Recently, low temperature solution-processed tin oxide (SnO2) as a versatile electron transport layer (ETL) for efficient and robust planar heterojunction (PH) perovskite solar cells (PSCs) has attracted particular attention due to its outstanding properties such as high optical transparency, high electron mobility, and suitable band alignment. However, for most of the reported works, an annealing temperature of 180 °C is generally required. This temperature is reluctantly considered to be a low temperature, especially with respect to the flexible application where 180 °C is still too high for the polyethylene terephthalate flexible substrate to bear. In this contribution, low temperature (about 70 °C) UV/ozone treatment was applied to in situ synthesis of SnO2 films deposited on the fluorine-doped tin oxide substrate as ETL. This method is a facile photochemical treatment which is simple to operate and can easily eliminate the organic components. Accordingly, PH PSCs with UV-sintered SnO2 films as ETL were successfully fabricated for the first time. The device exhibited excellent photovoltaic performance as high as 16.21%, which is even higher than the value (11.49%) reported for a counterpart device with solution-processed and high temperature annealed SnO2 films as ETL. These low temperature solution-processed and UV-sintered SnO2 films are suitable for the low-cost, large yield solution process on a flexible substrate for optoelectronic devices.Keywords: electron transport layer; low temperature processing; perovskite solar cells; tin dioxide; UV irradiation;

The development of solution processable perovskite solar cells (PSCs) has progressed rapidly, and the their highest power conversion efficiency (PCE) has recently surpassed 22%. Further studies to promote market-oriented PSCs call for further reducing the manufacturing cost of the device and addressing the concerns about the possible outflow of toxic lead. To reduce the level of environmental pollution and prevent the health hazard caused by degraded devices (solid waste) and possible lead outflow and to conserve resources, we adopted low-temperature solution-processed, multirecycled glass/FTO/c-TiO2 (m-TiO2) substrates from the degraded devices to fabricate efficient planar heterojunction (PH) and mesoporous (M) PSCs in an environmentally friendly and energy-conserving manner. This is realized by simple and low-temperature processes, including organic solvent washing, ultrasonic cleaning, and UV–ozone treatment. After two rounds of substrate recycling, the PH PSC and M PSC still exhibited peak efficiencies of 11.87% and 11.03%, respectively, indicating the feasibility of recycling used substrates for sustainable, energy and resource conservation-oriented, and environmentally friendly energy production.Keywords: Degraded devices; Environmentally friendly energy production; High performance; Low-temperature processing; Perovskite solar cells; Recycled substrates;

•Pseudo-planar heterojunction perovskite solar cell was reported for the first time.•The device was fabricated by a solution-saving one-step dip-coating method.•The device shows PCE close to that of a conventional spin-coated one.•Models of the abnormal hysteresis, roll-over and current peak were proposed.Rough dense sol-gel-derived titanium dioxide (TiO2) electron-transport layers (ETLs) and smooth organolead halide perovskite (PVK) films for pseudo-planar heterojunction perovskite solar cells (P-PH PVKSCs) were fabricated by a facile one-step dip-coating method. The highly compact TiO2 ETLs and uniform PVK films endow the device a high power conversion efficiency (PCE) of over 11%, which was nearly identical to that of a reference device (12%) fabricated by conventional spin-coating. Furthermore, the device showed no pronounced hysteresis when tested by scanning the voltage in a forward and backward direction, showing the potential of facile and waste-free dip-coating in replacing of spin-coating for large area perovskite solar cells preparation. Lastly, the hysteresis was compared and discussed and models regarding the abnormal hysteresis, roll-over and current peak phenomena were proposed as well.Efficient and hysteresis-less pseudo-planar heterojunction perovskite solar cell fabricated by a facile and solution-saving one-step dip-coating method was proposed for the first time. The device shows a high power conversion efficiency of over 11%, closing to that of a reference device (12%) fabricated by conventional spin-coating. Models regarding the abnormal hysteresis, roll-over and current peak phenomenons were proposed as well.Download high-res image (301KB)Download full-size image

