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Thin-film polycrystalline-silicon solar cells offer a promising alternative to standard silicon solar cells. Until now, however, obtained efficiencies are too low to lead to a cost reduction in photovoltaics. To obtain cells with higher efficiencies, polycrystalline-silicon layers need large grains, good passivation and an optimized cell design. In this work we compare pc-Si solar cells with an amorphous silicon–crystalline silicon heterojunction emitter to cells with a diffused phosphorus emitter. In heterojunction cells, hydrogen passivation was carried out before emitter formation, which appears to make it more effective. Open-circuit voltages ( V oc ) were much higher for cells with a heterojunction emitter, reaching values up to 520 mV. A maximum efficiency of 5.3% was obtained on the heterojunction cells, while the homojunction cells led to substantially lower efficiencies. A cell concept based on a heterojunction emitter seems therefore most favorable to lead to highly efficient thin-film pc-Si solar cells. The high V oc and efficiency values obtained in this work form an important step towards cost-effective polycrystalline-silicon solar cells.

Thin-film polycrystalline-silicon (pc-Si) solar cells with a high efficiency could lower the price of photovoltaic electricity substantially. Efficient thin-film solar cells will not only lead to a cost reduction by the use of less silicon material, but will also reduce the module fabrication costs if a monolithic module process is used that integrates cell interconnection with cell contacting. Aluminium-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening recently proved to be a simple way to obtain large-grained pc-Si thin films with excellent properties for solar cells. However, cell processes different from those for bulk-Si cells have to be implemented to fully exploit the pc-Si material quality and to obtain working solar cells. In this work, we propose a simple monolithic module process for thin-film pc-Si solar cells, in which all contacts are on top of the cells in an interdigitated pattern. As a first step towards implementation of this process, we made single pc-Si solar cells with interdigitated top contacts, using pc-Si layers on ceramic substrates grown by AIC in combination with high-temperature epitaxy. Next, we made pc-Si modules using a simplified metallization scheme. The interdigitated pc-Si cells had much higher efficiencies than mesa cells with base contacts at the periphery of the cells due to a lower series resistance and a higher current density. The maximum obtained cell efficiency was 5.6%, which is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of Si was involved. First module results showed that good cell separation and isolation is crucial to obtain proper working modules. Our interdigitated cell results indicate that monolithic thin-film modules with interdigitated top contacts based on pc-Si layers made by AIC will most likely lead to high efficiencies.

Most types of thin-film solar cells imply a radical departure from the dominant bulk crystalline Si technology. This is not the case for epitaxial thin-film solar cells. In this technology, a high quality Si layer is deposited epitaxially on a low-cost Si substrate (e.g. cast Upgraded Metallurgical Grade silicon or high-throughput Si ribbons) and processed into a solar cell. This technology combines the well-known advantages of crystalline Si (high efficiency potential, stability and reliability) with a substantial cost reduction potential. Moreover, the similarity to the traditional Si technology lowers the threshold for adoption by the PV industry. This paper gives an overview of the field of epitaxial thin-film solar cells, covering substrates, deposition techniques and solar cell processing. Achievements reported in the literature are summarized and recent results are presented. Special attention is given to the crucial issue of achieving high currents by increasing absorbance in the active layer. This can be achieved by increased absorption through material engineering and through implementation of advanced light confinement schemes.

This study investigates the crystallographic properties of polycrystalline silicon films formed on doped (boron or phosphorus) or undoped silicate glass coated ceramic substrates using the chemical vapor deposition method at high temperatures (> 1000 °C). First, FT-IR analysis of the silicate oxide layers is presented in order to monitor the presence of the Si–O, B–O and/or P–O bonds in the layers prior to deposition. Then, the average grain size, crystalline orientation and boundary defects of these poly-Si films on silicate coated mullite and alumina were evaluated as a function of deposition parameters and boron/phosphorus content. The results show a significant increase of the grain size and narrower size distribution after CVD on BSG or PSG intermediate layers, whatever is the substrate. Large grains up to 7 μm are observed for 9 μm thick films. The enhancement in grain size is attributed to the nucleation change by defects or impurities rather due to the flowability of the silicate glass during the silicon deposition at high temperature. The open-circuit voltage measurements of these fine-grained poly-silicon films before hydrogenation shows a dependence on the grain size through the choice of the substrate. The open-circuit voltage increases substantially from 250 mV to 410 mV after 1 h plasma hydrogenation, despite partial etching of the emitter by the hydrogen atoms.

