Steve Hegedus

Effect of textured tin oxide and zinc oxide substrates on the current generation in amorphous silicon solar cells

The authors evaluate material properties of a number of textured SnO/sub 2/ and ZnO substrates and their effect on current generation in a-Si. Most of these TCOs have been used by others for a-Si solar cell research or module fabrication. Bulk optoelectronic and structural properties are reported for seven TCO films with haze from 1 to 14%. Their results show that increasing haze above /spl sim/5% has limited effectiveness for increasing the generation at long wavelengths. In presently available textured ZnO, current generation is about 0.6 mA/cm/sup 2/ greater than in textured SnO/sub 2/. There may be greater advantages to using ZnO in multijunction devices since much thinner i-layers may be used to give the same short-circuit current with improved stability, shorter deposition time and less GeH/sub 4/ usage.

The Effects of Device Geometry and TCO/Buffer Layers on Damp Heat Accelerated Lifetime Testing of Cu(In,Ga)Se

In Cu(In,Ga)Se2 solar cells encapsulated with polyethylene terephthalate (PET) or glass top sheets, the effects of damp heat (D-H) accelerated lifetime testing (ALT) depend on water vapor transmission rate (WVTR) of both transparent conducting oxide (TCO) and the intrinsic zinc oxide (i-ZnO) buffer, as well as device geometry. PET top sheets have a WVTR of ~10 g/m2·day, and glass has a WVTR of 0. Previously, coupons encapsulated with PET degraded to 50% of initial efficiency after 1000 h D-H ALT. We show that PET encapsulated coupons degrade at the same rate as glass encapsulated coupons after 2000 h D-H ALT to 92% of initial efficiency. The only change from previous work is that, here, i-ZnO covers the entire coupon surface, not the just active area. The WVTR of the i-ZnO/TCO stack is 2 × 10-3 g·H2O/m2·day. A set of unencapsulated devices went through D-H ALT, one where scribing was used to define the active area of the device and another without scribing; both were protected only by 50-nm i-ZnO. The bare-unscribed device performed as well as the previous glass and PET encapsulated coupons after 1500 h D-H ALT; the bare-scribed device degraded to 78% of initial efficiency, indicating that TCO integrity is a critical ALT parameter.

_{\bf 2}

The fundamental mechanisms governing carrier transport and in CdTe solar cells are not well established. Effects of diffusion length (L), depletion width (W), primary heterojunction vs. back junction are not well correlated with the CdTe thickness (t), or back contact. Bifacial analysis provides quantitative insight into CdTe device operation by separating the effects of front and back junction. Back spectral response (SRB) was analyzed to evaluate L and W. Front spectral response (SRF) is nearly unaffected by L or W. SR B and back Jsc are higher for thinner cells as SR B is limited by diffusion across the field free region that is smaller for thinner cells. Bifacial characterization results indicate a photosensitive back barrier. There is no evidence of a back junction and we conclude that a single junction determines recombination current. These results establish that performance for AM1.5 light through front contact is determined primarily by voltage dependent collection, not diffusion length

Solar Cells

Minority carrier lifetime measurements were used to optimize processes for amorphous/crystalline silicon heterojunction solar cells. A blue filter highlights surface lifetime and is used to determine the interaction between the front and rear depositions. On n-type substrates, depositing the front p-type layer first led to contamination of the rear surface such that a subsequent n-type deposition on the rear intended as a back surface field had no passivating quality and giving cells with low open-circuit voltages only 580 mV. Switching the order of deposition and depositing the rear n-layer first, improved the quality of the rear passivation and subsequently increased open circuit voltages to over 620 mV; without intrinsic buffer layers. Depositions of intrinsic material resulted in lifetimes of 2.4 ms, and wafer cleaning was found to have a significant impact on measured lifetime. Finally, immersion in hydrofluoric acid was found to be the easiest way to measure substrate lifetime above 1 ms

How CDTE Solar Cells Operate: Determining Collection using Bifacial Device Characterization

Based on potential high efficiency, low thermal budget and compatibility with very thin Si wafers, many research groups and industries are considering interdigitated all back contact silicon heterojunction (IBC-SHJ) solar cell technology. Compared to front junction silicon heterojunction (FJ-SHJ) solar cells, IBC-SHJ cells also have the unique advantages for simpler module integration. However, the IBC-SHJ solar cells to date suffer with low fill factors. Both simulation and experimental results have been conducted to understand the cause of the low FF. In this paper, the effects of processing conditions are discussed by comparing FJ-SHJ and IBC-SHJ solar cells. The fabrication of IBC-SHJ cells requires several photolithography steps to form the interdigitated back structure, whereas the FJ-SHJ requires no photolithography steps. The effect of processing temperatures, deposition sequence and photolithography processing are evaluated by using the FJ-SHJ as a test bed and base-line standard. Contamination and/or introduction of defects at intrinsic/doped layer interfaces resulting from photolithography steps is identified as one of the major contributors to reduced performance of IBC-SHJ devices fabricated using a low temperature process.

