Impact of In-Situ Cd Saturation MOCVD Grown CdTe Solar Cells on As Doping and VOC

In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic chemical vapour deposition at a low temperature of 350 °C, to investigate the impact on As doping and device VOC. Device characterization showed conversion efficiency of ∼14%, and VOC of 772 mV, which is an improvement to the baseline device with CdTe:As absorber layer grown at 390 °C under non-saturated conditions. When the low temperature Cd-saturated growth was combined with chlorine heat treatment at a higher temperature of 440 °C (in contrast with the standard 420 °C) for 10 min, device efficiency improved to ∼17% with a high VOC of 877 mV. As a result, ∼100 mV boost in VOC from baseline is demonstrated with Cd-saturated CdTe:As device. Micro-photoluminescence and time-resolved photoluminescence measurements performed on these Cd-saturated CdTe:As devices confirmed that minority carrier lifetime significantly improved

Investigations of Thermally Induced Morphology in P3HT/PCBM Thin Films: Influence of Composition and Thermal Annealing on Photovoltaic Properties

Organic solar cells based on P3HT: PCBM bulk heterojunctions show promise for high power conversion efficiencies. The properties of composite donor polymers and acceptor materials play a significant role; hence the need for optimised bulk heterojunctions active layer morphologies is critical for efficient devices. To achieve optimised bulk heterojunctions, compositional factors and processing conditions are two primary aspects to focus on. Thermal annealing has been demonstrated to be one of the most successful processing techniques for morphology optimisation in P3HT – based organic solar cells. However the crucial correlation between composite composition and thermal annealing in P3HT – based devices is not fully understood yet. Combining optical absorption spectroscopy, structural and electrical methods; the properties of P3HT: PCBM blend thin films, with different PCBM percentage weight ratios were studied in this work. Optical absorption spectra results for all three blend ratios, i.e., 1:1, 1:0.8, and 1:0.6, showed that the peak absorption intensity associated with PCBM reduced the most for the 1:1 ratio, after thermal annealing at 175°C. The impact of the correlation between PCBM composition and thermal annealing on photovoltaic performance parameters was demonstrated. For the three different PCBM compositions, the optimum power conversion efficiencies were determined at different optimum thermal annealing conditions. Optimum power conversion efficiency of 3.38% (1:1) was obtained at 175°C, whilst 2.27% (1:0.8) and 1.44% (1:0.6) were demonstrated at 125°C respectively. To further probe the influence of thermally induced PCBM molecular segregation and aggregations, three different thermal annealing strategies were employed; namely, annealing (i) gradually from 50°C – 175°C, in steps of 25°C, 10 minutes each (ii) at high temperature 175°C, for 10 minutes once, and (iii) at 175°C for a longer time, i.e., 60 minutes . Optical absorption spectroscopy results reveal the dependence of PCBM aggregation on different thermal annealing strategies. Employing Raman spectroscopy mapping methods, the surface of thin films were mapped revealing and confirming PCBM rich regions upon thermal annealing. Furthermore exciton generation rate studies proved useful in establishing a good correlation between the estimated excitons generated, with short circuit current densities. The observed increase in excitons generated was also consistent with the photoluminescence spectra results which showed an enhancement in intensities upon thermal annealing. ii Importantly, this work has shown the significance of employing different thermal annealing strategies in nanomorphology control of the bulk of active layers of organic photovoltaic devices. Importantly, it has also been demonstrated in the work of this thesis that gradual thermal annealing, in a controlled manner revealed a more stable and efficient control in tuning the nanomorphologies of P3HT – based solar cells. In addition, impedance spectroscopy and capacitance – voltage measurement techniques have been shown to be very useful tools for characterising organic photovoltaic devices. Herein, it was shown that after thermal annealing at the optimum temperature of 150°C, impedance spectroscopy characterisation revealed extended charge carrier lifetimes in devices. This highlights the significance of having an optimised interpenetrating network within active layers of organic solar cells, as this have a critical impact on charge carrier lifetimes. Capacitance – voltage measurements was used to demonstrate the thermally induced vertical segregation of PCBM molecular aggregates also. The decrease in measured built – on potential from 0.68V (at film/cathode interface) from as cast device to about 0.35V after thermal annealing at 150°C, was shown to be indicative of vertical segregation

Correlation between charge carriers mobility and nanomorphology in a blend of P3HT/PCBM bulk heterojunction solar cell: Impact on recombination mechanisms

Sandwich configurations consisting of hole-only and electron-only devices are employed to study the transport properties of polymer/fullerene blended material. Upon thermal annealing at 150 °C and subsequent device characterisations a direct correlation between solar cell device power conversion efficiency and charge carrier mobilities is identified. The highest power conversion efficiency reported herein resulted from the electron to hole mobility ratio of 4.78. This ratio further reduces to 0.357 at 175 °C as a result of the deterioration of electron mobility at the same time as enhancement in hole carrier mobility with no further improvement in PCE. Effects of thermal annealing on trap density reveal that the trap assisted recombination is more dominant as compared to the bimolecular recombination. Furthermore such a mechanism enhances as a result of increasing trap density at higher annealing temperatures. The effect of light intensity on the open circuit voltage VOC further verifies that the dominant recombination process is through trap assisted recombination

Properties of Arsenic–Doped ZnTe Thin Films as a Back Contact for CdTe Solar Cells

