Naphthonaphtho[2,1-b:3,4-b']dithiophene is a good electron-rich building block for p-type semiconducting materials and has been extensively studied in our group. Various donor-acceptor type copolymers based on naphtho[2,1-b:3,4-b']dithiophene were designed and synthesized for high-performance bulk heterojunction solar cell applications. The structure-property relationship of these copolymers with respect to the influence of alkyl side-chain position and chain length, fluorination substitution effect as well as acceptor group effect with different electron withdrawing ability were also investigated. The results of the PV device performance based on these copolymers indicated that the device efficiency was sensitive to the position of the alkyl side-chains attached, which could cause non-planarity of polymer backbone. The resulting naphthodithiophene-based copolymer with shorter side-chains afforded higher device efficiency. The incorporation of fluorine atoms into the copolymers leading to the poor solubility caused a decrease in the device performance compared to the non-fluorinated counterparts. The copolymer with thiadiazol[3,4-c]pyridine as an electron-deficient unit also showed promising device performance with efficiency up to 5.10%. Among all the copolymers designed and synthesized, three copolymers namely PNB-4, PNB-C2,6 and PNTP exhibited excellent preliminary device performance with a PCE more than 5.0% showing potential for further device optimization and development.

As a clean and non-exhaustible energy source, solar energy is becoming increasingly important in reducing energy prices and influencing the global climate change. Compared to the traditional inorganic solar cells, conjugated polymer-based organic solar cells (OSCs) have shown much promise as an alternative photovoltaic technology for producing solar cells on large scale at low-cost. However, despite the rapid progresses made in the development of new donor materials, fullerene derivatives and hybrid small molecule/polymer blends, the efficiency and stability of OSCs are still limitations on the potential applications. The performance of OSCs is primarily hampered by the limited light absorption, caused by the mismatch between light absorption depth and carrier transport scale, low carrier mobility and unstable electrode/organic interfacial properties. Improved utilization of light in solution-processed OSCs via different light trapping schemes is a promising approach. The feasibility of light trapping using surface plasmonic structures and textured surfaces to confine light more efficiently into OSCs has been demonstrated. However, plasmon excitations are localized only in the vicinity of metal/organic interface, while the absorption enhancement due to the textured surfaces improves light trapping irrespective of the wavelength. A generic approach towards improving light harvesting in the organic active layer thinner than optical absorption length is one of the key strategies to the success of OSCs. The aim of this PhD project is to undertake a comprehensive study to analyze broadband and omnidirectional light absorption enhancement in bulk heterojunction (BHJ) OSCs, to understand the dynamics of charge transport, charge recombination, charge collection, and to develop solutions to improve the stability of OSCs. In this work, the broadband light absorption enhancement in solution-processed BHJ OSCs is realized by incorporating 2-D photonic structures in the active layer, formed using a nano-imprinting method. The performance of photonic-structured OSCs and planar control cells, fabricated with the blend of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-bA] dithiophene-2,6-diyl] [3-fluoro-2-[(2ethylhexyl) carbonyl] thieno[3,4-b]-thiophenediyl] (PTB7): [6,6]-phenyl-C70-butyric-acid-methyl-ester (PC70BM) is analyzed. By introducing the photonic structures with 500 nm structure period, the performance of structured OSCs is optimized by adjusting the structure height in the active layer. With the comparison of the current densityvoltage (JV) characteristics, the incident photon to charge carrier efficiency (IPCE) spectra and also the finite-difference time-domain (FDTD) calculated electric field distributions, our results reveal that the photonic structures allow improving light absorption in PTB7:PC70BM layer, especially in the long wavelength region. It is shown that the photonic-structured OSCs possess a 6.15 % increase in power conversion efficiency (PCE) and a 7.53 % increase in short circuit current density (JSC) compared to that of a compositionally identical planar control cell. Light absorption in the 2-D photonic-structured OSCs is a function of the photonic structures and the optical properties of the active layer. The correlation between the choice of the photonic structures and the enhanced spectral response in photonic-structured OSCs is analysed systematically using theoretical simulation and experimental optimization. It is found that the integrated absorption of the active layer decreases slightly with increase in the period of the photonic structures. The results reveal that the photonic-structured OSCs exhibit a stronger absorption enhancement over a broader range of the angle of incident light. The incorporation of the appropriate periodic nano-structures in the active layer is apparently favourable for efficient cell operation as compared to light absorption in the planar control cells made with the same blend system, which decreases rapidly with an increase in the angle of the incident light. Omnidirectional and broadband light absorption enhancement observed in photonic-structured OSCs agrees well with the theoretical simulation. More than 11% increase in the PCE of photonic-structured OSCs is obtained compared to that of an optimized planar control cell, caused mainly by the absorption enhancement in the active layer. 