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Liquid phase crystallized silicon solar cells on glass have recently demonstrated 15.1% efficiency using a heterojunction interdigitated back contact cell architecture and an absorber thickness of 14 $\mu$ m. One of the key enabling developments was a new method to first passivate electron contact fingers with a-Si:H(i) and then locally laser fire them to maintain a low contact resistance. In this work, high resolution, light-beam induced current measurements (LBIC) were used to analyze this approach. Charge collection was observed to have increased from 0.13 ${\text{mAcm}}^{-2}$ to 0.9 ${\text{mAcm}}^{-2}$ under the electron contact which is a sevenfold increase. Using 520, 642, 932, and 1067 nm wavelengths of incident light, external quantum efficiency was mapped in regions including grain boundaries, dislocation defects, shunts, defect-free regions, and laser fired spots. Reduction of charge collection in the laser fired spots was limited to diameters of 50–20 $\mu$ m, depending on whether electrical recombination or optical losses dominated. Effective minority carrier diffusion length under the majority carrier contacts was obtained by fitting of LBIC measurements. It was observed to have improved from 20.5 $\mu$ m to 22.7–40.4 $\mu$ m and up to 89.0 $\mu$ m in the best case. Based on this, wider contact fingers and improved surface passivation at the electron contact is encouraged in the near future to achieve efficiencies $\geq$ 16%.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V-OC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V-OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

Liquid phase crystallization of silicon (LPC-Si) offers great potential for high-quality Si films and a cost effective fabrication technique for thin crystalline silicon solar cells on glass. In this work, we report on the progress on LPC-silicon at HZB in the past years. Beginning with a brief description of the fabrication process, we summarize the work on the different contact systems developed for these absorbers before focusing on the interdigitated back contact architecture on which the highest efficiencies were reported. State-of-the art cells form the basis for a detailed discussion of the status of this technology. We investigate the current loss mechanisms and explore the potential for further improvement. Finally, based on this comprehensive quality assessment, we develop a roadmap to increase the cell efficiencies to wafer-equivalent values.

Laser based patterning for monolithic serial interconnection of metal halide perovskite MHP solar cells is a key process for industrial manufacturing of large scale MHP solar panels. It requires reliable patterning process parameters to achieve low interconnection losses and, thus, high efficiencies. Here, P2 laser patterning of the perovskite layer was obtained by laser ablation using conventional nanosecond laser pulses at systematically varied laser fluences. The correlation of the laser impact to the morphology, composition, and electrical functionality was analyzed in detail by several surface analytical techniques. The occurrence of laser induced periodic surface structures and microdroplets at the bottom of the trenches indicates that material removal via stress assisted ablation is strongly influenced by thermal processes. The formation of PbI 2 containing residuals was evidenced, possibly causing contact resistance losses through the P2 interconnect. These results contribute to the identification of loss factors in laser based serial interconnection of perovskite solar cells and to further process optimization for upscaling to industrial module sizes

Liquid phase crystallization of silicon (LPC-Si) on glass is a promising method to produce high quality multi-crystalline Si films with macroscopic grains. In this study, we report on recent improvements of our interdigitated back-contact silicon heterojunction contact system (IBC-SHJ), which enabled open circuit voltages as high as 661 mV and efficiencies up to 14.2% using a 13 µm thin n-type LPC-Si absorbers on glass. The influence of the BSF width on the cell performance is investigated both experimentally and numerically. We combine 1D optical simulations using GenPro4 and 2D electrical simulations using Sentaurus™ TCAD to determine the optical and electrical loss mechanisms in order to estimate the potential of our current LPC-Si absorbers. The simulations reveal an effective minority carrier diffusion length of 26 µm and further demonstrate that a doping concentration of 4 × 10 16 cm −3 and a back surface field width of 60 µm are optimum values to further increase cell efficiencies.

Liquid-phase crystallization (LPC) of thin silicon layers on glass substrates is a technique to fabricate solar cells with low energy and material consumption and open-circuit voltages comparable to multicrystalline silicon wafer cells. We studied the impact of different passivation layers deposited with plasma-enhanced chemical vapor deposition (PECVD) on the cell quality. Silicon nitride (SiNx) and ultrathin silicon oxide (SiO x ) layers with varying thicknesses were used. In addition, we plasma-treated the SiN x surface to form a thin silicon oxynitride (SiO x N y ) passivation layer. Plasma oxidation is an attractive alternative to PECVD, since thicknesses of ultrathin layers can be controlled easier as compared with PECVD. We found that the cell performance is influenced by the passivation layer. Particularly, cells on the 9 nm PECVD SiO x passivation layer are worse than cells on the 11 nm SiO x N y layer. We modeled cell parameters employing ASPIN3 to estimate effective diffusion lengths. Using transmission electron microscopy as well as electron energy loss spectroscopy, we studied the effect of the plasma treatment on the morphology and elemental distribution at the interface between passivation layer and LPC-Si absorber. Our best interdigitated back contacted solar cell with an efficiency >14% is based on the SiO x N y layer.

