Si/SiGe hetero-junction solar cell with optimization design and theoretical analysis

Performances of solar cells, such as short circuit current density, open-circuit voltage, fill factor, and efficiency of solar cells on the multi-crystalline (mc)-SiGe on the Si with different Ge contents, are compared and investigated in this paper. The average Ge concentration was varied from 0% to ~ 20%. Appropriate addition of Ge in crystal Si is a very effective method to enhance the short circuit current density without degrading the open-circuit voltage owing to the modulation of the SiGe band-gap. The band-gap of the SiGe can be extracted by electron-hole plasma (EHP) model. With an optimization of Ge content and clean process condition, the overall efficiency of a Si/SiGe hetero-junction solar cell with Ge content of 8% is found to be ~ 16% and ~ 4% improvement achieved, as compared to the control multi-crystalline (mc)-Si solar cell. The theoretical simulations and analyses can help design the high efficiency Si/SiGe hetero-junction solar cell.     

Luminescence of solar cells with a-Si:H/c-Si heterojunctions

We have studied the electroluminescence (EL) and photoluminescence (PL) of solar cells containing a-Si:H/c-Si heterojunctions. It is established that both the EL and PL properties of these cells are determined by the radiative recombination of nonequilibrium carriers in crystalline silicon (c-Si). The external EL energy yield (efficiency) of solar cells with a-Si:H/c-Si heterojunctions at room temperature amounts to 2.1% and exceeds the value reached in silicon diode structures. This large EL efficiency can be explained by good passivation of the surface of crystalline silicon and the corresponding increase in lifetime of minority carrier s in these solar cells.

     

Surface photovoltage investigation of recombination at the a-Si/c-Si heterojunction

We investigate the use of time-resolved surface photovoltage (SPV) transients as a means to determine band bending and recombination properties at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions. Experimentally, it is shown that for a-Si:H film thicknesses above ~ 6 nm, SPV transients do not depend on the film thickness anymore. On this basis, a simple numerical model is proposed that consists of a single rechargeable gap state on the c-Si wafer surface, into which the properties of the a-Si:H/c-Si interface and the adjacent a-Si:H are lumped. It is shown that this model can reproduce all principal features of high excitation SPV transients, i.e. an initial fast decay shown to be due to Auger recombination, a plateau region for high injection conditions and a fast decay when the sample returns into low injection and the defect states are recharged. Under sufficiently high excitation, the SPV saturates at a value that is determined by the a-Si:H/c-Si interface band bending in the dark. From the slope of the transient decay, defect parameters (density, energetic position) can be extracted.     

Determination of band offsets in a-Si:H/c-Si heterojunctions from capacitance-voltage measurements: Capabilities and limits

The capabilities and limitations of the well-known C-V technique for the determination of the conduction band offsets in (n)a-Si:H/(p)c-Si heterojunctions are presented. In particular, the effects due to the presence of an inversion layer in c-Si and a non-negligible defect density at the a-Si:H/c-Si interface on the reliability of the C-V intercept method are discussed. The influence of the Fermi level positions in (p)c-Si and (n)a-Si:H on the inversion layer formation and the influence of the interface defect density have been analysed using numerical simulations and experimental measurements.     

Low-temperature preparation of flexible a-Si:H solar cells with hydrogenated nanocrystalline silicon p layer

In this paper we report on flexible a-Si:H solar cells prepared on polyethylene naphthalate (PEN) substrates using p-type hydrogenated nanocrystalline silicon thin films (p-nc-Si:H) as the window layer. The p-nc-Si:H films were prepared at low temperature (150 °C) using trimethylboron (TMB) as a dopant gas. The influence of the silane concentration (SC) on the electrical and structural properties of ultra-thin p-nc-Si:H as well as the performance of solar cells on PEN was investigated. The results show that the crystalline fraction and conductivity of p-nc-Si:H thin films diminished, while the deposition rate and RMS roughness of films increased, when the SC increases from 0.53% to 0.8%. For the a-Si:H solar cells on PEN with the non-textured electrodes, the best efficiency of 6.3% was achieved with the p-nc-Si:H thin films deposited at SC = 0.67%.     

Low-temperature sintering properties of the screen-printed silver paste for a-Si:H/c-Si heterojunction solar cells

For a-Si:H/c-Si heterojunction (SHJ) solar cells, low-temperature sintered silver paste is necessary to fabricate the metal electrodes on transparent conductive oxide layer. Here, the thermal characteristic, the conductivity, the adhesion strength on indium tin oxide substrate and the microstructure evolution of the screen-printed low-temperature sintered silver grid were investigated by varying the sintering temperature and the sintering time. The results show that the grid performance is closely related to its microstructure. A relatively high sintering temperature and a long sintering time are beneficial to make the organics in the Ag paste burn out and the adjacent Ag particles coalesce together to be larger ones. As a result, the Ag grid can get a dense microstructure and tightly adhere onto the substrate surface. Thus, low line and contact resistance is achieved. What is more, the evolution of the preferential orientation of Ag particles has some contributions to the improved grid conductivity. Specifically, for the SHJ solar cell fabrication, in order to be compatible with the low-temperature deposition of a-Si:H, a long sintering time larger than 60 min with the sintering temperature in the range of 200–230 °C is preferred to realize high performance Ag electrical contacts.     

