Dual‐Source Precursor Approach for Highly Efficient Inverted Planar Heterojunction Perovskite Solar Cells

The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium and formamidinium mixed cations. Currently, high‐quality mixed‐cation perovskite thin films are normally made by use of antisolvent protocols. However, the widely used “antisolvent”‐assisted fabrication route suffers from challenges such as poor device reproducibility, toxic and hazardous organic solvent, and incompatibility with scalable fabrication process. Here, a simple dual‐source precursor approach is developed to fabricate high‐quality and mirror‐like mixed‐cation perovskite thin films without involving additional antisolvent process. By integrating the perovskite films into the planar heterojunction solar cells, a power conversion efficiency of 20.15% is achieved with negligible current density–voltage hysteresis. A stabilized power output approaching 20% is obtained at the maximum power point. These results shed light on fabricating highly efficient perovskite solar cells via a simple process, and pave the way for solar cell fabrication via scalable methods in the near future.     

Low‐Temperature Soft‐Cover Deposition of Uniform Large‐Scale Perovskite Films for High‐Performance Solar Cells

Large‐scale high‐quality perovskite thin films are crucial to produce high‐performance perovskite solar cells. However, for perovskite films fabricated by solvent‐rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low‐temperature soft‐cover deposition (LT‐SCD) method is presented, where the thermal convection‐induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection‐induced‐defects‐free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low‐temperature processing. Hence, the present LT‐SCD technology provides a new non‐spin‐coating route to the deposition of large‐area uniform perovskite films for both rigid and flexible perovskite devices.     

Polymer‐Fullerene Bulk‐Heterojunction Solar Cells

Solution‐processed bulk‐heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low‐cost power production. This article reviews the highlights of the last few years, and summarizes today's state‐of‐the‐art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.     

The Role of Bulk and Interface Recombination in High‐Efficiency Low‐Dimensional Perovskite Solar Cells

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n?1Pb_nI_3n+1 perovskite cells with different numbers of [PbI₆]^4? sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V_OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V_OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.     

All‐Solution‐Processed Metal‐Oxide‐Free Flexible Organic Solar Cells with Over 10% Efficiency

All‐solution‐processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers the possibility of a safe and cost‐effective fabrication environment for the devices. Herein, an all‐solution‐processed flexible organic solar cell (OSC) using poly(3,4‐ethylenedioxythiophene):poly‐(styrenesulfonate) electrodes is reported. The all‐solution‐processed flexible devices yield the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal‐oxide‐free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high‐performance PEDOT:PSS/plastic underlying substrate with a matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibit an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high‐performance all‐solution‐processed flexible OSCs, which is important for the development of printing, blading, and roll‐to‐roll technologies.     

Efficient Organic Solar Cells with Non‐Fullerene Acceptors

Fullerene‐free OSCs employing n‐type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non‐fullerene acceptors exhibit great potential for realizing high‐performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non‐fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.