Liquid phase crystallization using line-shaped energy sources such as CW-diode lasers or electron beams has proven to form mc-Si layers on borosilicate or borosilicate/aluminosilicate glass that exhibit wafer equivalent grain sizes and electronic quality. In this work, we characterize the impact of the employed dielectric interlayer stack sandwiched between glass and absorber on the electronic quality. For this purpose, we investigate a large variety of test cell results achieved in the past on different interlayer stacks composed of silicon oxide, silicon nitride as well as silicon oxynitride deposited by means of plasma enhanced chemical vapour deposition or plasma oxidation and employ i(v), SunsVoc, quantum efficiency measurements, and photoluminescence imaging to assess the electronic properties of the crystallized absorbers. The results are compared with state-of the art interdigitated back-contact cells and literature values. Based on these findings, we conclude that at the present state of interlayer , the bulk quality imposes the limits in cell efficiency and investigates potential approaches to increase the bulk quality of LPC-Si absorbers.

Inorganic-organic perovskites like methylammonium-lead-iodide have proven to be an effective class of 17 materials for fabricating efficient solar cells. To improve their performance, light management techniques using textured surfaces, similar to those used in established solar cell technologies, should be considered. Here, we apply a light management foil created by UV nanoimprint lithography on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA cm(-2) increase in the short-circuit current density translates to a relative improvement in cell performance of 5%, which results in a power conversion efficiency of 17.1%. Optical 3D simulations based on experimentally obtained parameters were used to support the experimental findings. A good match between the simulated and experimental data was obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current density of up to 10% is possible for large-area devices. Therefore, our results present the potential of light management foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in the field of perovskite solar cells.

—Liquid phase crystallized silicon (LPC-Si) solar cells on glass have demonstrated 14.2 % efficiency using a heterojunction interdigitated back contact cell architecture and an absorber thickness of 13 µm. However, losses are incurred in charge collection under the majority carrier contact regions even after reducing their width to 60 −120 µm. Recently, we developed a new method to first passivate these contact fingers with a-Si:H(i) and then locally laser-fire them to maintain a low contact resistance. High resolution light beam induced current measurements were used to analyze these developments. Without a-Si:H(i), an effective minority carrier diffusion length around 19 µm under the majority carrier contacts lead to only 0.13 mA cm−2 being collected there. With the new method, the diffusion length was improved to 36 µm and up to 113.6 µm in the best case. The reduction of passivation in the laser-fired spots was limited to a diameter of 55 µm. Over-all, charge collection increased by 0.8 mA cm−2 under the electron contact. Spatial mapping of charge collection creates a clear outlook to achieve 15 % efficiency even with the current material quality by reducing the laser firing spot size and implementing the new approach on cells with larger and more uniform grains.

The passivation quality at the interface between liquid-phase crystallized silicon (LPC-Si) and a dielectric interlayer (IL) was investigated in terms of the defect state density at the IL/LPC-Si interface (Dit) as well as the effective fixed charge density in the IL (QIL,eff). Both parameters were obtained via high-frequency capacitance–voltage measurements on developed metal–insulator–semiconductor structures based on a molybdenum layer sandwiched between the IL and the glass substrate. Dit and QIL,eff were correlated to the open circuit voltage (Voc) and the integrated external quantum efficiency (Jsc,EQE) obtained on corresponding solar cell structures as well as to Voc and Jsc,EQE results based on two-dimensional simulations. We found that Dit was reduced by one order of magnitude using a hydrogen plasma treatment (HPT) at 400 °C. Irrespectively of the HPT, QIL,eff was > 1012 cm−2. We suggest that field-effect passivation dominates chemical passivation at the IL/n-type LPC-Si interface. We attribute the significant enhancement of Voc and Jsc,EQE observed after HPT on n-type LPC-Si solar cells mainly to improvements of the passivation quality in the n-type LPC-Si bulk rather than at the IL/n-type LPC-Si interface. For p-type absorbers, the HPT did not improve Voc and Jsc,EQE significantly. We propose that this is because of an insufficient passivation of bulk defects by positively charged hydrogen, which dominates in p-type silicon, in combination with an insufficient interface passivation. Copyright © 2016 John Wiley & Sons, Ltd.

