Advanced electrical characterisation of thin-film solar cells

The work presented in this thesis focuses on both standard and advanced characterisation techniques applied to solution-processed CIGS and vacuum-processed CdTe thin film solar cells. Characterisation techniques such as capacitance spectroscopy, tempeture dependent J-V measurements, PDL Hall effect and along with other fundamental measurement techniques are used to extract the key parameters and material properties of these solar cells. Capacitance spectroscopy offers valuable insight to material properties such as defect density, energy level of defects and carrier concentration, which is especially important to understand in the operation of thin film solar cells and their limiting processes. Antimony (Sb) was introduced into the solution-processing CIGS absorbers which led to an increase in EQE, hence an increase in J_sc. A bilayer structure of large-grain top layer and a fine-grain bottom layer was observed by SEM. The devices with Sb showed a low net carrier concentration and defect density compared to the devices with no Sb doping, indicating that adding Sb might be passivating the defect in the bulk, rather than doping the absorber. A shift in the long wavelength decay of EQE spectra for the device with Sb is observed, indicating a decrease in bandgap and only the change in In/Ga ratio able to explain this behaviour. From the V_oc (T) analysis, the main recombination mechanism for the device with Sb is found to be in the bulk of the absorber, whereas the device without Sb is dominated with the interface recombination. Adding Sb into the absorber layer has reduced the rollover seen at low temperatures in the J-V characteristics, and has removed the barrier at the CdS/CIGS interface. This may be due to the incorporation of Sb into the CIGS absorber, resulting an improved band alignment of CdS/CIGS. The admittance spectroscopy measurements for the device without Sb revealed an admittance step with an activation energy of E_A = 330 meV. This step is considered as a deep level defect, which has often been referred to as the N2 defect. The N2 defect has been removed with Sb doping and left with a shallow level defect, N1 (E_A = 42 meV). After 1 hour of light soaking under 100 mWcm-2 illumination at room temperature, the devices showed a significant increase in the cell performance especially in the junction capacitance and net carrier concentration. A detailed study of the defects for N1 and N2 steps were performed using capacitance spectroscopy for devices with and without tellurium (Te) at the back-contact, and devices with and without CdSeTe (CST) layer of CdTe devices. The admittance measurements revealed different activation energies, E_A between 20 meV and 279 meV at different temperatures for each device. Adding Te to the CST/CdTe devices removed the N1 step. The N1 step corresponds to a shallow defect with low a E_A, suggesting that the defect might be located at the absorber/back-contact, since Te is added at the back of the device. For the CdTe devices without CST layer, Te has removed the N2 defect and left only the N1 defects. The CST&CdTe with Te devices showed high efficiencies around ~14-15% with high FF. The `CST with Te` device has shown an `S` shaped J-V curve, which can relate to a conduction band offset (CBO) or a presence of a significant current barrier. Non-diode behaviour J-V characteristics have been observed with the `CST/CdTe without Te` and this is due to having a low FF, a large R_s and a low R_sh. The dominant recombination mechanism by the activation energy 〖(E〗_a) is extracted from the intercept of V_oc (T) at T=0 K and this found to be at or close to the interface for all the devices. Adding Te to the CdTe devices has slightly reduced the interface recombination, leading to a smaller barrier height and improved J-V characteristics. Both C-V and DLCP measurements showed typical `U` shaped depth profiles with similar net carrier concentrations, except with the “CST/CdTe without Te”. The difference in depth profiles is due to high level of deep level and shallow defects. Thus, measurements are affected by the N1 and N2 steps.The metastable behaviour found in CdTe devices with MZO buffer layers resulted in variations from the J-V characteristics depending on prior exposure history of light and environmental stresses, such as temperature and climate. Different preconditioning procedures have been studied that are used to recover the performance of the devices. J-V characteristics before preconditioning have shown an `S` shaped behaviour, with significant current loss in forward bias which is removed after preconditioning. Also, the depth profiles before preconditioning showed an unusual double minima, and the second minima towards the back of the device became less pronounced after preconditioning. The “Atmospheric” preconditioning procedure resulted in a significant recovery of the device performance compared to “Vacuum” preconditioning. Temperature dependent J-V and capacitance measurements before and after preconditioning revealed the presence of recombination centres and defect levels at the MZO/absorber interface. Light and voltage bias have improved the degree of these metastable behaviours by reducing the formation of a blocking layer at the interface. Despite all the preconditioning attempts, the recovery of PV parameters remained only for 3 days while the devices were maintained under vacuum in the dark. Since the temperature dependent capacitance and J-V measurements showed defects and recombination centres at the MZO/absorber interface, the Hall effect measurements have been studied in an attempt to extract carrier concentration, mobility and the conductivity of the MZO films. Conductivity measurements on MZO films showed no significant changes before and after preconditioning. However, CdCl2 treated MZO films showed slight improvement in the linearity of the I-V response. Although the response was improved, no linear sheet resistance or proper Hall signal were detected in order to extract reliable and accurate carrier concentration, or mobility of the MZO films. Alternatively, Ga as a dopant is used in MZO layer (GMZO), which should enhance the film conductivity and the carrier concentration. The annealed GMZO films showed an improved I-V response and a large improvement in conductivity after light soaking. However, a gradual decrease in Hall signal over time was observed and no reliable results could be extracted from the Hall effect measurements.</div

