Chemically Engineered Titanium Oxide Interconnecting Layer for Multijunction Polymer Solar Cells.

We report chemically tunable n-type titanium oxides using ethanolamine as a nitrogen dopant source. As the amount of ethanolamine added to the titanium oxide precursor during synthesis increases, the Fermi level of the resulting titanium oxides (ethanolamine-incorporated titanium oxides) significantly changes from -4.9 eV to -4.3 eV, and their free charge carrier densities are enhanced by two orders of magnitudes, reaching up to 5 × 1018 cm-3. Unexpectedly, a basic ethanolamine reinforces not only the n-type properties of titanium oxides, but also their basicity, which facilitates acid-base ionic junctions in contact with acidic materials. The enhanced charge carrier density and basicity of the chemically tuned titanium oxides enable multi-junction solar cells to have interconnecting junctions consisting of basic n-type titanium oxides and acidic p-type PEDOT:PSS to gain high open-circuit voltages of 1.44 V and 2.25 V from tandem and triple architectures, respectively.

Synthesis and application of amine-containing conjugated small molecules for the automatic formation of an electron transporting layer via spontaneous phase separation from the bulk-heterojunction layer.

Carbazole-based conjugated small molecule electrolytes (CSEs) containing different numbers of amine groups were synthesized and applied to bulk-heterojunction (BHJ) organic solar cells for the formation of a spontaneous self-assembled electron transporting layer (ETL). The active layer was spin-coated with a mixture solution containing the BHJ materials and a small amount of CSE, and a thin layer of CSE was formed underneath the active layer (CSE/BHJ bi-layer) via spontaneous phase separation, which is confirmed by the depth profile of the time of flight secondary ion mass spectroscopy (ToF-SIMS) spectrum. The amino groups in the CSEs form hydrogen-bonds with the surface of indium tin oxide (ITO), which acts as an ETL in BHJ solar cells. Moreover, the formed CSE layer is capable of changing the effective work function (WF) of ITO. An increasing number of amino groups in the CSEs (from Cz1N to Cz3N) provides more reduction of the effective WF of ITO, which results in a lower internal resistance and a higher power conversion efficiency (PCE). Furthermore, the enhanced hydrogen bonding between the amines and ITO with an increased number of amine groups has been studied by XPS. This result suggests that one-step processing provides a reduction of the manufacturing cost, which can provide an attractive design concept for ETL fabrication.

High-efficiency large-area perovskite photovoltaic modules achieved via electrochemically assembled metal-filamentary nanoelectrodes.

Realizing industrial-scale, large-area photovoltaic modules without any considerable performance losses compared with the performance of laboratory-scale, small-area perovskite solar cells (PSCs) has been a challenge for practical applications of PSCs. Highly sophisticated patterning processes for achieving series connections, typically fabricated using printing or laser-scribing techniques, cause unexpected efficiency drops and require complicated manufacturing processes. We successfully fabricated high-efficiency, large-area PSC modules using a new electrochemical patterning process. The intrinsic ion-conducting features of perovskites enabled us to create metal-filamentary nanoelectrodes to facilitate the monolithic serial interconnections of PSC modules. By fabricating planar-type PSC modules through low-temperature annealing and all-solution processing, we demonstrated a notably high module efficiency of 14.0% for a total area of 9.06 cm2 with a high geometric fill factor of 94.1%.

Achieving Large-Area Planar Perovskite Solar Cells by Introducing an Interfacial Compatibilizer.

Despite the recent unprecedented increase in the power conversion efficiencies (PCEs) of small-area devices (≤0.1 cm2 ), the PCEs deteriorate drastically for PSCs of larger areas because of the incomplete film coverage caused by the dewetting of the hydrophilic perovskite precursor solutions on the hydrophobic organic charge-transport layers (CTLs). Here, an innovative method of fabricating scalable PSCs on all types of organic CTLs is reported. By introducing an amphiphilic conjugated polyelectrolyte as an interfacial compatibilizer, fabricating uniform perovskite films on large-area substrates (18.4 cm2 ) and PSCs with the total active area of 6 cm2 (1 cm2 × 6 unit cells) via a single-turn solution process is successfully demonstrated. All of the unit cells exhibit highly uniform PCEs of 16.1 ± 0.9% (best PCE of 17%), which is the highest value for printable PSCs with a total active area larger than 1 cm2 .

A series connection architecture for large-area organic photovoltaic modules with a 7.5% module efficiency.

