Production technology for amorphous silicon-based flexible solar cells

We have developed a-Si-based solar cells with plastic film substrate and achieved a stabilized efficiency of 9% in a 40 cm×80 cm cell. The structure and fabrication process of flexible solar cells are presented. Then we discuss the merits and demerits of our process from the viewpoint of mass production, and clarify that the SCAF cell has a good adaptability to mass production.     

Optical properties of random amorphous multilayers a-Si:H/a-Si3N4+x:H

Anomalous reflection peaks and dips have been observed in optical reflectance spectra of random amorphous multilayers made of a-Si:H/a-Si₃N_4+xg:H. A model using the idea of localization of light propagation has been used to explain these anomalies. The localization length of light ξ is estimated experimentally and by simulation in this random system. The distribution of electric field strengths E² in multilayers are calculated. The distribution of E² in random multilayers is quite different from that in superlattices which are the regular multilayers. It is found that the bandwidth of high reflection region is broadened by the randomization of the layer thickness in ψ/4 reflection multilayers made of a-Si:H/a-Si₃N_4+x:H. If these random amorphous multilayers are applied to design the structure of amorphous solar cells, there is a possibility to improve the efficiency by confining the light in cells.     

Low-temperature a-Si:H/ZnO/Al back contacts for high-efficiency silicon solar cells

The paper analyses the electronic transport of high-efficiency silicon solar cells with high-quality back contacts that use a sequence of amorphous (a-Si) and microcrystalline (μc-Si) silicon layers prepared at a maximum temperature of 220 °C. Our best solar cells having diffused emitters with random texture and full-area a-Si/μc-Si contacts have an independently confirmed efficiency of 21.0%. An alternative concept uses a simplified a-Si layer sequence combined with Al-point contacts and yields a confirmed efficiency of 19.3%. Analysis of the internal quantum efficiency (IQE) shows that both types of back contacts lead to effective diffusion lengths L_eff exceeding the wafer thickness considerably. Fill factor limitations for the full area contacts result from non-ideal diode behavior, possibly due to the injection dependence of the interface recombination velocity.     

Limiting carrier effect in a-Si:H solar cells

The limiting carrier effect in a-Si:H p–i–n and n–i–p solar cells has been investigated computationally, by adjusting the values of the carrier band mobilities. Using a realistic optical generation rate profile, it was found that the effect is significant in both types of cell, with the electron identified as the limiting carrier in the p–i–n cell, and the hole in the n–i–p cell. However, using a uniform generation rate profile in the simulation indicated that the limiting carrier effect was much less significant, and that device performance in both cells seemed to be slightly more sensitive to the hole transport properties than to the electron transport properties.     

Computer modelling of current matching in a-Si : H/a-Si : H tandem solar cells on textured TCO substrates

Computer modelling is used as a tool for optimising a-Si : H/a-Si : H tandem cells on textured substrate in order to achieve current matching between the top and bottom cell. To take light scattering at the textured interfaces of the cell into account, we developed a multirough-interface optical model which was used for calculating the absorption profiles in the tandem cells. In order to simulate multi-junction solar cell as a complete device we implemented a novel model for tunnel/recombination junction (TRJ), which combines the trap-assisted tunnelling and enhanced carrier transport in the high-field region of the TRJ.We investigated the influence of light scattering and thickness of the intrinsic layer of the bottom cell on the optimal ratio i2/i1 between the thicknesses of the bottom (i2) and top (i1) intrinsic layers in the current-matched cell. The simulation results show that increasing amount of scattering at the textured interfaces leads to a lower ratio i2/i1 in the current-matched cell. This ratio depends on the thickness of the intrinsic layer of the bottom cell. The simulation results demonstrate that a-Si : H/a-Si : H tandem cell with 300 nm thick intrinsic layer in the bottom cell exhibits higher efficiency than the cell with 500 nm thick bottom intrinsic layer.     

Optically induced degradation in a-Si:H solar cells

Difference spectra of optically induced changes in the surface photo-voltage response of a-Si:H solar cells show that illumination generates two sets of near midgap states which have energy levels consistent with the two energy states for dangling bond defects in this material. Recombination of photo-generated carriers in these states leads to significant degradation of cell performance for photon energies larger than the bandgap. At low temperature (−160°C), the difference spectrum indicates that although the defect states are still present, such degradation does not occur.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

Low-temperature contacts through SixNy-antireflection coatings for inverted a-Si:H/c-Si hetero-contact solar cells

High-temperature contact firing of screen printable metal pastes is getting more problematic as silicon wafers used in solar cell production are becoming thinner. Besides, an electronic degradation of the Si_xN_y/c-Si interface occurs at these temperatures especially if the Si_xN_y layer is directly deposited onto high-quality absorbers as on the front side of a-Si:H/c-Si hetero-contact solar cells of inverted geometry. The latter structure has been proposed as an easy producible high-efficiency solar cell. Low-temperature alternatives such as local ablation of Si_xN_y with 355 nm laser radiation are examined with regard to the stability of the electronic quality of passivated areas between the openings in the Si_xN_y layer. Contactless time-resolved microwave conductivity measurements (TRMC) are applied to measure changes in electronic passivation after these treatments. Subsequent galvanic metallization of the openings is optimized for its use as ohmic contacts.     

Potential of VHF-plasmas for low-cost production of a-Si: H solar cells

Compared to the use of the standard glow discharge technique the production of amorphous silicon solar cells by the very high frequency glow discharge (VHF-GD) bears yet additional cost reduction potentials:Using VHF-GD at excitation frequencies higher than 13.56 MHz, a more efficient dissociation of silane gas is obtained; thus, higher deposition rates are achieved; this reduces considerably the deposition time for intrinsic amorphous and microcrystalline silicon layers.Furthermore, by itself and even more so, in combination with argon dilution, VHF-GD technique improves silane utilisation and leads, thus, to further cost reduction.Finally, by combining the VHF-GD technique and the “micromorph” concept “real” tandem cells (i.e. a superposition of two cells with distinctly different band gaps) can be deposited at low temperatures without the use of expensive germane gas.     

Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.     

Performance of superstrate multijunction amorphous silicon-based solar cells using optical layers for current management

The performance of multijunction amorphous silicon-based thin film solar cells has been reported using thin layers of TiO₂ and SiO_x acting as refractive index matching optical layers for different interfaces of the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/μc-Si cells is demonstrated with TiO₂ as anti-reflection layer at TCO/Si interface and SiO_x as intermediate-reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO₂ and SiO_x optical layers in a-Si/μc-Si solar cell.     

Calculation of the performances of the a-Si:H/poly-Si multistacked solar cells

We investigated multistacked solar cells with a structure of metal/a-si: H (n-i-p)/poly-Si (n-p)/metal. This cell consists of two component cells; top n-i-p junction a-Si : H cell and a bottom n-p junction poly-Si cell. The solar cell conversion influencing factors were investigated in terms of film thickness, doping concentration, minority carrier lifetime, diffusion length, surface recombination, surface potential, AR coating, and circuit parameters of solar cells. The optimization of material and solar cell was carried out by using a PC-1D simulator. The main stream lines of the studies were the p-n junction poly-Si bottom cell, the p-i-n junction top a-Si : H cell, and the equivalent circuit examination. The optimized simulation results indicates that the 22% efficiency of multistacked solar cells can be achieved by optimizing parameters in each layer.     

Measuring and modelling of a-Si, Ge:H solar cells

We have measured and modelled a-Si,Ge:H pin cells with the purpose of refining the existing modelling parameters, so that the model can be used for designing cells. In this work, we have employed the numerical device model AMPS. We have found that we can satisfactorily model the performance of as-deposited alloy cells by suitably changing the band gap, mobility gap, optical absorption spectrum, and electron affinity.     

