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The fullerene C60 is commonly applied as the electron transport layer in high‐efficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra‐sensitive near‐UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5–10 nm, the perovskite/C60 interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement‐induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C60 HOMO‐edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C60 interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C60.

In this research instrument article, the capabilities and details of a high‐throughput aging system for parallel maximum power point tracking (MPPT) of perovskite solar cells (PSCs), capable of assessing the operational stability of a large number of devices, are presented. The intention is to provide insights into the machine and the development process to engineers who want to create a similar system and eventually make MPPT stability testing of PSCs a standard procedure.

Formamidinium iodide FAI based perovskite absorbers have been shown to be ideal candidates for highly efficient and operationally stable perovskite solar cells PSC . A major challenge for formamidinium lead iodide FAPbI3 is to suppress the phase transition from the photoactive black phase into yellow nonperovskite amp; 948; phase. Several approaches to stabilize the black phase have been developed for solution based perovskites, whereas so far, vacuum deposited FAPbI3 has rarely been reported. This study demonstrates the preparation of FAPbI3 by co evaporation and discusses the influence of the subjacent hole transporting layer HTL on its phase stability. By using FAI excess in the evaporation process in combination with phosphonic acids groups from the HTL, the black perovskite phase is stabilized at room temperature. Further addition of 32 59 methylammonium iodide MAI during the co evaporation process leads to good absorption properties and high PSC efficiencies of 20.4 . In addition, excellent stability is achieved for optimized MAI to FAI ratios, maintaining 100 of the initial PSC performance after 1000 h under constant operation. This highly stable perovskite composition enables the first monolithic fully textured perovskite silicon tandem solar cells with co evaporated perovskite absorbers. Due to the conformally covered pyramid texture, these tandem cells show minimal reflection losses and reach an efficiency of 24.6

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long-term stability of devices. A detailed understanding of the ion migration-driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long-term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias-induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide-rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance.

Summary Monolithic [Cs0.05(MA0.17FA0.83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon., Graphical Abstract, Highlights • Halide perovskite sub-cells exhibit strong proton irradiation resiliency • Novel operando characterization distinguishes degradation of individual sub-cells • Perovskite/CIGS tandem solar cells retain 85% of their initial efficiency after irradiation • Perovskite/SHJ tandem solar cells degrade to 1% of their initial efficiency after irradiation, Context & Scale Monolithic perovskite/silicon and perovskite/CIGS tandem solar cells could facilitate large-scale decarbonization of the power sector, provided their long-term stability is proven. In this work, we test the stability of both technologies under high-energy proton irradiation. While this mimics the radiation environment in space, our versatile operando and ex situ methodology is also suitable for studying the long-term stability of multijunction solar cells for terrestrial applications. We find that perovskite/silicon tandem solar cells are unsuitable for space, whereas perovskite/CIGS tandems are radiation-hard, promising cheap, flexible, and ultra-lightweight space photovoltaics. Both the growing demand for smaller, cheaper satellites and the privatization of space exploration are revolutionizing space economics, providing an ideal niche for the commercialization of this new technology until the levelized cost-of-electricity can compete with current terrestrial photovoltaics., We propose and test monolithic perovskite/CIGS tandem solar cells for readily stowable, ultra-lightweight space photovoltaics. We design operando and ex situ measurements to show that perovskite/CIGS tandem solar cells retain over 85% of their initial power-conversion efficiency after high-energy proton irradiation. While the perovskite sub-cell is unaffected after this bombardment, we identify increased non-radiative recombination in the CIGS bottom cell and nickel-oxide-based recombination layer. By contrast, monolithic perovskite/silicon-heterojunction cells degrade to 1% of their initial efficiency due to radiation-induced defects in silicon.

Perovskite based tandem solar cells can increase the power conversion efficiency PCE of conventional single junction photovoltaic devices. Here, we present monolithic perovskite CIGSe tandem solar cells with a perovskite top cell fabricated directly on an as grown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiOx layer is conformally deposited via atomic layer deposition on the front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the polymer PTAA at the NiOx perovskite interface. This hole transport bilayer enables a 21.6 stabilized PCE of the tandem device at amp; 8764;0.8 cm2 active area. We use TEM EDX measurements to investigate the deposition uniformity and conformality of the NiOx and PTAA layers. By absolute photoluminescence measurements, the contribution of the individual subcells to the tandem VOC is determined, revealing that further fine tuning of the recombination layers might improve the tandem VOC. Finally, on the basis of the obtained results, we give guidelines to improve monolithic perovskite CIGSe tandems toward predicted PCE estimates above 30

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long‐term stability of devices. A detailed understanding of the ion migration‐driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long‐term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias‐induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide‐rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance.