2-D photonic structures allow achieving broadband absorption enhancement in OSCs over a wider range of the angle of the incident light from -45 deg to +45 deg with respect to the normal to the cell surface. For example, the higher light absorption in the active layer of photonic-structured OSCs, integrated over the visible light wavelength range from 380 nm to 780 nm, changes slightly from 70.1% (normal) to 67.7% (45 deg), remaining 96.6% of the absorption in the cells at the normal incidence. While for the control planar OSC, the integrated absorption follows a faster decrease from 66.2% (normal) to 62.2% (45 deg), revealing a quicker reduction in the absorption of the cells at an angle of the incident light away from the normal incidence. In addition to the absorption enhancement, charge transport, recombination and collection are also prominent factors for the efficient operation of OSCs. Thus, it is crucial to improve the understanding of these important processes and their impacts on the cell performance in order to design optimized device architectures. The charge recombination processes, the distribution of charge density under different operation conditions and charge collection at the organic/electrode interfaces in PTB7:PC70BM-based OSCs are studied systematically using a combination of theoretical calculation, transient photocurrent (TPC) measurements, morphology analyses and device optimization. The charge transport and recombination properties in the BHJ OSCs are investigated using the photo-induced charge extraction by linearly increasing voltage (Photo-CELIV) method. Combined with light intensity-dependent J--V characteristic and TPC measurements, it is shown that the use of the ZnO cathode interlayer has a profound effect on enhancing charge collection efficiency and thereby improving in the overall performance of OSCs. The origin of the improvement in the cell performance is mainly associated with improved electrical properties. The TPC results reveal that the presence of the ZnO interlayer helps to prevent the unfavourable interfacial exciton dissociation for achieving efficient charge collection at the active layer/electrode interface. Light intensity-dependent J--V characteristics and the photo-CELIV results support the findings in showing that the charge recombination at the organic/cathode interface can be effectively suppressed by inserting a thin ZnO cathode interlayer, leading to a significant improvement in the charge collection efficiency. A comprehensive study on the degradation mechanisms of solution-processed BHJ. OSCs has been performed. It is manifested that the suppression in bi-molecular recombination and enhancement in charge mobility, achieved through appropriate electrode modification, is one of the effective approaches for achieving stable and performance reproducible OSCs. The effect of the solution-processed anode interlayer, e.g. a mixture of MoO3 and Au nanoparticles (MoO3:Au NPs), on the performance of BHJ OSCs is also examined, with the aim to replace the acidic and hygroscopic poly(3,4-ethylenedioxylenethiophene): polystyrene sulfonate (PEDOT:PSS) hole extraction layer (HEL). A 14.3% enhancement in the PCE of OSCs with an anode interlayer of MoO3:Au NPs (7.78%) is obtained compared to that of the structurally identical devices with a pristine MoO3-based interlayer (6.72%), due to the simultaneous improvements in both JSC and fill factor (FF). The accelerated aging tests for as-prepared structurally identical OSCs fabricated with different HELs were carried out in the ambient condition. It is shown that the solution-processed MoO3:Au NPs and pristine MoO3 interlayers are superior to the frequently-used PEDOT:PSS HEL for efficient operation over the long-term. PCE of the MoO3-based OSCs maintains about 40% of their initial value, while a catastrophic failure in the control devices with a PEDOT:PSS HEL is observed after the accelerated aging test under the same condition, with a high relative humidity of 90% at room temperature for 180 min. The degradation behavior of different OSCs performed in the accelerated aging test correlates well with light-intensity JV characteristic and TPC measurements. The outcomes of this work help to the creation of device knowledge and process integration technologies for realization of high performance solution-processed OSCs. It is anticipated that the adoption of the affordable organic photovoltaic technology as one of the clean energy sources will contribute to the preservation of the environment.

