We present temperature-dependent measurements of I-V curves in the dark and under illumination in order to elucidate the dominant transport mechanisms in amorphous silicon-crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. ZnO:Al/(p)a-Si:H/(n)c-Si/(n+)a-Si:H cells are compared with inversely doped structures and the impact of thin undoped a-Si:H buffer layers on charge carrier transport is explored. The solar cell I-V curves are analyzed employing a generalized two-diode model which allows fitting of the experimental data for a broad range of samples. The results obtained from the fitting are discussed using prevalent transport models under consideration of auxiliary data from constant-final-state-yield photoelectron spectroscopy, surface photovoltage, and minority carrier lifetime measurements. Thus, an in-depth understanding of the device characteristics is developed in terms of the electronic properties of the interfaces and thin films forming the heterojunction. It is shown that dark I-V curve fit parameters can unequivocally be linked to the open circuit voltage under illumination which opens a way to a simplified device assessment.

Electrical transport mechanisms in a-Si:H/c-Si heterojunction solar cells

Measured current–voltage characteristics of undoped and p‐type hydrogenated amorphous silicon (a‐Si:H)/n‐type crystalline silicon (c‐Si) heterojunctions are used to discuss the carrier transport mechanisms. The forward current was characterized by two parts: The forward current increased with applied voltage exponentially (region 1), and nonexponentially (region 2). In region 1, it was found that the current was dominated by the tunneling process in which electrons tunneled from the c‐Si into gap states in the a‐Si:H and recombinated holes captured by the gap states in the a‐Si:H. In region 2, the current was found to be a space‐charge‐limited current due to both electrons injected from the c‐Si and holes injected from an ohmic contact. The carrier transport mechanism of reverse currents depended on the magnitude of boron doping in the a‐Si:H. The reverse current was considered to be mainly generated in the depletion layer of the a‐Si:H for the heterojunction with undoped a‐Si:H, generated in the depletion layer of both the a‐Si:H and the c‐Si for that with boron‐doped a‐Si:H of B2H6/SiH4=1×10−5, and generated in the depletion layer of the c‐Si for that with boron‐doped a‐Si:H of B2H6/SiH4=1×10−4.

Carrier transport mechanisms of p-type amorphous-n-type crystalline silicon heterojunctions

The high-bias behaviour of the dark forward and reverse current–voltage (I–V) characteristics of an undoped radio frequency (rf) magnetron sputter deposited boron carbide (p-B5C)/p-type crystalline silicon heterojunction has been investigated at different ambient temperatures (130–300 K). The experimental forward current–voltage–temperature (I–V–T) characteristics indicate that the non-ohmic bulk conduction mechanisms operable in the highly resistive polycrystalline B5C counterpart material of this heterojunction largely determine the behaviour of its forward current at high-bias voltages (>0.3 V). The hopping conduction model of Apsley and Hughes for a flat density of localized energy states (Apsley N and Hughes H P 1975 Phil. Mag. 31 1327) can be utilized to elucidate the bias dependence of the measured heterojunction forward current over an extended bias-voltage range (0.4–2.7 V) at temperatures below 260 K. On the other hand, the junction-like conduction processes occurring in the depletion region should limit the high-bias behaviour of the measured heterojunction reverse current. Bardeen's model for a modified Schottky-like barrier at the p-B5C/p+-Si interface can be satisfactorily applied to describe the reverse current–voltage characteristics at bias voltages larger than 0.4 V.

http://iopscience.iop.org/article/10.1088/0268-1242/18/1/302/pdf

High-bias current–voltage–temperature characteristics of undoped rf magnetron sputter deposited boron carbide (B5C)/p-type crystalline silicon heterojunctions

Rectifying p‐n junctions have been formed between n‐type Ge20Bi11Se69 bulk glass and p‐type chalcogenide films such as Ge20Se80 and As2Se3. The forward current in the power law increased with increasing bias voltage, suggesting that it was space charge limited. The spectral response of the short circuit currents showed a maximum and a shoulder at photon energies which corresponded to the optical band gaps of the p‐type films and the n‐type glass, respectively. This finding is indicative of the bending of the energy bands in both the p‐type films and the n‐type glass in the vicinity of the p‐type film/n‐type glass interface.

Formation of chalcogenide glass p‐n junctions

The series resistance of SnO 2 -Si heterojunction solar cells increases with time on-shelf after fabrication, but tends to stabilize after a few weeks. This degradation is enhanced at elevated temperatures. The open-circuit voltage may increase or decrease with time. It is concluded that the degradation results from the spontaneous growth of a high-resistance interfacial layer (probably SiO 2 ) at the SnO 2 -Si interface. Degradation proceeds in two steps. The first step is relatively rapid and accounts for the degradation on-shelf. The second step is slower, is observed only at elevated temperatures, and is believed to be limited by the rate of transport of oxygen across the existing SiO 2 layer. Under prolonged illumination, the series resistance and open-circuit Voltage both decrease, Although the second stage of degradation is too slow to affect the practical life of these cells, it is not clear if the initial rapid stage and/or degradation under illumination will preclude the practical use of these cells.

http://ieeexplore.ieee.org/iel5/16/31762/01478952.pdf

Accelerated life tests of SnO 2 &#8212;Si heterojunction solar cells

Sevroal properties of glass‐silicon heterojunctions have been studied as a function of glass composition. Oxide and chalcogenide glasses of both insulating and semiconducting behavior were investigated. The current‐voltage characteristic indicates space‐charge‐limited currents in the glass film regardless of composition. An exponential trap distribution is suggested and trap density calculations agree with literature results. Capacitance‐voltage measurements show that chalcogenide glass/silicon structures behave like ideal abrupt heterojunctions.

J. D. Mackenzie

Papers

Transport properties of glass‐silicon heterojunctions