In this paper, we report on the current status of absorber attributes in Liquid Phase Crystallized Silicon (LPC-Si) cells. To this end, an absorber doping series (ND = 2·1016/cm³ to 7·1017/cm³) and an absorber thickness variation (14 and 33 µm) are evaluated. Best cells from the batches for these series showed open circuit voltages up to 640 mV and fill factors above 70%. It is observed that for state-of-the-art cells, thicknesses over 15 µm are not beneficial due to limited diffusion lengths. Lower absorber doping concentrations tend to yield longer intra-grain diffusion lengths (Ldiff) and better passivated grain boundaries, which may be due to lower impurity precipitation. The longer Ldiff leads to higher short circuit current densities which over-compensate a decrease in open circuit voltage and fill factor with regards to efficiency. Both front and rear surfaces are sufficiently passivated and at the current status the bulk lifetime has most potential for improvement.

Singlet exciton fission is an exciton multiplication process that occurs in certain organic materials, converting the energy of single highly-energetic photons into pairs of triplet excitons. This could be used to boost the conversion efficiency of crystalline silicon solar cells by creating photocurrent from energy that is usually lost to thermalisation. An appealing method of implementing singlet fission with crystalline silicon is to incorporate singlet fission media directly into a crystalline silicon device. To this end, we developed a solar cell that pairs the electron-selective contact of a high-efficiency silicon heterojunction cell with an organic singlet fission material, tetracene, and a PEDOT:PSS hole extraction layer. Tetracene and n-type crystalline silicon meet in a direct organic–inorganic heterojunction. In this concept the tetracene layer selectively absorbs blue-green light, generating triplet pairs that can dissociate or resonantly transfer at the organo-silicon interface, while lower-energy light is transmitted to the silicon absorber. UV photoemission measurements of the organic–inorganic interface showed an energy level alignment conducive to selective hole extraction from silicon by the organic layer. This was borne out by current–voltage measurements of devices subsequently produced. In these devices, the silicon substrate remained well-passivated beneath the tetracene thin film. Light absorption in the tetracene layer created a net reduction in current for the solar cell, but optical modelling of the external quantum efficiency spectrum suggested a small photocurrent contribution from the layer. This is a promising first result for the direct heterojunction approach to singlet fission on crystalline silicon.

Perovskite solar cells have been shown to be of extraordinary radiation hardness, considering high energetic 68 MeV proton irradiation with doses up to 1013 p cm amp; 8722;2. In this study electrical and photoelectrical properties of perovskite solar cells with and without proton irradiation are analyzed in detail. The results reveal that proton irradiation improves the open circuit voltage, fill factor, and recombination lifetime of photogenerated charge carriers in perovskite solar cells. These enhancements are mainly a result of the lower nonradiative recombination losses in the proton irradiated devices. The proton treatment creates shallow traps, which may be associated with the proton induced point defects due to the displacements of atoms in the inorganic Pb I framework, which act as unintentional doping sources and partially compensate deep traps originated from the photodegradation of methylammonium molecules

Electron-beam evaporation (EBE) of silicon permits the high-rate deposition of photovoltaic thin-film devices at low costs. The directional, non-conformal growth characteristic of EBE is systematically investigated by varying the silicon flux angle of incidence γ on the substrate surface between 0° and 49°. After solid phase crystallization the micro-structural properties of these silicon films are investigated and correlated with the electronic quality of n + / p − /p + -type solar cell stacks. As γ exceeds 30°, the porosity and oxygen content of the silicon films increase significantly coming along with the break-down of the electronic material quality. At γ >40° the silicon crystallization process is even found to be suppressed resulting from a columnar film morphology infiltrated by oxygen-rich pores. The knowledge of this critical angle is essential when textured substrates, consisting of many tilted micro-areas, are used for enhanced light absorption in the silicon film simultaneously ensuring the growth of high-quality material. Furthermore, the inclination angle γ can serve as design parameter for tailored substrate templates for the fabrication of advanced light-harvesting structures by self-organized solid phase crystallization.