The performance of bulk heterojunction organic solar cells based on Poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) processed from chlorobenzene solution can be enhanced by a thermal treatment of the device. The morphology of films made from 1 / 2 wt. ratio blends of P3HT and PCBM reveals an unfavorable amorphous phase in the as-produced cells while after annealing a more favorable crystalline phase is obtained. Through variation of the processing solvent a power conversion efficiency of 3.6% is demonstrated without the necessity of an additional annealing step. It is shown that the as-produced active layer already has a crystalline morphology. We argue that the high boiling point of the solvent plays an important role in this by influencing the evaporation speed during deposition of the active layer. Further proof is delivered that indeed slowing down the evaporation speed can beneficially influence the solar cell performance.

Thin-film silicon solar cells on low-cost foreign substrates could lead to a large cost reduction of photovoltaic electricity if sufficiently high efficiencies could be obtained. A possible approach is to make polycrystalline-silicon solar cells by epitaxial thickening of large-grained thin seed layers made by aluminium-induced crystallization (AIC) of silicon. Until now however, obtained efficiencies are too low to lead to the desired cost reduction. We report on the influence of the AIC seed layer morphology (grain size and presence/absence of secondary crystallites on top of the surface) on the epitaxial growth of absorber layers and on the resulting cell parameters. To increase the grain size of the seed layers, we investigated the use of a nitric acid treatment to oxidize the Al layers prior to the amorphous silicon deposition. We compared seed layers oxidized by nitric acid treatment to seed layers oxidized by a short exposure to the ambient air. The nitric acid treatment led to a larger grain size and strongly affected the structure of the secondary crystallites on most investigated samples. Removing these crystallites before epitaxial growth clearly led to larger grains and increased the open-circuit voltages (Voc) of our solar cells. Using the nitric acid treatment, absorber layers with grain diameters up to 50 μm were made, that reached efficiencies of 4.9%, with Voc values around 500 mV. However, seed layers that were made through oxidation by air exposure and had grain diameters below 12 μm led to similar Voc values and even higher efficiencies (5.3%). We believe that regions with very small grain size on the large-grained samples are responsible for the relative poor behaviour of the large-grained solar cells. Our results show that increasing the average grain size of AIC seed layers does not automatically lead to better solar cells.

The constant photocurrent method has been used for many years to study the sub-bandgap absorption coefficient of inorganic amorphous semiconductors such as amorphous silicon. In this paper CPM is applied to conjugated polymers and blends of polymers and fullerenes. More precisely the polymer MEH-PPV, the fullerene derivative PCBM and blends of MEH-PPV and PCBM have been studied. Because of the spectral dependency of the quantum yield for unblended polymers, extra measurements of photocurrent transients were made to scale the CPM results. The carrier mobility is measured through time-of-flight measurements.

Pholomultiplication in conventional inorganic semiconductors has been known mitt used lor decades, the underlyim', mechanism being multiplication by impact ionization triggered by hot carriers. Since neither carrier heating by an electric field nor avalanche multiplication arc possible in strongly disordered organic solids. charge multiplication seems to be highly unlikely in these materials. However, here the photornultiplicalion observed in the hulk of a unipolar disordered organic semiconductor is reported. The proportion of extracted carriers to incident photons is experimentally determined to be in excess of 3000 % in a single-layer device of the air-stable, n-type organic semiconductor F 16 CuPc (PC: phthalocyanine). This effect is explained in terms of exciton quenching by localized charges. the subsequent promotion of these detrapped charges to the high-mobility energy band of the density-of-states (DOS) distribution, and subsequent slow equilibration within this broad intrinsic DOS. Such a mechanism allows multiple replenishment of the optically released charge by mobile carriers injected from an Ohmic electrode. Also shown is pholomultiplication in double-layer devices composed of layers ol donor and acceptor small-molecule materials. This result implies that. apart from exciton dissociation at a donor/acceptor interface, exciton energy. transfer to trapped carrier, is a complementary photoconductivity process in organic solar cells. This new insight paves the way to cheap, highly- efficient organic photodetectors on flexible substrates for numerous applications.

In recent years, research on epitaxial growth for photovoltaics became more important due to the increasing interest in thin-film silicon solar cells. Two significant challenges need to be resolved before this technique can become a competitive industrial alternative to the current dominating technology of bulk silicon solar cells: (i) the availability of a high-throughput and cost-effective epitaxial CVD reactor and (ii) efficiencies approaching those of bulk Si solar cells. In this paper, two CVD systems are studied: an AP-CVD commercial reactor, as a reference system, and an experimental LP-CVD system for optimization of a low-cost semi-industrial process. For low growth rates, an LP-CVD process is realized with a defect density around 5×103 defects/cm2, comparable with the layers grown in the commercial reactor with a growth rate of 3.9 μm/min. First solar cells, grown in the LP-CVD reactor show an efficiency of 8.2% on mono-crystalline samples. Cells on various low-cost substrates, grown in the reference reactor, show efficiencies between 12% and 13% with IMEC's industrial screen-printing process. The short-circuit current of epitaxial cells is limited to 28 mA/cm2 (typically 5 mA/cm2 less than for bulk Si cells). Therefore, the thin epitaxial cell concept requires optimal light trapping, increasing the optical path length. Experiments show that a porous silicon (PS) intermediate layer as an internal reflector can fulfill this role adequately, giving an internal reflectance up to 80%. However, the effectiveness of this reflector depends on its influence on the quality of the epi-layer. Measurements show a lower-quality epi-layer for samples with a PS intermediate layer, but indicate that further optimization of the pre-deposition bake could lead to a compromise between current gain by internal reflectance and losses caused by the increased defect density.

