Investigations to improve CdTe-based solar cell open circuit voltage and efficiency using a passivation and selectivity theoretical framework

Includes bibliographical references.2022 Fall.The voltage of CdTe-based solar cells has remained conspicuously low despite years of efforts focused directly on its improvement. The efforts here have been primarily in increasing the equilibrium carrier concentration of the CdTe or its alloys which are used to absorb the light. This direction has been guided by a theory of solar cells that views the cell only as a single p/n junction. The modelling which has been used to confirm this as an appropriate direction indicated that with a moderate carrier lifetime, relatively small front interface recombination velocity, and large equilibrium carrier concentration in the absorber, efficiencies greater than the current record of 22.1% will be possible with open circuit voltages reaching over 1V. However, cells with these properties have been measured and increases in Voc and efficiency have not been attained. In the c-Si community, notably, the "passivation – selectivity" framework has been developed. In particular, it rejects the view that a singular p/n junction is responsible for the function of a solar cell. Instead, this framework operates with the understanding that the potential in the cell which can be turned into useful electrical energy and an increase in open circuit voltage comes only from the excess carriers generated by sunlight forcing a deviation from the equilibrium condition. As such there are two main components: 1) passivation – which refers to the recombination behavior in the cell and development of a large internal potential difference and 2) selectivity – which refers to the asymmetry of conduction in the cell that allows for production of a unidirectional current and an external voltage approaching that within the cell. This framework tends to break the cell into 3, sometimes overlapping, regions: an absorber region that is used to produce as large a potential difference as possible, and two contact regions in which the transport properties are modified to prefer transport of one carrier or the other. Here this framework is applied to CdTe-based solar cells to determine what limits current cells and how to overcome these limitations. In the investigation of passivation, first the electron contact interface is evaluated, resulting in the determination that this interface is not currently limiting the recombination in the cell. As a result, the current baseline is compared to structures hypothesized to provide improvement in the recombination behavior. It is found that cells with CdSeTe as the only material in the bulk exhibit more ideal recombination behavior when compared to a CdSeTe/CdTe structure as is currently used. This comparison demonstrates a pathway for cells to overcome their current limitation due to recombination, with the possibility of reaching up to 25% efficiency and 970 mV Voc with the material that currently is produced at CSU. A native oxide of TeOx is found to passivate the surface, reducing the rate non-radiative recombination, and forms during dry air exposure, providing a pathway to passivate contacts that would be ideal if not for the recombination at the interface. In the investigation related to selectivity, the electron contact is evaluated and it is demonstrated that MgZnO is appropriately selective when deposited with the correct conditions. It therefore is expected that hole selectivity is the primary loss to open circuit voltage in structures determined to have longer excess carrier lifetimes and large radiative efficiencies. Efforts to investigate novel routes to hole selectivity by use of heterojunction contacts are presented. Such routes did not yield improvements in cell Voc and efficiency, and through this work it was determined that a major source of selectivity losses in these cells is the high resistance to hole transport through the bulk semiconductor. Increasing hole concentration or thinning the absorber provide pathways to overcome this specific limitation, but it is modelled that such cells will require structures with hole selective materials that internally cause a reduction of electron current to see improvement in Voc and efficiency