Supplementary information files for Investigation of solution-based synthesis of non-toxic perovskite materials using Mg, Ca, Mn, Fe, Cu, and Zn as the B-site cation for photovoltaic applications

Supplementary files for article Investigation of solution-based synthesis of non-toxic perovskite materials using Mg, Ca, Mn, Fe, Cu, and Zn as the B-site cation for photovoltaic applications  High-efficiency perovskite solar cells are reliant on lead-based materials, which causes toxicity issues for large-scale implementation. Current alternatives can contain similarly environmentally dangerous chemicals such as tin halide compounds. Computational studies have suggested a large array of different potential B-site metal cations that could produce suitable perovskite materials. In this work, simple, solution synthesis of 24 candidate materials is attempted with a focus on the environmental safety of the starting compounds. Of these 24 materials, 10 formed a new material from XRD characterisation, and 1 of the resulting films produces a material with an observable band-gap in UV/vis. This material, a combination of potassium bromide and copper bromide, failed to produce a solar cell of any notable efficiency. This work demonstrates that completely environmentally benign perovskite materials may require more energy-intensive synthesis such as solid-state methods, removing the benefits of simple, solution processing evident in lead-halide perovskite solar cells. </p

Investigation of solution-based synthesis of non-toxic perovskite materials using Mg, Ca, Mn, Fe, Cu, and Zn as the B-site cation for photovoltaic applications

High-efficiency perovskite solar cells are reliant on lead-based materials, which causes toxicity issues for large-scale implementation. Current alternatives can contain similarly environmentally dangerous chemicals such as tin halide compounds. Computational studies have suggested a large array of different potential B-site metal cations that could produce suitable perovskite materials. In this work, simple, solution synthesis of 24 candidate materials is attempted with a focus on the environmental safety of the starting compounds. Of these 24 materials, 10 formed a new material from XRD characterisation, and 1 of the resulting films produces a material with an observable band-gap in UV/vis. This material, a combination of potassium bromide and copper bromide, failed to produce a solar cell of any notable efficiency. This work demonstrates that completely environmentally benign perovskite materials may require more energy-intensive synthesis such as solid-state methods, removing the benefits of simple, solution processing evident in lead-halide perovskite solar cells.</p

Exploiting bi-modulated magnetic field and drive current modulation to achieve high-sensitivity Hall measurements on thermoelectric samples

Hall mobility measurements of samples with high carrier density are challenging due to the low Hall voltage. Increasing the drive current enhances the Hall voltage, but risks Joule heating at the contacts resulting in unreliable measurements. Measurement noise suppression can be achieved by modulating either the magnetic field or the drive current during the Hall measurement. The Parallel dipole line (PDL) AC Hall measurement technique is a very practical way to realize pure harmonic magnetic field modulation in a very compact design using rotating permanent magnets. In this work, we combine magnetic field modulation with alternating polarity drive current, i.e., using a simultaneous two-parameter modulation at very different frequencies to gain further noise reduction. It makes possible to determine the charge carrier mobility in materials featuring very high free carrier density (over 1 E21 cm−3). The reliability and the applicability of the bi-modulation method was tested on Sn doped NbCoSb thermoelectric samples. The results exhibited excellent sensitivity to the chemical composition of the samples and therefore proven to be an efficient method for the development of such materials. Graphical abstract: [Figure not available: see fulltext.]</p

A broadband multilayer antireflection coating for thin film CdSeTe/CdTe solar cells