Solar cells allow the energy from the sun to be converted into electrical energy; this makes solar energy much more environmentally friendly than fossil fuel energy sources. These solar cells are connected together in a photovoltaic (PV) module to provide the higher current, voltage and power outputs necessary for electrical applications. However, the performance of the PV module is limited by the performance of the individual cells. Cell mismatch occurs when some cells are damaged or shaded and produce lower current output than the other cells in the series connected string. The cell mismatch lowers the module performance and can result in further damage as the weak cells are reverse biased and dissipate heat. Bypass diodes can be connected into the module to increase the module current output and prevent further damage. Since cell mismatch results in a significant decrease in the performance of deployed modules it is important to fully understand and characterise its effect on PV modules. PV modules can be characterised using various techniques, each providing important information about the performance of the module. Most commonly the current-voltage (I-V) characteristic curve of a module is measured in outdoor, fully illuminated conditions. This allows performance parameters such as short circuit current (Isc), open circuit voltage (Voc) and maximum power (Pmax) to be determined. In addition to this the shape of the curve allows device parameters like series and shunt resistances to be determined using parameter extraction algorithms like Particle Swarm Optimisation (PSO). The extracted parameters can be entered into the diode equation to model the I-V curve of the module. The I-V characteristic of the module can also be used to identify poor current producing cells in the module by using the worst-case cell determination method. In this technique a cell is shaded and the greater the drop in current in the whole module the better the current production of the shaded cell. The photoresponse of cells in a module can be determined by the Large-area Light Beam Induced Current (LA-LBIC) technique which involves scanning a module with a laser beam and recording the current generated. Electroluminescence (EL) is emitted by a forward biased PV module and is used to identify defects in cell material. Defects such as cracks and broken fingers can be detected as well as material features such as grain boundaries. These techniques are used to in conjunction to characterise the modules used in this study. The modules investigated in this study each exhibit cell mismatch resulting from different causes. Each module is characterised using a combination of characterisation techniques which allows the effect of cell mismatch be investigated. EL imaging enabled cracks and defects, invisible to the naked eye, to be detected allowing the reduced performance observed in I-V curves to be explained. It was seen that the cracked cells have a significant effect on the current produced by a string, while the effect of delaminated areas is less severe. Hot spots are observed on weak cells indicating they are in reverse bias conditions and will degrade further with time. PSO parameter extraction from I-V curves revealed that the effect of module degradation of device parameters like series and shunt resistances. A module with cracked cells and degradation of the antireflective coating has low shunt resistance indicating current losses due to shunting. Similar shunting is observed in a module with delamination and moisture ingress. The extracted parameters are used to simulate the I-V curves of modules with reasonable fit. The fit could be improved around the “knee” of the I-V curve by improving the methods of parameter extraction. This study has shown the effects of cell mismatch on the performance and I-V curves of the PV modules. The different causes of cell mismatch are discussed and modules with different cell configuration and damage are characterised. The characterisation techniques used on each module provide information about the photoresponse, current generation, material properties and cell defects. A comprehensive understanding of these techniques allows the cell mismatch in the modules to be fully characterized.

Light Beam Induced Current (LBIC) measurement is a non-destructive technique used to perform localized characterization of solar cells using a light beam as a probe. The technique allows the determination of local photo response of a cell, the electrical parameters and defects that occur in the individual solar cell. The semiconductor materials used to create solar cells are not always defect free and these defects reduce the electrical performance of the device. It is therefore important to use a system that will allow the characterization and extract the solar cell parameters as can be done using the LBIC system. By analysing these parameters and cell defects, further studies can be done to enhance the cell’s lifetime and hence its efficiency. Light beam induced current (LBIC) is a technique that focuses light onto a solar cell device and thus creating a photo-generated current that can be measured in the external circuit for analyses. By scanning this beam probe across a solar cell while measuring the current-voltage characteristics, a map of various parameters can be obtained. This thesis presents the design of the LBIC system, the software interfacing of the data acquisition system and local photo-response within different solar cell technologies. In addition, this thesis represent two curve fitting algorithms namely: the Gradient Descent Optimisation and the Differential Evolution used for the extraction of solar cell device parameters. The algorithms are based on the one-diode solar cell model and make use of the light generated current-voltage (I-V) data obtained from the LBIC system. Different solar cell technologies namely; single crystalline (c-Si) and multicrystalline silicon (mc-Si) was used for analysis. LBIC maps and I-V characteristics of both technologies was obtained. The LBIC maps shows performance degrading defects present in the bulk and the surface of the solar cells as a function of spatial distribution. These localised defects acts as trapping mechanism for the charge carriers and therefore limits recombination within the solar cell and thus decreasing the performance of the solar cell device. The resulting I-V characteristics obtained from the LBIC system were used to determine the performance parameters using the two algorithms. The resultant effect of these parameters on the performance of the solar cells was observed. The overall results showed that LBIC is a useful tool for identifying and characterising defects in solar cells.

