Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Abstract

kesterite material with its optimal bandgap and absorption coefficient. [2] Alkali treatment of kesterite solar cells is one of the measures to reduce the high V OC -deficit and most of today's >10% efficiency kesterite devices utilize the beneficial effects of alkali elements on absorber layer morphology and optoelectronic properties. So far the most research attention has been paid to sodium, resulting in many thorough investigations which revealed grain size enhancement, passivation of grain boundaries, and an increase in net hole concentration as the major beneficial effects of sodium treatments. [3][4][5][6] Also lithium addition has shown to improve device performance by boosting the electronic quality of the CZTSSe absorber material and grain boundaries. [7] First studies on the effect of potassium addition confirmed advantageous effects on kesterite absorber growth and optoelectronic properties similar to Na. [8,9] Several studies comparing different alkali elements and their effect on solar cell properties and device performance have recently been published. [10][11][12][13] Table 1 compares the effects on device performance by extracting a ranking of the various alkali elements in each publication in the order of their capability to improve device performance. It is apparent from these rankings that no consistent experimental results have been obtained, which triggers two questions: (i) Why do the published results differ so much? (ii) Which alkali element possesses the highest potential for efficiency improvements?This paper aims to unveil the discrepancy between the recently published results comparing the effects of alkali treatments on device performance. Our hypothesis is that each alkali element requires a different absorber composition to achieve the highest PV performance and we therefore prepared a comprehensive set of samples with different alkali elements and alkali concentrations as well as various metal ratios. All samples were thoroughly characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma-mass spectrometry (ICP-MS), as well as current-voltage (J-V), capacitance-voltage (C-V), time-resolved photoluminescence (TRPL), and external quantum efficiency (EQE) measurements.The methodology used for absorber synthesis is based on the solution process described elsewhere, [14] allowing for accurate alkali incorporation by simply adding alkali chlorides to the solution. [15] Figure 1a illustrates the matrix of sample compositions prepared with five different alkali elements: lithium (Li), Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the r...

Enhanced external quantum efficiency from Cu2ZnSn(S,Se)4 solar cells prepared from nanoparticle inks

“…[20][21][22] The small network of grain boundaries in the FG layer may contribute high series resistance and reduce the fill factor of solar cells. 11,23,24) It is difficult to draw conclusions about the total thickness of the entire film due to the combination of uncertainty in the precursor thickness (nominally 1.0 ± 0.1 µm) and surface roughness however, it can be clearly seen that as the thickness of the FG layer increases, the MoSe 2 layer becomes thinner. This is somewhat surprising as other works report an increase in the thickness of MoSe 2 with increasing selenization pressure.…”

Doping and alloying of kesterites

“…Although the results and the explanation on the effect of Li doping/alloying differ and even contrast to each other, partially due to variation in material preparation conditions and matrix composition, some conclusions can be drawn based on the available reports [23][24][25][26][27][28][29][30][31][32]: (1) Li alloys with CZTS and widens the band gap of (Li x Cu 1−x ) 2 ZnSn(S,Se) 4 [24,25,27]; (2) incorporation of Li to kesterite is sensitive to Na so that Li alloying is more easily achieved without Na, in the ceramic route [25] or on quartz substrate, or using a blocking layer to prevent Na diffusion from SLG [24,27]. The presence of Na diffused from SLG greatly reduces the Li doping concentration [23,26]; (3) Li doping/alloying improves photovoltaic performance regardless the doping concentration, however, the mechanism on how Li doping/alloying improves device performance remains unclear.…”

“…Morphology improvement and apparent carrier concentration increase from 3×10 15 cm −3 to 5×10 16 cm −3 was observed when increasing x from 0 up to 0.07, and a corresponding device had an active area efficiency of 12.2%. In a comprehensive study on the effects of all alkali elements in CZTSSe, Haass et al [28] found that Li is the most favorable among all alkalis, it improves the crystallization and it requires a comparatively high Sn/ (Cu+Zn+Sn) content for the maximum device efficiency. The apparent carrier concentration estimated from C-V measurements varied in a broad range 10 15 K10 18 cm −3 when changing (decreasing) the Sn content yet keeping the same Li content, indicating that the intrinsic doping rather prevails.…”

“…However, only a fraction of the added sodium is eventually incorporated into the processed absorber layer, e.g. adding nominally 3.33 at% NaCl resulted in only 0.18 at% in the processed absorber layer as determined by ICP-MS measurements [28]. An 11.2%-efficient CZTSSe solar cell is fabricated using NaCl treatment (figure 5) and the device features a V OC -deficit (defined as Eg/e−V OC ) of only 0.57 V, which is amongst the lowest for kesterite solar cells [41].…”

Solution-based synthesis of kesterite thin film semiconductors

“…More recently in 2018, Haas et al [49] presented a large study of over 700 CZTS devices from DMSO-TU inks, examining the effects of Li, Na, Cs, and Rb doping and the cross-correlation with the metal ratios. The same study also found that Li yielded the best devices and reported a new record of 12.3% active area PCE for hydrazine-free CZTSSe figure 4(c) [49]. Also, the DMF-TU system has been used to spray-coat germanium-alloyed CZGTSSe devices [22] with 11.0% PCE and a record high open circuit voltage (V oc ) of 583 mV for the 1.15 eV bandgap, which is 63% of the Shockley-Queisser (SQ) limit V oc .…”