Overcoming Space‐Charge Effect for Efficient Thick‐Film Non‐Fullerene Organic Solar Cells

Abstract

Organic solar cells (OSCs) containing non‐fullerene acceptors have realized high power conversion efficiency (PCE) up to 14%. However, most of these high‐performance non‐fullerene OSCs have been reported with optimal active layer thickness of about 100 nm, mainly due to the low electron mobility (≈10−4–10−5 cm2 V−1 s−1) of non‐fullerene acceptors, which are not suitable for roll‐to‐roll large‐scale processing. In this work, an efficient non‐fullerene OSC based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD):EH‐IDTBR (consists of electron‐rich indaceno[1,2‐b:5,6‐b′]dithiophene as the central unit and an electron‐deficient 5,6‐benzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group) with thickness‐independent PCE (maintaining a PCE of 9.1% with an active layer thickness of 300 nm) is presented by optimizing device architectures to overcome the space‐charge effects. Optical modeling reveals that most of the incident light is absorbed near the transparent electrode side in thick‐film devices. The transport distance of electrons with lower mobility will therefore be shortened when using inverted device architecture, in which most of the excitons are generated close to the cathode side and therefore substantially reduces the accumulation of electrons in the device. As a result, an efficient thick‐film non‐fullerene OSC is realized. These results provide important guidelines for the development of more efficient thick‐film non‐fullerene OSCs.

Watching Space Charge Build Up in an Organic Solar Cell

“…These materials have opened a completely new parameter space for researchers to find efficient donor–acceptor combinations. However, still very few systems are known that maintain their full performance at technically relevant thicknesses of 300 nm and more . The reason is that the operation of modern OPVs resembles a competition between charge collection and charge recombination driven by the internal electric field .…”

Molecular Orientation Unified Nonfullerene Acceptor Enabling 14% Efficiency As‐Cast Organic Solar Cells

“…In particular, the compatibility of high‐efficiency value and thick blend film is a major challenge as normally the most of the highly efficient OSCs provided their optimal performance with photoactive layer thickness of ≈100 nm . Further increasing the thickness of BHJ film, the complexity of the BHJ morphology would be significantly enhanced, which made it challenging to construct effective charge transport paths in active layer . On the other hand, thin active layer greatly limits the utilization of incident light as well as the real‐world application based on the high‐throughput roll‐to‐roll or doctor‐blading process, where thick photoactive layer over 300 nm is typically required for the construction of uniform and trap‐free film with large area.…”

High Performance Thick‐Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm

“…As shown in Table 1 and Figure 2b, the J sc s continuously increased and the FFs decreased along with the increase of film-thicknesses, while the V oc s declined from 0.893 to 0.852 V. The decrease of FFs and V oc s in the thick-film devices is mainly caused by the more severe nongeminate recombination, which is consistent with the results of reported thick film devices. [36,41] The optimal performance with high PCE of 13.80%, J sc of 19.74 mA cm −2 and FF of 78.47% was obtained with the active layer thickness of 120 nm. When the film-thickness reached up to 350 nm, the J sc started to descend, but high performance over 10% was still maintained.…”

“…However, a lot of research work have proved that the charge collection efficiency of a device is inversely proportional to the square of the film thickness of the active layer. [41] Zhang and co-workers reported devices based on PM6:IDIC with PCEs of 11.9% under the film thickness of 150 nm and 11.3% under the condition of 255 nm condition. Also, the J sc will decrease due to the severe bimolecular recombination.…”

“…For acceptor F-2Cl, the elemental compositions are C (74.02%); H (7.30%); Cl (9.01%); N (3.56%); O (2.03%); S (4.07%). [41] So in the inverted structure device with thick-film, the lower proportion of acceptor F-2Cl near the substrate region would restrict the electron transport and collection. Blending films with thicknesses of 400 nm were casted on two kind of substrates: poly (3,4- rate around 1 nm s −1 , and the signal of F (1s) and Cl (2p) were collected.…”