A New Model for Thin Film Solar Cells Using Photon Cycling

The solar energy has emerged as one of the most promising and reliable renewable energy resources attracting much attention to the study of photovoltaics. A principal aim of solar cells is to maximize the absorption of light to increase the generation of electron-hole pairs and harnessing it to increase the power generated. An attractive approach of increasing the generation rate in a thin PV cell by employing photon cycling. In this thesis, I report the results of my study using the updated solar irradiance, and a new model for calculating the generation rate in thin film solar cells. I develop a multiple light path model to arrive at a generation rate using my approximation of absorption coefficient and other physical structures at the back and front contacts. By increasing the path lengths, the generation of photocarriers at each level results in enhanced photocurrent. In calculating this, I have used a new approximation of the absorption coefficient as a function of wavelength. The consequence of the bandgap narrowing effect on the absorption coefficient has been studied using an existing model to show its impact on the generation rate. Furthermore, an optimized design for thin film solar cells is introduced to examine the photon cycling effect on the generation rate. It shows that considering the impact of photon cycling efficiently leads to enhancing the total generation rate by 144%. This permits reducing the thickness of the solar cell, which eventually reduces the cost of the cell. A novel model for accurate computation of the photon cycling effect has been developed, applicable to different semiconductor PVs. Finally, the photon recycling and the luminescent coupling effect are investigated to show an improvement up to 65% for the GaAs generation rate

Ultrafast electronic processes in doped metal-halide perovskite semiconductors

Electronic doping of semiconductors is an important topic in optoelectronics. Introducing small amounts of electron-rich or -poor dopants into the crystal structure of the semiconductor increases its conductivity and shifts the Fermi level, enabling efficient transistor, diode and photovoltaic device architectures. This thesis discusses the effects of doping in metal-halide perovskites, a novel group of semiconducting materials highly relevant for optoelectronic applications. The properties of the doped materials are studied in this work with a variety of spectroscopic techniques, notably including terahertz (THz) spectroscopy. The analysis of terahertz transmission through thin films has proven to be an excellent tool for investigating optoelectronic properties of novel semiconductors. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from experimental data in the regime of optically thin samples. However, significant deviations of the calculated photoconductivity from its actual value are observed in this thesis in semiconductors with high background conductivity when using these expressions. An alternative analytical formula is therefore developed, which greatly improves the accuracy of the estimated value of the photoconductivity while remaining simple to implement experimentally. This improvement of data analysis methodology is highly relevant for studies of photoexcited charge-carrier dynamics in electrically-doped semiconductors, such as tin-iodide perovskites, where the commonly used expression for photoconductivity would result in an underestimate of charge-carrier mobility by over 50%. Tin-iodide perovskites are an important group of semiconductors for photovoltaic applications, as they exhibit higher intrinsic charge-carrier mobilities and lower toxicity than their lead-based counterparts. The spontaneous but controllable p-type doping in these materials provides an interesting opportunity to obtain novel insight into the effect of electronic doping on the dynamic processes, particularly on the intraband transitions of hot charge carriers in metal-halide perovskites. Using a new experimental technique, this thesis reveals that newly photogenerated charge carriers relax within the bands of FA0.83Cs0.17SnI3 on a sub-picosecond timescale when a large, already fully thermalized (cold) population of charge carriers is present. Such rapid dissipation of the initial charge-carrier energy suggests that the propensity of tin-halide perovskites towards unintentional self-doping resulting from tin vacancy formation makes these materials less suited to implementation in hot-carrier solar cells than their lead-based counterparts. Finally, an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth is presented in this thesis. Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behaviour in doped metal-halide perovskites. It is here found that the addition of bismuth has no effect on either the bandgap or exciton binding energy of the MAPbBr3 host. However, a substantial enhancement of electron-trapping defects upon bismuth doping is observed, which results in an ultrafast charge-carrier decay component, enhanced infrared emission and a notable decrease in charge-carrier mobility. Such defects arise from the current approach to bismuth-doping through the addition of BiBr3 salt, which may enhance the presence of bromide interstitials

