Discotic liquid crystal is one kind of self-assembled material, liquid crystal with regular alignment could be applied to many kinds of organic electro-optical devices. This novel discotic liquid crystal polymer DLC-PAM, we used polyacrylamide(PAM) as main chain of the novel discotic liquid crystal polymer DLC-PAM and grafted the discotic liquid crystal monomer Acid-6 onto PAM by chemical synthesis. There were two parts in this study, first we investigated the electro-optical properties of DLC-PAM. Observation the aligned property of DLC-PAM during temperature variation utilizing polarizing optical microscope, DLC-PAM exhibited the columnar alignment. We observed hexagonal columns which DLC-PAM aligned by X-ray diffraction, it benefited the carrier transporting. The absorption spectrum of DLC-PAM presented an absorption peak at 409 nm certifying that DLC-PAM could absorb the visible light. We measured the HOMO potential of DLC-PAM which is 5.47eV by PESA. Form absorption spectrum we calculated the band-gap of DLC-PAM which was 2.55~2.82eV, and then we used the HOMO potential and the band-gap to calculate the LUMO potential of DLC-PAM to be 2.65~2.92eV. The second part in this study we applied DLC-PAM to the organic solar cell. Because of the energy level of DLC-PAM and the different device structure tests we realized that DLC-PAM was suitable to be hole transporting layer. The device structure we used was ITO/PEDOT:PSS/DLC-PAM/P3HT:PCBM/Al, DLC-PAM layer was added between the PEDOT:PSS layer and the active layer. The power conversion efficiencies proved that DLC-PAM layer which benefited hole transporting raising the power conversion efficiency of the solar cell. The power efficiency of the device added DLC-PAM layer raised 16.2% comparing with the standard device.

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In this research we produce a multi-layer Dye-Sensitized Solar Cell (DSSC) and formulate electrolyte to reduce electric leakage. In general, DSSC compound from FTO/ dense layer/ porous layer with Dye / electrolyte / counter Pt electrode. In this study, we use commercial dye Ruthenium N719, and own Lab-synthesized Coumarin series as dye. Ordinary DSSC use singular size TiO2 and mono-layer as active layer, but we demonstrate a multi-layer and multi-scale TiO2 particle of DSSC for increasing IPCE (incident photon-to- electron conversion efficiency). Compare with standard mono-layer DSSC, multi-layer DSSC has successful gotten promotion about 15%. We use FTO (SnO2:F) as substrate, because after annealing it has low resistance, and it is better to anti-erosion from electrolyte compare with ITO. Ruthenium N719 absorb photon to generate exciton, that separate off into electron and hole. Electron deliver to the FTO substrate through TiO2. But electrons also can deliver to electrolyte result in electron leakage. In order to decrease electron leakage, one solution is to mixed electrolyte with some chemicals. For example, tert-butylpyridine (TBP), that can adhere to sphere of TiO2 reducing electron leakage and promote the IPCE. Because of liquid state of electrolyte is hard to seal DSSC. Our future work is making gel-state electrolyte and improving its efficiency.

A study has been made of a variety of factors influencing the efficiency and operational stability of front-wall CdS-Cu2S solar cells. In the course of this work -1 cm2 cells were fabricated with conversion efficiency of up to 8% without attempting to reduce reflection losses.The CdS films were produced by vacuum evaporation and the electrical and structural characteristics of these films were studied as a function of the rate and temperature of the deposition. Previously there had been some controversy concerning the nature of the CdS source material required for fabricating high performance CdS-based solar cells, but this work has shown that a variety of CdS sources can be employed successfully provided that the film deposition parameters are suitably chosen.A conventional chemical exchange technique was employed to convert the CdS film surface to Cu2SI with the thickness and stoichiometry of the resultant Cu2S layer being examined by means of electrochemical analysis.Changes in the electrical properties of the CdS-Cu2S cells due to post- fabrication anealing under a variety of different conditions were studied and correlated with structural changes monitored by means of Auger electron spectroscopy with the aid of argon ion etching. Depth profiles of the constituent element concentrations indicate that, for samples annealed in air, a deep penetration of copper into the CdS layer occurs together with a significant out-diffusion of cadmium from the CdS after only a few minutes at 1000C. In contrast, the copper penetration which results from vacuum or hydrogen annealing treatment is substantially less and no significant out-diffusion of cadmium is observed for annealing temperatures up to 4000C. Two different diffusion processes, one in the grain boundaries and one in the mid-grain regions, have been identified and their relative importance has been studied for annealing cycles performed under the same three different ambient atmospheres (air, vacuum or hydrogen). The normally rapid and undesirable grain boundary diffusion of copper was found to be significantly inhibited by the use of flowing hydrogen during annealing. A further technologically important observation concerns the effect of the deposition of a film of copper over the copper sulphide layer of a cell and subsequent annealing of it in air. The improved electrical stability which this treatment yields has been shown to be directly associated with reduced interdiffusion at the CdS-Cu2S interface. This interfacial diffusion has also been shown to be influenced by the CdS stoichiometry in the vicinity of the junction.Finally, a brief investigation was made into the use of the ion implantation technique as a means of doping the upper layer of the OdS film with copper without annealing the completed cell. The results have demonstrated the feasibility of this technique, with the best results being obtained using a copper ion fluence of 5.1014 ions cm-2 at 50 keV ion energy.

