Nanostructured composites are attracting intense interest for electronic and optoelectronic device applications, specifically as active elements in thin film photovoltaic (PV) device architectures. These systems implement fundamentally different concepts of enhancing energy conversion efficiencies compared to those seen in current commercial devices. This is possible through considerable flexibility in the manipulation of device-relevant properties through control of the interplay between the nanostructure and the optoelectronic response. In the present work, inorganic nanocomposites of semiconductor Ge embedded in transparent conductive indium tin oxide (ITO) as well as Ge in zinc oxide (ZnO) were produced by a single step RF-magnetron sputter deposition process.It is shown that, by controlling the design of the nanocomposites as well as heat treatment conditions, decreases in the physical dimensions of Ge nanophase size provided an effective tuning of the optical absorption and charge transport properties. This effect of changes in the optical properties of nanophase semiconductors with respect to size is known as the quantum confinement effect. Variation in the embedding matrix material between ITO and ZnO with corresponding characterization of optoelectronic properties exhibit notable differences in the presence and evolution of an interfacial oxide within these composites. Further studies of interfacial structures were performed using depth-profiling XPS and Raman spectroscopy, while study of the corresponding electronic effects were performed using room temperature and temperature-dependent Hall Effect. Optical absorption was noted to shift to higher onset energies upon heat treatment with a decrease in the observed Ge domain size, indicating quantum confinement effects within these systems. This contrasts to previous investigations that have involved the introduction of nanoscale Ge into insulating, amorphous oxides. Comparison of these different matrix chemistries highlights the overarching role of interfacial structures on quantum-size characteristics. The opportunity to tune the spectral response of these PV materials, via control of semiconductor phase assembly in the nanocomposite, directly impacts the potential for the use of these materials as sensitizing elements for enhanced solar cell conversion efficiency.

abstract: Transparent conductive oxides (TCOs) are used as electrodes for a number of optoelectronic devices including solar cells. Because of its superior transparent and conductive properties, indium (In) tin (Sn) oxide (ITO) has long been at the forefront for TCO research activities and high-volume product applications. However, given the limited supply of In and potential toxicity of Sn-based compounds, attention has shifted to alternative TCOs like ZnO doped with group-III elements such as Ga and Al. Employing a variety of deposition techniques, many research groups are striving to achieve resistivities below 1E-4 ohm-cm with transmittance approaching the theoretical limit over a wide spectral range. In this work, Ga-doped ZnO is deposited using pulsed laser deposition (PLD). Material properties of the films are characterized using a number of techniques. For deposition in oxygen at pressures >1 mTorr, post-deposition annealing in forming gas (FG) is required to improve conductivity. At these higher oxygen pressures, thermodynamic analysis coupled with a study using the Hall effect measurements and photoluminescence spectroscopy suggest that conductivity is limited by oxygen-related acceptor-like defects in the grains that compensate donors, effectively reducing the net carrier concentration and creating scattering centers that reduce electron mobility. Oxygen is also responsible for further suppression of conductivity by forming insulative metal oxide regions at the grain edges and oxygen-related electron traps at the grain boundaries. The hydrogen component in the FG is thought to passivate the intra-grain acceptor-like defects and improve carrier transport across these grain boundaries. Given this deleterious effect of oxygen on conductivity, depositions are performed in pure argon (Ar), i.e., the only oxygen species in the growth ambient are those ejected directly from the PLD solid source target. Ga-doped ZnO deposited in Ar at 200 °C and 10 mTorr have resistivities of 1.8E-4 ohm-cm without the need for post deposition annealing. Average transmittance of the Ga-doped films is 93% over the visible and near infrared (IR) spectral regions, but free carrier absorption is a limiting factor further into the IR. After annealing in FG at 500 °C, a 300 nm Ar film has a Haacke figure of merit of 6.61E-2 sq. ohm. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

The transparent conductive oxide (TCO) SnO2:Ta is developed as a selectively solar-transmitting coating for concentrated solar power (CSP) absorbers. Upon covering with an antireflective layer, a calculated absorptivity of 95% and an emissivity of 30% are achieved for the model configuration of SnO2:Ta on top of a perfect black body (BB). High-temperature stability of the developed TCO up to 1073 K is shown in situ by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The universality of the concept is demonstrated by transforming silicon and glassy carbon from non-selective into solar-selective absorbers by depositing the TCO on top of them. Finally, the energy conversion efficiencies of SnO2:Ta on top of a BB and an ideal non-selective BB absorber are extensively compared as a function of solar concentration factor C and absorber temperature TH. Equal CSP efficiencies can be achieved by the TCO on BB configuration with approximately 50% lower solar concentration. This improvement could be used to reduce the number of mirrors in a solar plant, and thus, the levelized costs of electricity for CSP technology.