Carrier Lifetime as a Developmental and Diagnostic Tool in Silicon Heterojunction Solar Cells

A new method has been developed to characterize the TCO/p contact in a-Si p-i-n superstrate solar cells and modules. The method is applied to a series of devices fabricated at BP Solar on commercial SnO/sub 2/ and ZnO-coated SnO/sub 2/ having different p-layer recipes and pre-deposition treatments. Values of the contact resistance (R/sub c/) of 1/spl plusmn/0.5 /spl Omega/-cm/sup 2/ were found for a wide range of TCO and p-layer processing including ZnO. Temperature dependence of R/sub c/ gave barrier height of 40-55 meV. Analysis of devices with a thin ZnO layer on SnO/sub 2/ and with different pre-deposition treatments indicates lower V/sub oc/ and FF observed with ZnO is not due to the contact but to changes in the p/i junction recombination. The solar cell performance is very sensitive to the ZnO surface treatment but R/sub c/ is not. No evidence was found for a blocking or high resistance ZnO/p contact.

Impact of back surface patterning process on FF in IBC-SHJ

Measurements of sub-band gap primary photocurrent spectra on p-i-n devices are used to characterize defects between 1.0 and 1.4 eV above the valence band. The effect of light soaking is to increase the absorption due to light-induced defects centered between 1.2 and 1.3 eV, consistent with the results from photothermal deflection spectroscopy. A degradation in FF (fill factor) is correlated with an increase in light-induced defects measured on the same solar cells. Although increasing the i-layer deposition temperature reduces FF, it is suggested that this reduction is due to p/i interface degradation because bulk i-layer defects decrease with the deposition temperature. The measurement is applicable to any a-Si:H or a-SiGe:H p-i-n solar cell and provides information about fundamental i-layer properties (defect density and Urbach energy) which can be related to the device performance.<<ETX>>

Characterization of the SnO/sub 2//p and ZnO/p contact resistance and junction properties in a-Si p-i-n solar cells and modules

The effects of substrate temperature and hydrogen dilution on mobilitylifetime product and hydrogen bonding are reported for amorphous silicongermanium films with bandgaps from 1.1 to 1.74 eV deposited by Hg-sensitized photo-CVD. The ratio of dihydride to monohydride bonding and preferential H attachment to silicon both decreased with hydrogen dilution. The best films were deposited with hydrogen dilution at substrate temperatures between 230 and 250 ° C. Model equations used to calculate radical fluxes to the substrate are described.

Characterization of defects in a-Si:H solar cells using sub-band gap photocurrent spectroscopy

We investigate the effects of p a-Si:H emitter layer defects on n-type silicon heterojunction solar cells (SHJ) by experimental and simulated device analysis. Simulation of interdigitated back contact heterojunction (IBC-SHJ) solar cells shows that reducing the p-layer dangling bond density (Ddb) > 5×1018 cm-3eV-1 is an effective way to simultaneously increase VOC and FF. We use front and rear heterojunction test structures to diagnose the trade-offs between VOC and FF. Analysis of the dark current-voltage (JV) characteristics and suns-VOC curves of IBC-SHJ cells with different emitter p-layers shows the benefits of a low-high doping stack emitter structure where the lowly doped layer has less defects, providing low surface recombination, while the high doped layer enhances carrier transport and contacting. By utilizing the stack emitter structure we achieved IBC-SHJ solar cell with 20.2% efficiency.

Amorphous Silicon-Germanium Deposited by Photo-CVD: Effect of Hydrogen Dilution and Substrate Temperature

Maximizing the lateral transport and collection of photogenerated carriers near the rear surface of the interdigitated-back-contact silicon heterojunction solar cell (IBC-SHJ) is critical for high efficiency. Specifically, this requires minimizing the recombination over the gap region between the well-passivated emitter and base contacts. In this paper, we investigate the potential of a p-type amorphous silicon and intrinsic hydrogenated amorphous silicon layer stack, which forms an inversion layer for n-type crystalline Si (n.c-Si) surfaces, to provide gap passivation for a scalable low-temperature IBC-SHJ fabrication process. We fabricate novel three terminal rear heterojunction silicon devices to understand the effect of this gap passivation structure in terms of carrier distribution and transport mechanisms when localized high recombination regions are introduced due to the simplified fabrication process. Voltage-modulated laser-beam-induced-current measurements are utilized to characterize the device performance before and after introducing intentional localized laser damage. Interpretation of results is aided by one-dimensional simulation of band alignments and charge distribution at different voltage-modulations under illumination. We confirmed that the gap passivation structure that induces an inversion layer at the surface should be avoided to eliminate any parasitic shunting or inversion-layer-assisted-recombination on n.c-Si IBC-SHJ, and this is applicable to any n-type IBC cells in general.