Towards an electronic model for CuIn1 − xGaxSe2 solar cells

We model some aspects of highly efficient CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 as well as wide band gap cells with x = 1 and ask for the dominant recombination mechanism which limits the V_oc of these devices. For CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3, interface recombination combined with Fermi-level pinning is a possible but unlikely recombination mechanism. We argue that these cells are rather limited by recombination in the quasi-neutral region (QNR) including the back contact. Using the expression for the QNR recombination rate we calculate the derivative of the collection function in the absorber at the space charge region edge which is in reasonable agreement with the experiment. It turns out that the diffusion length must approximate the absorber thickness. Based on this information, we draw a band diagram for a CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 and plot the simulated collection function. For cells with x = 1 (Cu-poor CuGaSe₂), the experimental activation energy of the recombination rate mostly equals the absorber band gap, i.e. E_a ≈ E_g,a = 1.67 eV. As the experimental interface band gap is smaller than E_a, interface recombination must be ruled out. Thus, the carrier lifetime in the Cu-poor CuGaSe₂ absorber should be so small that bulk recombination is more efficient than interface recombination. From this consideration, we postulate an electron lifetime value of 10⁻¹² s for CuGaSe₂.     

ZnO:Al films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells

Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are usually achieved by RF sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, on the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented.     

Double layer SiNx:H films for passivation and anti-reflection coating of c-Si solar cells

In this report, we present a cost effective simple innovative approach to fabricate double layer anti-reflection (DLAR) coatings using a single material which can provide high qualities of passivation and anti-reflection property. Two layers of SiN_x:H films with different refractive indices were deposited onto p-type c-Si wafer using plasma enhanced chemical vapor deposition reactor by controlling the NH₃ and SiH₄ gas ratio. Refractive indices of top and bottom layers were chosen as 1.9 and 2.3 respectively. The effect of passivation at the interface was investigated by effective carrier lifetime, hydrogen concentration and interface trapped density (D_it) measurements. The optical characteristic was analyzed by reflectance and transmittance measurements. A superior efficiency of 17.61% was obtained for solar cells fabricated with DLAR coating when compared to an efficiency of 17.24% for cells with SLAR coating. Further, J_sc and V_oc of solar cell with DLAR coating is increased by a value of ~ 1 mA/cm² and 4 mV respectively than cell with SLAR coating.     

Rheological properties and related screen-printing performance of low-temperature silver pastes for a-Si:H/c-Si heterojunction solar cells

Four different low-temperature silver pastes were utilized to make metal grids by screen printing for silicon heterojunction solar cells. The rheological behaviors of the low-temperature silver pastes were characterized by viscosity test, thixotropy test, oscillatory stress sweep test and creep-recovery test. The correlationship between the screen-printing performance and the rheological properties was investigated. It was found that the shear thinning behavior and the thixotropy behavior of the silver pastes were desirable for the screen-printing process. An obvious viscoelastic behavior of the silver paste was also helpful for improving its printability. Further, good recovery and low creep and recovery compliances could minimize the printing defects and the tendencies to bleed out during the screen-printing, and thus increase the aspect ratio of the printed grids.     

Progress in a-Si:H/c-Si heterojunction emitters obtained by Hot-Wire CVD at 200 °C

In this work, we investigate heterojunction emitters deposited by Hot-Wire CVD on p-type crystalline silicon. The emitter structure consists of an n-doped film (20 nm) combined with a thin intrinsic hydrogenated amorphous silicon buffer layer (5 nm). The microstructure of these films has been studied by spectroscopic ellipsometry in the UV-visible range. These measurements reveal that the microstructure of the n-doped film is strongly influenced by the amorphous silicon buffer. The Quasy-Steady-State Photoconductance (QSS-PC) technique allows us to estimate implicit open-circuit voltages near 700 mV for heterojunction emitters on p-type (0.8 Ω·cm) FZ silicon wafers. Finally, 1 cm² heterojunction solar cells with 15.4% conversion efficiencies (total area) have been fabricated on flat p-type (14 Ω·cm) CZ silicon wafers with aluminum back-surface-field contact.     

Studies on p-type copper (I) selenide crystalline thin films for hetero-junction solar cells

Deposition of polycrystalline copper (I) selenide thin films onto glass substrates at relatively low temperature (95 °C) have been carried out by chemical route using optimized preparative conditions. The XRD pattern confirmed the formation of copper (I) selenide semiconducting films with orthorhombic structure. A direct-type transition with band gap energy of 1.73 eV was reported from optical absorption studies. p-Type behavior confirmed from sign of thermally induced voltage (thermo-emf), which may find interesting applications in hetero-junction solar cells as an absorber layer.     