Solar photovoltaics (PV) is expected to be a critical contributor to mitigating the effects of climate change by helping to satisfy net zero emissions. Since crystalline silicon-based solar cells are close to their practical efficiency limit, further reducing the balance of system (BoS) costs is only possible by increasing the cell efficiencies. The most promising candidate is perovskite/silicon (Si) tandem solar cell technology, which allows efficient solar spectrum harvesting. This relatively new technology attracts attention due to its potential to dominate the PV market; however, it also brings challenges that must be overcome, like stability and scalability concerns. This thesis project focuses on optimizing and characterizing recombination junctions (RJs) for monolithic perovskite/Si tandem solar cells aimed at improved performance and stability. Tandem solar cell PV parameter measurements, encapsulated stability measurements, and thin film characterizations are performed for RJ developments. The optimizations are performed for tandem solar cells with solution-processing and hybrid methods. Self-assembled monolayer (SAM) molecules and transparent conductive oxide (TCO) recombination layer (RL) combinations are optimized to obtain tandems with hybrid technique. In addition, the influence of the thickness of TCO RL on the tandem devices’ performance is also investigated, particularly solution-processed tandems. The improvements are observed by thinning down the thickness of TCOs regardless of the material type. 3 Characterizations revealed that ultra-thin ( 5 nm) amorphous indium zinc oxide (IZO) RL allows more workfunction shift, homogeneous surface potential distribution with SAM deposition, and better carrier recombination suppression at the perovskite/hole transport layer (HTL) interface. Ultra-thin RL idea is combined with some optical improvements in the device architecture, and stable high-efficient perovskite/Si tandem solar cells with 32.5% power conversion efficiency (PCE) and 80% fill factor (FF) values are realized. In addition, the preliminary examples of tandem devices with a larger active area (4 cm2 ) are presented. Finally, the remaining challenges and alternative concepts are also discussed.

In this thesis, the structural, optical, and electrical properties of wurtzite MgxZn1-xO:Al and MgxZn1-xO:Ga thin films have been investigated in dependence on Mg and dopant concentration. Among the transparent conductive oxides (TCOs), ZnO based compounds have gained renewed interest as a transparent electrode for large scale applications such as defroster windows, at panel displays, touch screens, and thin film solar cells due to low material and processing cost, non-toxicity, and suitable physical properties. In general, these applications require transparent electrodes with lowest possible resistivity of rho < 10^-3 Ohmcm and lower [1]. Recently, it has been reported that Ga and Al doped ZnO thin films can be deposited with respective resistivity of 5x10^-5 Ohmcm [2] and 3 x10^-5 Ohmcm [3] which are similar to the data obtained for other practical TCOs, i.e. the resistivity of about 4x 10^-5 Ohmcm for Sn doped In2O3 (ITO) thin films [4]. Moreover, the bandgap of ZnO can be increased by alloying with Mg offering band alignment between transparent electrode and active (or buffer) layer of the device, e.g. Cu(In,Ga)Se2 solar cells [5]. The tunable bandgap of these transparent electrodes can further increase the efficiency of the devices by avoiding energy losses in the interface region of the layers. From this point of view, this work has been aimed to investigate the doping efficiency and limits in transparent conductive (Mg,Zn)O alloys. For this purpose, the samples investigated in this work have been grown by pulsed-laser deposition (PLD) using a novel, continuous composition spread method (CCS). In general, this method allows to grow thin films with lateral composition gradient(s) [6, 7]. All MgxZn1-xO:Al and MgxZn1-xO:Ga thin films have been deposited on 2-inch in diameter glass, c- or r-plane sapphire substrates using threefold segmented PLD targets in order to grow thin films with two perpendicular, lateral composition gradients, i.e. the Mg composition is varied in one direction whereas the Al/Ga concentration is varied in a perpendicular direction [7, 8]. In order to investigate the influence of the temperature, samples grown at different substrate temperatures in the range of 25 to 600 C were investigated. The optical and electrical measurements have been carried out on (5x 5)mm^2 samples that were cut from the CCS wafers along the respective composition gradients, i.e. Mg and Al/Ga contents. Subsequently, physical properties of thin films have been analyzed for a large range of Al/Ga content between 0.5 and 7 at.