  We present a sample of 21 hydrogen-free superluminous supernovae (SLSNe-I),
and one hydrogen-rich SLSN (SLSN-II) detected during the five-year Dark Energy
Survey (DES). These SNe, located in the redshift range 0.220<z<1.998, represent
the largest homogeneously-selected sample of SLSN events at high redshift. We
present the observed g,r, i, z light curves for these SNe, which we interpolate
using Gaussian Processes. The resulting light curves are analysed to determine
the luminosity function of SLSN-I, and their evolutionary timescales. The DES
SLSN-I sample significantly broadens the distribution of SLSN-I light curve
properties when combined with existing samples from the literature. We fit a
magnetar model to our SLSNe, and find that this model alone is unable to
replicate the behaviour of many of the bolometric light curves. We search the
DES SLSN-I light curves for the presence of initial peaks prior to the main
light-curve peak. Using a shock breakout model, our Monte Carlo search finds
that 3 of our 14 events with pre-max data display such initial peaks. However,
10 events show no evidence for such peaks, in some cases down to an absolute
magnitude of <-16, suggesting that such features are not ubiquitous to all
SLSN-I events. We also identify a red pre-peak feature within the light curve
of one SLSN, which is comparable to that observed within SN2018bsz.
