-performance PSCs by interface modification and bulk asymmetric passivation in a single step.

Ultrawide Band Gap Oxide Semiconductor-Triggered Performance Improvement of Perovskite Solar Cells via the Novel Ga2O3/SnO2 Composite Electron-Transporting Bilayer.

The performance of perovskite solar cells (PSCs), especially for the parameters of open-circuit voltage (Voc) and fill factor, is seriously restricted by the unavoidable interfacial charge recombination. In this study, an ultrawide band gap semiconductor material of Ga2O3 is introduced between fluorine-doped tin oxide and SnO2 to regulate the interfacial charge dynamics by forming the Ga2O3/SnO2 electron-transporting bilayer. Ga2O3 has an appropriate conduction band minimum which benefits the electron transport, and at the same time, it has a very deep valence band maximum which could be regarded as an effective blocking layer. Such an innovative structure triggers the advantages of a lower work function and a smoother surface of the electron-transporting bilayer which leads to a high-quality perovskite film. Furthermore, superior hole-blocking properties of the introduced Ga2O3 layer could effectively reduce the interfacial recombination. All the properties could help to improve the extracting and transporting ability of charge carriers synergistically. Finally, the efficiency and stability of PSCs are greatly enhanced. All results suggest that the performance of PSCs could be improved effectively by introducing the ultrawide band gap oxide semiconductor of Ga2O3.

Transparent Ultrathin Metal Electrode with Microcavity Configuration for Highly Efficient TCO-Free Perovskite Solar Cells.

Optical microcavity configuration is one optical strategy to enhance light trapping in devices using planar electrodes. In this work, the potential application of optical microcavity configuration with ultrathin metal electrodes in highly efficient perovskite solar cells (PSCs) was investigated. By comparing with the device with conventional indium-tin-oxide (ITO) electrodes, it is shown that by carefully designing the Ag/dielectric planar electrode, a device with an optical microcavity structure can achieve comparable-or even higher-power conversion efficiency than a conventional device. Moreover, there is a relative high tolerance for the Ag film thickness in the optical microcavity structure. When the thickness of the Ag film is increased from 8 to 12 nm, the device still can attain the performance level of a conventional device. This gives a process tolerance to fabricate devices with an optical microcavity structure and reduces process difficulty. This work indicates the great application potential of optical microcavities with ultrathin metal electrodes in PSCs; more research attention should be paid in this field.

Efficient Ni/Au Mesh Transparent Electrodes for ITO-Free Planar Perovskite Solar Cells.

Indium thin oxide (ITO)-free planar perovskite solar cells (PSCs) were fabricated at a low temperature (150 °C) in this work based on the transparent electrode of photolithography processed nickel/gold (Ni/Au) mesh and the high conductivity polymer, PH1000. Ultrathin Au was introduced to increase the conductivity of metal mesh, and the optimal hexagonal Ni (30 nm)/Au (10 nm) mesh (line width of 5 µm) shows a transmittance close to 80% in the visible light region and a sheet resistance lower than 16.9 Ω/sq. The conductive polymer PH1000 not only smooths the raised surface of the metal mesh but also enhances the charge collection ability of metal mesh. The fabricated PSCs have the typical planar structure (glass/Ni-Au mesh/PH1000/PEDOT:PSS/MAyFA1-yPbIxCl3-x/PCBM/BCP/Ag) and the champion PSC (0.09 cm2) obtains a power conversion efficiency (PCE) of 13.88%, negligible current hysteresis, steady current density and PCE outputs, and good process repeatability. Its photovoltaic performance and stability are comparable to the reference PSC based on the ITO electrodes (PCE = 15.70%), which demonstrates that the Ni/Au mesh transparent electrodes are a promising ITO alternative to fabricate efficient PSCs. The relatively lower performance of Ni/Au based PSC results from the relatively slower charge extraction and stronger charge recombination than the ITO based PSC. Further, we tried to fabricate the large area (1 cm2) device and achieve a PCE over 6% with negligible hysteresis and steady current density and PCE outputs. The improvements of perovskite film quality and interface modification should be an effective approach to further enhance the device performance of Ni/Au based PSCs, and the Ni/Au mesh electrode may find wider applications in PSCs and flexible devices.