This study investigates defect passivation in fine-grained polycrystalline silicon layers by means of hydrogenation. The polycrystalline silicon layers are deposited on oxidized Si wafers using high-temperature chemical vapour deposition. The hydrogenation treatment is performed in a direct plasma enhanced chemical vapour deposition system. The samples are characterized by resistivity vs. temperature measurements of p-type layers and by quasi-steady state open-circuit voltage measurements of p–n diodes made in the material. The results show a large increase of the measured open-circuit voltage by the hydrogenation treatment, with the open-circuit voltage of the samples at a light intensity of 1 Sun rising from 180 mV without hydrogenation to values up to 380 mV. This corresponds to a defect concentration decrease by a factor of three.

Thin polycrystalline “seed layers” have been created on ceramic substrates using aluminium-induced crystallization (AIC) of amorphous silicon. An intermediate spin-on-oxide between substrate and AIC layer suppresses excessive nucleation and leads to larger grains. Epitaxial growth with high-temperature chemical vapor deposition has been successfully performed on these seed layers, yielding an average grain size around 5 μm. In this way, solar cells with grain size larger than active layer thickness have been made, reaching V oc values up to 460 mV and energy conversion efficiencies around 4.5%.

We have studied the possibility of using (nano-)porous GaAs top layers as antireflective coating and optical diffusor in high-efficiency III-V solar cells. Antireflective coating behaviour of thin, nanoporous GaAs layers has been demonstrated, whereas initial experiments on applying these porous layers on top of a GaAs solar cell structure also suggest an optical diffusion of the incident light in the porous layer.

We have performed an extensive study of the porosification of germanium by anodization in HF-based electrolytes. Both n- and p-doped Ge substrates (with varying doping levels) were used, as well as different electrolyte concentrations, anodization currents and times. We will review the conclusions we were able to draw from this series of experiments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of temperature and composition on the I–V characteristics of blends of (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV), and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between a transparent indium tin oxide (ITO) electrode and an Al backside contact (solar cell structure) are investigated. The observed dark I–V curves agree well with the trap-controlled space charge limited transport theory. The illuminated I–V curves do not obey the theory of space charge limited currents. The short-circuit currents in the illuminated samples depend strongly on the composition. The current is maximum for the 80% PCBM concentration in the blend. The open circuit voltage also depends on the composition. It is maximum for 0% PCBM concentration and decreases monotonically with PCBM concentration. This might lead to the conclusion that the open circuit voltage is due to internal bulk heterojunctions (BHs). However, we have pointed out that this conclusion, arrived at by earlier workers also, is not necessarily valid. The superposition principle is not valid but the discrepancy is small.

Thin-film epitaxial silicon solar cells are an attractive future alternative for bulk silicon solar cells incorporating many of the process advantages of the latter, but on a potentially cheap substrate. Several challenges have to be tackled before this potential can be successfully exploited on a large scale. This paper describes the points of interest and how IMEC aims to solve them. It presents a new step forward towards our final objective: the development of an industrial cell process based on screen-printing for >15% efficient epitaxial silicon solar cells on a low-cost substrate. Included in the discussion are the substrates onto which the epitaxial deposition is done and how work is progressing in several research institutes and universities on the topic of a high-throughput epitaxial reactor. The industrial screen-printing process sequence developed at IMEC for these epitaxial silicon solar cells is presented, with emphasis on plasma texturing and improvement of the quality of the epitaxial layer. Efficiencies between 12 and 13% are presented for large-area (98cm 2 ) epitaxial layers on highly doped UMG-Si, off-spec and reclaim material. Finally, the need for an internal reflection scheme is explained. A realistically achievable internal reflection at the epi/substrate interface of 70% will result in a calculated increase of 3 mA/cm 2 in short-circuit current. An interfacial stack of porous silicon layers (Bragg reflectors) is chosen as a promising candidate and the challenges facing its incorporation between the epitaxial layer and the substrate are presented. Experimental work on this topic is reported and concentrates on the extraction of the internal reflection at the epi/substrate interface from reflectance measurements. Initial results show an internal reflectance between 30 and 60% with a four-layer porous silicon stack. Resistance measurements for majority carrier flow through these porous silicon stacks are also included and show that no resistance increase is measurable for stacks up to four layers.