Numerical study of InAs/GaAs quantum dot solar cells

Solar energy conversion is a promising way to provide future energy demand since it is a clean energy. Unfortunately, the photovoltaic (PV) conversion of the solar energy is expensive, therefore, making attempts to increase the efficiency of PV is essential. A conventional single junction solar cell presents an efficiency limit that is determined by the Shockley-Queisser detailed balance principle (i.e. 40.7% under full sun concentration). The limit comes from the fact that only photons with energy close to the energy bandgap are efficiently converted. Below energy gap, photons are not absorbed since the cell is transparent to them and high energy photons only contribute part of their energy that is equal to the energy bandgap. Many concepts have been developed in order to increase the efficiency limit of solar cells. Among them the intermediate band solar cell (IBSC) has gained considerable attention. In principle, IBSCs have the potential to overcome Shockley-Queisser (SQ) limit of single junction solar cells by providing high current while preserving large voltage. The theoretical limit calculated for an ideal IBSC under full sun concentration is 63.1%. One of the most promising ways to realize the IBSC is to incorporate a QD superlattice in the active region of p-i-n single junction solar cells. The nano-size QDs behave like 3D potential well for the carriers and create discrete energy levels within the forbidden bandgap that allows sub-bandgap photon absorption. Stranski-Krastanov (S-K) growth mode (also called 'layer-plus-island growth') is one of the most common methods to fabricate QDs. This method has been used in many experimental studies for InAs/GaAs heteroepitaxial system which has lattice mismatch of 7.2%. Although InAs/GaAs is not an optimal material system for the IBSC performance, its properties and parameters are well reported in literature compared to other material systems. The drift-diffusion model is the most widely used mathematical approach to describe semiconductor devices. However, in case of quantum dot solar cells, the physics governing the device performance is not sufficiently covered and up to now, modeling of QDSCs has been treated as IBSC modeling through detailed balance principle and semi-analytical or numerical drift diffusion approaches. In this dissertation, QDSCs are investigated in detail by numerical simulation using a QD-aware physics-based model. The influence of selective doping in QDSCs is investigated considering different scenarios in terms of crystal quality. Regarding high-quality crystal, close to radiative limit, large open circuit voltage recovery is predicted in doped cells, due to the suppression of radiative recombination through QD ground state. In case of defective crystal, significant photovoltage recovery is also attained owing to the suppression of both non-radiative and QD ground state radiative recombination. The interplay between non-radiative and QD radiative recombination channels, and their interplay with respect to doping are analyzed in detail. Moreover, a numerical study on the influence of wetting layer states on the photovoltage loss of InAs/GaAs quantum dot solar cells is presented. Almost full open circuit voltage recovery is predicted by combining wetting layer reduction and selective doping. After investigating the inherent limitations of InAs/GaAs QD solar cells regarding realization of the IBSC, a brief description of QDs with type-II staggered band alignment based on GaSb/GaAs material systems (whose interband and intraband dynamics are more promising in view of attaining the IB operating regime) is given and a preliminary study of the competition between thermal and optical escape processes is presented

Advances in quantum tunneling models for semiconductor optoelectronic device simulation

