For more than three decades, Cu has been critical to dope CdSeTe solar cells, form effective contacts, and maximize efficiency. At the same time, Cu defect chemistry has limited stability, carrier concentration, and further efficiency improvements. In this article, 22.3% world record efficiency is demonstrated without Cu by implementing As doping, which also improves stability, temperature coefficient, and energy yield. The efficiency crossing point of Group V technology relative to Cu has been driven by steady improvements in the open-circuit voltage. Here, the certified record cell reaches open-circuit voltage of 899 mV while retaining high photocurrent values of 31.4 mA/cm 2 ; the fill factor is relatively low at 78.9%. Coupling 80% fill factor with top open-circuit voltage values of 917 mV reported here offers a near-term path to 23% efficiency. Characterization indicates reducing recombination and improving activation provide viable paths to 25% efficiency.

Since its inception, CdTe-based solar technology has been manufactured by using Cu to dope the absorber and to form effective back contacts. Removing Cu and implementing group V dopants has significantly decreased degradation in accelerated lifetime tests. We demonstrate As-doped cells are now at a crossing point with longstanding Cu-based technology, reaching 22.2%–22.4% efficiency by internal testing and 22.0% certified efficiency. Fill factor and open-circuit voltage on exemplary samples have achieved values as high as 82% or 880–895 mV, respectively. At the same time, the normalized efficiency temperature coefficient is only −0.23%/°C, offering substantively greater energy yield.

In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic chemical vapour deposition at a low temperature of 350 °C, to investigate the impact on As doping and device V OC . Device characterization showed conversion efficiency of ∼14%, and V OC of 772 mV, which is an improvement to the baseline device with CdTe:As absorber layer grown at 390 °C under non-saturated conditions. When the low temperature Cd-saturated growth was combined with chlorine heat treatment at a higher temperature of 440 °C (in contrast with the standard 420 °C) for 10 min, device efficiency improved to ∼17% with a high V OC of 877 mV. As a result, ∼100 mV boost in V OC from baseline is demonstrated with Cd-saturated CdTe:As device. Micro-photoluminescence and time-resolved photoluminescence measurements performed on these Cd-saturated CdTe:As devices confirmed that minority carrier lifetime significantly improved.

Polycrystalline CdTe thin film solar cells grown under Cd-rich conditions has been shown to be a promising strategy for further device improvement [1]. A combination of the above with group-V element doping are of interest for maximising the p-type doping concentration, without compromising minority carrier lifetime [2]. For a long time now, increasing hole concentration of >1x1016 cm-3 in thin polycrystalline CdTe films has been limiting and challenging [3]. In-situ Cd saturation growth of polycrystalline CdTe:As thin films was performed by metal organic chemical vapour deposition (MOCVD) at a low temperature of 350°C, to investigate the impact on As doping and device VOC. SIMS measurements on the Cd-rich CdTe:As layers revealed high As concentration of 1. ${15 -1.20\times 1018}$ As cm-3. Device characterisation showed PCE of ~14%, a VOC of 772 mV, with a corresponding C-V derived acceptor concentration (NA) of ${2.0\times 1016}$ cm-3. This is a small improvement to the baseline device with CdTe:As absorber layer grown at 390 oC and not under Cd saturation conditions [4]. A much larger improvement in PCE and Voc was achieved when the low temperature Cd saturation growth was combined with higher temperature chlorine heat treatment (CHT). This was achieved for these Cd-saturated CdTe:As devices by performing a post-growth CdCl2 activation process at an elevated temperature of ${440}^{\circ}\mathrm{C}$ for 10 mins compared with the standard 420 oC. The higher CHT temperature, however, made no significant improvement with the non-saturated baseline growth. An efficiency of ~17% was measured with a high VOC of 835 mV. Compared to the baseline device, the VOC has been enhanced by ~13%. Micro - photoluminescence (micro-PL) spectra measurements performed on these Cd-rich CdTe:As samples indicated improvement in minority carrier lifetimes, which was consistent with time resolved photoluminescence (TRPL) measurements confirming that carrier lifetime almost doubled. Optimisation of As doping efficiency as a function of Cd/Te precursor partial pressure ratio and its impact on device PV characteristics, especially VOC is also investigated and will be reported. The results that will be presented in this report further lend strength to the merit of Cd-saturated growth ambient, for higher efficiency polycrystalline CdTe:As thin film solar cells.

The ability to profile and understand carrier concentration throughout thin film absorbers is important to advance solar cell technology. Here, the historical impediments of grain boundary potentials, lateral resistance, and high work functions to such measurements on polycrystalline solar cells are overcome by applying electrochemical capacitance-voltage profiling across the solar cell p-n junction and an electrolyte/semiconductor junction made at the back. Despite the presence of two junctions, modeling indicates that accurate carrier concentrations at the rear of the device can be measured under certain conditions. This is validated by experiments on CdTe solar cells.

Copper-free polycrystalline CdTe solar cells doped with either Phosphorus, Arsenic or Antimony are compared for dopability, device performance and defect properties. In-situ doping of CdTe with group-V elements was carried out using vapor transport deposition followed by CdCl2 heat treatment. Uniform carrier concentration in ~1016 cm-3 range is achieved for all three dopants. Compatibility with CdSeTe ternary alloy is demonstrated and comparable quantum efficiency for all three dopants suggests that either dopant could serve as a replacement for copper. These results are compared with previously reported carrier concentration values for group-V doping of sx- and px- CdTe. Theoretical calculation for maximum carrier concentration based on acceptor energy level suggests least ionization of Antimony. Compensation worsens the achievable majority carrier concentration despite high acceptor doping.

Polycrystalline CdTe solar cells with hole carrier-density greater than 10 16 cm −3 have been realized by doping the absorber with Arsenic. The high carrier concentration requires a large excess of Arsenic in the film implying only percent level doping activation. Appearance of a sub-bandgap peak in the photoluminescence (PL) spectrum is linked with the activation of the Arsenic dopant. Low temperature PL measurements confirm an acceptor level at 90 meV from the valence band, which is consistent with reported literature values. Additional features in the PL spectra suggests the presence of sub-band defects. Understanding the origin for these features may enable the high efficiency potential of group-V doping.