penghuang

Stability required! Perovskite solar cells have emerged as one of the most exciting fields of research, owing to their impressive rise in power conversion efficiency surpassing 22% in six short years of research. Current research is focused on ways to improve stability of perovskite-based devices, a key characteristic required to bring this technology from the lab into the market. In this Editorial, guest editor Prof. Mohammad Khaja Nazeeruddin describes the context of this Special Issue, and summarizes the work being performed in his research group toward this low-cost near-future photovoltaic technology.

The power conversion efficiency (PCE) of the perovskite solar cell is high enough to be commercially viable. The next important issue is the stability of the device. This article discusses the effect of the perovskite grain-size on the long-term stability of inverted perovskite solar cells. Perovskite films composed of various sizes of grains were prepared by controlling the solvent annealing time. The grain-size related stability of the inverted cells was investigated both in ambient atmosphere at relative humidity of approximately 30–40 % and in a nitrogen filled glove box (H₂O<0.1 ppm, O₂<10 ppm). The PCE of the solar cell based on a perovskite film having the grain size larger than 1 μm (D-10) decreases less than 10 % with storage in a glove box and less than 15 % when it was stored under an ambient atmosphere for 30 days. However, the cell using the perovskite film composed of small (∼100 nm) perovskite grains (D-0) exhibits complete loss of PCE after storage under the ambient atmosphere for only 15 days and a PCE loss of up to 70 % with storage in the glove box for 30 days. These results suggest that, even under H₂O-free conditions, the chemical- and thermal-induced production of pin holes at the grain boundaries of the perovskite film could be the reason for long-term instability of inverted perovskite solar cells.

Removing boundaries! Inverted perovskite solar cells are fabricated from perovskite films with varied grain size. The films with larger grain size show improved stability under both glove-box-controlled and ambient conditions compared with the films having smaller grain size, suggesting that, the chemical- and thermal-induced production of pin holes at grain boundaries could be the reason for long-term instability of inverted perovskite solar cells.

The interface between the perovskite (PVK, CH₃NH₃PbI₃) and hole-transport layers in perovskite solar cells is discussed. The device architecture studied is as follows: F-doped tin oxide (FTO)-coated glass/compact TiO₂/mesoporous TiO₂/PVK/2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-MeOTAD)/Au. After a thin layer of 4,4,4-trifluorobutylammonium iodide (TFBA) was inserted at the interface between PVK and Spiro-MeOTAD, the photovoltaic efficiency increased from 11.6–14.5 % to 15.1–17.6 %. TFBA (10 ppm) was added in the PVK solution before coating. Owing to the low surface tension of TFBA, TFBA rose to the surface of the PVK layer spontaneously during spin-coating to make a thin organic layer. The PVK grain boundaries also seemed to be passivated with the addition of TFBA. However, large differences in Urbach energies and valence band energy level were not observed for the PVK layer with and without the addition of TFBA. The charge recombination time constant between the PVK and the Spiro-MeOTAD became slower (from 8.4 to 280 μsec) after 10 ppm of TFBA was added in the PVK. The experimental results using TFBA conclude that insertion of a very thin layer at the interface between PVK and Spiro-MeOTAD is effective for suppressing charge recombination and increasing photovoltaic performances.

Smooth it over: The perovskite (PVK) grain boundaries were passivated with fluoro- and amine-substituted surfactant molecules, serving to provide low surface energy and as anchoring groups, respectively, such as 4,4,4-trifluorobutylamine hydroiodide (TFBA). Charge recombination at the interface between passivated PVK and the hole-transport layer was suppressed and the power conversion efficiency of a device fabricated with this passivated PVK layer was enhanced.

Inorganic metal oxide electron-transport layers (ETLs) have the potential to yield perovskite solar cells with improved stability, but generally need high temperature to form conductive and defect-less forms, which is not compatible with the fabrication of flexible and tandem solar cells. Here, we demonstrate a facile strategy for developing efficient inorganic ETLs by doping SnO₂ nanocrystals (NCs) with a small amount of Sb using a low-temperature solution-processed method. The electrical conductivity was remarkably enhanced by Sb-doping, which increased the carrier concentration in Sb:SnO₂ NCs. Moreover, the upward shift of the Fermi level owing to doping results in improved energy level alignment, which led to reduced charge recombination, and thus longer electron recombination lifetime and improved open-circuit voltage (V_OC). Therefore, Sb-doping of SnO₂ significantly enhanced the photovoltaic performance of planar perovskite devices by increasing the fill factor and V_OC, and reducing photocurrent hysteresis, extending the potential application of low-temperature-processed ETLs in future flexible and tandem solar cells.

Doping effect: Sb:SnO₂ nanocrystalline films are successfully prepared using a low-temperature solution-processed method as an efficient electrontransport layer for planar perovskite solar cells. Intentional Sb-doping is demonstrated as an effective approach to increase the electrical conductivity and upward shift the Fermi level of SnO₂, leading to dramatically enhanced photovoltaic performance and suppressed hysteresis.

In this work, an efficient inverted perovskite solar cell with decent ambient stability is successfully demonstrated by employing an n-type polymer, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200), as the electron-transporting layer (ETL). The device performance can be further enhanced from a power conversion efficiency (PCE) of 15 to 16.8 % by tailoring the electronic properties of N2200 with a polymeric additive, poly[9,9-bis(6′-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl) fluorene] (PFN-Ox). More importantly, the device derived from this hybrid ETL can maintain good ambient stability inherent from the pristine N2200 ETL, for which 60–70 % of initial PCE can be retained after being stored in air with 10–20 % humidity for 45 days.

Mix & match: An inverted perovskite solar cell with power conversion efficiency of 15 % and decent ambient stability is successfully demonstrated by employing an n-type polymer (N2200) as the electron-transporting layer (ETL), and can be further enhanced to 16.8 % by tailoring the electronic properties of N2200 with a polymeric additive (PFN-Ox). The device derived from this hybrid ETL can maintain good ambient stability.