In recent years, thin-film silicon solar cells on glass prepared by liquid-phase crystallization have made progress towards high efficiency solar cells. Current record cells reach wafer-equivalent material quality and morphology using thin-film technologies. However, short-circuit current densities and hence, efficiencies, are still limited. The reflection at the interface between glass superstrate and silicon absorber layer has been identified as one major loss mechanism. These optical losses can be reduced by nanostructuring of the interface. It is important, however, that this nanostructured interface does not lead to a deterioration of silicon material quality simultaneously. Recently, we introduced SMooth Anti-Reflective Three-dimensional (SMART) textures, which consist of temperature-stable SiO x sol-gel nanostructures and a smoothing layer of spin-coated TiO x . These SMART textures on glass superstrates exhibit a smooth interface morphology and hence allow growing high-quality silicon absorber layers by liquid phase crystallization. Here, we investigate the optical properties of the SMART textures with and without an additional SiN x layer in experiment and by 1-dimensional optical simulations. It is shown that both, the SMART textures with and without additional SiN x layer, can outperform the optimized planar interlayer system of current record solar cell devices. Very low mean reflectance values of 11.2% are found in the wavelength regime from 400 nm to 600 nm for an optimized texture consisting of a 50 nm thick SMART texture with additional 15 nm SiN x layer.

This work reports a laser firing technique applied to completed silicon heterojunction interdigitated back contact solar cells in order to lower contact resistance. Previously, the implementation of a-Si:H(i) at the electron contact of polycrystalline silicon solar cells on glass substrates led to an increase in series resistance. The cell architecture with the current record efficiency of 14.2% (with illumination through glass) utilizes only an a-Si:H(n+) layer and 2–2.9 mA cm−2 of short circuit current density is lost due to electrical shading under the electron contact [1,2]. The goal of implementing an a-Si:H(i) layer and laser firing at this contact is to achieve low contact resistance at fired spots while preserving a-Si:H(i) passivation in unfired regions. After the laser firing, VOC was retained, while up to 14% absolute increase in FF was obtained with a mere 0.2 mA cm−2 loss in JSC. In the best performing cell, a 72.1% FF was achieved with a 0.7 mA cm−2 loss in JSC. Two laser sources were used to first ablate a part of the silver contact metal, and then to laser fire through the Si(n)/a-Si:H(i/n+)/ITO/Ag contact. The optimal laser fluence was found to be 1.1–0.5 J cm−2 (355 nm, picosecond pulse duration) and 4.4–5.2 J cm−2 (532 nm, nanosecond pulse duration), respectively. The upper limit on specific contact resistance in the laser fired spots was calculated to be 38 ± 20 mΩ cm 2 as a conservative estimate.

P2 laser patterning of the perovskite layer for serial monolithic integration was systematically investigated by laser ablation using nanosecond laser pulses at varied laser fluences. The correlation of the laser impact to the morphology, composition and electrical functionality was analyzed in detail by several surface-analytical techniques. It is found that material removal via stress-assisted ablation is strongly influenced by thermal processes. The formation of PbI 2 containing residuals was evidenced, presumably causing contact resistance losses through the P2 interconnect. From these results loss factors in laser-based serial interconnection of perovskite solar cells were identified, allowing for process optimization for upscaling to industrial module sizes.

The liquid phase crystallization (LPC) of silicon is an emerging technology for fabricating 10 − 20 µm thin multi-crystalline silicon layers on glass. LPC silicon solar cells exhibit similar electronic performance to multi-crystalline wafer-based devices. Due to the reduced absorber thickness, however, effective measures for light trapping have to be taken. We present tailor-made micro-structures for light trapping at the LPC silicon back-side, whereby a nano-imprinted resist layer serves as a three-dimensional etching mask in subsequent reactive ion etching. Contrary to state-of-the-art random pyramid textures produced by wet-chemical etching, this method allows to produce tailor-made textures independent of grain orientation. Differently shaped micro-textures were replicated in LPC silicon. Absorptance and external quantum efficiency of periodic honeycomb patterns and random pyramids were found to be equivalent. Thus, the method enables the potential to further optimize light trapping in LPC silicon solar cells.