This thesis is dedicated to developing conjugated polymer and small-molecule donor materials for solution-processable organic solar cells. To begin with, a brief introduction of organic solar cells (OSCs) and an overview of donor materials development were presented in Chapter 1. In chapter 2, we used carbon-carbon triple bands as linkage of the TVT unit to develop a new building block, ATVTA. Small molecules S-03, S-04, and S-05 with ATVTA as building block showed broad absorption spectra and low-lying HOMO energy levels. S-01 with TVT unit and S-02 with AT2 as building block were also synthesized for clear comparison. OSCs devices based on S-01 and S-02 showed a Voc of 0.88 V and 0.89 V, respectively. The device based on S-03 exhibited a high Voc of 0.96 V, leading to a PCE of 2.19%. The devices based on S-04 and S-05 afforded a notable Voc over 1.0 V. The results demonstrate that ATVTA unit is a promising building block for extending π conjugation of the molecules without pulling up their HOMO energy levels. Chapter 3 focused on the development of 2D-conjugated small-molecule donor materials. The 2D-conjugated small molecule S-06 possesses excellent solution processability, broad absorption feature, respectable hole mobility and good film-forming morphology. The conjugated thiophene side chain not only effectively extends the absorption spectrum, but also lowers the HOMO energy level, which is desirable for obtaining high Voc. The BHJ OSCs based on S-06:PC70BM (1:0.5, w/w) afforded a high PCE of 4.0% and a notable FF of 0.63 without any special treatment needed. This preliminary work demonstrates that this kind of 2D-conjugated small molecules offer a good strategy to design new photovoltaic small molecule-based donor materials with high FF and Voc for high-efficiency OSCs. The consistently developed two 2D-conjugated small molecules S-07 and S-08 also possess low-lying HOMO energy levels. OSC device based on S-07:PC60BM (1:3, w/w) afforded a notable Voc of 0.96 V, with a PCE of 2.52%. BHJ devices based on S-08 will be fabricated and tested to investigate its photovoltaic properties in the near future. We developed a series of oligothiophenes with platinum(Ⅱ) as the building block in Chapter 4. These small metallated conjugated small molecules exhibited broad spectra and relatively low-lying HOMO energy levels in the range of –5.27 eV to –5.40 eV. Introducing platinum(Ⅱ) arylene ethynylenes as building block can be considered as an approach to obtain small-molecule donors with satisfactory absorption features and HOMO energy levels. Nevertheless, due to the low FF, the PCEs of these donor materials based devices are lower than 2%. Fine tuning the film morphologies of this kind of metallated small-molecule donor materials should be carried out to improve their photovoltaic performance. We addressed an efficient approach to improve the photovoltaic properties by side chain engineering in 2D-conjugated polymers in Chapter 5. Considering the fact that the Voc of PBDTTT based devices is less than 0.8 V, we introduced alkylthio substituent on the conjugated thiophene side chains of the 2D-conjugated copolymer to further improve the photovoltaic performance of the 2D-conjugated copolymers PBDTTTs. The weak electron-donating ability of the alkylthio side chains effectively down-shifted the HOMO energy level of PBDTT-S-TT by 0.11 eV in comparison to the corresponding polymer with alkyl substitution on the conjugated thiophene side chains. The PSC device based on PBDTT-S-TT showed an enhanced Voc of 0.84 V, which is among the highest one in the reported copolymers based on BDT and TT units, leading to an enhanced PCE of 8.42%. The results indicate that molecular modification by introducing alkylthio side chain will be a promising strategy to broaden the absorption, down-shift the HOMO energy level and increase the hole mobility of the low band gap 2D-conjugated polymers for further enhancing the photovoltaic performance of PSCs. PBDTT-O-TT-C and PBDTT-S-TT-C were developed to further study the conclusion. We found that OSC device based on PBDTT-S-TT-C with alkylthio side chain also demonstrated a high Voc of 0.89 V, with a PCE of 6.85% when processed with 3% DIO additive

Hybrid organic-inorganic perovskites has attracted much attention for its diverse optoelectronic applications. Many studies point out that hybrid organic-inorganic perovskites compounds have superior physical properties that can enable these materials to fabricate good performance solar cells. However, there is a lack of repeatable recipe for the fabrication of perovskite transistors with high mobilities. In this work, a detailed investigation has been conducted on the fabrication of Methylammonium-based perovskite compounds transistors on various polymer substrates. A group of methacrylate-based polymers has been chosen as the materials for gate dielectric layers. Generally, we found that the growth of perovskite crystals highly depends on the hydrophobicity of the substrates. More hydrophobic polymer layers yield larger crystal growth, but suppress the adhesion of perovskites crystals. Aromatic groups in methacrylate-based polymers have hydrophobic properties but it still gives better compact perovskite films with larger crystals. Poly(phenyl methacrylate) (PPhMA) enables the growth of the best perovskite films. The best performance of MAPbI3-xClx perovskite transistors was fabricated on PPhMA with an electron mobility µsat = 4.30 cm2 V−1 s−1 at 150 K. Photothermal deflection spectroscopy was used to investigate the subgap optical absorptions of the perovskite films.

