High Voc Values Research Articles

Organic Dyes Incorporating Bis-hexapropyltruxeneamino Moiety for Efficient Dye-Sensitized Solar Cells

We report here on the synthesis and photophysical/electrochemical properties of three functional triarylamine organic dyes (MXD5−7) as well as their application in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). For the designed dyes, the nonplanar structures of bis-hexapropyltruxeneamino take the role of electron donor. The introduction of bis-hexapropyltruxeneamino units brought about superior performance over the simple triphenylamine dye, in terms of light-capturing abilities and suppressing dye aggregation. Among three dyes, the DSSCs based on the dye MXD7 showed the best photovoltaic performance: a short-circuit photocurrent density (JSC) of 11.8 mA cm−2, an open-circuit photovoltage (VOC) of 772 mV, and a fill factor (ff) of 0.68, corresponding to an overall conversion efficiency of 6.18% under 100 mW cm−2 irradiation. These dyes exhibited high VOC values, possible origin for which was investigated regarding the TiO2 surface blocking, conduction band movement, and electrolyte-dye interaction.

Benzo[2,1‐b;3,4‐b′]dithiophene‐based low‐bandgap polymers for photovoltaic applications

AbstractThe synthesis of four alternating copolymers using benzo[2,1‐b;3,4‐b′]dithiophene (BDP) as the common donor unit is presented. Before the synthesis, theoretical calculations that we performed predicted that the incorporation of BDP, which consists of fused dithiophene units with a benzene ring, into these polymers would produce a low‐lying highest occupied molecular orbital (HOMO) energy level. Low‐lying HOMO levels are desirable to produce high open circuit voltages (VOC) in organic bulk heterojunction (BHJ) photovoltaic devices. The polymers' structural characterization, as well as the preliminary results of their performance in BHJ devices, using (6,6)‐phenyl C61‐butyric acid methyl ester as the electron acceptor, is presented. The VOC values follow the expected trend: increasing with decreasing HOMO level of the polymer. High VOC values of 0.81 and 0.82 V have been obtained from two polymers: PBDPBT and PBDPDPP. The initial power conversion efficiency achieved in these unoptimized devices was 1.11% because of relatively low JSC values. The variation observed in the JSC values between the four polymers is discussed. Device performance is expected to increase with optimization of processing conditions for the devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Electrostatic spray processes

Cosensitization based on two or multiple dyes as “dye cocktails” can hit the target on compensating and broadening light-harvesting region. Two indoline D-A-π-A motif sensitizers (WS-2 and WS-39) that possess similar light response area but distinctly reversed feature in photovoltaic performance are selected as the specific cosensitization couple. That is, WS-2 shows quite high photocurrent but low photovoltage, and WS-39 gives relatively low photocurrent but quite high photovoltage. Due to the obvious “barrel effect”, both dyes show medium PCE around 8.50%. In contrast with the previous cosensitization strategy mostly focused on the compensation of light response region, herein we perform different cosensitization sequence, for taking insight into the balance of photocurrent and photovoltage, and achieving the synergistic improvement in power conversion efficiency (PCE). Electronic impedance spectra (EIS) indicate that exploiting dye WS-39 with high VOC value as the primary sensitizer can repress the charge recombination more effectively, resulting in superior VOC rather than using dye WS-2 with high JSC as the primary sensitizer. As a consequence, a high PCE value of 9.48% is obtained with the delicate cosensitization using WS-39 as primary dye and WS-2 as accessory dye, which is higher than the corresponding devices sensitized by each individual dye (around 8.48–8.67%). It provides an effective optimizing strategy of cosensitization how to combine the individual dye advantages for developing highly efficient solar cells.

Effects of electrode film modifications on the open-circuit photovoltage in enhanced dye-sensitized solar cells

In this study, the open-circuit photovoltage (Voc) decay technique was used to investigate the relationship between the electrode film morphology and the open-circuit photovoltage. Results indicate that dye-sensitized solar cells (DSCs) based on ordered arrays of TiO2 nanostructures (100 nm external diam- eters and 20-50 nm internal diameters) generally show higher open-circuit photovoltage (Voc) values than those based on sintered TiO2 nanoparticles (20-40 nm diameters). In particular, cells based on thick nanotu- bules (wall thickness C 45 nm in our research) and on nanorods (100 nm diameters) show particularly high Voc values, indicating slow recombination kinetics under open-circuit conditions. It can be argued that the nanorods and the thick nanotubules act like singles crystals and therefore the injected electrons in the inner TiO2 molecules are shielded from holes in the electro- lyte under open-circuit conditions. The open-circuit recombination time constant of electrons accumulated in the TiO2 conduction band is therefore prolonged and resulting in high Voc values.

