Get the time difference from time varaibles stored as character

I have two character variables in my dataframe start_time and stop_time:

  start_time     stop_time
        <chr>        <chr>   
1     19:5:00     19:11:00 
2    20:33:37     20:34:39
3    20:23:00     20:23:38
4    20:12:00     20:13:00
5    20:00:39     20:00:39

I would like to calculate the difference between start_time and stop_time in terms of minutes with dplyr. My problem is that the two variables are expressed as a character.

expected output:

    diff_time    
        <dbl>         
1           6     
2         1.2    
3        0.38     
4           1   
5        0.39

library(dplyr)
library(lubridate)

df1 %>% 
  mutate(duration = seconds_to_period(as.numeric(difftime(
                                            strptime(stop_time, "%H:%M:%S"), 
                                            strptime(start_time, "%H:%M:%S"), 
                             units = "secs"))))

#>   start_time stop_time duration
#> 1    19:5:00  19:11:00    6M 0S
#> 2   20:33:37  20:34:39    1M 2S
#> 3   20:23:00  20:23:38      38S
#> 4   20:12:00  20:13:00    1M 0S
#> 5   20:00:39  20:00:39       0S

Data:

read.table(text="  start_time     stop_time
19:5:00     19:11:00 
20:33:37     20:34:39
20:23:00     20:23:38
20:12:00     20:13:00
20:00:39     20:00:39", stringsAsFactors=F, header=T) -> df1