Promising Hole Transport Materials Research Articles

Hole-transport materials based on benzodithiophene-thiazolothiazole-containing conjugated polymers for efficient perovskite solar cells

Conjugated polymers are promising hole-transport materials (HTMs) for designing efficient and stable perovskite solar cells (PSCs). The structure of polymer backbone and position of side substituents greatly influences their electronic properties, film morphology, and hence, the charge extraction and charge transport characteristics of HTMs. Herein, we design two novel donor-acceptor conjugated polymers based on benzodithiophene (BDT) bearing triisopropylsilyl substituents and thiophene-flanked thiazolothiazole block (Tz) with different side chain position. In polymer in-BDTTz alkyl chains are placed “toward” to the Tz moiety, while in aus-BDTTz “outward” to the Tz block. Polymeric HTM in-BDTTz enables 18.7% power conversion efficiency in devices outperforming that of PSCs with aus-BDTTz and reference devices employing poly(triarylamine) (PTAA). Furthermore, PSCs with in-BDTTz as HTM demonstrates stable operation comparable to that of PTAA-based devices. These findings feature alkylsilyl-substituted benzodithiophene-based polymers as promising dopant-free hole-transport materials for efficient and stable perovskite photovoltaics.

Effect of PTAA and PCBM concentrations on the electrical response of bilayer heterojunction PTAA/PCBM Diode

Poly[bis(4-phenyl) (2,4,6-trimethylphenylamine] (PTAA) is a new promising hole transport material, and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) is a suitable electron acceptor material. This study deposited thin-film PTAA and PCBM at different spin coatings ranging from 1000 rpm to 5000 rpm. Ultraviolet–visible (UV–vis) spectrometer, X-ray diffraction (XRD) machine, and scanning electron microscope (SEM) techniques were used to determine the optical properties and structural properties, respectively. Thermal annealing with temperatures of 80 °C, 100 °C, 120 °C, and 150 °C was introduced to improve the properties of the PTAA and PCBM thin film. The results of SEM images for PTAA annealed at 150 °C appeared to darken and melt, whereas PCBM annealed at 120 °C and 150 °C had granule properties. The ideality factors for bilayer heterojunction diodes and bulk heterojunction PTAA/PCBM diodes with increased layers were 5.8–60.2 and 30.8–77.2, respectively. The optimum spin rate for the bilayer heterojunction PTAA/PCBM was 5000 rpm. The most appropriate diode behavior of the bilayer was at 5.0/1.0 wt%, 1.0/5.0 wt%, and 3.0/3.0 wt% of PTAA/PCBM, with the calculated ideality factors of 5.8, 6.2, and 15.45 with turn-on voltage of 1.57 V, 1.70 V, and 1.82 V, respectively.

Modulating the strength of acceptor in D-A-D type hole transport materials for efficient inverted perovskite solar cells

We report herein a design strategy to improve the hole mobility of D-A-D-based hole transport material (HTM) by modifying the accepting core of the reference HTMs based on triphenylamine (TPA) and bithieno thiophene (BTTI) units. Eight HTMs are designed by replacing the BTTI unit in BTTI-TPA HTM with several commonly available diones and imides as acceptors. The structural, electronic, optical, and charge transport properties are studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The designed molecules are extensively examined for their structural, electronic, and absorption properties to extract the features that improve their hole mobility using the MPW1PW91/6–31 g(d,p) method. We identified four carefully designed BTTD-TPA, DBTT-TPA, NDTTI-TPA, and NDTI-TPA molecules as the most promising HTMs. This work exemplifies the role of the methyl group on electron acceptors on hole reorganization energies. Results obtained from this study provide valuable guidelines for designing efficient HTMs.

