Correlation between micro-roughness,surface chemistry,and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells

The correlation between micro-roughness, surface chemistry, and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells is discussed through a deposition study of amorphous Si:H:Cl (a-Si:H:Cl) films by rf plasma-enhanced chemical vapor deposition using a SiH₂Cl₂–H₂ mixture. The degree of H- and Cl-termination on the growing surface determined the degree of micro-roughness at the p-type a-Si:H:Cl/intrinsic a-Si:H:Cl interface and solar cell performance. A higher degree of Cl-termination compared to H-termination was effective to suppress the micro-roughness at the growing surface and oxygen incorporation into the film, as well as chemical reduction of the intrinsic a-Si:H:Cl layer during the underneath p-layer formation. The study showed that a-Si:H:Cl deposited from SiH₂Cl₂ is a potential material for c-Si hetero-junction solar cells with an intrinsic a-Si:H:Cl thin layer.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

p/i界面掺碳缓冲层沉积时间对非晶硅太阳电池性能的影响

本文研究了pin型非晶硅(a-Si)太阳电池p/i界面掺碳缓冲层(C-buffer layer)沉积时间对电池效率和稳定性的影响.研究发现,随着掺碳缓冲层沉积时间的增加,太阳电池的初始效率有所增加,当沉积时间增加到约60s时,电池的初始效率达最大值,而后随着沉积时间的继续增加,电池效率下降.而在太阳电池的稳定性方面,当缓冲层沉积时间小于50s时,随着沉积时间的增加,电池衰退率增大;大于50s后,电池的衰退率又随沉积时间的增大而减小.     

Advanced deposition phase diagrams for guiding Si:H-based multijunction solar cells

Phase diagrams have been established to describe very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated silicon (Si:H) and silicon–germanium alloy (Si_1?xGe_x:H) thin films on crystalline Si substrates that have been over-deposited with n-type amorphous Si:H (a-Si:H). The Si:H and Si_1?xGe_x:H films are prepared under conditions used for the top and middle i-layers of high efficiency triple-junction a-Si:H-based n–i–p solar cells. Identical n/i cell structures were co-deposited in this study on textured (stainless steel)/Ag/ZnO which serve as substrate/back-reflectors in order to relate the phase diagrams to the performance parameters of single-junction solar cells. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1?xGe_x:H solar cells are obtained when the i-layers are prepared under previously-described maximal H₂ dilution conditions.     

On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n–i–p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a ‘reverse’ H₂ profiling technique to achieve nc-Si:H single junction n–i–p cells with high performance. Thus far, the best AM1.5 efficiency reached for n–i–p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H₂ profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H n–i–p cells obtained so far in our lab and the reported best n–i–p cells with PECVD i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.     

Controlling the quality of nanocrystalline silicon made by hot-wire chemical vapor deposition by using a reverse H2 profiling technique

Hydrogen profiling, i.e., decreasing the H₂ dilution during deposition, is a well-known technique to maintain a proper crystalline ratio of the nanocrystalline (nc-Si:H) absorber layers of plasma-enhanced chemical vapor-deposited (PECVD) thin film solar cells. With this technique a large increase in the energy conversion efficiency is obtained. Compared to PECVD, the unique characteristics of hot-wire CVD (HWCVD), such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for material and device optimization. We report in this paper the results of using a reverse H₂ profiling technique, i.e., increasing the H₂ dilution of silane instead of decreasing it, to improve the quality of HWCVD intrinsic nc-Si:H and the performance of this material in single junction n-i-p cells. Thus far, the efficiency of nc-Si:H n-i-p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector has been improved to 8.5%, and the efficiency of triple junction solar cells with a structure of proto-Si:H(HWCVD) top cell/proto-SiGe:H (PECVD) middle cell/nc-Si:H (HWCVD, with reverse H₂ profiling) bottom cell has reached 10.9%. These efficiency values show the viability of n-i-p cells comprising HWCVD nanocrystalline i-layers.     