An analytical model has been developed to estimate the short-circuit current density of conjugated polymer/fullerene bulk heterojunction solar cells. The model takes into account the solvent-dependent molecular morphology of the donor/acceptor blend, which was revealed by transmission electron microscopy. Field-effect transistors based on single and composite organic layers were fabricated to determine values for the charge carrier mobilities of such films. These values served as input parameters of the model. It is shown that the difference in short-circuit current density that was measured between toluene-cast and chlorobenzene-cast conjugated polymer/fullerene photovoltaic cells (Appl. Phys. Lett. 78 (2001) 841) could be very well simulated with the model. Moreover, the calculations illustrate how increasing the hole and electron mobilities in the photoactive blend can improve the overall short-circuit current density of the solar cell.

Aluminum-induced crystallization of a-Si is a promising path to cheap and simple definition of a seeding layer for epitaxial growth of a large-grained active layer in thin-film polycrystalline silicon solar cells on foreign substrates. The course of the crystallization has been shown to depend on a variety of factors such as temperature and deposition conditions of the aluminum and silicon layers. We suggest a dependency on the characteristics of the materials involved in the reaction, focusing particularly on the different reactivity of a-Si deposited with different deposition techniques (e-beam evaporation vs. PECVD). First results of high-temperature layer growth on the seeding layers and cell fabrication are also reported.

Thin film monocrystalline silicon solar cells based on porous silicon (PS) layer transfer processes could be cost-effective because of their lower consumption of material use and the potential for high efficiency. The process is based on our ability to separate thin porous films (p + ) of desired thickness and area from Si substrate. Crystallinity of these porous films is studied with XRD for their suitability for Si epitaxy. AFM is employed to study the Si substrate surface roughness after one or more porous film separations. The roughness stabilizes after four film separations. Annealing of the porous films at temperatures above 1050 °C results in a closed surface, suitable for Si epitaxy. XRD analysis of epitaxially deposited Si layer on annealed PS film revealed its high mono-crystalline quality. A FMS solar cell of 12% efficiency has been realized in an 18 μm thin epitaxial layer.

The performance of organic bulk donor/acceptor heterojunction solar cells with different transparent electrodes is compared. Up to now, expensive material like indium-tinoxide (ITO) has been commonly used as a transparent electrode. Nevertheless, an interesting organic alternative in the form of highly conductive poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) is emerging. In this work, a comparison is made between cells with either ITO or highly conductive PEDOT/PSS as a transparent contact. Different techniques such as spin-coating and screen-printing are used to deposit this latter material. Owing to still limited conductivity of PEDOT/PSS with respect to ITO, an underlying metallic grid is introduced. A standard photographic technique optimised here to result in a metallic Ag-pattern is used. In this way, a comparable performance of the photovoltaic devices with either type of anodes is obtained. This newly developed PEDOT/PSS-based transparent anode is successfully applied onto flexible substrates. Furthermore, appropriate design of the device structure makes it possible to realise serially connected solar cells. The performance of such devices is also reported. These results show that industrial production of fully flexible organic solar cell modules will be possible at low costs.

The role of hydrogen in a recently developed one-step method for the formation and separation of a thin porous Si film from the initial Si substrate is investigated by means of Raman spectroscopy and detailed morphological analysis. It is shown that hydrogen plays a crucial role in the formation and separation of the porous Si film. Micro-Raman spectroscopy reveals that hydrogen in molecular form is accumulated in the micropores and creates hydrodynamic pressure which could be strong enough to break the thin pore walls in a high-porosity layer. As a result the upper porous Si film is separated from the original substrate. The kinetics of the process is confirmed by transmission and scanning electron microscopy.

Formation and separation of thin porous silicon films from the original substrate in combination with the transfer of such films onto a cheap foreign substrate for the photovoltaic application provides a straightforward way to cost reduction. Separation of the porous silicon film from the reusable p + silicon substrate is achieved by electrochemical etching of silicon under specific HF concentration and current densities. The effect of HF concentration on film separation is studied. It is found that film separation is possible only for HF concentration of 22% and lower. Addition of DI water in the solution results in the formation of a thick porous silicon layer without separation from the substrate during 10 minutes. A theoretical explanation has been given to describe the phenomena of porous silicon film separation.