Tandem solar cells consisting of perovskite and silicon absorbers have the potential to outperform respective state-of-the-art single junction efficiencies. However, their development requires the gentle deposition of a transparent electrode onto the hybrid perovskite and its organic layers. Implementation of large area graphene obtained by chemical vapor deposition seems to be an excellent solution. In this paper, we present the impact of graphene on perovskite solar cells and their organic layers. Direct application of graphene on CH3NH3PbI3 is limited by a highly defective interface but insertion of spiro-OMeTAD enables a defect free implementation. Solar cells containing transparent graphene contacts approach identical electrical performance compared to devices with standard Au contacts. Hall-effect measurements of graphene on various organic thin-films, revealed the importance of field-effect doping. Gained knowledge enabled the development of a strategy to increase the charge carrier density in graphene by 60%, while lowering graphene sheet resistance by 24%. This combined route of spiro-OMeTAD and stabilized adsorbent doping is an important step toward the targeted application in high performance monolithic perovskite/silicon tandem solar cells.

High-mobility hydrogen-doped indium oxide In 2 O 3 :H (IOH) were deposited by magnetron sputtering (radio frequency 13.56 MHz) from an In 2 O 3 target in Ar/O 2 /H 2 O gas mixtures onto unheated substrates. The as-deposited films were amorphous as shown by X-ray measurements and were crystallized in a post-annealing step in vacuum and air at 180 °C for 15 min. The optical, electrical, and morphological properties as well as the crystallization behavior of these transparent conductive oxides films were investigated in detail. A dependence of the annealing behavior in air on the total pressure during deposition could be shown. High carrier mobilities > 100 cm 2 /Vs allow for very low optical absorption and a low resistivity around 350 μΩcm. Amorphous IOH films were crystallized by short-term flash lamp annealing (FLA) in argon atmosphere for around 2.7 ms. Spatial temperature distributions in a typical layer stack (crystalline silicon, amorphous silicon, silicon oxide, and IOH film) were calculated within milliseconds after the FLA-treatment. Electron backscattering diffraction measurements of IOH films crystallized by FLA reveal a polycrystalline microstructure with an average lateral crystallite size of 333 nm. The crystallization process of these IOH films was studied by XRD and Hall measurements.

Recent progress in Liquid Phase Crystallization (LPC) of silicon enabled reaching a material quality of thin-film silicon solar cells on glass comparable to multicrystalline silicon wafers. However, decreasing the absorber thickness requires taking measures for efficient coupling and trapping light into the solar cell device. Here, we successfully integrated different periodic sub-micrometer sized structures at the sun-facing air-glass superstrate interface of 5 x 5 cm 2 LPC silicon thin-film solar cells by nanoimprint lithography. These structured superstrates were modularly attached to the device and found to provide both, anti-reflective and scattering properties. In this study, the influence of the structure type and geometry is examined. With the structures used an enhancement in short-circuit current density of + 5% (relative) compared to the planar device could be achieved.

Thin crystalline silicon solar cells prepared directly on glass substrates by means of liquid-phase crystallization of the absorber utilize only a small fraction of the silicon material used by standard wafer-based silicon solar cells. The material consists of large crystal grains of up to square centimeter area and results in solar cells with open-circuit voltages of 650 mV, which is comparable with results achieved with multi-crystalline silicon wafers. We give a brief status update and present new results on the electronic interface and bulk properties. The interrelation between surface passivation and additional hydrogen plasma passivation is investigated for p-type and n-type absorbers with different doping concentrations. Internal quantum efficiency measurements from both sides on bifacial solar cells are used to extract the bulk-diffusion length and surface-recombination velocity. Finally, we compare various types of solar cell devices based on 10 µm thin crystalline silicon, where conversion efficiencies of 11–12% were achieved with p–type and n-type liquid-phase crystallized absorbers on glass. Copyright © 2015 John Wiley & Sons, Ltd.

We recently demonstrated how the short-circuit current density of an a-Si:H/c-Si heterojunction solar cell can be significantly improved to above 40 mA/cm2 by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nanocrystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9%, compared with 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (∼3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1-D device simulation, we explain the prevalent loss mechanism, which originates mostly from poor tunnel recombination at the transparent conducting oxide/emitter interface rather than in the bulk of the emitter. We suspect that have their origin in the lower effective dopant concentration of the nanocrystalline silicon oxide emitter. From the model, implications for further developments can be derived.

The crystallization process of hydrogen doped In2O3:H (IOH) films is investigated with energy-dispersive X-ray diffraction measurements. At annealing temperatures between 125 and 150 °C crystallization of 220 nm thin films occurs within only 2 min, and the percentage of the crystalline phase does not change anymore when the temperature is raised above the crystallization temperature of 150 °C. Maximum electron mobilities above 100 cm2/Vs have been reached after crystallization. The IOH films were integrated as front contact into amorphous/crystalline silicon heterojunction cells and compared to In2O3:Sn (ITO) front contacts. Cells with ITO/IOH bilayer front contacts show a slightly lower open circuit voltage because of the a-Si:H passivation layer degradation caused by the longer annealing process needed for the crystallization of the bilayers, while all cells reach total area efficiencies around 20%. IOH films were also implemented as silver free back contact for μc-Si:H cells, and show higher short-circuit current densities than ZnO:Al back contacts because of the higher near-infra-red transmission of IOH.

