Polymer bulk heterojunction solar cells with PEDOT:PSS bilayer structure as hole extraction layer

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.

Thermodynamically self-organized hole transport layers for high-efficiency inverted-planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a popular and promising hole transport material for making efficient inverted-planar perovskite solar cells (IP-PSCs). However, the mismatch between the work function of conventional PEDOT:PSS and the valence band maximum of perovskite materials is still a challenge for efficient hole extraction. Here, we report systematic studies on the work-function modification and thermodynamic morphological evolution of PEDOT:PSS films by tuning the PSS/PEDOT ratio, along with its effects on the photovoltaic responses of IP-PSCs. We found that the open-circuit voltage (V_OC) of an IP-PSC could be enhanced by controlling the work function of PEDOT:PSS. Furthermore, the optical transmittance of the PEDOT:PSS film could be maximized by controlling the morphological evolution, which will further increase the short-circuit current density (J_SC) of the IP-PSC. The V_OC and J_SC of the IP-PSC with the optimized PEDOT:PSS composition increased from 0.88 to 0.93 V and from 17.11 to 20.77 mA cm^-2, respectively, compared with an IP-PSC containing commercial PEDOT:PSS, which results in a power conversion energy that is greatly improved from 12.39 to 15.24%.

Surface-Engineered Graphene Quantum Dots Incorporated into Polymer Layers for High Performance Organic Photovoltaics

Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices due to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a surface-engineered GQD-incorporated polymer photovoltaic device shows enhanced power conversion efficiency (PCE), where the oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole extraction layer, leading to significantly improved short circuit current density (J_sc) value. To maximize the PCE of the device, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer.

Time-resolved neutron reflectometry and photovoltaic device studies on sequentially deposited PCDTBT-fullerene layers

We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells.

Flexible and transparent metallic grid electrodes prepared by evaporative assembly

We propose a novel approach to fabricating flexible transparent metallic grid electrodes via evaporative deposition involving flow-coating. A transparent flexible metal grid electrode was fabricated through four essential steps including: (i) polymer line pattern formation on the thermally evaporated metal layer onto a plastic substrate; (ii) rotation of the stage by 90° and the formation of the second polymer line pattern; (iii) etching of the unprotected metal region; and (iv) removal of the residual polymer from the metal grid pattern. Both the metal grid width and the spacing were systematically controlled by varying the concentration of the polymer solution and the moving distance between intermittent stop times of the polymer blade. The optimized Au grid electrodes exhibited an optical transmittance of 92% at 550 nm and a sheet resistance of 97 Ω/sq. The resulting metallic grid electrodes were successfully applied to various organic electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs).

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.