%, which corresponds to doping concentrations between 2x 10^20 and 3x 10^21 cm^-3, for different Mg contents x(Mg) ranging from 0.01 to 0.1. It has been found that practically the limiting the dopant concentrations is about 2 x10^21 cm^-3. Further, the electrical data suggests, that the compensating intrinsic defect is doubly chargeable hinting to the zinc vacancy (V_Zn) as microscopic origin. Increasing the dopant concentration above 2 x10^21 cm^-3 leads to a degradation of electrical and structural properties [8]. Further, the influence of growth and annealing temperatures on structural, electrical and optical properties of the films has been studied. For that purpose, Al and Ga doped (2.5 at.% = 1x10^21 cm^-3) Mg0.05Zn0.95O thin films have been chosen from CCS samples grown at T_g = (25 - 600) C . For both doping series, the samples grown at higher temperatures exhibit better crystalline quality compared to the samples grown at lower growth temperatures. As a result, samples grown at higher temperatures reveal higher Hall mobility. For the Al-doping series, the highest free charge carrier density of n = 8.2x 10^20 cm^-3 was obtained for an Mg0.05Zn0.95O:Al thin film grown at 200 C, with corresponding Hall mobility of mu = 13.3 cm^2/Vs, a resistivity of rho = 5.7x10^-4 Ohmcm, and optical bandgap of E_g = 3.8 eV. Interestingly, the free charge carrier density of n = (5 - 8) x 10^20 cm^-3 for samples grown with T_g > 300 C is clearly higher than the value of n = 1.25 x 10^20 cm^-3 that was obtained for the high temperature grown sample, i.e. at T_g = 600 C. Furthermore, for all T_g, Al-doped films have a higher doping efficiency than the Ga-doped counterparts. In order to look deeper into the microscopic origin of this behavior, the samples were post-annealed in vacuum at 400 C. Experimental results showed that the free charge carrier density of Al-doped samples first decreased and saturated afterward with increasing annealing time. On the other hand, the free charge carrier density of the Ga-doped samples first slightly increased and saturated with increasing annealing time. For both doping series, the saturation value of n ~ 1 x 10^20 cm^-3 was very close to the data that has been observed for (i) high temperature grown samples and (ii) the solubility limit of Al in ZnO of 0.3 at.% = 1.2x 10^20 cm^-3, that has been determined by Shirouzu et al. for high temperature grown (T_g > 600 C) Al-doped ZnO [9]. Correspondingly, the optical bandgap also changed, i.e. increased (decreased) for Al- (Ga-) doping series, and approached a constant value of 3.5 0 +- 0.1 eV which is explained by generation of acceptor-like compensating defects, and the solubility limit of the dopants. From XRD data, no secondary phases were found for as-grown and post-annealed films. However, the slight improvement of crystalline quality has been observed on post-annealed samples. Further, it has been shown that the growth and annealing temperatures are important as they strongly affect the metastable state of the solid solution that samples grown at low temperature represent. The low solubility limit of the dopants, i.e. 0.3 at.% for Al in ZnO under equilibrium condition, can be increased by preparing samples by non-equilibrium growth techniques [10]. This is also consistent with experimental results of this work that Al- as well as Ga-doped metastable ZnO and (Mg,Zn)O thin films can be prepared with highest possible doping efficiency for the dopant concentration up to 2.5 at.% when growth or annealing temperatures below 400 C are used.