Thin film cadmium telluride (CdTe) photovoltaics (PV) is the most successful second-generation PV technology, with a current installed capacity of over 30 GWp, predominantly at utility scale. Recent improvements in the buffer layer of the device, switching from cadmium sulphide (CdS) to transparent magnesium-doped zinc oxide (MZO), tin oxide (SnO2), or zinc oxide (ZnO), and the addition of selenium to the absorber layer, have expanded the wavelength range over which CdTe devices operate, from 400–850 nm to 350–900 nm. These changes have resulted in higher efficiency devices. As a result, an optimized antireflection (AR) coating design is required to improve the efficiency further. A six-layer AR coating of SiO2 and ZrO2, building on a previous four-layer design for CdTe devices, has been designed, modeled, and fabricated on 3.8-mm thick fluorine-doped tin oxide coated TEC™15 substrates, reducing reflection by 3.38% absolute. Electrical measurements of a CdSeTe/CdTe device before and after addition of the AR coating show an increase in short-circuit current density (Jsc) of almost 1 mAcm−2, a relative increase of 3.45%, and a 0.6% increase in the conversion efficiency of the device, from 16.93% to 17.53%, which is a relative increase of 3.54%. Unlike conventional single layer AR coatings this multilayer coating is stable even under the high processing temperatures required in module manufacturing, so could be supplied by glass manufacturers. This newly optimized broadband AR coating on will enable significantly higher conversion efficiencies and help push CdSeTe/CdTe module efficiencies higher.</p

Transient metastable behavior caused by magnesium-doped zinc oxide emitters in CdSeTe/CdTe solar cells

Metastable behavior in highly efficient MZO/CdSeTe/CdTe solar cells has been reported previously. Different preconditioning procedures have been studied that are used to recover the performance of the devices. 11 wt% of MgO content in the MZO layer has shown to give optimized photovoltaic parameters in the device compared to other MZO compositions. J – V characteristics before preconditioning of the devices with higher MgO content show an “ S ” shaped behavior, which is removed during preconditioning. However, this recovery remained only for 3 days while the devices were stored under vacuum in the dark. Temperature-dependent J – V and capacitance measurements before and after preconditioning revealed the presence of recombination centers and defect levels at the MZO/absorber interface. Previous studies have shown degradation of MZO occurring if the layer is exposed to ambient atmosphere. Hall effect measurements on the MZO films showed no significant changes after any preconditioning or CdCl 2 treatment. Secondary-ion mass spectrometry images show diffusion of oxygen from the MZO layer into the CdSeTe region after CdCl 2 treatments. This likely enables the MZO to function as a buffer layer since it will increase the carrier concentration due to the formation of oxygen vacancies. As-deposited MZO thin films are insulating. However, the oxygen vacancies in the MZO layer also increase its reactivity and instability. </p

Ga-doping of MZO in CdSeTe/CdTe thin film solar cells

Metastable effects in high efficiency MZO/CdSeTe/CdTe solar cells have been studied in an attempt to recover the device performance. Devices with the MZO buffer layer have shown an 'S' shaped behaviour in the J-V characteristics before any preconditioning. This is removed after light soaking under 1000 Wm-2 at 25 °C. However, this recovery remained only for a short period of time while the devices were stored under vacuum in the dark. Recent studies with Ga doping of the buffer MZO has shown the removal of this metastability in the J-V characteristics of CdTe devices can be achieved without light soaking. A significant improvement in conductivity and Hall signal has been measured with Ga doped MZO layers compared to previously measured MZO films. However, a gradual decrease in the Hall signal has been observed over time after films were light soaked and removed from the desiccator.</p

Supporting Information Files for Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Supporting Information files for Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaClPoor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution-processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum-based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution-based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto-electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda-lime glass substrate. NaCl is non-toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution-processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter-grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na-doping strategy for CIGS absorbers and reveals the current limitations of the Na-doping in solution-processed atmospherically deposited cells.</div

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Poor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution-processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum-based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution-based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto-electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda-lime glass substrate. NaCl is non-toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution-processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter-grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na-doping strategy for CIGS absorbers and reveals the current limitations of the Na-doping in solution-processed atmospherically deposited cells

The microstructure of thin film CdSe following cadmium chloride activation treatment

Thin film CdSe is an important precursor layer for CdSeTe/CdTe photovoltaic devices and as a potential future top cell in multijunction devices. It is also used in photodetectors. In this paper we report on the improvements in the microstructure of CdSe thin films caused by the cadmium chloride (CdC12) treatment. Using high resolution cross-sectional HRTEM and EBSD, we show that the CdCl2 treatment leads to recrystallisation, grain growth, texture randomization and defect removal. These improvements in microstructure result in a dramatic increase in luminescence and carrier lifetime as measured using PL and TRPL.</p