LAUSR

DOI: 10.1002/admi.202000041 charge generation over a wide and tunable range,[4] but also exhibit high carrier mobilities and long diffusion lengths up to several microns.[5–7] In any light harvesting device, appropriate contacts are critical to efficiently collect the photogenerated charges and deliver them to the external circuit. The contacts are responsible for providing the built-in asymmetry needed to create a driving force for the extraction of photogenerated carriers;[8] this built-in asymmetry can either be established by kinetic selectivity (diffusion-controlled) or by an energetic mismatch (drift-controlled) between the electrodes. The generic thin-film solar cell is composed of an active layer, sandwiched between a hole-extracting anode contact and an electron-extracting cathode contact. Under illumination, charge carriers generated within the active layer will drift-diffuse to the contacts and be extracted by the builtin asymmetry, resulting in the production of a net photocurrent. In organic solar cells, characterized by low carrier mobilities and short diffusion lengths, a strong built-in electric field across the active layer is necessary to enhance the charge extraction rate and avoid recombination.[9–11] This field is induced by the built-in potential Vbi (or contact potential), originating from the work function difference between the anode and cathode and is largely unscreened due to the relatively low dielectric constants of organic semiconductors. Conversely, in perovskite solar cells, exhibiting carrier diffusion lengths of several microns, photogenerated charges should, in the absence of electric fields, be able to effortlessly traverse active layers of 200–500 nm without recombining. Subsequently, provided that kinetic selectivity at the contacts can be ensured,[12] the charge collection is expected to be diffusion controlled,[8,13] and a consensus is emerging along these lines. Kinetic selectivity is established by employing separate charge transport layers (CTLs) in-between the electrodes and the active layer, resulting in either an n–i–p or p–i–n type device architectures, with a hole transport layer (HTL, p-layer) at the anode and an electron transport layer (ETL, n-layer) at the cathode. In the ideal case, these layers are able to conduct majority carriers, while simultaneously preventing the extraction of minority carriers, thus creating a preferred direction for a diffusion-driven charge collection. Within this framework of charge extraction requirements, there is still some conjecture as to the exact role of the in-built potential and hence the precise nature of the driving force responsible for charge extraction. Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built-in potential plays any role is still of matter of some conjecture. This important question using phase-sensitive photocurrent measurements and theoretical device simulations based upon the drift-diffusion framework is addressed. In particular, the role of the built-in electric field and charge-selective transport layers in state-of-the-art p–i–n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built-in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.