Managing the Double-Edged Sword of Ni3+ in Sputter-Deposited NiOx by Interfacial Redox Reactions for Efficient Perovskite Solar Cells

Nickel oxide (NiOx) is widely used as a promising hole transport material for perovskite solar cells (PSCs). A high concentration of Ni3+ in the NiOx film is generally beneficial for charge transport of the PSCs; however, chemical redox reactions between surface Ni3+ and perovskite materials result in decomposition of perovskite materials, which causes carrier recombination and impedes charge transport at the perovskite–NiOx interface. Herein, we employ magnetron sputtering to fabricate NiOx thin films with adjustable Ni3+ concentrations to optimize the hole-transporting properties. A thin layer of phenylethylamine iodide (PEAI) is further introduced to reduce the detrimental Ni3+ at the surface of NiOx, which eliminates the formation of undesirable defects and chemical species when in contact with the perovskite layer, leading to a dramatic increase in the power conversion efficiency (PCE) from 16.37 to 20.01%, which is one of the best performance using sputtered charge transport layers. The unencapsulated devices retain 88% of their initial PCE after storage in a nitrogen atmosphere for 1000 h under light. We further perform solar cell capacitance simulator (SCAPS) simulation to understand the effects of bulk and interfacial charge transport on the performance of PSCs, which agree with our experimental results. This work not only highlights the double-edged sword effect of the Ni3+ content on the performance and stability of PSCs but also demonstrates a simple yet effective strategy to avoid the undesirable reaction between Ni3+ and perovskite materials in the fabrication of PSCs with sputter-deposited NiOx.

A π-conjugated polymer as dopant-free hole transporting material for perovskite solar cells with efficiency approaching 19%

Perovskite solar cells (PSCs) have attracted considerable attention due to their outstanding advantages, such as cost-effective, easy fabrication, and high efficiency. Herein, we designed and synthesized siloxane-functionalized π-conjugated polymer Nap-SiBTAz2. The rational structural including benzodithiophene, difluorobenzotriazol unit and alkyl-siloxane enable excellent hole mobility and good solubility. Interestingly, dopant-free Nap-SiBTz2 as hole transporting material (HTM) in PSC exhibited impressive power conversion efficiency of 19.0% with open-circuit-voltage 1.14 V, short-circuit current density 23.6 mA cm−2, and fill factor 70.7%. These results proved that Nap-SiBTz2 is a promising HTM candidate for PSCs, while providing a rational method to design dopant-free HTM for high-efficiency PSCs.

A chlorinated polythiophene-based polymer as a dopant-free hole transport material in perovskite solar cells

We report a chlorine-substituted polythiophene-based, dioxobenzodithiophene-containing conjugated polymer (P2T-Cl) as a promising dopant-free hole transport material in lead halide perovskite solar cells.

D-π-A-Type Pyrazolo[1,5-a]pyrimidine-Based Hole-Transporting Materials for Perovskite Solar Cells: Effect of the Functionalization Position.

Donor-acceptor (D-A) small molecules are regarded as promising hole-transporting materials for perovskite solar cells (PSCs) due to their tunable optoelectronic properties. This paper reports the design, synthesis and characterization of three novel isomeric D-π-A small molecules PY1, PY2 and PY3. The chemical structures of the molecules consist of a pyrazolo[1,5-a]pyrimidine acceptor core functionalized with one 3,6-bis(4,4'-dimethoxydiphenylamino)carbazole (3,6-CzDMPA) donor moiety via a phenyl π-spacer at the 3, 5 and 7 positions, respectively. The isolated compounds possess suitable energy levels, sufficient thermal stability (Td > 400 °C), molecular glass behavior with Tg values in the range of 127-136 °C slightly higher than that of the reference material Spiro-OMeTAD (126 °C) and acceptable hydrophobicity. Undoped PY1 demonstrates the highest hole mobility (3 × 10-6 cm2 V-1 s-1) compared to PY2 and PY3 (1.3 × 10-6 cm2 V-1 s-1). The whole isomers were incorporated as doped HTMs in planar n-i-p PSCs based on double cation perovskite FA0.85Cs0.15Pb(I0.85Br0.15)3. The non-optimized device fabricated using PY1 exhibited a power conversion efficiency (PCE) of 12.41%, similar to that obtained using the reference, Spiro-OMeTAD, which demonstrated a maximum PCE of 12.58% under the same conditions. The PY2 and PY3 materials demonstrated slightly lower performance in device configuration, with relatively moderate PCEs of 10.21% and 10.82%, respectively, and slight hysteresis behavior (-0.01 and 0.02). The preliminary stability testing of PSCs is also described. The PY1-based device exhibited better stability than the device using Spiro-OMeTAD, which could be related to its slightly superior hydrophobic character preventing water diffusion into the perovskite layer.