Currently, most efficient perovskite solar cells (PVKSCs) with a p–i–n structure require simultaneously electron transport layers (ETLs) and hole transport layers (HTLs) to help collecting photogenerated electrons and holes for obtaining high performance. ETL free planar PVKSC is a relatively new and simple structured solar cell that gets rid of the complex and high temperature required ETL (such as compact and mesoporous TiO2). Here, we demonstrate the critical role of high coverage of perovskite in efficient ETL free PVKSCs from an energy band and equivalent circuit model perspective. From an electrical point of view, we confirmed that the low coverage of perovskite does cause localized short circuit of the device. With coverage optimization, a planar p–i–n++ device with a power conversion efficiency of over 11% was achieved, implying that the ETL layer may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%.Keywords: electron-transport layer; energy band structure; equivalent circuit model; full coverage perovskite; perovskite solar cells; ultraviolet-ozone treatment;

In this work, a facile and low temperature processed anodic oxidation approach is proposed for fabricating compact and homogeneous titanium dioxide film (AO-TiO2). In order to realize morphology and thickness control of AO-TiO2, the theory concerning anodic oxidation (AO) is unveiled and the influence of relevant parameters during the process of AO such as electrolyte ingredient and oxidation voltage on AO-TiO2 formation is observed as well. Meanwhile, we demonstrate that the planar perovskite solar cells (p-PSCs) fabricated in ambient air and utilizing optimized AO-TiO2 as electron transport layer (ETL) can deliver repeatable power conversion efficiency (PCE) over 13%, which possess superior open-circuit voltage (Voc) and higher fill factor (FF) compared to its counterpart utilizing conventional high temperature processed compact TiO2 (c-TiO2) as ETL. Through a further comparative study, it is indicated that the improvement of device performance should be attributed to more effective electron collection from perovskite layer to AO-TiO2 and the decrease of device series resistance. Furthermore, hysteresis effect about current density–voltage (J–V) curves in TiO2-based p-PSCs is also unveiled.

•Three kinds of the non-uniform distribution in µc-Si1−xGex:H are explored.•The properties of µc-Si1−xGex:H are mainly governed by Ge composition.•The infrared response of bottom cell can be enhanced by raising Ge content.•The 1200 nm a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.5Ge0.5:H solar cell shows an efficiency of 11.35%.In this work, hydrogenated microcrystalline silicon germanium (µc-Si1−xGex:H) thin films were fabricated by plasma-enhanced chemical vapor deposition (PECVD) and developed as the infrared absorber for thin film silicon solar cells. Three kinds of the non-uniform distribution in µc-Si1−xGex:H thin films including: “the non-uniform distribution of Si and Ge”, the non-uniform distribution of crystallization”, and “the non-uniform distribution of H”, and how these affect the structural, optical and photoelectric properties of µc-Si1−xGex:H thin films have been explored. The results show that the good film quality of µc-Si1−xGex:H is associated with the low crystalline volume fraction and microstructure factor. The band gap of µc-Si1−xGex:H is determined by the proportion of the Ge-related crystalline networks. In addition, it is suggested that the deterioration of the photosensitivity of µc-Si1−xGex:H is mainly due to the increase of the Ge clusters with higher micro-void and defect density. Furthermore, by using µc-Si1−xGex:H bottom sub-cells, the comparable efficiency can be realized under the thin thickness condition. An efficiency of 11.35% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.5Ge0.5:H triple junction structure with total cell thickness as thin as 1200 nm was obtained. It is believed that the µc-Si1−xGex:H thin films can be a better candidate for effective infrared absorber by further improving its microstructure uniformity.

