Origin of passivation in hole-selective transition metal oxides for crystalline silicon heterojunction solar cells

  • PDF / 553,996 Bytes
  • 9 Pages / 584.957 x 782.986 pts Page_size
  • 14 Downloads / 251 Views

DOWNLOAD

REPORT


ransition metal oxides (TMOs) have recently demonstrated to be a good alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type c-Si (n-Si) and three thermally evaporated TMOs (MoO3, WO3, and V2O5) was investigated by transmission electron microscopy, secondary ion-mass, and x-ray photoelectron spectroscopy. For the oxides studied, surface passivation of n-Si was attributed to an ultra-thin (1.9–2.8 nm) SiOx;1.5 interlayer formed by chemical reaction, leaving oxygen-deficient species (MoO, WO2, and VO2) as by-products. Carrier selectivity was also inferred from the inversion layer induced on the n-Si surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function difference Df $ 1 eV). Therefore, the hole-selective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower the cost and simplify solar cell processing.

I. INTRODUCTION

As today’s photovoltaic market continues to be dominated by crystalline silicon (c-Si) technology, two cost-reduction efforts are being contemplated: thinner silicon wafers (,50 lm) and the use of novel materials deposited at low temperatures with high throughput processes. Maximization of the solar cell open-circuit voltage (VOC) to record values of 750 mV1 has been possible by combining two principles of c-Si solar cell design2: (i) passivation of surface dangling bonds using hydrogenated amorphous silicon (a-Si:H) or other dielectrics (SiO2, Al2O3, Si3N4, SiC); and (ii) carrier selectivity via electron- and hole-selective contacts, traditionally achieved by phosphorous/boron doping gradients. Eventually, the ideal and simplest strategy would require a passivating/selective contact, dual-functionality materials that effectively passivate the c-Si surface while selectively conduct a specific charge carrier.3,4 In parallel, thin-film photovoltaics including organic and perovskite solar cells, have introduced a wide variety of electron- and hole-selective materials whose optoelectronic properties are comparable or superior to the standard p- and n-doped layers used in c-Si. Such materials have been successfully applied to c-Si heterojunction devices including organics,5 alkali salts,6,7 and Contributing Editor: Don W. Shaw a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.453

transition metal oxides (TMOs),8–18 using either low temperature (T , 200 °C) or solution-based processes. In particular, TMOs offer themselves as excellent candidates to substitute traditional c-Si doped layers given their wide range of work function values (3–7 eV), large energy band gaps Egap . 3 eV (high transparency), and marked p- or n-type semiconductivity.19 Additionally, their semi-insulating properties allow for passivation of the c-Si surface without compromising carrier conductivity, although their passivation potential in TMO/c-Si heteroj

Data Loading...