even with spin coating, efficiencie

even with spin coating, efficiencies up to 19.7%have been achieved on areas larger than 1 cm2(25, 29). More work in deposition (e.g., printingor vacuum-processed PSCs) and device optimizationis needed to achieve large-area, upscalablePSCs with efficiencies of >20%.Device photophysicsPSCs are heterojunction devices in which the holeselectiveor electron-selective contacts (or both)introduce the selectivity required to harvest chargecarriers at their respective electrodes to createa photovoltage. After an initial period of controversy,consensus has been reached that theseinterfaces are not required to dissociate a photogeneratedelectron-hole pair (exciton), becausethe exciton binding energy is comparable (sometens of millielectron volts) (30) to the thermalenergy under operation. This property, pairedwith a relatively large diffusion length (31) andsufficiently high mobilities of electrons and holes(32), allowed for high internal quantum efficiencieseven for the first reported PSCs. The absorptioncoefficient itself, shown in Fig. 2A, is high(>105 cm−1) because of a direct transition at theband gap. One of the device optimization stepstoward higher efficiency was to increase the thicknessof the perovskite layer to reach a sharp onsetof the external quantum efficiency (EQE) nearthe band gap, which is ~1.6 eV for the most efficientMAFA-based compounds. The photocurrentof the device shown in Fig. 2A (14), 23 mA cm−2,is rather optimized and could only be slightly increased(by 1 to 2 mA cm−2) in the case of a sharperEQE onset at 780 nm.Nominally, most of the perovskite architectureslook like a metal(-like)/electron-selectivelayer/intrinsic perovskite/p-type hole conductorsystem (Fig. 1, A to C). The work function differenceof the electrodes including doped chargetransport layers could introduce a built-in potentialgenerating an electric field in the perovskitefilm in equilibrium, or the perovskite itselfcould be either n- or p-doped depending on itsstoichiometry (33) and forming a p-n junctionsomewhere in the device. Varying results havebeen reported on the energetics of perovskites,such as a work function located far in the conductionband and ionization energies that dependon the stoichiometry (34).The electric field distribution is further influencedby intrinsic defects induced by movementof ions, such as I. These tend to screen the electricfield by accumulating at interfaces, similar tomobile ions in an electrolyte. Low mobilitiescause ionic defects to respond slowly (secondsto minutes) to voltage changes and are believedto be responsible for the frequently observedhysteresis in the current-voltage curve (Fig. 2B)(35–37). The hysteresis reflects that the collectionefficiency of photogenerated charge carriersis dependent on the biasing history and on theelectric field. Thus, hysteresis can be minimizedby either immobilizing ionic charge or increasingcharge carrier transport and extraction to makethe latter less sensitive to the electric field (38).Here, charge extraction layers, trap densities atthe interfaces, and the morphology of the perovskiteare influential.RecombinationCharges that are not collected are lost by recombination.Open-circuit conditions are idealfor studying recombination processes becauseno photogenerated charges are extracted. Thevery high VOC of PSCs relative to the perovskiteband gap is indicative of low recombinationlosses. Deposition, composition, and device engineeringhave increased VOC to values as highas 1.24 V at a band gap of ~1.6 eV (21). An idea