•N, N-Dimethylformamide (DMF) can easily dissolve the perovskite films of the degraded devices.•The obtained glass/FTO substrates can be easily rinsed clean with low-energy solution processes.•The cleaned glass/FTO substrates can participate in the new round of devices preparation.•The best device showed an efficiency of 9.97% after 2 times of glass/FTO substrates recycling.Perovskite solar cells (PVKSCs) are an attractive technology that finds their potential in the field of renewable energy sources. Transparent conductive oxides including fluorine-doped tin dioxide (FTO) and indium tin oxide (ITO) with high optical transmittance and low electrical resistivity are key components in PVKSCs. While commercial FTO or ITO either requires high temperature and high vacuum process or contains rare indium element, which will increase the production cost of PVKSCs. Here we report efficient electron-transport layer (ETL) free planar PVKSCs using the recycled FTO/glass substrates from degraded devices. By simple and low-temperature processes including organic solvent washing, ultrasonic cleaning and UV ozone treatment, the discarded substrates can be readily reused for fabricating ETL-free planar PVKSCs. The UV–vis optical transmission, crystal structure, sheet resistance, surface morphology, elemental composition and static contact angles measurement of the original and recycled FTO/glass substrates (one time and two times) were measured and compared. Planar ETL-free devices with power conversion efficiencies of about 10% have been achieved by adopting the recycled FTO/glass substrates, which are comparable to that of the devices based on the original FTO substrates, suggesting the feasibility of recycling the FTO/glass substrates from degraded devices for fabricating ETL-free PVKSCs.In this article, efficient electron-transport layer-free planar perovskite solar cells with power conversion efficiencies of about 10% have been achieved by recycling low-energy solution processed one time and two times used Glass/FTO substrates from degraded devices, establishing an instructive model towards an attractive technology for sustainable, scalable, energy and resources-conservation-oriented as well as environmentally-friendly energy production.

•UVO treatment greatly affected perovskite coverage and structure.•UVO treatment of FTO substrate resulted in high-efficient perovskite solar cells.•The best device showed an efficiency of 10.67%.Planar perovskite solar cells with a p–i–n structure use both hole-transport layers and electron-transport layers to promote collection of photogenerated holes and electrons for achieving high performance, wherein a high temperature processed compact and mesoporous titanium dioxide are usually required. We report here efficient perovskite solar cells grown directly on ultraviolet–ozone treated fluorine-doped tin oxide (FTO) substrates without using any electron-transport layers. The morphology, structure, optical–electrical properties of perovskite films deposited on FTO substrates with and without ultraviolet–ozone treatment and their corresponding devices׳ performance have been studied and compared. Ultraviolet–ozone treatment of FTO substrates improves the smoothness and coverage of CH3NH3PbI3−xClx films, which avoids direct contact between FTO and hole-transport layer. A planar electron-transport layer free device with a power conversion efficiency of over 10% has been achieved, suggesting that the widely adopted electron-transport layer is not a requirement for an efficient device.In this article, simple-structured and electron-transport layer free planar perovskite solar cells with power conversion efficiency of over 10% by simple one-step solution process under sub-100 °C temperature has been achieved.

•Optical, electrical and morphological properties of ternary system were investigated.•The hole mobility was improved by incorporating a 20% content of PCDTBT to PTB7-Th.•The PCE and stability of ternary organic solar cells was enhanced compared with the binary devices.Ternary bulk heterojunctions (BHJs) are promising candidates that can improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this paper, a ternary OSC with two donors, including one wide bandgap polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), one low bandgap polymer Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th), and one acceptor [6,6]-phenyl C70 butyric acid methyl ester (PC70BM), is fabricated in atmospheric conditions. By incorporating a 20% content of PCDTBT, an optimized PCE of 7.86% for ternary OSC is characterized by a short-circuit current density (Jsc) of 15.21 mA cm−2, a fill factor of 69.70% and an open-circuit voltage (Voc) of 0.74 V. The Voc values increased steadily from 0.73 to 0.86 V as the increase of PCDTBT fraction, which indicates that the Voc of ternary OSC is not limited by the smallest one of the corresponding binary OSC. We show that the Jsc of the ternary OSC is better than those of the binary OSC in virtue of the complementary polymer absorption and cascade energy levels, as well as optimized morphology of the ternary system. Furthermore, the lifetime of the devices with PCDTBT is greatly enhanced. This work indicates that two donors (PTB7-Th/PCDTBT) ternary BHJs system provide a simple and effective method to improve the performance and also the stability of OSCs.