We have investigated the influence of substrate type (GaAs vs. germanium) and of group-V precursor (AsH 3 vs. TBAs) on the epitaxial quality of (In)(Al)GaAs layers. We evaluated these layers in terms of morphology, background contamination and doping characteristics. For final benchmarking of the individually optimised processes, we produced p-on-n single junction GaAs solar cells and compared their relative performance. This type of device is an excellent performance monitor for epitaxial layers as the fundamental operating mechanism is drift of minority carriers. The solar cell grown with TBAs on a germanium-substrate has a conversion efficiency under the AM1.5 solar spectrum, which compares favourably with the highest reported value for a p-on-n GaAs solar cell on Ge (Prog. Photovolt. Res. Appl. 8 (2000) 377).

In future, thin wafers (< 100µm) will be employed in silicon heterojunction solar cell to decrease modules cost-per-Watt-Peak. However, in order to maintain excellent cell efficiency a higher device surface/volume ratio will demand stricter requirements on surface passivation. In this frame, the status of the crystalline surface (c-Si) prior to amorphous silicon (a-Si:H(i)) plasma deposition (PECVD) plays an important role: the c-Si chemical termination influences the quality of the interface layer a-Si:H(i)/c-Si, and affect the open circuit voltage (Voc). Previous studies have shown that smooth and fully hydrogenated c-Si surface [ lead to best quality heterojunction. These surfaces can be obtained by different wet cleaning procedures, usually terminated by an immersion in diluted HF. However, after this step, the wafer surface is highly reactive and can re-oxidize rapidly: contaminants presents in air can be adsorbed and affect wafer passivation [. For this reason, in-situ Hydrogen (H2) plasma cleaning prior to a-Si:H(i) deposition might be an interesting option to decrease the amount of contaminant on the surface. However, the experimental window is extremely narrow, since phenomena like epitaxial growth and ion-bombardment damage can easily occur [[ and worsen the surface passivation operated by a-Si:H(i) layers. In this contribution, we present an in-situ H2 plasma clean and show a decrease of Oxygen and Carbon on wafer surface after a short time (

This Letter discusses an important difference between positively charged SiO2 and negatively charged Al2O3 rear-passivated p-type Si solar cells: their illumination level dependency. For positively charged SiO2 rear-passivated p-type Si solar cells, a loss in short circuit current (JSC) and open circuit voltage (VOC) as a function of illumination level is mainly caused by parasitic shunting and a decrease in surface recombination, respectively. Hence, the relative loss in cell conversion efficiency, JSC, and VOC as a function of the illumination level for SiO2 compared to Al2O3 rear-passivated p-type Si solar cells has been measured and discussed. Subsequently, an exponential decay fit of the loss in cell efficiency is applied in order to estimate the difference in the energy output for both cell types in three different territories: Belgium (EU), Seattle and Austin (US). The observed trends in the difference in energy output between both cells, as a function of time of the year and region, are as expected and discussed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present field-effect mobility measurements on a conjugated polymer poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) that was spin-cast from different solvents. The organic field-effect transistors were fabricated using an Al/p+-Si/SiO2/TiW/Au/MDMO-PPV configuration. The hole field-effect mobilities for MDMO-PPV deposited from a toluene and chlorobenzene solution were found to be 5×10−6 and 3×10−5 cm2/(V s), respectively, as calculated from the saturation regime. These results are explained on the basis of a modification of the polymer morphology caused by using different solvents, as evidenced by light scattering experiments on the MDMO-PPV solutions.

We improve the trapping model of carrier transport in conducting polymers by integrating the coupled Poisson and continuity equations numerically. In the case of exponentially distributed traps calculated log J vs 1/T plots are straight lines indicating that the activation of carriers to the extended states can be described by one single activation energy, Eeff. The improved trapping model agrees well with the recent experimental data at high temperatures (i.e., near room temperature). At low temperatures the trapping model does not work since the available thermal energy is not sufficient to ionize the traps. We use the hopping mobility model at low temperatures. The model explains the low-temperature J–V characteristics well. This mechanism of charge carrier transport is similar to that used for interpreting experiments on inorganic amorphous materials.

Basic solar cell characteristics are examined in bulk donor-acceptor-heterojunction devices. Therefore, spin-coated organic blends of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are used as active material sandwiched between a transparent IndiumTinOxide (ITO)-electrode and an Al backside contact. A comparison is made between cells with or without an extra interfacial layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. Furthermore, the effect of the thickness of the active layer on the photovoltaic performance of the devices is studied. It is seen that applying this extra PEDOT/PSS layer results in an important increase of the contact component of the series resistance of the cells. At the same time, open circuit voltage improved for devices with an interfacial layer while the fill factor was higher for cells with no PEDOT/PSS film. For thinner active layers, in both cases the bulk component of the series resistance decreased. Nevertheless, a decrease of short circuit current is seen at reduced illumination. Shunt resistance shows a slight increase resulting in an improvement of open circuit voltage. Also, the fill factor increases for thinner active layers. For the thinnest standard devices a power conversion efficiency of over 3% could be measured under AM1.5 conditions.