Facile synthesis of spiro-core-based hole-transporting material for high-performance and stable perovskite solar cells

Two new spirocore-based hole-transporting materials (HTMs), spiro-1 and spiro-2, with 5H-spiro(benzo[1,2-b:6,5-b']dithiophene-4,4′-cyclopenta[2,1-b:3,4-b′]dithiophen)-5-one moiety connected with multiple triphenylamine donor substituents are synthesized via two-step synthesis with low cost and simple procedures and used as HTM in perovskite solar cells (PSCs). The correlation between different HTM structures and the performance of the PSCs was investigated systematically. Experiments results on the thermal stability, photophysical, electrochemical, charge-transporting, as well as film-forming properties show that spiro-1 and spiro-2 are suitable candidate as HTMs for PSCs. The fabricated PSCs with spiro-1 and spiro-2 as HTMs achieved an excellent power conversion efficiency (PCE) of 21.67 % and 19.65 % respectively, both with negligible hysteresis. The obtained PCE of 21.67 % based on spiro-1 is higher than that of the reference solar cell using traditional HTM of spiroOMeTAD (20.59 %) under the same conditions. It may be ascribed to the higher hole mobility, the more efficient hole extraction at the HTM/perovskite interface, the better film morphology. Moreover, the devices based on the new HTM showed good long-term stability and maintained over 80 % of their initial efficiency under continuous stressing at 85 °C and light illumination at 50 °C for 500 h in air without any encapsulation, which were superior to that of spiroOMeTAD. This work demonstrates that the newly developed spirofused molecules are promising HTMs for highly efficient and stable PSCs for the future commercialization of PSCs.

Side chain engineering and film uniformity: two key parameters for the rational design of dopant-free polymeric hole transport materials for efficient and stable perovskite solar cells

Herein, we introduce a series of new (X–DADAD)n-type conjugated polymers comprised of benzo[1,2-b:4,5-b′]dithiophene (X), benzo[c] [1,2,5]thiadiazole (acceptor, A), and thiophene (donor, D) units as highly promising hole transport materials for perovskite solar cells (PSCs). The rational engineering of the backbone structure and the side chains enabled high power conversion efficiencies of up to 20% in n-i-p perovskite solar cells in combination with impressive operational stability: the devices preserved initial performance after >2500 h of continuous illumination at open-circuit conditions. The origins of the observed long-term perovskite solar cells stability enabled by the newly designed polymeric hole transport materials have been revealed through the use of infrared scattering-type scanning near-field optical microscopy and further elucidated by density functional theory calculations. The presented findings point to novel strategies of designing advanced charge transport materials to simultaneously endow the PSCs with high performance and durability.

Self-Powered and Low-Noise Perovskite Photodetector Enabled by a Novel Dopant-Free Hole-Transport Material with Bottom Passivation for Underwater Blue Light Communications.

Designing dopant-free hole-transport materials (HTMs) is a facile and effective strategy to realize high-performance organic-inorganic hybrid perovskite (OIHP) photodetectors. Herein, a novel phenothiazine polymer, poly[4-(10H-phenothiazin-10-yl)-N,N-bis(4-methoxyphenyl)aniline] (PPZ-TPA), was synthesized and employed as a promising HTM in OIHP photodetectors. The triphenylamine donor unit was combined with a phenothiazine core, furnishing the polymer with a suitable highest occupied molecular orbital level, favorable thermal stability, and appropriate film morphology. The sulfur atom in the phenothiazine functional group can intentionally passivate the undercoordinated Pb2+ of OIHP films, suppressing nonradiative recombination and yielding an ultralow dark current density of 1.26 × 10-7 A cm-2 under -0.1 V, as well as a low-noise current of 3.75 × 10-13 A Hz-1/2 at 70 Hz. Encouragingly, the self-powered PPZ-TPA-based OIHP photodetectors were successfully integrated into a blue light communication system for the first time, demonstrating their application for receiving and transmitting light signals with a transmission rate of 300 bps. In addition, the PPZ-TPA-based devices exhibit nearly 1 year shelf stability without obvious degradation. We believe that PPZ-TPA demonstrates great potential to achieve high-performance perovskite photodetectors, also providing a strategy for the design of novel HTMs.