•Randomly textured photonic crystal (RTPC) for efficient light-trapping is developed.•The influence of texture on the optical performance of RTPCs is discussed.•The influence of texture on RTPC-based solar-cell performances was examined.•High reflectivity and strong light scattering were observed for the RTPC.•A 9.6% efficiency was achieved for the RTPC-based amorphous silicon solar cell.One of the foremost challenges in achieving high-efficiency thin-film silicon solar cells is in devising an efficient light trapping system because of the short optical path length imposed by the inherent thin absorption layers. In this paper, an efficient light trapping system is proposed using a combination of randomly textured surfaces and a one-dimensional photonic crystal (randomly textured photonic crystal; RTPC). The influence of the texture on the optical performance of RTPCs is discussed using the results of an experiment and a finite-difference time-domain simulation. This RTPC back reflector (BR) can provide high reflectivity and strong light scattering, resulting in an increased photocurrent density of the hydrogenated amorphous silicon (a-Si:H) solar cell. As a result, the highly textured RTPC BR yielded an efficiency of 9.6% for a-Si:H solar cell, which is much higher than the efficiency of 7.6% on flat AZO/Ag BR and 9.0% on textured AZO/Ag BR. This RTPC BR provides a new approach for creating high-efficiency, low-cost thin-film silicon solar cells.

•Sol-gel AZO films used as ETL on FTO electrode for inverted polymer solar cells.•PVP film was inserted as AZO surface modifier to improve AZO surface properties.•PVP film show superior electron extraction due to improved morphology of ETL.•PCE increase from 2.52% to 4.14% of IPSCs, compared to without PVP layer device.Nano-textured transparent electrodes are commonly used to receive higher light absorption in inverted polymer solar cells (IPSC). However, the performance of the device is often restricted by the highly rough morphology of the textured transparent electrode. In this work, a Polyvinylpyrrolidone (PVP) interlayer was inserted as a surface modifier in IPSCs based on a fluorine-doped SnO2 electrode (FTO). The inserted layer facilitates electron extraction due to improved interface morphology of the AZO electron transport layer (ETL). This enhancement resulted in an approximately 63% increase in power conversion efficiency from 2.52% to 4.14% of IPSCs based on the FTO electrode compared to solar cells without PVP layer.

•The band gap grading profile can be implemented in μc-SiGe:H solar cells.•μc-SiGe:H solar cell with normal profile obtains a higher Jsc.•A high efficiency was achieved by μc-SiGe:H solar cell with this novel structure.•μc-SiGe:H solar cell shows a great potential as an infrared absorber.In this work, hydrogenated microcrystalline silicon germanium (μc-SiGe:H) thin film solar cells with a novel band gap grading profile have been designed. By comparing different profile types (normal profile, reverse profile and no profile), the normal profile was formed in sequence by the superposition of a high Ge content layer, a Ge content grading layer and a μc-Si:H layer has been proposed. This structure exhibits higher short-circuit current density (Jsc) than conventional cell design with the similarly Ge content owing to the enhancement of the infrared response. Finally, an initial efficiency of 6.53% was achieved by μc-SiGe:H solar cell with this novel cell structure. The results have demonstrated a great potential of the μc-SiGe:H solar cells as the infrared absorber in multi-junction silicon based thin film solar cells.