Plasma enhanced chemical vapor deposition of excellent a-Si:H passivation layers for a-Si:H/c-Si heterojunction solar cells at high pressure and high power

The intrinsic a-Si:H passivation layer inserted between the doped a-Si:H layer and the c-Si substrate is very crucial for improving the performance of the a-Si:H/c-Si heterojunction (SHJ) solar cell. The passivation performance of the a-Si:H layer is strongly dependent on its microstructure. Usually, the compact a-Si:H deposited near the transition from the amorphous phase to the nanocrystalline phase by plasma enhanced chemical vapor deposition (PECVD) can provide excellent passivation. However, at the low deposition pressure and low deposition power, such an a-Si:H layer can be only prepared in a narrow region. The deposition condition must be controlled very carefully. In this paper, intrinsic a-Si:H layers were prepared on n-type Cz c-Si substrates by 27.12 MHz PECVD at a high deposition pressure and high deposition power. The corresponding passivation performance on c-Si was investigated by minority carrier lifetime measurement. It was found that an excellent a-Si:H passivation layer could be obtained in a very wide deposition pressure and power region. Such wide process window would be very beneficial for improving the uniformity and the yield for the solar cell fabrication. The a-Si:H layer microstructure was further investigated by Raman and Fourier transform infrared (FTIR) spectroscopy characterization. The correlation between the microstructure and the passivation performance was revealed. According to the above findings, the a-Si:H passivation performance was optimized more elaborately. Finally, a large-area SHJ solar cell with an efficiency of 22.25% was fabricated on the commercial 156 mm pseudo-square n-type Cz c-Si substrate with the opencircuit voltage (V_oc) of up to 0.732 V.     

Triple-junction amorphous silicon-based flexible solar minimodule with integrated interconnects

We report in detail the fabrication of triple-junction amorphous silicon-based, monolithically interconnected, flexible photovoltaic submodules of size 10 cm×30 cm. The cells used in the submodules include a tie-coat layer for improved film adhesion, and were encapsulated using the terpolymer THV. A combination of laser and chemical processing was used for the series interconnection. An initial efficiency of 4.9% (5.34%) was obtained for an aperture area of 204 cm².     

A new perspective on the characterization of materials for a-Si:H solar cells

A study has been carried out on a-Si:H solar cell materials fabricated under a wide range of deposition conditions in different laboratories. The results on both thin films and corresponding Schottky barrier cell structures demonstrate that analysis and characterization based solely on the neutral dangling bonds are clearly inadequate. Contributions of charged defects to the properties of a-Si:H, their effect on light-induced changes are identified together with the limitations of methods commonly used to characterize the solar cell properties and stability of a-Si:H materials. Self-consistent fitting of a wide range of results on films and Schottky barrier cell structures is obtained with a gap state distribution in which charged defects are included.     

Charge transport in a-Si: H PIN solar cells under AC excitation

We have made I-V and impedance measurements on a-Si: H PIN solar cells as a function of the frequency and bias voltage to understand the mechanisms that limit the charge transport under AC excitation. The measurements covered the range of frequencies from 1 Hz to 1 MHz. It is concluded that the interaction between gap states and extended states rather than the band transport is the limiting mechanism on the charge transport.     

Modeling of a-Si/poly-Si and a-Si/poly-Si/poly-Si stacked solar cells

A computer model for the poly-Si thin film-related solar cells is established, with which the solar cells with the structure of single junction poly-Si cell, a-Si/poly-Si tandem cell and a-Si/poly-Si/poly-Si triple cell are simulated. The results indicate that the practical structure for poly-Si-related solar cell is a-Si/poly-Si/poly-Si triple cell with the best matched thickness of 0.23/0.95/3 μm and with optical confinement structure, which has the highest simulated efficiency of 22.74%.     

Incorporation and critical concentration of oxygen in a-Si:H solar cells

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH₄ and H₂. Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p_b. It is found that for a certain deposition regime defined by silane and H₂ flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p_b/r_d with r_d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10²⁰ cm⁻³) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p_b/r_d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.