The work presented in this thesis is concerned with photovoltaic cells formed by plating CdS single crystals and thin films, and Cd(_y) Zn(1 _ y)S single crystals, with copper sulphide. An electroplating technique has been used to control the phase of copper sulphide by changing the electric field during its formation. Different phases of Cu(_x)S have been identified directly using Reflection High Energy Diffraction (RHEED), and indirectly from spectral response measurements. A dramatic change in the spectral response accompanying the reduction in the covellite response associated with an increase in that from chalcocite following argon heat treatment has been achieved. The change from the djurleite phase to that of chalcocite has also been obtained by using argon heat treatment for 5 minutes at 200 C. This effect was found to be reversible in that layers of chalcocite were converted to djurleite when air was used as the ambient for the heat treatment. C-V measurements have demonstrated that with increasing plating bias the donor concentration decreases at first before it assumes a constant value. This led to the effect of decreasing the junction capacitance as the width of the depletion region changed. The problem of the stability of the CdS-Cu(_2)S photovoltaic devices formed by wet plating" is addressed by studying the combined effects of the substrate onto which the CdS is deposited and the ambient used during annealing. Thin film cells have been prepared on both Ag/Cr and SnO substrates, and the device characteristics for each have been investigated as a function of annealing ambient. The results have shown that devices formed on Ag/Cr substrates were more stable following annealing in air than in argon, while the converse was true for cells fabricated on SnO(_x) substrates. The degradation effects of CdS-Cu(_2) S photovoltaic cells have been investigated. While devices stored in the dark showed little or no degradation, those maintained under illumination exhibited a significant deterioration in all operational parameters over a four week period. As far as the combined effect of temperature and ambient on the stability of cells are concerned, it was found that the ageing of devices in argon at room temperature in the dark was negligible, and moreover the fill factor was observed to improve marginally. When the devices were stored in the same ambient conditions at 50 C, they showed a significant improvement in the fill factor, but simultaneously exhibited a considerable reduction in the short circuit current. This process was reversible, since the sensitivity of degraded devices could be restored by annealing them in a hydrogen/nitrogen mixture. By comparing Electron Spectroscopy for Chemical Analysis (ESCA) studies with solar cell device characteristics, it has been shown that the formation of copper oxide on the Cu(_2)S surface plays a significant role in the degradation of CdS-Cu(_2) S devices. The extent of the cross-over between the dark and light J-V characteristics is a function of the period of etching used prior to junction formation. The variation of current and diode factor has been established as a function of the bias value. The dependence of forward current on the temperature at fixed forward voltage has also been investigated. Finally this work has shown that an increase in V(_oc) can be achieved when Cd(_0◦8)Zn(_0◦2)S is used as a base material for solar cells instead of CdS. Different traps were identified through a photocapacitance investigation. An important trap was found at 0.78eV below the conduction band. It has been demonstrated that the effect of this level was found to be diminished much more slowly when the annealing was carried out in argon rather than in air. This level may play an important role in the Cd(0◦8) Zn(0◦2)S-Cu(_2)S solar cell properties.