Liquid-phase crystallization (LPC) has proven to be a suitable method to grow large-grained silicon films on commercially well-available glass substrates. Zone-melting crystallization with high-energy-density line sources such as lasers or electron beams enabled polycrystalline grain growth with wafer equivalent morphology. However, the electronic quality is strongly affected by the material used as the interlayer between the glass and the silicon absorber. Open-circuit voltages above 630 mV, and efficiencies up to 11.8% were demonstrated using n-type absorbers on a sputtered interlayer comprising a triple stack of SiO2/SiN $_{x}$ /SiO2. In this study, we present our results to further improve the device performance by investigating the influence of the interlayer on the open-circuit voltage of the devices and characterize the properties of the absorber and interface using bias light-dependent quantum efficiency data and transmission electron microscopy (TEM) images. Finally, we investigate the applicability of aluminum oxide (Al2 O3) for passivation of p-type LPC absorbers.

Liquid phase crystallization of silicon is a promising technology to grow crystalline silicon thin films on glass. It has already been demonstrated that open circuit voltages of up to 656 mV and efficiencies of up to 11.8% can be achieved by this technique. Nevertheless further improvements are required to become competitive with wafer based silicon solar cells. A possibility to improve the quality is to enlarge the grain size and to control the crystallographic orientation of the resulting layers. While a preferential {100} surface orientation can be triggered utilizing suitable crystallization parameters, the in-plane orientation still remains random. Also the grain size stays within the same range. By introducing a local monocrystalline seed at the beginning of the crystallization process, we are able to control both the surface and the in-plane orientation of the silicon films. In addition the grain size is significantly increased in scanning direction and only limited by the substrate size of 5 cm. This high morphological quality is accompanied by an improved electrical quality confirmed by photoluminescence imaging and Hall measurements. This is a big step towards the final goal to directly grow monocrystalline silicon thin films on glass substrates.

We fabricated interdigitated back-contact silicon hetero-junction solar cells based on thin-film absorbers on glass. The Si absorbers were directly deposited on the glass and crystallized using liquid phase crystallization. To compare whether our contact system is applicable to a wide range of initial absorber conditions two different types of precursors were prepared with absorber thicknesses between 2.5-9 μm. Furthermore, glass superstrates, doping densities and interlayer stacks were varied. With a KOH random pyramid light trapping texture at the back more than 30 mA/cm 2 were achieved on an electron beam evaporated precursor material and 655 mV on a PECVD precursor material.

We present grazing incidence X-ray fluorescence (GIXRF) experiments on 3D periodically textured interfaces of liquid phase crystallized silicon thin-film solar cells on glass. The influence of functional layers (SiOx or SiOx/SiCx) – placed between glass substrate and silicon during crystallization – on the final carbon and oxygen contaminations inside the silicon was analyzed. Baring of the buried structured silicon surface prior to GIXRF measurement was achieved by removal of the original nano-imprinted glass substrate by wet-chemical etching. A broad angle of incidence distribution was determined for the X-ray radiation impinging on this textured surface. Optical simulations were performed in order to estimate the incident radiation intensity on the structured surface profile considering total reflection and attenuation effects. The results indicate a much lower contamination level for SiOx compared to the SiOx/SiCx interlayers, and about 25% increased contamination when comparing structured with planar silicon layers, both correlating with the corresponding solar cell performances.

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.

Liquid phase crystallization of 10 μm thin silicon layers on glass substrates was performed with a line-shaped continuous wave laser beam. The process window was investigated in terms of the scanning velocity of the laser, the pre-heating of the specimens and the applied laser intensity. We have identified the entire process window, in which large-scale crystallization without deformation or destruction of the substrate and cracking of the silicon layer can be obtained. The grain orientations of the resulting Si layers were analyzed using both electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The influence of the critical crystallization parameters on the grain orientation of the silicon film was examined. EBSD and XRD measurements show that a preferential {100} surface texture and {100} and {101} orientations in scanning direction of the laser can be achieved if appropriate crystallization parameters are used. This texture formation is accompanied with a substantial decrease of high angle grain boundaries.