Polycrystalline silicon layers have been grown on various alumino-silicate substrates in arapid thermal chemical vapor deposition (RTCVD) system at high temperatures (>10001C).Structural analysis shows a columnar growth with grain sizes up to 15mm and growth rates upto 5mm/min.Solar cell devices on this fine-grained Si material result in a short-circuit currentof about 13mA/cm 2 but a poor open-circuit voltage (o0.4V). Larger grains obtained by thezone melting recrystallization (ZMR) technique boosted the current up to 26.1mA/cm 2 ,thanks to the light-trapping by the mullite substrate.Best efficiency is 8.2% on a 1cm 2 cellmade on a 20mm thick poly-Si layer. r 2002 Elsevier Science B.V. All rights reserved. Keywords:Thin silicon; Ceramic; Photovolta.iic; Light-trapping 1. IntroductionAmong the alternative approaches to bulk silicon-based cells, there is the use ofpolycrystalline silicon thin films (o30mm) on low-cost substrates.Important issuesin these developments are the choice of the substrate and the growth method sincereasonably high growth rates and an overall reduction of the number of steps areessential in order to obtain a cost-effective process.As for the silicon formation,direct deposition on foreign substrates by chemical vapor deposition at elevatedtemperature (>10001C) and atmospheric pressure is suitable since it allows high

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Organic solar cells based on an active layer of a spincast polymer donor/acceptor blend have proven to be very efficient. We present similar photovoltaic devices with organic layers that are formed using the technique of vacuum deposition. The donor and acceptor materials are, respectively, the five-ring PPV-type oligomer 2-methoxy-5-(2′-ethylhexyloxy)-1,4-bis((4′,4″-bisstyryl)styrylbenzene) (MEH-OPV5) and C60. An elevated substrate temperature during deposition of the single MEH-OPV5 layers on ITO-coated glass substrates yielded polycrystalline films with a rough surface, as was determined from AFM and XRD analysis. The co-evaporation of both materials, also at high substrate temperature, resulted in amorphous, but very smooth films exhibiting a good percolation of donor and acceptor. The dark I–V behaviour of single-layer and donor/acceptor-layer devices in an ITO/PEDOT/organic/Al configuration is compared. It was found that the PEDOT/C60 interface of an ITO/PEDOT/MEH-OPV5:C60/Al solar cell structure is responsible for the exponential rise of its dark I–V curve under forward bias. Sandblasting of the glass substrate was applied as a way to reduce the reflection of the incoming light and resulted in a significant increase of the short-circuit current. Standardised spectral response measurements confirmed this effect. The sandblasted bulk heterojunction photovoltaic devices were characterised under AM1.5 illumination and reached a power conversion efficiency of 2.2%.

This Work was funded by King Abdul-Aziz City of Science and Technology (KACST) and the Flemish government, department EWI. The authors would like to thank Tom De Geyter, Greetje Godiers and Guido Huyberechts from Flamac in Gent for sputtering of the metal layers. AGC is acknowledged for providing the Mo/SLG substrates.

Introducing nanophotonics into photovoltaics sets the path for scaling down the surface texture of crystalline-silicon solar cells from the micro-to the nanoscale, allowing to further boost the photon absorption while reducing silicon material loss. However, keeping excellent electrical performance has proven to be very challenging, as the absorber is damaged by the nanotexturing and the sensitivity to the surface recombination is dramatically increased. Here we realize a light-wavelength-scale nanotextured monocrystalline silicon cell with the confirmed efficiency of 8.6% and an effective thickness of only 830 nm. For this we adopt a self-assembled large-area and industry-compatible amorphous ordered nanopatterning, combined with an advanced surface passivation, earning strongly enhanced solar light absorption while retaining efficient electron collection. This prompts the development of highly efficient flexible and semitransparent photovoltaics, based on the industrially mature monocrystalline silicon technology.