With bandgap ranging from 1.74 to 2.67eV depending on composition, the ternary alloy (ZnCd)Se is an interesting system for optoelectronic applications. The main purpose of the work reported in this thesis was to characterise some of the electrical properties of crystals of Zn(_x) Cd(_1-x) Se and to assess the " potential in CdSe/Cu(_2)Se and zn(_x)Cd(_1-x)Se/Cu(_2)Se photovoltaic cells. Single crystals of this ternary compound have been grown from the vapour phase using two different methods. With each technique boules of graded composition were produced with the Cd/Zn ratio decreasing towards the end that was last to grow. The variation in composition was determined using atomic absorption spectroscopy and energy dispersive X-ray analysis. Lattice Parameters were determined using X-ray diffractometry and were found to vary linearly with composition over a wide range. This study showed that for x < 0.5 the crystal adopts the hexagonal Wurtzite structure, changing to the cubic sphalerite for higher values of x. The variation is bandgap energy with composition was determined for single crystals of Zn(_x)Cd(1-x)Se at 300K and 9OK and shows that the bandgap changes quadratically in x for x < 0.6. The barrier heights of Aūzn cd(_1-x)sc (x < 0.45) Schottky diodes were calculated from forward I-V characteristics, C-V and photoelectric measurements were also carried out. A good linear relationship with composition was obtained for barrier heights measured by the Photoelectric method. Deep levels were also investigated in these diodes using Photocapacitance, which revealed the presence of two dominant levels having activation energies of 0.4 - 0.5 eV and 0.9 - 1.0 eV (referred to the valence bandedge) that were independent of the composition The second part of the thesis described an investigation into CdSe/CU(_2)Se and Zn(_x)Cd(_1-x)Se/CU(_2)Se (x < 0,4) devices that had been prepared on orientated single crystal substrates by a chemiplating technique. Reflection high energy diffraction (RHEED) showed that the structure of the CU(_2)Se layer took the cubic modification. Cells formed on as-grown low resistivity substrates exhibited no rectification. However good Photovoltaic properties were produced by heating the devices in air or Argon at 200 C, However, for cells formed on higher resistivity CdSe, the resultant devices showed a Photovoltaic effect without any heat treatment. The Photovoltaic output characteristics were measured under simulated AMI illumination. The properties of the Photovoltaic cells prepared on Zn(_x)Cd(1-x)Se single crystals are closely related to those of devices fabricated on CdSe substrates. Cells formed on CdSe were found to have higher short circuit current densities (J(_sc)), but lower open circuit voltage (V(oc)) than those produced on the mixed Zn(_x)Cd(_1-x)Se crystal substrates. Thus the open circuit voltage was increased with zinc content to 420 mv with a Zng(_0.4)Cd(_0.6)Se based cell. However, there was a considerable decrease in the short circuit current. The characterisation of these cells has revealed the main threshold in all the devices Indicated a dominant level with an activation energy of between 1.0 and 1.1 eV with respect to the conduction band in both CdSe and Zn(_x)Cd(_1-x)Se.

New technologies will require more and more compliant materials capable of conforming to curved surfaces, i.e., able to stretch and mechanically resist body motions for wearable and on-skin applications. In this regard, this work discusses strategies to induce stretchability in materials. We focused our attention on improving the elasticity of transparent conducting electrodes (TCE) based on PEDOT:PSS and semiconductors (active layer) for organic solar cells. Firstly, the introduction of DMSO and Zonyl as additives into PEDOT:PSS was shown to produce highly transparent conducting electrodes (FoM > 35) with low Young’s modulus and high carrier density. We investigated the relationship between the transport properties of PEDOT:PSS and the morphology and microstructure of its films. The combination of the two additives enhances the fibrillary nature and the aggregations of both PEDOT and PSS components of the films. Secondly, stretchable TCEs based on PEDOT:PSS were fabricated using an innovative approach that combines an interpenetrated polymer network-based on polyethylene oxide and Zonyl. The presence of three-dimensional matrix provided high electrical conductivity, elasticity, and mechanical recoverability. The potential of this electrode was demonstrated with indium-tin-oxide (ITO)-free solar cells with a power conversion efficiency similar to ITO. Finally, the research was completed by integrating a cross-linker or an elastomer into the active layer to enhance its stretchability while maintaining excellent photovoltaic performance. In particular, SEBS elastomer exhibited a tailored elasticity with various fullerene and non-fullerene blends: P3HT:PC61BM, PCE10:PC71BM and PCE13:IT-4F. This versatile approach highlights the ease of manufacturing and scalability achieved by the solution casting processes along with a high compatibility of acceptor and donor blends.

