Facile Fabrication of Self-Assembly Functionalized Polythiophene Hole Transporting Layer for High Performance Perovskite Solar Cells

Crystallinity and crystal orientation have a predominant impact on a materials' semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra-uniform hole-transporting material (HTM) by self-assembling COOH-functionalized P3HT (P3HT-COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self-assembly approach facilitates the P3HT-COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down-shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self-assembled HTM show high open-circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim-light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.

Hybrid Hole Extraction Layer Enabled High Efficiency in Polymer Solar Cells

Charge extraction layers with excellent charge extraction capability are essential for achieving high photovoltaic performance in cells. In this work, a hole extraction layer (HEL) is developed by doping conductive polymer TFB into CuSCN (CuSCN:TFB(X)), which exhibits good light transparency and high affinity for the light absorber. Compared to the reference cell, the CuSCN:TFB(X) HEL-based cells show impressive enhancement owing to the increased exciton dissociation and charge extraction processes and weak recombination losses. Furthermore, matched work function, better interface contact, and appropriate domain size also contribute to the enhanced power conversion efficiency. As a consequence, the highest conversion efficiency of 15.28% is observed in a cell based on the PM6:Y6 blend film and CuSCN:TFB(1.0%) HEL, which is >16% higher than the efficiency of 13.13% in a cell with CuSCN HEL.

Performance and Uniformity Improvement in Ultrathin Cu(In,Ga)Se2 Solar Cells with a WO x Nanointerlayer at the Absorber/Transparent Back-Contact Interface

Thinning CIGSe absorber layer to less than 500 nm is desirable for reducing the cost per unit watt of photovoltaic-generated electricity, and also, the semitransparent solar cell based on such a thin absorber can be used in bifacial and superstrate configurations if the back electrode is transparent. In this study, a WO _x layer is inserted between Cu(In,Ga)Se₂ (CIGSe) absorber and tin-doped indium oxide back-contact to enhance the hole collection at the back electrode. A WO _x interlayer with a thickness of 6 nm is found to be optimum because it causes a ∼38% relative increase in the fill factor of a ∼450 nm thick CIGSe-based device compared to the reference device without a WO _x interlayer. While fixing the thickness of CIGSe, increasing the WO _x interlayer thickness to ≥6 nm results in decreases of solar cell parameters primarily because of the emergence of a GaO _x interfacial layer at the CIGSe/WO _x junction.

Enhancing the Performance of the Half Tin and Half Lead Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination

In this article it is investigated how the hole extraction layer (HEL) influence the charge recombination and performance in half tin and half lead (FASn_0.5 Pb_0.5 I₃ ) based solar cells (HPSCs). FASn_0.5 Pb_0.5 I₃ film grown on PEDOT:PSS displays a large number of pin-holes and open grain boundaries, resulting in a high defect density and shunts in the perovskite film causing significant bulk and interfacial charge recombination in the HPSCs. By contrast, FASn_0.5 Pb_0.5 I₃ films grown on PCP-Na, an anionic conjugated polymer, show compact and pin-hole free morphology over a large area, which effectively eliminates the shunts and trap states. Moreover, PCP-Na is characterized by a higher work function, which determines a favorable energy alignment at the anode interface, enhancing the charge extraction. Consequently, both the interfacial and bulk charge recombination in devices using PCP-Na HEL are considerably reduced giving rise to an overall improvement of all the device parameters. The HPSCs fabricated with this HEL display power conversion efficiency up to 16.27%, which is 40% higher than the efficiency of the control devices using PEDOT:PSS HEL (11.60%). Furthermore, PCP-Na as HEL offers superior performance in larger area devices compared to PEDOT:PSS.