The perovskite / silicon-heterojunction (SHJ) tandem solar cell can theoretically overcome the efficiencies of the single junction solar cells with experimental results not far behind. Here, we use optical simulations to analyze the potential of such currently feasible monolithic perovskite / SHJ tandem devices. We consider three different device designs in the optical simulations: planar device, device built on back-side textured Si wafer, and device with textured UV Nanoimprint Lithography light management foil. For each of these three designs, the current matching point is simulated to evaluate device efficiencies. While the device with back-side textured Si wafer causes light trapping and therefore enhanced photocurrent generation for longer wavelengths, the anti-reflection foils prove to be overall more efficient as the foil significantly reduces the reflection in almost full wavelength range.

Perovskites for tandem solar cells Improving the performance of conventional single-crystalline silicon solar cells will help increase their adoption. The absorption of bluer light by an inexpensive overlying solar cell in a tandem arrangement would provide a step in the right direction by improving overall efficiency. Inorganic-organic perovskite cells can be tuned to have an appropriate band gap, but these compositions are prone to decomposition. McMeekin et al. show that using cesium ions along with formamidinium cations in lead bromide–iodide cells improved thermal and photostability. These improvements lead to high efficiency in single and tandem cells. Science , this issue p. 151

The hybrid perovskite methylammonium lead iodide CH3NH3PbI3 recently revealed its potential for the manufacturing of low-cost and efficient photovoltaic cells. However, many questions remain unanswered regarding the physics of the charge carrier conduction. In this respect, it is known that two structural phase transitions, occurring at temperatures near 160 and 310 K, could profoundly change the electronic properties of the photovoltaic material, but, up to now, a clear experimental evidence has not been reported. In order to shed light on this topic, the low-temperature phase transition of perovskite solar cells has been thoroughly investigated by using electric noise spectroscopy. Here it is shown that the dynamics of fluctuations detect the existence of a metastable state in a crossover region between the room-temperature tetragonal and the low-temperature orthorhombic phases of the perovskite compound. Besides the presence of a noise peak at this transition, a saturation of the fluctuation amplitudes is observed induced by the external DC current or, equivalently, by light exposure. This noise saturation effect is independent on temperature, and may represent an important aspect to consider for a detailed explanation of the mechanisms of operation in perovskite solar cells.

In this study, we investigate three screen-printed metallization concepts for back-contact back-junction silicon solar cells with an edge length of 156 mm: 1) a busbar-less concept with periodically interrupted contact fingers for wire-based interconnection; 2) a single-layer concept with printed busbars and periodically interrupted contact fingers; and 3) a multilayer concept consisting of continuous contact fingers, an insulation layer, and busbar metallization. A comprehensive simulation study is presented for all investigated metallization concepts, yielding their respective performance loss mechanisms. The multilayer approach is found to provide superior conversion efficiencies η, compared with the single-layer approach. For the wire-based concept, we show that contact finger interruptions up to a width of 1 mm have no significant negative impact on cell performance ( $\Delta \eta ). Furthermore, peel force measurements on test structures between soldered cell interconnectors and screen-printed metallization are discussed for the multilayer and the wire-based concept. Successful proof of principles with peel forces exceeding 1 N/mm are demonstrated for both investigated metallization concepts.

A wide variety of liquid and solid phase crystallized silicon films are investigated in order to determine the performance limiting defect types in crystalline silicon thin-film solar cells. Complementary characterization methods, such as electron spin resonance, photoluminescence, and electron microscopy, yield the densities of dangling bond defects and dislocations which are correlated with the electronic material quality in terms of solar cell open circuit voltage. The results indicate that the strongly differing performance of small-grained solid and large-grain liquid phase crystallized silicon can be explained by intra-grain defects like dislocations rather than grain boundary dangling bonds. A numerical model is developed containing both defect types, dislocations and dangling bonds, describing the experimental results.

Combining orientation dependent electrically detected magnetic resonance and g tensor calculations based on density functional theory we assign microscopic structures to paramagnetic states involved in spin-dependent recombination at the interface of hydrogenated amorphous silicon crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. We find that (i) the interface exhibits microscopic roughness, (ii) the electronic structure of the interface defects is mainly determined by c-Si, (iii) we identify the microscopic origin of the conduction band tail state in the a-Si:H layer, and (iv) present a detailed recombination mechanism.