No description available.

Neste trabalho foram realizados e estudados tratamentos superficiais sobre óxidos transparentes condutivos (TCOs) depositados sobre vidro, cuja aplicação ou finalidade é a montagem de dispositivos poliméricos eletroluminescentes. A principal intenção da utilização destes processos é diminuir a tensão de limiar e também aumentar a luminância desses dispositivos, sem interferir na transmitância original dos filmes. Três diferentes técnicas de tratamentos superficiais foram utilizadas: (a) Plasma de oxigênio; (b) Água-régia e (c) UV-Ozônio. Neste último processo, um reator foi montado utilizando uma lâmpada de vapor de mercúrio a alta pressão (tipo alta intensidade de descarga), sem o bulbo externo para fornecer a disponibilidade de radiação UV para a obtenção de ozônio a partir do ar atmosférico. Este reator com baixo custo e fácil manuseio foi montado para realizar um processo alternativo comparado aos dois processos anteriormente citados (Plasma de oxigênio e Água-Régia) e constitui o principal foco, comparando a partir de resultados experimentais obtidos por dispositivos montados, utilizando diferentes TCOs. Foi possível confirmar que o procedimento a partir do UV-Ozônio é reprodutível, pois pode substituir com vantagens as outras duas técnicas que apresentam custo mais elevado ou que exige manuseio especial. Pela utilização de diferentes períodos de tratamento como a única variável, nas condições estabelecidas durante os experimentos, foi mantida uma amostra sem tratamento para comparação em cada resultado obtido. Em comparação aos outros tratamentos, a técnica de UV-ozônio apresentou reprodutibilidade. Neste caso, verificamos que houve eliminação de contaminantes indesejáveis como carbono e hidrocarbonetos detectadas pela técnica de DRIFT (Diffuse Reflectance Infra-Red Fourier Transformed) e melhor espalhamento de polímero (PEDOT:PSS) sobre a superfície através da técnica de ângulo de contato foi observado. Para os filmes de ITO e FTO o período ótimo foi observado durante 5 minutos e para o ZnO, durante 15 minutos. Os resultados das medições de resistência de folha, espessura e efeito Hall, não revelaram significantes modificações. Revelando que as superfícies foram influenciadas apenas atomicamente ou molecularmente. / In this work superficial treatment on transparent conductive oxides (TCOs) were carried out and studied by application or finality for the assembly of electroluminescent polymeric devices. The mean intention by use of these processes is to decrease the threshold voltage and also increase the luminance of the devices, without interfering in the original TCOs transmittances. Three different treatment techniques were used: (a) oxygen plasma; (b) aqua regia and (c) UV-ozozne. In the he last one, a reactor was assembled using a high-pressure mercury vapor lamp (high intensity discharge lamp type) without outer bulb to provide the available UV radiation to obtain ozone from atmospheric air. This reactor with low cost and easy handle was mounted to accomplish an alternative process compared by other (oxygen plasma and aqua regia) and it has the main focus of this work compared from experimental results obtained by mounted devices using different TCOs. It was possible to confirm that the procedure from the UV-Ozone is reproducible, because it can replaced with advantages the other techniques that have expansive costs or special handling. The use of different treatment times as only variable on the imposed condition in the experiments, a sample was reserved without treatment for comparison during each obtained result. In comparison with other treatments the UV-Ozônio presented reproducibility. In this case was verified the undesirable contaminants eliminated as carbon and hydrocarbon and detected by DRIFT (Diffuse Reflectance Infra-Red Fourier Transformed) technique and better scattering of polymer (PEDOT:PSS) on surface by contact angle was observed. For ITO and FTO films the optimum period was observed during 5 minutes and ZnO during 15 minutes. The measurements results of sheet resistance, thickness and Hall effect revealed no significant changes confirming that the surfaces were influenced only atomically or molecularly only.

