Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells

Abstract

After a focused effort over the last decade, order‐of‐magnitude improvements in doping and electro‐optical characteristics (radiative efficiency, carrier lifetime, and passivation) have been reported for polycrystalline CdSeTe solar cells. Surprisingly, this did not result in higher solar cell voltages regardless of device contacting layers, absorber grading profiles, and other changes in device architecture. From detailed evaluation of radiative emission and carrier dynamics in CdSeTe heterostructures and devices, it is shown that the complexity introduced to the absorber to achieve lifetime and passivation metrics resulted in charge carrier trapping, which now negatively affects CdSeTe absorbers. It is found that the defects with activation energy Ea ≈ 0.14‐0.22 eV dominate radiative emission and carrier dynamics in undoped CdSeTe, and electronic potential fluctuations with the amplitude γ ≈ 45–60 meV are present in As‐doped CdSeTe/CdTe. Because of potential fluctuations, radiative voltage is reduced by ≈ −100 mV, to = 1020‐1050 mV (for 1.4 eV bandgap). For record‐efficiency solar cells with VOC ≥900 mV, radiative and nonradiative recombination voltage losses are comparable, and future research needs to focus on reducing dopant compensation which causes potential fluctuations. This represents a paradigm shift for CdTe solar cells, with non‐radiative bulk recombination no longer representing a dominant voltage loss pathway.

Comprehensive Study of Carrier Recombination in High‐Efficiency CdTe Solar Cells Using Transient Photovoltage

“…[ 1,2 ] However, realizing high open‐circuit voltage ( V OC ) remains a considerable challenge. [ 3–7 ] Efficient power conversion in solar cells depends on the generation, transport, and collection of charge carriers, processes which are sensitive to the absorber layer's characteristics and to the device interfaces. [ 8–18 ] Charge carrier dynamics within this framework are described by the drift‐diffusion equation, [ 19 ] which for electrons in one dimension is:…”

“…In this scenario, τ rad , τ srh , and τ eff represent the radiative, nonradiative, and effective carrier lifetimes, and k denotes the radiative recombination coefficient. Equation (3) shows that the shortest lifetime at any given carrier concentration dominates the recombination. Figure 1 illustrates how the effective carrier lifetime τ eff varies with carrier concentration n. At low n, τ rad is long as it is inversely proportional to n; thus, the nonradiative recombination τ srh dominates in this region, which is governed by defect density, capture rate, and surface recombination velocity.…”

“…In CdTe devices, these lifetimes are typically probed using time-resolved photoluminescence (TRPL) measurements. [3,4,24] However, the measurements often being conducted on partial or fully completed devices in the CdTe community, interpretation of the TRPL data is difficult because in addition to the recombination, the electric field and carrier concentrations in the device also change with time. [25][26][27] Transient photovoltage (TPV) and current (TPC) measurements offer an alternative method in which the device is held under steady-state bias-light illumination and probed using a small perturbative laser pulse that does not result in changes to the electric field or carrier concentration.…”

Hierarchical Transparent Back Contacts for Bifacial CdTe PV

“…The beneficial impact of the back contact barrier reduction may outweigh any detrimentally reduced τ 2 lifetime. A recent study demonstrated that in undoped CdSeTe, high τ 2 lifetimes might correspond to trapping/detrapping mechanisms that not only increase τ 2 but also impart a lower hole mobility on the absorber, increasing the sensitivity of V OC to the magnitude of any back contact barriers . Despite the improved V OC with continuous gold contacts, AlGaO x cannot be used directly with CFL-templated nanogrids due to the high series resistance stemming from the low in-plane conductivity of the AlGaO x layer.…”

“…A recent study demonstrated that in undoped CdSeTe, high τ 2 lifetimes might correspond to trapping/detrapping mechanisms that not only increase τ 2 but also impart a lower hole mobility on the absorber, increasing the sensitivity of V OC to the magnitude of any back contact barriers. 23 Despite the improved V OC with continuous gold contacts, AlGaO x cannot be used directly with CFL-templated nanogrids due to the high series resistance stemming from the low in-plane conductivity of the AlGaO x layer. Au nanogrids deposited on this AlGaO x layer resulted in similarly improved V OC but a drastic reduction in fill-factor (70.2% to 41.4%) due to the introduction of additional series resistance ( Figure 4 a).…”