Superstrate p-i-n a-Si:H solar cells on textured ZnO:Al front transparent conduction oxide

Superstrate p-i-n amorphous silicon thin-film (a-Si:H) solar cells are prepared on SnO2:F and ZnO:Al transparent conducting oxides (TCOs) in order to see the effect of TCO/p-layers on a-Si:H solar cell operation. The solar cells prepared on textured ZnO:Al have higher open circuit voltage Voc than cells prepared on SnO2:F. The presence of a thin microcrystalline p-type silicon layer (μc-Si:H) between ZnO:Al and p a-SiC:H plays a major role by causing an improvement in the fill factor as well as in Voc of a-Si:H solar cells prepared on ZnO:Al TCO. Without any treatment of the p-i interface, we could obtain a high Voc of 994 mV while keeping the fill factor (72.7%) and short circuit current density Jsc at the same level as for the cells on SnO2:F TCO. This high Voc value can be attributed to modification in the current transport in this region due to creation of a potential barrier.

8% Efficient thin‐film polycrystalline‐silicon solar cells based on aluminum‐ induced crystallization and thermal CVD

AbstractA considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline‐silicon (pc‐Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc‐Si layers obtained by aluminum‐induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc‐Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk‐Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc‐Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc‐Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin‐on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a‐Si layers that yield much higher Voc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc‐Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc‐Si solar cells based on the AIC seed layer approach. Copyright © 2007 John Wiley & Sons, Ltd.

Multi‐crystalline Si solar cells with very fast deposited (180 nm/min) passivating hot‐wire CVD silicon nitride as antireflection coating

AbstractHot‐wire chemical vapor deposition (HWCVD) is a promising technique for very fast deposition of high quality thin films. We developed processing conditions for device‐ quality silicon nitride (a‐SiNx:H) anti‐reflection coating (ARC) at high deposition rates of 3 nm/s. The HWCVD SiNx layers were deposited on multicrystalline silicon (mc‐Si) solar cells provided by IMEC and ECN Solar Energy. Reference cells were provided with optimized parallel plate PECVD SiNx and microwave PECVD SiNx respectively. The application of HWCVD SiNx on IMEC mc‐Si solar cells led to effective passivation, evidenced by a Voc of 606 mV and consistent IQE curves. For further optimization, series were made with HW SiNx (with different x) on mc‐Si solar cells from ECN Solar Energy. The best cell efficiencies were obtained for samples with a N/Si ratio of 1·2 and a high mass density of >2·9 g/cm3. The best solar cells reached an efficiency of 15·7%, which is similar to the best reference cell, made from neighboring wafers, with microwave PECVD SiNx. The IQE measurements and high Voc values for these cells with HW SiNx demonstrate good bulk passivation. PC1D simulations confirm the excellent bulk‐ and surface‐passivation for HW SiNx coatings. Interesting is the significantly higher blue response for the cells with HWCVD SiNx when compared to the PECVD SiNx reference cells. This difference in blue response is caused by lower light absorption of the HWCVD layers (compared to microwave CVD; ECN) and better surface passivation (compared to parallel plate PECVD; IMEC). The application of HW SiNx as a passivating antireflection layer on mc‐Si solar cells leads to efficiencies comparable to those with optimized PECVD SiNx coatings, although HWCVD is performed at a much higher deposition rate. Copyright © 2007 John Wiley & Sons, Ltd.

High open-circuit voltage values on fine-grained thin-film polysilicon solar cells

Grain boundaries are known to be the main limiting factor for a high performance of polysilicon solar cells. Defects at these grain boundaries serve as recombination centers for minority and majority carriers. Grain boundaries are also known to be paths for enhanced hydrogen diffusion, which results in passivation of part of the defects. In this paper, we show that grain boundaries are also paths for an enhanced phosphorus diffusion that limits the effect of hydrogen passivation. Phosphorus spikes along the grain boundaries enhance the junction area and determine the collection and the recombination volumes. Avoiding this preferential diffusion of phosphorus atoms during emitter formation, we obtained open-circuit voltages (Voc) up to 536mV on polysilicon material with a grain size of only 0.2μm. These high Voc values can only be accounted for by theory if a much smaller grain boundary recombination velocity is assumed than what was previously accepted for p-n junctions on fine-grained polysilicon solar cells.

Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by Aluminum-Induced Crystallization and Thermal CVD

AbstractA considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain large-grained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates.