Regimes of current transport mechanisms in CdS/CdTe solar cells

Abstract

Forward bias recombination (current transport) mechanisms have been evaluated for thin film solar cells and correlated to the in-gap trap levels present. Here CdTe/CdS devices were chosen as an archetypal example of a modern thin film solar cell, and a set of devices with a range of design variables was used in order to reveal the full range of behaviours that may operate to limit current transport. Experimental current-voltage-temperature datasets were compared to mathematical models of transport, and the in-gap traps were evaluated by thermal admittance spectroscopy. The current transport mechanisms operating are presented on a temperaturevoltage diagram. Three regimes were identified: at 'intermediate' voltages, the behaviour was temperature dependent. From 300 K down to 240 K, thermally activated Shockley Read Hall recombination mediated by a 0.38 eV trap (V Cd ) dominated the transport. Between 200 and 240 K the transport was thermally activated but below 200 K the mechanism became dominated by tunnel assisted interface recombination. At 'low' voltages (and for all devices at all voltages when measured at T<200 K) band to band recombination is via multi-step tunnelling through in-gap states. At high voltage, the forward current is dominated by the well-known limiting effect of the back Schottky contact to the CdTe which is in reverse bias. The current transport behaviour is also correlated with the n-CdS thickness and CdCl 2 processing conditions, both of which are critical to device performance.

Scientific complications and controversies noted in the field of CdS/CdTe thin film solar cells and the way forward for further development

“…However, since the CdS/CdTe hetero-junction showed good rectification properties, CdTe was assumed to be p-type in electrical conduction to form a p-n junction rectifying device with its n-CdS partner. Therefore, all the experimentally observed results were analysed and interpreted as a simple p-n hetero-junction [1][2][3][4][5][6][7][8]. In parallel, theoretical calculations and simulations were also carried out for this solar cell assuming principles of p-n junction diodes [9][10][11][12].…”

The study on saturation current and ideality factor of CdTe solar cell based on CdS window layer deposited with hydrogen peroxide

“…According to Bayhan et al [11], the thermally activated behavior of J 0 based on SRH recombination can be subdivided into bulk and interfacial recombination for different temperature region. As is well known, the multernary alloys are widely existed in pn-junction deposited with II-VI materials, especially for CdS 1−y Te y [23].…”

“…Rastogi [9] grown a more compact CdS film with smaller grain size and higher transmittance. However, the detailed analysis about the effect of compact structure and the band gap of CBD CdS on the recombination mechanisms [10,11] related to the saturation current which affects V oc intensively has not been reported.…”

Recombination-induced voltage-dependent photocurrent collection loss in CdTe thin film solar cell

“…Current density (mA/cm The tunneling-enhanced interface recombinations widely exist in Cu(In,Ga)Se 2 , CuGaSe 2 , CdTe, and Cu 2 ZnSnS 4 thin film solar cells. [44][45][46][47] A quantitative analysis was introduced to describe the tunneling at the junction in a Schottky barrier. [48,49] Rau et al extended this model to describe the diode current from pure SRH recombination to tunneling-enhanced interface recombination.…”

“…CdS/CdTe solar cell only when the temperature is approximately below 240 K. [45,46] For the MZO/CdTe solar cell, tunneling-enhanced interface recombination is involved over the entire temperature range (80 K≤ T ≤320 K) we tested. In our previous work, the electron concentration of MZO film after being annealed is about 10 18 cm −3 , [13] which is higher than that in CdS film.…”