Improvement on the interface properties of p-GaAs/n-InP heterojunction for wafer bonded four-junction solar cells

The p-GaAs/n-InP heterojunction was fabricated by direct wafer bonding technology. The optimized atomic level contact between GaAs and InP is critical for getting good ohmic contact and removing the bubbles or voids at the interface, which is helpful to enhance the efficiency of wafer bonded multi-junction solar cells. Through the surface megasonic cleaning and the plasma treatment, we have achieved the high quality bonding interface without bubbles or voids and with interface resistivity of about 0.1 ohms/cm². A GaInP/GaAs//InGaAsP/InGaAs 4-junction solar cell was prepared with the high efficiency of 34.4% (AM0) at 1 sun.     

a-Si∶H/a-Ge∶H超晶格的界面特性研究

本文用红外(IR)和拉曼(Raman)谱研究了反应溅射 a-Si∶H/a-Ge∶H 超晶格的界面特性.定量分析发现,界面处没有因应力释放引起的 H 富集,不存在由超晶格结构引起的界面结构无序,界面处存在着～(5±2)(?)的 Si、Ge 组分混合层.我们对结果进行了初步分析.     

Small-signal response of nanocrystalline-titanium dioxide/poly(3-hexylthiophene) heterojunction solar cells

Admittance spectra of nanocrystalline titanium dioxide/poly(3-hexyl thiophene), heterojunction solar cells have been measured in air and under vacuum. In air, a 3-orders of magnitude increase in capacitance is observed when the measurement frequency is decreased from 1 MHz to 0.1 Hz. The frequency-dependence of the loss tangent with zero bias voltage indicates the presence of two processes, the weaker of which is attributed to adsorbed water molecules on the surface of the nanocrystalline titanium dioxide. The second process which becomes dominant in forward bias is believed to arise from the presence of an interfacial capacitance. In air, the capacitance is seen to depend strongly on applied forward bias consistent with the presence of an interfacial depletion region. However, a number of inconsistencies indicate that a mechanism related to carrier interaction with oxygen-related interface states may be the cause. At high forward bias and low frequencies, a negative incremental capacitance is observed similar to that seen in organic light emitting diodes following the onset of double-injection.     

Band offset of SnS solar cell structure measured by X-ray photoelectron spectroscopy

The energy band offset at the heterointerface is one of the most important properties of semiconductor heterostructures, particularly in solar photovoltaic devices. Band discontinuities of CdS/SnS and SnS/SnO₂ heterointerfaces were measured by X-ray photoelectron spectroscopy and capacitance-voltage measurements. The valence band offsets were determined to be approximately 1.5 eV for CdS/SnS and 3.5 eV for SnS/SnO₂ interfaces whereas the conduction band discontinuities for these junctions were respectively found to be 0.4 eV and 1.0 eV. Using these values and the energy band gaps of the corresponding layers, the energy band diagram was developed and it was considered to be a TYPE-II heterostructure. The Fermi level was found to be much closer to the valence band maximum for SnS, whereas it appeared in the upper half of the band gap for both CdS and SnO₂.     

An improved algorithm for solving equations for intra-band tunneling current in heterojunction solar cells

Numerical simulation of heterojunction solar cells is a useful approach to investigate the inner physics of the devices, and can be implemented by simulation tools today. An intra-band tunneling model has been described previously and applied to some heterojunction devices. However, the commonly used algorithm for solving the associated equations, the Gummel method, fails in the case of high recombination rates such as in amorphous Si solar cells. In this work, we present an improved algorithm that enhances convermicmgence when solving for intra-band tunneling in heterojunction solar cells with high defect state densities. The algorithm uses a sequential application of the Gummel and Newton methods tailored for optimal stability and convergence. As an example, simulation results for a heterojunction with intrinsic thin layer solar cell are discussed. The improved algorithm and related simulations in this work are implemented through a program we have developed, wxAMPS.     

P型(掺硼)晶硅太阳电池的光衰减机制和技术改进措施

介绍了近年来对掺硼晶硅(Cz-Si和mc-Si)太阳电池的光照衰减问题及衰减机制的研究结果。通过光照及退火处理前后少子寿命变化的研究以及光衰减与硼和氧浓度关系的研究．表明引起掺硼晶硅太阳电池光照衰减的主要因素是硼和间隙氧的存在。同时介绍了减小或避免衰减的技术措施。     

Cu-related recombination in CdS/CdTe solar cells

Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance-profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor.     

High efficiency microcrystalline silicon solar cells with Hot-Wire CVD buffer layer

Microcrystalline silicon (μc-Si:H) solar cells with i-layers deposited by hot wire chemical vapor deposition (HWCVD) exhibit higher open circuit voltage and fill factor than the cells with i-layers deposited by plasma enhanced (PE)-CVD. Inserting an intrinsic μc-Si:H p/i buffer layer prepared by HWCVD into PECVD cells nearly eliminates these differences. The influence of buffer layer properties on the performance of μc-Si:H solar cells was investigated. Using such buffer layers allows to apply high deposition rate processes for the μc-Si:H i-layer material yielding a high efficiency of 10.3% for a single junction μc-Si:H solar cell.