Ujjwal Das

Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation

This letter reports interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells which combine the performance benefits of both back contact and heterojunction technologies while reducing their limitations. Low temperature (<200°C) deposited p- and n-type amorphous silicon used to form interdigitated heteroemitter and contacts in the rear preserves substrate lifetime while minimizes optical losses in the front. The IBC-SHJ structure is ideal for diagnosing surface passivation quality, which is analyzed and measured by internal quantum efficiency and minority carrier lifetime measurements. Initial cells have independently confirmed efficiency of 11.8% under AM1.5 illumination. Simulations indicate efficiencies greater than 20% after optimization.

Surface passivation and heterojunction cells on Si (100) and (111) wafers using dc and rf plasma deposited Si:H thin films

The structure of hydrogenated silicon (Si:H) films deposited by rf and dc plasma process on Si (100) and (111) wafers is correlated with the surface passivation quality and heterojunction cell performance. Microstructural defects associated with SiH2 bonding and apparent ion bombardment in dc plasmas have little or no adverse effect on passivation or cell properties, while presence of crystallinity in Si:H i layer severely deteriorates surface passivation and cell open circuit voltage (Voc). Excellent surface passivation (lifetime of >1ms) and high efficiency cells (>18%) with Voc of 694mV are demonstrated on n-type textured Czochralski wafer using dc plasma process.

Role of hydrogen bonding environment in a-Si:H films for c-Si surface passivation

The search for an ideal surface passivation layer of crystalline silicon (c-Si) to be employed in a silicon heterojunction photovoltaic device has garnered much attention. The leading candidate is a few nanometers of intrinsic amorphous silicon ((i)a-Si:H) film. Reported dependencies of film surface passivation quality on substrate preparation, orientation, and deposition temperature have been extended in this work to include H2 to SiH4 dilution ratio and postdeposition annealing. Simple avoidance of the deposition regimes that lead to epitaxial growth of Si on the c-Si substrate produces decent lifetimes on the order of 500μs. Subsequent low temperature annealings cause an important restructuring of Si–H bonding at the a-Si:H∕c-Si interface increasing the amount of monohydride at the c-Si surface. This restructuring would reduce the c-Si surface defect density and cause an improvement of surface passivation as confirmed by effective lifetime measurements. Final effective carrier lifetimes up to 2550μs ar...

Single heterojunction solar cells on exfoliated flexible ∼25 μm thick mono-crystalline silicon substrates

Mono-crystalline silicon single heterojunction solar cells on flexible, ultra-thin (∼25 μm) substrates have been developed based on a kerf-less exfoliation method. Optical and electrical measurements demonstrate maintained structural integrity of these flexible substrates. Among several single heterojunction ∼25 μm thick solar cells fabricated with un-optimized processes, the highest open circuit voltage of 603 mV, short circuit current of 34.4 mA/cm2, and conversion efficiency of 14.9% are achieved separately on three different cells. Preliminary reliability test results that include thermal shock and highly accelerated stress tests are also shown to demonstrate compatibility of this technology for use in photovoltaic modules.

Investigation of hetero-interface and junction properties in silicon heterojunction solar cells

The amorphous silicon (a-Si:H) - crystalline silicon (c-Si) heterojunction (SHJ) solar cell fill factor (FF) is very sensitive to the properties of c-Si surface, process parameters of thin a-Si:H layers, properties of transparent conducting front electrode, and all of their interfaces. In this work, quality of hetero-interface and junction properties in n-type SHJ solar cells were investigated by; (i) suns VOC under white, blue, and infrared light; (ii) dark and light JV; and (iii) quantum efficiency (QE) with and without voltage and light bias. Analysis of all these measurements suggest an anomalous “S” shape JV curve can arise due to at least two separate reasons; (a) existence of a large barrier for hole transport with excellent front surface passivation, and (b) existence of an opposing diode/Schottky barrier in the hetero emitter side of a SHJ solar cell. Both of the above-mentioned interface and/or junction properties severely affect minority carrier collection in SHJ cells.

