Eco-Friendly Push-Coated Polymer Solar Cells with No Active Material Wastes Yield Power Conversion Efficiencies over 5.5.

Push-coating is a simple process that can be employed for extremely low-cost polymer electronic device production. Here, we demonstrate its application to the fabrication of poly(2,7-carbazole- alt-dithienylbenzothiadiazole) (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) active layers processed in air, yielding similar photovoltaic performances as thermally annealed spin-coated thin films when used in inverted polymer solar cells (PSCs). During push-coating, the polydimethylsiloxane layer temporarily traps the deposition solvent, resulting in simultaneous film formation and solvent annealing effect. This removes the necessity for a postdeposition thermal annealing step which is required for spin-coated PSCs to produce high photovoltaic performances. Optimized PSC active layers are produced with a push-coating time of 5 min at room temperature with 20 times less hazardous solvent and 40 times less active material than spin-coating. Annealed spin-coated active layers and active layers push-coated for 5 min both produce average power conversion efficiencies (PCEs) of 5.77%, while those push-coated for a shorter time of 1 min yield a slightly lower value of 5.59%. We demonstrate that, despite differences in their donor:acceptor vertical concentration gradients, unencapsulated PCDTBT:PC71BM active layers push-coated for 1 min produce PSCs with similar operational stability and upscaling capacity as thermally annealed spin-coated ones. As fast device fabrication can be achieved with short-time push-coating, we further demonstrate the potential of this deposition technique by manufacturing push-coated PSC-based semitransparent photovoltaic devices with a PCE of 4.23%, relatively neutral colors and an average visible transparency of 40.2%. Our work thus confirms that push-coating is not limited to the widely employed poly(3-hexylthiophene-2,5-diyl) but can also be used with low band gap copolymers and opens the path to low-cost and eco-friendly, yet efficient and stable PSCs.

Fabrication Processes to Generate Concentration Gradients in Polymer Solar Cell Active Layers.

Polymer solar cells (PSCs) are considered as one of the most promising low-cost alternatives for renewable energy production with devices now reaching power conversion efficiencies (PCEs) above the milestone value of 10%. These enhanced performances were achieved by developing new electron-donor (ED) and electron-acceptor (EA) materials as well as finding the adequate morphologies in either bulk heterojunction or sequentially deposited active layers. In particular, producing adequate vertical concentration gradients with higher concentrations of ED and EA close to the anode and cathode, respectively, results in an improved charge collection and consequently higher photovoltaic parameters such as the fill factor. In this review, we relate processes to generate active layers with ED-EA vertical concentration gradients. After summarizing the formation of such concentration gradients in single layer active layers through processes such as annealing or additives, we will verify that sequential deposition of multilayered active layers can be an efficient approach to remarkably increase the fill factor and PCE of PSCs. In fact, applying this challenging approach to fabricate inverted architecture PSCs has the potential to generate low-cost, high efficiency and stable devices, which may revolutionize worldwide energy demand and/or help develop next generation devices such as semi-transparent photovoltaic windows.

Low-Cost and Green Fabrication of Polymer Electronic Devices by Push-Coating of the Polymer Active Layers.

Because of both its easy processability and compatibility with roll-to-roll processes, polymer electronics is considered to be the most promising technology for the future generation of low-cost electronic devices such as light-emitting diodes and solar cells. However, the state-of-the-art deposition technique for polymer electronics (spin-coating) generates a high volume of chlorinated solution wastes during the active layer fabrication. Here, we demonstrate that devices with similar or higher performances can be manufactured using the push-coating technique in which a poly(dimethylsiloxane) (PDMS) layer is simply laid over a very small amount of solution (less than 1µL/covered cm2), which is then left for drying. Using mm thick PDMS provides a means to control the solvent diffusion kinetics (sorption/retention) and removes the necessity for additional applied pressure to generate the desired active layer thickness. Unlike spin-coating, push-coating is a slow drying process that induces a higher degree of crystallinity in the polymer thin film without the necessity for a post-annealing step. The polymer light-emitting diodes and solar cells prepared by push-coating exhibit slightly higher performances with respect to the reference spin-coated devices, whereas at the same time reduce the amounts of active layer materials and chlorinated solvents by 50 and 20 times, respectively. These increased performances can be correlated to the higher polymer crystallinities obtained without applying a post-annealing treatment. As push-coating is a roll-to-roll compatible method, the results presented here open the path to low-cost and eco-friendly fabrication of a wide range of emerging devices based on conjugated polymer materials.

Transfer-printing of active layers to achieve high quality interfaces in sequentially deposited multilayer inverted polymer solar cells fabricated in air.

Polymer solar cells (PSCs) are greatly influenced by both the vertical concentration gradient in the active layer and the quality of the various interfaces. To achieve vertical concentration gradients in inverted PSCs, a sequential deposition approach is necessary. However, a direct approach to sequential deposition by spin-coating results in partial dissolution of the underlying layers which decreases the control over the process and results in not well-defined interfaces. Here, we demonstrate that by using a transfer-printing process based on polydimethylsiloxane (PDMS) stamps we can obtain increased control over the thickness of the various layers while at the same time increasing the quality of the interfaces and the overall concentration gradient within the active layer of PSCs prepared in air. To optimize the process and understand the influence of various interlayers, our approach is based on surface free energy, spreading parameters and work of adhesion calculations. The key parameter presented here is the insertion of high quality hole transporting and electron transporting layers, respectively above and underneath the active layer of the inverted structure PSC which not only facilitates the transfer process but also induces the adequate vertical concentration gradient in the device to facilitate charge extraction. The resulting non-encapsulated devices (active layer prepared in air) demonstrate over 40% increase in power conversion efficiency with respect to the reference spin-coated inverted PSCs.