The undiscussed role of solid-state optoelectronics covers nowadays a wide range of applications. Within this scenario, infrared (IR) detection is becoming crucial by the technological point of view, as well as for scientific purposes, from biology to aerospace. Its commercial and strategic role, however, is confirmed by its spreading use for surveillance, clinical diagnostics, environmental analysis, national/private security, military purposes or quality control as in food industry. At the same time solid-state lighting is emerging among the most efficient electronic applications of the modern era, with a billion-dollar business which is just destined to increase in the next decades. The ongoing development of such technologies must be accompanied by a sufficiently fast scientific progress, which is able to meet the growing demand of high-quality production standards and, as immediate but not obvious consequence, the need of performances which would be the highest possible. One issue affecting both kinds of applications we mentioned is the quantum efficiency, no matter the signal they produce is coming from absorbed or emitted photons. At any rate, the balance between the stimulus coming from the surrounding environment is and the generated electrical current is absolutely crucial in each modern optoelectronic device. More in depth, since IR detectors are asked to convert photons into electrons, device designers must ensure that mechanisms concurring to this conversion should be dominant with respect to any opponent phenomenon. Symmetrically, light-emitting diodes should realize the inverse process, where electrons are converted into photons. In real life this mechanism never take place in a one-to-one electron-photon correspondence. Indeed tunneling, a quantum effect related to the probabilistic nature of particles and, thus, also of charges, contributes to unbalance this correspondence by degrading the signal produced within the device active region. In IR photodetectors this translates into of a current even in absence of light (and, by virtue of this fact, this current is known as "dark current") while in light-emitters tunneling is responsible for leakages that may undermine the quantum efficiency and the power consumption also below the optical turn-on. The present dissertation is part of such framework being the result of studying and modeling different tunneling mechanisms occurring in narrow-gap infrared photodetectors (IRPDs) for mid-wavelength IR (MWIR) applications (3 to 5 um) and in wide-gap blue LEDs (around 450 nm) based on nitride material system. This study has been possible thanks to the collaboration with several academic institutions (Boston University, Padua and Modena e Reggio Emilia Universities) and two important German industries, AIM Infrarot Module and OSRAM Opto Semiconductors, which provided the case-study devices here analyzed. After reviewing basic concepts of solid-state physics, the first part of this work deals with the description of the above cited optoelectronic devices, along with their constituent materials: the HgCdTe alloy, in the case of photodetectors, and GaN and its ternary alloys with In and Al, for what concerns blue LEDs. Since the literature focusing on this research area is still not mature enough, in the second part different tunneling mechanisms and models are proposed, described in detail and then tested for the first time, as in the case of a novel formulation intended for direct tunneling in IRPDs or the description of defect-assisted tunneling in LEDs which also includes elements coming from the microscopic theory of multiphonon emission (MPE) in solids. Simulations are carried out by means of several numerical simulation approaches, using either commercial TCAD (Technology Computer Aided Design) tools and codes developed ad hoc for this purpose. The encouraging and fully satisfying results of numerical modeling here proposed confirm, on the one hand, the widely accepted relevance of tunneling in modern electronics and, on the other hand, also propose a new perspective about possible tunneling mechanism in optoelectronic devices and their appropriate physical, mathematical and numerical investigation tools. Furthermore, the role of device modeling does not end here because many physical details and technological information can be inferred from simulations, with enormous beneficial effects for the electronic industry and the quality improvement of its fabrication processes such those invoked above