The electronic structure of the interface between the boron-doped oxygenated amorphous silicon “window layer” (a-SiOx:H(B)) and aluminum-doped zinc oxide (ZnO:Al) was investigated using hard x-ray photoelectron spectroscopy and compared to that of the boron-doped microcrystalline silicon (μc- Si:H(B))/ZnO:Al interface. The corresponding valence band offsets have been determined to be (−2.87 ± 0.27) eV and (−3.37 ± 0.27) eV, respectively. A lower tunnel junction barrier height at the μc-Si:H(B)/ZnO:Al interface compared to that at the a-SiOx:H(B)/ZnO:Al interface is found and linked to the higher device performances in cells where a μc-Si:H(B) buffer between the a-Si:H p-i-n absorber stack and the ZnO:Al contact is employed.

In this paper, interdigitated back contacted silicon heterojunction (IBC-SHJ) solar cell results as well as two dimensional device simulations are presented. The simulation indicates that for the minority contact (emitter) the coverage of the metallisation should be nearly 100%, while this is less critical for the majority contact (BSF). We present experimental results for an IBC-SHJ solar cell with a metallisation fraction of 100% for the emitter and approximately 65% for the BSF, with a high fill factor of 77.7% and an independently confirmed energy conversion efficiency of 19.4%.

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (>2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance.

The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si-O bonds takes place at the expense of Zn-O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3644084]

Electrically detected electron-spin-echo envelope modulations (ED-ESEEM) were employed to detect hyperfine interactions between nuclear spins and paramagnetic sites, determining spin-dependent transport processes in multilayer thin-film microcrystalline silicon solar cells. Electrical detection in combination with a modified Hahn-echo sequence was used to measure echo modulations induced by ${}^{29}$Si, ${}^{31}$P, and ${}^{1}$H nuclei weakly coupled to electron spins of paramagnetic sites in the amorphous and microcrystalline solar cell layers. In the case of CE centers in the $\ensuremath{\mu}$c-Si:H $i$-layer, the absence of ${}^{1}$H ESEEM modulations indicates that the adjacencies of CE centers are depleted from hydrogen atoms. On the basis of this result, we discuss several models for the microscopic origin of the CE center and conclusively assign those centers to coherent twin boundaries inside of crystalline grains in $\ensuremath{\mu}$c-Si:H.

International audience; Pulsed electrically detected magnetic resonance (pEDMR) was employed to study spin-dependent processes that influence charge transport in microcrystalline (μc-Si:H) pin solar cells. Special emphasis was put on the identification of the signals with respect to the individual layers of the cell structure. For this to achieve, we systematically modulated the morphology of the highly doped n- and p-layers from amorphous to microcrystalline. By combining the information obtained from low-temperature (T = 10 K) pEDMR spectra and from the deconvoluted time evolution of spectrally overlapping resonances, we found signals from conduction band tail states as well as phosphorus donor states in samples containing an amorphous n-type layer and a resonance associated with valence band tail states in samples with amorphous p-layer. Moreover, several signals from the intrinsic microcrystalline absorber layers could be identified. An additional resonance at g = 1.9675(5), which has not been observed in EDMR before, was found. We assign this signal to shallow donors in the Al-doped ZnO layer which is commonly used as transparent conducting oxide in thin-film solar cells. The experimental findings are discussed in the light of various spin-dependent transport mechanisms known to occur in the respective layers of the pin structure.

Transparent conductive oxides (TCOs) with tailor-made electrical and optical properties are essential for a variety of applications. The linkage of optical and electrical properties in these films is usually described by the Drude theory despite the fact, that earlier investigations have already shown that the Drude theory fails to accurately describe optical properties. In this work we use an extension to the Drude theory to model optical spectra of reactively sputtered ZnO:Al thin films. The model uses a simple analytic expression, which can describe the general course of the dielectric function as calculated by others with more elaborate models. Using this approach optical spectra can be accurately modeled with low computational effort and reliable values for plasma frequency and damping can be obtained. By comparing these results with Hall measurements we derived the effective mass of free carriers m ⁎ in the conduction band as a function of carrier concentration. The results clearly showed the non-parabolicity of the conduction band for high doping levels. Using the band structure proposed by Pisarkiewicz and assuming an effective mass of 0.24 electron masses at the conduction band minimum we derived a non-parabolicity parameter of ~ 0.27 (2) eV − 1 . Finally we used the values for the effective mass to determine carrier mobility from optical measurements. Contradictions to Hall measurements can be explained by the effect of grain boundary scattering.