We present tailor-made imprinted nanostructures for light management in liquid phase crystallized silicon thin-film solar cells providing both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values >640mV. All superstrate textures successfully enhance light in-coupling in 10-20µm thick liquid phase crystallized silicon thin-films. Moreover, the effect of combining imprinted textures at the front side with individually optimized light trapping schemes at the rear side of the absorber layers on the optical properties is analyzed. With a silicon absorber layer thickness of 17µm maximum achievable short-circuit current density of 37.0mA/cm2 is obtained, an increase by + 1.8mA/cm2 (or 5.1%) compared to the optimized planar reference.

Recently, liquid phase crystallization of thin silicon films has emerged as a candidate for thin-film photovoltaics. On 10 μm thin absorbers, wafer-equivalent morphologies and open-circuit voltages were reached, leading to 13.2% record efficiency. However, short-circuit current densities are still limited, mainly due to optical losses at the glass-silicon interface. While nano-structures at this interface have been shown to efficiently reduce reflection, up to now these textures caused a deterioration of electronic silicon material quality. Therefore, optical gains were mitigated due to recombination losses. Here, the SMooth Anti-Reflective Three-dimensional (SMART) texture is introduced to overcome this trade-off. By smoothing nanoimprinted SiO x nano-pillar arrays with spin-coated TiO x layers, light in-coupling into laser-crystallized silicon solar cells is significantly improved as successfully demonstrated in three-dimensional simulations and in experiment. At the same time, electronic silicon material quality is equivalent to that of planar references, allowing to reach V oc values above 630 mV. Furthermore, the short-circuit current density could be increased from 21.0 mA cm−2 for planar reference cells to 24.5 mA cm−2 on SMART textures, a relative increase of 18%. External quantum efficiency measurements yield an increase for wavelengths up to 700 nm compared to a state-of-the-art solar cell with 11.9% efficiency, corresponding to a j sc, EQE gain of 2.8 mA cm−2.

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

Liquid-phase crystallization (LPC) using line-shaped energy sources such as laser or electron beam has proven to be a suitable method to grow large grained high-quality silicon films onto commercially well-available glass substrates. In this study, we compare cw-diode laser-crystallized absorbers with electron beam-crystallized material using backcontacted backjunction so- lar cells. Furthermore, the influence of the absorber doping con- centration thickness on the solar cell performance is studied. Using experimental data obtained on test structures, as well as solar cells and 1-D device simulations, an ideal dopant concentration is deter- mined to be 2 − 6 × 10 16 cm −3 , in combination with an absorber thickness of 10-20 μm. Finally, we present a slightly modified cell process to reduce the optical losses, which resulted in conversion efficiencies of up to 11.8%.

We present a comprehensive theoretical analysis accompanying our experimental development of micro-structured thin film polycrystalline silicon solar cell absorbers. Our focus is on 2D-periodic arrays of holes which do not act as a superficial scattering grating but pierce the entire silicon absorber layer. We deviate from the commonly used model of vertical hole sidewalls by employing conical frustum shaped holes with a range of opening angles. Additionally, an experimentally motivated front texture is applied to the simulated solar cell absorbers and varied in texture aspect. Based on this parametric absorber model, for a single domain period of 2 μ m , we discuss light trapping and optical losses in thin film cells with absorptive front and back contacts and a perfectly transparent glass encapsulation layer. Implications from this study are that light trapping in square periodic arrays with periods larger than the wavelength of light can be enhanced by microholes but that it cannot replace a front texture which provides both antireflection and scattering.

On a-Si:H/µc-Si:H thin film solar cells, the back-contact and reflector typically consist of a thin ZnO:Al (AZO) buffer layer and a silver layer. The high cost of silver as well as plasmonic absorption make it desirable to use a silver-free backside reflector. We report that a backside contact composed of an AZO transparent conducting oxide (TCO) layer deposited by DC-magnetron-sputtering at 180 °C combined with a white paint backside reflector can achieve almost the same solar cell performance as a TCO/silver back contact. The properties of the reflector materials, i.e. silver and commercially available white paint were studied by total and diffuse reflection measurements. The effect of the reflector materials on the performance of the solar cells in mini-module design were evaluated by current–voltage (I–V) and external quantum efficiency-measurements. Moreover, the AZO layer used as back contact was optimized to obtain high near-infrared (NIR)-transparency while preserving a low resistivity. Altogether, the approach of constructing an a-Si:H/µc-Si:H solar cell with a silver-free backside reflector has been successfully implemented and can still be improved by using a TCO with higher carrier mobility and lower carrier density, thus improving NIR-light transmission.