The development of nonfullerene organic solar cells (OSCs) has attracted increasing interests because of the intrinsic advantages of nonfullerene acceptors, including their high absorption capability over the long wavelength region, tunable electronic properties, and excellent miscibility with polymer donors. Recently, power conversion efficiency (PCE) of >15 % for single-junction nonfullerene OSCs has been reported. Apart from the rapid progresses made in the cell efficiency, significant improvement in the stability of nonfullerene OSCs is required if the organic photovoltaic technology is to become a viable option for commercialization. The lifetime of OSCs is closely related to the intrinsic properties of the functional photoactive materials, e.g., the acceptors with suitable energy levels, morphology of bulk heterojunction (BHJ), formation of the active layer, interlayer engineering and device configuration. However, the comprehensive study of the impacts of the morphological properties and vertical phase separation in a BHJ on charge transport, built-in potential, charge recombination processes, PCE as well as the lifetime of nonfullerene OSCs has not been reported yet. This work has been focused on unraveling the stability of highly efficient OSCs using different nonfullerene acceptor/polymer blend systems, e.g., 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis (4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC): poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), ITIC:poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) -benzo[1,2-b:4,5- b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4', 5'-c'] dithiophene-4,8-dione)] (PBDB-T), and 3,9-bis(2-methylene-((3-(1,1 -dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene(IT-4F):poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T-2F). The lifetime of the nonfullerene OSCs has been analyzed systematically using a combination of morphology, photoelectron spectroscopy, light intensity-dependent current density-voltage measurements, transient photocurrent and aging studies. The effects of built-in potential (V0), charge extraction, and bimolecular recombination processes on the performance and stability of nonfullerene OSCs with regular and reverse configurations were studied. The results reveal that PTB7-Th:ITIC based OSCs with a reverse configuration are more favorable for efficient operation, due to the advantages of: (1) enhancement of charge collection by avoiding the holes passing through acceptor-rich region, which would otherwise occur in an OSC with a regular configuration, and (2) suppression of bimolecular recombination enabled by a higher V0. It shows that the PTB7-Th:ITIC based OSCs with a reverse configuration possess a slow degradation process, and >29% increase in PCE (8%) as compared to that of an optimized control OSC (6.1%). We found that a gradual decrease in V0 and hence the performance deterioration in the regular configuration PBDB-T:ITIC OSCs are caused mainly by the interfacial reaction between nonfullerene acceptor (ITIC) and poly(3,4-ethylenedioxythiophene) -poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer (HTL). The reduction in V0, due to the unavoidable interfacial reaction between ITIC and PEDOT:PSS at the BHJ/HTL interface in the OSCs, can be overcome through interfacial engineering, , e.g., introducing a thin molybdenum oxide (MoO3) passivation layer. The effect of the HTL on stability of PBDB-T:IT-4F based OSCs has been analyzed using different HTLs, e.g., a pristine PEDOT:PSS layer, a MoO3-doped PEDOT:PSS layer and a pure MoO3 layer. It shows that MoO3-induced oxidation doping of PEDOT:PSS favors the stable and efficient operation of nonfullerene OSCs. The results suggest that a stable and high V0 across the BHJ is a prerequisite for attaining high efficiency nonfullerene OSCs with long-term stability.

