Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH3NH3PbI3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current–voltage (J–V) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J–V hysteresis.

cell prepared using a co-sublimation technique in which the perovskite layer is sandwiched in between organic electron and hole blocking layers. [ 2a ] This confi guration leads to stable and reproducible photovoltaic devices that do not suffer from strong hysteresis effects and when optimized lead to effi ciencies close to 15%. [ 10 ] From previous studies, [ 7,11 ] it is known that charge trapping does occur in (solution-processed) perovskite layers. From time-resolved photoluminescence spectroscopy studies, it was concluded that trap assisted recombination is non-radiative. [ 12 ] This would indicate that traps infl uence the recombination mechanism in an actual device. Here, we investigate the charge recombination in working devices, taking advantage of the fact that they are diodes, by examining the low-voltage part of the J-V characteristics, from which the diode ideality factor can be determined. The diode ideality factor is a measure of the steepness of the J-V characteristics in the low-voltage region. As fi rst described theoretically by Sah et al. for a classical semiconductor p-n junction, trap-assisted recombination changes the diode ideality factor up to a value of 2, [ 13 ] where it normally has a value of 1 in the case that recombination is absent or when only direct free-carrier recombination occurs. Such behavior has been observed also in (undoped) organic-semiconductor diodes, as also expected theoretically for a metal-insulator-metal diode. [ 14 ] A typical perovskite solar cell consists of an intrinsic semiconductor (perovskite) layer sandwiched between (organic) charge-blocking layers. As such, charge recombination should be confi ned to the perovskite layer and trap-assisted recombination can be exposed by investigating the diode ideality factor.Using these devices based on co-sublimated CH 3 NH 3 PbI 3 perovskite layers, we have prepared double-carrier, hole-only and electron-only devices by changing the organic charge extraction/blocking layers and or electrode materials used. We fi nd that trap-assisted recombination via electron traps is present as a non-radiative loss mechanism in co-sublimated perovskite devices.In order to investigate the diode behavior of perovskite solar cells, co-sublimed perovskite layers were prepared and sandwiched in between polyTPD and PCBM electron-and holeblocking layers, respectively. The layout of the device (see experimental section for more details) and the typical performance of these devices under 1 sun illumination are shown in Figure 1 .As can be seen, there is only a small difference in the current density ( J ) versus voltage ( V ) curve as a function of scan sweep direction (Figure 1 B). It has been reported that the scan direction has a large infl uence on the J -V curve of metal-oxide containing perovskite devices, leading to a strong hysteresis

High-quality semiconducting perovskites can be easily synthesized through several methods. The ease of fabrication has favored the adoption of lab-scale solution-processing techniques, which have yielded the highest performing devices. Most of these processes, however, are not directly applicable to larger scale and volume preparations, hindering the consolidation and market entry of this technology. Vapor-based methods, a mature technology widely adopted in the coating and semiconductor industry, could change this trend. Their application to perovskite solar cells includes a large amount of fabrication approaches, offering versatility in the employed materials as well as in the characteristics of the resulting perovskite films. It is thus essential to review the fundamentals of perovskite vapor-related techniques in order to put their real potential and challenges into perspective.