Defect related radiative recombination in mono-like crystalline silicon wafers

The aim of this work was to investigate defect related luminescence emission in four mono-like silicon wafers. The seed-assisted silicon ingot is built by six Czochralski silicon slabs, with nine seed junctions. The discovered emission signals are due to Shockley-Read-Hall recombination. Each wafer originates from a mono-like silicon ingot grown at the Norwegian University of Science and Technology (NTNU), Trondheim. The master thesis work was conducted at the Norwegian University of Life Sciences (NMBU), Ås. Hyperspectral imaging has been used in multiple branches like medicine, industry and military purposes. In this investigation hyperspectral imaging is conducted on mono-like silicon wafers. Seed-assisted grown mono-like silicon are produced with the goal of increasing wafer efficiency at lesser cost. Spectrally resolved photoluminescence (SPL) has been used together with multivariate data analysis. This is a non-destructive method to examine defect related luminescence in each mono-like wafers. Each wafer was cooled to 88 ± 2 K before illuminated with an 810 nm laser. The photoluminescence emission from each wafer was captured by an HgCdTe hyperspectral camera. The individual D1-D4 and band to band PL emission signals were extracted with Multivariate Curve Resolution (MCR) algorithm and found in the seed junctions. It has also been found three other PL emission signals, either in the seed junctions, or from parasitic crystals penetrating into the main wafer ingot. The D07 PL emission signal is restricted to the parasitic crystals and can be related to interstitial iron Fei. A signal at 0.846 ± 0.01 eV, known as D5, has been found as a shoulder of D1 and D2 PL emission signals. These three PL signals have been related to dislocations with oxygen impurities in other studies. A new signal denoted D09 with the energy 0.904 ± 0.01 eV is discovered and is growing in intensity with increased height of the ingot. The signal seems to be centered in the seed junctions and has not been mentioned before. The D1 PL signal is strong in the A-108 wafer, then decreasing in strength with increasing height. This seems to strengthen the theory of the D1 PL emission signal related to oxygen. The D2 PL signal on the other hand increases in intensity with increased ingot height, and contradicts the oxygen theory. D3/D4 PL emission signals are found in the seed junctions and can be related to the same spatial position. The PL emission signals increases with ingot height and strengthen the suggestion that D3/D4 PL signals originates from iron-boron (FeB) complexes. The high intensity PL emission signal known as VID3 has not been found in this work. A tail on the D1 PL signal found at 0.95 eV and 1.00 eV have been discussed in other studies and can be explained by hydrogen-silicon (H-Si) bond. One parasitic crystal has been found with multiple impurities. That crystal may have another grain boundary and dislocation number than the parasitic crystals with only the D07 PL signal. Comparing this work with the work done by Ekstrøm et al. [1] has discovered some similarities. It mentions different tilt and misorientation angles in each seed junction. The investigation concluded that misorientation angles in the seed crystal junctions produced tilt around one or several axis, and would play a major part in the bulk lifetime. Comparing to the current work has found that low misorientation angle around the X-axis seems to produce none or weak defect related PL emission signals. Misorientation around Z-axis seems to produce more defect related luminescence. While misorientation around multiple axis seems to create chaotic junctions with high defect related luminescence. The explanation can be the number of vacancies ready for impurities are higher in multiple axis tilts than one single axis tilt. The conclusion is that the combined strength of SPL and MCR as a method to investigate mono-like silicon wafers has been used with success. The known D1-D4, D5 and D07 PL emission signals was found alongside a new PL emission signal at 0.904 ± 0.01 eV. The PL emission signals are not clearer than the emission signals found in mc-Si wafers, however, the D07 signal has been found separated from the rest of the other DRL signals and this can be a helpful in further experiments. The different PL emission signals are found to vary greatly throughout the ingot and logic answers can be made to explain the results based on known literature. Hyperspectral imaging and Multivariate curve resolution can strengthen and contribute to an increased quality of seed assisted mono-like wafers.I denne masteroppgaven har fire "as cut" skiver fra en mono-lik silisiumkrystall blitt