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (mu c-Si:H) has been systematically investigated. Single mu c-Si:H layers and complete mu c-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of similar to 1.2 mu m thick mu c-Si: H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2 x 10(19) to 2 x 10(19) cm(-3); in the case of nitrogen contamination the critical impurity level is lower ([N](critical)=6 x 10(18)-8 x 10(18) cm(-3)). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of mu c-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the mu c-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick mu c-Si:H solar cells are found to be more impurity-sensitive than thinner cells. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3104781]

An analysis of spin-dependent processes in microcrystalline silicon (μc-Si:H) pin solar cells is presented using pulsed electrically detected magnetic resonance (pEDMR). In this first study it is shown that by modulating the morphology of the n-type contact layer from amorphous to microcrystalline, pronounced changes in the pEDMR spectra may be observed. Due to the fact that pEDMR allows a deconvolution of the spin-dependent signals in time as well as in magnetic field domain, we were able to significantly reduce the complexity of the spectra compared to conventional EDMR. In the samples containing amorphous n-type contact layers we found signals from shallow localized conduction band tail states and phosphorous donor states. Upon replacement of this layer by its microcrystalline counterpart both signals disappeared. Possible spin-dependent transport mechanisms involving paramagnetic states in the various layers are discussed in view of sign and time evolution of the associated pEDMR signals.

The crystallization of thin silicon films at temperatures between 425 and 600 degrees C was investigated on glass substrates coated with Al-doped zinc oxide (ZnO:Al). Bare ZnO:Al layers degrade at the crystallization temperatures used. A silicon layer on top, however, efficiently prevents deterioration. The resistivity was even found to drop from 4.3x10(-4) Omega cm for the as deposited ZnO:Al to 2.2x10(-4) Omega cm in the case of aluminium induced crystallization and to 3.4x10(-4) Omega cm for solid phase crystallization. The temperature-stable conductivity of ZnO:Al films coated with Si opens up appealing options for the production of polycrystalline silicon thin-film solar cells with transparent front contacts. (C) 2007 American Institute of Physics.

This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been identified. Low amounts of target doping and low heater temperatures result in small and irregular features in the postetching surface topography, which does not scatter the light efficiently. At higher substrate temperatures and target doping levels, more regularly distributed craters evolve with mean opening angles between 120 degrees and 135 degrees and lateral sizes of 1-3 mu m. These layers are very effective in light scattering. In the third regime-at very high substrate temperatures and high doping levels-the postetching surface is rather flat and almost no light scattering is observed. We applied the ZnO:Al films as front contacts in thin-film silicon solar cells to study their light-trapping ability. While high transparency is a prerequisite, light trapping was improved by using front contacts with a surface topography consisting of relatively uniformly dispersed craters. We have identified a low amount of target doping (0.5-1 wt %) and relatively high substrate temperatures (about 350-450 degrees C as sputter parameters enabling short-circuit current densities as high as 26.8 mA/cm(2) in mu c-Si:H pin cells with an i-layer thickness of 1.9 mu m. Limitations on further improvements of light-trapping ability are discussed in comparison with the theoretical limitations and Monte Carlo simulations presented in the literature. (c) 2007 American Institute of Physics.

This paper addresses the plasma deposition of highly efficient microcrystalline silicon (μc-Si:H) p-i-n solar cells under conditions of high SiH4 utilization and low H2 dilution. It was established that the transient depletion of the initially present SiH4 source gas induces the formation of an amorphous incubation layer that prevents successful crystallite nucleation in the i-layer and leads to poor solar cell performance. The effect of this transient depletion induced incubation layer on solar cells was made visible through dedicated solar cell deposition series and selected area electron diffraction measurements. Applying a gas flow procedure at plasma ignition it was succeeded to prepare state-of-the-art μc-Si:H material and solar cells under low hydrogen dilution conditions, highlighted by μc-Si:H solar cells of up to 9·5% efficiency prepared using an undiluted source gas flow consisting solely of SiH4. Copyright © 2007 John Wiley & Sons, Ltd.