In order to improve the electrical performances of Cu2ZnSnSe4 (CZTSe) based solar cells, the standard CdS buffer layer was replaced by (Cd,Zn)S processed by chemical bath deposition. The morphology and composition of the (Cd,Zn)S thin films were studied as a function of [Zn]/([Zn] + [Cd]) ratio in the chemical bath (80, 85 and 90%). The CZTSe/(Cd,Zn)S solar cells with and without Cd partial electrolyte (Cd PE) treatment were compared to CZTSe/CdS reference devices using current–voltage and external quantum efficiency measurements. The (Cd,Zn)S thin films show a non-homogeneity of Zn distribution and phase formation, with a shift from Zn(O,OH)x to ZnS phase when increasing the deposition time and a decrease of the layers thicknesses when increasing the Zn concentration in chemical bath. A model for the growth of (Cd,Zn)S thin films is proposed. The resulting CZTSe/(Cd,Zn)S devices show an important reduction of the barrier at the hetero-interface, which is attributed to the lower density of O contamination in (Cd,Zn)S compared to CdS, inducing a lower density of deep p-type recombination centers. Despite the reduced compensation of the buffer layer, CZTSe/(Cd,Zn)S devices show a deterioration of the open circuit voltage and the fill factor with the increase of Zn content in (Cd,Zn)S. These electrical losses were avoided by Cd PE treatment prior to the deposition of (Cd,Zn)S. This work was supported in part by the Flemish government, Department of Economy, Science and Innovation (EWI). Tom De Geyter, Greetje Godiers and Guido Huyberechts from Flamac in Gent are acknowledged for sputtering of the metal layers. AGC is acknowledged for providing substrates.

The fabrication of high-quality cesium (Cs)/formamidinium (FA) double-cation perovskite films through a two-step interdiffusion method is reported. CsxFA1-xPbI3-y(1-x)Bry(1-x) films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs+ and Br− on the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm2) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs0.1FA0.9PbI2.865Br0.135) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).

Multijunction solar cells employing perovskite and crystalline-silicon (c-Si) light absorbers bear the exciting potential to surpass the efficiency limit of market-leading single-junction c-Si solar cells. However, scaling up this technology and maintaining high efficiency over large areas are challenging as evidenced by the small-area perovskite/c-Si multijunction solar cells reported so far. In this work, a scalable four-terminal multijunction solar module design employing a 4 cm2 semitransparent methylammonium lead triiodide perovskite solar module stacked on top of an interdigitated back contact c-Si solar cell of identical area is demonstrated. With a combination of optimized transparent electrodes and efficient module design, the perovskite/c-Si multijunction solar modules exhibit power conversion efficiencies of 22.6% on 0.13 cm2 and 20.2% on 4 cm2 aperture area. Furthermore, a detailed optoelectronic loss analysis along with strategies to enhance the performance is discussed.

Hydrogen passivation of laser-induced defects is shown to be essential for the fabrication of laser doped solar cells. On first generation laser doped selective emitter solar cells where open circuit voltages are predominately limited by the full area back surface field, a 10 mV increase and 0.4 % increase in pseudo fill factor is observed through hydrogen passivation of defects generated during the laser doping process resulting in an efficiency gain of 0.35 % absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices, and can result in improvements on test structures up to 25 mV. On n-type PERT solar cells, an efficiency gain of 0.7 % absolute is demonstrated with increases in open circuit voltage and pseudo fill factor by applying a short low temperature hydrogenation process incorporating minority carrier injection using only hydrogen within the device. This process is also shown to improve the rear surface passivation increasing the short circuit current density from long wavelengths 0.2 mA/cm2 compared to that achieved using an Alneal process. Subsequently an average efficiency of 20.54 % is achieved.

This work presents a detailed analysis of the microstructure and the composition of our record Cu 2ZnSnSe4 (CZTSe)-CdS-ZnO solar cell with a total area efficiency of 9.7%. The average composition of the CZTSe crystallites is Cu 1.94 Zn 1.12Sn0.95Se3.99. Large crystals of ZnSe secondary phase (up to 400 nm diameter) are observed at the voids between the absorber and the back contact, while smaller ZnSe domains are segregated at the grain boundaries and close to the surface of the CZTSe grains. An underlying layer and some particles of Cu xSe are observed at the Mo-MoSe2-Cu2ZnSnSe4 interface. The free surface of the voids at the back interface is covered by an amorphous layer containing Cu, S, O, and C, while the presence of Cd, Na, and K is also observed in this region.

Printed chalcopyrite thin films have attracted considerable attention in recent years due to their potential in the high-throughput production of photovoltaic devices. To improve the homogeneity of printed CuInSe2 (CISe) layers, chemical additives such as binder can be added to the precursor ink. In this contribution, we investigate the influence of the dicyandiamide (DCDA) content, used as a binder in the precursor ink, on the physical and electrical properties of printed CISe solar cells. It is shown that the use of the binder leads to a dense absorber, composed of large CISe grains close to the surface, while the bulk of the layer consists of CISe crystallites embedded in a CuxS particle based matrix, resulting from the limited sintering of the precursor in this region. The expected additional carbon contamination of the CISe layer due to the addition of the binder appears to be limited, and the optical properties of the CISe layer are similar to the reference sample without additive. The electrical characterization of the corresponding CISe/CdS solar cells shows a degradation of the efficiency of the devices, due to a modification in the predominant recombination mechanisms and a limitation of the space charge region width when using the binder; both effects could be explained by the inhomogeneity of the bulk of the CISe absorber and high defect density at the CISe/CuxS-based matrix interface.

