Comparison of amorphous silicon absorber materials: Light-induced degradation and solar cell efficiency

Abstract

Several amorphous silicon (a-Si:H) deposition conditions have been reported to produce films that degrade least under light soaking when incorporated into a-Si:H solar cells. However, a systematic comparison of these a-Si:H materials has never been presented. In the present study, different plasma-enhanced chemical vapor deposition conditions, yielding standard low-pressure VHF a-Si:H, protocrystalline, polymorphous, and high-pressure RF a-Si:H materials, are compared with respect to their optical properties and their behavior when incorporated into single-junction solar cells. A wide deposition parameter space has been explored in the same deposition system varying hydrogen dilution, deposition pressure, temperature, frequency, and power. From the physics of layer growth, to layer properties, to solar cell performance and light-induced degradation, a consistent picture of a-Si:H materials that are currently used for a-Si:H solar cells emerges. The applications of these materials in single-junction, tandem, and triple-junction solar cells are discussed, as well as their deposition compatibility with rough substrates, taking into account aspects of voltage, current, and charge collection. In sum, this contributes to answering the question, “Which material is best for which type of solar cell?”

Light-induced Voc increase and decrease in high-efficiency amorphous silicon solar cells

“…30,58 The part of the substrate dependence of V oc that was not linked to a different effective p-(a-SiC:H)-layer thickness could be correlated to the roughness dependence of porous zones. Such porous zones with voids contain more defects-dangling bonds-than dense aSi:H material.…”

“…More details about individual layers, the reactor, and gas precursors can be found elsewhere. 30 We measured current-voltage (I(V)) characteristics under a four-lamp (three halogen, one xenon) solar simulator from Wacom (class AAA) under standard conditions (AM1.5g, 1000 W/m 2 , 25 C). 41,42 The current was determined from external quantum efficiency (EQE) measurements, taken with a system built in-house.…”

“…Details about layer characterization are given elsewhere. 30,31 For the front electrodes, boron-doped ZnO (ZnO:B) was deposited by low-pressure chemical vapor deposition (LPCVD). For each p-(a-SiC:H) thickness, the solar cells were co-deposited on four different substrates with varying roughness: Three substrates consisted of co-deposited, 2.3-lm-thick ZnO:B on glass.…”

Surface passivation of c-Si for silicon heterojunction solar cells using high-pressure hydrogen diluted plasmas

“…18 At the same time it has been shown that such attempts to increase the hydrogen content and improve the passivation quality can significantly alter the nanostructure of a-Si:H leading to void-rich material and in some cases cause irreversible damage at the interface. In this work we demonstrate PECVD conditions using high hydrogen dilution (200 sccm H 2 compared to 2.5 to 10 sccm SiH 4 ), low power density (∼0.05 W/cm 2 ), and relatively high deposition pressure (8 mbar) 20 that result in material close to the a-Si:H-to-µc-Si:H transition regime, passivating c-Si and achieving effective lifetimes (τ eff ) > 10 ms. As a result of the low power density and high deposition pressure the ion bombardment energy is decreased, 21 thereby minimizing possible damage on and just below the surface of the c-Si substrate while shifting the a-Si:H-to-µc-Si:H transition to higher dilutions and allowing to deposit amorphous silicon at extremely high dilution conditions. We show how the τ eff correlates to changes in the nanostructure of the material, which in turn is varied by changing the hydrogen dilution and substrate temperature.…”

Light-induced performance increase of silicon heterojunction solar cells

“…The latter is usually attributed to the generation of deep defects that act as recombination centers, 26 and is commonly referred to as the Staebler-Wronski effect (SWE). [27][28][29][30][31] Crystalline Si surfaces coated with intrinsic a-Si:H films were found to suffer as well from a degradation in surface passivation during prolonged light soaking (LS). 32 Kinetically, this degradation follows so-called power laws of a similar form as for bulk a-Si:H, suggesting that the intrinsic a-Si:H/c-Si interface equally suffers from SWE.…”