Bulk heterojunction organic solar cells (BHJ OSCs) have been fascinated in recent years for the future green energy generation due to their most promising results of low-cost fabrication, great flexibility, and lightweight properties. Very recently small molecule donors in the BHJ active layers have shown prominent attention due to the synergistic advantages over the polymer counterparts, which possess easy purification, highly facile synthesis, and negligible batch-batch variations. To construct push-pull molecules for p-type semiconductors, acceptor-donor-acceptor (A-D-A) based backbone exalted so far. In addition, the most impressive small molecule electron-donor units (D) are like benzodithiophene (BDT), oligothiophene, 3-dithienosilole (DTS), and indacenedithiophene (IDT) and so on. Likewise, electron-acceptors (A), such as 3-alkylrhodanine, diketopyrrolopyrrole (DPP), and perylenediimide (PDI) have been utilized. Porphyrin derivatives show excellent photochemical and electrochemical properties. Interestingly, porphyrins can be easily modified by different substituents at the peripheral positions (meso- and β-) and metal insertions at the center of the porphyrin core. In this work, we design, synthesize and characterize visible-near infrared absorbing new porphyrin dimer based small molecules with acceptor-donor-acceptor (A-D-A) configuration for bulk heterojunction organic solar cells, and investigate their structure-property relationships, specifically the effect of conjugation and planarity of the backbone central units on the charge mobility, film morphology, and solar cell performances. Chapter 1 deals with an overview of the past and recent development of BHJ OSCs, particularly the key principles and photovoltaic characteristics. Furthermore, we focus on the detailed classification of porphyrin-based small molecules and their performances in OSCs. In chapter 2, two promising near-IR absorbing porphyrin-based dimeric small molecules were designed and synthesized, in which diketopyrrolopyrrole-ethynylene-bridged porphyrin dimers are capped with electron-deficient 3-ethylrhodanine (A2) via a π-bridge of phenylene ethynylene, with an optimal A2-π-D-A1-D-π-A2 architecture affording porphyrin dimers DPP-2TTP and DPP-2TP. They possess strong absorption in ranges of 400-550 (Soret bands) and 700-900 nm (Q bands). Their intrinsic absorption deficiency between the Soret and Q bands could be perfectly compensated by a wide bandgap small molecule DR3TBDTTF with absorption in 500-700 nm. Impressively, the optimal ternary device based on the blend films of DPP-2TPP, DR3TBDTTF (20 wt.%) and PC71BM, shows a PCE of 11.15%, while the binary devices based on DPP-2TTP/PC71BM and DPP-2TP/PC71BM blend films exhibit PCEs of 9.30% and 8.23%, respectively. The high compatibility of the low bandgap porphyrin dimers with the wide bandgap small molecule provides a new threesome with PC71BM for highly efficient panchromatic ternary organic solar cells. Chapter 3 describes another two new A-π-D-π-A structural porphyrin small molecules denoted as TDPP-2P and TDPPE-2P which are constructed from dimeric porphyrin linked by 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPP), and 2,5-bis(2-butyloctyl)-3,6-bis(5-ethynyl-2-thienyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPPE), respectively, further π-extended symmetrically with electron-deficient 4-[(3-ethyl-4-oxo-2-thioxo-5-thiazolidinylidene)methyl]-phenylethynyl fragments. Compared to the absorption spectra of TDPP-2P, astonishingly TDPPE-2P improves the range of near-infrared over 1000 nm due to the enhanced coplanarity of the central core. Moreover, the intrinsic absorption deficiency (500-700 nm) is perfectly compensated by IT-M small molecule acceptor. Remarkably the blend film TDPPE-2P:IT-M accomplished panchromatic photo-current absorption from 400-900 nm, as a result, the device exhibits a prominent PCE of 5.69%. Whereas, the film TDPP-2P:IT-M shows comparatively low PCE of 4.12%. Finally, we believe that such a combination of TDPPE-2P:IT-M device demonstrates synergetic compatibility of donor/acceptor domain to promote the complementary absorption spectrum and enhances through higher hole mobilities and better crystallinity of the surface and interface for non-fullerene small-molecule organic solar cells. In Chapter 4, we further modified and synthesized a new series of A*-π-D2-D1-D2-π-A* based porphyrin dimer (2P) (D2) small molecules flanked by 