The fabrication of organic photovoltaic modules via printing techniques has been the greatest challenge for their commercial manufacture. Current module architecture, which is based on a monolithic geometry consisting of serially interconnecting stripe-patterned subcells with finite widths, requires highly sophisticated patterning processes that significantly increase the complexity of printing production lines and cause serious reductions in module efficiency due to so-called aperture loss in series connection regions. Herein we demonstrate an innovative module structure that can simultaneously reduce both patterning processes and aperture loss. By using a charge recombination feature that occurs at contacts between electron- and hole-transport layers, we devise a series connection method that facilitates module fabrication without patterning the charge transport layers. With the successive deposition of component layers using slot-die and doctor-blade printing techniques, we achieve a high module efficiency reaching 7.5% with area of 4.15 cm(2).

Long-Term Stable Recombination Layer for Tandem Polymer Solar Cells Using Self-Doped Conducting Polymers.

Recently, the most efficient tandem polymer solar cells (PSCs) have used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) as a p-type component of recombination layer (RL). However, its undesirable acidic nature, originating from insulating PSS, of PEDOT: PSS drastically reduces the lifetime of PSCs. Here, we demonstrate the efficient and stable tandem PSCs by introducing acid-free self-doped conducting polymer (SCP), combined with zinc oxide nanoparticles (ZnO NPs), as RL for PEDOT: PSS-free tandem PSCs. Moreover, we introduce an innovative and versatile nanocomposite system containing photoactive and p-type conjugated polyelectrolyte (p-CPE) into the tandem fabrication of an ideal self-organized recombination layer. In our new RL, highly conductive SCP facilitates charge transport and recombination process, and p-CPE helps to achieve nearly loss-free charge collection by increasing effective work function of indium tin oxide (ITO) and SCP. Because of the synergistic effect of extremely low electrical resistance, ohmic contact, and pH neutrality, tandem devices with our novel RL performed well, exhibiting a high power conversion efficiency of 10.2% and a prolonged lifetime. These findings provide a new insight for strategic design of RLs using SCPs to achieve efficient and stable tandem PSCs and enable us to review and extend the usefulness of SCPs in various electronics research fields.

High-Performance Integrated Perovskite and Organic Solar Cells with Enhanced Fill Factors and Near-Infrared Harvesting.

Highly efficient P-I-N type perovskite/bulk-heterojunction (BHJ) integrated solar cells (ISCs) with enhanced fill factor (FF) (≈80%) and high near-infrared harvesting (>30%) are demonstrated by optimizing the BHJ morphology with a novel n-type polymer, N2200, and a new solvent-processing additive. This work proves the feasibility of highly efficient ISCs with panchromatic absorption as a new photovoltaic architecture and provides important design rules for optimizing ISCs.

A new architecture for printable photovoltaics overcoming conventional module limits.

A new architecture for manufacturing large-area polymer solar cells that does not produce concomitant aperture and Ohmic losses is presented. By introducing the innovative concept of metal-filamentary nanoelectrodes, which are vertically formed inside the main active layers, loss-free, widely expandable solar cells with the highest relative power conversion efficiency (ca. 90%) in organic photovoltaic systems are demonstrated.

Soluble transition metal oxide/polymeric acid composites for efficient hole-transport layers in polymer solar cells.

We report a new method for developing a low-temperature solution processed vanadium oxide (s-VOx) and poly(4-styrene sulfonic acid) (PSS) composite to act as an efficient hole-transport layer (HTL) in polymer solar cells (PSCs). By compositing the s-VOx and PSS (s-VOx:PSS), the work function values of the s-VOx:PSS changed from 5.0 to 5.3 eV. Therefore, the energy level barrier between the HTL and organic active layer decreased, facilitating charge injection/extraction at the interfaces. In addition, the s-VOx:PSS films were denser and had more pin-hole-free surfaces than pristine s-VOx films, resulting in enhanced PSC performance due to significantly decreased leakage currents and excellent device stability in ambient condition. Because our approach of combining soluble transition metal oxide (TMO) and polymeric acid shows dramatically better performance than pristine TMO, we expect that it can provide useful guidelines for the synthesis and application of TMOs for organic electronics in the future.

Long-term stable polymer solar cells with significantly reduced burn-in loss.

The inferior long-term stability of polymer-based solar cells needs to be overcome for their commercialization to be viable. In particular, an abrupt decrease in performance during initial device operation, the so-called 'burn-in' loss, has been a major contributor to the short lifetime of polymer solar cells, fundamentally impeding polymer-based photovoltaic technology. In this study, we demonstrate polymer solar cells with significantly improved lifetime, in which an initial burn-in loss is substantially reduced. By isolating trap-embedded components from pristine photoactive polymers based on the unimodality of molecular weight distributions, we are able to selectively extract a trap-free, high-molecular-weight component. The resulting polymer component exhibits enhanced power conversion efficiency and long-term stability without abrupt initial burn-in degradation. Our discovery suggests a promising possibility for commercial viability of polymer-based photovoltaics towards real solar cell applications.