This paper reports on the role of boron in situ doping on enhancing crystallization of silicon germanium deposited at 400 °C and 2 torr. The dependence of growth rate on germanium content and boron concentration is investigated. The minimum boron concentration and the minimum germanium content required for crystallizing the as-grown layers is experimentally determined. The texture and grain microstructure of doped and undoped poly SiGe layers has been investigated by means of x-ray diffraction spectroscopy and transmission electron microscopy. The low deposition temperature coupled with the low tensile stress of the polycrystalline material enable postprocessing of surface micromachined microelectromechanical systems on top of standard complementry metal oxide semiconductor wafers with Al interconnects. Furthermore, the resistivity of the as-grown layers is as low as 1 mΩ cm, and hence, it can be used as a seeding layer for polycrystalline Si solar cells compatible with glass substrates.

An overview is given concerning industrial technologies, IMECS's advanced pilot line crystalline silicon solar cell technologies and medium term developments for industrial crystalline silicon terrestrial solar cell fabrication. Also IMEC's work on thin film crystalline silicon solar cells is shortly presented, all of this taking into account the existing market and technology trends.

Most conducting organic materials have a background p-type doping varying in the range 1015–1017 cm−3. We report results of a theoretical and experimental study of carrier transport in p-doped organic Schottky diodes. The theory given in this article shows that in a doped organic material with ohmic contacts the current is ohmic at low voltages. If the ohmic contact at the cathode is replaced by an Al Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change takes place depends on the doping concentrations. In the SCLC regime the current varies according to the well-known V2 law if there are no traps and the mobility is independent of the electric field. If either trapping or effect of field on mobility is important, the current varies as Vm, where m>2. We have investigated experimentally the I–V characteristics of Schottky diodes fabricated using the PPV-based oligomer 2,5-di-n...

We have investigated the presence of defects and their role on the efficiency of thin-film SiGe/Si solar cells. Temperature-dependent I–V, and DLTS measurements revealed the presence of a main defect that introduces a deep acceptor level (0/-) near the mid-gap of SiGe, at E v +0.45 eV . We show that this level is responsible for the generation–recombination current observed in our solar cells. We demonstrate that this defect is sensitive to passivation and that its concentration reduces with hydrogenation. Finally, we show that this level plays an important role in the determination of the minority carrier lifetime in our devices.

An overview is given concerning current industrial technologies, near future improvements and medium-term developments in the field of industrially viable crystalline silicon terrestrial solar cell fabrication (without concentration).

The potential of porous silicon (PS) with dual porosity structure as an intermediate layer for ultra-thin film solar cells is described. It is shown that a double-layered PS with a porosity of 20 60 % allows to grow epitaxial Si film at medium temperature (725°–800°C) and at the same time serves as a gettering/diffusion barrier for impurities from potentially contaminated low-cost substrate. A 3.5 μm thin-film cell with reasonable efficiency is realized using such a PS intermediate layer.

We have developed a model for light propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin-film crystalline silicon solar cell realized in an ultrathin (1–3 μm) epitaxially grown Si layer on PS. Three layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150–1000 nm epitaxial silicon layers on silicon wafers of which 150–450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400–1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high porosity layer from about 60% to 20–30%. Copyright © 2001 John Wiley & Sons, Ltd.

In this paper, the emitter formation on polycrystalline silicon (with grain size of 0.5–10 μm) deposited by chemical vapour deposition (CVD) on foreign substrates (thermal SiO2 and mullite ceramic) is studied. Phosphorus doping efficiency by POCl3 diffusion, APCVD+drive-in diffusion, and also rapid-thermal diffusion (RTD) from spin-on doping (SOD) sources were compared. For the first time, we report on photovoltaic results obtained on RTD-diffused emitters on pc-Si active layers deposited by rapid-thermal CVD, thus opening the way to an all rapid-thermal process for solar cell fabrication.

We demonstrate the use of industrial-orientated screen-printed aluminum-alloyed emitter for high-efficiency n-type interdigitated back-contact silicon solar cells. Different cell designs with various pitch sizes and emitter fractions have been studied. With an improved front surface field (FSF), short-circuit current densities up to 40 can be obtained. By combining the best cell design and the improved FSF, a high conversion efficiency of 19.1% with Czochralski n-type material has been achieved.