Improved Ultrafast Carrier Relaxation and Charge Transfer Dynamics in CuI Films and Their Heterojunctions via Sn Doping.

CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I-V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.

Effect of surface treatment of sputtered nickel oxide in inverted perovskite solar cells

Nickel oxide (NiOx) is promising hole transport material for efficient and stable lead halide perovskite solar cell (PSC) due to its promising optoelectronic properties and superior moisture resistivity. Herein, we investigate the effect of surface treatment of sputtered NiOx film using ethanol (NiOx-EtOH) in device parameters of PSC. Ethanol treatment of NiOx facilitates the growth of high-quality perovskite film (larger grain size, better crystallinity, and faster carrier injection). The device with NiOx-EtOH demonstrated the champion power conversion efficiency of ∼13.52% (with open circuit voltage (VOC) ∼1.075 V) of 1 cm2 large-area device. While the device with pristine NiOx has a device efficiency of 11.78% (with VOC ∼0.99 V). This report corroborates that the PSC with NiOx-EtOH improves device performance owing to hydrophilicity, defect passivation of sputtered NiOx surface, and attenuation of recombination in the perovskite bulk.

Increasing the stability of perovskite solar cells with dibenzofulvene-based hole transporting materials

The hole transporting material (HTM) plays a critical role in the performance and stability of perovskite solar cells (PSCs). In PSCs with n-i-p architecture, Spiro-OMeTAD has been widely applied as HTM reaching the highest efficiency, however, its low stability slows down the long-term application of the devices. Thus, in order to enhance the performance of the devices, in this work we analyse, in n-i-p PSCs, three organic hole-transporting materials containing two and three amino redox centers bridged to a dibenzofulvene (DBF) backbone. The difference in the molecular structure of the three DBF-based HTMs lies in the substitution pattern on the exocyclic fulvene bond. Methodical studies of kinetics and morphology reveal that the nature of the substituent plays a vital role in the performance of the PSC, allowing to obtain an efficiency (16.08 %) comparable to reference Spiro-OMeTAD (17.75 %). In addition, the PSCs with DBF-based HTMs demonstrated better stability against the reference prepared with Spiro-OMeTAD under continuous illumination in ambient conditions (15 ± 2°C and 60 ± 5% RH), as well as under dark and low-humidity conditions. These results place our DBF-based organic molecules as promising HTMs to form part of highly efficient and long-term stability perovskite solar cell applications.

Successive ionic layer adsorption and reaction processed CuSCN for efficient inverted perovskite solar cells

Copper thiocyanate (CuSCN) is a promising inorganic hole transport material for perovskite solar cells (PSCs) due to its wide-bandgap, low cost, high carrier mobility and good transmittance. However, there are limited reports about CuSCN application in PSCs mainly due to its inferior solubility. Here, we prepared high-quality CuSCN films through a simple solution method named successive ionic layer adsorption and reaction (SILAR). Using SILAR method, high-quality CuSCN films could be obtained on ITO (indium tin oxide) substrate with good morphology and photoelectric properties. As a result, efficient inverted PSCs were fabricated with SILAR-prepared CuSCN and the efficiency can be increased from 11.10 % to 18.71 %.