Improved Stability of Interfacial Energy-Level Alignment in Inverted Planar Perovskite Solar Cells

Even though poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been commonly used as a hole extraction layer (HEL) for p-i-n perovskite solar cells (PSCs), the cells' photovoltaic performance deteriorates because of the low and unstable work functions (WFs) of PEDOT:PSS versus those of a perovskite layer. To overcome this drawback, we synthesized a copolymer (P(SS- co-TFPMA)) ionomer consisting of PSS and tetrafluoropropylmethacrylate (TFPMA) as an alternative to conventional PEDOT:PSS. The PEDOT:P(SS- co-TFPMA) copolymer solution and its film exhibited excellent homogeneity and high phase stability compared with a physical mixture of TFPMA with PEDOT:PSS solution. During spin coating, a self-organized conducting PEDOT:P(SS- co-TFPMA) HEL evolved and the topmost PEDOT:P(SS- co-TFPMA) film showed a hydrophobic surface with a higher WF compared to that of the pristine PEDOT:PSS film because of its chemically bonded electron-withdrawing fluorinated functional groups. Interestingly, the WF of the conventional PEDOT:PSS film dramatically deteriorated after being coated with a perovskite layer, whereas the PEDOT:P(SS- co-TFPMA) film represented a relatively small influence. Because of the superior energy-level alignment between the HEL and a perovskite layer even after the contact, the open-circuit voltage, short-circuit current, and fill factor of the inverted planar p-i-n PSCs (IP-PSCs) with PEDOT:P(SS- co-TFPMA) were improved from 0.92 to 0.98 V, 18.96 to 19.66 mA/cm², and 78.96 to 82.43%, respectively, resulting in a 15% improvement in the power conversion efficiency vs that of IP-PSCs with conventional PEDOT:PSS. Moreover, the IP-PSCs with PEDOT:P(SS- co-TFPMA) layer showed not only improved photovoltaic performance but also enhanced device stability due to hydrophobic surface of PEDOT:P(SS- co-TFPMA) film.

A Solution-Processed Transparent NiO Hole-Extraction Layer for High-Performance Inverted Perovskite Solar Cells

A highly transparent NiO layer was prepared by a solution processing method with nickel(II) 2-ethylhexanoate in non-polar solvent and utilized as HTM in perovskite solar cells. Excellent optical transmittance and the matched energy level lead to the enhanced power conversion efficiency (PCE, 18.15 %) than that of conventional sol-gel-processed NiO-based device (12.98 %).

Graphene Oxide by UV-Ozone Treatment as an Efficient Hole Extraction Layer for Highly Efficient and Stable Polymer Solar Cells

The hole extraction layer has a significant impact on the achievement of high-efficiency polymer solar cells (PSCs). Here, we report an efficient approach to direct UV-ozone treatment by larger device performance enhancement employing graphene oxide (GO). The dramatic performance enhancement of PSCs with the P3HT:PCBM blend as an active layer was demonstrated by the UV-ozone treatment of GO for 30 min: best power conversion efficiency (PCE) of 4.18%, fill factor of 0.63, J_sc of 10.94 mA cm^-2, and V_oc of 0.61 V, which are significantly higher than those of the untreated GO (1.82%) and highly comparable PEDOT:PSS-based PSCs (3.73%). In addition, PSCs with UV-ozone-treated GO showed a longer stability than PSCs with PEDOT:PSS. The significant enhancement of PCEs of PSCs can be attributed to the fact that ozone molecules can oxidize GO into CO₂ and leave highly conductive graphene particles. We suggest that this simple UV-ozone treatment can provide an efficient method for highly efficient GO hole extraction in high-performance PSCs.

Improvement in Half-Life of Organic Solar Cells by Using a Blended Hole Extraction Layer Consisting of PEDOT:PSS and Conjugated Polymer Electrolyte

In this study, we fabricated conventional structured organic solar cells (OSCs) by introducing a hole extraction layer (HEL) that consisted of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and conjugated polymer electrolyte (CPE) poly[9,9-bis(4'-sulfonatobutyl)fluorene-alt-thiophene] (PFT-D). PFT-D has a -SO₃^- functional group that acts as a conjugate base against the -SO₃H of PSS. In addition, the molecular dipole of PFT-D can screen the Coulombic attraction between PEDOT chains and PSS chains. By blending PEDOT:PSS and PFT-D, the acidity of the HEL solution and changes to the surface morphology and potential of the HEL film as a function of exposure time in air were reduced. As a result, the half-life of the OSC fabricated with blended HEL was five times better at room temperature and 40% humidity without encapsulation as compared to the pristine PEDOT:PSS-based device.