Lead halide perovskite solar cells (PSCs) are considered the fastest growing photovoltaic technology, reaching an outstanding certified power conversion efficiency of 24.2% in just 10 years. The best performing PSCs are based on polycrystalline films, where the presence of grain boundaries and ultra-fast crystallization limit the further development of their performance by increasing the bulk and surface defects. Compared to their polycrystalline counterparts, single crystals of lead halide perovskites have been shown to possess much lower trap-state densities and diffusion lengths exceeding 100𝜇m. In this thesis, using a solution space-limited inverse temperature crystallization method, twenty-microns thick single crystals of MAPbI3 are grown directly on the charge selective contact to construct highly reproducible p-i-n inverted type solar cells with fill factors(FF) as high as 84.3% and power conversion efficiencies (PCEs) exceeding 21% under 1 sun illumination (AM 1.5G). A key requisite for high PCEs is avoiding surface hydration, in which moisture attacks the perovskite/transporting layer interface and causes a significant decrease in short-circuit current. These solar cells set a record for single crystal PSCs, and highlight the potential of single crystal PSCs in furthering perovskite photovoltaic technology.

This thesis presents advancements on the modeling of quantum dots using Fourier-space k.p theory and on the use of InGaN quantum dots for ratchet band solar cells. Fourier-space based methods have generally assumed sharp material interfaces for electronic structure, strain and piezoelectric potential calculations in quantum dot systems. Additionally, standard Fourier-space methods have often assumed uniform elastic and dielectric constants for the strain and piezoelectric potential calculations. We present generalized methods to include smoothly varying alloy profiles for the quantum dots, including spatially varying elastic and dielectric constants for the strain and piezoelectric potential calculations. For the case of InGaN/GaN quantum dots, we show that the elastic and dielectric constants corrections are important for accurate strain, piezoelectric potentials, and electronic structure. The smooth alloy profiles are constructed by convolving sharp alloy profiles with a Gaussian, and we show that the electronic structure strongly depends on the smoothing kernel, indicating the need for precise alloy profiles for accurate electronic structures. We also present a new method that facilitates the coupling of strain into the k.p Hamiltonian when considering isolated dots, greatly reducing the computational costs of calculating the Hamiltonian matrix elements. Using the methods, we investigate the use of InGaN/GaN quantum dot superlattices as ratchet band solar cells, where we propose to use the piezoelectric potential to generate a ratchet. The piezoelectric potential can spatially separate confined electron and hole states, creating a spatial ratchet in order to reduce recombination. From our quantum dot k.p model, we calculate optical light absorption cross sections and present an improved method to calculate bound-to-continuum absorption, where electrons are excited out of the dots. In this method, we approximate the continuum states as bulk k.p states for GaN. By coupling our k.p model absorptions into a detailed balance model, we predict power conversion efficiencies. We find that a large number of QD layers is necessary to achieve sufficiently strong absorption as to reach high efficiencies, highlighting one of the key issues of QD-based solar cells. We consider systems which consists of up to 131000 layers of quantum dots and an ideal Lambertian back reflector with two different QD geometries. The first geometry possesses a strong spatial ratchet and can reach a maximum efficiency of 36% under a 1-sun 6000 K black-body spectrum. We verify the existence of the spatial ratchet through the optical properties of the system, showing that it truly has the potential to block recombination. We also present an efficiency optimized system that reaches a 42% detailed balance efficiency but does not have a spatial ratchet.

Wide-bandgap metal halide perovskite solar cells have become an alluring next-generation solar panel technology because of their simple manufacturing and rising efficiencies by up to 25.7%. When the single junction devices face the ultimate S-Q limit, the incorporation of wide-bandgap perovskite materials with low-bandgap absorbers to form multi-junction cells offers a promising route to surpass the theoretical efficiency. Monolithic perovskite-organic tandem cells are appealing among other compositions owing to the combination of the sub-cells advantages: low-cost, flexibility, and solution processing. In this work, we focused on optimizing the hole transporting materials (HTMs) separately for the two components in tandem devices. In the 1.76 eV perovskite subcell, three commonly seen HTMs are selected (2PACz, NiOx and PTAA) to investigate the influence on device performance. An MgF2 interlayer at perovskite/C60 is deposited as passivation to enhance the voltage and overall performance. It is found that 2PACz is most suitable for triple cation FA0.7MA0.15Cs0.15Pb(I0.6Br0.4)3, giving good crystallinity, energy match and absorption with a champion PCE of 16.12%. Then, we performed a similar optimization for ternary PM6: BTP-eC9: PC70BM with MoOx, MoOx/2PACz, and PEDOT: PSS as HTMs, where MoOx/2PACz present the best statistics. Finally, two terminal tandem devices were fabricated based on the two optimized subcells, and a promising efficiency of 23.6% and a Voc of 2.09V were reached free of hysteresis. More passivation methods or perovskite bandgap engineering are expected to further improve the performance and break the record.