Charge collection in amorphous silicon solar cells: Cell analysis and simulation of high-efficiency pin devices

The drift length L_drift = μτE within the i layer of a-Si:H solar cells is a crucial parameter for charge collection and efficiency. It is strongly reduced not only by light-induced reduction of μτ, but also by electric field deformation ΔE by charges near the p–i and i–n interfaces. Here, a simple model is presented to estimate contributions of free carriers, charges trapped in band tails and charged dangling bonds to ΔE. It is shown that the model reproduces correctly trends observed experimentally and by ASA simulations: charged dangling bonds contribute most to ΔE of meta-stable cells. Electrons trapped in the conduction band tail near the i–n interface lead to the strongest field deformation in the initial state, while positively charged dangling bonds near the p–i interface get more important with degradation under AM1.5g spectrum. The measurable parameter V_coll is proposed as an indirect parameter to estimate the electric field, and an experimental technique is presented that could enable the distinction of defects near the p–i and the i–n interfaces.     

Hole mobilities and the physics of amorphous silicon solar cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.     

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

Thirty years trajectory of amorphous and nanocrystalline silicon materials and their optoelectronic devices

A review is given on research trajectory of hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si) materials with their device applications ongoing since the period of 1970. A brief explanation on the motivation to start amorphous semiconductors research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability. Due to the result of some basic research on the film quality improvement of a-Si:H and nc-Si, some innovative devices had been developed since middle of 1980s in R&D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/nc-Si tandem type solar cells. Finally, the state of the art on the industrialization of the new devices is introduced and discussed.     

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.     

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

Origin of the Voc enhancement with a p-doped nc-SiOx:H window layer in n-i-p solar cells

The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to a V_oc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the V_oc, both the higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well with the experimental V_oc increase with the oxygen content. Using the differential temperature method, the built-in voltage (V_bi) of the cells with the two different p-layers is measured to be similar, agreeing well with the CBO model. Thus we attribute the improvement of the V_oc to the reduction of recombination at the i/p interface, as a consequence of the CBO in this region.     

Light-induced changes in hydrogenated amorphous silicon solar cells deposited at the edge of crystallinity

A series of hydrogenated amorphous silicon (a-Si:H) films were deposited in the transition region from amorphous to nanocrystalline phases by changing hydrogen dilution ratio R, deposition gas pressure, and RF power. Single junction a-Si:H solar cells were made using these materials as the intrinsic layers in the structure of n–i–p type on ZnO/Ag/stainless steel substrates. Light-induced degradations in the photovoltaic parameters were characterized on these cells after 1 Sun solar illumination for 150 h. The stabilized efficiencies were compared in conjunction with the structures in the intrinsic layers, which were revealed by high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry (FTIR). It was found that the solar cells incorporated protocrystalline intrinsic layer as the i-layer give a better initial efficiency, while solar cells made from nanostructured i-layers have a better stability of ～7% degradation against light soaking, as a result, both have nearly the same final stabilized efficiency. The best device stabilized efficiency reaches ～10.2% (0.25 cm², AM1.5G) for the intrinsic layer deposited at a high pressure of 2 Torr.     

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2 MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density J_SC for both types of absorber layers. In most cases both V_OC and J_SC recover after the final annealing step (at 160 °C) for both types of cells.     

Comparison of amorphous silicon solar cell performance following light and high-energy electron-beam induced degradation

In this article a comparison is reported between amorphous silicon (a-Si:H) solar cells that have been degraded using light soaking and 1 MeV electron-beam irradiation. Solar cells were degraded in open- and short-circuit condition, with the aim to change the recombination profile in the cell. For light-soaked solar cells a clear difference is found between open- and short-circuit conditions. Under open-circuit condition the solar cells degrade much more, which is explained by a much higher recombination rate under illumination in this case. These recombination events are believed to initiate defect formation. The performance of thin solar cells degrades less, as expected. For solar cells degraded under electron-beam irradiation no difference is found between open- and short-circuit conditions. Therefore we think that during electron-beam irradiation defect creation is not initiated by recombination events, but by energy transfer during collisions. The fill factor of thin solar cells degrades more after electron-beam irradiation. This effect is ascribed to a significant increase of the activation energy of the doped layers after irradiation.     