Low-temperature, solution-processed perovskite solar cells (PSCs), which utilized organic poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) as a hole-transport layer (HTL), achieved a power conversion efficiency (PCE) as high as 13.29% when fabricated in ambient air. Through a comparative study, we demonstrate this PCE value to be superior compared to its counterparts with spiro-OMeTAD or P3HT as the HTL; the superiority consists of a higher fill factor (FF) and open-circuit voltage (Voc). By probing the absorption spectrum of CH3NH3PbI3−xClx before and after spin-coating the PTB7, it is discovered that the spin coating of PTB7 has little influence on the quality of the perovskite films. Furthermore, it is shown that PTB7 possesses higher conductivity compared with conventional HTLs, including spiro-OMeTAD, P3HT and PCDTBT. Moreover, in order to further improve device performance, the prevalent additives lithium bis (trifluoromethylsulphony) imide (LiTFSI) and 4-tert-butylpyridine (t-Bp) are investigated, along with a post-annealing process that is applied to the whole device. The results presented here and the overall fabrication method represent a helpful new approach for fabricating highly efficient perovskite-based photovoltaic devices.

The effect of the thickness of the poly(N-vinylpyrrolidone) interface modifier on the photovoltaic performance of inverted organic solar cells was investigated. Superior interface properties provided efficient charge transport and decreased the charge recombination due to PVP interlayer, which reduced the energy barrier for electron extraction by lowering the hydroxide radical amount. We obtained an enhanced efficiency of 4.55% (for the P3HT:PCBM device) and 6.18% (for the PTB7:PC71BM device).

•Sol–gel derived AZO films with flat and textured morphologies were used as cathode buffer layers for inverted polymer solar cells.•The textured-AZO together with reflective back electrode provides efficient light-trapping inducing further light absorption in active layer at the near-UV/blue light region.•The textured-AZO devices show 18.7% and 17.6% enhancement in short circuit current density and efficiency, respectively, compared with the flat-AZO devices.The effects of sol–gel textured Al-doped ZnO (AZO) cathode buffer layer on the performance of the inverted polymer solar cells have been investigated in this work. Textured AZO, together with a reflective back electrode provides efficient light-trapping, inducing further light absorption in the active layer. The AZO films were characterized with respect to their morphology, optical and electrical properties. Due to the enhanced absorption at near-UV/blue light region (300–500 nm), the short current density of the textured-AZO device is increased from 9.52 to 11.30 mA/cm2 compared with the reference cell based on the flat-AZO. Power conversion efficiency is increased substantially from 2.44% to 2.87% since without loss in the fill factor and open circuit voltage.

•AZO layer has been modified by PVP in inverted polymer solar cells.•PVP provides a strong adhesion and wettability between AZO and active layer.•Facilitated electron transport and suppressed charge carriers recombination.•This results in an increased device efficiency from 2.86% to 4.08%.A polyvinylpyrrolidone (PVP) thin film (9 nm) prepared on the top of Al-doped ZnO (AZO) electron transport layer by spin-coating, was developed as an interface modifier in inverted polymer solar cells (IPSCs). The PVP with excellent alcohol solubility provided a strong adhesion and wettability, leading to an improved interface quality between the AZO and active layer. Combined with a reduced work function of the AZO layer, the resulting devices showed a significant increase in power conversion efficiency from 2.86% to 4.08%, benefiting from the dramatic enhancement in fill factor (35%). Due to the ease of use and remarkable boost in efficiency, our results indicated that PVP was a promising candidate for surface modification material in IPSCs.

► The performance of µc-SiGe:H solar cells with varied Ge content is investigated.► µc-SiGe:H solar cells show higher infrared response than µc-Si:H solar cell.► A high efficiency of a-Si:H/a-SiGe:H/µc-SiGe:H triple junction solar cell is achieved.► Application of µc-SiGe:H absorber is a promising way to reduce the cell thickness.Hydrogenated microcrystalline silicon germanium (µc-Si1–xGex:H), with the advantage of its narrower variable band gap and higher absorption coefficient over the conventional hydrogenated microcrystalline silicon (µc-Si:H), has been implemented as the bottom sub-cell absorber of the triple junction solar cells. By replacing µc-Si:H i-layer with µc-Si0.91Ge0.09:H i-layer in the triple junction solar cell, the bottom sub-cell thickness (Dbottom) could be reduced by almost a half, meanwhile a higher efficiency was attained. As a result, an initial efficiency of 12.02% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.91Ge0.09:H triple junction structure with a total cell thickness as small as 1800 nm was achieved. It is demonstrated that the triple junction solar cell incorporating µc-Si1−xGex:H bottom sub-cell with high efficiency and a relatively low thickness has a high potential for cost-effective photovoltaic applications.

•ZnO prepared by MOCVD was used in inverted polymer solar cells.•The effects of ZnO thickness on device performance were investigated.•The effects of boron doped ZnO on device performance were investigated.•ZnO layer controls the tradeoff between Jsc, Voc and FF.We report on the photovoltaic properties of inverted polymer solar cells (IPSCs) where the transparent indium tin oxide (ITO) electrode was modified by a ZnO layer using metal organic chemical vapor deposition (MOCVD). The intrinsic ZnO (i-ZnO) layers were deposited with varying thicknesses from 0 to 1500 nm. The work function and surface morphology of ITO/i-ZnO were found to be dependent on the i-ZnO thickness. When the thickness of the i-ZnO layer was 80 nm, optimized IPSCs with a power conversion efficiency (PCE) of 2.93% was achieved. Furthermore, the i-ZnO layer doped with boron (BZO) was investigated. The best IPSC with a BZO layer showed a PCE of 3.26%, which is higher than that (2.93%) of the device with the i-ZnO layer. The better performance is due to combined effects of improvement in charge collection and conductivity of BZO/ITO electrode.

•Sol–gel derived zinc oxide (ZnO) thin films with different morphologies are prepared using two annealing rates.•ZnO films with different morphologies and thus different scattering properties.•Compared with the fast annealed film, ZnO film formed at 9 °C/min possesses the better scattering effect.•The resulting inverted polymer solar cell (IPSC) shows a 12.6% improvement in short-current density.•Performance of IPSCs could be effectively improved by controlling the morphology of ZnO films by a simple post-annealing process.Different morphology zinc oxide (ZnO) thin films were prepared by a sol–gel method using two annealing rates and used as the electron transport layers in inverted polymer solar cells. The morphology, optical and structure properties were performed by AFM, UV–vis and XRD in order to study the effect of annealing rates. The undulating morphology of ZnO film fabricated at a slow heating rate of 9 °C/min possesses a rougher surface than that of ZnO film annealed at the fast heating rate of 56 °C/min, which provides efficient light-trapping and increases photon absorption.The resulting device shows 12.6% and 6.7% improvement in short current density and fill factor, respectively, compared with the device based on the rapidly annealed ZnO; a maximum power conversion efficiency of 2.55% was achieved.

We have used dip coating method to fabricate polymer solar cells (PSC) with an active layer composed of a blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester. With optimized conditions, pinhole-free and highly self-assembly active films were obtained, resulting in a narrow distribution of the device parameter. The performance of the dip coated devices (4.03%) is comparable to that of the spin coated devices (4.00%). Moreover, we show that dip coating is a feasible technology to deposit all solution processable layers of PSC, including the hole-transporting layer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as well and demonstrate dip coated hole-transporting layer and active layer devices with an efficiency of 3.49%. Our research may provide an alternative way for preparation of roll-to-roll processed organic electronics.Graphical abstractHigh efficiency, air-processed and annealing-free polymer solar cells fabricated by dip coating process have been achieved. This process may provide an alternative way for preparation of roll-to-roll processed organic electronics.Highlights► High-efficiency PSCs by dip coating were obtained in air condition. ► High-quality active films result in a narrow distribution of the device parameter. ► The dip coated device performance is comparable to that of the spin coated device. ► Dip coating provides a way for roll-to-roll processed organic electronics.

