Electrical Properties of n--p Amorphous-Crystalline Silicon Heterojunctions

Abstract

Electrotechni cal_ Laboratory L-1,-4 Umezono, Sakura-mura, N'ihari-gun, Ibaraki 305 * 0n leave from Sharp Corporation, 26L3-L lchinomoto-cho, Tenri-c]-ty, Nara 632 We have measured C-V characteristics and temperature dependence of J-V characteristics of undoped a-Si:H heterojunctions formed on p-type c-Si substrates with different resistivities. It has been found out that an abrupt heterojunction model is valid for a-Si:H/p c-Si heterojunctions, and the electron affinity of a-Si:H has been estimated as 3.93 t 0.07 eV from C-V characteristics. The forward cument of all the junctions studied.shows voltage and temperature dependence expressed as exp(-M ^/kT) x exp(Av), where ot". and A are constants independent of vg]{age and templratu?5. in. reverse current is-p*roportional to exp(-AE^-/kT) x (V^_V)'r', where M is constant independent of voltage and temperature and Vo-is the diffusion voltage.

Capacitance Analysis of the Structures with the a-Si:H(i)/c-Si(p) Heterojunction for Solar-Cell Applications

“…The measured value of the electron affinity is close to the value χ a-Si:H = 3.93 eV, which was published in the work of Matsuura et al [9].…”

“…This value corresponds with the observations made by Matsuura et al [9] on related a-Si:H(i)/c-Si(p) heterostructures. When we take into account the electron affinity of crystalline silicon χ c-Si = 4.05 eV, then following the relation coming out from from Anderson's model for heterostructure…”

Obtaining a higherVoc in HIT cells

“…In order to collect generated carriers efficiently, the (p/i)-layer should have low barrier for minority carriers and (i/n)-layer should have a large one in the valence band. However, applying the experimentally determined optical bandgap of the a-Si layers (1Á6 eV for p-type, 1Á7 eV for i-and n-type) and the electron affinity 17,18 from the literature to the Anderson model, 19 it is found that there is a rather large band discontinuity in the valence band at the (p/i)-a-Si/n-type c-Si heterojunction (Figure 5b). The numerical analysis of HIT cells pointed out that it is important to control the band offset in the valence band at the heterointerface in order to achieve high performance.…”

Current Losses at the Front of Silicon Heterojunction Solar Cells

“…Such large values are possible because the architecture of heterojunction cells capitalizes on two unique properties of nanometer-thin intrinsic hydrogenated amorphous silicon (a-Si:H) films: their ability to both passivate [6] and drain charge [7] from crystalline silicon surfaces. The former results in several-millisecond effective lifetimes when high-quality wafers are used [5]; the latter permits photogenerated carriers to be extracted without recombinationactive contacts that ruin the high lifetimes [8], [9].…”