Dual role of novel hole-transporting and deep-blue emitting materials based on twisted binaphthyl

Three novel deep blue-emitting materials from binaphthyl derivatives, namely BFS, BNS and BNF, were designed and synthesized. Due to their rigid structure, all the materials exhibit high thermal stability with Td exceed 440 °C and Tg as high as 153 °C, 149 °C and 140 °C, respectively. They also show appropriate HOMO and LUMO energy level, excellent amorphous stability and outstanding solubility in common organic solvents. These novel materials are highly emissive in both solution and solid states, with deep blue emissions peaked at ∼450 nm. Deep-blue OLEDs using the novel materials as emitter resulted in efficient CE, EQE and luminance. The BFS, BNS and BNF based devices show good EL performances with the Lmax of 857 cd/m2 (CEmax of 0.3 cd/A), Lmax of 1038 cd/m2 (CEmax of 0.5 cd/A), and Lmax of 3717 cd/m2 (CEmax of 0.5 cd/A), respectively. They should be promising hole-transport and deep blue-emitting materials for OLED devices.

Understanding the p-doping of spiroOMeTAD by tris(pentafluorophenyl)borane

The solid-state organization of photoabsorber, hole and electron transporting layers, and interfaces between them plays an important role in governing the performance and stability of emerging optoelectronic devices such as perovskite solar cells (PSCs). The molecular organic semiconductor (OSC) 2,2′,7,7′-tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiroOMeTAD) is a promising hole-transporting material (HTM) for PSCs, which is p-doped by molecular dopants to augment the charge carrier mobility. Here, the p-type doping of spiroOMeTAD by tris(pentafluorophenyl)borane (BCF) is investigated by a combination of techniques including optical spectroscopy, X-ray diffraction, Fourier transform infrared (FTIR), solid-state (ss)NMR, and electron paramagnetic resonance (EPR) spectroscopy. BCF molecules interact with traces of water molecules to form BCF-water complexes. Optical spectroscopy analysis suggests that the BCF/BCF-water complexes oxidize spiroOMeTAD molecules and facilitate p-type doping of spiroOMeTAD molecules. The different distributions of BCF and BCF-water molecules in doped spiroOMeTAD are characterized by FTIR and 11B NMR spectroscopy. An NMR crystallography approach which combines two-dimensional (2D) ssNMR and crystallography modeling is employed to unravel the packing interactions in spiroOMeTAD, and this analysis is extended to probe the morphological and structural changes in spiroOMeTAD:BCF blends. The hyperfine interactions are characterized by 2D hyperfine sub-level correlation (HYSCORE) spectroscopy. In this way, insight into the complex spiroOMeTAD:BCF blend morphology is obtained and compared for different dopant concentrations. Molecular-level analysis of doped HTMs enabled by this study has much wider relevance for further investigation, for example, chemical design and interfacial engineering of p-type doped HTMs for stable and efficient hybrid perovskite photovoltaics.

Overcome Low Intrinsic Conductivity of NiOx Through Triazinyl Modification for Highly Efficient and Stable Inverted Perovskite Solar Cells

Nickel oxide (NiOx) is a promising hole transport material in inverted organic‐inorganic metal halide perovskite solar cells. However, its low intrinsic conductivity hinders its further improvement in device performance. Here, we employ a trimercapto‐s‐triazine trisodium salt (TTTS) as a chelating agent of Ni2+ in the NiOx layer to improve its conductivity. Due to the electron‐deficient triazine ring, the TTTS complexes with Ni2+ in NiOx via a strong Ni2+‐N coordination bond and increases the ratio of Ni3+:Ni2+. The increased Ni3+ concentration adjusts the band structure of NiOx, thus enhancing hole density and mobility, eventually improving the intrinsic conductivity of NiOx. As a result, the device with TTTS modification displays a champion power conversion efficiency (PCE) of 22.81%. The encapsulated device based on a modified‐NiOx layer maintains 94% of its initial power output at the maximum power point and continuous one‐sun illumination for 1000 h at 45 °C. In addition, the unencapsulated target devices also maintain 92% at 60 ± 5% relative humidity and 25 °C in the air for 5000 h; and 91% at 85 °C in a nitrogen atmosphere for 1000 h. The research provides an effective strategy to enhance PCE and stability of inverted PSCs via modifying NiOx films with triazine molecule.