As-doped polycrystalline ZnTe layers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated as a back contact for CdTe solar cells. While undoped ZnTe films were essentially insulating, the doped layers showed significant rise in conductivity with increasing As concentration. High p-type carrier densities up 4.5 × 1018 cm−3 was measured by the Hall-effect in heavily doped ZnTe:As films, displaying electrical properties comparable to epitaxial ZnTe single crystalline thin films in the literature. Device incorporation with as-deposited ZnTe:As yielded lower photovoltaic (PV) performance compared to reference devices, due to losses in the open-circuit potential (VOC) and fill factor (FF) related to reducing p-type doping density (NA) in the absorber layer. Some minor recovery observed in absorber doping following a Cl-free post–ZnTe:As deposition anneal in hydrogen at 420 °C contributed to a slight improvement in VOC and NA, highlighting the significance of back contact activation. A mild CdCl2 activation process on the ZnTe:As back contact layer via a sacrificial CdS cap layer has been assessed to suppress Zn losses, which occur in the case of standard CdCl2 anneal treatments (CHT) via formation of volatile ZnCl2. The CdS sacrificial cap was effective in minimising the Zn loss. Compared to untreated and non-capped, mild CHT processed ZnTe:As back contacted devices, mild CHT with a CdS barrier showed the highest recovery in absorber doping and an ~10 mV gain in VOC, with the best cell efficiency approaching the baseline devices

Development of arsenic doped Cd(Se,Te) absorbers by MOCVD for thin film solar cells

Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm2. The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells

Development of a Wireless Sensor Node for Building Information Management Systems

An increasing number of internet of things (IoT) devices are being deployed long term and therefore need to be self-powered in order to reduce maintenance costs. This paper reports on the design and implementation of a low power wireless sensor node for use in a building information management system powered by an organic solar module. Detailed analysis of the power requirements of the various sensors and the methods used to reduce the power consumption are given. The suitability of organic photovoltaic modules for indoor energy harvesting is examined. Early results from the deployment of these modules are shown

Impact of In-Situ Cd Saturation MOCVD Grown CdTe Solar Cells on As Doping and VOC

In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic chemical vapour deposition at a low temperature of 350 °C, to investigate the impact on As doping and device VOC. Device characterization showed conversion efficiency of ∼14%, and VOC of 772 mV, which is an improvement to the baseline device with CdTe:As absorber layer grown at 390 °C under non-saturated conditions. When the low temperature Cd-saturated growth was combined with chlorine heat treatment at a higher temperature of 440 °C (in contrast with the standard 420 °C) for 10 min, device efficiency improved to ∼17% with a high VOC of 877 mV. As a result, ∼100 mV boost in VOC from baseline is demonstrated with Cd-saturated CdTe:As device. Micro-photoluminescence and time-resolved photoluminescence measurements performed on these Cd-saturated CdTe:As devices confirmed that minority carrier lifetime significantly improved

Effects of PCBM loading and thermal annealing on nanomorphology of blend of polymer/fullerene thin films solar cells: Impact on charge carrier mobility and efficiency.

Blend of P3HT/Fullerene thin films solar cell with two different percentage ratio of PCBM loading is investigated. Optical absorption spectroscopy is employed to elucidate the nature of PCBM cluster formation upon thermal annealing. Sandwich structures comprising of ITO/Cs2CO3/P3HT: PCBM/LiF/Al (electron only device), and ITO/PEDOT:PSS/P3HT: PCBM/Au (hole only device) are fabricated using spin coating for the investigations concerning electron and hole mobilities. The impact of charge carrier mobilities on bimolecular recombination and ultimately the power conversion efficiency for two different PCBM loading is also investigated. A direct correlation between Langevin recombination rate and short circuit current density as a function of thermal annealing is realized. The maximum power conversion efficiency is measured at 150C for P3HT: PCBM (1:1) solar cell

A study of donor/acceptor interfaces in a blend of P3HT/PCBM solar cell: Effects of annealing and PCBM loading on optical and electrical properties

Optical, structural and electrical properties based on blend of P3HT: PCBM thin films have been investigated for several PCBM loading whilst exposed to air. Experimental evidence indicates that the peak absorption for PCBM with highest loading concentration show greater reduction in intensity upon annealing temperature of 175 °C. This provides useful information regarding PCBM segregation associated with annealing process. Comparison of GIXRD patterns indicates an increase in intraplanar d spacing and polymer grain size upon thermal annealing. Variation in open circuit voltage and short circuit current with annealing temperatures, suggests that the latter remains almost constant up to around 100 °C and improves gradually with temperature thereafter, whilst the former improves with temperature and the onset of deterioration in current density is linked to percentage PCBM loading. These results are further explained or supported by optical absorption spectroscopy to account for PCBM clustering. A power conversion efficiency of 3.38% was achieved at 175 °C for devices fabricated and tested in air

Data related to the PC71BM loading and it's impact on nanostructuring for blend of PBDTTT-EFT:PC71BM bulk heterojunction solar cell

The data included in this article is based on additional supporting information presented in our recent publication Komilian et al. [1]. The role of acceptor material (PC71BM) in restructuring copolymer PBDTTT-EFT from its relaxed pristine structure to interfaces suitable for exciton dissociation is discussed. The analysis of data indicates that the impact of acceptor material on nanostructuring initiates concurrent processes some of which supports and some impedes charge extractions. Therefore, this manuscript is designed to identify these processes and give and account of their impact on power conversion efficiency