undersøkt. Frø assistert mono-lik silisium ingot er en produksjonsmetode for å skape høyeffektivitetsskiver med den rimlige multikrystallinske størkningsprosessen. Prosessen er under utvikling, med mål om å kunne forbedre solcellene i et sluttprodukt. Denne masteroppgaven går ut på å avdekke defekter i disse skivene både i romlig og spektral posisjon. For å kunne finne defekter i silisiumskivene registreres fotonutslippet til eksiterte elektroner som rekombinerer etter Shockley-Read-Hall metoden. I dette eksperimentet skjer dette ved å la en 810 nm laser belyse hver av de nedkjølte skivene. Skivene er nedkjølt til 88 ± 2 K med flytende nitrogen. Fotonene fra de eksiterte elektronene registreres og fordeles til sine spektrale områder i et HgCdTe hyperspektralt kamera. For å kunne hente ut de interessante defektrelaterte spektrumene brukes Matlab og et statistisk verktøy som heter Multivariate curve resolution (MCR). I denne oppgaven har alle de fire kjente emisjonslinjene D1, D2, D3 og D4 blitt funnet sammen med bånd til bånd emisjonslinjen. Disse defektrelaterte emisjonslinjene er bare funnet i frø krystallgrensene, foruten ett sted: En parasittisk krystall nær en av sidekantene. I tillegg er det funnet tre andre emisjonslinjer. Den ene er kalt D5 som ser ut til å være en skulder av det sterkere D2 emisjonlinjen med energien 0.846 ± 0.01 eV. Det andre signalet er kalt D09 med 0.904 ± 0.01 eV og er funnet sentrert i krystallgrensene. Dette signalet ser ut til å utvikle seg fra D2 signalet i A-108 skiven og videre fram til et eget signal i A-45 skiven. Det tredje signalet er et signal nylig publisert som D07 og er bare funnet i parasittiske krystaller som virker å gro inn fra digelkantene. Denne emisjonslinjen kan stamme fra interstitielt jern Fei. D1 emisjonslinjen har høyest intensitet i nær bunn av ingoten og minsker gradvis i styrke med økende ingot høyde. D2 emisjonslinjen derimot ser ut til å styrke seg mot toppen av ingoten og er sterkest i A-45 skiven. Både D1, D2 og D5 er betegnet i litteraturen som å kunne relateres til oksygen. Ut i fra oppførselen til D2 emisjonslinjen, motsier resultatet oksygen teorien, mens D1 og D5 ser ut til å forsterke den samme teorien. D3/D4 emisjonslinjene er funnet i bare ett av frøkrystallgrensene i A-108 skiven. Derimot, i de resterende tre skivene er signalet tilstede i alle frø grensene. MCR algoritmen betrakter dette signalet som ett signal og styrker ideen om at D3/D4 har samme romlig opprinnelse. D4 emisjonslinjen er nevnt å oppstå fra jern utfellinger fra smeltediglen og ovnen, hvor D3 emisjonslinjen kan være en fononreplica av D4. I denne undersøkelsen øker D3/D4 emisjonslinjene i intensitet med økt ingot høyde. Metallutfellingsteorien ser ut til å holde og kan forklares med feller i båndgapet fra jern-bor (FeB) komplekser. Et signal som er diskutert i litteraturen er en emisjonslinje kalt VID3. Dette signalet er ikke funnet noen steder i noen av skivene undersøkt. I D1 emisjonslinjen er det funnet en hale som har to toppunkt i området 0.95 eV og 1.00 eV. Disse toppunktene er sett i andre studier av tynnfilm silisium og kan forklares med hydrogen-silisium feller i båndgapet. I en undersøkelse gjort av Ekstrøm et al. [1] på samme ingot ble det konkludert med at misorienteringsvinkler rundt en eller flere akser hadde stor innvirkning på antall dislokasjoner og levetid over frøkrystallgrensene. I denne undersøkelsen har den konklusjonen blitt undersøkt for å se om mulig korrelasjon kan bekreftes. Når det gjelder emisjonslinjene over frøkrystallgrenser er det funnet klar korrelasjon med misorienteringsvinkel om en eller flere akser. Der det er liten misorientering rundt X-aksen er det ingen eller lite defektrelaterte emisjonslinjer. Ved misorientering om Z-aksen er det mer defektrelaterte emisjonslinjer. Ved misorientering i flere akser er det funnet kaotiske frøkrystallgrenser med sterke defektrelaterte emisjonslinjer, selv om vinklene er små. En forklaring på dette er at det er flere ledige områder for urenheter å feste seg i ved misorientering i flere dimensjoner. Det konkluderes med at et samarbeid med SPL og MCR som en metode for å forbedre mono-lik silisium skiver har blitt brukt med suksess. De kjente D1-D4, D5 og D07 emisjonslinjene er funnet sammen med en ny emisjonslinje ved 0.904 ± 0.01 eV. Emisjonslinjene oppfattes ikke klarere enn emisjonslinjer i mc-Si, men D07 emisjonslinjen er funnet separert fra de andre emisjonslinjene. Dette kan være til god hjelp i framtidig forskning. Emisjonslinjene er funnet å variere mye mellom høyden til ingoten og logiske slutninger kan trekkes for å forklare hendelsene basert på kjent litteratur. Hyperspektral bildebehandling sammen med MCR kan styrke og bidra til en økt kvalitet på frø assistert mono-lik silisium skiver.M-M