Microcrystalline silicon solar cells with an open-circuit voltage surpassing the 600 mV barrier were prepared by combining the use of a hot wire deposited p-i interface with that of an i layer deposited by radio-frequency parallel plate plasma deposition using controlled SiH4 flow profiling. The control of the bulk and interface properties facilitated effective charge carrier collection from i layers with a crystalline volume fraction as low as similar to 30%. Judging from the absorption in the infrared and the excellent charge carrier transport, the optoelectronic properties of this material were still dominated by the crystalline rather than the amorphous phase. (c) 2007 American Institute of Physics.

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.

This contribution discusses recent scientific and technological challenges for the development of highly efficient microcrystalline silicon (μc-Si:H) based thin film solar cells. Aluminium doped ZnO films prepared by sputtering and post deposition etching serve as transparent conductive oxide (TCO) material, which provide excellent light trapping properties. Challenges are the transfer of this approach to cost-effective reactive sputtering from metallic targets and the reduction of optical absorption losses in the front TCO films. We developed μc-Si:H i-layers by plasma-enhanced chemical vapour deposition (PECVD) using 13.56 and 40.68 MHz excitation frequency, high deposition pressures and high RF-powers. These conditions provide sufficiently “soft” deposition for the growth of high quality μc-Si:H material and yield high deposition rates. Stable aperture area module efficiencies of 10.1% and 8.1% were obtained for a-Si:H/μc-Si:H and μc-Si:H modules, respectively, on 10 × 10 cm2 substrate size. Subsequent up-scaling to ∼1 m2 coating area was performed and a-Si:H/μc-Si:H modules exceeding 10% initial aperture area efficiency were obtained. We discuss the questions of process stability and process reproducibility with respect to large area PECVD production systems.

A 13.56 MHz parallel plate hydrogen-diluted silane plasma, operated at high pressure and high power, was used to deposit microcrystalline silicon solar cells with efficiencies of 6–9% at high deposition rates of 0.4–1.2 nm/s. In this regime new challenges arise regarding temperature control, since the high plasma power causes the substrate to heat up significantly during film deposition. We investigated this effect of plasma-induced substrate heating experimentally by means of pyrometric substrate temperature measurements and spectroscopic gas temperature measurements. The substrate temperature was observed to increase by up to 100 K during film deposition, depending on power density and deposition time. Performance of deposited solar cells decreased whenever the plasma induced heating caused a drift outside the ideal temperature window, of around 475 K (∼200 °C). Further analysis related this decrease in performance to the substrate temperature's influence on film crystallinity and open circuit voltage.

Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are Usually achieved by RF Sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, oil the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented. (c) 2005 Elsevier B.V. All rights reserved.

Absorption losses at a nanorough silver back reflector of a solar cell were measured with high accuracy by photothermal deflection spectroscopy. Roughness was characterized by atomic force microscopy. The observed increase of absorption, compared to the smooth silver, was explained by the surface plasmon absorption. Two series of silver back reflectors (one covered with thin ZnO layer) were investigated and their absorption related to surface morphology. (C) 2004 American Institute of Physics.

This article reports on the use of ZnO films in silicon thin film p–i–n solar cells and modules. It summarizes the status in the final phase of a joint research project aiming at the development of high-quality ZnO/glass substrates feasible for an industrial solar module production. The samples were prepared by reactive mid-frequency (mf) sputtering on large area (60×100 cm2) glass sheets using low-cost metallic Zn:Al targets. These ZnO films exhibit resistivities down to 2.6×10−4 Ω cm and high optical transmittance. Variation of the sputter pressure leads to films with significantly differing etching behavior in diluted acids. ZnO layers prepared in the high pressure regime develop strongly textured light scattering surfaces after etching, which is necessary to obtain highly efficient solar cells. Initial efficiencies of small area amorphous silicon (a-Si:H) cells on texture-etched ZnO-films prepared by mf-sputtering on 60×100 cm2 soda-lime glass (3 mm thick) range from 8 to 9% (highest efficiency 9.2%, i-layer thickness 350 nm). First 0.6 m2 modules on ZnO prove the principal applicability of the films for an industrial manufacturing process.

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (pc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for pc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Angstrom/s, respectively. These pc-Si: H solar cells were successfully up-scaled to a substrate area of 30 x 30 cm(2) and applied in a-Si:H/muc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%. (C) 2002 Elsevier Science B.V. All rights reserved.