MoS2 Quantum Dots with a Tunable Work Function for High-Performance Organic Solar Cells

An efficient hole extraction layer (HEL) is critical to achieve high-performance organic solar cells (OSCs). In this study, we developed a pinhole-free and efficient HEL based on MoS₂ quantum dots (QDs) combined with UV-ozone (UVO) treatment. The optophysical properties and morphology of MoS₂ QDs and their photovoltaic performance are investigated. The results showed that MoS₂ QDs can form homogeneous films and can be applied as an interfacial layer not only for donors with shallow highest occupied molecular orbital (HOMO) but also for those with deep HOMO energy levels after UVO treatment (O-MoS₂ QDs). The solar cells based on O-MoS₂ QDs yield a power conversion efficiency (PCE) of 8.66%, which is 71% and 12% higher than those of the OSCs with pristine MoS₂ QD and O-MoS₂ nanosheets, respectively, and the highest PCEs for OSCs containing MoS₂ materials. Furthermore, the stability of solar cells based on MoS₂ QDs is greatly improved in comparison with state-of-the-art PEDOT:PSS. These results demonstrate the great potential of O-MoS₂ QDs as an efficient HEL for high-performance OSCs.

Facile Preparation of Molybdenum Bronzes as an Efficient Hole Extraction Layer in Organic Photovoltaics

We proposed a facile and green one-pot strategy to synthesize Mo bronzes nanoparticles to serve as an efficient hole extraction layer in polymer solar cells. Mo bronzes were obtained through reducing the fractional self-aggregated ammonium heptamolybdate with appropriate reducing agent ascorbic acid, and its optoelectronic properties were fully characterized. The synthesized Mo bronzes displayed strong n-type semiconductor characteristics with a work function of 5.2-5.4 eV, matched well with the energy levels of current donor polymers. The presented gap states of the Mo bronzes near the Fermi level were beneficial for facilitating charge extraction. The as-synthesized Mo bronzes were used as hole extraction layer in polymer solar cells and significantly enhanced the photovoltaic performance and stability. The power conversion efficiency was increased by more than 18% compared with the polyethylene dioxythiophene:polystyrenesulfonate-based reference cell. The excellent performance and facile preparation render the as-synthesized solution-processed Mo bronzes nanoparticles a promising candidate for hole extraction layer in low-cost and efficient polymer solar cells.

Enhanced performance and stability of polymer BHJ photovoltaic devices from dry transfer of PEDOT:PSS

Polymer solar cells with enhanced initial cell performances and long-term stability were fabricated by performing a simple dry transfer of a hole extraction layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] onto an indium tin oxide (ITO) substrate. Due to the very flat surface of the polyurethane acrylate/polycarbonate (PUA/PC) film, which was used as a mold and resembled the surface of the original substrate (silicon wafer), the transferred layer had a very smooth surface morphology, resulting in enhancement of the interfacial characteristics. The work function of the PEDOT:PSS layer and the morphology of bulk hetero junction (BHJ) layer were tuned by controlling the position of PSS enrichment in the PEDOT:PSS layer using the dry transfer. The power conversion efficiency of PTB7:PC71 BM BHJ device prepared by the dry transfer was 8.06%, which was significantly higher than that of the spin-cast device (7.32%). By avoiding direct contact between the ITO substrate and the PEDOT:PSS solution in the dry transfer system, etching and diffusion of indium in the ITO substrate were greatly reduced, thereby improving the stability.

Sulfated Graphene Oxide as a Hole-Extraction Layer in High-Performance Polymer Solar Cells

In this study, we have rationally designed and successfully developed sulfated graphene oxide (GO-OSO3H) with -OSO3H groups attached to the carbon basal plane of reduced GO surrounded with edge-functionalized -COOH groups. The resultant GO-OSO3H is demonstrated to be an excellent hole extraction layer (HEL) for polymer solar cells (PSCs) because of its proper work function for Ohmic contact with the donor polymer, its reduced basal plane for improving conductivity, and its -OSO3H/-COOH groups for enhancing solubility for solution processing. Compared with that of GO, the much improved conductivity of GO-OSO3H (1.3 S m(-1) vs 0.004 S m(-1)) leads to greatly improved fill factor (0.71 vs 0.58) and power conversion efficiency (4.37% vs 3.34%) of the resulting PSC devices. Moreover, the device performance of GO-OSO3H is among the best reported for intensively studied poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) devices. Our results imply that judiciously functionalized graphene materials can be used to replace existing HEL materials for specific device applications with outstanding performance.