Thin film formation of wet chemically deposited NiOx and parasitic oxides at NiOx/Si interfaces during high-temperature annealing of selective contacts in perovskite/Si tandem solar cells

Wet chemically deposited NiOx is a promising hole-transport material in perovskite/Si tandem solar cells due to its high work function, chemical stability, and low cost. However, it requires subsequent annealing that may lead to the formation of SiOz at the Si interface to the bottom cell. These layers of nm thickness exhibit non-stoichiometric chemical composition and are detrimental for charge transport and passivation. In this contribution, we investigate the atomic and electronic structure as well as chemical properties of NiOx layers on Si after wet chemical deposition and annealing in air between 200 °C and 600 °C. For this purpose, photoelectron spectroscopy, X-ray diffraction, differential thermal analysis/thermogravimetric analysis coupled with Fourier transform infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and electron-energy loss spectroscopy in a scanning transmission electron microscope are applied. After deposition and annealing at 200 °C, a mixed bulk phase consisting of NiOxHy is found which transforms to pure NiOx above 300 °C. A parasitic SiOz layer is observed at the NiOx/Si interface after this process, with a thickness of 3–4 nm after annealing at 500 °C. The impact of this interfacial layer for electrical current transport will be discussed concerning optimized performance and integrability in perovskite/silicon tandems.

Fluorene-Modified Zinc Porphyrin as Low-Cost Hole-Transporting Material for Efficient Perovskite Solar Cells

The potential of porphyrin derivatives as hole-transporting materials (HTMs) for perovskite solar cells (PSCs) has been demonstrated. The structural engineering of porphyrin HTMs provides an important means for further improvement of the performance of PSCs. Herein, a zinc-porphyrin derivative (ZnP-FL) decorated with four fluorene-terminated triarylamines is presented. The lab synthesis cost of ZnP-FL is estimated to be around $32.2/g. It exhibits good charge-transport ability and thermal stability. A high power conversion efficiency (PCE) of 19.31% is achieved by using ZnP-FL HTM (V oc = 1.08 V; J sc = 24.08 mA · cm−2), which is distinctly higher than that of a control HTM without the fluorene groups (PCE = 17.75%; V oc = 0.97 V; J sc = 24.04 mA · cm−2). This performance enhancement is mainly attributed to the improved open-circuit voltage, which benefits from the stabilized HOMO level of ZnP-FL. In addition, the porphyrin HTM-based PSCs show superior air and thermal stability to the device with the standard HTM spiro-OMeTAD. These results demonstrate that the low-cost and easily accessible porphyrin derivatives are promising HTMs for efficient and stable PSCs.

A facile one‐step solution synthesis of Cs2SnI6−xBrx using less‐toxic methanol solvent for application in dye‐sensitized solar cells

SummaryCs2SnX6 (X = I, Br, and Cl), a vacancy‐ordered double perovskite, has recently emerged as a promising lead‐free perovskite for optoelectronic applications. The low quality of solution‐processed Cs2SnX6 perovskites, on the other hand, precludes further improvement of their performance in the devices. In this work, we developed a facile one‐step solution strategy for synthesizing high‐quality Cs2SnI6−xBrx material with the less‐toxic methanol solvent. A series of Cs2SnI6−xBrx perovskites was synthesized by carefully tuning each stage of the process and using thorough structural and optical characterizations as feedback. We identified two critical parameters that affect the quality of perovskite material through this detailed characterization: the reactant ratio and the annealing temperature. The optimal precursor ratio for the synthesized perovskite was determined to be 1:1 with an annealing temperature of 100°C. Leveraging this, we investigated the optimized perovskite material as a hole transport material (HTM) in a dye‐sensitized solar cell (ss‐DSSC) and achieved an outstanding PCE of 3.2% without using any modifiers. We envisage that our findings may open the door for the growth of nonhazardous synthesis approaches for Sn‐based optoelectronic devices.Novelty StatementThe preparation methods and high costs associated with the organic HTMs and the use of toxic solvents are the major obstacles to developing efficient materials for photovoltaic applications. With the possibility of commercialization of solution‐processable solar cells, it is critical to find less hazardous perovskite formation approaches. Leveraging this, we developed a facile one‐step solution method and investigated the potential of Cs2SnI6−xBrx perovskite as a promising hole transport material (HTM) for solid‐state dye‐sensitized solar cell (ss‐DSSC).