A combination of hydrogen and helium dilutions was introduced when the microcrystalline silicon germanium (μc-SiGe:H) thin films were prepared by very high frequency plasma enhanced chemical vapor deposition on a low-temperature substrate. An optimum helium flow rate was found to achieve the structural uniformity in the growth direction, while Ge content was found to nearly keep constant with varying flow rates of helium. An abundance of atomic H was detected in plasma due to the attendance of helium and no obvious photosensitivity deterioration was observed on the thin film with a high crystalline volume fraction. The active roles of helium were identified by analyzing the mechanism in the plasma, where both metastable Hem⁎ and He+ can accelerate the diffusion of Ge related radicals and passivation of the dangling bonds on the growth surface, respectively. These phenomena have been revealed by experimental results. Therefore, a combination of hydrogen and helium dilutions can improve the structure of the μc-SiGe:H thin films with little degradation of photo-electronic properties.Highlights► Helium is beneficial for the deposition of microcrystalline silicon germanium. ► An abundance of atomic H was detected in plasma. ► The structural uniformity was improved in the growth direction. ► No obvious photosensitivity deterioration on high crystallized films was observed.

ZnO nanopillars are grown on a ZnO seed layer on fluorine-doped tin oxide (FTO)-coated glass substrates for the fabrication of inverted organic photovoltaic devices based on poly(3-hexylthiophene) and (6,6)-phenyl C61-butyric acid methyl ester. It is found that the oriented ZnO nanopillars play an important role in collecting photogenerated electrons and act as an electron-transport path to the cathode. OPV devices with a ZnO nanopillar layer grown on a ZnO seed layer exhibit a threefold increase in power conversion efficiency compared with that of devices with a ZnO seed layer only.

► The performance of µc-SiGe:H solar cells with varied Ge content is investigated.► µc-SiGe:H solar cells show higher infrared response than µc-Si:H solar cell.► A high efficiency of a-Si:H/a-SiGe:H/µc-SiGe:H triple junction solar cell is achieved.► Application of µc-SiGe:H absorber is a promising way to reduce the cell thickness.Hydrogenated microcrystalline silicon germanium (µc-Si1–xGex:H), with the advantage of its narrower variable band gap and higher absorption coefficient over the conventional hydrogenated microcrystalline silicon (µc-Si:H), has been implemented as the bottom sub-cell absorber of the triple junction solar cells. By replacing µc-Si:H i-layer with µc-Si0.91Ge0.09:H i-layer in the triple junction solar cell, the bottom sub-cell thickness (Dbottom) could be reduced by almost a half, meanwhile a higher efficiency was attained. As a result, an initial efficiency of 12.02% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.91Ge0.09:H triple junction structure with a total cell thickness as small as 1800 nm was achieved. It is demonstrated that the triple junction solar cell incorporating µc-Si1−xGex:H bottom sub-cell with high efficiency and a relatively low thickness has a high potential for cost-effective photovoltaic applications.

•The initial deposition process of μc-Ge:H films is studied by different methods.•The effect of crystal nucleus in the initial layer is studied.•A power gradient method is proposed to optimize deposition process.•The μc-Ge:H thin films with less-than 6 nm incubation layer are prepared.•TFT structures are prepared to illustrate electrical propriety.This paper studies the microstructure evolution of hydrogenated microcrystalline germanium (μc-Ge:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD). There is an amorphous incubation layer formed in the initial deposition stage of μc-Ge:H thin film. It is demonstrated that the thickness of incubation layer can be reduced by high hydrogen dilution and high discharge power method. However, at high hydrogen dilution, the deposition rate of μc-Ge:H appears a sharply decrease. Using a high discharge power can compensate the deposition rate decrease but lead to decrease of average grain size and appearance of micro-void in the μc-Ge:H thin film. In addition, by comparing two thickness groups of μc-Ge:H thin films deposited at different discharge powers, it is noticed that the evolution process relates to the formation of crystal nucleuses. Thus, a power gradient method is proposed to understand the mechanism of nucleation and crystal growth in the initial deposition process of μc-Ge:H films. Finally, by power gradient method, the incubation layer thickness of μc-Ge:H thin films has been decreased to less than 6 nm. Moreover, Raman scattering spectra shows a 38 nm μc-Ge:H film has a crystal fraction (XC) of 62.4%. Meanwhile, the mobility of TFT devices shows the improved electrical property of μc-Ge:H film deposited by power gradient method.