Neste trabalho foram realizados e estudados tratamentos superficiais sobre óxidos transparentes condutivos (TCOs) depositados sobre vidro, cuja aplicação ou finalidade é a montagem de dispositivos poliméricos eletroluminescentes. A principal intenção da utilização destes processos é diminuir a tensão de limiar e também aumentar a luminância desses dispositivos, sem interferir na transmitância original dos filmes. Três diferentes técnicas de tratamentos superficiais foram utilizadas: (a) Plasma de oxigênio; (b) Água-régia e (c) UV-Ozônio. Neste último processo, um reator foi montado utilizando uma lâmpada de vapor de mercúrio a alta pressão (tipo alta intensidade de descarga), sem o bulbo externo para fornecer a disponibilidade de radiação UV para a obtenção de ozônio a partir do ar atmosférico. Este reator com baixo custo e fácil manuseio foi montado para realizar um processo alternativo comparado aos dois processos anteriormente citados (Plasma de oxigênio e Água-Régia) e constitui o principal foco, comparando a partir de resultados experimentais obtidos por dispositivos montados, utilizando diferentes TCOs. Foi possível confirmar que o procedimento a partir do UV-Ozônio é reprodutível, pois pode substituir com vantagens as outras duas técnicas que apresentam custo mais elevado ou que exige manuseio especial. Pela utilização de diferentes períodos de tratamento como a única variável, nas condições estabelecidas durante os experimentos, foi mantida uma amostra sem tratamento para comparação em cada resultado obtido. Em comparação aos outros tratamentos, a técnica de UV-ozônio apresentou reprodutibilidade. Neste caso, verificamos que houve eliminação de contaminantes indesejáveis como carbono e hidrocarbonetos detectadas pela técnica de DRIFT (Diffuse Reflectance Infra-Red Fourier Transformed) e melhor espalhamento de polímero (PEDOT:PSS) sobre a superfície através da técnica de ângulo de contato foi observado. Para os filmes de ITO e FTO o período ótimo foi observado durante 5 minutos e para o ZnO, durante 15 minutos. Os resultados das medições de resistência de folha, espessura e efeito Hall, não revelaram significantes modificações. Revelando que as superfícies foram influenciadas apenas atomicamente ou molecularmente. / In this work superficial treatment on transparent conductive oxides (TCOs) were carried out and studied by application or finality for the assembly of electroluminescent polymeric devices. The mean intention by use of these processes is to decrease the threshold voltage and also increase the luminance of the devices, without interfering in the original TCOs transmittances. Three different treatment techniques were used: (a) oxygen plasma; (b) aqua regia and (c) UV-ozozne. In the he last one, a reactor was assembled using a high-pressure mercury vapor lamp (high intensity discharge lamp type) without outer bulb to provide the available UV radiation to obtain ozone from atmospheric air. This reactor with low cost and easy handle was mounted to accomplish an alternative process compared by other (oxygen plasma and aqua regia) and it has the main focus of this work compared from experimental results obtained by mounted devices using different TCOs. It was possible to confirm that the procedure from the UV-Ozone is reproducible, because it can replaced with advantages the other techniques that have expansive costs or special handling. The use of different treatment times as only variable on the imposed condition in the experiments, a sample was reserved without treatment for comparison during each obtained result. In comparison with other treatments the UV-Ozônio presented reproducibility. In this case was verified the undesirable contaminants eliminated as carbon and hydrocarbon and detected by DRIFT (Diffuse Reflectance Infra-Red Fourier Transformed) technique and better scattering of polymer (PEDOT:PSS) on surface by contact angle was observed. For ITO and FTO films the optimum period was observed during 5 minutes and ZnO during 15 minutes. The measurements results of sheet resistance, thickness and Hall effect revealed no significant changes confirming that the surfaces were influenced only atomically or molecularly only.