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-2,6-diethynylbenzo[1,2-b:4,5-b']dithiophene (TBDTE) and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (TBDT) as TBDTE-2P and TBDT-2P respectively, in which benzodithiophene (BDT) (D1) based analog was constructed as a central unit because of extended coplanarity conjugation length. Finally, TBDTE and TBDT units end-capped with 3-ethylrhodanine (A*) via a π-bridge of phenyl ethynyl linker and 2-octyldodecan-1-al long alkyl chain was used on vertical meso-porphyrins to improve the material solubility for the solution-processed OSCs. The compound TBDTE-2P accomplishes absorption range from 400-800 nm in the vis-near-infrared region, whereas TBDT-2P compound absorbs 400-700 nm range. The higher absorption range of TBDTE-2P arises from more planar backbone orientation and strong intramolecular charge transfer (ICT) within the donor molecules. Further, we focus on the OPV performances of binary devices TBDTE-2P / TBDT-2P: IDIC under AM 1.5G 1-Sun and 300 lux LED illuminations. The champion device TBDTE-2P: IDIC was accumulated a PCE of 7.46% under 1-Sun whereas a PCE of 12.34% was obtained under indoor light illuminations. The exploit of superior properties, charge generation and collection, hole and electron mobilities, and atomic force microscopy (AFM) were also examined. In Chapter 5, we synergistically designed and synthesized two new porphyrin dimers triply fused at meso-meso, β-β and βꞌ-βꞌ positions, from the corresponding meso-meso singly-linked porphyrin arrays. These fused porphyrin tapes differ by two metal atoms at the porphyrin core, such as zinc and nickel, termed as F-C19ZnP and F-C19NiP, respectively. With the purpose for design new acceptor-donor-acceptor small molecules for OSCs, the two fused porphyrin tapes were investigated in detail on the photophysical and electrochemical properties. Both fused porphyrins exhibit a strong and wide Soret-band absorption from 400-570 nm. Interestingly, the compound F-C19ZnP is recorded a larger red-shift absorption than the compound F-C19NiP consistent with cyclic voltammetry (CV) measurements, because the Zn-porphyrin attains more planar conjugated geometry. Finally, the dissertation was completed with a summary in chapter 6

Bulk heterojunction organic solar cells (BHJ OSCs) have been fascinated in recent years for the future green energy generation due to their most promising results of low-cost fabrication, great flexibility, and lightweight properties. Very recently small molecule donors in the BHJ active layers have shown prominent attention due to the synergistic advantages over the polymer counterparts, which possess easy purification, highly facile synthesis, and negligible batch-batch variations. To construct push-pull molecules for p-type semiconductors, acceptor-donor-acceptor (A-D-A) based backbone exalted so far. In addition, the most impressive small molecule electron-donor units (D) are like benzodithiophene (BDT), oligothiophene, 3-dithienosilole (DTS), and indacenedithiophene (IDT) and so on. Likewise, electron-acceptors (A), such as 3-alkylrhodanine, diketopyrrolopyrrole (DPP), and perylenediimide (PDI) have been utilized. Porphyrin derivatives show excellent photochemical and electrochemical properties. Interestingly, porphyrins can be easily modified by different substituents at the peripheral positions (meso- and β-) and metal insertions at the center of the porphyrin core. In this work, we design, synthesize and characterize visible-near infrared absorbing new porphyrin dimer based small molecules with acceptor-donor-acceptor (A-D-A) configuration for bulk heterojunction organic solar cells, and investigate their structure-property relationships, specifically the effect of conjugation and planarity of the backbone central units on the charge mobility, film morphology, and solar cell performances. Chapter 1 deals with an overview of the past and recent development of BHJ OSCs, particularly the key principles and photovoltaic characteristics. Furthermore, we focus on the detailed classification of porphyrin-based small molecules and their performances in OSCs. In chapter 2, two promising near-IR absorbing porphyrin-based dimeric small molecules were designed and synthesized, in which diketopyrrolopyrrole-ethynylene-bridged porphyrin dimers are capped with electron-deficient 3-ethylrhodanine (A2) via a π-bridge of phenylene ethynylene, with an optimal A2-π-D-A1-D-π-A2 architecture affording porphyrin dimers DPP-2TTP and DPP-2TP. They possess strong absorption in ranges of 400-550 (Soret bands) and 700-900 nm (Q bands). Their intrinsic absorption deficiency between the Soret and Q bands could be perfectly compensated by a wide bandgap small molecule DR3TBDTTF with absorption in 500-700 nm. Impressively, the optimal ternary device based on the blend films of DPP-2TPP, DR3TBDTTF (20 wt.