Properties of light-induced degradation and the electronic properties of nanocrystalline silicon solar cells grown under functionally graded hydrogen dilutions

The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using drive-level capacitance profiling (DLCP) to obtain defect density profiles as well as transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopies to study the spectra of defect related optical transitions. These measurements were performed on a series of n–i–p solar cell devices with intrinsic layer thickness of roughly 1 μm. The nc-Si:H intrinsic layers were deposited using RF or MVHF glow discharge with various hydrogen dilution profiles predominantly on specular stainless steel substrates (SS/n⁺/i nc-Si:H/p⁺/ITO), but also on textured back reflectors (SS/Ag/ZnO/n⁺/i nc-Si:H/p⁺/ITO) in some cases. Crystallite fractions were estimated using Raman spectroscopy. The electronic properties determined by our measurements could be correlated with variations in structural device parameters and with the degree of hydrogen dilution profiling during growth. We also found, depending on the growth conditions, that the devices exhibited markedly different behaviors after prolonged light exposure (100 h using light at 610 nm and 500 mW/cm² intensity). We discuss one specific microscopic mechanism that may be responsible for the light-induced changes that we have observed.     

Why does the open-circuit voltage in a micro-crystalline silicon PIN solar cell decrease with increasing crystalline volume fraction?

In hydrogenated micro-crystalline silicon (μc-Si:H) thin film solar cells, the open-circuit voltage (V_OC) shows a decline when the crystalline volume fraction (F_c) of the intrinsic μc-Si:H layer increases from 60% to over 90%. In this article we have simulated the experimental characteristics of solar cells, having intrinsic layers of different degrees of crystallinity to understand the reasons why. In order to model all aspects of the characteristics, we had to assume (a) wider band tails, (b) a higher mid-gap defect density and (c) a lower band gap for the more crystallized material. Modeling reveals that all three factors lower the field in the volume of the device and hence V_OC, due to higher photo-generated hole-trapping close to the P/I interface. The third factor brings the quasi-Fermi levels closer to the band-edges, resulting in higher free and trapped carrier densities throughout the device, with the trapped hole population particularly high at the P/I interface. We further show that V_OC is higher in a crystalline silicon PN cell, in spite of a sharply reduced band gap, because the lower effective density of states at the band-edges and sharply reduced band gap defect density overcome the effect of the smaller band gap.     

A novel structured plastic substrate for light confinement in thin film silicon solar cells by a geometric optical effect

We present a novel method to achieve light trapping in thin film silicon solar cells. Unlike the commonly used surface textures, such as Asahi U-type TCO, that rely on light scattering phenomena, we employ embossed periodically arranged micro-pyramidal structures with feature sizes much larger than the wavelength of visible light. Angular resolved transmission of light through these substrates indeed showed diffraction patterns, unlike in the case of Asahi U-type substrates, which show angular resolved scattering. Single junction amorphous silicon (a-Si) solar cells made at 125 °C on the embossed structured polycarbonate (PC) substrates showed an increase in current density by 24% compared to a similar solar cell on a flat substrate. The band gap and thickness of the i-layer made by VHF PECVD are 1.9 eV and 270 nm respectively. A double p-layer (nc-Si:H/a-Si:H) was used to make proper contact with ZnO:Al TCO.Numerical modeling, called DokterDEP was performed to fit the dark and light current–voltage parameters and understand the characteristics of the cell. The output parameters from the modeling suggest that the cells have excellent built-in potential (V_bi). However, a rather high recombination voltage, V_μ, affects the FF and short circuit current density (J_sc) for the cells on Asahi as well as for the cells on PC. A rather high parallel resistance ? 1 MΩ cm² (obtained from the modeling) infers that there is no significant shunt leakage, which is often observed for solar cells made at low temperatures on rough substrates. An efficiency of more than 6% for a cell on PC shows enormous potential of this type of light trapping structures.     

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175 °C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N₂O), hydrogen (H₂), silane (SiH₄), and diborane (B₂H₆) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71 eV to 2.0 eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7 × 10^− 5 S/cm to 5.1 × 10^− 7 S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3 eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (V_oc) = 800 mV, short circuit current density (J_sc) = 16.3 mA/cm², fill-factor (FF) = 72%, and photovoltaic conversion efficiency (η) = 9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. J_sc, FF and V_oc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.