Transparent conductive oxides (TCOs) are key functional materials in existing and future electro-optical devices in the fields of energy efficiency, energy generation and information technology. The main application of TCOs is as thin films transparent electrodes where a combination of maximum electrical conductivity and transmittance in the visible to nearinfrared spectral range is required. However, due to the interdependence of the optical properties and the free electron density and mobility, respectively, these requirements cannot be achieved simultaneously in degenerately doped wide band-gap oxide semiconductors. Therefore, a detailed understanding of the mechanisms governing the generation of free charge carriers by extrinsic doping and the charge transport in these materials is essential for further development of high performance TCOs and corresponding deposition methods. The present work is aimed at a comprehensive investigation of the electrical, optical and structural properties as well as the elemental composition of (Al,Ga) doped ZnO and Nb doped TiO2 thin films prepared by pulsed DC reactive magnetron sputtering. The evolution of the film properties is studied in dependence of various deposition parameters through a combination of characterization techniques including Hall-effect, spectroscopic ellipsometry, spectral photometry, X-ray diffraction, X-ray near edge absorption, Rutherford backscattering spectrometry and particle induced X-ray emission. This approach resulted in the development of an alternative process control method based on the material specific current-voltage pressure characteristics of the reactive magnetron discharge which allows to precisely control the oxygen deficiency of the sputter deposited films. Based on the experimental data, models have been established that describe the room temperature charge transport properties and the dielectric function of the obtained ZnO and TiO2 based transparent conductors. On the one hand, these findings allow the prediction of material specific electron mobility limits by identifying the dominating charge carrier scattering mechanisms. On the other hand, new insight is gained into the origin of the observed transition from highly conductive to electrically insulating ZnO layers upon the incorporation of increasing concentrations of Al at elevated growth temperatures. Moreover, the Al and Ga dopant activation in ZnO have been quantified systematically for a wide range of Al concentrations and deposition conditions. A direct comparison of the Ga and Al doping efficiency demonstrates that Ga is a more efficient electron donor in ZnO. Further, it has been shown that high free electron mobilities in polycrystalline and epitaxial Nb:TiO2 layers can be achieved by reactive magnetron sputtering of TiNb alloy targets. The suppression of rutile phase formation and the control of the Nb dopant activation by fine tuning the oxygen deficiency have been identified as crucial for the growth of high quality TiO2 based TCO layers.

In this thesis, the structural, optical, and electrical properties of wurtzite MgxZn1-xO:Al and MgxZn1-xO:Ga thin films have been investigated in dependence on Mg and dopant concentration. Among the transparent conductive oxides (TCOs), ZnO based compounds have gained renewed interest as a transparent electrode for large scale applications such as defroster windows, at panel displays, touch screens, and thin film solar cells due to low material and processing cost, non-toxicity, and suitable physical properties. In general, these applications require transparent electrodes with lowest possible resistivity of rho < 10^-3 Ohmcm and lower [1]. Recently, it has been reported that Ga and Al doped ZnO thin films can be deposited with respective resistivity of 5x10^-5 Ohmcm [2] and 3 x10^-5 Ohmcm [3] which are similar to the data obtained for other practical TCOs, i.e. the resistivity of about 4x 10^-5 Ohmcm for Sn doped In2O3 (ITO) thin films [4]. Moreover, the bandgap of ZnO can be increased by alloying with Mg offering band alignment between transparent electrode and active (or buffer) layer of the device, e.g. Cu(In,Ga)Se2 solar cells [5]. The tunable bandgap of these transparent electrodes can further increase the efficiency of the devices by avoiding energy losses in the interface region of the layers. From this point of view, this work has been aimed to investigate the doping efficiency and limits in transparent conductive (Mg,Zn)O alloys. For this purpose, the samples investigated in this work have been grown by pulsed-laser