%) and PC71BM, shows a PCE of 11.15%, while the binary devices based on DPP-2TTP/PC71BM and DPP-2TP/PC71BM blend films exhibit PCEs of 9.30% and 8.23%, respectively. The high compatibility of the low bandgap porphyrin dimers with the wide bandgap small molecule provides a new threesome with PC71BM for highly efficient panchromatic ternary organic solar cells. Chapter 3 describes another two new A-π-D-π-A structural porphyrin small molecules denoted as TDPP-2P and TDPPE-2P which are constructed from dimeric porphyrin linked by 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPP), and 2,5-bis(2-butyloctyl)-3,6-bis(5-ethynyl-2-thienyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPPE), respectively, further π-extended symmetrically with electron-deficient 4-[(3-ethyl-4-oxo-2-thioxo-5-thiazolidinylidene)methyl]-phenylethynyl fragments. Compared to the absorption spectra of TDPP-2P, astonishingly TDPPE-2P improves the range of near-infrared over 1000 nm due to the enhanced coplanarity of the central core. Moreover, the intrinsic absorption deficiency (500-700 nm) is perfectly compensated by IT-M small molecule acceptor. Remarkably the blend film TDPPE-2P:IT-M accomplished panchromatic photo-current absorption from 400-900 nm, as a result, the device exhibits a prominent PCE of 5.69%. Whereas, the film TDPP-2P:IT-M shows comparatively low PCE of 4.12%. Finally, we believe that such a combination of TDPPE-2P:IT-M device demonstrates synergetic compatibility of donor/acceptor domain to promote the complementary absorption spectrum and enhances through higher hole mobilities and better crystallinity of the surface and interface for non-fullerene small-molecule organic solar cells. In Chapter 4, we further modified and synthesized a new series of A*-π-D2-D1-D2-π-A* based porphyrin dimer (2P) (D2) small molecules flanked by 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-2,6-diethynylbenzo[1,2-b:4,5-b']dithiophene (TBDTE) and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (TBDT) as TBDTE-2P and TBDT-2P respectively, in which benzodithiophene (BDT) (D1) based analog was constructed as a central unit because of extended coplanarity conjugation length. Finally, TBDTE and TBDT units end-capped with 3-ethylrhodanine (A*) via a π-bridge of phenyl ethynyl linker and 2-octyldodecan-1-al long alkyl chain was used on vertical meso-porphyrins to improve the material solubility for the solution-processed OSCs. The compound TBDTE-2P accomplishes absorption range from 400-800 nm in the vis-near-infrared region, whereas TBDT-2P compound absorbs 400-700 nm range. The higher absorption range of TBDTE-2P arises from more planar backbone orientation and strong intramolecular charge transfer (ICT) within the donor molecules. Further, we focus on the OPV performances of binary devices TBDTE-2P / TBDT-2P: IDIC under AM 1.5G 1-Sun and 300 lux LED illuminations. The champion device TBDTE-2P: IDIC was accumulated a PCE of 7.46% under 1-Sun whereas a PCE of 12.34% was obtained under indoor light illuminations. The exploit of superior properties, charge generation and collection, hole and electron mobilities, and atomic force microscopy (AFM) were also examined. In Chapter 5, we synergistically designed and synthesized two new porphyrin dimers triply fused at meso-meso, β-β and βꞌ-βꞌ positions, from the corresponding meso-meso singly-linked porphyrin arrays. These fused porphyrin tapes differ by two metal atoms at the porphyrin core, such as zinc and nickel, termed as F-C19ZnP and F-C19NiP, respectively. With the purpose for design new acceptor-donor-acceptor small molecules for OSCs, the two fused porphyrin tapes were investigated in detail on the photophysical and electrochemical properties. Both fused porphyrins exhibit a strong and wide Soret-band absorption from 400-570 nm. Interestingly, the compound F-C19ZnP is recorded a larger red-shift absorption than the compound F-C19NiP consistent with cyclic voltammetry (CV) measurements, because the Zn-porphyrin attains more planar conjugated geometry. Finally, the dissertation was completed with a summary in chapter 6

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