Performance enhancement of perovskite solar cells via material quality improvement assisted by MAI/IPA solution post-treatment.

The quality of the perovskite layer plays a key role in the performance of perovskite solar cells (PSCs). In this work, we demonstrate a facile way to improve the quality of perovskite films by using a MAI/IPA solution to post-treat the perovskite material after the film formation. XRD, SEM and AFM show that an optimal, uniform, dense and highly crystalline perovskite film is achieved after the post-treatment. The transient photocurrent and transient photovoltage also confirm that the devices after the post-treatment have both the outstanding ability of charge extraction and suppressed charge recombination. By using the post-treatment, a maximum PCE of 19.08%, which is 12% higher than that of the control device, is obtained with a Jsc of 23.12 mA cm-2, a Voc of 1.08 V and an FF of 76.03%. The results show that the MAI/IPA solution post-treatment after the perovskite film formation is a facile way to improve the film quality and the PSC performance.

Improving Electron Extraction Ability and Device Stability of Perovskite Solar Cells Using a Compatible PCBM/AZO Electron Transporting Bilayer.

Due to the low temperature fabrication process and reduced hysteresis effect, inverted p-i-n structured perovskite solar cells (PSCs) with the PEDOT:PSS as the hole transporting layer and PCBM as the electron transporting layer have attracted considerable attention. However, the energy barrier at the interface between the PCBM layer and the metal electrode, which is due to an energy level mismatch, limits the electron extraction ability. In this work, an inorganic aluminum-doped zinc oxide (AZO) interlayer is inserted between the PCBM layer and the metal electrode so that electrons can be collected efficiently by the electrode. It is shown that with the help of the PCBM/AZO bilayer, the power conversion efficiency of PSCs is significantly improved, with negligible hysteresis and improved device stability. The UPS measurement shows that the AZO interlayer can effectively decrease the energy offset between PCBM and the metal electrode. The steady state photoluminescence, time-resolved photoluminescence, transient photocurrent, and transient photovoltage measurements show that the PSCs with the AZO interlayer have a longer radiative carrier recombination lifetime and more efficient charge extraction efficiency. Moreover, the introduction of the AZO interlayer could protect the underlying perovskite, and thus, greatly improve device stability.

Efficient Bifacial Semitransparent Perovskite Solar Cells Using Ag/V2O5 as Transparent Anodes.

Bifacial semitransparent inverted planar structured perovskite solar cells (PSCs) based on Cs0.05FA0.3MA0.7PbI2.51Br0.54 using an Ag thin film electrode and V2O5 optical coupling layer are investigated theoretically and experimentally. It is shown that the introduction of the cesium (Cs) ions in the perovskite could obviously improve the device performance and stability. When only the bare Ag film electrode is used, the PSCs show a bifacial performance with the power conversion efficiency (PCE) of 14.62% illuminated from the indium tin oxide (ITO) side and 5.45% from the Ag film side. By introducing a V2O5 optical coupling layer, the PCE is enhanced to 8.91% illuminated from the Ag film side, which is 63% improvement compared with the bare Ag film electrode, whereas the PCE illuminated from the ITO side remains almost unchanged. Moreover, when a back-reflector is employed, the PCE of device could be further improved to 15.39% by illumination from the ITO side and 12.44% by illumination from the Ag side. The devices also show superior semitransparent properties and exhibit negligible photocurrent hysteresis, irrespective of the side from which the light is illuminated. In short, the Ag/V2O5 double layer is a promising semitransparent electrode due to its low cost and simple preparation process, which also point to a new direction for the bifacial PSCs and tandem solar cells.