Alternative approaches for low temperature front surface passivation of interdigitated back contact silicon heterojunction solar cell

In this work, we investigated two alternative approaches for the front surface passivation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells: (1) with plasma enhanced chemical vapor deposited (PEVCD) a-Si-based stack structure consisting of a-Si:H/a-SiNx:H/a-SiC:H, and (2) with physical vapor deposited (PVD) zinc sulfide (ZnS) film. The processing temperatures for both the approaches are under 300°C. Effective surface recombination velocities (SRV) of < 6.2cm/s and < 35cm/s are obtained with stack structure and ZnS respectively on n-type float zone (FZ) crystalline silicon (c-Si) wafers. The anti-reflection (AR) properties of these two passivation approaches are studied and the optimization procedure of the stack structure was discussed and shown to improve the photo-generated current. The IBC-SHJ solar cells were fabricated using both the front surface passivation approaches and a 15% cell efficiency was achieved on 150µm thick FZ c-Si wafer without surface texturing and optical optimization.

Fast hydrogen diffusion in hydrogenated amorphous silicon observed by in situ ESR

Abstract A high diffusion coefficient (> 10 −10 cm 2 s −1 ) of free H atoms in hydrogenated amorphous silicon (a-Si:H) has been observed using in situ electron spin resonance (ESR). The experimental results show that dual H-diffusion paths for free-H diffusion and H inter-diffusion exist in an a-Si:H network. The light-soaking effect in a-Si:H is discussed in relation to these dual H-diffusion paths.

Creation and annihilation mechanism of dangling bonds within the a-Si:H growth surface studied by in situ ESR technique

In situ electron-spin-resonance (ESR) measurements of a-Si:H film growth and hydrogen plasma treatments have been performed at different temperatures. The results showed that the surface dangling-bond density during film growth is almost independent of the growth temperature in the temperature range below 200°C. It suggests that during film growth the interaction between the film surface and impinging plasma species controls the number of surface-region dangling bonds rather than the thermal relaxation processes. It is also shown that during hydrogen plasma treatments hydrogen atoms create dangling bonds, rather than terminate them. These dangling bonds are not confined to the film surface but are spatially distributed within the deeper layers of the film (around 100 nm from the top-surface), whose depth decreases with the increase in the treatment temperature.

Optimization of interdigitated back contact silicon heterojunction solar cells by two-dimensional numerical simulation

In this paper, two-dimensional (2D) simulation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells is presented using Sentaurus Device, a software package of Synopsys TCAD. A model is established incorporating a distribution of trap states of amorphous-silicon material and thermionic emission across the amorphous-silicon / crystalline-silicon hetero-interface. The 2D nature of IBC-SHJ device is evaluated and current density-voltage (J-V) curves are generated. Optimization of IBC-SHJ solar cells is then discussed through simulation. It is shown that the open circuit voltage (VOC) and short circuit current density (JSC) of IBC-SHJ solar cells increase with decreasing front surface recombination velocity. The JSC improves further with the increase of relative coverage of p-type emitter contacts, which is explained by the simulated and measured position dependent laser beam induced current (LBIC) line scan. The S-shaped J-V curves with low fill factor (FF) observed in experiments are also simulated, and three methods to improve FF by modifying the intrinsic a-Si buffer layer are suggested: (i) decreased thickness, (ii) increased conductivity, and (iii) reduced band gap. With all these optimizations, an efficiency of 26% for IBC-SHJ solar cells is potentially achievable.

Amorphous and Microcrystalline Silicon Solar Cells Grown by Pulsed PECVD Technique

The Pulsed PECVD technique involves modulating the standard 13.56 MHz RF plasma, in the kHz range. This allows an increase in the electron density during the ‘ON’ cycle, while in the ‘OFF’ cycle neutralizing the ions responsible for dust formation in the plasma. In this work, we report the increase of i-layer growth rate and silane gas utilization rate (GUR) for amorphous Si p-i-n solar cells grown in a large area (30 cm × 40 cm) single chamber deposition system. The i-layer growth rate of 5.4 A/sec with a GUR of >15% has been achieved, which shows a device efficiency of 8.3% (almost same as of our conventional PECVD grown a-Si:H solar cell with ilayer growth rate of ∼1 A/sec). We also deposited microcrystalline Si p-i-n devices using the Pulsed PECVD technique. The crystallite orientation of the films changes from a random to a (220) orientation near the microcrystalline-to-amorphous transition. The effects of crystallite orientation, grain boundaries and ion bombardment during growth on the solar cell performances are investigated. An efficiency of 4.8% for single junction μc-Si:H p-i-n device has been achieved for the i-layer thickness of 0.9 μm.