Liquid phase crystallization (LPC) using e-beam or CW-laser line sources of amorphous or nanocrystalline silicon films has emerged as new method to form high quality absorbers on cheap substrates. A disadvantage of electron beam crystallization was the necessity to use amorphous SiC x layers in contact with the silicon to maintain a stable crystallization process, resulting in high surface recombination. In this work we investigate capping layers to enable e-beam induced LPC on alternative interlayer materials especially SiO 2 and characterize the morphological and electrical properties of these layer stacks. Photoluminescence measurements of LPC absorbers on SiO 2 layers exhibit significantly increased radiative recombination compared to SiC x . Fabricated hetero-junction cells achieved a new emitter area efficiency record for electron beam induced LPC solar cells of 9% using the FrontERA contact system and H passivation.

The present article gives a summary of recent technological and scientific developments in the field of polycrystalline silicon (poly-Si) thin-film solar cells on foreign substrates. Cost-effective fabrication methods and cheap substrate materials make poly-Si thin-film solar cells promising candidates for photovoltaics. However, it is still the challenge for research and development to achieve the necessary high electrical material quality known from crystalline Si wafers on glass as a prerequisite to harvest the advantages of thin-film technologies. A wide variety of poly-Si thin-film solar cell approaches has been investigated in the past years, such as thermal solid phase crystallization – the only technology that had already been matured to industrial production so far – the seed layer concept where a large-grained seed layer is epitaxially thickened, direct growth of fine grained material, and liquid phase crystallization methods by laser or electron beam. In the first part of this paper, the status of these four different poly-Si thin-film solar cell concepts is summarized, by comparing the technological fabrication methods, as well as the structural and electrical properties and solar cell performances of the respective materials. In the second part, three promising technologies are described in more detail due to their highly auspicious properties regarding material quality and throughput aspects during fabrication: (1) High-rate electron–beam evaporation of silicon for the low-cost deposition of high-quality material, (2) large-area periodic nano- and micro-structuring of poly-Si by the use of imprinted substrates providing a large absorption enhancement by a factor of six at a wavelength of 900 nm, (3) liquid-phase crystallization of silicon thin-film solar cells by electron–beam, yielding an excellent poly-Si material quality reflected by an open-circuit voltage of 582 mV which has been achieved only very recently. A successful combination of these three complementary technologies is envisaged to be the basis for a prospective low-cost and highly efficient poly-Si solar cell device.

This paper addresses the achievements and challenges in a-Si:H/µc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4–0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1 cm² solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed.

A simple and fast 1D effective medium simulation model for polycrystalline silicon thin film solar cells was defined, by using the device simulator AFORS-HET. The model was calibrated with current–voltage measurements of solar cells under illumination and in t he dark. The experimentally determined variation of the solar cell characteristics with varying absorber doping and absorber thickness can be reproduced by the model. Through inverse modeling, the effective defect density in the absorber layer was determined and it was shown that the open circuit voltage V OC of the cell is limited by defects localized in the bulk of the absorber layer. Using the calibrated simulation model for sensitivity analyses, different optimization strategies for this type of solar cells have been assessed. Firstly, the optimum absorber thickness as a function of absorber doping and defect densities has been determined and secondly, the V OC - increase when using a layer of hydrogenated amorphous silicon (a-Si:H) to form a heterojunction back surface field has been simulated as a function of the absorber defect density.

Polycrystalline silicon thin films were prepared by depositing amorphous or microcrystalline silicon layers onto glass substrates and subsequent crystallization via solid or liquid phase crystallization approaches. Differences in layer morphology and quality were characterized using low temperature photoluminescence (PL) spectroscopy and electron backscatter diffraction spectroscopy. The evaluation of spectral and spatially resolved PL response was used to conclude about the relative density of defects in these layers. Liquid phase crystallization using a line shaped electron beam proves to be the most promising technique in order to obtain large-grained high quality polycrystalline silicon thin films.

Back-contact silicon heterojunction solar cells with an efficiency of 22% were manufactured, featuring a simple aluminium metallisation directly on the doped amorphous silicon films. Both the open-circuit voltage and the fill factor heavily depend on the parameters of the annealing step after aluminium layer deposition. Using numerical device simulations and in accordance with the literature, we demonstrate that the changes in solar cell parameters with annealing can be explained by the formation of an aluminium silicide layer at temperatures as low as 150 degrees C, improving the contact resistance and thus enhancing the fill factor. Further annealing at higher temperatures initialises the crystallisation of the amorphous silicon layers, yielding even lower contact resistances, but also introduces more defects, diminishing the open-circuit voltage.

