Micro- and macroscopic characterization of recombination losses in high efficiency Cu(In,Ga)Se2 thin film solar cells
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- Doctoral Thesis
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Production of electric energy from sunlight has become a cost competitive alternative to conventional, non-renewable energy sources such as combustion of fossil fuels or nuclear fission. Due to the constantly decreasing costs of module production, the balance of system (BOS) costs make up an increasing share of the total price of a PV system. As most of the BOS costs scale with the system area, increasing the module efficiency has become the most important lever to reduce the cost for power generation. With lab scale device efficiencies approaching 23% photovoltaic (PV) devices based on Cu(In,Ga)Se2 (CIGS) absorber layers show a high potential for cost-effective energy production. Furthermore, absorber layers of only 2 μm to 3 μm thickness are required and devices with efficiencies above 20% can be fabricated at low temperatures <500 C. This allows roll-to-roll manufacturing on flexible substrates and promises comparably low energy consumption for production and material economy, all adding up to potentially short energy payback times. This thesis aims to elucidate optical and electrical loss mechanisms in state of the art, low temperature deposited devices and to derive strategies to overcome these limitations. Therefore electrical and optical characterization is performed on band gap graded absorbers and devices. The measurements are combined with numerical simulations that allow to estimate and quantify a variety of device and absorber properties. In Chapter 3 the unknown origin of losses in the near infrared region observed in the quantum efficiency of devices is investigated. Essentially, such losses can originate from insufficient collection of the photo generated charge carriers and from parasitic absorptionin other layers than the absorber, e.g. the transparent oxide front electrode or incomplete absorption of photons in the absorber and consequent absorption in the back contact. The electron beam induced current technique (EBIC) is applied on cross sections of finished devices to study the efficiency with which carriers are collected along the depth of the absorber layer. The measurements are combined with a model for the local optical absorption that includes the depth dependent compositional grading. This analysis provides a first indication that optical losses rather than extensive deficiencies in carrier collection are limiting the device EQE. As discussed in Chapter 3, the measurement principle of EBIC is adapted to a Helium ion microscope where He ions (instead of electrons as used for EBIC) are used as excitation particles to allow increased lateral resolution as compared to EBIC. This would allow to study the electrostatics at the absorber/CdS buffer layer interface for instance in view of a deeper understanding of absorber post deposition treatments. In a series of measurements the practicability of the approach for electrical characterization of CIGS devices is investigated. Optical measurements by means of photo spectrometry on finished devices and on delaminated slabs of absorber are investigated and described in Chapter 4. The measurements are analyzed by means of a numerical model for the optical properties of the device based on transfer matrix method (TMM) calculations. From the combination of simulations and measurements, the parasitic losses in the different layers of the device are studied. Most importantly, it is shown that the absorber layers are not completely absorbing the incident photons and those transmitted photons are absorbed in the Mo back contact. It is shown, that the cause for the residual transmission is an insufficient optical thickness of the low band gap region of the composition graded absorber layer. These insights support the conclusions drawn from the EBIC measurements. As a consequence of these findings, three viable approaches to increase the absorber NIR absorbtance are proposed and their potentials are discussed. One of these approaches, namely the implementation of a highly reflective back contact to substitute the absorptive Mo is investigated in detail. In a screening study, a number of potential rear contact structures to increase the rear contact reflectance are investigated. A Mo/Al/InZnO based rear contact structure is identified as most promising candidate as it is shown to increase the NIR response of the device while having benign influence on the photovoltaic properties of devices. The approach allows to reach efficiencies of 19.5% with a top efficiency of 19.9% which is comparable or even higher than reference devices fabricated for comparison. Consequently, the observed NIR optical gains are analyzed by comparison to TMM simulations in order propose next steps towards improved device EQEs. In Chapter 5, time resolved photo luminescence (TRPL) studies of absorber layers are described. The investigations were performed to estimate carrier mobility and minority carrier lifetime parameters of the absorber layers. Reliable estimates of these parameters are relevant for device modeling as they could provide a criterion for an assessment of the absorber quality before absorbers are processed into devices. In order to establish a measurement methodology, interpretation and modeling, the studies are performed on samples with simplified band gap grading i.e. pure CuInSe2 without band gap gradient and CuInGaSe2 with a conduction band increase towards the absorber back side. These systems show a reduced complexity as compared to the highly efficient absorbers that feature a conduction band increase towards the absorber front- and backside. Therefore they allow an analytical treatement that gives insight into the involved transport phenomena. The studies were aimed to establish a basic understanding of these processes for a future extension to double graded absorbers. To estimate the bulk and surface parameters, the sample surfaces were intentionally modified by surface etching or the absorber layers were delamination from the Mo substrates. Consequently TRPL measurements were performed and the influence of the surface state was analyzed. To that end, an analytical simplification of the underlying transport equations is presented that allows to derive constraints for the bulk and surface parameters (mobility, lifetime and surface recombination velocities). Consequently, numerical simulation of the charge carrier transport and recombination process are performed and compared to measurements in order to discuss the accuracy of the parameter estimation. Finally, in Chapter 6 the key findings of the thesis are summarized and an outlook for future investigations is given Show more
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ContributorsExaminer: Tiwari, Ayodhya N.
Examiner: Wood, Vanessa
Examiner: Siebentritt, Susanne
Examiner: Buecheler, Stephan
Organisational unit02140 - Dep. Inf.technologie und Elektrotechnik / Dep. of Inform.Technol. Electrical Eng.
03895 - Wood, Vanessa / Wood, Vanessa
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