Capturing the spectra of solar cells

Abstract

In broad terms, this thesis explores unchartered territory in solar cell characterisation and proposes several new techniques based on the spectral, injection and temperature dependence of three measurable parameters: short-circuit current, open-circuit voltage, and photoconductance. The work in this t hesis is a thorough study of all facets of these techniques and their comparison to existing diagnostic tools. The thesis proves the validity of these techniques through theoretical derivations and computer simulations, and illustrates their applicability with a broad range of experimental devices. The spectral response of the short-circuit current has significant explanatory power in disclosing recombination processes. Its measurement is used to reveal some of the most important device characteristics, among them the diffusion length, surface recombination velocity and emitter quantum efficiency. Spectral responsivity measurements are generally performed using a small signal technique and a cost-intensive lock-in detection. In this thesis, a novel approach to spectral response measurements is presented: the quasi-steadystate short-circuit current response (QSSJsc-λ). It employs large signals and cost-effective data-acquisition. Thus, in technical aspects, it stands out for its very simple design. Measurements of a wide range of different solar cell structures showed an excellent agreement with the conventional method. QSSJsc-λ could therefore prove useful as an alternative diagnostic tool to the traditional technique. The spectral response of the open-circuit voltage is a concept that was not deemed to be important in the past. The surface photovoltage technique (SVP) harnesses the colour sensitivity of the voltage in order to determine the diffusion length. However, the voltage response has only been used as a tool to extract the diffusion length, whilst no importance has been attached to its own meaning. In this work we show analytically, with simulations and also experimentally, that the spectral photovoltage perfectly mimics the characteristics of the spectral photocurrent. The technique devised for this purpose, the quasi-steady-state voltage response (QSSV₀c-λ), also uses large light intensities and a simple data acquisition. The distinct advantage of the new method over the spectral photocurrent method is its applicability directly after junction formation. It out-competes SPV in the sense that it offers the possibility of studying devices under real operating conditions. The similarity of the current and voltage responses provided the impetus to extend models used to analyse the spectral photocurrent to the voltage case. In particular, two models not previously employed in the SVP technique, the model of Isenberg and the model for rear-illuminated cells, could gain a great deal of importance in the quest to determine recombination parameters in silicon devices. By adding temperature variation to the spectral response of the short-circuit current, it becomes a technique for defect identification. This is rooted in the fact that the temperature dependence of the diffusion length can disclose valuable information about defects. The temperature dependent quantum efficiency of the current, TQEJsc, takes advantages of this property. We enrich the range of spectroscopic tools by studying the temperature dependent quantum efficiency of the voltage instead (TQEVoc)· Additionally, this work sounds the potential of the white light QSSVoc method as a spectroscopic tool. The temperature and injection-level dependent voltage spectroscopy (TIVS) is the outcome of this idea. This work demonstrates that TQEVoc is able to unveil defect properties equally as well as TQEJsc TIVS, in contrast, has been found to be hampered by the difficulty of predicting the bulk lifetime from the open-circuit voltage. The spectral photoconductance is a versatile technique widely employed in the investigation of photodetectors. In solar cell research, the use of two different wavelengths was occasionally used to separate between surface and bulk recombination. This work aims to establish the spectral photoconductance (QSSPC-λ) as a valuable and powerful tool for device characterisation. The underlying measurement technique to detect the photoconductance, the quasi-steady-state photoconductance (QSSPC), outranks similar techniques for its simple design and cost-effective signal detection. In non-diffused silicon wafers, QSSPC-λ offers the scope to separate between surface and bulk recombination and to act as a depth resolution of deeply injected defects. In emitter structures, the method promises to widen the realm of characteristic tools that trace the emitter quantum efficiency. Show more