deposition (PLD) using a novel, continuous composition spread method (CCS). In general, this method allows to grow thin films with lateral composition gradient(s) [6, 7]. All MgxZn1-xO:Al and MgxZn1-xO:Ga thin films have been deposited on 2-inch in diameter glass, c- or r-plane sapphire substrates using threefold segmented PLD targets in order to grow thin films with two perpendicular, lateral composition gradients, i.e. the Mg composition is varied in one direction whereas the Al/Ga concentration is varied in a perpendicular direction [7, 8]. In order to investigate the influence of the temperature, samples grown at different substrate temperatures in the range of 25 to 600 C were investigated. The optical and electrical measurements have been carried out on (5x 5)mm^2 samples that were cut from the CCS wafers along the respective composition gradients, i.e. Mg and Al/Ga contents. Subsequently, physical properties of thin films have been analyzed for a large range of Al/Ga content between 0.5 and 7 at.%, which corresponds to doping concentrations between 2x 10^20 and 3x 10^21 cm^-3, for different Mg contents x(Mg) ranging from 0.01 to 0.1. It has been found that practically the limiting the dopant concentrations is about 2 x10^21 cm^-3. Further, the electrical data suggests, that the compensating intrinsic defect is doubly chargeable hinting to the zinc vacancy (V_Zn) as microscopic origin. Increasing the dopant concentration above 2 x10^21 cm^-3 leads to a degradation of electrical and structural properties [8]. Further, the influence of growth and annealing temperatures on structural, electrical and optical properties of the films has been studied. For that purpose, Al and Ga doped (2.5 at.% = 1x10^21 cm^-3) Mg0.05Zn0.95O thin films have been chosen from CCS samples grown at T_g = (25 - 600) C . For both doping series, the samples grown at higher temperatures exhibit better crystalline quality compared to the samples grown at lower growth temperatures. As a result, samples grown at higher temperatures reveal higher Hall mobility. For the Al-doping series, the highest free charge carrier density of n = 8.2x 10^20 cm^-3 was obtained for an Mg0.05Zn0.95O:Al thin film grown at 200 C, with corresponding Hall mobility of mu = 13.3 cm^2/Vs, a resistivity of rho = 5.7x10^-4 Ohmcm, and optical bandgap of E_g = 3.8 eV. Interestingly, the free charge carrier density of n = (5 - 8) x 10^20 cm^-3 for samples grown with T_g > 300 C is clearly higher than the value of n = 1.25 x 10^20 cm^-3 that was obtained for the high temperature grown sample, i.e. at T_g = 600 C. Furthermore, for all T_g, Al-doped films have a higher doping efficiency than the Ga-doped counterparts. In order to look deeper into the microscopic origin of this behavior, the samples were post-annealed in vacuum at 400 C. Experimental results showed that the free charge carrier density of Al-doped samples first decreased and saturated afterward with increasing annealing time. On the other hand, the free charge carrier density of the Ga-doped samples first slightly increased and saturated with increasing annealing time. For both doping series, the saturation value of n ~ 1 x 10^20 cm^-3 was very close to the data that has been observed for (i) high temperature grown samples and (ii) the solubility limit of Al in ZnO of 0.3 at.% = 1.2x 10^20 cm^-3, that has been determined by Shirouzu et al. for high temperature grown (T_g > 600 C) Al-doped ZnO [9]. Correspondingly, the optical bandgap also changed, i.e. increased (decreased) for Al- (Ga-) doping series, and approached a constant value of 3.5 0 +- 0.1 eV which is explained by generation of acceptor-like compensating defects, and the solubility limit of the dopants. From XRD data, no secondary phases were found for as-grown and post-annealed films. However, the slight improvement of crystalline quality has been observed on post-annealed samples. Further, it has been shown that the growth and annealing temperatures are important as they strongly affect the metastable state of the solid solution that samples grown at low temperature represent. The low solubility limit of the dopants, i.e. 0.3 at.% for Al in ZnO under equilibrium condition, can be increased by preparing samples by non-equilibrium growth techniques [10]. This is also consistent with experimental results of this work that Al- as well as Ga-doped metastable ZnO and (Mg,Zn)O thin films can be prepared with highest possible doping efficiency for the dopant concentration up to 2.5 at.% when growth or annealing temperatures below 400 C are used.