We report on the design and manufacturing of interdigitated back contact cells based on the silicon heterojunction technology. The influence of geometry and overlap of the doped amorphous silicon layers forming the contact fingers on device performance have been investigated by simulation. Two contact formation concepts, with and without a TCO interlayer - an indium tin oxide/silver (ITO/Ag) stack, and a direct aluminum (Al) metallization - are experimentally evaluated. The former retains good passivation but leads to a too high contact resistivity, the latter shows the opposite behavior, but yields a slight benefit in terms of overall performance achieving more than 20% of efficiency. We show that in this case a contact system is formed whose properties can be tuned by annealing, enabling a trade-off between V-OC and FF. [GRAPHICS] . Structure of the presented solar cell; a SiNX layer covers the front side, the rear side is passivated by overlapping layer stacks of intrinsic and doped amorphous silicon, the latter are contacted by aluminum.

Light trapping enhancement by plasmonic-active metal nanoparticles (NPs) is believed to be a promising approach to increase silicon-based solar cell efficiency. Therefore, we investigated TiO2 films in situ doped by Au NPs (TiO2:AuNPs) deposited by spin coating on a silicon substrate. Photoconductivity and optical properties of the TiO2:AuNPs/Si structures were studied in comparison with those of TiO2/Si reference samples. We found that an introduction of the 40–50 nm diameter AuNPs into the antireflective TiO2 layer deteriorates the antireflection properties and decreases the external yield of photogeneration of charge carriers. This is due to an increase of the layer reflection in the red-IR part of the spectrum, and due to the parasitic absorption of light by AuNPs in the blue-green part. Charge carrier recombination effect at the TiO2:AuNPs/Si interface is also found to decrease the external yield. We conclude that the TiO2:AuNPs layers could potentially be applied on the rear but not on the front side of Si solar cells.

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 chemical and electronic properties of a-Si:H(B)/ZnO:Al and μc- Si:H(B)/ZnO:Al thin-film solar cell structures are studied by hard X-ray photoelectron spectroscopy (HAXPES). Using a combination of different X-ray excitation energies and deliberate sample design, we were able to select the probed volume, i.e., the silicon capping layer only or the silicon and zinc oxide layer (including the buried interface). For the a-Si:H(B) material, we find a higher deposition rate and a smaller value for the modified Auger parameter than for μc-Si:H(B). In addition, we find indications of a pronounced band bending limited to the very surface of the a-Si:H(B) and the μc-Si:H(B) layers, which is more distinct in the latter case.

Rigorous finite element optical simulations have been used to examine the absorption of light in various crystalline silicon based, nanostructured solar cell architectures. The compared structures can all be produced on glass substrates using a periodically structured dielectric coating and a combination of electron-beam evaporation of silicon and subsequent solid phase crystallization. A required post-treatment by selective etching of non-compact silicon regions results in an absorber material loss. We show that by adequately tailoring the optical design around the processed silicon layer, the absorptance loss due to material removal can be completely overcome. The resulting silicon structure, which is an array of holes with non-vertical sidewalls, shows promising light path enhancement and features an even higher absorptance than the initial nanodome structure of the unetched absorber.

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.

Backside contacted cells were fabricated on p- and n-type, liquid phase crystallized silicon on glass. The heterojunction emitter and transparent contacting layer were opened locally using an inkjet printer and the absorber was contacted with laser fired point contacts. An analytical resistance model was derived to determine the resistance losses for the point contact geometry. Using test structures and the model it was found that for p-type cells the main contribution to the cell resistance came from the absorber contact and the absorber. Both were strongly reduced by increasing the absorber doping. For n-type cells the main contributions originated from the emitter and emitter-TCO contact, which was improved by changing the emitter type, resulting in an 11.6 % solar cell.

The structural properties of crystalline Si nanodots embedded in a SiO2 matrix are investigated with respect to the exploitation of quantum confinement effects (QCE) in Si solar cells. The nanostructures are grown on crystalline Si (c-Si) wafers by decomposition of substoichiometric SiOxNy layers with various [O]/[Si] ratios. Cross-sectional high-resolution transmission electron microscopy investigations reveal the formation of separated single crystalline nanodots with diameters below 5 nm inside the SiOxNy volume and directly on the c-Si wafer. The density and diameter of the nanodots decreases with increasing [O]/[Si] ratio, leading to inter-dot distances above 10 nm for [O]/[Si]>1.3. Photoluminescence (PL) spectra are blue-shifted relative to the Si bulk PL, which is in good agreement with theoretical QCE models. It is found that for observing the PL signal the nanodots must be covered by a SiO2 shell to reduce charge carrier recombination via defects at the nanodot surface. This requires an [O]/[Si] ratio >0.5 for which the inter-dot distance becomes too large for charge carrier transport between the nanodots. It is concluded that a better control over the nanodot formation at high [O]/[Si] ratios has to be achieved before QCE can be successfully applied in Si solar cell devices.